Shaping of IPSCs by endogenous calcineurin activity

Molecular modelling reveals how abundance of α4 sub-type in synaptic GABARA receptor can lead to refractoriness toward GABA and BZ-type drugs

Epilepsy is a complex neurological disorder with genetic and acquired causes, and the drugs presently used to treat epilepsy are not effective in about 30% of the cases. Identification of the molecular mechanisms of resistance will help in the development of newer molecules for treatment. Recent clinical data indicate increased expression of α4- and γ2-containing synaptic GABARA receptors in patients of focal cortical dysplasia (FCD), which is associated with refractory epilepsy pathology. We have investigated, by molecular modelling and docking, the structure and ligand-binding efficiency of the α4-containing hetero-pentameric synaptic GABARA receptor. Though the overall conformation is similar to that of the α1-containing receptor, local conformational changes are seen due to differences between aligned α1 and α4 sub-type residues. The overlaps ALA209(α1)/PRO215(α4) and PHE73(α1)/TYR79(α4) have together caused conformational changes in ARG100(α4) (aligned with ARG94 in α1) thereby affecting key hydrogen bonding interactions with the inhibitory neurotransmitter GABA. This may influence the nature of seizures as strength of GABA-binding is known to affect the nature of Inhibitory Post-Synaptic Currents (IPSCs) from GABAergic neurons. The residue ARG135 (α4) aligns with the residue HIS129 (α1) in the benzodiazapine binding pocket. Molecular modelling also shows that a steric clash between benzodiazapine-type (BZ-type) drugs and ARG135 would reduce the binding of BZ-type drugs to α4-containing receptor. These two findings rationalize the observed association between over-expression of α4-containing synaptic GABARA receptors and refractory epilepsy pathology in FCD. The accurate three-dimensional geometry of the receptor-drug complex made available by these modelling studies will help in designing effective drugs.Communicated by Ramaswamy H. Sarma.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Signaling Mechanism for Modulation by GLP-1 and Exendin-4 of GABA Receptors on Rat Retinal Ganglion Cells

Glucagon-like peptide-1 (GLP-1) is expressed in retinal neurons, but its role in the retina is largely unknown. Here, we demonstrated that GLP-1 or the GLP-1 receptor (GLP-1R; a G protein-coupled receptor) agonist exendin-4 suppressed γ-aminobutyric acid receptor (GABAR)-mediated currents through GLP-1Rs in isolated rat retinal ganglion cells (GCs). Pre-incubation with the stimulatory G protein (G_s) inhibitor NF 449 abolished the exendin-4 effect. The exendin-4-induced suppression was mimicked by perfusion with 8-Br-cAMP (a cAMP analog), but was eliminated by the protein kinase A (PKA) inhibitor Rp-cAMP/KT-5720. The exendin-4 effect was accompanied by an increase in [Ca²⁺]_i of GCs through the IP₃-sensitive pathway and was blocked in Ca²⁺-free solution. Furthermore, when the activity of calmodulin (CaM) and CaM-dependent protein kinase II (CaMKII) was inhibited, the exendin-4 effect was eliminated. Consistent with this, exendin-4 suppressed GABAR-mediated light-evoked inhibitory postsynaptic currents in GCs in rat retinal slices. These results suggest that exendin-4-induced suppression may be mediated by a distinct G_s/cAMP-PKA/IP₃/Ca²⁺/CaM/CaMKII signaling pathway, following the activation of GLP-1Rs.

Inhibitory inputs to an inhibitory interneuron: Spontaneous postsynaptic currents and GABAA receptors of A17 amacrine cells in the rat retina

Amacrine cells constitute a large and heterogenous group of inhibitory interneurons in the retina. The A17 amacrine plays an important role for visual signaling in the rod pathway microcircuit of the mammalian retina. It receives excitatory input from rod bipolar cells and provides feedback inhibition to the same cells. However, from ultrastructural investigations, there is evidence for input to A17s from other types of amacrine cells, presumably inhibitory, but there is a lack of information about the identity and functional properties of the synaptic receptors and how inhibition contributes to the integrative properties of A17s. Here, we studied the biophysical and pharmacological properties of GABAergic spontaneous inhibitory postsynaptic currents (spIPSCs) and GABA_A receptors of A17 amacrines, using whole-cell and outside-out patch recordings from rat retinal slices. The spIPSCs displayed fast onsets (10-90% rise time ~740 μs) and double-exponential decays (τ_fast ~4.5 ms [43% of amplitude]; τ_slow ~22 ms). Ultrafast application of brief pulses of GABA (3 mM) to patches evoked responses with deactivation kinetics best fitted by a triple-exponential function (τ₁ ~5.3 ms [55% of amplitude]; τ₂ ~48 ms [32% amplitude]; τ₃ ~187 ms). Non-stationary noise analysis of spIPSCs and patch responses yielded single-channel conductances of ~21 and ~25 pS, respectively. Pharmacological analysis suggested that the spIPSCs are mediated by receptors with an α1βγ2 subunit composition and the somatic receptors have an α2βγ2 and/or α3βγ2 composition. These results demonstrate the presence of synaptic GABA_A receptors on A17s, which may play an important role in signal integration in these cells.

Molecular and Regulatory Mechanisms of Desensitization and Resensitization of GABAA Receptors with a Special Reference to Propofol/Barbiturate

It is known that desensitization of GABA_A receptor (GABA_AR)-mediated currents is paradoxically correlated with the slowdown of their deactivation, i.e., resensitization. It has been shown that an upregulation of calcineurin enhances the desensitization of GABA_AR-mediated currents but paradoxically prolongs the decay phase of inhibitory postsynaptic currents/potentials without appreciable diminution of their amplitudes. The paradoxical correlation between desensitization and resensitization of GABA_AR-mediated currents can be more clearly seen in response to a prolonged application of GABA to allow more desensitization, instead of brief pulse used in previous studies. Indeed, hump-like GABA_AR currents were produced after a strong desensitization at the offset of a prolonged puff application of GABA in pyramidal cells of the barrel cortex, in which calcineurin activity was enhanced by deleting phospholipase C-related catalytically inactive proteins to enhance the desensitization/resensitization of GABA_AR-mediated currents. Hump-like GABA_AR currents were also evoked at the offset of propofol or barbiturate applications in hippocampal or sensory neurons, but not GABA applications. Propofol and barbiturate are useful to treat benzodiazepine/alcohol withdrawal syndrome, suggesting that regulatory mechanisms of desensitization/resensitization of GABA_AR-mediated currents are important in understanding benzodiazepine/alcohol withdrawal syndrome. In this review, we will discuss the molecular and regulatory mechanisms underlying the desensitization and resensitization of GABA_AR-mediated currents and their functional significances.

Morphine Differentially Alters the Synaptic and Intrinsic Properties of D1R- and D2R-Expressing Medium Spiny Neurons in the Nucleus Accumbens

Exposure to opioids reshapes future reward and motivated behaviors partially by altering the functional output of medium spiny neurons (MSNs) in the nucleus accumbens shell. Here, we investigated how morphine, a highly addictive opioid, alters synaptic transmission and intrinsic excitability on dopamine D1-receptor (D1R) expressing and dopamine D2-receptor (D2R) expressing MSNs, the two main output neurons in the nucleus accumbens shell. Using whole-cell electrophysiology recordings, we show, that 24 h abstinence following repeated non-contingent administration of morphine (10 mg/kg, i.p.) in mice reduces the miniature excitatory postsynaptic current (mEPSC) frequency and miniature inhibitory postsynaptic current (mIPSC) frequency on D2R-MSNs, with concomitant increases in D2R-MSN intrinsic membrane excitability. We did not observe any changes in synaptic or intrinsic changes on D1R-MSNs. Last, in an attempt to determine the integrated effect of the synaptic and intrinsic alterations on the overall functional output of D2R-MSNs, we measured the input-output efficacy by measuring synaptically-driven action potential firing. We found that both D1R-MSN and D2R-MSN output was unchanged following morphine treatment.

