Protein kinase C phosphorylation regulates membrane insertion of GABAA receptor subtypes that mediate tonic inhibition

Neurosteroids as Novel Anticonvulsants for Refractory Status Epilepticus and Medical Countermeasures for Nerve Agents: A 15-Year Journey to Bring Ganaxolone from Bench to Clinic

This article describes recent advances in the use of neurosteroids as novel anticonvulsants for refractory status epilepticus (RSE) and as medical countermeasures (MCs) for organophosphates and chemical nerve agents (OPNAs). We highlight a comprehensive 15-year journey to bring the synthetic neurosteroid ganaxolone (GX) from bench to clinic. RSE, including when caused by nerve agents, is associated with devastating morbidity and permanent long-term neurologic dysfunction. Although recent approval of benzodiazepines such as intranasal midazolam and intranasal midazolam offers improved control of acute seizures, novel anticonvulsants are needed to suppress RSE and improve neurologic function outcomes. Currently, few anticonvulsant MCs exist for victims of OPNA exposure and RSE. Standard-of-care MCs for postexposure treatment include benzodiazepines, which do not effectively prevent or mitigate seizures resulting from nerve agent intoxication, leaving an urgent unmet medical need for new anticonvulsants for RSE. Recently, we pioneered neurosteroids as next-generation anticonvulsants that are superior to benzodiazepines for treatment of OPNA intoxication and RSE. Because GX and related neurosteroids that activate extrasynaptic GABA-A receptors rapidly control seizures and offer robust neuroprotection by reducing neuronal damage and neuroinflammation, they effectively improve neurologic outcomes after acute OPNA exposure and RSE. GX has been selected for advanced, Biomedical Advanced Research and Development Authority-supported phase 3 trials of RSE and nerve agent seizures. In addition, in mechanistic studies of neurosteroids at extrasynaptic receptors, we identified novel synthetic analogs with features that are superior to GX for current medical needs. Development of new MCs for RSE is complex, tedious, and uncertain due to scientific and regulatory challenges. Thus, further research will be critical to fill key gaps in evaluating RSE and anticonvulsants in vulnerable (pediatric and geriatric) populations and military persons. SIGNIFICANCE STATEMENT: Following organophosphate and nerve agent intoxication, refractory status epilepticus (RSE) occurs despite benzodiazepine treatment. RSE occurs in 40% of status epilepticus patients, with a 35% mortality rate and significant neurological morbidity in survivors. To treat RSE, neurosteroids are better anticonvulsants than benzodiazepines. Our pioneering use of neurosteroids for RSE and nerve agents led us to develop ganaxolone as a novel anticonvulsant and neuroprotectant with significantly improved neurological outcomes. This article describes the bench-to-bedside journey of bringing neurosteroid therapy to patients, with ganaxolone leading the way.

Neurosteroids: mechanistic considerations and clinical prospects

Like other classes of treatments described in this issue's section, neuroactive steroids have been studied for decades but have risen as a new class of rapid-acting, durable antidepressants with a distinct mechanism of action from previous antidepressant treatments and from other compounds covered in this issue. Neuroactive steroids are natural derivatives of progesterone but are proving effective as exogenous treatments. The best understood mechanism is that of positive allosteric modulation of GABAA receptors, where subunit selectivity may promote their profile of action. Mechanistically, there is some reason to think that neuroactive steroids may separate themselves from liabilities of other GABA modulators, although research is ongoing. It is also possible that intracellular targets, including inflammatory pathways, may be relevant to beneficial actions. Strengths and opportunities for further development include exploiting non-GABAergic targets, structural analogs, enzymatic production of natural steroids, precursor loading, and novel formulations. The molecular mechanisms of behavioral effects are not fully understood, but study of brain network states involved in emotional processing demonstrate a robust influence on affective states not evident with at least some other GABAergic drugs including benzodiazepines. Ongoing studies with neuroactive steroids will further elucidate the brain and behavioral effects of these compounds as well as likely underpinnings of disease.

Cyclin D1-Cdk4 regulates neuronal activity through phosphorylation of GABAA receptors

Nuclear Cyclin D1 (Ccnd1) is a main regulator of cell cycle progression and cell proliferation. Interestingly, Ccnd1 moves to the cytoplasm at the onset of differentiation in neuronal precursors. However, cytoplasmic functions and targets of Ccnd1 in post-mitotic neurons are unknown. Here we identify the α4 subunit of gamma-aminobutyric acid (GABA) type A receptors (GABAARs) as an interactor and target of Ccnd1-Cdk4. Ccnd1 binds to an intracellular loop in α4 and, together with Cdk4, phosphorylates the α4 subunit at threonine 423 and serine 431. These modifications upregulate α4 surface levels, increasing the response of α4-containing GABAARs, measured in whole-cell patch-clamp recordings. In agreement with this role of Ccnd1-Cdk4 in neuronal signalling, inhibition of Cdk4 or expression of the non-phosphorylatable α4 decreases synaptic and extra-synaptic currents in the hippocampus of newborn rats. Moreover, according to α4 functions in synaptic pruning, CCND1 knockout mice display an altered pattern of dendritic spines that is rescued by the phosphomimetic α4. Overall, our findings molecularly link Ccnd1-Cdk4 to GABAARs activity in the central nervous system and highlight a novel role for this G1 cyclin in neuronal signalling.

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.

Inhibitory and excitatory synaptic neuroadaptations in the diazepam tolerant brain

Benzodiazepine (BZ) drugs treat seizures, anxiety, insomnia, and alcohol withdrawal by potentiating γ2 subunit containing GABA type A receptors (GABAARs). BZ clinical use is hampered by tolerance and withdrawal symptoms including heightened seizure susceptibility, panic, and sleep disturbances. Here, we investigated inhibitory GABAergic and excitatory glutamatergic plasticity in mice tolerant to benzodiazepine sedation. Repeated diazepam (DZP) treatment diminished sedative effects and decreased DZP potentiation of GABAAR synaptic currents without impacting overall synaptic inhibition. While DZP did not alter γ2-GABAAR subunit composition, there was a redistribution of extrasynaptic GABAARs to synapses, resulting in higher levels of synaptic BZ-insensitive α4-containing GABAARs and a concomitant reduction in tonic inhibition. Conversely, excitatory glutamatergic synaptic transmission was increased, and NMDAR subunits were upregulated at synaptic and total protein levels. Quantitative proteomics further revealed cortex neuroadaptations of key pro-excitatory mediators and synaptic plasticity pathways highlighted by Ca2+/calmodulin-dependent protein kinase II (CAMKII), MAPK, and PKC signaling. Thus, reduced inhibitory GABAergic tone and elevated glutamatergic neurotransmission contribute to disrupted excitation/inhibition balance and reduced BZ therapeutic power with benzodiazepine tolerance.

Activation of α 6 -containing GABA A receptors induces antinociception under physiological and pathological conditions

ABSTRACT

The loss of GABAergic inhibition is a mechanism that underlies neuropathic pain. Therefore, rescuing the GABAergic inhibitory tone through the activation of GABA A receptors is a strategy to reduce neuropathic pain. This study was designed to elucidate the function of the spinal α 6 -containing GABA A receptor in physiological conditions and neuropathic pain in female and male rats. Results show that α 6 -containing GABA A receptor blockade or transient α 6 -containing GABA A receptor knockdown induces evoked hypersensitivity and spontaneous pain in naive female rats. The α 6 subunit is expressed in IB4 + and CGRP + primary afferent neurons in the rat spinal dorsal horn and dorsal root ganglia but not astrocytes. Nerve injury reduces α 6 subunit protein expression in the central terminals of the primary afferent neurons and dorsal root ganglia, whereas intrathecal administration of positive allosteric modulators of the α 6 -containing GABA A receptor reduces tactile allodynia and spontaneous nociceptive behaviors in female, but not male, neuropathic rats and mice. Overexpression of the spinal α 6 subunit reduces tactile allodynia and restores α 6 subunit expression in neuropathic rats. Positive allosteric modulators of the α 6 -containing GABA A receptor induces a greater antiallodynic effect in female rats and mice compared with male rats and mice. Finally, α 6 subunit is expressed in humans. This receptor is found in CGRP + and P2X3 + primary afferent fibers but not astrocytes in the human spinal dorsal horn. Our results suggest that the spinal α 6 -containing GABA A receptor has a sex-specific antinociceptive role in neuropathic pain, suggesting that this receptor may represent an interesting target to develop a novel treatment for neuropathic pain.

