Zinc inhibits miniature GABAergic currents by allosteric modulation of GABAA receptor gating

GABAA receptor proline 273 at the interdomain interface of the β2 subunit regulates entry into desensitization and opening/closing transitions

AIMS

GABA_A receptors belong to Cys-loop ion channel family and mediate inhibition in the brain. Despite the abundance of structural data on receptor structure, the molecular scenarios of activation are unknown. In this study we investigated the role of a β₂P273 residue in channel gating transitions. This residue is located in a central position of the M2-M3 linker of the interdomain interface, expected to be predisposed to interact with another interfacial element, the β1-β2 loop of the extracellular side. The interactions occurring on this interface have been reported to couple agonist binding to channel gating.

MAIN METHODS

We recorded micro- and macroscopic current responses of recombinant GABA_A receptors mutated at the β₂P273 residue (to A, K, E) to saturating GABA. Electrophysiological data served as basis to kinetic modeling, used to decipher which gating transition were affected by mutations.

KEY FINDINGS

Mutations of this residue impaired macroscopic desensitization and accelerated current deactivation with P273E mutant showing greatest deviation from wild-type. Single-channel analysis revealed alterations mainly in short-lived shut times and shortening of openings, resulting in dramatic changes in intraburst open probability. Kinetic modeling indicated that β₂P273 mutants show diminished entry into desensitized and open states as well as faster channel closing transitions.

SIGNIFICANCE

In conclusion, we demonstrate that β₂P273 of the M2-M3 linker is a crucial element of the ECD-TMD interface regulating the receptor's ability to undergo late gating transitions. Henceforth, this region could be an important target for new pharmacological tools affecting GABA_AR-mediated inhibition.

Neurosteroid replacement therapy for catamenial epilepsy, postpartum depression and neuroendocrine disorders in women

Neurosteroids are involved in the pathophysiology of many neuroendocrine disorders in women. This review describes recent advancements in pharmacology of neurosteroids and emphasizes the benefits of neurosteroid replacement therapy for the management of neuroendocrine disorders such as catamenial epilepsy (CE), postpartum depression (PPD) and premenstrual brain conditions. Neurosteroids are endogenous modulators of neuronal excitability. A variety of neurosteroids are present in the brain including allopregnanolone (AP), allotetrahydro-deoxycorticosterone and androstanediol. Neurosteroids interact with synaptic and extrasynaptic GABA_A receptors in the brain. AP and related neurosteroids, which are positive allosteric modulators of GABA_A receptors, are powerful anticonvulsants, anxiolytic, antistress and neuroprotectant agents. In CE, seizures are most often clustered around a specific menstrual period in women. Neurosteroid withdrawal-linked plasticity in extrasynaptic receptors has been shown to play a key role in catamenial seizures, anxiety and other mood disorders. Based on our extensive research spanning two decades, we have proposed and championed neurosteroid replacement therapy as a rational strategy for treating disorders marked by neurosteroid-deficiency, such as CE and other related ovarian or menstrual disorders. In 2019, AP (renamed as brexanolone) was approved for treating PPD. A variety of synthetic neurosteroids are in clinical trials for epilepsy, depression and other brain disorders. Recent advancements in our understanding of neurosteroids have entered a new era of drug discovery and one that offers a high therapeutic potential for treating complex brain disorders.

The Function and Regulation of Zinc in the Brain

Nearly sixty years ago Fredrich Timm developed a histochemical technique that revealed a rich reserve of free zinc in distinct regions of the brain. Subsequent electron microscopy studies in Timm- stained brain tissue found that this "labile" pool of cellular zinc was highly concentrated at synaptic boutons, hinting a possible role for the metal in synaptic transmission. Although evidence for activity-dependent synaptic release of zinc would not be reported for another twenty years, these initial findings spurred decades of research into zinc's role in neuronal function and revealed a diverse array of signaling cascades triggered or regulated by the metal. Here, we delve into our current understanding of the many roles zinc plays in the brain, from influencing neurotransmission and sensory processing, to activating both pro-survival and pro-death neuronal signaling pathways. Moreover, we detail the many mechanisms that tightly regulate cellular zinc levels, including metal binding proteins and a large array of zinc transporters.

Endogenous antagonists of N-methyl-d-aspartate receptor in schizophrenia

Schizophrenia is a chronic neuropsychiatric brain disorder that has devastating personal impact and rising healthcare costs. Dysregulation of glutamatergic neurotransmission has been implicated in the pathobiology of the disease, attributed largely to the hypofunction of the N-methyl-d-aspartate (NMDA) receptor. Currently, there is a major gap in mechanistic analysis as to how endogenous modulators of the NMDA receptors contribute to the onset and progression of the disease. We present a systematic review of the neurobiology and the role of endogenous NMDA receptor antagonists in animal models of schizophrenia, and in patients. We discuss their neurochemical origin, release from neurons and glia with action mechanisms, and functional effects, which might contribute toward the impairment of neuronal processes underlying this complex pathological state. We consider clinical evidence suggesting dysregulations of endogenous NMDA receptor in schizophrenia, and highlight the pressing need in future studies and emerging directions, to restore the NMDA receptor functions for therapeutic benefits.

GABAA Receptor β2E155 Residue Located at the Agonist-Binding Site Is Involved in the Receptor Gating

GABA_A receptors (GABA_ARs) play a crucial role in mediating inhibition in the adult brain. In spite of progress in describing (mainly) the static structures of this receptor, the molecular mechanisms underlying its activation remain unclear. It is known that in the α₁β₂γ_2L receptors, the mutation of the β₂E155 residue, at the orthosteric binding site, strongly impairs the receptor activation, but the molecular and kinetic mechanisms of this effect remain elusive. Herein, we investigated the impact of the β₂E155C mutation on binding and gating of the α₁β₂γ_2L receptor. To this end, we combined the macroscopic and single-channel analysis, the use of different agonists [GABA and muscimol (MSC)] and flurazepam (FLU) as a modulator. As expected, the β₂E155C mutation caused a vast right shift of the dose–response (for GABA and MSC) and, additionally, dramatic changes in the time course of current responses, indicative of alterations in gating. Mutated receptors showed reduced maximum open probability and enhanced receptor spontaneous activity. Model simulations for macroscopic currents revealed that the primary effect of the mutation was the downregulation of the preactivation (flipping) rate. Experiments with MSC and FLU further confirmed a reduction in the preactivation rate. Our single-channel analysis revealed the mutation impact mainly on the second component in the shut times distributions. Based on model simulations, this finding further confirms that this mutation affects mostly the preactivation transition, supporting thus the macroscopic data. Altogether, we provide new evidence that the β₂E155 residue is involved in both binding and gating (primarily preactivation).

Localization of Free and Bound Metal Species through X-Ray Synchrotron Fluorescence Microscopy in the Rodent Brain and Their Relation to Behavior

Biometals in the brain, such as zinc, copper, and iron, are often discussed in cases of neurological disorders; however, these metals also have important regulatory functions and mediate cell signaling and plasticity. With the use of synchrotron X-ray fluorescence, our lab localized total, both bound and free, levels of zinc, copper, and iron in a cross section of one hemisphere of a rat brain, which also showed differing metal distributions in different regions within the hippocampus, the site in the brain known to be crucial for certain types of memory. This review discusses the several roles of these metals in brain regions with an emphasis on hippocampal cell signaling, based on spatial mapping obtained from X-ray fluorescence microscopy. We also discuss the localization of these metals and emphasize different cell types and receptors in regions with metal accumulation, as well as the potential relationship between this physiology and behavior.

