Brain Research, 324 (1984) 165-170 165 Elsevier BRE 20536 Origin and termination of dorsal raphe-median eminence projection MARCELO J. VILLAR, SARA R. CHIOCCHIO and JUAN H. TRAMEZZANI Instituto de Neurobiologfa, Serrano 665, Buenos Aires 1414 (Argentina) (Accepted August 10th, 1984) Key words: dorsal raphe nucleus -- median eminence --serotonergic innervation --serotonin The origin and termination of the dorsal raphe (DR)-median eminence (ME) projection were studied by combining well-limited small electrolytic lesions of different parts of the DR, the axonal degeneration and the serotonin content in the median eminence. Our results show that the rostral third of the DR is a source of serotonergic innervation to the ME and that the rest of the nucleus does not contribute to this projection. The light microscopy study revealed that degenerating fibers occupy the internal layer of the ME. These results were confirmed by electron microscopy which in addition showed degenerating terminals. The dorsal raphe nucleus (DR) was proposed as a source of hypothalamic serotonergic innervation 5. Lesioning experiments and radioenzymatic assays as well as electron microscopy of anterograde degener- ation showed that the DR innervates various hypo- thalamic nuclei and the median eminence (ME)12A 3. Nevertheless, a careful analysis of this study reveals that lesions were not always restricted to the target nucleus and that neighboring areas were also dam- aged. Ascending fibers to the medial basal hypothalamus which originate in the DR leave the nucleus through the ventral ascending pathway and are incorporated into the medial forebrain bundle 4,7,14,19. The DR is a very extensive nucleus formed by several subgroups of serotonergic cell bodies which have differential projections to other areas of the CNS 15,16,21. So, it seemed very probable that the neu- rons projecting to the ME should be located in a spe- cial zone within the DR. The presence of serotonergic (5-hydroxytrypta- mine, 5-HT) fibers in all layers of ME has been shown by several techniques: histofluorescence 5, au- toradiography 2, neurotoxins combined with trans- mission electron microscopy 1, microspectrofluoro- metric analysis 10 and immunofluorescence s,20. In order to clarify the precise origin of the DR-ME projection and its site of termination within the medi- an eminence, well-limited small lesions of different parts of the DR nucleus were performed in rats, and axonal degeneration as well as 5-HT content in the ME were studied following several time intervals. Male adult Holtzman rats (220-240 g) were placed in a stereotaxic apparatus after being anesthetized with pentobarbital (40 mg/kg). Very small electro- lytic lesions restricted to different areas of the DR (the rostral, medial and caudal third) were made with tungsten electrodes by a dorsal approach, applying a 2 mA constant current during 10 s. Sham-operated animals were treated in the same way, except that the electrode was inserted just to the level of the aque- duct, and no current was applied. All the DR lesions were histologically checked in all their extent, and particular attention was paid that the medial longitudinal fasciculus (FLM) was left un- damaged. It is important to bear in mind this fact since when the laterodorsal parts of the FLM are le- sioned, an incrementation in the number of degener- ated fibers may be found in the hypothalamus 22. All animals which did not fulfill these requirements were discarded. For the light microscopic study (20 rats), the ani- mals were perfused with saline followed by 10% for- malin after 4-7 days of survival. Then, the brains Correspondence: M. J. Villar, Serrano 665, 1414 Buenos Aires, Argentina. 0006-8993/84/$03.00 (~ 1984 Elsevier Science Publishers B.V.