Positive association between dehydroepiandrosterone (DHEA) and gene expression of the gamma-aminobutyric acid (GABA-A) receptor δ subunit

Gamma-aminobutyric acid A (GABA-A) receptors in the cells of the immune system enhance anti-inflammatory responses by regulating cytokine secretion, cytotoxic responses, and cell activation. In the CNS, the formation of GABA-A subunits into a pentameric structure has been extensively studied; however, no such study has been conducted in the immune system. The objective of the present study was to examine associations between the levels of steroid hormones and GABA-A receptor δ subunit expression in the immune system. We focused on this subunit because GABA-A receptors that contain it become significantly more sensitive to steroid hormones. We collected 80 blood samples from reproductive age women for the purpose of analyzing dehydroepiandrosterone (DHEA), 17β-estradiol, progesterone, and allopregnanolone using liquid chromatography-mass spectrometry (LC-MS). Furthermore, we extracted peripheral blood mononuclear cells (PBMCs) for determining mRNA expression levels of GABA-A receptor genes encoding the δ and ε subunits. We constructed linear mixed effect models for each GABA-A receptor subunit with all 4 steroid hormones, age, and age of menarche as predictors. Whereas DHEA was significantly associated with δ subunit expression (t-value = 2.981; p = 0.003), in line with our hypothesis, none of the steroid hormones were significantly associated with the expression of the ε subunit. Results of this study indicate that significant interactions between hormones from the steroid hormone biosynthesis pathway and GABAergic machinery from the immune cells may be utilized to expand models examining the molecular basis of inflammatory conditions.

Allopregnanolone and its antagonist modulate neuroinflammation and neurological impairment

Neuroinflammation accompanies several brain disorders, either as a secondary consequence or as a primary cause and may contribute importantly to disease pathogenesis. Neurosteroids which act as Positive Steroid Allosteric GABA-A receptor Modulators (Steroid-PAM) appear to modulate neuroinflammation and their levels in the brain may vary because of increased or decreased local production or import from the systemic circulation. The increased synthesis of steroid-PAMs is possibly due to increased expression of the mitochondrial cholesterol transporting protein (TSPO) in neuroinflammatory tissue, and reduced production may be due to changes in the enzymatic activity. Microglia and astrocytes play an important role in neuroinflammation, and their production of inflammatory mediators can be both activated and inhibited by steroid-PAMs and GABA. What is surprising is the finding that both allopregnanolone, a steroid-PAM, and golexanolone, a novel GABA-A receptor modulating steroid antagonist (GAMSA), can inhibit microglia and astrocyte activation and normalize their function. This review focuses on the role of steroid-PAMs in neuroinflammation and their importance in new therapeutic approaches to CNS and liver disease.

Neuroendocrine insights into neurosteroid therapy for postpartum depression

Postpartum depression (PPD) is associated with a decline in progesterone-derived anxiolytic-antidepressant neurosteroids after delivery. Neurosteroid replacement therapy (NRT) with GABA-A receptor-modulating allopregnanolone (brexanolone) shows promise as the first drug treatment for PPD. Here we describe the molecular insights of the neurosteroid approach for rapid relief of PPD symptoms compared with traditional antidepressants.

Preclinical and clinical pharmacology of brexanolone (allopregnanolone) for postpartum depression: a landmark journey from concept to clinic in neurosteroid replacement therapy

This article describes the critical role of neurosteroids in postpartum depression (PPD) and outlines the landmark pharmacological journey of brexanolone as a first-in-class neurosteroid antidepressant with significant advantages over traditional antidepressants. PPD is a neuroendocrine disorder that affects about 20% of mothers after childbirth and is characterized by symptoms including persistent sadness, fatigue, dysphoria, as well as disturbances in cognition, emotion, appetite, and sleep. The main pathology behind PPD is the postpartum reduction of neurosteroids, referred to as neurosteroid withdrawal, a concept pioneered by our preclinical studies. We developed neurosteroid replacement therapy (NRT) as a rational approach for treating PPD and other conditions related to neurosteroid deficiency, unveiling the power of neurosteroids as novel anxiolytic-antidepressants. The neurosteroid, brexanolone (BX), is a progesterone-derived allopregnanolone that rapidly relieves anxiety and mood deficits by activating GABA-A receptors, making it a transformational treatment for PPD. In 2019, the FDA approved BX, an intravenous formulation of allopregnanolone, as an NRT to treat PPD. In clinical studies, BX significantly improved PPD symptoms within hours of administration, with tolerable side effects including headache, dizziness, and somnolence. We identified the molecular mechanism of BX in a neuronal PPD-like milieu. The mechanism of BX involves activation of both synaptic and extrasynaptic GABA-A receptors, which promote tonic inhibition and serve as a key target for PPD and related conditions. Neurosteroids offer several advantages over traditional antidepressants, including rapid onset, unique mechanism, and lack of tolerance upon repeated use. Some limitations of BX therapy include lack of aqueous solubility, limited accessibility, hospitalization for treatment, lack of oral product, and serious adverse events at high doses. However, the unmet need for synthetic neurosteroids to address this critical condition supersedes these limitations. Recently, we developed novel hydrophilic neurosteroids with a superior profile and improved drug delivery. Overall, approval of BX is a major milestone in the field of neurotherapeutics, paving the way for the development of novel synthetic neurosteroids to treat depression, epilepsy, and status epilepticus.

The GABA system, a new target for medications against cognitive impairment-Associated with neuroactive steroids

The prevalence of cognitive dysfunction, dementia, and neurodegenerative disorders such as Alzheimer's disease (AD) is increasing in parallel with an aging population. Distinct types of chronic stress are thought to be instrumental in the development of cognitive impairment in central nervous system (CNS) disorders where cognitive impairment is a major unmet medical need. Increased GABAergic tone is a mediator of stress effects but is also a result of other factors in CNS disorders. Positive GABA-A receptor modulating stress and sex steroids (steroid-PAMs) such as allopregnanolone (ALLO) and medroxyprogesterone acetate can provoke impaired cognition. As such, ALLO impairs memory and learning in both animals and humans. In transgenic AD animal studies, continuous exposure to ALLO at physiological levels impairs cognition and increases degenerative AD pathology, whereas intermittent ALLO injections enhance cognition, indicating pleiotropic functions of ALLO. We have shown that GABA-A receptor modulating steroid antagonists (GAMSAs) can block the acute negative cognitive impairment of ALLO on memory in animal studies and in patients with cognitive impairment due to hepatic encephalopathy. Here we describe disorders affected by steroid-PAMs and opportunities to treat these adverse effects of steroid-PAMs with novel GAMSAs.

In Vivo Evaluation of Self-assembled nano-Saikosaponin-a for Epilepsy Treatment

Saikosaponin-a (SSa) exhibits antiepileptic effects. However, its poor water solubility and inability to pass through the blood-brain barrier greatly limit its clinical development and application. In this study, SSa-loaded Methoxy poly (ethylene glycol)-poly(ε-caprolactone) (MePEG-SSa-PCL) NPs were successfully prepared and characterized. Our objective was to further investigate the effect of this composite on acute seizure in mice. First, we confirmed the particle size and surface potential of the composite (51.00 ± 0.25 nm and - 33.77 ± 2.04 mV, respectively). Further, we compared the effects of various MePEG-SSa-PCL doses (low, medium, and high) with those of free SSa, valproic acid (VPA - positive control), and saline only (model group) on acute seizure using three different acute epilepsy mouse models. We observed that compared with the model group, the three MePEG-SSa-PCL treatments showed significantly lowered seizure frequency in mice belonging to the maximum electroconvulsive model group. In the pentylenetetrazol and kainic acid (KA) acute epilepsy models, MePEG-SSa-PCL increased both clonic and convulsion latency periods and shortened convulsion duration more effectively than equivalent SSa-only doses. Furthermore, hematoxylin-eosin and Nissl staining revealed considerably less neuronal damage in the hippocampal CA3 area of KA mice in the SSa, VPA, and three MePEG-SSa-PCL groups relative to mice in the model group. Hippocampal gamma-aminobutyric acid-A (GABA-A) receptor and cleaved caspase-3 expression levels in KA mice were significantly higher and lower, respectively, in the three MePEG-SSa-PCL treatment groups than in the model group. Thus, MePEG-SSa-PCL exhibited a more potent antiepileptic effect than SSa in acute mouse epilepsy models and could alleviate neuronal damage in the hippocampus following epileptic seizures, possibly via GABA-A receptor expression upregulation.

Bibliometric Analysis on GABA-A Receptors Research Based on CiteSpace and VOSviewer

Background

GABA-A receptors are the primary mediators of brain inhibitory neurotransmission. In the past years, many studies focused on this channel to decipher the pathogenesis of related diseases but lacked bibliometric analysis research. This study aims to explore the research status and identify the research trends of GABA-A receptor channels.

Methods

Publications related to GABA-A receptor channels were retrieved from the Web of Science Core Collection from 2012 to 2022. After screening, the VOSviewer 1.6.18 and Citespace 5.8 R3 were used for bibliometric analysis from journals, countries, institutions, authors, co-cited references and keywords.

Results

We included 12,124 publications in the field of GABA-A receptor channels for analysis. The data shows that although there was a slight decrease in annual publications from 2012 to 2021, it remained at a relatively high level. Most publications were in the domain of neuroscience. Additionally, the United States was the most prolific country, followed by China. Univ Toronto was the most productive institution, and James M Cook led essential findings in this field. Furthermore, brain activation, GABAAR subunits expression, modulation mechanism in pain and anxiety behaviors and GABA and dopamine were paid attention to by researchers. And top research frontiers were molecular docking, autoimmune encephalitic series, obesity, sex difference, diagnosis and management, EEG and KCC2.

Conclusion

Taken together, academic attention on GABA-A receptor channels was never neglected since 2012. Our analysis identified key information, such as core countries, institutions and authors in this field. Molecular docking, autoimmune encephalitic series, obesity, sex difference, diagnosis and management, EEG and KCC2 will be the future research direction.

