Alterations in the expression of GABAA receptor subunits in cerebellar granule cells after the disruption of the alpha6 subunit gene

Genetic manipulations of GABAA receptor in mice make inhibition exciting

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) plays an important role in brain development and behavior. GABA(A) receptor subunits knock-out and knock-in mice have proven that GABA(A) receptors are involved in control of motor coordination, learning, and memory and play a role in anxiety, panic, and epileptogenesis. In addition, these receptors are involved in the molecular mechanisms of action of many drugs and participate actively in cortical plasticity. The use of genetically engineered mice has perhaps never been as successful as in understanding the importance of the heterogeneity of GABA(A) receptors. We review these findings and speculate on the new directions that the use of mice with altered expression of GABA(A) receptor subunits may provide.

Pontine GABAergic pathways: role and plasticity in the hypoxic ventilatory response

The hypoxic ventilatory response (HVR) was compared before and after uni- and bi-lateral injections of bicuculline, a GABA(A) receptor antagonist, into the ventrolateral (vl) pons and before and after conditioning animals to chronic sustained hypoxia (CSH). The HVR was assessed by recording phrenic nerve activity (PNA) during and after brief exposures to hypoxia (8% O(2) and 92% N(2) for 45s). Inspiratory (T(I)) and expiratory (T(E)) durations were averaged before hypoxia, at the peak breathing frequency during hypoxia, before the end of hypoxia, immediately after hypoxia, and 60s after hypoxia. Blocking GABA(A) receptors in the vl pons prolonged T(E) during, but not after hypoxia. After CSH induced by 14 days in a hypobaric chamber (0.5atm), the HVR was attenuated compared to that in the naive animals. This plasticity of HVR was associated with selective induction of alpha6 and delta GABA(A) receptor subunit mRNAs specifically in the pons compared to the medulla. These physiological and molecular results illustrate the importance of pontine GABAergic pathways in shaping the response to hypoxia.

γ-Aminobutyric acid A receptor subunit mutant mice: new perspectives on alcohol actions

gamma-Aminobutyric acid A (GABA(A)) receptors are believed to mediate a number of alcohol's behavioral actions. Because the subunit composition of GABA(A) receptors determines receptor pharmacology, behavioral sensitivity to alcohol (ethanol) may depend on which subunits are present (or absent). A number of knock-out and/or transgenic mouse models have been developed (alpha1, alpha2, alpha5, alpha6, beta2, beta3, gamma2S, gamma2L, delta) and tested for behavioral sensitivity to ethanol. Here we review the current GABA(A) receptor subunit knock-out and transgenic literature for ethanol sensitivity, and integrate these results into those obtained using quantitative trait loci (QTL) analysis and gene expression assays. Converging evidence from these three approaches support the notion that different behavioral actions of ethanol are mediated by specific subunits, and suggest that new drugs that target specific GABA(A) subunits may selectively alter some behavioral actions of ethanol, without altering others. Current data sets provide strongest evidence for a role of alpha1-subunits in ethanol-induced loss of righting reflex, and alpha5-subunits in ethanol-stimulated locomotion. However, three-way validation is hampered by the incomplete behavioral characterization of many of the mutant mice, and additional subunits are likely to be linked to alcohol actions as behavioral testing progresses.

Tonic inhibition in mouse hippocampal CA1 pyramidal neurons is mediated by alpha5 subunit-containing gamma-aminobutyric acid type A receptors

The principal inhibitory neurotransmitter in the mammalian brain, gamma-aminobutyric acid (GABA), is thought to regulate memory processes by activating transient inhibitory postsynaptic currents. Here we describe a nonsynaptic, tonic form of inhibition in mouse CA1 pyramidal neurons that is generated by a distinct subpopulation of GABA type A receptors (GABA(A)Rs). This tonic inhibitory conductance is predominantly mediated by alpha5 subunit-containing GABA(A)Rs (alpha5GABA(A)Rs) that have different pharmacological and kinetic properties compared to postsynaptic receptors. GABA(A)Rs that mediate the tonic conductance are well suited to detect low, persistent, ambient concentrations of GABA in the extracellular space because they are highly sensitive to GABA and desensitize slowly. Moreover, the tonic current is highly sensitive to enhancement by amnestic drugs. Given the restricted expression of alpha5GABA(A)Rs to the hippocampus and the association between reduced alpha5GABA(A)R function and improved memory performance in behavioral studies, our results suggest that tonic inhibition mediated by alpha5GABA(A)Rs in hippocampal pyramidal neurons plays a key role in cognitive processes.

ANALYSIS OFGABAARECEPTORFUNCTION ANDDISSECTION OF THEPHARMACOLOGY OFBENZODIAZEPINES ANDGENERALANESTHETICSTHROUGHMOUSEGENETICS

GABAA receptors are molecular substrates for the regulation of vigilance, anxiety, muscle tension, epileptogenic activity, and memory functions, and the enhancement of GABAA receptor-mediated fast synaptic inhibition is the basis for the pharmacotherapy of various neurological and psychiatric disorders. Two kinds of GABAA receptor-targeted mutant mice have been generated: (a) knockout mice that lack individual GABAA receptor subunits (alpha1, alpha5, alpha6, beta2, beta3, gamma2, delta, and rho1) and (b) knockin mice that carry point mutations affecting the action of modulatory drugs [alpha1(H101R), alpha2(H101R), alpha3(H126R), alpha5(H105R), and beta3(N265M)]. Whereas the knockout mice have provided information primarily with respect to the regulation of subunit gene transcription, receptor assembly, and some physiological functions of individual receptor subtypes, the point-mutated knockin mice in which specific GABAA receptor subtypes are insensitive to diazepam or some general anesthetics have revealed the specific contribution of individual receptor subtypes to the pharmacological spectrum of diazepam and general anesthetics.

