Inward-rectifying anion currents, similar to those carried by ClC-2 channels, have been identified in choroid plexus epithelial cells (Kotera & Brown, 1994; Kajita et al. 2000). mRNA encoding ClC-2 has also been identified in the choroid plexus epithelium (Smith et al. 1995). The aim of this study was to determine whether these currents are carried by ClC-2 channels, by examining channel activity ClC-2 knock-out mice (Bösl et al. 2001).

The fourth ventricle choroid plexus was isolated from three adult control (heterozygote) mice and three knock-out animals, humanely killed by an overdose of anaesthetic (Halothane inhalation). Channel activity was measured by whole-cell recording methods using K⁺-free solutions. The whole-cell capacitances in control (51.3 ± 2.9 pF; mean ± S.E.M., n = 11) and knock-out (50.0 ± 1.9 pF, n = 13) animals were not significantly different (P > 0.1 by Students’ t test for unpaired data). Inward rectifying currents were observed in choroid plexus cells from both control and knock-out animals. There was no significant difference (P > 0.1 by unpaired t test) between the mean conductance (measured between V_m = -100 and -140) in control (19.6 ± 3.6 nS; n = 8) and knock-out animals (22.5 ± 3.1 nS; n = 10). Channel properties were further investigated by replacing some of the Cl^– in the electrode solution with I^–. The channels in both the control and knock-out animals were more permeable to I^– than Cl^–, P_I : P_Cl = 3.4 ± 1 (n = 3) and 4.0 ± 2 (n = 3), respectively.

These data show that the inward-rectifying Cl^– currents in choroid plexus epithelial cells are not carried by ClC-2 channels.

This work was supported by The Wellcome Trust.