Collateral axonal projections to limbic structures from ventrolateral medullary A1 noradrenergic neurons

Experiments were done to investigate whether catecholaminergic neurons within the ventrolateral medulla (VLM) send collateral axonal projections to the central nucleus of the amygdala (ACe) and the bed nucleus of the stria terminalis (BST). Unilateral microinjections of the fluorescent retrograde tracers fluorogold (FG) or rhodamine labelled latex micro-beads (Rd) were made into either ACe or BST in the rat. Brainstem sections were then processed immunohistochemically for the identification of cell bodies containing the catecholamine biosynthetic enzymes tyrosine hydroxylase, dopamine beta-hydroxylase (DBH) or phenylethanolamine-N-methyltransferase (PNMT). Retrogradely labelled cell bodies projecting to either ACe or BST were found throughout the rostrocaudal extent of VLM, bilaterally. Approximately 44% of these retrogradely labelled neurons were found to contain both retrograde tracers. In addition, approximately 91% of the VLM neurons that send collateral axonal projections to ACe and BST were also immunoreactive to DBH. None were found to contain PNMT immunoreactivity. These results demonstrate that noradrenergic neurons of the A1 cell group in VLM innervate ACe and BST via collateral axonal projections and suggest that these VLM neurons may be directly involved in relaying cardiovascular afferent and/or visceral afferent information directly to these limbic structures.