Effect of a synthetic pyrethroid deltamethrin on excitability changes following a nerve impulse

Excitability changes following a nerve impulse were studied in the rat tail. Supernormal nerve excitability in control animals was present, as in other vertebrate nerve fibres, from 4-30 ms and was followed by a period of subnormal excitability extending up to 100-200 ms. Two to seven hours after intravenous administration of 1.5 mg/kg deltamethrin, supernormality was increased in degree and prolonged in duration for up to 400 ms. A minor effect was still detectable 24 hours after injection. This effect of deltamethrin on nerve excitability is probably due to persistent slight depolarisation of the nerve membrane resulting from its known effect of maintaining a proportion of sodium channels open for up to several hundred milliseconds following a nerve impulse.