Glycine uptake regulates hippocampal network activity via glycine receptor-mediated tonic inhibition

Supraspinal glycinergic neurotransmission in pain: A scoping review of current literature

The neurotransmitter glycine is an agonist at the strychnine-sensitive glycine receptors. In addition, it has recently been discovered to act at two new receptors, the excitatory glycine receptor and metabotropic glycine receptor. Glycine's neurotransmitter roles have been most extensively investigated in the spinal cord, where it is known to play essential roles in pain, itch, and motor function. In contrast, less is known about supraspinal glycinergic functions, and their contributions to pain circuits are largely unrecognized. As glycinergic neurons are absent from cortical regions, a clearer understanding of how supraspinal glycine modulates pain could reveal new pharmacological targets. This review aims to synthesize the published research on glycine's role in the adult brain, highlighting regions where glycine signaling may modulate pain responses. This was achieved through a scoping review methodology identifying several key regions of supraspinal pain circuitry where glycine signaling is involved. Therefore, this review unveils critical research gaps for supraspinal glycine's potential roles in pain and pain-associated responses, encouraging researchers to consider glycinergic neurotransmission more widely when investigating neural mechanisms of pain.

Dynamic role of GlyT1 as glycine sink or source: Pharmacological implications for the gain control of NMDA receptors

Glycine transporter 1 (GlyT1) mediates the termination of inhibitory glycinergic receptor signaling in the spinal cord and brainstem, and is also present diffusely in the forebrain. Here, it regulates the ambient glycine concentration and influences the 'glycine' site occupancy of N-methyl-d-aspartate receptors (NMDARs). GlyT1 is a reversible transporter with a substantial, but not excessive, sodium-motive force for uphill transport. This study investigates its role as a potential source of glycine supply, either by reverse uptake or heteroexchange. Indeed, glutamate alone does not induce NMDAR current in "naive" oocytes co-expressing GluN1/GluN2A and GlyT1, a previously characterized cellular model. However, after substantial intracellular glycine accumulation, GlyT1 reverses its transport mode, and begins to release glycine into the external compartment, allowing NMDAR activation by glutamate alone. These uptake-dependent glutamate currents were blocked by ALX-5407 and potentiated by sarcosine, a specific inhibitor and substrate of GlyT1, respectively, suggesting a higher occupancy of the co-agonist site when GlyT1 functions as a glycine source either by reversed-uptake or by heteroexchange. These two glycine release mechanisms can be distinguished by their voltage dependence, as the reversed-uptake cycle decreases at hyperpolarized potentials, whereas heteroexchange electroneutrality preserves glycine efflux and NMDAR activation at these potentials. These results establish GlyT1-mediated efflux as a positive regulator of NMDAR coagonist site occupancy, and demonstrate the efficacy of sarcosine heteroexchange in enhancing coagonist site occupancy. Because NMDAR facilitation by GlyT1-inhibitors and sarcosine relies on different transport mechanisms, their actions may be a source of variability in reversing NMDAR hypofunction in schizophrenia.

Alcohol and the dopamine system

The mesolimbic dopamine pathway plays a major role in drug reinforcement and is likely involved also in the development of drug addiction. Ethanol, like most addictive drugs, acutely activates the mesolimbic dopamine system and releases dopamine, and ethanol-associated stimuli also appear to trigger dopamine release. In addition, chronic exposure to ethanol reduces the baseline function of the mesolimbic dopamine system. The molecular mechanisms underlying ethanol´s interaction with this system remain, however, to be unveiled. Here research on the actions of ethanol in the mesolimbic dopamine system, focusing on the involvement of cystein-loop ligand-gated ion channels, opiate receptors, gastric peptides and acetaldehyde is briefly reviewed. In summary, a great complexity as regards ethanol´s mechanism(s) of action along the mesolimbic dopamine system has been revealed. Consequently, several new targets and possibilities for pharmacotherapies for alcohol use disorder have emerged.

Neonatal Maternal Separation Induces Sexual Dimorphism in Brain Development: The Influence on Amino Acid Levels and Cognitive Disorders

Repeated maternal separation (MS) is a useful experimental model in rodents for studying the long-term influence of early-life stress on brain neurophysiology. In our work, we assessed the effect of repeated MS (postnatal day (PND)1-21, 180 min/day) on the postnatal development of rat brain regions involved in memory using proton magnetic resonance spectroscopy (1HMRS) for tissue volume and the level of amino acids such as glutamate, aspartate, glutamine, glycine and gamma-aminobutyric acid (GABA) in the hippocampus. We assessed whether these effects are sex dependent. We also use novel object recognition (NOR) task to examine the effect of MS on memory and the effect of ethanol on it. Finally, we attempted to ameliorate postnatal stress-induced memory deficits by using VU-29, a positive allosteric modulator (PAM) of the metabotropic glutamate type 5 (mGlu5) receptor. In males, we noted deficits in the levels of glutamate, glycine and glutamine and increases in GABA in the hippocampus. In addition, the values of perirhinal cortex, prefrontal cortex and insular cortex and CA3 were decreased in these animals. MS females, in contrast, demonstrated significant increase in glutamate levels and decrease in GABA levels in the hippocampus. Here, the CA1 values alone were increased. VU-29 administration ameliorated these cognitive deficits. Thus, MS stress disturbs amino acids levels mainly in the hippocampus of adult male rats, and enhancement of glutamate neurotransmission reversed recognition memory deficits in these animals.

Loss of Extrasynaptic Inhibitory Glycine Receptors in the Hippocampus of an AD Mouse Model Is Restored by Treatment with Artesunate

Alzheimer's disease (AD) is characterized by synaptic failure and neuronal loss. Recently, we demonstrated that artemisinins restored the levels of key proteins of inhibitory GABAergic synapses in the hippocampus of APP/PS1 mice, a model of cerebral amyloidosis. In the present study, we analyzed the protein levels and subcellular localization of α2 and α3 subunits of GlyRs, indicated as the most abundant receptor subtypes in the mature hippocampus, in early and late stages of AD pathogenesis, and upon treatment with two different doses of artesunate (ARS). Immunofluorescence microscopy and Western blot analysis demonstrated that the protein levels of both α2 and α3 GlyRs are considerably reduced in the CA1 and the dentate gyrus of 12-month-old APP/PS1 mice when compared to WT mice. Notably, treatment with low-dose ARS affected GlyR expression in a subunit-specific way; the protein levels of α3 GlyR subunits were rescued to about WT levels, whereas that of α2 GlyRs were not affected significantly. Moreover, double labeling with a presynaptic marker indicated that the changes in GlyR α3 expression levels primarily involve extracellular GlyRs. Correspondingly, low concentrations of artesunate (≤1 µM) also increased the extrasynaptic GlyR cluster density in hAPPswe-transfected primary hippocampal neurons, whereas the number of GlyR clusters overlapping presynaptic VIAAT immunoreactivities remained unchanged. Thus, here we provide evidence that the protein levels and subcellular localization of α2 and α3 subunits of GlyRs show regional and temporal alterations in the hippocampus of APP/PS1 mice that can be modulated by the application of artesunate.

