Maternal responsiveness to suckling is modulated by time post-nursing: a behavioral and c-FOS/oxytocin immunocytochemistry study in rabbits.

Doe rabbits nurse once/day for ca. three min, with circadian periodicity, inside the nest box. The amount of suckling received at each bout regulates this behavior as reducing litter size to four kits or less disrupts nursing circadian periodicity and increases suckling bout duration. Additionally, the likelihood that does will nurse kits at a given time of day depends on the time elapsed since the last suckling episode and the litter size nursed then. We hypothesized that the time elapsed since the last nursing would impact the number of c-FOS immunoreactive (IR) cells observed after suckling five kits and also that observed before the next nursing ('no kits'). Suckling significantly increased, relative to 'no kits', the number of c-FOS-IR cells in the medial preoptic area and lateral septum but not in the bed nucleus of the stria terminalis (BNST), suprachiasmatic nucleus or ventromedial hypothalamus in does nursing at 18 hrs or 24 hrs after the previous bout. No effects of suckling were observed in mothers nursing at 6 hrs. Does given kits at 3 hrs post last suckling refused to nurse but in the remaining three groups all does nursed normally. At 'no kits' more c-FOS-IR cells were seen (in all regions except the BNST) in does given kits at 24 hrs after the last nursing and sacrificed one hour later (i.e., 4 hrs after lights-on) than in those killed earlier. The percentage of oxytocinergic (OT) cells co-expressing c-FOS was not modified by nursing in the paraventricular or supraoptic nuclei but in the latter the largest number of total OT-IR cells occurred at 18 and 24 hrs post-last nursing. Conclusion: the responsiveness of particular forebrain regions involved in regulating circadian rhythms, lactation, and maternal behavior is modulated by suckling and time of day.