The cerebellum: a model system for studying GABAA receptor diversity

GABAergic mechanisms in alcohol dependence

The target of alcohol's effect on the central nervous system has been sought for more than 50 years in the brain's GABA system. The behavioral and emotional effects of alcohol in humans and rodents are very similar to those of barbiturates and benzodiazepines, and GABAA receptors have been shown to be one of the sites of alcohol action. The mechanisms of GABAergic inhibition have been a hotspot of research but have turned out to be complex and controversial. Genetics support the involvement of some GABAA receptor subunits in the development of alcohol dependence and in alcohol use disorders (AUD). Since the effect of alcohol on the GABAA system resembles that of a GABAergic positive modulator, it may be possible to develop GABAergic drug treatments that could substitute for alcohol. The adaptation mechanisms of the GABA system and the plasticity of the brain are a big challenge for drug development: the drugs that act on GABAA receptors developed so far also may cause adaptation and development of additional addiction. Human polymorphisms should be studied further to get insight about how they affect receptor function, expression or other factors to make reasonable predictions/hypotheses about what non-addictive interventions would help in alcohol dependence and AUD.

Cerebellar α6GABAA Receptors as a Therapeutic Target for Essential Tremor: Proof-of-Concept Study with Ethanol and Pyrazoloquinolinones

Ethanol has been shown to suppress essential tremor (ET) in patients at low-to-moderate doses, but its mechanism(s) of action remain unknown. One of the ET hypotheses attributes the ET tremorgenesis to the over-activated firing of inferior olivary neurons, causing synchronic rhythmic firings of cerebellar Purkinje cells. Purkinje cells, however, also receive excitatory inputs from granule cells where the α6 subunit-containing GABAA receptors (α6GABAARs) are abundantly expressed. Since ethanol is a positive allosteric modulator (PAM) of α6GABAARs, such action may mediate its anti-tremor effect. Employing the harmaline-induced ET model in male ICR mice, we evaluated the possible anti-tremor effects of ethanol and α6GABAAR-selective pyrazoloquinolinone PAMs. The burrowing activity, an indicator of well-being in rodents, was measured concurrently. Ethanol significantly and dose-dependently attenuated action tremor at non-sedative doses (0.4-2.4 g/kg, i.p.). Propranolol and α6GABAAR-selective pyrazoloquinolinones also significantly suppressed tremor activity. Neither ethanol nor propranolol, but only pyrazoloquinolinones, restored burrowing activity in harmaline-treated mice. Importantly, intra-cerebellar micro-injection of furosemide (an α6GABAAR antagonist) had a trend of blocking the effect of pyrazoloquinolinone Compound 6 or ethanol on harmaline-induced tremor. In addition, the anti-tremor effects of Compound 6 and ethanol were synergistic. These results suggest that low doses of ethanol and α6GABAAR-selective PAMs can attenuate action tremor, at least partially by modulating cerebellar α6GABAARs. Thus, α6GABAARs are potential therapeutic targets for ET, and α6GABAAR-selective PAMs may be a potential mono- or add-on therapy.

The Cerebellar Dopaminergic System

In the central nervous system (CNS), dopamine (DA) is involved in motor and cognitive functions. Although the cerebellum is not been considered an elective dopaminergic region, studies attributed to it a critical role in dopamine deficit-related neurological and psychiatric disorders [e.g., Parkinson's disease (PD) and schizophrenia (SCZ)]. Data on the cerebellar dopaminergic neuronal system are still lacking. Nevertheless, biochemical studies detected in the mammalians cerebellum high dopamine levels, while chemical neuroanatomy studies revealed the presence of midbrain dopaminergic afferents to the cerebellum as well as wide distribution of the dopaminergic receptor subtypes (DRD1-DRD5). The present review summarizes the data on the cerebellar dopaminergic system including its involvement in associative and projective circuits. Furthermore, this study also briefly discusses the role of the cerebellar dopaminergic system in some neurologic and psychiatric disorders and suggests its potential involvement as a target in pharmacologic and non-pharmacologic treatments.

Mechanisms of GABAB receptor enhancement of extrasynaptic GABAA receptor currents in cerebellar granule cells

Many neurons, including cerebellar granule cells, exhibit a tonic GABA current mediated by extrasynaptic GABA_A receptors. This current is a critical regulator of firing and the target of many clinically relevant compounds. Using a combination of patch clamp electrophysiology and photolytic uncaging of RuBi-GABA we show that GABA_B receptors are tonically active and enhance extrasynaptic GABA_A receptor currents in cerebellar granule cells. This enhancement is not associated with meaningful changes in GABA_A receptor potency, mean channel open-time, open probability, or single-channel current. However, there was a significant (~40%) decrease in the number of channels participating in the GABA uncaging current and an increase in receptor desensitization. Furthermore, we find that adenylate cyclase, PKA, CaMKII, and release of Ca²⁺ from intracellular stores are necessary for modulation of GABA_A receptors. Overall, this work reveals crosstalk between postsynaptic GABA_A and GABA_B receptors and identifies the signaling pathways and mechanisms involved.

Neurexophilin4 is a selectively expressed α-neurexin ligand that modulates specific cerebellar synapses and motor functions

Neurexophilins are secreted neuropeptide-like glycoproteins, and neurexophilin1 and neurexophilin3 are ligands for the presynaptic cell adhesion molecule α-neurexin. Neurexophilins are more selectively expressed in the brain than α-neurexins, however, which led us to ask whether neurexophilins modulate the function of α-neurexin in a context-specific manner. We characterized the expression and function of neurexophilin4 in mice and found it to be expressed in subsets of neurons responsible for feeding, emotion, balance, and movement. Deletion of Neurexophilin4 caused corresponding impairments, most notably in motor learning and coordination. We demonstrated that neurexophilin4 interacts with α-neurexin and GABAARs in the cerebellum. Loss of Neurexophilin4 impaired cerebellar Golgi-granule inhibitory neurotransmission and synapse number, providing a partial explanation for the motor learning and coordination deficits observed in the Neurexophilin4 null mice. Our data illustrate how selectively expressed Neurexophilin4, an α-neurexin ligand, regulates specific synapse function and modulates cerebellar motor control.

Methylmercury induces an initial increase in GABA-evoked currents in Xenopus oocytes expressing α1 and α6 subunit-containing GABAA receptors

Early onset effects of methylmercury (MeHg) on recombinant α₁β₂γ_2S or α₆β₂γ_2S subunit-containing GABA_A receptors were examined. These are two of the most prevalent receptor types found in cerebellum-a consistent target of MeHg-induced neurotoxicity. Heterologously expressed receptors were used in order to: (1) isolate receptor-mediated events from extraneous effects of MeHg due to stimulation of the receptor secondary to increased release of GABA seen with MeHg in neurons in situ and (2) limit the phenotypes of GABA_A receptors present at one time. Initial changes in I_GABA in Xenopus laevis oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors were compared during continuous bath application of MeHg. A time-dependent increase in I_GABA mediated by both receptor subtypes occurred following the first 25-30min of MeHg (5μM) exposure. In α₆β₂γ_2S containing receptors, the MeHg-induced increase in I_GABA was less pronounced compared to that mediated by α₁β₂γ_2S containing receptors, although the pattern of effects was generally similar. Washing with MeHg-free solution reversed the increase in current amplitude. Application of bicuculline at the time of peak potentiation of I_GABA rapidly and completely reversed the MeHg-induced currents. Therefore these MeHg-increased inward currents are mediated specifically by the two subtypes of GABA_A receptors and appear to entail direct actions of MeHg on the receptor. However bicuculline did not affect stimulation by MeHg of oocyte endogenous Cl^- -mediated current, which presumably results from increased [Ca²⁺]_i. Thus, MeHg initially potentiates I_GABA in oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors prior to its more defined later effects, suggesting that MeHg may initially interact directly with GABA_A receptors in a reversible manner to cause this potentiation.

