High Fat Diet‐Induced Neuroplasticity in Cardiac‐Projecting Dorsal Motor Nucleus of the Vagus (DMV) Motoreurons

Abstract

The DMV is critical in the maintenance of gastrointestinal (GI) function and metabolism through its axonal projections to subdiaphragmatic organs. However, there is a subpopulation of choline acetyltransferase (ChAT) positive DMV neurons that send projections to cardiac tissue. Given their anatomical apposition to nuclei regulating GI and metabolic function, it is likely that cardiac vagal motorneurons (CVNs) in the DMV are sensitive to nutrient signals such as high fat diet (HFD). We hypothesized that HFD exposure for 15 days modulates the electrophysiological properties of DMV CVNs, thus restraining their ability to discharge.Mice underwent retrograde tracing to identify DMV CVNs, were challenged with HFD (60%) or control chow, and were used for whole‐cell patch‐clamp electrophysiology. When comparing DMV CVNs from control and HFD‐fed animals under current clamp conditions, no significant differences in firing frequency, resting membrane potential, or input resistance were found under normal conditions. However, DMV CVNs from HFD‐fed mice exhibited a significant increase in evoked action potentials (APs) following blockade of inhibitory, gamma‐aminobutyric acid type‐A receptor (GABAAR) neurotransmission with bicuculline methiodide (BIC; 30 µM; n=10; 5.02±1.23 APs) compared to the previous artificial cerebrospinal fluid conditions (aCSF; 4.2±0.92 APs; p<0.0001). CVNs from controls demonstrated no significant differences between BIC and aCSF conditions (n=10; p=0.1194). Taken together, HFD increased CVN sensitivity to antagonism of GABAARs. Additional studies using voltage clamp configuration assessed differences in the two signaling modalities of GABA: phasic/synaptic and tonic/extrasynaptic. Spontaneous phasic inhibitory post‐synaptic currents were not different in frequency, amplitude, or decay time. However, DMV CVNs from HFD animals had a significant increase in tonic GABAAR current after the application of BIC (n=9; 1.35±0.19 pA/pF) compared to controls (n=7; 0.8±0.11 pA/pF; p=0.0361). Given that δ subunit‐containing GABAARs (GABAA(δ)Rs) are theorized to mediate tonic GABAergic current, follow‐up experiments assessed the application of the specific GABAA(δ)R agonist, THIP (3 µM) followed by BIC. Although BIC‐sensitive increases in tonic current remained in CVNs from HFD mice, no significant currents were induced in either group after THIP application, suggesting that all GABAA(δ)Rs are actively contributing to tonic currents under these conditions. Since no antagonists exist for GABAA(δ)Rs, we developed a transgenic mouse line where the δ subunit is knocked out of ChAT motorneurons (i.e. ChAT/GABAA(δ)R‐null mice) to determine whether the electrophysiological changes induced by HFD could be prevented. Preliminary findings indicate that ablation of GABAA(δ)Rs in ChAT positive DMV CVNs was enough to prevent HFD‐induced increases in evoked APs following BIC application (n=6; 4.58±1.05 APs) compared to aCSF (4.5±0.98 APs; p=0.9559). These data suggest that DMV CVNs express more GABAA(δ)Rs responsible for the generation of long‐term inhibition after HFD exposure, thus restraining their discharge and potentially reducing their capacity for cardiovascular regulation. These early changes in CVN electrophysiology could impact not only cardiovascular function, but lead to other comorbidities, including obesity, since exercise capacity is linked to vagal regulation from the DMV. Further studies are warranted to determine the mechanistic underpinnings of HFD‐induced CVN neuroplasticity.