Hetero-pentamerization determines mobility and conductance of Glycine receptor α3 splice variants

Glycine receptors (GlyRs) are ligand-gated pentameric chloride channels in the central nervous system. GlyR-α3 is a possible target for chronic pain treatment and temporal lobe epilepsy. Alternative splicing into K or L variants determines the subcellular fate and function of GlyR-α3, yet it remains to be shown whether its different splice variants can functionally co-assemble, and what the properties of such heteropentamers would be. Here, we subjected GlyR-α3 to a combined fluorescence microscopy and electrophysiology analysis. We employ masked Pearson’s and dual-color spatiotemporal correlation analysis to prove that GlyR-α3 splice variants heteropentamerize, adopting the mobility of the K variant. Fluorescence-based single-subunit counting experiments revealed a variable and concentration ratio dependent hetero-stoichiometry. Via cell-attached single-channel electrophysiology we show that heteropentamers exhibit currents in between those of K and L variants. Our data are compatible with a model where α3 heteropentamerization fine-tunes mobility and activity of GlyR-α3 channels, which is important to understand and tackle α3 related diseases.

Action of the Photochrome Glyght on GABAergic Synaptic Transmission in Mouse Brain Slices

Glyght is a new photochromic compound described as an effective modulator of glycine receptors at heterologous expression, in brain slices and in zebrafish larvae. Glyght also caused weak inhibition of GABA_A-mediated currents in a cell line expressing α1/β2/γ2 GABA_A receptors. However, the effects of Glyght on GABAergic transmission in the brain have not been analysed, which does not allow a sufficiently comprehensive assessment of the effects of the compound on the nervous system. Therefore, in this study using whole-cell patch-clamp recording, we analysed the Glyght (100 µM) action on evoked GABAergic inhibitory postsynaptic currents (eIPSCs) in mice hippocampal slices. Two populations of cells were found: the first responded by reducing the GABAergic eIPSCs’ amplitude, whereas the second showed no sensitivity to the compound. Glyght did not affect the ionic currents’ amplitude induced by GABA application, suggesting the absence of action on postsynaptic GABA receptors. Additionally, Glyght had no impact on the paired-pulse modulation of GABAergic eIPSCs, indicating that Glyght does not modulate the neurotransmitter release mechanisms. In the presence of strychnine, an antagonist of glycine receptors, the Glyght effect on GABAergic synaptic transmission was absent. Our results suggest that Glyght can modulate GABAergic synaptic transmission via action on extrasynaptic glycine receptors.

Distributed genetic architecture across the hippocampal formation implies common neuropathology across brain disorders

Despite its major role in complex human functions across the lifespan, most notably navigation, learning and memory, much of the genetic architecture of the hippocampal formation is currently unexplored. Here, through multivariate genome-wide association analysis in volumetric data from 35,411 white British individuals, we reveal 177 unique genetic loci with distributed associations across the hippocampal formation. We identify genetic overlap with eight brain disorders with typical onset at different stages of life, where common genes suggest partly age- and disorder-independent mechanisms underlying hippocampal pathology.

Neuronal Nsun2 deficiency produces tRNA epitranscriptomic alterations and proteomic shifts impacting synaptic signaling and behavior

Epitranscriptomic mechanisms linking tRNA function and the brain proteome to cognition and complex behaviors are not well described. Here, we report bi-directional changes in depression-related behaviors after genetic disruption of neuronal tRNA cytosine methylation, including conditional ablation and transgene-derived overexpression of Nsun2 in the mouse prefrontal cortex (PFC). Neuronal Nsun2-deficiency was associated with a decrease in tRNA m5C levels, resulting in deficits in expression of 70% of tRNAGly isodecoders. Altogether, 1488/5820 proteins changed upon neuronal Nsun2-deficiency, in conjunction with glycine codon-specific defects in translational efficiencies. Loss of Gly-rich proteins critical for glutamatergic neurotransmission was associated with impaired synaptic signaling at PFC pyramidal neurons and defective contextual fear memory. Changes in the neuronal translatome were also associated with a 146% increase in glycine biosynthesis. These findings highlight the methylation sensitivity of glycinergic tRNAs in the adult PFC. Furthermore, they link synaptic plasticity and complex behaviors to epitranscriptomic modifications of cognate tRNAs and the proteomic homeostasis associated with specific amino acids.

Presence of ethanol sensitive and insensitive glycine receptors in the ventral tegmental area and prefrontal cortex in mice

BACKGROUND AND PURPOSE

Previous studies showed that glycine receptors (GlyRs) composed of α1 and β subunits are primarily found in spinal cord and brainstem and are potentiated by ethanol (10-100 mM). However, much less is known about the presence, composition, and ethanol sensitivity of GlyRs in higher CNS regions. In the present study, we examined two regions of the brain reward system, the ventral tegmental area (VTA) and the prefrontal cortex (PFC), to determine their GlyR subunit composition and sensitivity to ethanol.

EXPERIMENTAL APPROACH

To achieve these aims, we used Western blot, immunohistochemistry and electrophysiological techniques in three different models: Wild-type C57BL/6, GlyR α1 knock-in and GlyR α2 knockout mice.

KEY RESULTS

Similar levels of α and β GlyR subunits were detected in both brain regions, and electrophysiological recordings demonstrated the presence of glycine-activated currents in both areas. The sensitivity of GlyRs to glycine was lower in the PFC compared to VTA. Picrotoxin blocked the glycine-activated current in the PFC and VTA only partially, indicating that both regions express heteromeric αβ receptors. Interestingly, GlyRs in VTA neurons, but not in PFC neurons, were potentiated by ethanol.

CONCLUSION AND IMPLICATIONS

GlyRs in VTA neurons from WT and α2 KO mice were potentiated by ethanol, but not in neurons from the α1 KI mice, supporting the conclusion that α1 GlyRs are predominantly expressed in the VTA. By contrast, GlyRs in PFC neurons were not potentiated in any of the mouse models studied, suggesting the presence of either α2/α3/α4 rather than α1 GlyR subunits.

