| Abstract: | The discriminative stimulus effects of ethylketocyclazocine (EKC) were characterized in Fischer and Sprague-Dawley rats trained to discriminate 0.3 mg/kg of EKC (s. c.) from saline in a two-choice, discrete-trial avoidance paradigm. The putative mu-opioid receptor agonists morphine and fentanyl, as well as the putative kappa-opioid receptor agonists EKC and U50,488H generalized completely with the EKC cue in both strains of rats. Only small quantitative differences between strains were observed in the generalization of these agonists with EKC. However, Sprague-Dawley rats were notably more sensitive than Fischer rats to the depressant effects of fentanyl. Only small quantitative differences between strains were also observed in the dose of naloxone necessary for complete antagonism of the EKC-like stimulus effects of the mu- and kappa-opioid agonists. The mu-opioid agonists were approximately 2–6 times more sensitive to naloxone antagonism than the kappa-opioid agonists in both Fischer and Sprague-Dawley rats. However, due to inter-animal variability, this difference was not statistically significant. The results of this study suggest that one or more opioid receptor subtype(s) may be involved in the production of the EKC cue in both strains of rats. |
