Compensatory alteration of inhibitory synaptic circuits in cerebellum and thalamus of gamma-aminobutyric acid type A receptor alpha1 subunit knockout mice

The gephyrin scaffold modulates cortical layer 2/3 pyramidal neuron responsiveness to single whisker stimulation

Gephyrin is the main scaffolding protein at inhibitory postsynaptic sites, and its clusters are the signaling hubs where several molecular pathways converge. Post-translational modifications (PTMs) of gephyrin alter GABAA receptor clustering at the synapse, but it is unclear how this affects neuronal activity at the circuit level. We assessed the contribution of gephyrin PTMs to microcircuit activity in the mouse barrel cortex by slice electrophysiology and in vivo two-photon calcium imaging of layer 2/3 (L2/3) pyramidal cells during single-whisker stimulation. Our results suggest that, depending on the type of gephyrin PTM, the neuronal activities of L2/3 pyramidal neurons can be differentially modulated, leading to changes in the size of the neuronal population responding to the single-whisker stimulation. Furthermore, we show that gephyrin PTMs have their preference for selecting synaptic GABAA receptor subunits. Our results identify an important role of gephyrin and GABAergic postsynaptic sites for cortical microcircuit function during sensory stimulation.

Role of α1-GABAA receptors in the serotonergic dorsal raphe nucleus in models of opioid reward, anxiety, and depression

BACKGROUND

The serotonin (5-hydroxytryptamine (5-HT))-mediated system plays an important role in stress-related psychiatric disorders and substance abuse. Our previous studies showed that stress and drug exposure can modulate the dorsal raphe nucleus (DRN)-5-HT system via γ-aminobutyric acid (GABA)A receptors. Moreover, GABAA receptor-mediated inhibition of serotonergic DRN neurons is required for stress-induced reinstatement of opioid seeking.

AIM/METHODS

To further test the role of GABAA receptors in the 5-HT system in stress and opioid-sensitive behaviors, our current study generated mice with conditional genetic deletions of the GABAA α1 subunit to manipulate GABAA receptors in either the DRN or the entire population of 5-HT neurons. The GABAA α1 subunit is a constituent of the most abundant GABAA subtype in the brain and the most highly expressed subunit in 5-HT DRN neurons.

RESULTS

Our results showed that mice with DRN-specific knockout of α1-GABAA receptors exhibited a normal phenotype in tests of anxiety- and depression-like behaviors as well as swim stress-induced reinstatement of morphine-conditioned place preference. By contrast, mice with 5-HT neuron-specific knockout of α1-GABAA receptors exhibited an anxiolytic phenotype at baseline and increased sensitivity to post-morphine withdrawal-induced anxiety.

CONCLUSIONS

Our data suggest that GABAA receptors on 5-HT neurons contribute to anxiety-like behaviors and sensitivity of those behaviors to opioid withdrawal.

Recent Progress on Genetically Modified Animal Models for Membrane Skeletal Proteins: The 4.1 and MPP Families

The protein 4.1 and membrane palmitoylated protein (MPP) families were originally found as components in the erythrocyte membrane skeletal protein complex, which helps maintain the stability of erythrocyte membranes by linking intramembranous proteins and meshwork structures composed of actin and spectrin under the membranes. Recently, it has been recognized that cells and tissues ubiquitously use this membrane skeletal system. Various intramembranous proteins, including adhesion molecules, ion channels, and receptors, have been shown to interact with the 4.1 and MPP families, regulating cellular and tissue dynamics by binding to intracellular signal transduction proteins. In this review, we focus on our previous studies regarding genetically modified animal models, especially on 4.1G, MPP6, and MPP2, to describe their functional roles in the peripheral nervous system, the central nervous system, the testis, and bone formation. As the membrane skeletal proteins are located at sites that receive signals from outside the cell and transduce signals inside the cell, it is necessary to elucidate their molecular interrelationships, which may broaden the understanding of cell and tissue functions.

Suprachiasmatic to paraventricular nuclei interaction generates normal food searching rhythms in mice

Searching for food follows a well-organized decision process in mammals to take up food only if necessary. Moreover, scavenging is preferred during their activity phase. Various time-dependent regulatory processes have been identified originating from the suprachiasmatic nuclei (SCN), which convert external light information into synchronizing output signals. However, a direct impact of the SCN on the timing of normal food searching has not yet been found. Here, we revisited the function of the SCN to affect when mice look for food. We found that this process was independent of light but modified by the palatability of the food source. Surprisingly, reducing the output from the SCN, in particular from the vasopressin releasing neurons, reduced the amount of scavenging during the early activity phase. The SCN appeared to transmit a signal to the paraventricular nuclei (PVN) via GABA receptor A1. Finally, the interaction of SCN and PVN was verified by retrograde transport-mediated complementation. None of the genetic manipulations affected the uptake of more palatable food. The data indicate that the PVN are sufficient to produce blunted food searching rhythms and are responsive to hedonistic feeding. Nevertheless, the search for normal food during the early activity phase is significantly enhanced by the SCN.

Virally-induced expression of GABAA receptor δ subunits following their pathological loss reveals their role in regulating GABAA receptor assembly

Decreased expression of the δ subunit of the GABA_A receptor (GABA_AR) has been found in the dentate gyrus in several animal models of epilepsy and other disorders with increased excitability and is associated with altered modulation of tonic inhibition in dentate granule cells (GCs). In contrast, other GABA_AR subunits, including α4 and γ2 subunits, are increased, but the relationship between these changes is unclear. The goals of this study were to determine if viral transfection of δ subunits in dentate GCs could increase δ subunit expression, alter expression of potentially-related GABA_AR subunits, and restore more normal network excitability in the dentate gyrus in a mouse model of epilepsy. Pilocarpine-induced seizures were elicited in DOCK10-Cre mice that express Cre selectively in dentate GCs, and two weeks later the mice were injected unilaterally with a Cre-dependent δ-GABA_AR viral vector. At 4-6 weeks following transfection, δ subunit immunolabeling was substantially increased in dentate GCs on the transfected side compared to the nontransfected side. Importantly, α4 and γ2 subunit labeling was downregulated on the transfected side. Electrophysiological studies revealed enhanced tonic inhibition, decreased network excitability, and increased neurosteroid sensitivity in slices from the δ subunit-transfected side compared to those from the nontransfected side of the same pilocarpine-treated animal, consistent with the formation of δ subunit-containing GABA_ARs. No differences were observed between sides of eYFP-transfected animals. These findings are consistent with the idea that altering expression of key subunits, such as the δ subunit, regulates GABA_AR subunit assemblies, resulting in substantial effects on network excitability.

GABAA α subunit control of hyperactive behavior in developing zebrafish

GABAA receptors mediate rapid responses to the neurotransmitter gamma-aminobutyric acid and are robust regulators of the brain and spinal cord neural networks that control locomotor behaviors, such as walking and swimming. In developing zebrafish, gross pharmacological blockade of these receptors causes hyperactive swimming, which is also a feature of many zebrafish epilepsy models. Although GABAA receptors are important to control locomotor behavior, the large number of subunits and homeostatic compensatory mechanisms have challenged efforts to determine subunit-selective roles. To address this issue, we mutated each of the 8 zebrafish GABAA α subunit genes individually and in pairs using a CRISPR-Cas9 somatic inactivation approach and, then, we examined the swimming behavior of the mutants at 2 developmental stages, 48 and 96 h postfertilization. We found that disrupting the expression of specific pairs of subunits resulted in different abnormalities in swimming behavior at 48 h postfertilization. Mutation of α4 and α5 selectively resulted in longer duration swimming episodes, mutations in α3 and α4 selectively caused excess, large-amplitude body flexions (C-bends), and mutation of α3 and α5 resulted in increases in both of these measures of hyperactivity. At 96 h postfertilization, hyperactive phenotypes were nearly absent, suggesting that homeostatic compensation was able to overcome the disruption of even multiple subunits. Taken together, our results identify subunit-selective roles for GABAA α3, α4, and α5 in regulating locomotion. Given that these subunits exhibit spatially restricted expression patterns, these results provide a foundation to identify neurons and GABAergic networks that control discrete aspects of locomotor behavior.

