Neurotransmitters

Neurotransmitters are molecules that act as chemical signals between nerve cells

Nerve cells communicate with each other and with target tissues by secreting chemical messengers, called neurotransmitters. This chapter describes the various classes of neurotransmitters and how they interact with their target cells. It will discuss their effects on the body, how alterations in their signaling may cause disease, and how pharmacologic manipulation of their concentrations may be used therapeutically.

Several transmitters may be found in one nerve

An early dogma of nerve function held that one nerve contained one transmitter. However, this is now known to be an oversimplification, and combinations of transmitters are the rule. The pattern of cellular transmitters may characterize a particular functional role, but details of this also remain unclear. A major low-molecular-weight transmitter such as an amine is often present, along with several peptides, an amino acid, and a purine. Sometimes, there may even be more than one possible transmitter in a particular vesicle, as is believed to be the case for adenosine triphosphate (ATP) and norepinephrine in sympathetic nerves. In some cases, the intensity of stimulation may control which transmitter is released, peptides often requiring greater levels of stimulus. Furthermore, different transmitters may have a different timescale of action. Sympathetic nerves are good examples of nerves for which this is the case: it is believed that ATP causes their rapid excitation, whereas norepinephrine and the neuromodulator neuropeptide Y (NPY) cause a slower phase of action. In some tissues, NPY on its own may be able to produce a very slow excitation.

Action potentials are caused by changes in ion flows across cell membranes

The signal carried by a nerve cell reflects an abrupt change in the voltage potential difference across the cell membrane. The normal resting potential difference is a few millivolts, with the inside of the cell being negative, and is caused by an imbalance of ions across the plasma membrane: the concentration of K⁺ ion is much greater inside cells than outside, whereas the opposite is true for Na⁺ ion. This difference is maintained by the action of the Na⁺/K⁺-ATPase (Chapter 24). Only those ions to which the membrane is permeable can affect the potential, as they can come to an electrochemical steady state under the combined influence of concentration and voltage differences. Because the membrane in all resting cells is comparatively permeable to K⁺ as a result of the presence of voltage-independent (leakage) K⁺ channels, this ion largely controls the resting potential.

A change in voltage which tends to drive the resting potential towards zero from the normal negative voltage is known as a depolarization, whereas a process that increases the negative potential is called hyperpolarization

So far, this picture is common to all cells. However, nerve cells contain voltage-dependent sodium channels that open very rapidly when a depolarizing change in voltage is applied. When they open, they allow the inward passage of huge numbers of Na⁺ ions from the extracellular fluid (Fig. 41.1.2), which swamps the resting voltage and drives the membrane potential to positive values. This reversal of voltage is the action potential. Almost immediately afterwards, the sodium channels close and so-called delayed potassium channels open. These restore the normal resting balance of ions across the membrane and, after a short refractory period, the cell can conduct another action potential. Meanwhile, the action potential has spread by electrical conductance to the next segment of nerve membrane, and the entire cycle starts again.

Neurotransmitters alter the activity of various ion channels to cause changes in the membrane potential

Excitatory neurotransmitters cause a depolarizing change in voltage, in which case an action potential is more likely to occur. In contrast, inhibitory transmitters hyperpolarize the membrane and an action potential is then less likely to occur.

Neurotransmitters act at synapses

Neurotransmitters are released into the space between cells at a specialized area known as a synapse (Fig. 41.1.3). In the simplest case, they diffuse from the presynaptic membrane across the synaptic space or cleft, and bind to receptors at the postsynaptic membrane. However, many neurons, particularly those containing amines, have several varicosities along the axon, containing transmitter. These varicosities may not be close to any neighboring cell, so transmitter released from them has the possibility of affecting many neurons. Nerves innervating smooth muscle are commonly of this kind.

When the action potential arrives at the end of the axon, the change in voltage opens calcium channels. Calcium entry is essential for mobilization of vesicles containing transmitter, and for their eventual fusion with the synaptic membrane and release through it.

