Kinetic analysis of recombinant mammalian ?1 and ?1? glycine receptor channels

Characterization of the subunit composition and structure of adult human glycine receptors

The strychnine-sensitive pentameric glycine receptor (GlyR) mediates fast inhibitory neurotransmission in the mammalian nervous system. Only heteromeric GlyRs mediate synaptic transmission, as they contain the β subunit that permits clustering at the synapse through its interaction with scaffolding proteins. Here, we show that α2 and β subunits assemble with an unexpected 4:1 stoichiometry to produce GlyR with native electrophysiological properties. We determined structures in multiple functional states at 3.6-3.8 Å resolutions and show how 4:1 stoichiometry is consistent with the structural features of α2β GlyR. Furthermore, we show that one single β subunit in each GlyR gives rise to the characteristic electrophysiological properties of heteromeric GlyR, while more β subunits render GlyR non-conductive. A single β subunit ensures a univalent GlyR-scaffold linkage, which means the scaffold alone regulates the cluster properties.

A System for Assessing Dual Action Modulators of Glycine Transporters and Glycine Receptors

Reduced inhibitory glycinergic neurotransmission is implicated in a number of neurological conditions such as neuropathic pain, schizophrenia, epilepsy and hyperekplexia. Restoring glycinergic signalling may be an effective method of treating these pathologies. Glycine transporters (GlyTs) control synaptic and extra-synaptic glycine concentrations and slowing the reuptake of glycine using specific GlyT inhibitors will increase glycine extracellular concentrations and increase glycine receptor (GlyR) activation. Glycinergic neurotransmission can also be improved through positive allosteric modulation (PAM) of GlyRs. Despite efforts to manipulate this synapse, no therapeutics currently target it. We propose that dual action modulators of both GlyTs and GlyRs may show greater therapeutic potential than those targeting individual proteins. To show this, we have characterized a co-expression system in Xenopus laevis oocytes consisting of GlyT1 or GlyT2 co-expressed with GlyRα₁. We use two electrode voltage clamp recording techniques to measure the impact of GlyTs on GlyRs and the effects of modulators of these proteins. We show that increases in GlyT density in close proximity to GlyRs diminish receptor currents. Reductions in GlyR mediated currents are not observed when non-transportable GlyR agonists are applied or when Na⁺ is not available. GlyTs reduce glycine concentrations across different concentration ranges, corresponding with their ion-coupling stoichiometry, and full receptor currents can be restored when GlyTs are blocked with selective inhibitors. We show that partial inhibition of GlyT2 and modest GlyRα₁ potentiation using a dual action compound, is as useful in restoring GlyR currents as a full and potent single target GlyT2 inhibitor or single target GlyRα₁ PAM. The co-expression system developed in this study will provide a robust means for assessing the likely impact of GlyR PAMs and GlyT inhibitors on glycine neurotransmission.

Alfaxalone Causes Reduction of Glycinergic IPSCs, but Not Glutamatergic EPSCs, and Activates a Depolarizing Current in Rat Hypoglossal Motor Neurons

We investigated effects of the neuroactive steroid anesthetic alfaxalone on intrinsic excitability, and on inhibitory and excitatory synaptic transmission to hypoglossal motor neurons (HMNs). Whole cell recordings were made from HMNs in brainstem slices from 7 to 14-day-old Wistar rats. Spontaneous, miniature, and evoked inhibitory post-synaptic currents (IPSCs), and spontaneous and evoked excitatory PSCs (EPSCs) were recorded at –60 mV. Alfaxalone did not alter spontaneous glycinergic IPSC peak amplitude, rise-time or half-width up to 10 μM, but reduced IPSC frequency from 3 μM. Evoked IPSC amplitude was reduced from 30 nM. Evoked IPSC rise-time was prolonged and evoked IPSC decay time was increased only by 10 μM alfaxalone. Alfaxalone also decreased evoked IPSC paired pulse ratio (PPR). Spontaneous glutamatergic EPSC amplitude and frequency were not altered by alfaxalone, and evoked EPSC amplitude and PPR was also unchanged. Alfaxalone did not alter HMN repetitive firing or action potential amplitude. Baseline holding current at −60 mV with a CsCl-based pipette solution was increased in an inward direction; this effect was not seen when tetrodotoxin (TTX) was present. These results suggest that alfaxalone modulates glycine receptors (GlyRs), causing a delayed and prolonged channel opening, as well as causing presynaptic reduction of glycine release, and activates a membrane current, which remains to be identified. Alfaxalone selectively reduces glycinergic inhibitory transmission to rat HMNs via a combination of pre- and post-synaptic mechanisms. The net effect of these responses to alfaxalone is to increase HMN excitability and may therefore underlie neuro-motor excitation during neurosteroid anesthesia.

