Cloning of a glycine receptor subtype expressed in rat brain and spinal cord during a specific period of neuronal development

Comprehensive analysis of cellular specializations that initiate parallel auditory processing pathways in mice

The cochlear nuclear complex (CN) is the starting point for all central auditory processing and comprises a suite of neuronal cell types that are highly specialized for neural coding of acoustic signals. To examine how their striking functional specializations are determined at the molecular level, we performed single-nucleus RNA sequencing of the mouse CN to molecularly define all constituent cell types and related them to morphologically- and electrophysiologically-defined neurons using Patch-seq. We reveal an expanded set of molecular cell types encompassing all previously described major types and discover new subtypes both in terms of topographic and cell-physiologic properties. Our results define a complete cell-type taxonomy in CN that reconciles anatomical position, morphological, physiological, and molecular criteria. This high-resolution account of cellular heterogeneity and specializations from the molecular to the circuit level illustrates molecular underpinnings of functional specializations and enables genetic dissection of auditory processing and hearing disorders with unprecedented specificity.

Structure and Mechanism of Glycine Receptor Elucidated by Cryo-Electron Microscopy

Glycine receptors (GlyRs) are pentameric ion channels that mediate fast inhibitory neurotransmission. GlyRs are found in the central nervous system including the spinal cord, brain stem, and cerebellum, as well as in the retina, sperm, macrophages, hippocampus, cochlea, and liver. Due to their crucial roles in counter-balancing excitatory signals and pain signal transmission, GlyR dysfunction can lead to severe diseases, and as a result, compounds that modify GlyR activity may have tremendous therapeutic potential. Despite this potential, the development of GlyR-specific small-molecule ligands is lacking. Over the past few years, high-resolution structures of both homomeric and heteromeric GlyRs structures in various conformations have provided unprecedented details defining the pharmacology of ligand binding, subunit composition, and mechanisms of channel gating. These high-quality structures will undoubtedly help with the development of GlyR-targeted therapies.

Identification of the hypertension drug niflumic acid as a glycine receptor inhibitor

Glycine is one of the major neurotransmitters in the brainstem and the spinal cord. Glycine binds to and activates glycine receptors (GlyRs), increasing Cl^- conductance at postsynaptic sites. This glycinergic synaptic transmission contributes to the generation of respiratory rhythm and motor patterns. Strychnine inhibits GlyR by binding to glycine-binding site, while picrotoxin blocks GlyR by binding to the channel pore. We have previously reported that bath application of strychnine to zebrafish embryos causes bilateral muscle contractions in response to tactile stimulation. To explore the drug-mediated inhibition of GlyRs, we screened a chemical library of ~ 1,000 approved drugs and pharmacologically active molecules by observing touch-evoked response of zebrafish embryos in the presence of drugs. We found that exposure of zebrafish embryos to nifedipine (an inhibitor of voltage-gated calcium channel) or niflumic acid (an inhibitor of cyclooxygenase 2) caused bilateral muscle contractions just like strychnine-treated embryos showed. We then assayed strychnine, picrotoxin, nifedipine, and niflumic acid for concentration-dependent inhibition of glycine-mediated currents of GlyRs in oocytes and calculated IC₅₀s. The results indicate that all of them concentration-dependently inhibit GlyR in the order of strychnine > picrotoxin > nifedipine > niflumic acid.

The Glycine Receptor-A Functionally Important Primary Brain Target of Ethanol

Identification of ethanol's (EtOH) primary molecular brain targets and determination of their functional role is an ongoing, important quest. Pentameric ligand-gated ion channels, that is, the nicotinic acetylcholine receptor, the γ-aminobutyric acid type A receptor, the 5-hydroxytryptamine₃ , and the glycine receptor (GlyR), are such targets. Here, aspects of the structure and function of these receptors and EtOH's interaction with them are briefly reviewed, with special emphasis on the GlyR and the importance of this receptor and its ligands for EtOH pharmacology. It is suggested that GlyRs are involved in (i) the dopamine-activating effect of EtOH, (ii) regulating EtOH intake, and (iii) the relapse preventing effect of acamprosate. Exploration of the GlyR subtypes involved and efforts to develop subtype specific agonists or antagonists may offer new pharmacotherapies for alcohol use disorders.

Developmental Shift of Inhibitory Transmitter Content at a Central Auditory Synapse

Synaptic inhibition in the CNS is mostly mediated by GABA or glycine. Generally, the use of the two transmitters is spatially segregated, but there are central synapses employing both, which allows for spatial and temporal variability of inhibitory mechanisms. Spherical bushy cells (SBCs) in the mammalian cochlear nucleus receive primary excitatory inputs through auditory nerve fibers arising from the organ of Corti and non-primary inhibition mediated by a dual glycine-GABA transmission. Slow kinetics IPSCs enable activity dependent tonic-like conductance build up, functioning as a gain control by filtering out small or temporally imprecise EPSPs. However, it remained elusive whether GABA and glycine are released as content of the same vesicle or from distinct presynaptic terminals. The developmental profile of quantal release was investigated with whole cell recordings of miniature inhibitory postsynaptic currents (mIPSCs) from P1–P25 SBCs of Mongolian gerbils. GABA is the initial transmitter eliciting slow-rising and -decaying events of relatively small amplitudes, occurring only during early postnatal life. Around and after hearing onset, the inhibitory quanta are predominantly containing glycine that—with maturity—triggers progressively larger and longer mIPSC. In addition, GABA corelease with glycine evokes mIPSCs of particularly large amplitudes consistently occurring across all ages, but with low probability. Together, these results suggest that GABA, as the primary transmitter released from immature inhibitory terminals, initially plays a developmental role. In maturity, GABA is contained in synaptic vesicles only in addition to glycine to increase the inhibitory potency, thereby fulfilling solely a modulatory function.

Review : Glycine Receptors: Molecular Heterogeneity and Implications for Disease

The inhibitory glycine receptor is a ligand-gated chloride channel that exists in developmentally regulated isoforms. These oligomeric transmembrane proteins are composed of variants of the ligand binding α subunit and structural β polypeptides. The agonist and antagonist sites of the α subunits are formed by discontinuous sequence motifs. In the murine genome, the genes encoding the α1 ( Glra1), α3 ( Glra3), and β ( Glyrb) subunit are autosomally located, whereas the α2 ( Glra2) and α4 ( Glra4) genes reside on the X-chromosome. Mutations of glycine receptor genes have been found to underly hypertonic motor disorders in mice and humans. The mouse mutants spasmodic (spd) and oscillator ( spdot) carry recessive mutations of the Glra 1 gene. In the phenotypically similar mouse mutant spastic ( spa), the intronic insertion of a LINE-1 transposable element into the Gyrb gene results in the aberrant splicing and a consecutive loss of glycine receptors. The human neurological disorder hyperekplexia (startle disease, stiff baby syndrome) is caused by point mutations within the α1 subunit gene ( GLRA1) localized in the human chromosomal region 5q31.3. The Neuroscientist 1:130- 141,1995

Effects of propofol on glycinergic neurotransmission in a single spinal nerve synapse preparation

The effects of the intravenous anesthetic, propofol, on glycinergic transmission and on glycine receptor-mediated whole-cell currents (IGly) were examined in the substantia gelatinosa (SG) neuronal cell body, mechanically dissociated from the rat spinal cord. This "synaptic bouton" preparation, which retains functional native nerve endings, allowed us to evaluate glycinergic inhibitory postsynaptic currents (IPSCs) and whole-cell currents in a preparation in which experimental solution could rapidly access synaptic terminals. Synaptic IPSCs were measured as spontaneous (s) and evoked (e) IPSCs. The eIPSCs were elicited by applying paired-pulse focal electrical stimulation, while IGly was evoked by a bath application of glycine. A concentration-dependent enhancement of IGly was observed for ≥10µM propofol. Propofol (≥3µM) significantly increased the frequency of sIPSCs and prolonged the decay time without altering the current amplitude. However, propofol (≥3µM) also significantly increased the mean amplitude of eIPSCs and decreased the failure rate (Rf). A decrease in the paired-pulse ratio (PPR) was noted at higher concentrations (≥10µM). The decay time of eIPSCs was prolonged only at the maximum concentration tested (30µM). Propofol thus acts at both presynaptic glycine release machinery and postsynaptic glycine receptors. At clinically relevant concentrations (<1μM) there was no effect on IGly, sIPSCs or eIPSCs suggesting that at anesthetic doses propofol does not affect inhibitory glycinergic synapses in the spinal cord.

