Reviews of Physiology, Biochemistry and Pharmacology 147

Ligand-gated ion channels mediate fast synaptic transmission in the central nervous s- tem (CNS) and at ganglionic and neuromuscular synapses. The nicotinic acetylcholine - ceptor (nAChR) is a member of the ligand-gated ion channel superfamily, which includes the 5-HT , glycine and GABA type A and C receptors. These receptors are known as Cys- 3 loop receptors, as all of them have a conserved sequence containing a pair of cysteines separated by 13 residues and linked by a disulfide bridge. nAChRs can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system. Many of the early studies carried out on the subunit composition and structure of the nAChRs were performed on receptors isolated from the electric organ of Torpedo californica, as this tissue is very rich in nAChRs, and they were found to have ahigh degree of homology with the embryonic v- tebrate muscle type receptor. Muscle nAChRs are made up of five subunits arranged around a central pore (Fig. 1A, B). In Torpedo electric organ and vertebrate fetal muscle, the subunit composition is (a1) b1gd, and in adult muscle the g subunit is replaced by the e to give an (a1)b1ed 2 2 composition (Mishina et al. 1986).