Endogenous Clock Gene Expression in the Suprachiasmatic Nuclei of Previsual Newborn Rabbits is Entrained by Nursing

The rabbit is particularly suitable for investigating the development of mammalian circadian function. Blind at birth. the pups are only visited by the mother to be nursed once every 24 It for about 3 min and so call be studied largely without maternal interference. They anticipate the mother's visit with increased behavioral arousal and with it rise in body temperature, both of which represent endogenous circadian rhythms. We now report that in newborn pups the suprachiasmatic matic of the hypothalamus (SCN, the main circadian pacemaker in mammals) show endogenous 24-h rhythmicity in the expression of the clock genes Per1, Per2. and Bmall. Pups nursed front postnatal days I to 7 and fasted to day 9 showed the same rhythms of clock gene expression as normally nursed controls. We also report that these rhythms are entrained by nursing.Pups killed on postnatal (lays 3-4 showed the same rhythms in gene expression as pups in the previous experiment, whereas littermates subsequently nursed front postnatal days 4 to 7 with nursing delayed 6 It showed a corresponding shift in the diurnal pattern of clock gene expression. Consistent with this, two groups of pups implanted with telemetric thermal sensors and nursed 6 h apart had daily patterns in body temperature synchronized with the two different nursing times. We conclude that the expression of clock genes associated with the newborn rabbit's circadian system is entrained by nonphotic cues accompanying nursing, the exact nature of which now needs to be clarified. (C) 2008 Wiley Periodicals. Inc.Develop Neurobiol 69: 47-59.2009