Non-selective COX inhibitors impair memory formation and short-term but not long-term synaptic plasticity

Cyclooxygenase (COX) plays a critical role in synaptic plasticity. Therefore, long-term administration of acetylsalicylic acid (ASA) and its main metabolite, salicylate, as a COX inhibitor may impair synaptic plasticity and subsequently memory formation. Although different studies have tried to explain the effects of ASA and sodium salicylate (SS) on learning and memory, the results are contradictory and the mechanisms are not exactly known. The present study was designed to investigate the effects of long-term low-dose (equivalent to prophylactic dose) and short-term high-dose (equivalent to analgesic dose) administration of ASA and SS respectively, on spatial learning and memory and hippocampal synaptic plasticity. Animals were treated with a low dose of ASA (2 mg/ml solvated in drinking water, 6 weeks) or a high dose of SS, a metabolite of ASA, (300 mg/kg, 3 days, twice-daily, i.p). Spatial memory and synaptic plasticity were assessed by water maze performance and in vivo field potential recording from CA1, respectively. Animals treated with ASA but not SS showed a significant increase in escape latency and distance moved. Furthermore, in the probe test, animals treated with both drugs spent less time in the target quadrant zone. The paired-pulse ratio (PPR) at 20-ms inter-pulse intervals (IPI) as an index of short-term plasticity in both treated groups was significantly higher than of the control group. Interestingly, none of the administered drugs affected long-term potentiation (LTP). These data suggested that long-term inhibition of COX disrupted memory acquisition and retrieval. Interestingly, cognitive impairments happened along with short-term but not long-term synaptic plasticity disturbance.

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

KEY POINTS

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

ABSTRACT

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

Glycine Transporters and Its Coupling with NMDA Receptors

Glycine plays two roles in neurotransmission. In caudal areas like the spinal cord and the brainstem, it acts as an inhibitory neurotransmitter, but in all regions of the CNS, it also works as a co-agonist with L-glutamate at N-methyl-D-aspartate receptors (NMDARs). The glycine fluxes in the CNS are regulated by two specific transporters for glycine, GlyT1 and GlyT2, perhaps with the cooperation of diverse neutral amino acid transporters like Asc-1 or SNAT5/SN2. While GlyT2 and Asc-1 are neuronal proteins, GlyT1 and SNAT5 are mainly astrocytic, although neuronal forms of GlyT1 also exist. GlyT1 has attracted considerable interest from the medical community and the pharmaceutical industry since compelling evidence indicates a clear association with the functioning of NMDARs, whose activity is decreased in various psychiatric illnesses. By controlling extracellular glycine, transporter inhibitors might potentiate the activity of NMDARs without activating excitotoxic processes. Physiologically, GlyT1 is a central actor in the cross talk between glutamatergic, glycinergic, dopaminergic, and probably other neurotransmitter systems. Many of these relationships begin to be unraveled by studies performed in recent years using genetic and pharmacological models. These studies are also clarifying the interactions between glycine, glycine transporters, and other co-agonists of the glycine site of NMDARs like D-serine. These findings are also relevant to understand the pathophysiology of devastating diseases like schizophrenia, depression, anxiety, epilepsy, stroke, and chronic pain.

Glycine receptors are involved in hippocampal neuronal damage caused by oxygen-glucose deficiency

Glycine receptors (GlyRs) belong to the family of ligand-gated cys-loop receptors and effectuate fast inhibitory neurotransmission in central nervous system (CNS). They are involved in numerous physiological processes, such as movement, respiration, and processing of sensory information, as well as in regulation of neuronal excitability in different brain regions. GlyRs play important role in the maintenance of excitatory/inhibitory balance in the hippocampus and participate in the development of various brain pathologies. In the present study, we have examined a surface expression of GlyRs by pyramidal neurons and astrocytes in control and after 30 min of oxygen-glucose deprivation (OGD) in the organotypic culture of hippocampal slices. Our investigation has demonstrated a decrease in GlyR-positive staining associated with pyramidal neurons and relative stability of GlyRs expression at the surface of astrocytes 4 hs after OGD. These data indicate that GlyRs dysfunction may represent a significant additional factor leading to enhanced neuronal damage induced by OGD. Pharmacological modulation of GlyRs is a promising venue of research for the correction of negative consequences of oxygen-glucose deficiency.

Tonically Active α2 Subunit-Containing Glycine Receptors Regulate the Excitability of Striatal Medium Spiny Neurons

Medium spiny neurons (MSNs) of the dorsal striatum represent the first relay of cortico–striato–thalamic loop, responsible for the initiation of voluntary movements and motor learning. GABAergic transmission exerts the main inhibitory control of MSNs. However, MSNs also express chloride-permeable glycine receptors (GlyRs) although their subunit composition and functional significance in the striatum is unknown. Here, we studied the function of GlyRs in MSNs of young adult mice. We show that MSNs express functional GlyRs, with α2 being the main agonist binding subunit. These receptors are extrasynaptic and depolarizing at resting state. The pharmacological inhibition of GlyRs, as well as inactivation of the GlyR α2 subunit gene hyperpolarize the membrane potential of MSNs and increase their action potential firing offset. Mice lacking GlyR α2 showed impaired motor memory consolidation without any changes in the initial motor performance. Taken together, these results demonstrate that tonically active GlyRs regulate the firing properties of MSNs and may thus affect the function of basal ganglia.

Alcohol and the Brain: Neuronal Molecular Targets, Synapses, and Circuits

Ethanol is one of the most commonly abused drugs. Although environmental and genetic factors contribute to the etiology of alcohol use disorders, it is ethanol's actions in the brain that explain (1) acute ethanol-related behavioral changes, such as stimulant followed by depressant effects, and (2) chronic changes in behavior, including escalated use, tolerance, compulsive seeking, and dependence. Our knowledge of ethanol use and abuse thus relies on understanding its effects on the brain. Scientists have employed both bottom-up and top-down approaches, building from molecular targets to behavioral analyses and vice versa, respectively. This review highlights current progress in the field, focusing on recent and emerging molecular, cellular, and circuit effects of the drug that impact ethanol-related behaviors. The focus of the field is now on pinpointing which molecular effects in specific neurons within a brain region contribute to behavioral changes across the course of acute and chronic ethanol exposure.

