DEVELOPMENT OF THE SUPRACHIASMATIC NUCLEUS IN RATS DURING ONTOGENY - SEROTONIN-IMMUNOPOSITIVE FIBERS

The innervation of the suprachiasmatic nucleus by serotoninergic fiber s has been studied in rats from the 22nd embryonic until the 21st post natal day. The serotoninergic fibers were detected with the pre-embedd ing immunocytochemical technique at the electron-microscopic level usi ng antibodies to serotonin. Serotonin-immunopositive fibers were alway s identified as axons as they contained numerous synaptic vesicles bot h in fetuses and postnatal rats. Moreover, immunopositive dense core v esicles appeared in the axons after birth. From the end of fetal life onwards, the serotonin-immunopositive axons made specialized contacts with the immunonegative neurons, mainly with their dendrites and to a lesser extent with cell bodies and axons. In fetuses, only immature sy napses (presynapses) were observed. After birth, presynapses were repl aced by typical synapses with the thickened pre- and postsynaptic memb ranes, accumulations of dense materials at the pre- and postsynaptic m embranes as well as with an accumulation of synaptic vesicles at the p resynaptic membrane. The functional significance of the serotoninergic input to the suprachiasmatic nucleus in ontogenesis is discussed.