Glycine receptors and GABA receptor alpha 1 and gamma 2 subunits during the development of mouse hypoglossal nucleus

Alfaxalone does not have long-term effects on goldfish pyramidal neuron action potential properties or GABAA receptor currents

Anesthetics have varying physiological effects, but most notably alter ion channel kinetics. Alfaxalone is a rapid induction and washout neuroactive anesthetic, which potentiates γ-aminobutyric acid (GABA)-activated GABAA receptor (GABAA-R) currents. This study aims to identify any long-term effects of alfaxalone sedation on pyramidal neuron action potential and GABAA-R properties, to determine if its impact on neuronal function can be reversed in a sufficiently short timeframe to allow for same-day electrophysiological studies in goldfish brain. The goldfish (Carassius auratus) is an anoxia-tolerant vertebrate and is a useful model to study anoxia tolerance mechanisms. The results show that alfaxalone sedation did not significantly impact action potential properties. Additionally, the acute application of alfaxalone onto naive brain slices caused the potentiation of whole-cell GABAA-R current decay time and area under the curve. Following whole-animal sedation with alfaxalone, a 3-h wash of brain slices in alfaxalone-free saline, with saline exchanged every 30 min, was required to remove any potentiating impact of alfaxalone on GABAA-R whole-cell currents. These results demonstrate that alfaxalone is an effective anesthetic for same-day electrophysiological experiments with goldfish brain slices.

Development of γ-aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on the rat genioglossal motoneurons

Detailed information about the development of excitatory and inhibitory synapses on the genioglossal (GG) motoneuron may help to understand the mechanism of fine control of GG motoneuron firing and the coordinated tongue movement during postnatal development. For this, we investigated the development of γ-aminobutyric acid (GABA)-immunopositive (GABA +), glycine + (Gly +), and glutamate + (Glut +) axon terminals (boutons) on the somata of rat GG motoneurons at a postnatal day 2 (P2), P6 and P18 by retrograde labeling of GG motoneurons with horseradish peroxidase, electron microscopic postembedding immunogold staining with GABA, Gly, and Glut antisera, and quantitative analysis. The number of boutons per GG motoneuron somata and the mean length of bouton apposition, measures of bouton size and synaptic covering percentage, were significantly increased from P2/P6 to P18. The number and fraction of GABA + only boutons of all boutons decreased significantly, whereas those of Gly + only boutons increased significantly from P2/P6 to P18, suggesting developmental switch from GABAergic to glycinergic synaptic transmission. The fraction of mixed GABA +/Gly + boutons of all boutons was the highest among inhibitory bouton types throughout the postnatal development. The fractions of excitatory and inhibitory boutons of all boutons remained unchanged during postnatal development. These findings reveal a distinct developmental pattern of inhibitory synapses on the GG motoneurons different from that on spinal or trigeminal motoneurons, which may have an important role in the regulation of the precise and coordinated movements of the tongue during the maturation of the oral motor system.

Dual synaptic inhibitions of brainstem neurons by GABA and glycine with impact on Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disease caused mostly by mutations in the MECP2 gene. People with RTT show breathing dysfunction attributable to the high rate of sudden death. Previous studies have shown that insufficient GABA synaptic inhibition contributes to the breathing abnormalities in mouse models of RTT, while it remains elusive how the glycine system is affected. We found that optogenetic stimulation of GAD-expressing neurons in mice produced GABAergic and glycinergic postsynaptic inhibitions of neurons in the hypoglossal nucleus (XII) and the dorsal motor nucleus of vagus (DMNV). By sequential applications of bicuculline and strychnine, such inhibition appeared approximately 44% GABA_A ergic and 52% glycinergic in XII neurons, and approximately 49% GABA_A ergic and 46% glycinergic in DMNV neurons. Miniature inhibitory postsynaptic potentials (mIPSCs) in these neurons were approximately 47% GABA_A ergic and 49% glycinergic in XII neurons, and approximately 48% versus 50% in DMNV neurons, respectively. Consistent with the data, our single-cell polymerase chain reaction studies indicated that transcripts of GABA_A receptor γ2 subunit (GABA_A Rγ2) and glycine receptor β subunit (GlyRβ) were simultaneously expressed in these cells. In MeCP2^R168X mice, proportions of GABA_A ergic and glycinergic mIPSCs became approximately 28% versus 69% in XII neurons, and approximately 31% versus 66% in DMNV cells. In comparison with control mice, the GABA_A ergic and glycinergic mIPSCs decreased significantly in the XII and DMNV neurons from the MeCP2^R168X mice, so did the transcripts of GABA_A Rγ2 and GlyRβ. These results suggest that XII and DMNV neurons adopt dual GABA_A ergic and glycinergic synaptic inhibitions, and with Mecp2 disruption these neurons rely more on glycinergic synaptic inhibition.

The synthetic cannabinoid dehydroxylcannabidiol restores the function of a major GABAA receptor isoform in a cell model of hyperekplexia

The functions of the glycine receptor (GlyR) and GABAA receptor (GABAAR) are both impaired in hyperekplexia, a neurological disorder usually caused by GlyR mutations. Although emerging evidence indicates that cannabinoids can directly restore normal GlyR function, whether they affect GABAAR in hyperekplexia remains unknown. Here we show that dehydroxylcannabidiol (DH-CBD), a synthetic nonpsychoactive cannabinoid, restores the GABA- and glycine-activated currents (IGABA and IGly , respectively) in HEK293 cells coexpressing a major GABAAR isoform (α1β2γ2) and GlyRα1 carrying a human hyperekplexia-associated mutation (GlyRα1R271Q). Using coimmunoprecipitation and FRET assays, we found that DH-CBD disrupts the protein interaction between GABAAR and GlyRα1R271Q Furthermore, a point mutation of GlyRα1, changing Ser-296 to Ala-296, which is critical for cannabinoid binding on GlyR, significantly blocked DH-CBD-induced restoration of IGABA and IGly currents. This S296A substitution also considerably attenuated DH-CBD-induced disruption of the interaction between GlyRα1R271Q and GABAAR. These findings suggest that, because it restores the functions of both GlyRα1 and GABAAR, DH-CBD may represent a potentially valuable candidate drug to manage hyperekplexia.