Increased Motor-Impairing Effects of the Neuroactive Steroid Pregnanolone in Mice with Targeted Inactivation of the GABAA Receptor γ2 Subunit in the Cerebellum

Endogenous neurosteroids and neuroactive steroids have potent and widespread actions on the brain via inhibitory GABA_A receptors. In recombinant receptors and genetic mouse models their actions depend on the α, β, and δ subunits of the receptor, especially on those that form extrasynaptic GABA_A receptors responsible for non-synaptic (tonic) inhibition, but they also act on synaptically enriched γ2 subunit-containing receptors and even on αβ binary receptors. Here we tested whether behavioral sensitivity to the neuroactive steroid agonist 5β-pregnan-3α-ol-20-one is altered in genetically engineered mouse models that have deficient GABA_A receptor-mediated synaptic inhibition in selected neuronal populations. Mouse lines with the GABA_A receptor γ2 subunit gene selectively deleted either in parvalbumin-containing cells (including cerebellar Purkinje cells), cerebellar granule cells, or just in cerebellar Purkinje cells were trained on the accelerated rotating rod and then tested for motor impairment after cumulative intraperitoneal dosing of 5β-pregnan-3α-ol-20-one. Motor-impairing effects of 5β-pregnan-3α-ol-20-one were strongly increased in all three mouse models in which γ2 subunit-dependent synaptic GABA_A responses in cerebellar neurons were genetically abolished. Furthermore, rescue of postsynaptic GABA_A receptors in Purkinje cells normalized the effect of the steroid. Anxiolytic/explorative effects of the steroid in elevated plus maze and light:dark exploration tests in mice with Purkinje cell γ2 subunit inactivation were similar to those in control mice. The results suggest that, when the deletion of γ2 subunit has removed synaptic GABA_A receptors from the specific cerebellar neuronal populations, the effects of neuroactive steroids solely on extrasynaptic αβ or αβδ receptors lead to enhanced changes in the cerebellum-generated behavior.

N-methyl-d-aspartate inhibits cerebellar Purkinje cell activity via the excitation of molecular layer interneurons under in vivo conditions in mice

N-methyl-d-aspartate (NMDA) receptors play a key role in synaptic transmission, and are widely expressed on the membrane of granule cells, parallel fibers, and molecular layer interneurons (MLIs) in the cerebellar cortex of mammals. In cerebellar slices, activation of NMDA receptors increases inhibitory postsynaptic currents (IPSCs) of Purkinje cells (PCs). However, the effects of NMDA on the cerebellar network under in vivo conditions are currently unclear. In the present study, we examined the effects of NMDA on the spontaneous activity of PCs and MLIs in urethane-anesthetized mice by electrophysiological, pharmacological, and juxtacellular labeling methods. Our results revealed that cerebellar surface application of NMDA (5-200μM) reduced the PC simple spike (SS) firing rate in a dose-dependent manner. Application of GABAA receptor antagonist, SR95531 (20μM) abolished NMDA-induced inhibition of PCs spontaneous activity, and revealed NMDA-induced excitation of cerebellar PCs. NMDA receptor antagonist, DAP-V (250µM) did not affect the mean frequency of SS firing, but the SS firing rate of PCs became more regular than the control. In addition, NMDA increased the spike firing of both basket-type and stellate-type MLIs. Overall, these results indicated that NMDA-induced excitation of MLIs at the cerebellar surface may inhibit PC activity. Thus, NMDA receptors of MLIs may play a key role in regulating the spontaneous activity of PCs, and in information transmission and integration in cerebellar cortex.

Regulation and functional roles of rebound potentiation at cerebellar stellate cell-Purkinje cell synapses

Purkinje cells receive both excitatory and inhibitory synaptic inputs and send sole output from the cerebellar cortex. Long-term depression (LTD), a type of synaptic plasticity, at excitatory parallel fiber-Purkinje cell synapses has been studied extensively as a primary cellular mechanism of motor learning. On the other hand, at inhibitory synapses on a Purkinje cell, postsynaptic depolarization induces long-lasting potentiation of GABAergic synaptic transmission. This synaptic plasticity is called rebound potentiation (RP), and its molecular regulatory mechanisms have been studied. The increase in intracellular Ca(2+) concentration caused by depolarization induces RP through enhancement of GABAA receptor (GABAAR) responsiveness. RP induction depends on binding of GABAAR with GABAAR associated protein (GABARAP) which is regulated by Ca(2+)/calmodulin-dependent kinase II (CaMKII). Whether RP is induced or not is determined by the balance between phosphorylation and de-phosphorylation activities regulated by intracellular Ca(2+) and by metabotropic GABA and glutamate receptors. Recent studies have revealed that the subunit composition of CaMKII has significant impact on RP induction. A Purkinje cell expresses both α- and β-CaMKII, and the latter has much higher affinity for Ca(2+)/calmodulin than the former. It was shown that when the relative amount of α- to β-CaMKII is large, RP induction is suppressed. The functional significance of RP has also been studied using transgenic mice in which a peptide inhibiting association of GABARAP and GABAAR is expressed selectively in Purkinje cells. The transgenic mice show abrogation of RP and subnormal adaptation of vestibulo-ocular reflex (VOR), a type of motor learning. Thus, RP is involved in a certain type of motor learning.

Expression of GABAA α2-, β1- and ε-receptors are altered significantly in the lateral cerebellum of subjects with schizophrenia, major depression and bipolar disorder

There is abundant evidence that dysfunction of the γ-aminobutyric acid (GABA)ergic signaling system is implicated in the pathology of schizophrenia and mood disorders. Less is known about the alterations in protein expression of GABA receptor subunits in brains of subjects with schizophrenia and mood disorders. We have previously demonstrated reduced expression of GABA(B) receptor subunits 1 and 2 (GABBR1 and GABBR2) in the lateral cerebella of subjects with schizophrenia, bipolar disorder and major depressive disorder. In the current study, we have expanded these studies to examine the mRNA and protein expression of 12 GABA(A) subunit proteins (α1, α2, α3, α5, α6, β1, β2, β3, δ, ε, γ2 and γ3) in the lateral cerebella from the same set of subjects with schizophrenia (N=9-15), bipolar disorder (N=10-15) and major depression (N=12-15) versus healthy controls (N=10-15). We found significant group effects for protein levels of the α2-, β1- and ε-subunits across treatment groups. We also found a significant group effect for mRNA levels of the α1-subunit across treatment groups. New avenues for treatment, such as the use of neurosteroids to promote GABA modulation, could potentially ameliorate GABAergic dysfunction in these disorders.

Robust photoregulation of GABA(A) receptors by allosteric modulation with a propofol analogue

Photochemical switches represent a powerful method for improving pharmacological therapies and controlling cellular physiology. Here we report the photo-regulation of GABA_A receptors (GABA_ARs) by a derivative of propofol (2,6-diisopropylphenol), a GABA_AR allosteric modulator, that we have modified to contain photo-isomerizable azobenzene. Using α₁β₂γ₂ GABA_ARs expressed in Xenopus laevis oocytes and native GABA_ARs of isolated retinal ganglion cells, we show that the trans-azobenzene isomer of the new compound (trans-MPC088), generated by visible light (wavelengths ~440 nm), potentiates the GABA-elicited response and at higher concentrations directly activates the receptors. cis-MPC088, generated from trans-MPC088 by UV light (~365 nm), produces little if any receptor potentiation/activation. In cerebellar slices, MPC088 co-applied with GABA affords bidirectional photo-modulation of Purkinje cell membrane current and spike-firing rate. The findings demonstrate photo-control of GABA_ARs by an allosteric ligand and open new avenues for fundamental and clinically oriented research on GABA_ARs, a major class of neurotransmitter receptors in the central nervous system.

Tonic inhibition enhances fidelity of sensory information transmission in the cerebellar cortex

Tonic inhibition is a key regulator of neuronal excitability and network function in the brain, but its role in sensory information processing remains poorly understood. The cerebellum is a favorable model system for addressing this question as granule cells, which form the input layer of the cerebellar cortex, permit high-resolution patch-clamp recordings in vivo, and are the only neurons in the cerebellar cortex that express the α6δ-containing GABA(A) receptors mediating tonic inhibition. We investigated how tonic inhibition regulates sensory information transmission in the rat cerebellum by using a combination of intracellular recordings from granule cells and molecular layer interneurons in vivo, selective pharmacology, and in vitro dynamic clamp experiments. We show that blocking tonic inhibition significantly increases the spontaneous firing rate of granule cells while only moderately increasing sensory-evoked spike output. In contrast, enhancing tonic inhibition reduces the spike probability in response to sensory stimulation with minimal effect on the spontaneous spike rate. Both manipulations result in a reduction in the signal-to-noise ratio of sensory transmission in granule cells and of parallel fiber synaptic input to downstream molecular layer interneurons. These results suggest that under basal conditions the level of tonic inhibition in vivo enhances the fidelity of sensory information transmission through the input layer of the cerebellar cortex.

