Hippocampal α5 subunit-containing GABAA receptors are involved in the development of the latent inhibition effect

The fate of interneurons, GABAA receptor sub-types and perineuronal nets in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurological disease, which is associated with gradual memory loss and correlated with synaptic hyperactivity and abnormal oscillatory rhythmic brain activity that precedes phenotypic alterations and is partly responsible for the spread of the disease pathology. Synaptic hyperactivity is thought to be because of alteration in the homeostasis of phasic and tonic synaptic inhibition, which is orchestrated by the GABA_A inhibitory system, encompassing subclasses of interneurons and GABA_A receptors, which play a vital role in cognitive functions, including learning and memory. Furthermore, the extracellular matrix, the perineuronal nets (PNNs) which often go unnoticed in considerations of AD pathology, encapsulate the inhibitory cells and neurites in critical brain regions and have recently come under the light for their crucial role in synaptic stabilisation and excitatory-inhibitory balance and when disrupted, serve as a potential trigger for AD-associated synaptic imbalance. Therefore, in this review, we summarise the current understanding of the selective vulnerability of distinct interneuron subtypes, their synaptic and extrasynaptic GABA_A R subtypes as well as the changes in PNNs in AD, detailing their contribution to the mechanisms of disease development. We aim to highlight how seemingly unique malfunction in each component of the interneuronal GABA inhibitory system can be tied together to result in critical circuit dysfunction, leading to the irreversible symptomatic damage observed in AD.

GABAA and NMDA receptor density alterations and their behavioral correlates in the gestational methylazoxymethanol acetate model for schizophrenia

Hippocampal hyperactivity driven by GABAergic interneuron deficits and NMDA receptor hypofunction is associated with the hyperdopaminergic state often observed in schizophrenia. Furthermore, previous research in the methylazoxymethanol acetate (MAM) rat model has demonstrated that repeated peripubertal diazepam administration can prevent the emergence of adult hippocampal hyperactivity, dopamine-system hyperactivity, and associated psychosis-relevant behaviors. Here, we sought to characterize hippocampal GABAA and NMDA receptors in MAM-treated rats and to elucidate the receptor mechanisms underlying the promising effects of peripubertal diazepam exposure. Quantitative receptor autoradiography was used to measure receptor density in the dorsal hippocampus CA1, ventral hippocampus CA1, and ventral subiculum. Specifically, [3H]-Ro15-4513 was used to quantify the density of α5GABAA receptors (α5GABAAR), [3H]-flumazenil to quantify α1-3;5GABAAR, and [3H]-MK801 to quantify NMDA receptors. MAM rats exhibited anxiety and schizophrenia-relevant behaviors as measured by elevated plus maze and amphetamine-induced hyperlocomotion (AIH), although diazepam only partially rescued these behaviors. α5GABAAR density was reduced in MAM-treated rats in all hippocampal sub-regions, and negatively correlated with AIH. Ventral hippocampus CA1 α5GABAAR density was positively correlated with anxiety-like behavior. Dorsal hippocampus CA1 NMDA receptor density was increased in MAM-treated rats, and positively correlated with AIH. [3H]-flumazenil revealed no significant effects. Finally, we found no significant effect of diazepam treatment on receptor densities, potentially related to the only partial rescue of schizophrenia-relevant phenotypes. Overall, our findings provide first evidence of α5GABAAR and NMDA receptor abnormalities in the MAM model, suggesting that more selective pharmacological agents may become a novel therapeutic mechanism in schizophrenia.

Hippocampal α5-GABAA Receptors Modulate Dopamine Neuron Activity in the Rat Ventral Tegmental Area

Background

Aberrant dopamine neuron activity is attributable to hyperactivity in hippocampal subfields driving a pathological increase in dopamine neuron activity, which is positively correlated with psychosis in humans. Evidence indicates that hippocampal hyperactivity is due to loss of intrinsic GABAergic (gamma-aminobutyric acidergic) inhibition. We have previously demonstrated that hippocampal GABAergic neurotransmission can be modulated by targeting α5-GABA_A receptors, which are preferentially expressed in hippocampal regions. Positive and negative allosteric modulators of α5-GABA_A receptors (α5-PAMs and α5-NAMs) elicit effects on hippocampal-dependent behaviors. We posited that the selective manipulation of hippocampal inhibition, using α5-PAMs or α5-NAMs, would modulate dopamine activity in control rats. Further, α5-PAMs would reverse aberrant dopamine neuron activity in a rodent model with schizophrenia-related pathophysiologies (methylazoxymethanol acetate [MAM] model).

Methods

We performed in vivo extracellular recordings of ventral tegmental area dopamine neurons in anesthetized rats to compare the effects of two novel, selective α5-PAMs (GL-II-73, MP-III-022), a nonselective α-PAM (midazolam), and two selective α5-NAMs (L-655,708, TB 21007) in control and MAM-treated male Sprague Dawley rats (n = 5-9).

