Discriminative stimulus properties of (+)-N-allylnormetazocine in the rat: correlations with (+)-N-allylnormetazocine and phencyclidine receptor binding.

Recent studies have identified a stereospecific (+)-NANM binding site that binds psychotomimetic opioids and phencyclidine (PCP) but has a distribution in brain different from the PCP binding site. Since (+)-NANM has no opioid activity and (-)-NANM has opioid activity, rats were trained to discriminate (+)-NANM from saline in order to develop an ability to distinguish the (+)-NANM cues from other opioid agonist and antagonist activities. Cyclazocine, PCP, and ketamine all produced (+)-NANM-like stimuli in a dose-dependent manner. Behaviorally, cyclazocine and PCP are equipotent to (+)-NANM whereas ketamine is 6.7 times less potent than (+)-NANM. Pentazocine had the highest affinity for the (+)-[3H]NANM binding site, yet did not produce (+)-NANM-like discriminative stimuli. By contrast, ketamine had the lowest binding affinity for the (+)-[3H]NANM binding site and did produce (+)-NANM-like discriminative stimuli. Drug discrimination potencies relative to (+)-NANM were not predictive of relative binding affinities at (+)-NANM or PCP binding sites, although there was a trend toward a stronger correlation with the PCP binding site. Therefore, the discriminative stimulus properties of (+)-NANM cannot be explained by pharmacologic actions at either (+)-NANM or PCP binding sites alone, and may involve concurrent actions at both sites.