Chronic Intermittent Ethanol-Induced Switch of Ethanol Actions from Extrasynaptic to Synaptic Hippocampal GABA_AReceptors

Abstract

Alcohol withdrawal syndrome (AWS) symptoms include hyperexcitability, anxiety, and sleep disorders. Chronic intermittent ethanol (CIE) treatment of rats with subsequent withdrawal of ethanol (EtOH) reproduced AWS symptoms in behavioral assays, which included tolerance to the sleep-inducing effect of acute EtOH and its maintained anxiolytic effect. Electrophysiological assays demonstrated a CIE-induced long-term loss of extrasynaptic GABA A receptor (GABA A R) responsiveness and a gain of synaptic GABA A R responsiveness of CA1 pyramidal and dentate granule neurons to EtOH that we were able to relate to behavioral effects. After CIE treatment, the ␣4 subunit-preferring GABA A R ligands 4,5,6,7 tetrahydroisoxazolo[5,4-c]pyridin-3-ol, La 3ϩ , and Ro15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5␣][1,4]benzodiazepine-3-carboxylate) exerted decreased effects on extrasynaptic currents but had increased effects on synaptic currents. Electron microscopy revealed an increase in central synaptic localization of ␣4 but not ␦ subunits within GABAergic synapses on the dentate granule cells of CIE rats. Recordings in dentate granule cells from ␦ subunit-deficient mice revealed that this subunit is not required for synaptic GABA A R sensitivity to low [EtOH]. The profound alterations in EtOH sensitivity and ␣4 subunit localization at hippocampal GABA A Rs of CIE rats suggest that such changes in these and other relevant brain circuits may contribute to the development of tolerance to the sleep-inducing effects and long-term dependence on alcohol.

Altered Localization of GABAAReceptor Subunits on Dentate Granule Cell Dendrites Influences Tonic and Phasic Inhibition in a Mouse Model of Epilepsy

“…Methods for freeze substitution and low-temperature embedding have been described previously (Wei et al, 2003;Liang et al, 2006). Briefly, cryoprotected sections were rapidly plunged into liquid propane cooled by liquid nitrogen to Ϫ190°C in a cryofixation unit (EM CPC; Leica, Wien, Austria).…”

“…Ultrathin sections were cut on a microtome (Reichert-Jung, Vienna, Austria), picked up on nickel mesh grids that were freshly coated with a Coat-Quick "G" pen (Electron Microscopy Sciences), and air-dried at room temperature. The tissue was processed with slight modifications of the immunogold labeling methods of Matsubara et al (1996), and the current methods have been used previously for GABA A R subunit localization (Wei et al, 2003;Liang et al, 2006). Briefly, ultrathin sections were treated with 0.2% sodium hydroxide in distilled water for 5 min, and then with 0.1% sodium borohydride in 0.01 M Tris-buffered saline (TBS), pH 7.4, for 10 min.…”

“…Interestingly, changes in these subunits could alter both tonic and phasic inhibition in the dentate gyrus. Normally, in the forebrain, the ␦ and ␣4 subunits are preferential partners in many GABA A Rs that mediate tonic inhibition (Sur et al, 1999;Jia et al, 2005) and are located primarily at perisynaptic and extrasynaptic sites (Nusser et al, 1998b;Wei et al, 2003;Sun et al, 2004;Liang et al, 2006). In contrast, the ␥2 subunit plays a major role in phasic inhibition (Nusser and Mody, 2002) and is located directly at many GABAergic synapses, as well as some extrasynaptic sites (Somogyi et al, 1996;Nusser et al, 1998b;Sassoè-Pognetto et al, 2000).…”

The role of GABAA receptors in the development of alcoholism

“…Moreover, the balance between tonic and phasic inhibition may shift towards the direction of phasic inhibition during chronic ethanol consumption. Rats exposed to the CIE model of binge drinking (described later) showed a net loss of extrasynaptic, and a gain of synaptic, hippocampal GABA A receptor responsiveness to ethanol concomitant with an increase in central synaptic localization of α 4 , but not δ subunits (Liang et al, 2006). The fact that this switch corresponds with the development of tolerance to the sedating / hypnotic effects of alcohol suggests that it may be important in the pathway to addiction (Liang et al, 2006).…”

The effects of chronic ethanol administration on amygdala neuronal firing and ethanol withdrawal seizures

“…Thus, the tolerance of LAMG neuronal firing to the first dose of binge ethanol observed in the present study may be related to adaptive changes of NMDA receptors induced by CIE administration. It has been observed that chronic ethanol treatment also reduces the action of GABA (Kang et al, 1996;Faingold et al, 1998), which may involve GABA A receptor subunit alterations in the brain including the amygdala (Devaud et al, 1995;Grobin et al, 2000;Floyd et al, 2004;Liang et al, 2006). Therefore, a decrease in GABA A receptor function may also contribute to the tolerance of LAMG neuronal firing to ethanol following CIE administration.…”