Serotoninergic synapses on ependymal and hypendymal cells of the rat subcommissural organ

The nervous input to the subcommissural organ (SCO) of the rat has been investigated with Falck-Hillarp fluorescence histochemistry and electron microscopical techniques. Previous fluorescence histochemical observations of a dense plexus of serotoninergic nerve fibres in relation to the basal SCO were confirmed. Electron microscopically, unmyelinated fine varicose axons ranging in size from 0.1--0.6 micrometer were observed to penetrate into the SCO hypendyma. Boutons and presynaptic varicosities filled with a diversity of round and elongated clear vesicles, and occasional large dense cored vesicles establish asymmetric (Gray's type I) synaptic contacts with the basal processes and somata of the SCO ependymal and hypendymal cells. A typical varicosity in synaptic contact with an SCO cell contains a population of approximately 85% clear, elongated vesicles 45 X 60 nm in diameter, 15% clear, round vesicles 50 nm in diameter, and 1--2% large dense cored vesicles with a vesicle diameter of about 85 nm and a dense core diameter of 50--55 nm. The mean length of the postsynaptic membrane specialization was found to be 0.5 micrometer. Experiments with specific neurotoxic drugs revealed that the nerve terminals in synaptic contact with the SCO cells are identical to the fibres of the serotoninergic plexus identified fluorescence histochemically. Thus, an intraventricular injection of either 5,6-dihydroxytryptamine or 5,7-dihydroxytryptamine induced typical degenerative changes in most of the boutons in synaptic contact with the SCO cells, and also a disappearance of the yellow fluorescent nerve plexus. It is concluded that the SCO of the rat receives a dense plexus of serotonin-containing nerve fibres which form typical synaptic contacts with the specialized ependymal cells of the SCO and that these fibres may constitute the only direct nervous input to the organ. The degeneration of the serotoninergic synapses elicited a long-lasting, pronounced increase in the secretory activity of the SCO. Despite long survival times after the treatment with neurotoxic drugs, we found no evidence of regenerative restitution of the serotoninergic innervation nor normalization of the secretory activity of the SCO. The observed inverse relationship between secretory activity and serotoninergic innervation is in line with previous observations which indicate that the 5-hydroxytryptamine input to the SCO ependymal and hypendymal cells exerts a powerful inhibition on their protein synthetic machinary.