Results

Systemic or intracranial administration of selective α5-GABA_A receptor modulators regulated dopamine activity. Specifically, both α5-NAMs increased dopamine neuron activity in control rats, whereas GL-II-73, MP-III-022, and L-655,708 attenuated aberrant dopamine neuron activity in MAM-treated rats, an effect mediated by the ventral hippocampus.

Conclusions

This study demonstrated that α5-GABA_A receptor modulation can regulate dopamine neuron activity under control or abnormal activity, providing additional evidence that α5-PAMs and α5-NAMs may have therapeutic applications in psychosis and other psychiatric diseases where aberrant hippocampal activity is present.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

GABA-A receptor differences in schizophrenia: a positron emission tomography study using [11C]Ro154513

A loss of GABA signaling is a prevailing hypothesis for the pathogenesis of schizophrenia. Preclinical studies indicate that blockade of the α5 subtype of the GABA receptor (α5-GABA_ARs) leads to behavioral phenotypes associated with schizophrenia, and postmortem evidence indicates lower hippocampal α5-GABA_ARs protein and mRNA levels in schizophrenia. However, it is unclear if α5-GABA_ARs are altered in vivo or related to symptoms. We investigated α5-GABA_ARs availability in antipsychotic-free schizophrenia patients and antipsychotic-medicated schizophrenia patients using [¹¹C]Ro15-4513 PET imaging in a cross-sectional, case-control study design. Thirty-one schizophrenia patients (n = 10 antipsychotic free) and twenty-nine matched healthy controls underwent a [¹¹C]Ro15-4513 PET scan and MRI. The α5 subtype GABA-A receptor availability was indexed using [¹¹C]Ro15-4513 PET imaging. Dynamic PET data were analyzed using the two-tissue compartment model with an arterial plasma input function and total volume of distribution (V_T) as the outcome measure. Symptom severity was assessed using the PANSS scale. There was significantly lower [11C]Ro15-4513 V_T in the hippocampus of antipsychotic-free patients, but not in medicated patients (p = 0.64), relative to healthy controls (p < 0.05; effect size = 1.4). There was also a significant positive correlation between [¹¹C]Ro15-4513 V_T and total PANSS score in antipsychotic-free patients (r = 0.72; p = 0.044). The results suggest that antipsychotic-free patients with schizophrenia have lower α5-GABAARs levels in the hippocampus, consistent with the hypothesis that GABA hypofunction underlies the pathophysiology of the disorder.

Region-specific and dose-specific effects of chronic haloperidol exposure on [3H]-flumazenil and [3H]-Ro15-4513 GABAA receptor binding sites in the rat brain

Postmortem studies suggest that schizophrenia is associated with abnormal expression of specific GABAA receptor (GABAAR) α subunits, including α5GABAAR. Positron emission tomography (PET) measures of GABAAR availability in schizophrenia, however, have not revealed consistent alterations in vivo. Animal studies using the GABAAR agonist [3H]-muscimol provide evidence that antipsychotic drugs influence GABAAR availability, in a region-specific manner, suggesting a potential confounding effect of these drugs. No such data, however, are available for more recently developed subunit-selective GABAAR radioligands. To address this, we combined a rat model of clinically relevant antipsychotic drug exposure with quantitative receptor autoradiography. Haloperidol (0.5 and 2 mg/kg/day) or drug vehicle were administered continuously to adult male Sprague-Dawley rats via osmotic mini-pumps for 28 days. Quantitative receptor autoradiography was then performed postmortem using the GABAAR subunit-selective radioligand [3H]-Ro15-4513 and the non-subunit selective radioligand [3H]-flumazenil. Chronic haloperidol exposure increased [3H]-Ro15-4513 binding in the CA1 sub-field of the rat dorsal hippocampus (p<0.01; q<0.01; d=+1.3), which was not dose-dependent. [3H]-flumazenil binding also increased in most rat brain regions (p<0.05; main effect of treatment), irrespective of the haloperidol dose. These data confirm previous findings that chronic haloperidol exposure influences the specific binding of non-subtype selective GABAAR radioligands and is the first to demonstrate a potential effect of haloperidol on the binding of a α1/5GABAAR-selective radioligand. Although caution should be exerted when extrapolating results from animals to patients, our data support a view that exposure to antipsychotics may be a confounding factor in PET studies of GABAAR in the context of schizophrenia.

Bidirectional regulation of distinct memory domains by α5-subunit-containing GABAA receptors in CA1 pyramidal neurons

Reduction in the expression or function of α5-subunit-containing GABA_A receptors (α5GABA_ARs) leads to improvement in several hippocampus-dependent memory domains. However, studies thus far mostly lack anatomical specificity in terms of neuronal circuits and populations. We demonstrate that mice with a selective knockdown of α5GABA_ARs in CA1 pyramidal neurons (α5CA1KO mice) show improved spatial and trace fear-conditioning memory. Unexpectedly, α5CA1KO mice were comparable to controls in contextual fear-conditioning but showed an impairment in context discrimination, suggesting fine-tuning of activity in CA1 pyramidal cell dendrites through α5-mediated inhibition might be necessary for distinguishing highly similar contexts.

