EFFERENT PROJECTIONS FROM THE PARABRACHIAL NUCLEUS DEMONSTRATED WITH THE ANTEROGRADE TRACER PHASEOLUS-VULGARIS LEUKOAGGLUTININ

Efferent projections from the parabrachial complex (PBN) were studied in the rat using the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). Projections to the hypothalamus (ventromedial, dorsomedial, paraventricular, and supraoptic nuclei) originate primarily in the lateral PBN (lPBN). The amygdalar central nucleus (ACE) receives strong projections from all parts of the PBN although the external lPBN projects primarily to the lateral ACE. Whereas the projections to the lateral bed nucleus of the stria terminalis, median preoptic nucleus, diagonal band of Broca, and lateral preoptic area originate primarily from the lPBN, those to the insular cortex arise from the medial PBN (mPBN). The mPBN projects to the ventral posteromedial thalamus and the IPBN and mPBN project to the zona incerta. Descending projections from the mPBN and Kolliker-Fuse area target the commissural nucleus tractus solitarius (NTS); the mPBN projects to the more rostral NTS. Similarly, the caudal parvicellular reticular formation (RF) receives projections from the mPBN and lPBN, whereas input to the rostral RF arises from the former. All compartments of the PBN project to the ventrolateral medulla, although the projections arising from the lPBN are densest. Finally, the raphe nuclei and periaqueductal gray receive some projections from most PBN divisions. These pathways provide a potential means whereby autonomic information can be relayed through the PBN to other structures important in regulating autonomic functions.