CHOLECYSTOKININ (CCK) POTENTIATES THE RATE-DECREASING EFFECTS OF MORPHINEON SCHEDULE-CONTROLLED BEHAVIOR IN RATS

Cholecystokinin (CCK), a brain-gut peptide, has been shown to antagonize the effects of morphine on a variety of measures, but similar effects have not been demonstrated on schedule-controlled responding. The present research examined the interaction of CCK with cumulative doses of morphine on operant licking in rats. In general, as the dose of morphine increased (3.2-18 mg/kg) the overall response rate decreased. In two of three subjects, CCK alone (10-32 $\mu$g/kg) produced little or no effect. When these doses of CCK were given in combination with morphine, however, the morphine dose-effect curve tended to shift to the left. In contrast, after pretreatment with maltrexone (1 mg/kg), the same doses of morphine produced little or no effect, i.e., the curve shifted to the right. Accordingly, these results would indicate that not only does CCK interact with morphine differently on differently types of behavior, but also different from naltrexone.