Hypothalamic Projections to the Raphe Nuclei of the Medulla oblongata.

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The present study was undertaken using the anterograde and retograde labeling techniques to clarify the hypothalamic descending projections tothe medullary raphe nuclei in the rat.1. Anterograde labeling technique. A small amount of [ <^3H> ] leucine was injected unilaterally into various parts of the hypothalamus. Abundant and characteristic terminal labelings were observed bialterally in the nucleus raphe pallidus, the vental surface of the pyramidal tract and the nucleus interfascicularis hypoglossi, after injections into the dorsal posterior hypothalamic area caudal to the paraventricular hypothalamic nucleus. Conspicuous, but less numerous labelings were observed in the nucleus raphe obscurus and the ipsilateral raphe magnus.2. Retrograde labeling techniques. Following an injection of WGA-HRP in the nucleus raphe pallidus many labelled smalll neurons (6-13X9-22 <micro> m) with an oval shape were found in the ventromedial part of the dorsal hypothalamic area. At the level through the dorsomedial hypothalamic nucleus, they formed a distinct and compact cell cluster. Labelled neurons, which were large in size (9-22X11-36 <micro> m) with oval and triangular shapes, were found mainly in the dorsolateal part of the dorsal hypothalamic area after injections of HRP in the spinal cord. In a double-labeling experiment, Fast Blue or True Blue, and Nuclear Yellow were injected in the nucleus raphe pallidus and the spinal cord, respectively. A large number of blue-fluorescent neurons were located mostly in the ventromedial part of the dorsal hypothalamic area, while yellow-fluorescent ones were found in the dorsolateal part of the dorsal hypothalamic area.These results show that the dorsal hypothalamic area projects directly to the nucleus raphe pallidus, and that the neurons which project to the spinal cord differ from those which project to the nucleus raphe pallidus.