Autonomic Nervous System: Introduction

Acetylcholine is the neurotransmitter at all autonomic ganglia, all parasympathetic postganglionic synapses, the neuromuscular junction, and some central nervous system pathways. Its synthesis, storage, release, and inactivation each represent discrete pharmacological targets that are exploited by drugs in clinical use.

Synthesis occurs in the presynaptic terminal when choline acetyltransferase catalyzes the combination of choline (taken up from the extracellular fluid by a sodium-dependent choline transporter) with acetyl-coenzyme A to form acetylcholine. The newly synthesized acetylcholine is then packaged into synaptic vesicles by the vesicular acetylcholine transporter. Hemicholinium-3 is an experimental drug that blocks the choline transporter, preventing choline uptake and depleting acetylcholine stores — it is not used clinically but illustrates the vulnerability of this synthesis step.

Release is triggered when an action potential depolarizes the nerve terminal, opening voltage-gated calcium channels. The influx of calcium drives fusion of acetylcholine-containing vesicles with the presynaptic membrane, releasing acetylcholine into the synaptic cleft. Botulinum toxin — produced by Clostridium botulinum — is a protease that cleaves proteins required for vesicular fusion (the SNARE complex components), blocking acetylcholine release at all cholinergic synapses. Clinically, botulinum toxin is used in purified form for focal muscle spasticity, hyperhidrosis, cosmetic applications, and certain movement disorders.

Inactivation of acetylcholine is achieved almost entirely by enzymatic hydrolysis, not by reuptake. Acetylcholinesterase, located on the postsynaptic membrane and within the synaptic cleft, rapidly hydrolyzes acetylcholine into choline and acetate, terminating its action within milliseconds. Choline is then transported back into the presynaptic terminal for resynthesis. Drugs that inhibit acetylcholinesterase — the anticholinesterases — prolong and amplify cholinergic transmission at all acetylcholine-releasing synapses.

After synthesis, dopamine and norepinephrine are actively transported into dense-core vesicles by the vesicular monoamine transporter-2 (VMAT-2), a membrane protein powered by a proton gradient. Vesicular storage serves two purposes: it concentrates the neurotransmitter to achieve high local release concentrations during exocytosis, and it protects the monoamines from degradation by monoamine oxidase in the cytoplasm. Reserpine irreversibly blocks VMAT-2, causing catecholamines to leak out of vesicles into the cytoplasm where they are degraded by monoamine oxidase. The result is depletion of norepinephrine stores over days, producing antihypertensive effects and, at higher doses, depression. Tetrabenazine (VMAT-2 inhibitor) is used to reduce abnormal involuntary movements in Huntington disease.

Release is triggered by action potentials that depolarize the presynaptic terminal and open voltage-gated calcium channels. Calcium influx drives vesicle-membrane fusion and exocytosis of catecholamine into the synaptic cleft. Indirectly acting sympathomimetics such as amphetamine work by entering the presynaptic terminal — they are substrates for the norepinephrine transporter — and then reversing the direction of the transporter and VMAT-2, causing non-exocytotic release of catecholamine stores into the cleft. This mechanism is independent of calcium and accounts for the ability of amphetamine to release catecholamines even in the absence of action potentials.

Release is finely regulated by presynaptic autoreceptors — alpha-2 adrenergic receptors on the sympathetic nerve terminal that detect norepinephrine in the cleft and, when activated, reduce further release through inhibition of adenylyl cyclase and decreased calcium channel opening. This negative-feedback mechanism limits the magnitude of adrenergic activation. The drug clonidine — an alpha-2 agonist — exploits these presynaptic autoreceptors to reduce sympathetic outflow, producing its antihypertensive effect.

The neurotransmitter lifecycle — synthesis, storage, release, receptor activation, and inactivation — provides a systematic map of drug targets. Every major class of autonomic drug acts at one of these steps, and knowing which step a drug targets immediately predicts its mechanism of action and its likely adverse effects.

For the cholinergic system: acetylcholinesterase inhibitors block inactivation (the most clinically important target class), botulinum toxin blocks release, and direct muscarinic or nicotinic agonists bypass the entire presynaptic apparatus to activate receptors directly. For the adrenergic system: the most important targets are the postsynaptic adrenergic receptors themselves (alpha and beta agonists and antagonists), followed by the presynaptic reuptake transporter (norepinephrine transporter inhibitors such as cocaine and tricyclic antidepressants), storage depletion (reserpine, tetrabenazine via VMAT-2), and biosynthesis inhibition (metyrosine, carbidopa).

One principle worth noting: drugs acting at postsynaptic receptors produce immediate, predictable effects whose direction depends on the receptor subtype and organ. Drugs acting presynaptically (depleting stores, blocking synthesis, blocking reuptake) have effects that evolve over hours to days as stores change and often produce paradoxical early responses. Reserpine, for example, may briefly release catecholamines as vesicles are disrupted before depletion takes hold. Understanding which side of the synapse a drug acts on is as important as knowing its receptor target.