Because transmitters are released from vesicles, impulses arrive at the postsynaptic cell in individual packets, or quanta. At the neuromuscular junction between nerves and skeletal muscle cells, a large number of vesicles are discharged at a time, and a single impulse may therefore be enough to stimulate contraction of the muscle cell. The number of vesicles released at synapses between neurons, however, is much smaller; consequently, the recipient cell will be stimulated only if the total algebraic sum of the various positive and negative stimuli exceeds its threshold. As each cell in the brain receives input from a huge number of neurons, this implies that there is a far greater capability for the fine control of responses in the central nervous system (CNS) than there is at the neuromuscular junction.

Neurotransmitters act by binding to specific receptors, and opening or closing ion channels

There are several mechanisms by which receptors for excitatory neurotransmitters can cause the propagation of an action potential in a postsynaptic neuron. Directly or indirectly, they cause changes in ion flow across the membrane, until the potential reaches the critical point, or threshold, for initiation of an action potential. Receptors that directly control the opening of an ion channel are called ionotropic, whereas metabotropic receptors cause changes in second messenger systems, which in turn alter the function of channels that are separate from the receptor.

All known metabotropic receptors are coupled to G-proteins

Metabotropic receptors are coupled to second messenger pathways and act more slowly than ionotropic receptors. All known metabotropic receptors are coupled to G-proteins (Chapter 40) and, like hormone receptors, have seven transmembrane regions. Typically, they then couple either to adenylate cyclase, altering the production of cyclic adenosine monophosphate (cAMP), or to the phosphatidyl inositol pathway, which alters calcium fluxes. Ion channels that are separate from the receptor are then usually modified by phosphorylation. For instance, the β-adrenergic receptor, which responds to norepinephrine and epinephrine (Fig. 13.5), causes an increase in cAMP, which stimulates a kinase to phosphorylate and activate a calcium channel. Some of the muscarinic class of ACh receptors have similar effects on K⁺ channels.

The action of transmitters must be halted by their removal from the synaptic cleft

When transmitters have served their function, they must be removed from the synaptic space. Simple diffusion is probably the major mechanism of removal of neuropeptides. Enzymes such as acetylcholinesterase, which cleaves ACh, may destroy any remaining transmitter. Surplus transmitters may also be taken back up into the presynaptic neuron for reuse, and this is a major route of removal for catecholamines and amino acids. Interference with uptake causes an increase in the concentration of transmitter in the synaptic space; this often has useful therapeutic consequences.

Concentrations of neurotransmitters may be manipulated

The effects of neurotransmitters can be altered by changing their effective concentrations or the number of receptors. Concentrations can be altered by:

Changes in the number of receptors may be involved in long-term adaptations to the administration of drugs.

Glutamate is the most important excitatory transmitter in the CNS

Glutamate acts on both ionotropic and metabotropic receptors. Clinically, the receptor characterized in vitro by N-methyl-D-aspartate (NMDA) binding is particularly important (Fig. 41.1.5).

The hippocampus (Fig. 41.1.6) is an area of the limbic system of the brain that is involved in emotion and memory. Certain synaptic pathways there become more active when chronically stimulated, a phenomenon known as long-term potentiation. This represents a possible model of how memory is laid down, and it requires activation of the NMDA receptor and the consequent influx of calcium.

Glutamate is recycled by high-affinity transporters into both neurons and glial cells. The glial cells convert it into glutamine, which then diffuses back into the neuron. Mitochondrial glutaminase in the neuron regenerates glutamate for reuse.

Excess glutamate is toxic to nerve cells

The activation of NMDA receptor allows calcium entry into cells. This activates various proteases, which in turn initiate the pathway of programmed cell death or apoptosis (see Chapter 42). There may, in addition, be changes in other ionotropic glutamate receptors that also cause aberrant calcium uptake. Uptake of sodium ions is also implicated and causes swelling of cells. Activation of NMDA receptors also increases the production of nitric oxide, which may in itself be toxic. Cell death in some models of excitotoxicity can be prevented by inhibitors of nitric oxide production, but the mechanism of toxicity is not clear.