Development of glycinergic innervation to the murine LSO and SPN in the presence and absence of the MNTB

Neurons in the superior olivary complex (SOC) integrate excitatory and inhibitory inputs to localize sounds in space. The majority of these inhibitory inputs have been thought to arise within the SOC from the medial nucleus of the trapezoid body (MNTB). However, recent work demonstrates that glycinergic innervation of the SOC persists in Egr2; En1^CKO mice that lack MNTB neurons, suggesting that there are other sources of this innervation (Jalabi et al., 2013). To study the development of MNTB- and non-MNTB-derived glycinergic SOC innervation, we compared immunostaining patterns of glycine transporter 2 (GlyT2) at several postnatal ages in control and Egr2; En1^CKO mice. GlyT2 immunostaining was present at birth (P0) in controls and reached adult levels by P7 in the superior paraolivary nucleus (SPN) and by P12 in the lateral superior olive (LSO). In Egr2; En1^CKO mice, glycinergic innervation of the LSO developed at a similar rate but was delayed by one week in the SPN. Conversely, consistent reductions in the number of GlyT2⁺ boutons located on LSO somata were seen at all ages in Egr2; En1^CKO mice, while these numbers reached control levels in the SPN by adulthood. Dendritic localization of GlyT2+ boutons was unaltered in both the LSO and SPN of adult Egr2; En1^CKO mice. On the postsynaptic side, adult Egr2; En1^CKO mice had reduced glycine receptor α1 (GlyRα1) expression in the LSO but normal levels in the SPN. GlyRα2 was not expressed by LSO or SPN neurons in either genotype. These findings contribute important information for understanding the development of MNTB- and non-MNTB-derived glycinergic pathways to the mouse SOC.

Glycine receptors in nervous tissue and their functional role

The literature data on glycine metabolism in neural tissue, mitochondrial Gly-cleaving system, Gly-catching system in neural and glial cells are summarized. The peculiarities of localization and distribution of specific glycine receptors and binding-sites in nervous tissue of mammals are described. Four types of glycine-binding receptors are described: own specific glycine receptor (Gly-R), ionotropic receptor, which binds N-methyl-D-aspartate selectively (NMDA-R), and ionotropic receptors of g-aminobutyrate (GABA A -R, GABA С -R). The feutures of glycine effects in neuroglial cultures are discussed

Distinct phenotypes in zebrafish models of human startle disease

Startle disease is an inherited neurological disorder that causes affected individuals to suffer noise- or touch-induced non-epileptic seizures, excessive muscle stiffness and neonatal apnea episodes. Mutations known to cause startle disease have been identified in glycine receptor subunit (GLRA1 and GLRB) and glycine transporter (SLC6A5) genes, which serve essential functions at glycinergic synapses. Despite the significant successes in identifying startle disease mutations, many idiopathic cases remain unresolved. Exome sequencing in these individuals will identify new candidate genes. To validate these candidate disease genes, zebrafish is an ideal choice due to rapid knockdown strategies, accessible embryonic stages, and stereotyped behaviors. The only existing zebrafish model of startle disease, bandoneon (beo), harbors point mutations in glrbb (one of two zebrafish orthologs of human GLRB) that cause compromised glycinergic transmission and touch-induced bilateral muscle contractions. In order to further develop zebrafish as a model for startle disease, we sought to identify common phenotypic outcomes of knocking down zebrafish orthologs of two known startle disease genes, GLRA1 and GLRB, using splice site-targeted morpholinos. Although both morphants were expected to result in phenotypes similar to the zebrafish beo mutant, our direct comparison demonstrated that while both glra1 and glrbb morphants exhibited embryonic spasticity, only glrbb morphants exhibited bilateral contractions characteristic of beo mutants. Likewise, zebrafish over-expressing a dominant startle disease mutation (GlyR α1(R271Q)) exhibited spasticity but not bilateral contractions. Since GlyR βb can interact with GlyR α subunits 2-4 in addition to GlyR α1, loss of the GlyR βb subunit may produce more severe phenotypes by affecting multiple GlyR subtypes. Indeed, immunohistochemistry of glra1 morphants suggests that in zebrafish, alternate GlyR α subunits can compensate for the loss of the GlyR α1 subunit. To address the potential for interplay among GlyR subunits during development, we quantified the expression time-course for genes known to be critical to glycinergic synapse function. We found that GlyR α2, α3 and α4a are expressed in the correct temporal pattern and could compensate for the loss of the GlyR α1 subunit. Based on our findings, future studies that aim to model candidate startle disease genes in zebrafish should include measures of spasticity and synaptic development.

Changes in glycine receptor subunit expression in forebrain regions of the Wistar rat over development

Glycine receptors (GlyRs) are pentameric membrane proteins in the form of either α-homomers or α-β heteromers. Four out of five subunits; α1-3 and β, have been found in the mammalian brain. Early studies investigating subunit composition and expression patterns of this receptor have proposed a developmental switch from α2 homomers to α1β heteromers as the CNS matures, a conclusion primarily based on results from the spinal cord. However, our previous results indicate that this might not apply to e.g. the forebrain regions. Here we examined alterations in GlyR expression caused by developmental changes in selected brain areas, focusing on reward-related regions. Animals of several ages (P2, P21 and P60) were included to examine potential changes over time. In accordance with previous reports, a switch in expression was observed in the spinal cord. However, the present results indicate that a decrease in α2 subunit expression is not replaced by α1 subunit expression since the generally low levels, and modest increases, of α1 could hardly replace the reduction in α2-mRNA. Instead mRNA measurements indicate that α2 continues to be the dominating α-subunit also in adult animals, usually in combination with high and stable levels of β-subunit expression. This indicates that alterations in GlyR subunit expression are not simply a maturation effect common for the entire CNS, but rather a unique pattern of transition depending on the region at hand.

