GABAergic and glutamatergic terminals differentially influence the organization of GABAergic synapses in rat cerebellar granule cells in vitro

Knockout of the γ-aminobutyric acid receptor subunit α4 reduces functional δ-containing extrasynaptic receptors in hippocampal pyramidal cells at the onset of puberty

Increased plasmalemmal localization of α4βδ GABA(A) receptors (GABARs) occurs in the hippocampal pyramidal cells of female mice at pubertal onset (Shen et al., 2010). This increase occurs on both dendritic spines and shafts of CA1 pyramidal cells and is in response to hormone fluctuations that occur at pubertal onset. However, little is known about how the α4 and δ subunits individually mediate the formation of functional, plasmalemmal α4βδ GABARs. To determine whether expression of the α4 subunit is necessary for plasmalemmal δ subunit localization at pubertal onset, electron microscopic-immunocytochemistry (EM-ICC) was employed. CA1 pyramidal cells of female α4 knockout (KO) mice were tested for plasmalemmal levels of the δ subunit within dendritic spine and shaft profiles at the onset of puberty. EM-ICC revealed that the α4 and δ subunits localize on dendritic spines and shafts at sites extrasynaptic to GABAergic input at pubertal onset in tissue of wild-type (WT) mice. At pubertal onset, plasmalemmal localization of the δ subunit is reduced 45.9% on dendritic spines, and 56.7% on dendritic shafts with KO of the α4 subunit, as compared to WT tissue, yet levels of intracellular δ immunoreactivity remain unchanged. The decline in plasmalemmal localization is manifested as decreased responsiveness to the GABA agonist gaboxadol at concentrations that are selective for δ-containing GABARs. Additionally, α4 KO mice have larger dendritic spine and shaft profiles. Our findings demonstrate that α4 subunit expression strongly influences the pubertal increase of δ subunits at the plasma membrane, and that genetic deletion of α4 serves as a functional knock-down of δ-containing GABARs.

Inhibitory synapse dynamics: coordinated presynaptic and postsynaptic mobility and the major contribution of recycled vesicles to new synapse formation

Dynamics of GABAergic synaptic components have been studied previously over milliseconds to minutes, revealing mobility of postsynaptic scaffolds and receptors. Here we image inhibitory synapses containing fluorescently tagged postsynaptic scaffold Gephyrin, together with presynaptic vesicular GABA transporter (VGAT) or postsynaptic GABA(A) receptor γ2 subunit (GABA(A)Rγ2), over seconds to days in cultured rat hippocampal neurons, revealing modes of inhibitory synapse formation and remodeling. Entire synapses were mobile, translocating rapidly within a confined region and exhibiting greater nonstochastic motion over multihour periods. Presynaptic and postsynaptic components moved in unison, maintaining close apposition while translocating distances of several micrometers. An observed flux in the density of synaptic puncta partially resulted from the apparent merging and splitting of preexisting clusters. De novo formation of inhibitory synapses was observed, marked by the appearance of stably apposed Gephyrin and VGAT clusters at sites previously lacking either component. Coclustering of GABA(A)Rγ2 supports the identification of such new clusters as synapses. Nascent synapse formation occurred by gradual accumulation of components over several hours, with VGAT clustering preceding that of Gephyrin and GABA(A)Rγ2. Comparing VGAT labeling by active uptake of a luminal domain antibody with post hoc immunocytochemistry indicated that recycling vesicles from preexisting boutons significantly contribute to vesicle pools at the majority of new inhibitory synapses. Although new synapses formed primarily on dendrite shafts, some also formed on dendritic protrusions, without apparent interconversion. Altogether, the long-term imaging of GABAergic presynaptic and postsynaptic components reveals complex dynamics and perpetual remodeling with implications for mechanisms of assembly and synaptic integration.

Emerging themes in GABAergic synapse development

Glutamatergic synapse development has been rigorously investigated for the past two decades at both the molecular and cell biological level yet a comparable intensity of investigation into the cellular and molecular mechanisms of GABAergic synapse development has been lacking until relatively recently. This review will provide a detailed overview of the current understanding of GABAergic synapse development with a particular emphasis on assembly of synaptic components, molecular mechanisms of synaptic development, and a subset of human disorders which manifest when GABAergic synapse development is disrupted. An unexpected and emerging theme from these studies is that glutamatergic and GABAergic synapse development share a number of overlapping molecular and cell biological mechanisms that will be emphasized in this review.

