Serotonergic afferents to the rat olfactory bulb: I. Origins and laminar specificity of serotonergic inputs in the adult rat

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Abstract
This is a study of the chemoanatomical organization of the projection from the raphe nuclei to the main olfactory bulb in the rat. A heavy projection from the dorsal and median raphe nuclei to the main olfactory bulb was shown by both retrograde and anterograde tracing techniques. Following injections of 1% wheat germ agglutinin- horseradish peroxidase (WGA-HRP) into the main olfactory bulb, up to 1300 neurons were retrogradely labeled in the dorsal and median raphe nuclei, a much larger number than has been suggested by previous studies. By combining 5-HT immunofluorescence with True blue retrograde fluorescent labeling, it was determined that the vast majority of raphe neurons that project to the olfactory bulb contain serotonin. Injections of WGA-HRP into dorsal and/or median raphe produced dense anterograde labeling in the glomeruli, while fewer labeled fibers were observed in the external plexiform layer, internal plexiform layer, and granule cell layer. In contrast to the number of other centrifugal afferents to the bulb, a significant contingent of fibers from the raphe nuclei enters the main olfactory bulb (MOB) from outside in, i.e., via the olfactory nerve layer. Serotonergic fibers in MOB were visualized by immunocytochemistry, and the distribution of specific 5- HT fibers closely matched the distribution of anterograde terminal label resulting from injections of WGA-HRP in the raphe nuclei. The serotonergic fibers have a specific laminar distribution and morphology in MOB. Thus, the density of the serotonergic innervation to the glomerular layer is 2–3 times that of any other layer in MOB. In addition to their preferential innervation of the glomeruli, glomerular and infraglomerular serotonergic fibers are morphologically different. Serotonergic fibers located in the glomerular layer are generally thick (0.25–0.60 micron) compared to the thinner (0.25–0.35 micron) fibers that predominate in infraglomerular layers. Another difference is that the glomerular fibers often contain varicosities that are greater than 1 micron in diameter, while varicosities along infraglomerular fibers are usually barely larger than the axonal diameter. Finally, glomerular fibers are much more intensely stained than infraglomerular fibers. Electrolytic lesions of the dorsal and median raphe caused a total depletion of serotonin fiber staining in the bulb, demonstrating that the raphe nuclei are the sole source of the serotonergic input to the main olfactory bulb. Thus, it has been demonstrated that serotonergic neurons in dorsal and median raphe project very heavily to glomeruli and less heavily to other layers in the main olfactory bulb.(ABSTRACT TRUNCATED AT 400 WORDS)

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