Zinc reduces antiseizure activity of neurosteroids by selective blockade of extrasynaptic GABA-A receptor-mediated tonic inhibition in the hippocampus

Zinc is an abundant trace metal in the hippocampus nerve terminals. Previous studies demonstrate the ability of zinc to selectively block neurosteroid-sensitive, extrasynaptic GABA-A receptors in the hippocampus (Carver et al, 2016). Here we report that zinc prevents the seizure protective effects of the synthetic neurosteroid ganaxolone (GX) in an experimental model of epilepsy. GABA-gated and tonic currents were recorded from dissociated dentate gyrus granule cells (DGGCs), CA1 pyramidal cells (CA1PCs), and hippocampal slices from adult mice. Antiseizure effects of GX and the reversal of these effects by zinc were evaluated in fully-kindled mice expressing generalized (stage 5) seizures. In electrophysiological studies, zinc blocked the GABA-evoked and GX-potentiated GABA-gated chloride currents in DGGCs and CA1PCs in a concentration-dependent fashion similar to the competitive GABA-A receptor antagonists bicuculline and gabazine. Zinc completely blocked GX potentiation of extrasynaptic tonic currents, but not synaptic phasic currents. In hippocampus kindling studies, systemic administration of GX produced a dose-dependent suppression of behavioral and electrographic seizures in fully-kindled mice with complete seizure protection at the 10 mg/kg dose. However, the antiseizure effects of GX were significantly prevented by intrahippocampal administration of zinc (ED₅₀, 150 μM). The zinc antagonistic response was reversible as animals responded normally to GX administration 24 h post-zinc blockade. These results demonstrate that zinc reduces the antiseizure effects of GX by selectively blocking extrasynaptic δGABA-A receptors in the hippocampus. These pharmacodynamic interactions have clinical implications in neurosteroid therapy for brain conditions associated with zinc fluctuations.

Distinct Modulation of Spontaneous and GABA-Evoked Gating by Flurazepam Shapes Cross-Talk Between Agonist-Free and Liganded GABAA Receptor Activity

GABA_A receptors (GABA_ARs) play a crucial inhibitory role in the CNS. Benzodiazepines (BDZs) are positive modulators of specific subtypes of GABA_ARs, but the underlying mechanism remains obscure. Early studies demonstrated the major impact of BDZs on binding and more recent investigations indicated gating, but it is unclear which transitions are affected. Moreover, the upregulation of GABA_AR spontaneous activity by BDZs indicates their impact on receptor gating but the underlying mechanisms remain unknown. Herein, we investigated the effect of a BDZ (flurazepam) on the spontaneous and GABA-induced activity for wild-type (WT, α₁β₂γ₂) and mutated (at the orthosteric binding site α₁F64) GABA_ARs. Surprisingly, in spite of the localization at the binding site, these mutations increased the spontaneous activity. Flurazepam (FLU) upregulated this activity for mutants and WT receptors to a similar extent by affecting opening/closing transitions. Spontaneous activity affected GABA-evoked currents and is manifested as an overshoot after agonist removal that depended on the modulation by BDZs. We explain the mechanism of this phenomenon as a cross-desensitization of ligand-activated and spontaneously active receptors. Moreover, due to spontaneous activity, FLU-pretreatment and co-application (agonist + FLU) protocols yielded distinct results. We provide also the first evidence that GABA_AR may enter the desensitized state in the absence of GABA in a FLU-dependent manner. Based on our data and model simulations, we propose that FLU affects agonist-induced gating by modifying primarily preactivation and desensitization. We conclude that the mechanisms of modulation of spontaneous and ligand-activated GABA_AR activity concerns gating but distinct transitions are affected in spontaneous and agonist-evoked activity.

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

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

Menthol enhances phasic and tonic GABAA receptor-mediated currents in midbrain periaqueductal grey neurons

BACKGROUND AND PURPOSE

Menthol, a naturally occurring compound in the essential oil of mint leaves, is used for its medicinal, sensory and fragrant properties. Menthol acts via transient receptor potential (TRPM8 and TRPA1) channels and as a positive allosteric modulator of recombinant GABAA receptors. Here, we examined the actions of menthol on GABAA receptor-mediated currents in intact midbrain slices.

EXPERIMENTAL APPROACH

Whole-cell voltage-clamp recordings were made from periaqueductal grey (PAG) neurons in midbrain slices from rats to determine the effects of menthol on GABAA receptor-mediated phasic IPSCs and tonic currents.

KEY RESULTS

Menthol (150-750 μM) produced a concentration-dependent prolongation of spontaneous GABAA receptor-mediated IPSCs, but not non-NMDA receptor-mediated EPSCs throughout the PAG. Menthol actions were unaffected by TRPM8 and TRPA1 antagonists, tetrodotoxin and the benzodiazepine antagonist, flumazenil. Menthol also enhanced a tonic current, which was sensitive to the GABAA receptor antagonists, picrotoxin (100 μM), bicuculline (30 μM) and Zn(2+) (100 μM), but unaffected by gabazine (10 μM) and a GABAC receptor antagonist, 1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid hydrate (TPMPA; 50 μM). In addition, menthol potentiated currents induced by the extrasynaptic GABAA receptor agonist THIP/gaboxadol (10 μM).

CONCLUSIONS AND IMPLICATIONS

These results suggest that menthol positively modulates both synaptic and extrasynaptic populations of GABAA receptors in native PAG neurons. The development of agents that potentiate GABAA -mediated tonic currents and phasic IPSCs in a manner similar to menthol could provide a basis for novel GABAA -related pharmacotherapies.

Abnormalities in the zinc-metalloprotease-BDNF axis may contribute to megalencephaly and cortical hyperconnectivity in young autism spectrum disorder patients

Whereas aberrant brain connectivity is likely the core pathology of autism-spectrum disorder (ASD), studies do not agree as to whether hypo- or hyper-connectivity is the main underlying problem. Recent functional imaging studies have shown that, in most young ASD patients, cerebral cortical regions appear hyperconnected, and cortical thickness/brain size is increased. Collectively, these findings indicate that developing ASD brains may exist in an altered neurotrophic milieu. Consistently, some ASD patients, as well as some animal models of ASD, show increased levels of brain-derived neurotrophic factor (BDNF). However, how BDNF is upregulated in ASD is unknown. To address this question, we propose the novel hypothesis that a putative zinc-metalloprotease-BDNF (ZMB) axis in the forebrain plays a pivotal role in the development of hyperconnectivity and megalencephaly in ASD. We have previously demonstrated that extracellular zinc at micromolar concentrations can rapidly increase BDNF levels and phosphorylate the receptor tyrosine kinase TrkB via the activation of metalloproteases. The role of metalloproteases in ASD is still uncertain, but in fragile X syndrome, a monogenic disease with an autistic phenotype, the levels of MMP are increased. Early exposure to lipopolysaccharides (LPS) and other MMP activators such as organic mercurials also have been implicated in ASD pathogenesis. The resultant increases in BDNF levels at synapses, especially those involved in the zinc-containing, associative glutamatergic system may produce abnormal brain circuit development. Various genetic mutations that lead to ASD are also known to affect BDNF signaling: some down-regulate, and others up-regulate it. We hypothesize that, although both up- and down-regulation of BDNF may induce autism symptoms, only BDNF up-regulation is associated with the hyperconnectivity and large brain size observed in most young idiopathic ASD patients. To test this hypothesis, we propose to examine the ZMB axis in animal models of ASD. Synaptic zinc can be examined by fluorescence zinc staining. MMP activation can be measured by in situ zymography and Western blot analysis. Finally, regional levels of BDNF can be measured. Validating this hypothesis may shed light on the central pathogenic mechanism of ASD and aid in the identification of useful biomarkers and the development of preventive/therapeutic strategies.

