Action of the polyamine beta-philanthotoxin on neuromuscular transmission in insects.

The beta-fraction of the venom of the solitary wasp Philanthus triangulum inhibited glutamate-induced potentials in muscle fibres of the locust in a dose-dependent manner (50% block at 2 micrograms/ml). Single channel behaviour and miniature excitatory postsynaptic currents were not affected. The blocking effect disappeared after treatment of the muscle with the lectin concanavalin A which prevents desensitization of glutamate receptors. Block of potentials induced by iontophoretic application of glutamate was strongly dependent on the frequency of stimulation: no block occurred at stimulus intervals of 30 sec or more. The nature of this inhibition is discussed and proposed to be a delay of recovery from desensitization.