Strychnine-sensitive glycine receptors depress hyperexcitability in rat dentate gyrus

The mRNA expression profile of glycine receptor subunits alpha 1, alpha 2, alpha 4 and beta in female and male mice

Glycine receptors are ligand-gated chloride-selective channels that control excitability in the central nervous system (CNS). Herein, we have investigated the mRNA expression of the glycine receptor alpha 1 (Glra1), alpha 2 (Glra2), alpha 4 (Glra4) and the beta (Glrb) subunits, in adult female and male mice. Single-cell RNA sequencing data re-analysis of the Zeisel et al. (2018) dataset indicated widespread expression of Glra1, Glra2 and Glrb in the CNS, while only a few cells in the cortex, striatum, thalamus, midbrain and the spinal cord expressed Glra4. Highest occurrence of Glra1, Glra2 and Glrb were found in the brainstem. Moreover, Glra1 and Glrb were revealed to have the highest occurrences in the spinal cord of the investigated subunits. However, both Glra2 and Glrb had a more widespread expression in the CNS compared with Glra1 and Glra4. Bulk quantitative real-time-PCR (qRT-PCR) analysis revealed Glra1 expression in the hypothalamus, thalamus, brainstem and the spinal cord, and widespread, but low, Glra2 and Glrb expression in the CNS. Moreover, Glrb could be detected in a few visceral organs. Additionally, females and males were found to express Glra1, Glra2 and Glrb differently in certain brain areas such as the brainstem. Expression levels of Glra4 were too low to be detected using qRT-PCR. Lastly, RNAscope spatially validated the expression of Glra1, Glra2 and Glrb in the areas indicated by the single-cell and bulk analyses, and further revealed that Glra4 can be detected in the cortex, amygdala, hypothalamus, thalamus, brainstem, especially the cochlear nucleus, and in the spinal cord.

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.

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.

Schisandrin B, a dual positive allosteric modulator of GABAA and glycine receptors, alleviates seizures in multiple mouse models

Epilepsy is a prevalent and severe neurological disorder and approximately 30% of patients are resistant to existing medications. It is of utmost importance to develop alternative therapies to treat epilepsy. Schisandrin B (SchB) is a major bioactive constituent of Schisandra chinensis (Turcz.) Baill and has multiple neuroprotective effects, sedative and hypnotic activities. In this study, we investigated the antiseizure effect of SchB in various mouse models of seizure and explored the underlying mechanisms. Pentylenetetrazole (PTZ), strychnine (STR), and pilocarpine-induced mouse seizure models were established. We showed that injection of SchB (10, 30, 60 mg/kg, i.p.) dose-dependently delayed the onset of generalized tonic-clonic seizures (GTCS), reduced the incidence of GTCS and mortality in PTZ and STR models. Meanwhile, injection of SchB (30 mg/kg, i.p.) exhibited therapeutic potential in pilocarpine-induced status epilepticus model, which was considered as a drug-resistant model. In whole-cell recording from CHO/HEK-239 cells stably expressing recombinant human GABAA receptors (GABAARs) and glycine receptors (GlyRs) and cultured hippocampal neurons, co-application of SchB dose-dependently enhanced GABA or glycine-induced current with EC50 values at around 5 μM, and application of SchB (10 μM) alone did not activate the channels in the absence of GABA or glycine. Furthermore, SchB (10 μM) eliminated both PTZ-induced inhibition on GABA-induced current (IGABA) and strychnine (STR)-induced inhibition on glycine-induced current (Iglycine). Moreover, SchB (10 μM) efficiently rescued the impaired GABAARs associated with genetic epilepsies. In addition, the homologous mutants in both GlyRs-α1(S267Q) and GABAARs-α1(S297Q)β2(N289S)γ2L receptors by site-directed mutagenesis tests abolished SchB-induced potentiation of IGABA and Iglycine. In conclusion, we have identified SchB as a natural positive allosteric modulator of GABAARs and GlyRs, supporting its potential as alternative therapies for epilepsy.

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.

Combined alcohol and cannabinoid exposure leads to synergistic toxicity by affecting cerebellar Purkinje cells

Combined use of cannabis and alcohol results in greater psychoactive toxicity than either substance alone, but the underlying central mechanisms behind this worsened outcome remain unclear. Here we show that the synergistic effect of Δ⁹-tetrahydrocannabinol (THC) and ethanol on motor incoordination in mice is achieved by activating presynaptic type 1 cannabinoid receptors (CB₁R) and potentiating extrasynaptic glycine receptors (GlyR) within cerebellar Purkinje cells (PCs). The combination of ethanol and THC significantly reduces miniature excitatory postsynaptic current frequency in a CB₁R-dependent manner, while increasing the extrasynaptic GlyR-mediated chronic chloride current, both leading to decreased PC activity. Ethanol enhances THC actions by boosting the blood-brain-barrier permeability of THC and enriching THC in the cell membrane. Di-desoxy-THC, a designed compound that specifically disrupts THC-GlyR interaction without affecting the basic functions of CB₁R and GlyR, is able to restore PC function and motor coordination in mice. Our findings provide potential therapeutic strategies for overcoming the synergistic toxicity caused by combining cannabis and alcohol use.