Regulation of output spike patterns by phasic inhibition in cerebellar granule cells

The complex interplay of multiple molecular mechanisms taking part to synaptic integration is hard to disentangle experimentally. Therefore, we developed a biologically realistic computational model based on the rich set of data characterizing the cerebellar glomerulus microcircuit. A specific issue was to determine the relative role of phasic and tonic inhibition in dynamically regulating granule cell firing, which has not been clarified yet. The model comprised the excitatory mossy fiber—granule cell and the inhibitory Golgi cell—granule cell synapses and accounted for vesicular release processes, neurotransmitter diffusion and activation of different receptor subtypes. Phasic inhibition was based on stochastic GABA release and spillover causing activation of two major classes of postsynaptic receptors, α1 and α6, while tonic inhibition was based on steady regulation of a Cl⁻ leakage. The glomerular microcircuit model was validated against experimental responses to mossy fiber bursts while metabotropic receptors were blocked. Simulations showed that phasic inhibition controlled the number of spikes during burst transmission but predicted that it specifically controlled time-related parameters (firing initiation and conclusion and first spike precision) when the relative phase of excitation and inhibition was changed. In all conditions, the overall impact of α6 was larger than that of α1 subunit-containing receptors. However, α1 receptors controlled granule cell responses in a narrow ±10 ms band while α6 receptors showed broader ±50 ms tuning. Tonic inhibition biased these effects without changing their nature substantially. These simulations imply that phasic inhibitory mechanisms can dynamically regulate output spike patterns, as well as calcium influx and NMDA currents, at the mossy fiber—granule cell relay of cerebellum without the intervention of tonic inhibition.

Endogenous zinc depresses GABAergic transmission via T-type Ca(2+) channels and broadens the time window for integration of glutamatergic inputs in dentate granule cells

Abstract Zinc actions on synaptic transmission span the modulation of neurotransmitter receptors, transporters, activation of intracellular cascades and alterations in gene expression. Whether and how zinc affects inhibitory synaptic signalling in the dentate gyrus remains largely unexplored. We found that mono- and di-synaptic GABAergic inputs onto dentate granule cells were reversibly depressed by exogenous zinc application and enhanced by zinc chelation. Blocking T-type Ca²⁺ channels prevented the effect of zinc chelation. When recording from dentate fast-spiking interneurones, zinc chelation facilitated T-type Ca²⁺ currents, increased action potential half-width and decreased spike threshold. It also increased the offset of the input–output relation in a manner consistent with enhanced excitability. In granule cells, chelation of zinc reduced the time window for the integration of glutamatergic inputs originating from perforant path synapses, resulting in reduced spike transfer. Thus, zinc-mediated modulation of dentate interneurone excitability and GABA release regulates information flow to local targets and hippocampal networks.

Inhibitory effects of oenanthotoxin analogues on GABAergic currents in cultured rat hippocampal neurons depend on the polyacetylenes' polarity

Oenanthotoxin (OETX) and dihydro-OETX are polyacetylenic diols occurring in Oenanthe crocata and are known to exert proconvulsant effects. We have recently demonstrated that these compounds downregulated GABAergic currents (Appendino et al., 2009) and that OETX induced open channel block and allosterically modulated GABA(A) receptors (Wyrembek et al., 2010). O. crocata also contains several minor OETX analogues and in the present study we tested whether their effect on GABA(A) receptors depends on the compounds' polarity. We investigated a series of five polyacetylenes characterized by a higher lipophylicity than OETX, (1-acetyl-2,3-dihydrooenanthotoxin - X1, 14-acetyloenanthotoxin-X2, 1-deoxyoenanthotoxin - X3, 14-deoxyoenanthotoxin - X4, 14-dehydro-1-deoxyOETX - X5, polarity sequence: X1>X2>X3>X4>X5). Their effects were tested first on miniature inhibitory postsynaptic currents (mIPSCs). All but X3, significantly decreased the mIPSC amplitudes while X1, X2, X4 decreased, and X3 and X5 increased the mIPSC frequency. The lack of a clear correlation between the compounds' polarity and their effect on mIPSCs might result from their presynaptic effects. We thus considered their impact on current responses to exogenous GABA applications. Amplitude reduction of current responses was most prominent for X1 and virtually absent for X5 indicating a dependence on the compound's polarity. Only X1 and X2 showed open channel block, while the kinetics of currents were affected only by X1 which further supports a dependence of the drug's effects on their polarity. In conclusion, GABA(A) receptors are inhibited and allosterically modulated by naturally occurring OETX analogues (except X5) and these effects are positively correlated with the compounds' polarity.

Metal toxicity at the synapse: presynaptic, postsynaptic, and long-term effects

Metal neurotoxicity is a global health concern. This paper summarizes the evidence for metal interactions with synaptic transmission and synaptic plasticity. Presynaptically metal ions modulate neurotransmitter release through their interaction with synaptic vesicles, ion channels, and the metabolism of neurotransmitters (NT). Many metals (e.g., Pb(2+), Cd(2+), and Hg(+)) also interact with intracellular signaling pathways. Postsynaptically, processes associated with the binding of NT to their receptors, activation of channels, and degradation of NT are altered by metals. Zn(2+), Pb(2+), Cu(2+), Cd(2+), Ni(2+), Co(2+), Li(3+), Hg(+), and methylmercury modulate NMDA, AMPA/kainate, and/or GABA receptors activity. Al(3+), Pb(2+), Cd(2+), and As(2)O(3) also impair synaptic plasticity by targeting molecules such as CaM, PKC, and NOS as well as the transcription machinery involved in the maintenance of synaptic plasticity. The multiple effects of metals might occur simultaneously and are based on the specific metal species, metal concentrations, and the types of neurons involved.

Three arginines in the GABAA receptor binding pocket have distinct roles in the formation and stability of agonist- versus antagonist-bound complexes

Binding of the agonist GABA to the GABA(A) receptor causes channel gating, whereas competitive antagonists that bind at the same site do not. The details of ligand binding are not well understood, including which residues interact directly with ligands, maintain the structure of the binding pocket, or transduce the action of binding into opening of the ion channel gate. Recent work suggests that the amine group of the GABA molecule may form a cation-π bond with residues in a highly conserved "aromatic box" within the binding pocket. Although interactions with the carboxyl group of GABA remain unknown, three positively charged arginines (α(1)Arg67, α(1)Arg132, and β(2)Arg207) just outside of the aromatic box are likely candidates. To explore their roles in ligand binding, we individually mutated these arginines to alanine and measured the effects on microscopic ligand binding/unbinding rates and channel gating. The mutations α(1)R67A or β(2)R207A slowed agonist binding and sped unbinding with little effect on gating, demonstrating that these arginines are critical for both formation and stability of the agonist-bound complex. In addition, α(1)R67A sped binding of the antagonist 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide (SR-95531), indicating that this arginine poses a barrier to formation of the antagonist-bound complex. In contrast, β(2)R207A and α(1)R132A sped antagonist unbinding, indicating that these arginines stabilize the antagonist-bound state. α(1)R132A also conferred a new long-lived open state, indicating that this arginine influences the channel gate. Thus, each of these arginines plays a unique role in determining interactions with agonists versus antagonists and with the channel gate.

