Deletion of the gabra2 Gene Results in Hypersensitivity to the Acute Effects of Ethanol but Does Not Alter Ethanol Self Administration

Claire I Dixon,David N Stephens,Sarah L King,Sophie E Walker

doi:10.1371/journal.pone.0047135

Abstract

Human genetic studies have suggested that polymorphisms of the GABRA2 gene encoding the GABAA α2-subunit are associated with ethanol dependence. Variations in this gene also convey sensitivity to the subjective effects of ethanol, indicating a role in mediating ethanol-related behaviours. We therefore investigated the consequences of deleting the α2-subunit on the ataxic and rewarding properties of ethanol in mice. Ataxic and sedative effects of ethanol were explored in GABAA α2-subunit wildtype (WT) and knockout (KO) mice using a Rotarod apparatus, wire hang and the duration of loss of righting reflex. Following training, KO mice showed shorter latencies to fall than WT littermates under ethanol (2 g/kg i.p.) in both Rotarod and wire hang tests. After administration of ethanol (3.5 g/kg i.p.), KO mice took longer to regain the righting reflex than WT mice. To ensure the acute effects are not due to the gabra2 deletion affecting pharmacokinetics, blood ethanol concentrations were measured at 20 minute intervals after acute administration (2 g/kg i.p.), and did not differ between genotypes. To investigate ethanol’s rewarding properties, WT and KO mice were trained to lever press to receive increasing concentrations of ethanol on an FR4 schedule of reinforcement. Both WT and KO mice self-administered ethanol at similar rates, with no differences in the numbers of reinforcers earned. These data indicate a protective role for α2-subunits, against the acute sedative and ataxic effects of ethanol. However, no change was observed in ethanol self administration, suggesting the rewarding effects of ethanol remain unchanged.

Highlights

Human genetic studies have suggested that polymorphisms of the GABRA2 gene encoding the GABAA a2-subunit are associated with ethanol dependence in European American, white American, American plains Indian tribe and Russian populations [1,2,3,4], though, unsurprisingly, this association is not evident in all populations [5,6]
Since facilitated GABAergic transmission via a2subunit containing receptors contributes to the anxiolytic action of benzodiazepines [17,18] and barbiturates [19] and deletion of the a2-subunit gives rise to an anxious phenotype [19], genetic variations may contribute to the ability to cope with early life stress, and lead to an increased likelihood of addiction
There is evidence that the same haplotypic variations in GABRA2 may alter the subjective effects of ethanol ingestion as measured by self-assessment of ethanol-related sensations [20] and mood [21], and studies using a2-subunit mutant mice suggest that alcohol consumption is influenced by the manipulation even at socially relevant concentrations of ethanol [22]. These findings suggest a more direct relationship between ethanol and a2-subunit containing GABAA receptors and may be resolved by considering the role of intermediate neurotransmitters

Summary

Introduction

Human genetic studies have suggested that polymorphisms of the GABRA2 gene encoding the GABAA a2-subunit are associated with ethanol dependence in European American, white American, American plains Indian tribe and Russian populations [1,2,3,4], though, unsurprisingly, this association is not evident in all populations [5,6]. There is emerging evidence that the influence of GABRA2 haplotypes on the development of addictions is due to an interaction with early life stress [11]. Since facilitated GABAergic transmission via a2subunit containing receptors contributes to the anxiolytic action of benzodiazepines [17,18] and barbiturates [19] and deletion of the a2-subunit gives rise to an anxious phenotype [19], genetic variations may contribute to the ability to cope with early life stress, and lead to an increased likelihood of addiction

Methods

Results

Conclusion