Cannabinoid Receptors

Two cannabinoid receptors (CB₁ and CB₂) have been identified to date (for review see Howlett et al 2002). Additional receptor subtypes (e.g., the G-protein receptor GPR55) that share little homology with CB₁ and CB₂ have also been recently postulated to represent novel cannabinoid receptors (Lauckner et al 2008).

In the early days of cannabinoid pharmacology it was hypothesized that lipophilic cannabinoids exerted their effects by perturbing neuronal membranes in a fashion similar to a theory proposed for general anesthetics. However, the demonstration of cannabinoid receptor binding sites (labeled with [³H]CP55,940) in the brain that possessed the characteristics of a G protein–coupled receptor (Devane et al 1988) established the existence of a cannabinoid transmitter system. This discovery was followed by cloning of the CB₁ receptor from a rat cDNA library (Matsuda et al 1990). Rat CB₁ has, in common with many other G protein–coupled receptors, seven transmembrane-spanning α-helices, a C-terminal that couples to G proteins, an extracellular N-terminal, and three potential glycosylation sites. It is 473 amino acids in length and has a molecular weight of 53 kDa, although variants of 59 and 64 kDa also exist. Cloning of human CB₁ (hCB₁) (472 amino acids; Gerard et al 1991) and mouse (473 amino acids; Chakrabarti et al 1995) homologues that share close sequence homology (>97%) with rat CB₁ followed. Later, an immune cell human cannabinoid receptor (hCB₂) was identified, initially in a human promyelocytic leukemia cell line that has 44% sequence homology (68% in the transmembrane regions) with hCB₁ (Munro et al 1993). hCB₂ is also a G protein–coupled receptor but is shorter than CB₁ (360 amino acids, 40 kDa). Subsequently, the murine and rat homologues of CB₂ have been cloned and found to have 82% (Shire et al 1995) and 81% (Griffin et al 2000) sequence homology with hCB₂, respectively, and 90% homology between them. The presence of CB₂ isoforms with different distributions in different tissues and species has also been documented; in the brain, CB_2A has been reported to exist at levels of 0.1 or 1% of those expressed in the spleen (Liu et al 2009). Transgenic mice have been created in which the genes encoding CB₁ (Ledent et al 1999, Zimmer et al 1999) or CB₂ (Buckley et al 2000) have been disrupted.

Circumstantial evidence suggests the existence of further, hitherto uncharacterized, cannabinoid receptors predominantly based on residual pharmacological activity in cannabinoid receptor knockout mice or following the administration of receptor antagonists to naïve rodents (Breivogel et al 2001). An obvious strategy for identifying such receptors is to search databases for structural homology to CB₁ or CB₂. More recently, GPR55 has been identified as a putative third cannabinoid receptor, although it lacks the functional fingerprint of a cannabinoid receptor. GPR55 is highly sensitive to lysophosphatidylinositol (which does not bind cannabinoid receptors) and only some (e.g., tetrahydrocannabinol [THC], methanandamide, JWH015) cannabinoid and endocannabinoid ligands. Activation of GPR55 increases calcium and M currents, which engages signaling mechanisms distinct from CB₁ and CB₂ (Lauckner et al 2008). Intriguingly, this receptor may play a pro-nociceptive role in the nervous system inasmuch as mechanical hyperalgesia fails to develop in GPR55^−/− mice following treatment with an inflammatory agent, complete Freund’s adjuvant (CFA), or traumatic nerve injury produced by partial sciatic nerve ligation (Staton et al 2008).

Molecular and Cellular Consequences of Cannabinoid Receptor Activation

CB₁ receptors, via G_i/o proteins, influence several signal transduction pathways, including negative coupling to adenylate cyclase (AC) and activation of mitogen-activated protein kinase (MAPK) and immediate–early gene signaling pathways, including Krox-24 actions (see Howlett et al 2002). Additionally, CB₁ receptors modulate the activity of a number of ion channels via a G_i/o protein also, including negative coupling to presynaptic voltage-dependent Ca²⁺ channels (particularly L, N, and P/Q types) (Mackie et al 1993) and positive coupling to inwardly rectifying K⁺ channels (Mackie et al 1995). Thus, the net consequence of the CB₁ receptor is to augment membrane hyperpolarization and inhibit release of neurotransmitters. The conformational state of CB₁ may determine which transduction mechanism will be operative (Howlett 1998). On binding to a ligand, the CB₁ receptor–ligand complex rapidly internalizes, a process that may underlie the observed development of tolerance to cannabinoids (Coutts et al 2001, Kouznetsova et al 2002).

Activation of CB₂ receptors engages multiple signaling mechanisms, including negative coupling to AC via G_i/o and activation of extracellular signal–regulated kinase (ERK) and voltage-dependent calcium channels (for review see Atwood et al 2012). Understanding these signaling mechanisms is further complicated by the fact that different agonists can traffic through distinct signaling mechanisms (Schatz et al 1997) and differ in their capacity to produce receptor internalization (Atwood et al 2012).

Tissue Expression of Cannabinoid Receptors

Expression of the CB₁ receptor is largely restricted to neurons, and expression is high, similar to opioid receptors, at several loci important for nociceptive processing in the brain, spinal cord, and peripheral nervous system (Herkenham et al 1991, Tsou et al 1998, Egertova and Elphick 2000). Expression of CB₂ is predominantly restricted to cells of the immune system, including glia, and is highly induced by insult or injury in the central nervous system (CNS) (for review see Guindon and Hohmann 2008, Atwood and Mackie 2010).

Endocannabinoids

A number of endogenous ligands at cannabinoid receptors (“endocannabinoids”) have been identified, including anandamide (Devane et al 1992), 2-AG (Mechoulam et al 1995, Sugiura et al 1995), noladin ether (Hanus et al 2001), virodhamine (Porter et al 2002), and N-arachidonoyldopamine (Huang et al 2002). Anandamide and 2-AG remain the best studied of the endocannabinoids isolated to date. Endocannabinoid synthesis is not restricted to neurons; synthesis is also reported in immune cells, especially basophils, microglia, and macrophages (Bisogno et al 1997, De Petrocellis et al 2000, Walter et al 2003).

Plant-Derived Cannabinoids

Herbal cannabis contains a complex and variable mixture of cannabinoids, with Δ⁹- and Δ⁸-THC, cannabidiol, and cannabinol being the major pharmacologically active molecules (Gaoni and Mechoulam 1964). The high lipophilicity of plant-derived cannabinoids is associated with limited gastrointestinal absorption and bioavailability (Gaoni and Mechoulam 1964), which represents a barrier to drug development. Inhalation provides the most predictable and titratable route of administration. Oral and rectal absorption of Δ⁹-THC and other plant-derived cannabinoids tends to be unpredictable and of limited bioavailability, but the rectal route does appear to have some advantage over the oral route, at least for the Δ⁹-THC prodrug THC hemisuccinate (Walker et al 1999b). An oromucosal spray containing Δ⁹-THC and cannabidiol (Sativex) has been shown clinically to have efficacy in relieving the pain caused by multiple sclerosis (for review see Rahn and Hohmann 2009). The lipid solubility of these compounds means that they are rapidly sequestered in fatty tissue, and the elimination half-life is considerable, from days to weeks. Δ⁹-THC is a substrate for the P450 mixed-function oxidases and is biotransformed in the liver; metabolites are excreted in feces and urine as glucuronide conjugates.

Synthetic Cannabinoids

Recent advances in medicinal chemistry have yielded a number of synthetic cannabinoids with different chemical backgrounds that are broadly classified as follows:

Brain

The CB₁ receptor is widely distributed in the brain (Herkenham et al 1991). Direct evidence implicates a role for the PAG, RVM, amygdala, thalamus, and noradrenergic A5 nucleus in CB₁-mediated antinociception.

Early studies addressing how the brain mediates the antinociceptive actions of cannabinoids showed that intracerebroventricular administration of synthetic cannabinoids results in increased tail flick latency (Martin et al 1993). Microinjection experiments identified cannabinoid receptors in the PAG and dorsal raphe as the principal periventricular structures involved in cannabinoid-mediated antinociception (Martin et al 1995, Lichtman et al 1996). The nucleus reticularis gigantocellularis pars alpha of the RVM is also associated with CB₁ receptor–dependent antinociception in the formalin test (Monhemius et al 2001). Microdialysis experiments subsequently revealed that anandamide concentrations are elevated in the PAG following local electrical stimulation or subcutaneous formalin injection (Walker et al 1999a). This elevation is observed in concert with signs of CB₁ receptor–dependent antinociception. In vitro electrophysiological studies have revealed CB₁-dependent modulation of excitatory post-synaptic currents in the PAG (Vaughan et al 2000). This modulation is effected by a decrease in the probability of release of neurotransmitters from the presynaptic terminal rather than by a direct effect on the post-synaptic membrane, as is the case for opioids. Glutamate spillover also modulates synaptic transmission at the level of the PAG through an endocannabinoid mechanism (Drew et al 2008). Here, orexin also produces antinociception through endocannabinoid-dependent mechanisms likely to involve 2-AG (Ho et al 2011). Microinjection of HU210 into the PAG also suppresses formalin-evoked Fos protein expression, consistent with suppression of nociceptive processing (Finn et al 2003).

A similar CB₁-mediated effect has been shown in the RVM, where cannabinoids induce presynaptic inhibition of GABAergic neurotransmission (Vaughan et al 1999). Pharmacological inactivation of RVM neurons by microinjection of the GABA_A agonist muscimol also eliminated cannabinoid antinociception (Meng et al 1998), an effect achieved by modulation of both “off” and “on” cell activity of RVM neurons. Consistent with the descending pain modulatory effects of cannabinoids, spinal transection reduces both the antinociceptive (Lichtman and Martin 1991b) and electrophysiological (on spinal nociceptive neuron) effects of cannabinoids (Hohmann et al 1999b).

Cannabinoid modulation of noxious stimulus–evoked activity in neurons of the ventroposterolateral nucleus of the thalamus correlates with the antinociceptive but not with the motor effects of cannabinoids (Martin et al 1996). Microinjection and lesion studies have also implicated roles for regions rostral to the brain stem (i.e., the amygdala, lateral posterior and submedial regions of the thalamus, superior colliculus, and noradrenergic A5 region) in cannabinoid antinociception (Martin et al 1999a). For example, bilateral lesions of the amygdala in primates reduce both opioid and cannabinoid antinociception (Manning et al 2001). Similarly, pharmacological (muscimol) lesions of the central nucleus of the amygdala reduce the ability of systemic WIN55,212-2 to produce antinociception (tail flick, formalin tests) and suppress the accompanying spinal Fos protein (Manning et al 2003). Endocannabinoid analgesic mechanisms were discussed previously.

Spinal Cord

Electrophysiological studies indicate that spinal CB₁ receptors modulate the passage of nociceptive traffic. Walker’s group first showed that noxious thermal- and mechanical-evoked activity in spinal wide–dynamic range neurons is attenuated by systemically and intrathecally administered WIN55,212-2 (Hohmann 1995, 1998, 1999b), and systemic administration of WIN55,212-2 reduces the augmented activity (“wind-up”) in dorsal horn neurons evoked by a sustained noxious input (Strangman and Walker 1999). Chapman’s group also showed that cannabinoids, as well as endocannabinoid modulators, suppress evoked responses in spinal nociceptive neurons in rodent models of inflammation and neuropathic pain (for review see Walker and Hohmann 2005).

Evidence of spinal mechanisms of cannabinoid antinociception was first provided by intrathecal administration studies. Biodisposition studies also revealed that intrathecally administered cannabinoids tend to remain at the site of action and are not rapidly redistributed to the brain (Lichtman and Martin 1991b). Furthermore, intrathecal administration of the potent synthetic cannabinoid HU210 was more effective than morphine in attenuating formalin-induced pain behavior, and these effects could not be attributed to motor impairment (Guhring et al 2001). Intrathecal cannabinoids also reversed established mechanical hyperalgesia in the partial sciatic nerve ligation model of neuropathic pain (Fox et al 2001).

Systemic administration of Δ⁹-THC accompanied by the intrathecal administration of an α₂-noradrenergic antagonist suggested that the antinociceptive effects of cannabinoids are mediated, at least in part, by a descending noradrenergic mechanism (Lichtman and Martin 1991a). Lesions of descending noradrenergic systems also attenuated the antinociceptive effects of systemically administered cannabinoids in animal models of acute and tonic nociception, as well as the accompanying changes in Fos protein expression (Gutierrez et al 2003).

