Glycine receptors containing the alpha4 subunit in the embryonic sympathetic nervous system, spinal cord and male genital ridge

Investigation of central pattern generators in the spinal cord of chicken embryos

For most quadrupeds, locomotion involves alternating movements of the fore- and hindlimbs. In birds, however, while walking generally involves alternating movements of the legs, to generate lift and thrust, the wings are moved synchronously with each other. Neural circuits in the spinal cord, referred to as central pattern generators (CPGs), are the source of the basic locomotor rhythms and patterns. Given the differences in the patterns of movement of the wings and legs, it is likely that the neuronal components and connectivity of the CPG that coordinates wing movements differ from those that coordinate leg movements. In this study, we used in vitro preparations of embryonic chicken spinal cords (E11-E14) to compare the neural responses of spinal CPGs that control and coordinate wing flapping with those that control alternating leg movements. We found that in response to N-methyl-D-aspartate (NMDA) or a combination of NMDA and serotonin (5-HT), the intact chicken spinal cord produced rhythmic outputs that were synchronous both bilaterally and between the wing and leg segments. Despite this, we found that this rhythmic output was disrupted by an antagonist of glycine receptors in the lumbosacral (legs), but not the brachial (wing) segments. Thus, our results provide evidence of differences between CPGs that control the wings and legs in the spinal cord of birds.

The effect of psychoactive bacteria, Bifidobacterium longum Rosell®-175 and Lactobacillus rhamnosus JB-1, on brain proteome profiles in mice

RATIONALE

The gut microbiota may play an important role in the development and functioning of the mammalian central nervous system. The assumption of the experiment was to prove that the use of probiotic bacterial strains in the diet of mice modifies the expression of brain proteins involved in metabolic and immunological processes.

OBJECTIVES AND RESULTS

Albino Swiss mice were administered with Bifidobacterium longum Rosell®-175 or Lactobacillus rhamnosus JB-1 every 24 h for 28 days. Protein maps were prepared from hippocampal homogenates of euthanized mice. Selected proteins that were statistically significant were purified and concentrated and identified using MALDI-TOF mass spectrometry. Among the analysed samples, 13 proteins were identified. The mean volumes of calcyon, secreted frizzled-associated protein 3, and catalase in the hippocampus of mice from both experimental groups were statistically significantly higher than in the control group. In mice supplemented with Lactobacillus rhamnosus JB-1, a lower mean volume of fragrance binding protein 2, shadow of prion protein, and glycine receptor α4 subunit was observed compared to the control.

CONCLUSION

The psychobiotics Bifidobacterium longum Rosell®-175 and Lactobacillus rhamnosus JB-1enhances expression of proteins involved in the activation and maturation of nerve cells, as well as myelination and homeostatic regulation of neurogenesis in mice. The tested psychobiotics cause a decrease in the expression of proteins associated with CNS development and in synaptic transmission, thereby reducing the capacity for communication between nerve cells. The results of the study indicate that psychobiotic bacteria can be used in auxiliary treatment of neurological disorders.

Glycine: The Smallest Anti-Inflammatory Micronutrient

Glycine is a non-essential amino acid with many functions and effects. Glycine can bind to specific receptors and transporters that are expressed in many types of cells throughout an organism to exert its effects. There have been many studies focused on the anti-inflammatory effects of glycine, including its abilities to decrease pro-inflammatory cytokines and the concentration of free fatty acids, to improve the insulin response, and to mediate other changes. However, the mechanism through which glycine acts is not clear. In this review, we emphasize that glycine exerts its anti-inflammatory effects throughout the modulation of the expression of nuclear factor kappa B (NF-κB) in many cells. Although glycine is a non-essential amino acid, we highlight how dietary glycine supplementation is important in avoiding the development of chronic inflammation.

Comprehensive behavioral analyses of mice with a glycine receptor alpha 4 deficiency

Glycine receptors (GlyRs) are ligand-gated chloride channels comprising alpha (α1-4) and β subunits. The GlyR subunits play major roles in the mammalian central nervous system, ranging from regulating simple sensory information to modulating higher-order brain function. Unlike the other GlyR subunits, GlyR α4 receives relatively little attention because the human ortholog lacks a transmembrane domain and is thus considered a pseudogene. A recent genetic study reported that the GLRA4 pseudogene locus on the X chromosome is potentially involved in cognitive impairment, motor delay and craniofacial anomalies in humans. The physiologic roles of GlyR α4 in mammal behavior and its involvement in disease, however, are not known. Here we examined the temporal and spatial expression profile of GlyR α4 in the mouse brain and subjected Glra4 mutant mice to a comprehensive behavioral analysis to elucidate the role of GlyR α4 in behavior. The GlyR α4 subunit was mainly enriched in the hindbrain and midbrain, and had relatively lower expression in the thalamus, cerebellum, hypothalamus, and olfactory bulb. In addition, expression of the GlyR α4 subunit gradually increased during brain development. Glra4 mutant mice exhibited a decreased amplitude and delayed onset of the startle response compared with wild-type littermates, and increased social interaction in the home cage during the dark period. Glra4 mutants also had a low percentage of entries into open arms in the elevated plus-maze test. Although mice with GlyR α4 deficiency did not show motor and learning abnormalities reported to be associated in human genomics studies, they exhibited behavioral changes in startle response and social and anxiety-like behavior. Our data clarify the spatiotemporal expression pattern of the GlyR α4 subunit and suggest that glycinergic signaling modulates social, startle, and anxiety-like behaviors in mice.

