Lack of blockade of central dopaminergic receptors by narcotics: comparison with chlorpromazine.

d-Amphetamine and saline were used as discriminative stimuli in rats. After 0.6 mg/kg d-amphetamine administration, food reinforcement was contingent upon pressin 1 lever in 2-lever operant chamber. Responding on the other lever was reinforced only after saline injection of equal volume. The animals learned to discriminate between d-amphetamine and saline to a criterion of 85% correct. Administration of 2.5, 5.0 and 7.5 mg/kg morphine, and 0.02 and 0.04 mg/kg fentanyl produced saline-appropriate responses. Pretreatment with morphine and fentanyl had no significant effect on the d-amphetamine induced discriminative stimulus. In contrast, 1.25 and 2.5 mg/kg chlorpromazine antagonized d-amphetamine discrimination. These results indicate that despite their similar effect on brain dopamine turnover, morphine and fentanyl do not act on the same receptor site(s) as the neuroleptics, that is, they probably do not block brain dopamine receptors.