Hypothalamic projections to cardiovascular centers of the medulla

The purpose of this project was to identify hypothalamic neurons having projections to two cardiovascular centers of the medulla, the rostral ventrolateral medulla (RVLM; a vasopressor region) and the nucleus of the solitary tract (NTS; a vasodepressor region). To accomplish this, fluorescent tracers (fast blue and diamidino yellow) were injected into NTS and RVLM, after each site had been physiologically identified in rats. In each case, one of the tracers was injected into the RVLM and another was injected into the NTS. Labelled neurons were subsequently observed along the entire rostral-caudal extent of the hypothalamus, where they were found in nuclei having known cardiovascular functions. Although the two groups of hypothalamomedullary neurons were largely overlapped in their distributions, less than 0.1% of the neurons were double labelled. In addition to this overlapping distribution of neurons, there were some areas within the hypothalamus where the two groups of hypothalamomedullary neurons were somewhat segregated. This clustering pattern was observed in the posterolateral hypothalamus (PLH) and, to a much lesser degree, in the paraventricular nucleus (PVN). Within the PLH, lying medial to the subthalamic nucleus, virtually all the labelled neurons projected exclusively to the NTS. Within the PVN, neurons projecting to the NTS were more numerous ventrally, whereas neurons projecting to the RVLM were more evenly dispersed within the PVN. In addition to hypothalamic labeling, clusters of labelled neurons were also observed in the zona incerta and the interstitial nucleus of the stria terminalis. Within the zona incerta, almost all the labelled neurons projected to the RVLM. Within the interstitial nucleus of the stria terminalis, neurons projecting to NTS were much more abundant in the dorsal portion of this nucleus; whereas, neurons projecting to the RVLM were more abundant ventrally. The findings of this study provide additional support to the notion that hypothalamic influences upon cardiovascular functions are in part mediated through hypothalamomedullary projections.