Fos expression induced by milk ingestion in the caudal brainstem of neonatal rats.

Prominent Fos expression in the nucleus of the solitary tract (NTS) related to feeding has been reported in the brainstem of adult animals. In this study, we used a Fos-guided immunohistochemical approach to determine the brainstem areas activated specifically in response to milk ingestion in rat pups at two different ages. Rats at 9 or 18 days postpartum were isolated from the mother for a 6-h period, after which they were returned to the mother for a suckling period of either 5 or 90 min and then perfused at 90 min after the beginning of suckling. Control groups were sacrificed before or after the 6-h-deprivation period and showed little or no Fos-ir. In contrast, a 90-min-suckling episode after 6 h of deprivation induced strong Fos-ir in the caudal regions of the NTS and in the spinal nucleus of the trigeminal (SPV). Moderate expression was observed in the rostral NTS and in the nucleus raphé obscurus. In rat pups that suckled for only 5 min, the main area activated was the SPV. Fos immunostaining was detected in only 1% of the catecholaminergic neurons from the NTS after milk ingestion. The experimental design employed here allowed us to distinguish brainstem areas activated by milk ingestion from those activated by suckling action in rat pups. In contrast to adult rats, catecholaminergic neurons from the caudal NTS seem to contribute little to the regulation of feeding at this age.