Modulation of GABAergic and glutamatergic transmission by ethanol in the developing neocortex: an in vitro test of the excessive inhibition hypothesis of fetal alcohol spectrum disorder.
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Exposure to ethanol during development triggers neuronal cell death and this is thought to play a central role in the pathophysiology of fetal alcohol spectrum disorder (FASD). Studies suggest that ethanol-induced neurodegeneration during the period of synaptogenesis results from widespread potentiation of GABA(A) receptors and inhibition of NMDA receptors throughout the brain, with neocortical layer II being particularly sensitive. Here, we tested whether ethanol modulates the function of these receptors during this developmental period using patch-clamp electrophysiological and Ca(2+) imaging techniques in acute slices from postnatal day 7-9 rats. We focused on pyramidal neurons in layer II of the parietal cortex (with layer III as a control). Ethanol (70mM) increased spontaneous action potential-dependent GABA release in layer II (but not layer III) neurons without affecting postsynaptic GABA(A) receptors. Protein and mRNA expression for both the Cl(-) importer, NKCC1, and the Cl(-) exporter, KCC2, were detected in layer II/III neurons. Perforated-patch experiments demonstrated that E(Cl)((-)) is shifted to the right of E(m); activation of GABA(A) receptors with muscimol depolarized E(m), decreased action potential firing, and minimally increased [Ca(2+)](i). However, the ethanol-induced increase of GABAergic transmission did not affect neuronal excitability. Ethanol had no effect on currents exogenously evoked by NMDA or AMPA receptor-mediated spontaneous excitatory postsynaptic currents. Acute application of ethanol in the absence of receptor antagonists minimally increased [Ca(2+)](i). These findings are inconsistent with the excessive inhibition model of ethanol-induced neurodegeneration, supporting the view that ethanol damages developing neurons via more complex mechanisms that vary among specific neuronal populations.