Lobster neuromuscular junctions treated with black widow spider venom: correlation between ultrastructure and physiology.

Black widow spider venom (BWSV) causes marked physiological and morphological alterations at the lobster neuromuscular junction. BWSV is also active at vertebrate neuromuscular junctions but the component which acts on the lobster preparation is different from the one which affects vertebrates. Following exposure to BWSV, lobster neuromuscular junctions showed elevated frequencies of spontaneous miniature synaptic potentials for 15-30 min. Nerve-evoked synaptic potentials became blocked during this period. Subsequently, spontaneous miniature potentials disappeared and less frequent 'giant' spontaneous potentials appeared. Ultrastructural examination of excitatory and inhibitory nerve terminals showed that both types were affected by venom treatment. In untreated terminals, synaptic vesicles were grouped near the dense specialized membranes of the synapses. Soon after venom treatment, the synaptic vesicles were dispersed throughout the terminals and many larger and elongated vesicular structures were apparent. At the time of appearance of 'giant' spontaneous potentials, few synaptic vesicles were seen in the terminals, but large irregular vacuoles were present. Many mitochondria within the nerve terminals were swollen or disrupted, while nearby muscle mitochondria remained normal in size and appearance. Very few presynaptic dense bodies ('active zones') were seen at synapses of affected terminals. The observations are consistent with the hypothesis that BWSV allows an abnormal amount of Ca2+ to enter the nerve terminals, causing the various physiological and morphological changes.