Central and Systemic Effects of Subdiaphragmatic Vagus Nerve Stimulation during the Development of Hypertension in the SHR
Authors: Dirr Elliott W. , Martyniuk Christopher J. , Otto Kevin J. , Zubcevic Jasenka 
Published: May 2022

Introduction

The vagus nerve facilitates bidirectional communication between the gut and brain. A low vagal tone has been described in many conditions characterized by inflammation and gut dysbiosis, including hypertension (HTN). While linked with clinical observations, the mechanisms of vagal gut‐brain communication are not fully understood. Moreover, the role of the sub‐diaphragmatic gut‐projecting vagal nerve branches in control of blood pressure (BP) remains unclear. The objective of this study was to examine the systemic and central effects of augmenting vagus nerve activity using subdiaphragmatic vagus nerve stimulation (sdVNS) during the development of HTN in spontaneously hypertensive rat (SHR).

Methods

Male SHR (8 wo) were implanted with BP telemeters and either a silicone cuff electrode (STIM, N=5) or sham cuff (SHAM, N=5) on the ventral subdiaphragmatic vagal trunk. Following recovery and baseline BP recordings, STIM rats were connected to a stimulus generator, and subjected to biphasic (600 μA, 500 us/phase, 25 Hz) vagus nerve stimulation (10 minutes/ session, 3 sessions/ day, 5 days/ week) for 7 weeks. SHAM rats were subjected to similar handling but with no stimulation. Blood pressure was recorded with a 24‐hour continuous recording once per week for eight weeks in all rats. Rats were sacrificed and cecal content, plasma, proximal colon, and nucleus tractus solitarius (NTS) were collected. Bacterial 16s sequencing was used to determine abundance and composition of gut bacteria. Inflammatory cytokines including INF‐γ, IL‐1β, IL‐4, IL‐5, IL‐6, KC/GRO, IL‐10, IL‐13, TNF‐α, IL‐17α, and corticosterone were measured in serum and proximal colon. To further discern mechanisms, the NTS transcriptome was measured at endpoint using RNA‐seq.

Results

In SHAM SHR, both the mean BP (MBP) and diastolic BP (DBP) were significantly elevated in week one (ΔMBP= +13.6 mmHg, p = 0.01; ΔDBP= +11.5 mmHg, p = 0.03) when compared to baseline BP. In contrast, in STIM SHR, the MBP and DBP were not significantly elevated until week five of sdVNS (ΔMBP= +25.0 mmHg, p < 0.001; ΔDBP= +20.8 mmHg, p < 0.001) when compared to baseline BP. Systolic BP (SBP) values were similarly elevated compared to baseline at all time points examined. At endpoint, we observed no difference in any of the 11 proinflammatory cytokines measured in either plasma or colonic lysates between SHAM and STIM SHR. In addition, no difference was observed in the composition or abundance of major gut bacterial phyla. However, chronic intermittent sdVNS resulted in 387 up‐regulated and 236 down‐regulated genes in the NTS. Gene set enrichment analysis revealed differential regulation of central inflammatory and neural signaling pathways in the NTS of SHR following sdVNS.

Conclusions

Chronic intermittent sdVNS can delay but not prevent the onset of HTN in the SHR. This delay does not coincide with changes in systemic inflammatory markers or gut bacterial composition; however, changes in the NTS transcriptome indicate afferent rather than efferent effects of chronic intermittent sdVNS in the SHR. Future studies will dissect precise central effects of afferent gut‐brain communication.