Synaptic and extrasynaptic gamma -aminobutyric acid type A receptor clusters in rat hippocampal cultures during development

The α1, α2, α3, and γ2 subunits of GABAA receptors show characteristic spatial and temporal expression patterns in rhombencephalic structures during normal human brain development

γ-Aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in adult mammalian brain, mediating its actions chiefly via a pentameric chloride ion channel, the GABAA receptor. Nineteen different subunits (α1-6, β1-3, γ1-3, δ, ε, π, θ, ρ1-3) can give rise to multiple receptor subtypes that are the site of action of many clinically important drugs. In the developing brain, however, GABAA receptors mediate excitatory actions due to an increased chloride concentration within neurons and seem to control cell proliferation, migration, differentiation, synapse maturation, and cell death. Little is known about the distribution of single subunits in the human brain. Here we describe developmental changes in the immunohistochemical distribution of four subunits (α1, α2, α3, and γ2) in the human rhombencephalon. The γ2 was the most abundant subunit in all rhombencephalic structures during development and in adults, whereas α subunits showed a structure- and age-characteristic distribution. The α1 was expressed prenatally in the molecular and Purkinje cell layer, but only postnatally in the granule cell layer and the dentate nucleus. Expression was completely absent in the inferior olivary nucleus. The α2 gradually increased during development, showing some layer specificity in the cerebellar cortex. The α3-immunoreactivity in the cerebellar cortex was relatively weak, but it was abundantly observed in different cell populations in the subcortical cerebellar structures. Structure- and age-characteristic colocalization between subunits during development suggests differences in GABAA receptor composition. Interestingly, subunit expression in several instances differed between human and rodent brain, underlining the importance of immunohistochemical studies in humans.

The short splice variant of the gamma 2 subunit acts as an external modulator of GABA(A) receptor function

GABA(A) receptors (GABA(A)Rs) regulate the majority of fast inhibition in the mammalian brain and are the target for multiple drug types, including sleep aids, anti-anxiety medication, anesthetics, alcohol, and neurosteroids. A variety of subunits, including the highly distributed gamma2, allow for pharmacologic and kinetic differences in particular brain regions. The two common splice variants gamma2S (short) and gamma2L (long) show different patterns of regional distribution both in adult brain and during the course of development, but show few notable differences when incorporated into pentameric receptors. However, results presented here show that the gamma2S variant can strongly affect both GABA(A)R pharmacology and kinetics by acting as an external modulator of fully formed receptors. Mutation of one serine residue can confer gamma2S-like properties to gamma2L subunits, and addition of a modified gamma2 N-terminal polypeptide to the cell surface recapitulates the pharmacological effect. Thus, rather than incorporation of a separate accessory protein as with voltage-gated channels, this is an example of an ion channel using a common subunit for dual purposes. The modified receptor properties conferred by accessory gamma2S have implications for understanding GABA(A)R pharmacology, receptor kinetics, stoichiometry, GABAergic signaling in the brain during development, and altered function in disease states such as epilepsy.

Altered GABAA,slow inhibition and network oscillations in mice lacking the GABAA receptor beta3 subunit

Phasic GABAergic inhibition in hippocampus and neocortex falls into two kinetically distinct categories, GABA(A,fast) and GABA(A,slow). In hippocampal area CA1, GABA(A,fast) is generally believed to underlie gamma oscillations, whereas the contribution of GABA(A,slow) to hippocampal rhythms has been speculative. Hypothesizing that GABA(A) receptors containing the beta(3) subunit contribute to GABA(A,slow) inhibition and that slow inhibitory synapses control excitability as well as contribute to network rhythms, we investigated the consequences of this subunit's absence on synaptic inhibition and network function. In pyramidal neurons of GABA(A) receptor beta(3) subunit-deficient (beta(3)(-/-)) mice, spontaneous GABA(A,slow) inhibitory postsynaptic currents (IPSCs) were much less frequent, and evoked GABA(A,slow) currents were much smaller than in wild-type mice. Fittingly, long-lasting recurrent inhibition of population spikes was less powerful in the mutant, indicating that receptors containing beta(3) subunits contribute substantially to GABA(A,slow) currents in pyramidal neurons. By contrast, slow inhibitory control of GABA(A,fast)-producing interneurons was unaffected in beta(3)(-/-) mice. In vivo hippocampal network activity was markedly different in the two genotypes. In beta(3)(-/-) mice, epileptiform activity was observed, and theta oscillations were weaker, slower, less regular and less well coordinated across laminae compared with wild-type mice, whereas gamma oscillations were weaker and faster. The amplitude modulation of gamma oscillations at theta frequency ("nesting") was preserved but was less well coordinated with theta oscillations. With the caveat that seizure-induced changes in inhibitory circuits might have contributed to the changes observed in the mutant animals, our results point to a strong contribution of beta(3) subunits to slow GABAergic inhibition onto pyramidal neurons but not onto GABA(A,fast) -producing interneurons and support different roles for these slow inhibitory synapses in the generation and coordination of hippocampal network rhythms.