Glycine receptor α3 and α2 subunits mediate tonic and exogenous agonist-induced currents in forebrain

Neuronal inhibition can occur via synaptic mechanisms or through tonic activation of extrasynaptic receptors. In spinal cord, glycine mediates synaptic inhibition through the activation of heteromeric glycine receptors (GlyRs) composed primarily of α1 and β subunits. Inhibitory GlyRs are also found throughout the brain, where GlyR α2 and α3 subunit expression exceeds that of α1, particularly in forebrain structures, and coassembly of these α subunits with the β subunit appears to occur to a lesser extent than in spinal cord. Here, we analyzed GlyR currents in several regions of the adolescent mouse forebrain (striatum, prefrontal cortex, hippocampus, amygdala, and bed nucleus of the stria terminalis). Our results show ubiquitous expression of GlyRs that mediate large-amplitude currents in response to exogenously applied glycine in these forebrain structures. Additionally, tonic inward currents were also detected, but only in the striatum, hippocampus, and prefrontal cortex (PFC). These tonic currents were sensitive to both strychnine and picrotoxin, indicating that they are mediated by extrasynaptic homomeric GlyRs. Recordings from mice deficient in the GlyR α3 subunit (Glra3^-/-) revealed a lack of tonic GlyR currents in the striatum and the PFC. In Glra2^-/Y animals, GlyR tonic currents were preserved; however, the amplitudes of current responses to exogenous glycine were significantly reduced. We conclude that functional α2 and α3 GlyRs are present in various regions of the forebrain and that α3 GlyRs specifically participate in tonic inhibition in the striatum and PFC. Our findings suggest roles for glycine in regulating neuronal excitability in the forebrain.

Phenylalanine derivatives with modulating effects on human α1-glycine receptors and anticonvulsant activity in strychnine-induced seizure model in male adult rats

The critical role of α1-glycine receptor (α1-GLYRs) in pathological conditions such as epilepsy is well known. In the present study, structure-activity relations for a series of phenylalanine derivatives carrying selected hydrogen bond acceptors were investigated on the functional properties of human α1-GLYR expressed in Xenopus oocytes. The results indicate that one particular substitution position appeared to be of special importance for control of ligand activity. Among tested ligands (1-8), the biphenyl derivative (2) provided the most promising antagonistic effect on α1-GLYRs, while its phenylbenzyl analogue (5) exhibited the highest potentiation effect. Moreover, ligand 5 with most promising potentiating effect showed in-vivo moderate protection when tested in strychnine (STR)-induced seizure model in male adult rats, whereas ligand 2 with highest antagonistic effect failed to provide appreciable anti(pro)convulsant effect. Furthermore, ligands 2 and 5 with the most promising effects on human α1-GLYRs were examined for their toxicity and potential neuroprotective effect against neurotoxin 6-hydroxydopamine (6-OHDA). The results show that ligands 2 and 5 possessed neither significant antiproliferative effects, nor necrotic and mitochondrial toxicity (up to concentration of 50μM). Moreover, ligand 2 showed weak neuroprotective effect at the 50μM against 100μM toxic dose of 6-OHDA. Our results indicate that modulatory effects of ligands 2 and 5 on human α1-GLYRs as well as on STR-induced convulsion can provide further insights for the design of therapeutic agents in treatment of epilepsy and other pathological conditions requiring enhanced activity of inhibitory glycine receptors.

Glycine Transporters in Glia Cells: Structural Studies

Glycine, besides exerting essential metabolic functions, is an important inhibitory neurotransmitter in caudal areas of the central nervous system and also a positive neuromodulator at excitatory glutamate-mediated synapses. Glial cells provide metabolic support to neurons and modulate synaptic activity. Six transporters belonging to three solute carrier families (SLC6, SLC38, and SLC7) are capable of transporting glycine across the glial plasma membrane. The unique glial glycine-selective transporter GlyT1 (SLC6) is the main regulator of synaptic glycine concentrations, assisted by the neuronal GlyT2. The five additional glycine transporters ATB^0,+, SNAT1, SNAT2, SNAT5, and LAT2 display broad amino acid specificity and have differential contributions to glial glycine transport. Glial glycine transporters are divergent in sequence but share a similar architecture displaying the 5 + 5 inverted fold originally characterized in the leucine transporter LeuT. The availability of protein crystals solved at high resolution for prokaryotic and, more recently, eukaryotic homologues of this superfamily has advanced significantly our understanding of the mechanism of glycine transport.

Hippocampal Characteristics and Invariant Sequence Elements Distribution of GLRA2 and GLRA3 C-to-U Editing

Glycine receptor α2 and α3 subunit (GLRA2/GLRA3) high-affinity variants, of which the subjacent amino acid substitutions issue from C-to-U RNA editing, are thought to influence tonic inhibition and pathophysiology. In light of the detection of GLRA3 NM_006529:r.1157C>U and GLRA2 NM_002063:r.1416C>U exchanges in hippocampus explants of temporal lobe epilepsy patients, we now examine the healthy situation and relate it to the epileptic situation by ascertaining controls in a legitimate reanalysis. The GLRA2 and GLRA3 editing events that would ultimately result in a glycine receptor with increased affinity occur in the postmortem nonepileptic hippocampus. Most notably, their relative amounts do not significantly differ from those in increased damaged hippocampus explants, whereas curbed relative amounts in epileptic explants without cell loss come out statistically significant. Local sequence alignment reveals invariant sequence stretches consistent in GLRA2/ GLRA3 and other edited transcripts that coincide with known APOB sequence elements. Concerning the essential mooring element, GLRA2/GLRA3 comply strictly only with the motif's 5' part. While this lack of canonical mooring elements and uncertain action of the famous deaminase APOBEC1 suggest a specific regulation of GLRA2/GLRA3 editing, its reduction in the less-damaged epileptic hippocampus could be attributed to anomalous epileptic neurogenesis.

Time-dependent impact of glutamatergic modulators on the promnesiant effect of 5-HT6R blockade on mice recognition memory

Selective antagonists at serotonin 5-HT₆ receptors (5-HT₆R) improve memory performance in rodents and are currently under clinical investigations. If blockade of 5-HT₆R is known to increase glutamate release, only two studies have so far demonstrated an interaction between 5-HT₆R and glutamate transmission, but both, using the non-competitive NMDA antagonist MK-801, insensitive to variations of glutamate concentrations. In a place recognition task, we investigated here in mice the role of glutamate transmission in the beneficial effects of 5-HT₆R blockade (SB-271046). Through the use of increasing intervals (2, 4 and 6h) between acquisition and retrieval, we investigated the time-dependent impact of two different glutamatergic modulators. NMDAR-dependant glutamate transmission (NMDA Receptors) was either blocked by the competitive antagonist at NMDAR, CGS 19755, or potentiated by the glycine transporter type 1 (GlyT1) inhibitor, NFPS. Results showed that neither SB-271046, nor CGS 19755, nor NFPS, alter behavioural performances after short intervals, i.e. when control mice displayed significant memory performances (2h and 4h) (respectively 10, 3, and 0.625mg.kg^-1). Conversely, with the 6h-interval, a situation in which spontaneous forgetting is observed in control mice, SB-271046 improved recognition memory performances. This beneficial effect was prevented when co-administered with either CGS 19755 or NFPS, which themselves had no effect. Interestingly, a dose-dependent effect was observed with NFPS, with promnesic effect observed at lower dose (0.156mg.kg^-1) when administrated alone, whereas it did no modify promnesic effect of SB-271046. These results demonstrate that promnesiant effect induced by 5-HT₆R blockade is sensitive to the competitive blockade of NMDAR and underline the need of a fine adjustment of the inhibition of GlyT1. Overall, our findings support the idea of a complex crosstalk between serotonergic and glutamatergic systems in the promnesic properties of 5-HT₆R antagonists.