Dual Molecules Targeting 5-HT6 and GABA-A Receptors as a New Approach to Combat Depression Associated with Neuroinflammation

While monoaminergic deficits are evident in all depressed patients, nonresponders are characterized by impaired GABA-ergic signaling and the simultaneous presence of the inflammatory component. Pharmacological agents able to curb pathological immune responses and modulate ineffective GABA-ergic neurotransmission are thought to improve therapeutic outcomes in the treatment-resistant subgroup of depressed patients. Here, we report on a set of dually acting molecules designed to simultaneously modulate GABA-A and 5-HT₆ receptor activity. The serotonin 5-HT₆ receptor was chosen as a complementary molecular target, due to its promising antidepressant-like activities reported in animal studies. Within the study we identified that lead molecule 16 showed a desirable receptor profile and physicochemical properties. In pharmacological studies, 16 was able to reduce the secretion of proinflammatory cytokines and decrease oxidative stress markers. In animal studies, 16 exerted antidepressant-like activity deriving from a synergic interplay between 5-HT₆ and GABA-A receptors. Altogether, the presented findings point to hybrid 16 as an interesting tool that interacts with pharmacologically relevant targets, matching the pathological dysfunction of depression associated with neuroinflammation.

2-(4-Fluorophenyl)-1H-benzo[d]imidazole as a Promising Template for the Development of Metabolically Robust, α1β2γ2GABA-A Receptor-Positive Allosteric Modulators

Modulation of α1β2γ2GABA-A receptor subpopulation expressed in the basal ganglia region is a conceptually novel mode of pharmacological strategy that offers prospects to tackle a variety of neurological dysfunction. Although clinical findings provided compelling evidence for the validity of this strategy, the current chemical space of molecules able to modulate the α1/γ2 interface of the GABA-A receptor is limited to imidazo[1,2-a]pyridine derivatives that undergo rapid biotransformation. In response to a deficiency in the chemical repertoire of GABA-A receptors, we identified a series of 2-(4-fluorophenyl)-1H-benzo[d]imidazoles as positive allosteric modulators (PAMs) with improved metabolic stability and reduced potential for hepatotoxicity, where lead molecules 9 and 23 displayed interesting features in a preliminary investigation. We further disclose that the identified scaffold shows a preference for interaction with the α1/γ2 interface of the GABA-A receptor, delivering several PAMs of the GABA-A receptor. The present work provides useful chemical templates to further explore the therapeutic potential of GABA-A receptor ligands and enriches the chemical space of molecules suitable for the interaction with the α1/γ2 interface.

Projections from the Rostral Zona Incerta to the Thalamic Paraventricular Nucleus Mediate Nociceptive Neurotransmission in Mice

Zona incerta (ZI) is an integrative subthalamic region in nociceptive neurotransmission. Previous studies demonstrated that the rostral ZI (ZIR) is an important gamma-aminobutyric acid-ergic (GABAergic) source to the thalamic paraventricular nucleus (PVT), but whether the ZIR-PVT pathway participates in nociceptive modulation is still unclear. Therefore, our investigation utilized anatomical tracing, fiber photometry, chemogenetic, optogenetic and local pharmacological approaches to investigate the roles of the ZIR^GABA+-PVT pathway in nociceptive neurotransmission in mice. We found that projections from the GABAergic neurons in ZIR to PVT were involved in nociceptive neurotransmission. Furthermore, chemogenetic and optogenetic activation of the ZIR^GABA+-PVT pathway alleviates pain, whereas inhibiting the activities of the ZIR^GABA+-PVT circuit induces mechanical hypersensitivity and partial heat hyperalgesia. Importantly, in vivo pharmacology combined with optogenetics revealed that the GABA-A receptor (GABA_AR) is crucial for GABAergic inhibition from ZIR to PVT. Our data suggest that the ZIR^GABA+-PVT pathway acts through GABA_AR-expressing glutamatergic neurons in PVT mediates nociceptive neurotransmission.

GABRG1 variant as a potential novel cause of epileptic encephalopathy, hypotonia, and global developmental delay

Epileptic encephalopathies (EEs) are severe brain disorders with excessive ictal (seizure) and interictal (electrographic epileptiform discharges) activity in developing brain which may result in progressive cognitive and neuropsychological deterioration. In contrast to regular epilepsy where the treatment goal is to prevent the seizure (ictal) recurrence, in patients with EE the goal is to treat both ictal as well as interictal activity to prevent further progression. With the introduction of genetic sequencing technologies over the past 20 years, there is growing recognition of the genetic basis of EE, with the majority due to monogenic causes. Monogenic etiologies of EE include pathogenic variants in the γ-aminobutyric acid type A receptor (GABA-A) encoding gene family. We present a 2-year-old patient with EE, hypotonia, and global developmental delays. Clinical trio exome sequencing showed a novel, de novo variant in GABRG1. GABRG1 encodes the γ1 subunit of the GABA-A receptor. To date, there has not been an association of EE with pathogenic variants in GABRG1. This variant is predicted to be damaging to protein structure and function, and the patient's phenotype is similar to those with pathogenic variants in other members of the GABA-A receptor encoding gene family.

Gut Inflammation Induced by Finasteride Withdrawal: Therapeutic Effect of Allopregnanolone in Adult Male Rats

The treatment with finasteride (i.e., an inhibitor of 5α-reductase) may be associated with different side effects (i.e., depression, anxiety, cognitive impairment and sexual dysfunction) inducing the so-called post finasteride syndrome (PFS). Moreover, previous observations in PFS patients and an experimental model showed alterations in gut microbiota populations, suggesting an inflammatory environment. To confirm this hypothesis, we have explored the effect of chronic treatment with finasteride (i.e., for 20 days) and its withdrawal (i.e., for 1 month) on the levels of steroids, neurotransmitters, pro-inflammatory cytokines and gut permeability markers in the colon of adult male rat. The obtained data demonstrate that the levels of allopregnanolone (ALLO) decreased after finasteride treatment and after its withdrawal. Following the drug suspension, the decrease in ALLO levels correlates with an increase in IL-1β and TNF-α, serotonin and a decrease in dopamine. Importantly, ALLO treatment is able to counteract some of these alterations. The relation between ALLO and GABA-A receptors and/or pregnenolone (ALLO precursor) could be crucial in their mode of action. These observations provide an important background to explore further the protective effect of ALLO in the PFS experimental model and the possibility of its translation into clinical therapy.

Medroxyprogesterone acetate positively modulates specific GABAA-receptor subtypes - affecting memory and cognition

Medroxyprogesterone acetate (MPA) is a progestin widely used in humans as hormone replacement therapy and at other indications. Many progestin metabolites, as the progesterone metabolite allopregnanolone, have GABA_A-receptor modulatory effects and are known to affect memory, learning, appetite, and mood. In women, 4 years chronic treatment with MPA doubles the frequency of dementia and in rats, MPA causes cognitive impairment related to the GABAergic system. Activation of the membrane bound GABA_A receptor results in a chloride ion flux that can be studied by whole-cell patch-clamp electrophysiological recordings. The purpose of this study was to clarify the modulatory effects of MPA and specific MPA metabolites, with structures like known GABA_A-receptor modulators, on different GABA_A-receptor subtypes. An additional aim was to verify the results as steroid effects on GABA response in single cells taken from rat hypothalamus. HEK-293 cell-lines permanently expressing the recombinant human GABA_A-receptor subtype α1β2γ2L or α5β3γ2L or α2β3γ2S were created. The MPA metabolites 3α5α-MPA,3β5α-MPA and 3β5β-MPA were synthesised and purified for electrophysiological patch-clamp measurements with a Dynaflow system. The effects of MPA and tetrahydrodeoxycorticosterone were also studied. None of the studied MPA metabolites affected the responses mediated by α1β2γ2L or α5β3γ2L GABA_A receptors. Contrary, MPA clearly acted both as a positive modulator and as a direct activator of the α5β3γ2L and α2β3γ2S GABA_A receptors. However, in concentrations up to 10 μM, MPA was inactive at the α1β2γ2L GABA_A receptor. In the patch-clamp recordings from dissociated cells of the preoptic area in rats, MPA increased the amplitude of responses to GABA. In addition, MPA alone without added GABA, evoked a current response. In conclusion, MPA acts as a positive modulator of specific GABA_A receptor subtypes expressed in HEK cells and at native GABA receptors in single cells from the hypothalamic preoptic area.

High Fat Diet Increases [3H] Flunitrazepam Binding in the Mouse Brain that is Dependent on the Expression of the Dopamine D2 Gene

Dopamine is an important neuromodulator in the brain that binds to dopamine D1-like receptors (D1, D5) as well as dopamine D2-like receptors (D2, D3, D4). The D2 receptor is known to play an integral role in a variety of physiological processes including addictive behaviors, locomotion, motivation, feeding behavior, and more. It was recently reported that dopamine is a direct-acting modulator of mammalian GABA(A) receptors. To this end, we wanted to examine how the expression of the dopamine D2 gene impacts the expression of GABA(A) receptors in the brain under different dietary conditions. Adult female Drd2 wild-type (WT), heterozygous (HT), and knockout (KO) mice were given either normal or high-fat diet for a period of 30 weeks. Following this, their brains were collected for [³H] Flunitrazepam binding in order to assess GABA(A) receptor expression. A high fat diet significantly increased [³H] Flunitrazepam binding in the regions of the somatosensory cortex, striatum, and various other cortical areas within WT mice. In contrast, no effect of diet was observed in HT or KO mice. As such, HT and KO mice displayed reduced [³H] Flunitrazepam binding in these areas relative to WT mice under high-fat dietary conditions. The effect of a high-fat diet on [³H] Flunitrazepam binding is consistent with recent evidence showing increases in GABA neurotransmitter levels following a high-fat diet. We demonstrate for the first time that the expression of the D2 gene plays a prominent role in the ability of a high-fat diet to impact GABA(A) receptors in the mouse brain.

