Specific inhibition of the PVN to nTS pathway attenuates cardiorespiratory responses to hypoxia

Abstract

The hypothalamic paraventricular nucleus (PVN) contributes to cardiorespiratory responses to peripheral chemoreflex activation. Our previous data indicate that acute hypoxia (Hx) activates PVN neuropeptidergic neurons that project to the nucleus tractus solitarii (nTS), including corticotropin releasing hormone and oxytocin (OT) neurons. However, the functional role of these projections to the nTS during Hx has not been fully delineated. We hypothesized PVN inputs to the nTS facilitate cardiorespiratory responses to Hx, and this requires activation of neuropeptidergic, including OT, receptors. Male Sprague Dawley rats received bilateral microinjections of AAV‐hSyn‐Gi‐DREADD‐mCherry (inhibitory DREADD) or AAV‐hSyn‐GFP (control) into the PVN. 3–5 weeks were allowed for expression in PVN neurons and in their projections. Rats were anesthetized, and cardiorespiratory parameters [blood pressure, (BP), heart rate (HR) splanchnic sympathetic nerve activity (sSNA) and phrenic nerve activity (PhrNA)] were measured at baseline and during Hx (10% O2, 45s). Gi‐DREADD‐expressing PVN terminals specifically in the nTS were inhibited via bilateral nTS microinjection of a synthetic selective DREADD ligand, Compound 21 (C21, 0.1mM; 90nl/side). Cardiorespiratory responses to Hx were measured before and 1 hr after DREADD‐mediated inhibition of PVN terminals in the nTS. After the experiment, brains were removed, and immunohistochemistry was performed on PVN and nTS tissue to verify AAV injection sites in the PVN and terminal expression in the nTS. We observed robust expression of mCherry and GFP containing cells in the PVN and dense fibers throughout the nTS in Gi‐DREADD and GFP rats, respectively. A portion of these fibers also were OT immunoreactive. Under control conditions, Hx significantly decreased BP and increased HR, sSNA and PhrNA frequency, amplitude (PhrAmp) and minute activity (MinPhrNA). Responses were similar in GFP and Gi‐DREADD rats. nTS microinjection of C21 had minor effects on baseline cardiorespiratory parameters, which were not significantly different between the two groups. In Gi‐DREADD rats, inhibition of PVN terminals in the nTS (C21 microinjection) decreased the MinPhrNA response to Hx by 35±13% (p<0.05, n=6). The effect was primarily due to reduced PhrAmp (41±7%, p<0.07, n=6). In contrast, the Hx‐induced increase in MinPhrNA was not significantly affected by C21 in the nTS of GFP rats (% increase MinPhrNA, Ctrl Hx, 425±13; Hx 60’ post‐C21, 535±142, n=3). Comparison between groups indicated that PhrNA during Hx 1 hr post‐C21 was significantly less in Gi‐DREADD vs GFP rats. To evaluate a role for OT in the nTS, in separate animals (n=2) the OT receptor antagonist OTA (0.1mM) was bilaterally microinjected in the nTS. OTA blunted the Hx‐induced increase in MinPhrNA (mV.s: Ctrl Hx, 78±16; Hx post‐OTA, 45±20). Thus, inhibition of PVN terminals and OT receptors in the nTS both blunted Hx‐evoked chemoreflex cardiorespiratory output. Together, these results are consistent with the concept that PVN inputs to the nTS are required for full expression of cardiorespiratory chemoreflex responses, and OTergic mechanisms in the nTS may contribute.Support or Funding InformationNIH RO1 HL98602This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.