Nuclear factor I and cerebellar granule neuron development: an intrinsic-extrinsic interplay

Granule neurons have a central role in cerebellar function via their synaptic interactions with other neuronal cell types both within and outside this structure. Establishment of these synaptic connections and its control is therefore essential to their function. Both intrinsic as well as environmental mechanisms are required for neuronal development and formation of neuronal circuits, and a key but poorly understood question is how these various events are coordinated and integrated in maturing neurons. In this review, we summarize recent work on the role of the Nuclear Factor I family in the transcriptional programming of cerebellar granule neuron maturation and synapse formation. In particular, we describe (1) the involvement of this family of factors in key developmental steps occurring throughout postmitotic granule neuron development, including dendrite and synapse formation and synaptic receptor expression, and (2) the mediation of these actions by critical downstream gene targets that control cell-cell interactions. These findings illustrate how Nuclear Factor I proteins and their regulons function as a “bridge” between cell-intrinsic and cell-extrinsic interactions to control multiple phases of granule neuron development.

Impact of synaptic neurotransmitter concentration time course on the kinetics and pharmacological modulation of inhibitory synaptic currents

The time course of synaptic currents is a crucial determinant of rapid signaling between neurons. Traditionally, the mechanisms underlying the shape of synaptic signals are classified as pre- and post-synaptic. Over the last two decades, an extensive body of evidence indicated that synaptic signals are critically shaped by the neurotransmitter time course which encompasses several phenomena including pre- and post-synaptic ones. The agonist transient depends on neurotransmitter release mechanisms, diffusion within the synaptic cleft, spill-over to the extra-synaptic space, uptake, and binding to post-synaptic receptors. Most estimates indicate that the neurotransmitter transient is very brief, lasting between one hundred up to several hundreds of microseconds, implying that post-synaptic activation is characterized by a high degree of non-equilibrium. Moreover, pharmacological studies provide evidence that the kinetics of agonist transient plays a crucial role in setting the susceptibility of synaptic currents to modulation by a variety of compounds of physiological or clinical relevance. More recently, the role of the neurotransmitter time course has been emphasized by studies carried out on brain slice models that revealed a striking, cell-dependent variability of synaptic agonist waveforms ranging from rapid pulses to slow volume transmission. In the present paper we review the advances on studies addressing the impact of synaptic neurotransmitter transient on kinetics and pharmacological modulation of synaptic currents at inhibitory synapses.

Ro 15-4513 Antagonizes Alcohol-Induced Sedation in Mice Through αβγ2-type GABA(A) Receptors

Ethyl alcohol (ethanol) has many molecular targets in the nervous system, its potency at these sites being low compared to those of sedative drugs. This has made it difficult to discover ethanol's binding site(s). There are two putative binding sites at γ-aminobutyric acid (GABA) type A receptor subtypes for the proposed ethanol antagonist Ro 15-4513, the established γ2 subunit-dependent benzodiazepine site and the recently reported δ subunit-dependent Ro 15-4513/ethanol binding site. Here, we aimed at clarifying the in vivo role of Ro 15-4513 at these two sites. We found that the antagonism of ethanol actions by Ro 15-4513 in wildtype mice was dependent on the test: an open field test showed that light sedation induced by 1.5–1.8 g/kg ethanol was sensitive to Ro 15-4513, whereas several tests for ethanol-induced anxiolytic effects showed that the ethanol-induced effects were insensitive to Ro 15-4513. Antagonism of ethanol-induced sedation by Ro 15-4513 was unaffected in GABA_A receptor δ subunit knockout mice. By contrast, when testing the GABA_A receptor γ2 subunit F77I knock-in mouse line (γ2I77 mice) with its strongly reduced affinity of the benzodiazepine sites for Ro 15-4513, we found that the ethanol-induced sedation was no longer antagonized by Ro 15-4513. Indeed, γ2I77 mice had only a small proportion of high-affinity binding of [³H]Ro 15-4513 left as compared to wildtype mice, especially in the caudate–putamen and septal areas, but these residual sites are apparently not involved in ethanol antagonism. In conclusion, we found that Ro 15-4513 abolished the sedative effect of ethanol by an action on γ2 subunit-dependent benzodiazepine sites.

GABA(A) receptor subunit alteration-dependent diazepam insensitivity in the cerebellum of phospholipase C-related inactive protein knockout mice

The GABA(A) receptor, a pentamer composed predominantly of alpha, beta, and gamma subunits, mediates fast inhibitory synaptic transmission. We have previously reported that phospholipase C-related inactive protein (PRIP) is a modulator of GABA(A) receptor trafficking and that knockout (KO) mice exhibit a diazepam-insensitive phenotype in the hippocampus. The alpha subunit affects diazepam sensitivity; alpha1, 2, 3, and 5 subunits assemble with any form of beta and the gamma2 subunits to produce diazepam-sensitive receptors, whereas alpha4 or alpha6/beta/gamma2 receptors are diazepam-insensitive. Here, we investigated how PRIP is implicated in the diazepam-insensitive phenotype using cerebellar granule cells in animals expressing predominantly the alpha6 subunit. The expression of alpha1/beta/gamma2 diazepam-sensitive receptors was decreased in the PRIP-1 and 2 double KO cerebellum without any change in the total number of benzodiazepine-binding sites as assessed by radioligand-binding assay. Since levels of the alpha6 subunit were increased, the alpha1/beta/gamma2 receptors might be replaced with alpha6 subunit-containing receptors. Then, we further performed autoradiographic and electrophysiologic analyses. These results suggest that the expression of alpha6/delta receptors was decreased in cerebellar granule neurons, while that of alpha6/gamma2 receptors was increased. PRIP-1 and 2 double KO mice exhibit a diazepam-insensitive phenotype because of a decrease in diazepam-sensitive (alpha1/gamma2) and increase in diazepam-insensitive (alpha6/gamma2) GABA(A) receptors in the cerebellar granule cells.

Multiple roles for the first transmembrane domain of GABAA receptor subunits in neurosteroid modulation and spontaneous channel activity

Neurosteroids exert potent physiological effects by allosterically modulating synaptic and extrasynaptic GABA(A) receptors. Some endogenous neurosteroids, such as 3alpha, 21-dihydroxy-5beta-pregnan-20-one (5alpha, 3alpha-THDOC), potentiate GABA(A) receptor function by interacting with a binding pocket defined by conserved residues in the first and fourth transmembrane (TM) domains of alpha subunits. Others, such as pregnenolone sulfate (PS), inhibit GABA(A) receptor function through as-yet unidentified binding sites. Here we investigate the mechanisms of PS inhibition of mammalian GABA(A) receptors, based on studies of PS inhibition of the UNC-49 GABA receptor, a GABA(A)-like receptor from Caenorhabditis elegans. In UNC-49, a 19 residue segment of TM1 can be mutated to increase or decrease PS sensitivity over a 20-fold range. Surprisingly, substituting these UNC-49 sequences into mammalian alpha(1), beta(2), and gamma(2) subunits did not produce the corresponding effects on PS sensitivity of the resulting chimeric receptors. Therefore, it is unlikely that a conserved PS binding pocket is formed at this site. However we observed several interesting unexpected effects. First, chimeric gamma2 subunits caused increased efficacy of 5alpha, 3alpha-THDOC potentiation; second, spontaneous gating of alpha(6)beta(2)delta receptors was blocked by PS, and reduced by chimeric beta(2) subunits; and third, direct activation of alpha(6)beta(2)delta receptors by 5alpha, 3alpha-THDOC was reduced by chimeric beta(2) subunits. These results reveal novel roles for non-alpha subunits in neurosteroid modulation and direct activation, and show that the beta subunit TM1 domain is important for spontaneous activity of extrasynaptic GABA(A) receptors.

