Dexamethasone blocks morphine-induced hypothermia in restrained rats

Rats restrained in small plexiglass restraining cages responded to morphine, 30mg/kg, with a pronounced hypothermia. However, when this same dose of morphine was administered to unrestrained freely moving rats a marked hyperthermia resulted. The absolute magnitude and time course for body temperature changes after morphine were similar in both groups. When restrained rats pretreated with dexamethasone were administered morphine the hypothermic response was not only prevented but a subsequent hyperthermia occured. Morphine injected into unrestrained dexamethasone pretreated rats, however, still elicited a hyperthermia although of slightly lesser magnitude. These findings indicate that restraint is a potent modifier of morphine's effects on thermoregulation and that this effect most probably results from a stress-related activation of anterior pituitary hormone release. The possibility that ACTH or ß-endorphin released by the stress of restraint are responsible for this modification is discussed.