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.

Effects of Salt Loading on the Regulation of Rat Hypothalamic Magnocellular Neurosecretory Cells by Ionotropic GABA and Glycine Receptors

Synaptic and extrasynaptic transmission mediated by ionotropic GABA and glycine receptors plays a critical role in shaping the action potential firing (spiking) activity of hypothalamic magnocellular neurosecretory cells and therefore determines the rate at which vasopressin and oxytocin are released from the neurohypophysis. The inhibitory effect of these transmitters relies on the maintenance of a low concentration of intracellular chloride ions such that, when activated by GABA or glycine, a hyperpolarisation of the neuronal membrane potential results. In this review, we highlight the various ways by which the two types of inhibitory receptors contribute to homeostasis by fine-tuning the spiking rate of vasopressin-releasing magnocellular neurosecretory cells in a manner dependent on the hydration state of the animal. In addition, we review the currently available evidence on how the strength of these inhibitory pathways can be regulated during chronic hypernatraemia via a form of activity-dependent depolarisation of the chloride reversal potential, leading to an abolition of these inhibitory pathways potentially causing sodium-dependent elevations in blood pressure.

Glycine transporter 1 is a target for the treatment of epilepsy

Glycine is the major inhibitory neurotransmitter in brainstem and spinal cord, whereas in hippocampus glycine exerts dual modulatory roles on strychnine-sensitive glycine receptors and on the strychnine-insensitive glycineB site of the N-methyl-D-aspartate receptor (NMDAR). In hippocampus, the synaptic availability of glycine is largely under control of glycine transporter 1 (GlyT1). Since epilepsy is a disorder of disrupted network homeostasis affecting the equilibrium of various neurotransmitters and neuromodulators, we hypothesized that changes in hippocampal GlyT1 expression and resulting disruption of glycine homeostasis might be implicated in the pathophysiology of epilepsy. Using two different rodent models of temporal lobe epilepsy (TLE)--the intrahippocampal kainic acid model of TLE in mice, and the rat model of tetanic stimulation-induced TLE--we first demonstrated robust overexpression of GlyT1 in the hippocampal formation, suggesting dysfunctional glycine signaling in epilepsy. Overexpression of GlyT1 in the hippocampal formation was corroborated in human TLE samples by quantitative real time PCR. In support of a role of dysfunctional glycine signaling in the pathophysiology of epilepsy, both the genetic deletion of GlyT1 in hippocampus and the GlyT1 inhibitor LY2365109 increased seizure thresholds in mice. Importantly, chronic seizures in the mouse model of TLE were robustly suppressed by systemic administration of the GlyT1 inhibitor LY2365109. We conclude that GlyT1 overexpression in the epileptic brain constitutes a new target for therapeutic intervention, and that GlyT1 inhibitors constitute a new class of antiictogenic drugs. These findings are of translational value since GlyT1 inhibitors are already in clinical development to treat cognitive symptoms in schizophrenia.

Glycine transporter-1 controls nonsynaptic inhibitory actions of glycine receptors in the neonatal rat hippocampus

Although functional glycinergic synapses have not been identified in the hippocampus, neurons in this area express Cl(-) permeable extrasynaptic glycine receptors (GlyRs). In experiments on CA3 pyramidal neurons on postnatal day 0-6 rat hippocampal slices, we detected robust GlyR activity as a tonic current and as single-channel events. Glycine release was independent of neuronal activity or extracellular Ca(2+). The endogenous GlyR activity was strongly enhanced by inhibition of the glycine-transporter-1 (GlyT1). Blockade of GlyT1 also caused a profound increase in the baseline current induced by exogenous glycine. Inhibition of GlyT1 reduced the frequency of spontaneous network events known as field giant depolarizing potentials (fGDPs) and of the unit activity in the absence of synaptic transmission. This inhibitory action on fGDPs was mimicked by applying 2 μm glycine or 0.1 μm isoguvacine, a GABAA-receptor agonist. Furthermore, 2 μm glycine suppressed unit spiking in the absence of synaptic transmission. Hence, despite the well known depolarizing Cl(-) equilibrium potential of neonatal hippocampal neurons, physiologically relevant extracellular glycine concentrations can exert an inhibitory action. The present data show that, akin to GABA uptake, GlyT1 exerts a powerful modulatory action on network events in the newborn hippocampus.

Glycine induces bidirectional modifications in N-methyl-D-aspartate receptor-mediated synaptic responses in hippocampal CA1 neurons

Glycine can persistently potentiate or depress AMPA responses through differential actions on two binding sites: NMDA and glycine receptors. Whether glycine can induce long-lasting modifications in NMDA responses, however, remains unknown. Here, we report that glycine induces long-term potentiation (LTP) or long-term depression (LTD) of NMDA responses (Gly-LTP(NMDA) or Gly-LTD(NMDA)) in a dose-dependent manner in hippocampal CA1 neurons. These modifications of NMDA responses depend on NMDAR activation. In addition, the induction of Gly-LTP(NMDA) requires binding of glycine with NMDARs, whereas Gly-LTD(NMDA) requires that glycine bind with both sites on NMDARs and GlyRs. Moreover, activity-dependent exocytosis and endocytosis of postsynaptic NMDARs underlie glycine-induced bidirectional modification of NMDA excitatory postsynaptic currents. Thus, we conclude that glycine at different levels induces bidirectional plasticity of NMDA responses through differentially regulating NMDA receptor trafficking. Our present findings reveal important functions of the two glycine binding sites in gating the direction of synaptic plasticity in NMDA responses.