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.

Trafficking and synaptic anchoring of ionotropic inhibitory neurotransmitter receptors

Neurotransmitter receptors are subject to microtubule-based transport between intracellular organelles and the neuronal plasma membrane. Receptors that arrive at plasma membrane compartments diffuse laterally within the plane of the cellular surface. To achieve immobilization at their sites of action, cytoplasmic receptor residues bind to submembrane proteins, which are coupled to the underlying cytoskeleton by multiprotein scaffolds. GABA(A)Rs (gamma-aminobutyric type A receptors) and GlyRs (glycine receptors) are the major inhibitory receptors in the central nervous system. At inhibitory postsynaptic sites, all GlyRs and the majority of GABA(A)Rs directly or indirectly couple to gephyrin, a multimeric PSD (postsynaptic density) component. In addition to cluster formations at axo-dendritic contacts, individual GABA(A)R subtypes also anchor and concentrate at extrasynaptic positions, either through association with gephyrin or direct interaction with the ERM (ezrin/radixin/moesin) family protein radixin. In addition to their role in diffusion trapping of surface receptors, scaffold components also undergo rapid exchange to/from and between postsynaptic specializations, leading to a dynamic equilibrium of receptor-scaffold complexes. Moreover, scaffold components serve as adaptor proteins that mediate specificity in intracellular transport complexes. In the present review, we discuss the dynamic delivery, stabilization and removal of inhibitory receptors at synaptic sites.

Regulation of excitation by GABA(A) receptor internalization

Neuronal inhibition is of paramount importance in maintaining the delicate and dynamic balance between excitatory and inhibitory influences in the central nervous system. GABA (gamma-aminobutyric acid), the primary inhibitory neurotransmitter in brain, exerts its fast inhibitory effects through ubiquitously expressed GABA(A) receptors. Activation of these heteropentameric receptors by GABA results in the gating of an integral chloride channel leading to membrane hyperpolarization and neuronal inhibition. To participate in neurotransmission, the receptor must reside on the cell surface. The trafficking of nascent receptors to the cell surface involves posttranslational modification and the interaction of the receptor with proteins that reside within the secretory pathway. The subsequent insertion of the receptor into specialized regions of the plasma membrane is dictated by receptor composition and other factors that guide insertion at synaptic or perisynaptic/extrasynaptic sites, where phasic and tonic inhibition are mediated, respectively. Once at the cell surface, the receptor is laterally mobile and subject to both constitutive and regulated endocytosis. Following endocytosis the receptor undergoes either recycling to the plasma membrane or degradation. These dynamic processes profoundly affect the strength of GABAergic signaling, neuronal inhibition, and presumably synaptic plasticity. Heritable channelopathies that affect receptor trafficking have been recently recognized and compelling evidence exists that mechanisms underlying acquired epilepsy involve GABA(A) receptor internalization. Additionally, GABA(A) receptor endocytosis has been identified as an early event in the ischemic response that leads to excitotoxicity and cell death. This chapter summarizes what is known regarding the regulation of receptor trafficking and cell surface expression and its impact on nervous system function from both cell biology and disease perspectives.

Development of gamma-aminobutyric acidergic synapses in cultured hippocampal neurons

The formation and maturation of gamma-aminobutyric acid (GABA)-ergic synapses was studied in cultured hippocampal pyramidal neurons by both performing immunocytochemistry for GABAergic markers and recording miniature inhibitory postsynaptic currents (mIPSCs). Nascent GABAergic synapses appeared between 3 and 8 days in vitro (DIV), with GABAA receptor subunit clusters appearing first, followed by GAD-65 puncta, then functional synapses. The number of GABAergic synapses increased from 7 to 14 DIV, with a corresponding increase in frequency of mIPSCs. Moreover, these new GABAergic synapses formed on neuronal processes farther from the soma, contributing to decreased mIPSC amplitude and slowed mIPSC 19-90% rise time. The mIPSC decay quickened from 7 to 14 DIV, with a parallel change in the distribution of the alpha5 subunit from diffuse expression at 7 DIV to clustered expression at 14 DIV. These alpha5 clusters were mostly extrasynaptic. The alpha1 subunit was expressed as clusters in none of the neurons at 7 DIV, in 20% at 14 DIV, and in 80% at 21 DIV. Most of these alpha1 clusters were expressed at GABAergic synapses. In addition, puncta of GABA transporter 1 (GAT-1) were localized to GABAergic synapses at 14 DIV but were not expressed at 7 DIV. These studies demonstrate that mIPSCs appear after pre- and postsynaptic elements are in place. Furthermore, the process of maturation of GABAergic synapses involves increased synapse formation at distal processes, expression of new GABAA receptor subunits, and GAT-1 expression at synapses; these changes are reflected in altered frequency, kinetics, and drug sensitivity of mIPSCs.

