Postnatal changes in Fos-like immunoreactivity evoked by hypoxia in the rat brainstem and hypothalamus

We have recently used Fos expression in adult rats to map neuronal populati ons activated in the brainstem and hypothalamus during the acute ventilator y response to moderate hypoxia (O-2 11%). Although present at birth, this r esponse evolves postnatally. The present investigation aimed at a better un derstanding of these maturational processes by delineating structures that might functionally develop after birth. The developmental pattern Fos expre ssion evoked by hypoxia was analysed in rats aged from 0 to 26 postnatal da ys. The numbers of Fos positive neurons markedly increased with the age in the medullary areas related to respiratory control during the 2 first postn atal weeks. Thereafter, the response plateaued in the nucleus tractus solit arius and attenuated in the ventral medulla. In the upper brainstem (parabr achial area, central grey) and the hypothalamus (posterior and dorsomedial nuclei, ventral zone), Fos response to hypoxia was absent or weak at birth and increased until late development. The significance of the development o f evoked Fos expression in these rostral sites is discussed together with t heir possible contribution to the maturation of O-2-sensitive chemoreflex p athways. (C) 2000 Elsevier Science B.V. All rights reserved.