Modulation of GABAA receptor activity by phosphorylation and receptor trafficking: implications for the efficacy of synaptic inhibition

Neurosteroids and GABA(A) Receptor Interactions: A Focus on Stress

Since the pioneering discovery of the rapid CNS depressant actions of steroids by the "father of stress," Hans Seyle 70 years ago, brain-derived "neurosteroids" have emerged as powerful endogenous modulators of neuronal excitability. The majority of the intervening research has focused on a class of naturally occurring steroids that are metabolites of progesterone and deoxycorticosterone, which act in a non-genomic manner to selectively augment signals mediated by the main inhibitory receptor in the CNS, the GABA(A) receptor. Abnormal levels of such neurosteroids associate with a variety of neurological and psychiatric disorders, suggesting that they serve important physiological and pathophysiological roles. A compelling case can be made to implicate neurosteroids in stress-related disturbances. Here we will critically appraise how brain-derived neurosteroids may impact on the stress response to acute and chronic challenges, both pre- and postnatally through to adulthood. The pathological implications of such actions in the development of psychiatric disturbances will be discussed, with an emphasis on the therapeutic potential of neurosteroids for the treatment of stress-associated disorders.

Perinatal intermittent hypoxia alters γ-aminobutyric acid: a receptor levels in rat cerebellum

Perinatal hypoxia commonly causes brain injury in infants, but the time course and mechanisms underlying the preferential male injury are unclear. Intermittent hypoxia disturbs cerebellar γ-aminobutyric (GABA)-A receptor profiles during the perinatal period, possibly responding to transient excitatory processes associated with GABA(A) receptors. We examined whether hypoxic insults were particularly damaging to the male rodent cerebellum during a specific developmental time window. We evaluated cerebellar injury and GABA(A) receptor profiles following 5-h intermittent hypoxia (IH: 20.8% and 10.3% ambient oxygen, switched every 240s) or room-air control in groups of male and female rat pups on postnatal d 1-2, wk 1, or wk 3. The cerebella were harvested and compared between groups. The mRNA levels of GABA(A) receptors α6, normalized to a house-keeping gene GAPDH, and assessed using real-time reverse-transcriptase PCR assays were up-regulated by IH at wk 1, more extensively in male rats, with sex influencing the regulatory time-course. In contrast, GABA(A) α6 receptor protein expression levels, assessed using Western blot assays, reached a nadir at wk 1 in both male and female rats, possibly indicating involvement of a post-transcriptional mechanism. The extent of cerebellar damage and level of apoptosis, assessed by DNA fragmentation, were greatest in the wk 3 IH-exposed group. The findings suggest partial protection for female rats against early hypoxic insult in the cerebellum, and that down-regulation of GABA(A) receptors, rather than direct neural injury assessed by DNA fragmentation may modify cerebellar function, with potential later motor and other deficits.

The dynamic modulation of GABA(A) receptor trafficking and its role in regulating the plasticity of inhibitory synapses

Inhibition in the adult mammalian central nervous system (CNS) is mediated by γ-aminobutyric acid (GABA). The fast inhibitory actions of GABA are mediated by GABA type A receptors (GABA(A)Rs); they mediate both phasic and tonic inhibition in the brain and are the principle sites of action for anticonvulsant, anxiolytic, and sedative-hypnotic agents that include benzodiazepines, barbiturates, neurosteroids, and some general anesthetics. GABA(A)Rs are heteropentameric ligand-gated ion channels that are found concentrated at inhibitory postsynaptic sites where they mediate phasic inhibition and at extrasynaptic sites where they mediate tonic inhibition. The efficacy of inhibition and thus neuronal excitability is critically dependent on the accumulation of specific GABA(A)R subtypes at inhibitory synapses. Here we evaluate how neurons control the number of GABA(A)Rs on the neuronal plasma membrane together with their selective stabilization at synaptic sites. We then go on to examine the impact that these processes have on the strength of synaptic inhibition and behavior.

GABA exerts protective and regenerative effects on islet beta cells and reverses diabetes

Type 1 diabetes (T1D) is an autoimmune disease characterized by insulitis and islet β-cell loss. Thus, an effective therapy may require β-cell restoration and immune suppression. Currently, there is no treatment that can achieve both goals efficiently. We report here that GABA exerts antidiabetic effects by acting on both the islet β-cells and immune system. Unlike in adult brain or islet α-cells in which GABA exerts hyperpolarizing effects, in islet β-cells, GABA produces membrane depolarization and Ca(2+) influx, leading to the activation of PI3-K/Akt-dependent growth and survival pathways. This provides a potential mechanism underlying our in vivo findings that GABA therapy preserves β-cell mass and prevents the development of T1D. Remarkably, in severely diabetic mice, GABA restores β-cell mass and reverses the disease. Furthermore, GABA suppresses insulitis and systemic inflammatory cytokine production. The β-cell regenerative and immunoinhibitory effects of GABA provide insights into the role of GABA in regulating islet cell function and glucose homeostasis, which may find clinical application.

Glycine receptor internalization by protein kinases activation

Although glycine-induced currents in the central nervous system have been proven to be modulated by protein kinases A (PKA) and C (PKC), the mechanism is not well understood. In order to better comprehend the mechanism involved in this phenomenon, we tested the PKA and PKC activation effect on the specific [(3) H]glycine and [(3) H]strychnine binding to postsynaptic glycine receptor (GlyR) in intact rat retina. The specific binding constituted about 20% of the total radioligand binding. Kinetic analysis of the specific binding exhibited a sigmoidal behavior with three glycine and two strychnine binding sites and affinities of 212 nM for [(3) H]glycine and 50 nM for [(3) H]strychnine. Specific radioligand binding was decreased (60-85%) by PKA and PKC activation, an effect that was blocked by specific kinases inhibitors, as well as by cytochalasin D. GlyR expressed in the plasma membrane decreased about 50% in response to kinases activation, which was consistent with an increase of the receptor in the microsomal fraction when PKA was activated. Moreover, immunoprecipitation studies indicated that these kinases lead to a time-dependent receptor phosphorylation. Our results suggest that in retina, GlyR is cross-regulated by G protein-coupled receptors, activating PKA and PKC.

Impact of synaptic neurotransmitter concentration time course on the kinetics and pharmacological modulation of inhibitory synaptic currents

The time course of synaptic currents is a crucial determinant of rapid signaling between neurons. Traditionally, the mechanisms underlying the shape of synaptic signals are classified as pre- and post-synaptic. Over the last two decades, an extensive body of evidence indicated that synaptic signals are critically shaped by the neurotransmitter time course which encompasses several phenomena including pre- and post-synaptic ones. The agonist transient depends on neurotransmitter release mechanisms, diffusion within the synaptic cleft, spill-over to the extra-synaptic space, uptake, and binding to post-synaptic receptors. Most estimates indicate that the neurotransmitter transient is very brief, lasting between one hundred up to several hundreds of microseconds, implying that post-synaptic activation is characterized by a high degree of non-equilibrium. Moreover, pharmacological studies provide evidence that the kinetics of agonist transient plays a crucial role in setting the susceptibility of synaptic currents to modulation by a variety of compounds of physiological or clinical relevance. More recently, the role of the neurotransmitter time course has been emphasized by studies carried out on brain slice models that revealed a striking, cell-dependent variability of synaptic agonist waveforms ranging from rapid pulses to slow volume transmission. In the present paper we review the advances on studies addressing the impact of synaptic neurotransmitter transient on kinetics and pharmacological modulation of synaptic currents at inhibitory synapses.