The α3 subunit of GABAA receptors promotes formation of inhibitory synapses in the absence of collybistin

Signaling at nerve cell synapses is a key determinant of proper brain function, and synaptic defects—or synaptopathies—are at the basis of many neurological and psychiatric disorders. Collybistin (CB), a brain-specific guanine nucleotide exchange factor, is essential for the formation of γ-aminobutyric acidergic (GABAergic) postsynapses in defined regions of the mammalian forebrain, including the hippocampus and basolateral amygdala. This process depends on a direct interaction of CB with the scaffolding protein gephyrin, which leads to the redistribution of gephyrin into submembranous clusters at nascent inhibitory synapses. Strikingly, synaptic clustering of gephyrin and GABA_A type A receptors (GABA_ARs) in several brain regions, including the cerebral cortex and certain thalamic areas, is unperturbed in CB-deficient mice, indicating that the formation of a substantial subset of inhibitory postsynapses must be controlled by gephyrin-interacting proteins other than CB. Previous studies indicated that the α3 subunit of GABA_ARs (GABA_AR-α3) binds directly and with high affinity to gephyrin. Here, we provide evidence (i) that a homooligomeric GABA_AR-α3^A343W mutant induces the formation of submembranous gephyrin clusters independently of CB in COS-7 cells, (ii) that gephyrin clustering is unaltered in the neuronal subpopulations endogenously expressing the GABA_AR-α3 in CB-deficient brains, and (iii) that exogenous expression of GABA_AR-α3 partially rescues impaired gephyrin clustering in CB-deficient hippocampal neurons. Our results identify an important role of GABA_AR-α3 in promoting gephyrin-mediated and CB-independent formation of inhibitory postsynapses.

Expanding GABAAR pharmacology via receptor-associated proteins

Drugs directly targeting γ-aminobutyric acid type A receptors (GABA_ARs), the major mediators of fast synaptic inhibition, contribute significantly to today's neuropharmacology. Emerging evidence establishes intracellularly GABA_AR-associated proteins as the central players in determining cellular and subcellular GABAergic input sites, thereby providing pharmacological opportunities to affect distinct receptor populations and address discrete neuronal dysfunctions. Here, we report on recently studied GABA_AR-associated proteins and highlight challenges and newly available methods for their functional and physical mapping. We anticipate these efforts to contribute to decipher the complexity of GABAergic signalling in the brain and eventually enable therapeutic avenues for, so far, untreatable neuronal disorders and diseases.

Tonic GABAergic inhibition, via GABAA receptors containing αβƐ subunits, regulates excitability of ventral tegmental area dopamine neurons

The activity of midbrain dopamine neurons is strongly regulated by fast synaptic inhibitory γ‐Aminobutyric acid (GABA)ergic inputs. There is growing evidence in other brain regions that low concentrations of ambient GABA can persistently activate certain subtypes of GABA_A receptor to generate a tonic current. However, evidence for a tonic GABAergic current in midbrain dopamine neurons is limited. To address this, we conducted whole‐cell recordings from ventral tegmental area (VTA) dopamine neurons in brain slices from mice. We found that application of GABA_A receptor antagonists decreased the holding current, indicating the presence of a tonic GABAergic input. Global increases in GABA release, induced by either a nitric oxide donor or inhibition of GABA uptake, further increased this tonic current. Importantly, prolonged inhibition of the firing activity of local GABAergic neurons abolished the tonic current. A combination of pharmacology and immunohistochemistry experiments suggested that, unlike common examples of tonic inhibition, this current may be mediated by a relatively unusual combination of α4βƐ subunits. Lastly, we found that the tonic current reduced excitability in dopamine neurons suggesting a subtractive effect on firing activity.

Looking for Novelty in an "Old" Receptor: Recent Advances Toward Our Understanding of GABAARs and Their Implications in Receptor Pharmacology

Diverse populations of GABA_A receptors (GABA_ARs) throughout the brain mediate fast inhibitory transmission and are modulated by various endogenous ligands and therapeutic drugs. Deficits in GABA_AR signaling underlie the pathophysiology behind neurological and neuropsychiatric disorders such as epilepsy, anxiety, and depression. Pharmacological intervention for these disorders relies on several drug classes that target GABA_ARs, such as benzodiazepines and more recently neurosteroids. It has been widely demonstrated that subunit composition and receptor stoichiometry impact the biophysical and pharmacological properties of GABA_ARs. However, current GABA_AR-targeting drugs have limited subunit selectivity and produce their therapeutic effects concomitantly with undesired side effects. Therefore, there is still a need to develop more selective GABA_AR pharmaceuticals, as well as evaluate the potential for developing next-generation drugs that can target accessory proteins associated with native GABA_ARs. In this review, we briefly discuss the effects of benzodiazepines and neurosteroids on GABA_ARs, their use as therapeutics, and some of the pitfalls associated with their adverse side effects. We also discuss recent advances toward understanding the structure, function, and pharmacology of GABA_ARs with a focus on benzodiazepines and neurosteroids, as well as newly identified transmembrane proteins that modulate GABA_ARs.

Selective loss of the GABAAα1 subunit from Purkinje cells is sufficient to induce a tremor phenotype

Previously, an essential tremor-like phenotype has been noted in animals with a global knockout of the GABA_Aα1 subunit. Given the hypothesized role of the cerebellum in tremor, including essential tremor, we used transgenic mice to selectively knock out the GABA_Aα1 subunit from cerebellar Purkinje cells. We examined the resulting phenotype regarding impacts on inhibitory postsynaptic currents, survival rates, gross motor abilities, and expression of tremor. Purkinje cell specific knockout of the GABA_Aα1 subunit abolished all GABA_A-mediated inhibition in Purkinje cells, while leaving GABA_A-mediated inhibition to cerebellar molecular layer interneurons intact. Selective loss of GABA_Aα1 from Purkinje cells did not produce deficits on the accelerating rotarod, nor did it result in decreased survival rates. However, a tremor phenotype was apparent, regardless of sex or background strain. This tremor mimicked the tremor seen in animals with a global knockout of the GABA_Aα1 subunit, and, like essential tremor in patients, was responsive to ethanol. These findings indicate that reduced inhibition to Purkinje cells is sufficient to induce a tremor phenotype, highlighting the importance of the cerebellum, inhibition, and Purkinje cells in tremor.NEW & NOTEWORTHY Animals with a global knockout of the GABA_Aα1 subunit show a tremor phenotype reminiscent of essential tremor. Here we show that selective knockout of GABA_Aα1 from Purkinje cells is sufficient to produce a tremor phenotype, although this tremor is less severe than seen in animals with a global knockout. These findings illustrate that the cerebellum can play a key role in the genesis of the observed tremor phenotype.

Dystroglycan Mediates Clustering of Essential GABAergic Components in Cerebellar Purkinje Cells

Muscle dystrophin–glycoprotein complex (DGC) links the intracellular cytoskeleton to the extracellular matrix. In neurons, dystroglycan and dystrophin, two major components of the DGC, localize in a subset of GABAergic synapses, where their function is unclear. Here we used mouse models to analyze the specific role of the DGC in the organization and function of inhibitory synapses. Loss of full-length dystrophin in mdx mice resulted in a selective depletion of the transmembrane β-dystroglycan isoform from inhibitory post-synaptic sites in cerebellar Purkinje cells. Remarkably, there were no differences in the synaptic distribution of the extracellular α-dystroglycan subunit, of GABA_A receptors and neuroligin 2. In contrast, conditional deletion of the dystroglycan gene from Purkinje cells caused a disruption of the DGC and severely impaired post-synaptic clustering of neuroligin 2, GABA_A receptors and scaffolding proteins. Accordingly, whole-cell patch-clamp analysis revealed a significant reduction in the frequency and amplitude of spontaneous IPSCs recorded from Purkinje cells. In the long-term, deletion of dystroglycan resulted in a significant decrease of GABAergic innervation of Purkinje cells and caused an impairment of motor learning functions. These results show that dystroglycan is an essential synaptic organizer at GABAergic synapses in Purkinje cells.