Intrathecal administration of exogenous anandamide blocks the thermal hyperalgesia associated with intraplantar carrageenan at doses that did not alter basal nociceptive thresholds (Richardson et al 1998a). In vitro superfusion studies in the rat spinal cord revealed that anandamide prevents both capsaicin- and K⁺-evoked release of CGRP (Richardson et al 1998a) whereas the CB₁ receptor antagonist SR141716A enhances capsaicin-evoked release of substance P, thus suggesting that release of this neuropeptide may be under tonic control by endocannabinoids (Lever and Malcangio 2002). However, the inverse agonist properties of SR141716A may also play a role.

Biochemical markers have also been used to detect the spinal cellular activity associated with central sensitization. Intrathecal administration of WIN55,212-2, which reversed mechanical allodynia following subcutaneous injection of CFA into the hindpaw, also prevented the associated appearance of Fos-like immunoreactivity in spinal cord neurons (Martin et al 1999b). Likewise, a reduction in dorsal horn Fos-like immunoreactivity by WIN55,212-2 and anandamide following intravesicular turpentine, injection of NGF (Farquhar-Smith et al 2002), intraplantar injection of carrageenan (Nackley et al 2003b), or subcutaneous formalin injection (Tsou et al 1996) has been demonstrated. The selective CB₂ agonist AM1241 also reduces spinal Fos protein–like immunoreactivity following carrageenan-induced inflammation (Nackley et al 2003a).

Non-neuronal cells (e.g., microglia) also play a role in the CNS responses to inflammation and peripheral nerve injury that underlie persistent pain (DeLeo and Yezierski 2001, Watkins and Maier 2003). Microglia express functional CB₂ receptors, synthesize endocannabinoids, and influence microglial migration (Walter et al 2003), and an increase in spinal CB₂ expression is observed following peripheral nerve injury, but not following an inflammatory insult (Zhang et al 2003). Effects of endocannabinoids on microglial activity may be restricted to the process of migration because microglial proliferation, nitric oxide production, and phagocytosis are unaffected. Furthermore, cannabinoids block cytokine mRNA expression in cultured microglial cells through cannabinoid receptor–independent mechanisms (Puffenbarger et al 2000). Both the CB₁- and CB₂-mediated anti-inflammatory and neuroprotective effects of cannabinoids are also mediated by release of interleukin-1 receptor antagonist, which abrogates the effects of the key inflammatory cytokine interleukin-1 released from both glia and neurons (Molina-Holgado et al 2003). Nonetheless, spinally mediated cannabinoid antinociception is largely via CB₁, although pharmacological specificity may be dependent on the model.

Peripheral Analgesic Effects

Peripheral cannabinoid antinociceptive mechanisms represent a promising target for analgesic drug development. Anti-hyperalgesic and anti-allodynic effects of locally delivered cannabinoids are observed at doses that are not systemically effective. Locally administered anandamide attenuated carrageenan-induced thermal hyperalgesia in a CB₁-dependent manner, although a CB₂-mediated component could not be excluded (Richardson et al 1998b). A peripheral CB₂-mediated suppression of mechanical and heat hypersensitivity is an observed effect in carrageenan (Nackley et al 2003a, Quartilho et al 2003) and capsaicin models of inflammatory pain (Hohmann et al 2004). Suppression of noxious stimulus–evoked Fos protein expression (Nackley et al 2003a), as well as electrophysiological measures of spinal neuronal sensitization (Elmes et al 2004, Nackley et al 2004), accompanies the anti-allodynic effects of CB₂ agonists. Local administration of various cannabinoids attenuated both phases of the behavioral response to subcutaneous formalin injection via CB₁-dependent–mediated mechanisms (Calignano et al 1998). In the partial sciatic nerve ligation model of neuropathic pain, an infusion of WIN55,212-2 (30 μg) at the site of nerve injury prevented mechanical- and cold-induced hypersensitivity in a locally mediated effect that was a CB₁ and, to a certain extent, CB₂ receptor–dependent process (Lever et al 2007). Further evidence of a peripheral analgesic effect of cannabinoids comes from experiments in which the carrageenan model of cutaneous inflammatory hyperalgesia was examined (Nackley et al 2003a, 2003b). Locally administered WIN55,212-2 attenuated the behavioral signs of inflammatory hyperalgesia and the associated spinal Fos expression in an effect mediated by both CB₁ and CB₂ receptors. The same dose of WIN55,212-2 administered systemically or into the contralateral paw was ineffective (Nackley et al 2003a).

Evidence of functionally active CB₁ receptors in the peripheral nervous system was provided by demonstrating the inhibitory effect of WIN55,212-2 on voltage-activated Ca²⁺ currents and antagonism of this effect by the CB₁ antagonist SR141716A (Ross et al 2001). Cannabinoids are also known to inhibit the capsaicin-evoked influx of Ca²⁺ into cultured dorsal root ganglion cells (Millns et al 2001). Other authors have provided data that attenuation of the K⁺-evoked influx of Ca²⁺ in dissociated rat dorsal root ganglion cells by cannabinoids (in a CB₁-dependent manner) occurs predominantly in intermediate-sized cells (somal area, 800–1500 μm²) (Khasabova et al 2002). This finding is in agreement with the expression of CB₁ in medium to large dorsal root and trigeminal ganglion cells (Hohmann and Herkenham 1999a, Bridges et al 2003, Price et al 2003). Further evidence of a peripherally mediated cannabinoid effect comes from in vitro studies in which the synthetic cannabinoid CP55,940 attenuated the capsaicin-evoked release of CRGP in skin taken from normal and diabetic (streptozocin treated) rats in a CB₁-dependent–mediated manner (Ellington et al 2002).

The literature supports the existence of both CB₁- (Agarwal et al 2007) and CB₂-mediated peripheral antinociception (for review see Guindon and Hohmann 2008). Genetic deletion of CB₁ from nociceptive neurons attenuates the antinociception produced by systemic and local but not by intrathecal cannabinoids (Agarwal et al 2007), thus suggesting that peripheral CB₁ receptors remain a viable analgesic target. Indeed, AZ11713908, a peripherally restricted CB₁ agonist (with CB₂ inverse agonist and partial agonist properties), produces CB₁-mediated anti-hyperalgesic effects in a CFA model, and these effects are fully preserved in CB₂^−/− mice (Yu et al 2010). Since the majority of CB₂ expression is on cells of immune origin, the periphery is an obvious site to investigate such effects. Agonist activity at CB₂ generally down-regulates the activity of immune cells, and this effect contributes to the observed local anti-hyperalgesic effects of cannabinoids in inflammation and following peripheral nerve injury. For instance, the suppressive effect of locally administered anandamide on the enhanced, mechanically evoked responses observed in spinal dorsal horn neurons of rodents with carrageenan-induced inflammation of the hindpaw is mediated via peripheral CB₂ receptors (Sokal et al 2003), as are the anti-hyperalgesic actions of AM1241 in inflammatory (Nackley et al 2003a, Quartilho et al 2003) and neuropathic pain (Ibrahim et al 2003). Both neuronal (via primary afferent) and non-neuronal (via mast cells, neutrophils, macrophages) mechanisms may contribute to these effects.

NGF contributes to the generation and maintenance of inflammatory hyperalgesia; NGF-induced mast cell degranulation amplifies the NGF “signal,” and cannabinoids may attenuate this effect (Rice 2001). Neutrophil migration is also a key component of the NGF-mediated component of inflammatory hyperalgesia (Bennett et al 1998). Palmitoylethanolamide (PEA), a fatty acid amide that is structurally related to anandamide but does not bind to cannabinoid receptors, decreases NGF-evoked cutaneous thermal hyperalgesia and neutrophil accumulation, as measured with the myeloperoxidase assay, by an SR144528-sensitive mechanism (Farquhar-Smith and Rice 2001). Efficacy of locally administered CB₁-selective (Gutierrez et al 2007), CB₂-selective (Quartilho et al 2003, Hohmann et al 2004, Nackley et al 2004), and mixed CB₁/CB₂ agonists (Nackley et al 2003b), as well as endocannabinoid modulators (e.g., MGL inhibitors; Guindon et al 2011), has been demonstrated (Table 38-1). A peripherally restricted FAAH inhibitor has also been shown to suppress inflammatory and neuropathic pain through CB₁ mechanisms, whereas anti-inflammatory effects involve CB₁ as well as CB₂ receptors and peroxisome proliferator–associated receptor (PPAR-γ) (Clapper et al 2010).

Efficacy

Of seven RCTs included in updates of a systematic review of cannabinoids for neuropathic pain, five examined patients with multiple sclerosis, including more than 1000 patient episodes of pain. These trials compared either various extracts of herbal cannabis or Δ⁹-THC with placebo. In four of these studies, pain relief was greater in patients randomized to cannabinoid than to placebo. Two RCTs reported responder rates (50% pain relief), and similar “numbers need to treat” (NNTs) of 3.5 (Svendsen et al 2004) and 3.7 (Rog et al 2005) were calculated.

For HIV-related peripheral neuropathy, a condition that does not respond to conventional antineuropathic pain agents such as tricyclic antidepressants (Phillips et al 2010), the evidence from RCTs also indicates that cannabis has analgesic properties. Two RCTs compared smoked cannabis with smoked placebo and demonstrated the clear efficacy of cannabis (Abrams et al 2007, Ellis et al 2009). Responder rates (NNTs [30% pain relief] of 3.6 [Abrams et al 2007] and 3.5 [Ellis et al 2009]) compared favorably with those of other agents for pain relief in this condition (Phillips et al 2010). Although smoking is unlikely to become a practical method of drug administration, these RCTs do provide encouragement for further examination of cannabinoids in patients with HIV neuropathy when administered by more acceptable methods.

For other neuropathic pain conditions, in two RCTs that evaluated 181 episodes in patients with peripheral neuropathic pain (brachial plexus avulsion and a mixture of conditions), no appreciable evidence of efficacy was found for cannabinoids (Rice and Hill 2006). Another more recently published study suggested a modest degree of analgesic efficacy when two concentrations of smoked cannabis were compared with a placebo preparation in subjects with a range of central and peripheral neuropathic pain conditions, mainly complex regional pain syndrome type 1 (Wilsey et al 2008). A small (23 participants) four-period crossover RCT examining four potencies of smoked cannabis for pain associated with peripheral nerve injury reported that only the highest potency tested (9.4% THC) had analgesic efficacy when compared with placebo, and this group also reported significant CNS-mediated adverse events (Ware et al 2010). Finally, a small RCT of oromucosal cannabis extract in patients with painful diabetic neuropathy did not demonstrate any efficacy of this preparation over placebo (Selvarajah et al 2010).

A small number of reports have assessed cannabinoids for efficacy in relieving postoperative pain; RCTs of oral THC for pain following pelvic surgery (Buggy et al 2003) and nabilone for pain after a variety of surgical procedures (Beaulieu 2006) did not demonstrate evidence of analgesic effects. One RCT examined the effect of oral cannabis extract in a complicated design of dose escalation of patient-controlled analgesia; even though there was perhaps some sign of efficacy, the study was terminated early because of a serious adverse event (Holdcroft et al 2006).

Another systematic review also identified reports of cannabis preparations evaluated in clinical trials of non-neuropathic chronic pain conditions, including cancer pain (six RCTs), rheumatoid arthritis (one), and fibromyalgia (one) (Martin-Sanchez et al 2009). However, although some evidence of modest efficacy was suggested, the review identified significant methodological flaws in the design of the studies included, and therefore further high-quality RCTs are required before evidence of efficacy can be concluded for these conditions.

In summary, there is good evidence of the analgesic efficacy of cannabinoids for multiple sclerosis and HIV-associated polyneuropathy, but not for other neuropathic pain conditions. There is little substantial evidence of cannabinoid analgesia for postoperative or non-neuropathic chronic pain. More work is necessary to determine whether adjunctive treatment with cannabinoids enhances the efficacy and reduces the side effects of other analgesics.