Mass spectrometric analysis of GABAA receptor subtypes and phosphorylations from mouse hippocampus

The brain GABA(A) receptor (GABA(A) R) is a key element of signaling and neural transmission in health and disease. Recently, complete sequence analysis of the recombinant GABA(A) R has been reported, separation and mass spectrometrical (MS) characterisation from tissue, however, has not been published so far. Hippocampi were homogenised, put on a sucrose gradient 10-69% and the layer from 10 to 20% was used for extraction of membrane proteins by a solution of Triton X-100, 1.5 M aminocaproic acid in the presence of 0.3 M Bis-Tris. This mixture was subsequently loaded onto blue native PAGE (BN-PAGE) with subsequent analysis on denaturing gel systems. Spots from the 3-DE electrophoretic run were stained with Colloidal Coomassie Brilliant Blue, and spots with an apparent molecular weight between 40 and 60 kDa were picked and in-gel digested with trypsin, chymotrypsin and subtilisin. The resulting peptides were analysed by nano-LC-ESI-MS/MS (ion trap) and protein identification was carried out using MASCOT searches. In addition, known GABA(A) R-specific MS information taken from own previous studies was used for searches of GABA(A) R subunits. β-1, β-2 and β-3, θ and ρ-1 subunits were detected and six novel phosphorylation sites were observed and verified by phosphatase treatment. The method used herein enables identification of several GABA(A) R subunits from mouse hippocampus along with phosphorylations of β-1 (T227, Y230), β-2 (Y215, T439) and β-3 (T282, S406) subunits. The procedure forms the basis for GABA(A) R studies at the protein chemical rather than at the immunochemical level in health and disease.

Long-term plasticity at inhibitory synapses

Experience-dependent modifications of neural circuits and function are believed to heavily depend on changes in synaptic efficacy such as LTP/LTD. Hence, much effort has been devoted to elucidating the mechanisms underlying these forms of synaptic plasticity. Although most of this work has focused on excitatory synapses, it is now clear that diverse mechanisms of long-term inhibitory plasticity have evolved to provide additional flexibility to neural circuits. By changing the excitatory/inhibitory balance, GABAergic plasticity can regulate excitability, neural circuit function and ultimately, contribute to learning and memory, and neural circuit refinement. Here we discuss recent advancements in our understanding of the mechanisms and functional relevance of GABAergic inhibitory synaptic plasticity.

The reciprocal regulation of stress hormones and GABA(A) receptors

Stress-derived steroid hormones regulate the expression and function of GABA(A) receptors (GABA(A)Rs). Changes in GABA(A)R subunit expression have been demonstrated under conditions of altered steroid hormone levels, such as stress, as well as following exogenous steroid hormone administration. In addition to the effects of stress-derived steroid hormones on GABA(A)R subunit expression, stress hormones can also be metabolized to neuroactive derivatives which can alter the function of GABA(A)Rs. Neurosteroids allosterically modulate GABA(A)Rs at concentrations comparable to those during stress. In addition to the actions of stress-derived steroid hormones on GABA(A)Rs, GABA(A)Rs reciprocally regulate the production of stress hormones. The stress response is mediated by the hypothalamic-pituitary-adrenal (HPA) axis, the activity of which is governed by corticotropin releasing hormone (CRH) neurons. The activity of CRH neurons is largely controlled by robust GABAergic inhibition. Recently, it has been demonstrated that CRH neurons are regulated by neurosteroid-sensitive, GABA(A)R δ subunit-containing receptors representing a novel feedback mechanism onto the HPA axis. Further, it has been demonstrated that neurosteroidogenesis and neurosteroid actions on GABA(A)R δ subunit-containing receptors on CRH neurons are necessary to mount the physiological response to stress. Here we review the literature describing the effects of steroid hormones on GABA(A)Rs as well as the importance of GABA(A)Rs in regulating the production of steroid hormones. This review incorporates what we currently know about changes in GABA(A)Rs following stress and the role in HPA axis regulation.

GABAA receptor trafficking is regulated by protein kinase C(epsilon) and the N-ethylmaleimide-sensitive factor

Disturbances in GABA(A) receptor trafficking contribute to several neurological and psychiatric disorders by altering inhibitory neurotransmission. Identifying mechanisms that regulate GABA(A) receptor trafficking could lead to better understanding of disease pathogenesis and treatment. Here, we show that protein kinase Cε (PKCε) regulates the N-ethylmaleimide-sensitive factor (NSF), an ATPase critical for membrane fusion events, and thereby promotes the trafficking of GABA(A) receptors. Activation of PKCε decreased cell surface expression of GABA(A) receptors and attenuated GABA(A) currents. Activated PKCε associated with NSF, phosphorylated NSF at serine 460 and threonine 461, and increased NSF ATPase activity, which was required for GABA(A) receptor downregulation. These findings identify new roles for NSF and PKCε in regulating synaptic inhibition through downregulation of GABA(A) receptors. Reducing NSF activity by inhibiting PKCε could help restore synaptic inhibition in disease states in which it is impaired.

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

Tonic inhibition in the brain is mediated largely by specialized populations of extrasynaptic receptors, γ-aminobutyric acid receptors (GABA(A)Rs). In the dentate gyrus region of the hippocampus, tonic inhibition is mediated primarily by GABA(A)R subtypes assembled from α4β2/3 with or without the δ subunit. Although the gating of these receptors is subject to dynamic modulation by agents such as anesthetics, barbiturates, and neurosteroids, the cellular mechanisms neurons use to regulate their accumulation on the neuronal plasma membrane remain to be determined. Using immunoprecipitation coupled with metabolic labeling, we demonstrate that the α4 subunit is phosphorylated at Ser(443) by protein kinase C (PKC) in expression systems and hippocampal slices. In addition, the β3 subunit is phosphorylated on serine residues 408/409 by PKC activity, whereas the δ subunit did not appear to be a PKC substrate. We further demonstrate that the PKC-dependent increase of the cell surface expression of α4 subunit-containing GABA(A)Rs is dependent on Ser(443). Mechanistically, phosphorylation of Ser(443) acts to increase the stability of the α4 subunit within the endoplasmic reticulum, thereby increasing the rate of receptor insertion into the plasma membrane. Finally, we show that phosphorylation of Ser(443) increases the activity of α4 subunit-containing GABA(A)Rs by preventing current run-down. These results suggest that PKC-dependent phosphorylation of the α4 subunit plays a significant role in enhancing the cell surface stability and activity of GABA(A)R subtypes that mediate tonic inhibition.