The long activations of α2 glycine channels can be described by a mechanism with reaction intermediates ("flip")

The α2 glycine receptor (GlyR) subunit, abundant in embryonic neurons, is replaced by α1 in the adult nervous system. The single-channel activity of homomeric α2 channels differs from that of α1-containing GlyRs, as even at the lowest glycine concentration (20 µM), openings occurred in long (>300-ms) groups with high open probability (P_open; 0.96; cell-attached recordings, HEK-expressed channels). Shut-time intervals within groups of openings were dominated by short shuttings of 5–10 µs. The lack of concentration dependence in the groups of openings suggests that they represent single activations, separated by very long shut times at low concentrations. Several putative mechanisms were fitted by maximizing the likelihood of the entire sequence of open and shut times, with exact missed-events allowance (program hjcfit). Records obtained at several glycine concentrations were fitted simultaneously. The adequacy of the different schemes was judged by the accuracy with which they predicted not only single-channel data but also the time course and concentration dependence of macroscopic responses elicited by rapid glycine applications to outside-out patches. The data were adequately described only with schemes incorporating a reaction intermediate in the activation, and the best was a flip mechanism with two binding sites and one open state. Fits with this mechanism showed that for α2 channels, the opening rate constant is very fast, ∼130,000 s⁻¹, much as for α1β GlyRs (the receptor in mature synapses), but the estimated true mean open time is 20 times longer (around 3 ms). The efficacy for the flipping step and the binding affinity were lower for α2 than for α1β channels, but the overall efficacies were similar. As we previously showed for α1 homomeric receptors, in α2 glycine channels, maximum P_open is achieved when fewer than all five of the putative binding sites in the pentamer are occupied by glycine.

The biological role of the glycinergic synapse in early zebrafish motility

Glycine mediates fast inhibitory neurotransmission in the spinal cord, brainstem and retina. Loss of synaptic glycinergic transmission in vertebrates leads to a severe locomotion defect characterized by an exaggerated startle response accompanied by transient muscle rigidity in response to sudden acoustic or tactile stimuli. Several molecular components of the glycinergic synapse have been characterized as an outcome of genetic and physiological analyses of synaptogenesis in mammals. Recently, the glycinergic synapse has been studied using a forward genetic approach in zebrafish. This review aims to discuss molecular components of the glycinergic synapse, such as glycine receptor subunits, gephyrin, gephyrin-binding proteins and glycine transporters, as well as recent studies relevant to the genetic analysis of the glycinergic synapse in zebrafish.

Defective glycinergic synaptic transmission in zebrafish motility mutants

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem. Recently, in vivo analysis of glycinergic synaptic transmission has been pursued in zebrafish using molecular genetics. An ENU mutagenesis screen identified two behavioral mutants that are defective in glycinergic synaptic transmission. Zebrafish bandoneon (beo) mutants have a defect in glrbb, one of the duplicated glycine receptor (GlyR) beta subunit genes. These mutants exhibit a loss of glycinergic synaptic transmission due to a lack of synaptic aggregation of GlyRs. Due to the consequent loss of reciprocal inhibition of motor circuits between the two sides of the spinal cord, motor neurons activate simultaneously on both sides resulting in bilateral contraction of axial muscles of beo mutants, eliciting the so-called 'accordion' phenotype. Similar defects in GlyR subunit genes have been observed in several mammals and are the basis for human hyperekplexia/startle disease. By contrast, zebrafish shocked (sho) mutants have a defect in slc6a9, encoding GlyT1, a glycine transporter that is expressed by astroglial cells surrounding the glycinergic synapse in the hindbrain and spinal cord. GlyT1 mediates rapid uptake of glycine from the synaptic cleft, terminating synaptic transmission. In zebrafish sho mutants, there appears to be elevated extracellular glycine resulting in persistent inhibition of postsynaptic neurons and subsequent reduced motility, causing the 'twitch-once' phenotype. We review current knowledge regarding zebrafish 'accordion' and 'twitch-once' mutants, including beo and sho, and report the identification of a new alpha2 subunit that revises the phylogeny of zebrafish GlyRs.

Regulation of mouse embryonic stem cell neural differentiation by retinoic acid

Pluripotent mouse embryonic stem cells (ESCs) derived from the early blastocyst can differentiate in vitro into a variety of somatic cell types including lineages from all three embryonic germ layers. Protocols for ES cell neural differentiation typically involve induction by retinoic acid (RA), or by exposure to growth factors or medium conditioned by other cell types. A serum-free differentiation (SFD) medium completely lacking exogenous retinoids was devised that allows for efficient conversion of aggregated mouse ESCs into neural precursors and immature neurons. Neural cells produced in this medium express neuronal ion channels, establish polarity, and form functional excitatory and inhibitory synapses. Brief exposure to RA during the period of cell aggregation speeds neuronal maturation and suppresses cell proliferation. Differentiation without RA yields neurons and neural progenitors with apparent telencephalic identity, whereas cells differentiated with exposure to RA express markers of hindbrain and spinal cord. Transcriptional profiling indicates a substantial representation of transit amplifying neuroblasts in SFD cultures not exposed to RA.

Glycinergic synaptic currents in the deep cerebellar nuclei

Despite evidence of local glycinergic circuits in the mature cerebellar nuclei the result of their activation remains unknown. Here, using whole cell recordings in rat cerebellar slices we demonstrated that after postnatal day 17 (>P17) glycinergic IPSCs can be readily evoked in large deep cerebellar nuclear neurons (DCNs), in the same way as in neonatal DCNs (P7-P10). Spontaneous glycinergic IPSCs were very rare but direct presynaptic depolarization by superfusion with elevated potassium concentration or application of 4-aminopyridine consistently evoked strychnine sensitive IPSCs. Glycinergic IPSCs showed fast kinetics in >P17 DCNs while were significantly slower in neonatal DCNs. Immuno-histochemical investigations using a specific marker for glycinergic fibers and terminals showed low density of immuno-fluorescent puncta, putative glycinergic boutons surrounding P18-P23 DCNs, in agreement with the rare spontaneous synaptic activity. But putative glycinergic boutons were present in critical areas for the control of spike generation. In contrast to adult and neonatal DCNs, glycinergic IPSCs could not be induced in juvenile DCNs (P13-P17) despite similar perisomatic immuno-staining pattern and expression of glycinergic receptors to >P17 DCNs. The latter results demonstrate substantial postnatal development of glycinergic cerebellar nuclei circuits. The cerebellum is involved in rapidly controlling ongoing movements. For that function, it is thought important the temporal and spatial precision of its output, which is carried to target structures by DCNs. The present study, by demonstrating fast glycinergic IPSCs in mature DCNs, points to the activation of glycinergic microcircuits as one of the possible mechanism involved in the spatio-temporal control of cerebellar output.

Kinetics and functional characterization of the glycine receptor alpha2 and alpha3 subunit

In recent studies one serine residue (Ser-346) within the protein-kinase-A (R-E-S-R) consensus sequence of the GlyRalpha3 intracellular loop has proven to be an essential target for prostaglandin-E(2)-mediated phosphorylation, which further modulates spinal nociceptive transmission and central inflammatory pain sensitization. In the present study we investigated the effect of Ser-346 phosphorylation and Ser-346 mutation on receptor kinetics and function using whole-cell patch-clamp recordings in transfected HEK 293 T cells. We compared biophysical properties of wild type GlyRalpha3 and two site-directed mutants, where Ser-346 was replaced by alanine or aspartate, in the absence and presence of prostaglandin-E(2). The mutation to alanine was accompanied by significantly altered dose-response and desensitization properties. Mutation to aspartate had only minor effects on receptor kinetics and function. Phosphorylation of Ser-346 slowed desensitization and decreased glycinergic currents in GlyRalpha3/mEP2 transfected cells. In addition, we demonstrated that prostaglandin-E(2) also had an effect on the GlyRalpha2 subunit. Exposure to prostaglandin-E(2) decreased the maximum peak current amplitude of glycinergic currents in GlyRalpha2/mEP2 transfected cells in the same manner as phosphorylation of the GlyRalpha3 subunit. It led to a significant increase of the desensitization time constants and thus significantly affected the desensitization behaviour. These results indicate that the GlyRalpha2 and the GlyRalpha3 subunits act as important subunits for the modulation of glycine receptor kinetics and function.

