Abstract
The μ-opioid receptor antagonist effects of naloxonazine on levorphanol-induced thermal antinociception and respiratory depression were examined in rhesus monkeys. Levorphanol (0.032–3.2 mg/kg) produced dose-dependent increases in tail-withdrawal latencies from 50°C water in a warm-water tail-withdrawal assay and dose-dependent decreases in ventilation in both air and 5% CO 2 mixed in air. Naloxonazine (0.1–3.0 mg/kg) antagonized both the antinociceptive and ventilatory effects of levorphanol to a similar degree, and the antagonist effects of naloxonazine were greater after 1 h than after 24 h. Under all conditions, the antagonist effects of naloxonazine were fully surmountable. Schild analysis of the antagonist effects of naloxonazine after 1 h pretreatment in the antinociception assay yielded a pA 2 value of 7.6 and a slope of −0.50; by comparison, quadazocine yielded a pA 2 value of 7.5 and a slope of −1.05. These results suggest that naloxonazine acts as a potent and fully reversible μ-opioid receptor antagonist with a moderately long duration of action in rhesus monkeys. In addition, these results suggest that the antinociceptive and ventilatory effects of μ-opioid receptor agonists in rhesus monkeys are mediated by pharmacologically similar popolations of μ-opioid receptors.
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