Divergent electrophysiology between two cardiac vagal motor populations despite similar chronotropic effects
Authors: Strain Misty , Espinoza Liliana , Fedorchak Stephanie , Boychuk Carie 
Published: May 2023

Cardiac vagal motor output is generated by brainstem cardiac vagal motorneurons (CVNs) that send axonal projections, through the vagus nerve, to cardiac ganglia. CVNs have a well-established cardioinhibitory influence through reductions in heart rate (HR) and studies using vagotomy confirm cardiac vagal drive originates in CVNs. CVNs can be found in two brainstem regions: nucleus ambiguus (NA) and dorsal motor nucleus of the vagus (DMV). Although the cardioinhibitory role of CVNs in NA (CVNNA) is widely accepted, effects of CVNs in DMV (CVNDMV) on HR regulation is controversial. The present study aims to provide evidence of CVNDMV existence and identify the unique role DMV plays in HR regulation. We hypothesized that CVNDMV cause robust bradycardias and associate with unique electrophysiological properties making them hyperexcitable compared to CVNNA. Experiments were performed on adult (9-11 week old) male mice. Using a transgenic mouse line where light activated channelrhodopsin (ChR) is expressed in choline acetyltransferase neurons, activation of ChR in DMV using fiber optic probes resulted in a 359±43 bpm bradycardia (n=8) in awake behaving animals. DMV-mediated ChR-dependent bradycardia remained after i.p. administration of atenolol (256±31 bpm). However, i.p. scopolamine-methyl nitrate abolished the ability of ChR activation of DMV to impact HR (12±2 bpm, p=0.0002). ChR stimulation of NA resulted in a similar bradycardia (447±6 bpm; p<0.0001) that was also blocked by scopolamine-methyl nitrate (9±7 bpm, p=0.005). Retrograde tracing from cardiac tissue using rhodamine and cholera-toxin B (CTB) confirmed CVNs in DMV (50±15 labeled neurons, n=6; CTB: 12±4 labeled neurons, n=8). However, the number of CVNDMV was significantly lower than CVNNA (rhodamine: 116±24 labeled neurons, n=6; CTB: 83.33±9.548 labeled neurons, n=6, p<0.0001 for both). Unilateral vagotomy eliminated tracer presence in DMV ipsilateral (2±1 neurons) compared to contralateral (32±8 neurons; n=4; p=0.048). Using whole cell patch clamp, CVNDMV demonstrated spontaneous action potential firing in vitro (0.7±0.3 Hz, n=14 neurons/10 mice) whereas CVNNA were devoid of spontaneous firing (0.0±0.0 Hz, n=10 neurons/9 mice). CVNDMV (-58.6±2.7 Hz, n=14 neurons/10 mice) and CVNNA (-62.2±1.8 mV, n=10 neurons/9 mice, p=0.3) demonstrated similar resting membrane potential. However, CVNDMV (492.0±22.0 MΩ, n=14 neurons/10 mice) had higher input resistance compared to CVNNA (158.2±10.6 MΩ, n=10 neurons/9 mice, p<0.0001). Taken together, these data support the hypothesis that DMV contains a population of CVNs capable of producing cardioinhibitory actions with unique hyperexcitability compared to CVNNA. Future studies will examine both intracellular mechanism(s) of these differences, but also unique microcircuit pathways which utilize CVNDMV, and not CVNNA. NIH T32 HL007446 for MS; NIH R01HL157366 to CRB This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.