Immunohistochemical demonstration of serotonin-containing CSF-contacting neurons in the submammalian paraventricular organ.

The distribution and morphological aspects of the serotonin-containing neurons in the paraventricular organ of the carp, frog, turtle and chicken were studied by means of an immunoperoxidase technique using serotonin antiserum. In all species the serotonin-containing neurons were seen to have the appearance of the CSF-contacting neurons and to be distributed in the pars ependymalis and the pars hypendymalis of the organ. Particularly, in the frog, the serotonin-containing CSF-contacting neurons, mostly bipolar in shape, were also observed in the pars distalis. Their proximal processes protruded into the ventricular lumen through the ependymal layer with a globular- and triangular-shape. The distal processes projected ependymofugally to the pars distalis and formed a fine plexus in the neuropil of this part. The density of the serotonin fibers in the pars distalis was greater in the carp than in the other species.