GABAA receptor subunit composition regulates circadian rhythms in rest-wake and synchrony among cells in the suprachiasmatic nucleus.

The mammalian circadian clock located in the suprachiasmatic nucleus (SCN) produces robust daily rhythms including rest-wake. SCN neurons synthesize and respond to γ-aminobutyric acid (GABA), but its role remains unresolved. We tested the hypothesis that γ2- and δ-subunits of the GABAA receptor in the SCN differ in their regulation of synchrony among circadian cells. We used two approaches: 1) shRNA to knock-down (KD) the expression of either γ2 or δ subunits in the SCN or 2) knock-in mice harboring a point mutation in the M2 domains of the endogenous GABAA γ2 or δ subunits. KD of either γ2 or δ subunits in the SCN increased daytime running and reduced nocturnal running by reducing their circadian amplitude by a third. Similarly, δ subunit knock-in mice showed decreased circadian amplitude, increased duration of daily activity, and decreased total daily activity. Reduction, or mutation of either γ2 or δ subunits halved the synchrony among, and amplitude of, circadian SCN cells as measured by firing rate or expression of the PERIOD2 protein, in vitro. Surprisingly, overexpression of the γ2 subunit rescued these phenotypes following KD or mutation of the δ subunit, and overexpression of the δ subunit rescued deficiencies due to γ2 subunit KD or mutation. We conclude that γ2 and δ GABAA receptor subunits play similar roles in maintaining circadian synchrony in the SCN and amplitude of daily rest-wake rhythms, but that modulation of their relative densities can change the duration and amplitude of daily activities.

Algorithmic assessment reveals functional implications of GABRD gene variants linked to idiopathic generalized epilepsy.

OBJECTIVE: The primary objective of this study is to address the challenge posed by the increasing number of variants of unknown clinical significance (VUS) within the GABRD gene, which encodes the δ subunit of γ-Aminobutyric acid type A receptors. The focus is on predicting the most pathogenic GABRD VUS to enhance precision medicine and improve our understanding of relevant pathophysiology. METHODS: The study employs a combination of in silico algorithms to analyze 82 variants of unknown clinical significance of GABRD gene sourced from the ClinVar database. Initially, separate algorithms based on sequence homology are utilized to assess this variant set. Subsequently, consensus variants predicted as pathogenic undergo further evaluation through a web server employing an algorithm based on structural homology. The resulting 11 variants are then validated using in silico tools that assess variant effects based on genetic and molecular data. The evaluation includes consideration of disease association and protein stability due to amino acid substitutions. RESULTS: The study identifies specific variants (L111R, R114C, D123N, G150S, and L243P) in the coding region of the GABRD gene, which are predicted as deleterious by multiple algorithms. These variants are evolutionarily conserved, mapped onto the extracellular domain of the δ subunit, and associated with idiopathic generalized epilepsy. CONCLUSIONS: The findings suggest structural or functional consequences that lead to pathogenicity, offering valuable insights for wet-lab experimentation. Besides, the findings contribute to the validation of clinically significant genetic variants in the GABRD gene, which is critical for epilepsy precision medicine.

Triplications of chromosome 1p36.3, including the genes GABRD and SKI, are associated with a developmental disorder and a facial gestalt.

Triplication of chromosomal region 1p36.3 is a rare genomic rearrangement. In this report, we delineate the phenotypic spectrum associated with 1p36.3 triplications. We describe four patients with microtriplications of variable size, but with a strong phenotypic overlap, and compare them to previously described patients with an isolated triplication or duplication of this region. The 1p36.3 triplication syndrome is associated with a distinct phenotype, characterized by global developmental delay, moderate intellectual disability, seizures, behavioral problems, and specific facial dysmorphic features, including ptosis, hypertelorism, and arched eyebrows. The de novo occurrence of these microtriplications demonstrates the reduced reproductive fitness associated with this genotype, in contrast to 1p36.3 duplications which are mostly inherited and can be associated with similar facial features but with a less severe developmental phenotype. The shared triplicated region encompasses four disease-related genes of which GABRD and SKI are most likely to contribute to the phenotype.