Contribution of GABAA receptor subunits to attention and social behavior

INTRODUCTION

GABA dysfunction is associated with a number of psychiatric conditions including schizophrenia, autism and depression. Blocking cortical GABA_A receptors in rodents causes behavioral deficits, including impaired attention and sociability, that are consistent with the symptoms of these conditions. The subunit composition of GABA_A receptors is diverse and can affect receptor function. The current experiment examined the role of GABA_A receptors containing different α-subunits in social behavior and attention.

METHODS

Male Sprague-Dawley rats were administered FG7142 (0.0-5.0 mg/kg; a non-selective GABA_A receptor inverse agonist), L-655,708 (0-1.0 mg/kg; a low efficacy inverse agonist at α₅-containing GABA_A receptors), MRK-016 (0.0-2.0 mg/kg; a high efficacy inverse agonist at α₅-containing GABA_A receptors), or L-838,417 (0.0-3.0 mg/kg; an antagonist at α₁-containing receptors and a partial agonist at α₂, α₃, α₅-containing GABA_A receptors) and either tested on the social interaction and social preference tests or the 5-choice serial reaction time task.

RESULTS

FG7142 decreased social interactions and impaired attention. MRK-016 impaired attention but did not affect social behavior. Neither L-655,708 nor L-838,417 significantly affected either social behavior or attention.

DISCUSSION

Systemic reduction in GABA_A receptor signaling decreased sociability and attention, a result consistent with past research demonstrating cortical GABA_A receptor blockade impairs social behavior and attention. Overall, the effects of the receptor subtype selective ligands were minimal; α₅-containing GABA_A receptors may contribute to the attentional deficit but do not contribute to the decrease in sociability. Further research is needed to determine the GABA_A receptor subunits that contribute to social behavior and attention.

Modulation of extrasynaptic GABAA alpha 5 receptors in the ventral hippocampus normalizes physiological and behavioral deficits in a circuit specific manner

Hippocampal hyperactivity is correlated with psychosis in schizophrenia patients and likely attributable to deficits in GABAergic signaling. Here we attempt to reverse this deficit by overexpression of the α5-GABAA receptor within the ventral hippocampus (vHipp). Indeed, this is sufficient to normalize vHipp activity and downstream alterations in dopamine neuron function in the MAM rodent model. This approach also attenuated behavioral deficits in cognitive flexibility. To understand the specific pathways that mediate these effects, we used chemogenetics to manipulate discrete projections from the vHipp to the nucleus accumbens (NAc) or prefrontal cortex (mPFC). We found that inhibition of the vHipp-NAc, but not the vHipp-mPFC pathway, normalized aberrant dopamine neuron activity. Conversely, inhibition of the vHipp-mPFC improved cognitive function. Taken together, these results demonstrate that restoring GABAergic signaling in the vHipp improves schizophrenia-like deficits and that distinct behavioral alterations are mediated by discrete projections from the vHipp to the NAc and mPFC.

GABAA receptor subunit expression changes in the human Alzheimer's disease hippocampus, subiculum, entorhinal cortex and superior temporal gyrus

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system. GABA type A receptors (GABA_A Rs) are severely affected in Alzheimer's disease (AD). However, the distribution and subunit composition of GABA_A Rs in the AD brain are not well understood. This is the first comprehensive study to show brain region- and cell layer-specific alterations in the expression of the GABA_A R subunits α1-3, α5, β1-3 and γ2 in the human AD hippocampus, entorhinal cortex and superior temporal gyrus. In late-stage AD tissue samples using immunohistochemistry we found significant alteration of all investigated GABA_A Rs subunits except for α3 and β1 that were well preserved. The most prominent changes include an increase in GABA_A R α1 expression associated with AD in all layers of the CA3 region, in the stratum (str.) granulare and hilus of the dentate gyrus. We found a significant increase in GABA_A R α2 expression in the str. oriens of the CA1-3, str. radiatum of the CA2,3 and decrease in the str. pyramidale of the CA1 region in AD cases. In AD there was a significant increase in GABA_A R α5 subunit expression in str. pyramidale, str. oriens of the CA1 region and decrease in the superior temporal gyrus. We also found a significant decrease in the GABA_A R β3 subunit immunoreactivity in the str. oriens of the CA2, str. granulare and str. moleculare of the dentate gyrus. In conclusion, these findings indicate that the expression of the GABA_A R subunits shows brain region- and layer-specific alterations in AD, and these changes could significantly influence and alter GABA_A R function in the disease. Cover Image for this issue: doi: 10.1111/jnc.14179.

