Abstract

Possible commonalities in the discriminative stimulus properties of phencyclidine (PCP) and opioids were investigated in rats trained to discriminate between i.p. injections of saline and 2.0 mg/kg of PCP in a two-choice discrete-trial avoidance paradigm. Behavior was considered to be under stimulus control when a rat reliably completed at least 18 trials of a 20-trial session on the appropriate choice lever after receiving PCP or saline. Tests of stimulus generalization were performed over an 8- to 32-fold range of doses with ketamine, a PCP analog, and eight opioids. Dose- and time-dependent stimulus control of behavior comparable to that produced by 2.0 mg/kg of PCP (defined by the number of trials completed on the PCP-appropriate choice lever) were produced by ketamine and the opioids cyclazocine, SKF 10,047 and dextrorphan. These drugs also produced orderly increases in responses during the interval between trials suggestive of a relationship between this effect and PCP-like stimulus control. In contrast, after ethylketocyclazocine, ketocyclazocine, pentazocine and dextromethorphan trials were completed primarily on the saline-appropriate lever and responding between trials produced did not change. Neither the PCP-like stimulus control of behavior nor the increased responding between trials produced by cyclazocine were prevented by pretreatment with naltrexone (1.0 mg/kg). These results provide further evidence that PCP and certain opioids share a common component of action that is probably mediated by neuronal substrates not usually associated with the activity of opioids.