PCB-47, PBDE-47, and 6-OH-PBDE-47 differentially modulate human GABAA and alpha4beta2 nicotinic acetylcholine receptors

Polychlorinated biphenyls (PCBs) and the structurally related polybrominated diphenyl ethers (PBDEs) are abundant persistent organic pollutants that exert several comparable neurotoxic effects. Importantly, hydroxylated metabolites of PCBs and PBDEs have an increased neurotoxic potency. Recently, we demonstrated that PCBs can act as (partial) agonist on GABA(A) neurotransmitter receptors, with PCB-47 being the most potent congener. It is, however, unknown whether PBDE-47 and its metabolite 6-OH-PBDE-47 exert similar effects and if these effects are limited to GABA(A) receptors only. We therefore investigated effects of PCB-47, PBDE-47, and 6-OH-PBDE-47 on the inhibitory GABA(A) and excitatory α(4)β(2) nicotinic acetylcholine (nACh) receptor expressed in Xenopus oocytes using the two-electrode voltage-clamp technique. Since human exposure is generally not limited to individual compounds, experiments with binary mixtures were also performed. The results demonstrate that PCB-47 and 6-OH-PBDE-47 act as full and partial agonist on the GABA(A) receptor. However, both congeners act as antagonist on the nACh receptor. PBDE-47 does not affect either type of receptor. Binary mixtures of PCB-47 and 6-OH-PBDE-47 induced an additive activation as well as potentiation of GABA(A) receptors, whereas this mixture resulted in an additive inhibition of nACh receptors. Binary mixtures of PBDE-47 and 6-OH-PBDE-47 yielded similar effects as 6-OH-PBDE-47 alone. These findings demonstrate that GABA(A) and nACh receptors are affected differently by PCB-47 and 6-OH-PBDE-47, with inhibitory GABA(A)-mediated signaling being potentiated and excitatory α(4)β(2) nACh-mediated signaling being inhibited. Considering these opposite actions and the additive interaction of the congeners, these effects are likely to be augmented in vivo.

Alterations in fear response and spatial memory in pre- and post-natal zinc supplemented rats: remediation by copper

The role of zinc in the nervous system is receiving increased attention. At a time when dietary fortification and supplementation have increased the amount of zinc being consumed, little work has been done on the effects of enhanced zinc on behavior. Both zinc and copper are essential trace minerals that are acquired from the diet; under normal conditions the body protects against zinc overload, but at excessive dosages, copper deficiency has been seen. In order to examine the effect of enhanced metal administration on learning and memory, Sprague Dawley rats were given water supplemented with 10ppm Zn, 10ppm Zn+0.25ppm Cu, or normal lab water, during pre- and post-natal development. Fear conditioning tests at 4months showed significantly higher freezing rates during contextual retention and extinction and cued extinction for rats drinking water supplemented with zinc, suggesting increased anxiety compared to controls raised on lab water. During the MWM task at 9months, zinc-enhanced rats had significantly longer latencies to reach the platform compared to controls. The addition of copper to the zinc supplemented water brought freezing and latency levels closer to that of controls. These data demonstrate the importance of maintaining appropriate intake of both metals simultaneously, and show that long-term supplementation with zinc may cause alterations in memory.

Determining the neurotransmitter concentration profile at active synapses

Establishing the temporal and concentration profiles of neurotransmitters during synaptic release is an essential step towards understanding the basic properties of inter-neuronal communication in the central nervous system. A variety of ingenious attempts has been made to gain insights into this process, but the general inaccessibility of central synapses, intrinsic limitations of the techniques used, and natural variety of different synaptic environments have hindered a comprehensive description of this fundamental phenomenon. Here, we describe a number of experimental and theoretical findings that has been instrumental for advancing our knowledge of various features of neurotransmitter release, as well as newly developed tools that could overcome some limits of traditional pharmacological approaches and bring new impetus to the description of the complex mechanisms of synaptic transmission.

New insights on the role of gephyrin in regulating both phasic and tonic GABAergic inhibition in rat hippocampal neurons in culture

Gephyrin is a tubulin-binding protein that acts as a scaffold for clustering glycine and GABA(A) receptors at postsynaptic sites. In this study, the role of gephyrin on GABA(A) receptor function was assessed at the post-translational level, using gephyrin-specific single chain antibody fragments (scFv-gephyrin). When expressed in cultured rat hippocampal neurons as a fusion protein containing a nuclear localization signal, scFv-gephyrin were able to remove endogenous gephyrin from GABA(A) receptor clusters. Immunocytochemical experiments revealed a significant reduction in the number of synaptic gamma2-subunit containing GABA(A) receptors and a significant decrease in the density of the GABAergic presynaptic marker vesicular GABA transporter (VGAT). These effects were associated with a slow down of the onset kinetics, a reduction in the amplitude and in the frequency of miniature inhibitory postsynaptic currents (mIPSCs). The quantitative analysis of current responses to ultrafast application of GABA suggested that changes in onset kinetics resulted from modifications in the microscopic gating of GABA(A) receptors and in particular from a reduced entry into the desensitized state. In addition, hampering gephyrin function with scFv-gephyrin induced a significant reduction in GABA(A) receptor-mediated tonic conductance. This effect was probably dependent on the decrease in GABAergic innervation and in GABA release from presynaptic nerve terminals. These results indicate that gephyrin is essential not only for maintaining synaptic GABA(A) receptor clusters in the right position but also for regulating both phasic and tonic inhibition.

Electrophysiological description of mechanisms determining synaptic transmission and its modulation

Signal integration in neurons is a complex process that depends on e.g. the kinetics of synaptic currents, distribution of synaptic connections as well as passive and excitatory membrane properties. The time course of synaptic currents is largely determined by the kinetics of the postsynaptic receptors and the time course of synaptic neurotransmitter concentration. The analysis of current responses to rapid agonist applications provides the means to study the ligand-gated receptor gating but experimentally based estimation of neurotransmitter transient at central synapses was an important challenge during the last decade. Both theoretical as well as experimentally based approaches indicated that synaptic agonist transient is very brief, implying that the activation of postsynaptic receptors occurs in conditions of extreme non-equilibrium. Such a dynamic pattern of activation of postsynaptic receptors has a crucial impact not only on the kinetics of synaptic currents but also on their susceptibility to pharmacological modulation.

Zinc modulates GABAergic neurotransmission in rat globus pallidus

The globus pallidus plays a critical role in the regulation of movement, and abnormal activity of its neurons is associated with some basal ganglia motor diseases. A relatively high level of zinc has been reported in the globus pallidus, which is increased significantly after 6-OHDA treatments. To elucidate the action of zinc on GABAergic neurotransmission in the globus pallidus, whole-cell patch-clamp recordings were made from rat globus pallidus neurons. Superfusion of zinc significantly reduced both spontaneous and miniature inhibitory postsynaptic currents. The inhibition was selective to the amplitude with no change in the frequency, decay time and rise time. Furthermore, the reduction of spontaneous inhibitory postsynaptic currents (34.1+/-4.0%) was stronger than that of miniature inhibitory postsynaptic currents (19.7+/-3.2%). These results suggest that spontaneous inhibitory postsynaptic currents generated mainly by axonal collaterals and miniature inhibitory postsynaptic currents generated mainly by striatopallidal inputs may be mediated by different GABA(A) receptor combinations.

Effect of extracellular pH on recombinant alpha1beta2gamma2 and alpha1beta2 GABAA receptors

Recently, we have reported that extracellular protons allosterically modulated neuronal GABA(A) receptors [Mozrzymas, J.W., Zarnowska, E.D., Pytel, M., Mercik, K., 2003a. Modulation of GABA(A) receptors by hydrogen ions reveals synaptic GABA transient and a crucial role of desensitiztion process. Journal of Neuroscience 23, 7981-7992]. However, GABAARs in neurons are heterogeneous and the effect of hydrogen ions depends on the receptor subtype. In particular, gamma2 subunit sets the receptor sensibility to several modulators including protons. However, the mechanisms whereby protons modulate gamma2-containing and gamma2-free GABAARs have not been fully elucidated. To this end, current responses to ultrafast GABA applications were recorded for alpha1beta2gamma2 and alpha1beta2 receptors at different pH values. For both receptor types, increase in pH induced a decrease in amplitudes of currents elicited by saturating [GABA] but this effect was stronger for alpha1beta2 receptors. In the case of alpha1beta2gamma2 receptors, protons strongly affected the current time course due to a down regulation of binding and desensitization rates. This effect was qualitatively similar to that described in neurons. Protons strongly influenced the amplitude of alpha1beta2 receptor-mediated currents but the effect on their kinetics was weak suggesting a predominant direct non-competitive inhibition with a minor allosteric modulation. In conclusion, we provide evidence that extracellular protons strongly affect GABAA receptors and that, depending on the presence of the gamma2 subunit, the modulatory mechanisms show profound quantitative and qualitative differences.