Suppressive Modulation of the Chick Forebrain Network for Imprinting by Thyroid Hormone: An in Vitro Study

The thyroid hormone 3,5,3'-triiodothyronine (T3) is considered to act acutely in the chick forebrain because focal infusion of T3 to the intermediate medial mesopallium (IMM) causes 4 to 6-day-old hatchlings to become imprintable approximately 30 min after the infusion. To understand the mechanism of this acute T3 action, we examined synaptic responses of IMM neurons in slice preparations in vitro. Extracellular field potential responses to local electrical stimulation were pharmacologically dissociated to synaptic components mediated by AMPA and NMDA receptors, as well as GABA-A and -B receptors. Bath-applied T3 (20-40 μM) enhanced the positive peak amplitude of the field potential, which represented the GABA-A component. Bicuculline induced spontaneous epileptic bursts by NMDA receptor activation, and subsequent application of T3 suppressed the bursting frequency. Pretreatment of slices with T3 failed to influence the synaptic potentiation caused by tetanic stimulation. Intracellular whole-cell recording using a patch electrode confirmed the T3 actions on the GABA-A and NMDA components. T3 enhanced the GABA-A response and suppressed the NMDA plateau potential without changes in the resting membrane potential or the threshold of action potentials. Contrary to our initial expectation, T3 suppressed the synaptic drives of IMM neurons, and did not influence activity-dependent synaptic potentiation. Imprinting-associated T3 influx may act as an acute suppressor of the IMM network.

Genetic Code Expansion and Click-Chemistry Labeling to Visualize GABA-A Receptors by Super-Resolution Microscopy

Fluorescence labeling of difficult to access protein sites, e.g., in confined compartments, requires small fluorescent labels that can be covalently tethered at well-defined positions with high efficiency. Here, we report site-specific labeling of the extracellular domain of γ-aminobutyric acid type A (GABA-A) receptor subunits by genetic code expansion (GCE) with unnatural amino acids (ncAA) combined with bioorthogonal click-chemistry labeling with tetrazine dyes in HEK-293-T cells and primary cultured neurons. After optimization of GABA-A receptor expression and labeling efficiency, most effective variants were selected for super-resolution microscopy and functionality testing by whole-cell patch clamp. Our results show that GCE with ncAA and bioorthogonal click labeling with small tetrazine dyes represents a versatile method for highly efficient site-specific fluorescence labeling of proteins in a crowded environment, e.g., extracellular protein domains in confined compartments such as the synaptic cleft.

GABAergic signaling in human and murine NK cells upon challenge with Toxoplasma gondii

Protective cytotoxic and proinflammatory cytokine responses by NK cells impact the outcome of infections by Toxoplasma gondii, a common parasite in humans and other vertebrates. However, T. gondii can also sequester within NK cells and downmodulate their effector functions. Recently, the implication of GABA signaling in infection and inflammation-related responses of mononuclear phagocytes and T cells has become evident. Yet, the role of GABAergic signaling in NK cells has remained unknown. Here, we report that human and murine NK cells synthesize and secrete GABA in response to infection challenge. Parasitized NK cells secreted GABA, whereas activation stimuli, such as IL-12/IL-18 or parasite lysates, failed to induce GABA secretion. GABA secretion by NK cells was associated to a transcriptional up-regulation of GABA synthesis enzymes (glutamate decarboxylases [GAD65/67]) and was abrogated by GAD inhibition. Further, NK cells expressed GABA-A receptor subunits and GABA signaling regulators, with transcriptional modulations taking place upon challenge with T. gondii. Exogenous GABA and GABA-containing supernatants from parasitized dendritic cells (DCs) impacted NK cell function by reducing the degranulation and cytotoxicity of NK cells. Conversely, GABA-containing supernatants from NK cells enhanced the migratory responses of parasitized DCs. This enhanced DC migration was abolished by GABA-A receptor antagonism or GAD inhibition and was reconstituted by exogenous GABA. Jointly, the data show that NK cells are GABAergic cells and that GABA hampers NK cell cytotoxicity in vitro. We hypothesize that GABA secreted by parasitized immune cells modulates the immune responses to T. gondii infection.

Axonal GABAA stabilizes excitability in unmyelinated sensory axons secondary to NKCC1 activity

KEY POINTS

GABA depolarized sural nerve axons and increased the electrical excitability of C-fibres via GABAA receptor. Axonal excitability responses to GABA increased monotonically with the rate of action potential firing. Action potential activity in unmyelinated C-fibres is coupled to Na-K-Cl cotransporter type 1 (NKCC1) loading of axonal chloride. Activation of axonal GABAA receptor stabilized C-fibre excitability during prolonged low frequency (2.5 Hz) firing. NKCC1 maintains intra-axonal chloride to provide feed-forward stabilization of C-fibre excitability and thus support sustained firing.

ABSTRACT

GABAA receptor (GABAA R)-mediated depolarization of dorsal root ganglia (DRG) axonal projections in the spinal dorsal horn is implicated in pre-synaptic inhibition. Inhibition, in this case, is predicated on an elevated intra-axonal chloride concentration and a depolarizing GABA response. In the present study, we report that the peripheral axons of DRG neurons are also depolarized by GABA and this results in an increase in the electrical excitability of unmyelinated C-fibre axons. GABAA R agonists increased axonal excitability, whereas GABA excitability responses were blocked by GABAA R antagonists and were absent in mice lacking the GABAA R β3 subunit selectively in DRG neurons (AdvillinCre or snsCre ). Under control conditions, excitability responses to GABA became larger at higher rates of electrical stimulation (0.5-2.5 Hz). However, during Na-K-Cl cotransporter type 1 (NKCC1) blockade, the electrical stimulation rate did not affect GABA response size, suggesting that NKCC1 regulation of axonal chloride is coupled to action potential firing. To examine this, activity-dependent conduction velocity slowing (activity-dependent slowing; ADS) was used to quantify C-fibre excitability loss during a 2.5 Hz challenge. ADS was reduced by GABAA R agonists and exacerbated by either GABAA R antagonists, β3 deletion or NKCC1 blockade. This illustrates that activation of GABAA R stabilizes C-fibre excitability during sustained firing. We posit that NKCC1 acts in a feed-forward manner to maintain an elevated intra-axonal chloride in C-fibres during ongoing firing. The resulting chloride gradient can be utilized by GABAA R to stabilize axonal excitability. The data imply that therapeutic strategies targeting axonal chloride regulation at peripheral loci of pain and itch may curtail aberrant firing in C-fibres.

The dose makes the poison: Non-linear behavioural response to CO2-induced aquatic acidification in zebrafish (Danio rerio)

CO₂-induced aquatic acidification is predicted to affect fish neuronal GABA_A receptors leading to widespread behavioural alterations. However, the large variability in the magnitude and direction of behavioural responses suggests substantial species-specific CO₂ threshold differences, life history and parental acclimation effects, experimental artifacts, or a combination of these factors. As an established model organism, zebrafish (Danio rerio) can be reared under stable conditions for multiple generations, which may help control for some of the variability observed in wild-caught fishes. Here, we used two standardized tests to investigate the effect of 1-week acclimatization to four pCO₂ levels on zebrafish anxiety-like behaviour, exploratory behaviour, and locomotion. Fish acclimatized to 900 μatm CO₂ demonstrated increased anxiety-like behaviour compared to control fish (~480 μatm), however, the behaviour of fish exposed to 2200 μatm CO₂ was indistinguishable from that of controls. In addition, fish acclimatized to 4200 μatm CO₂ had decreased anxiety-like behaviour; i.e. the opposite response than the 900 μatm CO₂ treatment. On the other hand, exploratory behaviour did not differ among any of the pCO₂ exposures that were tested. Thus, zebrafish behavioural responses to elevated pCO₂ are not linear; with potential important implications for physiological, environmental, and aquatic acidification studies.

Microglial Cell Morphology and Phagocytic Activity Are Critically Regulated by the Neurosteroid Allopregnanolone: A Possible Role in Neuroprotection

Microglial cells are key players in neural pathogenesis and microglial function regulation appears to be pivotal in controlling neuroinflammatory/neurological diseases. Here, we investigated the effects and mechanism of action of neurosteroid allopregnanolone (ALLO) on murine microglial BV-2 cells and primary microglia in order to determine ALLO-induced immunomodulatory potential and to provide new insights for the development of both natural and safe neuroprotective strategies targeting microglia. Indeed, ALLO-treatment is increasingly suggested as beneficial in various models of neurological disorders but the underlying mechanisms have not been elucidated. Therefore, the microglial cells were cultured with various serum concentrations to mimic the blood-brain-barrier rupture and to induce their activation. Proliferation, viability, RT-qPCR, phagocytosis, and morphology analyzes, as well as migration with time-lapse imaging and quantitative morphodynamic methods, were combined to investigate ALLO actions on microglia. BV-2 cells express subunits of GABA-A receptor that mediates ALLO activity. ALLO (10µM) induced microglial cell process extension and decreased migratory capacity. Interestingly, ALLO modulated the phagocytic activity of BV-2 cells and primary microglia. Our results, which show a direct effect of ALLO on microglial morphology and phagocytic function, suggest that the natural neurosteroid-based approach may contribute to developing effective strategies against neurological disorders that are evoked by microglia-related abnormalities.

GABA-A receptor signaling in the anterior cingulate cortex modulates aggression and anxiety-related behaviors in socially isolated mice

Dysfunctional modulation of brain circuits that regulate the emotional response to potentially threatening stimuli is associated to an inappropriate representation of the emotional salience. Reduced top-down control by cortical areas is assumed to underlie several behavioral abnormalities including aggression and anxiety related behaviors. Previous studies have identified disrupted GABA signaling in the anterior cingulate cortex (ACC) as a possible mechanism underlying the top-down regulation of aggression and anxiety. In this study, we investigate a role for GABA-A receptor in the ACC in the regulation of aggression and anxiety related behaviors in socially isolated mice. We evaluated the effects of site directed injections of the GABA-A receptor agonist, muscimol or the GABA-A receptor antagonist, bicuculline into the ACC on these behaviors. Results showed that hyper-aggressive behavior, the anxiety and avoidance behavior in socially isolated mice were increased by muscimol microinfusion into ACC, while the sociability was not affected. In contrast, hyper-aggressive behavior in socially isolated mice was inhibited following bicuculline microinfusion without affecting anxiety. Furthermore, microinfusion of bicuculline into ACC decreased avoidance intensity and significantly reinforced social behavior, suggesting that GABA-A receptor inhibition in ACC specifically regulated aggression and sociability. Together, our results confirm a role for GABA-A receptor signaling in the ACC in the regulation of aggressive, social and anxiety related behaviors in socially isolated mice.

