Abstract
Facilitatory and inhibitory actions of morphine on the discriminative stimulus properties ofD-amphetamine were examined in rats trained to discriminateD-amphetamine (1 mg kg−1) from saline in a two–lever food reinforced operant task. Morphine (0.625–2.5 mg kg−1) did not itself generalize to amphetamine. When the percentages of rats selecting the drug–appropriate lever were plotted as a function of the individual lowest generalized dose (LGD), a log–linear model fitted to cell frequency indicated a ‘morphine×amphetamine×lever selection’ interaction. Subsequent comparisons indicated that morphine significantly increased or decreased drug lever selection in rats receiving one–half LGD or LGD, respectively. In both cases no difference was found among morphine doses. The present study confirms that the discriminative stimulus properties of amphetamine can be both potentiated and attenuated by morphine. The direction to which morphine modulates the interoceptive effects of 1 mg kg−1amphetamine seems to be dependent on the individual sensitivity of animals to amphetamine rather than on the overall effect of the test dose of amphetamine alone.
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