GluRdelta2 expression in the mature cerebellum of hotfoot mice promotes parallel fiber synaptogenesis and axonal competition

Glutamate receptor delta 2 (GluRdelta2) is selectively expressed in the cerebellum, exclusively in the spines of the Purkinje cells (PCs) that are in contact with parallel fibers (PFs). Although its structure is similar to ionotropic glutamate receptors, it has no channel function and its ligand is unknown. The GluRdelta2-null mice, such as knockout and hotfoot have profoundly altered cerebellar circuitry, which causes ataxia and impaired motor learning. Notably, GluRdelta2 in PC-PF synapses regulates their maturation and strengthening and induces long term depression (LTD). In addition, GluRdelta2 participates in the highly territorial competition between the two excitatory inputs to the PC; the climbing fiber (CF), which innervates the proximal dendritic compartment, and the PF, which is connected to spiny distal branchlets. Recently, studies have suggested that GluRdelta2 acts as an adhesion molecule in PF synaptogenesis. Here, we provide in vivo and in vitro evidence that supports this hypothesis. Through lentiviral rescue in hotfoot mice, we noted a recovery of PC-PF contacts in the distal dendritic domain. In the proximal domain, we observed the formation of new spines that were innervated by PFs and a reduction in contact with the CF; ie, the pattern of innervation in the PC shifted to favor the PF input. Moreover, ectopic expression of GluRdelta2 in HEK293 cells that were cocultured with granule cells or in cerebellar Golgi cells in the mature brain induced the formation of new PF contacts. Collectively, our observations show that GluRdelta2 is an adhesion molecule that induces the formation of PF contacts independently of its cellular localization and promotes heterosynaptic competition in the PC proximal dendritic domain.

Transmitter-receptor mismatch in GABAergic synapses in the absence of activity

Competition among different axons to reach the somatodendritic region of the target neuron is an important event during development to achieve the final architecture typical of the mature brain. Trasmitter-receptor matching is a critical step for the signaling between neurons. In the cerebellar cortex, there is a persistent competition between the two glutamatergic inputs, the parallel fibers and the climbing fibers, for the innervation of the Purkinje cells. The activity of the latter input is necessary to maintain its own synaptic contacts on the proximal dendritic domain and to confine the parallel fibers in the distal one. Here, we show that climbing fiber activity also limits the distribution of the GABAergic input in the proximal domain. In addition, blocking the activity by tetrodotoxin infusion in Wistar rat cerebellum, a synapse made by GABAergic terminals onto the recently formed Purkinje cell spines appear in the proximal dendrites. The density of GABAergic terminals is increased, and unexpected double symmetric/asymmetric postsynaptic densities add to the typical symmetric phenotype of the GABAergic shaft synapses. Moreover, glutamate receptors appear in these ectopic synapses even in the absence of glutamate transmitter inside the presynaptic terminal and close to GABA receptors. These results suggest that the Purkinje cell has an intrinsic tendency to develop postsynaptic assemblies of excitatory types, including glutamate receptors, over the entire dendritic territory. GABA receptors are induced in these assemblies when contacted by GABAergic terminals, thus leading to the formation of hybrid synapses.

Synaptogenesis in the cerebellar cortex: differential regulation of gephyrin and GABAA receptors at somatic and dendritic synapses of Purkinje cells

In rodent cerebellar cortex, synaptogenesis occurs entirely postnatally, allowing study of the mechanisms of synapse formation in vivo. Here we monitored the clustering of GABA(A) receptors and the scaffolding protein gephyrin at GABAergic postsynaptic sites during rat cerebellar development. We found that GABA(A) receptors and gephyrin co-aggregate at nascent synapses in the molecular and Purkinje cell layers with a similar time course. With few exceptions, gephyrin and GABA(A) receptor subunits clustered selectively in front of presynaptic boutons expressing the vesicular inhibitory amino acid transporter VIAAT and no ectopic localization of these molecules was observed. Surprisingly, gephyrin clusters outlining the cell body of Purkinje cells were transient, and disappeared rapidly at the end of the second postnatal week. The loss of gephyrin from perisomatic synapses was coincident with a significant reduction in the size of GABA(A) receptor clusters. Furthermore, these changes were accompanied by a developmental decrease in the size of synaptic appositions, as documented by electron microscopy. These findings suggest that gephyrin takes part in the initial assembly of postsynaptic specializations and reveal an unsuspected heterogeneity in the molecular organization of the postsynaptic apparatus at somatic and dendritic synapses of mature Purkinje cells.