Early central cardiovagal dysfunction after high fat diet in a murine model

High fat diet (HFD) promotes cardiovascular disease and blunted cardiac vagal regulation. Temporal onset of loss of cardiac vagal control and its underlying mechanism are presently unclear. We tested our hypothesis that reduced central vagal regulation occurs early after HFD and contributes to poor cardiac regulation using cardiovascular testing paired with pharmacology in mice, molecular biology, and a novel bi-transgenic mouse line. Results show HFD, compared to normal fat diet (NFD), significantly blunted cardio/pulmonary chemoreflex bradycardic responses after 15 days, extending as far as tested (> 30 days). HFD produced resting tachycardia by day 3, reflected significant loss of parasympathetic tone. No differences in bradycardic responses to graded electrical stimulation of the distal cut end of the cervical vagus indicated diet-induced differences in vagal activity were centrally mediated. In nucleus ambiguus (NA), surface expression of δ-subunit containing type A gamma-aminobutyric acid receptors (GABAA(δ)R) increased at day 15 of HFD. Novel mice lacking δ-subunit expression in vagal motor neurons (ChAT-δnull) failed to exhibit blunted reflex bradycardia or resting tachycardia after two weeks of HFD. Thus, reduced parasympathetic output contributes to early HFD-induced HR dysregulation, likely through increased GABAA(δ)Rs. Results underscore need for research on mechanisms of early onset increases in GABAA(δ)R expression and parasympathetic dysfunction after HFD.

Preventing Phosphorylation of the GABAAR β3 Subunit Compromises the Behavioral Effects of Neuroactive Steroids

Neuroactive steroids (NASs) have potent anxiolytic, anticonvulsant, sedative, and hypnotic actions, that reflect in part their efficacy as GABA_AR positive allosteric modulators (PAM). In addition to this, NAS exert metabotropic effects on GABAergic inhibition via the activation of membrane progesterone receptors (mPRs), which are G-protein coupled receptors. mPR activation enhances the phosphorylation of residues serine 408 and 409 (S408/9) in the β3 subunit of GABA_ARs, increasing their accumulation in the plasma membrane leading to a sustained increase in tonic inhibition. To explore the significance of NAS-induced phosphorylation of GABA_ARs, we used mice in which S408/9 in the β3 subunit have been mutated to alanines, mutations that prevent the metabotropic actions of NASs on GABA_AR function while preserving NAS allosteric potentiation of GABAergic current. While the sedative actions of NAS were comparable to WT, their anxiolytic actions were reduced in S408/9A mice. Although the induction of hypnosis by NAS were maintained in the mutant mice the duration of the loss of righting reflex was significantly shortened. Finally, ability of NAS to terminate diazepam pharmacoresistant seizures was abolished in S408/9A mice. In conclusion, our results suggest that S408/9 in the GABA_AR β3 subunit contribute to the anxiolytic and anticonvulsant efficacy of NAS, in addition to their ability to regulate the loss of righting reflex.

Deorphanization of novel biogenic amine-gated ion channels identifies a new serotonin receptor for learning

Pentameric ligand-gated ion channels (LGICs) play conserved, critical roles in both excitatory and inhibitory synaptic transmission and can be activated by diverse neurochemical ligands. We have performed a characterization of orphan channels from the nematode C. elegans, identifying five new monoamine-gated LGICs with diverse functional properties and expression postsynaptic to aminergic neurons. These include polymodal anion channels activated by both dopamine and tyramine, which may mediate inhibitory transmission by both molecules in vivo. Intriguingly, we also find that a novel serotonin-gated cation channel, LGC-50, is essential for aversive olfactory learning of pathogenic bacteria, a process known to depend on serotonergic neurotransmission. Remarkably, the redistribution of LGC-50 to neuronal processes is modulated by olfactory conditioning, and lgc-50 point mutations that cause misregulation of receptor membrane expression interfere with olfactory learning. Thus, the intracellular trafficking and localization of these receptors at synapses may represent a molecular cornerstone of the learning mechanism.

Activity- and sleep-dependent regulation of tonic inhibition by Shisa7

Tonic inhibition mediated by extrasynaptic γ-aminobutyric acid type A receptors (GABAARs) critically regulates neuronal excitability and brain function. However, the mechanisms regulating tonic inhibition remain poorly understood. Here, we report that Shisa7 is critical for tonic inhibition regulation in hippocampal neurons. In juvenile Shisa7 knockout (KO) mice, α5-GABAAR-mediated tonic currents are significantly reduced. Mechanistically, Shisa7 is crucial for α5-GABAAR exocytosis. Additionally, Shisa7 regulation of tonic inhibition requires protein kinase A (PKA) that phosphorylates Shisa7 serine 405 (S405). Importantly, tonic inhibition undergoes activity-dependent regulation, and Shisa7 is required for homeostatic potentiation of tonic inhibition. Interestingly, in young adult Shisa7 KOs, basal tonic inhibition in hippocampal neurons is unaltered, largely due to the diminished α5-GABAAR component of tonic inhibition. However, at this stage, tonic inhibition oscillates during the daily sleep/wake cycle, a process requiring Shisa7. Together, these data demonstrate that intricate signaling mechanisms regulate tonic inhibition at different developmental stages and reveal a molecular link between sleep and tonic inhibition.

Molecular insights into the role of AMPA receptors in the synaptic plasticity, pathogenesis and treatment of epilepsy: therapeutic potentials of perampanel and antisense oligonucleotide (ASO) technology

Glutamate is considered as the predominant excitatory neurotransmitter in the mammalian central nervous systems (CNS). Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are the main glutamate-gated ionotropic channels that mediate the majority of fast synaptic excitation in the brain. AMPARs are highly dynamic that constitutively move into and out of the postsynaptic membrane. Changes in the postsynaptic number of AMPARs play a key role in controlling synaptic plasticity and also brain functions such as memory formation and forgetting development. Impairments in the regulation of AMPAR function, trafficking, and signaling pathway may also contribute to neuronal hyperexcitability and epileptogenesis process, which offers AMPAR as a potential target for epilepsy therapy. Over the last decade, various types of AMPAR antagonists such as perampanel and talampanel have been developed to treat epilepsy, but they usually show limited efficacy at low doses and produce unwanted cognitive and motor side effects when administered at higher doses. In the present article, the latest findings in the field of molecular mechanisms controlling AMPAR biology, as well as the role of these mechanism dysfunctions in generating epilepsy will be reviewed. Also, a comprehensive summary of recent findings from clinical trials with perampanel, in treating epilepsy, glioma-associated epilepsy and Parkinson's disease is provided. Finally, antisense oligonucleotide therapy as an alternative strategy for the efficient treatment of epilepsy is discussed.