Gephyrin regulates the cell surface dynamics of synaptic GABAA receptors

The efficacy of fast synaptic inhibition is critically dependent on the accumulation of GABAA receptors at inhibitory synapses, a process that remains poorly understood. Here, we examined the dynamics of cell surface GABAA receptors using receptor subunits modified with N-terminal extracellular ecliptic pHluorin reporters. In hippocampal neurons, GABAA receptors incorporating pHluorin-tagged subunits were found to be clustered at synaptic sites and also expressed as diffuse extrasynaptic staining. By combining FRAP (fluorescence recovery after photobleaching) measurements with live imaging of FM4-64-labeled active presynaptic terminals, it was evident that clustered synaptic receptors exhibit significantly lower rates of mobility at the cell surface compared with their extrasynaptic counterparts. To examine the basis of this confinement, we used RNAi to inhibit the expression of gephyrin, a protein shown to regulate the accumulation of GABAA receptors at synaptic sites. However, whether gephyrin acts to control the actual formation of receptor clusters, their stability, or is simply a global regulator of receptor cell surface number remains unknown. Inhibiting gephyrin expression did not modify the total number of GABAA receptors expressed on the neuronal cell surface but significantly decreased the number of receptor clusters. Live imaging revealed that clusters that formed in the absence of gephyrin were significantly more mobile compared with those in control neurons. Together, our results demonstrate that synaptic GABAA receptors have lower levels of lateral mobility compared with their extrasynaptic counterparts, and suggest a specific role for gephyrin in reducing the diffusion of GABAA receptors, facilitating their accumulation at inhibitory synapses.

Neurobiological Effects of Bisphenol A May Be Mediated by Somatostatin Subtype 3 Receptors in Some Regions of the Developing Rat Brain

Considerable attention has been focused on environmental disruptors such as the xenoestrogen bisphenol A, which influences reproductive, developmental, and cognitive activities through its interaction with specific neuromediating systems in an estrogen-like fashion. In the present study, the effects of this xenoestrogen proved to be preferentially directed toward hypothalamic and extrahypothalamic somatostatin receptor subtype 3, which displayed a higher binding affinity of its specific nonpeptide agonist L-796-778 than that of L-779-976 (subtype 2). One type of action, with respect to animals treated with vehicle alone, consisted of a very strong (p < 0.001) decrease of somatostatin receptor subtype 3 mRNA levels in layer V of the frontoparietal cortex of adult rats (Sprague-Dawley) after transplacental and lactational exposure to bisphenol A (400 microg/kg/day). Similarly, such treatment in 7-day-old rats was responsible for a very strong reduction of the subtype 3 mRNA levels in the hypothalamic periventricular nuclei and a strong (p < 0.01) increase of the subtype 3 mRNA levels in the ventromedial nuclei. Moreover, even greater upregulated and downregulated activities were reported when subtype 3 mRNA levels were determined in the presence of receptor agonists specific for distinct alpha GABA(A) receptor subunits (alpha(1,5)). The predominant effects of bisphenol A on somatostatin receptor subtype 3 mRNA levels occurring in an alpha GABA(A) subunit-dependent manner tend to suggest the early modulatory importance of this environmental disruptor on cross-talking mechanisms that are implicated in the plasticity of neural circuits, with consequential influence on neuroendocrine/sociosexual behaviors.

GABAA receptor-associated protein traffics GABAA receptors to the plasma membrane in neurons

The trafficking of GABA(A) receptors is an important component of the pathway that regulates plasticity of inhibitory synapses. The 17 kDa GABA(A) receptor-associated protein (GABARAP) has been implicated in the trafficking of GABA(A) receptors because of its ability to interact not only with the gamma2 subunit of the receptor but also with microtubules and the N-ethylmaleimide-sensitive factor (NSF). To elucidate the role of GABARAP in the trafficking of GABA(A) receptors, we have constructed a yellow fluorescent protein (YFP) fusion protein of GABARAP and expressed it in neurons using adenovirus, so that its function may be examined. YFP-GABARAP colocalized with gamma2 subunit-containing GABA(A) receptors and NSF to the perinuclear cytoplasm in cultured hippocampal neurons and to the proximal regions of dendrites that are making synaptic contact. Expression of YFP-GABARAP in Cos7 cells and cultured hippocampal neurons was able to increase the level of GABA(A) receptors detected at the plasma membrane, even at low levels of YFP-GABARAP expression. This effect is specific to the function of GABARAP on GABA(A) receptor trafficking, because point mutations in the gamma2-binding domain of YFP-GABARAP interfered with the ability of YFP-GABARAP to increase GABA(A) receptor surface levels. These mutations also disrupted the colocalization of YFP-GABARAP with the gamma2 subunit and with NSF in hippocampal neurons. The results of this study show for the first time that GABARAP has a functional effect on the trafficking of GABA(A) receptors and provide decisive evidence for the role of GABARAP in transporting GABA(A) receptors to the plasma membrane in neurons.