Positive Allosteric Modulators of Glycine Receptors and Their Potential Use in Pain Therapies

Glycine receptors are ligand-gated ion channels that mediate synaptic inhibition throughout the mammalian spinal cord, brainstem, and higher brain regions. They have recently emerged as promising targets for novel pain therapies due to their ability to produce antinociception by inhibiting nociceptive signals within the dorsal horn of the spinal cord. This has greatly enhanced the interest in developing positive allosteric modulators of glycine receptors. Several pharmaceutical companies and research facilities have attempted to identify new therapeutic leads by conducting large-scale screens of compound libraries, screening new derivatives from natural sources, or synthesizing novel compounds that mimic endogenous compounds with antinociceptive activity. Advances in structural techniques have also led to the publication of multiple high-resolution structures of the receptor, highlighting novel allosteric binding sites and providing additional information for previously identified binding sites. This has greatly enhanced our understanding of the functional properties of glycine receptors and expanded the structure activity relationships of novel pharmacophores. Despite this, glycine receptors are yet to be used as drug targets due to the difficulties in obtaining potent, selective modulators with favorable pharmacokinetic profiles that are devoid of side effects. This review presents a summary of the structural basis for how current compounds cause positive allosteric modulation of glycine receptors and discusses their therapeutic potential as analgesics. SIGNIFICANCE STATEMENT: Chronic pain is a major cause of disability, and in Western societies, this will only increase as the population ages. Despite the high level of prevalence and enormous socioeconomic burden incurred, treatment of chronic pain remains limited as it is often refractory to current analgesics, such as opioids. The National Institute for Drug Abuse has set finding effective, safe, nonaddictive strategies to manage chronic pain as their top priority. Positive allosteric modulators of glycine receptors may provide a therapeutic option.

Characterization of the Zebrafish Glycine Receptor Family Reveals Insights Into Glycine Receptor Structure Function and Stoichiometry

To study characterization of zebrafish glycine receptors (zGlyRs), we assessed expression and function of five α- and two ß-subunit encoding GlyR in zebrafish. Our qPCR analysis revealed variable expression during development, while in situ hybridizations uncovered expression in the hindbrain and spinal cord; a finding consistent with the reported expression of GlyR subunits in these tissues from other organisms. Electrophysiological recordings using Xenopus oocytes revealed that all five α subunits form homomeric receptors activated by glycine, and inhibited by strychnine and picrotoxin. In contrast, ß subunits only formed functional heteromeric receptors when co-expressed with α subunits. Curiously, the second transmembranes of both ß subunits were found to lack a phenylalanine at the sixth position that is commonly associated with conferring picrotoxin resistance to heteromeric receptors. Consistent with the absence of phenylalanines at the sixth position, heteromeric zGlyRs often lacked significant picrotoxin resistance. Subsequent efforts revealed that resistance to picrotoxin in both zebrafish and human heteromeric GlyRs involves known residues within transmembrane 2, as well as previously unknown residues within transmembrane 3. We also found that a dominant mutation in human GlyRα1 that gives rise to hyperekplexia, and recessive mutations in zebrafish GlyRßb that underlie the bandoneon family of motor mutants, result in reduced receptor function. Lastly, through the use of a concatenated construct we demonstrate that zebrafish heteromeric receptors assemble with a stoichiometry of 3α:2ß. Collectively, our findings have furthered our knowledge regarding the assembly of heteromeric receptors, and the molecular basis of ß subunit-conferred picrotoxin resistance. These results should aid in future investigations of glycinergic signaling in zebrafish and mammals.

Structure/Function Studies of the α4 Subunit Reveal Evolutionary Loss of a GlyR Subtype Involved in Startle and Escape Responses

Inhibitory glycine receptors (GlyRs) are pentameric ligand-gated anion channels with major roles in startle disease/hyperekplexia (GlyR α1), cortical neuronal migration/autism spectrum disorder (GlyR α2), and inflammatory pain sensitization/rhythmic breathing (GlyR α3). However, the role of the GlyR α4 subunit has remained enigmatic, because the corresponding human gene (GLRA4) is thought to be a pseudogene due to an in-frame stop codon at position 390 within the fourth membrane-spanning domain (M4). Despite this, a recent genetic study has implicated GLRA4 in intellectual disability, behavioral problems and craniofacial anomalies. Analyzing data from sequenced genomes, we found that GlyR α4 subunit genes are predicted to be intact and functional in the majority of vertebrate species—with the exception of humans. Cloning of human GlyR α4 cDNAs excluded alternative splicing and RNA editing as mechanisms for restoring a full-length GlyR α4 subunit. Moreover, artificial restoration of the missing conserved arginine (R390) in the human cDNA was not sufficient to restore GlyR α4 function. Further bioinformatic and mutagenesis analysis revealed an additional damaging substitution at K59 that ablates human GlyR α4 function, which is not present in other vertebrate GlyR α4 sequences. The substitutions K59 and X390 were also present in the genome of an ancient Denisovan individual, indicating that GLRA4 has been a pseudogene for at least 30,000–50,000 years. In artificial synapses, we found that both mouse and gorilla α4β GlyRs mediate synaptic currents with unusually slow decay kinetics. Lastly, to gain insights into the biological role of GlyR α4 function, we studied the duplicated genes glra4a and glra4b in zebrafish. While glra4b expression is restricted to the retina, using a novel tol2-GAL4FF gene trap line (SAIGFF16B), we found that the zebrafish GlyR α4a subunit gene (glra4a) is strongly expressed in spinal cord and hindbrain commissural neurones. Using gene knockdown and a dominant-negative GlyR α4a^R278Q mutant, we found that GlyR α4a contributes to touch-evoked escape behaviors in zebrafish. Thus, although GlyR α4 is unlikely to be involved in human startle responses or disease states, this subtype may contribute to escape behaviors in other organisms.

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.

