HIGHER ENVIRONMENTAL TEMPERATURE-INDUCED INCREASE OF BODY-TEMPERATURE - INVOLVEMENT OF CENTRAL OPIOIDERGIC-GABAERGIC INTERACTION

Exposure (2 h) of male albino rats to higher environmental temperature (HET, 40 degrees C) significantly increased the body temperature (BT). Administration of bicuculline (1 mg/kg, IP), physostigmine (0.2 mg/kg, IP), or their combination significantly raised the BT of normal rats (kept at 28 degrees C) or of HET-exposed rats. Atropine (5 mg/kg, IP) abolished the hyperthermic effect of bicuculline in normal and HET-exposed rats. The BT of normal and HET-exposed rat was increased with morphine (1 mg/kg, IP) and was reduced with naloxone (1 mg/kg, IP). Bicuculline or physostigmine-induced rise in BT of HET-exposed rats was potentiated following cotreatment of physostigmine with morphine. Atropine-induced hypothermia was abolished due to the cotreatment of atropine with morphine in HET-exposed rats. Further, (morphine + bicuculline)induced hyperthermia in HET-exposed rats was potentiated with physostigmine but was attenuated with atropine. In normal rats (kept at 28 degrees C), only atropine attenuated(morphine + bicuculline)-induced hyperthermia. L-Dopa + carbidopa or haloperidol did not significantly affect the BT of rats under similar conditions. These results suggest that short-term (2 h) exposure to HET activates the opioidergic neuron, which activates cholinergic activity through the inhibition of GABAergic system and, thus, enhances the BT.