Role of alpha1 adrenergic signaling in nucleus ambiguus regulation of heart rate

Traditionally, adrenergic receptors (ARs) are G-protein-coupled receptors (GPCR) that initiate a diverse range of intracellular signaling cascades based on their receptor subtypes (α1, α2, and β). Notably, both α1- and β- subtypes are known to increase cellular excitability. In particular, α1-ARs were implicated in increased excitability in the brainstem’s nucleus ambiguus (NA). However, this is not in line with previous electrophysiological work demonstrating a robust increase in inhibitory network drive to cardiac vagal motor neurons (CVNs) through α1-AR activation, or the known impacts of several adrenergic regions to increase heart rate (HR; a possible functional readout of decreased CVN activity). Therefore, the present studies were undertaken to elucidate the role of α1-ARs in CVN mediated cardiac regulation. We hypothesized that α1A-ARs in NA suppress vagal activity. Initial studies used both RNA sequencing and immunohistochemistry to qualitatively confirm that all three isoforms (A, B and D) of the α1-AR are present within NA, and that CVNs contain the A-isoform (α1A-AR) at high levels. Next, under urethane, WT mice were bilaterally nanoinjected with the α1-AR agonist, phenylephrine (PE; 60 nL; 10 nL/s), into NA while continuously recording HR. PE produced a significant tachycardic response (Before: 564.2 ± 32.1 BPM; After: 625.7 ± 20.9; n=5, paired t-test p=0.03). To confirm this response was mediated by the α1A-AR isoform, PE nanoinjections were repeated in a transgenic mouse model where α1A-AR is knocked down (α1A-ARNull; B6.129X1-Adra1atm1Pcs/J strain# 005039). Unlike controls, PE induced a significant bradycardic response in all mice (Before: 593.1 ± 7.1 BPM; After: 555.5 ± 8.7; n=4 paired t-test p=0.01). Since previous work suggests that α1A-AR mediated inhibitory responses act though increases in GABAAR functional activation, PE nanoinjections were repeated in our unique mouse model lacking GABAAR-δ-subunits in ChAT-positive motor neurons. ChAT- δNull mice exhibited a smaller, non-significant bradycardic response (Before: 604.2 ± 31.1 BPM; After: 591.4 ± 39.7 BPM; n=4). Comparisons of the overall change in HR across all genotypes confirmed a significant difference between groups (WT: +61.3 ± 39.3 BPM; α1A-ARNull: –37.6 ± 13.0 BPM; ChAT- δNull: –12.2 ± 26.1 BPM; One-way ANOVA, p=0.001), with post-hoc analysis revealing significant differences between WT and α1A-ARNull (p=0.001) and WT and ChAT- δNull (p=0.01), but not α1A-ARNull and ChAT- δNull. These findings indicate that α1A-AR signaling in NA plays a key role in suppressing vagal control of HR to elicit tachycardia. Attenuated PE response in ChAT- δNull mice further implicates inhibitory GABAergic mechanisms in this regulation. Since PE in α1A-ARNull mice produced a bradycardia as expected if CVNs were excited, these results highlight a novel role for the α1A-AR isoform in central cardiac control through modulation of inhibitory neurotransmission to restrain vagal regulation of HR. Future studies will explore how endogenous release of catecholamines uses α1A-AR through optogenetic stimulation of dopamine β-hydroxylase positive terminals in NA. Additionally, whole-cell patch-clamp recordings from CVNs will determine whether α1A-AR effects originate from receptors on CVNs themselves or are mediated indirectly through local interneurons. These studies provide new insight into central regulation of CVN activity and inform development of novel therapeutic strategies to restore autonomic balance, which is disrupted in cardiovascular diseases such as heart failure, where decreased CVN activity contributes to disease progression. 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.