β-funaltrexamine antagonizes the discriminative stimulus effects of morphine but not naltrexone in pigeons

Antagonistic actions of the irreversible, μ-selective antagonist β-funaltrexamine (β-FNA) were evaluated in pigeons trained to discriminate among intramuscular injections of morphine (5.6 mg/kg), saline, and naltrexone (10.0 mg/kg). β-FNA administered alone (1.0 or 10.0 mg/kg) failed to mimic the discriminative stimulus effects of morphine or naltrexone. β-FNA attenuated the discriminative stimulus effects of morphine. A three-fold larger dose of morphine was required for complete generalization when pigeons were pretreated with a dose of 1.0 mg/kg β-FNA. A dose of 10.0 mg/kg β-FNA completely antagonized the morphine discriminative stimulus, so that pigeons responded predominantly on the saline key up to doses of morphine that suppressed responding. Doses of β-FNA that attenuated the effects of morphine had no effect on the discriminative stimulus effects of naltrexone. These results demonstrate that, like naltrexone, β-FNA attenuates the discriminative stimulus effects of morphine in pigeons and, at sufficiently large doses, antagonizes morphine in an unsurmountable manner. β-FNA does not, however, share discriminative stimulus properties with naltrexone in these pigeons, and fails to attenuate the discriminative stimulus effects of naltrexone, lending support to the suggestion that naltrexone exerts discriminative stimulus effects under these experimental conditions predominantly by a non-mu opioid mechanism.