GABAergic miniature postsynaptic currents in septal neurons show differential allosteric sensitivity after binge-like ethanol exposure

Binge-like ethanol treatment of septal neurons blunts GABAAR-mediated miniature postsynaptic currents (mPSCs), suggesting it arrests synaptic development. Ethanol may disrupt postsynaptic maturation by blunting feedback signaling through immature GABAARs. Here, the impact of ethanol on the sensitivity of mPSCs to zolpidem, zinc and 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha-OH-DHP) was tested. The decay phase of mPSCs showed concentration-dependent potentiation by zolpidem (0.03-100 microM), which was substantially blunted after ethanol exposure. Since zolpidem potentiation exhibited a substantial age-dependent increase in untreated neurons, this finding supported the idea that ethanol arrests synaptic development. GABAAR alpha1 subunit protein also increased with age in untreated neurons, paralleling enhanced sensitivity to zolpidem. Surprisingly, alpha1 levels were not reduced by binge ethanol even though mPSCs were relatively zolpidem-insensitive. Zinc (3-30 microM) decreased mPSC parameters in a concentration- and age-related manner with older untreated cells showing less inhibition. However, there was no increase in mPSC zinc sensitivity after binge ethanol as would be expected if a general arrest of synaptic maturation had occurred. 3alpha-OH-DHP (3-1000 nM) induced concentration-dependent potentiation of mPSC decay. Although potentiation was age-independent, binge ethanol treatment exaggerated sensitivity to this neurosteroid. Finally, chronic picrotoxin pretreatment (100 microM) intended to mimic GABAAR inhibition from ethanol pretreatment did not significantly change mPSC modulation by zolpidem, zinc or 3alpha-OH-DHP. These results suggest that binge ethanol treatment selectively arrests a subset of processes important for maturation of postsynaptic GABAA Rs. However, it is unlikely that ethanol causes a broad arrest of postsynaptic development through a direct inhibition of GABAAR signaling.

Functional characterization of Zn2(+)-sensitive GABA transporter expressed in primary cultures of astrocytes from rat cerebral cortex

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity Na(+)/Cl(-) dependent transporters (GATs). GAT1 mainly expressed in cerebrocortical neurons is thought to play an important role for clearance of GABA in the extracellular fluid, whereas there is a little information available for pharmacological importance for astrocytic GABA transporters. In the present study, we therefore described the functional characterization of GABA transport in primary cultures of astrocytes from rat cerebral cortex and the identification of GABA transporter subtype(s). GABA transport was Na(+) and Cl(-) dependent and saturable with a Michaelis constant (K(t)) of 9.3+/-2.8 microM. Na(+)- and Cl(-)- activation kinetics revealed that the Na(+)-Cl(-)-to-GABA stoichiometry was 2:1:1 and concentrations of Na(+) and Cl(-) necessary for half-maximal transport (K(0.5)(Na) and K(0.5)(Cl)) were 78+/-28 mM and 9.6+/-2.6 mM, respectively. Na(+)-dependent GABA transport was competitively inhibited by various GABA transport inhibitors, especially GAT2- or GAT3-selective inhibitor. In addition, Zn(2+), which has been reported to be a potent inhibitor of GAT3, was found to have a significantly but partially inhibitory effect on the Na(+)-dependent GABA transport in a concentration-dependent manner. Furthermore, reverse transcription-PCR and Western blot analyses revealed that GAT2 and GAT3 are expressed in primary cultures of astrocytes. These results clearly showed that zinc is a useful reagent for separating GAT3 activity from GAT1- and GAT2-activities in CNS. To our knowledge, the present study represents the first report on the inhibitory effect of zinc on the Na(+)-dependent GABA transport in rat cerebrocortical astrocytes.

Membrane voltage modulates the GABAA receptor gating in cultured rat hippocampal neurons

The kinetics of GABAergic currents in neurons is known to be modulated by the membrane voltage but the underlying mechanisms have not been fully explored. In particular, the impact of membrane potential on the GABA(A) receptor gating has not been elucidated. In the present study, the effect of membrane voltage on current responses elicited by ultrafast GABA applications was studied in cultured hippocampal neurons. The current to voltage relationship (I-V) for responses to saturating [GABA] (10 mM) showed an inward rectification (slope conductance at positive voltages was 0.62 +/- 0.05 of that at negative potentials). On the contrary, I-V for currents evoked by low [GABA] (1 microM) showed an outward rectification. The onset of currents elicited by saturating [GABA] was significantly accelerated at positive potentials. Analysis of currents evoked by prolonged applications of saturating [GABA] revealed that positive voltages significantly increased the rate and extent of desensitization. The onsets of current responses to non-saturating [GABA] were significantly accelerated at positive voltages indicating an enhancement of the binding rate. However, at low [GABA] at which the onset rate is expected to approach an asymptote set by opening/closing and unbinding rates, no significant modification of current onset by voltage was observed. Quantitative analysis based on model simulations indicated that the major effect of membrane depolarization was to increase the rates of binding, desensitization and of opening as well as to slightly reduce the rate of exit from desensitization. In conclusion, we provide evidence that membrane voltage affects the GABA(A) receptor microscopic gating.

Analysis of macroscopic ionic currents mediated by GABArho1 receptors during lanthanide modulation predicts novel states controlling channel gating

Lanthanide-induced modulation of GABA(C) receptors expressed in Xenopus oocytes was studied. We obtained two-electrode voltage-clamp recordings of ionic currents mediated by recombinant homomeric GABArho(1) receptors and performed numerical simulations of kinetic models of the macroscopic ionic currents.GABA-evoked chloride currents were potentiated by La(3+), Lu(3+) and Gd(3+) in the micromolar range. Lanthanide effects were rapid, reversible and voltage independent. The degree of potentiation was reduced by increasing GABA concentration.Lu(3+) also induced receptor desensitization and decreased the deactivation rate of GABArho(1) currents. In the presence of 300 microM Lu(3+), dose-response curves for GABA-evoked currents showed a significant enhancement of the maximum amplitude and an increase of the apparent affinity. The rate of onset of TPMPA and picrotoxin antagonism of GABArho(1) receptors was modulated by Lu(3+). These results suggest that the potentiation of the anionic current was the result of a direct lanthanide-receptor interaction at a site capable of allosterically modulating channel properties. Based on kinetic schemes, which included a second open state and a nonconducting desensitized state that closely reproduced the experimental results, two nonexclusive probable models of GABArho(1) channels gating are proposed.

Zinc inhibition of gamma-aminobutyric acid transporter 4 (GAT4) reveals a link between excitatory and inhibitory neurotransmission

gamma-Aminobutyric acid (GABA) transporters (GATs) play an important role in inhibitory neurotransmission by clearing synaptically released GABA and by maintaining low resting levels of GABA in synaptic and extrasynaptic regions. In certain brain regions, vesicular zinc is colocalized and coreleased with glutamate and modulates the behavior of a number of channels, receptors, and transporters. We examined the effect of zinc on expressed GATs (GAT1, GAT2, GAT3, and GAT4) in Xenopus laevis oocytes by using tracer flux and electrophysiological methods. We show that zinc is a potent inhibitor of GAT4 (K(i) of 3 muM). Immunolocalization of GAT4 in the hippocampus revealed dense localization in the CA1 and CA3 regions of the hippocampus, regions which are known to be heavily populated by zinc-containing glutamatergic neurons. The results suggest a physiological role of synaptically released zinc in the hippocampus, because zinc released from hyperactive glutamatergic neurons may simultaneously bring about elevated GABAergic inhibition. Therefore, this mode of zinc function signifies a link between excitatory and inhibitory neurotransmission and may play a neuroprotective role against glutamate-induced excitotoxicity.

