Cholinergic Transmission

The neurochemical steps that mediate cholinergic neurotransmission are summarized in Figure 10-1. ACh is synthesized from choline and acetyl coenzyme A (AcCoA) by the enzyme choline acetyltransferase (ChAT). ChAT is not rate-limiting for ACh synthesis, and thus ChAT inhibitors have little or no effect on ACh concentrations. The AcCoA used for ACh synthesis is synthesized in mitochondria from its immediate precursor, pyruvate, and is transported out of mitochondria into the cytoplasm.

FIGURE 10–1 Cholinergic neurotransmission and its perturbation by drugs. Acetylcholine (ACh) is synthesized from choline and acetyl coenzyme A (AcCoA) by the enzyme choline acetyltransferase (ChAT). Choline gains access to the nerve terminal by active transport from the extracellular fluid via the high-affinity choline transporter (HAChT). AcCoA is synthesized in mitochondria from pyruvate (Pyr) by the enzyme pyruvate dehydrogenase (PDH). AcCoA is transported out of the mitochondria for the synthesis of ACh. ACh is transported into synaptic vesicles by a vesicular ACh transporter. The arrival of an action potential at the nerve terminal causes the synaptic vesicles to fuse with the nerve membrane, followed by the exocytotic release of ACh. Upon release, ACh can activate postjunctional muscarinic (M) or nicotinic (N) receptors as well as presynaptic muscarinic receptors (M); stimulation of the latter inhibits the release of ACh. The actions of ACh are rapidly terminated by acetylcholinesterase (AChE), which hydrolyzes ACh into choline and acetate. The choline can be transported back into the nerve terminal for the synthesis of new ACh. Sites at which drugs act to alter cholinergic transmission are identified by numbers in circles.

Drugs that enhance cholinergic transmission:

1. Nicotinic receptor agonists (e.g., nicotine)

2. Muscarinic receptor agonists (e.g., bethanechol)

3. Cholinesterase inhibitor (e.g., physostigmine)

Drugs that inhibit cholinergic transmission:

4. Inhibitors of vesicular ACh transport (e.g., vesamicol)

5. Inhibitors of exocytotic release (e.g., botulinum toxin)

6. Nicotinic receptor antagonists (e.g., mecamylamine)

7. Muscarinic receptor antagonists (e.g., atropine)

8. Inhibitors of high-affinity choline transport (e.g., hemicholinium)

9. Inhibitors of pyruvate dehydrogenase (e.g., bromopyruvate)

Most of the choline used for ACh synthesis is provided to the cytoplasm by a high-affinity, Na⁺-dependent choline transporter; a small amount of choline is also generated from the hydrolysis of phospholipids within the neuron. The choline transporter provides choline from the extracellular fluid, which is ultimately derived from both the extraneuronal catabolism of ACh and the blood. Choline exists in the circulation primarily in an esterified form as phosphatidylcholine (lecithin) and, to a much lesser extent, as free choline. Circulating phosphatidylcholine is derived from both dietary sources and from hepatic methylation of phosphatidylethanolamine.

Once formed in the cytosol, ACh is transported actively into synaptic vesicles by the vesicular ACh transporter. The arrival of an action potential at the nerve terminal causes Ca⁺⁺ influx, triggering the release of ACh from storage vesicles. The toxin from Clostridium botulinum (botulinum toxin) prevents the exocytotic release of ACh from synaptic vesicles and blocks cholinergic neurotransmission. This mechanism underlies the clinical and cosmetic use of botulinum toxin in conditions that involve involuntary skeletal muscle activity or increased muscle tone including strabismus, blepharospasm, hemifacial spasm, and facial wrinkles (see Chapter 12). After release from the nerve terminal, ACh elicits cellular responses by activating postsynaptic muscarinic or nicotinic receptors. Muscarinic receptors are also located presynaptically, where they inhibit further neurotransmitter release. The effects of ACh are rapidly terminated by the enzyme acetylcholinesterase (AChE), which hydrolyzes ACh to acetate and choline. Choline is transported back into nerve terminals by the high-affinity choline transporter, where it can be reused for ACh synthesis.

Drugs can modify the effectiveness of parasympathetic nerve activity by increasing or decreasing cholinergic neurotransmission. Parasympathomimetics mimic cholinergic transmission by acting directly on postsynaptic receptors (muscarinic receptor agonists) or by prolonging the action of released ACh (AChE inhibitors). Drugs that block cholinergic transmission inhibit either the storage of ACh (vesamicol), vesicular exocytosis (botulinum toxin), or the high-affinity transport of choline (hemicholinium). Drugs can also block muscarinic receptors (muscarinic receptor antagonists). The sites of action of all these compounds are depicted in Figure 10-1.

Activation of Cholinergic Receptors

Drugs that activate cholinergic receptors may be classified as either directly or indirect-acting. Direct-acting compounds produce their effects by binding to and activating either muscarinic or nicotinic receptors and are termed muscarinic or nicotinic receptor agonists, respectively. Indirect-acting cholinomimetics produce their effects by inhibiting AChE, thereby increasing the concentration and prolonging the action of ACh at cholinergic synapses. The directly and indirect-acting cholinomimetics and their clinical uses are presented in Table 10-1.

TABLE 10–1 Direct- and Indirect-Acting Cholinomimetic Drugs and their Uses

* ChE inhibitors used for Alzheimer’s disease are listed in Chapter 28.