Structural difference between heteromeric somatic and homomeric axonal glycine receptors in the hypothalamo-neurohypophysial system

Glycine receptors are ionotropic receptors formed by either the homomeric assembly of ligand-binding alpha subunits or the heteromeric combination of an alpha subunit and the auxiliary beta subunit. Glycine receptors in the brain are found at either pre- or post-synaptic sites. Rat supraoptic nucleus neurons express glycine receptors on the membrane of both their soma and dendrites within the supraoptic nucleus, and their axon terminals in the neurohypophysis. Taking advantage of the well-separated cellular compartments of this system, we correlated the structural properties of the receptors to their subcellular localization. Immunohistochemical study using the generic mAb4a antibody revealed that somatodendritic receptors were clustered, whereas axonal glycine receptors showed a more diffuse distribution. This was paralleled by the presence of clusters of the glycine receptor aggregating protein gephyrin in the supraoptic nucleus and its complete absence in the neurohypophysis. Moreover, another antibody recognizing the alpha1/alpha2 subunits similarly labeled the axonal glycine receptors, but did not recognize the somatodendritic receptor clusters of supraoptic nucleus neurons, indicative of structural differences between somatic and axonal glycine receptors. Furthermore, the subunits composing the somatic and axonal receptors have different molecular weight. Functional study further differentiated the two types of glycine receptors on the basis of their sensitivity to picrotoxin, identifying somatic receptors as alpha/beta heteromers, and axonal receptors as alpha homomers. These results indicate that targeting of glycine receptors to axonal or somatodendritic compartment is directly related to their subunit composition, and set the hypothalamo-neurohypophysial system as an excellent model to study the mechanisms of targeting of proteins to various neuronal cellular compartments.

Brain-type creatine kinase activates neuron-specific K+-Cl- co-transporter KCC2

GABA, a major inhibitory neurotransmitter in the adult CNS, is excitatory at early developmental stages as a result of the elevated intracellular Cl- concentration ([Cl-]i). This functional switch is primarily attributable to the K+-Cl- co-transporter KCC2, the expression of which is developmentally regulated in neurons. Previously, we reported that KCC2 interacts with brain-type creatine kinase (CKB). To elucidate the functional significance of this interaction, HEK293 cells were transfected with KCC2 and glycine receptor alpha2 subunit, and gramicidin-perforated patch-clamp recordings were performed to measure the glycine reversal potential (Egly), giving an estimate of [Cl-]i. KCC2-expressing cells displayed the expected changes in Egly following alterations in the extracellular K+ concentration ([K+]o) or administration of an inhibitor of KCCs, suggesting that the KCC2 function was being properly assessed. When added into KCC2-expressing cells, dominant-negative CKB induced a depolarizing shift in Egly and reduced the hyperpolarizing shift in Egly seen in response to a lowering of [K+]o compared with wild-type CKB. Moreover, 2,4-dinitrofluorobenzene (DNFB), an inhibitor of CKs, shifted Egly in the depolarizing direction. In primary cortical neurons expressing CKB, the GABA reversal potential was also shifted in the depolarizing direction by DNFB. Our findings suggest that, in the cellular micro-environment, CKB activates the KCC2 function.

Postsynaptic receptor mechanisms underlying developmental speeding of synaptic transmission

As animals mature the decay of postsynaptic currents become faster at a variety of synapses. This change is thought to contribute to a refinement of motor co-ordination and to an increase in the precision of sensory perception and cognition. At cholinergic neuromuscular synapses and glycinergic and GABAergic inhibitory synapses, the developmental speeding of synaptic currents depends upon switches of receptor subunits and an ensuing acceleration in the kinetics of channel gating. At glutamatergic excitatory synapses, speeding in the decay time of NMDA receptor (NMDAR)-mediated excitatory postsynaptic currents (NMDA-EPSCs) is also dependent on developmental switches in NMDAR subunits. However, developmental speeding in the kinetics of AMPA receptor (AMPAR)-mediated EPSCs (AMPA-EPSCs) is caused by multiple factors. The decay time of AMPA-EPSCs can be shaped by the kinetics of channel gating or desensitization of AMPA receptors, depending upon the speed of transmitter clearance from the synaptic cleft. During postnatal development AMPAR channel gating and desensitization as well as the transmitter clearance speed up in kinetics. Given that the developmental speeding of synaptic currents play critical roles in the maturation of sensory and motor functions, any defect in this mechanism may seriously affect neuronal function.

The GABA rho1 subunit interacts with a cellular retinoic acid binding protein in mammalian retina

Interactions between the intracellular domain of ligand-gated membrane receptors and cytoplasmic proteins play important roles in their assembly, clustering, and function. In addition, protein-protein interactions may provide an alternative mechanism by which neurotransmitters activate intracellular pathways. In this study, we report a novel interaction between the GABA rho1 subunit and cellular retinoic acid binding protein in mammalian retina that could serve as a link between the GABA signaling pathway and the control of gene expression in neurons. The interaction between the intracellular loop of the human GABA rho subunit and cellular retinoic acid binding protein was identified using a CytoTrap XR yeast two-hybrid system, and was further confirmed by co-precipitation of the human GABA rho subunit and cellular retinoic acid binding protein from baboon retinal samples. The cellular retinoic acid binding protein binding domain on the human rho1 subunit was located to the C-terminal region of human GABA rho subunit, and the interaction of the human GABA rho subunit with cellular retinoic acid binding protein could be antagonized by a peptide derived from within the binding domain of the rho1 subunit. Since cellular retinoic acid binding protein is a carrier protein for retinoic acid, we investigated the effect of GABA on retinoic acid activity in neuroblastoma cells containing endogenously expressed cellular retinoic acid binding protein. In the absence of the rho1 receptor, these cells showed enhanced neurite outgrowth when exposed to retinoic acid and GABA had no effect on their response to retinoic acid. In contrast, cells stably transfected with the human rho1 subunit showed a significantly reduced sensitivity to retinoic acid when exposed to GABA. These results suggest that the GABA receptor subunit effectively altered gene expression through its interaction with the cellular retinoic acid binding protein pathway.

Characterization of newly cloned variant of rat glycine receptor α1 subunit

Responses to glycine, a major inhibitory neurotransmitter within the nervous system, are mediated by glycine receptors (GlyRs). Here, we report the cloning and analysis of a novel splicing variant of the GlyRalpha1 subunit. This variant, named GlyRalpha1del, has a truncated cytoplasmic region between transmembrane domains (TM)3 and TM4, and compared to other variants, the truncation is contributed by a different acceptor site in exon 9. We transfected GlyRalpha1 or GlyRalpha1del into HEK293 cells, and then examined the glycine-activated currents using a whole-cell patch-clamp recording technique. Maximal currents and current-voltage relationships showed no clear difference between GlyRalpha1del and GlyRalpha1. Moreover, dose-response curves indicated that the EC50 values for glycine differed significantly between the two GlyRalpha1 derivatives, although their Hill coefficients were similar. When present with other isoforms, GlyRalpha1del might alter the response to glycine or to other agonists, as this variant expands the potential heterogeneity among glycine receptors.

