Development of the suprachiasmatic nucleus in rats during ontogenesis: serotonin-immunopositive fibers.

The innervation of the suprachiasmatic nucleus by serotoninergic fibers has been studied in rats from the 22nd embryonic until the 21st postnatal day. The serotoninergic fibers were detected with the pre-embedding immunocytochemical technique at the electron-microscopic level using antibodies to serotonin. Serotonin-immunopositive fibers were always identified as axons as they contained numerous synaptic vesicles both in fetuses and postnatal rats. Moreover, immunopositive dense core vesicles 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 synapses (presynapses) were observed. After birth, presynapses were replaced by typical synapses with the thickened pre- and postsynaptic membranes, accumulations of dense materials at the pre- and postsynaptic membranes as well as with an accumulation of synaptic vesicles at the presynaptic membrane. The functional significance of the serotoninergic input to the suprachiasmatic nucleus in ontogenesis is discussed.