Proglumide (0.02 mg/kg), a cholecystokinin antagonist, was administered to rats either together with or without morphine (0, 5, 15, or 45 mg/kg). Whereas proglumide in the absence of morphine showed a trend towards enhanced behavioral activation, it potentiated the hypokinesia induced by morphine. These results are consistent with the hypothesis that endogenous cholecystokinin tonically antagonizes opiate modulation of motility, irrespective of whether such modulation is produced by opiates and endogenous or exogenous origin.

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Rats were trained in a Y-maze on a two-choice simultaneous black-white discrimination with either black or white as S+. Animals were then transferred to one of three discrimination tasks. In task 1 (New S-), a new stimulus, either vertical or horizontal stripes, was substituted for the original S-. In task 2 (New S+), a new stimulus, either vertical or horizontal stripes as in task 1, was substituted for the original S+. In task 3 (New S+/S-) animals were trained on horizontal-vertical discrimination. The pre-trial administration of 1 mg/kg d-amphetamine facilitated the acquisition of the original black-white discrimination with both black as S+ and white as S+. Likewise, the drug improved performance in all three transfer conditions. However, the course of learning in the three transfer tasks was different in the placebo- and amphetamine-treated animals. Amphetamine-treated animals were disrupted more by a change in S+ than by a change in S-, whereas the opposite pattern was evident in the placebo controls. When both discriminative stimuli were changed, placebo animals exhibited pronounced decrement in performance, whereas amphetamine animals exhibited excellent learning. The implications of these findings for the effects of amphetamine on discrimination learning are discussed.