GABA(A) receptor beta(2) subunit mRNA content is differentially regulated in ethanol-dependent DBA/2J and C57BL/6J mice

Ethanol-induced locomotor sensitization in DBA/2J mice is associated with alterations in GABA(A) subunit gene expression and behavioral sensitivity to GABA(A) acting drugs

Repeated exposure to ethanol may produce increased sensitivity to its acute locomotor stimulant actions, a process referred to as locomotor sensitization. Neuroadaptation within certain brain circuits, including those possessing GABA(A) receptors, may underlie locomotor sensitization to ethanol. Indeed, GABA(A) receptors are documented mediators of ethanol's cellular and behavioral actions. Moreover, because subunit composition of this receptor is predictive of its pharmacology, it is possible that alterations in subunit composition contribute to the expression of locomotor sensitization to ethanol. The goal of the present study was to determine if alterations in GABA(A) subunit composition are associated with the expression of locomotor sensitization in DBA/2J mice, a strain known to be particularly susceptible to the development of this behavioral phenomenon. Following a modified 14day sensitization procedure (Phillips et al., 1994) relative changes in GABA(A) subunit gene expression were assessed in discrete mesolimbic brain regions. To determine if the observed changes in gene expression produced functional changes in the locomotor responses to drugs known to either preferentially or generally activate GABA(A) receptors normally possessing the significantly altered subunits, separate cohorts of animals were challenged with one of several low doses of zolpidem (alpha1-selective), etomidate (beta2/3-selective), or flurazepam (gamma2-directed) and assessed for locomotor alterations. Sensitized animals displayed increased expression of the alpha1, beta2, and gamma2 (v1) subunits in the Nucleus Accumbens (NAc) but not Ventral Tegmental Area (VTA). Additionally, sensitized animals displayed altered sensitivity to the locomotor actions of etomidate and flurazepam. These results support the hypothesis that neuroadaptive changes in GABA(A) subunit composition participate in the expression of locomotor sensitization.

Association between GABRA1 and drinking behaviors in the collaborative study on the genetics of alcoholism sample

BACKGROUND

A wealth of literature supports the role of gamma-aminobutyric acid (GABA) in neurobiological pathways contributing to alcohol dependence and related phenotypes. Animal studies have consistently tied rodent homologs of the GABAA receptor genes on human chromosome 5q to alcohol-related behaviors; however, human studies have produced mixed results. Family-based association analyses previously conducted in the Collaborative Study on the Genetics of Alcoholism (COGA) sample yielded no evidence of association with Diagnostic and Statistical Manual of Mental Disorder-fourth edition (DSM-IV) alcohol dependence and these genes. As a follow-up to that study, we examined several alcohol-related behaviors in the COGA sample as follows: (1) a broader definition of alcohol dependence, including DSM-III-R symptoms and Feighner criteria (referred to as COGA alcohol dependence); (2) withdrawal; (3) history of alcohol-induced blackouts; (4) level of response to alcohol; (5) age of onset of regular drinking; and (6) age at first drunkenness.

METHODS

Family-based association tests were conducted, using multiple single-nucleotide polymorphisms (SNPs) in each of the 4 GABAA receptor genes on chromosome 5q.

RESULTS

In GABRA1, we found evidence of association with several of the drinking behavior phenotypes, including COGA alcohol dependence, history of blackouts, age at first drunkenness, and level of response to alcohol. We did not find consistent evidence of association with the remaining genes and any of the phenotypes.

CONCLUSIONS

We found evidence for association between GABRA1 and COGA alcohol dependence, history of blackouts, age at first drunkenness, and level of response to alcohol. These analyses suggest that efforts to characterize genetic contributions to alcohol dependence may benefit by examining alcohol-related behaviors in addition to clinical alcohol dependence diagnoses.

Alcohol and gene expression in the central nervous system

AIMS

To describe recent research focusing on the analysis of gene and protein expression relevant to understanding ethanol consumption, dependence and effects, in order to identify common themes.

