Behavioral thermoregulatory response to chlorpyrifos in the rat

Chlorpyrifos (CHP) is a heavily used organophosphorous-based insecticide that elicits thermoregulatory dysfunction in the rat characterized by an initial period of hypothermia followed by a delayed hyperthermia lasting 24-72 h after exposure. The purpose of the present study was to determine (1) if the delayed hyperthermia is linked to CHP-induced hypothermia and (2) if the hypothermia and delayed hyperthermia are regulated by the CNS thermoregulatory centers. Core temperature (Tc) and motor activity (MA) of female Long-Evans rats were monitored via radiotelemetry. Rats housed in a temperature gradient were administered the control vehicle or CHP (25 mg/kg (p.o.)) while Tc, MA and ambient temperature (Ta) preferred by rats in the gradient (i.e. selected Ta) were recorded. There was an initial reduction in Tc concomitant with a decrease in selected Taa A gradual recovery in Tc occurred during the first night along with a preference for warmer Ta's and depressed MA. The day after CHP there was an elevation in Tc but no change in selected Ta, suggesting that the delayed rise in Tc was regulated. In another experiment, the hypothermic effects of CHP (25 mg/kg (p.o.)) were blocked by raising Ta from 22 to 31 degrees C immediately after CHP administration. Non-heated rats administered CHP underwent a marked period of hypothermia followed by an elevation in diurnal Tc for 2 days. Heated rats showed no hypothermic response but did undergo a hyperthermic response 48 h after CHP. MA was reduced during the first night after CHP in both non-heated and heated groups. Overall, the CHP-induced hyperthermia is not dependent on the development of hypothermia. Behavioral thermoregulatory observations suggest that both hypothermia and hyperthermia are regulated by CNS thermoregulatory centers.