Zinc ions are endogenous modulators of neurotransmitter-stimulated capacitative Ca2+entry in both cultured andin situmouse astrocytes

Astrocytes express a variety of metabotropic receptors and their activation leads to a biphasic Ca2+ response due to Ca2+ release from intracellular stores and subsequent capacitative Ca2+ entry. We performed Ca2+ imaging with Fura-2 on cultured mouse astrocytes and showed that extracellular zinc reversibly blocks the capacitative Ca2+ entry following application of the metabotropic ligands ATP, glutamate and endothelin-1. Zinc blocked the plateau phase of the ligand-triggered Ca2+ responses. When ligands were repetitively applied in the presence of zinc the calcium responses progressively decayed and even disappeared, indicating that capacitative Ca2+ entry is required to refill the stores. Zinc inhibited the capacitative Ca2+ entry with a K(i) of approximately 6 microM, which is well within the physiological concentration range of zinc found in the brain. Application of the reducing agent DTT prevented the blocking effect by zinc ions but not the inhibition elicited by the nonphysiological metal ions Gd3+ and La3+, indicating that zinc has a distinct binding site. To monitor the capacitative Ca2+ entry in astrocytes in situ and to determine the effect of zinc on this pathway we utilized X-rhod-1 imaging in hippocampal slices of a transgenic mouse line with green fluorescent astrocytes. Zinc affected the repetitive metabotropic Ca2+ response in the following fashion: (i) after depleting stores in Ca(2+)-free solution, re-addition of Ca2+ led to an influx of Ca2+ via a zinc-sensitive Ca2+ entry route; (ii) with repetitive application of metabotropic ligands, Ca2+ responses became smaller and even disappeared in the presence of zinc. We conclude that zinc, which is co-released from glutamatergic synaptic vesicles upon neuronal activity, has a major impact on shaping the astrocytic calcium responses.

Developmental Regulation of β-Carboline-Induced Inhibition of Glycine-Evoked Responses Depends on Glycine Receptor β Subunit Expression

In this work, we show that beta-carbolines, which are known negative allosteric modulators of GABA(A) receptors, inhibit glycine-induced currents of embryonic mouse spinal cord and hippocampal neurons. In both cell types, beta-carboline-induced inhibition of glycine receptor (GlyR)-mediated responses decreases with time in culture. Single-channel recordings show that the major conductance levels of GlyR unitary currents shifts from high levels (> or = 50 pS) in 2 to 3 days in vitro (DIV) neurons to low levels (<50 pS) in 11 to 14 DIV neurons, assessing the replacement of functional homomeric GlyR by heteromeric GlyR. In cultured spinal cord neurons, the disappearance of beta-carboline inhibition of glycine responses and high conductance levels is almost complete in mature neurons, whereas a weaker decrease in beta-carboline-evoked glycine response inhibition and high conductance level proportion is observed in hippocampal neurons. To confirm the hypothesis that the decreased sensitivity of GlyR to beta-carbolines depends on beta subunit expression, Chinese hamster ovary cells were permanently transfected either with GlyR alpha2 subunit alone or in combination with GlyR beta subunit. Single-channel recordings revealed that the major conductance levels shifted from high levels (> or = 50 pS) in GlyR-alpha2-transfected cells to low levels (<50 pS) in GlyR-alpha2+beta-containing cells. Consistently, both picrotoxin- and beta-carboline-induced inhibition of glycine-gated currents were significantly decreased in GlyR-alpha2+beta-transfected cells compared with GlyR-alpha2-containing cells. In summary, we demonstrate that the incorporation of beta subunits in GlyRs confers resistance not only to picrotoxin but also to beta-carboline-induced inhibition. Furthermore, we also provide evidence that hippocampal neurons undergo in vitro a partial maturation process of their GlyR-mediated responses.

Zn2+ ions: modulators of excitatory and inhibitory synaptic activity

The role of Zn(2+) in the CNS has remained enigmatic for several decades. This divalent cation is accumulated by specific neurons into synaptic vesicles and can be released by stimulation in a Ca(2+)-dependent manner. Using Zn(2+) fluorophores, radiolabeled Zn(2+), and selective chelators, the location of this ion and its release pattern have been established across the brain. Given the distribution and possible release under physiological conditions, Zn(2+) has the potential to act as a modulator of both excitatory and inhibitory neurotransmission. Excitatory N-methyl-D-aspartate (NMDA) receptors are directly inhibited by Zn(2+), whereas non-NMDA receptors appear relatively unaffected. In contrast, inhibitory transmission mediated via GABA(A)receptors can be potentiated via a presynaptic mechanism, influencing transmitter release; however, although some tonic GABAergic inhibition may be suppressed by Zn(2+), most synaptic GABA receptors are unlikely to be modulated directly by this cation. In the spinal cord, glycinergic transmission may also be affected by Zn(2+) causing potentiation. Recently, the penetration of synaptically released Zn(2+) into neurons suggests that this ion has the potential to act as a direct transmitter, by affecting postsynaptic signaling pathways. Taken overall, present studies are broadly supportive of a neuromodulatory role for Zn(2+) at specific excitatory and inhibitory synapses.

Developmental Changes of GABA Synaptic Transient in Cerebellar Granule Cells

The time course of synaptic currents is largely determined by the microscopic gating of the postsynaptic receptors and the temporal profile of the synaptic neurotransmitter concentration. Although several lines of evidence indicate that developmental changes of GABAergic synaptic current time course are clearly correlated with a switch in postsynaptic receptors, much less is known about the modification of GABA release during development. To address this issue, we studied the sensitivity of miniature inhibitory postsynaptic currents (mIPSCs) to a quickly dissociating competitive antagonist, 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA), in neurons cultured for 6 to 8 days in vitro (DIV) ("young") and for 12 to 14 DIV ("old"). mIPSCs recorded in young neurons were significantly more resistant to the block by TPMPA. This observation was interpreted as a consequence of a more efficient displacement of TPMPA from GABA(A) receptors caused by a stronger GABA release in young neurons. The change in mIPSC sensitivity to TPMPA during development was not affected by the deletion of alpha(1) subunit, supporting its presynaptic origin. The effects of a second quickly dissociating antagonist, SR-95103 [2-(carboxy-3'-propyl)-3-amino-4-methyl-6-phenylpyridazinium chloride], on young, old, and alpha(1) -/- neurons were qualitatively the same as those obtained with TPMPA. Moreover, the analysis of current responses to ultrafast GABA applications showed that the unbinding rates of TPMPA in DIV 6 to 8 and in DIV 12 to 14 neurons are not significantly different, ruling out the postsynaptic mechanism of differential TPMPA action. Thus, we provide evidence that presynaptic GABA uniquantal release is developmentally regulated.

