The pharmacology of α-bungarotoxin-resistant acetylcholine receptors on an identified cockroach motoneurone

The effects of a number of cholinergic agonists and antagonists have been examined on the cell body of the fast coxal depressor (D_f) of the cockroach Periplaneta americana using voltage-clamp techniques. Acetylcholine (ACh), when applied to this neurone voltage-clamped at its normal resting potential, results in the generation of an inward current through the activation of receptors which are blocked by α-bungarotoxin (α-bgt). At more depolarized membrane potentials acetylcholine induces an inward current which is strongly voltage-dependent and which is insensitive to α-bgt. An α-bgt-resistant current is also induced by the application of a number of cholinergic agonists. In order of potency these are: decamethonium>oxotremorine = McN-A-343 = (+)-muscarine = arecaidine propargyl ester >nicotine >ACh. This α-bgt-resistant response to cholinergic agonists is blocked by a range of antagonists. In order of potency these are : decamethonium >pirenzipine >quinuclidinyl benzilate = atropine = p-f-hexahydro-sila-difenidol = dexetimide >scopolamine = methoctramine. The receptors mediating this response have been termed 'mixed cholinergic' due to their sensitivity to both nicotinic and muscarinic ligands. Pharmacological similarities between these 'mixed' receptors and binding components found in high speed supernatant fractions of insect nervous tissue are discussed.