A Cellular Mechanism of Learning-Induced Enhancement of Synaptic Inhibition: PKC-Dependent Upregulation of KCC2 Activation

Long-term memory of complex olfactory learning is expressed by wide spread enhancement in excitatory and inhibitory synaptic transmission onto piriform cortex pyramidal neurons. A particularly interesting modification in synaptic inhibition is the hyperpolarization of the reversal potential of the fast post synaptic inhibitory potential (fIPSP). Here we study the mechanism underlying the maintenance of such a shift in the fIPSP. Blocking of the neuronal specific K+-Cl- co-transporter (KCC2) in neurons of trained rats significantly depolarized the averaged fIPSP reversal potential of the spontaneous miniature inhibitory post synaptic currents (mIPSCs), to the averaged pre-training level. A similar effect was obtained by blocking PKC, which was previously shown to upregulate KCC2. Accordingly, the level of PKC-dependent phosphorylation of KCC2, at the serine 940 site, was significantly increased after learning. In contrast, blocking two other key second messenger systems CaMKII and PKA, which have no phosphorylation sites on KCC2, had no effect on the fIPSP reversal potential. Importantly, the PKC inhibitor also reduced the averaged amplitude of the spontaneous miniature excitatory synaptic currents (mEPSCs) in neurons of trained rats only, to the pre-training level. We conclude that learning-induced hyper-polarization of the fIPSP reversal potential is mediated by PKC-dependent increase of KCC2 phosphorylation.

Metabotropic, but not allosteric, effects of neurosteroids on GABAergic inhibition depend on the phosphorylation of GABAA receptors

Neuroactive steroids (NASs) are synthesized within the brain and exert profound effects on behavior. These effects are primarily believed to arise from the activities of NASs as positive allosteric modulators (PAMs) of the GABA-type A receptor (GABAAR). NASs also activate a family of G protein-coupled receptors known as membrane progesterone receptors (mPRs). Here, using surface-biotinylation assays and electrophysiology techniques, we examined mPRs' role in mediating the effects of NAS on the efficacy of GABAergic inhibition. Selective mPR activation enhanced phosphorylation of Ser-408 and Ser-409 (Ser-408/9) within the GABAAR β3 subunit, which depended on the activity of cAMP-dependent protein kinase A (PKA) and protein kinase C (PKC). mPR activation did not directly modify GABAAR activity and had no acute effects on phasic or tonic inhibition. Instead, mPR activation induced a sustained elevation in tonic current, which was blocked by PKA and PKC inhibition. Substitution of Ser-408/9 to alanine residues also prevented the effects of mPR activation on tonic current. Furthermore, this substitution abolished the effects of sustained NAS exposure on tonic inhibition. Interestingly, the allosteric effects of NAS on GABAergic inhibition were independent of Ser-408/9 in the β3 subunit. Additionally, although allosteric effects of NAS on GABAergic inhibition were sensitive to a recently developed "NAS antagonist," the sustained effects of NAS on tonic inhibition were not. We conclude that metabotropic effects of NAS on GABAergic inhibition are mediated by mPR-dependent modulation of GABAAR phosphorylation. We propose that this mechanism may contribute to the varying behavioral effects of NAS.

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.

Pubertal hormones increase hippocampal expression of α4βδ GABAA receptors

CA1 hippocampal expression of α4βδ GABA_A receptors (GABARs) increases at the onset of puberty in female mice, an effect dependent upon the decline in hippocampal levels of the neurosteroid THP (3α-OH-5α-pregnan-20-one) which occurs at this time. The present study further characterized the mechanisms underlying α4βδ expression, assessed in vivo. Blockade of pubertal levels of 17β-estradiol (E₂) (formestane, 0.5 mg/kg, i.p. 3 d) reduced α4 and δ expression by 75-80% (P < 0.05) in CA1 hippocampus of female mice, assessed using Western blot techniques. Conversely, E₂ administration increased α4 and δ expression by 50-100% in adults, an effect enhanced by more than 2-fold by concomitant administration of the 5α-reductase blocker finasteride (50 mg/kg, i.p., 3d, P < 0.05), suggesting that both declining THP levels and increasing E₂ levels before puberty trigger α4βδ expression. This effect was blocked by ICI 182,780 (20 mg/kg, s.c., 3 d), a selective blocker of E₂ receptor-α (ER-α). These results suggest that both the rise in circulating levels of E₂ and the decline in hippocampal THP levels at the onset of puberty trigger maximal levels of α4βδ expression in the CA1 hippocampus.

Neuroactive Steroids Reverse Tonic Inhibitory Deficits in Fragile X Syndrome Mouse Model

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. A reduction in neuronal inhibition mediated by γ-aminobutyric acid type A receptors (GABA_ARs) has been implicated in the pathophysiology of FXS. Neuroactive steroids (NASs) are known allosteric modulators of GABA_AR channel function, but recent studies from our laboratory have revealed that NASs also exert persistent metabotropic effects on the efficacy of tonic inhibition by increasing the protein kinase C (PKC)-mediated phosphorylation of the α4 and β3 subunits which increase the membrane expression and boosts tonic inhibition. We have assessed the GABAergic signaling in the hippocampus of fragile X mental retardation protein (FMRP) knock-out (Fmr1KO) mouse. The GABAergic tonic current in dentate gyrus granule cells (DGGCs) from 3- to 5-week-old (p21–35) Fmr1KO mice was significantly reduced compared to WT mice. Additionally, spontaneous inhibitory post synaptic inhibitory current (sIPSC) amplitudes were increased in DGGCs from Fmr1 KO mice. While sIPSCs decay in both genotypes was prolonged by the prototypic benzodiazepine diazepam, those in Frm1-KO mice were selectively potentiated by RO15-4513. Consistent with this altered pharmacology, modifications in the expression levels and phosphorylation of receptor GABA_AR subtypes that mediate tonic inhibition were seen in Fmr1 KO mice. Significantly, exposure to NASs induced a sustained elevation in tonic current in Fmr1 KO mice which was prevented with PKC inhibition. Likewise, exposure reduced elevated membrane excitability seen in the mutant mice. Collectively, our results suggest that NAS act to reverse the deficits of tonic inhibition seen in FXS, and thereby reduce aberrant neuronal hyperexcitability seen in this disorder.

Single Ethanol Withdrawal Regulates Extrasynaptic δ-GABAA Receptors Via PKCδ Activation

Alcohol (ethanol, EtOH) is one of the most widely abused drugs with profound effects on brain function and behavior. GABA_A receptors (GABA_ARs) are one of the major targets for EtOH in the brain. Temporary plastic changes in GABA_ARs after withdrawal from a single EtOH exposure occur both in vivo and in vitro, which may be the basis for chronic EtOH addiction, tolerance and withdrawal symptoms. Extrasynaptic δ-GABA_AR endocytosis is implicated in EtOH-induced GABA_AR plasticity, but the mechanisms by which the relative abundance and localization of specific GABA_ARs are altered by EtOH exposure and withdrawal remain unclear. In this study, we investigated the mechanisms underlying rapid regulation of extrasynaptic δ-GABA_AR by a single EtOH withdrawal in cultured rat hippocampal neurons. Thirty-minutes EtOH (60 mM) exposure increased extrasynaptic tonic current (I_tonic) amplitude without affecting synaptic GABA_AR function in neurons. In contrast, at 30 min after withdrawal, I_tonic amplitude and responsiveness to acute EtOH were both reduced. Similar results occurred in neurons with okadaic acid (OA) or phorbol 12,13-dibutyrate (PDBu) exposure. Protein kinase C (PKC) inhibition prevented the reduction of I_tonic amplitude and the tolerance to acute EtOH, as well as the reduction of GABA_AR-δ subunit abundance induced by a single EtOH withdrawal. Moreover, EtOH withdrawal selectively increased PKCδ level, whereas PKCδ inhibition specifically rescued the EtOH-induced alterations in GABA_AR-δ subunit level and δ-GABA_AR function. Together, we provided strong evidence for the important roles of PKCδ in the rapid regulation of extrasynaptic δ-GABA_AR induced by a single EtOH withdrawal.

