Effect of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 on food-anticipatory activity rhythm in the rat

When feeding was restricted to a single meal scheduled at a fixed time (1300-1700 h) in the day, rats exhibited increased prefeeding locomotor activity 1-3 h before the feeding time, which increased over 6 days. The activity increase was seen for about 5 h (1100-1600 h) on the seventh day when no food was provided (fasting day), as the rat anticipated mealtime. Thus, mealtime-associated activity rhythm on the fasting day was considered to be another good index of food-anticipatory activity. In this study, we investigated the effects of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, which has been reported to cause impairment of spatial learning, on food-anticipatory activity in rats. Daily injection of MK-801 (0.2 mg/kg) at 1700 h for 6 successive days impaired the development of increased prefeeding activity. The appearance of mealtime-associated activity was impaired by daily injection of MK-801 (0.2 mg/kg) at 1700 h (immediately after food restriction, FR) or at 1900 h (2 h after FR) but not at 0900 (16 h after FR). In addition, mealtime-associated activity was impaired by MK-801 injected at 1700 h in a dose-dependent manner (0.1-1 mg/kg). The present results demonstrate that NMDA receptor mechanisms were involved in learning processes of food anticipation and suggest that an impairment of food-anticipatory activity may be considered an animal model of a deficit of "temporal learning.".