Postpartum inhibition of ovarian steroid action increases aspects of maternal caregiving and reduces medial preoptic area progesterone receptor expression in female rats

The rapid peripartum onset of maternal caregiving involves progesterone synergizing with estradiol, but prolonging progesterone exposure past this time can prevent the emergence of mothering. Interestingly, there is a 7–10 day-long rise in progesterone during mid-lactation, but its effects on mothering are unknown. Given progesterone's potential to inhibit mothering onset, this mid-lactational rise may contribute to the normal attenuation of caregiving behaviors across lactation. To evaluate this, recently-parturient rats were ovariectomized and caregiving observed from postpartum days (PPD) 7–18. Ovariectomized dams were found to lick, hover over, and nurse in kyphosis more frequently than controls. Ovariectomy also decreased medial preoptic area (mPOA) progesterone receptor (PR) mRNA, which was negatively correlated with pup licking and kyphosis, but it did not affect mPOA levels of oxytocin receptor or vasopressin V1a receptor mRNAs. In a second study, gonadally intact dams were given the PR antagonist, RU 486, and were found to display more kyphosis and less supine nursing compared to controls. Finally, progesterone sensitivity across lactation was examined by measuring numbers of PR immunoreactive (PR-ir) cells in the mPOA, ventral bed nucleus of the stria terminalis (BSTv) and periaqueductal gray (PAG). PR-ir was higher in the mPOA at parturition compared to virgins, while PR-ir in the mPOA and BSTv dropped from parturition to PPD 7 and remained low through PPD 18. The number of PR-ir cells in the PAG was constant. Thus, in addition to their well-known prepartum effects, ovarian hormones limit the display of some maternal behaviors during mid-to-late lactation and contribute to their decline as weaning approaches.