Insect tolerance to a neurotoxic polypeptide: pharmacokinetic and pharmacodynamic aspects

Androctonus australis insect toxin (AaIT) is an insect-selective neurotoxic polypeptide from scorpion venom used to probe insect Na+ channels and to design insecticidal recombinant baculoviruses. When injected into susceptible insects (such as flies or cockroaches), nanogram doses of the toxin induce a rapid paralysis within seconds. More tolerant insects respond to microgram doses by developing either a slow progressive paralysis, as in lepidopterous larvae, or a rapid but reversible paralysis, as in Trachyderma philistina, a tenebrionid beetle. Using toxicity and binding assays, microscopy and chromatography, we show that the tolerance of insects to AaIT occurs at both the pharmacokinetic and pharmacodynamic levels. Pharmacokinetic effects occur in Trachyderma philistina in which the toxin undergoes a progressive process of degradation and elimination from the hemolymph, resulting in the loss of 95­97 % of toxin activity 6 h after injection. The pharmacodynamic aspect was demonstrated in studies of the kinetics of binding dissociation of [125I]AaIT from neuronal membranes of susceptible and tolerant insects. Stable binding is shown in susceptible insects such as cockroaches and locusts, which have a dissociation half-time of approximately 9 and 5 min, respectively. This contrasts strongly with the fast half-time of dissociation of 7 s for Spodoptera littoralis larvae and 9 s for Trachyderma philistina, which are both relatively tolerant to AaIT. These differences in binding kinetics may reflect a structural and functional diversity of Na+ channels in different insects that is responsible for their diverse susceptibility to neurotoxic polypeptides.