Structure-Function Analysis of the GlyR α2 Subunit Autism Mutation p.R323L Reveals a Gain-of-Function

Glycine receptors (GlyRs) containing the α2 subunit regulate cortical interneuron migration. Disruption of the GlyR α2 subunit gene (Glra2) in mice leads to disrupted dorsal cortical progenitor homeostasis, leading to a depletion of projection neurons and moderate microcephaly in newborn mice. In humans, rare variants in GLRA2, which is located on the X chromosome, are associated with autism spectrum disorder (ASD) in the hemizygous state in males. These include a microdeletion (GLRA2∆ex8-9) and missense mutations in GLRA2 (p.N109S and p.R126Q) that impair cell-surface expression of GlyR α2, and either abolish or markedly reduce sensitivity to glycine. We report the functional characterization of a third missense variant in GLRA2 (p.R323L), associated with autism, macrocephaly, epilepsy and hypothyroidism in a female proband. Using heterosynapse and macroscopic current recording techniques, we reveal that GlyR α2^R323L exhibits reduced glycine sensitivity, but significantly increased inhibitory postsynaptic current (IPSC) rise and decay times. Site-directed mutagenesis revealed that the nature of the amino acid switch at position 323 is critical for impairment of GlyR function. Single-channel recordings revealed that the conductance of α2^R323Lβ channels was higher than α2β channels. Longer mean opening durations induced by p.R323L may be due to a change in the gating pathway that enhances the stability of the GlyR open state. The slower synaptic decay times, longer duration active periods and increase in conductance demonstrates that the GlyR α2 p.R323L mutation results in an overall gain of function, and that GlyR α2 mutations can be pathogenic in the heterozygous state in females.

Structure and Pharmacologic Modulation of Inhibitory Glycine Receptors

Glycine receptors (GlyR) are inhibitory Cys-loop ion channels that contribute to the control of excitability along the central nervous system (CNS). GlyR are found in the spinal cord and brain stem, and more recently they were reported in higher regions of the CNS such as the hippocampus and nucleus accumbens. GlyR are involved in motor coordination, respiratory rhythms, pain transmission, and sensory processing, and they are targets for relevant physiologic and pharmacologic modulators. Several studies with protein crystallography and cryoelectron microscopy have shed light on the residues and mechanisms associated with the activation, blockade, and regulation of pentameric Cys-loop ion channels at the atomic level. Initial studies conducted on the extracellular domain of acetylcholine receptors, ion channels from prokaryote homologs-Erwinia chrysanthemi ligand-gated ion channel (ELIC), Gloeobacter violaceus ligand-gated ion channel (GLIC)-and crystallized eukaryotic receptors made it possible to define the overall structure and topology of the Cys-loop receptors. For example, the determination of pentameric GlyR structures bound to glycine and strychnine have contributed to visualizing the structural changes implicated in the transition between the open and closed states of the Cys-loop receptors. In this review, we summarize how the new information obtained in functional, mutagenesis, and structural studies have contributed to a better understanding of the function and regulation of GlyR.

Defects of the Glycinergic Synapse in Zebrafish

Glycine mediates fast inhibitory synaptic transmission. Physiological importance of the glycinergic synapse is well established in the brainstem and the spinal cord. In humans, the loss of glycinergic function in the spinal cord and brainstem leads to hyperekplexia, which is characterized by an excess startle reflex to sudden acoustic or tactile stimulation. In addition, glycinergic synapses in this region are also involved in the regulation of respiration and locomotion, and in the nociceptive processing. The importance of the glycinergic synapse is conserved across vertebrate species. A teleost fish, the zebrafish, offers several advantages as a vertebrate model for research of glycinergic synapse. Mutagenesis screens in zebrafish have isolated two motor defective mutants that have pathogenic mutations in glycinergic synaptic transmission: bandoneon (beo) and shocked (sho). Beo mutants have a loss-of-function mutation of glycine receptor (GlyR) β-subunit b, alternatively, sho mutant is a glycinergic transporter 1 (GlyT1) defective mutant. These mutants are useful animal models for understanding of glycinergic synaptic transmission and for identification of novel therapeutic agents for human diseases arising from defect in glycinergic transmission, such as hyperekplexia or glycine encephalopathy. Recent advances in techniques for genome editing and for imaging and manipulating of a molecule or a physiological process make zebrafish more attractive model. In this review, we describe the glycinergic defective zebrafish mutants and the technical advances in both forward and reverse genetic approaches as well as in vivo visualization and manipulation approaches for the study of the glycinergic synapse in zebrafish.

Inter- and Intra-Subunit Butanol/Isoflurane Sites of Action in the Human Glycine Receptor

Glycine receptors (GlyRs) mediate inhibitory neurotransmission and are targets for alcohols and anesthetics in brain. GlyR transmembrane (TM) domains contain critical residues for alcohol/anesthetic action: amino acid A288 in TM3 forms crosslinks with TM1 (I229) in the adjacent subunit as well as TM2 (S267) and TM4 (Y406, W407, I409, Y410) in the same subunit. We hypothesized that these residues may participate in intra-subunit and inter-subunit sites of alcohol/anesthetic action. The following double and triple mutants of GLRA1 cDNA (encoding human glycine receptor alpha 1 subunit) were injected into Xenopus laevis oocytes: I229C/A288C, I229C/A288C/C290S, A288C/Y406C, A288C/W407C, A288C/I409C, and A288C/Y410C along with the corresponding single mutants and wild-type GLRA1. Butanol (22 mM) or isoflurane (0.6 mM) potentiation of GlyR-mediated currents before and after application of the cysteine crosslinking agent HgCl₂ (10 μM) was measured using two-electrode voltage clamp electrophysiology. Crosslinking nearly abolished butanol and isoflurane potentiation in the I229C/A288C and I229C/A288C/C290S mutants but had no effect in single mutants or wild-type. Crosslinking also inhibited butanol and isoflurane potentiation in the TM3-4 mutants (A288C/Y406C, A288C/W407C, A288C/I409C, A288C/Y410C) with no effect in single mutants or wild-type. We extracted proteins from oocytes expressing I229C/288C, A288C/Y410C, or wild-type GlyRs, used mass spectrometry to verify their expression and possible inter-subunit dimerization, plus immunoblotting to investigate the biochemical features of proposed crosslinks. Wild-type GlyR subunits measured about 50 kDa; after crosslinking, the dimeric/monomeric 100:50 kDa band ratio was significantly increased in I229C/288C but not A288C/Y410C mutants or wild-type, providing support for TM1-3 inter-subunit and TM3-4 intra-subunit crosslinking. A GlyR homology model based on the GluCl template provides further evidence for a multi-site model for alcohol/anesthetic interaction with human GLRA1.