3β-Methyl-Neurosteroid Analogs Are Preferential Positive Allosteric Modulators and Direct Activators of Extrasynaptic δ-Subunit γ-Aminobutyric Acid Type A Receptors in the Hippocampus Dentate Gyrus Subfield

Neurosteroids are powerful modulators of γ-aminobutyric acid (GABA)-A receptors. Ganaxolone (3α-hydroxy-3β-methyl-5α-pregnan-20-one, GX) and synthetic analogs of the neurosteroid allopregnanolone (AP) are designed to treat epilepsy and related conditions. However, their precise mechanism of action in native neurons remains unclear. Here, we sought to determine the mode of action of GX and its analogs at GABA-A receptors in native hippocampal neurons by analyzing extrasynaptic receptor-mediated tonic currents and synaptic receptor-mediated phasic currents. Concentration-response profiles of GX were determined in two cell types: δ-containing dentate gyrus granule cells (DGGCs) and γ2-containing CA1 pyramidal cells (CA1PCs). GX produced significantly greater potentiation of the GABA-A receptor-activated chloride currents in DGGCs (500%) than CA1PCs (200%). In the absence of GABA, GX evoked 2-fold greater inward currents in DGGCs than CA1PCs, which were 2-fold greater than AP within DGGCs. In hippocampus slices, GX potentiated and directly activated tonic currents in DGGCs. These responses were significantly diminished in DGGCs from δ-subunit knockout (δKO) mice, confirming GX's selectivity for δGABA-A receptors. Like AP, GX potentiation of tonic currents was prevented by protein kinase C inhibition. Furthermore, GX's protection against hippocampus-kindled seizures was significantly diminished in δKO mice. GX analogs exhibited greater potency and efficacy than GX on δGABA-A receptor-mediated tonic inhibition. In summary, these results provide strong evidence that GX and its analogs are preferential allosteric modulators and direct activators of extrasynaptic δGABA-A receptors regulating network inhibition and seizures in the dentate gyrus. Therefore, these findings provide a mechanistic rationale for the clinical use of synthetic neurosteroids in epilepsy and seizure disorders.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

Genetic and Molecular Regulation of Extrasynaptic GABA-A Receptors in the Brain: Therapeutic Insights for Epilepsy

GABA-A receptors play a pivotal role in many brain diseases. Epilepsy is caused by acquired conditions and genetic defects in GABA receptor channels regulating neuronal excitability in the brain. The latter is referred to as GABA channelopathies. In the last two decades, major advances have been made in the genetics of epilepsy. The presence of specific GABAergic genetic abnormalities leading to some of the classic epileptic syndromes has been identified. Advances in molecular cloning and recombinant systems have helped characterize mutations in GABA-A receptor subunit genes in clinical neurology. GABA-A receptors are the prime targets for neurosteroids (NSs). However, GABA-A receptors are not static but undergo rapid changes in their number or composition in response to the neuroendocrine milieu. This review describes the recent advances in the genetic and neuroendocrine control of extrasynaptic and synaptic GABA-A receptors in epilepsy and its impact on neurologic conditions. It highlights the current knowledge of GABA genetics in epilepsy, with an emphasis on the neuroendocrine regulation of extrasynaptic GABA-A receptors in network excitability and seizure susceptibility. Recent advances in molecular regulation of extrasynaptic GABA-A receptor-mediated tonic inhibition are providing unique new therapeutic approaches for epilepsy, status epilepticus, and certain brain disorders. The discovery of an extrasynaptic molecular mechanism represents a milestone for developing novel therapies such as NS replacement therapy for catamenial epilepsy.

Neuroactive Steroids: Receptor Interactions and Responses

Neuroactive steroids (NASs) are naturally occurring steroids, which are synthesized centrally as de novo from cholesterol and are classified as pregnane, androstane, and sulfated neurosteroids (NSs). NASs modulate many processes via interacting with gamma-aminobutyric acid (GABA), N-methyl-d-aspartate, serotonin, voltage-gated calcium channels, voltage-dependent anion channels, α-adrenoreceptors, X-receptors of the liver, transient receptor potential channels, microtubule-associated protein 2, neurotrophin nerve growth factor, and σ1 receptors. Among these, NSs (especially allopregnanolone) have high potency and extensive GABA-A receptors and hence demonstrate anticonvulsant, anesthetic, central cytoprotectant, and baroreflex inhibitory effects. NSs are also involved in mood and learning via serotonin and anti-nociceptive activity via T-type voltage-gated Ca2+ channels. Moreover, they are modulators of mitochondrial function, synaptic plasticity, or regulators of apoptosis, which have a role in neuroprotective via voltage-dependent anion channels receptors. For proper functioning, NASs need to be in their normal level, whereas excess and deficiency may lead to abnormalities. When they are below the normal, NSs could have a part in development of depression, neuro-inflammation, multiple sclerosis, experimental autoimmune encephalitis, epilepsy, and schizophrenia. On the other hand, stress and attention deficit disorder could occur during excessive level. Overall, NASs are very important molecules with major neuropsychiatric activity.

The Influence of Regional Distribution and Pharmacologic Specificity of GABAAR Subtype Expression on Anesthesia and Emergence

Anesthetics produce unconsciousness by modulating ion channels that control neuronal excitability. Research has shown that specific GABA_A receptor (GABA_AR) subtypes in particular regions of the central nervous system contribute to different hyperpolarizing conductances, and behaviorally to distinct components of the anesthetized state. The expression of these receptors on the neuron cell surface, and thus the strength of inhibitory neurotransmission, is dynamically regulated by intracellular trafficking mechanisms. Pharmacologic or activity-based perturbations to these regulatory systems have been implicated in pathology of several neurological conditions, and can alter the individual response to anesthesia. Furthermore, studies are beginning to uncover how anesthetic exposure itself elicits enduring changes in subcellular physiology, including the processes that regulate ion channel trafficking. Here, we review the mechanisms that determine GABA_AR surface expression, and elaborate on influences germane to anesthesia and emergence. We address known trafficking differences between the intrasynaptic receptors that mediate phasic current and the extra-synaptic receptors mediating tonic current. We also describe neurophysiologic consequences and network-level abnormalities in brain function that result from receptor trafficking aberrations. We hypothesize that the relationship between commonly used anesthetic agents and GABA_AR surface expression has direct consequences on mature functioning neural networks and by extension ultimately influence the outcome of patients that undergo general anesthesia. Rational design of new anesthetics, anesthetic techniques, EEG-based monitoring strategies, or emergence treatments will need to take these effects into consideration.

Decreased surface expression of the δ subunit of the GABAA receptor contributes to reduced tonic inhibition in dentate granule cells in a mouse model of fragile X syndrome

While numerous changes in the GABA system have been identified in models of Fragile X Syndrome (FXS), alterations in subunits of the GABA_A receptors (GABA_ARs) that mediate tonic inhibition are particularly intriguing. Considering the key role of tonic inhibition in controlling neuronal excitability, reduced tonic inhibition could contribute to FXS-associated disorders such as hyperactivity, hypersensitivity, and increased seizure susceptibility. The current study has focused on the expression and function of the δ subunit of the GABA_AR, a major subunit involved in tonic inhibition, in granule cells of the dentate gyrus in the Fmr1 knockout (KO) mouse model of FXS. Electrophysiological studies of dentate granule cells revealed a marked, nearly four-fold, decrease in tonic inhibition in the Fmr1 KO mice, as well as reduced effects of two δ subunit-preferring pharmacological agents, THIP and DS2, supporting the suggestion that δ subunit-containing GABA_ARs are compromised in the Fmr1 KO mice. Immunohistochemistry demonstrated a small but statistically significant decrease in δ subunit labeling in the molecular layer of the dentate gyrus in Fmr1 KO mice compared to wildtype (WT) littermates. The discrepancy between the large deficits in GABA-mediated tonic inhibition in granule cells in the Fmr1 KO mice and only modest reductions in immunolabeling of the δ subunit led to studies of surface expression of the δ subunit. Cross-linking experiments followed by Western blot analysis demonstrated a small, non-significant decrease in total δ subunit protein in the hippocampus of Fmr1 KO mice, but a four-fold decrease in surface expression of the δ subunit in these mice. No significant changes were observed in total or surface expression of the α4 subunit protein, a major partner of the δ subunit in the forebrain. Postembedding immunogold labeling for the δ subunit demonstrated a large, three-fold, decrease in the number of symmetric synapses with immunolabeling at perisynaptic locations in Fmr1 KO mice. While α4 immunogold particles were also reduced at perisynaptic locations in the Fmr1 KO mice, the labeling was increased at synaptic sites. Together these findings suggest that, in the dentate gyrus, altered surface expression of the δ subunit, rather than a decrease in δ subunit expression alone, could be limiting δ subunit-mediated tonic inhibition in this model of FXS. Finding ways to increase surface expression of the δ subunit of the GABA_AR could be a novel approach to treatment of hyperexcitability-related alterations in FXS.

