Hypoxia activates neurons in the hypothalamic paraventricular nucleus (PVN) that project to the nucleus tractus solitarii (nTS) in the brainstem

Abstract

The PVN is critical to arterial chemoreflex function and projects to brainstem cardiorespiratory regions. Our group recently showed that efferent projections to the rostral ventrolateral medulla and the intermediolateral cell column are minimally activated by acute hypoxia (AH), suggesting other efferent pathways are involved. We hypothesized that PVN neurons with nTS projections are activated by AH. Rats were microinjected with a retrograde tracer in the nTS to label nTS‐projecting PVN neurons. Conscious rats were subjected to either two hours of normoxia (Norm; 21% O2) or hypoxia (AH; 10% O2). Brains were perfused and immunohistochemistry performed to identify activated neurons (Fos‐IR) and determine the phenotype (nNOS‐, AVP‐, CRH‐ and Oxy‐IR) of nTS‐projecting PVN neurons. AH significantly activated nTS‐projecting PVN neurons (Norm: 26±4; AH: 46±7 cells). Similar to our previous work, AH significantly activated nNOS‐IR and CRH‐IR neurons, but few AVP‐IR cells. OXY‐IR PVN neurons were significantly activated by AH but very few of the activated OXY‐IR cells projected to the nTS (1.6±0.9%). Subpopulations of nTS‐projecting cells contained nNOS (17±1%) or CRH (26±4%). Furthermore, these projecting cells were significantly activated by AH (nNOS, Norm: 16±3; AH: 24±3%; CRH, Norm: 13±0.05; AH: 29±2%). These results suggest the PVN contributes to chemoreflex output via efferent projections of CRH‐ and nNOS‐IR neurons to the nTS. (HL98602)