Study on a Three-Step Rapid Assembly of Zolpidem and Its Fluorinated Analogues Employing Microwave-Assisted Chemistry

We developed an efficient microwave-assisted three-step synthesis of zolpidem and its fluorinated analogues 1–3. The procedure relays on the utilization of easily accessible and inexpensive starting materials. Our protocol shows superior performance in terms of yield and purity of products, compared to conventional heating systems. Notably, the total time needed for reaction accomplishment is significantly lower comparing to oil bath heating systems. Finally, we have performed a detailed study on the preparation of zolpidem tartrate salt I, and we assessed its particle-sizes using a polarizing microscope. Our goal was to select the appropriate method that generates the acceptable particle-size, since the solid-size directly influences solubility in biological fluids and further bioavailability. We believe that the disclosed procedure will help to produce a lab-scale quantity of zolpidem and its fluorinated derivatives 1–3, as well as zolpidem tartrate salt I, with suitable fine-particle size for further biological experimentation.

Photoaffinity labeling identifies an intersubunit steroid-binding site in heteromeric GABA type A (GABAA) receptors

Allopregnanolone (3α5α-P), pregnanolone, and their synthetic derivatives are potent positive allosteric modulators (PAMs) of GABA_A receptors (GABA_ARs) with in vivo anesthetic, anxiolytic, and anti-convulsant effects. Mutational analysis, photoaffinity labeling, and structural studies have provided evidence for intersubunit and intrasubunit steroid-binding sites in the GABA_AR transmembrane domain, but revealed only little definition of their binding properties. Here, we identified steroid-binding sites in purified human α1β3 and α1β3γ2 GABA_ARs by photoaffinity labeling with [³H]21-[4-(3-(trifluoromethyl)-3H-diazirine-3-yl)benzoxy]allopregnanolone ([³H]21-pTFDBzox-AP), a potent GABA_AR PAM. Protein microsequencing established 3α5α-P inhibitable photolabeling of amino acids near the cytoplasmic end of the β subunit M4 (β3Pro-415, β3Leu-417, and β3Thr-418) and M3 (β3Arg-309) helices located at the base of a pocket in the β⁺-α^- subunit interface that extends to the level of αGln-242, a steroid sensitivity determinant in the αM1 helix. Competition photolabeling established that this site binds with high affinity a structurally diverse group of 3α-OH steroids that act as anesthetics, anti-epileptics, and anti-depressants. The presence of a 3α-OH was crucial: 3-acetylated, 3-deoxy, and 3-oxo analogs of 3α5α-P, as well as 3β-OH analogs that are GABA_AR antagonists, bound with at least 1000-fold lower affinity than 3α5α-P. Similarly, for GABA_AR PAMs with the C-20 carbonyl of 3α5α-P or pregnanolone reduced to a hydroxyl, binding affinity is reduced by 1,000-fold, whereas binding is retained after deoxygenation at the C-20 position. These results provide a first insight into the structure-activity relationship at the GABA_AR β⁺-α^- subunit interface steroid-binding site and identify several steroid PAMs that act via other sites.

Differential Coassembly of α1-GABAARs Associated with Epileptic Encephalopathy

GABA_A receptors (GABA_ARs) are profoundly important for controlling neuronal excitability. Spontaneous and familial mutations to these receptors feature prominently in excitability disorders and neurodevelopmental deficits following disruption to GABA-mediated inhibition. Recent genotyping of an individual with severe epilepsy and Williams-Beuren syndrome identified a frameshifting de novo variant in a major GABA_AR gene, GABRA1 This truncated the α1 subunit between the third and fourth transmembrane domains and introduced 24 new residues forming the mature protein, α1^Lys374Serfs*²⁵ Cell surface expression of mutant murine GABA_ARs is severely impaired compared with WT, due to retention in the endoplasmic reticulum. Mutant receptors were differentially coexpressed with β3, but not with β2, subunits in mammalian cells. Reduced surface expression was reflected by smaller IPSCs, which may underlie the induction of seizures. The mutant does not have a dominant-negative effect on native neuronal GABA_AR expression since GABA current density was unaffected in hippocampal neurons, although mutant receptors exhibited limited GABA sensitivity. To date, the underlying mechanism is unique for epileptogenic variants and involves differential β subunit expression of GABA_AR populations, which profoundly affected receptor function and synaptic inhibition.SIGNIFICANCE STATEMENT GABA_ARs are critical for controlling neural network excitability. They are ubiquitously distributed throughout the brain, and their dysfunction underlies many neurologic disorders, especially epilepsy. Here we report the characterization of an α1-GABA_AR variant that results in severe epilepsy. The underlying mechanism is structurally unusual, with the loss of part of the α1 subunit transmembrane domain and part-replacement with nonsense residues. This led to compromised and differential α1 subunit cell surface expression with β subunits resulting in severely reduced synaptic inhibition. Our study reveals that disease-inducing variants can affect GABA_AR structure, and consequently subunit assembly and cell surface expression, critically impacting on the efficacy of synaptic inhibition, a property that will orchestrate the extent and duration of neuronal excitability.

A potent photoreactive general anesthetic with novel binding site selectivity for GABAA receptors

The pentameric γ-aminobutyric acid type A receptors (GABA_ARs) are the major inhibitory ligand-gated ion channels in the central nervous system. They mediate diverse physiological functions, mutations in them are associated with mental disorders and they are the target of many drugs such as general anesthetics, anxiolytics and anti-convulsants. The five subunits of synaptic GABA_ARs are arranged around a central pore in the order β-α-β-α-γ. In the outer third of the transmembrane domain (TMD) drugs may bind to five homologous intersubunit binding sites. Etomidate binds between the pair of β - α subunit interfaces (designated as β⁺/α^-) and R-mTFD-MPAB binds to an α⁺/β^- and an γ⁺/β^- subunit interface (a β^- selective ligand). Ligands that bind selectively to other homologous sites have not been characterized. We have synthesized a novel photolabel, (2,6-diisopropyl-4-(3-(trifluoromethyl)-3H-diazirin-3-yl)phenyl)methanol or pTFD-di-iPr-BnOH). It is a potent general anesthetic that positively modulates agonist and benzodiazepine binding. It enhances GABA-induced currents, shifting the GABA concentration-response curve to lower concentrations. Photolabeling-protection studies show that it has negligible affinity for the etomidate sites and high affinity for only one of the two R-mTFD-MPAB sites. Exploratory site-directed mutagenesis studies confirm the latter conclusions and hint that pTFD-di-iPr-BnOH may bind between the α⁺/β^- and α⁺/γ^- subunits in the TMD, making it an α⁺ ligand. The latter α⁺/γ^- site has not previously been implicated in ligand binding. Thus, pTFD-di-iPr-BnOH is a promising new photolabel that may open up a new pharmacology for synaptic GABA_ARs.

Novel effect of Arthrocen (avocado/soy unsaponifiables) on pentylenetetrazole-induced seizure threshold in mice: Role of GABAergic pathway

Arthrocen, an avocado/soy unsaponifiable (ASU)-containing agent, is now used in the clinic and has potentially to decrease joint inflammation and pain associated with mild to severe osteoarthritis. Phytosterols are the major component of Arthrocen with documented anti-inflammatory properties, antioxidant, and analgesic effects. Here, we evaluated ASU anticonvulsant effect by its oral administration in pentylenetetrazole (PTZ)-induced seizure threshold and Maximal Electroshock Seizure (MES) Models. Also, the involvement of N-methyl-d-aspartate (NMDA) receptor, benzodiazepine receptor, and nitric oxide (NO) pathway were studied in anticonvulsant effect of ASU in male NMRI mice. Acute administration of Arthrocen (150, 75, 30, 10 mg/kg) by oral gavage significantly (p < 0.001) increased the clonic seizure threshold induced by intravenous administration of PTZ. Nonspecific inducible NO synthase (NOS) inhibitor L-NAME (10 mg/kg) and a specific NMDA receptor antagonist MK-801 (0.05 mg/kg) did not affect the anticonvulsant effect of Arthrocen, while pretreatment with flumazenil (0.25 mg/kg), a selective benzodiazepine receptor antagonist, reversed this effect (p < 0.01). Also, Arthrocen treated mice did not affect tonic hindlimb extension in the MES model. The data showed that Arthrocen might produce its anticonvulsant effect by enhancing GABAergic neurotransmission and/or action in the brain.

Microwave Assisted Synthesis and Molecular Docking Studies of Some 4- (3H)-quinazolinone Derivatives as Inhibitors of Human Gamma- Aminobutyric Acid Receptor, the GABA (A)R-BETA3 Homopentamer

BACKGROUND

Quinazolines and quinazolinones constitute a major class of biologically active molecules, both from natural and synthetic sources. The quinazolinone moiety is an important pharmacophore showing many types of pharmacological activities as shown in recent exhaustive review on the chemistry of 2-heteroaryl & heteroalkyl-4-quinazolinones4-quinazolinones that are the formal condensation products of anthranilic acid and amides. They can also be prepared in this fashion through the Niementowski quinazolinone synthesis, named after it's discoverer Stefan Niementowski. Quinazoline and condensed quinazoline exhibit potent central nervous system (CNS) activities like anti-anxiety, analgesic, anti-inflammatory and anticonvulsant. Quinazolin-4- ones with 2, 3-disubstitution is reported to possess significant analgesic, anti-inflammatory and anticonvulsant activities.

