Changes in Intracellular Ca2+ Induced by GABAA Receptor Activation and
Reduction in Cl- Gradient in Neonatal Rat Neocortex.
ATSUO FUKUDA, KANJI MURAMATSU, AKIHITO OKABE, YASUNOBU SHIMANO, HIDEKI HIDA,
ICHIRO FUJIMOTO, AND HITOO NISHINO.
Department of Physiology, Nagoya City University Medical School, Nagoya,
Aichi 467, Japan.
APStracts 4:0241N, 1997.
ABSTRACT
We have studied the effects of g-aminobutyric acid (GABA) and of reducing the
Cl- gradient on the intracellular concentration of Ca2+ ([Ca2+]i) in pyramidal
neurons of rat somatosensory cortex. The Cl- gradient was reduced either with
furosemide or by oxygen-glucose deprivation. Immature slices taken at
postnatal day (P) 7-14 were labeled with fura-2, and [Ca2+]i was monitored in
identified pyramidal cells in layer II/III as the ratio of fluorescence
intensities (RF340/F380). The magnitude of the [Ca2+]i increases induced by
oxygen-glucose deprivation was significantly reduced (by 44%) by bicuculline
(10 mM), a GABAA receptor antagonist. Under normal conditions, GABA generally
did not raise [Ca2+]i, though in some neurons a small and transient [Ca2+]i
increase was observed. These transient [Ca2+]i increases were blocked by Ni2+
(1 mM), a blocker of voltage-dependent Ca2+ channels (VDCCs). Continuous
perfusion with GABA did not cause a sustained elevation of [Ca2+]i, but
bicuculline caused [Ca2+]i oscillations. After inhibition of Cl- extrusion
with furosemide (1.5 mM), GABA induced a large [Ca2+]i increase consisting of
an initial peak followed by a sustained phase. Both the initial and the
sustained phases were eliminated by bicuculline (10 mM). The initial but not
the sustained phase was abolished by Ni2+. In the presence of Ni2+, the
remaining sustained response was inhibited by the addition of 2-amino-5-
phosphonopentanoic acid (AP5, 20 mM), a selective N-methyl-D-aspartate (NMDA)
receptor antagonist. Thus, the initial peak and the sustained phase of the
GABA-evoked [Ca2+]i increase were mediated by Ca2+ influx through VDCCs and
NMDA receptor-channels, respectively, while both phases were initiated via the
GABAA receptor. These results indicate that, in neocortical pyramidal neurons,
a reduction in the Cl- gradient converts the GABAA receptor-mediated action
from nothing or virtually nothing to a large and sustained accumulation of
cellular Ca2+. This accumulation is due to Ca2+ influx mainly through the NMDA
receptor-channel. Thus GABA, normally an inhibitory transmitter, may play an
aggravating role in excitotoxicity if a shift in the Cl- equilibrium potential
occurs, as reported previously, during cerebral ischemia.
Received 27 February 1997; accepted in final form 10 September 1997.
APS Manuscript Number J172-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 October 1997