A neurotoxin able to increase the spontaneous release of transmitter was found in the venom glands of the polychaete annelid Glycera convoluta. We studied the effect of this venom on the frog cutaneous pectoris muscle, where its application produced a prolonged (20-h), high-frequency discharge of miniature potentials. After 5 h of action, the initial store was renewed several times but no detectable ultrastructural changes were observed. After 19 h of sustained activity, nerve terminals with their normal vesicular contents were infrequent; others were fragmented and contained swollen mitochondria, abnormal inclusions, and vesicles of various sizes. In the noncholinergic crayfish neuromuscular preparation, the venom triggered an important increase in spontaneous quantal release that subsided in 1 h. An activity higher than that in resting conditions then persisted for many hours. This high electrical activity was not accompanied by any detectable structural modifications after 3 h. In the torpedo electric organ preparation, the venom elicited a burst of activity that returned to control levels in 1 h. The release of ACh (evaluated by the efflux of radioactive acetate) paralleled the high electrical activity. No morphological changes or significant depletion of tissue stores were detected. The venom of Glycera convoluta appears to enhance considerably the release of transmitter without impairing its turnover. The venom effect is Ca++ dependent and reversible by washing, at least during the first hour of action. Because the high rate of transmitter release appears dissociated from the later-occurring structural modifications, it is possible that the venom mimics one component of the double mode of action proposed for black widow spider venom.

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