An Emerging Circuit Pharmacology of GABAA Receptors

In the past 20 years we have learned a great deal about GABA_A receptor (GABA_AR) subtypes, and which behaviors are regulated or which drug effects are mediated by each subtype. However, the question of where GABA_ARs involved in specific drug effects and behaviors are located in the brain remains largely unanswered. We review here recent studies taking a circuit pharmacology approach to investigate the functions of GABA_AR subtypes in specific brain circuits controlling fear, anxiety, learning, memory, reward, addiction, and stress-related behaviors. The findings of these studies highlight the complexity of brain inhibitory systems and the importance of taking a subtype-, circuit-, and neuronal population-specific approach to develop future therapeutic strategies using cell type-specific drug delivery.

GABAergic inhibitory neurons as therapeutic targets for cognitive impairment in schizophrenia

Schizophrenia is considered primarily as a cognitive disorder. However, functional outcomes in schizophrenia are limited by the lack of effective pharmacological and psychosocial interventions for cognitive impairment. GABA (gamma-aminobutyric acid) interneurons are the main inhibitory neurons in the central nervous system (CNS), and they play a critical role in a variety of pathophysiological processes including modulation of cortical and hippocampal neural circuitry and activity, cognitive function-related neural oscillations (eg, gamma oscillations) and information integration and processing. Dysfunctional GABA interneuron activity can disrupt the excitatory/inhibitory (E/I) balance in the cortex, which could represent a core pathophysiological mechanism underlying cognitive dysfunction in schizophrenia. Recent research suggests that selective modulation of the GABAergic system is a promising intervention for the treatment of schizophrenia-associated cognitive defects. In this review, we summarized evidence from postmortem and animal studies for abnormal GABAergic neurotransmission in schizophrenia, and how altered GABA interneurons could disrupt neuronal oscillations. Next, we systemically reviewed a variety of up-to-date subtype-selective agonists, antagonists, positive and negative allosteric modulators (including dual allosteric modulators) for α5/α3/α2 GABA_A and GABA_B receptors, and summarized their pro-cognitive effects in animal behavioral tests and clinical trials. Finally, we also discuss various representative histone deacetylases (HDAC) inhibitors that target GABA system through epigenetic modulations, GABA prodrug and presynaptic GABA transporter inhibitors. This review provides important information on current potential GABA-associated therapies and future insights for development of more effective treatments.

Disinhibition, an emerging pharmacology of learning and memory

Learning and memory are dependent on interactive excitatory and inhibitory mechanisms. In this review, we discuss a mechanism called disinhibition, which is the release of an inhibitory constraint that effectively results in an increased activity in the target neurons (for example, principal or projection neurons). We focus on discussing the role of disinhibition in learning and memory at a basic level and in disease models with cognitive deficits and highlight a strategy to reverse cognitive deficits caused by excess inhibition, through disinhibition of α5-containing GABA _A receptors mediating tonic inhibition in the hippocampus, based on subtype-selective negative allosteric modulators as a novel class of drugs.

Formation of GABAA receptor complexes containing α1 and α5 subunits is paralleling a multiple T-maze learning task in mice

There is limited information on the role of GABA type A receptors (GABA_ARs) containing α1, α5 and γ2 subunits in learning and memory. Here, we assessed the possible role of such receptors in spatial learning using the multiple T-maze (MTM) paradigm. C57BL/6J mice were trained in the MTM which induced elevated levels of α1 and α5 subunit-containing hippocampal GABA_AR complexes. Moreover, spatial learning evoked a significant increase in the colocalization of α1 and α5 subunits in both, CA1 and dentate gyrus regions of the hippocampus suggesting the formation of complexes containing both subunits. Additionally, the presence of α1, α5 and γ2 subunits in high molecular weight GABA_ARs was detected and significant correlation in the level of α1-containing complexes with those containing α5 and γ2 subunits was demonstrated. Accordingly, α1 deficiency led to decreased levels of γ2 subunit-containing complexes, however, had no effect on α5-containing ones. On the other hand, α1 knockout mice showed impaired performance in the MTM correlating with increased levels of α5 subunit-containing GABA_ARs in comparison to trained floxed control animals which quickly learned the task. Taken together, these results suggest that α1, α5 and γ2-containing hippocampal GABA_AR complexes play an essential role in spatial learning and memory in which targeted disruption of the α1 subunit produces profound deficits.

Tonic Inhibitory Control of Dentate Gyrus Granule Cells by α5-Containing GABAA Receptors Reduces Memory Interference

Interference between similar or overlapping memories formed at different times poses an important challenge on the hippocampal declarative memory system. Difficulties in managing interference are at the core of disabling cognitive deficits in neuropsychiatric disorders. Computational models have suggested that, in the normal brain, the sparse activation of the dentate gyrus granule cells maintained by tonic inhibitory control enables pattern separation, an orthogonalization process that allows distinct representations of memories despite interference. To test this mechanistic hypothesis, we generated mice with significantly reduced expression of the α5-containing GABAA (α5-GABAARs) receptors selectively in the granule cells of the dentate gyrus (α5DGKO mice). α5DGKO mice had reduced tonic inhibition of the granule cells without any change in fast phasic inhibition and showed increased activation in the dentate gyrus when presented with novel stimuli. α5DGKO mice showed impairments in cognitive tasks characterized by high interference, without any deficiencies in low-interference tasks, suggesting specific impairment of pattern separation. Reduction of fast phasic inhibition in the dentate gyrus through granule cell-selective knock-out of α2-GABAARs or the knock-out of the α5-GABAARs in the downstream CA3 area did not detract from pattern separation abilities, which confirms the anatomical and molecular specificity of the findings. In addition to lending empirical support to computational hypotheses, our findings have implications for the treatment of interference-related cognitive symptoms in neuropsychiatric disorders, particularly considering the availability of pharmacological agents selectively targeting α5-GABAARs.