Attempts are being made to develop drugs to inhibit NMDA activation and suppress excitotoxicity. The hope is that damage caused by stroke can be limited or even reversed. Unfortunately, many of the drugs have side effects because they bind to the phencyclidine-binding site and have unpleasant psychologic effects such as paranoia and delusions.

GABA is synthesized from glutamate by the enzyme glutamate decarboxylase

GABA (Fig. 41.1.7) is the major inhibitory transmitter in the brain. There are two known GABA receptors: the GABA_A receptor is ionotropic and the GABA_B receptor is metabotropic. The GABA_A receptor consists of five subunits that arise from several gene families, giving an enormous number of potential receptors with different binding affinities. This receptor is the target for several useful therapeutic drugs. Benzodiazepines bind to it and cause a potentiation of the response to endogenous GABA; these drugs reduce anxiety and also cause muscle relaxation. Barbiturates also bind to the GABA receptor and stimulate it directly in the absence of GABA; because of this lack of dependence on endogenous ligand, they are more likely to cause toxic side effects in overdose.

Norepinephrine (also known as noradrenaline) is a major transmitter in the sympathetic nervous system

Sympathetic nerves arise in the spinal cord and run to ganglia situated close to the cord, from which postganglionic nerves run to the target tissues. Norepinephrine (Fig. 41.1.7) is the transmitter for these postganglionic nerves, whereas the transmitter at the intermediate ganglia is ACh. Stimulation of these nerves is responsible for various features of the ‘fight or flight’ response, such as stimulation of the heart rate, sweating, vasoconstriction in the skin, and bronchodilation.

There are also norepinephrine-containing neurons in the CNS, largely in the brainstem (Fig. 41.1.8). Their axons extend in a wide network throughout the cortex and alter the overall state of alertness or attention. The stimulatory effects of amphetamines are caused by their close chemical similarity to catecholamines.

Epinephrine (also known as adrenaline) is produced by the adrenal medulla under the influence of ACh-containing nerves, analogous to the sympathetic preganglionic nerves

Epinephrine is more active than norepinephrine on the heart and lungs, causes redirection of blood from the skin to skeletal muscle, and has important stimulatory effects on glycogen metabolism in the liver. In response to epinephrine, a sudden extra supply of glucose is delivered to muscle, the heart and lungs work harder to pump oxygen round the circulation, and the body is then prepared to run or to defend itself (Chapter 21). Epinephrine is not essential for life, however, as it is possible to remove the adrenal medulla without serious consequences.

The receptors for norepinephrine and epinephrine are called adrenoceptors (see Fig. 13.5). They are divided into α- and β-receptor classes and subclasses on the basis of their pharmacology. Epinephrine acts on all classes of the receptors but norepinephrine is more specific for α-receptors. β-Blockers, such as atenolol, are used to treat hypertension and chest pain (angina) in ischemic heart disease because they antagonize the stimulatory effects of catecholamines on the heart. Nonspecific α-blockers have limited use, although the more specific β₁-blockers, such as prazosin, and α₂-blockers, such as clonidine, can be used to treat hypertension. Certain subclasses of β-receptors are found in particular tissues; for instance, the β₂-receptor is present in lung and β₂-receptor agonists such as salbutamol are therefore used to produce bronchial dilatation in asthma without stimulating the β₁-receptor in the heart.

Norepinephrine is taken up into cells by a high-affinity transporter and catabolized by the enzyme monoamine oxidase (MAO). Further oxidation and methylation by catecholamine-O-methyltransferase (COMT) convert the products to metanephrines and vanillylmandelic acid (4-hydroxy-3-methoxymandelic acid) (Fig. 41.1.9), which can be measured in the urine as indices of the function of the adrenal medulla. They are particularly increased in patients who have the tumor of the adrenal medulla known as pheochromocytoma. This tumor causes hypertension because of the vasoconstrictor action of the catecholamines it produces.

