ARTICLES

Development of the circadian rhythm of jejunal sucrase activity in the weanling rat

In the adult rat, fed ad libitum, the activity of jejunal sucrase shows a diurnal rhythm with a peak during the dark period. Ontogenically, sucrase activity appears around the 16th postnatal day, then rises rapidly, and reaches adult levels by the 25th day. Our aim was to determine the developmental stage at which the diurnal rhythm appears and to elucidate its physiological basis. When sampled every 3 h on the 19th day, jejunal sucrase showed no discernible pattern with time. In contrast, at 22 days (1 day after completion of weaning) a circadian rhythm very similar to that of adults had appeared. Weights of stomach contents showed that the adult pattern of nocturnal feeding also matured between days 19 and 22. When weaning was prevented, no sucrase rhythm was detected on day 22, despite an imposed rhythm of suckling. Thus the sucrase rhythm is normally cued by some aspect of weaning other than rhythmic ingestion per se. Although corticosterone seemed a likely mediator of the sucrase rhythm, studies in adult rats showed that the rhythm persists in adrenalectomized animals.