Morphine discrimination in the pigeon using a color tracking procedure.

Pigeons were trained to discriminate 5.0 mg/kg morphine from saline. After morphine, subjects tracked the location of red response keys and after saline, the location of green keys. When stimulus generalization to other drugs was investigated dl-methadone produced morphine-like responding and this response generalization was primarily due to the l-isomer. Pretreatment with 1.0 mg/kg naloxone shifted the morphine generalization curve 10-fold to the right but only shifted the rate suppression curve 3-fold to the right. dl-Cyclazocine generated dose-related increases in responding on the red key location and in 3 of 5 birds, responses after 1.0 mg/kg were indistinguishable from those after morphine training doses. Meperidine did not produce responding on the red keys, nor did diazepam, cocaine, d-amphetamine, phencyclidine or pentobarbital. The discriminative stimulus effects of morphine are thus stereo-selective and pharmacologically specific. Generalization of responding to dl-cyclazocine but not to phencyclidine suggests that the morphine-like discriminative dl-cyclazocine cue was not due to interaction at sigma opiate receptors.