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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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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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