Subunit composition and quantitative importance of GABA(A) receptor subtypes in the cerebellum of mouse and rat

In cerebellum, 13 different GABA(A) receptor subunits are expressed. The number of different receptor subtypes formed in this tissue, their subunit composition and their quantitative importance so far has not been determined. In the present study, immunodepletion by immunoaffinity chromatography, as well as immunoprecipitation and western blot analysis was performed using 13 different subunit-specific antibodies to provide an overview on the subunit composition and abundance of GABA(A) receptor subtypes in mouse and rat cerebellum. Results obtained indicate that alpha1betaxgamma2, alpha1alpha6betaxgamma2, alpha6betaxgamma2, alpha6betaxdelta and alpha1alpha6betaxdelta are the major GABA(A) receptor subtypes present in the cerebellum. In addition, small amounts of alpha1betaxdelta receptors and a series of minor receptor subtypes containing alpha2, alpha3, alpha4, alpha5, gamma1 or gamma3 subunits are also present in the cerebellum. Whereas the abundance of alpha1alpha6betaxgamma2, alpha6betaxdelta and alpha1alpha6betaxdelta receptors is different in mouse and rat cerebellum, that of other receptors is quite similar in these tissues. Data obtained for the first time provide an overview on the GABA(A) receptor subtypes present in the cerebellum and represent the basis for further studies investigating changes in receptor expression and composition under pathological conditions.

Actions and interactions of extracellular potassium and kainate on expression of 13 gamma-aminobutyric acid type A receptor subunits in cultured mouse cerebellar granule neurons

Cerebellar granule neurons in culture are a popular model for studying neuronal signaling and development. Depolarizing concentrations of K(+) are routinely used to enhance cell survival, and kainate is sometimes added to eliminate GABAergic neurons. We have investigated the effect of these measures on expression of mRNA for gamma-aminobutyric acid type A (GABA(A)) receptor alpha1-6, beta1-3, gamma1-3, and delta subunits in cultures of mouse cerebellar granule neurons grown for 7 or 12 days in vitro (DIV) using semiquantitative reverse transcription-PCR. We detected mRNA for the alpha1, alpha2, alpha5, alpha6, beta2, beta3, gamma2, and delta subunits in all the cell cultures, but the expression levels of the alpha5-, alpha6-, and beta2-subunit mRNAs were significantly dependent on the composition of the culture medium. Both an increase of the extracellular K(+) concentration from 5 to 25 mm and the addition of 50 microm kainate immediately depolarized the neurons but prolonged exposure (7-8 DIV)-induced compensatory hyperpolarization. 25 mm K(+) caused a shift from alpha6 to alpha5 expression measured at 7 and 12 DIV, which was mimicked by kainate in 12 DIV cultures. The expression of beta2 was decreased by 25 mm K(+) in 7 DIV cultures and by kainate in 12 DIV cultures. The effects on beta2 expression could not be ascribed to depolarization. Alterations of alpha6 mRNA expression were reflected in altered sensitivity to GABA and furosemide of the resulting receptors. Our study has shown that a depolarizing K(+) concentration as well as kainate in the culture medium significantly disturbs maturation of GABA(A) receptor subunit expression.

Intact synaptic GABAergic inhibition and altered neurosteroid modulation of thalamic relay neurons in mice lacking delta subunit

Robust GABA-mediated inhibitory postsynaptic currents (IPSCs) in neurons of the thalamic relay (TC) nuclei are important in sustaining oscillatory activity within thalamic and thalamocortical circuits. The biophysical properties and pharmacological sensitivities of these IPSCs both depend on the subunit combination of postsynaptic gamma-aminobutyric acid-A (GABA(A)) receptors. Recombinant GABA(A) receptors containing the delta subunit (heavily expressed in TC nuclei) have been shown to exhibit slowed desensitization rates and high affinity for GABA in heterologous expression systems. We tested whether the GABA(A)-mediated synaptic inhibition in TC neurons would be affected by loss of the delta subunit. Spontaneous and evoked IPSCs were recorded from neurons in the ventral basal complex (VB) of the thalamus from brain slices of wild-type (delta(+/+)) and homozygous delta subunit deficient mice (delta(-/-)). Spontaneous IPSCs (sIPSCs) from delta(-/-) mice had no significant differences in amplitude, duration, or frequency compared with their delta(+/+) counterparts. However, baseline noise (63% of control) and the relative contribution of the slow component to overall decay (79% of control) were significantly lower in delta(-/-) VB recordings. Evoked IPSCs (eIPSCs) in delta(-/-) neurons showed no difference in peak amplitude, but had an accelerated slow decay component (40- vs. 55-ms time constant). We further tested whether neurosteroid modulation of GABA(A) receptors was dependent on the presence of the delta subunit, as previously reported in recombinant systems. Pregnenolone sulfate (PS) significantly reduced eIPSC peak amplitude (-30%) and increased duration in delta(-/-), but not in delta(+/+) mice. sIPSCs were not affected in any neurons, delta(-/-) or delta(+/+). In contrast, 3-alpha,5-alpha-tetrahydrodeoxycorticosterone (THDOC) increased the durations of eIPSCs and sIPSCs in both delta(-/-) and delta(+/+) VB neurons. Our findings show that although the delta subunit confers a striking PS insensitivity to eIPSCs in VB neurons, it plays only a minor role in the synaptic inhibition of VB neurons. This suggests delta subunit containing GABA(A) receptors may be functionally limited to an extrasynaptic locus in VB neurons.

