Submitted on March 3, 2008
Accepted on May 13, 2008
Hormonal and metabolic rhythms associated with the daily scheduled nursing in rabbit pups
Elvira Morgado1, M Kathleen Gordon2, Maria del Carmen Minana-Solis3, Enrique Meza1, Seymour Levine4, Carolina Escobar5, and Mario Caba1*
1 Lab. biol. of Reproduction, IIB, University of Veracruz, Xalapa, Veracruz, Mexico
2 Department of Psychology, University of Delaware, Newark, Delaware, United States
3 Anatomia, UNAM, Mexico City, Mexico; , Mexico
4 Psychiatry, University of California at Davis, Sacramento, Davis (Sacramento), California, United States
5 Anatomia / Facultad de Medicina, Universidad Nacional Autonoma de Mexico, Mexico DF, Mexico
* To whom correspondence should be addressed. E-mail: mcaba{at}uv.mx.
Young rabbits are nursed every 24 h for a period of three to five minutes. As a consequence, pups are synchronized to this nursing event, characterized by increased locomotor activity and a peaking of core temperature and plasma corticosterone in anticipation to the daily meal. Ghrelin is a hormone suggested to play a role in meal initiation and to promote food-intake. Present study explored the role of ghrelin in food entrained conditions. Newborn rabbits were maintained in constant darkness and were nursed once daily at 10:00 h by the lactating dam. On postnatal day 7 rabbits were sacrificed at 6 different time points to complete a 24 h cycle. All pups developed locomotor rhythms entrained by mealtime and exhibited anticipatory activity. Food entrained rhythms were observed in plasma corticosterone and free fatty acids and such rhythms were still observed if two meals were omitted. In contrast daily food driven rhythms were observed in stomach weight, plasma glucose, liver glycogen and ghrelin and they did not persist when two meals were omitted. Peak values of ghrelin were observed at the moment when the stomach attained the lowest values in the cycle, thus before rabbits started anticipating. Present data are in agreement with previous data reported from rabbit pups maintained in light /dark conditions and provide evidence that in constant darkness seven to nine day old rabbits can exhibit metabolic and hormonal rhythms mainly driven by the restricted daily nursing.