Environmental enrichment restores the reduced expression of cerebellar synaptophysin and the motor coordination impairment in rats prenatally treated with betamethasone

Preterm babies treated with synthetic glucocorticoids in utero exhibit behavioural alterations and disturbances in brain maturation during postnatal life. Accordingly, it has been shown in preclinical studies that SGC exposure at a clinical dose alters the presynaptic and postsynaptic structures and results in synaptic impairments. However, the precise mechanism by which SGC exposure impairs synaptic protein expression and its implications are not fully elucidated. Therefore, the purpose of this study was to investigate the effect of prenatal exposure to a clinical dose of betamethasone on the pre- and postsynaptic proteins expression in the developing rat cerebellum and prefrontal cortex, whose synchronized synaptic activity is crucial for motor control and learning. Consequently, the first objective of the present study was to determine whether prenatal betamethasone -equivalent to the clinically used dose- alters cerebellar vermal and cortical expression of synaptophysin, synaptotagmin I, post-synaptic density protein 95 and gephyrin - four important pre- and post-synaptic proteins, respectively- at a relevant adolescent stage. In addition, our second objective was to assess whether prenatal betamethasone administration induced coordination impairment using a rotarod test. On the other hand, it has been shown that the environmental enrichment is capable of improving synaptic transmission and recovering various behavioural impairments. Nevertheless, there is not enough information about the effect of this non-pharmacological preclinical approach on the regulation of this cerebellar and cortical synaptic proteins. Therefore, the third objective of this study was to examine whether environmental enrichment exposure could recover the possible molecular and behavioural impairments in the offspring at the same developmental stage. The principal data showed that adolescent rats prenatally treated with betamethasone exhibited underexpression of synaptophysin in the vermal cerebellum, but not change in levels of synaptotagmin I, post-synaptic density protein 95 and gephyrin. Analysis of the same pre- and post-synaptic proteins no showed differences in the frontal cortex of the same rats. These results were accompanied by an increase in the number of falls in the rotarod test, when the speed of rotation was fixed and when it was in acceleration, which means motor coordination impairments. Importantly, we found that environmental enrichment restores the betamethasone-induced reduction in the cerebellar synaptophysin together with a recover in the motor coordination impairments in prenatally betamethasone-exposed adolescent rats.

GABAA Receptors Are Well Preserved in the Hippocampus of Aged Mice

GABA is the primary inhibitory neurotransmitter in the nervous system. GABA_A receptors (GABA_ARs) are pentameric ionotropic channels. Subunit composition of the receptors is associated with the affinity of GABA binding and its downstream inhibitory actions. Fluctuations in subunit expression levels with increasing age have been demonstrated in animal and human studies. However, our knowledge regarding the age-related hippocampal GABA_AR expression changes is limited and based on rat studies. This study is the first analysis of the aging-related changes of the GABA_AR subunit expression in the CA1, CA2/3, and dentate gyrus regions of the mouse hippocampus. Using Western blotting and immunohistochemistry we found that the GABAergic system is robust, with no significant age-related differences in GABA_AR α1, α2, α3, α5, β3, and γ2 subunit expression level differences found between the young (6 months) and old (21 months) age groups in any of the hippocampal regions examined. However, we detected a localized decrease of α2 subunit expression around the soma, proximal dendrites, and in the axon initial segment of pyramidal cells in the CA1 and CA3 regions that is accompanied by a pronounced upregulation of the α2 subunit immunoreactivity in the neuropil of aged mice. In summary, GABA_ARs are well preserved in the mouse hippocampus during normal aging although GABA_ARs in the hippocampus are severely affected in age-related neurological disorders, including Alzheimer’s disease.

Functional connectivity impairment of postcentral gyrus in relapsing-remitting multiple sclerosis with somatosensory disorder

PURPOSE

To characterize the spatial patterns of functional connectivity(FC) changes of whole brain in RRMS with somatosensory disorder(RRMS-SS) and to investigate the correlation between abnormal FC and clinical scores.

METHODS

Twenty-six RRMS-SS patients and 23 healthy controls(HC) underwent resting-state functional magnetic resonance imaging(RS-fMRI) scanning. The clinical scores were collected including Expanded Disability Status Scores(EDSS), Disease Duration and Somatosensory Evaluation by the Fugl-Meyer sensory score(FMSS). With the voxel-wise methods, RS-fMRI data were analyzed using REST software, to assess the FC of the postcentral gyrus(PoCG). Correlation between clinical variables and the strength of FC was analyzed.

RESULTS

Compared with HC, the left postcentral-based FC showed decreased FC of the right cerebellum_8, lingual lobe and Rolandic operculum gyrus, and increased FC of the left middle frontal lobe. The right postcentral-based FC revealed decreased FC with the right Heschl's gyrus lobule, and increased FC with bilateral middle frontal lobe (p <  0.001, AlphaSim corrected). Correlation analysis revealed that the FC of altered brain regions was associated with FMSS, EDSS and disease duration.

CONCLUSION

The functional connectivity of PoCG at RS-fMRI has multi-network changes in patients with RRMS-SS. This suggests a complex pattern of abnormal connections between the somatosensory network regions and the whole brain. Moreover, the correlation between the FC and the FMSS, such as the left middle frontal lobe and the right PoCG, indicate that these two brain regions play an important role in RRMS-SS.

Role of cerebellar GABAergic dysfunctions in the origins of essential tremor

Essential tremor (ET) is among the most prevalent movement disorders, but its origins are elusive. The inferior olivary nucleus (ION) has been hypothesized as the prime generator of tremor because of the pacemaker properties of ION neurons, but structural and functional changes in ION are unlikely under ET. Abnormalities have instead been reported in the cerebello-thalamo-cortical network, including dysfunctions of the GABAergic projections from the cerebellar cortex to the dentate nucleus. It remains unclear, though, how tremor would relate to a dysfunction of cerebellar connectivity. To address this question, we built a computational model of the cortico-cerebello-thalamo-cortical loop. We simulated the effects of a progressive loss of GABA_A α₁-receptor subunits and up-regulation of α_2/3-receptor subunits in the dentate nucleus, and correspondingly, we studied the evolution of the firing patterns along the loop. The model closely reproduced experimental evidence for each structure in the loop. It showed that an alteration of amplitudes and decay times of the GABAergic currents to the dentate nucleus can facilitate sustained oscillatory activity at tremor frequency throughout the network as well as a robust bursting activity in the thalamus, which is consistent with observations of thalamic tremor cells in ET patients. Tremor-related oscillations initiated in small neural populations and spread to a larger network as the synaptic dysfunction increased, while thalamic high-frequency stimulation suppressed tremor-related activity in thalamus but increased the oscillation frequency in the olivocerebellar loop. These results suggest a mechanism for tremor generation under cerebellar dysfunction, which may explain the origin of ET.

