PVN-projecting NTS Neurons is Involved in Sympathetic Long-Term Facilitation After Acute Intermittent Optogenetic Stimulation

Acute intermittent optogenetic (AIO) stimulation to the caudal NTS, a visceral sensory nucleus with projections to, among others, the paraventricular nucleus (PVN), induces sympathetic long-term facilitation (LTF), which is a progressive increase of sympathetic nerve activity. The contribution of the NTS to PVN pathway in sympathetic LTF is unknown. We investigated the effects of AIO stimulation of PVN-projecting NTS neurons in rats that were previously injected with rAAV2/Ef1a-DIO-hchR2(H134R)-mCherry in the NTS and retrograde AAV9.hSyn.HI.eGFP-cre.WPRE.SV40 in the PVN. We measured splanchnic sympathetic nerve activity (SSNA), mean arterial pressure (MAP), and heart rate (HR) in anesthetized (urethane and cloralose) and mechanically ventilated adult male Sprague-Dawley rats. Twenty minutes after AIO, defined as train of 10 stimuli each consisting of pulse of light of 10-ms duration delivered at 20 Hz for 1 min every 6 min, SSNA amplitude increased by 26.8 ± 7.09% (n=7), MAP increased by 16.9 ± 11.72% (n=7). This increase in SSNA and MAP occurred whether singular optogenetic train of stimulation increased (n=3) or decreased (n=4) of SSNA. These results suggest that the splanchnic sympathetic LTF induced by acute intermittent stimulation of NTS neurons might be mediated by the PVN-projecting neurons, independently of baroreflex or chemoreflex circuits. Further histological study is needed to clarify whether the differential responses to single optogenetic stimulation is due to activation of collaterals from PVN-projecting NTS neurons that are connected to different circuits or due to activation of different downstream projections from the PVN.