Abstract
Neonatal mammals initiate suckling after birth and transiently change their feeding behavior from suckling to mastication along with the eruption of molars. The transition of food ingestive behavior during the developmental period is thought to be governed by the maturation of feeding center in the brain. In the present experiment, feeding-associated activation of neurons in the medulla oblongata was examined in both developing and adult rats, using the c-fos gene-encoded protein (Fos) as a marker. Neonates were isolated from mothers for 12 h during the dark phase of the 12: 12 light-dark cycle on postnatal days 3,7 and 14. After lights on, they were either breast-fed intensively or further isolated for another 3 h, and then sacrificed. Adult rats fasted for 24 hours by depriving them of solid food and water, and were either given food and water before sacrifice or sacrificed without food intake. In the breast-fed pups, Fos-immunoreactive (FI) neurons were predominantly localized in the nucleus of the solitary tract (NST) and in the dorsal medullary reticular formation (dRF). Satiated adult rats with food showed similar patterns of Fos expression in both NST and dRF, although the FI cells were less prominent than those found in the neonates. In contrast, many fewer Fl cells were observed in the NST and dRF in the pups and adults which had fasted. The distinct localization of Fos in both fed pups and adults indicates the functional significance of the NST and dRF in the feeding behavior.
