Effects of time of day on chlorpyrifos-induced alterations in body temperature

Chlorpyrifos (CPF) is an organophosphate (OP) insecticide and is among the most common and widely used commercial insecticides. Human intoxication is reported to result in a typical set of responses, which include an immediate and long lasting hyperthermic response (fever). Rodents exposed to similar doses exhibit a biphasic body temperature response: short-term hypothermia followed by subtle hyperthermia several days after administration. Time of day of administration has been suggested to alter the body temperature effect of CPF. In the present study, it is shown that adult male Sprague-Dawley rats exposed to CPF via (oral) gavage at four different times of the day demonstrate a hypothermic response, the timing and magnitude of which is independent of time of exposure and that is blocked by atropine pretreatment. However, a delayed (hyperthermic) response seems to be exhibited only when dosing occurs during the light phase. Our findings support existing theories that the hypothermic and hyperthermic effects of CPF work through independent mechanisms. It is also suggested that humans may indeed exhibit a biphasic temperature response to CPF intoxication, but that it is not typically detected.