Dopamine is both an intermediate in the synthesis of norepinephrine and a neurotransmitter

It is a major transmitter in nerves that interconnect the nuclei of the basal ganglia in the brain and control voluntary movement (Fig. 41.1.10). Damage to these nerves causes Parkinson's disease, which is characterized by tremor and difficulties in initiating and controlling movement. Dopamine is also found in pathways affecting the limbic systems of the brain, which are involved in emotional responses and memory. Defects in dopaminergic systems are implicated in schizophrenia, because many antipsychotic drugs used to treat this disease have been found to bind to dopamine receptors.

In the periphery, dopamine causes vasodilatation and it is therefore used clinically to stimulate renal blood flow, and is important in the treatment of renal failure (Chapter 23). The catabolism of dopamine is comparable to norepinephrine. However, the major metabolite formed is homovanillic acid (HVA).

Serotonin, also called 5-hydroxytryptamine (5-HT), is derived from tryptophan (Fig. 41.1.7)

In addition, serotonin biosynthesis has a number of biochemical similarities to dopamine synthesis. Thus, tryptophan hydroxylase, like tyrosine hydroxylase, displays a cofactor requirement for tetrahydrobiopterin (BH₄) (see below). Furthermore, 5-hydroxytryptophan is converted to serotonin by dopa decarboxylase (also known as aromatic amino acid decarboxylase).

Serotoninergic neurons are concentrated in the raphe nuclei in the upper brainstem (Fig. 41.1.11), but project up to the cerebral cortex and down to the spinal cord. They are more active when subjects are awake than when they are asleep, and serotonin may control the degree of responsiveness of motor neurons in the spinal cord. In addition, it is implicated in so-called vegetative behaviors such as feeding, sexual behavior, and temperature control.

Acetylcholine (ACh) is the transmitter of the parasympathetic autonomic nervous system and of the sympathetic ganglia (Fig. 41.1.3)

Stimulation of the parasympathetic system produces effects that are broadly opposite to those of the sympathetic system, such as slowing of the heart rate, bronchoconstriction, and stimulation of intestinal smooth muscle. ACh also acts at neuromuscular junctions, where motor nerves contact skeletal muscle cells and cause them to contract. Apart from these roles, ACh may be involved in learning and memory, as neurons containing this transmitter also exist in the brain.

ACh is synthesized from choline by the enzyme choline acetyl transferase. After it is secreted into the synaptic cleft, it is largely broken down by acetylcholinesterase. The remainder is taken back up into the nerve cell by transporters similar to those for amines.

There are two main classes of ACh receptors: nicotinic and muscarinic (see Chapter 41.2, Fig. 41.2.3).

Both respond to ACh but can be distinguished by their associated agonists and antagonists; they are quite different structurally and differ in their mechanisms of action.

Clinically, ACh agonists, in common with acetylcholinesterase inhibitors, are used to treat glaucoma, an eye disease characterized by high intraocular pressure, by increasing the tone of the muscles of accommodation of the eye. They are also used to stimulate intestinal function after surgery. On the other hand, when acetylcholinesterase is inhibited by organophosphate insecticides or nerve gases, a toxic syndrome is caused by the resulting excess of ACh. There may be diarrhea, increased secretory activity of several glands, and bronchoconstriction. This syndrome can be antagonized by atropine, although longer-term treatment involves the use of drugs that can remove the insecticide from the enzyme, such as pralidoxime.

In autonomic and enteric nerves, nitric oxide (NO) is produced from arginine by the tetrahydrobiopterin-dependent nitric oxide synthases

NO has a number of attributed physiologic functions including relaxation of both vascular and intestinal smooth muscle, and the possible regulation of mitochondrial energy production. Furthermore, within the brain, NO may have a role in memory formation. However, excessive NO formation has been implicated in the neurodegenerative process associated with Parkinson's and Alzheimer's disease. Whilst the exact mechanism whereby excessive NO causes neuronal death is not known, a growing body of evidence suggests that irreversible damage to the mitochondrial electron transport chain may be an important factor.

