, Volume 434, Issue 6, pp 735-741

Cholinergic agonists decrease quantal output at the frog neuromuscular junction by targeting a calcium channel blocked by ω-conotoxin

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Abstract

 Nicotinic cholinergic agonists are known to decrease synchronous evoked quantal output at the frog neuromuscular junction [Van der Kloot 1993, J Physiol (Lond) 468:567–589]. Here we also show that carbachol decreases the frequency of miniature endplate potentials (F MEPP) in solutions containing elevated levels of K+ and Ca2+. Carbachol did not decrease F MEPP in hypertonic solutions or in solutions containing the Ca2+ ionophore ionomycin and Ca2+. We conclude that the nicotinic agonists decrease Ca2+ influx through voltage-gated Ca2+ channels. Carbachol did not alter two-pulse facilitation. A blocker of N-type Ca2+ channels, ω-conotoxin GVIA, antagonized the nicotinic agonist-induced decrease in evoked quantal output. The effect of carbachol was not altered by ω-conotoxin MVIIC, a blocker of P-type and certain other Ca2+channels. The Ca2+ channel targeted by the nicotinic agonists appears to be of the N-type.

Received: 6 March 1997 / Received after revision: 6 May 1997 / Accepted: 4 June 1997