Pathophysiology of and therapeutic options for a GABRA1 variant linked to epileptic encephalopathy

We report the identification of a de novo GABRA1 (R214C) variant in a child with epileptic encephalopathy (EE), describe its functional characterization and pathophysiology, and evaluate its potential therapeutic options. The GABRA1 (R214C) variant was identified using whole exome sequencing, and the pathogenic effect of this mutation was investigated by comparing wild-type (WT) α1 and R214C α1 GABA_A receptor-expressing HEK cells. GABA-evoked currents in these cells were recorded using whole-cell, outside-out macro-patch and cell-attached single-channel patch-clamp recordings. Changes to surface and total protein expression levels of WT α1 and R214C α1 were quantified using surface biotinylation assay and western blotting, respectively. Finally, potential therapeutic options were explored by determining the effects of modulators, including diazepam, insulin, and verapamil, on channel gating and receptor trafficking of WT and R214C GABA_A receptors. We found that the GABRA1 (R214C) variant decreased whole-cell GABA-evoked currents by reducing single channel open time and both surface and total GABA_A receptor expression levels. The GABA-evoked currents in R214C GABA_A receptors could only be partially restored with benzodiazepine (diazepam) and insulin. However, verapamil treatment for 24 h fully restored the function of R214C mutant receptors, primarily by increasing channel open time. We conclude that the GABRA1 (R214C) variant reduces channel activity and surface expression of mutant receptors, thereby contributing to the pathogenesis of genetic EE. The functional restoration by verapamil suggests that it is a potentially new therapeutic option for patients with the R214C variant and highlights the value of precision medicine in the treatment of genetic EEs.

A Friend or Foe: Calcineurin across the Gamut of Neurological Disorders

The serine/threonine phosphatase calcineurin (CaN) is a unique but confounding calcium/calmodulin-mediated enzyme. CaN has shown to play essential roles from regulating calcium homeostasis to being an intricate part of learning and memory formation. Neurological disorders, despite differing in their etiology, share similar pathological outcomes, such as mitochondrial dysfunction and apoptotic signaling brought about by excitotoxic elements. CaN, being deeply integrated in vital neuronal functions, may be implicated in various neurological disorders. Understanding the enzyme and its physiological niche in the nervous system is vital in uncovering its roles in the spectrum of brain disorders. By reviewing the crosstalk in different neurological pathologies, a possible grasp of CaN's complex signaling may lead to forming better neurotherapy. This Outlook attempts to explore the various neuronal functions of CaN and investigate its pervasive role through the gamut of neurological disorders.

Balancing tonic and phasic inhibition in hypothalamic corticotropin-releasing hormone neurons

KEY POINTS

GABA transporter (GAT) blockade recruits extrasynaptic GABA_A receptors (GABA_A Rs) and amplifies constitutive presynaptic GABA_B R activity. Extrasynaptic GABA_A Rs contribute to a tonic current. Corticosteroids increase the tonic current mediated by extrasynaptic GABA_A Rs.

ABSTRACT

Corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) are integratory hubs that regulate the endocrine response to stress. GABA inputs provide a basal inhibitory tone that constrains this system and circulating glucocorticoids (CORT) are important feedback controllers of CRH output. Surprisingly little is known about the direct effects of CORT on GABA synapses in PVN. Here we used whole-cell patch clamp recordings from CRH neurons in mouse hypothalamic brain slices to examine the effects of CORT on synaptic and extrasynaptic GABA signalling. We show that GABA transporters (GATs) limit constitutive activation of presynaptic GABA_B receptors and ensure high release probability at GABA synapses. GATs in combination with GABA_B receptors also curtail extrasynaptic GABA_A R signalling. CORT has no effect on synaptic GABA signalling, but increases extrasynaptic GABA tone through upregulation of postsynaptic GABA_A receptors. These data show that efficient GABA clearance and autoinhibition control the balance between synaptic (phasic) and extrasynaptic (tonic) inhibition in PVN CRH neurons. This balance is shifted towards increased extrasynaptic inhibition by CORT.

Mechanisms of Neuronal Silencing After Cortical Spreading Depression

Cortical spreading depression (CSD) is associated with migraine, stroke, and traumatic brain injury, but its mechanisms remain poorly understood. One of the major features of CSD is an hour-long silencing of neuronal activity. Though this silencing has clear ramifications for CSD-associated disease, it has not been fully explained. We used in vivo whole-cell recordings to examine the effects of CSD on layer 2/3 pyramidal neurons in mouse somatosensory cortex and used in vitro recordings to examine their mechanism. We found that CSD caused a reduction in spontaneous synaptic activity and action potential (AP) firing that lasted over an hour. Both pre- and postsynaptic mechanisms contributed to this silencing. Reductions in frequency of postsynaptic potentials were due to a reduction in presynaptic transmitter release probability as well as reduced AP activity. Decreases in postsynaptic potential amplitude were due to an inhibitory shift in the ratio of excitatory and inhibitory postsynaptic currents. This inhibitory shift in turn contributed to the reduced frequency of APs. Thus, distinct but complementary mechanisms generate the long neuronal silence that follows CSD. These cellular changes could contribute to wider network dysfunction in CSD-associated disease, while the pre- and postsynaptic mechanisms offer separate targets for therapy.

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

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

GABA modulation of SVZ-derived progenitor ventral cell migration

The subventricular zone (SVZ) is a proliferative region that provides neurons to olfactory bulb throughout life. The new neurons undergo cell migration from SVZ and travel until they reach their final destination. We previously showed in the early postnatal mouse a ventral migratory subpopulation from SVZ targets the Islands of Calleja (ICC) in the basal forebrain. However, unlike the well-characterized rostral migratory stream, little is known about the guidance mechanisms operating in the ventrally directed migratory pathway. In this study, we examined the role of neurotransmitter γ-aminobutyric acid (GABA) in SVZ-derived progenitor ventral migration and the involvement of this neurotransmitter in the cytoarchitectual organization of dispersed cells into the tight clusters of the ICC. Our results show that the ventral SVZ cell migration rate was enhanced by GABA acting through a GABAA receptor and that GABA acts as a directional guidance cue for ventral migrating cells. Furthermore, disruption of GABA signaling inhibited the formation of Island clusters in vitro. Taken together, these data suggest that GABA is an important guidance and organizational cue for the Island of Calleja.

Diffusion dynamics of synaptic molecules during inhibitory postsynaptic plasticity

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

Modulation of GABAergic transmission in development and neurodevelopmental disorders: investigating physiology and pathology to gain therapeutic perspectives

During mammalian ontogenesis, the neurotransmitter GABA is a fundamental regulator of neuronal networks. In neuronal development, GABAergic signaling regulates neural proliferation, migration, differentiation, and neuronal-network wiring. In the adult, GABA orchestrates the activity of different neuronal cell-types largely interconnected, by powerfully modulating synaptic activity. GABA exerts these functions by binding to chloride-permeable ionotropic GABA_A receptors and metabotropic GABA_B receptors. According to its functional importance during development, GABA is implicated in a number of neurodevelopmental disorders such as autism, Fragile X, Rett syndrome, Down syndrome, schizophrenia, Tourette's syndrome and neurofibromatosis. The strength and polarity of GABAergic transmission is continuously modulated during physiological, but also pathological conditions. For GABAergic transmission through GABA_A receptors, strength regulation is achieved by different mechanisms such as modulation of GABA_A receptors themselves, variation of intracellular chloride concentration, and alteration in GABA metabolism. In the never-ending effort to find possible treatments for GABA-related neurological diseases, of great importance would be modulating GABAergic transmission in a safe and possibly physiological way, without the dangers of either silencing network activity or causing epileptic seizures. In this review, we will discuss the different ways to modulate GABAergic transmission normally at work both during physiological and pathological conditions. Our aim is to highlight new research perspectives for therapeutic treatments that reinstate natural and physiological brain functions in neuro-pathological conditions.

Enhanced desensitization followed by unusual resensitization in GABAA receptors in phospholipase C-related catalytically inactive protein-1/2 double-knockout mice

Phospholipase C-related catalytically inactive proteins (PRIP-1/2) are previously reported to be involved in the membrane trafficking of GABAA receptor (GABAAR) and the regulation of intracellular Ca(2+) stores. GABAAR-mediated currents can be regulated by the intracellular Ca(2+). However, in PRIP-1/2 double-knockout (PRIP-DKO) mice, it remains unclear whether the kinetic properties of GABAARs are modulated by the altered regulation of intracellular Ca(2+) stores. Here, we investigated whether GABAAR currents (IGABA) evoked by GABA puff in layer 3 (L3) pyramidal cells (PCs) of the barrel cortex are altered in PRIP-DKO mice. The deletion of PRIP-1/2 enhanced the desensitization of IGABA but induced a hump-like tail current (tail-I) at the GABA puff offset. IGABA and the hump-like tail-I were suppressed by GABAAR antagonists. The enhanced desensitization of IGABA and the hump-like tail-I in PRIP-DKO PCs were mediated by increases in the intracellular Ca(2+) concentration and were largely abolished by a calcineurin inhibitor and ruthenium red. Calcium imaging revealed that Ca(2+)-induced Ca(2+) release (CICR) and subsequent store-operated Ca(2+) entry (SOCE) are more potent in PRIP-DKO PCs than in wild-type PCs. A mathematical model revealed that a slowdown of GABA-unbinding rate and an acceleration of fast desensitization rate by enhancing its GABA concentration dependency are involved in the generation of hump-like tail-Is. These results suggest that in L3 PCs of the barrel cortex in PRIP-DKO mice, the increased calcineurin activity due to the potentiated CICR and SOCE enhances the desensitization of GABAARs and slows the GABA-unbinding rate, resulting in their unusual resensitization following removal of GABA.

