Hypobaric hypoxia induces Fos and neuronal nitric oxide synthase expression in the paraventricular and supraoptic nucleus in rats

This study examined the effects of high altitude exposure on neurons in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothal amus in adult and neonatal rats. In adult control rats, occasional Fos-like immunoreactive neurons were localized in both the hypothalamic nuclei, A m arked increase in Fos positive cells was induced at 1-4 h following altitud e exposure but it was reduced to levels comparable to the controls at 24 h. The expression of neuronal nitric oxide synthase (nNOS) immunoreactivity i n the PVN and SON followed a similar temporal pattern, The nNOS immunoreact ivity, which was constitutively expressed in the hypothalamic neurons in th e control rats, was noticeably augmented at 1-4 h, but it was comparable to the controls at 24 h following altitude exposure. In postnatal rats, Fos e xpression was not detected in the hypothalamic neurons of the controls. Ind uction of Fos expression was observed in some neurons at 1-4 h following al titude exposure but it was diminished at 24 h. There was no noticeable chan ge in nNOS expression in both the control and altitude exposed postnatal ra ts; in both instances, it was ba rely detectable. It is concluded that both the PVN a nd SON of the adult rats are activated at high altitude exposure and that they may be involved in the regulation of neuroendocrine, cardiov ascular and respiratory functions in hypobaric hypoxia. Th is study has als o shown the differential response of the hypothalamus neurons between the t wo age groups to the hypoxic insult. Our results suggest that the adult neu rons are probably more sensitive to the reduced oxygen levels in hypobaric hypoxia, as reflected by the upregulated NOS expression in this age group b ut not in the postnatal rats. (C) 2000 Elsevier Science Ireland Ltd. All ri ghts reserved.