The role of tonic glycinergic conductance in cerebellar granule cell signalling and the effect of gain-of-function mutation

KEY POINTS

A T258F mutation of the glycine receptor increases the receptor affinity to endogenous agonists, modifies single-channel conductance and shapes response decay kinetics. Glycine receptors of cerebellar granule cells play their functional role not continuously, but when the granule cell layer starts receiving a high amount of excitatory inputs. Despite their relative scarcity, tonically active glycine receptors of cerebellar granule cells make a significant impact on action potential generation and inter-neuronal crosstalk, and modulate synaptic plasticity in neural networks; extracellular glycine increases probability of postsynaptic response occurrence acting at NMDA receptors and decreases this probability acting at glycine receptors. Tonic conductance through glycine receptors of cerebellar granule cells is a yet undiscovered element of the biphasic mechanism that regulates processing of sensory inputs in the cerebellum. A T258F point mutation disrupts this biphasic mechanism, thus illustrating the possible role of the gain-of-function mutations of the glycine receptor in development of neural pathologies.

ABSTRACT

Functional glycine receptors (GlyRs) have been repeatedly detected in cerebellar granule cells (CGCs), where they deliver exclusively tonic inhibitory signals. The functional role of this signalling, however, remains unclear. Apart from that, there is accumulating evidence of the important role of GlyRs in cerebellar structures in development of neural pathologies such as hyperekplexia, which can be triggered by GlyR gain-of-function mutations. In this research we initially tested functional properties of GlyRs, carrying the yet understudied T258F gain-of-function mutation, and found that this mutation makes significant modifications in GlyR response to endogenous agonists. Next, we clarified the role of tonic GlyR conductance in neuronal signalling generated by single CGCs and by neural networks in cell cultures and in living cerebellar tissue of C57Bl-6J mice. We found that GlyRs of CGCs deliver a significant amount of tonic inhibition not continuously, but when the cerebellar granule layer starts receiving substantial excitatory input. Under these conditions tonically active GlyRs become a part of neural signalling machinery allowing generation of action potential (AP) bursts of limited length in response to sensory-evoked signals. GlyRs of CGCs support a biphasic modulatory mechanism which enhances AP firing when excitatory input intensity is low, but suppresses it when excitatory input rises to a certain critical level. This enables one of the key functions of the CGC layer: formation of sensory representations and their translation into motor output. Finally, we have demonstrated that the T258F mutation in CGC GlyRs modifies single-cell and neural network signalling, and breaks a biphasic modulation of the AP-generating machinery.

Expression of glycine receptor alpha 3 in the rat trigeminal neurons and central boutons in the brainstem

Increasing evidence shows that the homomeric glycine receptor is expressed in axon terminals and is involved in the presynaptic modulation of transmitter release. However, little is known about the expression of the glycine receptor, implicated in the presynaptic modulation of sensory transmission in the primary somatosensory neurons and their central boutons. To address this, we investigated the expression of glycine receptor subunit alpha 3 (GlyRα3) in the neurons in the trigeminal ganglion and axon terminals in the 1st relay nucleus of the brainstem by light- and electron-microscopic immunohistochemistry. Trigeminal primary sensory neurons were GlyRα3-immunopositive/gephyrin-immunonegative (indicating homomeric GlyR), whereas GlyRα3/gephyrin immunoreactivity (indicating heteromeric GlyR) was observed in dendrites. GlyRα3 immunoreactivity was also found in the central boutons of primary afferents but far from the presynaptic site and in dendrites at subsynaptic sites. Boutons expressing GlyRα3 contained small round vesicles, formed asymmetric synapses with dendrites and were immunoreactive for glutamate. These findings suggest that trigeminal primary afferent boutons receive presynaptic modulation via homomeric, extrasynaptic GlyRα3, and that different subtypes of GlyR may be involved in pre- and postsynaptic inhibition.