SIGNIFICANCE STATEMENT

Interference between similar memories poses a significant limitation on the hippocampal declarative memory system, and impaired interference management is a cognitive symptom in many disorders. Thus, understanding mechanisms of successful interference management or processes that can lead to interference-related memory problems has high theoretical and translational importance. This study provides empirical evidence that tonic inhibition in the dentate gyrus (DG), which maintains sparseness of neuronal activation in the DG, is essential for management of interference. The specificity of findings to tonic, but not faster, more transient types of neuronal inhibition and to the DG, but not the neighboring brain areas, is presented through control experiments. Thus, the findings link interference management to a specific mechanism, proposed previously by computational models.

GABA receptor subunit distribution and FMRP-mGluR5 signaling abnormalities in the cerebellum of subjects with schizophrenia, mood disorders, and autism

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. GABAergic receptor abnormalities have been documented in several major psychiatric disorders including schizophrenia, mood disorders, and autism. Abnormal expression of mRNA and protein for multiple GABA receptors has also been observed in multiple brain regions leading to alterations in the balance between excitatory/inhibitory signaling in the brain with potential profound consequences for normal cognition and maintenance of mood and perception. Altered expression of GABAA receptor subunits has been documented in fragile X mental retardation 1 (FMR1) knockout mice, suggesting that loss of its protein product, fragile X mental retardation protein (FMRP), impacts GABAA subunit expression. Recent postmortem studies from our laboratory have shown reduced expression of FMRP in the brains of subjects with schizophrenia, bipolar disorder, major depression, and autism. FMRP acts as a translational repressor and, under normal conditions, inhibits metabotropic glutamate receptor 5 (mGluR5)-mediated signaling. In fragile X syndrome (FXS), the absence of FMRP is hypothesized to lead to unregulated mGluR5 signaling, ultimately resulting in the behavioral and intellectual impairments associated with this disorder. Our laboratory has identified changes in mGluR5 expression in autism, schizophrenia, and mood disorders. In the current review article, we discuss our postmortem data on GABA receptors, FMRP, and mGluR5 levels and compare our results with other laboratories. Finally, we discuss the interactions between these molecules and the potential for new therapeutic interventions that target these interconnected signaling systems.

Behavioral effects of the benzodiazepine-positive allosteric modulator SH-053-2'F-S-CH₃ in an immune-mediated neurodevelopmental disruption model

BACKGROUND

Impaired γ-aminobutyric acid (GABA) signaling may contribute to the emergence of cognitive deficits and subcortical dopaminergic hyperactivity in patients with schizophrenia and related psychotic disorders. Against this background, it has been proposed that pharmacological interventions targeting GABAergic dysfunctions may prove useful in correcting such cognitive impairments and dopaminergic imbalances.

METHODS

Here, we explored possible beneficial effects of the benzodiazepine-positive allosteric modulator SH-053-2'F-S-CH₃, with partial selectivity at the α2, α3, and α5 subunits of the GABAA receptor in an immune-mediated neurodevelopmental disruption model. The model is based on prenatal administration of the viral mimetic polyriboinosinic-polyribocytidilic acid [poly(I:C)] in mice, which is known to capture various GABAergic, dopamine-related, and cognitive abnormalities implicated in schizophrenia and related disorders.

RESULTS

Real-time polymerase chain reaction analyses confirmed the expected alterations in GABAA receptor α subunit gene expression in the medial prefrontal cortices and ventral hippocampi of adult poly(I:C) offspring relative to control offspring. Systemic administration of SH-053-2'F-S-CH₃ failed to normalize the poly(I:C)-induced deficits in working memory and social interaction, but instead impaired performance in these cognitive and behavioral domains both in control and poly(I:C) offspring. In contrast, SH-053-2'F-S-CH₃ was highly effective in mitigating the poly(I:C)-induced amphetamine hypersensitivity phenotype without causing side effects in control offspring.

CONCLUSIONS

Our preclinical data suggest that benzodiazepine-like positive allosteric modulators with activity at the α2, α3, and α5 subunits of the GABAA receptor may be particularly useful in correcting pathological overactivity of the dopaminergic system, but they may be ineffective in targeting multiple pathological domains that involve the co-existence of psychotic, social, and cognitive dysfunctions.

