Neurotransmitter Chemistry of the Autonomic Nervous System

NEUROTRANSMITTER CHEMISTRY OF THE AUTONOMIC NERVOUS SYSTEM

An important traditional classification of autonomic nerves is based on the primary transmitter molecules—acetylcholine or norepinephrine—released from their terminal boutons and vari-cosities. A large number of peripheral ANS fibers synthesize and release acetylcholine; they are cholinergic fibers; that is, they work by releasing acetylcholine. As shown in Figure 6–1, these include all preganglionic efferent autonomic fibers and the somatic (non-autonomic) motor fibers to skeletal muscle as well. Thus, almost all efferent fibers leaving the CNS are cholinergic. In addition, most parasympathetic postganglionic and a few sympatheticpostganglionic fibers are cholinergic. A significant number of parasympathetic postganglionic neurons utilize nitric oxide or peptides as the primary transmitter or cotransmitters.

Most postganglionic sympathetic fibers release norepinephrine (also known as noradrenaline); they are noradrenergic (often called simply “adrenergic”) fibers; that is, they work by releasing norepinephrine (noradrenaline). These transmitter characteristics are presented schematically in Figure 6–1. As noted, some sympa-thetic fibers release acetylcholine. Dopamine is a very important transmitter in the CNS, and there is evidence that it may be released by some peripheral sympathetic fibers. Adrenal medullary cells, which are embryologically analogous to postganglionic sym-pathetic neurons, release a mixture of epinephrine and norepi-nephrine. Finally, most autonomic nerves also release several cotransmitter substances (described in the text that follows), inaddition to the primary transmitters just described.

Five key features of neurotransmitter function provide poten-tial targets for pharmacologic therapy: synthesis, storage, release, and termination of action of the transmitter, and receptoreffects. These processes are discussed next.

Cholinergic Transmission

The terminals and varicosities of cholinergic neurons contain large numbers of small membrane-bound vesicles concentrated near the synaptic portion of the cell membrane (Figure 6–3) as well as a smaller number of large dense-cored vesicles located farther from the synaptic membrane. The large vesicles contain a high concen-tration of peptide cotransmitters (Table 6–1), whereas the smaller clear vesicles contain most of the acetylcholine. Vesicles are ini-tially synthesized in the neuron cell body and carried to the termi-nal by axonal transport. They may also be recycled several times within the terminal. Vesicles are provided with vesicle-associatedmembrane proteins (VAMPs), which serve to align them withrelease sites on the inner neuronal cell membrane and participate in triggering the release of transmitter. The release site on the inner surface of the nerve terminal membrane contains synapto-somal nerve-associated proteins (SNAPs), which interact withVAMPs.

Acetylcholine is synthesized in the cytoplasm from acetyl-CoA and choline through the catalytic action of the enzyme cholineacetyltransferase (ChAT). Acetyl-CoA is synthesized in mito-chondria, which are present in large numbers in the nerve ending. Choline is transported from the extracellular fluid into the neuron terminal by a sodium-dependent membrane choline transporter (CHT; Figure 6–3). This symporter can be blocked by a group of research drugs called hemicholiniums. Once synthesized, acetyl-choline is transported from the cytoplasm into the vesicles by a vesicle-associated transporter (VAT) that is driven by protonefflux (Figure 6–3). This antiporter can be blocked by the research drug vesamicol. Acetylcholine synthesis is a rapid process capable of supporting a very high rate of transmitter release. Storage of acetylcholine is accomplished by the packaging of “quanta” of acetylcholine molecules (usually 1000 to 50,000 molecules in each vesicle). Most of the vesicular acetylcholine (ACh) is bound to negatively chargedvesicular proteoglycan (VPG).

Vesicles are concentrated on the inner surface of the nerve terminal facing the synapse through the interaction of so-called SNARE proteins on the vesicle (a subgroup of VAMPs called v-SNAREs, especially synaptobrevin) and on the inside of theterminal cell membrane (SNAPs called t-SNAREs, especially syntaxin and SNAP-25). Physiologic release of transmitter fromthe vesicles is dependent on extracellular calcium and occurs when an action potential reaches the terminal and triggers sufficient

influx of calcium ions via N-type calcium channels. Calcium interacts with the VAMP synaptotagmin on the vesicle mem-brane and triggers fusion of the vesicle membrane with the termi-nal membrane and opening of a pore into the synapse. The opening of the pore and inrush of cations results in release of the acetylcholine from the proteoglycan and exocytotic expulsion into the synaptic cleft. One depolarization of a somatic motor nerve may release several hundred quanta into the synaptic cleft. One depolarization of an autonomic postganglionic nerve varicosity or terminal probably releases less and releases it over a larger area. In addition to acetylcholine, several cotransmitters are released at the same time (Table 6–1). The acetylcholine vesicle release process is blocked by botulinum toxin through the enzymatic removal of two amino acids from one or more of the fusion proteins.

