Topographical distribution of dorsal and median raphe neurons projecting to motor, sensorimotor, and visual cortical areas in the rat

The present study was conducted to examine the spatial organization of dorsal (DR) and median (MR) raphe neurons that project to rostrocaudally aligned areas of the rat cerebral cortex. An additional goal was to determine if individual DR cells that send efferents to forelimb sensorimotor or visual regions of the neocortex also send axon collaterals to forelimb (crus II) or visual (paraflocculus) areas of the cerebellum. Long‐Evans hooded rats received unilateral pressure injections of horseradish peroxidase (HRP) in either motor n = 4) or sensorimotor (n = 5) or visual (n = 4) cortex to determine the intranuclear location of DR and MR neurons that project to specific neocortical regions. Coronal sections (40‐100 μm) through the pons and midbrain were examined by light microscopy after the tetramethyl benzidine reaction and neutral red counterstaining were carried out. The locations of retrogradely labeled cells were recorded relative to a threedimensional biological coordinate system maintained by a computer linked to the light microscope. For double labeling studies, unilateral injections of fast blue and nuclear yellow were made in paired motor (sensorimotor cortex and crus II of the lateral cerebellum) or visual (cortical area 17 and paraflocculus) areas of the CNS. Coronal tissue sections (35 μm) were collected on coverslips and examined on a Leitz fluorescence microscope (wavelength = 365 nm). DR neurons labeled from cerebrocortical injections of HRP were concentrated in the rostral two‐thirds of the nucleus. HRP‐filled neurons were distributed such that individual groups of neurons projecting to motor, sensorimotor, or visual cortex were aligned in a partially overlapping, rostral to caudal array. In the dorsoventral dimension, retrogradely labeled cells were clustered in three distinct groupings such that neurons projecting to the motor, sensorimotor, and visual areas were concentrated in dorsal, intermediate, and ventral portions of the DR nucleus, respectively. For all cases, the majority of HRP‐filled cells were positioned along the midline or displaced to the side of the nucleus that was ipsilateral to the cortical injection site. A small number of retrogradely labeled neurons were observed in the MR following injections in the motor cortex. Computer‐assisted reconstruction of th z neuroanatomical data facilitated the visualization of spatial relationships between groups of DR neocortical projection neurons. Twenty to 30% of the cells labeled in the DR after paired injections of fast blue and nuclear yellow in sensorimotor or visual areas of the neocortex and cerebellum were doubled labeled indicating that individual DR neurons can send axon collaterals to functionally analogous regions of the CNS. Overall, the results indicate that, within the DR, a topographic ordering exists with respect to rostrocaudally aligned terminal fields in the neocortex. In addition, a portion of the DR cerebrocortical projection neurons also send axon collaterals to specific regions of the cerebellar cortex. The intranuclear organization observed in this study suggests that neuronal activity in distinct regions of the DR nucleus may independently influence discrete populations of cerebrocortical cells. Furthermore, the possibility must now be considered that a substantial number of single DR neurons provide a common input via axon collerals to portions of the neocortical and cerebellar circuitry that receive afferent information related to the same sensory or motor function.