Presynaptic source of quantal size variability at GABAergic synapses in rat hippocampal neurons in culture

The variability of quantal size depends on both presynaptic (profile of the neurotransmitter concentration in the cleft) and postsynaptic (number and gating properties of postsynaptic receptors) factors. Here we have examined the possibility that at nonsaturated synapses in cultured hippocampal neurons, changes in both the transmitter concentration peak and its clearance from the synaptic cleft may influence the variability of spontaneous miniature synaptic GABAergic currents (mIPSCs). We found that, in contrast to the slow-off GABAA receptor antagonist bicuculline, fast-off competitive antagonists such as SR-95103 and TPMPA differentially blocked small and large mIPSCs. In the presence of flurazepam, a drug believed to increase the affinity of GABA for GABAAR, small mIPSCs were enhanced more efficiently than large events. Moreover, the addition of dextran, which increases the viscosity of the extracellular fluid, preferentially increased small mIPSCs with respect to large ones. These observations suggest that changes in the concentration peak and the speed of GABA clearance in the cleft may be an important source of synaptic variability. The study of the correlation between peak amplitude and kinetics of mIPSCs allowed determination of the relative contribution of transmitter peak concentration vs. time of GABA clearance. Small synaptic responses were associated with fast onset and decay kinetics while large amplitude currents were associated with slow kinetics, indicating a crucial role for GABA synaptic clearance in variability of mIPSCs. By using model simulations we were able to estimate the range of variability of both the concentration and the speed of clearance of the GABA transient in the synaptic cleft.

Dynamism of GABAA receptor activation shapes the “personality” of inhibitory synapses

The kinetics of synaptic currents is largely determined by the postsynaptic receptor gating and the concentration time course of synaptic neurotransmitter. While the analysis of current responses to rapid agonist application provides the means to study the ligand-gated receptor gating, no direct tools are available to measure the neurotransmitter transient at GABAergic and glutamatergic synapses. Several lines of evidence indicate that the synaptic agonist transient is very brief suggesting that the activation of postsynaptic receptors occurs in conditions of extreme non-equilibrium. Such a dynamic pattern of activation has a crucial impact not only on the kinetics of synaptic currents but also on their susceptibility to pharmacological modulation. Thus, changes in the synaptic agonist waveform due to, for example modulation of the release machinery or uptake system may considerably alter both kinetics and pharmacology of synaptic currents. The use of modifiers of GABA(A) receptor gating and low-affinity antagonists provides a tool to estimate the time course of the agonist transient revealing that synaptic neurotransmitter is not saturating and that the agonist clearance occurs at a sub-millisecond time scale. It is proposed that dynamic conditions of synaptic receptor activation assure a broad spectrum of performance rendering the synapse extremely susceptible to a variety of modulatory processes.

Modulation of GABA(A) receptors by hydrogen ions reveals synaptic GABA transient and a crucial role of the desensitization process

Protons are the most ubiquitous and very potent modulators of the biological systems. Hydrogen ions are known to modulate GABA(A) receptors (GABA(A)Rs), but the mechanism whereby these ions affect IPSCs and the gating of GABA(A)Rs is not clear. In the present study we examined the effect of protons on miniature IPSCs (mIPSCs) and found that hydrogen ions strongly affected both their amplitude and time course. To explore the underlying mechanisms with resolution adequate to the time scale of synaptic transmission, we recorded current responses to ultrafast GABA applications at various pH. These experiments revealed that the major effect of protons on GABA(A)R gating is a strong enhancement of desensitization and binding rates at increasing pH. This analysis also indicated that desensitization rate is the fastest ligand-independent transition in the GABA(A)R gating scheme. Although proton effects on the time course of mIPSCs and current responses to saturating [GABA] were similar, the pH dependencies of amplitudes were almost opposite. Our quantitative analysis, based on model simulations, indicated that this difference resulted from a much shorter receptor exposure to agonist in the case of mIPSCs. Modeling of IPSCs as current responses to brief exponentially decaying GABA applications was sufficient to reproduce correctly the pH dependence of mIPSCs, and optimal fit was obtained for peak [GABA] of 1.5-3 mm and a clearance time constant of 0.075-0.125 msec. Our analysis indicates that, for these parameters of GABA transient, in control conditions (pH 7.2) mIPSCs are not saturated.

Declusterization of GABAA receptors affects the kinetic properties of GABAergic currents in cultured hippocampal neurons

Speed and reliability of synaptic transmission are essential for information coding in neuronal networks and require the presence of clustered neurotransmitter receptors at the plasma membrane in precise apposition to presynaptic terminals. Receptor clusterization is the result of highly regulated processes involving functional and structural proteins. Among the structural elements, microtubules are known to play a crucial role in anchoring of gamma-aminobutyric acid, type A (GABA(A)) receptors. Here we show that microtubule depolymerization with nocodazole induces the declusterization of GABA(A) receptors and modifies the kinetic properties of GABAergic currents in cultured hippocampal neurons. In particular, this drug, applied either in the bath or via the patch pipette, induced the acceleration of the onset kinetics of miniature inhibitory postsynaptic currents (mIPSCs) without significantly affecting their frequency, thus suggesting a main postsynaptic site of action. After nocodazole treatment, current responses to ultrafast applications of GABA exhibited a faster rise time and an accelerated onset of desensitization. A quantitative analysis of GABA-evoked currents and model simulations suggest that declusterization affects the gating properties of GABA(A) receptors. In particular, a faster entry into the desensitized state of declustered GABA(A) receptors may account for the changes in the kinetic properties of mIPSCs after nocodazole treatment. Hence it appears that the clustered condition of GABA(A) receptors contributes in shaping GABAergic currents.

Dentate granule cell GABA(A) receptors in epileptic hippocampus: enhanced synaptic efficacy and altered pharmacology

The dentate gyrus (DG) normally functions as a filter, preventing propagation of synchronized activity into the seizure-prone hippocampus. This filter or 'gatekeeper' attribute of the DG is compromised in various pathological states, including temporal lobe epilepsy (TLE). This study examines the role that altered inhibition may play in the deterioration of this crucial DG function. Using the pilocarpine animal model of TLE, we demonstrate that inhibitory synaptic function is altered in principal cells of the DG. Spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in dentate granule cells (DGCs) from epileptic animals were larger, more sensitive to blockade by zinc and less sensitive to augmentation by the benzodiazepine type site 1 modulator zolpidem. Furthermore, mIPSCs examined during a quiescent period following injury but preceding onset of epilepsy were significantly smaller than those present either in control or in TLE DGCs, and had already acquired sensitivity to blockade by zinc prior to the onset of spontaneous seizures. Rapid agonist application experiments demonstrated that prolonged (>35 ms) exposure to zinc is required to block GABAA receptors (GABAARs) in patches pulled from epileptic DGCs. Therefore, zinc must be tonically present to block DGC GABAARs and alter DG function. This would occur only during repetitive activation of mossy fibres. Thus, in the pilocarpine animal model of TLE, an early, de novo, expression of zinc-sensitive GABAARs is coupled with delayed, epilepsy-induced development of a zinc delivery system provided by aberrant sprouting of zinc-containing mossy fibre recurrent collaterals. The temporal and spatial juxtaposition of these pathophysiological alterations may compromise normal 'gatekeeper' function of the DG through dynamic zinc-induced failure of inhibition, predisposing the hippocampal circuit to generate seizures.

Binding sites, singly bound states, and conformation coupling shape GABA-evoked currents

The time course of GABA-evoked currents is the main source of information on the GABA(A) receptor gating. Since the kinetics of these currents depends on the transitions between several receptor conformations, it is a major challenge to define the relations between current kinetics and the respective rate constants of the microscopic gating scheme. The aim of this study was to further explore the impact of different GABA(A) receptor conformations on the kinetics of currents elicited by ultra-fast GABA applications. We show that the rising phase and amplitude of GABA-evoked currents depend on desensitization and singly bound states. The occupancy of bound receptors depends not only on binding properties but also on opening/closing and desensitization. The impact of such functional coupling between channel states is critical in conditions of high non-equilibrium typical for synaptic transmission. The concentration dependence of the rising phase of the GABA-elicited current indicates positive cooperativity between agonist binding sites. We provide evidence that preequilibration at low GABA concentrations reduce GABA-evoked currents due to receptor trapping in a singly bound desensitized state.