Ethanol effects on glycinergic transmission: From molecular pharmacology to behavior responses

It is well accepted that ethanol is able to produce major health and economic problems associated to its abuse. Because of its intoxicating and addictive properties, it is necessary to analyze its effect in the central nervous system. However, we are only now learning about the mechanisms controlling the modification of important membrane proteins such as ligand-activated ion channels by ethanol. Furthermore, only recently are these effects being correlated to behavioral changes. Current studies show that the glycine receptor (GlyR) is a susceptible target for low concentrations of ethanol (5-40mM). GlyRs are relevant for the effects of ethanol because they are found in the spinal cord and brain stem where they primarily express the α1 subunit. More recently, the presence of GlyRs was described in higher regions, such as the hippocampus and nucleus accumbens, with a prevalence of α2/α3 subunits. Here, we review data on the following aspects of ethanol effects on GlyRs: (1) direct interaction of ethanol with amino acids in the extracellular or transmembrane domains, and indirect mechanisms through the activation of signal transduction pathways; (2) analysis of α2 and α3 subunits having different sensitivities to ethanol which allows the identification of structural requirements for ethanol modulation present in the intracellular domain and C-terminal region; (3) Genetically modified knock-in mice for α1 GlyRs that have an impaired interaction with G protein and demonstrate reduced ethanol sensitivity without changes in glycinergic transmission; and (4) GlyRs as potential therapeutic targets.

Glycine receptors containing α2 or α3 subunits regulate specific ethanol-mediated behaviors

Glycine receptors (GlyRs) are broadly expressed in the central nervous system. Ethanol enhances the function of brain GlyRs, and the GlyRα1 subunit is associated with some of the behavioral actions of ethanol, such as loss of righting reflex. The in vivo role of GlyRα2 and α3 subunits in alcohol responses has not been characterized despite high expression levels in the nucleus accumbens and amygdala, areas that are important for the rewarding properties of drugs of abuse. We used an extensive panel of behavioral tests to examine ethanol actions in mice lacking Glra2 (the gene encoding the glycine receptor alpha 2 subunit) or Glra3 (the gene encoding the glycine receptor alpha 3 subunit). Deletion of Glra2 or Glra3 alters specific ethanol-induced behaviors. Glra2 knockout mice demonstrate reduced ethanol intake and preference in the 24-hour two-bottle choice test and increased initial aversive responses to ethanol and lithium chloride. In contrast, Glra3 knockout mice show increased ethanol intake and preference in the 24-hour intermittent access test and increased development of conditioned taste aversion to ethanol. Mutants and wild-type mice consumed similar amounts of ethanol in the limited access drinking in the dark test. Other ethanol effects, such as anxiolysis, motor incoordination, loss of righting reflex, and acoustic startle response, were not altered in the mutants. The behavioral changes in mice lacking GlyRα2 or α3 subunits were distinct from effects previously observed in mice with knock-in mutations in the α1 subunit. We provide evidence that GlyRα2 and α3 subunits may regulate ethanol consumption and the aversive response to ethanol.

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.

Principles of agonist recognition in Cys-loop receptors

Cys-loop receptors are ligand-gated ion channels that are activated by a structurally diverse array of neurotransmitters, including acetylcholine, serotonin, glycine, and GABA. After the term "chemoreceptor" emerged over 100 years ago, there was some wait until affinity labeling, molecular cloning, functional studies, and X-ray crystallography experiments identified the extracellular interface of adjacent subunits as the principal site of agonist binding. The question of how subtle differences at and around agonist-binding sites of different Cys-loop receptors can accommodate transmitters as chemically diverse as glycine and serotonin has been subject to intense research over the last three decades. This review outlines the functional diversity and current structural understanding of agonist-binding sites, including those of invertebrate Cys-loop receptors. Together, this provides a framework to understand the atomic determinants involved in how these valuable therapeutic targets recognize and bind their ligands.

Intracellular glycine receptor function facilitates glioma formation in vivo

The neuronal function of Cys-loop neurotransmitter receptors is established; however, their role in non-neuronal cells is poorly defined. As brain tumors accumulate the neurotransmitter glycine, we studied expression and function of glycine receptors (GlyR) in glioma cells. Human brain tumor biopsies selectively expressed GlyR subunits with nuclear import signal (NLS, α1 and α3). The mouse glioma cell line GL261 expressed GlyR α1, and knock-down of α1 protein expression impaired self-renewal capacity and tumorigenicity of GL261 glioma cells as evidenced by the neurosphere assay and GL261 cell inoculation in vivo, respectively. We furthermore show that the pronounced tumorigenic effect of GlyR α1 relies on a new intracellular signaling function that depends on the NLS region in the large cytosolic loop and impacts on GL261 glioma cell gene regulation. Stable expression of GlyR α1 and α3 loops rescued self-renewal capacity of GlyR α1 knock-down cells, which demonstrates their functional equivalence. The new intracellular signaling function identified here goes beyond the well-established role of GlyRs as neuronal ligand-gated ion channels and defines NLS-containing GlyRs as novel potential targets for brain tumor therapies.