NMDA receptors regulate GABAA receptor lateral mobility and clustering at inhibitory synapses through serine 327 on the γ2 subunit

Modification of the number of GABA(A) receptors (GABA(A)Rs) clustered at inhibitory synapses can regulate inhibitory synapse strength with important implications for information processing and nervous system plasticity and pathology. Currently, however, the mechanisms that regulate the number of GABA(A)Rs at synapses remain poorly understood. By imaging superecliptic pHluorin tagged GABA(A)R subunits we show that synaptic GABA(A)R clusters are normally stable, but that increased neuronal activity upon glutamate receptor (GluR) activation results in their rapid and reversible dispersal. This dispersal correlates with increases in the mobility of single GABA(A)Rs within the clusters as determined using single-particle tracking of GABA(A)Rs labeled with quantum dots. GluR-dependent dispersal of GABA(A)R clusters requires Ca(2+) influx via NMDA receptors (NMDARs) and activation of the phosphatase calcineurin. Moreover, the dispersal of GABA(A)R clusters and increased mobility of individual GABA(A)Rs are dependent on serine 327 within the intracellular loop of the GABA(A)R γ2 subunit. Thus, NMDAR signaling, via calcineurin and a key GABA(A)R phosphorylation site, controls the stability of synaptic GABA(A)Rs, with important implications for activity-dependent control of synaptic inhibition and neuronal plasticity.

Metabotropic glutamate receptors in the trafficking of ionotropic glutamate and GABA(A) receptors at central synapses

The trafficking of ionotropic glutamate (AMPA, NMDA and kainate) and GABA(A) receptors in and out of, or laterally along, the postsynaptic membrane has recently emerged as an important mechanism in the regulation of synaptic function, both under physiological and pathological conditions, such as information processing, learning and memory formation, neuronal development, and neurodegenerative diseases. Non-ionotropic glutamate receptors, primarily group I metabotropic glutamate receptors (mGluRs), co-exist with the postsynaptic ionotropic glutamate and GABA(A) receptors. The ability of mGluRs to regulate postsynaptic phosphorylation and Ca(2+) concentration, as well as their interactions with postsynaptic scaffolding/signaling proteins, makes them well suited to influence the trafficking of ionotropic glutamate and GABA(A) receptors. Recent studies have provided insights into how mGluRs may impose such an influence at central synapses, and thus how they may affect synaptic signaling and the maintenance of long-term synaptic plasticity. In this review we will discuss some of the recent progress in this area: i) long-term synaptic plasticity and the involvement of mGluRs; ii) ionotropic glutamate receptor trafficking and long-term synaptic plasticity; iii) the involvement of postsynaptic group I mGluRs in regulating ionotropic glutamate receptor trafficking; iv) involvement of postsynaptic group I mGluRs in regulating GABA(A) receptor trafficking; v) and the trafficking of postsynaptic group I mGluRs themselves.

Dopamine-dependent tuning of striatal inhibitory synaptogenesis

Dopaminergic projections to the striatum, crucial for the correct functioning of this brain region in adulthood, are known to be established early in development, but their role is currently uncharacterized. We demonstrate here that dopamine, by activating D(1)- and/or D(2)-dopamine receptors, decreases the number of functional GABAergic synapses formed between the embryonic precursors of the medium spiny neurons, the principal output neurons of the striatum, with associated changes in spontaneous synaptic activity. Activation of these receptors reduces the size of postsynaptic GABA(A) receptor clusters and their overall cell-surface expression, without affecting the total number of clusters or the size or number of GABAergic nerve terminals. These changes result from an increased internalization of GABA(A) receptors, and are mediated by distinct signaling pathways converging at the level of GABA(A) receptors to cause a transient PP2A/PP1-dependent dephosphorylation. Thus, tonic D(1)- and D(2)-receptor activity limits the extent of collateral inhibitory synaptogenesis between medium spiny neurons, revealing a novel role of dopamine in controlling the development of intrinsic striatal microcircuits.

Identification and characterisation of a Maf1/Macoco protein complex that interacts with GABAA receptors in neurons

The majority of fast inhibitory synaptic transmission in the mammalian nervous system is mediated by GABA(A) receptors (GABA(A)Rs). Here we report a novel interaction between the protein Maf1 and GABA(A)R beta-subunit intracellular domains. We find Maf1 to be highly expressed in brain and enriched in the hippocampus and cortex. In heterologous cells and neurons we show Maf1 co-localises with GABA(A)Rs in intracellular compartments and at the cell surface. In neurons, Maf1 is found localised in the cytoplasm in dendrites, partially overlapping with GABA(A)Rs and inhibitory synapses and in addition is enriched in the neuronal nucleus. We also report that Maf1 interacts with a novel coiled-coil domain containing protein that we have called Macoco (for Maf1 interacting coiled-coil protein). Like Maf1, Macoco can also be found localised to inhibitory synapses and directly interacts with GABA(A)Rs. Expressing Macoco in neurons increases surface GABA(A)R levels. Our results suggest that Maf1 and Macoco are novel GABA(A)R interacting proteins important for regulating GABA(A)R surface expression and GABA(A)R signalling in the brain.

Increase of GABAA receptor-mediated tonic inhibition in dentate granule cells after traumatic brain injury

Traumatic brain injury (TBI) can result in altered inhibitory neurotransmission, hippocampal dysfunction, and cognitive impairments. GABAergic spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) and tonic (extrasynaptic) whole cell currents were recorded in control rat hippocampal dentate granule cells (DGCs) and at 90days after controlled cortical impact (CCI). At 34 degrees C, in CCI DGCs, sIPSC frequency and amplitude were unchanged, whereas mIPSC frequency was decreased (3.10+/-0.84Hz, n=16, and 2.44+/-0.67Hz, n=7, p<0.05). At 23 degrees C, 300nM diazepam increased peak amplitude of mIPSCs in control and CCI DGCs, but the increase was 20% higher in control (26.81+/-2.2pA and 42.60+/-1.22pA, n=9, p=0.031) compared to CCI DGCs (33.46+/-2.98pA and 46.13+/-1.09pA, n=10, p=0.047). At 34 degrees C, diazepam did not prolong decay time constants (6.59+/-0.12ms and 6.62+/-0.98ms, n=9, p=0.12), the latter suggesting that CCI resulted in benzodiazepine-insensitive pharmacology in synaptic GABA(A) receptors (GABA(A)Rs). In CCI DGCs, peak amplitude of mIPSCs was inhibited by 100microM furosemide (51.30+/-0.80pA at baseline and 43.50+/-5.30pA after furosemide, n=5, p<0.001), a noncompetitive antagonist of GABA(A)Rs with an enhanced affinity to alpha4 subunit-containing receptors. Potentiation of tonic current by the GABA(A)R delta subunit-preferring competitive agonist THIP (1 and 3microM) was increased in CCI DGCs (47% and 198%) compared to control DGCs (13% and 162%), suggesting the presence of larger tonic current in CCI DGCs; THIP (1microM) had no effect on mIPSCs. Taken together, these results demonstrate alterations in synaptic and extrasynaptic GABA(A)Rs in DGCs following CCI.

Stress and GABA receptors

GABA(A) receptors are sensitive to subtle changes in the environment in both early-life and adulthood. These neurochemical responses to stress in adulthood are sex-dependent. Acute stress induces rapid changes in GABA(A) receptors in experimental animals, with the direction of the changes varying according to the sex of the animals and the stress-paradigm studied. These rapid alterations are of particular interest as they provide an example of fast neurotransmitter system plasticity that may be mediated by stress-induced increases in neurosteroids, perhaps via effects on phosphorylation and/or receptor trafficking. Interestingly, some studies have also provided evidence for long-lasting changes in GABA(A) receptors as a result of exposure to stressors in early-life. The short- and long-term stress sensitivity of the GABAergic system implicates GABA(A) receptors in the non-genetic etiology of psychiatric illnesses such as depression and schizophrenia in which stress may be an important factor.

