Organization of Central Adrenergic Pathways: I. Relationships of Ventrolateral Medullary Projections to the Hypothalamus and Spinal Cord

By Diane Tucker, Clifford Saper, David Ruggiero and Donald Reis

This experiment had multiple purposes including:

- Determining whether PNMT-positive C1 cells contain other marker of catecholamine biosynthesis

- To determine the extent to which C1 neurons in the VLM send collaterals to both the spinal cord and the hypothalamus

- To examine the collateralization of VLM neurons of the A1 cell group that project to PVH and median preoptic nucleus (MnPO)

This summary will only focus on objective #2, since the others are not the relevant to what I am doing.

Dye was injected into rat spinal cord and hypothalamus. Rats were perfused, sacrificed, and their brain tissue was obtained and immunohistochemically processed. Cell counts were performed of:

- Total TH and PNMT positive cells

- The number of cells retrogradely labeled with each fluorescent dye

- The number of cells retrogradely labeled with both fluorescent dyes

- The number of cells labeled with each fluorescent dye that showed immunoreactivity for TH or PNMT

- The number of cells labeled with both fluorescent dyes that showed immunoreactivity for both TH and PNMT

Results showed that about half of the spinal projection cells in the RVLM were PNMT-positive. Catecholaminergic neurons throughout the length of the VLM project to the hypothalamus. The PVH also receives catecholaminergic afferents from the entire VLM. While there is sometimes considerable spatial overlap between cells with different projections (spinal cord, hypothalamus, PVH), very few VLM neurons are found to innervate more than one region.

There was no evidence for noradrenergic projection from the MVLM or CVLM to the spinal cord.

It should be noted that about half of the neurons in the RVLM that project to the spinal cord did not stain for PNMT. These other cell populations may play a role in blood pressure modulation as well.

-BH