Chapter 6. Introduction to Autonomic Pharmacology

The autonomic nervous system (ANS) is the major involuntary, unconscious, automatic portion of the nervous system and contrasts in several ways with the somatic (voluntary) nervous system. The anatomy, neurotransmitter chemistry, receptor characteristics, and functional integration of the ANS are discussed in this chapter. Major autonomic drug groups are discussed in Chapters 7, 8, 9, and 10. Drugs in many other groups have significant autonomic effects, many of which are undesirable.

The motor (efferent) portion of the ANS is the major pathway for information transmission from the central nervous system (CNS) to the involuntary effector tissues (smooth muscle, cardiac muscle, and exocrine glands; Figure 6–1). Its 2 major subdivisions are the parasympathetic ANS (PANS) and the sympathetic ANS (SANS). The enteric nervous system (ENS) is a semiautonomous part of the ANS located in the gastrointestinal tract, with specific functions for the control of this organ system. The ENS consists of the myenteric plexus (plexus of Auerbach) and the submucous plexus (plexus of Meissner); these neurons send sensory input to the parasympathetic and sympathetic nervous systems and receive motor output from them.

There are many sensory (afferent) fibers in autonomic nerves. These are of considerable importance for the physiologic control of the involuntary organs but are directly influenced by only a few drugs.

The parasympathetic preganglionic motor fibers originate in cranial nerve nuclei III, VII, IX, and X and in sacral segments (usually S2–S4) of the spinal cord. The sympathetic preganglionic fibers originate in the thoracic (T1–T12) and lumbar (L1–L5) segments of the cord.

Most of the sympathetic ganglia are located in 2 paravertebral chains that lie along the spinal column. A few (the prevertebral ganglia) are located on the anterior aspect of the abdominal aorta. Most of the parasympathetic ganglia are located in the organs innervated and more distant from the spinal cord. Because of the locations of the ganglia, the preganglionic sympathetic fibers are short and the postganglionic fibers are long. The opposite is true for the parasympathetic system: preganglionic fibers are longer and postganglionic fibers are short.

Some receptors that respond to autonomic transmitters and drugs receive no innervation. These include muscarinic receptors on the endothelium of blood vessels, some presynaptic receptors, and, in some species, the adrenoceptors on apocrine sweat glands and α2 and β adrenoceptors in blood vessels.

The synthesis, storage, release, receptor interactions, and termination of action of the neurotransmitters all contribute to the action of autonomic drugs (Figure 6–2).

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).

Synthesis and Storage

Acetylcholine is synthesized in the nerve terminal by the enzymecholine 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 Acetylcholine

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 enzymatically alter synaptobrevin or one of the other docking or fusion proteins to prevent the release process.

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.

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.

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.

Synthesis and Storage

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.

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). Termination of action, however, is quite different. 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 and has both therapeutic and toxic potential. Inhibition of COMT in the brain is useful in Parkinson's disease (Chapter 28).

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 amphetamine-like agents) and predictably cause sympathomimetic effects.

Cotransmitters

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.

See Chapter 1.

Botulinum toxin is a very large protein molecule and does not diffuse readily when injected into tissue. In spite of this property, it is able to enter cholinergic nerve endings from the extracellular space and block the release of acetylcholine. How might it cross the lipid membrane barrier? The Skill Keeper Answer appears at the end of the chapter.

The major receptor systems in the ANS include cholinoceptors, adrenoceptors, and dopamine receptors, which have been studied in some detail. The numerous receptors for cotransmitter substances have not been as fully defined.

Cholinoceptors

Also referred to as cholinergic receptors, these molecules respond to acetylcholine and its analogs. Cholinoceptors are subdivided as follows (Table 6–1):

Muscarinic Receptors

As their name suggests, these receptors respond to muscarine (an alkaloid) as well as to acetylcholine. The effects of activation of these receptors resemble those of postganglionic parasympathetic nerve stimulation. Muscarinic receptors are located primarily on autonomic effector cells (including heart, vascular endothelium, smooth muscle, presynaptic nerve terminals, and exocrine glands). Evidence (including their genes) has been found for 5 subtypes, of which 3 appear to be important in peripheral autonomic transmission. All 5 are G-protein-coupled receptors (see Chapter 2).

