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Journal of Neuroscience, Vol 15, 2720-2732, Copyright © 1995 by Society for Neuroscience
GABAA/benzodiazepine receptors in acutely isolated hippocampal astrocytes
DD Fraser, S Duffy, KJ Angelides, JL Perez-Velazquez, H Kettenmann and BA MacVicar
Neuroscience Research Group, University of Calgary, Alberta, Canada.
The properties of GABA receptor-mediated responses were examined in
noncultured astrocytes, acutely isolated from the mature rat hippocampus.
Whole-cell patch clamping revealed a GABA-activated Cl- conductance that
was mimicked by the GABAA receptor agonist muscimol and depressed by the
GABAA antagonists bicuculline and picrotoxin. The GABAA-activated currents
were potentiated by the barbiturate pentobarbital and the benzodiazepine
diazepam. The benzodiazepine inverse agonist DMCM either enhanced or
depressed the astrocytic GABAA- mediated responses, suggesting receptor
heterogeneity with respect to pharmacologic profiles. In addition, GABA
evoked an increase in [Ca2+]n measured by indo-1 fluorometry, which was
depressed in the presence of verapamil or picrotoxin. A GABAA-induced
depolarization, therefore, causes Ca2+ influx through voltage-gated Ca2+
channels. The expression and subcellular localization of GABAA receptors
and its subunits were examined using immunohistochemical and fluorescent
benzodiazepine binding techniques. Polyclonal antisera raised against the
GABAA/benzodiazepine receptor, which recognizes multiple subunit isoforms,
labeled receptors on the astrocytic cell body and most large processes. In
contrast, antisera generated against either alpha 1 or beta 1 subunit
peptides revealed immunoreactivity predominantly on a subset of processes.
To determine the subcellular distribution of membrane-bound receptors, a
fluorescent benzodiazepine derivative was superfused over live astrocytes
and visualized with laser-scanning confocal microscopy. Specific
fluorescence was distributed in discrete clusters on the cell soma and a
subset of distal processes. Collectively, these data support the view that
astrocytes, like neurons, express GABAA receptors and target subunit
isoforms to distinct cellular localizations. Astrocytic GABAA receptors may
be involved in both [Cl-]o and [pH]o homeostasis, and a GABA-evoked
increase in [Ca2+]i could serve as a signal between GABAergic neurons and
astrocytes.
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