Competitive GABA(A) receptor antagonists increase the proportion of functional high-affinity alpha6 subunit-containing receptors in granule cells of adult rat cerebellum

To investigate the properties of alpha6 subunit-containing GABA(A) receptors, whole-cell patch-clamp recordings were made from granule cells in adult rat cerebellar slices. In control, only currents evoked by low concentrations of GABA were significantly reduced in amplitude by furosemide, the alpha6 subunit-containing receptor antagonist. However, in the presence of competitive GABA(A) receptor antagonists, the furosemide block of currents evoked by higher GABA concentrations was markedly increased. Zinc, which preferentially blocks alpha6 subunit-containing receptors, also produced an increased block in the presence of bicuculline. To investigate whether similar effects occurred at synaptic receptors, inhibitory postsynaptic currents (IPSCs) were recorded. In most cells, furosemide produced little or no reduction in evoked IPSC amplitude. However in the presence of SR95531, a competitive antagonist, furosemide markedly reduced IPSC amplitude. One hypothesis, which could account for these observations, is that competitive antagonists prevent the continual activation of alpha6beta2/3gamma2 receptors by endogenous GABA and thus prevent their desensitisation. This hypothesis appears feasible as prolonged applications of low concentrations of GABA to recombinant alpha6beta2gamma2s receptors resulted in their desensitisation.

Molecular biology and ontogeny of gamma-aminobutyric acid (GABA) receptors in the mammalian central nervous system

gamma-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. After release from nerve terminals, GABA binds to at least two classes of postsynaptic receptors (ie, GABAA and GABAB), which are nearly ubiquitous in the brain. GABAA receptors are postsynaptic heteropentameric complexes that display unique physiologic and pharmacologic properties based on subunit composition. Activation of GABAA receptors in mature neurons results in membrane hyperpolarization, which is mediated principally by inward chloride flux, whereas in early stages of brain development, GABAA receptor activation causes depolarization of the postsynaptic membrane. GABA, receptors reside both presynaptically and postsynaptically, exist as heterodimers and are coupled to voltage-dependent ion channels through interactions with heterotrimeric G proteins. This review summarizes the molecular biology and ontogeny of GABAA and GABAB receptors, highlighting some of their putative roles during normal brain development as well as in disease states such as epilepsy.

Ectopic expression of the GABA(A) receptor alpha6 subunit in hippocampal pyramidal neurons produces extrasynaptic receptors and an increased tonic inhibition

We generated transgenic (Thy1alpha6) mice in which the GABA(A) receptor alpha6 subunit, whose expression is usually confined to granule cells of cerebellum and cochlear nuclei, is ectopically expressed under the control of the pan-neuronal Thy-1.2 promoter. Strong Thy1alpha6 subunit expression occurs, for example, in deep cerebellar nuclei, layer V iscocortical and hippocampal pyramidal cells and dentate granule cells. Ligand binding and protein biochemistry show that most forebrain alpha6 subunits assemble as alpha6betagamma2-type receptors, and some as alpha1alpha6betagamma2 and alpha3alpha6betagamma2 receptors. Electron microscopic immunogold labeling shows that most Thy1-derived alpha6 immunoreactivity is in the extrasynaptic plasma membrane of dendrites and spines in both layer V isocortical and CA1pyramidal cells. Synaptic immunolabeling is rare. Consistent with the alpha6 subunits' extrasynaptic localization, Thy1alpha6 CA1 pyramidal neurons have a five-fold increased tonic GABA(A) receptor-mediated current compared with wild-type cells; however, the spontaneous IPSC frequency and the mIPSC amplitude in Thy1alpha6 mice decrease 37 and 30%, respectively compared with wild-type. Our results strengthen the idea that GABA(A) receptors containing alpha6 subunits can function as extrasynaptic receptors responsible for tonic inhibition and further suggest that a homeostatic mechanism might operate, whereby increased tonic inhibition causes a compensatory decrease in synaptic GABA(A) receptor responses.

Amiloride inhibition of gamma-aminobutyric acid(A) receptors depends upon the alpha subunit subtype

gamma-Aminobutyric acid(A) (GABA(A)) receptors (GABARs) are responsible for most fast inhibitory neurotransmission in the mammalian brain. The GABARs contain several allosteric modulatory sites, many of which are useful clinically. The activity of most of these modulators depends upon the subunit composition of the receptor. The diuretic amiloride was previously reported to inhibit GABARs in frog sensory neurons. We measured its effects on recombinant GABARs to determine its mechanism of action at mammalian receptors and to examine the effect of subunit composition. Amiloride acted primarily as a competitive antagonist, reducing the sensitivity of the receptor to GABA without affecting the maximal current amplitude. Receptors containing an alpha6 subunit were about 10-fold more sensitive to amiloride than those containing other alpha subunits. In contrast, the identity of the beta or gamma subtype had little effect on amiloride sensitivity. Although several other modulators have specific effects at alpha6-containing receptors, amiloride is the first inhibitor to be reported with no additional dependence on the identity of the beta or gamma subunit. Therefore, it probably represents a unique modulatory site on the GABAR, which could be useful for developing drugs targeting these receptors. The selective activity of amiloride could also be helpful for isolating the contribution of receptors composed of alpha6 subtypes in heterogeneous native GABAR populations.

