Cholinergic inhibition of acetylcholine release in the electric organ of Torpedo.

The possibility that acetylcholine (ACh) may inhibit its own release from nerve terminals by acting on presynaptic receptors has been investigated using the electric organ of Torpedo marmorata. ACh release was analysed by electrophysiological and biochemical methods conjointly. Oxotremorine, at micromolar concentrations, depressed nerve-electroplaque transmission by reducing the amount of ACh released by nerve impulses. This effect was competitively antagonised by nanomolar concentrations of atropine or methylatropine. Other muscarinic agonists, betanechol, pilocarpine and muscarine, however, failed to depress transmission but choline was effective at high concentrations. Anticholinesterase drugs, physostigmine, neostigmine or fluostigmine (diisopropylfluorophosphate, given as pretreatment and subsequently washed out) markedly depressed evoked ACh release. When cholinesterase was inhibited, the addition of oxotremorine or exogenous ACh caused a further depression of ACh release. Atropine was found to be very effective in reversing the depression of transmitter release produced by anticholinesterases. Looking for the mechanism of these presynaptic changes, we found that oxotremorine had little, if any, effect on the size of the ACh store of the tissue, on the compartmentation of ACh inside and outside synaptic vesicles, or on the rate of ACh turnover. The changes induced by oxotremorine cannot be explained by a reduction in calcium entry, since the presence of oxotremorine did not change the uptake of 45Ca observed after repetitive stimulation. Electrophysiological techniques were used to test for an effect of atropine in experiments where transmission of one impulse was expected to depress ACh release by subsequent impulses. This depression was not affected atropine, making it unlikely that the 'muscarinic' inhibition of ACh release has a role as a short-term feedback regulation of transmission. A second possibility is that oxotremorine (and external non-hydrolysed ACh) can enter the presynaptic membrane and interfere with the mechanism of transmitter release.