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 dors al horn (SpDH), the origin and termination of descending tract cells w ere examined by the anterograde and retrograde transport methods. Phas eolus vulgaris leucoagglutinin (PHA-L) and Fluorogold (FG) or dextran- tetramethylrhodamine (Rho) were used as neuronal tracers for the anter ograde and retrograde transport, respectively. The ventrolateral PB, i ncluding Kolliker-Fuse nucleus (KF), sent axons terminating mainly in the ventrolateral parts of rostral trigeminal nuclei of the principali s (Vp), oralis (Vol, and interpolaris (Vi) as well as in the inner lam ina II of the medullary (nucleus caudalis, Vc) and SpDH. Although the descending projections were bilateral with an ipsilateral dominance, T SNC received a more dominant ipsilateral projection than SpDH. The cel ls of origin of the descending tracts were located mainly in KF, but T SNC received fewer projections from the KF than SpDH. Namely, TSNC rec eived a considerable projection from the medial subnucleus of PB and t he ventral parts of lateral subnuclei of PB, such as the central later al subnucleus and lateral crescent area. The other difference noted be tween TSNC and SpDH was that the former received projections mainly fr om the caudal two thirds of KF and the latter from the rostral two thi rds of KF. These results demonstrate the existence of direct parabrach ial projections to TSNC and SpDH that are organized in a distinct mann er and suggest that both pathways are involved in the control of nocic eption. (C) 1997 Wiley-Liss, Inc.