Blocking L-type voltage-gated Ca2+ channels with dihydropyridines reduces gamma-aminobutyric acid type A receptor expression and synaptic inhibition

Gamma-aminobutyric acid type A receptors (GABA(A)Rs) are the major sites of fast inhibitory neurotransmission in the brain, and the numbers of these receptors at the cell surface can determine the strength of GABAergic neurotransmission. Chronic changes in neuronal activity lead to an adaptive modulation in the efficacy of GABAergic synaptic inhibition, brought about in part by changes in the number of synaptic GABA(A)Rs, a mechanism known as homeostatic synaptic plasticity. Reduction in the number of GABA(A)Rs in response to prolonged neuronal activity blockade is dependent on the ubiquitin-proteasome system. The underlying biochemical pathways linking chronic activity blockade to proteasome-dependent degradation of GABA(A)Rs are unknown. Here, we show that chronic blockade of L-type voltage-gated calcium channels (VGCCs) with nifedipine decreases the number of GABA(A)Rs at synaptic sites but not the overall number of inhibitory synapses. In parallel, blockade of L-type VGCCs decreases the amplitude but not the frequency of miniature inhibitory postsynaptic currents or expression of the glutamic acid decarboxylase GAD65. We further reveal that the activation of L-type VGCCs regulates the turnover of newly translated GABA(A)R subunits in a mechanism dependent upon the activity of the proteasome and thus regulates GABA(A)R insertion into the plasma membrane. Together, these observations suggest that activation of L-type VGCCs can regulate the abundance of synaptic GABA(A)Rs and the efficacy of synaptic inhibition, revealing a potential mechanism underlying the homeostatic adaptation of fast GABAergic inhibition to prolonged changes in activity.

Cellular distribution of the nicotinic acetylcholine receptor alpha7 subunit in rat hippocampus

The hippocampus is a region of the mammalian brain that has been extensively studied due to its role in many forms of memory. To better understand hippocampal function, significant attention has focused upon the cellular distribution of ligand-gated ion channels. Despite strong cholinergic innervation from the basal forebrain and a dense expression of nicotinic acetylchoine receptors (nAChRs), the cellular distribution of subunits forming these receptors has received little attention. We used organotypic hippocampal slice cultures (OHSCs) to study native alpha7 subunits, which, unlike other nAChR subunits, form a homomeric receptor. Cell-surface biotinylation, cross-linking of surface proteins, and sub-cellular fractionation all revealed a very limited presence of the subunit at the plasma membrane. In contrast, subunits of other receptors displayed significant surface expression. Notably, subunits in adult hippocampal tissue were distributed in a fashion similar to that observed in OHSCs. To monitor alpha7 subunits contained in functional nAChRs, a colourimetric assay using alpha-bungarotoxin (a specific alpha7 nAChR antagonist) was developed, and revealed a majority of binding at the cell surface. To change alpha7 subunit distribution, OHSCs were treated with compounds known to affect other ionotropic receptors-insulin, genistein, and elevated external K(+); however, neither subunit surface expression nor antagonist binding was affected. Our data reveal that hippocampal neurons possess a large internal population of alpha7 subunits under basal conditions, which persists during stimuli affecting tyrosine phosphorylation or neuronal activity. The nature of the internal pool of alpha7 subunits remains to be determined, but should have important implications for hippocampal activity.

Activity-dependent tuning of inhibitory neurotransmission based on GABAAR diffusion dynamics

An activity-dependent change in synaptic efficacy is a central tenet in learning, memory, and pathological states of neuronal excitability. The lateral diffusion dynamics of neurotransmitter receptors are one of the important parameters regulating synaptic efficacy. We report here that neuronal activity modifies diffusion properties of type-A GABA receptors (GABA(A)R) in cultured hippocampal neurons: enhanced excitatory synaptic activity decreases the cluster size of GABA(A)Rs and reduces GABAergic mIPSC. Single-particle tracking of the GABA(A)R gamma2 subunit labeled with quantum dots reveals that the diffusion coefficient and the synaptic confinement domain size of GABA(A)R increases in parallel with neuronal activity, depending on Ca(2+) influx and calcineurin activity. These results indicate that GABA(A)R diffusion dynamics are directly linked to rapid and plastic modifications of inhibitory synaptic transmission in response to changes in intracellular Ca(2+) concentration. This transient activity-dependent reduction of inhibition would favor the onset of LTP during conditioning.

Dopamine modulation of GABA tonic conductance in striatal output neurons

We previously reported greater GABAA receptor-mediated tonic currents in D2+ striatopallidal than D1+ striatonigral medium spiny neurons (MSNs) are mediated by alpha5-subunit-containing receptors. Here, we used whole-cell recordings in slices from bacterial artificial chromosome transgenic mice to investigate the link between subunit composition, phosphorylation, and dopamine receptor activation. Whole-cell recordings in slices from delta-subunit knock-out mice demonstrate that while MSNs in wild-type mice do express delta-subunit-containing receptors, this receptor subtype is not responsible for tonic conductance observed in the acute slice preparation. We assessed the contribution of the beta1- and beta3-subunits expressed in MSNs by their sensitivity to etomidate, an agonist selective for beta2- or beta3-subunit-containing GABAA receptors. Although etomidate produced substantial tonic current in D2+ neurons, there was no effect in D1+ neurons. However, with internal PKA application or dopamine modulation, D1+ neurons expressed tonic conductance and responded to etomidate application. Our results suggest that distinct phosphorylation of beta3-subunits may cause larger tonic current in D2+ striatopallidal MSNs, and proper intracellular conditions can reveal tonic current in D1+ cells.

CaMKII phosphorylation of the GABA(A) receptor: receptor subtype- and synapse-specific modulation

As a major inhibitory neurotransmitter, GABA plays a vital role in the brain by controlling the extent of neuronal excitation. This widespread role is reflected by the ubiquitous distribution of GABA(A) receptors throughout the central nervous system. To regulate the level of neuronal inhibition requires some endogenous control over the release of GABA and/or its postsynaptic response. In this context, Ca(2+) ions are often used as primary or secondary messengers frequently resulting in the activation of protein kinases and phosphatases. One such kinase, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), can target the GABA(A) receptor to cause its phosphorylation. Evidence is now emerging, which is reviewed here, that GABA(A) receptors are indeed substrates for CaMKII and that this covalent modification alters the expression of cell surface receptors and their function. This type of regulation can also feature at inhibitory synapses leading to long-term inhibitory synaptic plasticity. Most recently, CaMKII has now been proposed to differentially phosphorylate particular isoforms of GABA(A) receptors in a synapse-specific context.

