[Effect of cholinergic compounds on spontaneous quantal mediator release at the neuromuscular synapse of the frog].

The action of cholinergic drugs on spontaneous quantal transmitter release has been investigated in the frog sartorius muscle. Acetylcholine and carbacholine decreased the miniature end-plate potential frequency. These presynaptic effects had no dependence on the potassium concentration in the bath solution. Nicotinic agonists--nicotine, tetramethylammonium and suberyldicholine had a similar effect, while muscarinic agents--methylfurmetide, oxotremorine and F-2268 (L- and D-stereoisomers) did not affect the transmitter release. The presynaptic effects of carbacholine and acetylcholine were abolished neither by atropine nor by d-tubocurarine and bensohexonium. It is suggested that there are nicotinic receptors on the frog motor nerve terminals that modify spontaneous quantal transmitter release and differ pharmacologically from nicotinic end-plate, ganglionic and presynaptic receptors of higher vertebrates.