Chlorpyrifos-induced hypothermia and vasodilation in the tail of the rat: Blockade by scopolamine

Organophosphate pesticides such as chlorpyrifos reduce core temperature (T- c) in laboratory rodents. The mechanism(s) responsible for the chlorpyrifos -induced hypothermia are not well known. This study assessed the role of a key effector for thermoregulation in the rat, vasomotor control of heat los s from the tail, and its possible cholinergic control during chlorpyrifos-i nduced hypothermia. T-c and motor activity were monitored by telemetry in f emale Long-Evans rats maintained at an ambient temperature (T-a) of 25 degr ees. Tail skin temperature (T-sk(t)) was measured hourly. Rats were dosed w ith chlorpyrifos (0 or 25 mg/kg orally). Two hr later the rats were dosed w ith saline or scopolamine (1.0 mg/kg intraperitoneally). Two hr after chlor pyrifos treatment there was a marked elevation in T-sk(t) concomitant with a 0.5 degrees reduction in T-c. Scopolamine administered to control rats le d to a marked elevation in T-c with little change in T-sk(t). Rats treated with chlorpyrifos and administered scopolamine underwent a marked vasoconst riction and elevation in T-c. Vasodilation of the tail is an important ther moeffector to reduce T-c during the acute stages of chlorpyrifos exposure. The blockade of the response by scopolamine suggests that the hypothermic a nd vasodilatory response to chlorpyrifos is mediated via a cholinergic musc arinic pathway in the CNS.