Glycine receptors control the generation of projection neurons in the developing cerebral cortex

The development of the cerebral cortex requires coordinated regulation of proliferation, specification, migration and differentiation of cortical progenitors into functionally integrated neurons. The completion of the neurogenic program requires a dynamic interplay between cell intrinsic regulators and extrinsic cues, such as growth factor and neurotransmitters. We previously demonstrated a role for extrasynaptic glycine receptors (GlyRs) containing the α2 subunit in cerebral cortical neurogenesis, revealing that endogenous GlyR activation promotes interneuron migration in the developing cortical wall. The proliferative compartment of the cortex comprises apical progenitors that give birth to neurons directly or indirectly through the generation of basal progenitors, which serve as amplification step to generate the bulk of cortical neurons. The present work shows that genetic inactivation of Glra2, the gene coding the α2 subunit of GlyRs, disrupts dorsal cortical progenitor homeostasis with an impaired capability of apical progenitors to generate basal progenitors. This defect results in an overall reduction of projection neurons that settle in upper or deep layers of the cerebral cortex. Overall, the depletion of cortical neurons observed in Glra2-knockout embryos leads to moderate microcephaly in newborn Glra2-knockout mice. Taken together, our findings support a contribution of GlyR α2 to early processes in cerebral cortical neurogenesis that are required later for the proper development of cortical circuits.

Strychnine-sensitive glycine receptors on pyramidal neurons in layers II/III of the mouse prefrontal cortex are tonically activated

Processing of signals within the cerebral cortex requires integration of synaptic inputs and a coordination between excitatory and inhibitory neurotransmission. In addition to the classic form of synaptic inhibition, another important mechanism that can regulate neuronal excitability is tonic inhibition via sustained activation of receptors by ambient levels of inhibitory neurotransmitter, usually GABA. The purpose of this study was to determine whether this occurs in layer II/III pyramidal neurons (PNs) in the prelimbic region of the mouse medial prefrontal cortex (mPFC). We found that these neurons respond to exogenous GABA and to the α4δ-containing GABAA receptor (GABA(A)R)-selective agonist gaboxadol, consistent with the presence of extrasynaptic GABA(A)R populations. Spontaneous and miniature synaptic currents were blocked by the GABA(A)R antagonist gabazine and had fast decay kinetics, consistent with typical synaptic GABA(A)Rs. Very few layer II/III neurons showed a baseline current shift in response to gabazine, but almost all showed a current shift (15-25 pA) in response to picrotoxin. In addition to being a noncompetitive antagonist at GABA(A)Rs, picrotoxin also blocks homomeric glycine receptors (GlyRs). Application of the GlyR antagonist strychnine caused a modest but consistent shift (∼15 pA) in membrane current, without affecting spontaneous synaptic events, consistent with the tonic activation of GlyRs. Further investigation showed that these neurons respond in a concentration-dependent manner to glycine and taurine. Inhibition of glycine transporter 1 (GlyT1) with sarcosine resulted in an inward current and an increase of the strychnine-sensitive current. Our data demonstrate the existence of functional GlyRs in layer II/III of the mPFC and a role for these receptors in tonic inhibition that can have an important influence on mPFC excitability and signal processing.

Curcumol from Rhizoma Curcumae suppresses epileptic seizure by facilitation of GABA(A) receptors

Rhizoma Curcumae is a common Chinese dietary spice used in South Asia and China for thousands of years. As the main extract, Rhizoma Curcumae oil has attracted a great interest due to its newly raised therapeutic activities including its pharmacological effects upon central nervous system such as neuroprotection, cognitive enhancement, and anti-seizure efficacy; however the molecular mechanisms and the target identification remain to be established. Here we characterize an inhibitory effect of curcumol, a major bioactive component of Rhizoma Curcumae oil, on the excitability of hippocampal neurons in culture, the basal locomotor activity of freely moving animals, and the chemically induced seizure activity in vivo. Electrophysiological recording showed that acute application of curcumol significantly facilitated the γ-aminobutyric acid (GABA)-activated current in cultured mouse hippocampal neurons and in human embryonic kidney cells expressing α1- or α5-containing A type GABA (GABAA) receptors in a concentration-dependent manner. Measurement of tonic and miniature inhibitory postsynaptic GABAergic currents in hippocampal slices indicated that curcumol enhanced both forms of inhibition. In both pentylenetetrazole and kainate seizure models, curcumol suppressed epileptic activity in mice by prolonging the latency to clonic and tonic seizures and reducing the mortality as well as the susceptibility to seizure, presumably by facilitating the activation of GABAA receptors. Taken together, our results identified curcumol as a novel anti-seizure agent which inhibited neuronal excitability through enhancing GABAergic inhibition.

Glycine transporters as novel therapeutic targets in schizophrenia, alcohol dependence and pain

Glycine transporters are endogenous regulators of the dual functions of glycine, which acts as a classical inhibitory neurotransmitter at glycinergic synapses and as a modulator of neuronal excitation mediated by NMDA (N-methyl-D-aspartate) receptors at glutamatergic synapses. The two major subtypes of glycine transporters, GlyT1 and GlyT2, have been linked to the pathogenesis and/or treatment of central and peripheral nervous system disorders, including schizophrenia and related affective and cognitive disturbances, alcohol dependence, pain, epilepsy, breathing disorders and startle disease (also known as hyperekplexia). This Review examines the rationale for the therapeutic potential of GlyT1 and GlyT2 inhibition, and surveys the latest advances in the biology of glycine reuptake and transport as well as the drug discovery and clinical development of compounds that block glycine transporters.

Glycine receptors and brain development

Glycine receptors (GlyRs) are ligand-gated chloride ion channels that mediate fast inhibitory neurotransmission in the spinal cord and the brainstem. There, they are mainly involved in motor control and pain perception in the adult. However, these receptors are also expressed in upper regions of the central nervous system, where they participate in different processes including synaptic neurotransmission. Moreover, GlyRs are present since early stages of brain development and might influence this process. Here, we discuss the current state of the art regarding GlyRs during embryonic and postnatal brain development in light of recent findings about the cellular and molecular mechanisms that control brain development.

Presynaptic glycine receptors increase GABAergic neurotransmission in rat periaqueductal gray neurons

The periaqueductal gray (PAG) is involved in the central regulation of nociceptive transmission by affecting the descending inhibitory pathway. In the present study, we have addressed the functional role of presynaptic glycine receptors in spontaneous glutamatergic transmission. Spontaneous EPSCs (sEPSCs) were recorded in mechanically dissociated rat PAG neurons using a conventional whole-cell patch recording technique under voltage-clamp conditions. The application of glycine (100 µM) significantly increased the frequency of sEPSCs, without affecting the amplitude of sEPSCs. The glycine-induced increase in sEPSC frequency was blocked by 1 µM strychnine, a specific glycine receptor antagonist. The results suggest that glycine acts on presynaptic glycine receptors to increase the probability of glutamate release from excitatory nerve terminals. The glycine-induced increase in sEPSC frequency completely disappeared either in the presence of tetrodotoxin or Cd²⁺, voltage-gated Na⁺, or Ca²⁺ channel blockers, suggesting that the activation of presynaptic glycine receptors might depolarize excitatory nerve terminals. The present results suggest that presynaptic glycine receptors can regulate the excitability of PAG neurons by enhancing glutamatergic transmission and therefore play an important role in the regulation of various physiological functions mediated by the PAG.

