Alkaloid actions as test for synapse-function in insects

Injection of strychnine solution, even at saturated concentration (0.5 c.c.), in a series of sphingid caterpillars (genera Samia, Automeris, Ceratomia), fails to induce “reversal of inhibition”; and save in the case of those species normally the most excitable it fails to induce any opisthotonic symptoms. Opisthotonic curvature (spasmodic) can be induced, however, by tetraethylammonium chloride. General excitation is produced by a variety of neurophil substances (but not by creatin). Only with atropine is one able to bring about reversal of inhibition in the use of antagonistic muscle groups; it is in this case very clearly shown in the behavior of the prolegs, which no longer react to embrace an object touching the skin between the members of a pair, but instead are pulled widely apart after such stimulation, with their terminal combs retracted.

By the action particularly of pilocarpine, it can be shown that in species normally sluggish, responding mildly to external excitation, the much more violent type of behavior characteristic of species armed with urticant spines may be induced through the effect of neurophil drugs. Therefore the effect of these substances is brought about in relation to nervous pathways already existing. And a suggestion is had as to the basis of behavior differences in species structurally related.

The failure of strychnine to produce its “typical” effects, in these insects, coupled with the observed “reversal” under atropine, points to possible chemical differentiation of the synaptic homologues in insects, and argues for caution in the use of drugs as a test for synapse-function in invertebrates.