Properties of pentazocine as a discriminative stimulus in the squirrel monkey

Squirrel monkeys were trained to discriminate i.m. injections of pentazocine (3.0 mg/kg) from vehicle. Stimulus control was considered to have been established when the monkeys reliably completed at least 22 of the 25 trials (i.e. 88%) on the appropriate lever in both pentazocine and vehicle sessions. In cross-generalization tests, two groups of drugs were shown to produce pentazocine-like discriminative effects: 1) the pure opiate agonists morphine, levorphanol and levomethorphan and 2) cyclazocine, phencyclidine and dextrorphan, drugs previously shown to have common discriminative effects. In addition, l-pentazocine proved to be about twice as potent as the racemic mixture. Butorphanol, SKF 10,047 and dextromethorphan produced pentazocine-appropriate responding in 80% of the trials, whereas ketocyclazocine, ethylketocyclazocine and nalbuphine produced less than 50% pentazocine-appropriate responding. Four nonopioid drugs from a variety of pharmacologic classes (apomorphine, d-amphetamine, secobarbital and mescaline) failed to show any discriminative properties in common with pentazocine. In antagonism studies, naltrexone completely blocked the discriminative effects of pentazocine, whereas haloperidol produced only a partial blockade. The discriminative effects of l-pentazocine and levorphanol could also be antagonized by naltrexone, but those of dextrorphan could not. The results suggest the pentazocine has both a morphine-like component of action and a component shared with the nonmorphine-like opioids.