Supraspinal connections and termination patterns of the parabrachial complex determined by the biocytin anterograde tract-tracing technique in the rat

We have re-evaluated, using the anterograde tracer biocytin, supraspinal efferent projections from the parabrachial complex (PBN) to gain new information about the nature of its connections and nerve terminal patterns. We selectively injected biocytin into the 3 main regions of the nucleus (lateral PEN, medial PEN and Kolliker-Fuse nucleus). We observed distinct groups of ascending and descending fibres of different calibre from the PEN running throughout the brain and reaching many brain areas involved in the regulation of autonomic function. Here we detected labelled bouton-like terminals and fibres with en-passage varicosities. The ascending efferents from the lateral PEN mainly reached the reticular, raphe and thalamic nuclei, the zona incerta (ZI), central nucleus of the amygdala (CeA) and lateral area of the periaqueductal grey (PAG). Thin descending efferents reached the ventral region of the solitary tract nucleus (STN). The ascending efferents from the medial PEN were seen in the raphe nuclei, reticular nuclei, ventral and lateral areas of the FAG, thalamic nuclei, and in the medial and lateral nuclei of the amygdala. Descending efferents were seen in the STN and in some reticular nuclei. The ascending projections from the KollikerFuse targeted the ventral area of FAG, CeA, ZI, lateral hypothalamic area, ventromedial thalamic nucleus and, with only a few terminals, the ipsi and contralateral reticular area. A large number of descending efferents reached STN, caudal and paragigantocellular reticular nuclei. The higher sensitivity of biocytin compared with other types of markers allowed us to determine more effectively the distribution, nature and extent of the supraspinal PEN connections. This suggested that in several nerve circuits the PEN probably plays a more important role than previously thought.