Dynamic mobility of functional GABAA receptors at inhibitory synapses

Importing functional GABAA receptors into synapses is fundamental for establishing and maintaining inhibitory transmission and for controlling neuronal excitability. By introducing a binding site for an irreversible inhibitor into the GABAA receptor alpha1 subunit channel lining region that can be accessed only when the receptor is activated, we have determined the dynamics of receptor mobility between synaptic and extrasynaptic locations in hippocampal pyramidal neurons. We demonstrate that the cell surface GABAA receptor population shows no fast recovery after irreversible inhibition. In contrast, after selective inhibition, the synaptic receptor population rapidly recovers by the import of new functional entities within minutes. The trafficking pathways that promote rapid importation of synaptic receptors do not involve insertion from intracellular pools, but reflect receptor diffusion within the plane of the membrane. This process offers the synapse a rapid mechanism to replenish functional GABAA receptors at inhibitory synapses and a means to control synaptic efficacy.

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.

Distribution of alpha1, alpha4, gamma2, and delta subunits of GABAA receptors in hippocampal granule cells

GABAA receptors are pentamers composed of subunits derived from the alpha, beta, gamma, delta, theta, epsilon, and pi gene families. alpha1, alpha4, gamma2, and delta subunits are expressed in the dentate gyrus of the hippocampus, but their subcellular distribution has not been described. Hippocampal sections were double-labeled for the alpha1, alpha4, gamma2, and delta subunits and GAD65 or gephyrin, and their subcellular distribution on dentate granule cells was studied by means of confocal laser scanning microscopy (CLSM). The synaptic versus extrasynaptic localization of these subunits was inferred by quantitative analysis of the frequency of colocalization of various subunits with synaptic markers in high-resolution images. GAD65 immunoreactive clusters colocalized with 26.24+/-0.86% of the alpha1 subunit immunoreactive clusters and 32.35+/-1.49% of the gamma2 subunit clusters. In contrast, only 1.58+/-0.13% of the alpha4 subunit immunoreactive clusters and 1.92+/-0.15% of the delta subunit clusters colocalized with the presynaptic marker GAD65. These findings were confirmed by studying colocalization with immunoreactivity of a postsynaptic marker, gephyrin, which colocalized with 27.61+/-0.16% of the alpha1 subunit immunoreactive clusters and 23.45+/-0.32% of the gamma2 subunit immunoreactive clusters. In contrast, only 1.90+/-0.13% of the alpha4 subunit immunoreactive clusters and 1.76+/-0.10% of the delta subunit clusters colocalized with gephyrin. These studies demonstrate that a subset of alpha1 and gamma2 subunit clusters colocalize with synaptic markers in hippocampal dentate granule cells. Furthermore, all four subunits, alpha1, alpha4, gamma2, and delta, are present in the extrasynaptic locations.

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.

Cultured Hippocampal Pyramidal Neurons Express Two Kinds of GABAA Receptors

We combined a study of the subcellular distribution of the alpha1, alpha2, alpha4, beta1, beta2/3, gamma2, and delta subunits of the GABAA receptor with an electrophysiological analysis of GABAA receptor currents determine the to types of receptors expressed on cultured hippocampal pyramidal neurons. The immunocytochemistry study demonstrated that alpha1, alpha2, beta2/3, and gamma2 subunits formed distinct clusters of various sizes, which were colocalized with clusters of glutamate decarboxylase (GAD) immunoreactivity at rates ranging from 22 to 58%. In contrast, alpha4, beta1, and delta subunits were distributed diffusely over the cell soma and neuronal processes of cultured neurons and did not colocalize with the synaptic marker GAD. Whole-cell GABA receptor currents were moderately sensitive to GABAA and were modulated by diazepam. The whole-cell currents were also enhanced by the neurosteroid allopregnanolone (10 nM). Tonic currents, measured as changes in baseline current and noise, were sensitive to Zn2+, furosemide, and loreclezole; they were insensitive to diazepam. These studies suggest that two kinds of GABAA receptors are expressed on cultured hippocampal neurons. One kind of receptor formed clusters, which were present at GABAergic synapses and in the extrasynaptic membrane. The alpha1, alpha2, beta2/3, and gamma2 subunits were contained in clustered receptors. The second kind was distributed diffusely in the extrasynaptic membrane. The alpha4, beta1, and delta subunits were contained in these diffusely distributed receptors. The properties of tonic currents recorded from these neurons were similar to those from recombinant receptors containing alpha4, beta1, and delta subunits.

