Discriminative stimulus effects of nalbuphine in nontreated and morphine-treated pigeons

In the present study, the stimulus effects of the low efficacy agonist nalb uphine were examined under two conditions: nontreated and morphine treated. In the first experiment, five pigeons were trained to discriminate among 3 .2 mg/kg morphine, 5.6 mg/kg nalbuphine, and saline. Nalbuphine produced na lbuphine-like responding. Low doses of morphine produced nalbuphine-like re sponding, whereas high doses produced morphine-like responding. Naltrexone produced saline-like re spending and reversed the stimulus effects produced by the training doses of morphine and nalbuphine. Five different pigeons w ere treated daily with 10 mg/kg morphine (IM) and trained 6 h later to disc riminate among 10 mg/kg morphine, 1.0 mg/kg nalbuphine and saline. In these pigeons, morphine produced morphine-like responding and nalbuphine produce d nalbuphine-like responding. Morphine abstinence produced nalbuphine-like responding that was reversed by morphine. Additionally, naltrexone produced nalbuphine-like responding. These data suggest that the discrimination bet ween morphine and nalbuphine in the nontreated and morphine-treated pigeons may be based on the relative efficacy differences between morphine, a high er efficacy mu-agonist, and nalbuphine a lower efficacy mu-agonist. (C) 199 9 Elsevier Science Inc.