GABA(A) receptor dephosphorylation followed by internalization is coupled to neuronal death in in vitro ischemia

Cerebral ischemia is characterized by an early disruption of GABAergic neurotransmission contributing to an imbalance of the excitatory/inhibitory equilibrium and neuronal death, but the molecular mechanisms involved are not fully understood. Here we report a downregulation of GABA(A) receptor (GABA(A)R) expression, affecting both mRNA and protein levels of GABA(A)R subunits, in hippocampal neurons subjected to oxygen-glucose deprivation (OGD), an in vitro model of ischemia. Similar alterations in the abundance of GABA(A)R subunits were observed in in vivo brain ischemia. OGD reduced the interaction of surface GABA(A)R with the scaffold protein gephyrin, followed by clathrin-dependent receptor internalization. Internalization of GABA(A)R was dependent on glutamate receptor activation and mediated by dephosphorylation of the β3 subunit at serine 408/409. Expression of phospho-mimetic mutant GABA(A)R β3 subunits prevented receptor internalization and protected hippocampal neurons from ischemic cell death. The results show a key role for β3 GABA(A)R subunit dephosphorylation in the downregulation of GABAergic synaptic transmission in brain ischemia, contributing to neuronal death. GABA(A)R phosphorylation might be a therapeutic target to preserve synaptic inhibition in brain ischemia.

Tonic GABA inhibition in hippocampal dentate granule cells: its regulation and function in temporal lobe epilepsies

Both human and experimental evidence strongly supports the view of brain region- and cell-specific changes in tonic GABA inhibition in temporal lobe epilepsies (TLE). This 'tonic' form of signalling is not time-locked to presynaptic action potentials, which depends upon detection of ambient GABA by extrasynaptic GABAA receptors (GABAA Rs). Extrasynaptic GABAA Rs have distinct physiological and pharmacological features, including high GABA-binding affinity and low desensitization and a variety of the specific subunit combinations (α4δ-,α6δ-,α5γ-,ε-containing receptors). These features closely contribute to the function of tonic GABA current, which is preserved properly or increased in dentate gyrus in models of TLE, even in the face of a loss of synaptic inhibition and inhibitory interneurones. Markedly reduced tonic GABA inhibition may facilitate an episode of epilepsy, while persistent elevated tonic inhibition may contribute to the onset of spontaneous recurrent seizures. In dentate granule cells, tonic GABA inhibition is positively modulated by endogenous neurosteroids and other factors, which undergo changes related to hormonal status after TLE. Tonic inhibition regulates neuronal excitability through its effects on membrane potential by both offsetting the threshold and reducing the frequency of action potentials and input resistance. Therefore, extrasynaptic GABAA Rs are expected to be the most important pharmacological targets in TLE. It is likely that both elevate the ambient GABA concentration and potentiate the tonic currents, contributing to the antiepileptic effects.

Protein kinase C regulates tonic GABA(A) receptor-mediated inhibition in the hippocampus and thalamus

Tonic inhibition mediated by extrasynaptic GABA_A receptors (GABA_ARs) is an important regulator of neuronal excitability. Phosphorylation by protein kinase C (PKC) provides a key mode of regulation for synaptic GABA_ARs underlying phasic inhibition; however, less attention has been focused on the plasticity of tonic inhibition and whether this can also be modulated by receptor phosphorylation. To address this issue, we used whole-cell patch clamp recording in acute murine brain slices at both room and physiological temperatures to examine the effects of PKC-mediated phosphorylation on tonic inhibition. Recordings from dentate gyrus granule cells in the hippocampus and dorsal lateral geniculate relay neurons in the thalamus demonstrated that PKC activation caused downregulation of tonic GABA_AR-mediated inhibition. Conversely, inhibition of PKC resulted in an increase in tonic GABA_AR activity. These findings were corroborated by experiments on human embryonic kidney 293 cells expressing recombinant α4β2δ GABA_ARs, which represent a key extrasynaptic GABA_AR isoform in the hippocampus and thalamus. Using bath application of low GABA concentrations to mimic activation by ambient neurotransmitter, we demonstrated a similar inhibition of receptor function following PKC activation at physiological temperature. Live cell imaging revealed that this was correlated with a loss of cell surface GABA_ARs. The inhibitory effects of PKC activation on α4β2δ GABA_AR activity appeared to be mediated by direct phosphorylation at a previously identified site on the β2 subunit, serine 410. These results indicate that PKC-mediated phosphorylation can be an important physiological regulator of tonic GABA_AR-mediated inhibition.

Sniffer patch laser uncaging response (SPLURgE): an assay of regional differences in allosteric receptor modulation and neurotransmitter clearance

Allosteric modulators exert actions on neurotransmitter receptors by positively or negatively altering the effective response of these receptors to their respective neurotransmitter. γ-Aminobutyric acid (GABA) type A ionotropic receptors (GABAARs) are major targets for allosteric modulators such as benzodiazepines, neurosteroids, and barbiturates. Analysis of substances that produce similar effects has been hampered by the lack of techniques to assess the localization and function of such agents in brain slices. Here we describe measurement of the sniffer patch laser uncaging response (SPLURgE), which combines the sniffer patch recording configuration with laser photolysis of caged GABA. This methodology enables the detection of allosteric GABAAR modulators endogenously present in discrete areas of the brain slice and allows for the application of exogenous GABA with spatiotemporal control without altering the release and localization of endogenous modulators within the slice. Here we demonstrate the development and use of this technique for the measurement of allosteric modulation in different areas of the thalamus. Application of this technique will be useful in determining whether a lack of modulatory effect on a particular category of neurons or receptors is due to insensitivity to allosteric modulation or a lack of local release of endogenous ligand. We also demonstrate that this technique can be used to investigate GABA diffusion and uptake. This method thus provides a biosensor assay for rapid detection of endogenous GABAAR modulators and has the potential to aid studies of allosteric modulators that exert effects on other classes of neurotransmitter receptors, such as glutamate, acetylcholine, or glycine receptors.

Endogenous positive allosteric modulation of GABA(A) receptors by diazepam binding inhibitor

Benzodiazepines (BZs) allosterically modulate γ-aminobutyric acid type-A receptors (GABAARs) to increase inhibitory synaptic strength. Diazepam binding inhibitor (DBI) protein is a BZ site ligand expressed endogenously in the brain, but functional evidence for BZ-mimicking positive modulatory actions has been elusive. We demonstrate an endogenous potentiation of GABAergic synaptic transmission and responses to GABA uncaging in the thalamic reticular nucleus (nRT) that is absent in both nm1054 mice, in which the Dbi gene is deleted, and mice in which BZ binding to α3 subunit-containing GABAARs is disrupted. Viral transduction of DBI into nRT is sufficient to rescue the endogenous potentiation of GABAergic transmission in nm1054 mice. Both mutations enhance thalamocortical spike-and-wave discharges characteristic of absence epilepsy. Together, these results indicate that DBI mediates endogenous nucleus-specific BZ-mimicking ("endozepine") roles to modulate nRT function and suppress thalamocortical oscillations. Enhanced DBI signaling might serve as a therapy for epilepsy and other neurological disorders.