Type A GABA-receptor-dependent synaptic transmission sculpts dendritic arbor structure in Xenopus tadpoles in vivo

The emergence of dendritic arbor structure in vivo depends on synaptic inputs. We tested whether inhibitory GABAergic synaptic transmission regulates Xenopus optic tectal cell dendritic arbor development in vivo by expressing a peptide corresponding to an intracellular loop (ICL) of the gamma2 subunit of type A GABA receptors (GABA(A)R), which is required to anchor GABA(A) receptors to the postsynaptic scaffold. Enhanced green fluorescent protein (EGFP)-tagged ICL (EGFP-ICL) was distributed in a punctate pattern at putative inhibitory synapses, identified by vesicular GABA transporter immunoreactive puncta. ICL expression completely blocked GABA(A)R-mediated transmission in 36% of transfected neurons and significantly reduced GABA(A)R-mediated synaptic currents relative to AMPA receptor-mediated synaptic currents in the remaining transfected neurons without altering release probability or neuronal excitability. Further analysis of ICL-expressing neurons with residual GABA(A)R-mediated inputs showed that the capacity of benzodiazepine to enhance GABAergic synaptic responses was reduced in ICL-expressing neurons, indicating that they were likely depleted of gamma2 subunit-containing GABA(A)R. Neurons expressing a mutant form of ICL were comparable to controls. In vivo time-lapse images showed that ICL-expressing neurons have more sparsely branched dendritic arbors, which expand over larger neuropil areas than EGFP-expressing control neurons. Analysis of branch dynamics indicated that ICL expression affected arbor growth by reducing rates of branch addition. Furthermore, we found that decreasing GABAergic synaptic transmission with ICL expression blocked visual experience dependent dendritic arbor structural plasticity. Our findings establish an essential role for inhibitory GABAergic synaptic transmission in the regulation of dendritic structural plasticity in Xenopus in vivo.

Neuroactive steroids inhibit spinal reflex potentiation by selectively enhancing specific spinal GABA(A) receptor subtypes

Recently, we demonstrated a spinal GABA(A) receptor (GABA(A)R)-dependent inhibition on the induction of repetitive stimulation-induced spinal reflex potentiation. However, it remains unclear whether steroid hormones modulate such an inhibition. Here, we show that progesterone is capable of producing GABA(A)Rs-dependent inhibition of the induction of spinal reflex potentiation by actions through neurosteroid metabolites. Progesterone (5mg/kg, twice daily for 4 days) up-regulates the expression of GABA(A)R alpha2, alpha3, alpha4 and delta subunits, and is associated with attenuated repetitive stimulation-induced spinal reflex activity in ovariectomized rats. These changes were blocked by finasteride (50mg/kg, twice daily), an antagonist of neurosteroid synthesis from progesterone, but not by the progesterone receptor antagonist, RU486 (100mg/kg, twice daily). The induction of spinal reflex potentiation was attenuated after a short (30 min) intrathecal treatment with the neurosteroids, allopregnanolone (ALLOP, 10 microM, 10 microL) and 3 alpha,5 alpha-tetrahydrodeoxycorticosterone (THDOC, 10 microM, 10 microL). Acute intrathecal administration of the GABA(A)R antagonist, bicuculline (10 microM, 10 microL) reversed the inhibition produced by progesterone, THDOC and allopregnanolone. These results imply that progesterone-mediated effects on GABA(A)R expression and neural inhibition are regulated by neurosteroids synthesis rather than progesterone receptor activation.

Dopamine D4 Receptors Regulate GABAA Receptor Trafficking via an Actin/Cofilin/Myosin-dependent Mechanism

The GABA(A) receptor-mediated inhibitory transmission in prefrontal cortex (PFC) is implicated in cognitive processes such as working memory. Our previous study has found that GABA(A)R current is subject to the regulation of dopamine D(4) receptors, a PFC-enriched neuromodulator critically involved in various mental disorders associated with PFC dysfunction. In this study, we have investigated the cellular mechanism underlying D(4) modulation of GABA(A)Rs. We found that the density of surface clusters of GABA(A)R beta2/3 subunits was reduced by D(4), suggesting that the D(4) reduction of GABA(A)R current is associated with a decrease in functional GABA(A)Rs at the plasma membrane. Moreover, the D(4) reduction of GABA(A)R current was blocked by the actin stabilizer phalloidin and was occluded by the actin destabilizer latrunculin, suggesting that D(4) regulates GABA(A)R trafficking via an actin-dependent mechanism. Cofilin, a major actin depolymerizing factor whose activity is strongly increased by dephosphorylation at Ser(3), provides the possible link between D(4) signaling and the actin dynamics. Because myosin motor proteins are important for the transport of vesicles along actin filaments, we also tested the potential involvement of myosin in D(4) regulation of GABA(A)R trafficking. We found that dialysis with a myosin peptide, which competes with endogenous myosin proteins for actin-binding sites, prevented the D(4) reduction of GABA(A)R current. These results suggest that D(4) receptor activation increases cofilin activity presumably via its dephosphorylation, resulting in actin depolymerization, thus causing a decrease in the myosin-based transport of GABA(A)R clusters to the surface.

Huntingtin associated protein 1 and its functions

Huntington disease (HD) is caused by a polyglutamine expansion in the protein huntingtin (Htt). Several studies suggest that Htt and huntingtin associated protein 1 (HAP1) participate in intracellular trafficking and that polyglutamine expansion affects vesicular transport. Understanding the function of HAP1 and its related proteins could help elucidate the pathogenesis of HD. The present review focuses on HAP1, which has proved to be involved in intracellular trafficking. Unlike huntingtin, which is expressed ubiquitously throughout the brain and body, HAP1 is enriched in neurons, suggesting that its dysfunction could contribute to the selective neuropathology in HD. We discuss recent evidence for the involvement of HAP1 and its binding proteins in potential functions.

Modulation of SK channel trafficking by beta adrenoceptors enhances excitatory synaptic transmission and plasticity in the amygdala

Emotionally arousing events are particularly well remembered. This effect is known to result from the release of stress hormones and activation of beta adrenoceptors in the amygdala. However, the underlying cellular mechanisms are not understood. Small conductance calcium-activated potassium (SK) channels are present at glutamatergic synapses where they limit synaptic transmission and plasticity. Here, we show that beta adrenoceptor activation regulates synaptic SK channels in lateral amygdala pyramidal neurons, through activation of protein kinase A. We show that SK channels are constitutively recycled from the postsynaptic membrane and that activation of beta adrenoceptors removes SK channels from excitatory synapses. This results in enhanced synaptic transmission and plasticity. Our findings demonstrate a novel mechanism by which beta adrenoceptors control synaptic transmission and plasticity, through regulation of SK channel trafficking, and suggest that modulation of synaptic SK channels may contribute to beta adrenoceptor-mediated potentiation of emotional memories.

