The discriminative stimulus properties of U50,488 and morphine are not shared by fedotozine

Fedotozine is a kappa opioid receptor agonist having antinociceptive properties but devoid of diuretic effects. The aim of the study was to evaluate the discriminative stimulus effects of fedotozine at doses previously reported to produce maximal effects in in vivo assays measuring kappa-mediated analgesia. By using a two-lever drug discrimination task, two groups of rats were trained to discriminate either a 3 mg/kg i.p. dose of the kappa opioid agonist, U50,488, or a 5 mg/kg i.p. dose of the mu opioid agonist, morphine, from saline. Once trained, rats were used to conduct tests of stimulus generalization with morphine, U50,488 and fedotozine along with another kappa agonist, CI-977, and another mu agonist, fentanyl. The stimulus effect of U50,488 was shared by CI-977 but not by morphine. Conversely, the stimulus effect of morphine was shared by fentanyl but not by U50,488. Fedotozine (1–10 mg/kg) failed to substitute to either U50,488 or morphine. These results indicate that, when administered at doses fully effective in producing antinociception, the interoceptive stimulus effects of fedotozine, if any, can be distinguished from those produced by U50,488 and morphine.