Shift from phasic to tonic GABAergic transmission following laser-lesions in the rat visual cortex

Reduction in the strength of GABAergic neurotransmission has often been reported following brain lesions. This weakened inhibition is believed to influence neurological deficits, neuronal hyperexcitability and functional recovery after brain injuries. Uncovering the mechanisms underlying the altered inhibition is therefore crucial. In the present study we used an ex vivo-in vitro model of laser lesions in the rat visual cortex to characterize the cellular correlates of changes in GABAergic transmission in the tissue adjacent to the injury. In the first week post-injury the number of VGAT positive GABAergic terminals as well as the expression level of the GABA synthesizing enzymes GAD67 and GAD65 remained unaltered. However, a reduced frequency of miniature inhibitory postsynaptic currents (mIPSCs) together with an increased paired-pulse ratio (PPR) of evoked IPSCs suggested a functional reduction of phasic GABA release. In parallel, we found an enhancement in the GABAA receptor-mediated tonic inhibition. On the basis of these findings, we propose that cortical lesions provoke a shift in GABAergic transmission, decreasing the phasic and reinforcing the tonic component. We therefore suggest that it is not, as traditionally assumed, the overall inhibitory strength to be primarily compromised by a cortical lesion but rather the temporal accuracy of the GABAergic synaptic signaling.

Neural functions of calcineurin in synaptic plasticity and memory

Major brain functions depend on neuronal processes that favor the plasticity of neuronal circuits while at the same time maintaining their stability. The mechanisms that regulate brain plasticity are complex and engage multiple cascades of molecular components that modulate synaptic efficacy. Protein kinases (PKs) and phosphatases (PPs) are among the most important of these components that act as positive and negative regulators of neuronal signaling and plasticity, respectively. In these cascades, the PP protein phosphatase 2B or calcineurin (CaN) is of particular interest because it is the only Ca(2+)-activated PP in the brain and a major regulator of key proteins essential for synaptic transmission and neuronal excitability. This review describes the primary properties of CaN and illustrates its functions and modes of action by focusing on several representative targets, in particular glutamate receptors, striatal enriched protein phosphatase (STEP), and neuromodulin (GAP43), and their functional significance for synaptic plasticity and memory.

Cell-specific expression of calcineurin immunoreactivity within the rat basolateral amygdala complex and colocalization with the neuropeptide Y Y1 receptor

Neuropeptide Y (NPY) produces potent anxiolytic effects via activation of NPY Y1 receptors (Y1r) within the basolateral amygdaloid complex (BLA). The role of NPY in the BLA was recently expanded to include the ability to produce stress resilience and long-lasting reductions in anxiety-like behavior. These persistent behavioral effects are dependent upon activity of the protein phosphatase, calcineurin (CaN), which has long been associated with shaping long-term synaptic signaling. Furthermore, NPY-induced reductions in anxiety-like behavior persist months after intra-BLA delivery, which together indicate a form of neuronal plasticity had likely occurred. To define a site of action for NPY-induced CaN signaling within the BLA, we employed multi-label immunohistochemistry to determine which cell types express CaN and if CaN colocalizes with the Y1r. We have previously reported that both major neuronal cell populations in the BLA, pyramidal projection neurons and GABAergic interneurons, express the Y1r. Therefore, this current study evaluated CaN immunoreactivity in these cell types, along with Y1r immunoreactivity. Antibodies against calcium-calmodulin kinase II (CaMKII) and GABA were used to identify pyramidal neurons and GABAergic interneurons, respectively. A large population of CaN immunoreactive cells displayed Y1r immunoreactivity (90%). Nearly all (98%) pyramidal neurons displayed CaN immunoreactivity, while only a small percentage of interneurons (10%) contained CaN immunoreactivity. Overall, these anatomical findings provide a model whereby NPY could directly regulate CaN activity in the BLA via activation of the Y1r on CaN-expressing, pyramidal neurons. Importantly, they support BLA pyramidal neurons as prime targets for neuronal plasticity associated with the long-term reductions in anxiety-like behavior produced by NPY injections into the BLA.

Effect of the calcineurin inhibitor FK506 on K+-Cl- cotransporter 2 expression in the mouse hippocampus after kainic acid-induced status epilepticus

Calcineurin (CaN)-mediated excitotoxicity impairs γ-aminobutyric acid (GABA) transmission and induces neuronal apoptosis. Ca(2+)-dependent K(+)-Cl(-) cotransporter 2 (KCC2) participates in GABAergic inhibitory transmission. However, the mechanism by which CaN mediates GABA receptor-mediated KCC2 in seizures is not fully understood. In the present study, we investigated the altered expression of KCC2 and the effects of the CaN inhibitor FK506 on KCC2 expression in the mouse hippocampus following kainic acid (KA) treatment. FK506 was injected twice 24 h and 30 min before KA treatment and then mice were treated with KA and killed 2 days later. FK506 had anticonvulsant effect on KA-induced seizure activities. CaN cleavage was evident in the hippocampus 24 h after KA treatment. FK506 pretreatment blocked the truncation of CaN in the KA-treated hippocampus. Cresyl violet and TUNEL staining showed that FK506 prevented KA-induced hippocampal cell death. In particular, Western blot analysis showed that KCC2 expression was time dependent, with a peak at 6 h and a return to decreased levels at 48 h, whereas FK506 pretreatment inhibited the KA-induced decrease in KCC2 expression in the hippocampus. Immunofluorescence showed that FK506 pretreatment protected the loss of inhibitory GABAergic KCC2-expressing neurons following KA treatment. Taken together, these results provide evidence that altered KCC2 expression may be associated with Ca(2+)-mediated seizure activity and indicate that neuron-specific KCC2 may be involved in neuroprotection after seizures.

GABA neuron alterations, cortical circuit dysfunction and cognitive deficits in schizophrenia

Schizophrenia is a brain disorder associated with cognitive deficits that severely affect the patients' capacity for daily functioning. Whereas our understanding of its pathophysiology is limited, postmortem studies suggest that schizophrenia is associated with deficits of GABA-mediated synaptic transmission. A major role of GABA-mediated transmission may be producing synchronized network oscillations which are currently hypothesized to be essential for normal cognitive function. Therefore, cognitive deficits in schizophrenia may result from a GABA synapse dysfunction that disturbs neural synchrony. Here, we highlight recent studies further suggesting alterations of GABA transmission and network oscillations in schizophrenia. We also review current models for the mechanisms of GABA-mediated synchronization of neural activity, focusing on parvalbumin-positive GABA neurons, which are altered in schizophrenia and whose function has been strongly linked to the production of neural synchrony. Alterations of GABA signaling that impair gamma oscillations and, as a result, cognitive function suggest paths for novel therapeutic interventions.

Sex difference in sensitivity to allopregnanolone neuroprotection in mice correlates with effect on spontaneous inhibitory post synaptic currents

Allopregnanolone (ALLO) is a neurosteroid that has many functions in the brain, most notably neuroprotection and modulation of gamma-amino butyric acid (GABA) neurotransmission. Using a mouse model of cardiac arrest and cardiopulmonary resuscitation, we have previously demonstrated that ALLO protects cerebellar Purkinje cells (PCs) from ischemia in a GABA(A) receptor-dependent manner. In this study we examined the effect of sex on ALLO neuroprotection, observing that low dose ALLO (2 mg/kg) provided greater neuroprotection in females compared to males. At a higher dose of ALLO (8 mg/kg), both sexes were significantly protected from ischemic damage. Using an acute cerebellar slice preparation, whole cell voltage clamp recordings were made from PCs. Spontaneous inhibitory post synaptic currents (IPSCs) were analyzed and the response to physiological ALLO (10 nM) was significantly greater in female PCs compared to male. In contrast, recordings of miniature IPSCs, did not exhibit a sex difference in response to ALLO, suggesting that ALLO affects males and females differentially through a mechanism other than binding postsynaptic GABA(A) receptors. We conclude that the female brain has greater sensitivity to ALLO mediated potentiation of GABAergic neurotransmission, contributing to increased neuroprotection.

The mechanisms of the strong inhibitory modulation of long-term potentiation in the rat dentate gyrus

The hippocampus is essential for the formation of certain types of memory, and synaptic plasticity such as long-term potentiation (LTP) is widely accepted as a cellular basis of hippocampus-dependent memory. Although LTP in both perforant path-dentate gyrus (DG) granule cell and CA3-CA1 pyramidal cell synapses is similarly dependent on activation of postsynaptic N-methyl-D-aspartate receptors, several reports suggest that modulation of LTP by γ-aminobutyric acid (GABA) receptor-mediated inhibitory inputs is stronger in perforant path-DG granule cell synapses. However, little is known about how different the mechanism and physiological relevance of the GABAergic modulation of LTP induction are among different brain regions. We confirmed that the action of GABA(A) receptor antagonists on LTP was more prominent in the DG, and explored the mechanism introducing such difference by examining two types of GABA(A) receptor-mediated inhibition, i.e. synaptic and tonic inhibition. As synaptic inhibition, we compared inhibitory vs. excitatory monosynaptic responses and their summation during an LTP-inducing stimulus, and found that the balance of the summated postsynaptic currents was biased toward inhibition in the DG. As tonic inhibition, or sustained activation of extrasynaptic GABA(A) receptors by ambient GABA, we measured the change in holding currents of the postsynaptic cells induced by GABA(A) receptor antagonists, and found that the tonic inhibition was significantly stronger in the DG. Furthermore, we found that tonic inhibition was associated with LTP modulation. Our results suggest that both the larger tonic inhibition and the larger inhibitory/excitatory summation balance during conditioning are involved in the stronger inhibitory modulation of LTP in the DG.