Insulin as a physiological modulator of glucagon secretion

Glucose homeostasis is regulated primarily by the opposing actions of insulin and glucagon, hormones that are secreted by pancreatic islets from beta-cells and alpha-cells, respectively. Insulin secretion is increased in response to elevated blood glucose to maintain normoglycemia by stimulating glucose transport in muscle and adipocytes and reducing glucose production by inhibiting gluconeogenesis in the liver. Whereas glucagon secretion is suppressed by hyperglycemia, it is stimulated during hypoglycemia, promoting hepatic glucose production and ultimately raising blood glucose levels. Diabetic hyperglycemia occurs as the result of insufficient insulin secretion from the beta-cells and/or lack of insulin action due to peripheral insulin resistance. Remarkably, excessive secretion of glucagon from the alpha-cells is also a major contributor to the development of diabetic hyperglycemia. Insulin is a physiological suppressor of glucagon secretion; however, at the cellular and molecular levels, how intraislet insulin exerts its suppressive effect on the alpha-cells is not very clear. Although the inhibitory effect of insulin on glucagon gene expression is an important means to regulate glucagon secretion, recent studies suggest that the underlying mechanisms of the intraislet insulin on suppression of glucagon secretion involve the modulation of K(ATP) channel activity and the activation of the GABA-GABA(A) receptor system. Nevertheless, regulation of glucagon secretion is multifactorial and yet to be fully understood.

Distinct regulation of beta2 and beta3 subunit-containing cerebellar synaptic GABAA receptors by calcium/calmodulin-dependent protein kinase II

Modulation of GABA(A) receptor function and inhibitory synaptic transmission by phosphorylation has profound consequences for the control of synaptic plasticity and network excitability. We have established that activating alpha-calcium/calmodulin-dependent protein kinase II (alpha-CaMK-II) in cerebellar granule neurons differentially affects populations of IPSCs that correspond to GABA(A) receptors containing different subtypes of beta subunit. By using transgenic mice, we ascertained that alpha-CaMK-II increased IPSC amplitude but not the decay time by acting via beta2 subunit-containing GABA(A) receptors. In contrast, IPSC populations whose decay times were increased by alpha-CaMK-II were most likely mediated by beta3 subunit-containing receptors. Expressing alpha-CaMK-II with mutations that affected kinase function revealed that Ca(2+) and calmodulin binding is crucial for alpha-CaMK-II modulation of GABA(A) receptors, whereas kinase autophosphorylation is not. These findings have significant consequences for understanding the role of synaptic GABA(A) receptor heterogeneity within neurons and the precise regulation of inhibitory transmission by CaMK-II phosphorylation.

International Union of Pharmacology. LXX. Subtypes of gamma-aminobutyric acid(A) receptors: classification on the basis of subunit composition, pharmacology, and function. Update

In this review we attempt to summarize experimental evidence on the existence of defined native GABA(A) receptor subtypes and to produce a list of receptors that actually seem to exist according to current knowledge. This will serve to update the most recent classification of GABA(A) receptors (Pharmacol Rev 50:291-313, 1998) approved by the Nomenclature Committee of the International Union of Pharmacology. GABA(A) receptors are chloride channels that mediate the major form of fast inhibitory neurotransmission in the central nervous system. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. GABA(A) receptors are assembled from a family of 19 homologous subunit gene products and form numerous, mostly hetero-oligomeric, pentamers. Such receptor subtypes with properties that depend on subunit composition vary in topography and ontogeny, in cellular and subcellular localization, in their role in brain circuits and behaviors, in their mechanisms of regulation, and in their pharmacology. We propose several criteria, which can be applied to all the members of the LGIC superfamily, for including a receptor subtype on a list of native hetero-oligomeric subtypes. With these criteria, we develop a working GABA(A) receptor list, which currently includes 26 members, but will undoubtedly be modified and grow as information expands. The list is divided into three categories of native receptor subtypes: "identified," "existence with high probability," and "tentative."

Altered phosphorylation and localization of the A-type channel, Kv4.2 in status epilepticus

Extracelluar signal-regulated kinase (ERK) pathway activation has been demonstrated following convulsant stimulation; however, little is known about the molecular targets of ERK in seizure models. Recently, it has been shown that ERK phosphorylates Kv4.2 channels leading to down-regulation of channel function, and substantially alters dendritic excitability. In the kainate model of status epilepticus (SE), we investigated whether ERK phosphorylates Kv4.2 and whether the changes in Kv4.2 were evident at a synaptosomal level during SE. Western blotting was performed on rat hippocampal whole cell, membrane, synaptosomal, and surface biotinylated extracts following systemic kainate using an antibody generated against the Kv4.2 ERK sites and for Kv4.2, ERK, and phospho-ERK. ERK activation was associated with an increase in Kv4.2 phosphorylation during behavioral SE. During SE, ERK activation and Kv4.2 phosphorylation were evident at the whole cell and synaptosomal levels. In addition, while whole-cell preparations revealed no alterations in total Kv4.2 levels, a decrease in synaptosomal and surface expression of Kv4.2 was evident after prolonged SE. These results demonstrate ERK pathway coupling to Kv4.2 phosphorylation. The finding of decreased Kv4.2 levels in hippocampal synaptosomes and surface membranes suggest additional mechanisms for decreasing the dendritic A-current, which could lead to altered intrinsic membrane excitability during SE.

Post-hypoxic changes in rat cortical neuron GABA A receptor function require L-type voltage-gated calcium channel activation

Hypoxia modifies GABA(A) receptor (GABA(A)R) function and can cause seizures, encephalopathy or myoclonus. To characterize the effects of hypoxia on neuronal GABA(A)Rs, we subjected rat cortical neurons to 1% O2 for 2, 4 or 8h, followed by recovery times of 0-96h, and used whole-cell and perforated patch-clamp recording to assess GABA(A)R currents and pharmacology. Hypoxic exposure for 4h caused downregulation of maximal GABA current immediately following hypoxia and after 48h recovery without changing the EC50 for GABA. Two- and eight-hour hypoxic exposures had inconsistent effects on GABA(A)R currents. Maximal diazepam potentiation was increased immediately following 4h hypoxia, while potentiation by zolpidem was increased after 48h recovery. Pentobarbital enhancement and zinc inhibition of GABA currents were unchanged. Hypoxia also caused a depolarizing shift in the reversal potential of GABA-induced Cl(-) currents after 24h recovery. The L-type voltage-gated calcium channel (L-VGCC) blocker, nitrendipine, during hypoxia or control treatment prevented the reduction in GABA(A)R currents, and increased control currents over baseline. Nitrendipine also prevented the increase in zolpidem potentiation 48h after hypoxia, and blocked the depolarizing shift in Cl(-) reversal potential 24h after hypoxia. The effects of hypoxia on maximal GABA(A)R currents, zolpidem pharmacology and Cl(-) reversal potential thus require depolarization-induced calcium entry via L-VGCCs, and constitutive L-VGCC activity appears to reduce maximal GABA(A)R currents in control neurons via a calcium-dependent process. Calcium-dependent modulation of GABA(A)R currents via L-VGCCs may be a fundamental regulatory mechanism for GABA receptor function.

Regulation of inhibitory synaptic transmission by a conserved atypical interaction of GABA(A) receptor beta- and gamma-subunits with the clathrin AP2 adaptor

The number of surface and synaptic GABA(A) receptors is an important determinant of inhibitory synapse strength. Surface receptor number is in part controlled by removal of receptors from the membrane by interaction with the clathrin adaptor AP2. Here we demonstrate that there are two binding sites for AP2 in the gamma2-subunit: a Yxxvarphi type motif specific to gamma2-subunits and a basic patch AP2 binding motif, that is also found in GABA(A) receptor beta-subunits. Blocking GABA(A) receptor-AP2 interactions using a peptide that inhibits AP2 binding to GABA(A) receptors via the conserved basic patch mechanism increases synaptic responses within minutes, whereas simultaneously blocking both binding mechanisms has an additive effect. These data suggest that multiple AP2 internalization signals control the levels of surface and synaptic GABA(A) receptors to regulate synaptic inhibition.