Human Hyperekplexic Mutations in Glycine Receptors Disinhibit the Brainstem by Hijacking GABAA Receptors

Hyperekplexia disease is usually caused by naturally occurring point mutations in glycine receptors (GlyRs). However, the γ-aminobutyric acid type A receptor (GABA_AR) seems to be also involved regarding the therapeutic basis for hyperekplexia using benzodiazepines, which target GABA_ARs but not GlyRs. Here, we show that the function of GABA_ARs was significantly impaired in the hypoglossal nucleus of hyperekplexic transgenic mice. Such impairment appeared to be mediated by interaction between GABA_AR and mutant GlyR. The GABA_AR dysfunction was caused only by mutant GlyR consisting of homomeric α₁ subunits, which locate primarily at pre- and extra-synaptic sites. In addition, the rescue effects of diazepam were attenuated by Xli-093, which specifically blocked diazepam-induced potentiation on α₅-containing GABA_AR, a major form of pre- and extra-synaptic GABA_AR in the brainstem. Thus, our results suggest that the pre- and extra-synaptic GABA_ARs could be a potential therapeutic target for hyperekplexia disease caused by GlyR mutations.

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Hyperekplexic mutant GlyRs interact with GABA_ARs and disrupt the GABA_AR function

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Pre- and extra-synaptic GABA_ARs are deficient in the hyperekplexia disease

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α₅-Containing GABA_AR is a potential therapeutic target for the hyperekplexia disease

Distribution of excitatory and inhibitory axon terminals on the rat hypoglossal motoneurons

Detailed information about the excitatory and inhibitory synapses on the hypoglossal motoneurons may help understand the neural mechanism for control of the hypoglossal motoneuron excitability and hence the precise and coordinated movements of the tongue during chewing, swallowing and licking. For this, we investigated the distribution of GABA-, glycine (Gly)- and glutamate (Glut)-immunopositive (+) axon terminals on the genioglossal (GG) motoneurons by retrograde tracing, electron microscopic immunohistochemistry, and quantitative analysis. Small GG motoneurons (< 400 μm² in cross-sectional area) had fewer primary dendrites, significantly higher nuclear/cytoplasmic ratio, and smaller membrane area covered by synaptic boutons than large GG motoneurons (> 400 μm²). The fraction of inhibitory boutons (GABA + only, Gly + only, and mixed GABA +/Gly + boutons) of all boutons was significantly higher for small GG motoneurons than for large ones, whereas the fraction of Glut + boutons was significantly higher for large GG motoneurons than for small ones. Almost all boutons (> 95%) on both small and large GG motoneurons were GABA + , Gly + or Glut + . The frequency of mixed GABA +/Gly + boutons was the highest among inhibitory boutons types for both small and large GG motoneurons. These findings may elucidate the anatomical substrate for precise regulation of the motoneuron firing required for the fine movements of the tongue, and also suggest that the excitability of small and large GG motoneurons may be regulated differently.

Extrasynaptic δ-GABAA receptors are high-affinity muscimol receptors

Muscimol, the major psychoactive ingredient in the mushroom Amanita muscaria, has been regarded as a universal non‐selective GABA‐site agonist. Deletion of the GABA_A receptor (GABA_AR) δ subunit in mice (δKO) leads to a drastic reduction in high‐affinity muscimol binding in brain sections and to a lower behavioral sensitivity to muscimol than their wild type counterparts. Here, we use forebrain and cerebellar brain homogenates from WT and δKO mice to show that deletion of the δ subunit leads to a > 50% loss of high‐affinity 5 nM [³H]muscimol‐binding sites despite the relatively low abundance of δ‐containing GABA_ARs (δ‐GABA_AR) in the brain. By subtracting residual high‐affinity binding in δKO mice and measuring the slow association and dissociation rates we show that native δ‐GABA_ARs in WT mice exhibit high‐affinity [³H]muscimol‐binding sites (K_D ~1.6 nM on α4βδ receptors in the forebrain and ~1 nM on α6βδ receptors in the cerebellum at 22°C). Co‐expression of the δ subunit with α6 and β2 or β3 in recombinant (HEK 293) expression leads to the appearance of a slowly dissociating [³H]muscimol component. In addition, we compared muscimol currents in recombinant α4β3δ and α4β3 receptors and show that δ subunit co‐expression leads to highly muscimol‐sensitive currents with an estimated EC₅₀ of around 1–2 nM and slow deactivation kinetics. These data indicate that δ subunit incorporation leads to a dramatic increase in GABA_AR muscimol sensitivity. We conclude that biochemical and behavioral low‐dose muscimol selectivity for δ‐subunit‐containing receptors is a result of low nanomolar‐binding affinity on δ‐GABA_ARs.

GABA beyond the synapse: defining the subtype-specific pharmacodynamics of non-synaptic GABAA receptors

KEY POINTS

Physiologically relevant combinations of recombinant GABAA receptor (GABAR) subunits were expressed in HEK293 cells. Using whole-cell voltage clamp and rapid drug application, we measured the GABAR-subtype-specific properties to convey either synaptic or extrasynaptic signalling in a range of physiological contexts. α4βδ GABARs are optimally tuned to submicromolar tonic GABA and transient surges of micromolar GABA concentrations. α5β2γ2l GABARs are better suited to higher tonic GABA levels, but also convey robust responses to brief synaptic and perisynaptic GABA fluctuations. α1β2/3δ GABARs function well at prolonged, micromolar (>2 μm) GABA levels, but not to low tonic (<1 μm GABA) or synaptic/transient GABAergic signalling. These results help illuminate the context- and isoform-specific modes of GABAergic signalling in the brain.

