Ontogeny of the O2-sensitive pathway in medulla oblongata of postnatal rat

Fos protein, the product of the immediate early gene c-fos, has been used as a metabolic marker to map the O2 chemosensory pathway activated by hypoxia in the adult rat (Erickson and Millhorn, Brain Res. 567: 11-24, 1991). The current study provides evidence that the O2 chemoreceptor pathway develops during the first postnatal month. Rats at postnatal ages (P) 3, 7, 10, 14, 21, and 28 days were exposed for 3 h to 21% (control) or 10% (hypoxia) O2. Pups were transcardially fixed, brain stems were frozen, sectioned, then reacted with Fos primary antibody, a secondary antibody, avidin-biotin peroxidase, then Ni-DAB as chromogen. Cells showing Fos-like immunoreactivity (Fos-LI) under control and hypoxic conditions were counted in the nucleus tractus solitarii (NTS) and the ventrolateral medulla (VLM). In both areas there was initially a low basal level of Fos-LI, a peak at P10 and a decline to P28. At all ages there was a significant increase in the number of Fos-LI cells in pups exposed to hypoxia. The high basal level of Fos expression at P10 and the high induced level at P14 may correlate with periods of terminal differentiation and maximum synaptogenesis, respectively.