Synaptic and Extrasynaptic Neuronal Nicotinic Receptors of Insects

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The nicotinic acetylcholine receptors of muscle and muscle-derived tissue (such as the electric organ) have been well characterized and are pentamers with four types of subunit (α2βγδ) (Conti-Tronconi and Raftery 1982; Noda et al 1983). Though much less is known of the properties and functions of nervous system nicotinic acetylcholine receptors, techniques of molecular biology and physiology have recently established the existence of 3 types of non-muscle receptor genes (α2, α3, α4) and one non-muscle gene (β2) (Barnard et al 1986; Lindstrom et al 1987). Expression studies with Xenopus oocytes have employed electrophysiological studies to monitor the appearance of functional neuronal nicotinic receptors. In this way various combinations of a and s subunits have been shown to yield functional receptors and their pharmacology is that expected for vertebrate neuronal nicotinic receptors. For example, they are blocked by neuronal bungarotoxin (= k-bungarotoxin) but not by α-bungarotoxin (Boulter et al 1987). Thus, evidence is emerging for a neuronal nicotinic receptor superfamily encoded by a separate set of genes from those encoding subunits of muscle nicotinic receptors.