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Anatomical and functional analysis of the subdivisions of the dorsal raphe nucleus in the Mongolian gerbil (Meriones unguiculatus)

Skirmantas Janusonis, University of Massachusetts Amherst


Recent anatomical and neurochemical studies suggest that the dorsal raphe nucleus (DRN) of the brainstem is composed of several subdivisions that have different afferent and efferent connections and are different neurochemically. In the present study, DRN subdivisions projecting to visual system structures were studied in a highly visual rodent, the Mongolian gerbil (Meriones unguiculatus). ^ In one group of gerbils, the fluorescent neural tract tracer Fluoro-Gold was injected into the superior colliculus (SC), and brain sections also were double-labeled for serotonin (5-hydroxytryptamine, 5-HT). In another group of gerbils, the neural tract tracer cholera toxin subunit B (CTB) was injected into the SC, lateral geniculate nucleus (LGN), or primary visual cortex (V1). The location of retrogradely labeled cells in the DRN was analyzed at five rostro-caudal levels to determine DRN subdivisions projecting to these visual structures. At the three most rostral coronal levels, virtually all cells projecting to the SC occurred in the lateral DRN; most of these cells were serotonergic. At these rostral levels, virtually all cells projecting to the LGN also were observed in the lateral DRN. At the two most caudal levels, retrogradely labeled cells were observed in the ventromedial and interfascicular DRN following tracer injections into the SC or LGN. Following CTB injections into the primary visual cortex, virtually all retrogradely labeled cells occurred in the ventromedial DRN at all rostro-caudal levels. Therefore, the ventral caudal region of the DRN appeared to be the only DRN region projecting to all three visual areas (SC, LGN, and V1). ^ In order to determine whether functional differences might exist among DRN subdivisions, the expression of c-fos, an immediate-early gene, was analyzed in all DRN subdivisions throughout the light-dark cycle. In all subdivisions, the highest c-Fos expression was observed one hour after the onset of the dark period. However, ANOVA analysis revealed that the diurnal variation in c-Fos expression was statistically significant only in the caudal ventral region of the DRN, and gradually became less prominent in the caudal-to-rostral and ventral-to-dorsal directions. ^ A portion of this Dissertation has been previously published in the Journal of Comparative Neurology (Janušonis et al., 1999). ^

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Recommended Citation

Janusonis, Skirmantas, "Anatomical and functional analysis of the subdivisions of the dorsal raphe nucleus in the Mongolian gerbil (Meriones unguiculatus)" (2001). Doctoral Dissertations Available from Proquest. AAI3012142.