Neurogenic mechanisms underlying the rapid onset of sympathetic responses to intermittent hypoxia

Sleep apnea (SA) leads to metabolic abnormalities and cardiovascular dysfunction. Rodent models of nocturnal intermittent hypoxia (IH) are used to mimic arterial hypoxemias that occur during SA. This mini-review focuses on our work examining central nervous system (CNS) mechanisms whereby nocturnal IH results in increased sympathetic nerve discharge (SND) and hypertension (HTN) that persist throughout the 24-h diurnal period. Within the first 1-2 days of IH, arterial pressure (AP) increases even during non-IH periods of the day. Exposure to IH for 7 days biases nucleus tractus solitarius (NTS) neurons receiving arterial chemoreceptor inputs toward increased discharge, providing a substrate for persistent activation of sympathetic outflow. IH HTN is blunted by manipulations that reduce angiotensin II (ANG II) signaling within the forebrain lamina terminalis suggesting that central ANG II supports persistent IH HTN. Inhibition of the hypothalamic paraventricular nucleus (PVN) reduces ongoing SND and acutely lowers AP in IH-conditioned animals. These findings support a role for the PVN, which integrates information ascending from NTS and descending from the lamina terminalis, in sustaining IH HTN. In summary, our findings indicate that IH rapidly and persistently activates a central circuit that includes the NTS, forebrain lamina terminalis, and the PVN. Our working model holds that NTS neuromodulation increases transmission of arterial chemoreceptor inputs, increasing SND via connections with PVN and rostral ventrolateral medulla. Increased circulating ANG II sensed by the lamina terminalis generates yet another excitatory drive to PVN. Together with adaptations intrinsic to the PVN, these responses to IH support rapid onset neurogenic HTN.