Origin of monoaminergic innervation of the nucleus raphe magnus--a combined monoamine histochemistry and fluorescent retrograde tracing study in the rat.

By the use of the fluorescent retrograde tracer, Fast Blue (FB), in combination with monoamine fluorescence histochemistry, the origin of monoaminergic input to the region of the nucleus raphe magnus (NRM) was investigated in the rat. After microinjection of FB into the NRM, a great number of FB-labeled NA-containing cells were found in the region of the nucleus reticularis lateralis (corresponding to A1 NA areas), the reticular formation just dorsolateral to the nucleus olivaris inferior (corresponding to the A3 NA area), the ventral part of the locus coeruleus (A6 NA area), and the lateral parts of the nucleus raphe dorsalis (B7 area). In the other NA cell groups of the brain stem, FB-labeled cells could not be observed. Serotoninergic input originating from nucleus raphe obscurus (B2), nucleus raphe pallidus (B1) and nucleus raphe pontis (B5-B6) to the NRM was also observed. A large number of cells containing serotonin (5-HT) in the B2 and B6 areas were labeled by FB, while only a few FB-labeled 5-HT cells in B8, B9 were seen. In addition to the FB-labeled monoaminergic (NA and 5-HT) cells, many FB-labeled non-monoamine containing neurons were observed in the nucleus olivaris inferior and various parts of the reticular formation (FR), particularly in the reticular formation of the pons. In conclusion, our study has extended wider the previous HRP finding. It is shown that the NRM receives noradrenergic, serotoninergic and non-monoaminergic innervation from many regions of the brain stem. The afferent projections containing different neurotransmitters provided an important structural basis for studying the function of the NRM.