Feeding-induced c-fos expression in the nucleus of the solitary tract and dorsal medullary reticular formation in neonatal rats.

We investigated feeding-associated activation of neurons in the medulla oblongata during the suckling period in rats, using the c-fos gene-encoded protein (Fos) immunohistochemistry. After an isolation from mothers for 12 h, neonates were either breast-fed intensively or further isolated for another 3 h, and sacrificed on postnatal day 1, 3, 5, 7 and 14 (P1-14). In the former pups, Fos-immunoreactive (FI) neurons were predominantly localized in the nucleus of the solitary tract (NST) and in the dorsal medullary reticular formation (RF). The number of FI cells peaked on P5-7 and decreased on P14 in the NST, and increased remarkably on P3 and was consistently high until P14 in the dorsal RF. In contrast, much fewer FI cells were found in the NST and RF in the latter pups. The results indicated that not only the NST but also the dorsal RF were implicated in feeding behavior in the suckled pups.