Studying Cerebellar Circuits by Remote Control of Selected Neuronal Types with GABA(A) Receptors

Although GABA_A receptor-mediated inhibition of cerebellar Purkinje cells by molecular layer interneurons (MLIs) has been studied intensely at the cellular level, it has remained unclear how this inhibition regulates cerebellum-dependent behaviour. We have implemented two complementary approaches to investigate the function of the MLI-Purkinje cell synapse on the behavioural level. In the first approach we permanently disrupted inhibitory fast synaptic transmission at the synapse by genetically removing the postsynaptic GABA_A receptors from Purkinje cells (PC-Δγ2 mice). We found that chronic disruption of the MLI-Purkinje cell synapse strongly impaired cerebellar learning of the vestibular occular reflex (VOR), presumably by disrupting the temporal patterns of Purkinje cell activity. However, in PC-Δγ2 mice the baseline VOR reflex was only mildly affected; indeed PC-Δγ2 mice show no ataxia or gait abnormalities, suggesting that MLI control of Purkinje cell activity is either not involved in ongoing motor tasks or that the system compensates for its loss. To investigate the latter possibility we developed an alternative genetic technique; we made the MLI-Purkinje cell synapse selectively sensitive to rapid manipulation with the GABA_A receptor modulator zolpidem (PC-γ2-swap mice). Minutes after intraperitoneal zolpidem injection, these PC-γ2-swap mice developed severe motor abnormalities, revealing a substantial contribution of the MLI-Purkinje cell synapses to real time motor control. The cell-type selective permanent knockout of synaptic GABAergic input and the fast reversible modulation of GABAergic input at the same synapse illustrate how pursuing both strategies gives a fuller view.

The ataxia (axJ) mutation causes abnormal GABAA receptor turnover in mice

Ataxia represents a pathological coordination failure that often involves functional disturbances in cerebellar circuits. Purkinje cells (PCs) characterize the only output neurons of the cerebellar cortex and critically participate in regulating motor coordination. Although different genetic mutations are known that cause ataxia, little is known about the underlying cellular mechanisms. Here we show that a mutated ax^J gene locus, encoding the ubiquitin-specific protease 14 (Usp14), negatively influences synaptic receptor turnover. Ax^J mouse mutants, characterized by cerebellar ataxia, display both increased GABA_A receptor (GABA_AR) levels at PC surface membranes accompanied by enlarged IPSCs. Accordingly, we identify physical interaction of Usp14 and the GABA_AR α1 subunit. Although other currently unknown changes might be involved, our data show that ubiquitin-dependent GABA_AR turnover at cerebellar synapses contributes to ax^J-mediated behavioural impairment.

Cerebellar ataxia describes a combination of motor symptoms and uncoordinated movements that originates from various hereditary and non-hereditary diseases. Although functional disturbances of cerebellar inhibitory output signals are thought to cause ataxia, the underlying molecular mechanisms are barely understood and medical treatment therefore remains difficult. We analysed a behavioural abnormality up to the molecular level in a mouse mutant (ax^J) representing a model for ataxia. The ax^J mutation reduces the expression level of a ubiquitin protease (Usp14) leading to an abnormal turnover of neurotransmitter receptors. Despite other yet unknown changes in ax^J mutants, our data show that intracellular protein turnover contributes to a motor behavioural syndrome.

GABA release from cerebellar stellate cells is developmentally regulated by presynaptic GABA(B) receptors in a target-cell-specific manner

Transmitter release from boutons along a common axon is often regulated depending on the postsynaptic target. Here, GABA release from cerebellar stellate cells onto Purkinje cells and other stellate cells was examined in acute cerebellar slices of 2- and 4-week-old mice. Consistent with previous findings on action potential-dependent GABA release, we found a developmental decrease in inhibitory inputs onto Purkinje cells but not onto stellate cells when recording miniature inhibitory postsynaptic currents (mIPSCs). Although amplitudes of mIPSCs were developmentally reduced in both cell types, mIPSC frequencies were decreased in Purkinje cells but were increased in stellate cells. Similarly, modulation of GABA release by presynaptic GABA(B) receptors changed during development in Purkinje cells but not in stellate cells, as demonstrated by the baclofen-mediated depression of mIPSC frequency and evoked IPSC (eIPSC) amplitudes. The selectively diminished baclofen effect in 4-week-old Purkinje cells correlated with a selective downregulation of presynaptic GABA(B) receptors at stellate cell-to-Purkinje cell synapses observed by immunoelectron microscopy analysis. Thus, expression of GABA(B) receptors in stellate cell axons and presynaptic modulation of GABA release appear to change during development in a target-cell-specific manner.

Ketamine, but not phencyclidine, selectively modulates cerebellar GABA(A) receptors containing alpha6 and delta subunits

Phencyclidine (PCP) and ketamine are dissociative anesthetics capable of inducing analgesia, psychomimetic behavior, and a catatonic state of unconsciousness. Despite broad similarities, there are notable differences between the clinical actions of ketamine and PCP. Ketamine has a lower incidence of adverse effects and generally produces greater CNS depression than PCP. Both noncompetitively inhibit NMDA receptors, yet there is little evidence that these drugs affect GABA(A) receptors, the primary target of most anesthetics. alpha6beta2/3delta receptors are subtypes of the GABA(A) receptor family and are abundantly expressed in granular neurons within the adult cerebellum. Here, using an oocyte expression system, we show that at anesthetically relevant concentrations, ketamine, but not PCP, modulates alpha6beta2delta and alpha6beta3delta receptors. Additionally, at higher concentrations, ketamine directly activates these GABA(A) receptors. Comparatively, dizocilpine (MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]), a potent noncompetitive antagonist of NMDA receptors that is structurally unrelated to PCP, did not produce any effect on alpha6beta2delta receptors. Of the recombinant GABA(A) receptor subtypes examined (alpha1beta2, alpha1beta2gamma2, alpha1beta2delta, alpha4beta2gamma2, alpha4beta2delta, alpha6beta2gamma2, alpha6beta2delta, and alpha6beta3delta), the actions of ketamine were unique to alpha6beta2delta and alpha6beta3delta receptors. In dissociated granule neurons and cerebellar slice recordings, ketamine potentiated the GABAergic conductance arising from alpha6-containing GABA(A) receptors, whereas PCP showed no effect. Furthermore, ketamine potentiation was absent in cerebellar granule neurons from transgenic functionally null alpha6(-/-) and delta(-/-)mice. These findings suggest that the higher CNS depressant level achieved by ketamine may be the result of its selective actions on alpha6beta2/3delta receptors.

Isoniazid-induced reduction in GABAergic neurotransmission alters the function of the cerebellar cortical circuit

The cerebellar cortex contributes to the control of movement, coordination, and certain cognitive functions. The cerebellar network is composed of five different types of neurons that are wired together in a repetitive module. Given that four of these five neurons synthesize and release GABA, this inhibitory neurotransmitter plays a central role in regulation of the excitability and correct functioning of the cerebellar cortex. We have now used isoniazid, an inhibitor of glutamic acid decarboxylase, the enzyme responsible for the synthesis of GABA, to evaluate the contribution of GABAergic transmission in different types of cerebellar cortical neurons to the functioning of the cerebellar circuit. Parasagittal cerebellar slices were prepared from 28- to 40-day-old male rats and were subjected to patch-clamp recording in the voltage- or current-clamp mode. Exposure of the tissue slices to isoniazid (10 mM) resulted in a decrease in the level of GABAergic transmission in Purkinje cells and a consequent increase in the firing rate of spontaneous action potentials that was apparent after 40 min. In granule neurons, isoniazid reduced both tonic and phasic GABAergic currents and thereby altered the flow of information across the cerebellar cortex. Our data support the notion that the amount of GABA at the synaptic level is a major determinant of the excitability of the cerebellar cortex, and they suggest that isoniazid may be a useful tool with which to study the function of the cerebellar network.