Postsynaptic dopamine D3 receptor modulation of evoked IPSCs via GABA(A) receptor endocytosis in rat hippocampus

Dopamine is known to be an important modulator of learning and memory processes, but its mechanisms of action at the cellular level are diverse and are not fully characterized. In the hippocampus, pharmacologically isolated monosynaptic IPSCs were measured using the whole-cell voltage-clamp recording technique. Both electrically evoked and spontaneous miniature GABA(A) receptor currents were recorded from CA1 pyramidal neurons in slices obtained from mature rats in the presence of the D3-selective agonist PD128907. The activation of D3 receptors inhibited synaptic GABAergic input without affecting presynaptic function or passive membrane properties. Inhibition of IPSCs evoked from stratum radiatum occurred via regulation of dynamin-dependent trafficking of the GABA(A) receptor, as inclusion of dynamin inhibitory peptide (50 microM) in the recording solution prevented the inhibitory effects of PD128907 (1 microM). This effect of D3 receptor activation could be prevented by intracellular application of either an inhibitor of protein kinase A (PKI, 20 microM) or an activator of protein kinase A (8-OH-cAMP, 50 microM). Neither synchronous IPSCs evoked from the stratum oriens nor asynchronous miniature IPSCs recorded from the stratum radiatum were affected by D3 agonist. The induction of long-term potentiation (LTP) of the extracellular field response in both the stratum radiatum and stratum oriens demonstrated that only potentiation in the stratum radiatum was significantly enhanced by PD128907 (1 microM). Our results suggest that the activation of D3 receptors can modulate GABA(A) receptor endocytosis in the hippocampus in a lamina specific manner, and thereby alter the efficacy of GABAergic transmission in the stratum radiatum of the CA1 region through a postsynaptic mechanism of action.

The ubiquitin-like protein Plic-1 enhances the membrane insertion of GABAA receptors by increasing their stability within the endoplasmic reticulum

Gamma-aminobutyric acid receptors (GABA(A)R) are the major sites of fast inhibitory neurotransmission in the brain, and a critical determinant for the efficacy of neuronal inhibition is the number of these receptors that are expressed on the neuronal cell surface. GABA(A)Rs are heteropentamers that can be constructed from seven subunit classes with multiple members; alpha, beta, gamma(1-3), delta, epsilon(1-3), theta, and pi. Receptor assembly occurs within the endoplasmic reticulum, and it is evident that transport-competent combinations exiting this organelle can access the cell surface, whereas unassembled subunits are ubiquitinated and subject to proteasomal degradation. In a previous report the ubiquitin-like protein Plic-1 was shown to directly interact with GABA(A)Rs and promote their accumulation at the cell surface. In this study we explore the mechanisms by which Plic-1 regulates the membrane trafficking of GABA(A)Rs. Using both recombinant and neuronal preparations it was apparent that Plic-1 increased the stability of endoplasmic reticulum resident GABA(A)Rs together with an increase in the abundance of poly-ubiquitinated receptor subunits. Furthermore, Plic-1 elevated cell surface expression levels by selectively increasing their rates of membrane insertion. Thus, Plic-1 may play a significant role in regulating the strength of synaptic inhibition by increasing the stability of GABA(A)Rs within the secretory pathway and thereby promoting their insertion into the neuronal plasma membrane.

GABA(A) receptor trafficking and its role in the dynamic modulation of neuronal inhibition

GABA (gamma-aminobutyric acid) type A receptors (GABA(A)Rs) mediate most fast synaptic inhibition in the mammalian brain, controlling activity at both the network and the cellular levels. The diverse functions of GABA in the CNS are matched not just by the heterogeneity of GABA(A)Rs, but also by the complex trafficking mechanisms and protein-protein interactions that generate and maintain an appropriate receptor cell-surface localization. In this Review, we discuss recent progress in our understanding of the dynamic regulation of GABA(A)R composition, trafficking to and from the neuronal surface, and lateral movement of receptors between synaptic and extrasynaptic locations. Finally, we highlight a number of neurological disorders, including epilepsy and schizophrenia, in which alterations in GABA(A)R trafficking occur.

State-dependent bidirectional modification of somatic inhibition in neocortical pyramidal cells

Cortical pyramidal neurons alter their responses to input signals depending on behavioral state. We investigated whether changes in somatic inhibition contribute to these alterations. In layer 5 pyramidal neurons of rat visual cortex, repetitive firing from a depolarized membrane potential, which typically occurs during arousal, produced long-lasting depression of somatic inhibition. In contrast, slow membrane oscillations with firing in the depolarized phase, which typically occurs during slow-wave sleep, produced long-lasting potentiation. The depression is mediated by L-type Ca2+ channels and GABA(A) receptor endocytosis, whereas potentiation is mediated by R-type Ca2+ channels and receptor exocytosis. It is likely that the direction of modification is mainly dependent on the ratio of R- and L-type Ca2+ channel activation. Furthermore, somatic inhibition was stronger in slices prepared from rats during slow-wave sleep than arousal. This bidirectional modification of somatic inhibition may alter pyramidal neuron responsiveness in accordance with behavioral state.

Epilepsy, E/I Balance and GABA(A) Receptor Plasticity

GABA_A receptors mediate most of the fast inhibitory transmission in the CNS. They form heteromeric complexes assembled from a large family of subunit genes. The existence of multiple GABA_A receptor subtypes differing in subunit composition, localization and functional properties underlies their role for fine-tuning of neuronal circuits and genesis of network oscillations. The differential regulation of GABA_A receptor subtypes represents a major facet of homeostatic synaptic plasticity and contributes to the excitation/inhibition (E/I) balance under physiological conditions and upon pathological challenges. The purpose of this review is to discuss recent findings highlighting the significance of GABA_A receptor heterogeneity for the concept of E/I balance and its relevance for epilepsy. Specifically, we address the following issues: (1) role for tonic inhibition, mediated by extrasynaptic GABA_A receptors, for controlling neuronal excitability; (2) significance of chloride ion transport for maintenance of the E/I balance in adult brain; and (3) molecular mechanisms underlying GABA_A receptor regulation (trafficking, posttranslational modification, gene transcription) that are important for homoeostatic plasticity. Finally, the relevance of these findings is discussed in light of the involvement of GABA_A receptors in epileptic disorders, based on recent experimental studies of temporal lobe epilepsy (TLE) and absence seizures and on the identification of mutations in GABA_A receptor subunit genes underlying familial forms of epilepsy.