GABA(A) receptor gamma 2 subunit mutations linked to human epileptic syndromes differentially affect phasic and tonic inhibition

GABA acts on GABA(A) receptors to evoke both phasic inhibitory synaptic events and persistent, tonic currents. The gamma2 subunit of the GABA(A) receptor is involved in both phasic and tonic signaling in the hippocampus. Several mutations of this subunit are linked to human epileptic syndromes with febrile seizures, yet it is not clear how they perturb neuronal activity. Here, we examined the expression and functional impact of recombinant gamma2 in hippocampal neurons. We show that the K289M mutation has no effect on membrane trafficking and synaptic aggregation of recombinant gamma2, but accelerates the decay of synaptic currents. In contrast, the R43Q mutation primarily reduces surface expression of recombinant gamma2. However, it has no dominant effect on synaptic currents but instead reduces tonic GABA currents, at least in part by reducing surface expression of the alpha5 subunit. Our data suggests that the phenotypic specificity of mutations affecting the GABA(A) receptor gamma2 gene may result from different actions specific to distinct modes of GABAergic signaling.

Gephyrin alterations due to protein accumulation stress are reduced by the lysosomal modulator Z-Phe-Ala-diazomethylketone

Inhibitory neurotransmission is important for brain function and requires specific transmitter receptors that are organized in synaptic domains. Gephyrin is a cytoskeletal organization protein that binds tubulin and plays an important role in clustering and organizing select inhibitory neurotransmitter receptors. Here, we tested if gephyrin is altered by protein accumulation stress that is common in age-related neurodegenerative disorders. For this, we used the hippocampal slice model that has been shown to exhibit chloroquine (CQN)-induced protein accumulation, microtubule destabilization, transport failure, and declines in excitatory neurotransmitter receptors and their responses. In addition to the decreases in excitatory receptor subunits and other glutamatergic markers, we found that gephyrin isoforms were reduced across the CQN treatment period. Associated with this decline in gephyrin levels was the production of three gephyrin breakdown products (GBDPs) of 30, 38, and 48 kDa. The induced effects on gephyrin were tested for evidence of recovery through enhancement of lysosomal function that is known to promote protein clearance and microtubule integrity. Using the lysosomal modulator Z-Phe-Ala-diazomethylketone (PADK), gephyrin levels were completely restored in correspondence with the recovery of excitatory glutamatergic components. In addition, GBDPs were significantly reduced after the 2-day PADK treatment, to levels that were at or below those measured in control cultures. These findings suggest that receptor-clustering mechanisms for inhibitory synapses are compromised during protein accumulation events. They also indicate that a lysosomal enhancement strategy can protect gephyrin integrity, which may be vital for the balance between inhibitory and excitatory signaling during age-related diseases.

Matching of pre- and postsynaptic specializations during synaptogenesis

Formation of chemical synapses in the central nervous system is a highly regulated, multistep process that requires bidirectional communication across the synaptic cleft. Neurotransmitter receptors, scaffolding proteins, and signaling molecules need to be concentrated in the postsynaptic density, a specialized membrane microdomain apposed to the active zone of presynaptic terminals, where transmitter release occurs. This precise, synapse-specific matching implicates that sorting and targeting mechanisms exist for the molecular constituents of different types of synapses to ensure correct formation of neuronal circuits in the brain. There is considerable evidence from in vitro and in vivo studies that neurotransmitter signaling is not required for proper sorting during synapse formation, whereas active neurotransmission is essential for long-term synapse maintenance. Here, the authors review recent studies on the role of cell adhesion molecules in synaptogenesis and on possible mechanisms ensuring correct matching of pre- and postsynaptic sites. They discuss the role of neurotransmitter receptors and scaffolding proteins in these processes, focusing on fundamental differences between synapse formation during development and synapse maintenance and plasticity in adulthood.