Gain-of-function variants in GABRD reveal a novel pathway for neurodevelopmental disorders and epilepsy.

A potential link between GABRD encoding the δ subunit of extrasynaptic GABAA receptors and neurodevelopmental disorders has largely been disregarded due to conflicting conclusions from early studies. However, we identified seven heterozygous missense GABRD variants in 10 patients with neurodevelopmental disorders and generalized epilepsy. One variant occurred in two sibs of healthy parents with presumed somatic mosaicism, another segregated with the disease in three affected family members, and the remaining five occurred de novo in sporadic patients. Electrophysiological measurements were used to determine the functional consequence of the seven missense δ subunit variants in receptor combinations of α1ß3δ and α4ß2δ GABAA receptors. This was accompanied by analysis of electroclinical phenotypes of the affected individuals. We determined that five of the seven variants caused altered function of the resulting α1ß3δ and α4ß2δ GABAA receptors. Surprisingly, four of the five variants led to gain-of-function effects, whereas one led to a loss-of-function effect. The stark differences between the gain-of-function and loss-of function effects were mirrored by the clinical phenotypes. Six patients with gain-of-function variants shared common phenotypes: neurodevelopmental disorders with behavioural issues, various degrees of intellectual disability, generalized epilepsy with atypical absences and generalized myoclonic and/or bilateral tonic-clonic seizures. The EEG showed qualitative analogies among the different gain-of-function variant carriers consisting of focal slowing in the occipital regions often preceding irregular generalized epileptiform discharges, with frontal predominance. In contrast, the one patient carrying a loss-of-function variant had normal intelligence and no seizure history, but has a diagnosis of autism spectrum disorder and suffers from elevated internalizing psychiatric symptoms. We hypothesize that increase in tonic GABA-evoked current levels mediated by δ-containing extrasynaptic GABAA receptors lead to abnormal neurotransmission, which represent a novel mechanism for severe neurodevelopmental disorders. In support of this, the electroclinical findings for the gain-of-function GABRD variants resemble the phenotypic spectrum reported in patients with missense SLC6A1 (GABA uptake transporter) variants. This also indicates that the phenomenon of extrasynaptic receptor overactivity is observed in a broader range of patients with neurodevelopmental disorders, because SLC6A1 loss-of-function variants also lead to overactive extrasynaptic δ-containing GABAA receptors. These findings have implications when selecting potential treatment options, as a substantial portion of available antiseizure medication act by enhancing GABAergic function either directly or indirectly, which could exacerbate symptoms in patients with gain-of-function GABRD variants.

DEPDC1B collaborates with GABRD to regulate ESCC progression.

BACKGROUND: Esophageal squamous cell carcinoma (ESCC) is the leading cause of cancer-related death worldwide with a poor prognosis. Given that DEPDC1B plays a key role in multiple cancers, the role of this molecule in ESCC was explored to identify potential targets for ESCC patients. METHOD: The expression level of DEPDC1B in ESCC was revealed based on the TCGA database and immunohistochemical experiments on clinical tissues. The correlation between DEPDC1B and survival of ESCC patients was analyzed by Kaplan-Meier method. Small hairpin RNA (shRNA)-mediated silencing of DEPDC1B expression in ESCC cells and performed a series of in vitro and in vivo functional validations. RESULT: DEPDC1B was overexpressed in ESCC. High expression of DEPDC1B was significantly negatively correlated with overall survival in patients with ESCC. Moreover, knockdown of DEPDC1B inhibited ESCC cell proliferation, clone formation, migration, tumor formation and promoted apoptosis. Furthermore, knockdown of DEPDC1B leaded to significant downregulation of GABRD in ESCC cells. Meanwhile, GABRD expression was upregulated in ESCC, and its silencing can inhibit the proliferation and migration of the tumor cells. Interestingly, there was a protein interaction between DEPDC1B and GABRD. Functionally, GABRD knockdown partially reversed the contribution of DEPDC1B to ESCC progression. In addition, GABRD regulated ESCC progression may depend on PI3K/AKT/mTOR signaling pathway. CONCLUSION: DEPDC1B collaborated with GABRD to regulate ESCC progression, and inhibition of this signaling axis may be a potential therapeutic target for ESCC.

