The organization of the sympathetic nervous system: shining new light on historic views

Abstract

The organization and spatial distribution of neurons within the sympathetic nervous system (SNS) have been long-standing questions in autonomic neuroscience, particularly concerning the functional innervation of target organs. First understood as an ‘alarm reaction’ system turned on en masse, advancements in nerve recording techniques have revealed sympathetic outflow to be differentially controlled to distinct functional subunits within (DiBona 2000; Incognito et al. 2019) and between (Rundqvist et al. 1997) organ systems. Our modern understanding of sympathetic control is now regionand function-specific, governed by an intricate topography within central autonomic nuclei (e.g. rostral ventrolateral medulla; RVLM). Despite intensive efforts to elucidate the regulation of sympathetic outflow, a consensus amongst investigators regarding how topographic arrangements mediate unique activation patterns remains to be reached. Expanding the organizational framework of the SNS is crucial for understanding pathophysiological manifestations. Specifically, cardiovascular disease states present with hallmark sympathetic overactivation, predictive of disease severity (Rundqvist et al. 1997), and with characteristic region-specific alterations in sympathetic outflow to morphologically distinct target organs (e.g. heart, kidney, muscle; Rundqvist et al. 1997). Thus, investigating how the central autonomic circuitry mediates sympathoexcitatory patterns may elucidate cardiovascular (or other) disease mechanisms of neurogenic origin. In a recent article in The Journal of Physiology, Farmer & colleagues (2019) used viral vector tracing and optogenetic manipulation, in vivo, of spinally projecting RVLM neurons to test if these neurons possess the anatomical chassis and functional capacity to elicit sympathoexcitation to regionally and morphologically distinct target organs. The primary objectives were to (1) identify RVLM neurons synapsing at both rostral and caudal thoracic spine regions (i.e. axon collaterals at T2 and T10 spinal segments) and (2) identify if selective stimulation of RVLM neurons with known projections to the caudal thoracic spine (i.e. T12) elicits parallel activation of post-ganglionic nerves in regionally (i.e. rostral thoracic spine) and/or morphologically (i.e. non-vascular) distinct target organs. The authors suggest that evidence of RVLM neurons with functioning axon collaterals would revive older views of generalized/global SNS activation properties, adding an intriguing supplement to the well-established modern view of differential control. Using healthy Sprague–Dawley rats, the first objective was to define the anatomical organization of RVLM projections using retrograde viral tracing. Investigators performed bilateral pressure injections of recombinant herpes simplex retrograde viral vectors into the interomediolateral column (IML) at T2 and T10. Differences in fluorescent tags enabled neurons projecting from the two sites of injection (green fluorescent protein (GFP) at T2 and mCherry at T10) to be distinguished. Of the labelled RVLM neurons, 53% expressed GFP (projecting to T2), 26% expressed mCherry (projecting to T10), and 21% co-expressed GFP and mCherry (projecting to T2 and T10) – the latter being evidence for axon collateralization, and therefore the required substrate for generalized sympathetic activation, in a subgroup of spinally projecting sympathetic RVLM neurons. The second objective of Farmer and colleagues was to investigate if post-ganglionic nerves arising from the upper thoracic spine are activated in response to optogenetic stimulation of select RVLM neurons with known projections to the lower thoracic spine. The optogenetic methods involved an intersectional approach whereby the AAV-ChAR2 vector was injected into the RVLM, and the CAV2-CMV-Cre promoter was injected into the lower thoracic spine to ensure Channelrhodopsin expression in the RVLM would be only in neurons projecting to the IML at the level of T12. After 11–50 days post-injection, simultaneous recordings from post-ganglionic nerve pairings were obtained from (1) forelimb muscle sympathetic nerve activity (SNA) and hindlimb muscle SNA, and (2) left cardiac SNA and hindlimb muscle SNA. Upon optogenetic stimulation of these select RVLM neurons, there was an increase in hindlimb muscle SNA, as expected, but also in forelimb muscle SNA and cardiac SNA. These results demonstrate the functional capacity of RVLM neurons to evoke generalized sympathoexcitation to distant, yet morphologically similar, target organs (i.e. forelimb and hindlimb muscle vasculature), as well as to morphologically distinct, yet potentially functionally similar, target organs (i.e. muscle vasculature and heart). These findings demonstrate the existence of axon collateralization in a subpopulation of spinally projecting RVLM neurons and their capacity to mediate generalized sympathoexcitation. The results of the current study well support the primary conclusions and have sparked important physiological and pathophysiological considerations. A critical point raised by the authors is whether collateralized RVLM neurons regulate functionally similar subsets of pre-ganglionic neurons. From post-ganglionic recordings, there exist single-unit subgroups that are differentially controlled in response to stress (DiBona 2000; Incognito et al. 2019). Differential control within post-ganglionic nerves (likely driven by pre-ganglionic innervation) is required for selective control of functionally distinct target-organ subgroups. This is particularly apparent in the kidney, where there is sympathetic innervation to the renal tubules, vasculature and juxtaglomerular granular cells (DiBona