Oxytocin and corticotropin‐releasing hormone augment synaptic activity in the nTS of rats subjected to chronic intermittent hypoxia

Abstract

Individuals with obstructive sleep apnea often develop hypertension. Chronic intermittent hypoxia (CIH) in rodents mimics the hypoxia‐induced hypertension seen in these individuals. Alterations in synaptic and neuronal activity within the nucleus tractus solitarii (nTS), the central integration site of sensory afferents, are involved in the CIH‐induced hypertension. The paraventricular nucleus of the hypothalamus (PVN) has a modulating role in controlling nTS activity to influence blood pressure. The PVN sends direct neuronal projections containing oxytocin (OT) and/or corticotropin‐releasing hormone (CRH) to the nTS, and these neuropeptides alter nTS synaptic signaling. During hypoxia, there is an increase in the production of these neuropeptides, yet their contribution to nTS activity after CIH is not understood. We hypothesized OT and CRH will individually increase nTS synaptic activity, their influence will be enhanced after CIH, and co‐application of OT and CRH will further magnify synaptic transmission in CIH. Male Sprague‐Dawley rats (3‐4 weeks) were exposed to either 10 days normoxia (21% O2) or CIH (30‐40 sec of 6% oxygen, 10 episodes/h, 8 h/day). Horizontal brainstem slices were generated, and using whole‐cell patch‐clamp recordings, we examined synaptic neurotransmission in monosynaptic nTS neurons. nTS neurons were also acutely dissociated and calcium (Ca2+) influx was examined via Fura‐2 imaging to isolate postsynaptic effects. Spontaneous (s) and afferent (TS)‐evoked excitatory postsynaptic currents (EPSCs) in slices or Ca2+ fluorescence in isolated cells were examined during sequential aCSF control, OT (600 nM) alone, CRH (300 nM) alone, and the combination of OT (600 nM) and CRH (300 nM). Vehicle (aCSF) application during the equivalent time period served as controls. In nTS normoxic slices, vehicle application (1 hr) did not change sEPSC frequency or amplitude, or TS‐evoked EPSC amplitude. During neuropeptide application (n=9), OT alone did not alter s‐ or TS‐EPSCs. CRH alone increased sEPSC and TS‐EPSC amplitude, and elevated overall currents in response to 20 Hz stimulation. Co‐application of OT and CRH did not further alter TS‐ or sEPSCs. After CIH, as in normoxia rats, vehicle did not alter synaptic currents. OT and CRH elevated TS‐EPSC amplitude after CIH (n=9), although sEPSCs were unaffected. Co‐application of OT and CRH significantly augmented TS‐EPSC amplitude evoked in response to 0.5 and 20Hz stimulation. Comparing the OT+CRH responses in both groups demonstrated the relative increase in current amplitude during 20 Hz stimulation was greater in CIH than Norm. In isolated nTS neurons, individual OT and CRH application increased Ca2+ relative to baseline and vehicle in normoxia and CIH. The co‐application of OT and CRH enhanced the increase of cytosolic Ca2+ in CIH rats compared to their normoxic controls. In summary, these data show that following CIH, OT and CRH enhanced afferent‐driven neurotransmission to nTS neurons via an increase in postsynaptic responses. The data predicate an increase in their receptors and a shift in CRH receptors after CIH.