Variability among insect sodium channels revealed by selective neurotoxins

Abstract

Binding assays with the radioiodinated depressant (LqhIT 2) and excitatory (AaIT) insect selective neurotoxins derived from scorpion venoms to neuronal membrane preparations derived from cockroach, fly and lepidopterous larvae coupled with the employment of sodium channel site-directed antibodies resulted in the following informatio. (1) The two toxins were shown to bind with high affinity to the insect neuronal preparations and revealed similar binding constants in each of the various preparations. (2) Reciprocal competitive displacement assays between the two toxins in the various neuronal preparations revealed symmetrical and asymmetrical binding interactions. The depressant toxin was able to fully displace the excitatory toxin in all preparations with, however, different affinities. The excitatory toxin fully displaced the depressant toxin from its high affinity binding sites in the locust and cockroach preparations; partially in the fly head preparation and no displacement was obtained in the lepidopterous larvae neuronal preparation. (3) The antibody mediated binding inhibition differentially affected the binding of each of the two toxins in both the cockroach and fly neuronal preparations. The above data suggest that sodium channels of various insects possess separate, high affinity binding sites for each of the two groups of insect selective toxins but, however, consist of pharmacologically distinguishable subtypes.