Rationale: Opioid abuse remains a serious public health problem. The pseudoirreversible mu opioid receptor antagonist methocinnamox (MCAM) might be useful for treating opioid abuse and overdose. Because endogenous opioid systems can modulate cognition and decision-making, it is important to evaluate whether long-term blockade of mu opioid receptors by MCAM adversely impacts complex operant behavior involving memory.
Objective: This study tested the effects of MCAM in rhesus monkeys responding under a delayed matching-to-sample task, with correct responses reinforced by sucrose pellets. Because MCAM did not alter performance, antagonism of the rate-decreasing effects of morphine was used to confirm that an effective dose of MCAM was administered. Moreover, the muscarinic receptor antagonist scopolamine and the N-methyl-D-aspartate antagonist phencyclidine were studied as positive controls to demonstrate sensitivity of this procedure to memory disruption.
Results: Neither MCAM (0.32 mg/kg) nor morphine (1-5.6 mg/kg) impaired delayed matching-to-sample accuracy. Morphine dose-dependently decreased the number of trials completed before MCAM administration, and a single injection of MCAM blocked the behavioral suppressant effects of morphine for at least 7 days. Scopolamine (0.01-0.056 mg/kg) and phencyclidine (0.1-0.56 mg/kg) dose-dependently decreased delayed matching-to-sample accuracy and the number of trials completed.
Conclusions: MCAM did not impair memory (as measured by accuracy in a delayed matching-to-sample task) and did not decrease responding for or consumption of sucrose pellets. This dose of MCAM attenuates self-administration of opioids and reverses as well as prevents opioid-induced respiratory depression. These results provide further support for a favorable adverse effect profile for MCAM.
Keywords: Antagonism; Cognition; Memory; Operant behavior; Opioid; Rhesus monkey.