AMPA and kainate receptors mediate mutually exclusive effects on GABA(A) receptor expression in cultured mouse cerebellar granule neurones

Studies on animal models of epilepsy and cerebellar ataxia, e.g., stargazer mice (stg) have identified changes in the GABAergic properties of neurones associated with the affected brain loci. Whether these changes contribute to or constitute homeostatic adaptations to a state of altered neuronal excitability is as yet unknown. Using cultured cerebellar granule neurones from control [+/+; alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptor (AMPAR)-competent, Kainate receptor (KAR)-competent] and stg (AMPAR-incompetent, KAR-competent), we investigated whether non-NMDA receptor (NMDAR) activity regulates GABA(A) receptor (GABAR) expression. Neurones were maintained in 5 mmol/L KCl-containing basal media or depolarizing media containing either 25 mmol/L KCl or the non-NMDAR agonist kainic acid (KA) (100 micromol/L). KCl- and KA-mediated depolarization down-regulated GABAR alpha1, alpha6 and beta2, but up-regulated alpha4, beta3 and delta subunits in +/+ neurones. The KCl-evoked but not KA-evoked effects were reciprocated in stg neurones compatible with AMPAR-regulation of GABAR expression. Conversely, GABAR gamma2 expression was insensitive to KCl-mediated depolarization, but was down-regulated by KA-treatment in a 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-reversible manner in +/+ and stg neurones compatible with a KAR-mediated response. KA-mediated up-regulation of GABAR alpha4, beta3 and delta was inhibited by L-type voltage-gated calcium channel (L-VGCC) blockers and the Ca2+/calmodulin-dependent protein kinase inhibitor, 4-[(2S)-2-[(5-isoquinolinylsulfonyl)methylamino]-3-oxo-3-(4-phenyl-1-piperazinyl)propyl] phenyl isoquinoline sulfonic acid ester (KN-62). Up-regulation of GABAR alpha4 and beta3 was also prevented by calcineurin (CaN) inhibitors, FK506 and cyclosporin A. Down-regulation of GABAR alpha1, alpha6 and beta2 was independent of L-VGCC activity, but was prevented by inhibitors of CaN. Thus, we provide evidence that a KAR-mediated and at least three mutually exclusive AMPAR-mediated signalling mechanisms regulate neuronal GABAR expression.

Ethanol sensitivity of GABAergic currents in cerebellar granule neurons is not increased by a single amino acid change (R100Q) in the alpha6 GABAA receptor subunit

Cerebellar granule neurons (CGNs) extrasynaptically express GABA(A) receptors containing alpha(6)beta(x)delta subunits, which mediate tonic inhibitory currents. Although it has been shown that the function of these receptors is potently and directly enhanced by ethanol, this finding has not been reproducible across different laboratories. In outbred Sprague-Dawley rats, a naturally occurring arginine (R) to glutamine (Q) mutation in position 100 of the alpha(6) subunit was reported to increase the ethanol sensitivity of these receptors. However, we did not detect an action of this mutation in selectively bred rats (alcohol-tolerant and alcohol-nontolerant). Consequently, we reexamined the effect of the mutation on ethanol sensitivity in Sprague-Dawley rats. Using patch-clamp electrophysiological techniques in cerebellar vermis parasagittal slices, we found that 25 mM ethanol increases the tonic current amplitude, tonic current noise, and spontaneous inhibitory postsynaptic current (sIPSC) frequency to a similar extent in alpha(6)-100R/100R and alpha(6)-100Q/100Q CGNs. Exposure to 80 mM ethanol increased the tonic current amplitude to a significantly greater extent in alpha(6)-100R/100R than in alpha(6)-100Q/100Q CGNs; however, the effects of 80 mM ethanol on the tonic current noise and sIPSC frequency were not significantly different between these groups. In the presence of tetrodo-toxin, a non-N-methyl-d-aspartate receptor antagonist, exogenous GABA, and a GABA transporter inhibitor, neither 8 nor 40 mM ethanol consistently affected tonic current amplitude or noise in alpha(6)-100R/100R or alpha(6)-100Q/100Q CGNs. Thus, the alpha(6)-R100Q GABA(A) receptor subunit polymorphism does not in-crease the acute ethanol sensitivity of extrasynaptic receptors, lending further support to the hypothesis that ethanol modulates these currents indirectly via a presynaptic mechanism.

Modulation of GABAA receptors in cerebellar granule neurons by ethanol: a review of genetic and electrophysiological studies

Cerebellar granule neurons (CGNs) receive inhibitory input from Golgi cells in the form of phasic and tonic currents that are mediated by postsynaptic and extrasynaptic gamma-aminobutyric acid type A (GABAA) receptors, respectively. Extrasynaptic receptors are thought to contain alpha6betaxdelta subunits. Here, we review studies on ethanol (EtOH) modulation of these receptors, which have yielded contradictory results. Although studies with recombinant receptors expressed in Xenopus oocytes indicate that alpha6beta3delta receptors are potently enhanced by acute exposure to low (>or=3 mM) EtOH concentrations, this effect was not observed when these receptors were expressed in Chinese hamster ovary cells. Slice recordings of CGNs have consistently shown that EtOH increases the frequency of phasic spontaneous inhibitory postsynaptic currents (sIPSCs), as well as the tonic current amplitude and noise. However, there is a lack of consensus as to whether EtOH directly acts on extrasynaptic receptors or modulates them indirectly; that is, via an increase in spillover of synaptically released GABA. It was recently demonstrated that an R to Q mutation of amino acid 100 of the alpha6 subunit increases the effect of EtOH on both sIPSCs and tonic current. These electrophysiological findings have not been reproducible in our hands. Moreover, it was shown the alpha6-R100Q mutation enhances sensitivity to the motor-impairing effects of EtOH in outbred Sprague-Dawley rats, but this was not observed in a line of rats selectively bred for high sensitivity to EtOH-induced motor alterations (Alcohol Non-Tolerant rats). We conclude that currently there is insufficient evidence conclusively supporting a direct potentiation of extrasynaptic GABAA receptors following acute EtOH exposure in CGNs.

Rat alpha6beta2delta GABAA receptors exhibit two distinct and separable agonist affinities

The onset of motor learning in rats coincides with exclusive expression of GABAA receptors containing alpha6 and delta subunits in the granule neurons of the cerebellum. This development temporally correlates with the presence of a spontaneously active chloride current through alpha6-containing GABAA receptors, known as tonic inhibition. Here we report that the coexpression of alpha6, beta2, and delta subunits produced receptor-channels which possessed two distinct and separable states of agonist affinity, one exhibiting micromolar and the other nanomolar affinities for GABA. The high-affinity state was associated with a significant level of spontaneous channel activity. Increasing the level of expression or the ratio of beta2 to alpha6 and delta subunits increased the prevalence of the high-affinity state. Comparative studies of alpha6beta2delta, alpha1beta2delta, alpha6beta2gamma2, alpha1beta2gamma2 and alpha4beta2delta receptors under equivalent levels of expression demonstrated that the significant level of spontaneous channel activity is uniquely attributable to alpha6beta2delta receptors. The pharmacology of spontaneous channel activity arising from alpha6beta2delta receptor expression corresponded to that of tonic inhibition. For example, GABAA receptor antagonists, including furosemide, blocked the spontaneous current. Further, the neuroactive steroid 5alpha-THDOC and classical glycine receptor agonists beta-alanine and taurine directly activated alpha6beta2delta receptors with high potency. Specific mutation within the GABA-dependent activation domain (betaY157F) impaired both low- and high-affinity components of GABA agonist activity in alpha6betaY157Fdelta receptors, but did not attenuate the spontaneous current. In comparison, a mutation located between the second and third transmembrane segments of the delta subunit (deltaR287M) significantly diminished the nanomolar component and the spontaneous activity. The possibility that the high affinity state of the alpha6beta2delta receptor modulates the granule neuron activity as well as potential mechanisms affecting its expression are discussed.

Confirmation of region-specific patterns of gene expression in the human brain

The human brain is divided and categorized in different ways, yet a molecular genetic approach to region specificity does not exist. Using data from 12 healthy control subjects across 18 brain regions, we performed a microarray analysis using both the HG-U133AB and HG-U133 plus 2 chips for each subject to determine molecular targets showing region specificity. Using a previously published data as our guide, we confirm SIX3, GPR6, SH3RF2, and hSyn as molecular markers of the nucleus accumbens and gamma-aminobutyric-acid A receptor alpha-6, Nik-related kinase, and eomesodermin as molecular markers of the cerebellum.