Alterations of GABAA-Receptor Function and Allosteric Modulation During Development of Status Epilepticus

Partial limbic seizures in rodents induced by pilocarpine progress from stages I–II (mouth and facial movements; head nodding) to stage III (forelimb clonus) and then progress rapidly to stages IV–V (generalized limbic seizures; rearing, and rearing with falling) followed by status epilepticus (SE). Although limbic seizures in rodents are terminated by benzodiazepines, a group of γ-aminobutyric acid type A (GABAA)–receptor positive modulators, significant pharmacoresistance to benzodiazepines develops within minutes during SE. The alterations of GABAA-receptor function and allosteric modulation during development of SE are poorly understood. We induced seizures in juvenile rats by administration of lithium followed by pilocarpine, and whole cell recordings of miniature inhibitory postsynaptic currents (mIPSCs) were obtained from hippocampal dentate granule cells in brain slices. Compared with a sham-treated group, mIPSC amplitude was reduced and decay was accelerated at onset of the first occurrence of stage III (S3) seizures [S3(0)], resulting in a reduction in the total charge transfer at S3(0). Recovery of mIPSC amplitude and prolongation of mIPSC decay occurred 30 min after onset of S3 seizures [S3(30)]. The mIPSC frequency was not altered for S3(0) and S3(30) neurons compared with sham neurons. The net enhancement of total charge transfer by diazepam was smaller for S3(30) than that for sham and S3(0) neurons; however, the net reduction of total charge transfer by zinc was greater for S3(30) than that for sham and S3(0) neurons. These findings suggest that substantial plastic changes of GABAA-receptor function and allosteric modulation occur rapidly in neurons from juvenile animals during development of SE.

Developmental maturation of synaptic and extrasynaptic GABAA receptors in mouse thalamic ventrobasal neurones

Thalamic ventrobasal (VB) relay neurones express multiple GABA(A) receptor subtypes mediating phasic and tonic inhibition. During postnatal development, marked changes in subunit expression occur, presumably reflecting changes in functional properties of neuronal networks. The aims of this study were to characterize the properties of synaptic and extrasynaptic GABA(A) receptors of developing VB neurones and investigate the role of the alpha(1) subunit during maturation of GABA-ergic transmission, using electrophysiology and immunohistochemistry in wild type (WT) and alpha(1)(0/0) mice and mice engineered to express diazepam-insensitive receptors (alpha(1H101R), alpha(2H101R)). In immature brain, rapid (P8/9-P10/11) developmental change to mIPSC kinetics and increased expression of extrasynaptic receptors (P8-27) formed by the alpha(4) and delta subunit occurred independently of the alpha(1) subunit. Subsequently (> or = P15), synaptic alpha(2) subunit/gephyrin clusters of WT VB neurones were replaced by those containing the alpha(1) subunit. Surprisingly, in alpha(1)(0/0) VB neurones the frequency of mIPSCs decreased between P12 and P27, because the alpha(2) subunit also disappeared from these cells. The loss of synaptic GABA(A) receptors led to a delayed disruption of gephyrin clusters. Despite these alterations, GABA-ergic terminals were preserved, perhaps maintaining tonic inhibition. These results demonstrate that maturation of synaptic and extrasynaptic GABA(A) receptors in VB follows a developmental programme independent of the alpha(1) subunit. Changes to synaptic GABA(A) receptor function and the increased expression of extrasynaptic GABA(A) receptors represent two distinct mechanisms for fine-tuning GABA-ergic control of thalamic relay neurone activity during development.

Diversity of GABA(A) receptor synaptic currents on individual pyramidal cortical neurons

Miniature GABA(A) receptor-mediated inhibitory postsynaptic currents (mIPSCs) in cortical pyramidal neurons have previously been categorized into two types: small amplitude mIPSCs with a mono-exponential deactivation (mono-mIPSCs) and relatively larger mIPSCs with bi-exponential deactivation (bi-mIPSCs). The aim of this study was to determine if the GABA(A) channels that underlie these mIPSCSs are molecularly distinct. We found, using non-stationary noise analysis, that the difference in their amplitude could be not accounted for by their single channel conductance (both were 40 pS). Next, using alpha subunit selective GABA(A) receptor modulators, we examined the identity of the alpha subunits that may be expressed in the synapses that give rise to these mIPSCs. Zolpidem (100 and 500 nM, alpha1 selective) affected the deactivation of a subset of the mono-mIPSCs, indicating that alpha1 subunits are not highly expressed in these synapses. However, zolpidem (100 nM) prolonged the deactivation of all bi-mIPSCs, indicating a high abundance of alpha1 subunits in these synapses. SB-205384 (alpha3 selective) had no effect on the mono-mIPSCs but the bi-mIPSCs were prolonged. Furosemide (alpha4 selective) reduced the amplitude of only the mono-mIPSCs. L655,708 (alpha5 selective) reduced the amplitude of both populations and shortened the duration of the mono-mIPSCs. Finally, we found that the neuroactive steroid pregesterone sulphate reduced the amplitude of both mIPSC types. These results provide pharmacological evidence that synapses on cortical pyramidal neurons are molecularly distinct. The purpose of these different types of synapses may be to provide different inhibitory timing patterns on these cells.

Kindling-induced alterations in GABAAreceptor-mediated inhibition and neurosteroid activity in the rat piriform cortex

The piriform cortex makes strong interconnections with limbic structures (amygdala, entorhinal cortex and hippocampus) that are involved in memory processing. These connections have also been implicated in the development of temporal lobe epilepsy. However, little is known about how neurones in this region may change during seizure genesis. Here we tested the hypothesis that in the kindling model of temporal lobe epilepsy GABAA receptor-mediated inhibition is altered in the piriform cortex. To do this we performed whole-cell patch-clamp recordings in piriform cortex brain slices obtained from non-kindled and amygdala-kindled adult rats. We found that kindling coincided with an increase in the amplitude and duration of miniature inhibitory post-synaptic currents (mIPSCs) recorded from non-pyramidal neurones, whereas the mIPSCs occurring on pyramidal (excitatory) cells did not change. Non-stationary noise analysis of mIPSCs occurring on the non-pyramidal neurones showed that inferred unitary conductance of synaptic channels were the same before and after kindling, implying that the channel number increased significantly. Immunocytochemical analysis of the inhibitory innervation showed that it was also unaltered by seizure induction. We also found that the effect of the positive modulator tetrahydrodeoxycorticosterone was reduced on the pyramidal neurones after kindling. In contrast, the potentiating effects of tetrahydrodeoxycorticosterone on non-pyramidal cells were about the same after kindling as in control (sham) rats. These data indicate that amygdala kindling causes a shift in the inhibition 'balance' between the pyramidal and non-pyramidal cells, perhaps leading to the disinhibition of pyramidal cells.

Interaction between cyclodextrin and neuronal membrane results in modulation of GABA(A) receptor conformational transitions

Cyclodextrins (CDs) are nanostructures widely applied in biotechnology and chemistry. Owing to partially hydrophobic character, CDs interact with biological membranes. While the mechanisms of CDs interactions with lipids were widely studied, their effects on proteins are less understood. In the present study we investigated the effects of beta cyclodextrin (betaCD) on GABA(A) receptor (GABA(A)R) gating. To reliably resolve the kinetics of conformational transitions, currents were elicited by ultrafast gamma-aminobutyric acid (GABA) applications to outside-out patches from rat cultured hippocampal neurons. betaCD increased the amplitude of responses to saturating GABA concentration ([GABA]) in a dose-dependent manner and this effect was accompanied by profound alterations in the current kinetics. Current deactivation was slowed down by betaCD but this effect was biphasic with a maximum at around 0.5 mM betaCD. While the fast deactivation time constant was monotonically slowed down within considered betaCD concentration range, the slow component first increased and then, at millimolar betaCD concentration, decreased. The rate and extent of desensitization was decreased by betaCD in a dose-dependent manner. The analysis of current responses to nonsaturating [GABA] indicated that betaCD affected the GABA(A)R agonist binding site by slowing down the unbinding rate. Modulation of GABA(A)R desensitization and binding showed different concentration-dependence suggesting different modualtory sites with higher affinity of the latter one. All the betaCD effects were fully reversible indicating that cholesterol uptake into betaCD was not the primary mechanism. We conclude that betaCD is a strong modulator of GABA(A)R conformational transitions.