ABSTRACT

GABAA receptors (GABARs) mediate a remarkable diversity of signalling modalities in vivo. Yet most published work characterizing responses to GABA has focused on the properties needed to convey fast, phasic synaptic inhibition. We therefore aimed to characterize the most prevalent (α4βδ, α5β3γ2L) and least prevalent (α1β2δ) non-synaptic GABAR currents, using whole-cell voltage clamp recordings of recombinant GABAR expressed in HEK293 cells and drug application protocols to recapitulate the GABA concentration profiles occurring during both fast synaptic and slow extrasynaptic signalling. We found that α4βδ GABARs were very sensitive to submicromolar GABA, with a rank order potency of α4β2δ ≥ α4β1δ ≈ α4β3δ GABARs. In comparison, the GABA EC50 was up to 20 times higher for α1β2γ2L GABARs, with α1β2δ and α5β3γ2L GABARs having intermediate GABA potency. Both α4βδ and α5β3γ2L GABAR currents exhibited slow, but substantial, desensitization as well as prolonged rates of deactivation. These GABAR current properties defined distinct 'dynamic ranges' of responsiveness to changing GABA for α4β2δ (0.1-1 μm), α5β3γ2L (0.5-7 μm) and α1β2γ2L (0.6-9 μm) GABARs. Finally, α1β2δ GABARs were notable for their relative lack of desensitization and extremely quick deactivation. In summary, our results help delineate the roles that specific GABARs may play in mediating non-synaptic GABA signals. Since ambient GABA levels may be altered during development as well as by drugs and disease states, these findings may help future efforts to understand disrupted inhibition underlying a variety of neurological illnesses, such as epilepsy.

Developmental changes in the morphology of mouse hypoglossal motor neurons

Hypoglossal motor neurons (XII MNs) innervate tongue muscles important in breathing, suckling and vocalization. Morphological properties of 103 XII MNs were studied using Neurobiotin™ filling in transverse brainstem slices from C57/Bl6 mice (n = 34) from embryonic day (E) 17 to postnatal day (P) 28. XII MNs from areas thought to innervate different tongue muscles showed similar morphology in most, but not all, features. Morphological properties of XII MNs were established prior to birth, not differing between E17-18 and P0. MN somatic volume gradually increased for the first 2 weeks post-birth. The complexity of dendritic branching and dendrite length of XII MNs increased throughout development (E17-P28). MNs in the ventromedial XII motor nucleus, likely to innervate the genioglossus, frequently (42 %) had dendrites crossing to the contralateral side at all ages, but their number declined with postnatal development. Unexpectedly, putative dendritic spines were found in all XII MNs at all ages, and were primarily localized to XII MN somata and primary dendrites at E18-P4, increased in distal dendrites by P5-P8, and were later predominantly found in distal dendrites. Dye-coupling between XII MNs was common from E18 to P7, but declined strongly with maturation after P7. Axon collaterals were found in 20 % (6 of 28) of XII MNs with filled axons; collaterals terminated widely outside and, in one case, within the XII motor nucleus. These results reveal new morphological features of mouse XII MNs, and suggest that dendritic projection patterns, spine density and distribution, and dye-coupling patterns show specific developmental changes in mice.

Altered intrathalamic GABAA neurotransmission in a mouse model of a human genetic absence epilepsy syndrome

We previously demonstrated that heterozygous deletion of Gabra1, the mouse homolog of the human absence epilepsy gene that encodes the GABAA receptor (GABAAR) α1 subunit, causes absence seizures. We showed that cortex partially compensates for this deletion by increasing the cell surface expression of residual α1 subunit and by increasing α3 subunit expression. Absence seizures also involve two thalamic nuclei: the ventrobasal (VB) nucleus, which expresses only the α1 and α4 subtypes of GABAAR α subunits, and the reticular (nRT) nucleus, which expresses only the α3 subunit subtype. Here, we found that, unlike cortex, VB exhibited significantly reduced total and synaptic α1 subunit expression. In addition, heterozygous α1 subunit deletion substantially reduced miniature inhibitory postsynaptic current (mIPSC) peak amplitudes and frequency in VB. However, there was no change in the expression of the extrasynaptic α4 or δ subunits in VB and, unlike other models of absence epilepsy, no change in tonic GABAAR currents. Although heterozygous α1 subunit knockout increased α3 subunit expression in medial thalamic nuclei, it did not alter α3 subunit expression in nRT. However, it did enlarge the presynaptic vesicular inhibitory amino acid transporter puncta and lengthen the time constant of mIPSC decay in nRT. We conclude that increased tonic GABAA currents are not necessary for absence seizures. In addition, heterozygous loss of α1 subunit disinhibits VB by substantially reducing phasic GABAergic currents and surprisingly, it also increases nRT inhibition by prolonging phasic currents. The increased inhibition in nRT likely represents a partial compensation that helps reduce absence seizures.

