State-dependent effects of beta-endorphin on core temperature in stressed and non-stressed rats

beta-Endorphin, gamma-endorphin, delta-endorphin and morphine injected i.c.v. caused hyperthermia in non-stressed rats; however, the same dose of beta-endorphin and morphine caused hypothermia in stressed animals. These effects of beta-endorphin were antagonized by naloxone. The results suggest that effects of beta-endorphin are different depending on the animal's state and that these effects are mediated via opioid receptors.