LAUSR

Introduction: Gut dysbiosis is linked to hypertension (HTN), but the precise mechanisms of host-microbiota interactions are not known. Gut and brain communicate via the vagal axis, of which serotonin (5-HT) is an important mediator. 5-HT acts via vagal 5-HT3a receptor (5-HT3aR), stimulation of which causes a reflex bradycardia similar to activation of the baroreflex in the nucleus of the solitary tract (NTS). As the microbiota can exert powerful effects on gut 5-HT production, we investigated 5-HT3aR-dependent signaling in the spontaneously hypertensive rats (SHR). Methods: WKY and SHR were randomly injected with AAV-hSyn-gCAMP in the left or the right nodose ganglia. All rats also received an injection of CTB 594 (1ul of 1% solution) to the proximal colon and/or small intestine, to retrogradely label NG. In vivo Ca 2+ imaging was performed in all anesthetized rats using a two-photon microscope. High definition Z-stack images of the whole NG followed by a video of real time neuronal activity were made (~30 3um stacks every 2 seconds for 5 minutes). During this time, a bolus i.v. injection of 5-HT3aR agonist 1-Phenylbigunide (35ug) was delivered while simultaneously recording the real time activity of NG neurons and blood pressure/heart rate by telemetry. Data were analyzed using Fiji and Python script. qPCR for 5-HT3aRs was performed in RNA isolated from the proximal colon, nodose ganglia (NG) and NTS of adult male SHR and WKY rats. Results: Injection of 5-HT3aR agonist produced activation of the same number of gCAMP-labeled neurons in the SHR and WKY; however, a number of double-labeled gut-projecting neurons that were activated by the 5-HT3aR agonist were slightly lower in the SHR (11%) vs the WKY (13.5%). SHR NG neurons exhibited slower rate of activation compared to WKY NG neurons following the injection, which was associated with dampened effects of 5-HT3aR agonist on BP and HR in the SHR. Expression of 5-HT3aRs was completely diminished in the gut and NG (P<0.001) and significantly downregulated in the NTS (by ~40%, P<0.01) of SHR compared to WKY. Conclusions: Reduced 5-HT3aR NG signaling may be a consequence of gut dysbiosis in the SHR. Future studies will elucidate the functional significance of this pathway in HTN.