Changes in intracellular Ca2+ induced by GABAA receptor activation and reduction in Cl- gradient in neonatal rat neocortex

J Neurophysiol. 1998 Jan;79(1):439-46. doi: 10.1152/jn.1998.79.1.439.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • 2-Amino-5-phosphonovalerate / pharmacology
  • Animals
  • Animals, Newborn
  • Bicuculline / pharmacology*
  • Calcium / metabolism*
  • Cell Hypoxia / physiology*
  • Chlorides / metabolism*
  • Excitatory Amino Acid Antagonists / pharmacology
  • Fura-2
  • Furosemide / pharmacology*
  • Glucose / metabolism
  • In Vitro Techniques
  • Kinetics
  • Models, Neurological
  • Nickel / pharmacology
  • Pyramidal Cells / drug effects
  • Pyramidal Cells / physiology*
  • Rats
  • Rats, Wistar
  • Receptors, GABA-A / drug effects
  • Receptors, GABA-A / physiology*
  • Receptors, N-Methyl-D-Aspartate / physiology
  • Somatosensory Cortex / physiology*
  • gamma-Aminobutyric Acid / pharmacology*

Substances

  • Chlorides
  • Excitatory Amino Acid Antagonists
  • Receptors, GABA-A
  • Receptors, N-Methyl-D-Aspartate
  • gamma-Aminobutyric Acid
  • 2-Amino-5-phosphonovalerate
  • Furosemide
  • Nickel
  • Glucose
  • Calcium
  • Fura-2
  • Bicuculline