Hormonal and metabolic rhythms associated with the daily scheduled nursing in rabbit pups

1 Laboratorio de Biología de la Reproducción, IIB, Universidad Veracruzana, Xalapa, Veracruz, México; 2 Department of Psychology, University of Delaware, Newark, Delaware; 3 Departamento de Anatomía, Facultad de Medicina, Universidad Nacional Autónoma de México, México DF, Mexico; and 4 Center for Neurosciences, University of California, Davis, California Submitted 3 March 2008 ; accepted in final form 13 May 2008 Young rabbits are nursed every 24 h for a period of 3–5 min. As a consequence, pups are synchronized to this nursing event; this synchronization is characterized by increased locomotor activity and a peaking of core temperature and plasma corticosterone in anticipation of the daily meal. Ghrelin is a hormone suggested to play a role in meal initiation and to promote food intake. The present study explored the role of ghrelin in food-entrained conditions. Newborn rabbits were maintained in constant darkness and nursed once daily at 1000 by the lactating dam. On postnatal day 7 , rabbits were killed at six different time points to complete a 24-h cycle. All pups developed locomotor rhythms entrained by mealtime and exhibited anticipatory activity. Food-entrained rhythms in plasma corticosterone and free fatty acids were observed even if two meals were omitted. In contrast, daily food-driven rhythms in stomach weight, plasma glucose, liver glycogen, and ghrelin did not persist when two meals were omitted. Peak ghrelin levels were observed at the moment in the cycle when the stomach weight was lowest, i.e., before initiation of anticipation. The present data are in agreement with previous data from rabbit pups maintained in light-dark conditions and provide evidence that 7- to 9-day-old rabbits in constant darkness can exhibit metabolic and hormonal rhythms mainly driven by the restricted daily nursing. food entrainment; corticosterone; ghrelin; development; circadian rhythms Address for reprint requests and other correspondence: M. Caba, Lab. Biol. de la Reproducción, IIB, Universidad Veracruzana, AP 114, Xalapa 91000, Ver., Mexico (e-mail: mcaba{at}uv.mx )