The structures of several direct-acting cholinergic agonists are shown in Figure 10-2. Introduction of a methyl group in the β position of ACh yields agonists selective for muscarinic receptors such as methacholine and bethanechol. In addition, substitution of an amino group for the terminal methyl group yields corresponding carbamic acid ester derivatives (i.e., carbachol and bethanechol), which render these compounds relatively resistant to hydrolysis by AChE, prolonging their action as compared with ACh.

FIGURE 10–2 Structures of some direct-acting cholinergic agonists.

Muscarinic Receptor Agonists

Muscarinic receptors mediate cellular responses by interacting with heterotrimeric G proteins to affect ionic conductances and the cytosolic concentration of second messengers (see Chapters 1 9). As indicated in Chapter 9, five muscarinic receptor subtypes (M₁ to M₅) have been identified, with M₁ receptors located in the CNS, peripheral neurons, and gastric parietal cells, M₂ receptors located in the heart and on some presynaptic nerve terminals in the periphery and CNS, and M₃ receptors located on glands and smooth muscle. M₄ and M₅ receptors are located in the CNS and are less well understood than the others.

M₁, M₃, and M₅ receptors are coupled to G_q proteins, and stimulation of these receptors activates phospholipase C, leading to increased phosphoinositide hydrolysis and the release of intracellular Ca⁺⁺. In contrast, M₂ and M₄ receptors are coupled to G_i, and agonist activation leads to inhibition of adenylyl cyclase. The signaling pathways mediating the contraction of smooth muscle upon activation of M₂ or M₃ receptors is depicted in Figure 10-3, A. Activation of M₃ receptors promotes smooth muscle contraction directly, whereas activation of M₂ receptors promotes contraction by inhibiting the ability of β adrenergic receptor activation to relax smooth muscle. Activation of M₃ receptors on the endothelium of blood vessels (see Fig. 10-3, B) increases the production of nitric oxide (NO), which diffuses and causes relaxation of the muscle, while activation of M₁ receptors in parasympathetic ganglia (see Fig. 10-3, C) releases an inhibitory neurotransmitter such as adenosine triphosphate (ATP), NO, or vasoactive intestinal peptide to relax smooth muscle sphincters.

FIGURE 10–3 Muscarinic receptor-mediated contraction and relaxation in different types of smooth muscle. A, Most smooth muscle cells express M₂ and M₃ receptors. Activation of M₃ receptors elicits contraction through stimulation of phospholipase C-β (PLC-β), which cleaves phosphatidylinositol-4,5-bisphosphate into diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (IP₃). The IP₃ mobilizes Ca⁺⁺ and triggers contraction. β Adrenergic receptor activation (β) stimulates adenylyl cyclase (AC) to generate cyclic AMP (cAMP), which causes the relaxation of smooth muscle. M₂ receptors inhibit AC to prevent the relaxant effects on stimulation of the β adrenergic receptor. B, Most peripheral blood vessels express M₃ receptors on the endothelium, which trigger the synthesis of nitric oxide (NO). The NO diffuses into the smooth muscle, where it mediates relaxation through the production of cyclic guanosine monophosphate. C, Activation of M₁ receptors and nicotinic receptors (N) in parasympathetic ganglia causes the release of an inhibitory neurotransmitter (IN) from postganglionic neurons in gastrointestinal sphincters. This inhibitory neurotransmitter is usually ATP, NO, or VIP and causes the sphincter smooth muscle to relax.

In the myocardium, the activation of M₂ receptors decreases the force of contraction (negative inotropic effect) and heart rate (negative chronotropic effect). Thus M₂ receptor activation in the heart opposes the increased force of contraction elicited by activation of adrenergic β₁ receptors. M₂ and M₄ receptors also interact with G_i/o, mediating a presynaptic inhibition of neurotransmitter release.

Most, but not all, peripheral effects of muscarinic receptor agonists resemble those elicited by the activation of parasympathetic nerves (see Chapter 9). These effects include: (1) constriction of the iris sphincter; (2) contraction of the ciliary muscle, resulting in accommodation of the lens; (3) constriction of the airways; (4) an increase in GI motility; (5) contraction of the bladder reservoir and relaxation of its outlet; (6) a decrease in heart rate; and (7) an increase in secretions of sweat glands, salivary glands, lacrimal glands, and glands of the trachea and GI mucosa. Although the ventricular myocardium and most peripheral blood vessels lack cholinergic innervation, they express muscarinic receptors. When activated by exogenously administered agonists, these receptors decrease the force of contraction in the heart and dilate peripheral blood vessels. Muscarinic receptor agonists also activate receptors throughout the CNS, and muscarinic receptors in the brain play an important role in learning, memory, control of posture, and temperature regulation. Excessive activation of central muscarinic receptors causes tremor, convulsions, and hypothermia.

Cholinesterase Inhibitors

Two types of cholinesterase (ChE) enzymes are expressed in the body, AChE and butyrylcholinesterase (BuChE, also called plasma or pseudoChE), each with a different distribution and substrate specificity. AChE is located at synapses throughout the nervous system and is responsible for terminating the action of ACh. BuChE is located at non-neuronal sites, including plasma and liver, and is responsible for the metabolism of certain drugs, including ester-type local anesthetics (Chapter 13) and succinylcholine (Chapter 12). Most enzyme inhibitors used clinically do not discriminate between the two types of ChEs.