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.

Tonic and spillover inhibition of granule cells control information flow through cerebellar cortex

We show that information flow through the adult cerebellar cortex, from the mossy fiber input to the Purkinje cell output, is controlled by furosemide-sensitive, diazepam- and neurosteroid-insensitive GABA(A) receptors on granule cells, which are activated both tonically and by GABA spillover from synaptic release sites. Tonic activation of these receptors contributes a 3-fold larger mean inhibitory conductance than GABA released synaptically by high-frequency stimulation. Tonic and spillover inhibition reduce the fraction of granule cells activated by mossy fiber input, generating an increase of coding sparseness, which is predicted to improve the information storage capacity of the cerebellum.

A new benzodiazepine pharmacology

Classical benzodiazepine drugs are in wide clinical use as anxiolytics, hypnotics, anticonvulsants, and muscle relaxants. They act by enhancing the gamma-aminobutyric acid(A) (GABA(A)) receptor function in the central nervous system. The pharmacological relevance of the multitude of structurally diverse GABA(A) receptor subtypes has only recently been identified. Based on an in vivo point mutation strategy, alpha(1)-GABA(A) receptors were found to mediate sedation, anterograde amnesia, and part of the seizure protection, whereas alpha(2)-GABA(A) receptors, but not alpha(3)-receptors, mediate anxiolysis. Rational drug targeting to specific receptor subtypes has now become possible. Only restricted neuronal networks will be modulated by the new subtype-selective drugs. Promising new anxiolytics have already been developed. A new pharmacology of the benzodiazepine site is on the horizon.

Disruption of GABA(A) receptors on GABAergic interneurons leads to increased oscillatory power in the olfactory bulb network

Synchronized neural activity is believed to be essential for many CNS functions, including neuronal development, sensory perception, and memory formation. In several brain areas GABA(A) receptor-mediated synaptic inhibition is thought to be important for the generation of synchronous network activity. We have used GABA(A) receptor beta3 subunit deficient mice (beta3-/-) to study the role of GABAergic inhibition in the generation of network oscillations in the olfactory bulb (OB) and to reveal the role of such oscillations in olfaction. The expression of functional GABA(A) receptors was drastically reduced (>93%) in beta3-/- granule cells, the local inhibitory interneurons of the OB. This was revealed by a large reduction of muscimol-evoked whole-cell current and the total current mediated by spontaneous, miniature inhibitory postsynaptic currents (mIPSCs). In beta3-/- mitral/tufted cells (principal cells), there was a two-fold increase in mIPSC amplitudes without any significant change in their kinetics or frequency. In parallel with the altered inhibition, there was a significant increase in the amplitude of theta (80% increase) and gamma (178% increase) frequency oscillations in beta3-/- OBs recorded in vivo from freely moving mice. In odor discrimination tests, we found beta3-/- mice to be initially the same as, but better with experience than beta3+/+ mice in distinguishing closely related monomolecular alcohols. However, beta3-/- mice were initially better and then worse with practice than control mice in distinguishing closely related mixtures of alcohols. Our results indicate that the disruption of GABA(A) receptor-mediated synaptic inhibition of GABAergic interneurons and the augmentation of IPSCs in principal cells result in increased network oscillations in the OB with complex effects on olfactory discrimination, which can be explained by an increase in the size or effective power of oscillating neural cell assemblies among the mitral cells of beta3-/- mice.

Loss of the major GABA(A) receptor subtype in the brain is not lethal in mice

The alpha1beta2gamma2 is the most abundant subtype of the GABA(A) receptor and is localized in many regions of the brain. To gain more insight into the role of this receptor subtype in the modulation of inhibitory neurotransmission, we generated mice lacking either the alpha1 or beta2 subunit. In agreement with the reported abundance of this subtype, >50% of total GABA(A) receptors are lost in both alpha1-/- and beta2-/- mice. Surprisingly, homozygotes of both mouse lines are viable, fertile, and show no spontaneous seizures. Initially half of the alpha1-/- mice died prenatally or perinatally, but they exhibited a lower mortality rate in subsequent generations, suggesting some phenotypic drift and adaptive changes. Both adult alpha1-/- and beta2-/- mice demonstrate normal performances on the rotarod, but beta2-/- mice displayed increased locomotor activity. Purkinje cells of the cerebellum primarily express alpha1beta2gamma2 receptors, and in electrophysiological recordings from alpha1-/- mice GABA currents in these neurons are dramatically reduced, and residual currents have a benzodiazepine pharmacology characteristic of alpha2- or alpha3-containing receptors. In contrast, the cerebellar Purkinje neurons from beta2-/- mice have only a relatively small reduction of GABA currents. In beta2-/- mice expression levels of all six alpha subunits are reduced by approximately 50%, suggesting that the beta2 subunit can coassemble with alpha subunits other than just alpha1. Our data confirm that alpha1beta2gamma2 is the major GABA(A) receptor subtype in the murine brain and demonstrate that, surprisingly, the loss of this receptor subtype is not lethal.