Harmful Effects

A systematic review assessed acute safety data in 31 reports in which cannabinoids were used for analgesia (Wang et al 2008). Using internationally accepted criteria for defining the severity of adverse events, the authors reported that although “non-serious” acute adverse events were associated with cannabinoid treatment (rate ratio, 1.86), “serious” adverse events were not associated with cannabinoid treatment in this limited, short-term data set (1.04). CNS-mediated adverse events such as dizziness (15.5%) were most frequently reported with cannabinoid therapy. The authors drew attention to the paucity of safety data related to the effects of long-term therapeutic exposure to cannabinoids. A further confounder is that many RCTs in this area included dosing regimens that permitted self-titration. Conversely, another systematic review of treatment of chronic pain with cannabis found evidence of frequent significant adverse events, particularly related to the CNS (Martin-Sanchez et al 2009). The NNH (number needed to harm) for perception-, motor-, and cognitive-related adverse events was 7, 5, and 8, respectively, which when considered in relation to the above NNTs for efficacy suggests a narrow therapeutic index. Furthermore, a small study of healthy volunteers that compared orally administered single doses of Δ⁹-THC (20 mg) with inactive placebo (5 mg diazepam) reported noteworthy evidence of CNS-mediated adverse effects associated with Δ⁹-THC treatment (Kaufmann et al 2010). Finally, implications for the development of cannabinoids as therapeutics do need to take into account the impact of cannabis intoxication on the cognitive skills required for driving motor vehicles (Asbridge et al 2012).

Implications of the Risk for Mental Illness with Cannabis Abuse

In any discussion of the evidence of the clinical potential of cannabinoids, the issue of potential long-term mental health risks associated with therapeutic cannabinoid administration requires serious consideration. There is a now a substantial and consistent epidemiological literature supporting a dose-related association between cannabis abuse and subsequent long-term risk for the development of psychotic illness or schizophrenia (see Rice 2008a, 2008b). A systematic review encompassing nine studies consistently identified such findings in studies with different methodology in different settings (Semple et al 2005). Cannabis users are two to three times more likely for serious psychotic illness to subsequently develop, including schizophrenia, than non-cannabis users are. Similar findings were reported in a subsequent systematic review (Moore et al 2010). This observation is strengthened by a clear dose-response relationship between cannabis use and schizophrenia or the development of psychotic symptoms (Zammit et al 2002, Henquet et al 2005, Di Forti et al 2009). Populations at enhanced risk can be identified; for instance, there is an 18.2% higher risk for cannabis-associated psychotic symptoms in persons who possess baseline risk factors for the development of psychosis (Henquet et al 2005). There is also a genetic element inherent in such risk: individuals who carry a functional polymorphism in the gene encoding the enzyme catechol O-methyltransferase are more likely to exhibit psychotic symptoms and to develop schizophrenia if they use cannabis (Caspi et al 2005). Polymorphisms in this same enzyme are associated with variations in pain sensitivity and risk for the development of chronic pain conditions such as temporomandibular joint dysfunction (Diatchenko et al 2005). It should be noted that many of these data have emerged largely from studies in adolescents, so the extent to which they can be generalized to the wider population and specifically to patients is unknown. Finally, a frequent criticism of such epidemiological studies is the “self-medication” hypothesis whereby individuals with early preclinical psychosis could self-medicate with cannabis—but this hypothesis has recently been refuted (Henquet et al 2005). The therapeutic trials of cannabinoids for neuropathic pain reported to date are insufficiently powered to detect such infrequent but significant adverse events. Furthermore, although some therapeutic trials have monitored patients for up to 1 year, this may be an insufficient period to detect the long-term adverse effects of cannabinoids. It is impossible to disregard the implications of these facts for cannabinoids being developed for chronic regular administration in patients with long-term conditions such as neuropathic pain.

Another important lesson regarding the danger of premature clinical introduction of therapeutic interventions that perturb the CNS endocannabinoid system is the increased risk for serious psychiatric adverse events that was revealed in the clinical development of CB₁ receptor antagonists for the treatment of obesity and other indications (Christensen et al 2007, Hill and Gorzalka 2009, Topol et al 2010). These adverse events led to withdrawal from the market and termination of phase III clinical trials of these drugs. However, these data may reveal insight into the neurobiology of depression (Hill and Gorzalka 2009). More work is necessary to determine whether peripherally restricted modulators of the endocannabinoid system would produce a more circumscribed and beneficial spectrum of therapeutic efficacy in humans than brain-penetrant therapeutic agents would.

Conclusions for Clinical Studies

Justified by a strong basic science foundation, RCTs have been conducted that reveal evidence of analgesia for certain cannabinoids/cannabis in patients with multiple sclerosis and HIV-related polyneuropathy. The short-term adverse effect profiles of the cannabinoids evaluated in these RCTs are not dramatically different from those for other systemically administered neuropathic pain therapies. However, the implications of the strong and consistent epidemiological data associating cannabis misuse with a dose-dependent long-term risk for mental illness cannot be isolated from any discussion of the therapeutic use of potent cannabinoids, especially in the context of neuropathic pain, for which regular long-term therapy is required. The precise magnitude, relevance, and implications of this risk with therapeutic cannabinoid use are currently unknown. This should be borne in mind when taking informed consent in both RCT and therapeutic settings. It would seem prudent to exclude, both from clinical therapy and from RCTs, any patients with risk factors, including genetic ones, for psychosis or schizophrenia. Furthermore, arrangements for long-term follow-up of subjects who have been treated with cannabinoids should be made to ascertain whether such adverse events are revealed years after the intervention. From the foregoing discussion it would also seem sensible to avoid drugs that target brain CB₁ receptors—at least until the issue of cannabis-induced risk for psychosis is resolved. Furthermore, preparations of centrally acting CB₁ agonists or cannabis extracts will have significant misuse potential, which has implications for drug enforcement authorities and society. Encouragingly, there are some other avenues that could circumvent brain CB₁ while retaining cannabinoid analgesia, including CB₂ agonists (Malan et al 2003, Guindon and Hohmann 2008, Anand et al 2009), endocannabinoid-degrading enzyme inhibitors (Cravatt and Lichtman 2003, Hohmann 2007, Clapper et al 2010, Long et al 2009a), PEA analogues (LoVerme et al 2005, Wallace et al 2007b), and targeting CB₁ in the peripheral nervous system (Agarwal et al 2007). All these targets are being keenly pursued in academia and the pharmaceutical industry. For example, there are preliminary case reports and open-label studies (Indraccolo and Barbieri 2010, Phan et al 2010) and unverified clinical trials (see Rice and Mackie 2001) suggesting an analgesic effect of PEA in humans. However, a recent report of a clinical trial investigating the CB₂ agonist GW842166 indicated no superiority over placebo for this compound in relieving acute pain following third molar tooth extraction, and a number of other pain-related clinical trials, particularly for osteoarthritis, have been registered as completed (http://clinicaltrials.gov/ct2/results?term-GW842166 accessed March 5, 2012). Similarly, clinical trials of the FAAH inhibitor PF-04457845, which has shown promise in preclinical pain models of osteoarthritis (Ahn et al 2011), have been completed (http://clinicaltrials.gov/ct2/show/NCT00981357 [accessed March 5, 2012] and clinical trial gov/ct2/show/NCT00836082).” PF-04457845 failed to show analgesic efficacy in patients with pain due to osteoarthritis of the knee in the most recent reporting of a completed randomized placebo- controlled trial (Huggins et al 2012). A study of Sativex for the treatment of central pain due to multiple sclerosis has recently been completed (http://clinicaltrials.gov/ct2/show/NCT01604265?term=Sativex) and its evaluation in persistent pain due to cancer is ongoing (http://clinicaltrials.gov/ct2/show/NCT01604265?term=Sativex accessed September 2012). More work is necessary to determine whether adjunctive therapies that combine endocannabinoid modulators with exisiting analgesics may enhance the analgesic efficacy of available analgesics and reduce unwanted side-effects.

The references for this chapter can be found at www.expertconsult.com.

Abadji V., Lin S., Taha G., et al. (r)-Methanandamide: a chiral novel anandamide possessing higher potency and metabolic stability. Journal of Medicinal Chemistry. 1994;37:1889–1893.

Abrams D.I., Jay C.A., Shade S.B., et al. Cannabis in painful HIV-associated sensory neuropathy: a randomized placebo-controlled trial. Neurology. 2007;68:515–521.

Agarwal N., Pacher P., Tegeder I., et al. Cannabinoids mediate analgesia largely via peripheral type 1 cannabinoid receptors in nociceptors. Nature Neuroscience. 2007;10:870–879.

Ahn K., Smith S.E., Liimatta M.B., et al. Mechanistic and pharmacological characterization of PF-04457845: a highly potent and selective fatty acid amide hydrolase inhibitor that reduces inflammatory and noninflammatory pain. Journal of Pharmacology and Experimental Therapeutics. 2011;338:114–124.

Anand P., Whiteside G., Fowler C.J., et al. Targeting CB(2) receptors and the endocannabinoid system for the treatment of pain. Brain Research Reviews. 2009;60:255–266.

Anand U., Otto W.R., Sanchez-Herrera D., et al. Cannabinoid receptor CB2 localisation and agonist-mediated inhibition of capsaicin responses in human sensory neurons. Pain. 2008;138:667–680.

Asbridge M., Hayden J.A., Cartwright J.L. Acute cannabis consumption and motor vehicle collision risk: systematic review of observational studies and meta-analysis. British Medical Journal. 2012;344:e536.

Ates M., Hamza M., Seidel K., et al. Intrathecally applied flurbiprofen produces an endocannabinoid-dependent antinociception in the rat formalin test. European Journal of Neuroscience. 2003;17:597–604.

Atwood B.K., Mackie K. CB2: a cannabinoid receptor with an identity crisis. British Journal of Pharmacology. 2010;160:467–479.

Atwood B.K., Wager-Miller J., Haskins C., et al. Functional selectivity in CB2 cannabinoid receptor signaling and regulation: implications for the therapeutic potential of CB2 ligands. Molecular Pharmacology. 2012;81:250–263.

Barth F. Cannabinoid receptor agonists and antagonists. Expert Opinion on Therapeutic Patents. 1998;8:301–313.

Bayewitch M., Avidor Reiss T., Levy R., et al. The peripheral cannabinoid receptor: adenylate cyclase inhibition and G protein coupling. FEBS Letters. 1995;375:143–147.

Beaulieu P. Effects of nabilone, a synthetic cannabinoid, on postoperative pain. Canadian Journal of Anaesthesia/Journal Canadien d’Anesthesie. 2006;53:769–775.

Beltramo M., Stella N., Calignano A., et al. Functional role of high-affinity anandamide transport, as revealed by selective inhibition. Science. 1997;277:1094–1097.

Bennett G., al Rashed S., Hoult J.R., et al. Nerve growth factor induced hyperalgesia in the rat hind paw is dependent on circulating neutrophils. Pain. 1998;77:315–322.

Bisogno T., Maurelli S., Melck D., et al. Biosynthesis, uptake, and degradation of anandamide and palmitoylethanolamide in leukocytes. Journal of Biological Chemistry. 1997;272:3315–3323.

Blankman J.L., Simon G.M., Cravatt B.F. A comprehensive profile of brain enzymes that hydrolyze the endocannabinoid 2-arachidonoylglycerol. Chemistry & Biology. 2007;14:1347–1356.

Bouaboula M., Desnoyer N., Carayon P., et al. G_i protein modulation induced by a selective inverse agonist for the peripheral cannabinoid receptor CB2: implication for intracellular signalization cross-regulation. Molecular Pharmacology. 1999;55:473–480.

Breivogel C.S., Griffin G., Di Marzo V., et al. Evidence for a new G protein–coupled cannabinoid receptor in mouse brain. Molecular Pharmacology. 2001;60:155–163.

Bridges D., Rice A.S., Egertová M., et al. Localisation of cannabinoid receptor 1 in rat dorsal root ganglion using in situ hybridisation and immunohistochemistry. Neuroscience. 2003;119:803–812.

Brooks J.W., Pryce G., Bisogno T., et al. Arvanil-induced inhibition of spasticity and persistent pain: evidence for therapeutic sites of action different from the vanilloid VR1 receptor and cannabinoid CB1/CB2 receptors. European Journal of Pharmacology. 2002;439:83–92.

Buckley N.E., McCoy K.L., Mezey E., et al. Immunomodulation by cannabinoids is absent in mice deficient for the cannabinoid CB(2) receptor. European Journal of Pharmacology. 2000;396:141–149.