Nicotinic Receptors

These receptors are located on Na+-K+ ion channels and respond to acetylcholine and nicotine, another acetylcholine mimic (but not to muscarine) by opening the channel. The 2 major nicotinic subtypes are located in ganglia and in skeletal muscle end plates. The nicotinic receptors are the primary receptors for transmission at these sites.

Adrenoceptors

Also referred to as adrenergic receptors, adrenoceptors are divided into several subtypes (Table 6–2).

Alpha Receptors

These are located on vascular smooth muscle, presynaptic nerve terminals, blood platelets, fat cells (lipocytes), and neurons in the brain. Alpha receptors are further divided into 2 major types, α1 and α2. These 2 subtypes constitute different families and use different G-coupling proteins.

Beta Receptors

These receptors are located on most types of smooth muscle, cardiac muscle, some presynaptic nerve terminals, and lipocytes. Beta receptors are divided into 3 major subtypes, β1, β2, and β3. These subtypes are rather similar and use the same G-coupling protein.

Dopamine Receptors

Dopamine (D, DA) receptors are a subclass of adrenoceptors but with rather different distribution and function. Dopamine receptors are especially important in the renal and splanchnic vessels and in the brain. Although at least 5 subtypes exist, the D1 subtype appears to be the most important dopamine receptor on peripheral effector cells. D2 receptors are found on presynaptic nerve terminals. D1, D2, and other types of dopamine receptors also occur in the CNS.

Each division of the ANS has specific effects on organ systems. These effects, summarized in Table 6–3, should be memorized.

Dually innervated organs such as the iris of the eye and the sinoatrial node of the heart receive both sympathetic and parasympathetic innervation. The pupil has a natural, intrinsic diameter to which it returns when both divisions of the ANS are blocked. Pharmacologic ganglionic blockade, therefore, causes it to move to its intrinsic size. Similarly, the cardiac sinus node pacemaker rate has an intrinsic value (about 100–110/min) in the absence of both ANS inputs. How will these variables change (increase or decrease) if the ganglia are blocked? The answer is predictable if one knows which system is dominant. For example, both the pupil and, at rest, the sinoatrial node are dominated by the parasympathetic system. Thus, blockade of both systems, with removal of the dominant PANS and nondominant SANS effects, result in mydriasis and tachycardia.

Some nerve fibers in autonomic effector tissues do not show the histochemical characteristics of either cholinergic or adrenergic fibers. Some of these are motor fibers that cause the release of ATP and other purines related to it. Purine-evoked responses have been identified in the bronchi, gastrointestinal tract, and urinary tract. Other motor fibers are peptidergic, that is, they release peptides as the primary transmitters (see list in earlier Cotransmitters section).

Other nonadrenergic, noncholinergic fibers have the anatomic characteristics of sensory fibers and contain peptides, such as substance P, that are stored in and released from the fiber terminals. These fibers have been termed "sensory-efferent" or "sensory-local effector" fibers because, when activated by a sensory input, they are capable of releasing transmitter peptides from the sensory ending itself, from local axon branches, and from collaterals that terminate in the autonomic ganglia. In addition to their neurotransmitter roles, these peptides are potent agonists in many autonomic effector tissues, especially smooth muscle (see Chapter 17).

Because of the number of steps in the transmission of autonomic commands from the CNS to the effector cells, there are many sites at which autonomic drugs may act. These sites include the CNS centers; the ganglia; the postganglionic nerve terminals; the effector cell receptors; and the mechanisms responsible for transmitter synthesis, storage, release, and termination of action. The most selective effect is achieved by drugs acting at receptors that mediate very selective actions (Table 6–4). Many natural and synthetic toxins have significant effects on autonomic and somatic nerve function.

Functional integration in the ANS is provided mainly through the mechanism of negative feedback and is extremely important in determining the overall response to endogenous and exogenous ANS transmitters and their analogs. This process uses modulatory pre- and postsynaptic receptors at the local level and homeostatic reflexes at the system level.