Postsynaptic plasticity of GABAergic synapses

The flexibility of neuronal networks is believed to rely mainly on the plasticity of excitatory synapses. However, like their excitatory counterparts, inhibitory synapses also undergo several forms of synaptic plasticity. This review examines recent advances in the understanding of the molecular mechanisms leading to postsynaptic GABAergic plasticity. Specifically, modulation of GABAA receptor (GABAAR) number at postsynaptic sites plays a key role, with the interaction of GABAARs with the scaffold protein gephyrin and other postsynaptic scaffold/regulatory proteins having particular importance. Our understanding of these molecular interactions are progressing, based on recent insights into the processes of GABAAR lateral diffusion, gephyrin dynamics, and gephyrin nanoscale organization. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

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

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

Cell type-specific distribution of GABAA receptor subtypes in the mouse dorsal striatum

The striatum is the main input nucleus of the basal ganglia, mediating motor and cognitive functions. Striatal projection neurons are GABAergic medium spiny neurons (MSN), expressing either the dopamine receptor type 1 (D₁ -R MSN) and forming the direct, movement-promoting pathway, or dopamine receptor type 2 (D₂ -R MSN), forming the indirect movement-suppressing pathway. Locally, activity and synchronization of MSN are modulated by several subtypes of GABAergic and cholinergic interneurons. Overall, GABAergic circuits in the striatum remain poorly characterized, and little is known about the intrastriatal connectivity of interneurons and the distribution of GABA_A receptor (GABA_A R) subtypes, distinguished by their subunit composition, in striatal synapses. Here, by using immunofluorescence in mouse tissue, we investigated the distribution of GABA_A Rs containing the α₁ , α₂ , or α₃ subunit in perisomatic synapses of striatal MSN and interneurons, as well as the innervation pattern of D₁ R- and D₂ R-MSN soma and axonal initial segment (AIS) by GABAergic and cholinergic interneurons. Our results show that perisomatic GABAergic synapses of D₁ R- and D₂ R-MSN contain the GABA_A R α₁ and/or α₂ subunits, but not the α₃ subunit; D₂ R-MSN have significantly more α₁ -GABA_A Rs on their soma than D₁ R-MSN. Further, interneurons have few perisomatic synapses containing α₂ -GABA_A Rs, whereas α₃ -GABA_A Rs (along with the α₁ -GABA_A Rs) are abundant in perisomatic synapses of CCK⁺ , NPY⁺ /SOM⁺ , and vAChT⁺ interneurons. Each MSN and interneuron population analyzed received a distinct pattern of GABAergic and cholinergic innervation, complementing this postsynaptic heterogeneity. In conclusion, intra-striatal GABAergic circuits are distinguished by cell-type specific innervation patterns, differential expression and postsynaptic targeting of GABA_A R subtypes.

Sex- and age-related changes in GABA signaling components in the human cortex

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the nervous system. Previous studies have shown fluctuations in expression levels of GABA signaling components-glutamic acid decarboxylase (GAD), GABA receptor (GABAR) subunit, and GABA transporter (GAT)-with increasing age and between sexes; however, this limited knowledge is highly based on animal models that produce inconsistent findings. This study is the first analysis of the age- and sex-specific changes of the GAD, GABAA/BR subunits, and GAT expression in the human primary sensory and motor cortices; superior (STG), middle (MTG), and inferior temporal gyrus (ITG); and cerebellum. Utilizing Western blotting, we found that the GABAergic system is relatively robust against sex and age-related differences in all brain regions examined. However, we observed several sex-dependent differences in GABAAR subunit expression in STG along with age-dependent GABAAR subunit and GAD level alteration. No significant age-related differences were found in α1, α2, α5, β3, and γ2 subunit expression in the STG. However, we found significantly higher GABAAR α3 subunit expression in the STG in young males compared to old males. We observed a significant sex-dependent difference in α1 subunit expression: males presenting significantly higher levels compared to women across all stages of life in STG. Older females showed significantly lower α2, α5, and β3 subunit expression compared to old males in the STG. These changes found in the STG might significantly influence GABAergic neurotransmission and lead to sex- and age-specific disease susceptibility and progression.

An Essential Role for the Tetraspanin LHFPL4 in the Cell-Type-Specific Targeting and Clustering of Synaptic GABAA Receptors

Inhibitory synaptic transmission requires the targeting and stabilization of GABA_A receptors (GABA_ARs) at synapses. The mechanisms responsible remain poorly understood, and roles for transmembrane accessory proteins have not been established. Using molecular, imaging, and electrophysiological approaches, we identify the tetraspanin LHFPL4 as a critical regulator of postsynaptic GABA_AR clustering in hippocampal pyramidal neurons. LHFPL4 interacts tightly with GABA_AR subunits and is selectively enriched at inhibitory synapses. In LHFPL4 knockout mice, there is a dramatic cell-type-specific reduction in GABA_AR and gephyrin clusters and an accumulation of large intracellular gephyrin aggregates in vivo. While GABA_ARs are still trafficked to the neuronal surface in pyramidal neurons, they are no longer localized at synapses, resulting in a profound loss of fast inhibitory postsynaptic currents. Hippocampal interneuron currents remain unaffected. Our results establish LHFPL4 as a synapse-specific tetraspanin essential for inhibitory synapse function and provide fresh insights into the molecular make-up of inhibitory synapses.

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LHFPL4 is a tetraspanin enriched at inhibitory synapses that complexes with GABA_ARs

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LHFPL4 is important for GABA_AR clustering both in vitro and in vivo

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LHFPL4 is required for the surface clustering but not the trafficking of GABA_ARs

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GABAergic synaptic inputs on CA1 pyramidal neurons, but not interneurons, require LHFPL4

Expression of the eight GABAA receptor α subunits in the developing zebrafish central nervous system

GABA is a robust regulator of both developing and mature neural networks. It exerts many of its effects through GABA_A receptors, which are heteropentamers assembled from a large array of subunits encoded by distinct genes. In mammals, there are 19 different GABA_A subunit types, which are divided into the α, β, γ, δ, ε, π, θ and ρ subfamilies. The immense diversity of GABA_A receptors is not fully understood. However, it is known that specific isoforms, with their distinct biophysical properties and expression profiles, tune responses to GABA. Although larval zebrafish are well-established as a model system for neural circuit analysis, little is known about GABA_A receptors diversity and expression in this system. Here, using database analysis, we show that the zebrafish genome contains at least 23 subunits. All but the mammalian θ and ε subunits have at least one zebrafish ortholog, while five mammalian GABA_A receptor subunits have two zebrafish orthologs. Zebrafish contain one subunit, β4, which does not have a clear mammalian ortholog. Similar to mammalian GABA_A receptors, the zebrafish α subfamily is the largest and most diverse of the subfamilies. In zebrafish there are eight α subunits, and RNA in situ hybridization across early zebrafish development revealed that they demonstrate distinct patterns of expression in the brain, spinal cord, and retina. Some subunits were very broadly distributed, whereas others were restricted to small populations of cells. Subunit-specific expression patterns in zebrafish resembled were those found in frogs and rodents, which suggests that the roles of different GABA_A receptor isoforms are largely conserved among vertebrates. This study provides a platform to examine isoform specific roles of GABA_A receptors within zebrafish neural circuits and it highlights the potential of this system to better understand the remarkable heterogeneity of GABA_A receptors.

Differential role of GABAA receptors and neuroligin 2 for perisomatic GABAergic synapse formation in the hippocampus

Perisomatic GABAergic synapses onto hippocampal pyramidal cells arise from two populations of basket cells with different neurochemical and functional properties. The presence of the dystrophin-glycoprotein complex in their postsynaptic density (PSD) distinguishes perisomatic synapses from GABAergic synapses on dendrites and the axon-initial segment. Targeted deletion of neuroligin 2 (NL2), a transmembrane protein interacting with presynaptic neurexin, has been reported to disrupt postsynaptic clustering of GABA_A receptors (GABA_AR) and their anchoring protein, gephyrin, at perisomatic synapses. In contrast, targeted deletion of Gabra2 disrupts perisomatic clustering of gephyrin, but not of α1-GABA_AR, NL2, or dystrophin/dystroglycan. Unexpectedly, conditional deletion of Dag1, encoding dystroglycan, selectively prevents the formation of perisomatic GABAergic synapses from basket cells expressing cholecystokinin. Collectively, these observations suggest that multiple mechanisms regulate formation and molecular composition of the GABAergic PSD at perisomatic synapses. Here, we further explored this issue by investigating the effect of targeted deletion of Gabra1 and NL2 on the dystrophin-glycoprotein complex and on perisomatic synapse formation, using immunofluorescence analysis with a battery of GABAergic pre- and postsynaptic markers. We show that the absence of α1-GABA_AR increases GABAergic synapses containing the α2 subunit, without affecting the clustering of dystrophin and NL2; in contrast, the absence of NL2 produces highly variable effects postsynaptically, not restricted to perisomatic synapses and being more severe for the GABA_AR subunits and gephyrin than dystrophin. Altogether, the results confirm the importance of NL2 as organizer of the GABAergic PSD and unravel distinct roles for α1- and α2-GABA_ARs in the formation of GABAergic circuits in close interaction with the dystrophin-glycoprotein complex.