Buggy D.J., Toogood L., Maric S., et al. Lack of analgesic efficacy of oral [delta]-9-tetrahydrocannabinol in postoperative pain. Pain. 2003;106:169–172.

Bujalska M., Tatarkiewicz J., de Cordé A., et al. Effect of cyclooxygenase and nitric oxide synthase inhibitors on streptozotocin-induced hyperalgesia in rats. Pharmacology. 2008;81:151–157.

Calignano A., La Rana G., Giuffrida A., et al. Control of pain initiation by endogenous cannabinoids. Nature. 1998;394:277–281.

Carlson G., Wang Y., Ali Z. Endocannabinoids facilitate the induction of LTP in the hippocampus. Nature Neuroscience. 2002;5:723–724.

Caspi A., Moffitt T.E., Cannon M., et al. Moderation of the effect of adolescent-onset cannabis use on adult psychosis by a functional polymorphism in the catechol-O-methyltransferase gene: longitudinal evidence of a gene X environment interaction. Biological Psychiatry. 2005;57:1117–1127.

Chakrabarti A., Onaivi E.S., Chaudhuri G. Cloning and sequencing of a cDNA encoding the mouse brain-type cannabinoid receptor protein. DNA Sequence: The Journal of DNA Sequencing and Mapping. 1995;5:385–388.

Christensen R., Kristensen P.K., Bartels E.M., et al. Efficacy and safety of the weight-loss drug rimonabant: a meta-analysis of randomised trials. Lancet. 2007;370:1706–1713.

Cisneros J.A., Bjorklund E., Gonzalez-Gil I., et al. Structure-activity relationship of a new series of reversible dual monoacylglycerol lipase/fatty acid amide hydrolase inhibitors. Journal of Medicinal Chemistry. 2012;55:824–836.

Clapper J.R., Moreno-Sanz G., Russo R., et al. Anandamide suppresses pain initiation through a peripheral endocannabinoid mechanism. Nature Neuroscience. 2010;13:1265–1270.

Connell K., Bolton N., Olsen D., et al. Role of the basolateral nucleus of the amygdala in endocannabinoid-mediated stress-induced analgesia. Neuroscience Letters. 2006;397:180–184.

Coutts A.A., Anavi-Goffer S., Ross R.A., et al. Agonist-induced internalisation and trafficking of cannabinoid CB1 receptors in hippocampal neurons. Journal of Neuroscience. 2001;21:2425–2433.

Cravatt B.F., Demarest K., Patricelli M.P., et al. Supersensitivity to anandamide and enhanced endogenous cannabinoid signaling in mice lacking fatty acid amide hydrolase. Proceedings of the National Academy of Sciences of the United States of America. 2001;98:9371–9376.

Cravatt B.F., Giang D.K., Mayfield S.P., et al. Molecular characterization of an enzyme that degrades neuromodulatory fatty-acid amides. Nature. 1996;384:83–87.

Cravatt B.F., Lichtman A.H. Fatty acid amide hydrolase: an emerging therapeutic target in the endocannabinoid system. Current Opinion in Chemical Biology. 2003;7:469–475.

Cravatt B.F., Saghatelian A., Hawkins E.G., et al. Functional disassociation of the central and peripheral fatty acid amide signaling systems. Proceedings of the National Academy of Sciences of the United States of America. 2004;101:10821–10826.

Curto-Reyes V., Llames S., Hidalgo A., et al. Spinal and peripheral analgesic effects of the CB2 cannabinoid receptor agonist AM1241 in two models of bone cancer–induced pain. British Journal of Pharmacology. 2010;160:561–573.

DeLeo J.A., Yezierski R.P. The role of neuroinflammation and neuroimmune activation in persistent pain. Pain. 2001;90:1–6.

De Petrocellis L., Melck D., Bisogno T., et al. Endocannabinoids and fatty acid amides in cancer, inflammation and related disorders. Chemistry and Physics of Lipids. 2000;108:191–209.

Deutsch D.G., Chin S.A. Enzymatic synthesis and degradation of anandamide, a cannabinoid receptor agonist. Biochemical Pharmacology. 1993;46:791–796.

Devane W.A., Dysarz F.A., 3rd., Johnson M.R., et al. Determination and characterization of a cannabinoid receptor in rat brain. Molecular Pharmacology. 1988;34:605–613.

Devane W.A., Hanus L., Breuer A., et al. Isolation and structure of a brain constituent that binds to the cannabinoid receptor. Science. 1992;258:1946–1949.

Diana M.A., Marty A. Endocannabinoid-mediated short-term synaptic plasticity: depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE). British Journal of Pharmacology. 2004;142:9–19.

Diatchenko L., Slade G.D., Nackley A.G., et al. Genetic basis for individual variations in pain perception and the development of a chronic pain condition. Human Molecular Genetics. 2005;14:135–143.

Di Forti M., Morgan C., Dazzan P., et al. High-potency cannabis and the risk of psychosis. British Journal of Psychiatry. 2009;195:488–491.

Di Marzo V., Fontana A., Cadas H., et al. Formation and inactivation of endogenous cannabinoid anandamide in central neurons. Nature. 1994;372:686–691.

Dinh T.P., Carpenter D., Leslie F.M., et al. Brain monoglyceride lipase participating in endocannabinoid inactivation. Proceedings of the National Academy of Sciences of the United States of America. 2002;99:10819–10824.

Dinh T.P., Kathuria S., Piomelli D. RNA interference suggests a primary role for monoacylglycerol lipase in the degradation of the endocannabinoid 2-arachidonoylglycerol. Molecular Pharmacology. 2004;66:1260–1264.

Drew G.M., Lau B.K., Vaughan C.W. Substance P drives endocannabinoid-mediated disinhibition in a midbrain descending analgesic pathway. Journal of Neuroscience. 2009;29:7220–7229.

Drew G.M., Mitchell V.A., Vaughan C.W. Glutamate spillover modulates GABAergic synaptic transmission in the rat midbrain periaqueductal grey via metabotropic glutamate receptors and endocannabinoid signaling. Journal of Neuroscience. 2008;28:808–815.

Egertová M., Cravatt B.F., Elphick M.R. Comparative analysis of fatty acid amide hydrolase and cb(1) cannabinoid receptor expression in the mouse brain: evidence of a widespread role for fatty acid amide of endocannabinoid signaling. Neuroscience. 2003;119:481–496.

Egertova M., Elphick M.R. Localisation of cannabinoid receptors in the rat brain using antibodies to the intracellular C-terminal of CB 1. Journal of Comparative Neurology. 2000;422:159–171.

Egertová M., Giang D.K., Cravatt B.F., et al. A new perspective on cannabinoid signalling: complementary localization of fatty acid amide hydrolase and the CB1 receptor in rat brain. Proceedings of the Royal Society of London. Series B, Biological Sciences. 1998;265:2081–2085.

Ellington H.C., Cotter M.A., Cameron N.E., et al. The effect of cannabinoids on capsaicin-evoked calcitonin gene–related peptide (CGRP) release from the isolated paw skin of diabetic and non-diabetic rats. Neuropharmacology. 2002;42:966–975.

Ellis R.J., Toperoff W., Vaida F., et al. Smoked medicinal cannabis for neuropathic pain in HIV: a randomized, crossover clinical trial. Neuropsychopharmacology. 2009;34:672–680.

Elmes S.J., Jhaveri M.D., Smart D., et al. Cannabinoid CB2 receptor activation inhibits mechanically evoked responses of wide dynamic range dorsal horn neurons in naive rats and in rat models of inflammatory and neuropathic pain. European Journal of Neuroscience. 2004;20:2311–2320.

Farquhar-Smith W.P., Egertova M., Bradbury E.J., et al. Cannabinoid CB(1) receptor expression in rat spinal cord. Molecular and Cellular Neurosciences. 2000;15:510–521.

Farquhar-Smith W.P., Jaggar S.I., Rice A.S. Attenuation of nerve growth factor–induced visceral hyperalgesia via cannabinoid CB(1) and CB(2)-like receptors. Pain. 2002;97:11–21.

Farquhar-Smith W.P., Rice A.S.C. Palmitoylethanolamide (PEA) attenuates NGF-induced hyperalgesia by reducing neutrophil accumulation via cannabinoid CB2-like receptors. Society of Neuroscience Abstracts. 2001;27:510–512.

Felder C.C., Briley E.M., Axelrod J., et al. Anandamide, an endogenous cannabimimetic eicosanoid, binds to the cloned human cannabinoid receptor and stimulates receptor-mediated signal transduction. Proceedings of the National Academy of Sciences of the United States of America. 1993;90:7656–7660.

Finn D.P., Jhaveri M.D., Beckett S.R., et al. Effects of direct periaqueductal grey administration of a cannabinoid receptor agonist on nociceptive and aversive responses in rats. Neuropharmacology. 2003;45:594–604.

Fowler C.J., Stenstrom A., Tiger G. Ibuprofen inhibits the metabolism of the endogenous cannabimimetic agent anandamide. Pharmacology & Toxicology. 1997;80:103–107.

Fox A., Kesingland A., Gentry C., et al. The role of central and peripheral cannabinoid1 receptors in the antihyperalgesic activity of cannabinoids in a model of neuropathic pain. Pain. 2001;92:91–100.

Galiegue S., Mary S., Marchand J., et al. Expression of central and peripheral cannabinoid receptors in human immune tissues and leukocyte subpopulations. European Journal of Biochemistry. 1995;232:54–61.

Gallant M., Dufresne C., Gareau Y., et al. New class of potent ligands for the human peripheral cannabinoid receptor. Bioorganic & Medicinal Chemistry Letters. 1996;6:2263–2268.

Gao Y., Vasilyev D.V., Goncalves M.B., et al. Loss of retrograde endocannabinoid signaling and reduced adult neurogenesis in diacylglycerol lipase knock-out mice. Journal of Neuroscience. 2010;30:2017–2024.

Gaoni Y., Mechoulam R. Isolation, structure and partial synthesis of an active constituent of hashish. Journal of the American Chemical Society. 1964;86:1946–1947.

Gauldie S.D., McQueen D.S., Pertwee R., et al. Anandamide activates peripheral nociceptors in normal and arthritic rat knee joints. British Journal of Pharmacology. 2001;132:617–621.

Gerard C.M., Mollereau C., Vassart G., et al. Molecular cloning of a human cannabinoid receptor which is also expressed in testis. Biochemical Journal. 1991;279:129–134.

Gerdeman G.L., Ronesi J., Lovinger D.M., et al. Postsynaptic endocannabinoid release is critical to long-term depression in the striatum. Nature Neuroscience. 2002;5:446–450.

Glaser S.T., Abumrad N.A., Fatade F., et al. Evidence against the presence of an anandamide transporter. Proceedings of the National Academy of Sciences of the United States of America. 2003;100:4269–4274.

Gregg L.C., Jung K.M., Spradley J.M., et al. Activation of type-5 metabotropic glutamate receptors and diacylglycerol lipase-α initiates 2-arachidonoylglycerol formation and endocannabinoid-mediated analgesia. The Journal of Neuroscience. 2012;32:9457–9468.

Griffin G., Tao Q., Abood M.E. Cloning and pharmacological characterization of the rat CB(2) cannabinoid receptor. Journal of Pharmacology and Experimental Therapeutics. 2000;292:886–894.

Guhring H., Hamza M., Sergejeva M., et al. A role for endocannabinoids in indomethacin-induced spinal antinociception. European Journal of Pharmacology. 2002;454:153–163.

Guhring H., Schuster J., Hamza M., et al. HU-210 shows higher efficacy and potency than morphine after intrathecal administration in the mouse formalin test. Eur J Pharmacol. 2001;429:127–134.

Guindon J., Beaulieu P., Hohmann A.G. The pharmacology of the cannabinoid system. In: Beaulieu P., Lussier D., Porreca F., Dickenson A.H., eds. Pharmacology of Pain. Seattle WA: IASP Press; 2010:111–138.

Guindon J., Desroches J., Beaulieu P. The antinociceptive effects of intraplantar injections of 2-arachidonoyl glycerol are mediated by cannabinoid CB2 receptors. British Journal of Pharmacology. 2007;150:693–701.