Changes on the properties of glycine receptors during neuronal development

Glycine receptors (GlyRs) play a major role in the excitability of spinal cord and brain stem neurons. During development, several properties of these receptors undergo significant changes resulting in major modifications of their physiological functions. For example, the receptor structure switches from a monomeric alpha or heteromeric alpha 2 beta in immature neurons to an alpha 1 beta receptor type in mature neurons. Together with these changes in receptor subunits, the postsynaptic cluster size increases with development. Parallel to these modifications, the apparent receptor affinity to glycine and strychnine, as well as that of Zn(2+) and ethanol increases with time. The mature receptor is characterized by a slow desensitizing current and high sensitivity to modulation by protein kinase C. Also, the high level of glycinergic transmission in immature spinal neurons modulates neuronal excitability causing membrane depolarization and changes in intracellular calcium. Due to these properties, chronic inhibition of glycinergic transmission affects neurite outgrowth and produces changes in the level of synaptic transmission induced by GABA(A) and AMPA receptors. Finally, the high level of plasticity found in immature GlyRs is likely associated to changes in cytoskeleton dynamics.

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.

Staggered development of GABAergic and glycinergic transmission in the MNTB

Maturation of some brain stem and spinal inhibitory systems is characterized by a shift from GABAergic to glycinergic transmission. Little is known about how this transition is expressed in terms of individual axonal inputs and synaptic sites. We have explored this issue in the rat medial nucleus of the trapezoid body (MNTB). Synaptic responses at postnatal days 5-7 (P5-P7) were small, slow, and primarily mediated by GABA(A) receptors. By P8-P12, an additional, faster glycinergic component emerged. At these ages, GABA(A), glycine, or both types of receptors mediated transmission, even at single synaptic sites. Thereafter, glycinergic development greatly accelerated. By P25, evoked inhibitory postsynaptic currents (IPSCs) were 10 times briefer and 100 times larger than those measured in the youngest group, suggesting a proliferation of synaptic inputs activating fast-kinetic receptors. Glycinergic miniature IPSCs (mIPSCs) increased markedly in size and decay rate with age. GABAergic mIPSCs also accelerated, but declined slightly in amplitude. Overall, the efficacy of GABAergic inputs showed little maturation between P5 and P20. Although gramicidin perforated-patch recordings revealed that GABA or glycine depolarized P5-P7 cells but hyperpolarized P14-P15 cells, the young depolarizing inputs were not suprathreshold. In addition, vesicle-release properties of inhibitory axons also matured: GABAergic responses in immature rats were highly asynchronous, while in older rats, precise, phasic glycinergic IPSCs could transmit even with 500-Hz stimuli. Thus development of inhibition is characterized by coordinated modifications to transmitter systems, vesicle release kinetics, Cl- gradients, receptor properties, and numbers of synaptic inputs. The apparent switch in GABA/glycine transmission was predominantly due to enhanced glycinergic function.

A novel glycine receptor alpha Z1 subunit variant in the zebrafish brain

Alpha subunits of the inhibitory glycine receptor (GlyR) display genetic heterogeneity in mammals and zebrafish. This diversity is increased in mammals by the alternative splicing mechanism. We report here in zebrafish, the characterization of a new alphaZ1 subunit likely arising from alphaZ1 gene by an alternative splice process (alphaZ1L). This novel cDNA possesses 45 supplementary nucleotides at the putative exon2/exon3 boundary. The corresponding protein contains 15 additional amino acids in the NH2-terminal domain. Heterologous expression of homomeric GlyRalphaZ1L in human embryonic kidney-293 cells generates glycine-gated strychnine-sensitive chloride channels with no obvious discrepancy with pharmacological properties of GlyRalphaZ1. Moreover, zinc modulation of glycine-induced currents is identical in alphaZ1 and alphaZ1L glycine receptors. During ontogenesis, simultaneous alphaZ1 and alphaZ1L mRNA synthesis have been observed. Embryonic and adult alphaZ1 and alphaZ1L mRNA expressions are restricted to the CNS. Embryonic alphaZ1L mRNA anatomical pattern of expression is, however, highly restrained and strictly limited to the rostral part of the brain revealing a highly regionalized function of alphaZ1L in the CNS. This report contributes to the characterization of the diversity of glycine receptor isoforms in zebrafish and emphasizes the common mechanism used among vertebrates for creating GlyR variety and specificity.

Selective antagonism of rat inhibitory glycine receptor subunits

Retinal ganglion cells exhibit fast and slow inhibitory synaptic glycine currents that can be selectively inhibited by strychnine and 5,7-dichlorokynurenic acid (DCKA), respectively. In this study we examined whether strychnine and DCKA selectivity correlated with the subunit composition of the glycine receptor. Homomeric alpha1, alpha2 or alpha2* glycine subunits were in vitro expressed in human embryonic kidney cells (HEK 293). In cells expressing the alpha1 subunit, responses to 200 microm glycine were blocked by 1 microm strychnine but not by 500 microm DCKA. In cells expressing the alpha2 subunit, both 1 microm strychnine and 500 microm DCKA were effective antagonists of 200 microm glycine. In cells expressing alpha2* subunits, which are much less glycine-sensitive, 10 mm glycine was inhibited by 500 microm DCKA but not by 1 microm strychnine. A single amino acid mutation in the alpha1 subunit (R196G), converted this subunit from DCKA-insensitive to DCKA-sensitive. In conclusion, the comparative effectiveness of strychnine and DCKA can be used to distinguish between the alpha1, alpha2 and alpha2* receptor responses. Furthermore, a single amino acid near the glycine receptor's putative agonist binding site may account for differences in DCKA sensitivity amongst the alpha subunits.

A basic cluster determines topology of the cytoplasmic M3-M4 loop of the glycine receptor alpha1 subunit

The inhibitory glycine receptor is a member of the ligand-gated ion channel superfamily of neurotransmitter receptors, which are composed of homologous subunits with four transmembrane segments (M1-M4), each. Here, we demonstrate that the correct topology of the glycine receptor alpha1 subunit depends critically on six positively charged residues within a basic cluster, RFRRKRR, located in the large cytoplasmic loop (designated M3-M4 loop) following the C-terminal end of M3. Neutralization of one or more charges of this cluster, but not of other charged residues in the M3-M4 loop, led to an aberrant translocation into the endoplasmic reticulum lumen of the M3-M4 loop. However, when two of the three basic charges located in the ectodomain linking M2 and M3 were neutralized, in addition to two charges of the basic cluster, endoplasmic reticulum disposition of the M3-M4 loop was prevented. We conclude that a high density of basic residues C-terminal to M3 is required to compensate for the presence of positively charged residues in the M2-M3 ectodomain, which otherwise impair correct membrane integration of the M3 segment.

The actions of propofol on gamma-aminobutyric acid-A and glycine receptors in acutely dissociated spinal dorsal horn neurons of the rat

The spinal cord plays an important role in modulating anesthetic-induced suppression of nociceptive transmission. To gain some insight into the anesthetic mechanisms of propofol at the spinal level, we investigated the direct action of propofol and its modulation on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol induced Cl(-) currents (I(Cl)), which were sensitive to bicuculline and, to a lesser extent, to strychnine. The activation, desensitization, and deactivation of propofol-induced I(Cl) were slower than those of GABA- and glycine-induced I(Cl). In addition, this study revealed similar modulatory actions of propofol on GABA(A)R and GlyR. Propofol potentiated both GABA- and glycine-induced I(Cl) at small con-centrations and inhibited both GABA- and glycine-induced I(Cl) at large concentrations. The potentiation of propofol on I(Cl) was caused by slowing current desensitization and deactivation, whereas the inhibition actions might be involved in the cross-desensitization between GABA- and propofol-induced I(Cl) and the cross-inhibition between the GABA(A)R and GlyR. The results suggest that propofol facilitation of GABA(A)R and GlyR at the spinal level could contribute significantly to general anesthetic-induced analgesia and anesthesia.