MPTP-Induced Dopamine Depletion in Basolateral Amygdala via Decrease of D2R Activation Suppresses GABAA Receptors Expression and LTD Induction Leading to Anxiety-Like Behaviors

Anxiety disorders commonly occur in Parkinson’s disease. Using field potential recording and patch-clamp recording, we evaluated influence of MPTP-reduced dopaminergic afferent in basolateral amygdala (BLA), a main region for affective regulation, on excitatory–inhibitory circuits and synaptic plasticity. Field excitatory post-synaptic potential (fEPSP) slopes at external capsule-BLA synapses were increased in MPTP-mice with decreases in paired-pulse facilitation and long-term potentiation amplitude, which were corrected by bath-application of D2R agonist quinpirole or cannabinoid type 1 receptors agonist WIN55,212-2, but not D1R agonist SKF38393. Compared to single waveform fEPSP in control mice, a multi-spike waveform fEPSP was observed in MPTP-mice with prolongation of duration and an increase in paired-pulse inhibition, which were recovered by BLA-injection of quinpirole for 2 days rather than bath-application. Density of GABA-evoked current (I_GABA) in BLA principal neurons and GABA_AR-α2 subunit expression were reduced in MPTP-mice, which were recovered by administration of quinpirole. Decline of PKC phosphorylation in BLA of MPTP-mice was corrected by bath-application of quinpirole, but not SKF38393. In MPTP-mice, BLA-injection of quinpirole or PKC activator PMA could recover GABA_AR expression, which was sensitive to PKC inhibitor GF109203X. The impairment of long-term depression (LTD) in MPTP-mice was rescued by bath-application of GABA_AR agonist muscimol or BLA-injection of quinpirole and PMA. Finally, BLA-injection of muscimol, quinpirole or PMA relieved anxiety-like behaviors in MPTP-mice. The results indicate that the MPTP-induced dopamine depletion in BLA principal neurons through reducing D2R-mediated PKC phosphorylation suppresses GABA_AR expression and activity, which impairs GABA_AR-mediated inhibition and LTD induction leading to anxiety-like behaviors.

Regulating the Efficacy of Inhibition Through Trafficking of γ-Aminobutyric Acid Type A Receptors

Trafficking of anesthetic-sensitive receptors within the plasma membrane, or from one cellular component to another, occurs continuously. Changes in receptor trafficking have implications in altering anesthetic sensitivity. γ-Aminobutyric acid type A receptors (GABAARs) are anion-permeable ion channels and are the major class of receptor in the adult mammalian central nervous system that mediates inhibition. GABAergic signaling allows for precise synchronized firing of action potentials within brain circuits that is critical for cognition, behavior, and consciousness. This precision depends upon tightly controlled trafficking of GABAARs into the membrane. General anesthetics bind to and allosterically enhance GABAARs by prolonging the open state of the receptor and thereby altering neuronal and brain circuit activity. Subunit composition and GABAAR localization strongly influence anesthetic end points; therefore, changes in GABAAR trafficking could have significant consequences to anesthetic sensitivity. GABAARs are not static membrane structures but are in a constant state of flux between extrasynaptic and synaptic locations and are continually endocytosed and recycled from and to the membrane. Neuronal activity, posttranslational modifications, and some naturally occurring and synthetic compounds can influence the expression and trafficking of GABAARs. In this article, we review GABAARs, their trafficking, and how phosphorylation of GABAAR subunits can influence the surface expression and function of the receptor. Ultimately, alterations of GABAAR trafficking could modify anesthetic end points, both unintentionally through pathologic processes but potentially as a therapeutic target to adjust anesthetic-sensitive GABAARs.

Propofol Anesthesia Is Reduced in Phospholipase C-Related Inactive Protein Type-1 Knockout Mice

The GABA type A receptor (GABA_A-R) is a major target of intravenous anesthetics. Phospholipase C-related inactive protein type-1 (PRIP-1) is important in GABA_A-R phosphorylation and membrane trafficking. In this study, we investigated the role of PRIP-1 in general anesthetic action. The anesthetic effects of propofol, etomidate, and pentobarbital were evaluated in wild-type and PRIP-1 knockout (PRIP-1 KO) mice by measuring the latency and duration of loss of righting reflex (LORR) and loss of tail-pinch withdrawal response (LTWR). The effect of pretreatment with okadaic acid (OA), a protein phosphatase 1/2A inhibitor, on propofol- and etomidate-induced LORR was also examined. PRIP-1 deficiency provided the reduction of LORR and LTWR induced by propofol but not by etomidate or pentobarbital, indicating that PRIP-1 could determine the potency of the anesthetic action of propofol. Pretreatment with OA recovered the anesthetic potency induced by propofol in PRIP-1 KO mice. OA injection enhanced phosphorylation of cortical the GABA_A-R β3 subunit in PRIP-1 KO mice. These results suggest that PRIP-1-mediated GABA_A-R β3 subunit phosphorylation might be involved in the general anesthetic action induced by propofol but not by etomidate or pentobarbital.

Endogenous and synthetic neuroactive steroids evoke sustained increases in the efficacy of GABAergic inhibition via a protein kinase C-dependent mechanism

The neuroactive steroid (NAS) tetrahydrodeoxycorticosterone (THDOC) increases protein kinase C (PKC) mediated phosphorylation of extrasynaptic GABA_A receptor (GABA_AR) subunits leading to increased surface expression of α4/β3 subunit-containing extrasynaptic GABA_ARs, leading to a sustained increase in GABA_AR tonic current density. Whether other naturally occurring and synthetic NASs share both an allosteric and metabotropic action on GABA_ARs is unknown. Here, we examine the allosteric and metabotropic properties of allopregnanolone (ALLO), and synthetic NASs SGE-516 and ganaxolone. ALLO, SGE-516, and ganaxolone all allosterically enhanced prototypical synaptic and extrasynaptic recombinant GABA_ARs. In dentate gyrus granule cells (DGGCs) all three NASs, when applied acutely, allosterically enhanced tonic and phasic GABAergic currents. In separate experiments, slices were exposed to NASs for 15 min, and then transferred to a steroid naïve recording chamber followed by ≥ 30 min wash before tonic currents were measured. A sustained increase in tonic current was observed following exposure to ALLO, or SGE-516 and was prevented by inhibiting PKC with GF 109203X. No increase in tonic current was observed with exposure to ganaxolone. In agreement with the observations of an increased tonic current, the NASs ALLO and SGE-516 increased the phosphorylation and surface expression of the β3 subunit-containing GABA_ARs. Our studies demonstrate that neuroactive steroids have differential abilities to induce sustained increases in the efficacy of tonic inhibition by promoting GABA_AR phosphorylation and membrane trafficking dependent on PKC activity.

