Serotoninergic supraependymal plexus in the ventricular system of the hedgehog: organization principles and functional implications.

The distribution of serotonin (5-HT) immunoreactive fibers in the ventricular system of the hedgehog Erinaceus europaeus was studied by means of specific antibodies against serotonin. Light microscopic examination of immunocytochemically stained coronal sections showed abundant serotoninergic fibers, unevenly distributed along the ventricular wall, both supraependymally and subependymally. No immunoreactivity was evident in the choroid plexus. Serotonin fibers lying supraependymally exhibited distinct morphological and distributional features. They were either found to terminate freely in the ventricular lumen or contributed to the formation of a large number of interconnected basket-like formations, particularly abundant in the lateral and fourth ventricles, where they usually lay opposite to the choroid plexus. In the electron microscopic study of the 5-HT-stained thin sections, the ependymal monolayer was found to be composed of squamous or cuboidal cells protruding into the ventricle. Immunoreactive axons cut in favorable tangential planes formed typical baskets that encircled the apical cytoplasmic process of the ependymal cells. Immunoreactive varicosities containing clear and dense cored vesicles were found either to terminate as "free arborizations" within the ventricles, or contacted the ventricular surface of ependymal cells. In these contacts, junctional specializations resembling those of classical synapses were often observed. The functional significance of the 5-HT baskets which surround individual ependymal cells is briefly discussed.