THE NICOTINIC ACETYLCHOLINE RECEPTOR
The nicotinic ACh receptor mediates neurotransmission postsynaptically at the neuromuscular junction and peripheral autonomic ganglia; in the CNS, it largely modulates release of neurotransmitters from presynaptic sites. The receptor is called the nicotinic ACh receptor because both the alkaloid nicotine and the neurotransmitter ACh can stimulate the receptor. Distinct subtypes of nicotinic receptors exist at the neuromuscular junction (Nm), in autonomic ganglia, and in the CNS (the neuronal form, Nn). The binding of ACh to the nicotinic ACh receptor initiates an EPP in muscle or an EPSP in peripheral ganglia by directly mediating cation influx into the postsynaptic cell (see Chapter 8).
Classical studies of the actions of curare and nicotine defined the concept of the nicotinic ACh receptor over a century ago and made this the prototypical pharmacological receptor. By taking advantage of specialized structures that have evolved to mediate cholinergic neurotransmission and of natural toxins that block motor activity, nicotinic receptors were isolated and characterized. These accomplishments represent landmarks in the development of molecular pharmacology.
Cholinergic neurotransmission mediates motor activity in marine vertebrates and mammals, and a large number of peptide, terpinoid, and alkaloid toxins that block the nicotinic receptors have evolved to enhance predation or protect plant and animal species from predation (Taylor et al., 2007). Among these toxins are the α-toxins: peptides of about 7 kDa from venoms of the krait, Bungarus multicinctus, and varieties of the cobra, Naja naja. These toxins potently inhibit neuromuscular transmission, are readily radiolabeled, and provide excellent probes for the nicotinic receptor.
The electrical organs from the aquatic species of Electrophorus and Torpedo provide rich sources of nicotinic receptor; up to 40% of the surface of the electric organ’s membrane is excitable and contains cholinergic receptors, in contrast to vertebrate skeletal muscle, in which motor end plates occupy 0.1% or less of the cell surface. Using the α-toxin probes, the receptor from Torpedo was purified, the cDNAs of the subunits were isolated, and the genes were cloned for the multiple receptor subunits from mammalian neurons and muscle (Numa et al., 1983). By simultaneously expressing various permutations of the genes that encode the individual subunits in cellular systems and then measuring binding and the electrophysiological events that result from activation by agonists, researchers have been able to correlate functional properties with details of primary structures of the receptor subtypes (Changeux and Edelstein, 2005; Karlin, 2002; Sine et al., 2008).
Structure of Nicotinic Receptors
In vertebrates, the nicotinic receptors of skeletal muscle Nm are pentamers composed of four distinct subunits (α, β, γ, and δ) in a stoichiometric ratio of 2:1:1:1 (Changeux and Edelstein, 2005; Karlin, 2002; Unwin, 2005). In mature, innervated muscle end plates, the γ subunit is replaced by ε, a ...