Clustering of Extrasynaptic GABAA Receptors Modulates Tonic Inhibition in Cultured Hippocampal Neurons

Tonic inhibition plays a crucial role in regulating neuronal excitability because it sets the threshold for action potential generation and integrates excitatory signals. Tonic currents are known to be largely mediated by extrasynaptic gamma-aminobutyric acid type A (GABA(A)) receptors that are persistently activated by submicromolar concentrations of ambient GABA. We recently reported that, in cultured hippocampal neurons, the clustering of synaptic GABA(A) receptors significantly affects synaptic transmission. In this work, we demonstrated that the clustering of extrasynaptic GABA(A) receptors modulated tonic inhibition. Depolymerization of the cytoskeleton with nocodazole promoted the disassembly of extrasynaptic clusters of delta and gamma(2) subunit-containing GABA(A) receptors. This effect was associated with a reduction in the amplitude of tonic currents and diminished shunting inhibition. Moreover, diffuse GABA(A) receptors were less sensitive to the GAT-1 inhibitor NO-711 and to flurazepam. Quantitative analysis of GABA-evoked currents after prolonged exposure to submicromolar concentrations of GABA and model simulations suggest that clustering affects the gating properties of extrasynaptic GABA(A) receptors. In particular, a larger occupancy of the singly and doubly bound desensitized states can account for the modulation of tonic inhibition recorded after nocodazole treatment. Moreover, comparison of tonic currents recorded during spontaneous activity and those elicited by exogenously applied low agonist concentrations allows estimation of the concentration of ambient GABA. In conclusion, receptor clustering appears to be an additional regulating factor for tonic inhibition.

Expression of alpha 5 GABAA receptor subunit in developing rat hippocampus

The GABAergic system plays an important role in the hippocampal development. Here we have studied the developmental expression of the alpha 5 subunit of the GABA(A) receptor (from rat hippocampus) by RT-competitive PCR, immunoblot and immunocytochemistry. Our results demonstrated an early induction of the alpha 5 subunit expression (at mRNA and protein levels) during the first postnatal week, peaking at P5 and decreasing after this age. The peak of alpha 5 subunit expression precedes the peak of expression for the synaptophysin, GAD65 and GAD67. Thus, the increase in the alpha 5 GABA(A) receptor subunit expression may precede the GABAergic synaptogenesis. Importantly, between P0 and P7, the expression of the alpha 5 subunit was concentrated at the cell somata of the pyramidal and granular cells. After P10, its localization shifted from the cell bodies to the dendritic layers. This developmental pattern is similar to that reported for the Na(+)-K(+)-2Cl(-) system and it might be correlated with the transition from excitatory to inhibitory GABAergic activity.

Organization of GABA receptor alpha-subunit clustering in the developing rat neocortex and hippocampus

We compared the expression and co-expression of alpha1, alpha2, alpha3, and alpha5-subunit protein clusters of the gamma-aminobutyric acid (GABA)(A) receptor in the neocortex and hippocampus of rat at postnatal days (PND) 5-10 and 30-40 in order to understand how inhibitory receptors reorganize during brain maturation. The size, intensity, density and pattern of co-localization of fluorescently tagged subunit clusters were determined in deconvolved digital images using a novel 2D cross-correlational analysis. The cross-correlation analysis allowed an unbiased identification of GABA(A) receptor subunit clusters based on staining intensity. Cluster size increased through development; only the alpha2 clusters in dentate gyrus (DG) decreased in size. alpha5-subunit cluster density either increased or decreased with maturation depending on the brain region. For the other subunits, the cluster density remained rather constant, with noted exceptions (increase in alpha2 clusters in cortical layer 5 but a decrease of alpha3 clusters in hilus). The co-localization of alpha1-subunit with the others was unique and not correlated to overall changes in subunit abundance between developmental époques. So, although alpha2-subunit expression went up in the DG, the clusters became less co-localized with alpha1. In contrast, alpha5-subunit clusters became more co-localized with alpha1 as the alpha5-subunit expression declined in cortex and CA1. The co-localization of alpha3 with alpha1 also became greater in layer 6. In the adult brain not all clustering was associated with synapses, as many alpha-subunit clusters did not co-localize with synaptophysin. Overall, these data indicate that the regulation of GABA(A) receptor clustering is both synaptic and extrasynaptic, presumably reflecting complex cellular trafficking mechanisms.