The mechanism involved in the hydrolysis of ACh explains the biochemical basis of the actions of ChE inhibitors (Fig. 10-4). AChE is a member of the serine hydrolase family of enzymes and contains a serine (ser)-glutamate (glu)-histidine (hist) triad at the active site. The enzyme has an anionic and a catalytic domain. When ACh binds to AChE, the quaternary nitrogen of ACh binds to the anionic site, positioning the ester group near the catalytic site. The ester moiety undergoes a nucleophilic attack by the serine of the catalytic site, resulting in the hydrolysis of ACh and binding of acetate to the serine of the catalytic domain (acetylation). The acetylated serine is rapidly hydrolyzed, and the free enzyme is regenerated. This enzymatic reaction is one of the fastest known; approximately 10⁴ molecules of ACh are hydrolyzed per second by a single AChE molecule, requiring only about 100 μsec.

FIGURE 10–4 Hydrolysis of acetylcholine (ACh) by acetylcholinesterase (AChE). A major domain of the AChE enzyme contains an amino acid triad consisting of serine (ser), histidine (hist), and glutamate (glu). The (-) denotes the anionic region of the enzyme, and the ser-OH represents the catalytic region. (A) The quarternary (+) nitrogen of the choline portion of the ACh molecule is attracted to the anionic site positioning the ester portion of ACh in close proximity to the catalytic site; (B) the ester bond on ACh is cleaved, the enzyme is acetylated, and choline is released; (C) hydrolysis of the acetylated enzyme rapidly liberates the acetate, and the free enzyme can hydrolyze another ACh molecule.

ChE inhibitors are divided into two main types, reversible and irreversible. Reversible inhibitors are further subdivided into noncovalent and covalent enzyme inhibitors. Noncovalent inhibitors, such as edrophonium, bind reversibly to the anionic domain of AChE. The duration of action is determined in part by the way in which the inhibitor binds. Edrophonium binds weakly and has a rapid renal clearance, resulting in a brief duration of action (approximately 10 minutes). Tacrine and donepezil are also noncovalent ChE inhibitors used to treat Alzheimer’s disease (Chapter 28); they have higher affinities and partition into lipids, giving longer durations of action.

Covalent reversible ChE inhibitors, such as physostigmine and neostigmine, are sometimes referred to as carbamate inhibitors and are carbamic acid ester derivatives. These compounds bind to AChE and are hydrolyzed, but at a relatively slow rate (Fig. 10-5). The resultant carbamylated enzyme is more stable than the acetylated enzyme produced by the hydrolysis of ACh. Unlike the acetylated enzyme, which is deacetylated within seconds, the carbamylated enzyme takes 3 to 4 hours to decarbamylate, contributing to the moderate duration of action of these compounds in patients.

FIGURE 10–5 The interaction of a reversible acetylcholinesterase (AChE) antagonist, neostigmine, with the AChE enzyme. A major domain of AChE enzyme contains an amino acid triad that consists of serine (ser), histidine (hist), and glutamate (glu). The (-) denotes the anionic region of the enzyme, and the ser-OH represents the catalytic region. (A) The quarternary (+) nitrogen of neostigmine is attracted to the anionic site positioning the ester portion of the molecule in close proximity to the catalytic site; (B) the ester bond on neostigmine is cleaved, the enzyme is carbamylated, which prevents the enzyme from interacting with ACh, and the remainder of the neostigmine molecule is released; (C) hydrolysis of carbamylated enzyme occurs slowly, causing reversible inhibition of the enzyme.

Irreversible ChE inhibitors phosphorylate the serine in the active site of AChE (Fig. 10-6). These compounds are organophosphorus molecules and include the toxic nerve gases sarin, soman, and tabun; the insecticides parathion and malathion; and the therapeutic agents echothiophate and isoflurophate. Collectively these compounds are termed organophosphorus ChE inhibitors. The phosphorylated enzyme formed with these compounds is extremely stable, unlike that formed from the hydrolysis of ACh. Dephosphorylation of the phosphorylated enzyme takes hours, if it occurs at all. In the case of secondary (isoflurophate) and tertiary (soman) alkyl-substituted phosphates, the phosphorylated enzyme is so stable that it is not dephosphorylated, and new enzyme molecules must be synthesized for enzyme activity to recover. Many organophosphorus ChE inhibitors also irreversibly phosphorylate and inactivate other serine hydrolases, including trypsin and chymotrypsin. Enzyme reactivation with oximes used in clinical practice such as pralidoxime (2-PAM) is used in conjunction with respiratory support and muscarinic receptor antagonists.

FIGURE 10–6 The interaction of an irreversible acetylcholinesterase (AChE) antagonist, isoflurophate (DFP or diisopropyl flurophosphate), with the AChE enzyme. A major domain of AChE enzyme contains an amino acid triad that consists of serine (ser), histidine (hist), and glutamate (glu). The (-) denotes the anionic region of the enzyme, and the ser-OH represents the catalytic region. (A) The phosphate portion of DFP aligns with the catalytic site of the enzyme; (B) the enzyme is phosphorylated, representing a very stable moiety. Enzyme inhibition is considered irreversible; dephosphorylation, if it occurs, takes hours. After that time the phosphorylated enzyme undergoes a process termed “aging,” which involves hydrolysis of one of the isopropyl groups of the inhibitor, rendering the complex unable to dissociate. Note: An oxime such as 2-PAM, if administered before aging occurs, can bind to and release the phosphate moiety attached to the enzyme; this process reverses the enzyme inhibition.

