Direct projections from serotonergic neurons in the dorsal and median raphe nuclei of midbrain to the suprachiasmatic nucleus in Tupaia belangeri chinensis

This study investigated the direct serotonergic projections to the suprachi asmatic nucleus (SCN) from the dorsal and median raphe nuclei (DR/MR) of th e midbrain in Tupaia belangeri chinensis (TBC) by combined application of r etrograde horseradish peroxidase (HRP) tract tracing, immunohistochemistry, and electron microscope techniques. The results provide evidence for the d irect projections to the SCN from serotonergic neurons distributed predomin antly in the MR (mainly in its lateral portion) and to a lesser degree in t he DR (in its ventrolateral portion) more caudally in the midbrain, and the existence of abundant symmetrical and asymmetrical synaptic connections be tween the serotonergic terminals and the postsynaptic elements in the SCN T BC. The results also revealed that almost all DR neurons projecting to the SCN contained serotonin, whereas about one-half of MR neurons projecting to the SCN were immunoreactive for serotonin. NeuroReport 12:2341-2345 (C) 20 01 Lippincott Williams & Wilkins.