Actions of Cholinergic Pharmacological Agents on the Cell Body Membrane of the Fast Coxal Depressor Motoneurone of the Cockroach (Periplaneta Americana)

The pharmacological properties of cholinergic receptors on the cell body membrane of the fast coxal depressor motoneurone (Df) of the cockroach (Periplaneta americana) have been investigated. Parallel dose-response curves were obtained for the depolarizing actions of four bath-applied agonists, with the following order of effectiveness: nicotine>acetylcholine (ACh), in the presence of 1.0×10−7M neostigmine>carbamylcholine>tetramethylammonium. By contrast, dimethyl-4-phenyl piperazinium, suberyldicholine, D,L-muscarine, oxotremorine, acetyl-β-methylcholine and sebacinylcholine were practically ineffective. Of the three putative receptor-specific ligands used to date in binding studies on insect CNS tissues, α-bungarotoxin (α-BGTX) was much more effective (I50 = 6.4×10−8M) in blocking the depolarization resulting from ionophoretic application of ACh, than either quinuclidinyl benzilate (QNB) (I50 = 1.6×10−4 M) or decamethonium (I50 × 2.8×10−3 M). The order of effectiveness of ligands that were particularly effective in blocking ACh depolarization was α-BGTX>α-cobratoxin (α-COTX)>mecamylamine>dihydro-)β-erythroidine>benzoquinonium. Less potent and almost equally effective were atropine, d-tubocurarine, pancuronium and quinuclidinyl benzilate. Even less effective were hexamethonium, gallamine, decamethonium and succinylcholine, all requiring concentrations of 1.0×10−3M and higher to produce a significant block of the ACh response. Not all reversibly acting antagonists were equally effective in preventing irreversible block of the ACh-induced depolarization by α-BGTX. Whereas α-COTX protected the receptors, mecamylamine did not. With the cell body of Df voltage-clamped, the degree of antagonism of the ACh-induced current was assessed at potentials in the range −120 mV to −60 mV. α-BGTX, dihydro-β-erythroidine, benzoquinonium, QNB and decamethonium appeared to be voltage-independent over this potential range, whereas d-tubocurarine and atropine were strongly voltage-dependent in their blocking actions. Sites of action of cholinergic antagonists at the insect ACh receptor/ion channel complex are discussed.