Thiocolchicoside inhibits the activity of various subtypes of recombinant GABAA receptors expressed in Xenopus laevis oocytes

Thiocolchicoside is a myorelaxant drug with anti-inflammatory and analgesic properties as well as pronounced convulsant activity. To characterize the mechanisms of action of this drug at the molecular level, we examined its effects on the function of various recombinant neurotransmitter receptors expressed in Xenopus oocytes. Electrophysiological recordings from recombinant human gamma-aminobutyric acid type A (GABA(A)) receptors consisting of alpha1beta1gamma2L, alpha1beta2gamma2L, or alpha2beta2gamma2L subunit combinations revealed that thiocolchicoside inhibited GABA-evoked Cl(-) currents with similar potencies (median inhibitory concentrations of 0.13 to 0.2 microM) and in a competitive manner. Consistent with previous observations, thiocolchicoside also inhibited the binding of GABA to rat cerebral cortical membranes. Thiocolchicoside inhibited the function of recombinant human strychnine-sensitive glycine receptors composed of the alpha1 subunit with a potency (median inhibitory concentration of 47 microM) lower than that apparent with recombinant GABA(A) receptors. It also inhibited the function of human nicotinic acetylcholine receptors composed of the alpha4 and beta2 subunits, but this effect was only partial and apparent at high concentrations. In contrast, thiocolchicoside had no effect on the function of 5-HT(3A) serotonin receptors. Our results thus provide molecular evidence that the epileptogenic activity of thiocolchicoside might be due to inhibition of the function of inhibitory receptors in the central nervous system, especially that of GABA(A) receptors.

Molecular and synaptic organization of GABAA receptors in the cerebellum: Effects of targeted subunit gene deletions

GABAA receptors form heteromeric GABA-gated chloride channels assembled from a large family of subunit genes. In cerebellum, distinct GABAA receptor subtypes, differing in subunit composition, are segregated between cell types and synaptic circuits. The cerebellum therefore represents a useful system to investigate the significance of GABAA receptor heterogeneity. For instance, studies of mice carrying targeted deletion of major GABAA receptor subunit genes revealed the role of alpha subunit variants for receptor assembly, synaptic targeting, and functional properties. In addition, these studies unraveled mandatory association between certain subunits and demonstrated distinct pharmacology of receptors mediating phasic and tonic inhibition. Although some of these mutants have a profound loss of GABAA receptors, they exhibit only minor impairment of motor function, suggesting activation of compensatory mechanisms to preserve inhibitory networks in the cerebellum. These adaptations include an altered balance between phasic and tonic inhibition, activation of voltage-independent K+ conductances, and upregulation of GABAA receptors in interneurons that are not affected directly by the mutation. Deletion of the alpha1 subunit gene leads to complete loss of GABAA receptors in Purkinje cells. A striking alteration occurs in these mice, whereby presynaptic GABAergic terminals are preserved in the molecular layer but make heterologous synapses with spines, characterized by a glutamatergic-like postsynaptic density. During development of alpha1(0/0) mice, GABAergic synapses are initially formed but are replaced upon spine maturation. These findings suggest that functional GABAA receptors are required for long-term maintenance of GABAergic synapses in Purkinje cells.

CaMK-II modulation of GABAA receptors expressed in HEK293, NG108-15 and rat cerebellar granule neurons

The gamma-aminobutyric acid type A (GABA(A)) receptor is a pentameric ligand-gated ion channel responsible for fast synaptic inhibition in the brain. Phosphorylation of the GABA(A) receptor by serine/threonine protein kinases, at residues located in the intracellular loop between the third and fourth transmembrane domains of each subunit, can dynamically modulate receptor trafficking and function. In this study, we have assessed the effect that Ca(2+)-calmodulin-dependent protein kinase-II (CaMK-II) has on GABA(A) receptors. The intracellular application of preactivated CaMK-II failed to modulate the function of alphabeta and alphabetagamma subunit GABA(A) receptors heterologously expressed in human embryonic kidney (HEK)293 cells. However, application of similarly preactivated alpha-CaMK-II significantly potentiated the amplitudes of whole-cell GABA currents recorded from rat cultured cerebellar granule neurons and from recombinant GABA(A) receptors expressed in neuroblastoma, NG108-15, cells. The modulation by alpha-CaMK-II of current amplitude depended upon the subunit composition of GABA(A) receptors. alpha-CaMK-II potentiated GABA currents recorded from alpha1beta3 and alpha1beta3gamma2 GABA(A) receptors, but was unable to functionally modulate beta2 subunit-containing receptors. Similar results were obtained from beta2 -/- mouse cerebellar granule cell cultures and from rat granule cell cultures overexpressing recombinant alpha1beta2 or alpha1beta3 GABA(A) receptors. alpha-CaMK-II had a greater effect on the modulation of GABA responses mediated by alpha1beta3gamma2 compared with alpha1beta3 receptors, indicating a possible role for the gamma2 subunit in CaMK-II-mediated phosphorylation. In conclusion, CaMK-II can upregulate the function of GABA(A) receptors expressed in neurons or a neuronal cell line that is dependent on the beta subunit co-assembled into the receptor complex.

Sex- and age-specific effects of anabolic androgenic steroids on reproductive behaviors and on GABAergic transmission in neuroendocrine control regions

Illicit use of anabolic androgenic steroids (AAS) has become a prevalent health concern not only among male professional athletes, but, disturbingly, among a growing number of women and adolescent girls. Despite the increasing use of AAS among women and adolescents, few studies have focused on the effects of these steroids in females, and female adolescent subjects are particularly underrepresented. Among the hallmarks of AAS abuse are changes in reproductive behaviors. Here, we discuss work from our laboratories on the actions of AAS on the onset of puberty and sexual behaviors in female rodents, AAS interactions and sex- and age-specific effects of these steroids on neural transmission mediated by gamma-aminobutyric acid receptors within forebrain neuroendocrine control regions that may underlie AAS-induced changes in these behaviors.

Evidence that GABA rho subunits contribute to functional ionotropic GABA receptors in mouse cerebellar Purkinje cells

Ionotropic gamma-amino butyric acid (GABA) receptors composed of heterogeneous molecular subunits are major mediators of inhibitory responses in the adult CNS. Here, we describe a novel ionotropic GABA receptor in mouse cerebellar Purkinje cells (PCs) using agents reported to have increased affinity for rho subunit-containing GABA(C) over other GABA receptors. Exogenous application of the GABA(C)-preferring agonist cis-4-aminocrotonic acid (CACA) evoked whole-cell currents in PCs, whilst equimolar concentrations of GABA evoked larger currents. CACA-evoked currents had a greater sensitivity to the selective GABA(C) antagonist (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) than GABA-evoked currents. Focal application of agonists produced a differential response profile; CACA-evoked currents displayed a much more pronounced attenuation with increasing distance from the PC soma, displayed a slower time-to-peak and exhibited less desensitization than GABA-evoked currents. However, CACA-evoked currents were also completely blocked by bicuculline, a selective agent for GABA(A) receptors. Thus, we describe a population of ionotropic GABA receptors with a mixed GABA(A)/GABA(C) pharmacology. TPMPA reduced inhibitory synaptic transmission at interneurone-Purkinje cell (IN-PC) synapses, causing clear reductions in miniature inhibitory postsynaptic current (mIPSC) amplitude and frequency. Combined application of NO-711 (a selective GABA transporter subtype 1 (GAT-1) antagonist) and SNAP-5114 (a GAT-(2)/3/4 antagonist) induced a tonic GABA conductance in PCs; however, TPMPA had no effect on this current. Immunohistochemical studies suggest that rho subunits are expressed predominantly in PC soma and proximal dendritic compartments with a lower level of expression in more distal dendrites; this selective immunoreactivity contrasted with a more uniform distribution of GABA(A) alpha1 subunits in PCs. Finally, co-immunoprecipitation studies suggest that rho subunits can form complexes with GABA(A) receptor alpha1 subunits in the cerebellar cortex. Overall, these data suggest that rho subunits contribute to functional ionotropic receptors that mediate a component of phasic inhibitory GABAergic transmission at IN-PC synapses in the cerebellum.