NO is not stored in vesicles, but released directly into the extracellular space

Consequently, NO does not, in the strictest sense, meet all the current criteria to be labeled as a neurotransmitter. NO itself diffuses comparatively easily between cells, and binds directly to heme groups in the enzyme guanylate cyclase, stimulating the production of cyclic guanosine monophosphate.

ATP and other purine-containing molecules derived from it are now known to have transmitter functions

ATP is present in synaptic vesicles of sympathetic nerves, along with norepinephrine, and is responsible for rapid excitatory potentials in smooth muscle. Adenosine receptors are widespread in the brain and in vascular tissue. Adenosine is largely inhibitory in the CNS, and inhibition of adenosine receptors is believed to underlie the stimulatory effects of caffeine.

Study of histamine in nerves is complicated by the large amounts that are present in mast cells

Histamine is found in a small number of neurons, mainly in the hypothalamus, although their projections are widespread throughout the brain. It has been shown to control the release of pituitary hormones, arousal, and food intake. Antihistamines designed to control allergies caused by release from mast cells act on the H₁ receptor and tend to be sedative, suggesting that other central functions also probably exist. The histamine receptor in the stomach is of the H₂ class; therefore, the H₂ inhibitors, such as cimetidine and ranitidine, that are used to treat peptic ulcers have no effect on allergy.

Many peptides act as neurotransmitters

It is an open question whether all the peptides that have been described are really true neurotransmitters. Nevertheless, more than 50 small peptides have now been shown to influence neural function. All known peptide receptors are metabotropic and coupled to G-proteins (Chapter 40), and so act comparatively slowly. There are no specific uptake pathways or degradative enzymes, and the main route of disposal is simple diffusion followed by cleavage by a number of peptidases in the extracellular fluid. This allows a peptide to affect a number of neurons before it is finally degraded.

Vasoactive intestinal peptide (VIP) is one of many peptides that affect the function of the intestine through the enteric nervous system. It was originally described as a gut hormone that affected blood flow and fluid secretion, but it is now known to be an important enteric neuropeptide, inhibiting smooth muscle contraction. It also causes vasodilatation in several secretory glands, and potentiates stimulation by ACh.

Many neuropeptides belong to a multigene family

The opioid peptides and opioid receptors provide a good example of a multigene family. They are the endogenous ligands for opiate analgesics such as morphine and codeine. The control of pain is complex, and opioid peptides and receptors are found both in the spinal cord and in the brain itself. There are at least three genes that code for these peptides, and each contains the sequences for several active molecules:

Opiates affect pleasure pathways in the brain, which explains their euphoriant effects, and they also have side effects, such as respiratory depression, that limit their use. In excess, they cause contraction of the muscles of the eye, resulting in ‘pinpoint’ pupils. It has been shown that endorphins are released after strenuous exercise, giving the so-called ‘jogger’s high’. It is hoped that increased knowledge of the specific opioid receptors and neural opioid pathways will allow the development of analgesics with fewer side effects and less likelihood of abuse.

Substance P is another example of a member of a multigene family, known as the tachykinin family. It is present in afferent fibers of sensory nerves and transmits signals in response to pain. It is also involved in so-called neurogenic inflammation stimulated by nerve impulses, and is an important neurotransmitter in the intestine.

Neuropeptides can act as neuromodulators

Some peptides do act as true neurotransmitters, but they also have many other actions. They often alter the action of other transmitters, acting as neuromodulators, but have no action of their own. For instance, VIP enhances the effect of ACh on salivary gland secretion in cat submandibular glands (glands located under the jawbone) by causing vasodilatation and potentiating the cholinergic component. NPY causes inhibition of the release of norepinephrine at autonomic nerve terminals, acting at presynaptic autoreceptors, and potentiates the action of norepinephrine in certain arteries while having only weak actions itself. Opioid peptides also are capable of modulating neurotransmitter release.