IMPLICATIONS

The actions of propofol on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) were investigated in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol was found to potentiate the functions of GABA(A)R and GlyR at the spinal level, which might contribute to propofol-induced analgesia and anesthesia.

Role of Nova-1 in regulating alpha2N, a novel glycine receptor splice variant, in developing spinal cord neurons

Inhibitory glycine receptor (GlyR) subunits undergo developmental regulation, but the molecular mechanisms of GlyR regulation in developing neurons are little understood. Using RT-PCR, we investigated the regulation of GlyR alpha-subunit splice forms during the development of the spinal cord of the rat. Experiments to compare the amounts of mRNA for two known splice variants of the GlyR alpha2 subunit, alpha2A and alpha2B, in the developing rat spinal cord revealed the presence of an additional, novel variant that lacked any exon 3, herein named "alpha2N." Examination of the RNA from spinal cords of different-aged rats showed a dramatic down-regulation of alpha2N during prenatal development: alpha2N mRNA formed a significant portion of the alpha2 subunit pool at E14, but its relative level was reduced by 85% by birth and was undetectable in adults. Two proteins previously implicated in regulating the splicing of GlyR alpha2 pre-mRNA, the neurooncological ventral antigen-1 (Nova-1) and the brain isoform of the polypyrimidine tract binding protein (brPTB), underwent small changes over the same period that did not correlate directly with the changes in the level of alpha2N, calling into question their involvement in the developmental regulation of alpha2N. However, treatment of spinal cord neurons in culture with antisense oligonucleotides designed selectively to knock down one of three Nova-1 variants significantly altered the relative level of GlyR alpha2N, showing that Nova-1 isoforms can regulate GlyR alpha2 pre-mRNA splicing in developing neurons. These results provide evidence for a novel splice variant of the GlyR alpha2 subunit that undergoes dramatic developmental regulation, reveal the expression profiles of Nova-1 and brPTB in the developing spinal cord, and suggest that Nova-1 plays a role in regulating GlyR alpha2N in developing neurons.

Cloning and functional characterization of two glycine receptor alpha-subunits from the perch retina

Glycine receptors are ligand-gated ion channel proteins mediating synaptic inhibition in the spinal cord, retina and brain of vertebrates. We have cloned and functionally characterized two glycine receptor alpha-subunits from the perch (Roccus americana) retina. Based on sequence homology with the mammalian counterparts, we termed these subunits alpha 1 and alpha 3. RT-PCR revealed the presence of both subunits in retina and brain, whereas alpha1 was predominant in spinal cord. A short splice variant of alpha1 was detected in the brain but not in the retina. Functional expression of the perch subunits in HEK-293 cells yielded robust glycine-gated currents sensitive to strychnine. The perch receptors displayed a high efficacy for taurine and GABA and thus differ from the mammalian counterparts. Because the retina is a rich source for taurine, this finding could be of physiological importance. The structural features of the ligand binding domain strongly support the notion of increased glycine/GABA discrimination in higher vertebrates.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Ubiquitination precedes internalization and proteolytic cleavage of plasma membrane-bound glycine receptors

The inhibitory glycine receptor (GlyR) in developing spinal neurones is internalized efficiently upon antagonist inhibition. Here we used surface labeling combined with affinity purification to show that homopentameric alpha1 GlyRs generated in Xenopus oocytes are proteolytically nicked into fragments of 35 and 13 kDa upon prolonged incubation. Nicked GlyRs do not exist at the cell surface, indicating that proteolysis occurs exclusively in the endocytotic pathway. Consistent with this interpretation, elevation of the lysosomal pH, but not the proteasome inhibitor lactacystin, prevents GlyR cleavage. Prior to internalization, alpha1 GlyRs are conjugated extensively with ubiquitin in the plasma membrane. Our results are consistent with ubiquitination regulating the endocytosis and subsequent proteolysis of GlyRs residing in the plasma membrane. Ubiquitin-conjugating enzymes thus may have a crucial role in synaptic plasticity by determining postsynaptic receptor numbers.

Isolation and characterization of an alpha 2-type zebrafish glycine receptor subunit

The complementary DNA for a novel alpha subunit of the glycine receptor, alphaZ2, was isolated from a zebrafish adult brain library. The molecular characteristics, phylogenetic relationships and messenger RNA length of this alphaZ2 subunit show it to be an alpha2-type glycine receptor subunit isoform. The leader peptide however, diverges from those of known glycine receptor alpha isoforms. Recombinantly expressed in Xenopus oocytes, alphaZ2 formed functional glycine receptor channels. These homomeric channels were activated by glycine and taurine, with apparent affinities similar to those reported for zebrafish alphaZ1 glycine receptor, and were also effectively antagonized by nanomolar concentrations of strychnine. However, during prolonged applications of agonists, ionic currents of alphaZ2 receptor channels declined to a much lower steady-state level than those of alphaZ1, indicating different desensitization properties. Analysis of messenger RNA revealed that alphaZ2 is specifically expressed in adult brain tissue and present in both adult and embryonic zebrafish. This report contributes to the characterization of the diversity of glycine receptor isoforms in vertebrates.

Regional distribution of glycine receptor messenger RNA in the central nervous system of zebrafish

We report the cloning of the zebrafish beta subunit of the glycine receptor and compare the anatomical distribution of three glycine receptor subunit constituents in adult zebrafish brain (alphaZ1, alphaZ2 and betaZ) to the expression pattern of homologous receptor subunits (alpha1, alpha2 and beta) in the mammalian adult CNS. Non-radioactive hybridization was used to map the distribution of the alphaZ1, alphaZ2 and betaZ glycine receptor subunit messenger RNAs in the adult zebrafish brain. The anterior-posterior expression gradient found in adult zebrafish brain was similar to that reported in mammalian CNS. However, the glycine receptor transcripts, notably the alphaZ1 subunit, were more widely distributed in the anterior regions of the zebrafish than in the adult mammalian brain. The isoform-specific distribution pattern was less regionalized in zebrafish than in the rat mammalian CNS. Nevertheless, there was some regionalization of alphaZ1, alphaZ2 and betaZ transcripts in the diencephalic and mesencephalic nuclei where different sensory and motor centers express either alphaZ1/betaZ or alphaZ2 subunits. In contrast to the widespread distribution of the beta subunit in adult mammalian brain, alphaZ2 messenger RNA presented the widest expression territory of all three glycine receptor subunits tested. alphaZ2 messenger RNA was expressed in the absence of alphaZ1 and betaZ messenger RNA in the outer nuclear layer of the retina, the inferior olive and the raphe of the medulla oblongata, as well as in the nucleus of Cajal of the medulla spinalis. In contrast, an identified central neuron of the reticular formation, the Mauthner cell, expresses all three glycine receptor subunits (alphaZ1, alphaZ2 and betaZ). This report extends the already described glycine receptor expression in the vertebrate CNS and confirms the importance of glycine-mediated inhibition in spinal cord and brainstem.