ChE inhibitors amplify the effects of ACh at all sites throughout the nervous system, which results in both therapeutic and adverse effects. Thus ChE inhibitors indirectly activate nicotinic receptors at the neuromuscular junction and at sympathetic and parasympathetic ganglia. In skeletal muscle, low doses of ChE inhibitors tend to increase the force of contraction. This is not particularly noticeable in normal individuals, but it is the basis for the therapeutic effect of these compounds in patients with myasthenia gravis. Intermediate doses of ChE inhibitors elicit muscle fasciculations and fibrillations, and high doses cause depolarization blockade and muscle paralysis. ChE inhibitors indirectly activate muscarinic receptors at all postjunctional sites in the parasympathetic nervous system and sweat glands. In GI smooth muscle, ChE inhibitors increase motility by prolonging the action of ACh at muscarinic receptors on smooth muscle. ChE inhibitors also relax sphincters by enhancing neurotransmission through parasympathetic ganglia, causing the release of a noncholinergic inhibitory neurotransmitter that directly relaxes GI sphincter smooth muscle (see Fig. 10-3, C).

The effects of ChE inhibitors on the cardiovascular system are complicated by opposing effects resulting from the activation of both sympathetic and parasympathetic ganglia. Because the effects of the parasympathetic system predominate in the heart, ChE inhibitors decrease heart rate and cardiac output but have little effect on ventricular contraction. Moderate doses of ChE inhibitors have little effect on blood pressure because few blood vessels receive cholinergic innervation, but high doses decrease blood pressure.

Most organophosphorus ChE inhibitors and tertiary amine carbamate inhibitors readily penetrate the blood-brain barrier and indirectly activate central nicotinic and muscarinic receptors. ChE inhibitors that penetrate the blood-brain barrier (see Chapter 2) have been specifically developed and used for Alzheimer’s disease (see Chapter 28).

Blockade of Cholinergic Receptors

Cholinergic receptor blockers are classified as muscarinic or nicotinic receptor antagonists.

Muscarinic Receptor Antagonists

Muscarinic receptor antagonists block muscarinic receptors competitively. Because the effects of these drugs resemble those of agents that block postganglionic parasympathetic nerves, these compounds are sometimes referred to as parasympatholytics. Included in this group are the naturally occurring belladonna alkaloids atropine and scopolamine.¹ Muscarinic receptor antagonists and their clinical uses are presented in Table 10-2. Most antagonists used clinically are not selective for different muscarinic receptor subtypes.

TABLE 10–2 Muscarinic Receptor Antagonists and their Uses*

Agent	Use
Tertiary Amines
Atropine	Treatment of anti-ChE poisoning
Scopolamine	Treatment of motion sickness
Homatropine	Mydriatic and cycloplegic; for mild uveitis
Dicyclomine	Alleviates GI spasms, pylorospasm, and biliary distention
Oxybutynin	Treatment of urinary incontinence
Oxyphencyclimine	Antisecretory agent for peptic ulcer
Cyclopentolate	Mydriatic and cycloplegic
Tropicamide	Mydriatic and cycloplegic
Benztropine	Treatment of Parkinson’s disease
Trihexyphenidyl	Treatment of Parkinson’s disease
Pirenzepine	Antisecretory agent for peptic ulcer
Quaternary Ammonium Derivatives
Methylatropine	Mydriatic, cycloplegic and antispasmodic
Methylscopolamine	Antisecretory agent for peptic ulcer, antispasmodic
Ipratropium	Aerosol for COPD
Glycopyrrolate	Antisecretory agent for peptic ulcer, antispasmodic
Tolterodine	Treatment of urinary incontinence
Propantheline	GI antispasmodic
Tiotropium	Aerosol for COPD

* Muscarinic antagonists used for Parkinson’s disease are also listed in Chapter 28.

The structures of several muscarinic receptor antagonists are shown in Figure 10-7. The prototypical compound is atropine, which has been used for many years to define muscarinic responses. Scopolamine differs from atropine by the addition of an epoxide group that reduces the base strength and enables it to penetrate into the brain more readily than atropine. Other drugs, including some antihistamines, tricyclic antidepressants, and antipsychotics, are structurally similar to the muscarinic receptor antagonists and have prominent antimuscarinic side effects. The anticholinergic effects of these compounds are discussed in chapters specifically dealing with these drugs.

FIGURE 10–7 The structures of some muscarinic antagonists.

The effects of muscarinic receptor antagonists can be understood by knowledge of the distribution of muscarinic receptors throughout the body, as discussed in Chapter 9. For example, in the anterior segment of the eye, muscarinic receptor antagonists relax the iris sphincter and ciliary muscle, causing pupillary dilation (mydriasis) and a paralysis of the accommodation reflex (cycloplegia), resulting in blurred vision. Muscarinic receptor antagonists also relax nonvascular smooth muscle, including that in the lung airways, the GI tract, and the urinary bladder. Muscarinic receptor antagonists inhibit the secretion of substances from various exocrine glands, including sweat, salivary, lacrimal, and mucosal glands of the trachea and GI tract. In moderate to high doses, muscarinic receptor antagonists block the effects of ACh on the heart, resulting in an increased heart rate.

Muscarinic receptor antagonists that reach the brain (e.g., atropine and scopolamine) interfere with short-term memory and, in moderately high doses, cause delirium, excitement, agitation, and toxic psychosis. Quaternary amine versions of these compounds are available; they do not penetrate the blood-brain barrier very well, thereby reducing effects on the CNS (Chapter 2).

Atropine and other muscarinic receptor antagonists produce moderately selective dose-related effects after systemic administration. Low doses of scopolamine are more sedative than low doses of atropine. Low doses of atropine cause dry mouth, whereas high doses cause tachycardia and blockade of acid secretion by gastric parietal cells. These selective actions are attributed to the differential release of ACh at various synapses and junctions. Atropine antagonizes the effects of ACh more readily at sites where less neurotransmitter is released (e.g., salivary glands) than at sites where more is released (e.g., sinoatrial node).