Saturation and self-inhibition of rat hippocampal GABA(A) receptors at high GABA concentrations

Current responses to ultrafast gamma-aminobutyric acid (GABA) applications were recorded from excised patches in rat hippocampal neurons to study the gating properties of GABA(A) receptors at GABA concentrations close to saturating ones and higher. The amplitude of currents saturated at approximately 1 mm, while the onset rate of responses reached saturation at 4-6 mm GABA. At high GABA concentrations (> 10 mm), the amplitude of current responses was reduced in a dose-dependent manner with a half-blocking GABA concentration of approximately 50 mm. The peak reduction at high GABA doses was accompanied by a tendency to increase the steady-state to peak ratio. At concentrations higher than 30 mm, this effect took the form of a rebound current, i.e. during the prolonged GABA applications, the current firstly declined due to desensitization onset and then, instead of decreasing towards a steady-state value, clearly increased. Both the self-inhibition of GABA(A) receptors by high GABA doses and rebound were clearly voltage dependent, being larger at positive holding potentials. The fast desensitization component accelerated with depolarization at all saturating [GABA] tested. The rebound phenomenon indicates that the self-block of GABAA receptors is state dependent, and suggests that the sojourn in the desensitized conformation provides a 'rescue' from the block. We propose that high GABA concentrations inhibit the receptors by direct occlusion of the channel pore having no effect on the receptor gating.

Augmentation by zinc of NMDA receptor-mediated synaptic responses in CA1 of rat hippocampal slices: mediation by Src family tyrosine kinases

Normal neuronal activity results in the release of zinc from the synaptic vesicles of glutamatergic terminals and subsequent entry into postsynaptic neurons. Although the exact physiological role of zinc translocation is currently unknown, it is very likely that intracellular zinc exerts long-term modulatory effects upon synaptic transmission since zinc affects various molecules involved in signaling pathways. In this study we used rat hippocampal slices to examine the effect of zinc on glutamatergic synaptic transmission in the Schaffer collateral-CA1 synapses. Following a 10-min exposure to 0.3-1 mM zinc, the magnitude of NMDA receptor-mediated field excitatory postsynaptic potentials (fEPSP) gradually increased over the subsequent 30-40 min. In contrast, the magnitude of AMPA/kainate receptor-mediated fEPSPs remained unchanged. The selective potentiation of NMDA receptor-mediated fEPSPs by zinc was unlikely to be a presynaptic event, since the degree of paired-pulse facilitation was unaltered. Interestingly, the specific Src family tyrosine kinase inhibitor PP2 completely blocked zinc-induced potentiation of NMDA receptor-mediated fEPSP while the inactive analog PP3 had no effect, thereby suggesting the involvement of Src family tyrosine kinases. Furthermore, zinc exposure increased levels of total and tyrosine-phosphorylated forms of NR2A and NR2B in a PP2-dependent manner in both hippocampal slices and cell cultures. In addition, zinc treatment of hippocampal cultures increased the levels of tyrosine phosphorylation at the two positive regulatory sites of Src family tyrosine kinases. Our results demonstrate that zinc increases NMDA receptor function via Src family tyrosine kinase-mediated increases of NR2A and 2B tyrosine phosphorylation. We speculate that intense release of endogenous synaptic zinc may potentiate NMDA receptor-mediated transmission in zinc-containing glutamatergic pathways by a similar mechanism.

Potentiation of inhibitory glycinergic neurotransmission by Zn2+: a synergistic interplay between presynaptic P2X2 and postsynaptic glycine receptors

The divalent cation zinc is known to modulate chloride currents carried by native and recombinant mammalian glycine receptors (GlyRs). To unravel the effect of Zn2+ on glycinergic neurotransmission, inhibitory postsynaptic currents (IPSC) of rat spinal neurons grown in culture were analysed in the absence and presence of Zn2+. Low concentrations of Zn2+ (0.5 and 5 micro m) augmented the mean amplitude of miniature IPSCs by approximately 40% over values obtained in the absence of zinc, whereas higher concentrations of Zn2+ (50 micro m) significantly decreased mean amplitude values. Remarkably, low concentrations of Zn2+ also significantly increased the mean frequency of miniature IPSCs. This effect was blocked by the P2X receptor antagonists PPADS and suramin, indicating the presence of Zn2+-sensitive presynaptic P2X receptors on glycinergic terminals. Immunostaining with antibodies against different P2X receptor subtypes revealed that P2X2 receptors partially colocalize with the GlyR. Potentiating concentrations of Zn2+ also affected the kinetics of miniature and evoked IPSCs by significantly prolonging their decay time constants. Electrophysiological analysis of heterologously expressed glycine transporters (GlyT) revealed for GlyT2 zero, and for GlyT1 a modest (< 20%), reduction of glycine uptake in the presence of 5 micro m Zn2+, indicating that prolongation of glycinergic IPSCs by Zn2+ is not due to inhibition of glycine removal from the synaptic cleft. Together, these results suggest that Zn2+ is a potent modulator of glycinergic synaptic transmission which increases in a synergistic manner the agonist affinity of both presynaptic P2X2 receptors and postsynaptic GlyRs.

Allosteric interaction of zinc with recombinant alpha(1)beta(2)gamma(2) and alpha(1)beta(2) GABA(A) receptors

In a recent study we have provided evidence that inhibition of native GABA(A) receptors by zinc depends primarily on the allosteric modulation of receptor gating. Both the kinetics and the sensitivity of the GABA(A) receptor to zinc depend on subunit composition, especially on the presence of the gamma(2) subunit. To analyze the mechanism of action of zinc its effects have been tested on recombinant alpha(1)beta(2)gamma(2) and alpha(1)beta(2) receptors expressed in HEK 293 cells. The currents produced by ultrafast application of GABA have been measured to assess the impact of zinc ions on GABA(A) receptor gating with resolution corresponding to the time scale of synaptic currents. While, as expected, zinc markedly reduced the peak amplitude of alpha(1)beta(2)-mediated currents, its effect on kinetics was significantly different from that observed for alpha(1)beta(2)gamma(2). In particular, unlike alpha(1)beta(2)gamma(2), zinc did not affect the onset of alpha(1)beta(2)-mediated responses. Moreover, zinc increased the extent of desensitisation of alpha(1)beta(2)gamma(2) receptors and reduced desensitisation of alpha(1)beta(2) ones. Quantitative analysis suggests that zinc exerts an allosteric modulation on both alpha(1)beta(2)gamma(2) and alpha(1)beta(2) receptors. Zinc effects on alpha(1)beta(2)gamma(2) were qualitatively similar to those reported for native receptors.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Allosteric modulation of beta2-adrenergic receptor by Zn(2+)

Zn(2+) is abundant in the brain, where it plays a role in the function of a number of enzymes, structural proteins, and transcription factors. Zn(2+) is also found in synaptic vesicles and is released into synapses achieving concentrations in the range of 100 to 300 microM [Proc Natl Acad Sci USA 1997;94:13386-13387; Mol Pharmacol 1997;51:1015-1023]. Therefore, Zn(2+) may play a physiological role in regulating the function of postsynaptic channels and receptors. We characterized the effect of Zn(2+) on the functional properties of the beta2-adrenergic receptor (beta2AR). We found that physiological concentrations of Zn(2+) increased agonist affinity and enhanced cAMP accumulation stimulated by submaximal concentrations of the betaAR agonist isoproterenol. These results provide evidence that Zn(2+) released at nerve terminals may modulate signals generated by the beta2AR in vivo.