The neural plexus located on the ependyma’s apical surface of the cerebral ventricles may be an important link in the extra-barrier signal transmission and regulation of the cerebrospinal fluid composition, but to date it remains a poorly understood part of the central nervous system.

Aim. To investigate signs of functional activity of the supraependymal nerve plexus in early postnatal development using immunohistochemical methods.

Materials and methods. Coronal brain sections of adult rats and rats on postnatal days 1, 7, and 10 (n = 16) were used for immunohistochemical analysis. Antibodies to tyrosine hydroxylase, tryptophan hydroxylase, and synaptophysin were used to identify supraependymal fibers and structures exhibiting potential synaptic activity.

Results. There was no immunopositive reaction observed in the supraependymal plexus of neonatal rats regardless of the marker used. Synaptophysin-containing puncta or fibers with varicosities on the surface of ependymal cells were first detected by the end of the first week of postnatal development. Tryptophan hydroxylase-positive structures were also observed at this stage of postnatal development. Fibers and puncta were located throughout the walls of the lateral and third ventricles, with the exception of the infundibular recess. Only on the 10^th day of postnatal development were tyrosine hydroxylase-containing granules detected on the apical surface of the ependyma. A double immunofluorescence assay demonstrated that the tryptophan hydroxylase distribution matched with synaptophysin. Moreover, at none of the studied periods were neurons found on the surface of the ependyma.

Conclusion. The supraependymal plexus exhibits the capacity for synaptic transmission and serotonin synthesis by the end of the first week of postnatal development. By the 10^th day of postnatal development, supraependymal nerve fibers may participate in catecholaminergic neurotransmission.