Effect of the venom of the black widow spider, Latrodectus mactans tredecimguttatus, on evoked action potentials in the isolated nerve cord of Periplaneta americana.

INTRODUCTION NERI et al. (1965) found that Latrodectus tredecimguttatus venom at a minimal concentration of 400 hg of protein per ml upon application first enhanced and then suppressed the endogenous (spontaneous) activity recorded on one cereal nerve in the isolated nerve cord ofPeriplaneta americana. They also observed that the endogenous activity could be restored by immersing the cord in a specific antitoxic serum. GRasso and PAGGI (1967), using Crustacea, demonstrated that the venom blocked the activity of the slowly adapting stretch receptor of the crayfish Astacus astacus, after having induced a transient increase in the frequency of repetitive firing. Subsequently, Obara and Mauro (personal communication) observed by means of intracellular recordings that in the crayfish stretch receptor the venom brings about a gradual depolarization of the soma-dendritic region of the cell membrane ; firing rate increases with time and then stops abruptly, owing to the membrane potential having fallen to a level sufficient to effect a `cathodal block' . It is interesting to note that at no time was conduction along the axon impaired by the venom, thus indicating a localized sensitivity to the protein in a given neuron . Moreover, as was observed in the restoration ofendogenous activity in the cockroach cord (NERI et al., 1965), the stretch receptor function recovered in the presence of specific antisertun . In an attempt to further establish the action of the venom on the cockroach cord, namely, whether soma-dendritic structures or the axons are involved, we studied the effect of the venom on the sixth abdominal ganglion preparation .