Morphine-induced hyperthermia: lack of cross-tolerance with enkephalin analogs

Abstract Cats were made tolerant to the hyperthermic action of morphine by repeated intracerebroventricular injections. Tolerant animals exhibited little or no cross-tolerance to hyperthermic actions of d -Ala 2 -Met-enkephalinamide or FK 33–824. A delay in the hyperthermic response to pentazocine, however, indicated that some thermoregulatory changes may have occurred that were unrelated to naloxone-sensitive receptors stimulated by morphine. The results provide further evidence that neither enkephalin analog influences thermoregulation primarily by activation of morphine-sensitive receptors.