Abstract
The potential of centrally (ICV) or systemically (SC) administered M6G to substitute for morphine in a drug discrimination task was characterized in the present study. Rats with a cannula in the lateral cerebral ventricle were trained to discriminate between injections of morphine (3 mg/kg, SC) and saline using a discrete-trial avoidance/escape procedure. Substitution tests were conducted with SC or ICV morphine, morphine-3-β-d-glucuronide (M3G), or morphine-6-β-d-glucuronide (M6G) and response latency in a tail-flick test was measured before each session began. The stimulus effects of morphine (ED50=1.02 mg/kg SC or 2.1 μg/kg ICV) were fully shared by M6G, with potency dependent on route of administration (ED50=3.12 mg/kg SC or 0.34 μg/kg ICV). The stimulus effects of M6G were highly correlated with its antinociceptive activity (r=0.84 SC or 0.46 ICV) and, at equipotent systemic doses, they lasted longer (t1/2=391 min) than those of morphine (t1/2=185 min). M3G was inactive in both procedures by both routes of administration. Naltrexone SC, given 30 min prior to testing, completely attenuated the stimulus effects of ICV M6G (AD50=0.011 mg/kg), indicating that they are mediated by opioid receptors. The results of this study suggest that M6G might contribute to the interoceptive effects of morphine that underlie its potential for abuse.
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Received: 29 January 1998 / Final version: 21 March 1998
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Easterling, K., Holtzman, S. Comparison of the discriminative and antinociceptive effects of morphine and its glucuronide metabolites after central or systemic administration in the rat. Psychopharmacology 140, 91–97 (1998). https://doi.org/10.1007/s002130050743
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DOI: https://doi.org/10.1007/s002130050743