Regulation of synaptic inhibition by phospho-dependent binding of the AP2 complex to a YECL motif in the GABAA receptor gamma2 subunit

The regulation of the number of gamma2-subunit-containing GABA(A) receptors (GABA(A)Rs) present at synapses is critical for correct synaptic inhibition and animal behavior. This regulation occurs, in part, by the controlled removal of receptors from the membrane in clathrin-coated vesicles, but it remains unclear how clathrin recruitment to surface gamma2-subunit-containing GABA(A)Rs is regulated. Here, we identify a gamma2-subunit-specific Yxxvarphi-type-binding motif for the clathrin adaptor protein, AP2, which is located within a site for gamma2-subunit tyrosine phosphorylation. Blocking GABA(A)R-AP2 interactions via this motif increases synaptic responses within minutes. Crystallographic and biochemical studies reveal that phosphorylation of the Yxxvarphi motif inhibits AP2 binding, leading to increased surface receptor number. In addition, the crystal structure provides an explanation for the high affinity of this motif for AP2 and suggests that gamma2-subunit-containing heteromeric GABA(A)Rs may be internalized as dimers or multimers. These data define a mechanism for tyrosine kinase regulation of GABA(A)R surface levels and synaptic inhibition.

The clustering of GABA(A) receptor subtypes at inhibitory synapses is facilitated via the direct binding of receptor alpha 2 subunits to gephyrin

Classical benzodiazepine sensitive GABA(A) receptor subtypes, the major mediators of fast synaptic inhibition in the brain are heteropentamers that can be assembled from alpha1-3/5, beta1-3, and gamma2 subunits, but how neurons orchestrate their selective accumulation at synapses remains obscure. We have identified a 10 amino acid hydrophobic motif within the intracellular domain of the alpha2 subunit that regulates the accumulation of GABA(A) receptors at inhibitory synaptic sites on both axon initial segments and dendrites in a mechanism dependent on the inhibitory scaffold protein gephyrin. This motif was sufficient to target CD4 (cluster of differentiation molecule 4) molecules to inhibitory synapses, and was also critical in regulating the direct binding of alpha2 subunits to gephyrin in vitro. Our results thus reveal that the specific accumulation of GABA(A) receptor subtypes containing alpha2 subunits at inhibitory synapses is dependent on their ability to bind gephyrin.

Unifying electrostatic mechanism for receptor-ligand activity

A prior article in skeletal form proposed an electrostatic mechanism for receptor-ligand activity. The present review provides an elaboration, including supporting evidence. The fundamental aspect entails the presence of molecular electrostatic potential associated with ions and dipoles in the ligand. The ligand can be regarded as an electrical link that joins prevalent electrostatic fields present in the surrounding protein matrix. The exact role of these fields is speculative. One possibility is to function as conduits for electrons and radicals in cell signaling. There is increasing support for important participation of these species in signal transduction. There might also be a favorable influence on energetics involving the electron transfer process. A summary of receptor biology is also provided, including receptors for acetylcholine (nicotinic and muscarinic), GABA, adrenergic, and glutamate.

Deficits in phosphorylation of GABA(A) receptors by intimately associated protein kinase C activity underlie compromised synaptic inhibition during status epilepticus

Status epilepticus (SE) is a progressive and often lethal human disorder characterized by continuous or rapidly repeating seizures. Of major significance in the pathology of SE are deficits in the functional expression of GABA(A) receptors (GABA(A)Rs), the major sites of fast synaptic inhibition in the brain. We demonstrate that SE selectively decreases the phosphorylation of GABA(A)Rs on serine residues 408/9 (S408/9) in the beta3 subunit by intimately associated protein kinase C isoforms. Dephosphorylation of S408/9 unmasks a basic patch-binding motif for the clathrin adaptor AP2, enhancing the endocytosis of selected GABA(A)R subtypes from the plasma membrane during SE. In agreement with this, enhancing S408/9 phosphorylation or selectively blocking the binding of the beta3 subunit to AP2 increased GABA(A)R cell surface expression levels and restored the efficacy of synaptic inhibition in SE. Thus, enhancing phosphorylation of GABA(A)Rs or selectively blocking their interaction with AP2 may provide novel therapeutic strategies to ameliorate SE.

Activity-dependent ubiquitination of GABA(A) receptors regulates their accumulation at synaptic sites

GABA(A) receptors (GABA(A)Rs) are the major mediators of fast synaptic inhibition in the brain. In neurons, these receptors undergo significant rates of endocytosis and exocytosis, processes that regulate both their accumulation at synaptic sites and the efficacy of synaptic inhibition. Here we have evaluated the role that neuronal activity plays in regulating the residence time of GABA(A)Rs on the plasma membrane and their targeting to synapses. Chronic blockade of neuronal activity dramatically increases the level of the GABA(A)R ubiquitination, decreasing their cell surface stability via a mechanism dependent on the activity of the proteasome. Coincident with this loss of cell surface expression levels, TTX treatment reduced both the amplitude and frequency of miniature inhibitory synaptic currents. Conversely, increasing the level of neuronal activity decreases GABA(A)R ubiquitination enhancing their stability on the plasma membrane. Activity-dependent ubiquitination primarily acts to reduce GABA(A)R stability within the endoplasmic reticulum and, thereby, their insertion into the plasma membrane and subsequent accumulation at synaptic sites. Thus, activity-dependent ubiquitination of GABA(A)Rs and their subsequent proteasomal degradation may represent a potent mechanism to regulate the efficacy of synaptic inhibition and may also contribute to homeostatic synaptic plasticity.

The expression of GABAA beta subunit isoforms in synaptic and extrasynaptic receptor populations of mouse dentate gyrus granule cells

The subunit composition of GABA(A) receptors influences their biophysical and pharmacological properties, dictates neuronal location and the interaction with associated proteins, and markedly influences the impact of intracellular biochemistry. The focus has been on alpha and gamma subunits, with little attention given to beta subunits. Dentate gyrus granule cells (DGGCs) express all three beta subunit isoforms and exhibit both synaptic and extrasynaptic receptors that mediate 'phasic' and 'tonic' transmission, respectively. To investigate the subcellular distribution of the beta subunits we have utilized the patch-clamp technique to compare the properties of 'tonic' and miniature inhibitory postsynaptic currents (mIPSCs) recorded from DGGCs of hippocampal slices of P20-26 wild-type (WT), beta(2)(-/-), beta(2N265S) (etomidate-insensitive), alpha(1)(-/-) and delta(-/-) mice. Deletion of either the beta(2) or the delta subunit produced a significant reduction of the tonic current and attenuated the increase of this current induced by the delta subunit-preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). By contrast, mIPSCs were not influenced by deletion of these genes. Enhancement of the tonic current by the beta(2/3) subunit-selective agent etomidate was significantly reduced for DGGCs derived from beta(2N265S) mice, whereas this manipulation had no effect on the prolongation of mIPSCs produced by this anaesthetic. Collectively, these observations, together with previous studies on alpha(4)(-/-) mice, identify a population of extrasynaptic alpha(4)beta(2)delta receptors, whereas synaptic GABA(A) receptors appear to primarily incorporate the beta(3) subunit. A component of the tonic current is diazepam sensitive and is mediated by extrasynaptic receptors incorporating alpha(5) and gamma(2) subunits. Deletion of the beta(2) subunit had no effect on the diazepam-induced current and therefore these extrasynaptic receptors do not contain this subunit. The unambiguous identification of these distinct pools of synaptic and extrasynaptic GABA(A) receptors should aid our understanding of how they act in harmony, to regulate hippocampal signalling in health and disease.