Splice-Site Variants in the Gene Encoding GABA-A Receptor Delta Subunit Are Associated with Amphetamine Use in Patients under Methadone Maintenance Treatment.

Chronic opioid use disorder patients often also use other substances such as amphetamines. The gene-based analysis method was applied in the genomic database obtained from our previous study with 343 methadone maintenance treatment (MMT) patients. We found that the gene encoding gamma-aminobutyric acid type A receptors (GABA-A receptor) delta subunit isoforms (GABRD) was associated with amphetamine use in heroin dependent patients under MMT in Taiwan. A total of 15% of the 343 MMT patients tested positive for amphetamine in the urine toxicology test. Two genetic variants in the GABRD, rs2889475 and rs2376805, were found to be associated with the positive urine amphetamine test. They are located in the exon 1 of the splice variant and altered amino acid compositions (T126I, C/T, for rs2889475, and R252Q, G/A, for rs2376805). The CC genotype carriers of rs2889475 showed a four times higher risk of amphetamine use than those with TT genotype. The GG genotype carriers of rs2376805 showed a three times higher risk of amphetamine use than the AA genotype carriers. To our knowledge, this is the first report that demonstrated an association of the delta splice variant isoform in the GABA-A receptor with an increased risk of amphetamine use in MMT patients. Our results suggest that rs2889475 and rs2376805 may be indicators for the functional role and risk of amphetamine use in MMT patients.

Extrasynaptic δ-subunit containing GABAA receptors.

γ-Aminobutyric acid type A receptors (GABAARs) are GABA gated heteropentameric chloride channels responsible for the adult brain's primary inhibition. In specific brain cells, such as in the hippocampus, one of the subtypes of GABAARs, the δ subunit containing GABAARs (δ-GABAARs), is predominantly expressed and located in extrasynaptic or perisynaptic positions. δ-GABAARs mediate a slow constant inhibitory current called tonic inhibition. While δ-GABAARs and tonic inhibition is critical for the excitability of single neurons, accumulating data suggest that the function of δ-GABAARs are broader and includes an integrative role in the network oscillations. While these open new horizons on the neurobiology of δ-GABAARs, the complexity continues to challenge the analysis of GABAARs and their subtypes. This review will summarize the current knowledge of molecular, cellular and physiological characteristics of δ-GABAARs during health and disease.

Ameliorative effect of curcumin on altered expression of CACNA1A and GABRD in the pathogenesis of FeCl3-induced epilepsy.

The pivotal role played by ion-channel dysregulations in the pathogenesis of epilepsy has always garnered much attention. Since mutation of ion-channel proteins CACNA1A and GABRD have been associated with epilepsy, it is important to determine the post-traumatic epilepsy-associated changes in expression levels of these ion channel proteins. Additionally, curcumin is already known for its antiepileptic and neuroprotective potential in FeCl3-induced model of post-traumatic epilepsy. Thus, we investigated FeCl3-induced epilepsy mediated differential expression of CACNA1A and GABRD in the cortical region of the rat brain. Furthermore, we investigated the effect of curcumin on the expression of both proteins. For this, epilepsy was induced by intracortical FeCl3 injection (5 µl of 100 mM). Additionally, curcumin (conc. 1000 ppm; 75 mg/kg of b.wt.; for 14 and 28 days) was administered, mixed with normal food pellets. Results obtained from EEG-MUA and Morris water maze assay demonstrate the progression of epilepsy after FeCl3 injection. Additionally, western blotting and histological studies show the downregulation of CACNA1A and GABRD during epileptogenesis. It was observed that epilepsy-associated decline in learning and memory of animals might be linked with the dysregulation of both proteins. Results also demonstrated that curcumin administration ameliorated epilepsy-associated change in expression of both CACNA1A and GABRD proteins. In conclusion, the neuroprotective effect of curcumin against iron-induced epilepsy might be accompanied by the alleviated upregulation of these channel proteins.

A Targeted Bioinformatics Assessment of Adrenocortical Carcinoma Reveals Prognostic Implications of GABA System Gene Expression.

