Organization of the descending projections from the parabrachial nucleus to the trigeminal sensory nuclear complex and spinal dorsal horn in the rat.

To clarify direct descending projections from the parabrachial nucleus (PB) to the trigeminal sensory nuclear complex (TSNC) and spinal dorsal horn (SpDH), the origin and termination of descending tract cells were examined by the anterograde and retrograde transport methods. Phaseolus vulgaris leucoagglutinin (PHA-L) and Fluorogold (FG) or dextran-tetramethylrhodamine (Rho) were used as neuronal tracers for the anterograde and retrograde transport, respectively. The ventrolateral PB, including Kölliker-Fuse nucleus (KF), sent axons terminating mainly in the ventrolateral parts of rostral trigeminal nuclei of the principalis (Vp), oralis (Vo), and interpolaris (Vi) as well as in the inner lamina II of the medullary (nucleus caudalis, Vc) and SpDH. Although the descending projections were bilateral with an ipsilateral dominance, TSNC received a more dominant ipsilateral projection than SpDH. The cells of origin of the descending tracts were located mainly in KF, but TSNC received fewer projections from the KF than SpDH. Namely, TSNC received a considerable projection from the medial subnucleus of PB and the ventral parts of lateral subnuclei of PB, such as the central lateral subnucleus and lateral crescent area. The other difference noted between TSNC and SpDH was that the former received projections mainly from the caudal two thirds of KF and the latter from the rostral two thirds of KF. These results demonstrate the existence of direct parabrachial projections to TSNC and SpDH that are organized in a distinct manner and suggest that both pathways are involved in the control of nociception.