Chronic intermittent hypoxia (CIH) enhances nTS sensitivity to oxytocin (OT) and corticotropin-releasing hormone (CRH) via effects at the nTS soma and on presynaptic release
Authors: Gama de Barcellos Filho Procopio , Dantzler Heather A , Hasser Eileen M , Kline David D 
Published: May 2024

OT and CRH neurons from the hypothalamus project to nucleus tractus solitarii (nTS) to influence its activity and response to hypoxia. CIH increases sympathetic nervous system activity, blood pressure, and respiration, and the nTS contributes to these responses. Yet, the influence of these OT or CRH inputs on nTS activity during CIH is unclear. We hypothesized that OT or CRH individually increases nTS activity, and their combined influence would further enhance activity. We also expected these responses to be enhanced by CIH. Male Sprague-Dawley rats (~3 wks) were exposed to 10d normoxia (21% O2) or CIH (~30s of 6% O2, 10x/hr, 8 h/day). Recorded events were examined during exogenous OT and CRH or their receptor block (OTR-x, CRHR2-x). As a time control, aCSF only was examined, which did not alter any events over the 40-min recording period. In nTS slices, extracellular neuronal discharge from multielectrode array recording was increased by OT and its co-application with CRH throughout the nTS after normoxia. Following CIH, OT and CRH alone, and their co-application increased firing rate, and the response to OT+CRH after CIH was greater than in normoxia slices. Fura-2 calcium imaging in dissociated nTS neurons demonstrated enhanced responses to OT+ CRH following CIH vs normoxia. In second-order nTS neurons recorded via patch clamp, OT and CRH induced spontaneous action potential discharge after CIH in quiescent neurons during baseline periods. Excitatory postsynaptic currents (EPSCs) were recorded to examine synaptic transmission. Spontaneous (s)EPSCs were not altered by OT and CRH after normoxia or CIH. Yet, only after CIH OTR-x and/or CRHR2-x reduced sEPSC amplitude, indicating tonic activation of these receptors maintain current amplitude. Afferent (TS)-EPSCs evoked at 0.5 and 20 Hz were not altered by OT and CRH or their receptor blockers in normoxia. However, CIH elevated TS-EPSCs in response to CRH and CRH+OT. Block of OTR and CRHR2 after CIH did not alter TS-EPSC amplitude, indicating their minimal tonic influence on sensory integration. After a 20Hz stimulation, the peak of asynchronous EPSCs increased in presence of CRH and OT+CRH only after CIH. We examined presynaptic Ca2+ in pre-labeled GCaMP6m afferent terminals in response to a 20 Hz stimulation. In normoxia-exposed slices, OT and CRH increased Ca2+ fluorescence. However, after CIH, the application of CRH and the co-application of OT+CRH decreased the terminal Ca2+ response. The expression of OT but not CRH fibers within the nTS decreased (immunohistochemistry) while OTR and CRHR2 mRNA increased (RNA Scope). The CRH and OT neuron populations within PVN were not altered after CIH (immunohistochemistry). Altogether, our data show that following CIH, OT and CRH augment neuronal activity via postsynaptic mechanisms while CRH also increases synaptic transmission through enhancing presynaptic release. Those alterations may contribute to elevated sympathetic response and blood pressure observed after CIH. NIH R01 HL098602 & R01 HL128454 (DDK). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.