Cholinomimetics

The rapid hydrolysis of ACh eliminates its direct clinical utility. Among the ChE-resistant muscarinic receptor agonists, lipid solubility is important in influencing absorption and distribution. Quaternary ammonium agonists such as bethanechol do not penetrate into the brain and are poorly absorbed orally. Consequently, oral administration confines their action to the GI tract, making them useful for enhancing GI motility.

The tertiary amines pilocarpine and physostigmine are well absorbed from the GI tract and penetrate readily into the CNS. They are administered topically in the eye to treat glaucoma. Pilocarpine is available in a reservoir-type diffusional device, which is placed behind the lower eyelid to deliver the drug at a constant rate for up to 1 week. Pilocarpine is also used to stimulate salivary secretion in patients after laryngeal surgery.

Highly lipophilic organophosphorus ChE inhibitors are well absorbed from the GI tract, lung, eye, and skin, making these compounds extremely dangerous and accounting for their use as nerve gases. The carbamate insecticides are not highly absorbed transdermally. The organophosphorus insecticides malathion and parathion are prodrugs, that is, they are inactive but can be metabolized to the active ChE inhibitors paraoxon and malaoxon. Malathion is relatively safe in mammals because it is hydrolyzed rapidly by plasma carboxylesterases. This detoxification occurs much more rapidly in birds and mammals than in insects. Malathion is available outside the United States in the form of a lotion or shampoo for the treatment of head lice.

Cholinergic Blocking Drugs

The lipophilicity and degree of ionization of cholinergic antagonists also influences their absorption and distribution. Quaternary ammonium antagonists are poorly absorbed from the GI tract and do not penetrate readily into the CNS (Chapter 2). The passage of these agents across the blood-brain barrier is strongly influenced by their degree of ionization, even among the tertiary amines. Although atropine and scopolamine have a similar affinity for muscarinic receptors, scopolamine is approximately 10 times more potent at producing CNS effects. This is attributed to its weaker base strength (pK_a = 7.53) relative to that of atropine (pK_a = 9.65). Consequently, a greater fraction of scopolamine is present in a unionized form at physiological pH relative to that of atropine (see Chapter 2). The relatively low pK_a of scopolamine facilitates its absorption through the skin, making the transdermal route of administration of the free base feasible, and has led to the therapeutic use of this agent to treat motion sickness. The selective M₁ receptor antagonist pirenzepine contains three tertiary amine groups, with a resultant high degree of ionization, thereby preventing its entry into brain.

Central Nervous System

Scopolamine is effective for preventing motion sickness through an inhibitory effect on the vestibular apparatus. Scopolamine base is available in a transdermal patch that is placed behind the ear and delivers scopolamine for approximately 3 days. When administered in this fashion, scopolamine is effective against motion sickness without causing substantial anticholinergic side effects. It is more effective when administered prophylactically.

Both muscarinic receptor agonists and antagonists are used in the treatment of several neurodegenerative disorders. Muscarinic receptor antagonists, such as trihexyphenidyl and benztropine, are used for Parkinson’s disease with some benefit, and centrally acting ChE inhibitors have been noted to improve the cognitive deficits in Alzheimer’s disease without halting the underlying progression of the disease. These agents and uses are discussed in Chapter 28.

Ophthalmology

When left unchecked, high intraocular pressure (glaucoma) damages the optic nerve and retina, resulting in blindness. Usually, glaucoma is caused by impaired drainage of aqueous humor produced by the ciliary epithelium in the posterior chamber of the eye. Normally, aqueous humor flows into the anterior chamber by first passing between the lens and iris and then out through the pupil (Fig. 10-8, A). It leaves the anterior chamber by flowing through the fenestrated trabecular meshwork and into Schlemm’s canal, which lies at the vertex of the angle formed by the intersection of the cornea and the iris. This region is known as the ocular angle. In primary angle-closure glaucoma, pressure from the posterior chamber pushes the iris forward, closing the ocular angle and preventing the drainage of aqueous humor (Fig. 10-8, B). People with narrow angles are predisposed to closed-angle glaucoma. In primary open-angle glaucoma, the ocular angle remains open, but abnormalities in the trabecular meshwork cause the outflow of aqueous humor to be impeded. In secondary glaucoma, inflammation, trauma, or various ocular diseases can cause intraocular pressure to increase.

FIGURE 10–8 The anterior chamber of the eye and the pharmacological and surgical measures used to treat glaucoma. A, The flow of aqueous humor (arrow) from the ciliary body to the trabecular meshwork. B, In angle-closure glaucoma, pressure from the posterior chamber pushes the iris against the trabecular meshwork, closing the ocular angle and preventing the drainage of aqueous humor. C, Cholinomimetics constrict the iris sphincter, thereby opening up the ocular angle and causing a decrease in intraocular pressure in closed-angle glaucoma. Cholinomimetics also elicit contraction of the longitudinal and circular ciliary muscles. Contraction of the longitudinal ciliary muscle stretches open the trabecular meshwork and facilitates the drainage of aqueous humor, particularly in open-angle glaucoma. The circular ciliary muscle forms a sphincter-like ring around the lens, into which the zonules are attached. Constriction of the circular muscle relaxes the tension on the zonules and allows the lens to relax into a more convex shape, which increases its refractive power and enables near vision. D, The surgical treatment of closed-angle glaucoma entails the placement of a hole in the peripheral iris, either surgically or with a laser. The procedure provides a pathway for drainage of aqueous humor (arrow) and thus reduces intraocular pressure.

