Brainstem and hypothalamic areas involved in respiratory chemoreflexes: a Fos study in adult rats

The adaptation to hypoxia and hypercapnia requires the activation of severa l anatomical structures along the neuraxis. In this study, using Fos immuno reactivity, we sought to map neuronal populations involved in chemoreflex n etworks activated during the responses to moderate hypoxia (O-2, 11%), and hypercapnia (CO, 5%) in the brainstem and the hypothalamus of the rat. In t he: medulla, hypoxia elicited marked and significant staining in the nucleu s of the solitary tract (NTS), and in parapyramidal neurons located near th e ventral surface, whereas hypercapnia evoked significantly c-fos only near the ventral surface in paraolivar neurons. In contrast, within pontine and suprapontine structures, both hypoxia and hypercapnia evoked similarly Fos immunoreactivity in the lateral parabrachialis area, the central grey, the caudal hypothalamus (dorsomedial and posterior hypothalamic nuclei), and i n a ventro-lateral hypothalamic area, extending from the rostral limit of t he mammillary nuclei to the retrochiasmatic area. more rostrally, labelling was observed in the paraventricular nucleus of the hypothalamus in respons e to hypercapnia, and in the supraoptic nucleus in response to hypoxia. The se results support the hypothesis that chemoreflexes pathways are not only restricted to medulla and pens but also involved mesencephalic and hypothal amic regions. The parabrachialis area and the central grey may be key relay s between caudal and ventral hypothalamic neurons, and medullary neurons in volved in the response to hypoxia and hypercapnia, (C) 2000 Published by El sevier Science B.V. All rights reserved.