ATPM [(-)-3-amino-thiazolo[5,4-b]-N-cyclopropylmethylmorphinan hydrochloride] was found to have mixed κ- and μ-opioid activity and identified to act as a full κ-agonist and a partial μ-agonist by in vitro binding assays. The present study was undertaken to characterize its in vivo effects on morphine antinociceptive tolerance in mice and heroin self-administration in rats. ATPM was demonstrated to yield more potent antinociceptive effects than (-)U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide). It was further found that the antinociceptive effects of ATPM were mediated by κ- and μ-, but not δ-opioid, receptors. In addition to its agonist profile on the μ-receptor, ATPM also acted as a μ-antagonist, as measured by its inhibition of morphine-induced antinociception. It is more important that ATPM had a greater ratio of the ED50 value of sedation to that of antinociception than (-)U50,488 (11.8 versus 3.7), indicative of a less sedative effect than (-)U50,488H. In addition, ATPM showed less potential to develop antinociceptive tolerance relative to (-)U50,488H and morphine. Moreover, it dose-dependently inhibited morphine-induced antinociceptive tolerance. Furthermore, it was found that chronic treatment of rats for 8 consecutive days with ATPM (0.5 mg/kg s.c.) produced sustained decreases in heroin self-administration. (-)U50,488H (2 mg/kg s.c.) also produced similar inhibitory effect. Taken together, our findings demonstrated that ATPM, a novel mixed κ-agonist and μ-agonist/-antagonist, could inhibit morphine-induced antinociceptive tolerance, with less potential to develop tolerance and reduce heroin self-administration with less sedative effect. κ-Agonists with some μ-activity appear to offer some advantages over selective κ-agonists for the treatment of heroin abuse
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