Double mode of action of black widow spider venom on frog neuromuscular junction

Black widow spider venom (BWSV) contains a toxin, α-latrotoxin, which is capable of stimulating vesicle release, resulting eventually in depletion of vesicles and block of neuromuscular transmission at the frog neuromuscular junction. Since it has been shown that α-latrotoxin very markedly increases the cation conductance of artificial lipid bilayers, it was postulated that BWSV stimulates release by opening channels permeable to Ca2+ and, in the case of Ca2+-free Ringer's, to Na+ which would release Ca2+ from intracellular stores. To test this hypothesis we chose as a sodium substitute, glucosamine, which is impermeable to the venom-induced channels in the lipid bilayers and to the postsynaptic membrane of the frog neuromuscular junction. Electron microscopical analysis showed that up to 75 min perfusion in Na+ and Ca2+-free medium did not alter the ultrastructure of the nerve terminals. However when BWSV was applied in this medium a significant depletion was noticeable within 15 min and after 60 min the terminals were depleted of vesicles whereas the mitochondria were unchanged in number and structure. If BWSV is applied for 60 min in glucosamine Ringer's containing 1.8 mM Ca2+, most of the nerve terminals still have synaptic vesicles scattered in the cytoplasm or clustered around amorphous structures and the mitochondria are swollen. Application of large doses of BWSV in low Ca2+ Ringer's leads to damage of the mitochondria and to very pronounced swelling of the nerve endings, whereas this is not observed if the dose of venom is applied in Na+-free and Ca2+-free Ringer's. Electrophysiological recording showed that neuromuscular transmission is already blocked after 15 min treatment with BWSV in glucosamme-Ringer's. From these results we conclude that BWSV increases the conductance of the nerve terminal membrane to cations such as Na+ and Ca2+ and stimulates release by a mechanism which may not involve its ionophore property.