Differential alterations in GABA-A receptor expression and function induced by ethanol of hippocampal and cerebellar granule cells

Gamma-aminobutyric acid type A (GABAA) receptors have been implicated as main target sites for the acute and chronic actions of ethanol (EtOH) in the central nervous system. Prolonged exposure to and withdrawal of EtOH are associated to selective and opposite alterations in GABAA receptor subunit gene expression as well as in receptor function and pharmacological sensitivity in different in vivo and in vitro experimental models [1]. Here, we focus on the effects of chronic exposure to and withdrawal of EtOH on the expression of the 6. subunit of the GABAA receptor in cultured rat hippocampal (HP) and cerebellar granule (CG) neu- rons. GABAA receptor containing the 6. subunit are preferentially extrasynaptic, are responsible for the tonic inhibition [2], and possess an enhanced sensitivity to the agonist THIP as well as to neurosteroids and low concentrations of EtOH [3]. Long-term (5 days) exposure to 100 mM EtOH increased 6. subunit mRNA and peptide levels in HP cells, while it did not significantly modify its expression in CG cells. EtOH withdrawal was associated to a persistent enhancement of 6. subunit expression in HP cells, and to a marked reduction of in CG cells, compared to the relative vehicle-treated control neurons. Whole-cell patch clamp record- ings were carried out in order to determine whether these EtOH- induced changes in 6. subunit expression could be associated, in HP and CG cells, to related alterations in the efficacy of THIP and allopregnanolone (AP). Concentration-response curves for THIP revealed that in HP cells, chronic EtOH exposure and 6-hr withdrawal increased (EC50, 18±3 ~tM and 31±2~tM; p <0.05, respectively) the potency of this agonist compared to control cells (EC50, 72±3 ~tM). The modulatory efficacy of 0.3 ~tM AP on THIP-evoked C1- currents was increased by 2.7 fold in HP neurons subjected to chronic EtOH exposure, an effect that remained significant during withdrawal for 6 hr, compared to control cells. In GC cells, THIP potency was not altered following chronic EtOH exposure, but it was markedly reduced (EC50, 51±2 ~tM; p <0.05) compared to control cells (EC50, 18±1 ~tM). Chronic EtOH exposure did not change the effect of 1~tM AP on THIP- evoked C1- currents, whereas withdrawal of EtOH for 6 hr was associated to a significant reduction of AP efficacy. These data demonstrate that the changes in gene expression and function of GABAA receptors containing the 6. subunit induced by long-term EtOH exposure and withdrawal are opposite in HP and CG cells, and suggest a different role of these extrasynaptic receptors in controlling tonic inhibition in these two different cell types. References [1] Kumar S. et al. Pharmacol Ther. 2004 Mar; 101(3): 211 26. [2] Semyanov A et al. Trends Neurosci. 2004 May; 27(5): 262 9. [3] Wei W. et al. J Neurosci. 2004 Sep 22; 24(38): 8379 82.