Hyperthermic responses to central and peripheral injections of morphine sulphate in the cat

1 The effect of morphine on body temperature was studied in conscious, unrestrained cats provided with implanted third or lateral cerebral ventricular cannulae, jugular venous catheters and retroperitoneal thermocouples.2 Intraventricular injections of 2.5-50 mug and intravenous injections of 1-10 mg/kg morphine sulphate produced dose-related hyperthermic responses. Similar mean increases in body temperature after administration of a given dose were elicited in cats which had not previously received morphine and, provided that tolerance was avoided by spacing injections at least 72 h apart, in cats which received a series of injections of morphine. Morphine was at least 850 times more potent when injected into the third ventricle than when given intravenously. Increasing the dose of morphine sulphate injected into the third ventricle to 1250 mug only prolonged the hyperthermia. Morphine did not produce hypothermia at any dose tested.3 Injection of 10 mug morphine sulphate into the third ventricle produced similar hyperthermias at ambient temperatures (tas) of 4-6, 21-23 and 33-36 degrees C. The increase in body temperature was associated with shivering at the lower tas. At the highest ta, shivering was not evoked, but respiratory rate decreased after morphine if it was initially elevated. These results suggest that morphine increased the level at which body temperature was regulated.4 Neither metiamide nor indomethacin antagonized morphine so histamine and prostaglandins were apparently not required for the hyperthermic effect.