Daily anticipatory rhythms of behavior and body temperature in response to glucose availability in rats

When food is available recurrently at a particular time of day, several species increase their locomotion in the hours that precede food delivery, a phenomenon called food anticipatory activity (FAA). In mammals, many studies have shown that FAA is driven by a food-entrained circadian oscillator (FEO) that is distinct from the light-entrained pacemaker in the suprachiasmatic nucleus of the hypothalamus. Few studies have investigated the effect of sugar ingestion on food anticipatory rhythms and the FEO. We aimed to extend the understanding of the role of glucose on the emergence of food anticipatory rhythms by investigating whether glucose ingestion is sufficient to produce daily food anticipation, reflected by motor activity and core body temperature rhythms. Under a 12 h/12 h light/dark cycle, chow-deprived rats had glucose solution available between Zeitgeber Time (ZT) 6 and ZT 9 for 10 days (glucose restriction group), whereas control animals had chow available within the same time window (chow restriction group). Animals in both groups exhibited anticipatory motor activity and body temperature around the fourth day of the scheduled food restriction. Glucose-fed rats ingested ~15 kcal on the days immediately before FAA emergence and reached an intake of ~20 kcal/day, whereas chow-fed rats ingested ~40 kcal/day. The glucose restriction group exhibited a pattern of food anticipation (activity and temperature) that was extremely similar to that observed in the chow restriction group. We conclude that glucose ingestion is a sufficient temporal cue to produce recurrent food anticipation, reflected by activity and temperature rhythms, in rats.