After release from the presynaptic terminal, acetylcholine mol-ecules may bind to and activate an acetylcholine receptor (cholinoceptor). Eventually (and usually very rapidly), all of the acetylcholine released diffuses within range of an acetylcholinest-erase (AChE) molecule. AChE very efficiently splits acetylcholineinto choline and acetate, neither of which has significant transmit-ter effect, and thereby terminates the action of the transmitter (Figure 6–3). Most cholinergic synapses are richly supplied with acetylcholinesterase; the half-life of acetylcholine molecules in the synapse is therefore very short (a fraction of a second). Acetylcholinesterase is also found in other tissues, eg, red blood cells. (Other cholinesterases with a lower specificity for acetylcho-line, including butyrylcholinesterase [pseudocholinesterase], are found in blood plasma, liver, glia, and many other tissues.)

Adrenergic Transmission

Adrenergic neurons (Figure 6–4) transport a precursor amino acid (tyrosine) into the nerve ending, then synthesize the catecholamine transmitter (Figure 6–5), and finally store it in membrane-bound vesicles. In most sympathetic postganglionic neurons, norepi-nephrine is the final product. In the adrenal medulla and certain areas of the brain, some norepinephrine is further converted to epinephrine. In dopaminergic neurons, synthesis terminates with dopamine. Several processes in these nerve terminals are potential sites of drug action. One of these, the conversion of tyrosine to dopa, is the rate-limiting step in catecholamine transmitter syn-thesis. It can be inhibited by the tyrosine analog metyrosine. A high-affinity antiporter for catecholamines located in the wall of the storage vesicle (vesicular monoamine transporter, VMAT) can be inhibited by the reserpine alkaloids. Reserpine causes depletion of transmitter stores. Another transporter (norepi-nephrine transporter, NET) carries norepinephrine and similarmolecules back into the cell cytoplasm from the synaptic cleft (Figure 6–4; NET). NET is also commonly called uptake 1 or reuptake 1 and is partially responsible for the termination of syn-aptic activity. NET can be inhibited by cocaine and tricyclicantidepressant drugs, resulting in an increase of transmitter activ-ity in the synaptic cleft (see Box: Neurotransmitter Uptake Carriers).

Release of the vesicular transmitter store from noradrenergic nerve endings is similar to the calcium-dependent process previ-ously described for cholinergic terminals. In addition to the pri-mary transmitter (norepinephrine), adenosine triphosphate (ATP), dopamine-β-hydroxylase, and peptide cotransmitters are alsoreleased into the synaptic cleft. Indirectly acting and mixed sym-pathomimetics, eg, tyramine, amphetamines, and ephedrine, are capable of releasing stored transmitter from noradrenergic nerve endings by a calcium-independent process. These drugs are poor agonists (some are inactive) at adrenoceptors, but they are excellent substrates for monoamine transporters. As a result, they are avidly taken up into noradrenergic nerve endings by NET. In the nerve ending, they are then transported by VMAT into the vesicles, displacing norepinephrine, which is subsequently expelled into the synaptic space by reverse transport via NET. Amphetamines also inhibit monoamine oxidase and have other effects that result in increased norepinephrine activity in the synapse. Their action does not require vesicle exocytosis.

Norepinephrine and epinephrine can be metabolized by several enzymes, as shown in Figure 6–6. Because of the high activity of monoamine oxidase in the mitochondria of the nerve terminal, there is significant turnover of norepinephrine even in the resting terminal. Since the metabolic products are excreted in the urine, an estimate of catecholamine turnover can be obtained from labo-ratory analysis of total metabolites (sometimes referred to as “VMA and metanephrines”) in a 24-hour urine sample. However, metabolism is not the primary mechanism for termination of action of norepinephrine physiologically released from noradren-ergic nerves. Termination of noradrenergic transmission results from two processes: simple diffusion away from the receptor site (with eventual metabolism in the plasma or liver) and reuptake into the nerve terminal by NET (Figure 6–4) or into perisynaptic glia or other cells.

Cotransmitters in Cholinergic & Adrenergic Nerves

As previously noted, the vesicles of both cholinergic and adrener-gic nerves contain other substances in addition to the primary transmitter, sometimes in the same vesicles and sometimes in aseparate vesicle population. Some of the substances identified to date are listed in Table 6–1. Many of these substances are also primary transmitters in the nonadrenergic, noncholinergic nervesdescribed in the text that follows. They appear to play several roles in the function of nerves that release acetylcholine or norepineph-rine. In some cases, they provide a faster or slower action to sup-plement or modulate the effects of the primary transmitter. They also participate in feedback inhibition of the same and nearby nerve terminals.