Nocturnal hyperphagia in the rat is characterized by decreased postprandial satiety

Rats eat more at night than during the day. This work investigated whether this nocturnal hyperphagia is characterized by increased hunger, decreased postprandial satiety, or both. Rats were presented with liquid food after 3-hr food deprivation at the midpoint of the night or day phase of a 12:12 hr light/dark cycle. Quinine adulteration of food produced equal percentage suppression of first meal size (MS) and of 60-min intakes in the night and the day. This suggests that rats are equally hungry after 3-hr food deprivation in the night and the day. In contrast to apparently equal hunger, rats were less satiated by ingested food after 3 hr of food deprivation at night than during the day. This conclusion is based on the observations that the postprandial intermeal interval (IMI) was significantly shorter at night and that the satiety ratio (IMI/MS) was smaller at night. This nocturnal decrease in the satiating potency of ingested food was demonstrated for two specific preabsorptive satiety mechanisms: (a) the pregastric satiety mechanism(s) stimulated by sham feeding and (b) cholecystokinin, the putative satiety hormone released by food contacting the mucosal surface of the upper small intestine. All the results suggest that the diurnal variation of food intake in rats is primarily the result of diurnal variation in the potency of postprandial satiety mechanisms.