The synthesis, storage, release, receptor interactions, and termination of action of the neurotransmitters all contribute to the action of autonomic drugs (Figure 6–2).
Characteristics of transmitter synthesis, storage, release, and termination of action at cholinergic and noradrenergic nerve terminals are shown from the top downward. Circles represent transporters; ACh, acetylcholine; AChE, acetylcholinesterase; ChAT, choline acetyltransferase; DOPA, dihydroxyphenylalanine; NE, norepinephrine; NET, norepinephrine transporter; TCA, tricyclic antidepressant; TH, tyrosine hydroxylase.
A. Cholinergic Transmission
Acetylcholine (ACh) is the primary transmitter in all autonomic ganglia and at the synapses between parasympathetic postganglionic neurons and their effector cells. It is the transmitter at postganglionic sympathetic neurons to the thermoregulatory sweat glands. It is also the primary transmitter at the somatic (voluntary) skeletal muscle neuromuscular junction (Figure 6–1).
Acetylcholine is synthesized in the nerve terminal by the enzyme choline acetyltransferase (ChAT) from acetyl-CoA (produced in mitochondria) and choline (transported across the cell membrane) (Figure 6–2). The rate-limiting step is probably the transport of choline into the nerve terminal. This transport can be inhibited by the research drug hemicholinium. Acetylcholine is actively transported into its vesicles for storage by the vesicle-associated transporter, VAT. This process can be inhibited by another research drug, vesamicol.
Release of transmitter stores from vesicles in the nerve ending requires the entry of calcium through calcium channels and triggering of an interaction between SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor) proteins. SNARE proteins include v-SNARES associated with the vesicles (VAMPs, vesicle-associated membrane proteins: synaptobrevin, synaptotagmin) and t-SNARE proteins associated with the nerve terminal membrane (SNAPs, synaptosome-associated proteins: SNAP25, syntaxin, and others). This interaction results in docking of the vesicle to the terminal membrane and, with influx of calcium, fusion of the membranes of the vesicles with the nerve-ending membranes, the opening of a pore to the extracellular space, and the release of the stored transmitter. The several types of botulinum toxins are able to enter cholinergic nerve terminals and enzymatically alter synaptobrevin or one of the other docking or fusion proteins to prevent the release process.
3. Termination of action of acetylcholine
The action of acetylcholine in the synapse is normally terminated by metabolism to acetate and choline by the enzyme acetylcholinesterase in the synaptic cleft. The products are not excreted but are recycled in the body. Inhibition of acetylcholinesterase is an important therapeutic (and potentially toxic) effect of several drugs.
4. Drug effects on synthesis, storage, release, and termination of action of acetylcholine
Drugs that block the synthesis of acetylcholine (eg, hemicholinium), its storage (eg, vesamicol), or its release (eg, botulinum toxin) are not very useful for systemic therapy because their effects are not sufficiently selective (ie, PANS and SANS ganglia and somatic neuromuscular junctions all may be blocked). However, because botulinum toxin is a very large molecule and diffuses very slowly, it can be used by injection for relatively selective local effects.
B. Adrenergic Transmission
Norepinephrine (NE) is the primary transmitter at the sympathetic postganglionic neuron-effector cell synapses in most tissues. Important exceptions include sympathetic fibers to thermoregulatory (eccrine) sweat glands and probably vasodilator sympathetic fibers in skeletal muscle, which release acetylcholine. Dopamine may be a vasodilator transmitter in renal blood vessels, but norepinephrine is a vasoconstrictor of these vessels.
The synthesis of dopamine and norepinephrine requires several steps (Figure 6–2). After transport across the cell membrane, tyrosine is hydroxylated by tyrosine hydroxylase (the rate-limiting step) to DOPA (dihydroxyphenylalanine), decarboxylated to dopamine, and (inside the vesicle) hydroxylated to norepinephrine. Tyrosine hydroxylase can be inhibited by metyrosine. Norepinephrine and dopamine are transported into vesicles by the vesicular monoamine transporter (VMAT) and are stored there. Monoamine oxidase (MAO) is present on mitochondria in the adrenergic nerve ending and inactivates a portion of the dopamine and norepinephrine in the cytoplasm. Therefore, MAO inhibitors may increase the stores of these transmitters and other amines in the nerve endings (Chapter 30). VMAT can be inhibited by reserpine, resulting in depletion of transmitter stores.
2. Release and termination of action
Dopamine and norepinephrine are released from their nerve endings by the same calcium-dependent mechanism responsible for acetylcholine release (see prior discussion). In contrast to cholinergic neurons, noradrenergic and dopaminergic neurons lack receptors for botulinum and do not transport this toxin into the nerve terminal. Termination of action is also quite different from the cholinergic system. Metabolism is not responsible for termination of action of the catecholamine transmitters, norepinephrine and dopamine. Rather, diffusion and reuptake (especially uptake-1, Figure 6–2, by the norepinephrine transporter, NET, or the dopamine transporter, DAT) reduce their concentration in the synaptic cleft and stop their action. Outside the cleft, these transmitters can be metabolized—by MAO and catechol-O-methyltransferase (COMT)—and the products of these enzymatic reactions are excreted. Determination of the 24-h excretion of metanephrine, normetanephrine, 3-methoxy-4-hydroxymandelic acid (VMA), and other metabolites provides a measure of the total body production of catecholamines, a determination useful in diagnosing conditions such as pheochromocytoma. Inhibition of MAO increases stores of catecholamines in nerve endings and has both therapeutic and toxic potential. Inhibition of COMT in the brain is useful in Parkinson’s disease (Chapter 28).
3. Drug effects on adrenergic transmission
Drugs that block norepinephrine synthesis (eg, metyrosine) or catecholamine storage (eg, reserpine) or release (eg, guanethidine) were used in treatment of several diseases (eg, hypertension) because they block sympathetic but not parasympathetic functions. Other drugs promote catecholamine release (eg, the amphetamines) and predictably cause sympathomimetic effects.
Many (probably all) autonomic nerves have transmitter vesicles that contain other transmitter molecules in addition to the primary agents (acetylcholine or norepinephrine) previously described. These cotransmitters may be localized in the same vesicles as the primary transmitter or in a separate population of vesicles. Substances recognized to date as cotransmitters include ATP (adenosine triphosphate), enkephalins, vasoactive intestinal peptide, neuropeptide Y, substance P, neurotensin, somatostatin, and others. Their main role in autonomic function appears to involve modulation of synaptic transmission. The same substances function as primary transmitters in other synapses.