METHODS

A selective literature search was used to collate the relevant data.

RESULTS

Over 160 genes have been individually assessed before or after ethanol administration, as well as in genetically selected lines. Techniques for studying gene expression include northern blots, differential display, real time reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. More recently, high throughput functional genomic technology, such as DNA microarrays, has been used to examine gene expression. Recent gene expression analyses have dramatically increased the number of candidate genes (nine array papers have illuminated 600 novel gene transcripts that may contribute to alcohol abuse and alcoholism).

CONCLUSIONS

Although functional genomic experiments (transcriptome analysis) have failed to identify a single alcoholism gene, they have illuminated important pathways and gene products that may contribute to the risk of alcohol abuse and alcoholism.

The role of pregnane neurosteroids in ethanol withdrawal: behavioral genetic approaches

Within the last 20 years, rapid nongenomic actions of steroid hormones have been demonstrated to occur via an interaction with ligand-gated ion channels. For example, the pregnane neurosteroid allopregnanolone (ALLOP) is a potent positive modulator of gamma-aminobutyric acid(A) (GABA(A)) receptors. The physiological significance of fluctuations in endogenous ALLOP levels has been investigated with regard to disease states and the effect of therapeutic agents on ALLOP levels. Because the pharmacological profile of ALLOP is similar to that of ethanol (EtOH), the modulatory effect of pregnane neurosteroids on EtOH dependence and withdrawal will be the focus of this review. Data on the effects of chronic EtOH exposure and withdrawal on pregnane neurosteroid levels, biosynthetic enzymes, and changes in neurosteroid sensitivity will be summarized. Results from genetic animal models indicate that seizure-prone animals have a persistent decrease in endogenous ALLOP levels during EtOH withdrawal in conjunction with tolerance to ALLOP's anticonvulsant effect. Manipulation of endogenous ALLOP levels with finasteride also markedly reduced the severity of chronic EtOH withdrawal. Gene mapping studies provide a hint for an interaction between genes for GABA(A) receptor subunits and the biosynthetic enzyme 5alpha-reductase. Overall, the results are suggestive of a relationship between endogenous pregnane neurosteroid levels and behavioral changes in excitability during EtOH withdrawal, consistent with recent findings in humans. While the findings with ALLOP emphasize the therapeutic potential of neurosteroid treatment during EtOH withdrawal, the gene mapping studies suggest that pregnane neurosteroid biosynthesis may represent a target for therapeutic intervention in the treatment of alcohol dependence.

Differences in ionotropic glutamate receptor subunit expression are not responsible for strain-dependent susceptibility to excitotoxin-induced injury

Systemic administration of kainic acid in C57BL/6 and FVB/N mice induces a comparable level of seizure induction yet results in differential susceptibility to seizure-induced cell death. While kainate administration causes severe hippocampal damage in mice of the FVB/N strain, C57BL/6 mice display no demonstrable cell loss or damage. At present, while the cellular mechanisms underlying strain-dependent differences in susceptibility remain unclear, some of this variation is assumed to have a genetic basis. As glutamate receptors are thought to participate in seizure induction and the subsequent neuronal degeneration that ensues, previous studies have proposed that variation in the precise subunit composition of glutamate receptors may result in differential susceptibility to excitotoxic cell death. Thus, we chose to examine the relationship between the cellular distribution and expression of glutamate receptor subunit proteins and cell loss within the hippocampus in mouse strains resistant and susceptible to kainate-induced excitotoxicity. Using semi-quantitative Western blot techniques and immunohistochemistry with the use of antibodies that recognize subunits of the KA (GluR5,6,7), AMPA (GluR1, GluR2, and GluR4), and NMDA (NMDAR1 and NMDAR2A/2B) receptors, we found no significant strain-dependent differences in the expression or distribution of these glutamate receptor subunits in the intact hippocampus. Following kainate administration, expression changes in ionotropic glutamate receptor subunits paralleled the development of susceptibility to cell death in the FVB/N strain only. Strain differences in hippocampal vulnerability to kainate-induced status epilepticus are not due to glutamate receptor protein expression.

