Hyperthermic effects of morphine: set point manipulation by a direct spinal action.

U Rats were implanted with chronic indwelling catheters in the lumbar spinal subarachnoid space in order to study the effects of morphine on body temperature. 2 Morphine administered intrathecally produced a dose-dependent rise in body temperature that was antagonized by naloxone. 3 The rise in body temperature evoked by a given dose of morphine appeared to be independent of the environmental temperture ((4 degrees C to 32 degrees C) and was consistently associated with coordinated thermoregulatory responses (i.e. shivering and tail vasoconstriction). The fall in body temperature observed in these hyperthermic animals, following naloxone, was associated with a vasodilatation which coincided with the fall to the premorphine temperature level. 4 Morphine administered systemically also produced hyperthermia. This was only partially reversed by intrathecal naloxone. In animals made tolerant to the hyperthermic effects of systemic morphine, the intrathecal administration of naloxone produced a fall in body temperature. 5 Naloxone alone, administered either intrathecally or systemically, had no effect upon body temperature. 6 We suggest that morphine exerts a direct, pharmacologically specific effect on the spinal cord, which results in an altered thermoregulatory set point in the rat.