The effects ofα-latrotoxin of black widow spider venom on synaptosome ultrastructure. A morphometric analysis correlating its effects on transmitter release

Summary

The morphological effects of α-latrotoxin, the major component of black widow spider venom, were studied quantitatively in a crude synaptosome fraction (prepared from rat brain cortices) which was incubated at 37 ° C for 10 min in Ringer solutions. Two toxin concentrations were employed, one causing a very large stimulation of transmitter release (∼65% and ∼43% release of [³H]noradrenaline from preloaded synaptosomes, with and without Ca²⁺ in the incubation buffer), the other 50–60% as active. Incubated synaptosomes, fixed in suspension with aldehydes, were evenly dispersed in agarose before embedding, to assure randomized sampling in the subsequent morphometric analysis. In all the experimental conditions investigated, α-latrotoxin treatment caused a significant decrease in the density (number/unit area) of synaptic vesicles in synaptosome profiles. Such an effect was dose-dependent and partially Ca²⁺-dependent, in good agreement with the data on transmitter release. At high toxin concentration a moderate increase of synaptosome volume and surface area was observed, both with and without Ca²⁺. Mitochondrial swelling appeared only in synaptosomes treated in Ca²⁺ containing medium. These effects of α-latrotoxin are similar to those described previously at the neuromuscular junction. Thus, the toxin might be a tool of general use for studying vertebrate synapses.