Effects of chronic administration of naloxone on wheel running activity in rats under 2-h feeding schedule

The effects of chronic administration of naloxone (120-150 micrograms X kg-1 X h-1) on the wheel running activity (WR) and on food and water intakes were studied in male Wistar rats subjected to a 2-h restricted feeding (1200-1400 h) schedule at 24 +/- 1 degrees C and LD 12:12 (L: 0600-1800 h) cycle. The restricted feeding significantly increased WR before and after the feeding time. Food and water intakes per day were reduced and body weight gradually decreased for the 2-week food restriction period. Food and water intakes appeared to be suppressed by naloxone, particularly shortly after the administration. The chronically administered naloxone slightly increased the 24-h WR. In the naloxone-treated rats, the fraction of WR before the feeding time (anticipatory activity) was significantly increased compared with saline-treated rats. The fraction of WR after the feeding time (succeeding activity) did not change. These results suggest that the endogenous opioid system may play a role in suppressing the excess increase in the anticipatory locomotor activity in the food restricted rats.