Changes in Intracellular Ca2+ Induced by GABAA Receptor Activation and Reduction in Cl− Gradient in Neonatal Rat Neocortex

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Abstract
Fukuda, Atsuo, Kanji Muramatsu, Akihito Okabe, Yasunobu Shimano, Hideki Hida, Ichiro Fujimoto, and Hitoo Nishino. Changes in intracellular Ca2+ induced by GABAA receptor activation and reduction in Cl− gradient in neonatal rat neocortex. J. Neurophysiol. 79: 439–446, 1998. We have studied the effects of γ-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 μM), 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 μM). 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 μM), 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.