GABA receptors and T-type Ca2+ channels crosstalk in thalamic networks

Although the thalamus presents a rather limited repertoire of GABAergic cell types compare to other CNS area, this structure is a privileged system to study how GABA impacts neuronal network excitability. Indeed both glutamatergic thalamocortical (TC) and GABAergic nucleus reticularis thalami (NRT) neurons present a high expression of T-type voltage-dependent Ca²⁺ channels whose activation that shapes the output of the thalamus critically depends upon a preceding hyperpolarisation. Because of this strict dependence, a tight functional link between GABA mediated hyperpolarization and T-currents characterizes the thalamic network excitability. In this review we summarize a number of studies showing that the relationships between the various thalamic GABA_A/B receptors and T-channels are complex and bidirectional. We discuss how this dynamic interaction sets the global intrathalamic network activity and its long-term plasticity and highlight how the functional relationship between GABA release and T-channel-dependent excitability is finely tuned by the T-channel activation itself. Finally, we illustrate how an impaired balance between T-channels and GABA receptors can lead to pathologically abnormal cellular and network behaviours. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Altered postnatal maturation of striatal GABAergic interneurons in a phenotypic animal model of dystonia

GABAergic disinhibition has been suggested to play a critical role in the pathophysiology of several basal ganglia disorders, including dystonia, a common movement disorder. Previous studies have shown a deficit of striatal GABAergic interneurons (IN) in the dtsz mutant hamster, one of the few phenotypic animal models of dystonia. However, mechanisms underlying this deficit are largely unknown. In the present study, we investigated the migration and maturation of striatal IN during postnatal development (18days of age) and at age of highest severity of dystonia (33days of age) in this hamster model. In line with previous findings, the density of GAD67-positive IN and the level of parvalbumin mRNA, a marker for fast spiking GABAergic IN, were lower in the dtsz mutant than in control hamsters. However, an unaltered density of Nkx2.1 labeled cells and Nkx2.1 mRNA level suggested that the migration of GABAergic IN into the striatum was not retarded. Therefore, different factors that indicate maturation of GABAergic IN were determined. While mRNA of the KCC2 cation/chloride transporters and the cytosolic carboanhydrase VII, used as markers for the so called GABA switch, as well as BDNF were unaltered, we found a reduced number of IN expressing the alpha1 subunit of the GABAA-receptor (37.5%) in dtsz hamsters at an age of 33days, but not after spontaneous remission of dystonia at an age of 90days. Since IN shift expression from alpha2 to alpha1 subunits during postnatal maturation, this result together with a decreased parvalbumin mRNA expression suggest a delayed maturation of striatal GABAergic IN in this animal model, which might underlie abnormal neuronal activity and striatal plasticity.

Simultaneous impairment of neuronal and metabolic function of mutated gephyrin in a patient with epileptic encephalopathy

Synaptic inhibition is essential for shaping the dynamics of neuronal networks, and aberrant inhibition plays an important role in neurological disorders. Gephyrin is a central player at inhibitory postsynapses, directly binds and organizes GABA_A and glycine receptors (GABA_ARs and GlyRs), and is thereby indispensable for normal inhibitory neurotransmission. Additionally, gephyrin catalyzes the synthesis of the molybdenum cofactor (MoCo) in peripheral tissue. We identified a de novo missense mutation (G375D) in the gephyrin gene (GPHN) in a patient with epileptic encephalopathy resembling Dravet syndrome. Although stably expressed and correctly folded, gephyrin‐G375D was non‐synaptically localized in neurons and acted dominant‐negatively on the clustering of wild‐type gephyrin leading to a marked decrease in GABA_AR surface expression and GABAergic signaling. We identified a decreased binding affinity between gephyrin‐G375D and the receptors, suggesting that Gly375 is essential for gephyrin–receptor complex formation. Surprisingly, gephyrin‐G375D was also unable to synthesize MoCo and activate MoCo‐dependent enzymes. Thus, we describe a missense mutation that affects both functions of gephyrin and suggest that the identified defect at GABAergic synapses is the mechanism underlying the patient's severe phenotype.

Synaptic localization of α5 GABA (A) receptors via gephyrin interaction regulates dendritic outgrowth and spine maturation

GABAA receptor subunit composition is a critical determinant of receptor localization and physiology, with synaptic receptors generating phasic inhibition and extrasynaptic receptors producing tonic inhibition. Extrasynaptically localized α5 GABAA receptors are largely responsible for tonic inhibition in hippocampal neurons. However, we show here that inhibitory synapses also contain a constant level of α5 GABAA receptors throughout neuronal development, as measured by its colocalization with gephyrin, the inhibitory postsynaptic scaffolding protein. Immunoprecipitation of the α5 subunit from both cultured neurons and adult rat brain coimmunoprecipitated gephyrin, confirming this interaction in vivo. Furthermore, the α5 subunit can interact with gephyrin independent of other synaptically localized alpha subunits, as shown by immunoprecipitation experiments in HEK cells. By replacing the α5 predicted gephyrin binding domain (Residues 370-385) with either the high affinity gephyrin binding domain of the α2 subunit or homologous residues from the extrasynaptic α4 subunit that does not interact with gephyrin, α5 GABAA receptor localization shifted into or out of the synapse, respectively. These shifts in the ratio of synaptic/extrasynaptic α5 localization disrupted dendritic outgrowth and spine maturation. In contrast to the predominant view of α5 GABAA receptors being extrasynaptic and modulating tonic inhibition, we identify an intimate association of the α5 subunit with gephyrin, resulting in constant synaptic levels of α5 GABAA R throughout circuit formation that regulates neuronal development.

Synaptic localization of neurotransmitter receptors: comparing mechanisms for AMPA and GABAA receptors

Ionotropic neurotransmitter receptors mediate fast synaptic transmission by localizing at postsynapses. Changes in receptor number at synapses induce synaptic plasticity. Thus, mechanisms for the synaptic localization of receptors in basal transmission and synaptic plasticity have been investigated extensively. Recent findings reveal that synaptic localization of tetrameric AMPA receptors in basal transmission requires the PDZ binding of TARP auxiliary subunits, which modulate receptor properties and pharmacology. On the other hand, pentameric GABAA receptors require multiple receptor subunits for their synaptic localization in basal transmission. AMPA receptors seem to utilize distinct mechanisms for basal synaptic localization and synaptic insertion during plasticity. Revealing precise mechanisms for receptor synaptic localization may establish new approaches to control synaptic transmission.

Behavioral functions of GABAA receptor subtypes--the Zurich experience

GABAA receptors are the major inhibitory neurotransmitter receptors in the brain. They are heteropentamers that are typically classified according to their α subunits. By rendering each of the benzodiazepine-sensitive α subunits (α1, α2, α3, and α5) insensitive to modulation by classical benzodiazepines by His to Arg point mutations in knock-in mice, we were able to identify behavioral functions mediated by different GABAA receptor subtypes, which led to the development of novel therapeutic strategies. In this chapter, we provide a largely chronological overview on behavioral studies on GABAA receptor mutant mice at the Institute of Pharmacology and Toxicology at the University of Zurich.