Role of GABA abnormalities in the inferior colliculus pathophysiology - audiogenic seizures

gamma-Aminobutyric acid (GABA), acting at GABA(A) receptors, mediates inhibition in inferior colliculus (IC) central nucleus (ICc) neurons and plays a prominent role in mediating acoustically evoked non-monotonicity, offset inhibition, and binaural inhibition, and is also important in tonic inhibition. The IC plays an important role in a number of pathophysiological conditions that involve hearing, including tinnitus, age-related hearing loss, and audiogenic seizures (AGS). AGS are a major form of rodent neurological disorder that can be genetically mediated and can also be readily induced in both young and mature animals. A deficit in GABA-mediated inhibition in IC neurons has been shown to be a critical mechanism in genetic and induced forms of AGS. Thus, both endogenously evoked GABA-mediated inhibition and exogenously applied GABA are reduced in efficacy in IC neurons of rats that are susceptible to AGS. GABA-mediated inhibition in IC neurons is significantly more easily blocked by a GABA(A) antagonist in genetic and induced forms of AGS in vivo and in vitro. AGS can be induced in normal animals by treatments that reduce the effectiveness of GABA in the IC. Glutamate-mediated excitation is a critical element of neurotransmission in IC neurons, and excessive activation of glutamate receptors in the IC is also strongly implicated as the other major mechanism in the pathophysiology of AGS. These neurotransmitter abnormalities result in excessive firing of ICc neurons that acts as the critical initiation mechanism for triggering seizures in response to intense acoustic stimuli.

Expression profiling identifies strain-specific changes associated with ethanol withdrawal in mice

Mice that exhibit characteristics of physical dependence following ethanol exposure serve as useful models of alcoholism in humans. The DBA/2J and C57BL/6J inbred strains differ in their behavioral response to ethanol withdrawal. Alterations in gene expression are believed to underlie neuroadaptation to ethanol dependence and tolerance. Therefore, the differences in ethanol withdrawal severity observed between the DBA/2J and C57BL/6J strains may be related to differential regulation of gene expression. We have used cDNA microarrays to determine the gene expression profile in the hippocampus of DBA/2J and C57BL/6J mice during withdrawal after chronic and acute ethanol exposure. Of the 7634 genes surveyed, approximately 2% were consistently differentially expressed by at least 1.4-fold in DBA/2J mice during chronic ethanol withdrawal. Less than 1% of the genes showed altered expression in C57BL/6J mice under the same conditions, or in DBA/2J mice during acute ethanol withdrawal. Strain- and treatment-specific patterns of altered expression were observed for multiple genes associated with the Janus kinase/signal transducers and activators of transcription and the mitogen activated protein kinase pathways. Genes associated with both pathways are regulated in DBA/2J mice during chronic ethanol withdrawal, and to a lesser extent during acute ethanol withdrawal. Only those genes associated with the mitogen-activated protein kinase (MAPK) pathway exhibited changes in expression in C57BL/6J mice during ethanol withdrawal. Furthermore, genes associated with retinoic acid-mediated signaling show differential expression exclusively in C57BL/6J mice. These findings represent significant differences in cellular adaptation to ethanol between the DBA/2J and C57BL/6J strains.

Glutamate-mediated transmission, alcohol, and alcoholism

Glutamate-mediated neurotransmission may be involved in the range of adaptive changes in brain which occur after ethanol administration in laboratory animals, and in chronic alcoholism in human cases. Excitatory amino acid transmission is modulated by a complex system of receptors and other effectors, the efficacy of which can be profoundly affected by altered gene or protein expression. Local variations in receptor composition may underlie intrinsic regional variations in susceptibility to pathological change. Equally, ethanol use and abuse may bring about alterations in receptor subunit expression as the essence of the adaptive response. Such considerations may underlie the regional localization characteristic of the pathogenesis of alcoholic brain damage, or they may form part of the homeostatic change that constitutes the neural substrate for alcohol dependence.