Differential GABAergic and glycinergic inputs of inhibitory interneurons and Purkinje cells to principal cells of the cerebellar nuclei

The principal neurons of the cerebellar nuclei (CN), the sole output of the olivo-cerebellar system, receive a massive inhibitory input from Purkinje cells (PCs) of the cerebellar cortex. Morphological evidence suggests that CN principal cells are also contacted by inhibitory interneurons, but the properties of this connection are unknown. Using transgenic, tracing, and immunohistochemical approaches in mice, we show that CN interneurons form a large heterogeneous population with GABA/glycinergic phenotypes, distinct from GABAergic olive-projecting neurons. CN interneurons are found to contact principal output neurons, via glycine receptor (GlyR)-enriched synapses, virtually devoid of the main GABA receptor (GABAR) subunits α1 and γ2. Those clusters account for 5% of the total number of inhibitory receptor clusters on principal neurons. Brief optogenetic stimulations of CN interneurons, through selective expression of channelrhodopsin 2 after viral-mediated transfection of the flexed gene in GlyT2-Cre transgenic mice, evoked fast IPSCs in principal cells. GlyR activation accounted for 15% of interneuron IPSC amplitude, while the remaining current was mediated by activation of GABAR. Surprisingly, small GlyR clusters were also found at PC synapses onto principal CN neurons in addition to α1 and γ2 GABAR subunits. However, GlyR activation was found to account for <3% of the PC inhibitory synaptic currents evoked by electrical stimulation. This work establishes CN glycinergic neurons as a significant source of inhibition to CN principal cells, forming contacts molecularly distinct from, but functionally similar to, Purkinje cell synapses. Their impact on CN output, motor learning, and motor execution deserves further investigation.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Lack of an endogenous GABAA receptor-mediated tonic current in hypoglossal motoneurons

Tonic GABAA receptor-mediated current is an important modulator of neuronal excitability, but it is not known if it is present in mammalian motoneurons. To address this question studies were performed using whole-cell patch-clamp recordings from mouse hypoglossal motoneurons (HMs) in an in vitro slice preparation. In the presence of blockers of glutamatergic and glycinergic receptor-mediated transmission application of SR-95531 or bicuculline, while abolishing GABAA receptor-mediated phasic synaptic currents, did not reveal a tonic GABAA receptor-mediated current. Additionally, blockade of both GAT-1 and GAT-3 GABA transporters did not unmask this tonic current. In contrast, application of exogenous GABA (1 to 15 μm) resulted in a tonic GABAergic current that was observed when both GAT-1 and GAT-3 transporters were simultaneously blocked, and this current was greater than the sum of the current observed when each transporter was blocked individually. We also investigated which GABAA receptor subunits may be responsible for the current. Application of the δ subunit GABAA receptor agonist THIP resulted in a tonic GABAA receptor current. Application of the δ subunit modulator THDOC resulted in an enhanced tonic current. Application of the α5 subunit GABAA receptor inverse agonist L-655,708 did not modulate the current. In conclusion, these data show that HMs have tonic GABAA receptor-mediated current. The level of GABA in the vicinity of GABAA receptors responsible for this current is regulated by GABA transporters. In HMs a tonic current in response to exogenous GABA probably arises from activation of GABAA receptors containing δ subunits.

Maturation of the GABAergic transmission in normal and pathologic motoneurons

γ-aminobutyric acid (GABA) acting on Cl⁻-permeable ionotropic type A (GABA_A) receptors (GABA_AR) is the major inhibitory neurotransmitter in the adult central nervous system of vertebrates. In immature brain structures, GABA exerts depolarizing effects mostly contributing to the expression of spontaneous activities that are instructive for the construction of neural networks but GABA also acts as a potent trophic factor. In the present paper, we concentrate on brainstem and spinal motoneurons that are largely targeted by GABAergic interneurons, and we bring together data on the switch from excitatory to inhibitory effects of GABA, on the maturation of the GABAergic system and GABA_AR subunits. We finally discuss the role of GABA and its GABA_AR in immature hypoglossal motoneurons of the spastic (SPA) mouse, a model of human hyperekplexic syndrome.

Development of synaptic transmission to respiratory motoneurons

Respiratory motoneurons provide the exclusive drive to respiratory muscles and therefore are a key relay between brainstem neural circuits that generate respiratory rhythm and respiratory muscles that control moment of gases into and out of the airways and lungs. This review is focused on postnatal development of fast ionotropic synaptic transmission to respiratory motoneurons, with a focus on hypoglossal motoneurons (HMs). Glutamatergic synaptic transmission to HMs involves activation of both non-NMDA and NMDA receptors and during the postnatal period co-activation of these receptors located at the same synapse may occur. Further, the relative role of each receptor type in inspiratory-phase motoneuron depolarization is dependent on the type of preparation used (in vitro versus in vivo; neonatal versus adult). Respiratory motoneurons receive both glycinergic and GABAergic inhibitory synaptic inputs. During inspiration phrenic and HMs receive concurrent excitatory and inhibitory synaptic inputs. During postnatal development in HMs GABAergic and glycinergic synaptic inputs have slow kinetics and are depolarizing and with postnatal development they become faster and hyperpolarizing. Additionally shunting inhibition may play an important role in synaptic processing by respiratory motoneurons.

The specification of glycinergic neurons and the role of glycinergic transmission in development

Glycine's role as an inhibitory neurotransmitter in the adult vertebrate nervous system has been well characterized in a number of different model organisms. However, a full understanding of glycinergic transmission requires a knowledge of how glycinergic synapses emerge and the role of glycinergic signaling during development. Recent literature has provided a detailed picture of the developmental expression of many of the molecular components that comprise the glycinergic phenotype, namely the glycine transporters and the glycine receptor subunits; the transcriptional networks leading to the expression of this important neurotransmitter phenotype are also being elucidated. An equally important focus of research has revealed the critical role of glycinergic signaling in sculpting many different aspects of neural development. This review examines the current literature detailing the expression patterns of the components of the glycinergic phenotype in various vertebrate model organisms over the course of development and the molecular mechanisms governing the expression of the glycinergic phenotype. The review then surveys the recent work on the role of glycinergic signaling in the developing nervous system and concludes with an overview of areas for further research.