Altered intrathalamic GABAA neurotransmission in a mouse model of a human genetic absence epilepsy syndrome

We previously demonstrated that heterozygous deletion of Gabra1, the mouse homolog of the human absence epilepsy gene that encodes the GABAA receptor (GABAAR) α1 subunit, causes absence seizures. We showed that cortex partially compensates for this deletion by increasing the cell surface expression of residual α1 subunit and by increasing α3 subunit expression. Absence seizures also involve two thalamic nuclei: the ventrobasal (VB) nucleus, which expresses only the α1 and α4 subtypes of GABAAR α subunits, and the reticular (nRT) nucleus, which expresses only the α3 subunit subtype. Here, we found that, unlike cortex, VB exhibited significantly reduced total and synaptic α1 subunit expression. In addition, heterozygous α1 subunit deletion substantially reduced miniature inhibitory postsynaptic current (mIPSC) peak amplitudes and frequency in VB. However, there was no change in the expression of the extrasynaptic α4 or δ subunits in VB and, unlike other models of absence epilepsy, no change in tonic GABAAR currents. Although heterozygous α1 subunit knockout increased α3 subunit expression in medial thalamic nuclei, it did not alter α3 subunit expression in nRT. However, it did enlarge the presynaptic vesicular inhibitory amino acid transporter puncta and lengthen the time constant of mIPSC decay in nRT. We conclude that increased tonic GABAA currents are not necessary for absence seizures. In addition, heterozygous loss of α1 subunit disinhibits VB by substantially reducing phasic GABAergic currents and surprisingly, it also increases nRT inhibition by prolonging phasic currents. The increased inhibition in nRT likely represents a partial compensation that helps reduce absence seizures.

Single-cell genetic expression of mutant GABAA receptors causing Human genetic epilepsy alters dendritic spine and GABAergic bouton formation in a mutation-specific manner

Mutations in genes encoding for GABA_A receptor subunits is a well-established cause of genetic generalized epilepsy. GABA neurotransmission is implicated in several developmental processes including neurite outgrowth and synapse formation. Alteration in excitatory/inhibitory synaptic activities plays a critical role in epilepsy, thus here we investigated whether mutations in α1 subunit of GABA_A receptor may affect dendritic spine and GABAergic bouton formation. In particular, we examined the effects of three mutations of the GABRA1 gene (D219N, A322D and K353delins18X) that were found in a cohort of French Canadian families with genetic generalized epilepsy. We used a novel single-cell genetic approach, by preparing cortical organotypic cultures from GABRA1^flox/flox mice and simultaneously inactivating endogenous GABRA1 and transfecting mutant α1 subunits in single glutamatergic pyramidal cells and basket GABAergic interneurons by biolistic transfection. We found that GABRA1^−/− GABAergic cells showed reduced innervation field, which was rescued by co-expressing α1-A322D and α1-WT but not α1-D219N. We further found that the expression of the most severe GABRA1 missense mutation (α1-A322D) induced a striking increase of spine density in pyramidal cells along with an increase in the number of mushroom-like spines. In addition, α1-A322D expression in GABAergic cells slightly increased perisomatic bouton density, whereas other mutations did not alter bouton formation. All together, these results suggest that the effects of different GABA_AR mutations on GABAergic bouton and dendritic spine formation are specific to the mutation and cannot be always explained by a simple loss-of-function gene model. The use of single cell genetic manipulation in organotypic cultures may provide a better understanding of the specific and distinct neural circuit alterations caused by different GABA_A receptor subunit mutations and will help define the pathophysiology of genetic generalized epilepsy syndromes.

Gephyrin clusters are absent from small diameter primary afferent terminals despite the presence of GABA(A) receptors

Whereas both GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs) play a role in control of dorsal horn neuron excitability, their relative contribution to inhibition of small diameter primary afferent terminals remains controversial. To address this, we designed an approach for quantitative analyses of the distribution of GABA(A)R-subunits, GlyR α1-subunit and their anchoring protein, gephyrin, on terminals of rat spinal sensory afferents identified by Calcitonin-Gene-Related-Peptide (CGRP) for peptidergic terminals, and by Isolectin-B4 (IB4) for nonpeptidergic terminals. The approach was designed for light microscopy, which is compatible with the mild fixation conditions necessary for immunodetection of several of these antigens. An algorithm was designed to recognize structures with dimensions similar to those of the microscope resolution. To avoid detecting false colocalization, the latter was considered significant only if the degree of pixel overlap exceeded that expected from randomly overlapping pixels given a hypergeometric distribution. We found that both CGRP(+) and IB4(+) terminals were devoid of GlyR α1-subunit and gephyrin. The α1 GABA(A)R was also absent from these terminals. In contrast, the GABA(A)R α2/α3/α5 and β3 subunits were significantly expressed in both terminal types, as were other GABA(A)R-associated-proteins (α-Dystroglycan/Neuroligin-2/Collybistin-2). Ultrastructural immunocytochemistry confirmed the presence of GABA(A)R β3 subunits in small afferent terminals. Real-time quantitative PCR (qRT-PCR) confirmed the results of light microscopy immunochemical analysis. These results indicate that dorsal horn inhibitory synapses follow different rules of organization at presynaptic versus postsynaptic sites (nociceptive afferent terminals vs inhibitory synapses on dorsal horn neurons). The absence of gephyrin clusters from primary afferent terminals suggests a more diffuse mode of GABA(A)-mediated transmission at presynaptic than at postsynaptic sites.

Localisation and stress-induced plasticity of GABAA receptor subunits within the cellular networks of the mouse dorsal raphe nucleus

The dorsal raphe nucleus (DRN) provides the major source of serotonin to the central nervous system (CNS) and modulates diverse neural functions including mood. Furthermore, DRN cellular networks are engaged in the stress-response at the CNS level allowing for adaptive behavioural responses, whilst stress-induced dysregulation of DRN and serotonin release is implicated in psychiatric disorders. Therefore, identifying the molecules regulating DRN activity is fundamental to understand DRN function in health and disease. GABAA receptors (GABAARs) allow for brain region, cell type and subcellular domain-specific GABA-mediated inhibitory currents and are thus key regulators of neuronal activity. Yet, the GABAAR subtypes expressed within the neurochemically diverse cell types of the mouse DRN are poorly described. In this study, immunohistochemistry and confocal microscopy revealed that all serotonergic neurons expressed immunoreactivity for the GABAAR alpha2 and 3 subunits, although the respective signals were co-localised to varying degrees with inhibitory synaptic marker proteins. Only a topographically located sub-population of serotonergic neurons exhibited GABAAR alpha1 subunit immunoreactivity. However, all GABAergic as well as non-GABAergic, non-serotonergic neurons within the DRN expressed GABAAR alpha1 subunit immunoreactivity. Intriguingly, immunoreactivity for the GABAAR gamma2 subunit was enriched on GABAergic rather than serotonergic neurons. Finally, repeated restraint stress increased the expression of the GABAAR alpha3 subunit at the mRNA and protein level. The study demonstrates the identity and location of distinct GABAAR subunits within the cellular networks of the mouse DRN and that stress impacts on the expression levels of particular subunits at the gene and protein level.

Regulation of adult neurogenesis by GABAergic transmission: signaling beyond GABAA-receptors

In the adult mammalian brain, neurogenesis occurs in the olfactory bulb (OB) and in the dentate gyrus (DG) of the hippocampus. Several studies have shown that multiple stages of neurogenesis are regulated by GABAergic transmission with precise spatio-temporal selectivity, and involving mechanisms common to both systems or specific only to one. In the subgranular zone (SGZ) of the DG, GABA neurotransmitter, released by a specific population of interneurons, regulates stem cell quiescence and neuronal cell fate decisions. Similarly, in the subventricular zone (SVZ), OB neuroblast production is modulated by ambient GABA. Ambient GABA, acting on extrasynaptic GABAA receptors (GABAAR), is also crucial for proper adult-born granule cell (GC) maturation and synaptic integration in the OB as well as in the DG. Throughout adult-born neuron development, various GABA receptors and receptor subunits play specific roles. Previous work has demonstrated that adult-born GCs in both the OB and the DG show a time window of increased plasticity in which adult-born cells are more prone to modification by external stimuli. One mechanism that controls this "critical period" is GABAergic modulation. Indeed, depleting the main phasic GABAergic inputs in adult-born neurons results in dramatic effects, such as reduction of spine density and dendritic branching in adult-born OB GCs. In this review, we systematically compare the role of GABAergic transmission in the regulation of adult neurogenesis between the OB and the hippocampus, focusing on the role of GABA in modulating plasticity and critical periods of adult-born neuron development. Finally, we discuss signaling pathways that might mediate some of the deficits observed upon targeted deletion of postsynaptic GABAARs in adult-born neurons.