Distribution of the glycine receptor β-subunit in the mouse CNS as revealed by a novel monoclonal antibody

Inhibitory glycine receptors (GlyRs) are composed of homologous α- (α1-4) and β-subunits. The β-subunits (GlyRβ) interact via their large cytosolic loops with the postsynaptic scaffolding protein gephyrin and are therefore considered essential for synaptic localization. In situ hybridization studies indicate a widespread distribution of GlyRβ transcripts throughout the mammalian central nervous system (CNS), whereas GlyRα mRNAs and proteins display more restricted expression patterns. Here we report the generation of a monoclonal antibody that specifically recognizes rodent GlyRβ (mAb-GlyRβ) and does not exhibit crossreactivity with any of the GlyRα1-4 subunits. Immunostaining with this antibody revealed high densities of punctate GlyRβ immunoreactivity at inhibitory synapses in mouse spinal cord, brainstem, midbrain, and olfactory bulb but not in the neocortex, cerebellum, or hippocampus. This contrasts the abundance of GlyRβ transcripts in all major regions of the rodent brain and suggests that GlyRβ protein levels are regulated posttranscriptionally. When mAb-GlyRβ was used in double-labeling experiments with GlyRα1-, α2-, α3-, or α4-specific antibodies to examine the colocalization of GlyRβ with these GlyR subunits in the mouse retina, >90% of the GlyRα1-3 clusters detected were found to be GlyRβ-immunoreactive. A subset (about 50%) of the GlyRα4 puncta in the inner plexiform layer, however, was found to lack GlyRβ and gephyrin immunostaining. These GlyRα4-only clusters were apposed to bassoon immunoreactivity and hence synaptically localized. Their existence points to a gephyrin-independent synaptic localization mechanism for a minor subset of GlyRs.

Inhibitory glycine receptors: an update

Strychnine-sensitive glycine receptors (GlyRs) mediate synaptic inhibition in the spinal cord, brainstem, and other regions of the mammalian central nervous system. In this minireview, we summarize our current view of the structure, ligand-binding sites, and chloride channel of these receptors and discuss recently emerging functions of distinct GlyR isoforms. GlyRs not only regulate the excitability of motor and afferent sensory neurons, including pain fibers, but also are involved in the processing of visual and auditory signals. Hence, GlyRs constitute promising targets for the development of therapeutically useful compounds.

Glycine receptor mutants of the mouse: what are possible routes of inhibitory compensation?

Defects in glycinergic inhibition result in a complex neuromotor disorder in humans known as hyperekplexia (OMIM 149400) with similar phenotypes in rodents characterized by an exaggerated startle reflex and hypertonia. Analogous to genetic defects in humans single point mutations, microdeletions, or insertions in the Glra1 gene but also in the Glrb gene underlie the pathology in mice. The mutations either localized in the α (spasmodic, oscillator, cincinnati, Nmf11) or the β (spastic) subunit of the glycine receptor (GlyR) are much less tolerated in mice than in humans, leaving the question for the existence of different regulatory elements of the pathomechanisms in humans and rodents. In addition to the spontaneous mutations, new insights into understanding of the regulatory pathways in hyperekplexia or glycine encephalopathy arose from the constantly increasing number of knock-out as well as knock-in mutants of GlyRs. Over the last five years, various efforts using in vivo whole cell recordings provided a detailed analysis of the kinetic parameters underlying glycinergic dysfunction. Presynaptic compensation as well as postsynaptic compensatory mechanisms in these mice by other GlyR subunits or GABA_A receptors, and the role of extra-synaptic GlyRs is still a matter of debate. A recent study on the mouse mutant oscillator displayed a novel aspect for compensation of functionality by complementation of receptor domains that fold independently. This review focuses on defects in glycinergic neurotransmission in mice discussed with the background of human hyperekplexia en route to strategies of compensation.

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.

Linking of Glycine Receptor Transmembrane Segments Three and Four Allows Assignment of Intrasubunit-Facing Residues

Glycine receptors (GlyRs) are pentameric ligand-gated ion channels that mediate inhibitory neurotransmission in the brain and spinal cord and are targets of alcohols and anesthetics. The transmembrane (TM) domain of GlyR subunits is composed of four α-helical segments (TM1-4), but there are conflicting data about the orientation of TM3 and TM4 and, therefore, also the proximity of residues (e.g., A288) that are important for alcohol and anesthetic effects. In the present study, we investigated the proximity of A288 in TM3 to residues in TM4 from M404 to K411. We generated eight double mutant GlyRs (A288C/M404C, A288C/F405C, A288C/Y406C, A288C/W407C, A288C/I408C, A288C/I409C, A288C/Y410C, and A288C/K411C), as well as the corresponding single mutants, and expressed them in Xenopus laevis oocytes. To measure glycine responses, we used two-electrode voltage clamp electrophysiology. We built homology models of the GlyR using structures of the nicotinic acetylcholine receptor (nAChR) and a prokaryotic ion channel (Gloeobacter violaceus, GLIC) as templates, and asked which model best fit our experimental data. Application of the cross-linking reagent HgCl(2) in the closed state produced a leftward shift in the glycine concentration-response curves of the A288C/W407C and A288C/Y410C mutants, suggesting they are able to form cross-links. In addition, when HgCl(2) was coapplied with glycine, responses were changed in the A288C/Y406C, A288C/I409C, and A288C/Y410C double mutants, suggesting that agonist-induced rotation of TM4 allows A288C/Y406C and A288C/I409C to cross-link. These results are consistent with a model of GlyR, based on nAChR, in which A288, Y406, W407, I409, and Y410 face into a four-helical bundle.

