Serotonergic afferents to the rat olfactory bulb: II. Changes in fiber distribution during development.

The present studies have defined the developmental time course and distribution patterns of serotonergic fibers in the main olfactory bulb (MOB) using immunocytochemistry, anterograde tracing and image analysis. The results indicate that the deployment of serotonergic fibers to the main olfactory bulb is essentially a postnatal event in the rat. During the first 4 d after birth, 5-HT fibers infiltrate and begin to arborize in the MOB. The density of fibers in each layer is sparse during this period, but increases rapidly. By postnatal day 8 all layers are much more heavily innervated by 5-HT fibers. The surge of fiber growth into all layers is rapid; the fibers arborize earlier at caudal than at rostral levels. This may be related to the increased metabolic activity that is reported to occur selectively in the caudal parts of the immature olfactory bulb. After the second postnatal week, 5-HT fiber density increases much more gradually in all layers except the glomerular layer; in the glomerular layer, 5-HT fiber density continues to increase rapidly. It is also during this time that the olfactory bulb begins to grow substantially in volume. Bulb volume increases from the second week into adulthood (greater than 60 d); during the same period, the density of 5-HT remains relatively constant in the infraglomerular layers. Thus, from the second week onward, the growth of 5-HT fibers appears to be closely linked to the increasing volumes of these layers. The density of 5-HT fibers in the glomerular layer, however, continues to increase from the second postnatal week. Thus, the density of fibers in the glomerular layer increases more than the increase in glomerular size, indicating that the glomerular 5-HT fibers are proliferating more than could be accounted for by simple glomerular expansion. In the adult, 5-HT fibers are 2-3 times denser in the glomerular than the infraglomerular layers (McLean and Shipley, 1987). This preferential innervation of glomeruli may be the result of a protracted period of arborization by glomerular versus infraglomerular fibers. This could be due to the prolonged focal release of a trophic factor by glomerular-associated neurons or to the earlier production of an inhibitory factor by infraglomerular neurons.