Neurotoxic Actions of Pyrethroid Insecticides

Pyrethroid insecticides interact with a variety of neurochemical processes, but not all of these actions are likely to be involved in the disruption of nerve function. Several lines of evidence suggest that the voltage-sensitive sodium channel is the single principal molecular target site for all pyrethroids and DDT analogs in both insects and mammals. The alterations of sodium channel functions identified in both biophysical and biochemical studies are directly related to the effects of these compounds on intact nerves. The pyrethroid recognition site of the sodium channel exhibits the stringent stereospecificity predicted by in vivo estimates of intrinsic neurotoxicity in both insects and mammals. Type I and Type II compounds produce qualitatively different effects on sodium channel tail currents, divergent actions on intact nerves, and different effects on the excitability of vertebrate skeletal muscle. Moreover, compounds that are defined as intermediate in the Type I/Type II classification scheme are also intermediate in their effects on sodium channel kinetics. The range of different actions on sensory and motor nerve pathways arising from these qualitatively different effects at the level of the sodium channel appear to be sufficient to explain the distinct poisoning syndromes that have been identified in both insects and mammals. Thus, it does not appear necessary to invoke different primary target sites for Type I and Type II compounds to explain their actions in whole animals. Although the voltage-sensitive sodium channel is likely to be the principal site of pyrethroid action, it is probably not the only site involved in intoxication. Insect neurosecretory neurons are sensitive to very low concentrations of pyrethroids, and disruption of the neuroendocrine system has been implicated as a factor contributing to the irreversible effects of pyrethroid intoxication in insects. Since action potentials in these nerves are carried by calcium ions through TTX-insensitive voltage-gated cation channels, these findings provide evidence that pyrethroids can alter neuronal excitability through an action on voltage-sensitive channels other than the sodium channel. Actions on voltage-sensitive calcium channels may also be involved in the effects of pyrethroids on neurotransmitter release in mammals. The proconvulsant actions of pyrethroids mediated through the peripheral-type benzodiazepine receptor may also contribute to pyrethroid intoxication. Both Type I and Type II compounds are potent proconvulsants in vivo at doses well below those required to produce pyrethroid-dependent intoxication.(ABSTRACT TRUNCATED AT 400 WORDS)