[Locations and Functions of Water Selective Channels from the View-Points of Cerebrospinal Fluid Distribution]

Water not only exists in the ventricles and cisterns but is also contained in the cerebral parenchyma. Water movement in most tissues has been described to be mediated by water channels since the discovery of these channels(aquaporins: AQPs). As to the intracranial situation, AQP-1 is abundantly expressed in the epithelial cells of the choroid plexus. AQP-4 is also enriched in the ependyma, endfeet of astrocytes at the blood-brain barrier, and glia limitans. AQP-4 is involved in cerebrospinal fluid(CSF)absorption, while AQP-1 is involved in CSF production. Impairment of CSF absorption through AQP-4 is considered to be caused by an inflammatory mechanism and finally leads to the development of hydrocephalus. Recently, the glymphatic system was recognized as an intracranial lymphatic system and hypothesized to be involved in CSF absorption and waste removal into the CSF. Disturbances of this glymphatic system may induce Alzheimer's disease and idiopathic normal-pressure hydrocephalus. Although the mechanism of CSF production in the epithelial cells of the choroid plexus remains to be elucidated, the existence of glycolysis leading to the facilitation of AQP-1 expression is considered a key point in CSF production.