Expression of amino acid receptors and neural peptides in the weaver mouse brain

In the present study, we conducted: (i) in situ hybridization in order to investigate the expression of kainate and GABA(A) receptor subunits and the pre-proenkephalin and prodynorphin peptides in the brain of weaver mouse (a genetic model of dopamine deficiency) and (ii) immunocytochemistry in order to study the somatostatin-positive cells in weaver striatum. Our results indicated: (i) increases in mRNA levels of KA2 and GluR6 kainate receptor subunits, of alpha(4) and beta(3) GABA(A) receptor subunits and of pre-proenkephalin and prodynorphin in 6-month-old weaver striatum; (ii) a decrease in alpha(1) and beta(2) GABA(A) subunit mRNAs in 6-month-old weaver globus pallidus; (iii) increases in KA2, alpha(4) and beta(3) and decreases in alpha(2) and beta(2) mRNAs in the 6-month-old weaver somatosensory cortex; and (iv) an increase in somatostatin-immunopositive cells in 3-month-old weaver striatum. We suggest that: (i) in striatum, the alterations are induced by the induction of the transcription factor DeltafosB (for GluR6, pre-proenkephalin and prodynorphin mRNAs) and the suppression of transcription factors like NGF-IB (nerve growth factor inducible B; for the KA2 mRNA), in response to dopamine depletion; (ii) in striatum and cortex, the alterations in the expression of the GABA(A) subunits indicate an increase of extrasynaptic versus a decrease of synaptic GABA(A) receptors; and (iii) in globus pallidus, the increased striatopallidal GABAergic transmission leads to a decrease in the number of GABA(A) receptors. Our results further clarify the regulatory role of dopamine in the expression of amino acid receptors and striatal neuropeptides.

Presynaptic terminals independently regulate synaptic clustering and autophagy of GABAA receptors in Caenorhabditis elegans

Synaptic clustering of GABAA receptors is important for the function of inhibitory synapses, influencing synapse strength and, consequently, the balance of excitation and inhibition in the brain. Presynaptic terminals are known to induce GABAA receptor clustering during synaptogenesis, but the mechanisms of cluster formation and maintenance are not known. To study how presynaptic neurons direct the formation of GABAA receptor clusters, we have investigated GABAA receptor localization in postsynaptic cells that fail to receive presynaptic contacts in Caenorhabditis elegans. Postsynaptic muscles in C. elegans receive acetylcholine and GABA motor innervation, and GABAA receptors cluster opposite GABA terminals. Selective loss of GABA inputs caused GABAA receptors to be diffusely distributed at or near the muscle cell surface, confirming that GABA presynaptic terminals induce GABAA receptor clustering. In contrast, selective loss of acetylcholine innervation had no effect on GABAA receptor localization. However, loss of both GABA and acetylcholine inputs together caused GABAA receptors to traffic to intracellular autophagosomes. Autophagosomes normally transport bulk cytoplasm to the lysosome for degradation. However, we show that GABAA receptors traffic to autophagosomes after endocytic removal from the cell surface and that acetylcholine receptors in the same cells do not traffic to autophagosomes. Thus, autophagy can degrade cell-surface receptors and can do so selectively. Our results show that presynaptic terminals induce GABAA receptor clustering by independently controlling synaptic localization and surface stability of GABAA receptors. They also demonstrate a novel function for autophagy in GABAA receptor degradative trafficking.

Differential dependence of axo-dendritic and axo-somatic GABAergic synapses on GABAA receptors containing the alpha1 subunit in Purkinje cells