Glaucoma is treated with directly- and indirectly-acting cholinergic agonists. Open-angle glaucoma is also treated with carbonic anhydrase inhibitors, β receptor blockers, and epinephrine (Epi).² When applied topically to the eye, cholinomimetics constrict the pupil, contract the ciliary muscles, and decrease intraocular pressure. In open-angle glaucoma, the contraction of the longitudinal ciliary muscle decreases intraocular pressure by stretching the trabecular meshwork and opening its tubules. In closed-angle glaucoma, pupillary constriction lowers the intraocular pressure by pulling the iris away from the trabeculum and opening the angle (see Fig. 10-8, C). Closed-angle glaucoma is a medical emergency that is corrected surgically by placing a hole in the peripheral portion of the iris to release pressure in the posterior chamber (see Fig. 10-8, D). Cholinomimetics are used acutely to treat closed-angle glaucoma until surgery can be performed. Occasionally, patients with closed-angle glaucoma exhibit a paradoxical increase in intraocular pressure in response to cholinomimetics because constriction of the pupil causes the iris to be pressed against the lens, thereby blocking the flow of aqueous humor into the anterior chamber. Cholinomimetics are also used to treat a variety of noninflammatory secondary glaucomas.

Among the cholinomimetics, pilocarpine is most commonly used. Because its duration of action is approximately 6 hours, it must be administered topically approximately four times a day. Pilocarpine can also be delivered in a reservoir as described. Although pilocarpine can cause local irritation, it is tolerated better than other cholinomimetics. Other short-acting cholinomimetics used in the treatment of glaucoma are carbachol and physostigmine. The long-acting organophosphorus ChE inhibitors—echothiophate, and isoflurophate—are sometimes used to treat glaucoma; however, these agents can lead to cataracts with long-term use. Consequently, they are used only in aphakic patients (i.e., those lacking a lens) or in those cases in which other agents are ineffective.

Muscarinic receptor antagonists dilate the pupil and relax the ciliary muscle, making them useful for examination of the retina and measurement of refractive errors of the lens, which often requires complete paralysis of the ciliary muscle. Choice of a mydriatic depends on its effectiveness and duration of action. Young children have a powerful accommodation reflex, requiring a highly potent and long-acting mydriatic such as atropine to cause complete blockade (cycloplegia). In contrast, shorter-acting agents such as tropicamide are used in older children and adults. The muscarinic receptor antagonists used in ophthalmology for refraction and their durations of action are given in Table 10-3.

TABLE 10–3 Pharmacokinetic Parameters for Muscarinic Receptor Antagonists used for Refraction

Muscarinic receptor antagonists are also used in the treatment of inflammatory uveitis and its associated glaucoma. Although the mechanism is not completely understood, they reduce the pain and photophobia associated with inflammation by paralyzing the eye muscles. Muscarinic receptor antagonists are also useful for breaking adhesions (synechiae) between the lens and iris that may be produced by inflammation. When treating uveitis, it is desirable to achieve a continuous relaxation of the eye muscles. Consequently, the long-acting agents atropine and scopolamine are frequently used. It is important to note that a variety of other drugs are used in ophthalmology, including agents acting on adrenergic function (e.g., sympathomimetics, α and β receptor antagonists), agents affecting prostaglandins (e.g. latanoprost), topical carbonic anhydrase inhibitors (e.g., dorzolamide), and other agents recently approved for macular degeneration that are monoclonal antibodies (e.g., ranibizumab and pegaptanib), which act as antagonists of vascular endothelial growth factor (VEGF).

Gastrointestinal and Urinary Tracts

The muscarinic receptor agonist bethanechol and the ChE inhibitor neostigmine are used to treat urinary retention and decreased stomach and bowel motility when there is no obstruction (e.g., postoperatively) and to treat congenital megacolon. When administered orally, these drugs primarily affect the GI tract.

Muscarinic receptor antagonists are used to treat conditions characterized by excessive motility of the GI and urinary tracts. Oxybutynin and tolterodine are frequently used in the treatment of urge incontinence and overactive bladder, whereas dicyclomine and propantheline are often used in irritable bowel syndrome. The latter two antagonists are also sometimes used in the treatment of urinary incontinence. Propantheline and homatropine are quaternary ammonium compounds; consequently, their actions are limited to the peripheral nervous system (PNS). Oxybutynin, tolterodine, darifenacin, solifenacin, and trospium are relatively selective muscarinic receptor antagonists, acting at M₂/M₃ receptors, that are approved for the treatment of overactive bladder. These subtype-selective receptor antagonists relieve the symptoms of urgency, frequency, and incontinence and exhibit reduced inhibitory effect on salivation and GI motility.

Nonselective muscarinic receptor antagonists were occasionally used to inhibit gastric acid secretion in peptic ulcer disease but have now been supplanted by the histamine receptor antagonists and proton pump inhibitors (see Chapter 18). Pirenzepine, a selective M₁ receptor antagonist that causes fewer side effects, is used in Europe to treat peptic ulcer disease but is not currently available in the United States.

Respiratory Tract

COPD is characterized by a persistent narrowing of the airways and excessive vagal tone contributing to bronchoconstriction. Muscarinic receptor antagonists are useful in the treatment of COPD. The quaternary antagonists, ipratropium and tiotropium, are administered with the use of an inhaler (see Chapter 16). Because systemic absorption of these compounds by the lung is poor, their muscarinic receptor blocking effects are usually confined to the lung. Tiotropium has a much longer duration of action than ipratropium, but both ipratropium and tiotropium have little or no inhibitory effect on mucociliary clearance, unlike most other muscarinic receptor antagonists such as atropine, which typically inhibit this function.