Detection and binding properties of GABA(A) receptor assembly intermediates

Density gradient centrifugation of native and recombinant gamma-aminobutyric acid, type A (GABA(A)) receptors was used to detect assembly intermediates. No such intermediates could be identified in extracts from adult rat brain or from human embryonic kidney (HEK) 293 cells transfected with alpha(1), beta(3), and gamma(2) subunits and cultured at 37 degrees C. However, subunit dimers, trimers, tetramers, and pentamers were found in extracts from the brain of 8-10-day-old rats and from alpha(1)beta(3)gamma(2) transfected HEK cells cultured at 25 degrees C. In both systems, alpha(1), beta(3), and gamma(2) subunits could be identified in subunit dimers, indicating that different subunit dimers are formed during GABA(A) receptor assembly. Co-transfection of HEK cells with various combinations of full-length and C-terminally truncated alpha(1) and beta(3) or alpha(1) and gamma(2) subunits and co-immunoprecipitation with subunit-specific antibodies indicated that even subunits containing no transmembrane domain can assemble with each other. Whereas alpha(1)gamma(2), alpha(1)Ngamma(2), alpha(1)gamma(2)N, and alpha(1)Ngamma(2)N, combinations exhibited specific [(3)H]Ro 15-1788 binding, specific [(3)H]muscimol binding could only be found in alpha(1)beta(3) and alpha(1)beta(3)N, but not in alpha(1)Nbeta(3) or alpha(1)Nbeta(3)N combinations. This seems to indicate that a full-length alpha(1) subunit is necessary for the formation of the muscimol-binding site and for the transduction of agonist binding into channel gating.

GABA(A) receptor subtypes: dissecting their pharmacological functions

The enhancement of GABA-mediated synaptic transmission underlies the pharmacotherapy of various neurological and psychiatric disorders. GABA(A) receptors are pluripotent drug targets that display an extraordinary structural heterogeneity: they are assembled from a repertoire of at least 18 subunits (alpha1-6, beta1-3, gamma1-3, delta, epsilon, theta, rho1-3). However, differentiating defined GABA(A) receptor subtypes on the basis of function has had to await recent progress in the genetic dissection of receptor subtypes in vivo. Evidence that the various actions of allosteric modulators of GABA(A) receptors, in particular the benzodiazepines, can be attributed to specific GABA(A) receptor subtypes will be discussed. Such discoveries could open up new avenues for drug development.

Targeted disruption of the GABA(A) receptor delta subunit gene leads to an up-regulation of gamma 2 subunit-containing receptors in cerebellar granule cells

GABA(A) receptors are chloride channels composed of five subunits. Cerebellar granule cells express abundantly six subunits belonging to four subunit classes. These are assembled into a number of distinct receptors, but the regulation of their relative proportions is yet unknown. Here, we studied the composition of cerebellar GABA(A) receptors after targeted disruption of the delta subunit gene. In membranes and extracts of delta-/- cerebellum, [(3)H]muscimol binding was not significantly changed, whereas [(3)H]Ro15-4513 binding was increased by 52% due to an increase in diazepam-insensitive binding. Immunocytochemical and Western blot analysis revealed no change in alpha(6) subunits but an increased expression of gamma(2) subunits in delta-/- cerebellum. Immunoaffinity chromatography of cerebellar extracts indicated there was an increased coassembly of alpha(6) and gamma(2) subunits and that 24% of all receptors in delta-/- cerebellum did not contain a gamma subunit. Because 97% of delta subunits are coassembled with alpha(6) subunits in the cerebellum of wild-type mice, these results indicated that, in delta-/- mice, alpha(6)betagamma(2) and alphabeta receptors replaced delta subunit-containing receptors. The availability of the delta subunit, thus, influences the level of expression or the extent of assembly of the gamma(2) subunit, although these two subunits do not occur in the same receptor.

GABA(A) receptors: immunocytochemical distribution of 13 subunits in the adult rat brain

GABA(A) receptors are ligand-operated chloride channels assembled from five subunits in a heteropentameric manner. Using immunocytochemistry, we investigated the distribution of GABA(A) receptor subunits deriving from 13 different genes (alpha1-alpha6, beta1-beta3, gamma1-gamma3 and delta) in the adult rat brain. Subunit alpha1-, beta1-, beta2-, beta3- and gamma2-immunoreactivities were found throughout the brain, although differences in their distribution were observed. Subunit alpha2-, alpha3-, alpha4-, alpha5-, alpha6-, gamma1- and delta-immunoreactivities were more confined to certain brain areas. Thus, alpha2-subunit-immunoreactivity was preferentially located in forebrain areas and the cerebellum. Subunit alpha6-immunoreactivity was only present in granule cells of the cerebellum and the cochlear nucleus, and subunit gamma1-immunoreactivity was preferentially located in the central and medial amygdaloid nuclei, in pallidal areas, the substantia nigra pars reticulata and the inferior olive. The alpha5-subunit-immunoreactivity was strongest in Ammon's horn, the olfactory bulb and hypothalamus. In contrast, alpha4-subunit-immunoreactivity was detected in the thalamus, dentate gyrus, olfactory tubercle and basal ganglia. Subunit alpha3-immunoreactivity was observed in the glomerular and external plexiform layers of the olfactory bulb, in the inner layers of the cerebral cortex, the reticular thalamic nucleus, the zonal and superficial layers of the superior colliculus, the amygdala and cranial nerve nuclei. Only faint subunit gamma3-immunoreactivity was detected in most areas; it was darkest in midbrain and pontine nuclei. Subunit delta-immunoreactivity was frequently co-distributed with alpha4 subunit-immunoreactivity, e.g. in the thalamus, striatum, outer layers of the cortex and dentate molecular layer. Striking examples of complementary distribution of certain subunit-immunoreactivities were observed. Thus, subunit alpha2-, alpha4-, beta1-, beta3- and delta-immunoreactivities were considerably more concentrated in the neostriatum than in the pallidum and entopeduncular nucleus. In contrast, labeling for the alpha1-, beta2-, gamma1- and gamma2-subunits prevailed in the pallidum compared to the striatum. With the exception of the reticular thalamic nucleus, which was prominently stained for subunits alpha3, beta1, beta3 and gamma2, most thalamic nuclei were rich in alpha1-, alpha4-, beta2- and delta-immunoreactivities. Whereas the dorsal lateral geniculate nucleus was strongly immunoreactive for subunits alpha4, beta2 and delta, the ventral lateral geniculate nucleus was predominantly labeled for subunits alpha2, alpha3, beta1, beta3 and gamma2; subunit alpha1- and alpha5-immunoreactivities were about equally distributed in both areas. In most hypothalamic areas, immunoreactivities for subunits alpha1, alpha2, beta1, beta2 and beta3 were observed. In the supraoptic nucleus, staining of conspicuous dendritic networks with subunit alpha1, alpha2, beta2, and gamma2 antibodies was contrasted by perykarya labeled for alpha5-, beta1- and delta-immunoreactivities. Among all brain regions, the median emminence was most heavily labeled for subunit beta2-immunoreactivity. In most pontine and cranial nerve nuclei and in the medulla, only subunit alpha1-, beta2- and gamma2-immunoreactivities were strong, whereas the inferior olive was significantly labeled only for subunits beta1, gamma1 and gamma2. In this study, a highly heterogeneous distribution of 13 different GABA(A) receptor subunit-immunoreactivities was observed. This distribution and the apparently typical patterns of co-distribution of these GABA(A) receptor subunits support the assumption of multiple, differently assembled GABA(A) receptor subtypes and their heterogeneous distribution within the adult rat brain.

