Effects of opiates on the discriminative stimulus properties of dopamine agonists

Two groups of rats were trained to discriminate the dopamine agonists amphetamine (0.75 mg/kg) or apomorphine (0.38 mg/kg) from saline in a two-lever operant task. Various doses of morphine and pentazocine were tested for generalization to and interference with the discriminative stimulus complexes produced by the dopamine agonists. Low doses of morphine appeared to produce a stimulus complex which is similar to that produced by apomorphine, but which differs from the produced by amphetamine. Pentazocine showed no evidence of generalization to either the apomorphine or the amphetamine cue. Neither opiate interfered with discriminative stimuli produced by the dopamine agonists, although decreases in the number of animals responding occurred.