Fast IPSCs in rat thalamic reticular nucleus require the GABAA receptor beta1 subunit

Synchrony within the thalamocortical system is regulated in part by intranuclear synaptic inhibition within the reticular nucleus (RTN). Inhibitory postsynaptic currents (IPSCs) in RTN neurons are largely characterized by slow decay kinetics that result in powerful and prolonged suppression of spikes. Here we show that some individual RTN neurons are characterized by highly variable mixtures of fast, slow and mixed IPSCs. Heterogeneity arose largely through differences in the contribution of an initial decay component (tau(D) approximately 10 ms) which was insensitive to loreclezole, suggesting involvement of the GABA(A) receptor beta(1) subunit. Single-cell RT-PCR revealed the presence of beta(1) subunit mRNA only in those neurons whose IPSCs were dominated by a rapid and prominent initial decay phase. These data show that brief, beta(1)-dependent, loreclezole-insensitive IPSCs are present in a subpopulation of RTN neurons, and suggest that striking differences in IPSC heterogeneity within single neurons can result from of the presence or absence of a single GABA(A) receptor subunit.

Cell-type specific GABA synaptic transmission and activity-dependent plasticity in rat hippocampal stratum radiatum interneurons

Abstract In hippocampal pyramidal cells, the efficacy of synaptic transmission at gamma-aminobutyric acid (GABA)ergic synapses, is modulated by activity. However, whether such plasticity occurs at inhibitory synapses on interneurons remains largely unknown. Using whole-cell voltage-clamp recordings of inhibitory postsynaptic currents (IPSCs) in Sprague-Dawley rat hippocampal slices, we examined whether GABA synapses of stratum radiatum interneurons were affected by stimulation protocols known to alter efficacy at inhibitory synapses of CA1 pyramidal cells. Monosynaptically evoked IPSCs (eIPSCs) exhibited different properties with significantly faster kinetics, higher coefficients of variation, a current-voltage (I-V) relationship shifted to depolarized values and a smaller paired-pulse depression, in interneurons than in pyramidal cells. GABA synapses on interneurons also showed a different capacity for plasticity. Indeed, theta-burst stimulation induced a long-term potentiation of eIPSCs in both cell types, but the induction mechanisms differed in interneurons, as it was not affected by antagonists of GABAB receptors and group I/II metabotropic glutamate receptors (mGluRs). Furthermore, 100-Hz tetanization selectively elicited a short-term depression of eIPSCs in pyramidal cells. A postsynaptic depolarization produced a transient suppression of eIPSCs (depolarization-induced suppression of inhibition) in pyramidal cells but not in interneurons. Spontaneous IPSCs were similarly reduced following depolarization in pyramidal cells, but not in interneurons. These results indicate that GABA synapses of stratum radiatum interneurons exhibit different properties and capacity for activity-dependent plasticity than those of pyramidal cells. This cell-type specific mode of transmission and adaptive regulation of GABA synapses may contribute to hippocampal plasticity and functions.

The composition of the GABA receptor at the Caenorhabditis elegans neuromuscular junction

1. The unc-49 gene of the nematode Caenorhabditis elegans encodes three gamma-aminobutyric acid type A (GABA(A)) receptor subunits. Two of these, UNC-49B and UNC-49C, are expressed at high abundance and co-localize at the neuromuscular junction. 2. The UNC-49B subunit is sufficient to form a GABA(A) receptor in vitro and in vivo. Furthermore, all loss-of-function unc-49 alleles lack functional UNC-49B. No mutations specifically inactivate UNC-49C. Thus, UNC-49C appears to be dispensable for receptor function; however, UNC-49C has been conserved among different nematode species, suggesting it plays a necessary role. 3. To ascertain whether UNC-49C is part of the GABA(A) receptor in vivo, we performed patch-clamp electrophysiology on C. elegans muscle cells. Sensitivity to GABA, and to the antagonists picrotoxin and pregnenolone sulfate, matched the UNC-49B/C heteromer rather than the UNC-49B homomer, for both exogenous and synaptically-released GABA. 4. The synaptic localization of UNC-49C requires the presence of UNC-49B, indicative of a physical association between the two subunits in vivo. Thus, the in vivo receptor is an UNC-49B/C heteromer. 5. UNC-49C plays a negative modulatory role. Using the rapid ligand-exchange technique in vitro, we determined that UNC-49C causes accelerated receptor desensitization. Previously, UNC-49C was shown to reduce single-channel conductance in UNC-49B/C heteromers. Thus, the function of UNC-49B is to provide GABA responsiveness and localization to synapses, while the function of UNC-49C is to negatively modulate receptor function and precisely shape inhibitory postsynaptic currents.

Differential GABAA receptor clustering determines GABA synapse plasticity in rat oxytocin neurons around parturition and the onset of lactation

Expression, functional properties, and clustering of alpha 1-, alpha 2-, and alpha 3-subunit containing GABA(A) receptors (GABA(A)Rs) were studied in dorsomedial SON neurons of the adult female rat supraoptic nucleus (SON) around parturition. We show that, although the decay time constant (tau(decay)) of GABAergic postsynaptic currents between and within individual recordings was very diverse, ranging from fast (i.e., alpha 1-like) to significantly slower (i.e., non-alpha 1-like), there was an overall shift towards slower decaying synaptic currents during the onset of lactation. This shift is not due to changes in mRNA expression levels, because real-time quantitative PCR assays indicated that the relative contribution of alpha 1, alpha 2, and alpha 3 remained the same before and after parturition. Also, changes in phosphorylation levels are not likely to affect the tau(decay) of postsynaptic currents. In alpha-latrotoxin (alpha-LTX)-induced bursts of synaptic currents from individual synapses, the tau(decay) of consecutive synaptic events within bursts was very similar, but between bursts there were large differences in tau(decay). This suggested that different synapses within individual SON neurons contain distinct GABA(A)R subtypes. Using multilabeling confocal microscopy, we examined the distribution of postsynaptic alpha 1-, alpha 2-, and alpha 3-GABA(A)Rs, based on colocalization with gephyrin. We show that the three GABA(A)R subtypes occurred either in segregated clusters of one subtype as well as in mixed clusters of two or possibly even three receptor subtypes. After parturition, the density and proportion of clusters containing alpha 2- (or alpha 3-), but not alpha1-GABA(A)Rs, was significantly increased. Thus, the functional synaptic diversity at the postsynaptic level in dorsomedial SON neurons is correlated with a differential clustering of distinct GABA(A)R subtypes at individual synapses.

GluR- and TrkB-mediated maturation of GABA receptor function during the period of eye opening

Synapse maturation includes the shortening of postsynaptic currents, due to changes in the subunit composition of respective transmitter receptors. Patch clamp experiments revealed that GABAergic inhibitory postsynaptic currents (ISPCs) of superior colliculus neurons significantly shorten from postnatal day (P)1 to P21. The change started after P6 and was steepest between P12 and P15, i.e. around eye opening. It was accompanied by enhanced sensitivity to zolpidem and increased expression of GABAAR alpha1 mRNA, whereas the level of alpha3 mRNA decreased. This result is consistent with the hypothesis that the IPSC kinetics of developing collicular neurons is determined by the level of alpha1/alpha3. As alpha1/alpha3 peaked when N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic currents reached their maximum (P12) it was asked whether NMDAR activity can shape the kinetics of GABAergic IPSCs. Cultured collicular neurons were treated with NMDA or NMDAR block, and it was found that the former resulted in faster and the latter in slower IPSC decay. Group I mGluR blockade had no effect. Experiments with bdnf-/- mice revealed that, with some delay, the increase of alpha1/alpha3 mRNA also occurred in the chronic absence of brain-derived neurotrophic factor (BDNF) and, again, this was accompanied by the shortening of IPSCs. In addition, there was an age-dependent depression of IPSC amplitudes by endogenous BDNF, which might reflect the developmental increase in the expression of GABAAR gamma2L, as opposed to gamma2S. Together, these experiments suggest that the GABAAR alpha subunit switch and the associated change in the IPSC kinetics were specifically controlled by NMDAR activity and independent on the signalling through group I mGluRs or TrkB.