Synapse-specific contribution of the variation of transmitter concentration to the decay of inhibitory postsynaptic currents

Synaptic transmission is characterized by a remarkable trial-to-trial variability in the postsynaptic response, influencing the way in which information is processed in neuronal networks. This variability may originate from the probabilistic nature of quantal transmitter release, from the stochastic behavior of the receptors, or from the fluctuation of the transmitter concentration in the cleft. We combined nonstationary noise analysis and modeling techniques to estimate the contribution of transmitter fluctuation to miniature inhibitory postsynaptic current (mIPSC) variability. A substantial variability (approximately 30%) in mIPSC decay was found in all cell types studied (neocortical layer2/3 pyramidal cells, granule cells of the olfactory bulb, and interneurons of the cerebellar molecular layer). This large variability was not solely the consequence of the expression of multiple types of GABA(A) receptors, as a similar mIPSC decay variability was observed in cerebellar interneurons that express only a single type (alpha(1)beta(2)gamma(2)) of GABA(A) receptor. At large synapses on these cells, all variance in mIPSC decay could be accounted for by the stochastic behavior of approximately 36 pS channels, consistent with the conductance of alpha(1)beta(2)gamma(2) GABA(A) receptors at physiological temperatures. In contrast, at small synapses, a significant amount of variability in the synaptic cleft GABA transient had to be present to account for the additional variance in IPSC decay over that produced by stochastic channel openings. Thus, our results suggest a synapse-specific contribution of the variation of the spatiotemporal profile of GABA to the decay of IPSCs.

Adaptive regulation of neuronal excitability by a voltage-independent potassium conductance

Many neurons receive a continuous, or 'tonic', synaptic input, which increases their membrane conductance, and so modifies the spatial and temporal integration of excitatory signals. In cerebellar granule cells, although the frequency of inhibitory synaptic currents is relatively low, the spillover of synaptically released GABA (gamma-aminobutyric acid) gives rise to a persistent conductance mediated by the GABA A receptor that also modifies the excitability of granule cells. Here we show that this tonic conductance is absent in granule cells that lack the alpha6 and delta-subunits of the GABAA receptor. The response of these granule cells to excitatory synaptic input remains unaltered, owing to an increase in a 'leak' conductance, which is present at rest, with properties characteristic of the two-pore-domain K+ channel TASK-1 (refs 9,10,11,12). Our results highlight the importance of tonic inhibition mediated by GABAA receptors, loss of which triggers a form of homeostatic plasticity leading to a change in the magnitude of a voltage-independent K + conductance that maintains normal neuronal behaviour.

Role of the GABA(A)beta2, GABA(A)alpha6, GABA(A)alpha1 and GABA(A)gamma2 receptor subunit genes cluster in drug responses and the development of alcohol dependence

gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter of the central nervous system and it acts at the GABA(A) and GABA(B) receptors. A possible role for the GABA(A) receptors in alcohol action has been derived from in vitro cell models, animal studies and human research. GABA(A) subunit mRNA expression in cell models has suggested that the long form of the gamma2 subunit is essential for ethanol enhanced potentiation of GABA(A) receptors, by phosphorylation of a serine contained within the extra eight amino acids. Several animal studies have demonstrated that alterations in drug and alcohol responses may be caused by amino-acid differences at the GABA(A)alpha6 and GABA(A)gamma2 subunits. An Arg(100)/Glu(100) change at the GABA(A)alpha6 subunit conferring altered binding efficacy of the benzodiazepine inverse agonist Ro 15-4513, was found between the AT (alcohol tolerance) and ANT (alcohol non-tolerance) rats. Several loci related to alcohol withdrawal on mouse chromosome 11 which corresponds to the region containing four GABA(A) subunit (beta2, alpha6, alpha1 and gamma2) genes on human chromosome 5q33-34, were also identified. Gene knockout studies of the role of GABA(A)alpha6 and GABA(A)gamma2 subunit genes in mice have demonstrated an essential role in the modulation of other GABA(A) subunit expression and the efficacy of benzodiazepine binding. Absence of the GABA(A)gamma2 subunit gene has more severe effects with many of the mice dying shortly after birth. Disappointingly few studies have examined the effects of response to alcohol in these gene knockout mice. Human genetic association studies have suggested that the GABA(A)beta2, alpha6, alpha1 and gamma2 subunit genes have a role in the development of alcohol dependence, although their contributions may vary between ethnic group and phenotype. In summary, in vitro cell, animal and human genetic association studies have suggested that the GABA(A)beta2, alpha6, alpha1 and gamma2 subunit genes have an important role in alcohol related phenotypes (300 words).

Chronic ethanol treatment and GABA(A) receptor α6 subunit gene expression: a study using α6 subunit-deficient mice

Chronic alcohol administration increases the expression of cerebellum-specific GABA A receptor α6 subunit mRNA, protein and selective autoradiographical fingerprint on rat and mouse brain sections. We have tested whether the α6 gene is activated by chronic alcohol administration (daily p.o. injection of 2 g/kg during the first 3 days and 2.5 g/kg during the next 17 days) that produced tolerance in the rotarod test to motor impairment by acute challenge of ethanol (2 g/kg, i.p.). We utilized a mouse line engineered to express E. coli β-galactosidase enzyme and an unfunctional truncated α6 subunit under the control of the α6 gene promoter. Chronic ethanol treatment failed to alter the cerebellar β-galactosidase activity when compared with no treatment and isocaloric sucrose treatment in groups of α6 subunit-deficient mice. The results suggest that tolerance to motor-impairing effects of ethanol can be achieved in the absence of α6 subunit-containing GABA(A) receptors, but that the reported upregulation of α6 gene transcription by ethanol treatment requires functional α6 subunits.

Somato-synaptic variation of GABA(A) receptors in cultured murine cerebellar granule cells: investigation of the role of the alpha6 subunit

Electrophysiological investigation of cultured cerebellar murine granule cells revealed differences between the GABA(A) receptors at inhibitory synapses and those on the cell body. Specifically, mIPSCs decayed more rapidly than cell body receptors deactivated, the mean single channel conductance at the synapse (32 pS) was greater than that at cell body (21 pS) and only cell body receptors were sensitive to Zn(2+) (150 microM), which depressed response amplitude by 82+/-5% and almost doubled the rate of channel deactivation. The GABA(A) receptor alpha6 subunit is selectively expressed in cerebellar granule cells. Although concentrated at synapses, it is also found on extrasynaptic membranes. Using a mouse line (Deltaalpha6lacZ) lacking this subunit, we investigated its role in the somato-synaptic differences in GABA(A) receptor function. All differences between cell body and synaptic GABA(A) receptors observed in wild-type (WT) granule cells persisted in Deltaalpha6lacZ cells, thus demonstrating that they are not specifically due to the cellular distribution of the alpha6 subunit. However, mIPSCs from WT and Deltaalpha6lacZ cells differed in both their kinetics (faster decay in WT cells) and underlying single channel conductance (32 pS WT, 25 pS Deltaalpha6lacZ). This provides good evidence for a functional contribution of the alpha6 subunit to postsynaptic GABA(A) receptors in these cells. Despite this, deactivation kinetics of mIPSCs in WT and Deltaalpha6lacZ granule cells exhibited similar benzodiazepene (BDZ) sensitivity. This suggests that the enhanced BDZ-induced ataxia seen in Deltaalpha6lacZ mice may reflect physiological activity at extrasynaptic receptors which, unlike those at synapses, display differential BDZ-sensitivity in WT and Deltaalpha6lacZ granule cells (Jones, A.M., Korpi, E.R., McKernan, R.M., Nusser, Z., Pelz, R., Makela, R., Mellor, J.R., Pollard, S., Bahn, S., Stephenson, F.A., Randall, A.D., Sieghart, W., Somogyi, P., Smith, A.J.H., Wisden, W., 1997. Ligand-gated ion channel partnerships: GABA(A) receptor alpha(6) subunit inactivation inhibits delta subunit expression. Journal of Neuroscience 17, 1350-1362).

Long-range interactions in neuronal gene expression: evidence from gene targeting in the GABA(A) receptor beta2-alpha6-alpha1-gamma2 subunit gene cluster

Clustering of GABA(A) receptor alpha1, alpha6, beta2, and gamma2 subunit genes on mouse chromosome 11/human chromosome 5 may have functional significance for coordinating expression patterns, but until now there has been no evidence for cross-talk between the genes. However, altering the structure of the alpha6 gene, specifically expressed in the cerebellum, with neomycin gene insertions in two different experiments unexpectedly reduced the expression of the widespread alpha1 and beta2 genes in the forebrain. There were corresponding reductions in the levels of alpha1 and beta2 subunit proteins and in autoradiographic ligand binding densities to GABA(A) receptors in the forebrain of alpha6-/- mice. The gamma2 mRNA level was not changed, nor were beta3 and delta mRNAs. The data suggest that elements in the neo gene may have an influence over long distances in the GABA(A) subunit gene complex on as yet undefined structures coordinating the expression of the alpha1 and beta2 genes.

