The cerebrospinal fluid-contacting neuron: a peculiar cell type of the central nervous system. Immunocytochemical aspects.

Cerebrospinal fluid (CSF)-contacting neurons are located periventricularly or inside the brain ventricles; they contact the CSF via their dendrites, perikarya or axons. Most of these neurons form ciliated dendrite terminals in the internal CSF as do retinal and pineal photoreceptors in the optic ventricle and pineal recess. The peculiar localization, polarization and synaptic connections of the CSF-contacting neuronal elements suggest receptor and integrative functions. The present review pays special attention to vitamin A (retinoids) immunoreactivity in CSF-contacting neurons as compared with that present in retinal and pineal photoreceptor cells, common neurons, glial and adenohypophysial cells. The immunoreactivity of the dark-adapted photoreceptor outer segments was strong, but decreased after illumination, suggesting the functioning of vitamin A as the chromophore of the retinal and pineal photopigments. Retinoid immunoreaction was also found in the endoplasmic reticulum, nuclei, nucleoli and mitochondria of the cell types studied. This cytological localization suggests that vitamin A compounds may be involved in the function of these organelles. The CSF-contacting neurons contain varying amounts of bioactive materials. The intracellular distribution of immunoreactive serotonin (5-HT), substance P (SP) and gamma-aminobutyric acid (GABA) is compared with that of immunoreactive vitamin A. Immunogold labeling for SP was demonstrated in dense-core vesicles of preoptic neurons; 5-HT marking was found on the dense-core vesicles of subependymal CSF-contacting neurons of the paraventricular organ, while GABA immunoreaction was localized in the cytoplasm of distal infundibular CSF-contacting neurons. The CSF-contacting neurons are considered to synthesize and release their bioactive substances at transmitter synapses, and/or at neurohormonal terminals into the external CSF in accord with information received by their dendrites from the internal CSF and by afferent fiber connections from various brain areas.