The role of α5 GABAA receptor agonists in the treatment of cognitive deficits in schizophrenia

Currently available pharmacotherapies for the treatment of schizophrenia are ineffective in restoring the disrupted cognitive function associated with this disorder. As such, there is a continued search for more viable novel drug targets. Engaging in cognitive behaviors is associated with distinct coordinated oscillatory activity across brain regions, in particular the hippocampus and prefrontal cortex. In schizophrenia patients, pathological alterations in the functionality of GABAergic interneurons in the PFC and HPC responsible for generating network oscillations are thought to contribute to impaired cognition. Destabilized GABAergic interneuron activity in the HPC is further associated with aberrant increases in HPC output and enhanced dopamine neuron activity. Consequently, drugs directed at restoring HPC function could impact both oscillatory activity along with dopamine tone. There is compelling evidence from animal models of schizophrenia that allosteric modulation of the α5 subunit of the GABAA receptor is a viable means of resolving aberrant dopamine system activity through indirect alteration of HPC output. Consequently, these compounds are promising for their potential in also ameliorating cognitive deficits attributed to dysfunction in HPC network activity.

GABAA receptor subtypes: Therapeutic potential in Down syndrome, affective disorders, schizophrenia, and autism

The γ-aminobutyric acid (GABA) system plays a pivotal role in orchestrating the synchronicity of local networks and the functional coupling of different brain regions. Here we review the impact of the GABAA receptor subtypes on cognitive and emotional behavior, paying particular attention to five disease states: cognitive dysfunction and Down syndrome, anxiety disorders, depression, schizophrenia, and autism. Through the bidirectional modulation of tonic inhibition, α5-subunit-containing GABAA receptors permit the bidirectional modulation of cognitive processes, and a partial inverse agonist acting at the α5-subunit-containing GABAA receptor is in a clinical trial in individuals with Down syndrome. With regard to anxiety disorders, the viability of nonsedative anxiolytics based on the modulation of α2- and α3-subunit-containing GABAA receptors has been established in clinical proof-of-concept trials. Regarding the remaining three disease states, the GABA hypothesis of depression offers new options for antidepressant drug development; cognitive symptoms in schizophrenia are attributed to a cortical GABAergic deficit, and dysfunctional GABAergic inhibition is increasingly understood to contribute to the pathophysiology of autism spectrum disorders.

Negative modulation of GABAA α5 receptors by RO4938581 attenuates discrete sub-chronic and early postnatal phencyclidine (PCP)-induced cognitive deficits in rats

RATIONALE

A growing body of evidence suggests that negative modulation of γ-aminobutyric acid (GABA) GABA(A) α5 receptors may be a promising strategy for the treatment of certain facets of cognitive impairment; however, selective modulators of GABA(A) α5 receptors have not yet been tested in "schizophrenia-relevant" cognitive assay/model systems in animals.

OBJECTIVES

The objectives of this study were to investigate the potential of RO4938581, a negative modulator of GABA(A) α5 receptors, and to attenuate cognitive impairments induced following sub-chronic (sub-PCP) and early postnatal PCP (neo-PCP) administration in the novel object recognition (NOR) and intra-extradimensional shift (ID/ED) paradigms in rats. Complementary in vitro, ex vivo and in vivo studies were performed to confirm negative modulatory activity of RO4938581 and to investigate animal model validity, concept validity and potential side effect issues, respectively.

RESULTS

In vitro studies confirmed the reported negative modulatory activity of RO4938581, whilst immunohistochemical analyses revealed significantly reduced parvalbumin-positive cells in the prefrontal cortex of sub-PCP- and neo-PCP-treated rats. RO4938581 (1 mg/kg) ameliorated both sub-PCP- and neo-PCP-induced cognitive deficits in NOR and ID/ED performance, respectively. In contrast, QH-II-066 (1 and 3 mg/kg), a GABA(A) α5 receptor positive modulator, impaired cognitive performance in the NOR task when administered to vehicle-treated animals. Additional studies revealed that both RO4938581 (1 mg/kg) and QH-II-066 (1 and 3 mg/kg) attenuated amphetamine-induced hyperactivity in rats.

CONCLUSIONS

Taken together, these novel findings suggest that negative modulation of GABA(A) α5 receptors may represent an attractive treatment option for the cognitive impairments, and potentially positive symptoms, associated with schizophrenia.

