Adrenergic activation of cardiac preganglionic neurons in the Nucleus Ambiguus

Abstract

Cholinergic neurons within the nucleus ambiguus (NAm) are the major parasympathetic preganglionic neurons innervating heart. This study characterized the excitability of NAm neurons using in vitro acute brainstem slice preparations obtained from young adult mice. Cholinergic premotor NA neurons were identified based on their expression of tdTomato fluorescence in the Chat‐Cre:floxed‐tdTomato mouse line. Retrograde labeling of pericardiac axon terminals study confirmed cholinergic NAm neurons with large soma as preganglionic cells. In current‐clamp recordings, a brief hyperpolarizing current evoked rebound action potentials (APs). Low voltage‐activated, T‐type calcium current (ICaT) is required for this rebound discharge pattern of NAm neurons. Voltage‐clamp recordings detected a significant T‐current in NA neurons, and quantitative in situ hybridization analysis identified Cav3.1 and 3.2 channels as the major contributors. These preganglionic neurons are strongly activated by adrenergic agonists in vitro. Bath application of adrenergic agonists, epinephrine (10 μM) and norepinephrine (20 μM), induced spontaneous action potentials in these neurons. Inhibition of synaptic ionotropic receptors did not prevent the adrenergic activation, suggesting a minor contribution of synaptic mechanism. Instead, adrenergic stimulation significantly increased the intrinsic excitability of NA neurons; input resistance of NA neurons was significantly increased and single evoked rebound discharges (normally composed of 1–2 APs) converted to regenerative oscillations with burst of >10APs lasting up to a few seconds. Pharmacological tests suggest that persistent sodium current is an important contributor to the activated action potential generation. Together this study identified basic intrinsic membrane excitability mechanism of NAm and its activation by adrenergic agonists.Support or Funding InformationU01 NS090340 (JLN) AHA Career development award (IA)