ORGANIZATION OF EFFERENT PROJECTIONS FROM THE PARABRACHIAL AREA TO THE HYPOTHALAMUS - A PHASEOLUS-VULGARIS-LEUKOAGGLUTININ 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 Phase olus 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; Alden et al. [1994] J. Comp. Neu rol. 341:289-314) that examined PB projections onto the amygdala and t he 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 e ntire dorsomedial subdivision of the ipsilateral ventromedial hypothal amic nucleus; 2) the mesencephalic PB region, located in the medial po rtion of the superior lateral subnucleus and weakly overflowing into t he rostralmost dorsal lateral pontine subnucleus, projects densely to the retrochiasmatic area and, to a lesser extent, to the ipsilateral v entromedial nucleus of the hypothalamus; 3) the PB region, including t he central lateral, a portion of the superior lateral, and the outer e xternal lateral subnuclei, projects densely to the ipsilateral median, anteroventral, and periventricular preoptic hypothalamic nuclei and p rojects more weakly to the dorsal border of the paraventricular nucleu s (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, th e medial half of the PB area projects consistently to the posterior la teral hypothalamus. It is suggested that these pathways may be involve d in aversive-defensive behavior, in autonomic and neuroendocrine aspe cts of pain, and in feeding and energy metabolism regulation. (C) 1997 Wiley-Liss, Inc.