Behavioral Pharmacogenetic Analysis on the Role of the α4 GABAA Receptor Subunit in the Ethanol-Mediated Impairment of Hippocampus-Dependent Contextual Learning

Department of Psychology and Brain Research Institute, University of California, Los Angeles, California, USA.
Alcoholism Clinical and Experimental Research (Impact Factor: 3.21). 09/2011; 35(11):1948-59. DOI: 10.1111/j.1530-0277.2011.01546.x
Source: PubMed


A major effect of low-dose ethanol is impairment of hippocampus-dependent cognitive function. α4/δ -containing GABA(A) Rs are highly expressed within the dentate gyrus region of the hippocampus where they mediate a tonic inhibitory current that is sensitive to the enhancement by low ethanol concentrations. These receptors are also powerful modulators of learning and memory, suggesting that they could play an important role in ethanol's cognitive impairing effects. The goal of this study was to develop a high-throughput cognitive ethanol assay, amenable to use in genetically modified mice that could be used to test this hypothesis.
We developed a procedure where preexposure to a conditioning chamber is used to rescue the "immediate shock deficit." Using this task, ethanol can be specifically targeted at the hippocampus-dependent process of contextual learning without interfering with pain sensitivity or behavioral performance.
Validation of this task in C57BL/6 mice indicated that 1.0 g/kg ethanol and 10 mg/kg allopregnanolone disrupt contextual learning. Ro15-4513 reversed the effects of ethanol but not allopregnanolone, whereas it produced an impairment when given alone. The high-throughput nature of this task allowed for its application in a large cohort of α4 GABA(A) R KO mice. Loss of the α4 GABA(A) R subunit produced an enhanced sensitivity to the cognitive impairing effects of ethanol. This is consistent with the enhanced ethanol sensitivity of synaptic GABA(A) Rs that has been previously observed in the dentate gyrus in these mice, but inconsistent with the reduced ethanol sensitivity of extrasynaptic GABA(A) Rs observed in the same cells.
Overall, these findings are consistent with our hypothesis that ethanol acts directly at GABA(A) receptors to impair hippocampus-dependent cognitive function. Furthermore, validation of this high-throughput assay will allow for future studies to use anatomically and temporally restricted genetic manipulations to probe more deeply into the neural mechanisms of ethanol action on learning and memory circuits.