Neurosteroid administration and withdrawal alter GABAA receptor kinetics in CA1 hippocampus of female rats

Withdrawal from the GABA-modulatory steroid 3alpha-OH-5alpha-pregnan-20-one (3alpha,5alpha-THP) following exposure of female rats to the parent compound progesterone (P) produces a syndrome characterized by behavioural excitability in association with up-regulation of the alpha4 subunit of the GABA(A) receptor (GABAR) in the hippocampus. Similar changes are seen after 48 h exposure to its stereoisomer, 3alpha,5beta-THP. Here, we further characterize the effects of P withdrawal on GABAR kinetics, using brief (1 ms) application of 5-10 mm GABA to outside-out patches from acutely isolated CA1 hippocampal pyramidal cells. Under control conditions, GABA-gated current deactivated biexponentially, with tau(fast) = 12-19 ms (45-60% of the current), and tau(slow) = 80-140 ms. P withdrawal resulted in marked acceleration of deactivation (tau(fast) = 3-7 ms and tau(slow) = 30-100 ms), as did 48 h exposure to 3alpha,5beta-THP (tau(fast) = 5-8 ms; tau(slow) = 40-120 ms). When recombinant receptors were tested in HEK-293 cells, a similar acceleration in tau(fast) was observed for alpha4beta2delta and alpha4beta2gamma2 GABARs, compared to alpha1beta2gamma2 and alpha5beta2gamma2 receptors. In addition, tau(slow) was also accelerated for alpha4beta2delta receptors, which are increased following steroid withdrawal. As predicted by the Jones-Westbrook model, this change was accompanied by reduced receptor desensitization as well as an acceleration of the rate of recovery from rapid desensitization. A theoretical analysis of the data suggested that steroid treatment leads to receptors with a greater stability of the bound, activatable state. This was achieved by altering multiple parameters, including desensitization and gating rates, within the model. These results suggest that fluctuations in endogenous steroids result in altered GABAR kinetics which may regulate neuronal excitability.

Variations on an inhibitory theme: phasic and tonic activation of GABA(A) receptors

The proper functioning of the adult mammalian brain relies on the orchestrated regulation of neural activity by a diverse population of GABA (gamma-aminobutyric acid)-releasing neurons. Until recently, our appreciation of GABA-mediated inhibition focused predominantly on the GABA(A) (GABA type A) receptors located at synaptic contacts, which are activated in a transient or 'phasic' manner by GABA that is released from synaptic vesicles. However, there is growing evidence that low concentrations of ambient GABA can persistently activate certain subtypes of GABA(A) receptor, which are often remote from synapses, to generate a 'tonic' conductance. In this review, we consider the distinct roles of synaptic and extrasynaptic GABA receptor subtypes in the control of neuronal excitability.

Mice lacking the major adult GABAA receptor subtype have normal number of synapses, but retain juvenile IPSC kinetics until adulthood

There is a large variation in structurally and functionally different GABA(A) receptor subtypes. The expression pattern of GABA(A) receptor subunits is highly regulated, both temporarily and spatially. Especially during development, profound changes in subunit expression have been described. In most brain areas, the GABA(A) receptor alpha1 subunit replaces the alpha2 and/or alpha3 subunit as major alpha subunit. This is accompanied by a marked decrease in the open time of GABA(A) receptors and hence in the duration of postsynaptic responses. We describe here the development of GABAergic, synaptic transmission in mice lacking the alpha1 subunit. We show that alpha1 is to a large extent--but not entirely--responsible for the relatively short duration of postsynaptic responses in the developing and the mature brain. However, alpha1 already affects GABAergic transmission in the neonatal cerebral cortex when it is only sparsely expressed. It appears that the alpha1 -/- mice do not show a large reduction in GABAergic synapses but do retain long-lasting postsynaptic currents into adulthood. Hence, they form a good model to study the functional role of developmental GABA(A) receptor subunit switching.

Insulin exerts neuroprotection by counteracting the decrease in cell-surface GABAA receptors following oxygen-glucose deprivation in cultured cortical neurons

A loss of balance between excitatory and inhibitory signaling leads to excitoxicity, and contributes to ischemic cell death. Reduced synaptic inhibition as a result of dysfunction of the ionotropic GABAA receptor has been suggested as one of the major causes for this imbalance, although the underlying mechanisms remain poorly understood. In the present study, we investigated whether oxygen-glucose deprivation (OGD), an ischemia-like challenge, alters cell-surface expression of GABAA receptors in cultured hippocampal neurons, and thereby leads to excitotoxic cell death. Using cell culture ELISA as a cell surface receptor assay, we found that OGD produced a marked decrease in cell surface GABAA receptors, without altering the total amount of receptors. Furthermore, the reduction could be prevented by inhibition of receptor endocytosis with hypertonic sucrose treatment. Notably, insulin significantly limited OGD-induced changes in cell-surface GABAA receptors. In parallel, insulin protected cultured neurons against both glutamate toxicity and OGD, as assayed by mitochondrial reduction of Alamar Blue. Importantly, insulin-mediated neuroprotection was eliminated when bicuculline, a GABAA receptor antagonist, was co-applied with insulin during OGD. Together, our results strongly suggest that ischemia-like insults decrease cell surface GABAA receptors in neurons via accelerated internalization, and that insulin provides neuroprotection by counteracting this reduction.

Brain-derived neurotrophic factor modulates GABAergic synaptic transmission by enhancing presynaptic glutamic acid decarboxylase 65 levels, promoting asynchronous release and reducing the number of activated postsynaptic receptors

Brain-derived neurotrophic factor is known to modulate the function of GABAergic synapses, but the site of brain-derived neurotrophic factor action is still a matter of controversy. This study was aimed at further dissecting the functional alterations produced by brain-derived neurotrophic factor treatment of GABAergic synaptic connections in cultures of the murine superior colliculus. The functional consequences of long-term brain-derived neurotrophic factor treatment were assessed by analysis of unitary evoked and delayed inhibitory postsynaptic currents in response to high frequency stimulation of single axons. It was found that brain-derived neurotrophic factor facilitated the asynchronous release, but had no effect on the probability of evoked release, the size of the readily releasable pool, and the paired-pulse behavior of evoked inhibitory postsynaptic currents. However, the amplitudes of evoked inhibitory postsynaptic currents, delayed inhibitory postsynaptic currents and miniature inhibitory postsynaptic currents were significantly reduced. Non-stationary fluctuation analysis revealed a decrease in the open channel number at the miniature/evoked inhibitory postsynaptic current peak, but no effect on the mean GABA(A) receptor single channel conductance. Quantitative immunocytochemistry uncovered a significant elevation of presynaptic levels of glutamic acid decarboxylase 65. Together, these findings indicate that brain-derived neurotrophic factor treatment induces pre- as well as postsynaptic changes. What effect predominates will depend on the presynaptic activity pattern: at low activation rates brain-derived neurotrophic factor-treated synapses display a pronounced postsynaptic depression, but at high frequencies this depression is fully compensated by an enhancement of asynchronous release.

GABAA receptor kinetics in the cerebellar nuclei: evidence for detection of transmitter from distant release sites

Neurons of the cerebellar nuclei receive GABAergic input from Purkinje cells. Purkinje boutons have several closely spaced presynaptic densities without GABA transporters, raising the possibility that neurotransmitter released by one presynaptic site diffuses to multiple postsynaptic sites. To test whether such local spillover may contribute to transmission, we studied gating of GABA(A) receptors at 31-33 degrees C in cerebellar nuclear neurons acutely dissociated from mice. Currents were evoked by rapid application of long steps, brief pulses, and high-frequency trains of GABA to outside-out patches. Receptors desensitized and deactivated rapidly, and dose-response measurements estimated an EC(50) of approximately 30 microM. From these data, a kinetic scheme was developed that replicated the recorded currents. Next, we simulated diffusion of GABA in the synaptic cleft, constrained by previous electron microscopic data, and drove the kinetic GABA(A) receptor model with modeled concentration transients. Simulations predicted receptor occupancies of approximately 100% directly opposite the release site and approximately 50% at distant postsynaptic densities, such that receptors up to 700 nm from a release site opened on the timescale of the inhibitory postsynaptic currents before desensitizing. Further simulations of probabilistic release from multiple-site boutons suggested that local spillover-mediated transmission slows the onset and limits the extent of depression during high-frequency signaling.