Altered Pharmacology of Synaptic and Extrasynaptic GABAA Receptors on CA1 Hippocampal Neurons Is Consistent with Subunit Changes in a Model of Alcohol Withdrawal and Dependence

Previously, we reported (Cagetti, Liang, Spigelman, and Olsen, 2003) that chronic intermittent ethanol (CIE) treatment leads to signs of alcohol dependence, including anxiety and hyperactivity, accompanied by reduced synaptic gamma-aminobutyric acid (A) receptor (GABAAR) function and altered sensitivity to its allosteric modulators consistent with a measured switch in subunit composition. In this study, we separated the synaptic and extrasynaptic components of GABAAR activation in recordings from pyramidal CA1 cells of hippocampal slices and demonstrated marked differences in the responsiveness of synaptic and extrasynaptic GABAARs to agonists and allosteric modulators in control rats, and in the way they are altered following CIE treatment. Notably, tonic inhibition mediated by extrasynaptic GABAARs was differentially sensitive to the partial agonist gaboxadol (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; THIP) and the allosteric modulator zolpidem, compared with the miniature inhibitory synaptic currents (mIPSCs) in the same cells from saline-treated rats. After CIE treatment, potentiation of tonic currents by diazepam and zolpidem was lost, whereas potentiation by the alpha4 subunit-preferring benzodiazepine Ro15-4513 (ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a]-[1,4]benzodiazepine-3-carboxylate) and THIP was only partially reduced. Potentiation of synaptic GABAAR currents by zolpidem was eliminated after CIE, whereas THIP slightly inhibited mIPSCs from control rats and greatly enhanced them after CIE treatment. These results are consistent with alpha1 subunit decreases at synaptic and extrasynaptic GABAARs, whereas alpha4 subunits are increased at synaptic and decreased at extrasynaptic GABAARs. Behaviorally, THIP was active as a hypnotic and anxiolytic but not as an anti-convulsant against pentylenetetrazol seizures in control rats. Only slight tolerance was observed to the sleep time, but not to the anxiolytic, effect of THIP after CIE. Thus, differential alterations in synaptic and extrasynaptic GABAARs appear to play an important role in the brain plasticity of alcohol dependence, and withdrawal signs may be profitably treated with GABAergic drugs such as THIP, which does not show cross-tolerance with ethanol.

Hetero-oligomerization between GABAA and GABAB Receptors Regulates GABAB Receptor Trafficking

The neurotransmitter gamma-aminobutyric acid (GABA) mediates inhibitory signaling in the brain via stimulation of both GABA(A) receptors (GABA(A)R), which are chloride-permeant ion channels, and GABA(B) receptors (GABA(B)R), which signal through coupling to G proteins. Here we report physical interactions between these two different classes of GABA receptor. Association of the GABA(B) receptor 1 (GABA(B)R1) with the GABA(A) receptor gamma2S subunit robustly promotes cell surface expression of GABA(B)R1 in the absence of GABA(B)R2, a closely related GABA(B) receptor that is usually required for efficient trafficking of GABA(B)R1 to the cell surface. The GABA(B)R1/gamma2S complex is not detectably functional when expressed alone, as assessed in both ERK activation assays and physiological analyses in oocytes. However, the gamma2S subunit associates not only with GABA(B)R1 alone but also with the functional GABA(B)R1/GABA(B)R2 heterodimer to markedly enhance GABA(B) receptor internalization in response to agonist stimulation. These findings reveal that the GABA(B)R1/gamma2S interaction results in the regulation of multiple aspects of GABA(B) receptor trafficking, allowing for cross-talk between these two distinct classes of GABA receptor.

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.

Association of gephyrin with synaptic and extrasynaptic GABAA receptors varies during development in cultured hippocampal neurons

Several studies have reported extrasynaptic clusters of GABAA receptors in hippocampal neurons. Yet their functional relevance as well as their evolution in relation with gephyrin during synaptogenesis remain unknown. We have analyzed the expression pattern of the main proteins of the GABAergic synapses during synaptogenesis in cultured hippocampal neurons. We found that GABAergic terminals, characterized by VIAAT and GAD-65 expression, differentiated 3 to 7 days after the glutamatergic endings. At the postsynaptic side, the GABAAR- beta3 subunit was first diffuse and then clustered when GABAergic terminals differentiated and gephyrin formed large clusters. Colocalization of these proteins was high and increased with development. At later stages, GABAAR beta3 clusters colocalized with gephyrin at synaptic but also at extrasynaptic sites. GABAAR gamma2 subunits were directly expressed as clusters which were first extrasynaptic and not associated with gephyrin. Subsequently, the GABAAR gamma2 subunits associated with gephyrin at synaptic and/or extrasynaptic sites. Our data indicate that formation of GABAAR gamma2 subunit clusters is gephyrin independent.

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.

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.