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Available from: Michael Fanselow, Oct 13, 2015
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    • "First, enhanced δGABAA receptor activity constrains neuronal firing (Bonin et al., 2011), reduces network excitability (Maguire et al., 2009) and attenuates synaptic plasticity in the CA1 region of the hippocampus (Shen et al., 2010). Second, one of the primary molecular targets of THIP, the α4βδ GABAA receptor (Brown et al., 2002), constrains fear-associated memory (Wiltgen et al., 2005) as evidenced by studies of transgenic mice that lack either the δ subunit gene (Wiltgen et al., 2005) or the α4 subunit gene (Moore et al., 2010; Cushman et al., 2011). Interestingly, human studies have shown that THIP does not alter memory performance measured 12–24 h after drug treatment (Mathias et al., 2005; Boyle et al., 2009; Leufkens et al., 2009). "
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    ABSTRACT: Extrasynaptic γ-aminobutyric acid type A (GABAA) receptors that contain the δ subunit (δGABAA receptors) are expressed in several brain regions including the dentate gyrus (DG) and CA1 subfields of the hippocampus. Drugs that increase δGABAA receptor activity have been proposed as treatments for a variety of disorders including insomnia, epilepsy and chronic pain. Also, long-term pretreatment with the δGABAA receptor-preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) enhances discrimination memory and increases neurogenesis in the DG. Despite the potential therapeutic benefits of such treatments, the effects of acutely increasing δGABAA receptor activity on memory behaviors remain unknown. Here, we studied the effects of THIP (4 mg/kg, i.p.) on memory performance in wild-type (WT) and δGABAA receptor null mutant (Gabrd(-/-)) mice. Additionally, the effects of THIP on long-term potentiation (LTP), a molecular correlate of memory, were studied within the DG and CA1 subfields of the hippocampus using electrophysiological recordings of field potentials in hippocampal slices. The results showed that THIP impaired performance in the Morris water maze, contextual fear conditioning and object recognition tasks in WT mice but not Gabrd(-/-) mice. Furthermore, THIP inhibited LTP in hippocampal slices from WT but not Gabrd(-/-) mice, an effect that was blocked by GABAA receptor antagonist bicuculline. Thus, acutely increasing δGABAA receptor activity impairs memory behaviors and inhibits synaptic plasticity. These results have important implications for the development of therapies aimed at increasing δGABAA receptor activity.
    Frontiers in Neural Circuits 09/2013; 7:146. DOI:10.3389/fncir.2013.00146 · 3.60 Impact Factor
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    ABSTRACT: γ-Aminobutyric acid type A receptors (GABAA-Rs) are considered to be the primary molecular targets of injectable anesthetics such as propofol, etomidate and the neurosteriod, alphaxalone. A number of studies have sought to understand the specific GABAA-R subtypes involved in the mechanism of action of these three drugs. Here, we investigated the role of α4-subunit containing GABAA-Rs in the neurobehavioral responses to these drugs. Drug responses in α4 subunit knockout (KO) mice were compared to wild type (WT) littermate controls. While etomidate and propofol are currently used as injectable anesthetics, alphaxalone belongs to the class of neurosteroid drugs having anesthetic effects. Low dose effects of etomidate and alphaxalone were studied using an open field assay. The moderate and high dose effects of all three anesthetics were measured using the rotarod and loss of righting reflex assays, respectively. The locomotor stimulatory effect of alphaxalone was reduced significantly in α4 KO mice compared to WT controls. Neither the low dose sedating effect of etomidate, nor the moderate/high dose effect of any of the drugs differed between genotypes. These results suggest that α4 subunit-containing GABAA-Rs are required for the low dose, locomotor stimulatory effect of alphaxalone but are not required for the sedating effect of etomidate or the moderate/high dose effects of etomidate, propofol or alphaxalone on motor ataxia and loss of righting reflex.
    Neurochemical Research 09/2013; 39(6). DOI:10.1007/s11064-013-1148-3 · 2.59 Impact Factor
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    ABSTRACT: Objective Extrasynaptic γ‐aminobutyric acid type A receptors that contain the δ subunit (δGABAA receptors) are highly expressed in the dentate gyrus (DG) subfield of the hippocampus, where they generate a tonic conductance that regulates neuronal activity. GABAA receptor‐dependent signaling regulates memory and also facilitates postnatal neurogenesis in the adult DG; however, the role of the δGABAA receptors in these processes is unclear. Accordingly, we sought to determine whether δGABAA receptors regulate memory behaviors, as well as neurogenesis in the DG. Methods Memory and neurogenesis were studied in wild‐type (WT) mice and transgenic mice that lacked δGABAA receptors (Gabrd−/−). To pharmacologically increase δGABAA receptor activity, mice were treated with the δGABAA receptor‐preferring agonist 4,5,6,7‐tetrahydroisoxazolo(5,4‐c)pyridin‐3‐ol (THIP). Behavioral assays including recognition memory, contextual discrimination, and fear extinction were used. Neurogenesis was studied by measuring the proliferation, survival, migration, maturation, and dendritic complexity of adult‐born neurons in the DG. ResultsGabrd−/− mice exhibited impaired recognition memory and contextual discrimination relative to WT mice. Fear extinction was also impaired in Gabrd−/− mice, although the acquisition of fear memory was enhanced. Neurogenesis was disrupted in Gabrd−/− mice as the migration, maturation, and dendritic development of adult‐born neurons were impaired. Long‐term treatment with THIP facilitated learning and neurogenesis in WT but not Gabrd−/− mice. InterpretationδGABAA receptors promote the performance of certain DG‐dependent memory behaviors and facilitate neurogenesis. Furthermore, δGABAA receptors can be pharmacologically targeted to enhance these processes. Ann Neurol 2013;74:611–621
    Annals of Neurology 10/2013; 74(4). DOI:10.1002/ana.23941 · 9.98 Impact Factor
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