Adrenocortical carcinoma (ACC) is a rare but deadly cancer for which few treatments exist. Here, we have undertaken a targeted bioinformatics study of The Cancer Genome Atlas (TCGA) ACC dataset focusing on the 30 genes encoding the γ-aminobutyric acid (GABA) system-an under-studied, evolutionarily-conserved system that is an emerging potential player in cancer progression. Our analysis identified a subset of ACC patients whose tumors expressed a distinct GABA system transcriptome. Transcript levels of ABAT (encoding a key GABA shunt enzyme), were upregulated in over 40% of tumors, and this correlated with several favorable clinical outcomes including patient survival; while enrichment and ontology analysis implicated two cancer-related biological pathways involved in metastasis and immune response. The phenotype associated with ABAT upregulation revealed a potential metabolic heterogeneity among ACC tumors associated with enhanced mitochondrial metabolism. Furthermore, many GABAA receptor subunit-encoding transcripts were expressed, including two (GABRB2 and GABRD) prognostic for patient survival. Transcripts encoding GABAB receptor subunits and GABA transporters were also ubiquitously expressed. The GABA system transcriptome of ACC tumors is largely mirrored in the ACC NCI-H295R cell line, suggesting that this cell line may be appropriate for future functional studies investigating the role of the GABA system in ACC cell growth phenotypes and metabolism.

Targeted next-generation sequencing in the diagnosis of neurodevelopmental disorders.

We developed a next-generation sequencing (NGS) based mutation screening strategy for neurodevelopmental diseases. Using this system, we screened 284 genes in 40 patients. Several novel mutations were discovered. Patient 1 had a novel mutation in ACTB. Her dysmorphic feature was mild for Baraitser-Winter syndrome. Patient 2 had a truncating mutation of DYRK1A. She lacked microcephaly, which was previously assumed to be a constant feature of DYRK1A loss of function. Patient 3 had a novel mutation in GABRD gene. She showed Rett syndrome like features. Patient 4 was diagnosed with Noonan syndrome with PTPN11 mutation. He showed complete agenesis of corpus callosum. We have discussed these novel findings.

Impact of inhibitory constraint of interneurons on neuronal excitability.

Tonic inhibition is thought to dampen the excitability of principal neurons; however, little is known about the role of tonic GABAergic inhibition in interneurons and the impact on principal neuron excitability. In many brain regions, tonic GABAergic inhibition is mediated by extrasynaptic, δ-subunit-containing GABAA receptors (GABAARs). In the present study we demonstrate the importance of GABAAR δ-subunit-mediated tonic inhibition in interneurons. Selective elimination of the GABAAR δ-subunit from interneurons was achieved by crossing a novel floxed Gabrd mouse model with GAD65-Cre mice (Gabrd/Gad mice). Deficits in GABAAR δ-subunit expression in GAD65-positive neurons result in a decrease in tonic GABAergic inhibition and increased excitability of both molecular layer (ML) and stratum radiatum (SR) interneurons. Disinhibition of interneurons results in robust alterations in the neuronal excitability of principal neurons and decreased seizure susceptibility. Gabrd/Gad mice have enhanced tonic and phasic GABAergic inhibition in both CA1 pyramidal neurons and dentate gyrus granule cells (DGGCs). Consistent with alterations in hippocampal excitability, CA1 pyramidal neurons and DGGCs from Gabrd/Gad mice exhibit a shift in the input-output relationship toward decreased excitability compared with those from Cre(-/-) littermates. Furthermore, seizure susceptibility, in response to 20 mg/kg kainic acid, is significantly decreased in Gabrd/Gad mice compared with Cre(-/-) controls. These data demonstrate a critical role for GABAAR δ-subunit-mediated tonic GABAergic inhibition of interneurons on principal neuronal excitability and seizure susceptibility.

Differential expression of GABAA receptor subunits δ and α6 mediates tonic inhibition in parvalbumin and somatostatin interneurons in the mouse hippocampus.