Correlating structural and energetic changes in glycine receptor activation

Pentameric ligand-gated ion channels (pLGICs) mediate fast chemoelectrical transduction in the nervous system. The mechanism by which the energy of ligand binding leads to current-conducting receptors is poorly understood and may vary among family members. We addressed these questions by correlating the structural and energetic mechanisms by which a naturally occurring M1 domain mutation (α1(Q-26'E)) enhances receptor activation in homo- and heteromeric glycine receptors. We systematically altered the charge of spatially clustered residues at positions 19' and 24', in the M2 and M2-M3 linker domains, respectively, which are known to be critical to efficient receptor activation, on a background of α1(Q-26'E). Changes in the durations of single receptor activations (clusters) and conductance were used to determine interaction coupling energies, which we correlated with conformational displacements as measured in pLGIC crystal structures. Presence of the α1(Q-26'E) enhanced cluster durations and reduced channel conductance in homo- and heteromeric receptors. Strong coupling between α1(-26') and α1(19') across the subunit interface suggests an important role in receptor activation. A lack of coupling between α1(-26') and α1(24') implies that 24' mutations disrupt activation via other interactions. A similar lack of energetic coupling between α1(-26') and reciprocal mutations in the β subunit suggests that this subunit remains relatively static during receptor activation. However, the channel effects of α1(Q-26'E) on α1β receptors suggests at least one α1-α1 interface per pentamer. The coupling-energy change between α1(-26') and α1(19') correlates with a local structural rearrangement essential for pLGIC activation, implying it comprises a key energetic pathway in activating glycine receptors and other pLGICs.

The GLRA1 missense mutation W170S associates lack of Zn2+ potentiation with human hyperekplexia

Hyperekplexia is a neurological disorder associated primarily with mutations in the α1 subunit of glycine receptors (GlyRs) that lead to dysfunction of glycinergic inhibitory transmission. To date, most of the identified mutations result in disruption of surface expression or altered channel properties of α1-containing GlyRs. Little evidence has emerged to support an involvement of allosteric GlyR modulation in human hyperekplexia. Here, we report that recombinant human GlyRs containing α1 or α1β subunits with a missense mutation in the α1 subunit (W170S), previously identified from familial hyperekplexia, caused remarkably reduced potentiation and enhanced inhibition by Zn(2+). Interestingly, mutant α1(W170S)β GlyRs displayed no significant changes in potency or maximum response to glycine, taurine, or β-alanine. By temporally separating the potentiating and the inhibitory effects of Zn(2+), we found that the enhancement of Zn(2+) inhibition resulted from a loss of Zn(2+)-mediated potentiation. The W170S mutation on the background of H107N, which was previously reported to selectively disrupt Zn(2+) inhibition, showed remarkable attenuation of Zn(2+)-mediated potentiation and thus indicated that W170 is an important residue for the Zn(2+)-mediated GlyR potentiation. Moreover, overexpressing the α1(W170S) subunit in cultured rat neurons confirmed the results from heterologous expression. Together, our results reveal a new zinc potentiation site on α1 GlyRs and a strong link between Zn(2+) modulation and human disease.

