High fat diet impacts GABAergic neurotransmission in cardiac vagal motor neurons in nucleus ambiguus

A 15-day high-fat-diet (HFD) blunts cardiac parasympathetic output, contributing to early cardiac dysregulation. Cardiac vagal motor neurons in nucleus ambiguus (CVNNA) provide parasympathetic control of heart rate (HR), and their activity is regulated by inhibitory, GABAergic neurotransmission. However, how HFD impacts CVNNA GABAergic neurotransmission remains unclear. We hypothesize that GABAA receptors-containing δ subunits (GABAAδRs) in CVNNA contribute to blunted parasympathetic-mediated HR changes after HFD. To test this, we used ex vivo electrophysiology combined with retrograde cardiac tracing to label CVNNA in mice and examined both modalities of inhibition (phasic and tonic). Since CVNNA receives glycinergic and GABAergic inhibitory inputs, we first characterized the contribution of each to phasic and tonic currents in slice preparation. Application of strychnine (1µM), a glycine receptor antagonist, and GABAAR antagonists (picrotoxin, 100 µM and/or gabazine, 100 µM), revealed that glycine receptors contribute to ~91% of phasic current (charge transferGly vs GABA=49.6±19.9 vs 1.4±0.7pC, p=0.02, n=13). In contrast, GABAAR contributed to the majority of (~68%) of tonic inhibition (charge transferGly vs GABA=2283±472.3 vs 5336±856.5pC, p=0.005, n=18). Comparison of total charge transfer confirms that GABAAR neurotransmission (regardless of modality) confers greater inhibition than glycine neurotransmission (p< 0.0001). Furthermore, the majority of tonic GABAAR inhibition was generated by GABAAδRs (~59%; n=5) and application of GABAAδR-selective agonist THIP (3 µM) increased tonic current in ~40% of CVNNA (p=0.0005, n=10). Together, these data confirm the importance of GABAergic tonic currents in slice preparation. In HFD mice, no changes were observed in phasic currents. However, THIP induced greater tonic current density in HFD compared to NFD (HFD vs NFD=0.07±0.04 vs -0.02±0.02 pA/pF, p=0.048), with positive THIP-responses observed in 6/8 cells (75%) in HFD and 2/7 cells (26%) in NFD. These findings suggest that 15 days of HFD enhanced GABAAδR activity in CVNNA, potentially contributing to early cardiac parasympathetic dysfunction. Future studies will examine how manipulation of GABAAδR-dependent current impacts blunted cardiac parasympathetic output and early cardiac dysregulation. Funding: R01HL157366 NHLBI to CRB This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.