Inhibitory γ-aminobutyric acid (GABA)-ergic interneurons mediate inhibition in neuronal circuitry and support normal brain function. Consequently, dysregulation of inhibition is implicated in various brain disorders. Parvalbumin (PV) and somatostatin (SST) interneurons, the two major types of GABAergic inhibitory interneurons in the hippocampus, exhibit distinct morpho-physiological properties and coordinate information processing and memory formation. However, the molecular mechanisms underlying the specialized properties of PV and SST interneurons remain unclear. This study aimed to compare the transcriptomic differences between these two classes of interneurons in the hippocampus using the ribosome tagging approach. The results revealed distinct expressions of genes such as voltage-gated ion channels and GABAA receptor subunits between PV and SST interneurons. Gabrd and Gabra6 were identified as contributors to the contrasting tonic GABAergic inhibition observed in PV and SST interneurons. Moreover, some of the differentially expressed genes were associated with schizophrenia and epilepsy. In conclusion, our results provide molecular insights into the distinct roles of PV and SST interneurons in health and disease.

GABAA receptors as targets for treating affective and cognitive symptoms of depression.

In the past 20 years, our understanding of the pathophysiology of depression has evolved from a focus on an imbalance of monoaminergic neurotransmitters to a multifactorial picture including an improved understanding of the role of glutamatergic excitatory and GABAergic inhibitory neurotransmission. FDA-approved treatments targeting the glutamatergic [esketamine for major depressive disorder (MDD)] and GABAergic (brexanolone for peripartum depression) systems have become available. This review focuses on the GABAA receptor (GABAAR) system as a target for novel antidepressants and discusses the mechanisms by which modulation of δ-containing GABAARs with neuroactive steroids (NASs) or of α5-containing GABAARs results in antidepressant or antidepressant-like actions and discusses clinical data on NASs. Moreover, a potential mechanism by which α5-GABAAR-positive allosteric modulators (PAMs) may improve cognitive deficits in depression is presented.

Role of GABRD Gene Methylation in the Nucleus Accumbens in Heroin-Seeking Behavior in Rats.

Epigenetic modifications such as DNA methylation play important roles in regulating gene expression and may mediate neuroplasticity and lead to drug-induced aberrant behaviors. Although several brain regions and neurobiological mechanisms have been suggested to be involved in these processes, there is remarkably little known about the effects of DNA methylation on heroin-seeking behavior. Using a Sprague-Dawley rat model, we show that heroin self-administration resulted in gamma-aminobutyric acid type A receptor subunit delta (GABRD) gene hypomethylation, which was associated with transcriptional upregulation of GABRD in the nucleus accumbens (NAc). Systemic l-methionine (MET) administration significantly strengthened the reinstatement of heroin-seeking behavior induced by heroin priming, whereas intra-NAc injections of the DNA methyltransferase (DNMT) inhibitor 5-aza-2'-deoxycytidine (5-Aza-dC) had the opposite effect on heroin-seeking. Meanwhile, 5-Aza-dC treatment decreased DNA methylation and upregulated the expression of GABRD in the NAc, whereas MET had the opposite effect. Our results also reveal that 5-Aza-dC might alter the methylation landscape of the GABRD gene by directly repressing DNMT1 and DNMT3A expression. Furthermore, reinstatement of heroin-seeking behavior was significantly inhibited by directly overexpressing GABRD and remarkably reinforced by GABRD gene silencing in the NAc. Collectively, these results suggest that targeting the GABRD gene and its methylation might represent a novel pharmacological strategy for treating heroin addiction and relapse.

GABRD promotes progression and predicts poor prognosis in colorectal cancer.

Little is known about the functional roles of gamma-aminobutyric acid type A receptor subunit delta (GABRD) in colorectal cancer (CRC). The expression of GABRD between CRCs and adjacent normal tissues (NTs), metastasis and primary tumors was compared using public transcriptomic datasets. A tissue microarray and immunohistochemical staining (IHC) were used to determine the clinical and prognostic significance of the GABRD in CRC. We used gain-of-function and loss-of-function experiments to investigate the in vitro roles of GABRD in cultured CRC cells. We characterized the potential mechanism of GABRD's activities in CRC using a Gene Set Enrichment Analysis (GSEA) with The Cancer Genome Atlas Colon Adenocarcinoma (TCGA-COAD) dataset. We found that the GABRD expression was significantly increased in CRCs compared to that in NTs, but was similar between metastasis and primary tumors. Overexpression of GABRD was significantly associated with later pTNM stages and unfavorable patient survival. Overexpression of GABRD accelerated while knock-down of GABRD inhibited cell growth and migration. Mechanistically, the function of GABRD might be ascribed to its influence on major oncogenic events such as epithelial-mesenchymal transition (EMT), angiogenesis, and hedgehog signaling. Collectively, GABRD could be a novel prognostic predictor for CRC that deserves further investigation.

