An adrenergic link in the hypothalamic pathways which mediates morphine- and beta-endorphin-induced hyperthermia in the rat

Abstract

The mechanism underlying the hyperthermia induced by intrahypothalamic administration of either morphine or beta-endorphin has been investigated in conscious rats. Direct administration of morphine (1–8 μg in 1 μl) or beta-endorphin (1–3 μg in 1 μl) into the anterior hypothalamus caused hyperthermia in rats at the ambient temperature (8, 22 and 30°C) studied. The hyperthermia in response to opiods was brought about by both increased metabolism and cutaneous vasoconstriction. This hyperthermia, unlike the hypothermia induced by intraventricular administration of opiods was not blocked by naloxone nor did tolerance develop to the response. However, the hyperthermia induced by intrahypothalamic administration of opioids was greatly reduced by pretreatment with intrahypothalamic administration of either yohimbine (a blocking agent of alpha-adrenergic receptors) or dl-propranolol (a blocking agent of beta-adrenergic receptors) in the rat. These observations suggest that an adrenergic link occurs in the hypothalamic pathways which mediate morphine- and beta-endorphin-induced hyperthermia in the rat.