Guindon J., Guijarro A., Piomelli D., et al. Peripheral antinociceptive effects of inhibitors of monoacylglycerol lipase in a rat model of inflammatory pain. British Journal of Pharmacology. 2011;163:1464–1478.

Guindon J., Hohmann A.G. Cannabinoid CB2 receptors: a therapeutic target for the treatment of inflammatory and neuropathic pain. British Journal of Pharmacology. 2008;153:319–334.

Guindon J., Hohmann A.G. The endocannabinoid system and pain. CNS & Neurological Disorders Drug Targets. 2009;8:403–421.

Guindon J., Hohmann A.G. The endocannabinoid system and cancer: therapeutic implications. British Journal of Pharmacology. 2011;163:1447–1463.

Gutierrez T., Farthing J.N., Zvonok A.M., et al. Activation of peripheral cannabinoid CB1 and CB2 receptors suppresses the maintenance of inflammatory nociception: a comparative analysis. British Journal of Pharmacology. 2007;150:153–163.

Gutierrez T., Nackley A.G., Neely M.H., et al. Effects of neurotoxic destruction of descending noradrenergic pathways on cannabinoid antinociception in models of acute and tonic nociception. Brain Research. 2003;987:176–185.

Hanus L., Abu-Lafi S., Fride E., et al. 2-arachidonyl glyceryl ether, an endogenous agonist of the cannabinoid CB1 receptor. Proceedings of the National Academy of Sciences of the United States of America. 2001;98:3662–3665.

Hanus L., Breuer A., Tchilibon S., et al. HU 308: a specific agonist for CB(2), a peripheral cannabinoid receptor. Proceedings of the National Academy of Sciences of the United States of America. 1999;96:14228–14233.

Hasnie F.S., Breuer J., Parker S., et al. Further characterization of a rat model of varicella zoster virus–associated pain: relationship between mechanical hypersensitivity and anxiety-related behavior, and the influence of analgesic drugs. Neuroscience. 2007;144:1495–1508.

Henquet C., Krabbendam L., Spauwen J., et al. Prospective cohort study of cannabis use, predisposition for psychosis, and psychotic symptoms in young people. British Medical Journal. 2005;330:11.

Herkenham M., Lynn A.B., Johnson M.R., et al. Characterization and localization of cannabinoid receptors in rat brain: a quantitative in vitro autoradiographic study. Journal of Neuroscience. 1991;11:563–583.

Herzberg U., Eliav E., Bennett G.J., et al. The analgesic effects of R(+)-WIN 55,212-2 mesylate, a high affinity cannabinoid agonist, in a rat model of neuropathic pain. Neuroscience Letters. 1997;221:157–160.

Hill M.N., Gorzalka B.B. Impairments in endocannabinoid signaling and depressive illness. JAMA: The Journal of the American Medical Association. 2009;301:1165–1166.

Hillard C.J., Manna S., Greenberg M.J. Synthesis and characterization of potent and selective agonists of the neuronal cannabinoid receptor (CB1). J Pharmacol Exp Ther. 1999;289:1427–1433.

Ho Y.C., Lee H.J., Tung L.W., et al. Activation of orexin 1 receptors in the periaqueductal gray of male rats leads to antinociception via retrograde endocannabinoid (2-arachidonoylglycerol)-induced disinhibition. Journal of Neuroscience. 2011;31:14600–14610.

Hohmann A.G. Inhibitors of monoacylglycerol lipase as novel analgesics. British Journal of Pharmacology. 2007;150:673–675.

Hohmann A.G., Briley E.M., Herkenham M. Pre- and postsynaptic distribution of cannabinoid and mu opioid receptors in rat spinal cord. Brain Research. 1999;822:17–25.

Hohmann A.G., Farthing J.N., Zvonok A.M., et al. Selective activation of cannabinoid CB2 receptors suppresses hyperalgesia evoked by intradermal capsaicin. Journal of Pharmacology and Experimental Therapeutics. 2004;308:446–453.

Hohmann A.G., Herkenham M. Regulation of cannabinoid and mu opioid receptor binding sites following neonatal capsaicin treatment. Neuroscience Letters. 1998;252:13–16.

Hohmann A.G., Herkenham M. Localization of central cannabinoid CB1 receptor messenger RNA in neuronal subpopulations of rat dorsal root ganglia: a double-label in situ hybridization study. Neuroscience. 1999;90:923–931.

Hohmann A.G., Herkenham M. Cannabinoid receptors undergo axonal flow in sensory nerves. Neuroscience. 1999;92:1171–1175.

Hohmann A.G., Martin W.J., Tsou K., et al. Inhibition of noxious stimulus–evoked activity of spinal cord dorsal horn neurons by the cannabinoid WIN 55,212-2. Life Sciences. 1995;56:2111–2118.

Hohmann A.G., Suplita R.L., 2nd. Endocannabinoid mechanisms of pain modulation. AAPS Journal. 2006;8:E693–E708.

Hohmann A.G., Suplita R.L., Bolton N.M., et al. An endocannabinoid mechanism for stress-induced analgesia. Nature. 2005;435:1108–1112.

Hohmann A.G., Tsou K., Walker J.M. Cannabinoid modulation of wide dynamic range neurons in the lumbar dorsal horn of the rat by spinally administered WIN55,212-2. Neuroscience Letters. 1998;257:119–122.

Hohmann A.G., Tsou K., Walker J.M. Cannabinoid suppression of noxious heat–evoked activity in wide dynamic range neurons in the lumbar dorsal horn of the rat. Journal of Neurophysiology. 1999;81:575–583.

Holdcroft A., Maze M., Dore C., et al. A multicenter dose-escalation study of the analgesic and adverse effects of an oral cannabis extract (Cannador) for postoperative pain management. Anesthesiology. 2006;104:1040–1046.

Howlett A.C. The CB1 cannabinoid receptor in the brain. Neurobiology of Disease. 1998;5:405–416.

Howlett A.C., Barth F., Bonner T.I., et al. International Union of Pharmacology. XXVII. Classification of cannabinoid receptors. Pharmacological Reviews. 2002;54:161–202.

Huang S.M., Bisogno T., Trevisani M., et al. An endogenous capsaicin-like substance with high potency at recombinant and native vanilloid VR1 receptors. Proceedings of the National Academy of Sciences of the United States of America. 2002;99:8400–8405.

Huang S.M., Strangman N.M., Walker J.M. Liquid chromatographic–mass spectrometric measurement of the endogenous cannabinoid 2-arachidonylglycerol in the spinal cord and peripheral nervous system. Acta Pharmacologica Sinica. 1999;20:1098–1102.

Huffman J.W. The search for selective ligands for the CB2 receptor. Current Pharmaceutical Design. 2000;6:1323–1337.

Huggins J.P., Smart T.S., Langman S., et al. An efficient randomised, placebo-controlled clinical trial with the irreversible fatty acid amide hydrolase-1 inhibitor PF-04457845, which modulates endocannabinoids but fails to induce effective analgesia in patients with pain due to osteoarthritis of the knee. Pain. 2012;153:1837–1846.

Ibrahim M.M., Deng H., Zvonok A., et al. Activation of CB2 cannabinoid receptors by AM1241 inhibits experimental neuropathic pain: pain inhibition by receptors not present in the CNS. Proceedings of the National Academy of Sciences of the United States of America. 2003;100:10529–10533.

Indraccolo U., Barbieri F. Effect of palmitoylethanolamide-polydatin combination on chronic pelvic pain associated with endometriosis: preliminary observations. European Journal of Obstetrics, Gynecology, and Reproductive Biology. 2010;150:76–79.

Jain A.K., Ryan J.R., McMahon F.G., et al. Evaluation of intramuscular levonantradol and placebo in acute postoperative pain. Journal of Clinical Pharmacology. 1981;21(8–9 Suppl):320S–326S.

Jung K.M., Astarita G., Zhu C., et al. A key role for diacylglycerol lipase-alpha in metabotropic glutamate receptor–dependent endocannabinoid mobilization. Molecular Pharmacology. 2007;72:612–621.

Jung K.M., Mangieri R., Stapleton C., et al. Stimulation of endocannabinoid formation in brain slice cultures through activation of group I metabotropic glutamate receptors. Molecular Pharmacology. 2005;68:1196–1202.

Kaczocha M., Glaser S.T., Deutsh D.G. Identification of intracellular carriers for the endocannabinoid anandamide. Proceedings of the National Academy of Sciences USA. 2009;106:6375–6380.

Katona I., Freund T.F. Endocannabinoid signaling as a synaptic circuit breaker in neurological disease. Nature Medicine. 2008;14:923–930.

Katona I., Sperlagh B., Magloczky Z., et al. GABAergic interneurons are the targets of cannabinoid actions in the human hippocampus. Neuroscience. 2000;100:797–804.

Katona I., Urban G.M., Wallace M., et al. Molecular composition of the endocannabinoid system at glutamatergic synapses. Journal of Neuroscience. 2006;26:5628–5637.

Kaufmann R.M., Kraft B., Frey R., et al. Acute psychotropic effects of oral cannabis extract with a defined content of Δ⁹-tetrahydrocannabinol (THC) in healthy volunteers. Pharmacopsychiatry. 2010;43:24–32.

Khasabova I.A., Chandiramani A., Harding-Rose C., et al. Increasing 2-arachidonoyl glycerol signaling in the periphery attenuates mechanical hyperalgesia in a model of bone cancer pain. Pharmacological Research. 2011;64:60–67.

Khasabova I.A., Gielissen J., Chandiramani A., et al. CB1 and CB2 receptor agonists promote analgesia through synergy in a murine model of tumor pain. Behavioural Pharmacology. 2011;22:607–616.

Khasabova I.A., Harding-Rose C., Simone D.A., et al. Differential effects of CB1 and opioid agonists on two populations of adult rat dorsal root ganglion neurons. Journal of Neuroscience. 2004;24:1744–1753.

Khasabova I.A., Khasabov S.G., Harding-Rose C., et al. A decrease in anandamide signaling contributes to the maintenance of cutaneous mechanical hyperalgesia in a model of bone cancer pain. Journal of Neuroscience. 2008;28:11141–11152.

Khasabova I.A., Simone D.A., Seybold V.S. Cannabinoids attenuate depolarization-dependent Ca²⁺ influx in intermediate-size primary afferent neurons of adult rats. Neuroscience. 2002;115:613–625.

Kinsey S.G., Long J.Z., Cravatt B.F., et al. Fatty acid amide hydrolase and monoacylglycerol lipase inhibitors produce anti-allodynic effects in mice through distinct cannabinoid receptor mechanisms. Journal of Pain. 2010;11:1420–1428.

Kinsey S.G., Long J.Z., O’Neal S.T., et al. Blockade of endocannabinoid-degrading enzymes attenuates neuropathic pain. Journal of Pharmacology and Experimental Therapeutics. 2009;330:902–910.

Kouznetsova M., Kelley B., Shen M., et al. Desensitization of cannabinoid-mediated presynaptic inhibition of neurotransmission between rat hippocampal neurons in culture. Molecular Pharmacology. 2002;61:477–485.

Kreitzer A.C., Regehr W.G. Retrograde inhibition of presynaptic calcium influx by endogenous cannabinoids at excitatory synapses onto Purkinje cells. Neuron. 2001;29:717–727.

Landsman R.S., Burkey T.H., Consroe P., et al. SR141716A is an inverse agonist at the human cannabinoid CB1 receptor. European Journal of Pharmacology. 1997;334:R1–R2.

Lauckner J.E., Jensen J.B., Chen H.Y., et al. GPR55 is a cannabinoid receptor that increases intracellular calcium and inhibits M current. Proceedings of the National Academy of Sciences of the United States of America. 2008;105:2699–2704.

Ledent C., Valverde O., Cossu G., et al. Unresponsiveness to cannabinoids and reduced addictive effects of opiates in CB1 receptor knockout mice. Science. 1999;283:401–404.

Lee S.F., Newton C., Widen R., et al. Differential expression of cannabinoid CB2 receptor mRNA in mouse immune cell subpopulations and following B cell stimulation. European Journal of Pharmacology. 2001;423:235–241.

Leung D., Saghatelian A., Simon G.M., et al. Inactivation of N-acyl phosphatidylethanolamine phospholipase D reveals multiple mechanisms for the biosynthesis of endocannabinoids. Biochemistry. 2006;45:4720–4726.

