Maternal behavior stimulates c-fos activity within estrogen receptor alpha-containing neurons in lactating rats

Estradiol and other hormones are thought to be critical for the onset, but not maintenance, of maternal behavior in rats. Maternal behavior is instead maintained postpartum by tactile stimulation that dams receive during interactions with pups, and many neural sites implicated in the control of maternal behavior show elevated c-fos activity in response to this stimulation. Many of these sites also contain neurons that express the alpha subtype of the estrogen receptor (ERalpha). Because of possible interactions between tactile stimulation from pups, c-fos, and ERalpha in the lactating rat brain, we determined if populations of cells that show increased c-fos activity after maternal behavior in lactating rats also contain ERalpha. Dams were separated from their pups for 48 h beginning on day 5 postpartum. On day 7 postpartum, experimental dams were reunited with pups and mother-litter interactions were observed for 60 min. Control dams received no pup stimulation. Subjects were sacrificed 60 min later and brain sections were double immunolabeled for the Fos and ERalpha proteins. As expected, the number of ERalpha-immunoreactive (ERalpha-ir) neurons did not differ between the two groups in the eight areas analyzed (lateral region of the lateral septum, posterodorsal medial amygdala, dorsal and ventral medial preoptic area, dorsal and ventral bed nucleus of the stria terminalis, lateral habenula, and ventrolateral caudal periaqueductal gray). Consistent with previous reports, maternal dams had 2- to 7-fold more Fos-immunoreactive (Fos-ir) neurons in these sites compared with nonstimulated controls. Maternal dams had significantly more Fos-ir neurons that also contained ERalpha-ir in all sites, with the greatest increases in the ventral medial preoptic area, lateral habenula, and ventral bed nucleus of the stria terminalis. Between approximately 25 and 45% of the Fos-ir cells in the sites examined also expressed ERalpha. Thus, a substantial number of neurons that are genomically activated during maternal behavior contain ERalpha, raising the possibility that the postpartum display of maternal behavior can be influenced by ERalpha activity.