Changes in intracellular Ca2+ induced by GABAA receptor activation and reduction in Cl- gradient in neonatal rat neocortex.
We have studied the effects of gamma-aminobutyric acid (GABA) and of reducing the Cl- gradient on the [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 microM), a GABAA receptor antagonist. Under normal conditions, GABA generally did not raise [Ca2+]i, although 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 microM). 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 microM), 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, and 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 the result of 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.
Fukuda
A
A
Department of Physiology, Nagoya City University Medical School, Nagoya, Aichi 467, Japan.
Muramatsu
K
K
Okabe
A
A
Shimano
Y
Y
Hida
H
H
Fujimoto
I
I
Nishino
H
H
D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D001640:Bicuculline / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D015687:Cell Hypoxia / Q000502:physiology*; D002712:Chlorides / Q000378:metabolism*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D016257:Fura-2; D005665:Furosemide / Q000494:pharmacology*; D005947:Glucose / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008959:Models, Neurological; D009532:Nickel / Q000494:pharmacology; D017966:Pyramidal Cells / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000187:drug effects / Q000502:physiology*; D016194:Receptors, N-Methyl-D-Aspartate / Q000502:physiology; D013003:Somatosensory Cortex / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
D002712:Chlorides; D018691:Excitatory Amino Acid Antagonists; D011963:Receptors, GABA-A; D016194:Receptors, N-Methyl-D-Aspartate; D005680:gamma-Aminobutyric Acid; D015763:2-Amino-5-phosphonovalerate; D005665:Furosemide; D009532:Nickel; D005947:Glucose; D002118:Calcium; D016257:Fura-2; D001640:Bicuculline