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We previously showed that orexin neurons are activated by hypoxia and facilitate the peripheral chemoreflex (PCR)-mediated hypoxic ventilatory response (HVR), mostly by promoting the respiratory frequency response. Orexin neurons project to the nucleus of the solitary tract (nTS) and the paraventricular nucleus of the hypothalamus (PVN). The PVN contributes significantly to the PCR and contains nTS-projecting corticotropin-releasing hormone (CRH) neurons. We hypothesized that in male rats, orexin neurons contribute to the PCR by activating nTS-projecting CRH neurons. We used neuronal tract tracing and immunohistochemistry (IHC) to quantify the degree that hypoxia activates PVN-projecting orexin neurons. We coupled this with orexin receptor (OxR) blockade with suvorexant (Suvo, 20 mg/kg, i.p.) to assess the degree that orexin facilitates the hypoxia-induced activation of CRH neurons in the PVN, including those projecting to the nTS. In separate groups of rats, we measured the PCR following systemic orexin 1 receptor (Ox1R) blockade (SB-334867; 1 mg/kg) and specific Ox1R knockdown in PVN. OxR blockade with Suvo reduced the number of nTS and PVN neurons activated by hypoxia, including those CRH neurons projecting to nTS. Hypoxia increased the number of activated PVN-projecting orexin neurons but had no effect on the number of activated nTS-projecting orexin neurons. Global Ox1R blockade and partial Ox1R knockdown in the PVN significantly reduced the PCR. Ox1R knockdown also reduced the number of activated PVN neurons and the number of activated tyrosine hydroxylase neurons in the nTS. Our findings suggest orexin facilitates the PCR via nTS-projecting CRH neurons expressing Ox1R., Competing Interests: The authors declare no competing financial interests., (Copyright © 2024 the authors.)

Brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, are highly expressed in the nucleus tractus solitarius (nTS), the principal target of cardiovascular primary afferent input to the brainstem. However, little is known about the role of BDNF signaling in nTS in cardiovascular homeostasis. We examined whether BDNF in nTS modulates cardiovascular function in vivo and regulates synaptic and/or neuronal activity in isolated brainstem slices. Microinjection of BDNF into the rat medial nTS (mnTS), a region critical for baroreflex control of sympathetic outflow, produced dose-dependent increases in mean arterial pressure (MAP), heart rate (HR), and lumbar sympathetic nerve activity (LSNA) that were blocked by the tyrosine kinase inhibitor K252a. In contrast, immunoneutralization of endogenous BDNF (anti-BDNF), or microinjection of K252a alone, decreased MAP, HR, and LSNA. The effects of anti-BDNF were abolished by blockade of ionotropic glutamate receptors, indicating a role for glutamate signaling in the response to BDNF. In vitro, BDNF reduced the amplitude of miniature EPSCs as well as solitary tract (TS) evoked EPSC amplitude and action potential discharge (APD) in second-order nTS neurons. BDNF effects on EPSCs were independent of GABAergic signaling and abolished by AMPA receptor blockade. In contrast, K252a increased spontaneous EPSC frequency and TS evoked EPSC amplitude. BDNF also attenuated APD evoked by injection of depolarizing current into second-order neurons, indicating reduced intrinsic neuronal excitability. Our data demonstrate that BDNF signaling in mnTS plays a tonic role in regulating cardiovascular function, likely via modulation of primary afferent glutamatergic excitatory transmission and neural activity.