Synapse formation and maintenance require extensive transsynaptic interactions involving multiple signal transduction pathways. In the cerebellum, Purkinje cells (PCs) receive GABAergic, axo-dendritic synapses from stellate cells and axo-somatic synapses from basket cells, both with GABAA receptors containing the alpha1 subunit. Here, we investigated the effects of a targeted deletion of the alpha1 subunit gene on GABAergic synaptogenesis in PCs, using electrophysiology and immunoelectron microscopy. Whole-cell patch-clamp recordings in acute slices revealed that PCs from alpha1(0/0) mice lack spontaneous and evoked IPSCs, demonstrating that assembly of functional GABAA receptors requires the alpha1 subunit. Ultrastructurally, stellate cell synapses on PC dendrites were reduced by 75%, whereas basket cell synapses on the soma were not affected, despite the lack of GABAA-mediated synaptic transmission. Most strikingly, GABAergic terminals were retained in the molecular layer of adult alpha1(0/0) mice and formed heterologous synapses with PC spines characterized by a well differentiated asymmetric postsynaptic density. These synapses lacked presynaptic glutamatergic markers and postsynaptic AMPA-type glutamate receptors but contained delta2-glutamate receptors. During postnatal development, initial steps of GABAergic synapse formation were qualitatively normal, and heterologous synapses appeared in parallel with maturation of dendritic spines. These results suggest that synapse formation in the cerebellum is governed by neurotransmitter-independent mechanisms. However, in the absence of GABAA-mediated transmission, GABAergic terminals in the molecular layer apparently become responsive to synaptogenic signals from PC spines and form stable heterologous synapses. In contrast, maintenance of axo-somatic GABAergic synapses does not depend on functional GABAA receptors, suggesting differential regulation in distinct subcellular compartments.

Disruption of postsynaptic GABA receptor clusters leads to decreased GABAergic innervation of pyramidal neurons

We have used RNA interference (RNAi) to knock down the expression of the gamma2 subunit of the GABA(A) receptors (GABA(A)Rs) in pyramidal neurons in culture and in the intact brain. Two hairpin small interference RNAs (shRNAs) for the gamma2 subunit, one targeting the coding region and the other one the 3'-untranslated region (UTR) of the gamma2 mRNA, when introduced into cultured rat hippocampal pyramidal neurons, efficiently inhibited the synthesis of the GABA(A) receptor gamma2 subunit and the clustering of other GABA(A)R subunits and gephyrin in these cells. More significantly, this effect was accompanied by a reduction of the GABAergic innervation that these neurons received. In contrast, the gamma2 shRNAs had no effect on the clustering of postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, postsynaptic density protein 95 (PSD-95) or presynaptic glutamatergic innervation. A gamma2-enhanced green fluorescent protein (EGFP) subunit construct, whose mRNA did not contain the 3'-UTR targeted by gamma2 RNAi, rescued both the postsynaptic clustering of GABA(A)Rs and the GABAergic innervation. Decreased GABA(A)R clustering and GABAergic innervation of pyramidal neurons in the post-natal rat cerebral cortex was also observed after in utero transfection of these neurons with the gamma2 shRNAs. The results indicate that the postsynaptic clustering of GABA(A)Rs in pyramidal neurons is involved in the stabilization of the presynaptic GABAergic contacts.

Rapid modulation of inhibitory synaptic currents in cerebellar Purkinje cells by BDNF

The present study examined the acute effects of exogenous BDNF on inhibitory synaptic currents in Purkinje cells in cerebellar cultures. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded in cultures (20-30 days in vitro), using discontinuous single electrode voltage clamp (dSEVC) technique. The effects of BDNF were studied in untreated control cultures and in cultures in which the endogenous levels of BDNF were decreased by chronic block of neural activity with tetrodotoxin (TTX). Chronic activity deprivation did not alter the amplitude of mIPSCs in Purkinje cells, and acute application of BDNF (50 ng/ml) to Purkinje cells in TTX-treated cultures significantly potentiated the amplitude and frequency of mIPSCs. By contrast, acute application of BDNF (50 ng/ml) produced no significant changes on mIPSC activity in control neurons. At higher concentrations of BDNF (100 ng/ml), comparable effects on mIPSC activity were also observed in control neurons. Preincubation of cerebellar cultures with K252a, an inhibitor of tyrosine kinases, effectively blocked the effects of BDNF on mIPSCs. These results indicate that functional inhibitory synapses develop in the absence of neural activity, and that activation of TrkB receptors by BDNF modulates inhibitory neurotransmission in Purkinje cells at both pre- and postsynaptic sites.