Lever I.J., Malcangio M. CB1 receptor antagonist SR141716A increases capsaicin-evoked release of substance P from the adult mouse spinal cord. British Journal of Pharmacology. 2002;135:21–24.

Lever I.J., Pheby T.M., Rice A.S. Continuous infusion of the cannabinoid WIN 55,212-2 to the site of a peripheral nerve injury reduces mechanical and cold hypersensitivity. British Journal of Pharmacology. 2007;151:292–302.

Lever I.J., Robinson M., Cibelli M., et al. Localization of the endocannabinoid-degrading enzyme fatty acid amide hydrolase in rat dorsal root ganglion cells and its regulation after peripheral nerve injury. Journal of Neuroscience. 2009;29:3766–3780.

Lichtman A.H., Cook S.A., Martin B.R. Investigation of brain sites mediating cannabinoid-induced antinociception in rats: evidence supporting periaqueductal gray involvement. Journal of Pharmacology and Experimental Therapeutics. 1996;276:585–593.

Lichtman A.H., Martin B.R. Cannabinoid-induced antinociception is mediated by a spinal α₂-noradrenergic mechanism. Brain Research. 1991;559:309–314.

Lichtman A.H., Martin B.R. Spinal and supraspinal components of cannabinoid-induced antinociception. Journal of Pharmacology and Experimental Therapeutics. 1991;258:517–523.

Lichtman A.H., Shelton C.C., Advani T., et al. Mice lacking fatty acid amide hydrolase exhibit a cannabinoid receptor–mediated phenotypic hypoalgesia. Pain. 2004;109:319–327.

Liu Q.R., Pan C.H., Hishimoto A., et al. Species differences in cannabinoid receptor 2 (CNR2 gene): identification of novel human and rodent CB2 isoforms, differential tissue expression and regulation by cannabinoid receptor ligands. Genes, Brain, and Behavior. 2009;8:519–530.

Long J.Z., Li W., Booker L., et al. Selective blockade of 2-arachidonoylglycerol hydrolysis produces cannabinoid behavioral effects. Nature Chemical Biology. 2009;5:37–44.

Long J.Z., Nomura D.K., Vann R.E., et al. Dual blockade of FAAH and MAGL identifies behavioral processes regulated by endocannabinoid crosstalk in vivo. Proceedings of the National Academy of Sciences of the United States of America. 2009;106:20270–20275.

LoVerme J., La Rana G., Russo R., et al. The search for the palmitoylethanolamide receptor. Life Sciences. 2005;77:1685–1698.

Mackie K., Devane W.A., Hille B. Anandamide, an endogenous cannabinoid, inhibits calcium currents as a partial agonist in N18 neuroblastoma cells. Molecular Pharmacology. 1993;44:498–503.

Mackie K., Lai Y., Westenbroek R., et al. Cannabinoids activate an inwardly rectifying potassium conductance and inhibit Q-type calcium currents in AtT20 cells transfected with rat brain cannabinoid receptor. Journal of Neuroscience. 1995;15:6552–6561.

Mailleux P., Parmentier M., Vanderhaeghen J.J. Distribution of cannabinoid receptor messenger RNA in the human brain: an in situ hybridization histochemistry with oligonucleotides. Neuroscience Letters. 1992;143:200–204.

Malan T.P., Jr., Ibrahim M.M., Lai J., et al. CB2 cannabinoid receptor agonists: pain relief without psychoactive effects? Current Opinion in Pharmacology. 2003;3:62–67.

Manning B.H., Martin W.J., Meng I.D. The rodent amygdala contributes to the production of cannabinoid-induced antinociception. Neuroscience. 2003;120:1157–1170.

Manning B.H., Merin N.M., Meng I.D., et al. Reduction in opioid- and cannabinoid-induced antinociception in rhesus monkeys after bilateral lesions of the amygdaloid complex. Journal of Neuroscience. 2001;21:8238–8246.

Marsicano G., Goodenough S., Monory K., et al. CB1 cannabinoid receptors and on-demand defense against excitotoxicity. Science. 2003;302:84–88.

Martin W.J., Coffin P.O., Attias E., et al. Anatomical basis for cannabinoid-induced antinociception as revealed by intracerebral microinjections. Brain Research. 1999;822:237–242.

Martin W.J., Gupta N.K., Loo C.M., et al. Differential effects of neurotoxic destruction of descending noradrenergic pathways on acute and persistent nociceptive processing. Pain. 1999;80:57–65.

Martin W.J., Hohmann A.G., Walker J.M. Suppression of noxious stimulus–evoked activity in the ventral posterolateral nucleus of the thalamus by a cannabinoid agonist: correlation between electrophysiological and antinociceptive effects. Journal of Neuroscience. 1996;16:6601–6611.

Martin W.J., Lai N.K., Patrick S.L., et al. Antinociceptive actions of cannabinoids following intraventricular administration in rats. Brain Research. 1993;629:300–304.

Martin W.J., Patrick S.L., Coffin P.O., et al. An examination of the central sites of action of cannabinoid-induced antinociception in the rat. Life Sciences. 1995;56:2103–2109.

Martin-Sanchez E., Furukawa T.A., Taylor J., et al. Systematic review and meta-analysis of cannabis treatment for chronic pain. Pain Medicine. 2009;10:1353–1368.

Matsuda L.A., Bonner T.I., Lolait S.J. Localization of cannabinoid receptor mRNA in rat brain. Journal of Comparative Neurology. 1993;327:535–550.

Matsuda L.A., Lolait S.J., Brownstein M.J., et al. Structure of a cannabinoid receptor and functional expression of the cloned cDNA. Nature. 1990;346:561–564.

Mechoulam R., Ben Shabat S., Hanus L., et al. Identification of an endogenous 2-monoglyceride, present in canine gut, that binds to cannabinoid receptors. Biochemical Pharmacology. 1995;50:83–90.

Melck D., Bisogno T., De Petrocellis L., et al. Unsaturated long chain N-acyl-vanillyl-amides (N-AVAMs): vanilloid receptor ligands that inhibit anandamide-facilitated transport and bind to CB 1 cannabinoid receptors. Biochemical and Biophysical Research Communications. 1999;262:275–284.

Meng I.D., Manning B.H., Martin W.J., et al. An analgesia circuit activated by cannabinoids. Nature. 1998;395:381–383.

Millns P.J., Chapman V., Kendall D.A. Cannabinoid inhibition of the capsaicin-induced calcium response in rat dorsal root ganglion neurones. British Journal of Pharmacology. 2001;132:969–971.

Mitrirattanakul S., Ramakul N., Guerrero A.V., et al. Site-specific increases in peripheral cannabinoid receptors and their endogenous ligands in a model of neuropathic pain. Pain. 2006;126:102–114.

Molina-Holgado F., Pinteaux E., Moore J.D., et al. Endogenous interleukin-1 receptor antagonist mediates anti-inflammatory and neuroprotective actions of cannabinoids in neurons and glia. Journal of Neuroscience. 2003;23:6470–6474.

Monhemius R., Azami J., Green D.L., et al. CB1 receptor mediated analgesia from the nucleus reticularis gigantocellularis pars alpha is activated in an animal model of neuropathic pain. Brain Research. 2001;908:67–74.

Monory K., Massa F., Egertova M., et al. The endocannabinoid system controls key epileptogenic circuits in the hippocampus. Neuron. 2006;51:455–466.

Moore R.A., Derry S., McQuay H.J., et al. Clinical effectiveness: an approach to clinical trial design more relevant to clinical practice, acknowledging the importance of individual differences. Pain. 2010;149:173–176.

Munro S., Thomas K.L., Abu-Shaar M. Molecular characterization of a peripheral receptor for cannabinoids. Nature. 1993;365:61–65.

Nackley A.G., Makriyannis A., Hohmann A.G. Selective activation of cannabinoid CB2 receptors suppresses spinal fos protein expression and pain behavior in a rat model of inflammation. Neuroscience. 2003;119:747–757.

Nackley A.G., Suplita I.I., Hohmann A.G. A peripheral cannabinoid mechanism suppresses spinal fos protein expression and pain behavior in a rat model of inflammation. Neuroscience. 2003;117:659–670.

Nackley A.G., Zvonok A.M., Makriyannis A., et al. Activation of cannabinoid CB2 receptors suppresses C-fiber responses and windup in spinal wide dynamic range neurons in the absence and presence of inflammation. Journal of Neurophysiology. 2004;92:3562–3574.

Nomura D.K., Morrison B.E., Blankman J.L., et al. Endocannabinoid hydrolysis generates brain prostaglandins that promote neuroinflammation. Science. 2011;334:809–813.

Nyilas R., Gregg L.C., Mackie K., et al. Molecular architecture of endocannabinoid signaling at nociceptive synapses mediating analgesia. European Journal of Neuroscience. 2009;29:1964–1978.

Ohno-Shosaku T., Maejima T., Kano M. Endogenous cannabinoids mediate retrograde signals from depolarized postsynaptic neurons to presynaptic terminals. Neuron. 2001;29:729–738.

Olango W.M., Roche M., Ford G.K., et al. The endocannabinoid system in the rat dorsolateral periaqueductal grey mediates fear-conditioned analgesia and controls fear expression in the presence of nociceptive tone. British Journal of Pharmacology. 2012;165:2549–2560.

Pascual D., Goicoechea C., Suardâiaz M., et al. A cannabinoid agonist, WIN 55,212-2, reduces neuropathic nociception induced by paclitaxel in rats. Pain. 2005;118:23–34.

Pernia-Andrade A.J., Kato A., Witschi R., et al. Spinal endocannabinoids and CB1 receptors mediate C-fiber–induced heterosynaptic pain sensitization. Science. 2009;325:760–764.

Phan N.Q., Siepmann D., Gralow I., et al. Adjuvant topical therapy with a cannabinoid receptor agonist in facial postherpetic neuralgia. Journal der Deutschen Dermatologischen Gesellschaft. 2010;8:88–91.

Phillips T.J.C., Cherry C.L., Cox S., et al. A metaanalysis of randomised controlled clinical trials assessing the effectiveness of pharmacological interventions for painful HIV-associated sensory neuropathy. European Journal of Pain Supplements. 4, 2010. 141–141

Piomelli D. The molecular logic of endocannabinoid signalling. Nature Reviews. Neuroscience. 2003;4:873–884.

Porter A.C., Sauer J.M., Knierman M.D., et al. Characterization of a novel endocannabinoid, virodhamine, with antagonist activity at the CB1 receptor. Journal of Pharmacology and Experimental Therapeutics. 2002;301:1020–1024.

Price T.J., Helesic G., Parghi D., et al. The neuronal distribution of cannabinoid receptor type 1 in the trigeminal ganglion of the rat. Neuroscience. 2003;120:155–162.

Puffenbarger R., Boothe A.C., Cabral G.A. Cannabinoids inhibit LPS-inducible cytokine mRNA expression in rat microglial cells. Glia. 2000;29:58–69.

Quartilho A., Mata H.P., Ibrahim M.M., et al. Inhibition of inflammatory hyperalgesia by activation of peripheral CB2 cannabinoid receptors. Anesthesiology. 2003;99:955–960.

Racz I., Nadal X., Alferink J., et al. Crucial role of CB(2) cannabinoid receptor in the regulation of central immune responses during neuropathic pain. Journal of Neuroscience. 2008;28:12125–12135.

Racz I., Nadal X., Alferink J., et al. Interferon-gamma is a critical modulator of CB(2) cannabinoid receptor signaling during neuropathic pain. Journal of Neuroscience. 2008;28:12136–12145.

Rahn E.J., Hohmann A.G. Cannabinoids as pharmacotherapies for neuropathic pain: from the bench to the bedside. Neurotherapeutics. 2009;6:713–737.

Rahn E.J., Makriyannis A., Hohmann A.G. Activation of cannabinoid CB1 and CB2 receptors suppresses neuropathic nociception evoked by the chemotherapeutic agent vincristine in rats. British Journal of Pharmacology. 2007;152:765–777.

Rahn E.J., Zvonok A.M., Thakur G.A., et al. Selective activation of cannabinoid CB2 receptors suppresses neuropathic nociception induced by treatment with the chemotherapeutic agent paclitaxel in rats. Journal of Pharmacology and Experimental Therapeutics. 2008;327:584–591.

