Research ReportEffect of chronic administration of ethanol on the regulation of the δ-subunit of GABAA receptors in the rat brain
Introduction
Extensive structural heterogeneity exists among various subtypes of GABAA receptor since native pentameric receptor assemblies are derived from several subunits such as α1–6, β1–3, γ1–3, δ, ε, π and θ (see Mehta and Ticku, 1999a, Sieghart et al., 1999, Whiting, 1999). Most common subunits combination for GABAA receptors in the brain is α1β2γ2 (Fritschy et al., 1992). GABAA receptors gene expression is affected by physiological and pathological processes as well as by the drugs that modulate GABAA receptors (Aguayo et al., 2002, Cagetti et al., 2003, Follesa et al., 2003, Sanna et al., 2003). Ethanol is known to modulate the activity of a variety of receptors and ion channels (Lovinger et al., 1989, Lovinger and White, 1991, Lovinger, 1999, Lewohl et al., 1999, Mihic, 1999, Narahashi et al., 1999, Woodward, 1999, Walter and Messing, 1999, Weight et al., 1999, Lei et al., 2000, Aguayo et al., 2002, Carta et al., 2003, Davies et al., 2003, Roberto et al., 2003, Gehlert et al., 2007), and the role of GABAA receptors in the behavioral effects of ethanol is well documented (see Ticku and Mehta, 1995).
Chronic ethanol (CE) as well as chronic intermittent ethanol (CIE) treatment is known to modulate the expression of the major subunits of GABAA receptors in the brain (Montpied et al., 1991, Mhatre and Ticku, 1992, Mhatre and Ticku, 1994, Devaud et al., 1995, Devaud et al., 1997; see Mehta and Ticku, 1999a, Cagetti et al., 2003, Marutha Ravindran and Ticku, 2006a, Marutha Ravindran and Ticku, 2006b, Marutha Ravindran et al., 2007). Recently, it has been suggested that the GABAA receptor derived from δ-subunit is important site for the pharmacological actions of ethanol (Mihalek et al., 2001, Sundstrom-Poromaa et al., 2002, Cagetti et al., 2003, Wallner et al., 2003, Wei et al., 2004, Hanchar et al., 2004, Glykys et al., 2007). On the other hand, there are also reports contradicting the involvement of the δ-subunit-containing GABAA receptors in the pharmacology of clinically relevant low concentrations of ethanol (Borghese et al., 2006, Yamashita et al., 2006, Casagrande et al., 2007). Recently, we have reported that ethanol (up to 50 mM) does not affect [3H]muscimol binding to the immunoprecipitated δ-subunit-containing GABAA receptor assemblies in the rat cerebellum and hippocampus, thereby suggesting that the native δ-subunit-containing GABAA receptors do not play a major role in the pharmacology of clinically relevant low concentrations of ethanol (Mehta et al., 2007). In the present study, we investigated the effect of CE administration on the polypeptide levels of the δ-subunit of GABAA receptors in the rat cerebellum, hippocampus and cerebral cortex. Further, we investigated the effect of chronic administration of ethanol on the regulation of native δ-subunit-containing GABAA receptor assemblies in the rat cerebellum so as to examine whether δ-subunit-containing GABAA receptors are a potential target site for the chronic ethanol-induced phenomena.
Section snippets
Effect of CE administration on the polypeptide levels of the δ-subunit of GABAA receptors in the rat brain
Western blot experiments revealed that chronic administration of ethanol causes down-regulation of the polypeptide levels of the δ-subunit of GABAA receptors in the rat cerebellum (Fig. 1Aa, c; ANOVA F2,6 = 43.59, p = 0.0003; Dunnett's multiple comparison p < 0.01) and hippocampus (Fig. 1Ba, c; ANOVA F2,6 = 29.79, p = 0.0008; Dunnett's multiple comparison p < 0.01). These changes in the polypeptide levels of δ-subunit reverted back to control levels in the ethanol-withdrawn group (48 h) as shown in Figs. 1
Discussion
Role of GABAA receptors in the pharmacology of ethanol has been investigated in detail (see Ticku and Mehta, 1995, Mehta and Ticku, 1999a, Sieghart et al., 1999, Whiting, 1999, Hanchar et al., 2004). The subunit composition and stoichiometry of native GABAA receptors are currently unknown, but the most abundant population of native GABAA receptors in the mammalian brain is believed to be the α1β2γ2 subunit combination (Benke et al., 1991, Fritschy et al., 1992, McKernan and Whiting, 1996), and
Experimental procedures
Adult male Sprague–Dawley rats (Harlan, Indianapolis, IN, USA) weighing 200–250 g were maintained at a constant room temperature (22 °C) on a 12-h light/12-h dark cycle. All experiments were conducted in accordance with the Declaration of Helsinki and/or with the Guide for the Care and Use of Laboratory Animals, as adopted and promulgated by the National Institutes of Health. Adequate measures were taken to minimize pain or discomfort to the animals. Food and water were available ad libitum.
Acknowledgment
This research work was supported by National Institute on Alcohol and Alcohol Abuse (NIAAA) grant AA10552.
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