Membrane potential-regulated Ca2+ signalling in development and maturation of mammalian cerebellar granule cells

In many developing neuronal cell types, the resting membrane potential is relatively depolarized, then gradually hyperpolarizes during the early postnatal period. The regulatory roles of membrane potential changes in neuronal development and maturation have been extensively studied in developing cerebellar granule cells, using primary culture under depolarizing and non-depolarizing conditions in combination with in vivo analysis. Depolarization enhances calcium entry via voltage-sensitive Ca2+ channels (VSCCs) and activates Ca2+-calmodulin-dependent protein kinase (CaMK) and calcineurin phophatase (CaN). The activation of CaN induces many genes encoding extracellular and intracellular signalling molecules implicated in granule cell development. The inactivation of CaN in turn up-regulates many other genes characteristic of mature granule cells, including NR2C NMDA receptor and GABAAalpha1 and alpha6 receptors. The induction of NR2C also requires CaMK-up-regulated brain-derived neurotrophic factor (BDNF), indicating a convergence of signalling mechanism of the CaMK and CaN cascades. The inactivation of CaN maintains the phosphorylated and sumoylated form of a transcriptional myocyte enhances factor 2A (MEF2A) regulator. This form of MEF2A acts as a transcriptional repressor and is essential for the dendritic morphogenesis of differentiated granule cells. Collectively, the membrane potential change and the resulting Ca2+ signalling play a pivotal role in development and maturation of neuronal cells.

Ethanol modulation of GABAergic transmission: the view from the slice

For almost three decades now, the GABAergic synapse has been the focus of intense study for its putative role in mediating many of the behavioral consequences associated with acute and chronic ethanol exposure. Although it was initially thought that ethanol interacted solely with the postsynaptic GABAA receptors that mediate the majority of fast synaptic inhibition in the mammalian central nervous system (CNS), a number of recent studies have identified novel pre- and postsynaptic mechanisms that may contribute to the acute and long-term effects of ethanol on GABAergic synaptic inhibition. These mechanisms appear to differ in a brain region specific manner and may also be influenced by a variety of endogenous neuromodulatory factors. This article provides a focused review of recent evidence, primarily from in vitro brain slice electrophysiological studies, that offers new insight into the mechanisms through which acute and chronic ethanol exposures modulate the activity of GABAergic synapses. The implications of these new mechanistic insights to our understanding of the behavioral and cognitive effects of ethanol are also discussed.

Role of GABAB receptors in GABA and baclofen-induced inhibition of adult rat cerebellar interpositus nucleus neurons in vitro

Previous studies suggested that the postsynaptic GABA(B) receptors of deep cerebellar nuclear neurons of adult rats were not activated by selective GABA(B) receptor agonist baclofen or endogenous GABA released by cerebellar cortical Purkinje cells, although the receptors have been demonstrated to exist in the deep cerebellar nuclei. In this study, cerebellar slices of adult rats were prepared for testing effects of GABA, baclofen and muscimol (selective GABA(A) receptor agonist) on cerebellar interpositus nucleus (IN) neurons. Perfusing slices with GABA (10-1000 microM), baclofen (1-30 microM) and muscimol (1-100 microM) respectively produced a dose-dependent inhibitory response on the IN neurons (n = 39, 62 and 50), which was not blocked by low-Ca(2+)/high-Mg(2+) medium (n = 5, 6 and 6), supporting a direct postsynaptic action of these GABAergic agonists. Moreover, both selective GABA(B) receptor antagonist CGP35348 and selective GABA(A) receptor antagonist bicuculline were capable of partially blocking the inhibitory response of IN neurons to GABA (n = 14 and 11), suggesting that the GABA-induced inhibition may contain two components, a GABA(B) receptors-mediated component and a GABA(A) receptors-mediated one. Further experiments revealed that not only muscimol (n = 50) but also baclofen (n = 62) suppressed IN cells' activity. The baclofen-induced inhibition was selectively blocked by CGP35348 (n = 12) but not by bicuculline (n = 8), whereas the muscimol-induced inhibition was selectively antagonized by bicuculline (n = 8) instead of CGP35348 (n = 9). These results indicate that GABA(B) receptors in the IN neurons can be activated not only by GABA but also by baclofen, suggesting that besides GABA(A) receptors, GABA(B) receptors may also be involved in mediating the inhibitory effect of GABA on cerebellar IN neurons of adult rats.

Alternative splicing generates a smaller assortment of CaV2.1 transcripts in cerebellar Purkinje cells than in the cerebellum

P/Q-type calcium channels control many calcium-driven functions in the brain. The CACNA1A gene encoding the pore-forming CaV2.1 (alpha1A) subunit of P/Q-type channels undergoes alternative splicing at multiple loci. This results in channel variants with different phenotypes. However, the combinatorial patterns of alternative splice events at two or more loci, and hence the diversity of CaV2.1 transcripts, are incompletely defined for specific brain regions and types of brain neurons. Using RT-PCR and splice variant-specific primers, we have identified multiple CaV2.1 transcript variants defined by different pairs of splice events in the cerebellum of adult rat. We have uncovered new splice variations between exons 28 and 34 (some of which predict a premature stop codon) and a new variation in exon 47 (which predicts a novel extended COOH-terminus). Single cell RT-PCR reveals that each individual cerebellar Purkinje neuron also expresses multiple alternative CaV2.1 transcripts, but the assortment is smaller than in the cerebellum. Two of these variants encode different extended COOH-termini which are not the same as those previously reported in Purkinje cells of the mouse. Our patch-clamp recordings show that calcium channel currents in the soma and dendrites of Purkinje cells are largely inhibited by a concentration of omega-agatoxin IVA selective for P-type over Q-type channels, suggesting that the different transcripts may form phenotypic variants of P-type calcium channels in Purkinje cells. These results expand the known diversity of CaV2.1 transcripts in cerebellar Purkinje cells, and propose the selective expression of distinct assortments of CaV2.1 transcripts in different brain neurons and species.

Is chemical neurotransmission altered specifically during methylmercury-induced cerebellar dysfunction?

Methylmercury (MeHg) is an important environmental neurotoxicant that is present in seafood and affects the developing and mature nervous system. The neurotoxicity induced by MeHg is a concern, particularly for fish-eating populations and pregnant or nursing women. During MeHg-induced neurotoxicity, degeneration of the granule cell layer in the cerebellum occurs, which leads to deficits in motor function. I suggest that the action of MeHg on specific neurotransmitter receptors contributes to the selective vulnerability of granule cells. MeHg appears to stimulate M(3) muscarinic acetylcholine receptors and to inhibit GABA(A) receptor subtypes preferentially on cerebellar granule cells. This could lead to the loss of tonic inhibition of granule cells as a result of antagonism of GABA(A) receptors, and a M(3)-receptor-mediated increase in the intracellular concentration of Ca(2+) and block of a K(+)-dependent leak current. The net result would be increased spontaneous release of glutamate, which, coupled with a MeHg-induced impairment of glutamate uptake by astrocytes, could cause Ca(2+)-mediated cytotoxicity.

Alterations in GABAA receptor occupancy occur during the postnatal development of rat Purkinje cell but not granule cell synapses

The identification of synaptic GABA(A) receptors has proved difficult as neurones express multiple GABA(A) receptor subunits. For example, cerebellar granule cells express alpha1, alpha6, gamma2, delta and beta2/3 subunits and thus express many different GABA(A) receptor subtypes. Furthermore, the contribution of individual GABA(A) receptor subtypes is changed by developmental alterations in subunit expression. To further characterise the pharmacology of Golgi cell to granule cell synapses during development, the benzodiazepine-site ligand zolpidem was used. Zolpidem shows selectivity for alpha1betaxgamma2 receptors (x is any beta subunit) and slows the decay and enhances the amplitude of alpha1betaxgamma2 receptor-mediated synaptic currents, provided the receptors are not fully occupied. For comparison, zolpidem was applied to Purkinje cell synapses, since the synaptic receptors are of known composition (alpha1betaxgamma2). At immature and adult Golgi cell to granule cell synapses, the decay of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) was slowed by zolpidem but their amplitude and frequency were unaffected. At Purkinje cell synapses, although zolpidem slowed the decay of IPSCs at both immature and adult synapses, zolpidem only enhanced the amplitude of IPSCs at adult synapses. Thus during development, the level of receptor occupation remains constant at Golgi cell to granule cell synapses but falls at Purkinje cell synapses.