METHODS

To expand these views and application profiles, efforts have been made for the synthesis of a new class of quinazolinone by incorporating different amines into synthesized benzoxazinone ring by replacing O atom in the ring. Up till now, a great number of various procedures have been proposed for the synthesis of quinazolin-4-ones in the past few years. Using microwave radiation, this reaction could be easily and rapidly performed in very good yields, providing a large number of various 3-substituted-2- propyl-quinazolin-4-one derivatives which can be employed as useful bioactive compounds. We report a facile and efficient method for the synthesis of 3-substituted-2- propyl-quinazolin-4-one by the condensation reaction of anthranilic acid or halogen substituted anthranilic acid or methyl anthranilate, butanoic anhydride with various amines. We also report a drug/ligand or receptor/protein interactions by identifying the suitable active sites in the human gamma-aminobutyric acid receptor, the gaba (a)r-beta3 homopentamer human gammaaminobutyric acid receptor, and the gaba (a)r-beta3 homopentamer protein.

RESULTS

It was observed in the reaction, 3-alkyl/aryl-2-alkyl-quinazolin-4-one gave good yield as well as good quality of the product by using MW. All the synthesized compounds were subjected to grid-based molecular docking studies. The results show that compound 4t has good affinity to the active site residue of the human gamma-aminobutyric acid receptor, and the gaba (a)r-beta3 homopentamer.

CONCLUSION

The Microwave irradiation for the synthesis of the title compounds offers a reduction in reaction time, operation simplicity, cleaner reaction, easy work-up and improved yields. The procedure clearly highlights the advantages of green chemistry. The data reported in this article may be helpful for the medicinal chemists who are working in this area. The protein-ligand interaction plays a significant role in structural based drug designing. In the present work, we have docked the ligand, 2, 3-disubstituted quinazolinone with the proteins that are used as the target for GABA-A receptor.

GABA Receptors and the Pharmacology of Sleep

Current GABAergic sleep-promoting medications were developed pragmatically, without making use of the immense diversity of GABA_A receptors. Pharmacogenetic experiments are leading to an understanding of the circuit mechanisms in the hypothalamus by which zolpidem and similar compounds induce sleep at α2βγ2-type GABA_A receptors. Drugs acting at more selective receptor types, for example, at receptors containing the α2 and/or α3 subunits expressed in hypothalamic and brain stem areas, could in principle be useful as hypnotics/anxiolytics. A highly promising sleep-promoting drug, gaboxadol, which activates αβδ-type receptors failed in clinical trials. Thus, for the time being, drugs such as zolpidem, which work as positive allosteric modulators at GABA_A receptors, continue to be some of the most effective compounds to treat primary insomnia.

The role of galectin-3 in modulation of anxiety state level in mice

Galectin-3 (Gal-3), a member of lectin family that binds to oligosaccharides, is involved in several biological processes, including maturation and function of nervous system. It had been reported that Gal-3 regulates oligodendrocytes differentiation and that Gal-3/Toll-like receptor-4 (TLR4) axis is involved in neuroinflammation. As both, central nervous system (CNS) maturation and neuroinflammation may affect behavior, the principle aim of this study was to examine the effects of Gal-3 gene deletion on behavior. Here we provide the evidence that Gal-3 deficiency shows clear anxiogenic effect in mature untreated animals (basal conditions). This was accompanied with lower interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) relative gene expression and hippocampal content, with no effect on TLR4 expression. Gal-3 deficiency was also accompanied with lower brain-derived neurotrophic factor (BDNF) relative gene expression and immunoreactivity in hippocampus (predominantly in CA1 region). Besides, the Gal-3 gene deletion resulted in attenuation of the hippocampal relative gene expression of GABA-A receptor subunits 2 and 5 (GABA-AR2S and GABA-AR5S), On the other hand, Gal-3 deficiency attenuates LPS-induced neuroinflammation. The anxiogenic effect of acute neuroinflammation was accompanied with increased hippocampal IL-6, TNF-α and TLR4 gene expression, as well as decreased gene and immunohistochemical BDNF expression in hippocampus, with significant decline in GABA-AR2S in wild type (WT) mice in comparison to basal conditions. Gal-3 gene deletion prevented the increase in IL-6, the decline in BDNF gene expression and immunoreactivity, and reduction in hippocampal GABA-AR2S, and therefore attenuated the anxiogenic effect of neuroinflammation. In summary, our data demonstrate that apparently opposite effects of Gal-3 deficiency on anxiety levels (anxiogenic effect under basal conditions and anxiolytic action during neuroinflammation) seem to be related to the shift in IL-6, TNF-α and hippocampal BDNF.

The Functional Role of Spontaneously Opening GABAA Receptors in Neural Transmission

Ionotropic type of γ-aminobutyric acid receptors (GABA_ARs) produce two forms of inhibitory signaling: phasic inhibition generated by rapid efflux of neurotransmitter GABA into the synaptic cleft with subsequent binding to GABA_ARs, and tonic inhibition generated by persistent activation of extrasynaptic and/or perisynaptic GABA_ARs by GABA continuously present in the extracellular space. It is widely accepted that phasic and tonic GABAergic inhibition is mediated by receptor groups of distinct subunit composition and modulated by different cytoplasmic mechanisms. Recently, however, it has been demonstrated that spontaneously opening GABA_ARs (s-GABA_ARs), which do not need GABA binding to enter an active state, make a significant input into tonic inhibitory signaling. Due to GABA-independent action mode, s-GABA_ARs promise new safer options for therapy of neural disorders (such as epilepsy) devoid of side effects connected to abnormal fluctuations of GABA concentration in the brain. However, despite the potentially important role of s-GABA_ARs in neural signaling, they still remain out of focus of neuroscience studies, to a large extent due to technical difficulties in their experimental research. Here, we summarize present data on s-GABA_ARs functional properties and experimental approaches that allow isolation of s-GABA_ARs effects from those of conventional (GABA-dependent) GABA_ARs.

Overlapping Activities of Two Neuronal Splicing Factors Switch the GABA Effect from Excitatory to Inhibitory by Regulating REST

A truncating mutation in the mouse Srrm4 gene, which encodes a neuronal splicing factor, causes alternative splicing defects selectively in the ear. The mechanism by which splicing is preserved in the brain of these mice is not known. Here, we show that SRRM3 limits the Srrm4 mutation-associated defects to the ear and that, in cortical neurons, overlapping SRRM3-SRRM4 activity regulates the development of interneuronal inhibition. In vitro, SRRM3 and SRRM4 regulate the same splicing events, but a mutation in mouse Srrm3 causes tremors and mild defects in neuronal alternative splicing, demonstrating unique SRRM3 roles in vivo. Mice harboring mutations in both Srrm3 and Srrm4 die neonatally and exhibit severe splicing defects. In these mice, splicing alterations prevent inactivation of the gene repressor REST, which maintains immature excitatory GABAergic neurotransmission by repressing K-Cl cotransporter 2. Thus, our data reveal that SRRM3 and SRRM4 act redundantly to regulate GABAergic neurotransmission by inactivating REST.

Identification of a Functional Non-coding Variant in the GABA A Receptor α2 Subunit of the C57BL/6J Mouse Reference Genome: Major Implications for Neuroscience Research

GABA type-A (GABA-A) receptors containing the α2 subunit (GABRA2) are expressed in most brain regions and are critical in modulating inhibitory synaptic function. Genetic variation at the GABRA2 locus has been implicated in epilepsy, affective and psychiatric disorders, alcoholism and drug abuse. Gabra2 expression varies as a function of genotype and is modulated by sequence variants in several brain structures and populations, including F2 crosses originating from C57BL/6J (B6J) and the BXD recombinant inbred family derived from B6J and DBA/2J. Here we demonstrate a global reduction of GABRA2 brain protein and mRNA in the B6J strain relative to other inbred strains, and identify and validate the causal mutation in B6J. The mutation is a single base pair deletion located in an intron adjacent to a splice acceptor site that only occurs in the B6J reference genome. The deletion became fixed in B6J between 1976 and 1991 and is now pervasive in many engineered lines, BXD strains generated after 1991, the Collaborative Cross, and the majority of consomic lines. Repair of the deletion using CRISPR-Cas9-mediated gene editing on a B6J genetic background completely restored brain levels of GABRA2 protein and mRNA. Comparison of transcript expression in hippocampus, cortex, and striatum between B6J and repaired genotypes revealed alterations in GABA-A receptor subunit expression, especially in striatum. These results suggest that naturally occurring variation in GABRA2 levels between B6J and other substrains or inbred strains may also explain strain differences in anxiety-like or alcohol and drug response traits related to striatal function. Characterization of the B6J private mutation in the Gabra2 gene is of critical importance to molecular genetic studies in neurobiological research because this strain is widely used to generate genetically engineered mice and murine genetic populations, and is the most widely utilized strain for evaluation of anxiety-like, depression-like, pain, epilepsy, and drug response traits that may be partly modulated by GABRA2 function.

Extrasynaptic γ-aminobutyric acid type A receptor-mediated sex differences in the antiseizure activity of neurosteroids in status epilepticus and complex partial seizures

OBJECTIVE

Sex differences are evident in the antiseizure activity of neurosteroids; however, the potential mechanisms remain unclear. In this study, we sought to determine whether differences in target extrasynaptic δ-subunit γ-aminobutyric acid type A (GABA-A) receptor expression and function underlie the sex differences in seizure susceptibility and the antiseizure activity of neurosteroids.

METHODS

Sex differences in seizure susceptibility and protective activity of three distinct neurosteroids-allopregnanolone (AP), androstanediol (AD), and ganaxolone-were evaluated in the pilocarpine model of status epilepticus (SE) and kindling seizure test in mice. Immunocytochemistry was used for δGABA-A receptor expression analysis, and patch-clamp recordings in brain slices evaluated its functional currents.