An outline of desensitization in pentameric ligand-gated ion channel receptors

Pentameric ligand-gated ion channel (pLGIC) receptors exhibit desensitization, the progressive reduction in ionic flux in the prolonged presence of agonist. Despite its pathophysiological importance and the fact that it was first described over half a century ago, surprisingly little is known about the structural basis of desensitization in this receptor family. Here, we explain how desensitization is defined using functional criteria. We then review recent progress into reconciling the structural and functional basis of this phenomenon. The extracellular-transmembrane domain interface is a key locus. Activation is well known to involve conformational changes at this interface, and several lines of evidence suggest that desensitization involves a distinct conformational change here that is incompatible with activation. However, major questions remain unresolved, including the structural basis of the desensitization-induced agonist affinity increase and the mechanism of pore closure during desensitization.

Differential distribution of glycine receptor subtypes at the rat calyx of Held synapse

The properties of glycine receptors (GlyRs) depend upon their subunit composition. While the prevalent adult forms of GlyRs are heteromers, previous reports suggested functional α homomeric receptors in mature nervous tissues. Here we show two functionally different GlyRs populations in the rat medial nucleus of trapezoid body (MNTB). Postsynaptic receptors formed α1/β-containing clusters on somatodendritic domains of MNTB principal neurons, colocalizing with glycinergic nerve endings to mediate fast, phasic IPSCs. In contrast, presynaptic receptors on glutamatergic calyx of Held terminals were composed of dispersed, homomeric α1 receptors. Interestingly, the parent cell bodies of the calyces of Held, the globular bushy cells of the cochlear nucleus, expressed somatodendritic receptors (α1/β heteromers) and showed similar clustering and pharmacological profile as GlyRs on MNTB principal cells. These results suggest that specific targeting of GlyR β-subunit produces segregation of GlyR subtypes involved in two different mechanisms of modulation of synaptic strength.

4-Chloropropofol enhances chloride currents in human hyperekplexic and artificial mutated glycine receptors

BACKGROUND

The mammalian neurological disorder hereditary hyperekplexia can be attributed to various mutations of strychnine sensitive glycine receptors. The clinical symptoms of "startle disease" predominantly occur in the newborn leading to convulsive hypertonia and an exaggerated startle response to unexpected mild stimuli. Amongst others, point mutations R271Q and R271L in the α1-subunit of strychnine sensitive glycine receptors show reduced glycine sensitivity and cause the clinical symptoms of hyperekplexia.Halogenation has been shown to be a crucial structural determinant for the potency of a phenolic compound to positively modulate glycine receptor function.The aim of this in vitro study was to characterize the effects of 4-chloropropofol (4-chloro-2,6-dimethylphenol) at four glycine receptor mutations.

METHODS

Glycine receptor subunits were expressed in HEK 293 cells and experiments were performed using the whole-cell patch-clamp technique.

RESULTS

4-chloropropofol exerted a positive allosteric modulatory effect in a low sub-nanomolar concentration range at the wild type receptor (EC50 value of 0.08 ± 0.02 nM) and in a micromolar concentration range at the mutations (1.3 ± 0.6 μM, 0.1 ± 0.2 μM, 6.0 ± 2.3 μM and 55 ± 28 μM for R271Q, L, K and S267I, respectively).

CONCLUSIONS

4-chloropropofol might be an effective compound for the activation of mutated glycine receptors in experimental models of startle disease.

HSV delivery of a ligand-regulated endogenous ion channel gene to sensory neurons results in pain control following channel activation

Persistent pain remains a tremendous health problem due to both its prevalence and dearth of effective therapeutic interventions. To maximize pain relief while minimizing side effects, current gene therapy-based approaches have mostly exploited the expression of pain inhibitory products or interfered with pronociceptive ion channels. These methods do not enable control over the timing or duration of analgesia, nor titration to analgesic efficacy. Here, we describe a gene therapy strategy that potentially overcomes these limitations by providing exquisite control over therapy with efficacy in clinically relevant models of inflammatory pain. We utilize a herpes simplex viral (HSV) vector (vHGlyRα1) to express a ligand-regulated chloride ion channel, the glycine receptor (GlyR) in targeted sensory afferents; the subsequent exogenous addition of glycine provides the means for temporal and spatial control of afferent activity, and therefore pain. Use of an endogenous inhibitory receptor not normally present on sensory neurons both minimizes immunogenicity and maximizes therapeutic selectivity.

