COLLATERAL AXONAL PROJECTIONS TO LIMBIC STRUCTURES FROM VENTROLATERALMEDULLARY A1 NORADRENERGIC NEURONS

Experiments were done to investigate whether catecholaminergic neurons within the ventrolateral medulla (VLM) send collateral axonal project ions to the central nucleus of the amygdala (ACe) and the bed nucleus of the stria terminalis (BST). Unilateral microinjections of the fluor escent retrograde tracers fluorogold (FG) or rhodamine labelled latex micro-beads (Rd) were made into either ACe or BST in the rat. Brainste m sections were then processed immunohistochemically for the identific ation of cell bodies containing the catecholamine biosynthetic enzymes tyrosine hydroxylase, dopamine beta-hydroxylase (DBH) or phenylethano lamine-N-methyltransferase (PNMT). Retrogradely labelled cell bodies p rojecting to either ACe or BST were found throughout the rostrocaudal extent of VLM, bilaterally. Approximately 44% of these retrogradely la belled neurons were found to contain both retrograde tracers. In addit ion, 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 Al cell group in VLM innervate ACe and B ST via collateral axonal projections and suggest that these VLM neuron s may be directly involved in relaying cardiovascular afferent and/or visceral afferent information directly to these limbic structures.