The tolerance of lepidopterous larvae to an insect selective neurotoxin

The interaction of the fast-neurotoxic and insect selective polypeptide derived from scorpion venom (AaIT) with lepidopterous larvae tissues was studied through assays of toxicity, chromatography, binding and light microscopical autoradiography. The native and/or radioiodinated toxin was shown to:

• 1.(1) Induce a delayed, slow, progressive paralysis (within 24–48 h) of Spodoptera larvae by relatively high doses (paralytic unit = 2.4 μg/100 mg) corresponding to about only 10% of the total toxicity of the crude venom. Larvae of six species representing five families of Lepidoptera responded similarly to the toxin.
• 2.(2) Resist an in vitro incubation in the insect's hemolymph.
• 3.(3) Lose 80% of its toxicity in the insect's body within 24 h, accompanied by a progressive process of degradation and elimination by the excretory system.
• 4.(4) Specifically bind to a single class of non-interacting binding sites of high affinity and low capacity (0.2 pmol/mg protein, similar to tritiated saxitoxin) in an in vitro, homogenate derived, neuronal preparation.
• 5.(5) Specifically bind with high affinity to desheathed but otherwise intact nerves.
• 6.(6) Be devoid of accessibility to peripheral-terminal branches of Spodoptera motor nerves in situ—strongly contrasting those of the toxin susceptible Periplaneta nerves.

It may be thus concluded that the tolerance of the lepidopterous larvae to AaIT can be substantially attributed to pharmacokinetic aspects of toxin accessibility barriers and degradation processes.