Properties of glycine receptors underlying synaptic currents in presynaptic axon terminals of rod bipolar cells in the rat retina

The excitability of presynaptic terminals can be controlled by synaptic input that directly targets the terminals. Retinal rod bipolar axon terminals receive presynaptic input from different types of amacrine cells, some of which are glycinergic. Here, we have performed patch-clamp recordings from rod bipolar axon terminals in rat retinal slices. We used whole-cell recordings to study glycinergic inhibitory postsynaptic currents (IPSCs) under conditions of adequate local voltage clamp and outside-out patch recordings to study biophysical and pharmacological properties of the glycine receptors with ultrafast application. Glycinergic IPSCs, recorded in both intact cells and isolated terminals, were strychnine sensitive and displayed fast kinetics with a double-exponential decay. Ultrafast application of brief (approximately 1 ms) pulses of glycine (3 mM) to patches evoked responses with fast, double-exponential deactivation kinetics, no evidence of desensitization in double-pulse experiments, relatively low apparent affinity (EC(50) approximately 100 microM), and high maximum open probability (0.9). Longer pulses evoked slow, double-exponential desensitization and double-pulse experiments indicated slow, double-exponential recovery from desensitization. Non-stationary noise analysis of IPSCs and patch responses yielded single-channel conductances of approximately 41 pS and approximately 64 pS, respectively. Directly observed single-channel gating occurred at approximately 40-50 pS and approximately 80-90 pS in both types of responses, suggesting a mixture of heteromeric and homomeric receptors. Synaptic release of glycine leads to transient receptor activation, with about eight receptors available to bind transmitter after release of a single vesicle. With a low intracellular chloride concentration, this leads to either hyperpolarizing or shunting inhibition that will counteract passive and regenerative depolarization and depolarization-evoked transmitter release.

Slow glycinergic transmission mediated by transmitter pooling

Most fast-acting neurotransmitters are rapidly cleared from synaptic regions. This feature isolates synaptic sites, rendering the timecourse of synaptic responses independent of the number of active synapses. We describe a striking exception at glycinergic synapses on granule cells of the rat dorsal cochlear nucleus. The duration of IPSCs was dependent on the number of presynaptic axons that were stimulated and on the number of vesicles released from each axon. Increasing stimulus number or frequency, or blocking glycine uptake, slowed synaptic decays, while a low-affinity competitive antagonist of GlyRs accelerated IPSC decay. These effects could be explained by unique features of GlyRs when activated by pooling of glycine across synapses. Functionally, increasing the number of IPSPs markedly lengthened the period of spike inhibition following cessation of presynaptic stimulation. Thus, temporal properties of inhibition can be controlled by activity levels in multiple presynaptic cells or by adjusting release probability at individual synapses.

High intracellular chloride slows the decay of glycinergic currents

The time course of currents mediated by native and recombinant glycine receptors was examined with a combination of rapid agonist applications to outside-out patches and single-channel recording. The deactivation time constant of currents evoked by brief, saturating pulses of glycine is profoundly affected by the chloride concentration on the intracellular side of the cell membrane. Deactivation was threefold slower when intracellular chloride was increased from a low level (10 mm), similar to that observed in living mature neurons, to 131 mm ("symmetrical" chloride, often used in pipette internal solutions). Single-channel analysis revealed that high chloride has its greatest effect on the channel closing rate, slowing it by a factor of 2 compared with the value we estimated in the cell-attached mode (in which the channels are at physiological intracellular chloride concentrations). The same effect of chloride was observed when glycinergic evoked synaptic currents were recorded from juvenile rat spinal motoneurons in vitro, because the decay time constant was reduced from approximately 7 ms to approximately 3 ms when cells were dialyzed with 10 mm chloride intracellular recording solution. Our results indicate that the time course of glycinergic synaptic inhibition in intact neurons is much faster than is estimated by measurements in symmetrical chloride and can be modulated by changes in intracellular chloride concentration in the range that can occur in physiological or pathological conditions.