Glycinergic and GABAergic synaptic transmission are differentially affected by gephyrin in spinal neurons

In the present study, we have examined the physiological properties of synaptic currents mediated by GlyRs and GABAARs after culturing spinal neurons with a gephyrin antisense oligonucleotide. Application of gephyrin antisense, but not the sense, reduced the glycinergic mIPSC amplitude ( approximately 50%) and frequency ( approximately 85%), indicating the importance of gephyrin for GlyR anchoring at postsynaptic sites. On the other hand, the glycine-evoked current amplitude was unchanged indicating that functional GlyRs were still located in the extrasynaptic membrane. The analysis of the GABAergic transmission in the same neurons revealed approximately 70% reduction in the frequency of the GABAergic mIPSCs, without changes in the amplitude. Interestingly, the modulation of remaining GABAAR-mediated synaptic events by zinc and diazepam was significantly altered by the antisense. These results indicate that gephyrin is required for the membrane insertion/stabilization of the GABAAR gamma2 subunit as well as for its subsequent localization in the postsynaptic membrane.

Differential regulation of GABA(A) receptor and gephyrin postsynaptic clustering in immature hippocampal neuronal cultures

Gephyrin is a postsynaptic scaffolding protein involved in clustering of glycine- and GABA(A) receptors at inhibitory synapses. The role of gephyrin in GABAergic synapses, the nature of its interactions with GABA(A) receptors, and the mechanisms of targeting to GABAergic synapses are largely unknown. To gain further insights into these questions, the formation of GABA(A) receptor and gephyrin clusters and their distribution relative to presynaptic terminals were investigated in immature cultures of embryonic hippocampal neurons using triple immunofluorescence staining. GABA(A) receptor clusters, labeled for the alpha2 subunit, formed independently of gephyrin clusters, and were distributed on neurites at constant densities, either extrasynaptically or, to a lesser extent, postsynaptically, apposed to synapsin-I-positive axon terminals. In contrast, gephyrin clusters were always associated with GABA(A) receptors and were preferentially localized postsynaptically. Their density increased linearly with the extent of innervation, which developed rapidly during the first week in vitro. These results suggested that GABA(A) receptor clustering is mediated by cell-autonomous mechanisms independent of synapse formation. Their association with gephyrin is dynamically regulated and may contribute to stabilization at postsynaptic sites. Labeling for vesicular glutamate transporters revealed that most synapses in these immature cultures were presumably glutamatergic, implying that postsynaptic GABA(A) receptor and gephyrin clusters initially were located in "mismatched" synapses. However, clusters appropriately localized in GABAergic synapses were distinctly larger and more intensely stained. Altogether, these results demonstrate that the targeting of GABA(A) receptor and gephyrin clusters to GABAergic synapses occurs secondarily and is regulated by presynaptic factors that are not essential for clustering.

Lipid raft localization of GABAA receptor and Na+, K+-ATPase in discrete microdomain clusters in rat cerebellar granule cells

The microdomain localization of the GABA(A) receptor in rat cerebellar granule cells was studied by subcellular fractionation and fluorescence- and immunogold electron microscopy. The receptor resided in lipid rafts, prepared at 37 degrees C by extraction with the nonionic detergent Brij 98, but the raft fraction, defined by the marker ganglioside GM(1) in the floating fractions following density gradient centrifugation, was heterogeneous in density and protein composition. Thus, another major raft-associated membrane protein, the Na(+), K(+)-ATPase, was found in discrete rafts of lower density, reflecting clustering of the two proteins in separate membrane microdomains. Both proteins were observed in patchy "hot spots" at the cell surface as well as in isolated lipid rafts. Their insolubility in Brij 98 was only marginally affected by methyl-beta-cyclodextrin. In contrast, both the GABA(A) receptor and Na(+), K(+)-ATPase were largely soluble in ice cold Triton X-100. This indicates that Brij 98 extraction defines an unusual type of cholesterol-independent lipid rafts that harbour membrane proteins also associated with underlying scaffolding/cytoskeletal proteins such as gephyrin (GABA(A) receptor) and ankyrin G (Na(+), K(+)-ATPase). By providing an ordered membrane microenvironment, lipid rafts may contribute to the clustering of the GABA(A) receptor and the Na(+), K(+)-ATPase at distinct functional locations on the cell surface.