RESULTS

Sex differences were apparent in kindling epileptogenic seizures, with males exhibiting a faster progression to a fully kindled state. Neurosteroids AP, AD, or ganaxolone produced dose-dependent protection against SE and acute partial seizures. However, female mice exhibited strikingly enhanced sensitivity to the antiseizure activity of neurosteroids compared to males. Sex differences in neurosteroid protection were unrelated to pharmacokinetic factors, as plasma levels of neurosteroids associated with seizure protection were similar between sexes. Mice lacking extrasynaptic δGABA-A receptors did not exhibit sex differences in neurosteroid protection. Consistent with a greater abundance of extrasynaptic δGABA-A receptors, AP produced a significantly greater potentiation of tonic currents in dentate gyrus granule cells in females than males; however, such enhanced AP sensitivity was diminished in δGABA-A receptor knockout female mice.

SIGNIFICANCE

Neurosteroids exhibit greater antiseizure potency in females than males, likely due to a greater abundance of extrasynaptic δGABA-A receptors that mediate neurosteroid-sensitive tonic currents and seizure protection. These findings indicate the potential to develop personalized gender-specific neurosteroid treatments for SE and epilepsy in men and women, including catamenial epilepsy.

Analgesic potential of PF-06372865, an α2/α3/α5 subtype-selective GABAA partial agonist, in humans

BACKGROUND

This study investigated the analgesic effects of two doses (15 and 65 mg) of PF-06372865, a novel α2/α3/α5 gamma-aminobutyric acid A (GABA_A) subunit selective partial positive allosteric modulator (PAM), compared with placebo and pregabalin (300 mg) as a positive control.

METHODS

We performed a randomised placebo-controlled crossover study (NCT02238717) in 20 healthy subjects, using a battery of pain tasks (electrical, pressure, heat, cold and inflammatory pain, including a paradigm of conditioned pain modulation). Pharmacodynamic measurements were performed at baseline and up to 10 h after dose.

RESULTS

A dose of 15 mg PF-06372865 increased pain tolerance thresholds (PTTs) for pressure pain at a ratio of 1.11 (90% confidence interval [CI]: 1.02, 1.22) compared with placebo. A dose of 65 mg PF-06372865 led to an increase in PTT for the cold pressor at a ratio of 1.17 (90% CI: 1.03, 1.32), and pressure pain task: 1.11 (90% CI: 1.01, 1.21). Pregabalin showed an increase in PTT for pressure pain at a ratio of 1.15 (95% CI: 1.06, 1.26) and cold pressor task: 1.31 (90% CI: 1.16, 1.48).

CONCLUSION

We conclude that PF-06372865 has analgesic potential at doses that do not induce significant sedation or other intolerable adverse events limiting its clinical use. In addition, the present study established the potential role for this battery of pain tasks as a tool in the development of analgesics with a novel mechanism of action, for the treatment of various pain states including neuropathic pain and to establish proof-of-concept.

CLINICAL TRIALS REGISTRATION

NCT0223871.

GABA-A and GABA-B Receptors in Filial Imprinting Linked With Opening and Closing of the Sensitive Period in Domestic Chicks (Gallus gallus domesticus)

Filial imprinting of domestic chicks has a well-defined sensitive (critical) period lasting in the laboratory from hatching to day 3. It is a typical model to investigate the molecular mechanisms underlying memory formation in early learning. We recently found that thyroid hormone 3,5,3′-triiodothyronine (T₃) is a determinant of the sensitive period. Rapid increases in cerebral T₃ levels are induced by imprinting training, rendering chicks imprintable. Furthermore, the administration of exogenous T₃ makes chicks imprintable on days 4 or 6 even after the sensitive period has ended. However, how T₃ affects neural transmission to enable imprinting remains mostly unknown. In this study, we demonstrate opposing roles for gamma-aminobutyric acid (GABA)-A and GABA-B receptors in imprinting downstream of T₃. Quantitative reverse transcription polymerase chain reaction and immunoblotting showed that the GABA-A receptor expression increases gradually from days 1 to 5, whereas the GABA-B receptor expression gradually decreases. We examined whether neurons in the intermediate medial mesopallium (IMM), the brain region responsible for imprinting, express both types of GABA receptors. Immunostaining showed that morphologically identified putative projection neurons express both GABA-A and GABA-B receptors, suggesting that those GABA receptors interact with each other in these cells to modulate the IMM outputs. The roles of GABA-A and GABA-B receptors were investigated using various agonists and antagonists. Our results show that GABA-B receptor antagonists suppressed imprinting on day 1, while its agonists made day 4 chicks imprintable without administration of exogenous T₃. By contrast, GABA-A receptor agonists suppressed imprinting on day 1, while its antagonists induced imprintability on day 4 without exogenous T₃. Furthermore, both GABA-A receptor agonists and GABA-B receptor antagonists suppressed T₃-induced imprintability on day 4 after the sensitive period has ended. Our data from these pharmacological experiments indicate that GABA-B receptors facilitate imprinting downstream of T₃ by initiating the sensitive period, while the GABA-A receptor contributes to the termination of the sensitive period. In conclusion, we propose that opposing roles of GABA-A and GABA-B receptors in the brain during development determine the induction and termination of the sensitive period.

Nuclear Respiratory Factor 1 (NRF-1) Controls the Activity Dependent Transcription of the GABA-A Receptor Beta 1 Subunit Gene in Neurons

While the exact role of β1 subunit-containing GABA-A receptors (GABARs) in brain function is not well understood, altered expression of the β1 subunit gene (GABRB1) is associated with neurological and neuropsychiatric disorders. In particular, down-regulation of β1 subunit levels is observed in brains of patients with epilepsy, autism, bipolar disorder and schizophrenia. A pathophysiological feature of these disease states is imbalance in energy metabolism and mitochondrial dysfunction. The transcription factor, nuclear respiratory factor 1 (NRF-1), has been shown to be a key mediator of genes involved in oxidative phosphorylation and mitochondrial biogenesis. Using a variety of molecular approaches (including mobility shift, promoter/reporter assays, and overexpression of dominant negative NRF-1), we now report that NRF-1 regulates transcription of GABRB1 and that its core promoter contains a conserved canonical NRF-1 element responsible for sequence specific binding and transcriptional activation. Our identification of GABRB1 as a new target for NRF-1 in neurons suggests that genes coding for inhibitory neurotransmission may be coupled to cellular metabolism. This is especially meaningful as binding of NRF-1 to its element is sensitive to the kind of epigenetic changes that occur in multiple disorders associated with altered brain inhibition.

GABA-B1 Receptor-Null Schwann Cells Exhibit Compromised In Vitro Myelination

GABA-B receptors are important for Schwann cell (SC) commitment to a non-myelinating phenotype during development. However, the P0-GABA-B1^fl/fl conditional knockout mice, lacking the GABA-B1 receptor specifically in SCs, also presented axon modifications, suggesting SC non-autonomous effects through the neuronal compartment. In this in vitro study, we evaluated whether the specific deletion of the GABA-B1 receptor in SCs may induce autonomous or non-autonomous cross-changes in sensory dorsal root ganglia (DRG) neurons. To this end, we performed an in vitro biomolecular and transcriptomic analysis of SC and DRG neuron primary cultures from P0-GABA-B1^fl/fl mice. We found that cells from conditional P0-GABA-B1^fl/fl mice exhibited proliferative, migratory and myelinating alterations. Moreover, we found transcriptomic changes in novel molecules that are involved in peripheral neuron-SC interaction.

Probing GABAA receptors with inhibitory neurosteroids

γ-aminobutyric acid type A receptors (GABA_ARs) are important components of the central nervous system and they are functionally tasked with controlling neuronal excitability. These receptors are subject to post-translational modification and also to modulation by endogenous regulators, such as the neurosteroids. These modulators can either potentiate or inhibit GABA_AR function. Whilst the former class of neurosteroids are considered to bind to and act from the transmembrane domain of the receptor, the domains that are important for the inhibitory neurosteroids remain less clear. In this study, we systematically compare a panel of recombinant synaptic-type and extrasynaptic-type GABA_ARs expressed in heterologous cell systems for their sensitivity to inhibition by the classic inhibitory neurosteroid, pregnenolone sulphate. Generally, peak GABA current responses were inhibited less compared to steady-state currents, implicating the desensitised state in inhibition. Moreover, pregnenolone sulphate inhibition increased with GABA concentration, but showed minimal voltage dependence. There was no strong dependence of inhibition on receptor subunit composition, the exception being the ρ1 receptor, which is markedly less sensitive. By using competition experiments with pregnenolone sulphate and the GABA channel blocker picrotoxinin, discrete binding sites are proposed. Furthermore, by assessing inhibition using site-directed mutagenesis and receptor chimeras comprising α, β or γ subunits with ρ1 subunits, the receptor transmembrane domains are strongly implicated in mediating inhibition and most likely the binding location for pregnenolone sulphate in GABA_ARs.

This article is part of the “Special Issue Dedicated to Norman G. Bowery”.

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A range of GABA_A receptor subtypes are inhibited by pregenolone sulphate.

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Peak GABA curents are less sensitive to inhibition than steady-state currents.

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Desensitised state of GABA_A receptors most sensitive to neurosteroid inhibition.

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Inhibition increases with GABA concentration, but not strongly voltage-dependent.

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Pregnenolone sulphate binding site located within subunit transmembrane domains.

Norman Bowery's discoveries about extrasynaptic and asynaptic GABA systems and their significance

Before discovering the GABA-B receptor, Norman Bowery completed a series of studies on an extrasynaptic or asynaptic "GABA system" in the rat superior cervical sympathetic ganglion. First, he discovered an uptake system for GABA in neuroglial cells in the ganglia and in peripheral nerves, with a different substrate specificity than that in neurons. Second, he showed that accumulated GABA in sympathetic glial cells was metabolized to succinate by a transaminase enzyme. Third, he provided detailed structure-activity information about compounds activating an extrasynaptic GABA-A receptor on neurons in the rat sympathetic ganglion. Fourth, he showed that some amino acid substrates for the neuroglial transporter could indirectly stimulate neurons by releasing GABA from adjacent glial cells, and that GABA could also be released from neuroglial cells by membrane depolarization. In this review, these discoveries are briefly described and updated and some of their implications assessed. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Acute sleep deprivation preconditions the heart against ischemia/ reperfusion injury: the role of central GABA-A receptors

OBJECTIVES

Central γ-aminobutyric acid (GABA) neurotransmission modulates cardiovascular functions and sleep. Acute sleep deprivation (ASD) affects functions of various body organs via different mechanisms. Here, we evaluated the effect of ASD on cardiac ischemia/reperfusion injury (IRI), and studied the role of GABA-A receptor inhibition in central nucleus of amygdala (CeA) by assessing nitric oxide (NO) and oxidative stress.

