METABOLIC, RESPIRATORY, VASOMOTOR AND BODY-TEMPERATURE RESPONSES TO BETA-ENDORPHIN AND MORPHINE IN RABBITS

1. The effects of beta‐endorphin and morphine on thermoregulatory responses of unanaesthetized rabbits to different ambient temperatures (Ta) of 2, 22 and 32 degrees C were assessed. 2. Intraventricular administration of either beta‐endorphin or morphine produced dose‐dependent hypothermia at 2 and 22 degrees C Ta. At 2 degrees C Ta the hypothermia was brought about solely by a decrease in metabolic heat production. At 22 degrees C Ta the hypothermia was due to a decrease in metabolism and an increase in peripheral blood flow. However, at 32 degrees C Ta, there were no changes in rectal temperature in response to either beta‐endorphin or morphine application. 3. Hypothermic effects of the administration of beta‐endorphin or morphine were greatly antagonized by pretreatment of animals with either an opiate antagonist naloxone or a serotonin depletor 5,6‐dihydroxytryptamine. 4. These findings indicate that the hypothermic responses to beta‐endorphin or morphine in rabbits may be mediated through central serotonergic mechanisms. The hypothermia was due to a decrease in heat production and/or an increase in heat loss. © 1979 The Physiological Society