Central nuclei activated during long-term facilitation of blood pressure following acute exposures to intermittent hypoxia

Introduction: Acute intermittent hypoxia (AIH) is a protocol used to mimic the arterial hypoxemia that occurs during sleep apnea. AIH involves brief (1 min) exposures to systemic hypoxia (10% FIO2) repeated at 6 min intervals for an hour. Such exposures to AIH induce a phenomenon termed long-term facilitation (LTF). LTF is a long-lasting (at least 3 hr) increase in mean arterial pressure (MAP), heart rate (HR), sympathetic nerve discharge (SND) and phrenic nerve discharge (PND). As LTF represents a form of neuronal plasticity, we were interested in determining what sites within the CNS might be involved in the generation of LTF induced by AIH. Our hypothesis is that AIH will induce activity in central sites typically associated with cardiovascular regulation in the brain. Methods: After rats underwent tracheal intubation and were artificially ventilated, they had femoral artery and venous catheters implanted for measurement of arterial pressure and administration of drugs, respectively. AIH was induced as previously described (Yamamoto et al., 2015). The test rats (n=3) were maintained for 1 hour after the last hypoxic exposure. The control group (n=2) was surgically prepared exactly as the experimental group but was not exposed to hypoxia during the 2-hour experimental period. Following the AIH protocol, rats were sacrificed, transcardially perfused with paraformaldehyde and their brains sectioned on a cryostat and processed for immunohistochemical detection of c-Fos in alternate sections. Cardiovascular parameters were statistically analyzed using 2-way ANOVA. Brain sections containing the central nuclei of interest were examined for the presence of c-Fos. Results: Our protocol of AIH induced a significant increase in all measured cardiovascular parameters (MAP, HR, renal SND and PND) measured 1 hr after cessation of exposure to AIH. This LTF was associated with c-Fos immunoreactivity in neurons located with the NTS and the RVLM but not within the hypothalamic PVN. Conclusions & Future Studies: The generation of LTF appears to be dependent upon medullary sites involved in cardiorespiratory regulation (NTS) and sympathetic outflow (RVLM) but not within hypothalamic cardiorespiratory and sympathetic regulatory nuclei such as the PVN. Future experiments can also use neuronal inhibitors to determine the specific role of each area in AIH-induced LTF, and further clarify the statistical significance of this result. This could provide insights into central areas involved in the persistent sympatho-excitation and hypertension observed in sleep apnea patients.