[Anti-insect scorpion toxins: historical account, activities and prospects].

Abstract

Some toxins from scorpion venoms, much more toxic to insects than to other animal classes, possess high affinity to Na+ channels. These anti-insect scorpion toxins have been divided into: 1) alpha toxins which lack strict selectivity for insects, do not compete with following groups of anti-insect toxins, resemble other alpha scorpion toxins by their structure and their ability, as alpha anemone toxins, to prolong insect axonal action potential durations through a drastic slowing down of the Na+ current inactivation, 2) excitatory insect selective scorpion toxins which induce in blowfly larvae an immediate fast paralysis; in isolated cockroach axons, they depolarize and induce a sustained repetitive activity of short (normal) action potentials through a shift of Na+ activation mechanism towards more negative potentials and some decrease of inactivation at these potential values, 3) depressant insect selective neurotoxins which cause a slow progressive flaccid paralysis of larvae, depolarize insect axons and reduce or even suppress evoked action potentials; resting depolarizations which are antagonized by a post-application of TTX, are due to the opening of sodium channels at very negative potential values and to the suppression of their inactivation mechanism. The decrease of the maximal Na+ conductance following flaccid toxin action may be understood if toxin-modified channels opened at very negative potentials values remain open (or re-open) for much longer times than in control conditions and pass by substate less conductant states. Anti-insect scorpion toxins become of major interest into insect neurophysiology and also into insect pest control, due to their specific target sites and to the recent constructions of insecticidal baculovirus expressions of several of these toxins.