Structure and functions of inhibitory and excitatory glycine receptors

The strychnine-sensitive glycine receptor (GlyR) is a pentameric chloride channel protein that exists in several developmentally and regionally regulated isoforms in the CNS. These result from the differential expression of four genes encoding different variants (alpha 1-alpha 4) of the ligand-binding subunit of the GlyR. Their assembly with the structural beta subunit is governed by "assembly cassettes" within the extracellular domains of these proteins and creates chloride channels of distinct conductance properties. GlyR gating is potentiated by Zn2+, a metal ion co-released with different neurotransmitters. Site-directed mutagenesis has unraveled major determinants of agonist binding and Zn2+ potentiation. During development, glycine receptors mediate excitation that results in Ca2+ influx and neurotransmitter release. Ca2+ influx triggered by the activation of embryonic GlyRs is required for the synaptic localization of the GlyR and its anchoring protein gepyhrin. In the adult, mutations in GlyR-subunit genes result in motor disorders. The spastic and spasmodic phenotypes in mouse as well as human hereditary startle disease will be discussed.

Actions of 3-[2-phosphonomethyl[1,1-biphenyl]-3-yl]alanine (PMBA) on cloned glycine receptors

1. PMBA is a novel antagonist of strychnine-sensitive glycine receptors in the rat spinal cord, however, its mode of action is unknown. The actions of PMBA on rat glycine receptor alpha1 and alpha2 homomers in Xenopus oocytes were studied under two-electrode voltage-clamp. 2. Co-application of PMBA and glycine to both alpha1 and alpha2 homomers yielded inward currents which decayed to a steady-state. Responses rose slowly to the same steady-state amplitude following a 2 min pre-incubation in PMBA. Strychnine, but not picrotoxinin, showed similar antagonism to PMBA. The potency of PMBA was independent of membrane potential between -100 and 0 mV. 3. When tested against EC50 concentrations of glycine, PMBA was almost equally potent on alpha1 (IC50, 406+/-41 nM: Hill coefficient, 1.5+/-0.2) and alpha2 (IC50, 539+/-56 nM; Hill coefficient, 1.4+/-0.2) homomers. 4. PMBA (1-I0 microM) and strychnine (200 nM) reduced the potency of glycine and the amplitude of the maximal agonist response of alpha1 and alpha2 homomers. In 10 microM PMBA, two distinct classes of glycine response were observed on alpha2, only a single class of responses were observed on alpha1. 5. There are similarities in PMBA and strychnine antagonism, although these compounds are structurally distinct. The possibility that PMBA interacts at two binding sites which differ in alpha1 and alpha2 subunits is discussed. PMBA may provide a lead structure for novel antagonists with which to investigate structural differences in glycine receptor at alpha1 and alpha2 subunits.

Modulation of glycine-induced currents by zinc and other metal cations in neurons acutely dissociated from the dorsal motor nucleus of the vagus of the rat

Effects of Zn2+ and other polyvalent cations on glycine-induced currents in the freshly dissociated rat dorsal motor nucleus of the vagus neurons were investigated under voltage-clamp conditions by the use of the nystatin-perforated patch recording configuration. Glycine evoked a Cl- current in a concentration-dependent manner with a half-maximum effective concentration (EC50) of 3.3x10-5 M. Strychnine inhibited the 3x10-5 M glycine-induced current in a concentration-dependent manner with a half-maximal inhibitory concentration (IC50) of 6.8x10-7 M. At low concentrations (3x10-8 M-3x10-6 M), Zn2+ potentiated the current elicited by 3x10-5 M glycine. On the other hand, at concentrations higher than 10-5 M, Zn2+ inhibited the glycine response. The biphasic action of Zn2+ was mimicked by Ni2+, but La3+ and Co2+ had only potentiating effect. Zn2+ shifted the concentration-response curve for the glycine-induced current without changing the maximum response, and the EC50 values for the glycine response in the absence and presence of 10-6 M and 10-4 M Zn2+ were 3.3x10-5 M, 1.3x10-5 M and 1.3x10-4 M, respectively. These results suggest that the biphasic modulation of glycine response by Zn2+ results from changes in apparent glycine affinity.

Multiple picrotoxinin effect on glycine channels in rat hippocampal neurons

The effect of picrotoxinin on glycine-induced chloride currents was studied in dissociated rat hippocampal neuron culture in whole-cell and excised outside-out patches. Picrotoxinin blocked the glycine induced chloride currents. The picrotoxinin effect at 20 microM on glycine dose response relationship suggested a competitive mechanism. However, at 1 mM, the picrotoxinin effect was largely noncompetitive. In excised patches, glycine activated two types of channels distinguished by a difference in conductances. The first group had single channel conductances of around 47 pS and another around 100 pS. Occasionally, both types of channels were found in the same excised patch. Low concentration of picrotoxinin selectively blocked large conductance channels. At higher concentrations of 0.5 to 1 mM, picrotoxinin blocked the small conductance channels by a flickering block. These findings indicate that the whole-cell glycine current in rat hippocampal neurons is mediated by at least two types of channels. The two types of channels have distinct conductance, picrotoxinin sensitivity and different mechanism of picrotoxinin block.

Protein kinases modulate two glycine currents in salamander retinal ganglion cells

1. Protein kinase modulation of glycine-activated currents was examined in acutely dissociated ganglion cells from tiger salamander retina using whole-cell voltage-clamp techniques. 2. Glycine (100 microM) induced an outward chloride current in cells clamped at 0 mV. Co-application of 50 microM forskolin made the glycine-induced current more transient. The combination of forskolin and glycine reduced the later portion of current response without changing the initial peak amplitude. 3. 3-Isobutyl-1-methylxanthine (IBMX) or 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) produced effects similar to those of forskolin. H-89, a protein kinase A (PKA) inhibitor, blocked the effect of forskolin. 4. A protein kinase C (PKC) activator, OAG (1-oleoyl-2-acetyl-sn-glycerol), also made the glycine response more transient. Unlike PKA analogues, OAG enhanced the glycine peak response without changing the glycine late response. OAG effects were blocked by 1 microM GF-109203X, a PKC inhibitor. 5. Nanomolar concentrations of strychnine selectively blocked the fast phase of the glycine current and reversed the effect of OAG, but not that of forskolin. Conversely, forskolin occluded the effect of 5,7-dichlorokynurenic acid, which selectively suppresses the late phase of the glycine current. The action of OAG was not blocked by 5,7-dichlorokynurenic acid. 6. Thus, through a differential modulation, both protein kinase A and C shorten the decay time of the glycine current. PKA suppresses the slow component, while PKC potentiates the fast component.

