Trace and contextual fear conditioning is enhanced in mice lacking the alpha4 subunit of the GABA(A) receptor

Behavioral pharmacogenetic analysis on the role of the α4 GABA(A) receptor subunit in the ethanol-mediated impairment of hippocampus-dependent contextual learning

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

GABA transport modulates the ethanol sensitivity of tonic inhibition in the rat dentate gyrus

In recent years, the effect of ethanol on tonic inhibition mediated by extrasynaptic GABA(A) receptors (GABA(A)Rs) has become a topic of intensive investigation and some controversy. The high ethanol sensitivity of extrasynaptic GABA(A) receptors containing the δ subunit combined with the role of tonic inhibition in maintaining the background inhibitory "tone" in hippocampal circuits has suggested that they may play a key role mediating certain behavioral effects of ethanol, including those related to learning and memory. We have found that ethanol disrupts learning and learning-related hippocampal function more potently in adolescent animals than in adults and that ethanol promotes extrasynaptic receptor-mediated GABAergic tonic currents more potently in adolescents than in adults. However, there have been no studies of potential mechanisms that may underlie the enhanced ethanol sensitivity of the tonic current in adolescents. In this study, we recorded GABA(A) receptor-mediated tonic currents in dentate gyrus granule cells in hippocampal slices from adolescent and adult rats. As previously reported, we found that ethanol potentiated the currents more efficaciously in cells from adolescents than in those from adults. We also found that the GAT-1 blocker NO-711 eliminated this developmental difference in ethanol sensitivity. These findings suggest that regulation of ambient GABA by GABA transporters may contribute to the difference in ethanol sensitivity between adolescents and adults.

Mouse models of genetic effects on cognition: relevance to schizophrenia

Cognitive dysfunction is a core feature of schizophrenia. Growing evidence indicates that a wide variety of genetic mutations and polymorphisms impact cognition and may thus be implicated in various aspects of this mental disorder. Despite differences between human and rodent brain structure and function, genetic mouse models have contributed critical information about brain mechanisms involved in cognitive processes. Here, we summarize discoveries of genetic modifications in mice that impact cognition. Based on functional hypotheses, gene modifications within five model systems are described: 1) dopamine (D1, D2, D3, D4, D5, DAT, COMT, MAO); 2) glutamate (GluR-A, NR1, NR2A, NR2B, GRM2, GRM3, GLAST); 3) GABA (α(5), γ(2), α(4), δGABA(A), GABA(B(1)), GAT1); 4) acetylcholine (nAChRβ2, α7, CHRM1); and 5) calcium (CaMKII-α, neurogranin, CaMKKβ, CaMKIV). We also consider other risk-associated genes for schizophrenia such as dysbindin (DTNBP1), neuregulin (NRG1), disrupted-in-schizophrenia1 (DISC1), reelin and proline dehydrogenase (PRODH). Because of the presumed importance of environmental factors, we further consider genetic modifications within the stress-sensitive systems of corticotropin-releasing factor (CRF), brain-derived neurotrophic factor (BDNF) and the endocannabinoid systems. We highlight the missing information and limitations of cognitive assays in genetically modified mice models relevant to schizophrenia pathology.

Ablation of Fmrp in adult neural stem cells disrupts hippocampus-dependent learning

Deficiency in fragile X mental retardation protein (FMRP) results in fragile X syndrome (FXS), an inherited form of intellectual disability. Despite extensive research, it is unclear how FMRP deficiency contributes to the cognitive deficits in FXS. Fmrp-null mice show reduced adult hippocampal neurogenesis. As Fmrp is also enriched in mature neurons, we investigated the function of Fmrp expression in neural stem and progenitor cells (aNSCs) and its role in adult neurogenesis. Here we show that ablation of Fmrp in aNSCs by inducible gene recombination leads to reduced hippocampal neurogenesis in vitro and in vivo, as well as markedly impairing hippocampus-dependent learning in mice. Conversely, restoration of Fmrp expression specifically in aNSCs rescues these learning deficits in Fmrp-deficient mice. These data suggest that defective adult neurogenesis may contribute to the learning impairment seen in FXS, and these learning deficits can be rectified by delayed restoration of Fmrp specifically in aNSCs.

Inhaled anesthetic responses of recombinant receptors and knockin mice harboring α2(S270H/L277A) GABA(A) receptor subunits that are resistant to isoflurane

The mechanism by which the inhaled anesthetic isoflurane produces amnesia and immobility is not understood. Isoflurane modulates GABA(A) receptors (GABA(A)-Rs) in a manner that makes them plausible targets. We asked whether GABA(A)-R α2 subunits contribute to a site of anesthetic action in vivo. Previous studies demonstrated that Ser270 in the second transmembrane domain is involved in the modulation of GABA(A)-Rs by volatile anesthetics and alcohol, either as a binding site or a critical allosteric residue. We engineered GABA(A)-Rs with two mutations in the α2 subunit, changing Ser270 to His and Leu277 to Ala. Recombinant receptors with these mutations demonstrated normal affinity for GABA, but substantially reduced responses to isoflurane. We then produced mutant (knockin) mice in which this mutated subunit replaced the wild-type α2 subunit. The adult mutant mice were overtly normal, although there was evidence of enhanced neonatal mortality and fear conditioning. Electrophysiological recordings from dentate granule neurons in brain slices confirmed the decreased actions of isoflurane on mutant receptors contributing to inhibitory synaptic currents. The loss of righting reflex EC(50) for isoflurane did not differ between genotypes, but time to regain the righting reflex was increased in N(2) generation knockins. This effect was not observed at the N(4) generation. Isoflurane produced immobility (as measured by tail clamp) and amnesia (as measured by fear conditioning) in both wild-type and mutant mice, and potencies (EC(50)) did not differ between the strains for these actions of isoflurane. Thus, immobility or amnesia does not require isoflurane potentiation of the α2 subunit.