Opposite effects of morphine on feeding and drinking in rats relative to administration time.

The present study was undertaken to examine how morphine changes food and water intake in non-fasted or fasted rats with different administration times. Morphine (1, 3 and 10 mg/kg) was intraperitoneally administered at 10:45 (light period) or 18:45 (dark period). Morphine increased food and water intake in non-fasted rats 2 hr after the administration during the light period, whereas the total daily intakes were decreased. In contrast, morphine decreased food and water intake in non-fasted rats during the dark period and in fasted rats during both the light and dark period. These results suggest that morphine disorders the baseline levels of feeding and water drinking of naive rats.