Endogenous expression of C-Fos in hypothalamic nuclei of neonatal rabbits coincides with their circadian pattern of suckling-associated arousal.

Rabbits exhibit an unusually restricted form of maternal care. The newborn young are only nursed for about 3 min every 24 h and drink up to one third of their body weight in this time. They show a circadian increase in activity in anticipation of the doe's visit, which is entrained by suckling and appears analogous to feeding-entrained rhythms reported for other species. By investigating the expression of c-Fos protein in nursed and un-nursed pups at selected times before and after scheduled nursing, we were able to distinguish endogenous patterns of neuronal activation and patterns induced by suckling. Increased endogenous expression of c-Fos in the paraventricular thalamus and in the magnocellular neurons of the paraventricular hypothalamus coincided with the period of anticipatory arousal. By contrast, suckling induced c-Fos expression in the supraoptic nucleus and the parvocellular neurons of the paraventricular hypothalamus, whilst decreasing expression in the magnocellular neurons. Furthermore, the endogenous pattern of c-Fos expression was less distinct in 3-day-old compared to 7-day-old pups, suggesting maturational or experience-related influences on the neural correlates of the anticipatory arousal. In conclusion, the close correspondence between c-Fos expression and pups' endogenous rhythm of behaviour suggests that the rabbit's natural schedule of restricted nursing provides an ideal model in which to investigate the ontogeny of circadian function without disrupting the mother-offspring relationship or normal neonatal development.