Ethanol reduces neuronal excitability of lateral orbitofrontal cortex neurons via a glycine receptor dependent mechanism

Trauma-induced damage to the orbitofrontal cortex (OFC) often results in behavioral inflexibility and impaired judgment. Human alcoholics exhibit similar cognitive deficits suggesting that OFC neurons are susceptible to alcohol-induced dysfunction. A previous study from this laboratory examined OFC mediated cognitive behaviors in mice and showed that behavioral flexibility during a reversal learning discrimination task was reduced in alcohol-dependent mice. Despite these intriguing findings, the actions of alcohol on OFC neuron function are unknown. To address this issue, slices containing the lateral OFC (lOFC) were prepared from adult C57BL/6J mice and whole-cell patch clamp electrophysiology was used to characterize the effects of ethanol (EtOH) on neuronal function. EtOH (66 mM) had no effect on AMPA-mediated EPSCs but decreased those mediated by NMDA receptors. EtOH (11-66 mM) also decreased current-evoked spike firing and this was accompanied by a decrease in input resistance and a modest hyperpolarization. EtOH inhibition of spike firing was prevented by the GABAA antagonist picrotoxin, but EtOH had no effect on evoked or spontaneous GABA IPSCs. EtOH increased the holding current of voltage-clamped neurons and this action was blocked by picrotoxin but not the more selective GABAA antagonist biccuculine. The glycine receptor antagonist strychnine also prevented EtOH's effect on holding current and spike firing, and western blotting revealed the presence of glycine receptors in lOFC. Overall, these results suggest that acutely, EtOH may reduce lOFC function via a glycine receptor dependent process and this may trigger neuroadaptive mechanisms that contribute to the impairment of OFC-dependent behaviors in alcohol-dependent subjects.

Taurine release by astrocytes modulates osmosensitive glycine receptor tone and excitability in the adult supraoptic nucleus

Cells can release the free amino acid taurine through volume-regulated anion channels (VRACs), and it has been hypothesized that taurine released from glial cells is capable of inhibiting action potential (AP) firing by activating neuronal glycine receptors (GlyRs) (Hussy et al., 1997). Although an inhibitory GlyR tone is widely observed in the brain, it remains unknown whether this specifically reflects gliotransmission because most neurons also express VRACs and other endogenous molecules can activate GlyRs. We found that VRACs are absent in neurons of the rat supraoptic nucleus (SON), suggesting that glial cells are the exclusive source of taurine in this nucleus. Application of strychnine to rat hypothalamic explants caused a depolarization of SON neurons associated with a decrease of chloride conductance and could excite these cells in the absence of fast synaptic transmission. This inhibitory GlyR tone was eliminated by pharmacological blockade of VRACs, by cellular taurine depletion, by metabolic inactivation of glia with fluorocitrate, and after retraction of astrocytic processes that intercalate neuronal somata and dendrites. Finally, GlyR tone varied inversely with extracellular fluid tonicity to mediate the osmotic control of AP firing by SON neurons. These findings establish taurine as a physiological gliotransmitter and show that gliotransmission is a spatially constrained process that can be modulated by the morphological rearrangement of astrocytes.

Evaluation of the antiepileptic effect of curcumin and Nigella sativa oil in the pilocarpine model of epilepsy in comparison with valproate

The present study aimed to investigate the effect of curcumin and Nigella sativa oil (NSO) on amino acid neurotransmitter alterations and the histological changes induced by pilocarpine in the hippocampus and cortex of rats. Epilepsy was induced by i.p. injection of pilocarpine, and the animals were left for 22 days to establish spontaneous recurrent seizures. They were then treated with curcumin, NSO or valproate for 21 days. Pilocarpine induced a significant increase in hippocampal aspartate and a significant decrease in glycine and taurine levels. In the cortex, a significant increase in aspartate, glutamate, GABA, glycine, and taurine levels was obtained after pilocarpine injection. Treatment of pilocarpinized rats with curcumin and valproate ameliorated most of the changes in amino acid concentrations and reduced the histopathological abnormalities induced by pilocarpine. N. sativa oil failed to improve the pilocarpine-induced abnormalities. This may explain the antiepileptic effect of curcumin and suggest its use as an anticonvulsant.

Glycine transporter inhibitor attenuates the psychotomimetic effects of ketamine in healthy males: preliminary evidence

Enhancing glutamate function by stimulating the glycine site of the NMDA receptor with glycine, D-serine, or with drugs that inhibit glycine reuptake may have therapeutic potential in schizophrenia. The effects of a single oral dose of cis-N-methyl-N-(6-methoxy-1-phenyl-1,2,3,4-tetrahydronaphthalen-2-ylmethyl) amino-methylcarboxylic acid hydrochloride (Org 25935), a glycine transporter-1 (GlyT1) inhibitor, and placebo pretreatment on ketamine-induced schizophrenia-like psychotic symptoms, perceptual alterations, and subjective effects were evaluated in 12 healthy male subjects in a randomized, counter-balanced, within-subjects, crossover design. At 2.5 h after administration of the Org 25935 or placebo, subjects received a ketamine bolus and constant infusion lasting 100 min. Psychotic symptoms, perceptual, and a number of subjective effects were assessed repeatedly before, several times during, and after completion of ketamine administration. A cognitive battery was administered once per test day. Ketamine produced behavioral, subjective, and cognitive effects consistent with its known effects. Org 25935 reduced the ketamine-induced increases in measures of psychosis (Positive and Negative Syndrome Scale (PANSS)) and perceptual alterations (Clinician Administered Dissociative Symptoms Scale (CADSS)). The magnitude of the effect of Org 25935 on ketamine-induced increases in Total PANSS and CADSS Clinician-rated scores was 0.71 and 0.98 (SD units), respectively. None of the behavioral effects of ketamine were increased by Org 25935 pretreatment. Org 25935 worsened some aspects of learning and delayed recall, and trended to improve choice reaction time. This study demonstrates for the first time in humans that a GlyT1 inhibitor reduces the effects induced by NMDA receptor antagonism. These findings provide preliminary support for further study of the antipsychotic potential of GlyT1 inhibitors.