The small conductance Ca2+-activated K+ channel SK3 is localized in nerve terminals of excitatory synapses of cultured mouse hippocampal neurons

In the central nervous system small conductance Ca2+-activated K+ (SK) channels are important for generating the medium/slow afterhyperpolarization seen after single or trains of action potentials. Three SK channel isoforms (SK1,-2,-3) are differentially distributed throughout the brain, but little is known about their specific expression in particular neuronal compartments. In the hippocampus SK3 was found in the neuropil, predominantly in the terminal field of the mossy fibres and in fine varicose fibres, but excluded from the pyramidal and granule cell layers. Because this expression pattern suggested a presynaptic localization, we examined the subcellular distribution of SK3 in cultured hippocampal neurons using high-resolution immunofluorescence analysis. SK3 was localized in a punctate, synaptic pattern. The SK3 clusters were precisely colocalized with the presynaptic marker synapsin and at close range (0.4-0.5 microm) from NMDA-receptors and PSD-95. This arrangement is consistent with a localization of SK3 in the presynaptic nerve terminal, but not restricted to the synaptic membrane proper. In agreement with the increasing expression of SK3 during early postnatal development in vivo, the fraction of synapses containing SK3 increased from 14% to 57% over a six-week culture period. SK3-containing synapses were equally observed on spiny, glutamatergic and smooth GABAergic neurons. In contrast to its close association with NMDA-receptors and PSD-95, SK3 was rarely associated with GABAA-receptor clusters. Thus, SK3 is a presynaptic channel in excitatory hippocampal synapses, with no preference for glutamatergic or GABAergic postsynaptic neurons, and is probably involved in regulating neurotransmitter release.

Increased neurosteroid sensitivity of hippocampal GABAA receptors during postnatal development

To investigate developmental changes in neurosteroid modulation of GABA(A) receptors, whole-cell currents were elicited by applying GABA with allopregnanolone or pregnenolone sulfate (PS) to dentate granule cells (DGCs), acutely isolated from 7-14-day-old and adult rats. GABA evoked larger currents from dentate granule cells acutely isolated from adult rats (adult DGCs) than from neonatal DGCs, due to increased efficacy (1662+/-267 pA in adult DGCs versus 1094+/-198 pA in neonatal DGCs, P=0.004), and current density (0.072+/-0.01 pA/microm(2) in neonatal rat DGCs to 0.178+/-0.02 pA/microm(2) in adult DGCs), but unchanged potency (EC(50) was 18.5+/-2 microm in adult DGCs, and 26.6+/-7.9 microm in neonatal DGCs, P=0.21). Allopregnanolone sensitivity of GABA(A) receptor currents increased during development due to an increased potency (21.1+/-4.7 nM in adult DGCs versus 94.6+/-9 nM in neonatal DGCs, P=0.0002). The potency and efficacy of PS inhibition of GABA(A) receptor currents were remained unchanged during development (13+/-6 microm and 13.2+/-5.9 microm, P=0.71 and 85.5%+/-3.5% and 83.6%+/-0.8%, P=0.29, respectively). To investigate possible mechanism of developmental changes in GABA(A) receptor properties, in situ hybridization for alpha1, alpha4 and gamma2 subunit mRNAs was performed in dentate gyrus of the two age groups. Qualitatively, alpha1 subunit mRNA was expressed at low levels in neonatal rats while it was well expressed in adult rats. The alpha4 and gamma2 subunits were well expressed in the dentate gyrus of adult and neonatal rats. Immunohistochemical staining for alpha1 subunit in hippocampal slices from neonatal and adult rats was examined under confocal laser scanning microscope. This demonstrated that cell bodies and dendrites of granule cells are moderately positive for the alpha1 staining in adult rats but weakly so in neonatal rats. Higher-magnification images demonstrate large number of clusters of alpha1-subunit in the cell bodies of dentate granule cells of adult rat but rare clusters in granule cells of neonatal rats. Maturation of GABA(A) receptors in DGCs is characterized by increased number of GABA(A) receptors that are more sensitive to endogenous neurosteroid allopregnanolone, which might be related to increased expression of alpha1 subunit.

Benzodiazepine binding studies on living cells: application of small ligands for fluorescence correlation spectroscopy

We demonstrate the applicability of fluorescence correlation spectroscopy (FCS) for receptor binding studies using low molecular weight ligands on the membranes of living nerve cells. The binding of the benzodiazepine Ro 7-1986/602 (N-des-diethyl-fluorazepam), labeled with the fluorophore Alexa 532, to the benzodiazepine receptor was analyzed quantitatively at the membrane of single rat hippocampal neurons. The values obtained for the dissociation constant Kd = (9.9 +/- 1.9) nm and the rate constant for ligand-receptor dissociation kdisS = (1.28 +/- 0.08) x 10(-3) s(-1) show that there is a specific and high affinity interaction between the dye-labeled ligand (Ro-Alexa) and the receptor site. The binding was saturated at approx. 100 nM and displacement of 10 nM Ro-Alexa, with a 1,000-fold excess of midazolam, showed a non-specific binding of 7-10%. Additionally, two populations of the benzodiazepine receptor that differed in their lateral mobility were detected in the membrane of rat neurons. The diffusion coefficients for these two populations [D(bound1) = (1.32 +/- 0.26) microm2/s; D(bound2) = (2.63 +/- 0.63) x 10(-2) microm2/s] are related to binding sites, which shows a mono-exponential decay in a time-dependent dissociation of the ligand-receptor complex.

