Topology of ascending brainstem projections to nucleus parabrachialis in the cat

Abstract

The afferent projections to nucleus parabrachialis (NPB) and nearby pontine areas from the lower brainstem were studied in cats using retrograde horseradish peroxidase (HRP) and anterograde autoradiographic tracing techniques. Two groups of medullary neurons send major projections to NPB and the Kölliker‐Fuse nucleus (KF): (1) the solitary complex, especially the medial nucleus of the solitary tract (SM), nearby smaller cells of the dorsal motor nucleus of the vagus (DMV) and the commissural nucleus; and (2) the lateral tegmental field (FTL), or parvocellular reticular formation. Autoradiographic tracing from these areas demonstrated terminal fields in NPB/KF and emphasized a ventrolateral route to NPB from both sources, with axons ascending between the facial nerve and superior olive and passing rostral to the trigeminal nuclei. Minor projections to NPB/KF originate in the ventrolateral nucleus of the solitary tract, area subpostrema, the alaminar spinal trigeminal nucleus, the gigantocellular and magnocellular tegmental fields, and an area dorsal to the ipsilateral inferior olive. Topographical features of the major projections were studied by correlating the locus of overlap of injection sites with the locations of HRP‐positive cells. Medial areas of SM/DMV project mostly to medial parts of NPB, while lateral areas near the solitary tract project to lateral parts of NPB and KF. Crossing projections from SM/DMV favor dorsolateral NPB and KF. FTL neurons in dorsomedial areas project more to medial NPB, and ventrolateral areas project to lateral NPB/KF. Using a new coordinate system to locate and normalize the positions of FTL neurons, data from many brains were collated. FTL cells projecting to NPB/KF were found to be on discrete longitudinal sheets, running radially with respect to the fourth ventricle. This substructure and related evidence suggest a preferred pattern for neuroanatomical connections and information processing in the lateral reticular areas of the brainstem, and help in understanding the topography of the projections to NPB/KF.