Myasthenia Gravis

Myasthenia gravis is an autoimmune disease characterized by skeletal muscle weakness. Many people afflicted with this disorder have circulating autoantibodies against the nicotinic cholinergic receptor at the neuromuscular junction, leading to reduced numbers of these receptors, which causes the weakness. Most cases of this disease are acquired (with an element of genetic predisposition), but there are also congenital myasthenia gravis cases. The ability of ChE inhibitors to amplify the effects of neuronally released ACh makes these drugs useful for temporarily restoring muscle strength in patients with myasthenia. ChE inhibitors are used alone in mild cases and in combination with immunosuppressants, including corticosteroids (e.g., prednisone) and other immunosuppressive drugs (azathioprine, cyclosporine, mycophenolate) and treatments (intravenous immune globulin [IVG] and plasmapheresis [removal and exchange of plasma to remove antibodies]) in more severe cases (see Chapter 6). Myasthenia gravis is also successfully treated surgically by removal of the thymus gland (thymectomy), because there is an abnormality of the thymus (hyperplasia or more rarely tumor) in the vast majority of cases that is associated with the autoimmune disease process.

The reversible ChE inhibitors, particularly pyridostigmine and to a lesser extent neostigmine, and rarely ambenonium (when patients have a hypersensitivity reaction to bromide that is present in the other ChE inhibitors), are used because these are all quaternary ammonium compounds, limiting their pharmacological effects to the PNS. These agents can also enhance neurotransmission at muscarinic receptors throughout the parasympathetic nervous system. Consequently, they frequently elicit GI side effects, including abdominal cramping and diarrhea. The effects of these agents last for 3 to 4 hours. Thus they must be administered frequently. The use of organophosphorus ChE inhibitors is not beneficial, because frequent dose adjustments of the ChE inhibitor are necessary in patients with myasthenia gravis, making the long-lasting effects of irreversible agents inappropriate. In addition, an excess of an organophosporus ChE inhibitor could cause a long-lasting paralysis of muscle resulting from desensitization and depolarization blockade, leading to a potential lethal contraindication.

Edrophonium, the short-acting ChE inhibitor, is useful for the diagnosis of myasthenia gravis. Affected patients show a brief improvement in strength after intravenous administration of edrophonium, whereas patients with other muscular diseases do not. Muscle strength can suddenly deteriorate during therapy with ChE inhibitors as a consequence of insufficient ChE inhibition (myasthenic crisis) or too much ChE inhibition, resulting in depolarization blockade (cholinergic crisis). Edrophonium is used to distinguish between these two conditions. A resulting improvement in strength is diagnostic of a myasthenic crisis; no response or a worsening indicates a cholinergic crisis.

Hypertensive Emergencies

Ganglionic blockers were developed initially for the treatment of hypertension, but better agents are now available (see Chapter 20). Because blockade of both sympathetic and parasympathetic ganglia causes numerous side effects, ganglionic blockers are used only for treatment of hypertensive emergencies that occur during surgery or with an aortic aneurysm. Because of its quaternary ammonium structure, trimethaphan, which is used clinically, has effects only in the PNS. In addition, it selectively blocks nicotinic receptors at autonomic ganglia and not at the neuromuscular junction. Mecamylamine, a second ganglionic blocking drug, is rarely used and has both central and peripheral effects.

Miscellaneous Uses of Cholinesterase Inhibitors

ChE inhibitors are also used to rapidly reverse the neuromuscular blockade induced during surgery. Neuromuscular blocking agents are often administered as adjuncts during general anesthesia (see Chapter 12), and it is sometimes useful to rapidly reverse neuromuscular blockade after surgery. The reversible ChE inhibitors such as neostigmine and pyridostigmine are used to increase the availability of ACh, which competes with the competitive neuromuscular blockers and restores function at the neuromuscular junction, but these agents are not useful in reversing the depolarizing neuromuscular blockers. Atropine is often used in combination with the ChE inhibitor to counteract the muscarinic effects of ChE inhibition.

Muscarinic Receptor Agonists

The side effects of muscarinic receptor agonists are an extension of their parasympathomimetic actions and include miosis, blurred vision, lacrimation, excessive salivation and bronchial secretions, sweating, bronchoconstriction, bradycardia, abdominal cramping, increased gastric acid secretion, diarrhea, and polyuria. Muscarinic receptor agonists that can penetrate into the brain also cause tremor, hypothermia, and convulsions. Although most peripheral blood vessels lack cholinergic innervation, they have endothelial muscarinic receptors that trigger vasodilation and a decrease in blood pressure. Similarly, cardiac ventricles receive little parasympathetic innervation, yet they contain muscarinic receptors that decrease the force of contraction. If given in sufficient doses or administered parenterally, muscarinic receptor agonists can trigger acute circulatory failure with cardiac arrest. Atropine antagonizes all of these effects and is a useful antidote to poisoning with muscarinic receptor agonists.

Muscarinic receptor agonists are contraindicated in patients with diseases that make them susceptible to parasympathetic stimulation. The bronchoconstriction induced by muscarinic receptor agonists can have disastrous consequences in patients with asthma or COPD. Similarly, they are contraindicated in patients with peptic ulcer disease because they stimulate gastric acid secretion. Muscarinic receptor agonists are also contraindicated in patients with an obstruction in the GI or urinary tract because the stimulatory effect exacerbates the blockage, causing pressure to build up that may lead to perforation.