NMDA receptors increase the size of GABAergic terminals and enhance GABA release

In developing cerebellar interneurons, NMDA increases spontaneous GABA release by activating presynaptic NMDA receptors. We investigated the role of these receptors on differentiating basket/stellate cells in cerebellar cultures grown under conditions allowing functional synaptic transmission. Presynaptic GABAergic boutons were visualized either by GAD65 immunostaining or by using cells derived from GAD65-enhanced green fluorescent protein (eGFP) transgenic mice, in which cerebellar basket/stellate cells express eGFP. After the first week in culture, whole-cell recordings from granule cells reveal that acute application of NMDA increases miniature IPSC (mIPSC) frequency. Interestingly, after 2 weeks, the mIPSC frequency increases compared with the first week but is not modulated by NMDA. Furthermore, in cultures chronically treated with NMDA for 1 week, the size of the GABAergic boutons increases. This growth is paralleled by increased mIPSC frequency and the loss of NMDA sensitivity. Direct patch-clamp recording from these presynaptic terminals reveals single NMDA-activated channels, showing multiple conductance levels, and electronic propagation from the somatodendritic compartment. Our results demonstrate that NMDA receptors alter GABAergic synapses in developing cerebellar cultures by increasing the size of the terminal and spontaneous GABA release. These findings parallel changes in inhibitory synaptic efficacy seen in vivo in developing GABAergic interneurons of the molecular layer of the cerebellum.

Maturation of glutamatergic and GABAergic synapse composition in hippocampal neurons

It is commonly accepted that glutamatergic and GABAergic presynaptic terminals form perfectly matched appositions opposite their appropriate receptors and associated binding proteins. However, recent reports indicate that certain synaptic proteins that are commonly used to identify excitatory or inhibitory synapses can be mismatched, particularly during development. In order to construct a more comprehensive scheme of synapse composition during development, we co-immunolabeled for several principle excitatory and inhibitory proteins over the course of synaptogenesis in cultured hippocampal neurons. We find that although the majority of synaptic appositions are composed of matched clusters of pre- and postsynaptic proteins appropriate for a particular neurotransmitter, many are initially mismatched, even in dendrites receiving both glutamatergic and GABAergic innervation. Over time, the fidelity of GABAergic synapse composition increases such that, despite the persistence of some mismatched components at glutamatergic sites, the incidence of mismatch diminishes at both inhibitory and excitatory synapses. Activation of either GABA-A or NMDA receptors promotes fidelity at GABAergic sites, but NMDA receptor activation promotes mismatching among glutamatergic synapses. Thus, apposition of pre- and postsynaptic elements can occur independent of neurotransmitter specificity and synaptic activity modifies these associations. Our findings support the idea that synapse maturation occurs in several distinct stages, and that these stages are regulated by a combination of activity-dependent and -independent factors.

Deletion of the NR2A subunit prevents developmental changes of NMDA-mEPSCs in cultured mouse cerebellar granule neurones

We investigated the role N-methyl-d-aspartate (NMDA) receptor subunits play in shaping excitatory synaptic currents in cultures of cerebellar granule cells (CGCs) from NR2A knockout (NR2A-/-) and wild-type (+/+) mice. Cultures were maintained in a condition that facilitates the occurrence of functional synapses, allowing us to record NMDA-miniature excitatory postsynaptic currents (mEPSCs) in addition to NMDA receptor-mediated whole-cell currents at three ages in vitro. Whole-cell NMDA current density decreased with development in both strains though currents from NR2A-/- neurones demonstrated greater sensitivity to CP101 606, an NR2B subunit specific blocker. Sensitivity to Mg(2+) blockade decreased with age in vitro in +/+ but not in NR2A-/- CGCs. Immunocytochemistry revealed that dendrites and somas displayed distinct NR1 and NR2A subunit clusters which became increasingly colocalized in +/+ neurones. Qualitatively the overall NR2B subunit staining pattern was similar in +/+ and NR2A-/- neurones throughout development, suggesting that the NR2B subunit distribution is not mediated by the NR2A subunit. In addition, staining with markers for excitatory synapses showed that expression of NR2A subunit (but not NR2B) increases at both synaptic and extrasynaptic sites in +/+ neurones during development. In parallel, NMDA-mEPSCs were faster in +/+ compared with NR2A-/- neurones at all time points studied, suggesting that the NR2A subunit begins to replace NR2B-rich NMDA receptors even at early stages of development. Many NR2A-/- neurones were devoid of NMDA-mEPSCs at the later time point, and transfection of the NR2A subunit in these neurones restored fast decay and the occurrence of NMDA-mEPSCs. Taken together, our results indicate that the NR2A subunit is mainly responsible for the developmental changes observed in the maturation of excitatory synapses.