Glycine receptor expression in the forebrain of male AA/ANA rats

Ethanol is known to directly interact with the glycine receptor (GlyR). GlyRs are membrane proteins and are constituted as either alpha-homomers or alpha-beta heteromers with a subunit stoichiometry of 2 alpha 3 beta. Previous studies by our group have suggested a role for GlyRs and its endogenous ligands glycine and taurine in the mesolimbic dopamine activating and reinforcing effects of ethanol. Here we use quantitative PCR (qPCR) to compare the relative GlyR expression in Alko Alcohol/Non-Alcohol (AA/ANA) rats. These animals have been selectively bred to create distinct populations regarding alcohol consumption and preference, presumably mainly due to genetic differences. The aim of this study was to examine the relative gene expression of GlyR subunits (alpha1-3 and beta) in different brain areas and relate it to alcohol consumption. The hypothesis was that AA/ANA rats are differently disposed to ethanol consumption due to their GlyR set-ups and/or compositions. Results from the present study indicate that alpha2 is the most widely expressed alpha-subunit in the forebrain regions and that the alpha 2 beta-heteromer seems to be the most common subunit composition in this part of the CNS. Despite displaying different drinking behaviours the anticipated differences in mRNA expression were few. However, correlations found between alcohol consumption and/or preference and GlyR expression support a role for GlyRs in alcohol consumption. Tentative differences between AA and ANA animals related to GlyR transmission could therefore lie in, for example, the regulation of the levels of the endogenous ligand(s) for the receptor or in mechanisms downstream to GlyR activation.

Glycine and glycine receptor signalling in non-neuronal cells

Glycine is an inhibitory neurotransmitter acting mainly in the caudal part of the central nervous system. Besides this neurotransmitter function, glycine has cytoprotective and modulatory effects in different non-neuronal cell types. Modulatory effects were mainly described in immune cells, endothelial cells and macroglial cells, where glycine modulates proliferation, differentiation, migration and cytokine production. Activation of glycine receptors (GlyRs) causes membrane potential changes that in turn modulate calcium flux and downstream effects in these cells. Cytoprotective effects were mainly described in renal cells, hepatocytes and endothelial cells, where glycine protects cells from ischemic cell death. In these cell types, glycine has been suggested to stabilize porous defects that develop in the plasma membranes of ischemic cells, leading to leakage of macromolecules and subsequent cell death. Although there is some evidence linking these effects to the activation of GlyRs, they seem to operate in an entirely different mode from classical neuronal subtypes.

Glycinergic transmission in the Mammalian retina

Glycine and γ-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the retina. Approximately half of the amacrine cells release glycine at their synapses with bipolar, other amacrine, and ganglion cells. Glycinergic amacrine cells are small-field amacrine cells with vertically oriented dendrites and comprise more than 10 different morphological types. The retinal distributions of glycine receptor (GlyR) α1, α2, α3 and α4 subtypes have been mapped with subunit-specific antibodies. GlyRs were clustered at postsynaptic hot spots which showed selective distributions for the different subunits. As a rule, only one α subunit was expressed at a given postsynaptic site. The kinetic properties of GlyRs were measured by recording spontaneous inhibitory postsynaptic currents (sIPSCs) from identified retinal neurons in wild-type, Glra1^spd-ot, Glra2 and Glra3 knockout mice. From observed differences of sIPSCs in wild-type and mutant mice, the cell-type specific subunit composition of GlyRs could be defined. OFF-cone bipolar cells and A-type ganglion cells receive prominent glycinergic input with fast kinetics that is mainly mediated by α1β GlyRs (decay time constant τ ∼ 5 ms). By contrast, AII amacrine cells express α3β GlyRs with medium fast kinetics (τ ∼ 11 ms). Narrow-field (NF) and wide-field amacrine cells contain predominantly α2β GlyRs with slow kinetics (τ ∼ 27 ms). Lastly, ON-starburst, narrow-field and wide-field amacrine cells in Glra2 knockout mice express α4β GlyRs with very slow kinetics (τ ∼ 70 ms).

Glycinergic input of widefield, displaced amacrine cells of the mouse retina

Glycine receptors (GlyRs) of displaced amacrine cells of the mouse retina were analysed using whole cell recordings and immunocytochemical staining with subunit-specific antibodies. During the recordings the cells were filled with a fluorescent tracer and 11 different morphological types could be identified. The studies were performed in wild-type mice and in mutant mice deficient in the GlyRalpha1 (Glra1(spd-ot), 'oscillator' mouse), the GlyRalpha2 (Glra2(-/-)) and the GlyRalpha3 subunit (Glra3(-/-)). Based on their responses to the application of exogenous glycine in the retinas of wild-type and mutant mice, the cells were grouped into three major classes: group I cells (comprising the morphological types MA-S5, MA-S1, MA-S1/S5, A17, PA-S1, PA-S5 and WA-S1), group II cells (comprising the morphological types PA-S4, WA-S3 and WA-multi) and ON-starburst cells. For further analysis, spontaneous inhibitory postsynaptic currents (sIPSCs) were measured both in wild-type and mutant mouse retinas. Glycinergic sIPSCs and glycine induced currents of group I cells remained unaltered across wild-type and the three mutant mice (mean decay time constant of sIPSCs, tau approximately 25 ms). Group II cells showed glycinergic sIPSCs and glycine induced currents in wild-type, Glra1(spd-ot) and Glra3(-/-) mice (tau approximately 25 ms); however, glycinergic currents were absent in group II cells of Glra2(-/-) mice. Glycine induced currents and sIPSCs recorded from ON-starburst amacrine cells did not differ significantly between wild-type and the mutant mouse retinas (tau approximately 50-70 ms). We propose that GlyRs of group II cells are dominated by the alpha2 subunit; GlyRs of ON-starburst amacrine cells appear to be dominated by the alpha4 subunit.

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.