Mushrooms of the genera Inocybe and Clitocybe contain appreciable amounts of muscarine, which can cause rapid-type mushroom poisoning. Signs and symptoms occur within 30 to 60 minutes after ingestion of the mushrooms and are similar to the peripheral muscarinic effects described earlier. Atropine is administered as an antidote.

Cholinesterase Inhibitors

The side effects of the ChE inhibitors are similar to those of the muscarinic receptor agonists but also include toxic effects at the neuromuscular junction. For quaternary ammonium compounds, the cholinergic symptoms are confined to the PNS. Organophosphorus ChE inhibitors are particularly dangerous, because they are readily absorbed through the skin, lungs, and conjunctiva. They are used as insecticides and can be dangerous to agricultural workers. They are contraindicated in patients with asthma, COPD, peptic ulcer disease, or GI or urinary tract obstruction for the same reasons that muscarinic receptor agonists are contraindicated in these patients. A useful pneumonic for the common adverse effects of the ChE inhibitors has been proposed—the SLUD-B syndrome, which stands for salvation, lacrimation, urination, defecation, and bradycardia.

People can be exposed to irreversible ChE inhibitors accidentally when these agents are used as insecticides, or deliberately through use of these agents as poison gas in terror or battlefield attacks. Individuals exposed to these organophosphorus compounds experience miosis, blurred vision, profuse salivation, sweating, bronchoconstriction, difficulty breathing, bradycardia, abdominal cramping, diarrhea, polyuria, tremor, and muscle fasciculations that can progress to convulsions. With increased exposure, blood pressure decreases and skeletal muscles weaken as a result of depolarization blockade at the neuromuscular junction, causing paralysis of the diaphragm and respiratory failure, exacerbated by increased bronchial secretions and pulmonary edema. Death usually results from respiratory failure.

ChE inhibitor poisoning is diagnosed on the basis of the signs and symptoms and the patient’s history. Diagnosis can be verified by plasma or erythrocyte ChE determinations, if time permits. Atropine is used to reverse the effects of ACh at muscarinic synapses and must be continually administered as long as ChE is inhibited. Organophosphorus irreversible ChE inhibitors phosphorylate a serine hydroxyl group in the active site of the enzyme (see Fig. 10-6) and results in excessive accumulation of ACh, which causes the toxicities. With many organophosporus ChE inhibitors, an additional reaction occurs called “aging.” Aging involves a chemical separation of a part of the alkyl or alkoxy portion of the ChE inhibitor molecule. The resultant complex is then resistant to spontaneous hydrolysis or reactivation. Oxime compounds such as 2-PAM can be used to reactivate the ChE and treat the adverse effects of organophosphorus ChE inhibitor poisoning. 2-PAM is a site-directed nucleophile that reacts with the phosphorylated-ChE complex to regenerate the free enzyme. It is important that the oxime is administered as soon after organophosphorus exposure as possible before the enzyme ages. 2-PAM is ineffective against the toxicity caused by carbamate inhibitors. In the United States only 2-PAM is available, and it is in a fixed combination with atropine in an injectable form known as ATNAA (Antidote Treatment-Nerve Agent, Auto-Injector). Considerable interest has been generated in administering reversible ChE inhibitors, such as pyridostigmine, prophylactically in battlefield situations where exposure to a nerve gas is possible, but the efficacy of this use is not clear. Benzodiazepines, including diazepam, are used to treat the convulsive seizures caused by ChE inhibitors. Supportive therapy is also important, including airway maintenance, ventilatory support, and O₂ administration.

Muscarinic Receptor Antagonists

The side effects associated with the use of muscarinic receptor antagonists can be attributed to blockade of muscarinic receptors throughout the CNS and PNS. These agents cause mydriasis, cycloplegia, blurred vision, dry mouth, tachycardia, urinary retention, cutaneous vasodilation, and decreased motility of the stomach and intestines with constipation. In low doses muscarinic antagonists that enter the brain interfere with memory and are sedating; sedation is a prominent side effect of scopolamine. However, in moderate to high doses, centrally active muscarinic receptor antagonists cause excitation, hallucinations, delirium, stupor, toxic psychosis, and convulsions, which can be followed by respiratory depression and death.

The ophthalmological use of muscarinic receptor antagonists is contraindicated in the elderly and in patients with inherently narrow angles. In such patients topical application of muscarinic receptor antagonists to the eye can trigger acute angle-closure glaucoma. Muscarinic receptor antagonists are also contraindicated in patients with bowel atony, urinary retention, or prostatic hypertrophy. They are also contraindicated in patients receiving other drugs with prominent anticholinergic side effects.

Nicotine

Nicotine poisoning results from the ingestion of tobacco products or exposure to nicotine-containing insecticides. The pharmacological response is complex (see earlier discussion) because nicotine stimulates nicotinic receptors at sympathetic and parasympathetic ganglia, at the neuromuscular junction, and in the brain. Moreover, nicotine stimulates the neuromuscular junction but can produce a blockade, with muscle weakness, paralysis of the diaphragm, and respiratory failure. Signs and symptoms of acute nicotine intoxication include nausea, vomiting, salivation, diarrhea, perturbed vision, mental confusion, tremors, and convulsions, followed by depression and coma. The actions of nicotine on the cardiovascular system

include an initial increase in blood pressure, followed by a decrease, weak pulse, and variable heart rate. Treatment of toxicity after oral ingestion includes gastric lavage or inducement of vomiting with syrup of ipecac. Support of respiration and maintenance of blood pressure are also important.