Quantitative effects produced by modifications of neuronal activity on the size of GABAA receptor clusters in hippocampal slice cultures

The number and strength of GABAergic synapses needs to be precisely adjusted for adequate control of excitatory activity. We investigated to what extent the size of GABA(A) receptor clusters at inhibitory synapses is under the regulation of neuronal activity. Slices from P7 rat hippocampus were cultured for 13 days in the presence of bicuculline or 4-aminopyridine (4-AP) to increase neuronal activity, or DNQX to decrease activity. The changes provoked by these treatments on clusters immunoreactive for the alpha1 and alpha2 subunits of the GABA(A) receptor or gephyrin were quantitatively evaluated. While an increase in activity augmented the density of these clusters, a decrease in activity provoked, in contrast, a decrease in their density. An inverse regulation was observed for the size of individual clusters. Bicuculline and 4-AP decreased whilst DNQX increased the mean size of the clusters. When the pharmacological treatments were applied for 2 days instead of 2 weeks, no effects on the size of the clusters were observed. The variations in the mean size of individual clusters were mainly due to changes in the number of small clusters. Finally, a regulation of the size of GABA(A) receptor clusters occurred during development in vivo, with a decrease of the mean size of the clusters between P7 and P21. This physiological change was also the result of an increase in the number of small clusters. These results indicate that neuronal activity regulates the mean size of GABA(A) receptor- and gephyrin-immunoreactive clusters by modifying specifically the number of synapses with small clusters of receptors.

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.

Expression of distinct alpha subunits of GABAA receptor regulates inhibitory synaptic strength

Distinct alpha subunit subtypes in the molecular assembly of GABA(A) receptors are a critical determinant of the functional properties of inhibitory synapses and their modulation by a range of pharmacological agents. We investigated the contribution of these subunits to the developmental changes of inhibitory synapses in cerebellar granule neurons in primary cultures from wild-type and alpha1 subunit -/- mice. The decay time of miniature inhibitory postsynaptic currents (mIPSCs) halved between 6 days in vitro (DIV6) and DIV12. This was paralleled by the decrease of alpha2 and alpha3 subunits, the increase of alpha1 and alpha6 subunits expression at synapses, and changes in the action of selective alpha subunit modulators. A small but significant shortening of mIPSCs was observed with development in cells from -/- mice together with a decrease in the expression of alpha3 subunit. In contrast, the expression of alpha2 subunit at inhibitory synapses in -/- cells was significantly higher than in +/+ cells at DIV11-12. alpha5 subunit was not detected, and increased sensitivity to a selective alpha4/alpha6 subunit agonist suggests increased expression of extrasynaptic receptors in -/- mice. beta2/beta3 subunit expression and loreclezole sensitivity increased with development in +/+ but not in -/- cells, supporting the preferential association of the alpha1 with the beta2 subunit. Synaptic charge transfer strongly decreased with development but was not different between cells in the +/+ and -/- groups until DIV11-12. Our results uncover a pattern of sequential expression of alpha subunits underlying the changes in functional efficacy of GABAergic networks with development.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.

The enigma of transmitter-selective receptor accumulation at developing inhibitory synapses

The control of synaptic inhibition is crucial for normal brain function. More than 20 years ago, glycine and gamma-aminobutyric acid (GABA) were shown to be the two major inhibitory neurotransmitters. They can be released independently from different terminals or co-released from the same terminal to activate postsynaptic glycine and GABA(A) receptors. The anchoring protein gephyrin is involved in the postsynaptic accumulation of both glycine and GABA(A) receptors. In lower brain regions, both receptors can be concentrated in synapses, whereas in higher brain regions, glycine receptors are mostly excluded from postsynaptic sites. The activation of glycine and/or GABA(A) receptors determines the strength and precise timing of inhibition. Therefore, tight regulation of postsynaptic glycine versus GABA(A) receptor localization is crucial for optimizing synaptic inhibition in neurons. This review focuses on recent findings and discusses questions concerning the specificity of postsynaptic inhibitory neurotransmitter receptor accumulation during inhibitory synapse formation and development.