Native glycine receptor subtypes and their physiological roles

The glycine receptor chloride channel (GlyR), a member of the pentameric Cys-loop ion channel receptor family, mediates inhibitory neurotransmission in the spinal cord, brainstem and retina. They are also found presynaptically, where they modulate neurotransmitter release. Functional GlyRs are formed from a total of five subunits (alpha1-alpha4, beta). Although alpha subunits efficiently form homomeric GlyRs in recombinant expression systems, homomeric alpha1, alpha3 and alpha4 GlyRs are weakly expressed in adult neurons. In contrast, alpha2 homomeric GlyRs are abundantly expressed in embryonic neurons, although their numbers decline sharply by adulthood. Numerous lines of biochemical, biophysical, pharmacological and genetic evidence suggest the majority of glycinergic neurotransmission in adults is mediated by heteromeric alpha1beta GlyRs. Immunocytochemical co-localisation experiments suggest the presence of alpha2beta, alpha3beta and alpha4beta GlyRs at synapses in the adult mouse retina. Immunocytochemical and electrophysiological evidence also implicates alpha3beta GlyRs as important mediators of glycinergic inhibitory neurotransmission in nociceptive sensory neuronal circuits in peripheral laminae of the spinal cord dorsal horn. It is yet to be determined why multiple GlyR synaptic subtypes are differentially distributed in these and possibly other locations. The development of pharmacological agents that can discriminate strongly between different beta subunit-containing GlyR isoforms will help to address this issue, and thereby provide important insights into a variety of central nervous system functions including retinal signal processing and spinal pain mechanisms. Finally, agents that selectively potentiate different GlyR isoforms may be useful as therapeutic lead compounds for peripheral inflammatory pain and movement disorders such as spasticity.

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

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

Glycine receptors of A-type ganglion cells of the mouse retina

A-type ganglion cells of the mouse retina represent the visual channel that transfers temporal changes of the outside world very fast and with high fidelity. In this study we combined anatomical and physiological methods in order to study the glycinergic, inhibitory input of A-type ganglion cells. Immunocytochemical studies were performed in a transgenic mouse line whose ganglion cells express green fluorescent protein (GFP). The cells were double labeled for GFP and the four alpha subunits of the glycine receptor (GlyR). It was found that most of the glycinergic input of A-type cells is through fast, alpha1-expressing synapses. Whole-cell currents were recorded from A-type ganglion cells in retinal whole mounts. The response to exogenous application of glycine and spontaneous inhibitory postsynaptic currents (sIPSCs) were measured. By comparing glycinergic currents recorded in wildtype mice and in mice with specific deletions of GlyRalpha subunits (Glra1spd-ot, Glra2-/-, Glra3-/-), the subunit composition of GlyRs of A-type ganglion cells could be further defined. Glycinergic sIPSCs of A-type ganglion cells have fast kinetics (decay time constant tau = 3.9 +/- 2.5 ms, mean +/- SD). Glycinergic sIPSCs recorded in Glra2-/- and Glra3-/- mice did not differ from those of wildtype mice. However, the number of glycinergic sIPSCs was significantly reduced in Glra1spd-ot mice and the remaining sIPSCs had slower kinetics than in wildtype mice. The results show that A-type ganglion cells receive preferentially kinetically fast glycinergic inputs, mediated by GlyRs composed of alpha1 and beta subunits.

Spontaneous IPSCs and glycine receptors with slow kinetics in wide-field amacrine cells in the mature rat retina

The functional properties of glycine receptors were analysed in different types of wide-field amacrine cells, narrowly stratifying cells considered to play a role in larger-scale integration across the retina. The patch-clamp technique was used to record spontaneous IPSCs (spIPSCs) and glycine-evoked patch responses from mature rat retinal slices (4-7 weeks postnatal). Glycinergic spIPSCs were blocked reversibly by strychnine (300 nM). Compared to previously described spIPSCs in AII amacrine cells, the spIPSCs in wide-field amacrine cells displayed a very slow decay time course (tau(fast) approximately 15 ms; tau(slow) approximately 57 ms). The kinetic properties of spIPSCs in whole-cell recordings were paralleled by even slower deactivation kinetics of responses evoked by brief pulses of glycine (3 mm) to outside-out patches from wide-field amacrine cells (tau(fast) approximately 45 ms; tau(slow) approximately 350 ms). Non-stationary noise analysis of patch responses and spIPSCs yielded similar average single-channel conductances (approximately 31 and approximately 34 pS, respectively). Similar, as well as both lower- and higher-conductance levels could be identified from directly observed single-channel gating during the decay phase of spIPSCs and patch responses. These results suggest that the slow glycinergic spIPSCs in wide-field amacrine cells involve alpha2beta heteromeric receptors. Taken together with previous work, the kinetic properties of glycine receptors in different types of amacrine cells display a considerable range that is probably a direct consequence of differential expression of receptor subunits. Unique kinetic properties are likely to differentially shape the glycinergic input to different types of amacrine cells and thereby contribute to distinct integrative properties among these cells.

Characterization of glycinergic synapses in vertebrate retinas

Glycine is one of the essential neurotransmitters modulating visual signals in retina. Glycine activates Cl(-) permeable receptors that conduct either inhibitory or excitatory actions, depending on the Cl(-) electrical-chemical gradient (E (Cl)) positive or negative to the resting potential in the cells. Interestingly, both glycine-induced inhibitory and excitatory responses are present in adult retinas, and the effects are confined in the inner and outer retinal neurons. Glycine inhibits glutamate synapses in the inner plexiform layer (IPL), resulting in shaping light responses in ganglion cells. In contrast, glycine excites horizontal cells and On-bipolar dendrites in the outer plexiform layer (OPL). The function of glycinergic synapse in the outer retina represents the effect of network feedback from a group of centrifugal neurons, glycinergic interplexiform cells. Moreover, immunocytochemical studies identify glycine receptor subunits (alpha1, alpha2, alpha3 and beta) in retinas, forming picrotoxin-sensitive alpha-homomeric and picrotoxin-insensitive alpha/beta-heteromeric receptors. Glycine receptors are modulated by intracellular Ca(2+) and protein kinas C and A pathways. Extracellular Zn(2+) regulates glycine receptors in a concentration-dependent manner, nanomolar Zn(2+) enhancing glycine responses, and micromolar Zn(2+) suppressing glycine responses in retinal neurons. These studies describe the function and mechanism of glycinergic synapses in retinas.