Glutamatergic drive facilitates synaptic inhibition of dorsal vagal motor neurons after experimentally induced diabetes in mice

The role of central regulatory circuits in modulating diabetes-associated glucose dysregulation has only recently been under rigorous investigation. One brain region of interest is the dorsal motor nucleus of the vagus (DMV), which contains preganglionic parasympathetic motor neurons that regulate subdiaphragmatic visceral function. Previous research has demonstrated that glutamatergic and GABAergic neurotransmission are independently remodeled after chronic hyperglycemia/hypoinsulinemia. However, glutamatergic circuitry within the dorsal brain stem impinges on GABAergic regulation of the DMV. The present study investigated the role of glutamatergic neurotransmission in synaptic GABAergic control of DMV neurons after streptozotocin (STZ)-induced hyperglycemia/hypoinsulinemia by using electrophysiological recordings in vitro. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was elevated in DMV neurons from STZ-treated mice. The effect was abolished in the presence of the ionotropic glutamate receptor blocker kynurenic acid or the sodium channel blocker tetrodotoxin, suggesting that after STZ-induced hyperglycemia/hypoinsulinemia, increased glutamatergic receptor activity occurs at a soma-dendritic location on local GABA neurons projecting to the DMV. Although sIPSCs in DMV neurons normally demonstrated considerable amplitude variability, this variability was significantly increased after STZ-induced hyperglycemia/hypoinsulinemia. The elevated amplitude variability was not related to changes in quantal release, but rather correlated with significantly elevated frequency of sIPSCs in these mice. Taken together, these findings suggest that GABAergic regulation of central vagal circuitry responsible for the regulation of energy homeostasis undergoes complex functional reorganization after several days of hyperglycemia/hypoinsulinemia, including both glutamate-dependent and -independent forms of plasticity.

Proteomic Characterization of Inhibitory Synapses Using a Novel pHluorin-tagged γ-Aminobutyric Acid Receptor, Type A (GABAA), α2 Subunit Knock-in Mouse

The accumulation of γ-aminobutyric acid receptors (GABAARs) at the appropriate postsynaptic sites is critical for determining the efficacy of fast inhibitory neurotransmission. Although we know that the majority of synaptic GABAAR subtypes are assembled from α1-3, β, and γ2 subunits, our understanding of how neurons facilitate their targeting to and stabilization at inhibitory synapses is rudimentary. To address these issues, we have created knock-in mice in which the pH-sensitive green fluorescent protein (GFP) and the Myc epitope were introduced to the extracellular domain of the mature receptor α2 subunit (pHα2). Using immunoaffinity purification and mass spectroscopy, we identified a stable complex of 174 proteins that were associated with pHα2, including other GABAAR subunits, and previously identified receptor-associated proteins such as gephyrin and collybistin. 149 of these proteins were novel GABAAR binding partners and included G-protein-coupled receptors and ion channel subunits, proteins that regulate trafficking and degradation, regulators of protein phosphorylation, GTPases, and a number of proteins that regulate their activity. Notably, members of the postsynaptic density family of proteins that are critical components of excitatory synapses were not associated with GABAARs. Crucially, we demonstrated for a subset of these novel proteins (including cullin1, ephexin, potassium channel tetramerization domain containing protein 12, mitofusin2, metabotropic glutamate receptor 5, p21-activated kinase 7, and Ras-related protein 5A) bind directly to the intracellular domains of GABAARs, validating our proteomic analysis. Thus, our experiments illustrate the complexity of the GABAAR proteome and enhance our understanding of the mechanisms neurons use to construct inhibitory synapses.

Ethanol-induced GABAA receptor alpha4 subunit plasticity involves phosphorylation and neuroactive steroids

GABAA receptors containing α4 subunits are widely implicated in acute ethanol sensitivity, and their spatial and temporal regulation prominently contributes to ethanol-induced neuroplasticity in hippocampus and cortex. However, it is unknown if α4-containing GABAA receptors in the thalamus, an area of high α4 expression, display similar regulatory patterns following ethanol administration, and if so, by which molecular mechanisms. In the current study, thalamic GABAA receptor α4 subunit levels were increased following a 6-week-, but not a 2-week chronic ethanol diet. Following acute high-dose ethanol administration, thalamic GABAA receptor α4 subunit levels were regulated in a temporal fashion, as a decrease was observed at 2h followed by a delayed transient increase. PKCγ and PKCδ levels paralleled α4 temporal expression patterns following ethanol exposure. Initial decreases in α4 subunit expression were associated with reduced serine phosphorylation. Delayed increases in expression were not associated with a change in phosphorylation state, but were prevented by inhibiting neuroactive steroid production with the 5α-reductase inhibitor finasteride. Overall, these studies indicate that thalamic GABAA receptor α4 subunit expression following acute and chronic ethanol administration exhibits similar regulatory patterns as other regions and that transient expression patterns following acute exposure in vivo are likely dependent on both subunit phosphorylation state and neuroactive steroids.

Differential regulation of synaptic and extrasynaptic α4 GABA(A) receptor populations by protein kinase A and protein kinase C in cultured cortical neurons

The GABAA α4 subunit exists in two distinct populations of GABAA receptors. Synaptic GABAA α4 receptors are localized at the synapse and mediate phasic inhibitory neurotransmission, while extrasynaptic GABAA receptors are located outside of the synapse and mediate tonic inhibitory transmission. These receptors have distinct pharmacological and biophysical properties that contribute to interest in how these different subtypes are regulated under physiological and pathological states. We utilized subcellular fractionation procedures to separate these populations of receptors in order to investigate their regulation by protein kinases in cortical cultured neurons. Protein kinase A (PKA) activation decreases synaptic α4 expression while protein kinase C (PKC) activation increases α4 subunit expression, and these effects are associated with increased β3 S408/409 or γ2 S327 phosphorylation respectively. In contrast, PKA activation increases extrasynaptic α4 and δ subunit expression, while PKC activation has no effect. Our findings suggest synaptic and extrasynaptic GABAA α4 subunit expression can be modulated by PKA to inform the development of more specific therapeutics for neurological diseases that involve deficits in GABAergic transmission.

Regulation of Extrasynaptic GABAA α4 Receptors by Ethanol-Induced Protein Kinase A, but Not Protein Kinase C Activation in Cultured Rat Cerebral Cortical Neurons

Ethanol produces changes in GABAA receptor trafficking and function that contribute to ethanol dependence symptomatology. Extrasynaptic γ-aminobutyric acid A receptors (GABAA-R) mediate inhibitory tonic current and are of particular interest because they are potentiated by physiologically relevant doses of ethanol. Here, we isolate GABAA α4δ receptors by western blotting in subsynaptic fractions to investigate protein kinase A (PKA) and protein kinase C (PKC) modulation of ethanol-induced receptor trafficking, while extrasynaptic receptor function is determined by measurement of tonic inhibition and responses evoked by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). Rat cerebral cortical neurons were grown for 18 days in vitro and exposed to ethanol and/or PKA/PKC modulators. Ethanol exposure (1 hour) did not alter GABAA α4 receptor abundance, but it increased tonic current amplitude, an effect that was prevented by inhibiting PKA, but not PKC. Direct activation of PKA, but not PKC, increased the abundance and tonic current of extrasynaptic α4δ receptors. In contrast, prolonged ethanol exposure (4 hours) reduced α4δ receptor abundance as well as tonic current, and this effect was also PKA dependent. Finally, PKC activation by ethanol or phorbol-12,13-dibutyrate (PdBu) had no effect on extrasynaptic α4δ subunit abundance or activity. We conclude that ethanol alters extrasynaptic α4δ receptor function and expression in cortical neurons in a PKA-dependent manner, but ethanol activation of PKC does not influence these receptors. These results could have clinical relevance for therapeutic strategies to restore normal GABAergic functioning for the treatment of alcohol use disorders.