MATERIALS AND METHODS

The CeA in sixty male Wistar rats was cannulated for saline or bicuculline (GABA-A receptor antagonist) administration. All animals underwent 30 min of coronary occlusion (ischemia), followed by 2 hr reperfusion (IR). The five experimental groups (n=12) included are as follows: IR: received saline; BIC+IR: received Bicuculline; MLP+IR: received saline, followed by the placement of animals in an aquarium with multiple large platforms; ASD+IR: underwent ASD in an aquarium with multiple small platforms; and BIC+ASD+IR: received bicuculline prior to ASD.

RESULTS

Bicuculline administration increased the malondialdehyde levels and infarct size, and decreased the NO metabolites levels and endothelial nitric oxide synthase (eNOS) gene expression in infarcted and non-infarcted areas in comparison to IR group. ASD reduced malondialdehyde levels and infarct size and increased NO metabolites, corticosterone levels and eNOS expression in infarcted and non-infarcted areas as compared to the IR group. Levels of malondialdehyde were increased while levels of NO metabolites, corticosterone and eNOS expression in infarcted and non-infarcted areas were reduced in the BIC+ASD+IR as compared to the ASD+IR group.

CONCLUSION

Blockade of GABA-A receptors in the CeA abolishes ASD-induced cardioprotection by suppressing oxidative stress and NO production.

Bottom-Up versus Top-Down Induction of Sleep by Zolpidem Acting on Histaminergic and Neocortex Neurons

Zolpidem, a GABA_A receptor-positive modulator, is the gold-standard drug for treating insomnia. Zolpidem prolongs IPSCs to decrease sleep latency and increase sleep time, effects that depend on α2 and/or α3 subunit-containing receptors. Compared with natural NREM sleep, zolpidem also decreases the EEG power, an effect that depends on α1 subunit-containing receptors, and which may make zolpidem-induced sleep less optimal. In this paper, we investigate whether zolpidem needs to potentiate only particular GABAergic pathways to induce sleep without reducing EEG power. Mice with a knock-in F77I mutation in the GABA_A receptor γ2 subunit gene are zolpidem-insensitive. Using these mice, GABA_A receptors in the frontal motor neocortex and hypothalamic (tuberomammillary nucleus) histaminergic-neurons of γ2I77 mice were made selectively sensitive to zolpidem by genetically swapping the γ2I77 subunits with γ2F77 subunits. When histamine neurons were made selectively zolpidem-sensitive, systemic administration of zolpidem shortened sleep latency and increased sleep time. But in contrast to the effect of zolpidem on wild-type mice, the power in the EEG spectra of NREM sleep was not decreased, suggesting that these EEG power-reducing effects of zolpidem do not depend on reduced histamine release. Selective potentiation of GABA_A receptors in the frontal cortex by systemic zolpidem administration also reduced sleep latency, but less so than for histamine neurons. These results could help with the design of new sedatives that induce a more natural sleep.

SIGNIFICANCE STATEMENT

Many people who find it hard to get to sleep take sedatives. Zolpidem (Ambien) is the most widely prescribed "sleeping pill." It makes the inhibitory neurotransmitter GABA work better at its receptors throughout the brain. The sleep induced by zolpidem does not resemble natural sleep because it produces a lower power in the brain waves that occur while we are sleeping. We show using mouse genetics that zolpidem only needs to work on specific parts and cell types of the brain, including histamine neurons in the hypothalamus, to induce sleep but without reducing the power of the sleep. This knowledge could help in the design of sleeping pills that induce a more natural sleep.

α4βδ GABAA receptors reduce dendritic spine density in CA1 hippocampus and impair relearning ability of adolescent female mice: Effects of a GABA agonist and a stress steroid

Synaptic pruning underlies the transition from an immature to an adult CNS through refinements of neuronal circuits. Our recent study indicates that pubertal synaptic pruning is triggered by the inhibition generated by extrasynaptic α4βδ GABA_A receptors (GABARs) which are increased for 10 d on dendritic spines of CA1 pyramidal cells at the onset of puberty (PND 35-44) in the female mouse, suggesting α4βδ GABARs as a novel target for the regulation of adolescent synaptic pruning. In the present study we used a pharmacological approach to further examine the role of these receptors in altering spine density during puberty of female mice and the impact of these changes on spatial learning, assessed in adulthood. Two drugs were chronically administered during the pubertal period (PND 35-44): the GABA agonist gaboxadol (GBX, 0.1mg/kg, i.p.), to enhance current gated by α4βδ GABARs and the neurosteroid/stress steroid THP (3α-OH-5β-pregnan-20-one, 10mg/kg, i.p.) to decrease expression of α4βδ. Spine density was determined on PND 56 with Golgi staining. Spatial learning and relearning were assessed using the multiple object relocation task and an active place avoidance task on PND 56. Pubertal GBX decreased spine density post-pubertally by 70% (P<0.05), while decreasing α4βδ expression with THP increased spine density by twofold (P<0.05), in both cases, with greatest effects on the mushroom spines. Adult relearning ability was compromised in both hippocampus-dependent tasks after pubertal administration of either drug. These findings suggest that an optimal spine density produced by α4βδ GABARs is necessary for optimal cognition in adults.

5-(N, N-Hexamethylene) amiloride is a GABA-A ρ1 receptor positive allosteric modulator

Guanidine compounds act as ion channel modulators. In the case of Cys-loop receptors, the guanidine compound amiloride antagonized the heteromeric GABA-A, glycine, and nicotinic acetylcholine receptors. However, amiloride exhibits characteristics consistent with a positive allosteric modulator for the human GABA-A (hGABA-A) ρ1 receptor. Site-directed mutagenesis revealed that the positive allosteric modulation was influenced by the GABA-A ρ1 second transmembrane domain 15' position, a site implicated in ligand allosteric modulation of Cys-loop receptors. There are a variety of amiloride derivatives that provide opportunities to assess the significance of amiloride functional groups (e.g., the guanidine group, the pyrazine ring, etc.) in the modulation of the GABA-A ρ1 receptor activity. We utilized 3 amiloride derivatives (benzamil, phenamil, and 5-(N, N-Hexamethylene) amiloride) to assess the contribution of these groups toward the potentiation of the GABA-A ρ1 receptor. Benzamil and phenamil failed to potentiate on the wild type GABA-A ρ1 GABA-mediated current while HMA demonstrated efficacy only at the highest concentration studied. The hGABA-A ρ1 (I15'N) mutant receptor activity was potentiated by lower HMA concentrations compared to the wild type receptor. Our findings suggest that an exposed guanidine group on amiloride and amiloride derivatives is critical for modulating the GABA-A ρ1 receptor. The present study provides a conceptual framework for predicting which amiloride derivatives will demonstrate positive allosteric modulation of the GABA-A ρ1 receptor.

Flumazenil for the Treatment of Refractory Hypersomnolence: Clinical Experience with 153 Patients

STUDY OBJECTIVES

Patients with central disorders of hypersomnolence sometimes do not achieve satisfactory symptom control with currently available wake-promoting medications. Based on the finding that the cerebrospinal fluid from some patients with hypersomnolence demonstrates potentiation of gamma-aminobutyric acid (GABA)-A receptors in excess of that of controls, a finding that reverses with flumazenil, we initiated prescribing compounded flumazenil to carefully selected, treatment-refractory hypersomnolent patients.

METHODS

This retrospective chart review evaluated the first 153 consecutive patients treated with transdermal and/or sublingual flumazenil by physicians at our center from 2013 through January 2015.

RESULTS

Patients were 35.5 y old (± 14.4) and 92 (60.1%) were women. Mean Epworth Sleepiness Scale scores prior to flumazenil were 15.1 (± 4.5) despite prior or current treatment with traditional wake-promoting therapies. Symptomatic benefit was noted by 96 patients (62.8%), with a mean reduction in Epworth Sleepiness Scale score of 4.7 points (± 4.7) among responders. Of these, 59 remained on flumazenil chronically, for a mean of 7.8 mo (± 6.9 mo). Female sex and presence of reported sleep inertia differentiated flumazenil responders from nonresponders. Adverse events were common, but often did not result in treatment discontinuation. Serious adverse events included a transient ischemic attack and a lupus vasculopathy, although whether these events occurred because of flumazenil administration is unknown.

CONCLUSIONS

This chart review demonstrates that sublingual and transdermal flumazenil provided sustained clinical benefit to 39% of patients with treatment-refractory hypersomnolence. Prospective, controlled studies of this GABA-A receptor antagonist for the treatment of hypersomnolence are needed.

COMMENTARY

A commentary on this article appears in this issue on page 1321.

Insulin-Independent GABAA Receptor-Mediated Response in the Barrel Cortex of Mice with Impaired Met Activity

Autism spectrum disorder (ASD) is a neurodevelopmental disorder caused by genetic variants, susceptibility alleles, and environmental perturbations. The autism associated geneMETtyrosine kinase has been implicated in many behavioral domains and endophenotypes of autism, including abnormal neural signaling in human sensory cortex. We investigated somatosensory thalamocortical synaptic communication in mice deficient in Met activity in cortical excitatory neurons to gain insights into aberrant somatosensation characteristic of ASD. The ratio of excitation to inhibition is dramatically increased due to decreased postsynaptic GABAAreceptor-mediated inhibition in the trigeminal thalamocortical pathway of mice lacking active Met in the cerebral cortex. Furthermore, in contrast to wild-type mice, insulin failed to increase GABAAreceptor-mediated response in the barrel cortex of mice with compromised Met signaling. Thus, lacking insulin effects may be a risk factor in ASD pathogenesis.

