Narcotic discrimination in pigeons

Pigeons were trained to discriminate the intramuscular injection of 10 mg/kg of morphine from saline in a task in which 20 consecutive key pecks on either the left or right key, depending on whether morphine or saline had been administered, produced access to mixed grain. During sessions in which stimulus generalization to other drugs was evaluated, 20 consecutive responses on either the morphine-appropriate or saline-appropriate key produced access to mixed grain. Codeine (32 mg/kg) and FK 33824 (3.2 mg/kg), a systemically active analog of methionine enkephalin, produced stimulus control of behavior comparable to that of morphine. In addition, the κ agonists ketazocine (10 mg/kg) and ethylketazocine (3.2 mg/kg), as well as two N-furyl benzomorphans, UM 909 (10 mg/kg) and UM 1072 (1.0 mg/kg), produced morphine-appropriate responding. Naltrexone (1.0 mg/kg) completely antagonized the stimulus control of behavior exerted by morphine (10 mg/kg) and ethylketazocine (3.2 mg/kg). The mixed narcotic agonist-antagonists, cyclazocine and SKF-10,047, as well as the nonnarcotic drugs, pentobarbital, dextrorphan, ketamine, UM 1046 and clonidine, produced predominantly saline-appropriate responding. Pentazocine, nalorphine and meperidine occasioned levels of responding intermediate between those appropriate for morphine or saline. The inability of the pigeon to distinguish μ-like compounds (e.g., morphine) from κ-like compounds (e.g., ethylketazocine and ketazocine), in contrast to the rat and monkey, suggests that the discriminative effects of these two classes of drugs share common mechanisms of action in this species.