Serotonergic ependymal fibres in rat and monkey: Light and electron microscopic immunocytochemical study

The distribution of serotonin (5HT) immunoreactive fibres in the ependymal epithelium of aqueductus cerebri in adult rat and monkey was studied by mea ns of immunocytochemical methods using specific antibodies against 5HT. Lig ht microscopic examination of immunostained coronal sections of mesencephal on showed, in tryptophan and pargylin pretreated animals, abundant serotone rgic fibres distributed along the ependymal cells of the aqueduct, forming supraependymal and subependymal plexi. Serotonin fibres lying either suprae pendymally or ending freely in the aqueduct lumen contributed to the format ion of a rich 5HT containing network on the ependymal surface. Electron mic roscope images showed dense 5HT- immunoreactive (5HT-IR) profiles with ultr astructural characteristics of axon terminals ending on the ependymal cells . Dense diaminobenzidine (DAB) deposits were found in the axoplasm, on oute r mitochondrial surface and in vesicles. No synaptic contacts were observed between 5HT-IR terminals and ependymal cells. Unstained microvilli and cil ia were also observed in the aqueduct lumen. Serotonin immunoreactivity dis appeared from ependymal fibres in animals treated with parachlorophenylalan ine, an inhibitor of 5HT synthesis. 5HT containing fibres described in this paper may be the source of 5HT and its metabolite 5-hydroxyindoleacetic acid (5HIAA) found in cerebrospinal fl uid (CSF) and of clinical relevance in some psychiatric conditions such as depression, suicidal attempts, etc.