Cadherin regulates dendritic spine morphogenesis

Synaptic remodeling has been postulated as a mechanism underlying synaptic plasticity, and cadherin adhesion molecules are thought to be a regulator of such a process. We examined the effects of cadherin blockage on synaptogenesis in cultured hippocampal neurons. This blockade resulted in alterations of dendritic spine morphology, such as filopodia-like elongation of the spine and bifurcation of its head structure, along with concomitant disruption of the distribution of postsynaptic proteins. The accumulation of synapsin at presynaptic sites and synaptic vesicle recycling were also perturbed, although these synaptic responses to the cadherin blockade became less evident upon the maturation of the synapses. These findings suggest that cadherin regulates dendritic spine morphogenesis and related synaptic functions, presumably cooperating with cadherin-independent adhesive mechanisms to maintain spine-axon contacts.

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.

GABAergic innervation organizes synaptic and extrasynaptic GABAA receptor clustering in cultured hippocampal neurons

We have studied the effects of GABAergic innervation on the clustering of GABA(A) receptors (GABA(A)Rs) in cultured hippocampal neurons. In the absence of GABAergic innervation, pyramidal cells form small (0.36 +/- 0.01 micrometer diameter) GABA(A)R clusters at their surface in the dendrites and soma. When receiving GABAergic innervation from glutamic acid decarboxylase-containing interneurons, pyramidal cells form large (1.62 +/- 0.08 micrometer breadth) GABA(A)R clusters at GABAergic synapses. This is accompanied by a disappearance of the small GABA(A)R clusters in the local area surrounding each GABAergic synapse. Although the large synaptic GABA(A)R clusters of any neuron contained all GABA(A)R subunits and isoforms expressed by that neuron, the small clusters not localized at GABAergic synapses showed significant heterogeneity in subunit and isoform composition. Another difference between large GABAergic and small non-GABAergic GABA(A)R clusters was that a significant proportion of the latter was juxtaposed to postsynaptic markers of glutamatergic synapses such as PSD-95 and AMPA receptor GluR1 subunit. The densities of both the glutamate receptor-associated and non-associated small GABA(A)R clusters were decreased in areas surrounding GABAergic synapses. However, no effect on the density or distribution of glutamate receptor clusters was observed. The results suggest that there are local signals generated at GABAergic synapses that induce both assembly of large synaptic GABA(A)R clusters at the synapse and disappearance of the small GABA(A)R clusters in the surrounding area. In the absence of GABAergic innervation, weaker GABA(A)R-clustering signals, generated at glutamatergic synapses, induce the formation of small postsynaptic GABA(A)R clusters that remain juxtaposed to glutamate receptors at glutamatergic synapses.

GABAergic innervation organizes synaptic and extrasynaptic GABAA receptor clustering in cultured hippocampal neurons

We have studied the effects of GABAergic innervation on the clustering of GABA(A) receptors (GABA(A)Rs) in cultured hippocampal neurons. In the absence of GABAergic innervation, pyramidal cells form small (0.36 +/- 0.01 micrometer diameter) GABA(A)R clusters at their surface in the dendrites and soma. When receiving GABAergic innervation from glutamic acid decarboxylase-containing interneurons, pyramidal cells form large (1.62 +/- 0.08 micrometer breadth) GABA(A)R clusters at GABAergic synapses. This is accompanied by a disappearance of the small GABA(A)R clusters in the local area surrounding each GABAergic synapse. Although the large synaptic GABA(A)R clusters of any neuron contained all GABA(A)R subunits and isoforms expressed by that neuron, the small clusters not localized at GABAergic synapses showed significant heterogeneity in subunit and isoform composition. Another difference between large GABAergic and small non-GABAergic GABA(A)R clusters was that a significant proportion of the latter was juxtaposed to postsynaptic markers of glutamatergic synapses such as PSD-95 and AMPA receptor GluR1 subunit. The densities of both the glutamate receptor-associated and non-associated small GABA(A)R clusters were decreased in areas surrounding GABAergic synapses. However, no effect on the density or distribution of glutamate receptor clusters was observed. The results suggest that there are local signals generated at GABAergic synapses that induce both assembly of large synaptic GABA(A)R clusters at the synapse and disappearance of the small GABA(A)R clusters in the surrounding area. In the absence of GABAergic innervation, weaker GABA(A)R-clustering signals, generated at glutamatergic synapses, induce the formation of small postsynaptic GABA(A)R clusters that remain juxtaposed to glutamate receptors at glutamatergic synapses.