Exposure to 50 Hz magnetic field modulates GABAA currents in cerebellar granule neurons through an EP receptor-mediated PKC pathway

Previous work from both our lab and others have indicated that exposure to 50 Hz magnetic fields (ELF-MF) was able to modify ion channel functions. However, very few studies have investigated the effects of MF on γ-aminobutyric acid (GABA) type A receptors (GABA(A) Rs) channel functioning, which are fundamental to overall neuronal excitability. Here, our major goal is to reveal the potential effects of ELF-MF on GABA(A) Rs activity in rat cerebellar granule neurons (CGNs). Our results indicated that exposing CGNs to 1 mT ELF-MF for 60 min. significantly increased GABA(A) R currents without modifying sensitivity to GABA. However, activation of PKA by db-cAMP failed to do so, but led to a slight decrease instead. On the other hand, PKC activation or inhibition by PMA or Bis and Docosahexaenoic acid (DHA) mimicked or eliminated the field-induced-increase of GABA(A) R currents. Western blot analysis indicated that the intracellular levels of phosphorylated PKC (pPKC) were significantly elevated after 60 min. of ELF-MF exposure, which was subsequently blocked by application of DHA or EP1 receptor-specific (prostaglandin E receptor 1) antagonist (SC19220), but not by EP2-EP4 receptor-specific antagonists. SC19220 also significantly inhibited the ELF-MF-induced elevation on GABA(A) R currents. Together, these data obviously demonstrated for the first time that neuronal GABA(A) currents are significantly increased by ELF-MF exposure, and also suggest that these effects are mediated via an EP1 receptor-mediated PKC pathway. Future work will focus on a more comprehensive analysis of the physiological and/or pathological consequences of these effects.

Protein kinase C bidirectionally modulates Ih and hyperpolarization-activated cyclic nucleotide-gated (HCN) channel surface expression in hippocampal pyramidal neurons

KEY POINTS

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, particularly that of the HCN1 isoform, are enriched in the distal dendrites of hippocampal CA1 pyramidal neurons; these channels have physiological functions with respect to decreasing neuronal excitability. In the present study, we aimed to investigate phosphorylation as a mechanism controlling Ih amplitude and HCN1 surface expression in hippocampal principal neurons under normal physiological conditions. Tyrosine phosphorylation decreased Ih amplitude at maximal activation (maximal Ih ), without altering HCN1 surface expression, in two classes of hippocampal principal neurons. Inhibition of serine/threonine protein phosphatases 1 and 2A decreased maximal Ih and HCN1 surface expression in hippocampal principal neurons. Protein kinase C (PKC) activation irreversibly diminished Ih and HCN1 surface expression, whereas PKC inhibition augmented Ih and HCN1 surface expression. PKC activation increased HCN1 channel phosphorylation. These results demonstrate the novel finding of a phosphorylation mechanism, dependent on PKC activity, which bidirectionally modulates Ih amplitude and HCN1channel surface expression in hippocampal principal neurons under normal physiological conditions.

ABSTRACT

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) ion channels attenuate excitability in hippocampal pyramidal neurons. Loss of HCN channel-mediated current (Ih ), particularly that mediated by the HCN1 isoform, occurs with the development of epilepsy. Previously, we showed that, following pilocarpine-induced status epilepticus, there are two independent changes in HCN function in dendrites: decreased Ih amplitude associated with a loss of HCN1 surface expression and a hyperpolarizing shift in voltage-dependence of activation (gating). The hyperpolarizing shift in gating was attributed to decreased phosphorylation as a result of a loss of p38 mitogen-activated protein kinase activity and increased calcineurin activity; however, the mechanisms controlling Ih amplitude and HCN1 surface expression under epileptic or normal physiological conditions are poorly understood. We aimed to investigate phosphorylation as a mechanism regulating Ih amplitude and HCN1 surface expression (i.e. as is the case for HCN gating) in hippocampal principal neurons under normal physiological conditions. We discovered that inhibition of either tyrosine phosphatases or the serine/threonine protein phosphatases 1 and 2A decreased Ih at maximal activation in hippocampal CA1 pyramidal dendrites and pyramidal-like principal neuron somata from naïve rats. Furthermore, we found that inhibition of PP1/PP2A decreased HCN1 surface expression, whereas tyrosine phosphatase inhibition did not. Protein kinase C (PKC) activation reduced Ih amplitude and HCN1 surface expression, whereas PKC inhibition produced the opposite effect. Inhibition of protein phosphatases 1 and 2A and activation of PKC increased the serine phosphorylation state of the HCN1 protein. The effect of PKC activation on Ih was irreversible. These results indicate that PKC bidirectionally modulates Ih amplitude and HCN1 surface expression in hippocampal principal neurons.

Diabetes induces GABA receptor plasticity in murine vagal motor neurons

Autonomic dysregulation accompanies type-1 diabetes, and synaptic regulation of parasympathetic preganglionic motor neurons in the dorsal motor nucleus of the vagus (DMV) is altered after chronic hyperglycemia/hypoinsulinemia. Tonic gamma-aminobutyric acid A (GABAA) inhibition prominently regulates DMV neuron activity, which contributes to autonomic control of energy homeostasis. This study investigated persistent effects of chronic hyperglycemia/hypoinsulinemia on GABAA receptor-mediated inhibition in the DMV after streptozotocin-induced type-1 diabetes using electrophysiological recordings in vitro, quantitative (q)RT-PCR, and immunohistochemistry. Application of the nonspecific GABAA receptor agonist muscimol evoked an outward current of significantly larger amplitude in DMV neurons from diabetic mice than controls. Results from application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP), a δ-subunit agonist, suggested that GABAA receptors containing δ-subunits contributed to the enhanced inducible tonic GABA current in diabetic mice. Sensitivity to THIP of inhibitory postsynaptic currents in DMV neurons from diabetic mice was also increased. Results from qRT-PCR and immunohistochemical analyses indicated that the altered GABAergic inhibition may be related to increased trafficking of GABAA receptors that contain the δ-subunit, rather than an expression change. Overall these findings suggest increased sensitivity of δ-subunit containing GABAA receptors after several days of hyperglycemia/hypoinsulinemia, which dramatically alters GABAergic inhibition of DMV neurons and could contribute to diabetic autonomic dysregulation.

CaMKIIα and caveolin-1 cooperate to drive ATP-induced membrane delivery of the P2X3 receptor

The P2X3 receptor plays a vital role in sensory processing and transmission. The assembly and trafficking of the P2X3 receptor are important for its function in primary sensory neurons. As an important inflammation mediator, ATP is released from different cell types around primary sensory neurons, especially under pathological pain conditions. Here, we show that α, β-MeATP dramatically promoted membrane delivery of the P2X3 receptor both in HEK293T cells expressing recombinant P2X3 receptor and in rat primary sensory neurons. α, β-MeATP induced P2X3 receptor-mediated Ca²⁺ influx, which further activated Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα). The N terminus of the P2X3 receptor was responsible for CaMKIIα binding, whereas Thr³⁸⁸ in the C terminus was phosphorylated by CaMKIIα. Thr³⁸⁸ phosphorylation increased P2X3 receptor binding to caveolin-1. Caveolin-1 knockdown abrogated the α, β-MeATP-induced membrane insertion of the P2X3 receptor. Moreover, α, β-MeATP drove the CaMKIIα-mediated membrane coinsertion of the P2X2 receptor with the P2X3 receptor. The increased P2X3 receptors on the cell membrane that are due to Thr³⁸⁸ phosphorylation facilitated P2X3 receptor-mediated signal transduction. Together, our data indicate that CaMKIIα and caveolin-1 cooperate to drive ligand-induced membrane delivery of the P2X3 receptor and may provide a mechanism of P2X3 receptor sensitization in pain development.

Persistent CaMKII activation mediates learning-induced long-lasting enhancement of synaptic inhibition

Training rats in a particularly difficult olfactory-discrimination task results in acquisition of high skill to perform the task superbly, termed "rule learning" or "learning set." Such complex learning results in enhanced intrinsic neuronal excitability of piriform cortex pyramidal neurons, and in their excitatory synaptic interconnections. These changes, while subserving memory maintenance, must be counterbalanced by modifications that prevent overspreading of activity and uncontrolled synaptic strengthening. Indeed, we have previously shown that the average amplitude of GABAA-mediated miniature IPSCs (mIPSCs) in these neurons is enhanced for several days after learning, an enhancement mediated via a postsynaptic mechanism. To unravel the molecular mechanism of this long-term inhibition enhancement, we tested the role of key second-messenger systems in maintaining such long-lasting modulation. The calcium/calmodulin-dependent kinase II (CaMKII) blocker, KN93, significantly reduced the average mIPSC amplitude in neurons from trained rats only to the average pretraining level. A similar effect was obtained by the CaMKII peptide inhibitor, tatCN21. Such reduction resulted from decreased single-channel conductance and not in the number of activated channels. The PKC inhibitor, GF109203X, reduced the average mIPSC amplitude in neurons from naive, pseudo-trained, and trained animals, and the difference between the trained and control groups remained. Such reduction resulted from a decrease in the number of activated channels. The PKA inhibitor H89 dihydrochloride did not affect the average mIPSC amplitude in neurons from any of the three groups. We conclude that learning-induced enhancement of GABAA-mediated synaptic inhibition is maintained by persistent CaMKII activation.

