Giant miniature endplate potentials induced by 4-aminoquinoline

In experiments on the isolated extensor digitorum longus muscle of the rat it was shown that 4-aminoquinoline (125-250 micro M) altered the amplitude distribution of spontaneous miniature endplate potentials to include a large portion of giant miniature endplate potentials with slow rise and decay times. Similar, slow-rising giant miniature endplate potentials were induced by the drug at neuromuscular junctions with regenerating nerve terminals, i.e. in a condition where spontaneous as well as evoked transmitter release is depressed. The appearance of giant miniature endplate potentials was not correlated with inhibition of cholinesterase since neostigmine (3 micro M) failed to induce such potentials. Nerve impulse evoked endplate potentials of amplitudes similar to the spontaneous giant miniature endplate potentials had a faster and more uniform rise time. The results suggest that 4-amino-quinoline, by a direct action on the nerve terminal, causes the release of larger than normal quanta of acetylcholine. Quantitative assays of acetylcholine released before and in the presence of 4-aminoquinoline gave similar values showing that the amounts of acetylcholine which give rise to the giant miniature potentials contribute little to the total amount of acetylcholine liberated.