Increased Sensitivity of Mice Lacking Extrasynaptic δ-Containing GABAA Receptors to Histamine Receptor 3 Antagonists.

Histamine/gamma-aminobutyric acid (GABA) neurons of posterior hypothalamus send wide projections to many brain areas and participate in stabilizing the wake state. Recent research has suggested that GABA released from the histamine/GABA neurons acts on extrasynaptic GABAA receptors and balances the excitatory effect of histamine. In the current study, we show the presence of vesicular GABA transporter mRNA in a majority of quantified hypothalamic histaminergic neurons, which suggest vesicular release of GABA. As histamine/GABA neurons form conventional synapses infrequently, it is possible that GABA released from these neurons diffuses to target areas by volume transmission and acts on extrasynaptic GABA receptors. To investigate this hypothesis, mice lacking extrasynaptic GABAA receptor δ subunit (Gabrd KO) were used. A pharmacological approach was employed to activate histamine/GABA neurons and induce histamine and presumably, GABA, release. Control and Gabrd KO mice were treated with histamine receptor 3 (Hrh3) inverse agonists ciproxifan and pitolisant, which block Hrh3 autoreceptors on histamine/GABA neurons and histamine-dependently promote wakefulness. Low doses of ciproxifan (1 mg/kg) and pitolisant (5 mg/kg) reduced locomotion in Gabrd KO, but not in WT mice. EEG recording showed that Gabrd KO mice were also more sensitive to the wake-promoting effect of ciproxifan (3 mg/kg) than control mice. Low frequency delta waves, associated with NREM sleep, were significantly suppressed in Gabrd KO mice compared with the WT group. Ciproxifan-induced wakefulness was blocked by histamine synthesis inhibitor α-fluoromethylhistidine (αFMH). The findings indicate that both histamine and GABA, released from histamine/GABA neurons, are involved in regulation of brain arousal states and δ-containing subunit GABAA receptors are involved in mediating GABA response.

Systemic screening identifies GABRD, a subunit gene of GABAA receptor as a prognostic marker in adult IDH wild-type diffuse low-grade glioma.

Currently, no reliable prognostic biomarkers have been identified for adult IDH wild-type (WT) diffuse low grade glioma (LGG). With data from The Cancer Genome Atlas (TCGA)-LGG, we examined the prognostic value of GABAA receptor subunits in adult IDH WT LGG. Using 2016 WHO CNS tumor classification, we re-classified the TCGA-LGG and identified 95 IDH WT patients. Among 16 GABAA receptor subunit genes with RNA-seq data, eight genes showed significantly different expression in IDH WT LGG compared with IDH mutant (MT) cases. Among these genes, only GABRD expression was related to overall survival (OS) status. Preserved GABRD expression was independently associated with longer OS (HR: 0.799, 95%CI: 0.691-0.925, p = 0.003) in IDH WT LGG. GABRD expression showed a moderately negative correlation with tumor infiltration macrophage (TIM) and CSF1 expression. The methylation status of 34 CpG sites across GABRD gene was checked and only cg13916816 showed a moderately negative correlation with GABRD expression. In conclusion, GABRD expression might serve as a potential independent prognostic marker in patients with IDH WT LGG. Meanwhile, its expression was negatively correlated with the extent of TIM, which might help to explain the favorable survival outcome. Cg13916816 might be a critical CpG site influencing GABRD expression in IDH WT LGG.

Association of GABAA Receptor Gene with Epilepsy Syndromes.