Rice A.S., Farquhar-Smith W.P., Nagy I. Endocannabinoids and pain: spinal and peripheral analgesia in inflammation and neuropathy. Prostaglandins, Leukotrienes, and Essential Fatty Acids. 2002;66:243–256.

Rice A.S.C. Mechanisms of inflammatory pain: role of neurotrophins and cannabinoids. In: Soulsby L., Morton D., eds. Pain: its nature and management in man and animals. London: Royal Society of Medicine Press; 2001:35–45.

Rice A.S.C., Cannabinoids for neuropathic pain? Where next? Neuropathic Pain 2008;11:3–6. www neupsig.org

Rice A.S.C. Should cannabinoids be used as analgesics for neuropathic pain? Nature Clinical Practice Neurology. 2008;4:654–655.

Rice A.S.C., Mackie K. Analgesics: cannabinoids. In: Evers A.S., Maze M., eds. Anesthetic pharmacology: physiologic principles and clinical practise. St Louis: Harcourt Health Sciences, 2001.

Richardson J.D., Aanonsen L., Hargreaves K.M. Antihyperalgesic effects of spinal cannabinoids. European Journal of Pharmacology. 1998;345:145–153.

Richardson J.D., Kilo S., Hargreaves K.M. Cannabinoids reduce hyperalgesia and inflammation via interaction with peripheral CB1 receptors. Pain. 1998;75:111–119.

Rinaldi-Carmona M., Barth F., Heaulme M., et al. SR141716A, a potent and selective antagonist of the brain cannabinoid receptor. FEBS Letters. 1994;350:240–244.

Rinaldi-Carmona M., Barth F., Millan J., et al. SR 144528, the first potent and selective antagonist of the CB2 cannabinoid receptor. Journal of Pharmacology and Experimental Therapeutics. 1998;284:644–650.

Rog D.J., Nurmikko T.J., Friede T., et al. Randomized, controlled trial of cannabis-based medicine in central pain in multiple sclerosis. Neurology. 2005;65:812–819.

Ross R.A., Brockie H.C., Stevenson L.A., et al. Agonist–inverse agonist characterization at CB1 and CB2 cannabinoid receptors of L759633, L759656, and AM630. British Journal of Pharmacology. 1999;126:665–672.

Ross R.A., Coutt A.A., McFarlane S.M., et al. Actions of cannabinoid receptor ligands on rat cultured sensory neurones: implications for antinociception. Neuropharmacology. 2001;40:221–232.

Samson M.T., Small-Howard A., Shimoda L.M.N., et al. Differential roles of CB1 and CB2 cannabinoid receptors in mast cells. Journal of Immunology. 2003;170:4953–4962.

Sanudo-Pena M.C., Strangman N.M., Mackie K., et al. CB1 receptor localization in rat spinal cord and roots, dorsal root ganglion and peripheral nerve. Acta Pharmacologica Sinica. 1999;20:1115–1120.

Schatz A.R., Lee M., Condie R.B., et al. Cannabinoid receptors CB1 and CB2: a characterization of expression and adenylate cyclase modulation within the immune system. Toxicology and Applied Pharmacology. 1997;142:278–287.

Schlosburg J.E., Blankman J.L., Long J.Z., et al. Chronic monoacylglycerol lipase blockade causes functional antagonism of the endocannabinoid system. Nature Neuroscience. 2010;13:1113–1119.

Schuelert N., Zhang C., Mogg A.J., et al. Paradoxical effects of the cannabinoid CB2 receptor agonist GW405833 on rat osteoarthritic knee joint pain. Osteoarthritis and Cartilage. 2010;18:1536–1543.

Sciolino N.R., Zhou W., Hohmann A.G. Enhancement of endocannabinoid signaling with JZL184, an inhibitor of the 2-arachidonoylglycerol hydrolyzing enzyme monoacylglycerol lipase, produces anxiolytic effects under conditions of high environmental aversiveness in rats. Pharmacological Research. 2011;64:226–234.

Seidel K., Hamza M., Ates M., et al. Flurbiprofen inhibits capsaicin induced calcitonin gene related peptide release from rat spinal cord via an endocannabinoid dependent mechanism. Neuroscience Letters. 2003;338:99–102.

Selvarajah D., Gandhi R., Emery C.J., et al. Randomized placebo-controlled double-blind clinical trial of cannabis-based medicinal product (Sativex) in painful diabetic neuropathy. Diabetes Care. 2010;33:128–130.

Semple D.M., McIntosh A.M., Lawrie S.M. Cannabis as a risk factor for psychosis: systematic review. Journal of Psychopharmacology. 2005;19:187–194.

Shen M., Piser T.M., Seybold V.S., et al. Cannabinoid receptor agonists inhibit glutamatergic synaptic transmission in rat hippocampal cultures. Journal of Neuroscience. 1996;16:4322–4334.

Shire D., Carillon C., Kaghad M., et al, An amino-terminal variant of the central cannabinoid receptor resulting from alternative splicing. [published erratum appears in J Biol Chem 1996 Dec 27;271(52):33706]. Journal of Biological Chemistry. 1995;270:3726–3731.

Smart D., Gunthorpe M.J., Jerman J.C., et al. The endogenous lipid anandamide is a full agonist at the human vanilloid receptor (hVR1). British Journal of Pharmacology. 2000;129:227–230.

Sokal D.M., Elmes S.J.R., Kendall D.A., et al. Intraplantar injection of anandamide inhibits mechanically-evoked responses of spinal neurons via activation of CB2 receptors in anesthetized rats. Neuropharmacology. 2003;45:404–411.

Spradley J.M., Guindon J., Hohmann A.G. Inhibitors of monoacylglycerol lipase, fatty-acid amide hydrolase and endocannabinoid transport differentially suppress capsaicin-induced behavioral sensitization through peripheral endocannabinoid mechanisms. Pharmacological Research. 2010;62:249–258.

Staton P.C., Hatcher J.P., Walker D.J., et al. The putative cannabinoid receptor GPR55 plays a role in mechanical hyperalgesia associated with inflammatory and neuropathic pain. Pain. 2008;139:225–236.

Stella N., Schweitzer P., Piomelli D. A second endogenous cannabinoid that modulates long-term potentiation. Nature. 1997;388:773–778.

Strangman N.M., Walker J.M. The cannabinoid WIN 55,212-2 inhibits the activity-dependent facilitation of spinal nociceptive responses. Journal of Neurophysiology. 1999;81:472–477.

Sugiura T., Kondo S., Kishimoto S., et al. Evidence that 2-arachidonoylglycerol but not N-palmitoylethanolamine or anandamide is the physiological ligand for the cannabinoid CB2 receptor. Comparison of the agonistic activities of various cannabinoid receptor ligands in HL-60 cells. Journal of Biological Chemistry. 2000;275:605–612.

Sugiura T., Kondo S., Sukagawa A., et al. 2-Arachidonoylgylcerol: a possible endogenous cannabinoid receptor ligand in brain. Biochemical and Biophysical Research Communications. 1995;215:89–97.

Suplita R.L., 2nd., Farthing J.N., Gutierrez T., et al. Inhibition of fatty-acid amide hydrolase enhances cannabinoid stress-induced analgesia: sites of action in the dorsolateral periaqueductal gray and rostral ventromedial medulla. Neuropharmacology. 2005;49:1201–1209.

Svendsen K.B., Jensen T.S., Bach F.W. Does the cannabinoid dronabinol reduce central pain in multiple sclerosis? Randomised double blind placebo controlled crossover trial. British Medical Journal. 2004;329:253–257.

Tam J., Vemuri V.K., Liu J., et al. Peripheral CB1 cannabinoid receptor blockade improves cardiometabolic risk in mouse models of obesity. Journal of Clinical Investigation. 2010;120:2953–2966.

Tanimura A., Yamazaki M., Hashimotodani Y., et al. The endocannabinoid 2-arachidonoylglycerol produced by diacylglycerol lipase alpha mediates retrograde suppression of synaptic transmission. Neuron. 2010;65:320–327.

Topol E.J., Bousser M.G., Fox K.A., et al. Rimonabant for prevention of cardiovascular events (CRESCENDO): a randomised, multicentre, placebo-controlled trial. Lancet. 2010;376:517–523.

Tsou K., Brown S., Sañuedo-Peña M.C., et al. Immunohistochemical distribution of cannabinoid CB1 receptors in the rat central nervous system. Neuroscience. 1998;83:393–411.

Tsou K., Lowitz K.A., Hohmann A.G., et al. Suppression of noxious stimulus–evoked expression of FOS protein–like immunoreactivity in rat spinal cord by a selective cannabinoid agonist. Neuroscience. 1996;70:791–798.

Valenzano K.J., Tafesse L., Lee G., et al. Pharmacological and pharmacokinetic characterization of the cannabinoid receptor 2 agonist, GW405833, utilizing rodent models of acute and chronic pain, anxiety, ataxia and catalepsy. Neuropharmacology. 2005;48:658–672.

Van Sickle M.D., Duncan M., Kingsley P.J., et al. Identification and functional characterization of brainstem cannabinoid CB2 receptors. Science. 2005;310:329–332.

Vaughan C.W., Connor M., Bagley E.E., et al. Actions of cannabinoids on membrane properties and synaptic transmission in rat periaqueductal gray neurons in vitro. Molecular Pharmacology. 2000;57:288–295.

Vaughan C.W., McGregor I.S., Christie M.J. Cannabinoid receptor activation inhibits GABAergic neurotransmission in rostral ventromedial medulla neurons in vitro. British Journal of Pharmacology. 1999;127:935–940.

Vera G., Chiarlone A., Cabezos P.A., et al. WIN 55,212-2 prevents mechanical allodynia but not alterations in feeding behaviour induced by chronic cisplatin in the rat. Life Sciences. 2007;81:468–479.

Walker J.M., Hohmann A.G. Cannabinoid mechanisms of pain suppression. Handbook of Experimental Pharmacology. 2005;168:509–554.

Walker J.M., Huang S.M., Strangman N.M., et al. Pain modulation by release of the endogenous cannabinoid anandamide. Proceedings of the National Academy of Sciences of the United States of America. 1999;96:12198–12203.

Walker L.A., Harland E.C., Best A.M., et al. Delta 9 THC hemisuccinate in suppository form and an alternative to oral and smoked THC. In: Nahas G.G., Sutin K.M., Harvey D.J., et al, eds. Marihuana and medicine. Totowa, NJ: Humana Press; 1999:123–135.

Wallace V.C., Blackbeard J., Segerdahl A.R., et al. Characterization of rodent models of HIV-gp120 and anti-retroviral–associated neuropathic pain. Brain. 2007;130:2688–2702.

Wallace V.C., Cottrell D.F., Brophy P.J., et al. Focal lysolecithin-induced demyelination of peripheral afferents results in neuropathic pain behavior that is attenuated by cannabinoids. Journal of Neuroscience. 2003;23:3221–3233.

Wallace V.C., Segerdahl A.R., Lambert D.M., et al. The effect of the palmitoylethanolamide analogue, palmitoylallylamide (L-29) on pain behaviour in rodent models of neuropathy. British Journal of Pharmacology. 2007;151:1117–1128.

Walter L., Franklin A., Witting A., et al. Nonpsychotropic cannabinoid receptors regulate microglial cell migration. Journal of Neuroscience. 2003;23:398–1405.

Walter L.A., Franklin A., Myhre A., et al. Microglial cells produce endocannabinoids and express functional cannabinoid receptors. Society of Neuroscience Abstracts. 634:616, 2001.

Wang T., Collet J.-P., Shapiro S., et al. Adverse effects of medical cannabinoids: a systematic review. CMAJ: Canadian Medical Association Journal. 2008;178:1669–1678.

Ware M.A., Wang T., Shapiro S., et al. Smoked cannabis for chronic neuropathic pain: a randomized controlled trial. CMAJ: Canadian Medical Association Journal. 2010;182:E694–E701.

Watanabe H., Vriens J., Prenen J., et al. Anandamide and arachidonic acid use epoxyeicosatrienoic acids to activate TRPV4 channels. Nature. 2003;424:434–438.

Watkins L.R., Maier S.F. Glia: a novel drug discovery target for clinical pain. Nature Reviews Drug Discovery. 2003;2:973–985.

Wilsey B., Marcotte T., Tsodikov A., et al. A randomized, placebo-controlled, crossover trial of cannabis cigarettes in neuropathic pain. Journal of Pain. 2008;9:506–521.

