ORGANIZATION OF DIENCEPHALIC PROJECTIONS FROM THE MEDULLARY SUBNUCLEUS RETICULARIS DORSALIS AND THE ADJACENT CUNEATE NUCLEUS - A RETROGRADEAND ANTEROGRADE TRACER STUDY IN THE RAT

The distribution and organization of diencephalic projections from the subnucleus reticularis dorsalis (SRD) and the neighbouring cuneate nu cleus (Cu) were studied in the rat by using microinjections of Phaseol us vulgaris leucoagglutinin in SRD and Cu and wheat germ agglutinin-ap o horseradish peroxidase-gold in some selected thalamic areas. As prev iously reported, the efferent projections from the Cu were essentially contralateral and terminated mainly in the ventroposterolateral thala mic nucleus. Less dense terminals from the Cu were also observed in th e posterior thalamic group, the ventral aspect of the zona incerta and the caudal and dorsal portion of the reuniens area. Retrograde tracer injections in the medial ventroposterolateral thalamic nucleus labele d numerous cells in the contralateral Cu, with a smaller number in the gracile nucleus. From the SRD, terminals were observed in the lateral aspect of the ventromedial thalamic nucleus, the lateral parafascicul ar area and, to a lesser extent, in the ventral aspect of the zona inc erta and the core of the reuniens area. Retrograde tracer injections i n the lateral part of the ventromedial thalamic nucleus labeled cells in the caudal medulla? many of which were located in the dorsal-most a spect of the SRD throughout its caudo-rostral extent. The existence of SRD-thalamic connections reinforces the idea that the caudal reticula r formation is an important nociceptive relay to the thalamus. Our dat a shed new light on old hypotheses suggesting that, in addition to spi no-thalamic pathways, spino-reticulo-thalamic pathways may play an imp ortant role in distributing pain signals to the forebrain. (C) 1998 Wi ley-Liss, Inc.