Naloxazone pretreatment modifies cardiorespiratory, temperature, and behavioral effects of morphine

Behavioral and cardiorespiratory responses to a lethal dose of morphine were evaluated in rats pretreated with saline or naloxazone, an antagonist of high-affinity mu 1 opioid receptors. Pretreatment with naloxazone significantly blocked morphine analgesia, catalepsy and hypothermia at a dose which completely eliminated high-affinity binding in brain membranes. Moreover, naloxazone significantly attenuated the morphine-induced hypotension and respiratory depression, whereas morphine-induced bradycardia was less affected. Results indicate that subpopulations of mu receptors may mediate selective behavioral and cardiorespiratory responses to morphine.