Wilson R.I., Nicoll R.A. Endogenous cannabinoids mediate retrograde signalling at hippocampal synapses. Nature. 2001;410:588–592.

Yoshida T., Hashimoto K., Zimmer A., et al. The cannabinoid CB1 receptor mediates retrograde signals for depolarisation-induced suppression of inhibition in cerebellar Purkinje cells. Journal of Neuroscience. 2002;22:1690–1697.

Yu X.H., Cao C.Q., Martino G., et al. A peripherally restricted cannabinoid receptor agonist produces robust anti-nociceptive effects in rodent models of inflammatory and neuropathic pain. Pain. 2010;151:337–344.

Zammit S., Allebeck P., Andreasson S., et al. Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study. British Medical Journal. 2002;325:1199–2005.

Zhang J., Hoffert C., Vu H.K., et al. Induction of CB2 receptor expression in the rat spinal cord of neuropathic but not inflammatory chronic pain models. European Journal of Neuroscience. 2003;17:2750–2754.

Zimmer A., Zimmer A.M., Hohmann A.G., et al. Increased mortality, hypoactivity, and hypoalgesia in cannabinoid CB1 receptor knockout mice. Proceedings of the National Academy of Sciences of the United States of America. 1999;96:5780–5785.

Abrams D.I., Jay C.A., Shade S.B., et al. Cannabis in painful HIV-associated sensory neuropathy: a randomized placebo-controlled trial. Neurology. 2007;68:515–521.

Agarwal N., Pacher P., Tegeder I., et al. Cannabinoids mediate analgesia largely via peripheral type 1 cannabinoid receptors in nociceptors. Nature Neuroscience. 2007;10:870–879.

Bridges D., Rice A.S., Egertová M., et al. Localisation of cannabinoid receptor 1 in rat dorsal root ganglion using in situ hybridisation and immunohistochemistry. Neuroscience. 2003;119:803–812.

Calignano A., La Rana G., Giuffrida A., et al. Control of pain initiation by endogenous cannabinoids. Nature. 1998;394:277–281.

Caspi A., Moffitt T.E., Cannon M., et al. Moderation of the effect of adolescent-onset cannabis use on adult psychosis by a functional polymorphism in the catechol-O-methyltransferase gene: longitudinal evidence of a gene X environment interaction. Biological Psychiatry. 2005;57:1117–1127.

Clapper J.R., Moreno-Sanz G., Russo R., et al. Anandamide suppresses pain initiation through a peripheral endocannabinoid mechanism. Nature Neuroscience. 2010;13:1265–1270.

Cravatt B.F., Demarest K., Patricelli M.P., et al. Supersensitivity to anandamide and enhanced endogenous cannabinoid signaling in mice lacking fatty acid amide hydrolase. Proceedings of the National Academy of Sciences of the United States of America. 2001;98:9371–9376.

Cravatt B.F., Saghatelian A., Hawkins E.G., et al. Functional disassociation of the central and peripheral fatty acid amide signaling systems. Proceedings of the National Academy of Sciences of the United States of America. 2004;101:10821–10826.

Diatchenko L., Slade G.D., Nackley A.G., et al. Genetic basis for individual variations in pain perception and the development of a chronic pain condition. Human Molecular Genetics. 2005;14:135–143.

Dinh T.P., Carpenter D., Leslie F.M., et al. Brain monoglyceride lipase participating in endocannabinoid inactivation. Proceedings of the National Academy of Sciences of the United States of America. 2002;99:10819–10824.

Fox A., Kesingland A., Gentry C., et al. The role of central and peripheral cannabinoid-1 receptors in the antihyperalgesic activity of cannabinoids in a model of neuropathic pain. Pain. 2001;92:91–100.

Gregg L.C., Jung K.M., Spradley J.M., et al. Activation of type-5 metabotropic glutamate receptors and diacylglycerol lipase-α initiates 2-arachidonoylglycerol formation and endocannabinoid-mediated analgesia. The Journal of Neuroscience. 2012;32:9457–9468.

Gao Y., Vasilyev D.V., Goncalves M.B., et al. Loss of retrograde endocannabinoid signaling and reduced adult neurogenesis in diacylglycerol lipase knock-out mice. Journal of Neuroscience. 2010;30:2017–2024.

Guindon J., Guijarro A., Piomelli D., et al. Peripheral antinociceptive effects of inhibitors of monoacylglycerol lipase in a rat model of inflammatory pain. British Journal of Pharmacology. 2011;163:1464–1478.

Guindon J., Hohmann A.G. Cannabinoid CB2 receptors: a therapeutic target for the treatment of inflammatory and neuropathic pain. British Journal of Pharmacology. 2008;153:319–334.

Guindon J., Hohmann A.G. The endocannabinoid system and pain. CNS & Neurological Disorders Drug Targets. 2009;8:403–421.

Guindon J., Hohmann A.G. The endocannabinoid system and cancer: therapeutic implications. British Journal of Pharmacology. 2011;163:1447–1463.

Hasnie F.S., Breuer J., Parker S., et al. Further characterization of a rat model of varicella zoster virus–associated pain: relationship between mechanical hypersensitivity and anxiety-related behavior, and the influence of analgesic drugs. Neuroscience. 2007;144:1495–1508.

Herkenham M., Lynn A.B., Johnson M.R., et al. Characterization and localization of cannabinoid receptors in rat brain: a quantitative in vitro autoradiographic study. Journal of Neuroscience. 1991;11:563–583.

Herzberg U., Eliav E., Bennett G.J., et al. The analgesic effects of R(+)-WIN 55,212-2 mesylate, a high affinity cannabinoid agonist, in a rat model of neuropathic pain. Neuroscience Letters. 1997;221:157–160.

Hill M.N., Gorzalka B.B. Impairments in endocannabinoid signaling and depressive illness. JAMA: The Journal of the American Medical Association. 2009;301:1165–1166.

Hohmann A.G., Suplita R.L., Bolton N.M., et al. An endocannabinoid mechanism for stress-induced analgesia. Nature. 2005;435:1108–1112.

Ibrahim M.M., Deng H., Zvonok A., et al. Activation of CB2 cannabinoid receptors by AM1241 inhibits experimental neuropathic pain: pain inhibition by receptors not present in the CNS. Proceedings of the National Academy of Sciences of the United States of America. 2003;100:10529–10533.

Jung K.M., Astarita G., Zhu C., et al. A key role for diacylglycerol lipase-alpha in metabotropic glutamate receptor–dependent endocannabinoid mobilization. Molecular Pharmacology. 2007;72:612–621.

Katona I., Freund T.F. Endocannabinoid signaling as a synaptic circuit breaker in neurological disease. Nature Medicine. 2008;14:923–930.

Khasabova I.A., Khasabov S.G., Harding-Rose C., et al. A decrease in anandamide signaling contributes to the maintenance of cutaneous mechanical hyperalgesia in a model of bone cancer pain. Journal of Neuroscience. 2008;28:11141–11152.

Kinsey S.G., Long J.Z., Cravatt B.F., et al. Fatty acid amide hydrolase and monoacylglycerol lipase inhibitors produce anti-allodynic effects in mice through distinct cannabinoid receptor mechanisms. Journal of Pain. 2010;11:1420–1428.

Lever I.J., Robinson M., Cibelli M., et al. Localization of the endocannabinoid-degrading enzyme fatty acid amide hydrolase in rat dorsal root ganglion cells and its regulation after peripheral nerve injury. Journal of Neuroscience. 2009;29:3766–3780.

Martin W.J., Hohmann A.G., Walker J.M. Suppression of noxious stimulus–evoked activity in the ventral posterolateral nucleus of the thalamus by a cannabinoid agonist: correlation between electrophysiological and antinociceptive effects. Journal of Neuroscience. 1996;16:6601–6611.

Meng I.D., Manning B.H., Martin W.J., et al. An analgesia circuit activated by cannabinoids. Nature. 1998;395:381–383.

Mitrirattanakul S., Ramakul N., Guerrero A.V., et al. Site-specific increases in peripheral cannabinoid receptors and their endogenous ligands in a model of neuropathic pain. Pain. 2006;126:102–114.

Nackley A.G., Makriyannis A., Hohmann A.G. Selective activation of cannabinoid CB2 receptors suppresses spinal fos protein expression and pain behavior in a rat model of inflammation. Neuroscience. 2003;119:747–757.

Nyilas R., Gregg L.C., Mackie K., et al. Molecular architecture of endocannabinoid signaling at nociceptive synapses mediating analgesia. European Journal of Neuroscience. 2009;29:1964–1978.

Olango W.M., Roche M., Ford G.K., et al. The endocannabinoid system in the rat dorsolateral periaqueductal grey mediates fear-conditioned analgesia and controls fear expression in the presence of nociceptive tone. British Journal of Pharmacology. 2012;165:2549–2560.

Phillips T.J.C., Cherry C.L., Cox S., et al. A metaanalysis of randomised controlled clinical trials assessing the effectiveness of pharmacological interventions for painful HIV-associated sensory neuropathy. European Journal of Pain Supplements. 4, 2010. 141–141

Racz I., Nadal X., Alferink J., et al. Crucial role of CB(2) cannabinoid receptor in the regulation of central immune responses during neuropathic pain. Journal of Neuroscience. 2008;28:12125–12135.

Racz I., Nadal X., Alferink J., et al. Interferon-gamma is a critical modulator of CB(2) cannabinoid receptor signaling during neuropathic pain. Journal of Neuroscience. 2008;28:12136–12145.

Rahn E.J., Hohmann A.G. Cannabinoids as pharmacotherapies for neuropathic pain: from the bench to the bedside. Neurotherapeutics. 2009;6:713–737.

Rice A.S., Farquhar-Smith W.P., Nagy I. Endocannabinoids and pain: spinal and peripheral analgesia in inflammation and neuropathy. Prostaglandins, Leukotrienes, and Essential Fatty Acids. 2002;66:243–256.

Rog D.J., Nurmikko T.J., Friede T., et al. Randomized, controlled trial of cannabis-based medicine in central pain in multiple sclerosis. Neurology. 2005;65:812–819.

Schlosburg J.E., Blankman J.L., Long J.Z., et al. Chronic monoacylglycerol lipase blockade causes functional antagonism of the endocannabinoid system. Nature Neuroscience. 2010;13:1113–1119.

Spradley J.M., Guindon J., Hohmann A.G. Inhibitors of monoacylglycerol lipase, fatty-acid amide hydrolase and endocannabinoid transport differentially suppress capsaicin-induced behavioral sensitization through peripheral endocannabinoid mechanisms. Pharmacological Research. 2010;62:249–258.

Tanimura A., Yamazaki M., Hashimotodani Y., et al. The endocannabinoid 2-arachidonoylglycerol produced by diacylglycerol lipase alpha mediates retrograde suppression of synaptic transmission. Neuron. 2010;65:320–327.

Valenzano K.J., Tafesse L., Lee G., et al. Pharmacological and pharmacokinetic characterization of the cannabinoid receptor 2 agonist, GW405833, utilizing rodent models of acute and chronic pain, anxiety, ataxia and catalepsy. Neuropharmacology. 2005;48:658–672.

Wallace V.C., Cottrell D.F., Brophy P.J., et al. Focal lysolecithin-induced demyelination of peripheral afferents results in neuropathic pain behavior that is attenuated by cannabinoids. Journal of Neuroscience. 2003;23:3221–3233.

Ware M.A., Wang T., Shapiro S., et al. Smoked cannabis for chronic neuropathic pain: a randomized controlled trial. CMAJ: Canadian Medical Association Journal. 2010;182:E694–E701.

Yu X.H., Cao C.Q., Martino G., et al. A peripherally restricted cannabinoid receptor agonist produces robust anti-nociceptive effects in rodent models of inflammatory and neuropathic pain. Pain. 2010;151:337–344.

Zhang J., Hoffert C., Vu H.K., et al. Induction of CB2 receptor expression in the rat spinal cord of neuropathic but not inflammatory chronic pain models. European Journal of Neuroscience. 2003;17:2750–2754.

Zimmer A., Zimmer A.M., Hohmann A.G., et al. Increased mortality, hypoactivity, and hypoalgesia in cannabinoid CB1 receptor knockout mice. Proceedings of the National Academy of Sciences of the United States of America. 1999;96:5780–5785.