Behavioral animal models of antipsychotic drug actions

Basic research in animals represents a fruitful approach to study the neurobiological basis of brain and behavioral disturbances relevant to neuropsychiatric disease and to establish and evaluate novel pharmacological therapies for their treatment. In the context of schizophrenia, there are models employing specific experimental manipulations developed according to specific pathophysiological or etiological hypotheses. The use of selective lesions in adult animals and the acute administration of psychotomimetic agents are indispensable tools in the elucidation of the contribution of specific brain regions or neurotransmitters to the genesis of a specific symptom or collection of symptoms and enjoy some degrees of predictive validity. However, they may be inaccurate, if not inadequate, in capturing the etiological mechanisms or ontology of the disease needed for a complete understanding of the disease and may be limited in the discovery of novel compounds for the treatment of negative and cognitive symptoms of schizophrenia. Under the prevailing consensus of schizophrenia as a disease of neurodevelopmental origin, we have seen the establishment of neurodevelopmental animal models which aim to identify the etiological processes whereby the brain, following specific triggering events, develops into a "schizophrenia-like brain" over time. Many neurodevelopmental models such as the neonatal ventral hippocampus (vHPC) lesion, methylazoxymethanol (MAM), and prenatal immune activation models can mimic a broad spectrum of behavioral, cognitive, and pharmacological abnormalities directly implicated in schizophrenic disease. These models allow pharmacological screens against multiple and coexisting schizophrenia-related dysfunctions while incorporating the disease-relevant concept of abnormal brain development. The multiplicity of existing models is testimonial to the multifactorial nature of schizophrenia, and there are ample opportunities for their integration. Indeed, one ultimate goal must be to incorporate the successes of distinct models into one unitary account of the complex disorder of schizophrenia and to use such unitary approaches in the further development and evaluation of novel antipsychotic treatment strategies.

A novel α5GABA(A)R-positive allosteric modulator reverses hyperactivation of the dopamine system in the MAM model of schizophrenia

We have shown previously that aberrant hippocampal (HPC) output underlies the dopamine (DA) dysfunction observed in the methylazoxymethanol acetate (MAM) developmental model of schizophrenia in the rodent. This alteration of HPC activity was proposed to result from a reduction in parvalbumin (PV)-expressing GABAergic interneurons and consequent destabilization of the output of pyramidal neurons, as well as disrupted activation across a broad neural network. In vivo extracellular recordings were performed in the ventral tegmental area (VTA) and ventral HPC of saline- (SAL) and MAM-treated animals. A novel benzodiazepine-positive allosteric modulator (PAM), selective for the α5 subunit of the GABA(A) receptor, SH-053-2'F-R-CH3, was tested for its effects on the output of the HPC, leading to dopamine system hyperactivity in MAM-treated animals. In addition, the effect of SH-053-2'F-R-CH3 on the hyperactive locomotor response to amphetamine in MAM animals was examined. We demonstrate that treatment with the α5GABA(A)R PAM reduced the number of spontaneously active DA neurons in the VTA of MAM animals to levels observed in SAL rats, both when administered systemically and when directly infused into the ventral HPC. Moreover, HPC neurons in both SAL and MAM animals showed diminished cortical-evoked responses following α5GABA(A)R PAM treatment. In addition, the increased locomotor response to amphetamine observed in MAM rats was reduced following α5GABA(A)R treatment. This study supports a novel treatment of schizophrenia that targets abnormal HPC output, which in turn normalizes dopaminergic neuronal activity.

Alpha5GABAA receptor activity sets the threshold for long-term potentiation and constrains hippocampus-dependent memory

Synaptic plasticity, which is the neuronal substrate for many forms of hippocampus-dependent learning, is attenuated by GABA type A receptor (GABA(A)R)-mediated inhibition. The prevailing notion is that a synaptic or phasic form of GABAergic inhibition regulates synaptic plasticity; however, little is known about the role of GABA(A)R subtypes that generate a tonic or persistent inhibitory conductance. We studied the regulation of synaptic plasticity by alpha5 subunit-containing GABA(A)Rs (alpha5GABA(A)Rs), which generate a tonic inhibitory conductance in CA1 pyramidal neurons using electrophysiological recordings of field and whole-cell potentials in hippocampal slices from both wild-type and null mutant mice for the alpha5 subunit of the GABA(A)R (Gabra5(-/-) mice). In addition, the strength of fear-associated memory was studied. The results showed that alpha5GABA(A)R activity raises the threshold for induction of long-term potentiation in a highly specific band of stimulation frequencies (10-20 Hz) through mechanisms that are predominantly independent of inhibitory synaptic transmission. The deletion or pharmacological inhibition of alpha5GABA(A)Rs caused no change in baseline membrane potential or input resistance but increased depolarization during 10 Hz stimulation. The encoding of hippocampus-dependent memory was regulated by alpha5GABA(A)Rs but only under specific conditions that generate moderate but not robust forms of fear-associated learning. Thus, under specific conditions, alpha5GABA(A)R activity predominates over synaptic inhibition in modifying the strength of both synaptic plasticity in vitro and certain forms of memory in vivo.

