ORGANIZATION OF PROJECTIONS FROM THE DORSOMEDIAL NUCLEUS OF THE HYPOTHALAMUS - A PHA-L STUDY IN THE RAT

The axonal projections of the dorsomedial nucleus of the hypothalamus were investigated by using Phaseolous vulgaris-leucoagglutinin. The ma in conclusion of this work is that these projections are largely intra hypothalamic, with smaller components directed toward the brainstem an d telencephalon. Although the intrahypothalamic pathways are very comp lex and intermix at various levels, we conclude that dorsomedial nucle us outputs follow three distinct ascending pathways: periventricular, coursing through the hypothalamic periventricular zone; ventral, trave ling beneath the medial zone; and lateral, ascending in medial parts o f the lateral hypothalamic area. Within the hypothalamus, the most den sely innervated areas are the paraventricular nucleus, other dorsal re gions of the periventricular zone, the preoptic suprachiasmatic nucleu s, and the parastrial nucleus. Other significant terminal fields inclu de the median preoptic, anteroventral periventricular, lateral part of the medial preoptic, and anteroventral preoptic nuclei; and the retro chiasmatic (including perisuprachiasmatic) area. Descending projection s follow two pathways that also converge at various levels: a dorsal p athway in the midbrain periventricular system travels through, and pri marily innervates, the periaqueductal and pontine gray, and a ventral pathway extends through ventromedial regions of the brainstem. Althoug h sparse, fibers in the later pathway can be traced as far caudally as the nucleus of the solitary tract. The results are discussed relative to the pathways and properties of nearby hypothalamic medial zone nuc lei. Dorsomedial nucleus projections are similar to certain other nucl ei (e.g., anteroventral periventricular and parastrial) with predomina ntly intrahypothalamic projections, and different from those arising i n the medial zone nuclei (medial preoptic, anterior hypothalamic, vent romedial, and mammillary). (C) 1996 Wiley-Liss, Inc.