GABAA Receptor Subunit Expression in Early Adolescent Mice

poster abstractGamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system (CNS). Of the known receptors for GABA, the pentameric GABAA receptor appears to mediate fast GABA neurotransmission in the CNS. A number of different GABAA receptor subunits have been described to date, including α1-6, β1-3, γ1-2, and δ, with the specific functionality of any one receptor dictated by the combination of subunits. Although much is known about the pattern of GABAA subunit expression across the adult brain, the pattern of such expression during the critical developmental period of adolescence, which is a time of rapid neurobiological, hormonal, and behavioral change, remains largely unknown. GABAA receptor systems play a key role in normal neural, hormonal, and behavioral function, warranting a basic understanding of adolescent-specific alterations in the pattern of subunit expression. The current project focuses on determining the pattern of mRNA expression of GABAA receptor subunits in early adolescent (postnatal day 30) versus adult (postnatal day 65) mice. The prefrontal cortex, hippocampus, nucleus accumbens, midbrain, amygdala and cerebellum were harvested at these two ages; thus far the midbrain has been the focus due to the presence of the ventral tegmental area and its well-known role in motivation, reward and drug addiction. Midbrain tissue was processed for determination of GABAA receptor subunit mRNA expression using RT-PCR. We predict that early adolescence will be associated with a unique pattern of GABAA receptor subunit expression, suggesting an important role for GABAA receptors in the neurobiological, hormonal, and behavioral profile of this developmental period in mice. Preliminary findings indicate an increase in GABAA delta subunit expression in the adult midbrain

Activation of extrasynaptic δ-GABAA receptors globally or within the posterior-VTA has estrous-dependent effects on consumption of alcohol and estrous-independent effects on locomotion

Recent reports support higher than expected rates of binge alcohol consumption among women and girls. Unfortunately, few studies have assessed the mechanisms underlying this pattern of intake in females. Studies in males suggest that alcohol concentrations relevant to the beginning stages of binge intoxication may selectively target tonic GABAergic inhibition mediated by GABAA receptor subtypes expressing the δ-subunit protein (δ-GABAARs). Indeed, administration of agonists that interact with these δ-GABAARs prior to alcohol access can abolish binge drinking behavior in male mice. These δ-GABAARs have also been shown to exhibit estrous-dependent plasticity in regions relevant to drug taking behavior, like the hippocampus and periaqueductal gray. The present experiments were designed to determine whether the estrous cycle would alter binge drinking, or our ability to modulate this pattern of alcohol use with THIP, an agonist with high selectivity and efficacy at δ-GABAARs. Using the Drinking-in-the-Dark (DID) binge-drinking model, regularly cycling female mice were given 2h of daily access to alcohol (20%v/v). Vaginal cytology or vaginal impedance was assessed after drinking sessions to track estrous status. There was no fluctuation in binge drinking associated with the estrous cycle. Both Intra-posterior-VTA administration of THIP and systemic administration of the drug was also associated with an estrous cycle dependent reduction in drinking behavior. Pre-treatment with finasteride to inhibit synthesis of 5α-reduced neurosteroids did not disrupt THIP's effects. Analysis of δ-subunit mRNA from posterior-VTA enriched tissue samples revealed that expression of this GABAA receptor subunit is elevated during diestrus in this region. Taken together, these studies demonstrate that δGABAARs in the VTA are an important target for binge drinking in females and confirm that the estrous cycle is an important moderator of the pharmacology of this GABAA receptor subtype