Organization of efferent projections from the parabrachial area to the hypothalamus: a Phaseolus vulgaris-leucoagglutinin study in the rat.

The organization of projections from the parabrachial (PB) area to the hypothalamus was studied in the rat by using microinjections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into subregions of the PB area. The present study is a follow-up of two former studies (Bernard et al. [1993] J. Comp. Neurol. 329:201-229; Aldén et al. [1994] J. Comp. Neurol. 341:289-314) that examined PB projections onto the amygdala and the bed nucleus of the stria terminalis. The results demonstrate that 1) the mesencephalic PB region, centered in the lateral portion of the superior lateral subnucleus projects extremely densely to almost the entire dorsomedial subdivision of the ipsilateral ventromedial hypothalamic nucleus; 2) the mesencephalic PB region, located in the medial portion of the superior lateral subnucleus and weakly overflowing into the rostralmost dorsal lateral pontine subnucleus, projects densely to the retrochiasmatic area and, to a lesser extent, to the ipsilateral ventromedial nucleus of the hypothalamus; 3) the PB region, including the central lateral, a portion of the superior lateral, and the outer external lateral subnuclei, projects densely to the ipsilateral median, anteroventral, and periventricular preoptic hypothalamic nuclei and projects more weakly to the dorsal border of the paraventricular nucleus (PVN). No consistent projection was found in the magnocellular PVN. All of these PB regions also project diffusely to the dorsomedial area and to a small tuberal subfornical hypothalamic area. In addition, the medial half of the PB area projects consistently to the posterior lateral hypothalamus. It is suggested that these pathways may be involved in aversive-defensive behavior, in autonomic and neuroendocrine aspects of pain, and in feeding and energy metabolism regulation.