Pharmacological characterization of an epibatidine binding site in the nerve cord of Periplaneta americana

A binding site for [3H](+/-)-epibatidine was found to be present in American cockroach nerve cord homogenates. This binding site was saturable, with a Kd of 6.4 +/- 0.8 nM and a Bmax of 3.4 +/- 1.1 pmol/mg protein. The pharmacology of this site suggests it to be associated with the nicotinic acetylcholine receptor. (-)-Nicotine, imidacloprid, acetamiprid, (-)cytisine, and the isomers of epibatidine all displaced [3H](+/-)-epibatidine. Additionally, all these compounds activated currents from whole-cell patch-clamped cockroach neurons, although there were differences in the magnitudes of maximal currents produced. The nicotinic antagonist, methyllycaconatine, blocked the agonist actions of (-)-nicotine. (+)-epibatidine, and (-)-epibatidine. Methyllycaconatine and alpha-bungarotoxin also displaced [3H](+/-)-epibatidine in the cockroach nerve cord homogenates. Attempts at demonstrating specific binding of [3H](+/-)-epibatidine in house fly head membranes were unsuccessful and may suggest the existence of species-specific epibatidine binding sites or a much lower density of these sites in house fly head membranes. Taken as a whole, the data indicate that [3H](+/-) -epibatidine labels a site at the nicotinic acetylcholine receptor in cockroach nerve cord and, consequently, should prove useful in the continued characterization of insect nicotinic receptors