veterinary (in sheep): Oral (drench): 2 mg/kg

A two-micropipette current-clamp technique was used to study electrophysiological effects of the anthelmintics on: (1) acetylcholine responses in somatic muscle and; (2) on l-glutamate responses in pharyngeal preparations. On somatic muscle, derquantel (0.1-30μM) produced a potent (IC50 0.22, CI 0.18-0.28μM) reversible antagonism of acetylcholine depolarizations. Abamectin (0.3μM) produced a slow onset inhibition of acetylcholine depolarizations. It was compared effects of abamectin and derquantel on muscle preparations pretreated for 30 min with these drugs. The effect of the combination was significantly greater than the predicted additive effect of both drugs at higher acetylcholine concentrations. On the pharynx, application of derquantel produced no significant effect by itself or on responses to abamectin and l-glutamate. Abamectin increased the input conductance of the pharynx (EC50 0.42, CI 0.13-1.36μM). It was demontrsated that abamectin and derquantel interact at nicotinic acetylcholine receptors on the somatic muscle and suggested synergism can occur.