Loss of zolpidem efficacy in the hippocampus of mice with the GABAA receptor gamma2 F77I point mutation

Zolpidem is a hypnotic benzodiazepine site agonist with some gamma-aminobutyric acid (GABA)(A) receptor subtype selectivity. Here, we have tested the effects of zolpidem on the hippocampus of gamma2 subunit (gamma2F77I) point mutant mice. Analysis of forebrain GABA(A) receptor expression with immunocytochemistry, quantitative [(3)H]muscimol and [(35)S] t-butylbicyclophosphorothionate (TBPS) autoradiography, membrane binding with [(3)H]flunitrazepam and [(3)H]muscimol, and comparison of miniature inhibitory postsynaptic current (mIPSC) parameters did not reveal any differences between homozygous gamma2I77/I77 and gamma2F77/F77 mice. However, quantitative immunoblot analysis of gamma2I77/I77 hippocampi showed some increased levels of gamma2, alpha1, alpha4 and delta subunits, suggesting that differences between strains may exist in unassembled subunit levels, but not in assembled receptors. Zolpidem (1 microm) enhanced the decay of mIPSCs in CA1 pyramidal cells of control (C57BL/6J, gamma2F77/F77) mice by approximately 60%, and peak amplitude by approximately 20% at 33-34 degrees C in vitro. The actions of zolpidem (100 nm or 1 microm) were substantially reduced in gamma2I77/I77 mice, although residual effects included a 9% increase in decay and 5% decrease in peak amplitude. Similar results were observed in CA1 stratum oriens/alveus interneurons. At network level, the effect of zolpidem (10 microm) on carbachol-induced oscillations in the CA3 area of gamma2I77/I77 mice was significantly different compared with controls. Thus, the gamma2F77I point mutation virtually abolished the actions of zolpidem on GABA(A) receptors in the hippocampus. However, some residual effects of zolpidem may involve receptors that do not contain the gamma2 subunit.

GABAergic signaling in the developing cerebellum

In the adult central nervous system (CNS), GABA is a predominant inhibitory neurotransmitter that regulates glutamatergic activity. Recent studies have revealed that GABA serves as an excitatory transmitter in the immature CNS and acts as a trophic factor for brain development. Furthermore, synaptic transmission by GABA is also involved in the expression of higher brain functions, such as memory, learning and anxiety. These results indicate that GABA plays various roles in the expression of brain functions and GABAergic roles change developmentally in accordance with alterations in GABAergic transmission and signaling. We have investigated morphologically the developmental changes in the GABAergic transmission system and the key factors important for the formation of GABAergic synapses and networks using the mouse cerebellum, which provides an ideal system for the investigation of brain development. Here, we focus on GABA and GABA(A) receptors in the developing cerebellum and address the processes of how GABA exerts its effect on developing neurons and the mechanisms underlying the formation of functional GABAergic synapses.

Alcohol-induced motor impairment caused by increased extrasynaptic GABA(A) receptor activity

Neuronal mechanisms underlying alcohol intoxication are unclear. We find that alcohol impairs motor coordination by enhancing tonic inhibition mediated by a specific subtype of extrasynaptic GABA(A) receptor (GABAR), alpha6beta3delta, expressed exclusively in cerebellar granule cells. In recombinant studies, we characterize a naturally occurring single-nucleotide polymorphism that causes a single amino acid change (R100Q) in alpha6 (encoded in rats by the Gabra6 gene). We show that this change selectively increases alcohol sensitivity of alpha6beta3delta GABARs. Behavioral and electrophysiological comparisons of Gabra6(100R/100R) and Gabra6(100Q/100Q) rats strongly suggest that alcohol impairs motor coordination by enhancing granule cell tonic inhibition. These findings identify extrasynaptic GABARs as critical targets underlying low-dose alcohol intoxication and demonstrate that subtle changes in tonic inhibition in one class of neurons can alter behavior.

A role for zinc in cerebellar synaptic transmission?

There is considerable evidence that the transition metal zinc plays an important role in mammalian neural development, physiology and pathology. The most compelling evidence for a synaptic role for zinc comes from hippocampal studies: zinc is concentrated in the synaptic vesicles of some glutamatergic neurons, zinc can be released during neural activity and zinc can modulate postsynaptic GABA and glutamate receptors. The possibility that zinc is involved in cerebellar synaptic transmission is supported by the expression of specific zinc transporters (ZnT, including the synaptic vesicle transporter ZnT-3) in the cerebellar cortex. Furthermore, some subtypes of neurotransmitter receptors, expressed by cerebellar neurones, are highly sensitive to low concentrations of exogenous zinc. However there appears to be little chelatable (synaptic) zinc in the cerebellum, deletion of the ZnT-3 gene has no effect on motor phenotype and there is currently no evidence that zinc is released from cerebellar neurones to have physiological actions. Thus it is possible that the different types of zinc transporter found in the cerebellum play a neuroprotective rather than a signalling role.

A Role for Nuclear Factor I in the Intrinsic Control of Cerebellar Granule Neuron Gene Expression

Nervous system formation requires the elaboration of a complex series of differentiation events in both a spatially and maturation-regulated manner. A fundamental question is how neuronal subtype specification and developmental gene expression are controlled within maturing neurons. The alpha6 subunit of the gamma-aminobutyric acid type A (GABA(A)) receptor (GABRA6) is preferentially expressed in cerebellar granule neurons and is part of an intrinsic program directing their differentiation. We have employed a lentiviral approach to examine the transcriptional mechanisms controlling neuronal subtype-selective expression of this gene. These studies demonstrated that nuclear factor I (NFI) proteins are required for both transgenic GABRA6 promoter activity as well as endogenous expression of this gene in cerebellar granule neurons. Chromatin immunoprecipitation also showed that NFI proteins are bound to the GABRA6 promoter in these cells in vivo. Furthermore, analyses of gene knockout mice revealed that Nfia is specifically required for normal expression of the GABRA6 gene in cerebellar granule neurons. NFI expression and DNA binding activity are highly enriched in granule neurons, implicating this transcription factor family in the neuronal subtype-selective expression of the GABRA6 gene. These studies define a new role for NFI proteins as neuronal subtype-enriched transcriptional regulators that participate in an intrinsic transcriptional program directing the differentiation of cerebellar granule neurons.

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

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

Abolition of zolpidem sensitivity in mice with a point mutation in the GABAA receptor gamma2 subunit

Agonists of the allosteric benzodiazepine site of GABAA receptors bind at the interface of the alpha and gamma subunits. Here, we tested the in vivo contribution of the gamma2 subunit to the actions of zolpidem, an alpha1 subunit selective benzodiazepine agonist, by generating mice with a phenylalanine (F) to isoleucine (I) substitution at position 77 in the gamma2 subunit. The gamma2F77I mutation has no major effect on the expression of GABAA receptor subunits in the cerebellum. The potency of zolpidem, but not that of flurazepam, for the inhibition of [3H]flunitrazepam binding to cerebellar membranes is greatly reduced in gamma2I77/I77 mice. Zolpidem (1 microM) increased both the amplitude and decay of miniature inhibitory postsynaptic currents (mIPSCs) in Purkinje cells of control C57BL/6 (34% and 92%, respectively) and gamma2F77/F77 (20% and 84%) mice, but not in those of gamma2F77I mice. Zolpidem tartrate had no effect on exploratory activity (staircase test) or motor performance (rotarod test) in gamma2I77/I77 mice at doses up to 30 mg/kg (i.p.) that strongly sedated or impaired the control mice. Flurazepam was equally effective in enhancing mIPSCs and disrupting performance in the rotarod test in control and gamma2I77/I77 mice. These results show that the effect of zolpidem, but not flurazepam, is selectively eliminated in the brain by the gamma2F77I point mutation.