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When synaptic transmission depends upon acetylcholine as the primary neurotransmitter, it is labeled cholinergic. The termination of acetylcholine activity is mediated by the enzymeacetylcholinesterase. There are two subtypes of cholinergic receptors, muscarinic (M) and nicotinic (N). Agonists that mimic the effects of acetylcholine are defined as cholinomimetics. Some drugs are direct-acting agonists for the cholinergic receptors (Figure 5–1). Other drugs function as indirect-acting agonists by preventing the inactivation of acetylcholine. Antagonists that inhibit acetylcholine at muscarinic or nicotinic receptors are defined as anticholinergics. Drugs that selectively inhibit muscarinic receptors are called antimuscarinics (Figure 5–2), whereas those that selectively inhibit nicotinic receptors are antinicotinics.

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Figure 5–1.
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Algorithm of cholinomimetic drugs. Some drugs are direct-acting agonists that stimulate the muscarinic or nicotinic receptors. Alternatively, the drugs may be indirect-acting in that they inhibit the enzymeacetylcholinesterase that is responsible for terminating the action of acetylcholine.

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Figure 5–2.
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Grouping of cholinoreceptor direct-acting antagonists based on their inhibition of either muscarinic (M) or nicotinic (N) receptors. Further subdivisions for the muscarinic receptors include drugs that are specific antagonists of M1 receptors located on nerve endings or nonspecific muscarinic antagonists. Nicotinic antagonists are subdivided based on whether the drug inhibits postsynaptic receptors at the neuromuscular junction (NM) or postsynaptic receptors in the parasympathetic and sympathetic ganglia (NN).

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The subtypes of cholinoreceptors are set forth in Table 5–1. At present, subtype-selective agonists for the muscarinic receptors are not clinically available. Direct-acting nicotinic agonists may be classified on the basis of whether ganglionic (NN) or neuromuscular (NM) stimulation predominates, but agonist selectivity is very limited. Several molecular mechanisms for receptor signaling have been identified for muscarinic receptors (Table 5–1). In general, these receptors modulate the formation of second messengers or the activity of ion channels. In contrast, all nicotinic receptors cause the opening of a channel selective for sodium and potassium that results in cellular depolarization. This signaling mechanism occurs in the autonomic ganglia and at the neuromuscular junction.

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Table 5–1. Subtypes and Characteristics of Cholinoceptors

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