Ethanol enhances α ₄ β ₃ δ and α ₆ β ₃ δ γ-aminobutyric acid type A receptors at low concentrations known to affect humans

Abstract

␥-Aminobutyric acid type A receptors (GABARs) have long been implicated in mediating ethanol (EtOH) actions, but so far most of the reported recombinant GABAR combinations have shown EtOH responses only at fairly high concentrations (>60 mM). We show that GABARs containing the ␦-subunit, which are highly sensitive to ␥-aminobutyric acid, slowly inactivating, and thought to be located outside of synapses, are enhanced by EtOH at concentrations that are reached with moderate, social EtOH consumption. Reproducible ethanol enhancements occur at 3 mM, a concentration six times lower than the legal blood-alcohol intoxication (driving) limit in most states (0.08% wt͞vol or 17.4 mM). GABARs responsive to these low EtOH concentrations require the GABAR ␦-subunit, which is thought to be associated exclusively with ␣4-and ␣6-subunits in vivo, and the ␤3-subunit, which has recently been shown to be essential for the in vivo anesthetic actions of etomidate and propofol. GABARs containing ␤2-instead of ␤3-subunits in ␣4␤␦-and ␣6␤␦-receptor combinations are almost 10 times less sensitive to EtOH, with threshold enhancement at 30 mM. GABARs containing ␥2-instead of ␦-subunits with ␣4␤ and ␣6␤ are three times less sensitive to EtOH, with threshold responses at 100 mM, a concentration not usually reached with social EtOH consumption. These combined findings suggest that ''extrasynaptic'' ␦-subunit-containing GABARs, but not their ''synaptic'' ␥-subunit-containing counterparts, are primary targets for EtOH. Despite the fact that ethanol (EtOH) is the most widely used psychoactive agent, its actions on brain functions are poorly understood. Several types of receptors and channels have been shown to be functionally altered by EtOH, which include Nmethyl-D-aspartate (1) and non-N-methyl-D-aspartate glutamate receptors (2, 3), serotonin (4), glycine (5, 6), and GABARs (7,8), and G protein-coupled inwardly rectifying K ϩ channels (9, 10). With a few exceptions (3,(8)(9)(10)(11)(12), EtOH effects on these targets are seen only at fairly high concentrations (Ն60 mM).The GABAR, the major inhibitory neurotransmitter receptor, has been a long-time focus for studies on EtOH and anesthetic actions. For example, it has been shown that EtOH at low intoxicating concentrations was able to enhance Cl Ϫ flux in synaptoneurosomes (13,14) and cultured neurons (15). However, electrophysiological studies of GABARs in single neurons and recombinant receptors showed current enhancement only at fairly high concentrations (Ͼ50 mM) of EtOH (5,7,16), which now appears to be due to the fact that these studies focused on synaptic and͞or ␥-subunit-containing receptors. It is thought that replacement of the ␥-subunit in the GABAR 2␣-2␤-1␥ pentameric complex by the ␦-subunit changes not only the localization of the receptor from mainly postsynaptic to extrasynaptic, but also leads to up to a 50-fold increase in ␥-aminobutyric acid (GABA) affinity and slower desensitization (17)(18)(19). These functional properties are consistent with ␣␤␦ GABARs, which are activat...

A Conceptualization of Integrated Actions of Ethanol Contributing to its GABAmimetic Profile: A Commentary

“…As noted earlier, low ethanol concentrations (3-30 mM) enhance the effects of GABA from transfected a 4 b X d and a 6 b X d GABA A receptor subunits expressed in vitro in oocytes (Sundstrom-Poromaa et al, 2002;Wallner et al, 2003). Consistent with ethanol having an effect on neurons with BZD-insensitive receptors, low concentrations of ethanol also selectively augmented the tonic GABA inhibition of granule cells that contain the a 4 and d GABA A receptor subunits, but did not have an effect on CA1 pyramidal cells containing the a 5 b 3 g 2/3 subunits (Wei et al, 2004).…”

“…Roberto et al (2003) recently reported that 44 mM ethanol enhanced the effect of exogenously applied GABA to amygdaloid neurons, indicative that ethanol had a direct postsynaptic action on GABA A receptors at this site. Finally, the specific a 4 b X d and a 6 b X d GABA A receptor subunits (BZD-insensitive receptor subtypes), when expressed in oocytes, exhibited a direct action of low ethanol concentration enhancement of GABA responsiveness (Sundstrom-Poromaa et al, 2002;Wallner et al, 2003). More will be said later about this latter observation.…”

“…Regardless, with current evidence not being persuasive that ethanol has a direct effect on zolpidem-sensitive receptors to account for its GABAmimetic profile, new initiatives were needed to understand the overwhelming behavioral and in vivo electrophysiological studies linking ethanol to a GABAlike action. Of course, one proposed component that could contribute to its GABAmimetic profile would be a direct action of ethanol on selected BZD-insensitive GABA A receptors (Sundstrom-Poromaa et al, 2002;Wallner et al, 2003). However, as will be discussed, these BZD-insensitive GABA A receptors are believed to have limited localization in brain (Peng et al, 2002).…”

“…It is recognized that GABA has an extremely high affinity for some BZD-insensitive receptors containing the d subunit, but a relatively low efficacy (Wallner et al, 2003). In some cases, the BDZ-insensitive receptors carry a major portion of the inhibitory current (Mody, 2001;Nusser and Mody, 2002;Hamann et al, 2002).…”

“…Given the sensitivity of these BZD-insensitive receptor subtypes to ethanol, work like that previously undertaken with 200 mM ethanol to identify critical sites for ethanol action should be performed at low ethanol concentrations (ie o60 mM) on the BZDinsensitive GABA A receptor subtypes to define the molecular basis of the difference between these receptors and those subtypes only sensitive to a high level of ethanol (ie 4100 mM). (Hanchar et al, 2004;Sundstrom-Poromaa et al, 2002;Wallner et al, 2003) could be a means by which ethanol is capable of enhancing GABA transmission in selected brain regions. Additionally, systemic administration of ethanol is proposed to result in an increase in GABA release from presynaptic terminals, as illustrated (right side under ETOH).…”