Prepulse inhibition and genetic mouse models of schizophrenia

Mutant mouse models related to schizophrenia have been based primarily on the pathophysiology of schizophrenia, the known effects of antipsychotic drugs, and candidate genes for schizophrenia. Sensorimotor gating deficits in schizophrenia patients, as indexed by measures of prepulse inhibition of startle (PPI), have been well characterized and suggested to meet the criteria as a useful endophenotype in human genetic studies. PPI refers to the ability of a non-startling "prepulse" to inhibit responding to the subsequent startling stimulus or "pulse." Because of the cross-species nature of PPI, it has been used primarily in pharmacological animal models to screen putative antipsychotic medications. As techniques in molecular genetics have progressed over the past 15 years, PPI has emerged as a phenotype used in assessing genetic mouse models of relevance to schizophrenia. In this review, we provide a selected overview of the use of PPI in mouse models of schizophrenia and discuss the contribution and usefulness of PPI as a phenotype in the context of genetic mouse models. To that end, we discuss mutant mice generated to address hypotheses regarding the pathophysiology of schizophrenia and candidate genes (i.e., hypothesis driven). We also briefly discuss the usefulness of PPI in phenotype-driven approaches in which a PPI phenotype could lead to "bottom up" approaches of identifying novel genes of relevance to PPI (i.e., hypothesis generating).

The alpha5(H105R) mutation impairs alpha5 selective binding properties by altered positioning of the alpha5 subunit in GABAA receptors containing two distinct types of alpha subunits

GABA(A) receptors are pentameric ligand-gated ion channels that are major mediators of fast inhibitory neurotransmission. Clinically relevant GABA(A) receptor subtypes are assembled from alpha5(1-3, 5), beta1-3 and the gamma2 subunit. They exhibit a stoichiometry of two alpha, two beta and one gamma subunit, with two GABA binding sites located at the alpha/beta and one benzodiazepine binding site located at the alpha/gamma subunit interface. Introduction of the H105R point mutation into the alpha5 subunit, to render alpha5 subunit-containing receptors insensitive to the clinically important benzodiazepine site agonist diazepam, unexpectedly resulted in a reduced level of alpha5 subunit protein in alpha5(H105R) mice. In this study, we show that the alpha5(H105R) mutation did not affect cell surface expression and targeting of the receptors or their assembly into macromolecular receptor complexes but resulted in a severe reduction of alpha5-selective ligand binding. Immunoprecipitation studies suggest that the diminished alpha5-selective binding is presumably due to a repositioning of the alpha5(H105R) subunit in GABA(A) receptor complexes containing two different alpha subunits. These findings imply an important role of histidine 105 in determining the position of the alpha5 subunit within the receptor complex by determining the affinity for assembly with the gamma2 subunit.

The GABAA receptor RDL suppresses the conditioned stimulus pathway for olfactory learning

Assigning a gene's function to specific pathways used for classical conditioning, such as conditioned stimulus (CS) and unconditioned stimulus (US) pathway, is important for understanding the fundamental molecular and cellular mechanisms underlying memory formation. Prior studies have shown that the GABA receptor RDL inhibits aversive olfactory learning via its role in the Drosophila mushroom bodies (MBs). Here, we describe the results of further behavioral tests to further define the pathway involvement of RDL. The expression level of Rdl in the MBs influenced both appetitive and aversive olfactory learning, suggesting that it functions by suppressing a common pathway used for both forms of olfactory learning. Rdl knock down failed to enhance learning in animals carrying mutations in genes of the cAMP signaling pathway, such as rutabaga and NF1, suggesting that RDL works up stream of these functions in CS/US integration. Finally, knocking down Rdl or over expressing the dopamine receptor dDA1 in the MBs enhanced olfactory learning, but no significant additional enhancement was detected with both manipulations. The combined data suggest that RDL suppresses olfactory learning via CS pathway involvement.

GABAA receptor alpha5 subunits contribute to GABAA,slow synaptic inhibition in mouse hippocampus

gamma-Aminobutyric acid type A (GABA(A)) receptor alpha5 subunits, which are heavily expressed in the hippocampus, are potential drug targets for improving cognitive function. They are found at synaptic and extrasynaptic sites and have been shown to mediate tonic inhibition in pyramidal neurons. We tested the hypothesis that alpha5 subunits also contribute to synaptic inhibition by measuring the effect of diazepam (DZ) on spontaneous and stimulus-evoked inhibitory postsynaptic currents (IPSCs) in genetically modified mice carrying a point mutation in the alpha5 subunit (alpha5-H105R) that renders those receptors insensitive to benzodiazepines. In wild type mice, DZ (1 microM) increased the amplitude of spontaneous IPSCs (sIPSCs) and stimulus-evoked GABA(A,slow) IPSCs (eIPSCs) and prolonged the decay of GABA(A,fast) sIPSCs. In alpha5-mutant mice, DZ increased the amplitude of a small-amplitude subset of sIPSCs (<50 pA) and eIPSCs (<300 pA) GABA(A,slow) and prolonged the decay of GABA(A,fast) sIPSCs, but failed to increase the amplitude of larger sIPSCs and eIPSCs GABA(A,slow). These results indicate that alpha5 subunits contribute to a large-amplitude subset of GABA(A,slow) synapses and implicate these synapses in modulation of cognitive function by drugs that target alpha5 subunits.