Differential localization of gamma-aminobutyric acid type A and glycine receptor subunits and gephyrin in the human pons, medulla oblongata and uppermost cervical segment of the spinal cord: an immunohistochemical study

Gephyrin is a multifunctional protein responsible for the clustering of glycine receptors (GlyR) and gamma-aminobutyric acid type A receptors (GABA(A)R). GlyR and GABA(A)R are heteropentameric chloride ion channels that facilitate fast-response, inhibitory neurotransmission in the mammalian brain and spinal cord. We investigated the immunohistochemical distribution of gephyrin and the major GABA(A)R and GlyR subunits in the human light microscopically in the rostral and caudal one-thirds of the pons, in the middle and caudal one-thirds of the medulla oblongata, and in the first cervical segment of the spinal cord. The results demonstrate a widespread pattern of immunoreactivity for GlyR and GABA(A)R subunits throughout these regions, including the spinal trigeminal nucleus, abducens nucleus, facial nucleus, pontine reticular formation, dorsal motor nucleus of the vagus nerve, hypoglossal nucleus, lateral cuneate nucleus, and nucleus of the solitary tract. The GABA(A)R alpha(1) and GlyR alpha(1) and beta subunits show high levels of immunoreactivity in these nuclei. The GABA(A)R subunits alpha(2), alpha(3), beta(2,3), and gamma(2) present weaker levels of immunoreactivity. Exceptions are intense levels of GABA(A)R alpha(2) subunit immunoreactivity in the inferior olivary complex and high levels of GABA(A)R alpha(3) subunit immunoreactivity in the locus coeruleus and raphe nuclei. Gephyrin immunoreactivity is highest in the first segment of the cervical spinal cord and hypoglossal nucleus. Our results suggest that a variety of different inhibitory receptor subtypes is responsible for inhibitory functions in the human brainstem and cervical spinal cord and that gephyrin functions as a clustering molecule for major subtypes of these inhibitory neurotransmitter receptors.

Akt promotes chemoresistance in human ovarian cancer cells by modulating cisplatin-induced, p53-dependent ubiquitination of FLICE-like inhibitory protein

Although Akt is a determinant of cisplatin (cis-diaminedichloroplatinum (CDDP)) resistance in ovarian cancer cells, which is related in part to its inhibitory action on p53 activation, precisely how Akt confers CDDP resistance is unclear. In this study, we show that CDDP induced p53-dependent Fas-associated death domain-like interleukin-1beta-converting enzyme (FLICE)-like inhibitory protein (FLIP) degradation in chemosensitive ovarian cancer cells but not their resistant counterparts. CDDP induced FLIP-p53-Itch interaction, colocalization and FLIP ubiquitination in chemosensitive but not chemoresistant ovarian cancer cells. Moreover, although activated Akt inhibited CDDP-induced FLIP degradation and apoptosis in sensitive cells, these responses were facilitated by dominant-negative Akt expression in chemoresistant cells. Inhibition of Akt function also facilitated p53-FLIP interaction and FLIP ubiquitination, which were attenuated by p53 silencing. These results suggest that Akt confers resistance, in part, by modulating CDDP-induced, p53-dependent FLIP ubiquitination. Understanding the precise etiology of chemoresistance may improve treatment for ovarian cancer.

Despite GABAergic neurotransmission, GABAergic innervation does not compensate for the defect in glycine receptor postsynaptic aggregation in spastic mice

In the hypoglossal nucleus of wild-type mice, early mixed glycinergic-GABAergic inhibitory transmission becomes mainly glycinergic during postnatal maturation. In spastic mice (SPA), a model of human hyperekplexic syndrome, an insertion into the gene of the glycine receptor (GlyR) beta subunit results in a decreased accumulation of GlyRs at postsynaptic sites and an impaired glycinergic neurotransmission. In SPA mice displaying a mild phenotype (B6C3Fe strain), a compensatory process involving an increased aggregation of GABA(A) receptors (GABA(A)Rs) at postsynaptic sites was proposed to explain survival of mutant animals until adulthood. However, C57BL/6J strain SPA mice which express a lower amount of GlyR beta subunit die 2-3 weeks after birth, suggesting that GABAergic compensation does not necessarily take place. We performed a morphofunctional study of inhibitory synapses in the developing hypoglossal nucleus of C57BL/6J SPA mice. In this mutant, the inhibitory synaptic activity was mainly GABAergic. Accordingly, we observed a developmental loss of glycinergic presynaptic terminals and an increase in the density of GABAergic presynaptic terminals during the first two postnatal weeks. In addition, while C57BL/6J SPA mice displayed a strong impairment in GlyR aggregation at postsynaptic loci, the proportion of inhibitory presynaptic terminals facing diffuse GABA(A)Rs significantly increased during development. Our results suggest crosstalk between postsynaptic and presynaptic elements, leading to the developmental regulation of the presynaptic terminal neurotransmitter content according to the level of postsynaptic GlyR aggregation. They also indicate that GABAergic neurotransmission does not compensate for defects in GlyR postsynaptic aggregation leading to spastic syndrome in C57BL/6J SPA mice.

Colocalization of synaptic GABA(C)-receptors with GABA (A)-receptors and glycine-receptors in the rodent central nervous system

Fast inhibition in the nervous system is preferentially mediated by GABA- and glycine-receptors. Two types of ionotropic GABA-receptor, the GABA(A)-receptor and GABA(C)-receptor, have been identified; they have specific molecular compositions, different sensitivities to GABA, different kinetics, and distinct pharmacological profiles. We have studied, by immunocytochemistry, the synaptic localization of glycine-, GABA(A)-, and GABA(C)-receptors in rodent retina, spinal cord, midbrain, and brain-stem. Antibodies specific for the alpha1 subunit of the glycine-receptor, the gamma2 subunit of the GABA(A)-receptor, and the rho subunits of the GABA(C)-receptor have been applied. Using double-immunolabeling, we have determined whether these receptors are expressed at the same postsynaptic sites. In the retina, no such colocalization was observed. However, in the spinal cord, we found the colocalization of glycine-receptors with GABA(A)- or GABA(C)-receptors and the colocalization of GABA(A)- and GABA(C)-receptors in approximately 25% of the synapses. In the midbrain and brain-stem, GABA(A)- and GABA(C)-receptors were colocalized in 10%-15% of the postsynaptic sites. We discuss the possible expression of heteromeric (hybrid) receptors assembled from GABA(A)- and GABA(C)-receptor subunits. Our results suggest that GABA(A)- and GABA(C)-receptors are colocalized in a minority of synapses of the central nervous system.