Regulation of GABAARs by phosphorylation

γ-Aminobutyric acid type A receptors (GABAARs) are the principal mediators of fast synaptic inhibition in the brain as well as the low persistent extrasynaptic inhibition, both of which are fundamental to proper brain function. Thus unsurprisingly, deficits in GABAARs are implicated in a number of neurological disorders and diseases. The complexity of GABAAR regulation is determined not only by the heterogeneity of these receptors but also by its posttranslational modifications, the foremost, and best characterized of which is phosphorylation. This review will explore the details of this dynamic process, our understanding of which has barely scratched the surface. GABAARs are regulated by a number of kinases and phosphatases, and its phosphorylation plays an important role in governing its trafficking, expression, and interaction partners. Here, we summarize the progress in understanding the role phosphorylation plays in the regulation of GABAARs. This includes how phosphorylation can affect the allosteric modulation of GABAARs, as well as signaling pathways that affect GABAAR phosphorylation. Finally, we discuss the dysregulation of GABAAR phosphorylation and its implication in disease processes.

Modulation of neurosteroid potentiation by protein kinases at synaptic- and extrasynaptic-type GABAA receptors

GABAA receptors are important for inhibition in the CNS where neurosteroids and protein kinases are potent endogenous modulators. Acting individually, these can either enhance or depress receptor function, dependent upon the type of neurosteroid or kinase and the receptor subunit combination. However, in vivo, these modulators probably act in concert to fine-tune GABAA receptor activity and thus inhibition, although how this is achieved remains unclear. Therefore, we investigated the relationship between these modulators at synaptic-type α1β3γ2L and extrasynaptic-type α4β3δ GABAA receptors using electrophysiology. For α1β3γ2L, potentiation of GABA responses by tetrahydro-deoxycorticosterone was reduced after inhibiting protein kinase C, and enhanced following its activation, suggesting this kinase regulates neurosteroid modulation. In comparison, neurosteroid potentiation was reduced at α1β3(S408A,S409A)γ2L receptors, and unaltered by PKC inhibitors or activators, indicating that phosphorylation of β3 subunits is important for regulating neurosteroid activity. To determine whether extrasynaptic-type GABAA receptors were similarly modulated, α4β3δ and α4β3(S408A,S409A)δ receptors were investigated. Neurosteroid potentiation was reduced at both receptors by the kinase inhibitor staurosporine. By contrast, neurosteroid-mediated potentiation at α4(S443A)β3(S408A,S409A)δ receptors was unaffected by protein kinase inhibition, strongly suggesting that phosphorylation of α4 and β3 subunits is required for regulating neurosteroid activity at extrasynaptic receptors. Western blot analyses revealed that neurosteroids increased phosphorylation of β3(S408,S409) implying that a reciprocal pathway exists for neurosteroids to modulate phosphorylation of GABAA receptors. Overall, these findings provide important insight into the regulation of GABAA receptors in vivo, and into the mechanisms by which GABAergic inhibitory transmission may be simultaneously tuned by two endogenous neuromodulators.

Diffusion dynamics of synaptic molecules during inhibitory postsynaptic plasticity

The plasticity of inhibitory transmission is expected to play a key role in the modulation of neuronal excitability and network function. Over the last two decades, the investigation of the determinants of inhibitory synaptic plasticity has allowed distinguishing presynaptic and postsynaptic mechanisms. While there has been a remarkable progress in the characterization of presynaptically-expressed plasticity of inhibition, the postsynaptic mechanisms of inhibitory long-term synaptic plasticity only begin to be unraveled. At postsynaptic level, the expression of inhibitory synaptic plasticity involves the rearrangement of the postsynaptic molecular components of the GABAergic synapse, including GABA_A receptors, scaffold proteins and structural molecules. This implies a dynamic modulation of receptor intracellular trafficking and receptor surface lateral diffusion, along with regulation of the availability and distribution of scaffold proteins. This Review will focus on the mechanisms of the multifaceted molecular reorganization of the inhibitory synapse during postsynaptic plasticity, with special emphasis on the key role of protein dynamics to ensure prompt and reliable activity-dependent adjustments of synaptic strength.

The role of ovarian hormone-derived neurosteroids on the regulation of GABAA receptors in affective disorders

RATIONALE

Neuroactive derivatives of steroid hormones, neurosteroids, can act on GABAA receptors (GABAARs) to potentiate the effects of GABA on these receptors. Neurosteroids become elevated to physiologically relevant levels under conditions characterized by increased steroid hormones. There is considerable evidence for plasticity of GABAARs associated with altered levels of neurosteroids which may counteract the fluctuations in the levels of these allosteric modulators.

OBJECTIVES

The objective of this review is to summarize the current literature on GABAAR plasticity under conditions characterized by alterations in neurosteroid levels, such as over the estrous cycle, during puberty, and throughout pregnancy and the postpartum period.

RESULTS

The expression of specific GABAAR subunits is altered over the estrous cycle, at puberty, and throughout pregnancy and the postpartum period. Inability to regulate δ subunit-containing GABAARs throughout pregnancy and the postpartum period is associated with depression-like behavior restricted to the postpartum period.

CONCLUSIONS

GABAAR plasticity associated with alterations in neurosteroid levels represents a homeostatic compensatory mechanism to maintain an ideal level of inhibition to offset the potentiating effects of neurosteroids on GABAergic inhibition. Failure to properly regulate GABAARs under conditions of altered neurosteroid levels may increase vulnerability to mood disorders, such as premenstrual syndrome (PMS), premenstrual dysphoric disorder (PMDD), and postpartum depression.

Phosphorylation of GABAA receptors influences receptor trafficking and neurosteroid actions

RATIONALE

Gamma-aminobutyric acid type A receptors (GABAARs) are the principal mediators of inhibitory transmission in the mammalian central nervous system. GABAARs can be localized at post-synaptic inhibitory specializations or at extrasynaptic sites. While synaptic GABAARs are activated transiently following the release of GABA from presynaptic vesicles, extrasynaptic GABAARs are typically activated continuously by ambient GABA concentrations and thus mediate tonic inhibition. The tonic inhibitory currents mediated by extrasynaptic GABAARs control neuronal excitability and the strength of synaptic transmission. However, the mechanisms by which neurons control the functional properties of extrasynaptic GABAARs had not yet been explored.

OBJECTIVES

We review GABAARs, how they are assembled and trafficked, and the role phosphorylation has on receptor insertion and membrane stabilization. Finally, we review the modulation of GABAARs by neurosteroids and how GABAAR phosphorylation can influence the actions of neurosteroids.

CONCLUSIONS

Trafficking and stability of functional channels to the membrane surface are critical for inhibitory efficacy. Phosphorylation of residues within GABAAR subunits plays an essential role in the assembly, trafficking, and cell surface stability of GABAARs. Neurosteroids are produced in the brain and are highly efficacious allosteric modulators of GABAAR-mediated current. This allosteric modulation by neurosteroids is influenced by the phosphorylated state of the GABAAR which is subunit dependent, adding temporal and regional variability to the neurosteroid response. Possible links between neurosteroid actions, phosphorylation, and GABAAR trafficking remain to be explored, but potential novel therapeutic targets may exist for numerous neurological and psychological disorders which are linked to fluctuations in neurosteroid levels and GABAA subunit expression.