Direct projections to subfornical organ from catecholaminergic neurons in the caudal nucleus of the solitary tract.

Experiments were done to investigate the distribution of neurons within the nucleus of the solitary tract (NTS) that projected directly to the subfornical organ (SFO) and to determine whether these neurons were components of the catecholaminergic cell groups in the dorsal medial medulla. Microinjection of the fluorescent tract-tracers fluorogold or rhodamine latex micro-beads were made into the SFO of the rat. Brainstem sections were then processed immunocytochemically for the identification of neuronal cell bodies containing the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH) or phenylethanolamine-N-methyltransferase (PNMT). Retrogradely labelled cell bodies that projected to the SFO were found in the NTS, bilaterally, extending from approximately the level of the rostral aspect of the area postrema (obex) caudally to the calamus scriptorius. These neurons were located predominantly in the medial, lateral and dorsolateral subdivisions of NTS. Approximately one-third of these retrogradely labelled neurons were immunoreactive to TH and DBH. On the other hand, less than 7% of the retrogradely labelled neurons were found to contain TH and PNMT immunoreactivity. These data have demonstrated that noradrenergic neurons of the A2 cell group in the caudal NTS innervate the SFO and suggest that these neurons may be involved in relaying cardiovascular afferent information directly to a forebrain circumventricular organ involved in arterial pressure and body fluid homeostasis.