Evidence for a projection from the B9 serotonergic cell group to the median raphe nucleus

Following injections of several fluorescent retrograde tracers into the median raphe nucleus large numbers of retrogradely labeled cells were observed in the ventral mesencephalic tegmentum. Subsequent immunocytochemical processing for serotonin-like immunoreactivity suggested that a large component of this projection originates from serotonergic B9 cells. Although tracer injections into the dorsal raphe did not result in similar labeling of B9 cells, evidence was obtained suggesting the existence of a small serotonergic projection from the median to the dorsal raphe.