Paraventricular nucleus projections to the nucleus tractus solitarii are essential for full expression of hypoxia‐induced peripheral chemoreflex responses

Abstract

The hypothalamic paraventricular nucleus (PVN) is essential to peripheral chemoreflex neurocircuitry, but the specific efferent pathways utilized are not well defined. The PVN sends dense projections to the nucleus tractus solitarii (nTS), which exhibits neuronal activation following a hypoxic challenge. We hypothesized that nTS‐projecting PVN (PVN‐nTS) neurons contribute to hypoxia‐induced nTS neuronal activation and cardiorespiratory responses. To selectively target PVN‐nTS neurons, rats underwent bilateral nTS nanoinjection of retrogradely transported adeno‐associated virus (AAV) driving Cre recombinase expression. We then nanoinjected into PVN AAVs driving Cre‐dependent expression of Gq or Gi designer receptors exclusively activated by designer drugs (DREADDs) to test the degree that selective activation or inhibition, respectively, of the PVN‐nTS pathway affects the hypoxic ventilatory response (HVR) of conscious rats. We used immunohistochemistry for Fos and extracellular recordings to examine how DREADD activation influences PVN‐nTS neuronal activation by hypoxia. Pathway activation enhanced the HVR at moderate hypoxic intensities and increased PVN and nTS Fos immunoreactivity in normoxia and hypoxia. In contrast, PVN‐nTS inhibition reduced both the HVR and PVN and nTS neuronal activation following hypoxia. To further confirm selective pathway effects on central cardiorespiratory output, rats underwent hypoxia before and after bilateral nTS nanoinjections of C21 to activate or inhibit PVN‐nTS terminals. PVN terminal activation within the nTS enhanced tachycardic, sympathetic and phrenic (PhrNA) nerve activity responses to hypoxia whereas inhibition attenuated hypoxia‐induced increases in nTS neuronal action potential discharge and PhrNA. The results demonstrate the PVN‐nTS pathway enhances nTS neuronal activation and is necessary for full cardiorespiratory responses to hypoxia. imageKey points The hypothalamic paraventricular nucleus (PVN) contributes to peripheral chemoreflex cardiorespiratory responses, but specific PVN efferent pathways are not known. The nucleus tractus solitarii (nTS) is the first integration site of the peripheral chemoreflex, and the nTS receives dense projections from the PVN. Selective GqDREADD activation of the PVN‐nTS pathway was shown to enhance ventilatory responses to hypoxia and activation (Fos immunoreactivity (IR)) of nTS neurons in conscious rats, augmenting the sympathetic and phrenic nerve activity (SSNA and PhrNA) responses to hypoxia in anaesthetized rats. Selective GiDREADD inhibition of PVN‐nTS neurons attenuates ventilatory responses, nTS neuronal Fos‐IR, action potential discharge and PhrNA responses to hypoxia. These results demonstrate that a projection from the PVN to the nTS is critical for full chemoreflex responses to hypoxia.

Understanding the hypothalamic–brainstem bridge in the hypoxia puzzle

“…The findings presented by Ruyle et al. (2023) also hold significant implications that can potentially pave the way for further studies examining the long‐term effects of hypoxia exposure on the functioning of autonomic and respiratory systems. This is particularly relevant in situations of sustained exposure to low O 2 levels, as observed at high altitudes or in mines, and diseases associated with chronic hypoxia, including sleep‐disordered breathing, lung diseases, heart failure and others.…”

“…Examining these possibilities would further expand the relevance of the results presented by Ruyle et al. (2023).…”

“…Ruyle et al. (2023) reported that activating PVN projections to the NTS enhanced the ventilatory response to moderate hypoxia in conscious rats, increased the hypoxia‐induced fos expression in the PVN and NTS regions, and amplified the phrenic and sympathetic excitatory responses to hypoxia in anaesthetized animals. In contrast, the authors observed that inhibition of the PVN–NTS pathway attenuated the ventilatory and phrenic responses to moderate and intense hypoxia and reduced fos expression and firing frequency response of NTS neurons promoted by hypoxia exposure.…”

“…However, how the PVN neurons interact with other brain regions regulating respiratory and autonomic activities remains unclear. The study by Ruyle et al (2023), published in this issue of The Journal of Physiology, provides detailed information on how the neurons of the PVN can modulate the magnitude of cardiorespiratory responses to hypoxia by interacting with the NTS neurons. To achieve this goal, the authors combined retrograde and cre-dependent viral transfections to express excitatory or inhibitory DREADDs (designer receptors exclusively activated by designer drugs) in NTS-projecting PVN neurons, and performed measurements of pulmonary ventilation, electrophysiology and imaging studies.…”

“…However, the underlying mechanism is still a matter for future research. Interestingly, the study by Ruyle et al (2023) also invites additional questions about the operational details of this pathway. Among these are the phenotype of PVN neurons projecting to NTS, the characteristics of the NTS neurons receiving these inputs (2nd-order or high-order neurons), and the involvement of this pathway in modulating the reflex responses to carotid body activation by other mediators.…”