Metabolic, respiratory, vasomotor and body temperature responses to beta‐endorphin and morphine in rabbits.

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Abstract
The effects of .beta.-endorphin and morphine on thermoregulatory responses of unanesthetized rabbits to different ambient temperatures (Ta) of 2, 22 and 32.degree. C were assessed. Intraventricular administration of .beta.-endorphin or morphine produced dose-dependent hypothermia at 2 and 22.degree. C Ta. At 2.degree. C Ta the hypothermia was caused solely by a decrease in metabolic heat production. At 22.degree. C Ta the hypothermia was due to a decrease in metabolism and an increase in peripheral blood flow. At 32.degree. C Ta, there were no changes in rectal temperature in response to .beta.-endorphin or morphine application. Hypothermic effects of the administration of .beta.-endorphin or morphine were greatly antagonized by pretreatment of animals with an opiate antagonist naloxone or a serotonin depletor 5,6-dihydroxytryptamine. 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.