GABA has always been an inviting target in the etiology and treatment of epilepsy. The GABRA1, GABRG2, and GABRD genes provide instructions for making α1, ϒ2, and δ subunits of GABAA receptor protein respectively. GABAA is considered as one of the most important proteins and has found to play an important role in many neurological disorders. We explored the association of GABAA receptor gene mutation/SNPs in JME and LGS patients in Indian population. A total of 100 epilepsy syndrome patients (50 JME and 50 LGS) and 100 healthy control subjects were recruited and analyzed by AS-PCR and RFLP-PCR techniques. In our study, GABRA1 965 C > A mutation and 15 A > G polymorphism gene may play an important role in modulating the drug efficacy in LGS patients. The GABRA1 15 A > G polymorphism may also play an important role in the susceptibility of LGS and the inheritance of GG genotype of this polymorphism may provide an increased risk of development of LGS. The GABRG2 588 C > T polymorphism may decrease the duration of seizures in JME patients. The GABRD 659 G > A polymorphism may play an important role in the susceptibility of JME and LGS and this polymorphism may also increase the duration of postictal period in JME patients but may decrease the duration of seizure in LGS patients.

Temporal Regulation of GABAA Receptor Subunit Expression: Role in Synaptic and Extrasynaptic Communication in the Suprachiasmatic Nucleus.

Recent molecular studies suggest that the expression levels of δ and γ2 GABAA receptor (GABAAR) subunits regulate the balance between synaptic and extrasynaptic GABA neurotransmission in multiple brain regions. We investigated the expression of GABAAδ and GABAAγ2 and the functional significance of a change in balance between these subunits in a robust local GABA network contained within the suprachiasmatic nucleus of the hypothalamus (SCN). Muscimol, which can activate both synaptic and extrasynaptic GABAARs, injected into the SCN during the day phase advanced the circadian pacemaker, whereas injection of the extrasynaptic GABAA superagonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP) had no effect on circadian phase. In contrast, injection of either THIP or muscimol during the night was sufficient to block the phase shifting effects of light. Gene expression analysis of the whole SCN revealed different temporal patterns in GABAAδ and GABAAγ2 mRNA expression. When examined across all subregions of the SCN, quantitative immunohistochemical analysis found no significant variations in GABAAδ protein immunoreactivity (IR) but did find significant variations in GABAAγ2 protein-IR in hamsters housed in either LD cycles or in constant darkness. Remarkably, significant interactions in the ratio of GABAAδ:GABAAγ2 subunits between lighting condition and circadian phase occurred only within one highly discrete anatomical area of the SCN; a region that functions as the input for lighting information from the retina. Taken together, these data support the hypothesis that the balance between synaptic and extrasynaptic GABAARs determines the functional response to GABA, and that this balance is differentially regulated in a region-specific manner.

GABAergic regulation of the HPA and HPG axes and the impact of stress on reproductive function.

The hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes are regulated by GABAergic signaling at the level of corticotropin-releasing hormone (CRH) and gonadotropin-releasing hormone (GnRH) neurons, respectively. Under basal conditions, activity of CRH and GnRH neurons are controlled in part by both phasic and tonic GABAergic inhibition, mediated by synaptic and extrasynaptic GABAA receptors (GABAARs), respectively. For CRH neurons, this tonic GABAergic inhibition is mediated by extrasynaptic, δ subunit-containing GABAARs. Similarly, a THIP-sensitive tonic GABAergic current has been shown to regulate GnRH neurons, suggesting a role for δ subunit-containing GABAARs; however, this remains to be explicitly demonstrated. GABAARs incorporating the δ subunit confer neurosteroid sensitivity, suggesting a potential role for neurosteroid modulation in the regulation of the HPA and HPG axes. Thus, stress-derived neurosteroids may contribute to the impact of stress on reproductive function. Interestingly, excitatory actions of GABA have been demonstrated in both CRH neurons at the apex of control of the HPA axis and in GnRH neurons which mediate the HPG axis, adding to the complexity for the role of GABAergic signaling in the regulation of these systems. Here we review the effects that stress has on GnRH neurons and HPG axis function alongside evidence supporting GABAARs as a major interface between the stress and reproductive axes.