Effects of H1 receptor blocking drugs on the frog sartorius neuromuscular junction

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Abstract
1 The effects of H1-receptor blocking agents, pyrilamine, promethazine, and diphenhydramine, on the amplitude and time course of endplate potentials (e.p.ps) were studied in the sartorius muscles of frogs. 2 H1-receptor blockers (10−5-10−4M) reduced the amplitude of e.p.ps recorded intracellularly without affecting the resting membrane potential. 3 The acetylcholine potential was decreased by perfusion of H1-preceptor blockers. However, when the muscle fibre was stimulated directly, threshold and magnitude of action potential were not affected by H1-receptor blockers. 4 The time constant of decay of the e.p.ps recorded extracellularly was reduced by H1-receptor blockers and the decay remained exponential with a single time constant. 5 The quantal content was not reduced by perfusion of H1-receptor blockers at a concentration of 10−4 M. 6 It is assumed that the major site of action of these H1-receptor blocking agents is at the postsynaptic membrane.