# RVLM C1 Neurons Innervate Sacral as well as Thoracolumbar Autonomic Preganglionic Neurons in the Rat

## TL;DR

This study maps the spinal cord connections of C1 neurons in rats, showing they target both sympathetic and parasympathetic neurons, including those involved in the micturition reflex.

## Contribution

The study reveals that C1 neurons innervate sacral parasympathetic neurons, including those involved in micturition, in addition to thoracolumbar sympathetic neurons.

## Key findings

- C1 neurons and PNMT axons innervate most thoracolumbar sympathetic preganglionic neurons.
- C1 axons also innervate sacral parasympathetic preganglionic neurons involved in the micturition reflex.
- A few C1 axons make contact with somatic motor neurons in the sacral cord and ventral horn.

## Abstract

We examined the distribution of axons throughout the spinal cord of the rat that were either immunoreactive for the adrenaline‐synthesizing enzyme, phenylethanolamine N‐methyltransferase (PNMT), or derived from medullary C1 neurons, one of the three groups of neurons in the brain that synthesize PNMT. We observed that PMNT‐immunoreactive axons, as well as C1 axons labelled with GFP from viral transduction, innervate most, but not all, sympathetic preganglionic neurons in the thoracolumbar spinal cord. GFP‐positive C1 axons provided innervation to sympathetic preganglionic neurons that expressed cocaine and amphetamine regulated transcript, an accepted marker of sympathetic vasomotor neurons. In addition, we observed axons from PNMT‐containing and C1 neurons caudal to the distribution of sympathetic preganglionic neurons in the sacral spinal cord where they closely apposed parasympathetic preganglionic neurons retrogradely labelled from the major pelvic ganglion. We also found close appositions from PNMT‐immunoreactive or GFP‐labelled C1 axons on choline acetyltransferase‐stained parasympathetic preganglionic neurons activated by the micturition reflex, thus providing clear evidence of a non‐cardiovascular target for RVLM C1 neurons. Furthermore, we observed a few PNMT‐positive and GFP‐positive C1 axons making close appositions with somatic motor neurons in Onuf's nucleus in the sacral cord and in the ventral horn at more rostral levels. These data provide a comprehensive map of the distribution of adrenergic inputs to the spinal cord and identify parasympathetic preganglionic neurons, including those involved in the micturition reflex, as well as sympathetic preganglionic neurons as the major targets for these inputs.

Axons expressing the adrenaline‐synthesizing enzyme, phenylethanolamine N‐methyltransferase (PNMT), project widely throughout the spinal cord. Many of these axons are from C1 neurons of the rostral ventrolateral medulla. We confirm that dense arrays of PNMT‐expressing, C1‐derived axons surround thoracolumbar sympathetic preganglionic neurons, many of which are putative vasomotor neurons. The C1/PNMT axons are also more widespread, innervating sacral preganglionic neurons activated by the micturition reflex and, very sparsely, somatic motor neurons in the ventral horn.