Cyclic AMP-mediated regulation of GABA(A) receptor subunit expression in mature rat cerebellar granule cells: evidence for transcriptional and translational control

Exposure of rat cerebellar granule cells cultured to maturity in vitro to forskolin, N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (Bt2cAMP), and 3-isobutyl-1-methylxanthine (IBMX) down-regulated GABA(A) receptor alpha6 and beta3 subunits but up-regulated alpha1 and beta2 subunits with respect to vehicle-treated controls. Dideoxyforskolin had no effect on subunit expression. Protein kinase A inhibitors, H-89 and Rp-adenosine 3',5'cyclic monophosphothioate, prevented these effects on alpha1 but not alpha6 subunit expression. Flunitrazepam-sensitive [3H]Ro 15-4513 binding sites were increased by 144 +/- 20% following forskolin treatment. [3H]Ro 15-4513 photoaffinity labelling showed that the GABA(A) receptor alpha1 subunit was the principal locus of the increased flunitrazepam-sensitive [3H]Ro 15-4513 binding. Forskolin decreased flunitrazepam-insensitive [3H]Ro 15-4513 binding sites by 25 +/- 8% and resulted in a 20% decrease in the irreversible incorporation of radioactivity in the alpha6 subunit. Steady-state levels of GABA(A) receptor subunit mRNAs were determined by semiquantitative RT-PCR in forskolin-treated cultures. Forskolin, Bt2cAMP, and IBMX down-regulated GABA(A) receptor alpha6 subunit mRNA expression; alpha1 and beta3 mRNA levels were unaffected, whereas beta2 subunit mRNA was up-regulated. Dideoxyforskolin had no significant effect on alpha1, alpha6, beta2, and beta3 mRNA levels. Thus, in mature cerebellar granule cells, GABA(A) receptor expression can be regulated by intracellular cyclic AMP levels. This occurs at the level of gene transcription and/or translation by mechanisms that are only partially governed by protein kinase A.

Differential regulation of synaptic GABAA receptors by cAMP-dependent protein kinase in mouse cerebellar and olfactory bulb neurones

1. It has been demonstrated that the regulation of recombinant GABAA receptors by phosphorylation depends on the subunit composition. Here we studied the regulation of synaptic GABAA receptor function by cAMP-dependent protein kinase (PKA) in neurones expressing distinct receptor subtypes. 2. Light microscopic immunocytochemistry revealed that granule cells of the olfactory bulb express only the beta3 as the beta subunit variant, whereas cerebellar stellate and basket cells express only the beta2 as the beta subunit. 3. In cerebellar interneurones, intracellular application of 20 microM microcystin, a protein phosphatase 1/2A inhibitor, prolonged (63 +/- 14 %; mean +/- s.e.m.) the decay time course of miniature IPSCs (mIPSCs) without significantly affecting their amplitude, rise time and frequency. The effect of microcystin could be blocked by co-applying PKA inhibitory peptide (PKA-I, 1 microM). 4. No significant changes in any of the mIPSC parameters could be detected after intracellular application of PKA-I alone or following the inhibition of calcineurin with FK506 (50 nM). 5. In granule cells of the olfactory bulb expressing the beta3 subunit fast and slowly rising mIPSCs were detected, resulting in a bimodal distribution of the 10-90 % rise times, suggesting two distinct populations of events. Fast rising mIPSCs (mIPSCFR) had a 10-90 % rise time of 410 +/- 50 micros, an amplitude of 68 +/- 6 pA, and a weighted decay time constant (tauw) of 15.8 +/- 2.9 ms. In contrast, slowly rising mIPSCs (mIPSCSR) displayed an approximately threefold slower rise time (1.15 +/- 0.12 ms), 57 % smaller amplitude (29 +/- 1.7 pA), but had a tauw (16.8 +/- 3.0 ms) similar to that of the fast events. 6. mIPSCs in olfactory granule cells were not affected by the intracellular perfusion of microcystin. In spite of this, intracellular administration of constitutively active PKA caused a small, gradual, but significant increase (18 +/- 5 %) in the amplitude of the events without changing their time course. 7. These findings demonstrate a cell-type-dependent regulation of synaptic inhibition by protein phosphorylation. Furthermore, our results show that the effect of PKA-mediated phosphorylation on synaptic inhibition depends upon the subunit composition of postsynaptic GABAA receptors.

Loreclezole and La3+ differentiate cerebellar granule cell GABA(A) receptor subtypes

The effects of loreclezole and La3+ on native cerebellar GABA(A) receptors were compared between GABA(A) receptor alpha6 subunit-deficient (alpha6-/-) and wildtype mouse lines, produced through homologous recombination, using t-[35S]butylbicyclophosphorothionate ([35S]TBPS) autoradiography in brain sections. In the alpha6 subunit-deficient mice, the GABA receptor antagonistic ability of La3+ was abolished in the cerebellar granule cell layer, consistent with its opposite actions on alpha6- and of alpha1 subunit-containing receptors. La3+ significantly potentiated the action of GABA in the molecular layer of the alpha6-/- mice, but not in that of the wildtype mice. The potentiation of agonistic GABA inhibition of [35S]TBPS binding by loreclezole in alpha6-/- granule cells was reduced, suggesting an emergence of low-affinity GABA(A) receptors. The present results thus identified two ligands that may be useful in studying functional roles of cerebellar alpha1 and alpha6 subunit-containing GABA(A) receptor subtypes.