Differential expression of GABAA and glycine receptors in ALS-resistant vs. ALS-vulnerable motoneurons: possible implications for selective vulnerability of motoneurons

Summary Amyotrophic lateral sclerosis (ALS) is a devastating motoneuronal degenerative disease, which is inevitably fatal in adults. ALS is characterized by an extensive loss of motoneurons in the cerebrospinal axis, except for those motoneurons that control eye movements and bladder contraction. The reason for this selectivity is not known. Systematic differences have been found in the organization of excitatory synaptic transmission in ALS-resistant vs. ALS-susceptible motor nuclei. However, although motoneurons express high levels of glycine receptors (GlyR) and GABA(A) receptors (GABA(A)R), no such studies have been carried out yet for inhibitory synaptic transmission. In this study, we compared the subunit composition, patterns of expression, density and synaptic localization of inhibitory synaptic receptors in ALS-resistant (oculomotor, trochlear and abducens) and ALS-vulnerable motoneurons (trigeminal, facial and hypoglossi). Triple immunofluorescent stainings of the major GABA(A)R subunits (alpha1, alpha2, alpha3, and alpha5), the GlyR alpha1 subunit and gephyrin, were visualized by confocal microscopy and analysed quantitatively. A strong correlation was observed between the vulnerability of motoneurons and the subunit composition of GABA(A)R, the GlyR/GABA(A)R density ratios and the incidence of synaptic vs. extrasynaptic GABA(A)R. These differences contrast strikingly with the uniform gephyrin cluster density and synaptic GlyR levels recorded in all motor nuclei examined. These results suggest that the specific patterns of inhibitory receptor organization observed might reflect functional differences that are relevant to the physiopathology of ALS.

Glycine receptors in the striatum, globus pallidus, and substantia nigra of the human brain: An immunohistochemical study

Glycine receptors (GlyRs) are heteropentameric chloride ion channels that facilitate fast-response, inhibitory neurotransmission in the mammalian spinal cord and brain. GlyRs have four functional subunits, alpha1-3 and beta, which likely exist in heteromeric alphabeta combinations. Mutations in GlyR alpha1 and beta subunits are well known for their involvement in hyperekplexia, a paroxysmal motor disorder. In this study we present the first detailed immunohistochemical investigation at the regional, cellular, and subcellular levels of GlyRs in the human basal ganglia. The results show that GlyRs are present at the regional level in low concentrations in the striatum and globus pallidus and are present in the highest concentrations in the substantia nigra. At the cellular level, GlyRs are present only in discrete populations of neurons immunoreactive for choline acetyltransferase (ChAT), parvalbumin, and calretinin in the human striatum, on a subpopulation of parvalbumin- and calretinin-positive neurons in the globus pallidus, and in the substantia nigra GlyRs are present on approximately three-fourths of all pars compacta and one-third of all pars reticulata neurons. They also form a distinct band of immunoreactive neurons in the intermedullary layers of the globus pallidus. At the subcellular level in the substantia nigra pars reticulata (SNr), GlyRs show a localized distribution on the soma and dendrites that partially complements but does not overlap with the distribution of gamma-aminobutyric acid (GABA)A receptors. Our results demonstrate the precise cellular and subcellular localization of GlyRs in the human basal ganglia and suggest that glycinergic receptors may play an important complementary role to other inhibitory receptors in modulating cholinergic, dopaminergic, and GABAergic neuronal pathways in the basal ganglia.

Developmental dissociation of presynaptic inhibitory neurotransmitter and postsynaptic receptor clustering in the hypoglossal nucleus

At postsynaptic densities of mouse hypoglossal motoneurons, the proportion of glycine receptors co-clustered with GABAA receptors increases from neonatal to adult animals, suggesting that mixed synapses might play a greater role in adult synaptic inhibition. We visualized the presynaptic correlates of these developmental changes using immunocytochemistry. At P5, presynaptic terminals contained glycine and GlyT2 and/or GABA and GAD65, but at P15, the majority of inhibitory terminals contained glycine and GlyT2 only. The GABAergic component of evoked inhibitory postsynaptic currents in HMs decreased strongly between P5 and P15. Similarly, miniature inhibitory postsynaptic currents evolved from mainly glycinergic and mixed glycinergic/GABAergic events at P3-5 to predominantly glycinergic currents at P15. These results indicate that the decrease in the proportion of functional mixed inhibitory synapses with maturation results from a loss of the ability of presynaptic terminals to release both neurotransmitters during development while co-aggregation of GlyRs + GABAARs at postsynaptic loci remained.

The density of AMPA receptors activated by a transmitter quantum at the climbing fibre-Purkinje cell synapse in immature rats