Targeting inhibitory neurotransmission in tinnitus

Tinnitus perception depends on the presence of its neural correlates within the auditory neuraxis and associated structures. Targeting specific circuits and receptors within the central nervous system in an effort to relieve the perception of tinnitus and its impact on one's emotional and mental state has become a focus of tinnitus research. One approach is to upregulate endogenous inhibitory neurotransmitter levels (e.g., glycine and GABA) and selectively target inhibitory receptors in key circuits to normalize tinnitus pathophysiology. Thus, the basic functional and molecular properties of two major ligand-gated inhibitory receptor systems, the GABA(A) receptor (GABA(A)R) and glycine receptor (GlyR) are described. Also reviewed is the rationale for targeting inhibition, which stems from reported tinnitus-related homeostatic plasticity of inhibitory neurotransmitter systems and associated enhanced neuronal excitability throughout most central auditory structures. However, the putative role of the medial geniculate body (MGB) in tinnitus has not been previously addressed, specifically in terms of its inhibitory afferents from inferior colliculus and thalamic reticular nucleus and its GABA(A)R functional heterogeneity. This heterogeneous population of GABA(A)Rs, which may be altered in tinnitus pathology, and its key anatomical position in the auditory CNS make the MGB a compelling structure for tinnitus research. Finally, some selective compounds, which enhance tonic inhibition, have successfully ameliorated tinnitus in animal studies, suggesting that the MGB and, to a lesser degree, the auditory cortex may be their primary locus of action. These pharmacological interventions are examined in terms of their mechanism of action and why these agents may be effective in tinnitus treatment. This article is part of a Special Issue entitled: Tinnitus Neuroscience.

Intact working memory in the absence of forebrain neuronal glycine transporter 1

Glycine transporter 1 (GlyT1) is a potential pharmacological target to ameliorate memory deficits attributable to N-methyl-d-aspartate receptor (NMDAR) hypofunction. Disruption of glycine-reuptake near excitatory synapses is expected to enhance NMDAR function by increasing glycine-B site occupancy. Genetic models with conditional GlyT1 deletion restricted to forebrain neurons have yielded several promising promnesic effects, yet its impact on working memory function remains essentially unanswered because the previous attempt had yielded un-interpretable outcomes. The present study clarified this important outstanding lacuna using a within-subject multi-test approach. Here, a consistent lack of effects was convincingly demonstrated across three working memory tests - the radial arm maze, the cheeseboard maze, and the water maze. These null outcomes contrasted with the phenotype of enhanced working memory performance seen in mutant mice with GlyT1 deletion extended to cortical/hippocampal glial cells. It follows that glial-based GlyT1 might be more closely linked to the modulation of working memory function, and raises the possibility that neuronal and glial GlyT1 may regulate cognitive functions via dissociable mechanisms.

Role of glycine receptors in glycine-induced LTD in hippocampal CA1 pyramidal neurons

Glycine in the hippocampus can exert its effect on both synaptic NMDA receptors (NMDARs) and extrasynaptic functional glycine receptors (GlyRs) via distinct binding sites. Previous studies have reported that glycine induces long-term potentiation (LTP) through the activation of synaptic NMDARs. However, little is known about the potential role of the activated GlyRs that are largely located in extrasynaptic regions. We report here that relatively high levels of glycine achieved either by exogenous glycine application or by the elevation of endogenous glycine accumulation with an antagonist of the glycine transporter induced long-term depression (LTD) of excitatory postsynaptic currents (EPSCs) in hippocampal CA1 pyramidal neurons. The co-application of glycine with the selective GlyR antagonist strychnine changed glycine-induced LTD (Gly-LTD) to LTP. Blocking the postsynaptic GlyR-gated net chloride flux by manipulating intracellular chloride concentrations failed to elicit any changes in EPSCs. These results suggest that GlyRs are involved in Gly-LTD. Furthermore, this new form of chemical LTD was accompanied by the internalization of postsynaptic AMPA receptors and required the activation of NMDARs. Therefore, our present findings reveal an important function of GlyR activation and modulation in gating the direction of synaptic plasticity.

GABA(A) receptor and glycine receptor activation by paracrine/autocrine release of endogenous agonists: more than a simple communication pathway

It is a common and widely accepted assumption that glycine and GABA are the main inhibitory transmitters in the central nervous system (CNS). But, in the past 20 years, several studies have clearly demonstrated that these amino acids can also be excitatory in the immature central nervous system. In addition, it is now established that both GABA receptors (GABARs) and glycine receptors (GlyRs) can be located extrasynaptically and can be activated by paracrine release of endogenous agonists, such as GABA, glycine, and taurine. Recently, non-synaptic release of GABA, glycine, and taurine gained further attention with increasing evidence suggesting a developmental role of these neurotransmitters in neuronal network formation before and during synaptogenesis. This review summarizes recent knowledge about the non-synaptic activation of GABA(A)Rs and GlyRs, both in developing and adult CNS. We first present studies that reveal the functional specialization of both non-synaptic GABA(A)Rs and GlyRs and we discuss the neuronal versus non-neuronal origin of the paracrine release of GABA(A)R and GlyR agonists. We then discuss the proposed non-synaptic release mechanisms and/or pathways for GABA, glycine, and taurine. Finally, we summarize recent data about the various roles of non-synaptic GABAergic and glycinergic systems during the development of neuronal networks and in the adult.

Modulation of sensorimotor gating in prepulse inhibition by conditional brain glycine transporter 1 deletion in mice

Inhibition of glycine transporter 1 (GlyT1) augments N-methyl-D-aspartate receptor (NMDAR)-mediated transmission and represents a potential antipsychotic drug target according to the NMDAR hypofunction hypothesis of schizophrenia. Preclinical evaluation of GlyT1 inhibiting drugs using the prepulse inhibition (PPI) test, however, has yielded mixed outcomes. Here, we tested for the first time the impact of two conditional knockouts of GlyT1 on PPI expression. Complete deletion of GlyT1 in the cerebral cortices confers resistance to PPI disruption induced by the NMDAR blocker MK-801 (0.2mg/kg, i.p.) without affecting PPI expression in unchallenged conditions. In contrast, restricting GlyT1 deletion to neurons in forebrain including the striatum significantly attenuated PPI, and the animals remained sensitive to the PPI-disruptive effect of MK-801 at the same dose. These results demonstrate in mice that depending on the regional and/or cell-type specificity, deletion of the GlyT1 gene could yield divergent effects on PPI.