Effect of venom fromLatrodectus katipoandIxeuticus martius(Arachnida: Araneae) on insect neuromuscular transmission

Homogenates of spider venom glands were applied topically to fibres from the extensor tibiae muscles of adult male American cockroaches (Periplaneta americana) from which miniature end plate potentials (MEPPs) were recorded with an intracellular glass capillary microelectrode. Venom from Latrodectus katipo caused a sudden, transitory increase in MEPP frequency, which peaked and then decreased over the next hour until no MEPPs were recorded. Small, random fluctuations (3–5 mV) in the resting potential of the muscle fibres were also recorded. Ixeuticus martius venom caused a rapid, logarithmic decrease in MEPP frequency; no MEPPs were recorded 15–20 min after venom application. The resting potential decreased by 30–40 mV 5–10 min after application. L. katipo venom appears to act presynaptically, causing a massive release of transmitter. Although a presynaptic mechanism for I. martius venom has not been excluded, it is more likely that the observed effect results from a post-synaptic blockade of tra...