We aimed to estimate the number of AMPA receptors (AMPARs) bound by the quantal transmitter packet, their single-channel conductance and their density in the postsynaptic membrane at cerebellar Purkinje cell synapses. The synaptic and extrasynaptic AMPARs were examined in Purkinje cells in 2- to 4-day-old rats, when they receive synaptic inputs solely from climbing fibres (CFs). Evoked CF EPSCs and whole-cell AMPA currents displayed roughly linear current-voltage relationships, consistent with the presence of GluR2 subunits in synaptic and extrasynaptic AMPARs. The mean quantal size, estimated from the miniature EPSCs (MEPSCs), was approximately 300 pS. Peak-scaled non-stationary fluctuation analysis of spontaneous EPSCs and MEPSCs gave a weighted-mean synaptic channel conductance of approximately 5 pS (approximately 7 pS when corrected for filtering). By applying non-stationary fluctuation analysis to extrasynaptic currents activated by brief glutamate pulses (5 mM), we also obtained a small single-channel conductance estimate for extrasynaptic AMPARs (approximately 11 pS). This approach allowed us to obtain a maximum open probability (Po,max) value for the extrasynaptic receptors (Po,max = 0.72). Directly resolved extrasynaptic channel openings in the continued presence of glutamate exhibited clear multiple-conductance levels. The mean area of the postsynaptic density (PSD) of these synapses was 0.074 microm2, measured by reconstructing electron-microscopic (EM) serial sections. Postembedding immunogold labelling by anti-GluR2/3 antibody revealed that AMPARs are localised in PSDs. From these data and by simulating error factors, we estimate that at least 66 AMPARs are bound by a quantal transmitter packet at CF-Purkinje cell synapses, and the receptors are packed at a minimum density of approximately 900 microm-2 in the postsynaptic membrane.

Slow phases of GABA(A) receptor desensitization: structural determinants and possible relevance for synaptic function

GABA(A) receptor fast desensitization is thought to shape the time course of individual IPSCs. Although GABA(A) receptors also exhibit slower phases of desensitization, the possible role of slow desensitization in modifying synaptic function is poorly understood. In transiently transfected human embryonic kidney (HEK293T) cells, rat alpha1beta3delta and alpha1beta3gamma2L GABA(A) receptors showed distinct desensitization patterns during long (28 s) concentration jumps using a saturating (1 mM) GABA concentration. alpha1beta3gamma2L receptors desensitized extensively (approximately 90%), with four phases (tau(1) approximately 20 ms, tau(2) approximately 400 ms, tau(3) approximately 2 s, tau(4) approximately 10 s), while alpha1beta3delta receptors desensitized slowly and less extensively (approximately 35 %), with one or two slow phases with time constants similar to tau(3) and tau(4) of alpha1beta3gamma2L receptors. To determine the structural basis of subunit-specific desensitization, delta-gamma2L chimera subunits were expressed with alpha1 and beta3 subunits. Replacing the entire N-terminus of the gamma2L subunit with delta subunit sequence did not alter the number of phases or the extent of desensitization. Although extension of delta subunit sequence into transmembrane domain 1 (TM1) abolished the fast and intermediate components of desensitization, the two slow phases still accounted for substantial current loss (approximately 65 %). However, when delta subunit sequence was extended through TM2, the extent of desensitization was significantly decreased and indistinguishable from that of alpha1beta3delta receptors. The importance of TM2 sequence was confirmed by introducing gamma2 subunit TM2 residues into the delta subunit, which significantly increased the extent of desensitization, without introducing either the fast or intermediate desensitization phases. However, introducing delta subunit TM2 sequence into the gamma2L subunit had minimal effect on the rates or extent of desensitization. The results suggest that distinct delta subunit structures are responsible for its unique desensitization properties: lack of fast and intermediate desensitization and small contribution of the slow phases of desensitization. Finally, to investigate the possible role of slow desensitization in synaptic function, we used a pulse train protocol. We observed inhibition of peak current amplitude that depended on the frequency and duration of GABA pulses for receptors exhibiting extensive desensitization, whether fast phases were present or not. The minimally desensitizing alpha1beta3delta receptor exhibited negligible inhibition during pulse trains. Because receptors that desensitized without the fast and intermediate phases showed pulse train inhibition, we concluded that receptors can accumulate in slowly equilibrating desensitized states during repetitive receptor activation. These results may indicate a previously unrecognized role for the slow phases of desensitization for synaptic function under conditions of repeated GABA(A) receptor activation.

GABA and glycine co-release optimizes functional inhibition in rat brainstem motoneurons in vitro

Whole-cell patch clamp recordings of miniature inhibitory postsynaptic currents (mIPSCs) were obtained in identified abducens motoneurons (aMns) from young rats (P5-P13). Three types of mIPSC were distinguished according to their kinetics and their sensitivity to receptor antagonists: faster decaying events mediated by glycine receptors (glyRs), slower decaying events mediated by GABA(A) receptors (GABA(A)Rs), and mIPSCs displaying two components corresponding to GABA and glycine co-release. Dual component events accounted for approximately 30 % of mIPSCs, independently of the rat's age and were also identified during evoked transmitter release. In contrast, the kinetics of glyR- and GABA(A)R-mediated mIPSCs became faster during development. Monosynaptic inhibitory postsynaptic potentials (IPSPs) were able to fully inhibit motoneuron discharge elicited by current pulses. When the GABA(A)R-mediated component or the glyR-mediated component of the IPSP was blocked, the inhibition of motoneuron firing was reduced. The 20-80 % rise time and duration of GABA(A)R-mediated IPSPs were significantly longer than those mediated by glyRs. The time window of inhibition for each component was determined using single postsynaptic action potentials elicited with various delays from the onset of the IPSP. GlyR-mediated IPSPs induced fast transient inhibition whereas GABA(A)R-mediated IPSPs induced slow sustained suppression of firing. Using a modelling approach, we found that the two components summated non-linearly. We conclude that in developing aMns, co-release of GABA and glycine determines the strength and timing of inhibition through non-linear interactions between the two components, thus optimizing inhibition of motoneuron function.

