Journal of Neuroscience, Vol 7, 3016-3028, Copyright © 1987 by Society for Neuroscience
Serotonergic afferents to the rat olfactory bulb: I. Origins and laminar specificity of serotonergic inputs in the adult rat
JH McLean and MT Shipley
Department of Anatomy and Cell Biology, University of Cincinnati College of Medicine, Ohio 45267.
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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