A postpartum separation induces c-Fos expression in the supramammillary nucleus of lactating rats.

Aim: Elucidation of the neural mechanism of maternal behaviors is a medically and biologically important research task. The rat is the laboratory animal most extensively analyzed for maternal behaviors. However, the neural mechanism that maintains the motivation of postpartum rats for maternal behaviors has not yet been elucidated. In this study, we aimed to identify brain regions involved in the maintenance of motivation for maternal behaviors by detecting brain regions that exhibit changes in nerve activity when the mother rat is separated from her pups.

Methods: Lactating mother rats were separated from their pups on postpartum day 3 and kept away from the pups for a certain period of time, and brain regions that exhibited changes in nerve activity when the rats were separated from their pups and those that exhibited changes in nerve activity when the pups are returned were detected by immunohistochemistry using anti-c-Fos antibody, a marker for increased nerve activity.

Results: Rats that were separated from their pups and with the pups returned later showed increases in the number of c-Fos immunoreactive (c-Fos-IR) cells in the medial preoptic area (MPA), the bed nucleus of the stria terminalis (BST), the caudal portion of posterior hypothalamic area (PH) and the supramamillary nucleus (SUM). In mother rats permanently separated from their pups, only the PH and SUM exhibited an increase in the number of c-Fos-IR cells.

Conclusion: In rats, the SUM is involved in aversive memory and changes in the postpartum anxiety level. The observed increase in the number of c-Fos-IR cells in the SUM of mother rats separated from their pups suggests that the nerve activity change in the SUM, which is involved in aversive memory and anxiety, is involved in the maintenance of maternal behaviors.