Local Integration

Local feedback control has been found at the level of the nerve endings in all systems investigated. The best documented of these is the negative feedback of norepinephrine upon its own release from adrenergic nerve terminals. This effect is mediated by α2 receptors located on the presynaptic nerve membrane (Figure 6–3).

Presynaptic receptors that bind the primary transmitter substance and thereby regulate its release are called autoreceptors. Transmitter release is also modulated by other presynaptic receptors (heteroreceptors); in the case of adrenergic nerve terminals, receptors for acetylcholine, histamine, serotonin, prostaglandins, peptides, and other substances have been found. Presynaptic regulation by a variety of endogenous chemicals probably occurs in all nerve fibers.

Postsynaptic modulatory receptors, including M1 and M2 muscarinic receptors and at least 1 type of peptidergic receptor, have been found in ganglionic synapses, where nicotinic transmission is primary. These receptors may facilitate or inhibit transmission by evoking slow excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs).

Systemic Reflexes

System reflexes regulate blood pressure, gastrointestinal motility, bladder tone, airway smooth muscle, and other processes. The control of blood pressure—by the baroreceptor neural reflex and the renin-angiotensin-aldosterone hormonal response—is especially important (Figure 6–4). These homeostatic mechanisms have evolved to maintain mean arterial blood pressure at a level determined by the vasomotor center and renal sensors. Any deviation from this blood pressure "set point" causes a change in ANS activity and renin-angiotensin-aldosterone levels. For example, a decrease in blood pressure caused by hemorrhage causes increased SANS discharge and renin release. As a result, peripheral vascular resistance, venous tone, heart rate, and cardiac force are increased by norepinephrine released from sympathetic nerves. This ANS response can be blocked with ganglion-blocking drugs such as hexamethonium. Blood volume is replenished by retention of salt and water in the kidney under the influence of increased levels of aldosterone. These compensatory responses may be large enough to overcome some of the actions of drugs. For example, the chronic treatment of hypertension with a vasodilator such as hydralazine will be unsuccessful when the compensatory tachycardia (via the baroreceptor reflex) and the salt and water retention (via the renin system response) are not prevented through the use of additional drugs.

Complex Organ Control: The Eye

The eye contains multiple tissues, several of them under autonomic control (Figure 6–5). The pupil, discussed previously, is under reciprocal control by the SANS (via α receptors on the pupillary dilator muscle) and the PANS (via muscarinic receptors on the pupillary constrictor). The ciliary muscle, which controls accommodation, is under primary control of muscarinic receptors innervated by the PANS, with insignificant contributions from the SANS. The ciliary epithelium, on the other hand, has important β receptors that have a permissive effect on aqueous humor secretion. Each of these receptors is an important target of drugs that are discussed in the following chapters.

See Chapter 1.

Botulinum toxin is too large to cross membranes by means of lipid or aqueous diffusion. It must bind to membrane receptors and enter by endocytosis. Botulinum-binding receptors for endocytosis are present on cholinergic neurons but not adrenergic neurons.

When you complete this chapter, you should be able to:

• □ Describe the steps in the synthesis, storage, release, and termination of action of the major autonomic transmitters.
• □ Name 2 cotransmitter substances.
• □ Name the major types of autonomic receptors and the tissues in which they are found.
• □ Describe the organ system effects of stimulation of the parasympathetic and sympathetic systems.
• □ Name examples of inhibitors of acetylcholine and norepinephrine synthesis, storage, and release. Predict the effects of these inhibitors on the function of the major organ systems.
• □ List the determinants of blood pressure and describe the baroreceptor reflex response for the following perturbations: (1) blood loss, (2) administration of a vasodilator, (3) a vasoconstrictor, (4) a cardiac stimulant, (5) a cardiac depressant.
• □ Describe the results of transplantation of the heart (with interruption of its autonomic nerves) on cardiac function.
• □ Describe the actions of several toxins that affect nerve function: tetrodotoxin, saxitoxin, botulinum toxins, and latrotoxin.