Acetylcholine receptors have been classified into subtypes based on the pharmacology of the receptors

Initially, subtyping of the receptors in the cholinergic nervous system was based on the pharmacological activity of two alkaloids: nicotine and muscarine (Fig. 11-2). This classification occurred long before the structures of these naturally occurring agonists were determined (Fig. 11-3). The greatly different activities of the antagonists atropine on muscarinic receptors and d-tubocurarine on nicotinic receptors further supported the argument that multiple classes of receptors exist for ACh. Subsequently, it was found that all nicotinic receptors are not identical. Nicotinic receptors in the neuromuscular junction, sometimes denoted as N₁ receptors, show selectivity for phenyltrimethylam-monium as an agonist; elicit membrane depolarization in the presence of bisquaternary agents, with decamethonium being the most potent; are preferentially blocked by the competitive antagonist d-tubocurarine; and are blocked irreversibly by the snake α-toxins. Nicotinic receptors in ganglia, N₂ receptors, are stimulated preferentially by 1,1-dimethyl-4-phenylpiperazinium; blocked competitively by trimethaphan; blocked noncompetitively by bisquaternary agents, with hexamethonium being the most potent; and resistant to the snake α-toxins [9]. A large number of distinct neuronal nicotinic receptors are found in the central nervous system (CNS); they are closer relatives of the nicotinic receptors in ganglia than of those in muscle.

Figure 11-2

Classification of cholinergic receptors. The diagram shows a historical classification of receptors analyzed on the basis of distinct responses with crude alkaloids (stage 1), the partial resolution of receptor subtypes with chemically synthesized agonists (more...)

Figure 11-3

Structure of compounds important to the classification of receptor subtypes at cholinergic synapses. Compounds are subdivided as nicotinic (N) and muscarinic (M). The compounds interacting with nicotinic receptors are subdivided further according to whether (more...)

Muscarinic receptors also exhibit distinct subtypes. The antagonist pirenzepine (PZ) has the highest affinity for one subtype, M₁, which is found mainly in neuronal tissues. Another antagonist, methoctramine, has a higher affinity for M₂ receptors, which are the predominant muscarinic receptor subtype in mammalian heart. Hexahydrosiladifenidol is relatively selective for the M₃ receptors present in smooth muscle and glands, whereas himbacine exhibits high affinity for M₄ receptors. With this level of multiplicity of receptor subtypes, limitations on specificity preclude a single antagonist defining a distinct subtype.

The intrinsic complexity and the multiplicity of cholinergic receptors became evident upon elucidation of their primary structures

In the CNS, at least eight different sequences of α subunits and three different sequences of β subunits of the nicotinic receptor have been identified [10,11]. Expression of the cloned genes encoding certain subunit combinations yields functional receptors with different sensitivities toward various toxins and agonists.

At least five distinct muscarinic receptor genes have been cloned and sequenced. The genes are called m₁ to m₅. The m₁ to m₄ clones correlate with the M₁ to M₄ receptors identified pharmacologically. The subtypes differ in their ability to couple to different G proteins and, hence, to elicit cellular signaling events. The muscarinic and nicotinic receptor subtypes exhibit distinct regional locations of their mRNAs, based on in situ hybridization.

Thus, cholinergic receptor classification can be considered in terms of three stages of development. Initially, Dale [2] distinguished nicotinic and muscarinic receptor subtypes with crude alkaloids. Then, chemical synthesis and structure—activity relationships clearly revealed that nicotinic and muscarinic receptors were heterogeneous but could not come close to uncovering the true diversity of receptor subtypes. Lastly, analysis of subtypes comes from molecular cloning, which makes possible the classification of receptors on the basis of primary structure (Fig. 11-2).