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The Journal of Neuroscience, May 1, 2003, 23(9):3588
Functional Hemichannels in Astrocytes: A Novel Mechanism of
Glutamate Release
Zu-Cheng
Ye,
Megan S.
Wyeth,
Selva
Baltan-Tekkok, and
Bruce R.
Ransom
Department of Neurology, University of Washington School of
Medicine, Seattle, Washington 98195
Little is known about the expression and possible functions of
unopposed gap junction hemichannels in the brain. Emerging evidence
suggests that gap junction hemichannels can act as stand-alone functional channels in astrocytes. With immunocytochemistry, dye uptake, and HPLC measurements, we show that astrocytes in
vitro express functional hemichannels that can mediate robust
efflux of glutamate and aspartate. Functional hemichannels were
confirmed by passage of extracellular lucifer yellow (LY) into
astrocytes in nominal divalent cation-free solution (DCFS) and the
ability to block this passage with gap junction blocking agents.
Glutamate/aspartate release (or LY loading) in DCFS was blocked by
multivalent cations (Ca2+, Ba2+,
Sr2+, Mg2+, and
La3+) and by gap junction blocking agents
(carbenoxolone, octanol, heptanol, flufenamic acid, and
18 -glycyrrhetinic acid) with affinities close to those reported for
blockade of gap junction intercellular communication. Glutamate efflux
via hemichannels was also accompanied by greatly reduced glutamate
uptake. Glutamate release in DCFS, however, was not significantly
mediated by reversal of the glutamate transporter: release did not
saturate and was not blocked by glutamate transporter blockers. Control
experiments in DCFS precluded glutamate release by volume-sensitive
anion channels, P2X7 purinergic receptor pores, or
general purinergic receptor activation. Blocking intracellular Ca2+ mobilization by BAPTA-AM or thapsigargin did
not inhibit glutamate release in DCFS. Divalent cation removal also
induced glutamate release from intact CNS white matter (acutely
isolated optic nerve) that was blocked by carbenoxolone, suggesting the
existence of functional hemichannels in situ. Our
results indicated that astrocyte hemichannels could influence CNS
levels of extracellular glutamate with implications for normal and
pathological brain function.
Key words:
astrocyte; gap junction; hemichannel; glutamate; divalent cation; glutamate release; glutamate transport
Copyright © 2003 Society for Neuroscience 0270-6474/03/2393588-09$05.00/0
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