GABA(A) receptors are sensitive to ethanol in distinct brain regions and are clearly involved in the acute actions of ethanol, ethanol tolerance, ethanol dependence and ethanol self-administration. Data from a variety of perspectives such as molecular, cellular and behavioral analysis have elucidated the role of GABA(A) receptors in these processes. GABA(A) receptor activation mediates many of the behavioral effects of ethanol including motor incoordination, anxiolysis and sedation. The actions of ethanol at GABA(A) receptors are influenced by endogenous modulators such as the neuroactive steroids. Sensitization to these compounds influences ethanol dependence and withdrawal and may explain gender differences in the molecular effects of ethanol. Furthermore, GABA(A) receptors may also play a role in ethanol self-administration via the mesolimbic reward system. Ethanol tolerance and dependence may be explained, in part, by changes in the function of GABA(A) receptors. We have proposed that alterations in native GABA(A) receptor subunit assembly could alter the functional properties of these receptors. However, post-translational modifications or other post-synaptic mechanisms may also explain changes in GABA(A) receptor function. Genetic animal models of ethanol dependence have also identified GABA(A) receptor genes as likely mediators of the behavioral adaptations associated with ethanol dependence and withdrawal. A better understanding of the effects of ethanol at GABA(A) receptors has highlighted important potential mechanisms involved in the development of alcoholism.

Neuroactive steroids are synthesized de novo in brain, yet their physiological significance remains elusive. We provide biochemical, electrophysiological, and behavioral evidence that several specific actions of alcohol (ethanol) are mediated by the neurosteroid 3␣-hydroxy-5␣-pregnan-20-one (3␣,5␣-THP; allopregnanolone). Systemic alcohol administration elevates 3␣,5␣-THP levels in the cerebral cortex to pharmacologically relevant concentrations. The elevation of 3␣,5␣-THP is doseand time-dependent. Furthermore, there is a significant correlation between 3␣,5␣-THP levels in cerebral cortex and the hypnotic effect of ethanol. Blockade of de novo biosynthesis of 5␣-reduced steroids using the 5␣-reductase inhibitor finasteride prevents several effects of ethanol. Pretreatment with finasteride causes no changes in baseline bicuculline-induced seizure threshold but reverses the anticonvulsant effect of ethanol. Finasteride pretreatment also reverses ethanol inhibition of spontaneous neural activity in medial septal/diagonal band of Broca neurons while having no direct effect on spontaneous firing rates. Thus, elevation of 3␣,5␣-THP levels by acute ethanol administration represents a novel mechanism of ethanol action as well as an important modulatory role for neurosteroids in the CNS.

The pharmacological properties of y-aminobutyric acidA (GABAA) receptors are altered by prolonged exposure to ethanol both in viva and in vitro. We have shown previously that prolonged ethanol exposure elicits selective alterations in various GABAA receptor subunit mRNA levels in rat cerebral cortex. Some of these effects are rapidly reversed during ethanol withdrawal. The present study was conducted to determine the effects of prolonged ethanol exposure (dependence) and ethanol withdrawal on cerebral cortical peptide expression for several subunits. GABAA receptor cnl subunit peptide levels were decreased by nearly 40%, whereas cr4 subunit peptide levels were increased by 27% in both ethanol-dependent and withdrawn rats. These changes correlate well with observed alterations in mRNA levels following prolonged ethanol exposure in dependent rats, but do not match the effects on mRNA levels during ethanol withdrawal. /32/3 subunit peptide levels increased by -32% in both ethanol-dependent rats and rats undergoing ethanol withdrawal. We observed a 30-60% increase in yl subunit peptide levels in both dependent rats and those undergoing withdrawal, also correlating with the previous report on ethanol-induced alterations in mRNA levels.Peptide levels for y 2 subunits did not differ from control values in either condition. These findings show that specific alterations in GABAA receptor subunit peptide levels are associated with ethanol dependence in rats. GABAA receptor subunit peptide expression is more stable than mRNA expression, and mRNA levels are not representative of peptide expression during ethanol withdrawal. These findings are consistent with the suggestion that alterations in GABAA receptor gene expression underlie the functional properties of GABAA receptors in ethanoldependent rats and those undergoing ethanol withdrawal. Key Words: GABAA receptor-GABAA receptor subunit peptide expression-Ethanol.

Abstract:Previous research has shown that chronic ethanol consumption dramatically alters GABAA receptor a, and a 4 subunit gene expression in the cerebral cortex and GABAA receptor a, and a6 subunit gene expression in the cerebellum. However, it is not yet known if chronic ethanol consumption produces similar alterations in GA-BAA receptor gene expression in other brain regions. One brain region of interest is the hippocampus because it has recently been shown that a subset of GABAA receptors in the hippocampus is responsive to pharmacologically relevant concentrations of ethanol. Therefore, we directly compared the effects of chronic ethanol consumption on GABAA receptor subunit gene expression in the hippocampus and cerebral cortex. Furthermore, we investigated whether the duration of ethanol consumption (14 or 40 days) would influence regulation of GABAA receptor gene expression in these two brain regions. Chronic ethanol consumption produced a significant increase in the level of GABAA receptor a4 subunit peptide in the hippocampus following 40 days but not 14 days. The relative expression of hippocampal GABAA receptor a1, a2, a3, /32/3, or Y2 was not altered by either period of chronic ethanol exposure. In marked contrast, chronic ethanol consumption for 40 days significantly increased the relative expression of cerebral cortical GABAA receptor a4 subunits and significantly decreased the relative expression of cerebral cortical GABAA receptor a, subunits. This finding is consistent with previous results following 14 days of chronic ethanol consumption. Hence, chronic ethanol consumption alters GABAA receptor gene expression in the hippocampus but in a different manner from that in either the cerebral cortex or the cerebellum. Furthermore, these alterations are dependent on the duration of ethanol exposure.

Prolonged alcohol consumption leads to the development of tolerance to and dependence on ethanol, resulting in a decreased response to the sedative/hypnotic effects of ethanol, and by negative symptomatology following abrupt termination of use. One symptom associated with ethanol withdrawal in humans, as well as laboratory animals, is enhanced susceptibility to seizures. This study investigated the effects of the neurosteroid, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha-5 alpha-THP), on alterations in seizure sensitivity associated with ethanol withdrawal. 3 alpha-5 alpha-THP is a potent anxiolytic and anticonvulsant agent that acts via selective interactions with GABAA receptors. Extensive evidence suggests that some aspects of ethanol dependence and withdrawal are mediated by alterations in GABAA receptor function. Withdrawal from chronic ethanol exposure elicited dramatic increases in seizure susceptibility in male and female rats. Administration of 3 alpha-5 alpha-THP just before seizure threshold determinations blocked the increased seizure susceptibility induced by ethanol withdrawal. Ethanol-withdrawn animals were protected by 3 alpha-5 alpha-THP at a dose that had no effect on control animal seizure thresholds. Moreover, male and female rats displayed differential responses to the seizure-threshold lowering effects of ethanol withdrawal, as well as the protection by 3 alpha-5 alpha-THP pretreatment. These findings suggest that there are gender differences associated both with ethanol withdrawal as well as the protection by 3 alpha-5 alpha-THP in ethanol-dependent rats.