PROJECTION PATTERNS FROM THE RAPHE NUCLEAR-COMPLEX TO THE EPENDYMAL WALL OF THE VENTRICULAR SYSTEM IN THE RAT

The goal of the present study was to characterize the anatomical and n eurochemical relationships that the raphe nuclear complex maintains wi th respect to lateralized and centralized components of the ventricula r system. From this investigation, we 1) determined the ipsilateral vs , contralateral distribution of raphe efferents to the ependymal wall of the lateral ventricle, 2) assessed the degree of collateralization of individual ependymal projection neurons to other sites along the ve ntricular path, 3) compared the topography of raphe neurons that proje ct to the ventricular lining as well as the lumen of the fourth and la teral ventricles, and 4) evaluated the neurochemical identity of raphe neurons that innervate the ventricular system. After tracer injection s into the lateral ventricle, labeled cells were distributed evenly on both sides of the midline in the dorsomedial subregion of the interme diate dorsal raphe nucleus. Further rostrally, labeled cells were clus tered bilaterally above the medial longitudinal fasciculi and extended into the median raphe nucleus. Injections that involved the ependymal wall of the lateral ventricle resulted in prominent ipsilateral label ing within the dorsal raphe nucleus, just ventral to the cerebral aque duct. Most of the labeled cells in this latter group had collateral pr ojections to other sites along the ventricular path. Most of the ventr icle projection cells contained serotonin but not nicotinamide adenine dinucleotide phosphate diaphorase. These findings indicate that the r aphe nuclear complex is topographically organized with respect to the ventricular system. Selected subsets of serotoninergic dorsal raphe ne urons may influence discrete segments of the ventricular system indepe ndently as well as coordinate functions throughout the system through axon collaterals to other sites along the ventricular neuraxis. J. Com p. Neurol. 399:61-72, 1998. (C) 1998 Wiley-Liss, Inc.