Gephyrin: a master regulator of neuronal function?

The neurotransmitters GABA and glycine mediate fast synaptic inhibition by activating ligand-gated chloride channels--namely, type A GABA (GABA(A)) and glycine receptors. Both types of receptors are anchored postsynaptically by gephyrin, which self-assembles into a scaffold and interacts with the cytoskeleton. Current research indicates that postsynaptic gephyrin clusters are dynamic assemblies that are held together and regulated by multiple protein-protein interactions. Moreover, post-translational modifications of gephyrin regulate the formation and plasticity of GABAergic synapses by altering the clustering properties of postsynaptic scaffolds and thereby the availability and function of receptors and other signalling molecules. Here, we discuss the formation and regulation of the gephyrin scaffold, its role in GABAergic and glycinergic synaptic function and the implications for the pathophysiology of brain disorders caused by abnormal inhibitory neurotransmission.

The general anaesthetic etomidate inhibits the excitability of mouse thalamocortical relay neurons by modulating multiple modes of GABAA receptor-mediated inhibition

Modulation of thalamocortical (TC) relay neuron function has been implicated in the sedative and hypnotic effects of general anaesthetics. Inhibition of TC neurons is mediated predominantly by a combination of phasic and tonic inhibition, together with a recently described ‘spillover’ mode of inhibition, generated by the dynamic recruitment of extrasynaptic γ-aminobutyric acid (GABA)_A receptors (GABA_ARs). Previous studies demonstrated that the intravenous anaesthetic etomidate enhances tonic and phasic inhibition in TC relay neurons, but it is not known how etomidate may influence spillover inhibition. Moreover, it is unclear how etomidate influences the excitability of TC neurons. Thus, to investigate the relative contribution of synaptic (α1β2γ2) and extrasynaptic (α4β2δ) GABA_ARs to the thalamic effects of etomidate, we performed whole-cell recordings from mouse TC neurons lacking synaptic (α1^0/0) or extrasynaptic (δ^0/0) GABA_ARs. Etomidate (3 μm) significantly inhibited action-potential discharge in a manner that was dependent on facilitation of both synaptic and extrasynaptic GABA_ARs, although enhanced tonic inhibition was dominant in this respect. Additionally, phasic inhibition evoked by stimulation of the nucleus reticularis exhibited a spillover component mediated by δ-GABA_ARs, which was significantly prolonged in the presence of etomidate. Thus, etomidate greatly enhanced the transient suppression of TC spike trains by evoked inhibitory postsynaptic potentials. Collectively, these results suggest that the deactivation of thalamus observed during etomidate-induced anaesthesia involves potentiation of tonic and phasic inhibition, and implicate amplification of spillover inhibition as a novel mechanism to regulate the gating of sensory information through the thalamus during anaesthetic states.

Contribution of P2X4 receptors to ethanol intake in male C57BL/6 mice

P2X receptors (P2XRs) are a family of cation-permeable ligand-gated ion channels activated by synaptically released extracellular adenosine 5'-triphosphate. The P2X4 subtype is abundantly expressed in the central nervous system and is sensitive to low intoxicating ethanol concentrations. Genetic meta-analyses identified the p2rx4 gene as a candidate gene for innate alcohol intake and/or preference. The current study used mice lacking the p2rx4 gene (knockout, KO) and wildtype (WT) C57BL/6 controls to test the hypothesis that P2X4Rs contribute to ethanol intake. The early acquisition and early maintenance phases of ethanol intake were measured with three different drinking procedures. Further, we tested the effects of ivermectin (IVM), a drug previously shown to reduce ethanol's effects on P2X4Rs and to reduce ethanol intake and preference, for its ability to differentially alter stable ethanol intake in KO and WT mice. Depending on the procedure and the concentration of the ethanol solution, ethanol intake was transiently increased in P2X4R KO versus WT mice during the acquisition of 24-h and limited access ethanol intake. IVM significantly reduced ethanol intake in P2X4R KO and WT mice, but the degree of reduction was 50 % less in the P2X4R KO mice. Western blot analysis identified significant changes in γ-aminobutyric acidA receptor α1 subunit expression in brain regions associated with the regulation of ethanol behaviors in P2X4R KO mice. These findings add to evidence that P2X4Rs contribute to ethanol intake and indicate that there is a complex interaction between P2X4Rs, ethanol, and other neurotransmitter receptor systems.

Monogenic models of absence epilepsy: windows into the complex balance between inhibition and excitation in thalamocortical microcircuits

Absence epilepsy is a common disorder that arises in childhood and can be refractory to medical treatment. Single genetic mutations in mice, at times found in patients with absence epilepsy, provide the unique opportunity to bridge the gap between dysfunction at the genetic level and pathological oscillations within the thalamocortical circuit. Interestingly, unlike other forms of epilepsy, only genes related to ion channels have so far been linked to absence phenotypes. Here, we delineate a paradigm which attempts to unify the various monogenic models based on decades of research. While reviewing the particular impact of these individual mutations, we posit a framework involving fast feedforward disinhibition as one common mechanism that can lead to increased tonic inhibition in the cortex and/or thalamus. Enhanced tonic inhibition hyperpolarizes principal cells, deinactivates T-type calcium channels, and leads to reciprocal burst firing within the thalamocortical loop. We also review data from pharmacologic and polygenic models in light of this paradigm. Ultimately, many questions remain unanswered regarding the pathogenesis of absence epilepsy.

Altered localization of the δ subunit of the GABAA receptor in the thalamus of α4 subunit knockout mice

The α4 subunit of the GABAA receptor (GABAAR) is highly expressed in the thalamus where receptors containing the α4 and δ subunits are major mediators of tonic inhibition. The α4 subunit also exhibits considerable plasticity in a number of physiological and pathological conditions, raising questions about the expression of remaining GABAAR subunits when the α4 subunit is absent. Immunohistochemical studies of an α4 subunit knockout (KO) mouse revealed a substantial decrease in δ subunit expression in the ventrobasal nucleus of the thalamus as well as other forebrain regions where the α4 subunit is normally expressed. In contrast, several subunits associated primarily with phasic inhibition, including the α1 and γ2 subunits, were moderately increased. Intracellular localization of the δ subunit was also altered. While δ subunit labeling was decreased within the neuropil, some labeling remained in the cell bodies of many neurons in the ventrobasal nucleus. Confocal microscopy demonstrated co-localization of this labeling with an endoplasmic reticulum marker, and electron microscopy demonstrated increased immunogold labeling near the endoplasmic reticulum in the α4 KO mouse. These results emphasize the strong partnership of the δ and α4 subunit in the thalamus and suggest that the α4 subunit of the GABAAR plays a critical role in trafficking of the δ subunit to the neuronal surface. The findings also suggest that previously observed reductions in tonic inhibition in the α4 subunit KO mouse are likely to be related to alterations in δ subunit expression, in addition to loss of the α4 subunit.