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.

Glycinergic transmission shaped by the corelease of GABA in a mammalian auditory synapse

The firing pattern of neurons is shaped by the convergence of excitation and inhibition, each with finely tuned magnitude and duration. In an auditory brainstem nucleus, glycinergic inhibition features fast decay kinetics, the mechanism of which is unknown. By applying glycine to native or recombinant glycine receptors, we show that response decay times are accelerated by addition of GABA, a weak partial agonist of glycine receptors. Systematic variation in agonist exposure time revealed that fast synaptic time course may be achieved with submillisecond exposures to mixtures of glycine and GABA at physiological concentrations. Accordingly, presynaptic terminals generally contained both transmitters, and depleting terminals of GABA slowed glycinergic synaptic currents. Thus, coreleased GABA accelerates glycinergic transmission by acting directly on glycine receptors, narrowing the time window for effective inhibition. Packaging both weak and strong agonists in vesicles may be a general means by which presynaptic neurons regulate the duration of postsynaptic responses.

The effect of zinc on glycinergic inhibitory postsynaptic currents in rat spinal dorsal horn neurons

The effect of zinc on glycinergic spontaneous inhibitory postsynaptic currents (IPSCs) was investigated using the whole-cell patch-clamp technique in mechanically dissociated rat spinal dorsal horn neurons. Zinc at a concentration of 10 microM reversibly increased the spontaneous IPSC frequency without changing the current amplitudes, suggesting that zinc increases spontaneous glycine release from presynaptic nerve terminals. At a low concentration of 1 microM, on the other hand, zinc potentiated the amplitude of spontaneous IPSCs but had no effect on the frequency. At a high concentration of 100 microM, zinc increased the spontaneous IPSC frequency while it inhibited the IPSC amplitude. The current evoked by exogenously applied glycine was potentiated and inhibited by low and high concentrations of zinc, respectively. The increase in spontaneous IPSC frequency by 10 microM zinc was inhibited by blocking the voltage-dependent Ca(2+) channels in the presence of both omega-conotoxin-MVIIC and nifedipine. The facilitatory effect of zinc on spontaneous IPSC frequency was also inhibited in the presence of tetrodotoxin. In the slice preparation, 30 microM zinc potentiated the evoked IPSC amplitude and decreased the paired pulse ratio. These results suggest that, in addition to an action on the postsynaptic glycine receptors, zinc may depolarize the presynaptic nerve terminals, leading to an activation of voltage-dependent Na(+) and Ca(2+) channels that in turn increases glycine release. Since dorsal horn neurons receive nociceptive inputs, zinc may play an important role in the regulation of sensory transmission.

Identification of residues mediating inhibition of glycine receptors by protons

We previously identified H109 of the glycine alpha1 subunit as a putative proton binding site. In the present studies, we explored additional proton binding site(s) as well as the mechanism underlying modulation of glycine receptors by protons. Whole-cell glycine currents were recorded from HEK 293 cells transiently expressing wild type or mutant glycine receptors. Individual mutation of 3 of 4 remaining extracellular histidine residue into alanine (i.e., alpha1 H107A, H215A or H419A), reduced the receptor sensitivity to protons to a varying extent. In contrast, mutation of alpha1 H201A did not affect proton sensitivity. Double, triple or quadruple histidine mutation of these residues caused a further reduction of proton sensitivity, suggesting multiple binding sites for proton action on glycine receptors. Furthermore, the substitution T133A, which mediates Zn(2+) inhibition, virtually abolished the proton effect on peak amplitude and current kinetics of glycine response. Replacement of T with S on position 133 partially restored receptor sensitivity to protons, suggesting the hydroxyl group of residue T133 is essential for proton-mediated modulation. In heteromeric alpha1beta receptors, mutations beta H132A and S156A, which correspond to H109 and T133 of the alpha1 subunit, respectively, also affected proton inhibition. In conclusion, multiple extracellular histidine residues (H107, H109, H215 and H419) and threonine residues of the alpha1 and beta Zn(2+) coordination sites are critical for modulation of the glycine receptor by protons.