Functional properties of glycine receptors expressed in primary cultures of mouse cerebellar granule cells

Expression of the glycine receptor was investigated in membranes prepared from primary cultures of mouse cerebellar granule cells and postnatal mouse cerebellum using the antagonist [3H]strychnine for ligand binding. Scatchard analysis of the binding data obtained from P17 cerebellum showed a single population of binding sites (K(D) approximately 6 nM) and [3H]strychnine binding to membranes prepared from cultured neurons and P17 cerebellum was found to have the same sensitivity to the glycinergic agonists glycine, beta-alanine and taurine. The development of [3H]strychnine binding sites in cultured cerebellar granule cells and cerebellum showed opposing profiles. [3H]strychnine binding to primary cultures increased significantly during the culture period whereas during development in vivo the number of binding sites decreased over time and was hardly detectable in the adult cerebellum. Release of preloaded D-[3H]aspartate evoked by 40 mM K+ from granule cells cultured for seven days was inhibited by glycine by about 50%. Beginning after seven days in culture the ability of glycine to inhibit transmitter release declined to no inhibition after 17 days in culture. Experiments with the non-competitive antagonist, picrotoxinin, showed no blocking effect of 150 microM picrotoxinin on the glycine-induced inhibition of transmitter release. This contrasted with the inhibitory effect of 100 microM picrotoxinin in whole-cell patch-clamp recordings on responses to 500 microM glycine (56% block). Furthermore, it was demonstrated that the amplitude of the glycine activated peak current had the same size after six to seven days and after 16-17 days in culture. Northern blot analysis, and co-injection of messenger RNA plus antisense oligonucleotides into Xenopus oocytes revealed glycine receptor alpha2 and beta messenger RNAs in the cultured granule cells. These findings suggest that granule cells in culture express glycine receptor isoforms containing alpha2 picrotoxinin-sensitive and alpha2/beta picrotoxinin-insensitive receptors.

Age-related glycine receptor subunit changes in the cochlear nucleus of Fischer-344 rats

Previous studies have shown that levels of binding for the strychnine-sensitive glycine receptor in the cochlear nucleus (CN) of Fischer (F344) rats decrease with age. Given the major role glycine plays in normal CN function, changes in glycine-receptor activity may contribute to central presbycusis. To further evaluate the impact of age on glycine receptors, in situ hybridization was used to assess, in three age groups of F344 rats, changes in levels of gene expression for four of its subunits. When compared with the 3-month-old rats, expression of mRNAs for alpha1 and beta subunits in the anteroventral CN decreased significantly in the 18- and 27-month-old age groups, while mRNA expression for the alpha2 subunit increased. If protein expressions are similar, these subunit changes may alter the function of glycine receptors, thereby affecting binding to its ligands.

Inhibitory effects of corymine, an alkaloidal component from the leaves of Hunteria zeylanica, on glycine receptors expressed in Xenopus oocytes

We previously reported that corymine, an alkaloidal compound extracted from the leaves of Hunteria zeylanica native to Thailand, potentiated convulsions induced by either picrotoxin or strychnine. Therefore, to clarify the mechanism of action of corymine, the effects of corymine on gamma-aminobutyric acid (GABA) and glycine receptors were examined. We used Xenopus oocytes expressing these receptors and the two-electrode voltage-clamp method. The receptors expressed in oocytes injected with rat brain and spinal cord RNA showed the pharmacological properties of GABAA and glycine receptors, respectively. Corymine (1-100 microM) partially (20-30%) reduced the GABA responses in oocytes injected with rat brain RNA, while marked (up to 80%) dose-dependent reductions were observed in the glycine responses in oocytes injected with rat spinal cord RNA. These observations suggest that corymine was more effective against the glycine receptors than the GABA receptors. The ED50 of corymine on the glycine response was 10.8 microM. Corymine, at 30 microM, caused a shift to the right, with a lower maximal response, of the glycine concentration-response curve. This indicated that the action of corymine on glycine receptors is neither competitive nor purely non-competitive. These observations suggest that a binding site other than the glycine recognition site of the glycine receptors is the site of action of corymine.

Embryonic rat spinal cord neurons change expression of glycine receptor subtypes during development in vitro

The expression of functional glycine receptors (GlyRs) by embryonic rat spinal cord neurons during development in vitro was investigated using whole-cell patch-clamp recordings. Functional GlyRs were expressed by most neurons within 1 day in vitro, and by all neurons from 4 days onward. However, the extent to which responses to glycine were blocked by the antagonist strychnine differed significantly between the first few days and 8 days in culture. Responses to glycine by neurons during the first few days in culture exhibited significantly less blockade by strychnine than those in neurons after 1 week in culture. Responses to glycine at both ages reflected an increased conductance to chloride ions, ruling out involvement of N-methyl-D-aspartate type glutamate receptors, and were not due to cross activation of gamma-aminobutyric acid receptors. Monoclonal antibody 4a, which recognizes multiple subtypes of rat GlyR alpha subunits, labeled most neurons as early as 1 day in vitro, confirming that neurons express some form of GlyR alpha subunits by the first day in culture. These results show that rat spinal cord neurons express GlyRs early in their differentiation in vitro, and they suggest that individual neurons express as functional, cell-surface GlyRs a strychnine-insensitive isoform of the GlyR, possibly the previously described alpha 2* subunit. In addition, these results indicate that the expression of GlyR isoforms changes from predominantly a strychnine-insensitive isoform to other, strychnine-sensitive isoform(s) GlyR during development in vitro.

Expression and mRNA splicing of glycine receptor subunits and gephyrin during neuronal differentiation of P19 cells in vitro, studied by RT-PCR and immunocytochemistry

The mouse EC cell line P19, differentiating in vitro into neural cell types under the influence of retinoic acid, represents a well established model system for neurogenesis. In this system the expression of the alpha (alpha 1-alpha 3) and beta subunits of the inhibitory glycine receptor (GlyR) and of gephyrin as well as their mRNA splice variants was analyzed by RT-PCR and by immunocytochemistry. In the course of neuronal differentiation of P19 cells mRNA of GlyR beta is constitutively expressed, GlyR alpha 1 and alpha 2 are induced and GlyR alpha 3 was not detected. From the three gephyrin transcripts known to be differently spliced in the C3/C4 cassette region, the C3 transcript was found at all stages while the C4 transcript was not detectable. The insert-free form was measurable in P19 cells only 3-4 days post induction by retinoic acid. In addition a GlyR beta splice variant and a fourth gephyrin transcript were detected. Primary glial cells do not contain significant amounts of GlyR alpha subunits while in primary neuronal cells transcripts of GlyR alpha 2 were found as well as the mRNA of the GlyR beta subunit and of gephyrin. PC12 cells do not express glycine receptor genes but do express gephyrin. Immunocytochemistry confirmed the constitutive expression of gephyrin at the protein level, whereas GlyR antigens could only be detected in islets of the 'P19 neurons'. In conclusion, P19 and primary neuronal cells but not PC12 cells express the transcripts of glycine receptor components, necessary to generate functional receptors.

Glycine-activated whole cell and single channel currents in rat cerebellar granule cells in culture

The patch clamp technique was used to study whole cell and single channel currents evoked by glycine in cerebellar granule cells in culture. Whole cell concentration response curve gave a Kd value for glycine of 73 microM and a Hill slope of 1.58. Glycine-activated currents reversed close to the predicted Cl- equilibrium potential. The responses to glycine were antagonized by strychnine and picrotoxin with an IC50 of 58 nM and 172 microM, respectively. Furthermore, glycine-evoked currents were potentiated by zinc in a dose-dependent way. In outside-out membrane patches, glycine opened channels with conductances of 32, 52, 84 and 96 pS. The most frequently occurring was the 52 pS channel. The single channel current/voltage relationship was linear in the potential range between -60 and 60 mV. The 52, 84 and 96 pS channels exhibited prolonged openings whereas the 32 pS was characterized by fast (< 10 ms) openings. Open and closed time histograms of the 52 pS channel could be fitted with the sum of two or three exponentials, respectively, whereas burst duration histograms could be fitted with the sum of two exponentials. Glycine current density change drastically during days in culture, the maximal expression being between day 4 and 7, suggesting that the expression of glycine receptor channels is developmentally regulated.