Mechanisms of CSF secretion by the choroid plexus

The epithelial cells of the choroid plexus secrete cerebrospinal fluid (CSF ), by a process that involves the movement of Na+, Cl- and HCO3- from the b lood to the ventricles of the brain. This creates the osmotic gradient, whi ch drives the secretion of H2O. The unidirectional movement of the ions is achieved due to the polarity of the epithelium, i.e., the ion transport pro teins in the blood-facing (basolateral) are different to those in the ventr icular (apical) membranes. Saito and Wright (1983) proposed a model for sec retion by the amphibian choroid plexus, in which secretion was dependent on activity of HCO3- channels in the apical membrane. The patch clamp method has now been used to study the ion channels expressed in rat choroid plexus . Two potassium channels have been observed that have a role in maintaining the membrane potential of the epithelial cell, and in regulating the trans port of K+ across the epithelium. An inward-rectifying anion channel has al so been identified, which is closely related to ClC-2 channels, and has a s ignificant HCO3- permeability. This channel is expressed in the apical memb rane of the epithelium where it may play an important role in CSF secretion . A model of CSF secretion by the mammalian choroid plexus is proposed that accommodates these channels and other data on the expression of transport proteins in the choroid plexus. (C) 2001 Wiley-Liss, Inc.