Ascending and descending projections from the median and dorsal raphe nuclei of the midbrain were mapped in the albino rat, using reducedsilver stains after lesions or the autoradiographic technique following injections of tritiated proline. In all major respects the two techniques gave the same results. The majority of the ascending projections sweep ventrally from the raphe nuclei, then curve rostrally to course through the ventral tegmentum and into the medial forebrain bundle (MFB). Others radiate through the mesencephalic reticular formation (RF) and central grey, turning ventrally at the posterior thalamic border to enter the subthalamus. From the MFB, fibers branch into the hypothalamus, preoptic areas, anterior amygdala and olfactory tubercle, while some fibers enter the fornix or the stria terminalis. Many fibers continue rostrally in the MFB, joining the diagonal bands of Broca to reach the septa1 nuclei or, further rostrally, the cingulum bundle. Fibers in the cingulum bundle turn caudally around the genu of the corpus callosum, some branching into the cell-free layers of the pregenual cortex. Running caudally, then curving around the splenium, the cinguIum bundle projection sprays out into the subiculum and in some cases a projection into the hippocampus is seen.Other ascending projections include one to the habenular nuclei through the fasciculus retroflexus. Projections to the mediodorsal, parafascicular and reuniens nuclei of the thalamus were also noted.Descending projections were observed to the dorsal tegmental nucleus and locus coeruleus and diffusely to the pontine reticular formation and caudal central grey. No projections were seen below the level of the facial nerve nucleus and none were observed to the cerebellum, caudal raphe nuclei or cranial nerve nuclei.Since the discovery that much of the serotonin in the brain is localized within the cell bodies, axons and axon terminals of a smaI1 group of nuclei in the midbrain tegmentum -the n. dorsalis raphe and n. medianus raphe (n. centralis superioris of Bechterew) -it has been suggested that those nuclei play important roles in mediating the effects of serotonin on sleep, neuroendocrine regulation, sexuality, aggression and pain responses (Chase and Murphy, '73 for review). However, though serotonin can be linked to these behaviors, the neuroanatomical pathways on which such functions are based have remained unclear. Earlier anatomical studies dealing with midbrain connections, such as those by Brodal and co-workers ('60) and Nauta and Kuypers ('58) either have used retrograde degeneration methods or have not focused systematically on the raphe. Numerous studies employing the histochemical fluorescence technique for serotonin (Dahlstrom and Fuxe, '65) have led to the proposal of an ascending serotonergic system presumably originating in the raphe nuclei of the midbrain (Anden et al., '66; Ungerstedt, '71). However, axons of cells containing serotonin can rarely be visualized and the fluorescence technique has appeared less sensitive f...
Efferent projections from the ventromedial nucleus of the hypothalamus (VMN) were traced using tritiated amino acid autoradiography in albino rats. Ascending fibers passed through the anterior hypothalamus. Labelled fibers and terminal fields were seen in the preoptic area, bed nucleus of the stria terminalis, substantia innominata, the anterior amygdaloid area, diagonal bands of Broca and lateral septum. Fibers also projected laterally from VMN and entered the supraoptic commissures and zona incerta. These lateral projections were responsible for the fibers observed in the cerebral peduncle, the amygdala, the thalamus and the reticular formation. Fibers descending in a medial position projected through the posterior hypothalamus and then swept dorsally to terminate in the mesencephalic and pontine central grey. A projection from VMN into the median eminence was noted. The overall patterns of projection from different parts of VMN were similar; differences that existed were primarily in the relative strengths of the different projections. The efferent projections from VMN are extensive, well organized, and would appear capable of supporting significant physiological actions on extra-hypothalamic structures.
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