Regulation of GABAA receptor trafficking, channel activity, and functional plasticity of inhibitory synapses

Neural inhibition in the brain is mainly mediated by ionotropic gamma-aminobutyric acid type A (GABA(A)) receptors. Different subtypes of these receptors, distinguished by their subunit composition, are either concentrated at postsynaptic sites where they mediate phasic inhibition or found at perisynaptic and extrasynaptic locations where they prolong phasic inhibition and mediate tonic inhibition, respectively. Of special interest are mechanisms that modulate the stability and function of postsynaptic GABA(A) receptor subtypes and that are implicated in functional plasticity of inhibitory transmission in the brain. We will summarize recent progress on the classification of synaptic versus extrasynaptic receptors, the molecular composition of the postsynaptic cytoskeleton, the function of receptor-associated proteins in trafficking of GABA(A) receptors to and from synapses, and their role in post-translational signaling mechanisms that modulate the stability, density, and function of GABA(A) receptors in the postsynaptic membrane.

Calcineurin in memory and bidirectional plasticity

The molecular mechanisms of learning and memory, and the underlying bidirectional changes in synaptic plasticity that sustain them largely implicate protein kinases and phosphatases. Specifically, Ca(2+)-dependent kinases and phosphatases actively control neuronal processing by forming a tightly regulated balance in which they oppose each other. In this balance, calcineurin (PP2B) is a critical protein phosphatase whose main function is to negatively modulate learning, memory, and plasticity. It acts by dephosphorylating numerous substrates in different neuronal compartments. This review outlines some of CN neuronal targets and their implication in synaptic functions, and describes the role of CN in the acquisition, storage, retrieval, and extinction of memory, as well as in bidirectional plasticity.

Calcineurin regulation of neuronal plasticity

From the most basic of nervous systems to the intricate circuits found within the human brain, a fundamental requirement of neuronal function is that it be malleable, altering its output based upon experience. A host of cellular proteins are recruited for this purpose, which themselves are regulated by protein phosphorylation. Over the past several decades, research has demonstrated that the Ca(2+) and calmodulin-dependent protein phosphatase calcineurin (protein phosphatase 2B) is a critical regulator of a diverse array of proteins, leading to both short- and long-term effects on neuronal excitability and function. This review describes many of the influences of calcineurin on a variety of proteins, including ion channels, neurotransmitter receptors, enzymes, and transcription factors. Intriguingly, due to the bi-directional influences of Ca(2+) and calmodulin on calcineurin activity, the strength and duration of particular stimulations may cause apparently antagonistic functions of calcineurin to work in concert.

Simulation modeling of ligand receptor interactions at non-equilibrium conditions: processing of noisy inputs by ionotropic receptors

The first event in signal transduction at a synapse is the binding of transmitters to receptors. Because of rapidly changing transmitter levels this binding is unlikely to occur at equilibrium. We describe a mathematical approach that models complex receptor interactions in which the timing and amplitude of transmitter release are noisy. We show that exact solutions for simple bimolecular interactions and receptor transitions can be used to model complex reaction schemes by expressing them in sets of difference equations. Results from the difference equation method to describe binding and channel opening at extended time points compare well with standard solutions using ordinary differential equations. Because it is applicable to noisy systems we used the difference method to investigate the information processing capabilities of GABA receptors and predict how pharmacological agents may modify these properties. As previously demonstrated, the response to a single pulse of GABA is prolonged through entry into a desensitized state. During trains of stimuli the signal to noise ratio can change, and even increase progressively, but the overall transmitted fidelity of the signal decreases with increased driving frequency. The GABA modulator chlorpromazine (primarily affects agonist on and off rates) is predicated to increase receptor signal to noise ratio at all frequencies whereas pregnenolone sulfate (affects receptor desensitization) completely inhibits information transfer.

Ca2+–calmodulin signalling pathway up-regulates GABA synaptic transmission through cytoskeleton-mediated mechanisms

We investigated the role of calcium (Ca(2+))/calmodulin (CaM) signaling pathways in modulating GABA synaptic transmission at CA1 pyramidal neurons in hippocampal slices. Whole-cell pipettes were used to record type A GABA receptor (GABA(A)R)-gated inhibitory postsynaptic currents (IPSCs) and to perfuse intracellularly modulators in the presence of glutamate receptor antagonists. GABA(A)R-gated IPSCs were enhanced by the postsynaptic infusions of adenophostin (1 microM), a potent agonist of inositol-1,4,5-triphosphate receptor (IP(3)R) that induces Ca(2+) release. The enhancement was blocked by co-infusing either 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (10 mM) or CaM-binding peptide (100 microM). Moreover, the postsynaptic infusion of Ca(2+)-CaM (40/10 microM) enhanced both evoked and spontaneous GABA(A)R-gated IPSCs. The enhancement was attenuated by co-infusing 100 microM CaM-KII(281-301), an autoinhibitory peptide of CaM-dependent protein kinases. These results indicate that postsynaptic Ca(2+)-CaM signaling pathways essentially enhance GABAergic synaptic transmission. In the investigation of synaptic targets for the enhancement, we found that IP(3)R agonist-enhanced GABA(A)R-gated IPSCs were attenuated by co-infusing colchicine (30 microM), vincristine (3 microM) or cytochalasin D (1 microM) that inhibits tubulin or actin polymerization, implying that actin filament and microtubules are involved. We conclude that postsynaptic Ca(2+)-CaM signaling pathways strengthen the function of GABAergic synapses via a cytoskeleton-mediated mechanism, probably the recruitment of receptors in the postsynaptic membrane.

Beta subunit phosphorylation selectively increases fast desensitization and prolongs deactivation of alpha1beta1gamma2L and alpha1beta3gamma2L GABA(A) receptor currents

We studied the effects of phosphorylation by protein kinase A (PKA) on GABA(A) receptors (alpha1beta1gamma2L andalpha1beta3gamma2L) transiently expressed in HEK 293T cells. Under conditions favorable for PKA activation, currents obtained using whole-cell patch clamp of lifted cells displayed increased rate and extent of the fast phases of desensitization, decreased rate of current deactivation after GABA removal, and prolongation of brief IPSC-like currents. Mutation of serine residues (beta1 S409, beta3 S407, beta3 S408) revealed that only beta1 S409 and beta3 S408 were critical for the modulatory effect of PKA on GABA(A) receptor currents. Additionally, repeated pulse inhibition was increased in receptors after mutation of the critical serine to glutamate and decreased when the serine was mutated to alanine. These data demonstrate that PKA phosphorylation modulated GABA(A) receptor currents by increasing fast phases of macroscopic desensitization and suggest a role for PKA in regulating GABAergic IPSC duration.

Major differences in inhibitory synaptic transmission onto two neocortical interneuron subclasses

Locally projecting GABAergic interneurons are the major providers of inhibition in the neocortex and play a crucial role in several brain functions. Neocortical interneurons are connected via electrical and chemical synapses that may be crucial in modulating complex network oscillations. We investigated the properties of spontaneous and evoked IPSCs in two morphologically and physiologically identified interneuron subtypes, the fast-spiking (FS) and low threshold-spiking (LTS) cells in layer V of rodent sensorimotor cortex. We found that IPSCs recorded in FS cells were several orders of magnitude more frequent, larger in amplitude, and had faster kinetics than IPSCs recorded in LTS cells. GABA(A) receptor alpha- and beta-subunit selective modulators, zolpidem and loreclezole, had different effects on IPSCs in FS and LTS interneurons, suggesting differential expression of GABA(A) receptor subunit subtypes. These pharmacological data indicated that the alpha1 subunit subtype is poorly expressed by LTS cells but makes a large contribution to GABA(A) receptors on FS cells. This was confirmed by experiments performed in genetically modified mice in which the alpha1 subunit had been made insensitive to benzodiazepine-like agonists. These results suggest that differences in IPSC waveform are likely attributable to distinctive expression of GABA(A) receptor subunits in FS and LTS cells. The particular properties of GABAergic input on different interneuronal subtypes might have important consequences for generation and pacing of cortical rhythms underlying several brain functions. Moreover, selective pharmacological manipulation of distinct inhibitory circuits might allow regulation of pyramidal cell activities under specific physiological and pathophysiological conditions.