Extrasynaptic GABA(A) receptors couple presynaptic activity to postsynaptic inhibition in the somatosensory thalamus

Thalamocortical circuits govern cognitive, sensorimotor, and sleep-related network processes, and generate pathological activities during absence epilepsy. Inhibitory control of thalamocortical (TC) relay neurons is partially mediated by GABA released from neurons of the thalamic reticular nucleus (nRT), acting predominantly via synaptic α1β2γ2 GABA(A) receptors (GABA(A)Rs). Importantly, TC neurons also express extrasynaptic α4β2δ GABA(A)Rs, although how they cooperate with synaptic GABA(A)Rs to influence relay cell inhibition, particularly during physiologically relevant nRT output, is unknown. To address this question, we performed paired whole-cell recordings from synaptically coupled nRT and TC neurons of the ventrobasal (VB) complex in brain slices derived from wild-type and extrasynaptic GABA(A)R-lacking, α4 "knock-out" (α4(0/0)) mice. We demonstrate that the duration of VB phasic inhibition generated in response to nRT burst firing is greatly reduced in α4(0/0) pairs, suggesting that action potential-dependent phasic inhibition is prolonged by recruitment of extrasynaptic GABA(A)Rs. Furthermore, the influence of nRT tonic firing frequency on VB holding current is also greatly reduced in α4(0/0) pairs, implying that the α4-GABA(A)R-mediated tonic conductance of relay neurons is dynamically influenced, in an activity-dependent manner, by nRT tonic firing intensity. Collectively, our data reveal that extrasynaptic GABA(A)Rs of the somatosensory thalamus do not merely provide static tonic inhibition but can also be dynamically engaged to couple presynaptic activity to postsynaptic excitability. Moreover, these processes are highly sensitive to the δ-selective allosteric modulator, DS2 and manipulation of GABA transport systems, revealing novel opportunities for therapeutic intervention in thalamocortical network disorders.

Neurosteroid interactions with synaptic and extrasynaptic GABA(A) receptors: regulation of subunit plasticity, phasic and tonic inhibition, and neuronal network excitability

RATIONALE

Neurosteroids are steroids synthesized within the brain with rapid effects on neuronal excitability. Allopregnanolone, allotetrahydrodeoxycorticosterone, and androstanediol are three widely explored prototype endogenous neurosteroids. They have very different targets and functions compared to conventional steroid hormones. Neuronal γ-aminobutyric acid (GABA) type A (GABA(A)) receptors are one of the prime molecular targets of neurosteroids.

OBJECTIVE

This review provides a critical appraisal of recent advances in the pharmacology of endogenous neurosteroids that interact with GABA(A) receptors in the brain. Neurosteroids possess distinct, characteristic effects on the membrane potential and current conductance of the neuron, mainly via potentiation of GABA(A) receptors at low concentrations and direct activation of receptor chloride channel at higher concentrations. The GABA(A) receptor mediates two types of inhibition, now characterized as synaptic (phasic) and extrasynaptic (tonic) inhibition. Synaptic release of GABA results in the activation of low-affinity γ2-containing synaptic receptors, while high-affinity δ-containing extrasynaptic receptors are persistently activated by the ambient GABA present in the extracellular fluid. Neurosteroids are potent positive allosteric modulators of synaptic and extrasynaptic GABA(A) receptors and therefore enhance both phasic and tonic inhibition. Tonic inhibition is specifically more sensitive to neurosteroids. The resulting tonic conductance generates a form of shunting inhibition that controls neuronal network excitability, seizure susceptibility, and behavior.

CONCLUSION

The growing understanding of the mechanisms of neurosteroid regulation of the structure and function of the synaptic and extrasynaptic GABA(A) receptors provides many opportunities to create improved therapies for sleep, anxiety, stress, epilepsy, and other neuropsychiatric conditions.

Developmental changes in expression of inhibitory neuronal proteins in the Fragile X Syndrome mouse basolateral amygdala

In humans, Fragile X Syndrome (FXS) is characterized by enhanced fear, hyperactivity, social anxiety, and, in a subset of individuals, autism. Many of the emotional and social deficits point to defects in the amygdala. We have previously shown defects in inhibitory neuron drive onto excitatory projection neurons in the basolateral amygdala (BLA) of juvenile Fmr1(-/y) knockout (KO) mice. Using pharmacological approaches, we have also previously revealed dynamic functional deficits in α1, α2, and α3 subunit-containing GABAA receptors (GABAARs α1, α2, and α3) during early postnatal development. In this study, we sought to determine whether these defects in GABAAR function are accompanied by changes in protein expression of GABAARs α1, α2, and α3 and the post-synaptic GABAAR-clustering protein gephyrin. Interestingly, we found that while the expression of these proteins did not significantly differ between wildtype (WT) and KO mice at each time point, the timing of developmental expression of GABAAR α1, α2, and gephyrin was altered. Collectively, these data reveal novel defects in inhibitory synapse protein expression during critical periods of early postnatal development that could contribute to observed inhibitory neurotransmission deficits in the KO mouse BLA.

Homeostatic competition between phasic and tonic inhibition

The GABAA receptors are the major inhibitory receptors in the brain and are localized at both synaptic and extrasynaptic membranes. Synaptic GABAA receptors mediate phasic inhibition, whereas extrasynaptic GABAA receptors mediate tonic inhibition. Both phasic and tonic inhibitions regulate neuronal activity, but whether they regulate each other is not very clear. Here, we investigated the functional interaction between synaptic and extrasynaptic GABAA receptors through various molecular manipulations. Overexpression of extrasynaptic α6β3δ-GABAA receptors in mouse hippocampal pyramidal neurons significantly increased tonic currents. Surprisingly, the increase of tonic inhibition was accompanied by a dramatic reduction of the phasic inhibition, suggesting a possible homeostatic regulation of the total inhibition. Overexpressing the α6 subunit alone induced an up-regulation of δ subunit expression and suppressed phasic inhibition similar to overexpressing the α6β3δ subunits. Interestingly, blocking all GABAA receptors after overexpressing α6β3δ receptors could not restore the synaptic GABAergic transmission, suggesting that receptor activation is not required for the homeostatic interplay. Furthermore, insertion of a gephyrin-binding-site (GBS) into the α6 and δ subunits recruited α6(GBS)β3δ(GBS) receptors to postsynaptic sites but failed to rescue synaptic GABAergic transmission. Thus, it is not the positional effect of extrasynaptic α6β3δ receptors that causes the down-regulation of phasic inhibition. Overexpressing α5β3γ2 subunits similarly reduced synaptic GABAergic transmission. We propose a working model that both synaptic and extrasynaptic GABAA receptors may compete for limited receptor slots on the plasma membrane to maintain a homeostatic range of the total inhibition.

Altered cortical GABAA receptor composition, physiology, and endocytosis in a mouse model of a human genetic absence epilepsy syndrome

Patients with generalized epilepsy exhibit cerebral cortical disinhibition. Likewise, mutations in the inhibitory ligand-gated ion channels, GABAA receptors (GABAARs), cause generalized epilepsy syndromes in humans. Recently, we demonstrated that heterozygous knock-out (Hetα1KO) of the human epilepsy gene, the GABAAR α1 subunit, produced absence epilepsy in mice. Here, we determined the effects of Hetα1KO on the expression and physiology of GABAARs in the mouse cortex. We found that Hetα1KO caused modest reductions in the total and surface expression of the β2 subunit but did not alter β1 or β3 subunit expression, results consistent with a small reduction of GABAARs. Cortices partially compensated for Hetα1KO by increasing the fraction of residual α1 subunit on the cell surface and by increasing total and surface expression of α3, but not α2, subunits. Co-immunoprecipitation experiments revealed that Hetα1KO increased the fraction of α1 subunits, and decreased the fraction of α3 subunits, that associated in hybrid α1α3βγ receptors. Patch clamp electrophysiology studies showed that Hetα1KO layer VI cortical neurons exhibited reduced inhibitory postsynaptic current peak amplitudes, prolonged current rise and decay times, and altered responses to benzodiazepine agonists. Finally, application of inhibitors of dynamin-mediated endocytosis revealed that Hetα1KO reduced base-line GABAAR endocytosis, an effect that probably contributes to the observed changes in GABAAR expression. These findings demonstrate that Hetα1KO exerts two principle disinhibitory effects on cortical GABAAR-mediated inhibitory neurotransmission: 1) a modest reduction of GABAAR number and 2) a partial compensation with GABAAR isoforms that possess physiological properties different from those of the otherwise predominant α1βγ GABAARs.