In the present series of studies we examined the effect of butorphanol tartrate on food-reinforced operant responding in satiated rats. In the first experiment, 8.0 mg/kg butorphanol was administered subcutaneously, once per day for 4 days, to satiated rats responding under an fixed ratio 10 (FR 10) reinforcement schedule. In the second experiment, butorphanol (0, 0.3, 1.0, 3.0, 10.0 mg/kg) was administered to satiated rats responding under an FR 80 (first pellet) FR 3 (subsequent pellets) reinforcement schedule for 4 consecutive days. Repeated butorphanol administration increased total amount of food consumed over sessions in both experiments. Under the FR 80 schedule component, butorphanol initially increased latency to acquire the first pellet, an effect attenuated by repeated administration. Whereas vehicle administration was associated with consumption of relatively large quantities of food within the first 10 min of receiving the first pellet, butorphanol was associated with continued feeding as the session progressed. These data suggest that butorphanol-induced food intake is associated with maintenance rather than initiation of feeding.
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ACKNOWLEDGEMENTS This work was supported by the Department of Veterans Affairs, the National Institute of Drug Abuse DA03999, and the National Institutes of Health DK42698.