Diverse Morphology of Sympathetic Neuron Subpopulations in the Stellate Ganglia

Abstract

Sympathetic nerves innervate the heart and changes in sympathetic efferent activity can modulate cardiac function. The post‐ganglionic sympathetic neurons that innervate the heart are located primarily in the stellate ganglia (SG), which also contains neurons that project to brown adipose tissue (BAT), among other target tissues. We aim to examine morphological changes in SG neurons after myocardial infarction, and characterize the specificity of neural plasticity within the ganglion. To interpret morphological changes induced by cardiac injury, we must first fully understand the normal morphology of distinct groups of SG neurons. To that end, we examined SG neurons following injections of retrograde tracers into cardiac tissue and/or intrascapular brown adipose tissue (BAT) in adult male and female C57BL/6J mice (8 males, 2 females, 28‐32 weeks, 24‐39 g; The Jackson Laboratory). Using cholera toxin B (CTb) tracers conjugated to two different fluorophores (1 mg/mL in 0.01 M PBS; Alexa Fluor 488 or Alexa Fluor 555), we assessed the number, dispersion, pre‐synaptic cholinergic inputs and morphology of BAT‐ and cardiac‐projecting SG neurons. We also examined the quality of the CTb tracer detection at varying times after BAT injection (7, 14 and 28 days; n=2 mice per time point). BAT‐projecting SG neuron visualization was optimal 7 days post‐injection compared to 14 and 28 days, and could be visualized directly or with antibodies against CTb. BAT‐projecting SG neurons were located in both left and right SG, predominately ipsilateral to the injection site and were widely dispersed. In contrast, cardiac‐projecting SG neurons were more localized near the base of the inferior cardiac nerve. In BAT‐ and cardiac‐injected mice (n=4), we found that SG neurons were not dual‐labeled, supporting the notion that sympathetic innervation to target tissues is specific. We found cardiac‐projecting SG neuron somas have more volume (mm3) and cardiac‐projecting cells are less abundant within the SG than BAT‐projecting neurons with our tracer‐labeling paradigm. We also discovered a positive correlation between number of primary dendrites per neuron and soma volume in cardiac‐projecting SG neurons, though not in BAT‐projecting SG neurons. Additionally, in both SG subpopulations, the number of cholinergic inputs marked with vesicular acetylcholine transporter (VAChT) on the cell body is positively correlated to soma volume. Tract tracing distinct subpopulations of neurons will allow us to characterize morphological changes after cardiac injury in specific cell groups. This approach will allow insights into whether mechanisms of plasticity are cell specific or occur globally within the SG.