Fig. 9.

Discriminative-stimulus effects of morphine in combination with nalbuphine (left) and mitragynine (right). Abscissae: vehicle and morphine dose in milligrams per kilogram (intraperitoneal, log scale). Ordinates: top, percentage of responses on the morphine-appropriate lever. Bottom, mean rates of responding expressed as a percentage of vehicle control. Morphine and nalbuphine (1.78 mg/kg) were administered intraperitoneally 15 minutes before sessions, whereas mitragynine (5.6 mg/kg) was administered intraperitoneally 30 minutes before sessions. The morphine dose-effect function is replotted from Fig. 5. Each point represents the mean ± S.E.M. (N = 8) except for nalbuphine + morphine at 17.8 mg/kg (N = 7) and mitragynine + morphine at 10 mg/kg (N = 5). Details for statistical analyses are shown in Supplemental Tables 2–4 and Tables 5 and 6.