Anion blockers inhibit impulse-evoked quantal transmitter release at the crayfish neuromuscular junction

1. The effects of the anion-transport blocking reagents Probenecid, 4,4'diisothiocyanostilbene-2,2'-disulphonate (DIDS), and pyridoxal phosphate on the number of quanta released (m) and the time course of the probability of quantal release, alpha(t), following an action potential were studied at single synaptic sites on the opener muscle in the crayfish leg.2. Low concentrations (10-50 muM) of DIDS or Probenecid produced decreases in m to about 30-50% of the initial value; at higher concentrations (100 muM or greater), evoked release failed. Much greater concentrations (500 muM) of pyridoxal phosphate were necessary to produce decreases in m.3. Synaptic depression in 50 muM-Probenecid was reversible by removing the agent; after treatment with 50 muM-DIDS, however, m partially regained the initial value upon removal of the drug.4. The time course of alpha(t) after an action potential was unaffected by any of the reagents.5. It is concluded that the reagents decrease the number of quanta evoked by a nerve impulse by decreasing the magnitude, but not the time course, of the average rate of quantal release following an action potential; it is suggested that nerves and other secretory cells may share a common mechanism during exocytosis that is subject to modification by these agents.