CO2-induced c-fos expression in medullary neurons during early development

In this study, we characterized the responses of brainstem neurons to hypercapnic loading at 5, 15, and 40 postnatal days, using c-fos gene encoded protein (Fos), as a marker of neuronal activity. At any of these studied ages exposure to 10% CO2 for 1 h produced a significant increase in the number of activated neurons within the ventral and the dorsal aspects of the brainstem. In the ventrolateral aspect of the medulla oblongata, Fos positive cells were observed within the ventrolateral medulla, extending from the pontomedullary border to the decussation of the pyramids. In the most rostral regions, within the retrotrapezoid field, the number of Fos positive cells was lower than in caudal ventral medullary regions at the levels of the area postrema and the caudal to it. No age related differences were observed in the number of neurons exhibiting CO2-induced Fos expression. Fos positive cells were additionally observed in the lateral paragigantocellular and gigantocellular reticular nuclei, in the medullary midline complex, in the raphe pallidus and in the raphe obscurus. The number of activated cells in the midline neurons was higher at 5 than at 40 days of age. In the dorsal aspect of the medulla oblongata Fos positive neurons were observed mainly within the caudal nucleus tractus solitarius (nTS). Postnatal age had no effect on the distribution and number of nTS cells activated by hypercapnic loading. These findings indicate that neurons activated by increases in CO2/H+ concentrations appear to be well developed from the first days of postnatal life in maturing rat pups. (C) 1999 Elsevier Science B.V. All rights reserved.