Transporter-mediated L-glutamate elimination from cerebrospinal fluid: possible involvement of excitatory amino acid transporters expressed in ependymal cells and choroid plexus epithelial cells.
BACKGROUND
L-Glutamate (L-Glu) is the major excitatory neurotransmitter in the CNS, and its level in cerebrospinal fluid (CSF) is reported to be increased in neuroexcitatory diseases such as epilepsy. Since L-Glu concentration in the CSF is reported to be lower than that in plasma, it has been proposed that some mechanisms of L-Glu clearance from the CSF operate in the brain. The purpose of this study was to elucidate the major pathway of L-Glu elimination from rat CSF and the transporters responsible.


METHODS
Protein expression and localization of excitatory amino acid transporters were examined by immunohistochemical analysis using specific antibodies. In vivo elimination of L-Glu from rat CSF was evaluated by intracerebroventricular administration. An L-Glu uptake study by using primary-cultured rat ependymal cells and isolated rat choroid plexus was performed to characterize L-Glu transport mechanisms.


RESULTS
An immunohistochemical analysis has shown that excitatory amino acid transporter (EAAT) 1 and EAAT3, which are D-aspartate-sensitive and kainate-insensitive L-Glu transporters, are localized on the CSF-side of rat ependymal cells and choroid plexus epithelial cells, respectively. In contrast, the kainate-sensitive L-Glu transporter, EAAT2, is not expressed in these cells. In vivo L-Glu elimination clearance from the rat CSF (189 μL/(min · rat)) was 23-fold higher than the CSF bulk flow rate, indicating that facilitative process(es) are involved in L-Glu elimination from the CSF. The in vivo [(3)H]L-Glu elimination from the CSF was significantly inhibited by unlabeled L-Glu and D-aspartate, but not kainate. Moreover, unlabeled L-Glu and D-aspartate inhibited [(3)H]L-Glu uptake by rat ependymal cells and choroid plexus epithelial cells, whereas kainate had little effect.


CONCLUSIONS
It is suggested that EAAT1 in ependymal cells and EAAT3 in choroid plexus epithelial cells participate in L-Glu elimination from the CSF.
Department of Pharmaceutics, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, 2630 Sugitani, Toyama, 930-0194, Japan. akanumas@pha.u-toyama.ac.jp.
Shin-ichi
S
Akanuma
Department of Pharmaceutics, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, 2630 Sugitani, Toyama, 930-0194, Japan. d1361305@ems.u-toyama.ac.jp.
Tatsuhiko
T
Sakurai
Division of Membrane Transport and Drug Targeting, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai, Miyagi, 980-8578, Japan. tachikaw@m.tohoku.ac.jp.
Masanori
M
Tachikawa
Department of Pharmaceutics, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, 2630 Sugitani, Toyama, 930-0194, Japan. kuboyosh@pha.u-toyama.ac.jp.
Yoshiyuki
Y
Kubo
Department of Pharmaceutics, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, 2630 Sugitani, Toyama, 930-0194, Japan. hosoyak@pha.u-toyama.ac.jp.
Ken-ichi
K
Hosoya
D000818:Animals; D002478:Cells, Cultured; D002555:Cerebrospinal Fluid / Q000378:metabolism*; D002831:Choroid Plexus / Q000166:cytology / Q000378:metabolism*; D004847:Epithelial Cells / Q000378:metabolism*; D050491:Glutamate Plasma Membrane Transport Proteins / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D020547:Lateral Ventricles / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D050491:Glutamate Plasma Membrane Transport Proteins; D018698:Glutamic Acid