The effect of DDT and dieldrin on myelinated nerve fibres

Abstract

The effects of the chlorinated hydrocarbon insecticides, DDT and dieldrin, on myelinated nerve fibres of the clawed toad, Xenopus laevis, were studied by recording compound action nerve fibres, and membrane potentials of single nodes of Ranvier. The effect of DDT (5 × 10−4 M) was found to be exclusively on the falling phase of the action potential; either it slowed down the whole falling phase or induced a large negative afterpotential. DDT did not affect threshold voltage, rate of rise and amplitude of the action potential, nor the testing potential. Dieldrin, even in high concentrations (10−3 M), had no appreciable effect on myelinated nerve fibres.

DDT caused repetitive firing in sensory fibres only. The number of repetitive action potentials increased gradually with increasing amplitude of the negative afterpotential, while the interval between the action potentials decreased. These results indicate that the DDT-induced negative afterpotential is directly responsible for the repetitive activity. The possible mechanisms by which the negative afterpotential could induce a repetitive response were further investigated. It is suggested that the rate of recovery from sodium inactivation plays a major role in this process.