Proton sensitivity of rat cerebellar granule cell GABAA receptors: dependence on neuronal development

The effect of GABAA receptor development in culture on the modulation of GABA-induced currents by external H+ was examined in cerebellar granule cells using whole-cell and single-channel recording. Equilibrium concentration-response curves revealed a lower potency for GABA between 11 and 12 days in vitro (DIV) resulting in a shift of the EC50 from 10.7 to 2.4 uM. For granule cells before 11 DIV, the peak GABA-activated current was inhibited at low external pH and enhanced at high pH with a pKa of 6.6. For the steady-state response, low pH was inhibitory with a pKa of 5.56. After 11 DIV, the peak GABA-activated current was largely pH insensitive; however, the steady-state current was potentiated at low pH with a pKa of 6.84. Single GABA-activated ion channels were recorded from outside-out patches of granule cell bodies. At pH 5.4-9.4, single GABA channels exhibited multiple conductance states occurring at 22-26, 16-17 and 12-14 pS. The conductance levels were not significantly altered over the time period of study, nor by changing the external H+ concentration. Two exponential functions were required to fit the open-time frequency histograms at both early (< 11 DIV) and late (> 11 DIV) development times at each H+ concentration. The short and long open time constants were unaffected either by the extracellular H+ concentration or by neuronal development. The distribution of all shut times was fitted by the sum of three exponentials designated as short, intermediate and long. At acidic pH, the long shut time constant decreased with development as did the relative contribution of these components to the overall distribution. This was concurrent with an increase in the mean probability of channel opening. In conclusion, this study demonstrates in cerebellar granule cells that external pH can either reduce, have no effect on, or enhance GABA-activated responses depending on the stage of development, possibly related to the subunit composition of the GABAA receptors. The mode of interaction of H+ at the single-channel level and implications of such interactions at cerebellar granule cell GABAA receptors are discussed.

Single-channel properties of neuronal GABAA receptors from mice lacking the 2 subunit

1. The aim of this study was to define the biophysical properties contributed by the gamma2 subunit to native single GABAA receptors. 2. Single-channel activity was recorded from neurones of wild-type (gamma2+/+) mice and compared with that from mice which were heterozygous (gamma2+/-) or homozygous (gamma2-/-) for a targeted disruption in the gamma2 subunit gene of the GABAA receptor. Unitary currents were evoked by low concentrations of GABA (0.5-5 microM) in membrane patches from acutely isolated dorsal root ganglion (DRG) neurones (postnatal day 0) and by 1 microM GABA in patches from embryonic hippocampal neurones which were cultured for up to 3 weeks. 3. GABAA receptors from DRG and hippocampal neurones of gamma2+/+ and gamma2+/- mice displayed predominantly a conductance state of 28 pS and less frequently 18 and 12 pS states. In gamma2-/- mice, conductance states mainly of 12 pS and less frequently of 24 pS were found. 4. The mean open duration of the 28 pS state in gamma2+/+ GABAA receptors (1.5-2.6 ms) was substantially longer than for the 12 pS state of gamma2-/- GABAA receptors (0.9-1.2 ms) at all GABA concentrations. For gamma2+/+ and gamma2-/- channels, the mean open duration was increased at higher GABA concentrations. 5. Open duration frequency distributions of 28 and 12 pS receptors revealed the existence of at least three exponential components. Components with short mean durations declined and components with long mean durations increased in relative frequency at higher GABA concentration indicating at least two binding sites of GABA per 28 and 12 pS receptor. 6. Shut time frequency distributions revealed at least four exponential components of which two were identified as intraburst components in 28 pS and one in 12 pS GABAA receptors. 7. The mean burst duration and the mean number of openings per burst increased in 28 and 12 pS GABAA receptors with increasing GABA concentration. At least two burst types were identified: simple bursts consisting of single openings and complex bursts of five to six openings in 28 pS but only two to three openings in 12 pS GABAA receptors. 8. We conclude that the gamma2 subunit enhances the efficacy of GABA by determining open conformations of high conductance and long lifetime, and by prolonging the time receptors remain in the activated bursting state.

Slow deactivation kinetics of NMDA receptors containing NR1 and NR2D subunits in rat cerebellar Purkinje cells

We have examined the deactivation kinetics of native N-methyl-D-aspartate receptors (NMDARs) containing NR1 and NR2D subunits by patch-clamp recording from Purkinje cells in cerebellar slices from young rats. NMDAR-mediated whole-cell currents were elicited in response to bath application of 20 microM NMDA and 50 microM glycine. The NMDAR-mediated currents were small, with an average whole-cell conductance of approximately 750 pS. Following the rapid application of brief pulses (1-10 ms) of 1 mM glutamate to outside-out membrane patches, we observed a low-conductance type of single-channel activity which lasted up to 30 s after the removal of agonist. Analysis of individual channel openings revealed asymmetry of transitions between the main- and subconductance states - a characteristic of NR1/NR2D-containing NMDARs. The averaged macroscopic current exhibited a decay time course which was well described by a single exponential function with a time constant of approximately 3 s. We conclude that native NR1/NR2D-containing NMDARs, like their recombinant counterparts, display very slow deactivation kinetics. This feature should provide a means for identification of these receptors at synapses, and indicates that they do not contribute to the synaptic NMDAR currents so far described.

Distinct synaptic and extrasynaptic NMDA receptors identified in dorsal horn neurones of the adult rat spinal cord

1. Using patch-clamp recordings, properties of single-channel and synaptic currents mediated by N-methyl-D-aspartate receptors (NMDARs) were examin ed in substantia gelatinosa (SG) neurones of adult rat spinal cord slices. 2. In somatic outside-out patches, high- and low-conductance NMDAR channels were present. The low-conductance channels exhibited asymmetrical transitions between the main (44 pS) and subconductance (19 pS) levels, suggesting that they arise from NR2D subunit-containing receptors. The high-conductance channels (main conductance, 57 pS) were blocked by ifenprodil, an NR2B subunit selective blocker. 3. Ifenprodil had no effect on NMDA-EPSCs. The double-exponential decay time course and the apparent Kd for Mg2+ of NMDA-EPSCs suggested the expression of NR2A subunit-containing receptors at the synapse. 4. These results indicate that different NMDAR subtypes are expressed in subsynaptic and extrasynaptic regions of adult SG neurones, which may have differential roles in nociception.

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.