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

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. H1-receptor blockers (10(-5)-10(-4) M) reduced the amplitude of e.p.ps recorded intracellularly without affecting the resting membrane potential. The acetylcholine potential was decreased by perfusion of H1-receptor blockers. However, when the muscle fibre was stimulated directly, threshold and magnitude of action potential were not affected by H1-receptor blockers. 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. The quantal content was not reduced by perfusion of H1-receptor blockers at a concentration of 10(-4) M. It is assumed that the major site of action of these H1-receptor blocking agents is at the postsynaptic membrane.