Glycine receptors: recent insights into their structural organization and functional diversity

Strychnine-sensitive glycine receptors (GlyRs) are known to mediate synaptic inhibition in spinal cord, brainstem and other regions of the CNS. During the past 5 years, considerable progress has been made in delineating structural determinants of ligand binding and channel activation in recombinant GlyRs. Furthermore, immunohistochemical and gene inactivation studies have disclosed distinct distributions and functions of differentially expressed GlyR subtypes in retina, hippocampus and the dorsal horn of the spinal cord. Accordingly, GlyRs regulate not only the excitability of motor and sensory neurones, but are also essential for the processing of photoreceptor signals, neuronal development and inflammatory pain sensitization. Hence, these receptors constitute promising targets for the development of clinically useful compounds.

Immunocytochemical study of glycine receptors in the retina of the frog Xenopus laevis

The expression of glycine receptors in the retina of clawed frog, Xenopus laevis was studied immunocytochemically. Glycine receptors (GlyRs), as revealed by means of several different antibodies, were mainly distributed in the inner (IPL) and the outer plexiform layers. Their composition was determined to include alpha2 and alpha3 subunits. Typical punctate appearance and specific lamination in the IPL were seen with each of the antibodies directed against the different GlyRs' subunits. A notion for diversity of the glycine receptors was put forward, according to which the alpha2 and alpha3 subunits are located in different subtypes of glycine synapses.

Glycine receptors in a population of adult mammalian cones

Glycinergic interplexiform cells provide a feedback signal from the inner retina to the outer retina. To determine if cones receive such a signal, glycine was applied on cultured porcine cone photoreceptors recorded with the patch clamp technique. A minor population of cone photoreceptors was found to generate large currents in response to puff application of glycine. These currents reversed close to the calculated equilibrium potential for chloride ions. These glycine-elicited currents were sensitive to strychnine but not to picrotoxin consistent with the expression of alpha-beta-heteromeric glycine receptors. Glycine receptors were also activated by taurine and beta-alanine. The glycine receptor antibody mAb4a labelled a minority of the cone photoreceptors identified by an antibody specific for cone arrestin. Finally, expression of the beta subunit of the glycine receptor was demonstrated by single cell RT-PCR in a similar proportion (approximately 13%) of cone photoreceptors freshly isolated by lectin-panning. The identity of cone photoreceptors was assessed by their specific expression of the cone arrestin mRNA. The population of cone photoreceptors expressing the glycine receptor was not correlated to a specific colour-sensitive subtype as demonstrated by single cell RT-PCR experiments using primers for S opsin, cone arrestin and glycine receptor beta subunit. This glycine receptor expression in a minority of cones defines a new cone population suggesting an unexpected role for glycine in the visual information processing in the outer retina.

Diversity of glycine receptors in the mouse retina: Localization of the α4 subunit

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the retina. Approximately half of the amacrine cells release glycine at their synapses with bipolar, other amacrine, and ganglion cells. Whereas the retinal distributions of glycine receptor (GlyR) subunits alpha1, alpha2, and alpha3 have been mapped, the role of the alpha4 subunit in retinal circuitry remains unclear. A rabbit polyclonal antiserum was raised against a peptide that comprises the C-terminal 14 amino acids of the mouse GlyR alpha4 subunit. Using immunocytochemistry, we localized the alpha4 subunit in the inner plexiform layer (IPL) in brightly fluorescent puncta, which represent postsynaptically clustered GlyRs. This was shown by double-labeling sections for GlyR alpha4 and synaptic markers (bassoon, gephyrin). Double-labeling sections for GlyR alpha4 and the other GlyR alpha subunits shows that they are mostly clustered at different synapses; however, approximately 30% of the alpha4-containing synapses also express the alpha2 subunit. We also studied the pre- and postsynaptic partners at GlyR alpha4-containing synapses and found that displaced (ON-) cholinergic amacrine cells prominently expressed the alpha4 subunit. The density of GlyR alpha4-expressing synapses in wildtype, Glra1(ot/ot), and Glra3(-/-) mouse retinas did not differ significantly. Thus, there is no apparent compensation of the loss of alpha1 or alpha3 subunits by an upregulation of alpha4 subunit gene expression; however, the alpha2 subunit is moderately upregulated.

Mechanisms for picrotoxin block of alpha2 homomeric glycine receptors

It is well known that the convulsant alkaloid picrotoxin (PTX) can inhibit neuronal gamma-aminobutyric acid (GABA) and homomeric glycine receptors (GlyR). However, the mechanism for PTX block of alpha(2) homomeric GlyR is still unclear compared with that of alpha(1) homomeric GlyR, GABA(A), and GABA(C) receptors. Furthermore, PTX effects on GlyR kinetics have been poorly explored at the single-channel level. Hence, we used the patch-clamp technique in the outside-out configuration to investigate the mechanism of PTX suppression of currents carried by alpha(2) homomeric GlyRs stably transfected into Chinese hamster ovary cells. PTX inhibited the alpha(2) homomeric GlyR current elicited by glycine in a concentration-dependent and voltage-independent manner. Both competitive and noncompetitive mechanisms were observed. PTX decreased the mean open time of the GlyR channel in a concentration-dependent manner, suggesting that PTX can block channel openings and bind to the receptor in the open channel conformation. When PTX and glycine were co-applied, a small rebound current was observed during drug washout. Application of PTX during the deactivation phase of glycine-induced currents eliminated the rebound current and accelerated the deactivation time course in a concentration-dependent manner. PTX could not bind to the unbound conformation of GlyR, but could be trapped at its binding site when the channel closed during glycine dissociation. Based on these observations, we propose a kinetic Markov model in which PTX binds to the alpha(2) homomeric GlyR in both the open channel state and the fully liganded closed state. Our data suggest a new allosteric mechanism for PTX inhibition of wild-type homomeric alpha(2) GlyR.

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.