SIGNIFICANCE STATEMENT

A proposed common cause of neurodevelopmental disorders is an imbalance in excitatory neural transmission, provided by the glutamatergic neurons, and the inhibitory signals from the GABAergic interneurons. Many genes associated with autism spectrum disorders impair synaptic transmission in the expected cell type. Previously, inactivation of the autism-associated Met tyrosine kinase receptor in GABAergic interneurons led to decreased inhibition. In thus report, decreased Met signaling in glutamatergic neurons had no effect on excitation, but decimated inhibition. Further experiments indicate that loss of Met activity downregulates GABAAreceptors on glutamatergic neurons in an insulin independent manner. These data provide a new mechanism for the loss of inhibition and subsequent abnormal excitation/inhibition balance and potential molecular candidates for treatment or prevention.

Energy and Potassium Ion Homeostasis during Gamma Oscillations

Fast neuronal network oscillations in the gamma frequency band (30-100 Hz) occur in various cortex regions, require timed synaptic excitation and inhibition with glutamate and GABA, respectively, and are associated with higher brain functions such as sensory perception, attentional selection and memory formation. However, little is known about energy and ion homeostasis during the gamma oscillation. Recent studies addressed this topic in slices of the rodent hippocampus using cholinergic and glutamatergic receptor models of gamma oscillations (GAM). Methods with high spatial and temporal resolution were applied in vitro, such as electrophysiological recordings of local field potential (LFP) and extracellular potassium concentration ([K(+)]o), live-cell fluorescence imaging of nicotinamide adenine dinucleotide (phosphate) and flavin adenine dinucleotide [NAD(P)H and FAD, respectively] (cellular redox state), and monitoring of the interstitial partial oxygen pressure (pO2) in depth profiles with microsensor electrodes, including mathematical modeling. The main findings are: (i) GAM are associated with high oxygen consumption rate and significant changes in the cellular redox state, indicating rapid adaptations in glycolysis and oxidative phosphorylation; (ii) GAM are accompanied by fluctuating elevations in [K(+)]o of less than 0.5 mmol/L from baseline, likely reflecting effective K(+)-uptake mechanisms of neuron and astrocyte compartments; and (iii) GAM are exquisitely sensitive to metabolic stress induced by lowering oxygen availability or by pharmacological inhibition of the mitochondrial respiratory chain. These findings reflect precise cellular adaptations to maintain adenosine-5'-triphosphate (ATP), ion and neurotransmitter homeostasis and thus neural excitability and synaptic signaling during GAM. Conversely, the exquisite sensitivity of GAM to metabolic stress might significantly contribute the exceptional vulnerability of higher brain functions in brain disease.

Gastroprotective effect of (-)-myrtenol against ethanol-induced acute gastric lesions: possible mechanisms

OBJECTIVES

(-)-Myrtenol is a natural fragrance monoterpenoid structurally related to α-pinene found in diverse plant essential oils. This study was aimed to assess the anti-ulcerogenic potential of (-)-myrtenol against ethanol-induced gastric lesions and to elucidate the underlying mechanism(s).

METHODS

Gastroprotective activity of (-)-myrtenol was evaluated using the mouse model of ethanol-induced gastric damage. To elucidate the gastroprotective mechanism(s), the roles of GABA, prostaglandins, nitric oxide and KATP channels were assessed. Besides, the oxidative stress-related parameters and the mucus content in gastric tissues were analysed.

KEY FINDINGS

(-)-Myrtenol at oral doses of 25, 50 and 100 mg/kg significantly decreased the severity of ethanol-induced gastric lesions affording gastroprotection that was accompanied by a decrease in the activity of myeloperoxidase and malondialdehyde, an increase in GPx, SOD, and catalase activity in gastric tissues, and with well-maintained normal levels of nitrite/nitrate, gastric mucus and NP-SHs. Pretreatment with GABA-A receptor antagonist flumazenil, the COX inhibitor indomethacin, and NO synthesis inhibitor L-NAME but not with KATP channel blocker glibenclamide significantly blocked the (-)-myrtenol gastroprotection.

CONCLUSION

These results provide first-time evidence for the gastroprotective effect of (-)-myrtenol that could be related to GABAA -receptor activation and antioxidant activity.

Clinical Potential of Neurosteroids for CNS Disorders

Neurosteroids are key endogenous molecules in the brain that affect many neural functions. We describe here recent advances in US National Institutes of Health (NIH)-sponsored and other clinical studies of neurosteroids for CNS disorders. The neuronal GABA-A receptor chloride channel is one of the prime molecular targets of neurosteroids. Allopregnanolone-like neurosteroids are potent allosteric agonists as well as direct activators of both synaptic and extrasynaptic GABA-A receptors. Hence, neurosteroids can maximally enhance synaptic phasic and extrasynaptic tonic inhibition. The resulting chloride current conductance generates a form of shunting inhibition that controls network excitability, seizures, and behavior. Such mechanisms of neurosteroids are providing innovative therapies for epilepsy, status epilepticus (SE), traumatic brain injury (TBI), fragile X syndrome (FXS), and chemical neurotoxicity. The neurosteroid field has entered a new era, and many compounds have reached advanced clinical trials. Synthetic analogs have several advantages over natural neurosteroids for clinical use because of their superior bioavailability and safety trends.

Synaptic pruning in the female hippocampus is triggered at puberty by extrasynaptic GABAA receptors on dendritic spines

Adolescent synaptic pruning is thought to enable optimal cognition because it is disrupted in certain neuropathologies, yet the initiator of this process is unknown. One factor not yet considered is the α4βδ GABA_A receptor (GABAR), an extrasynaptic inhibitory receptor which first emerges on dendritic spines at puberty in female mice. Here we show that α4βδ GABARs trigger adolescent pruning. Spine density of CA1 hippocampal pyramidal cells decreased by half post-pubertally in female wild-type but not α4 KO mice. This effect was associated with decreased expression of kalirin-7 (Kal7), a spine protein which controls actin cytoskeleton remodeling. Kal7 decreased at puberty as a result of reduced NMDAR activation due to α4βδ-mediated inhibition. In the absence of this inhibition, Kal7 expression was unchanged at puberty. In the unpruned condition, spatial re-learning was impaired. These data suggest that pubertal pruning requires α4βδ GABARs. In their absence, pruning is prevented and cognition is not optimal.

DOI:http://dx.doi.org/10.7554/eLife.15106.001

Memories are formed at structures in the brain known as dendritic spines. These structures receive connections from other brain cells through regions called synapses. In humans, the number of these brain connections increases dramatically from birth to childhood, reflecting a period of rapid learning. However, the number of these brain connections halves after puberty, a dramatic reduction shown in many brain areas and for many species, including humans and rodents. This process is referred to as adolescent synaptic pruning and is thought to be important for optimal learning in adulthood because it is disrupted in autism and schizophrenia. Synaptic pruning is believed to remove unnecessary brain connections to make room for new relevant memories. However, the process that triggers synaptic pruning is not known.

Within the brain, proteins called inhibitory GABA receptors are targets for chemicals that reduce the activity of nerve cells. As brain connections must be kept active to survive, inhibitory receptors could help to trigger synaptic pruning.

Afroz, Parato et al. now show that, at puberty, the number of a particular type of GABA_A receptor increases in the brain of female mice. This triggers synaptic pruning in the hippocampus, a key brain area necessary for learning and memory. By reducing brain activity, these inhibitory receptors also reduce the levels of a protein in the dendritic spine that stabilizes the scaffolding of the spine to maintain its structure.

Mice that do not have these GABA_A receptors maintain a constant high level of brain connections throughout adolescence, and synaptic pruning does not occur in their brains. These mice were initially able to learn to avoid a specific location that provided a mild shock to their foot. However, when this location changed the mice were unable to re-learn where to avoid, suggesting that too many brain connections limits learning potential.

Brain connections are regulated by many factors, including the environment and stress. Future studies will test how these additional factors alter synaptic pruning in adolescence, and will test drugs that target these inhibitory receptors to manipulate adolescent pruning. These findings may suggest new treatments for “normalizing” synaptic pruning in conditions where this process occurs abnormally, such as autism and schizophrenia.

DOI:http://dx.doi.org/10.7554/eLife.15106.002

Catamenial Epilepsy: Discovery of an Extrasynaptic Molecular Mechanism for Targeted Therapy

Catamenial epilepsy is a type of refractory epilepsy characterized by seizure clusters around perimenstrual or periovulatory period. The pathophysiology of catamenial epilepsy still remains unclear, yet there are few animal models to study this gender-specific disorder. The pathophysiology of perimenstrual catamenial epilepsy involves the withdrawal of the progesterone-derived GABAergic neurosteroids due to the decline in progesterone level at the time of menstruation. These manifestations can be faithfully reproduced in rodents by specific neuroendocrine manipulations. Since mice and rats, like humans, have ovarian cycles with circulating hormones, they appear to be suitable animal models for studies of perimenstrual seizures. Recently, we created specific experimental models to mimic perimenstrual seizures. Studies in rat and mouse models of catamenial epilepsy show enhanced susceptibility to seizures or increased seizure exacerbations following neurosteroid withdrawal. During such a seizure exacerbation period, there is a striking decrease in the anticonvulsant effect of commonly prescribed antiepileptics, such as benzodiazepines, but an increase in the anticonvulsant potency of exogenous neurosteroids. We discovered an extrasynaptic molecular mechanism of catamenial epilepsy. In essence, extrasynaptic δGABA-A receptors are upregulated during perimenstrual-like neuroendocrine milieu. Consequently, there is enhanced antiseizure efficacy of neurosteroids in catamenial models because δGABA-A receptors confer neurosteroid sensitivity and greater seizure protection. Molecular mechanisms such as these offer a strong rationale for the clinical development of a neurosteroid replacement therapy for catamenial epilepsy.