Frequency-dependent modulation of glycine receptor activation recorded from the zebrafish larvae hindbrain

In vertebrates, most glycinergic inhibitory neurons discharge phasically at a relatively low frequency. Such a pattern of glycine liberation from presynaptic terminals may affect the kinetics of post-synaptic glycine receptors. To examine this influence, we have analyzed the behavior of glycine receptors in response to repetitive stimulation at frequencies at which consecutive outside-out currents did not superimpose (0.5-4 Hz). Neurotransmitter release was mimicked on outside-out patches from zebrafish hindbrain Mauthner cells using fast flow application techniques. The amplitude of outside-out currents evoked by short (1 ms) repetitive applications of a saturating concentration (3 mM) of glycine remained unchanged for application frequencies<or=1 Hz. When the application frequency was increased from 1 to 4 Hz, the amplitude of the outside-out currents decreased with time to reach a steady state level. This decrease in current amplitude was larger and occurred faster with increasing application frequencies. Recovery occurred when the stimulation frequency was decreased back to 1 Hz. The recovery time constant was independent on the application frequency. This frequency-dependent inhibition was also observed for non-saturating glycine concentrations. Our results indicate that glycine receptor activity is down-regulated when the stimulation frequency increases to values>1 Hz. Glycine-evoked current simulations using a simple Markov model describing zebrafish glycine receptor kinetic behavior, indicates that this down-regulation of glycine receptor efficacy is due to a progressive accumulation of the receptors in a long lasting desensitization state. Our simulations suggest that this down-regulation can occur even when spontaneous inhibitory currents were generated randomly at a frequency>1 Hz.

Functional recovery of glycine receptors in spastic murine model of startle disease

Clinical variability is common in inherited gene defects of the central nervous system in humans and in animal models of human disorders. Here, we used the homozygous spastic (spa) mutant mice, which resemble human hereditary hyperekplexia, to determine the molecular remodeling of the spinal cord through the course of the disease, and develop a model for clinical disparity between littermates. The spa mutation is an insertion of a LINE-1 element in the gene for the beta subunit of the glycine receptor, Glrb. The insertion causes aberrant splicing in the beta subunit of glycine receptor gene with a consequent important reduction of glycine receptors. At young ages, all homozygous spa animals were spastic, showed loss of glycine receptors, increased expression of vesicular glycine/GABA transporter and NMDA receptors, induction of activated caspase3, and preferential loss of glycinergic interneurons consistent with neurotransmitter toxicity model. Those littermates that recovered from symptoms showed strong over-expression of the glycine receptor alpha 1 subunit (Glra1), and increased myelination and synaptic plasticity. Littermates that showed a deteriorating clinical course failed to over-express Glra1, and also showed relative loss of gephyrin (receptor clustering). These molecular changes were associated with a preferential loss of GABAergic interneurons, and extensive motorneuron loss. These data suggest that functional recovery is likely due to expression of homomeric glycine receptors, rescue from excitotoxicity, and subsequent neuronal remodeling. We propose that human patients with hyperekplexia show remodeling similar to that of the recovering spa mice, as human patients also show a lessening of symptoms as a function of age.

Molecular analysis of the A322D mutation in the GABAAreceptor α1-subunit causing juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME) belongs to the most common forms of hereditary epilepsy, the idiopathic generalized epilepsies. Although the mode of inheritance is usually complex, mutations in single genes have been shown to cause the disease in some families with autosomal dominant inheritance. The first mutation in a multigeneration JME family has been recently found in the alpha1-subunit of the GABAA receptor (GABRA1), predicting the single amino acid substitution A322D. We further characterized the functional consequences of this mutation by coexpressing alpha1-, beta2- and gamma2-subunits in human embryonic kidney (HEK293) cells. By using an ultrafast application system, mutant receptors have shown reduced macroscopic current amplitudes at saturating GABA concentrations and a highly reduced affinity to GABA compared to the wild-type (WT). Dose-response curves for current amplitudes, activation kinetics, and GABA-dependent desensitization parameters showed a parallel shift towards 30- to 40-fold higher GABA concentrations. Both deactivation and resensitization kinetics were considerably accelerated in mutant channels. In addition, mutant receptors labelled with enhanced green fluorescent protein (EGFP) were not integrated in the cell membrane, in contrast to WT receptors. Therefore, the A322D mutation leads to a severe loss-of-function of the human GABAA receptor by several mechanisms, including reduced surface expression, reduced GABA-sensitivity, and accelerated deactivation. These molecular defects could decrease and shorten the resulting inhibitory postsynaptic currents (IPSCs) in vivo, which can induce a hyperexcitability of the postsynaptic membrane and explain the occurrence of epileptic seizures.

