Pharmacological characterization of morphine-6 beta-glucuronide, a very potent morphine metabolite

Morphine-6 beta-glucuronide is a major metabolite of morphine with potent analgesic actions. To define more fully the importance of this compound in morphine action, we have compared the analgesic actions of morphine and its 6 beta-glucuronide metabolite after both peripheral and central administration. Given s.c., morphine-6 beta-glucuronide elicited analgesia with an effect approximately twice that of morphine due, in part, to its long duration of action and also inhibited gastrointestinal motility. Both actions were easily reversed by naloxone (s.c.). However, when injected either i.c.v. or intrathecally, morphine-6 beta-glucuronide was approximately 90- and 650-fold more potent an analgesic than morphine, respectively. Whereas morphine in these studies was equipotent at both levels of the neuraxis as an analgesic, the 6 beta-glucuronide was approximately 5-fold more effective at the level of the spinal cord than supraspinally. The mu 1-selective antagonist naloxonazine blocked the analgesic effect of systemic and i.c.v. morphine-6 beta-glucuronide much as it blocked morphine, implying a role for mu1 receptors in these actions. Like morphine, morphine-6 beta-glucuronide analgesia after intrathecal injection was not sensitive to naloxonazine, suggesting a mu2 mechanism within the spinal cord. Together, these results imply that morphine-6 beta-glucuronide elicited its analgesic actions through the same receptor mechanisms as morphine. Mice highly tolerant to morphine after implantation of morphine pellets showed cross-tolerance to morphine-6 beta-glucuronide (s.c.). The high potency of morphine-6 beta-glucuronide strongly suggests that this metabolite plays an important role in morphine's actions.