Activation of nucleus tractus solitarii (nTS) neurons that project to the rostral ventrolateral medulla (RVLM) or hypothalamic paraventricular nucleus (PVN): Role of acute hypoxia (AH)

Abstract

The nTS receives and integrates chemoafferent input signaling arterial O2, and sends this information to the RVLM and PVN to produce coordinated cardiorespiratory responses to AH. The importance of the nTS in this reflex is understood, but the relative contribution of these two pathways is unknown. Retrograde tracers were microinjected to label RVLM‐ and PVN‐projecting nTS neurons. After recovery, conscious rats underwent 2 hr normoxia (21% O2) or AH (14, 12, 10 or 8% O2). We used Fos‐immunoreactivity (IR) as an index of neuronal activation and tyrosine hydroxylase (TH)‐IR to identify catecholaminergic neurons important in cardiorespiratory function in the nTS. AH progressively increased Fos‐IR in nTS cells. The number of Fos‐IR nTS‐RVLM cells was greater compared to nTS‐PVN cells after 14% ‐ 10% O2 (e.g. 12% O2: RVLM, 25±5; PVN, 14±4 cells) but was similar at 8% O2. However, the percentage of cells that were Fos‐IR was greater in nTS‐PVN neurons than nTS‐RVLM neurons after 12–8% (e.g. 10% O2: RVLM, 15±3; PVN, 25±4%). Very few nTS‐RVLM cells were catecholaminergic. However, 42±3% of nTS‐PVN cells was catecholaminergic; these cells were progressively activated by AH. We conclude that the nTS‐RVLM pathway is important as an early response to mild AH. The nTS‐PVN pathway has an important neuromodulatory role during severe AH, and these cells are likely catecholaminergic. HL98602