Interaction of topiramate with glycine receptor channels

Glycine receptor channels are pentameric ligand-gated ion channels that respond to the application of inhibitory neurotransmitters by opening of a chloride-selective central pore. Topiramate (TPM) is a broad-spectrum antiepileptic drug used as add-on or monotherapy for focal seizures. In the present study the interaction of TPM with glycine receptor channels was studied on outside-out patches from HEK293 cells expressing alpha1beta glycine receptor channels. The patch clamp techniques combined with ultra fast solution exchange enabled us to investigate the kinetics of receptor channels in presence of TPM. Our study showed no agonistic or potentiating effect for TPM on glycine receptor channels. However, in presence of 1 mM glycine + 1 mM TPM, the desensitization got faster and the peak current amplitude decreased. After the end of glycine + TPM pulses, off-currents occurred, suggestive for a specific channel block mechanism.

Two different modes of action of pentobarbital at glycine receptor channels

Glycine receptor channels are pentameric ligand-gated ion channels which respond to the binding of inhibitory transmitters by opening of a chloride-selective central pore. Pentobarbital is widely used as an anticonvulsive, hypnotic and anaesthetic drug. In the present study, the interaction between pentobarbital and glycine receptor channels was studied on outside-out patches of human embryonic kidney (HEK) 293 cells expressing alpha(1)beta glycine receptor channels. Currents elicited by 0.03 mM glycine were enhanced by pentobarbital showing potentiation of alpha(1)beta glycine receptor channels. In the presence of 1 mM glycine+pentobarbital (1 and 3 mM), desensitization was faster and the peak current amplitude decreased. After the end of glycine+pentobarbital pulses, off-currents occurred suggestive for a channel block mechanism. Pentobarbital had no agonistic effects at glycine receptor channels.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

2,6 di-tert-butylphenol, a nonanesthetic propofol analog, modulates alpha1beta glycine receptor function in a manner distinct from propofol

The anesthetic propofol (2,6 diisopropylphenol) mediates some of its effects by activating inhibitory chloride currents in the lower brainstem and spinal cord. The effects comprise direct activation of gamma-aminobutyric acid-A and glycine receptors in the absence of the natural agonist, as well as potentiation of the effect of submaximal agonist concentrations. Replacement of propofol's isopropyl groups by di-tert-butyl groups yields a compound without in vivo anesthetic effects. We have studied the effects of propofol and 2,6 di-tert-butylphenol on chloride inward currents via rat alpha1beta glycine receptors heterologously expressed in human embryonic kidney cells. Propofol, but not 2,6 di-tert-butylphenol, directly activated glycine receptors; half-maximal current activation was observed with propofol 114 +/- 27 microM. Both compounds potentiated the effect of a submaximal glycine concentration (10 microM) to a maximum value of 136% +/- 71% (propofol) and 279% +/- 109% (2,6 di-tert-butylphenol) of the response to glycine 10 microM. The 50% effective concentration for this effect was 12.5 +/- 6.4 microM and 9.4 +/- 10.2 microM for propofol and 2,6 di-tert-butylphenol, respectively. Propofol and its nonanesthetic structural analog do not differ in their ability to coactivate the glycine receptor but differ in their ability to directly activate the receptor in the absence of the natural agonist.