The anticonvulsant zonisamide positively modulates recombinant and native glycine receptors at clinically relevant concentrations

GABA_A and glycine receptors mediate fast synaptic inhibitory neurotransmission. Despite studies showing that activation of cerebral glycine receptors could be a potential strategy in the treatment of epilepsy, few studies have assessed the effects of existing anticonvulsant therapies on recombinant or native glycine receptors. We, therefore, evaluated the actions of a series of anticonvulsants at recombinant human homo-oligomeric glycine receptor α1, α2 and α3 subtypes expressed in Xenopus oocytes using two-electrode voltage-clamp methods, and then assessed the most effective drug at native glycine receptors from entorhinal cortex neurons using whole-cell voltage-clamp recordings. Ganaxolone, tiagabine and zonisamide positively modulated glycine induced currents at recombinant homomeric glycine receptors. Of these, zonisamide was the most efficacious and exhibited an EC₅₀ value ranging between 450 and 560 μM at α1, α2 and α3 subtypes. These values were not significantly different indicating a non-selective modulation of glycine receptors. Using a therapeutic concentration of zonisamide (100 μM), the potency of glycine was significantly shifted from 106 to 56 μM at α1, 185 to 112 μM at α2, and 245 to 91 μM at α3 receptors. Furthermore, zonisamide (100 μM) potentiated exogenous homomeric and heteromeric glycine mediated currents from layer II pyramidal cells of the lateral or medial entorhinal cortex. As therapeutic concentrations of zonisamide positively modulate recombinant and native glycine receptors, we propose that the anticonvulsant effects of zonisamide may, at least in part, be mediated via this action.

Sarcosine Suppresses Epileptogenesis in Rats With Effects on Hippocampal DNA Methylation

Epileptogenesis is a common consequence of brain insults, however, the prevention or delay of the epileptogenic process remains an important unmet medical challenge. Overexpression of glycine transporter 1 (GlyT1) is proposed as a pathological hallmark in the hippocampus of patients with temporal lobe epilepsy (TLE), and we previously demonstrated in rodent epilepsy models that augmentation of glycine suppressed chronic seizures and altered acute seizure thresholds. In the present study we evaluated the effect of the GlyT1 inhibitor, sarcosine (aka N-methylglycine), on epileptogenesis and also investigated possible mechanisms. We developed a modified rapid kindling model of epileptogenesis in rats combined with seizure score monitoring to evaluate the antiepileptogenic effect of sarcosine. We used immunohistochemistry and Western blot analysis for the evaluation of GlyT1 expression and epigenetic changes of 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) in the epileptogenic hippocampi of rats, and further evaluated expression changes in enzymes involved in the regulation of DNA methylation, ten-eleven translocation methylcytosine dioxygenase 1 (TET1), DNA-methyltransferase 1 (DNMT1), and DNMT3a. Our results demonstrated: (i) experimental evidence that sarcosine (3 g/kg, i.p. daily) suppressed kindling epileptogenesis in rats; (ii) the sarcosine-induced antiepileptogenic effect was accompanied by a suppressed hippocampal GlyT1 expression as well as a reduction of hippocampal 5mC levels and a corresponding increase in 5hmC; and (iii) sarcosine treatment caused differential expression changes of TET1 and DNMTs. Together, these findings suggest that sarcosine has unprecedented disease-modifying properties in a kindling model of epileptogenesis in rats, which was associated with altered hippocampal DNA methylation. Thus, manipulation of the glycine system is a potential therapeutic approach to attenuate the development of epilepsy.

Removal of hERG potassium channel affinity through introduction of an oxygen atom: Molecular insights from structure-activity relationships of strychnine and its analogs

Nux vomica has been effectively used in Traditional Chinese Medicine. The processing of Nux vomica is necessary to reduce toxicity before it can be used in clinical practice. However, the mechanism for processing detoxification is unclear. hERG channels have been subjected to a routine test for compound cardiac toxicity in the drug development process. Therefore, we examined the effects and mechanisms of strychnine and brucine, two main ingredients of Nux vomica, and their N-oxides on hERG channels. Strychnine and brucine exhibited concentration-dependent inhibition of hERG channels with IC₅₀ values of 25.9 μM and 44.18 μM, respectively. However, their nitrogen oxidative derivatives produced by processing of Nux vomica, strychnine N-oxide and brucine N-oxide, lost their activity on hERG channels. Compared to their parent compounds, only an oxygen atom was introduced in the nitrogen oxidative isoforms to compensate for the N⁺ - charge, suggesting that the protonated nitrogen is the key group for strychnine and brucine binding to hERG channel. Alanine-mutagenesis identified Y652 is the most important residue for strychnine and brucine binding to hERG channel. Y652A mutation increased the IC₅₀ for strychnine and brucine by 21.64-fold and 29.78-fold that of WT I_hERG, respectively. Docking simulations suggested that the protonated nitrogen of strychnine and brucine formed a cation-π interaction with the aromatic ring of Y652. This study suggests that introduction of an oxygen to compensate for the N⁺ - charge could be a useful strategy for reducing hERG potency and increasing the safety margin of alkaloid-type compounds in drug development.

Glycine Receptor Activation Impairs ATP-Induced Calcium Transients in Cultured Cortical Astrocytes

In central nervous system, glycine receptor (GlyR) is mostly expressed in the spinal cord and brainstem, but glycinergic transmission related elements have also been identified in the brain. Astrocytes are active elements at the tripartite synapse, being responsible for the maintenance of brain homeostasis and for the fine-tuning of synaptic activity. These cells communicate, spontaneously or in response to a stimulus, by elevations in their cytosolic calcium (calcium transients, Ca2+T) that can be propagated to other cells. How these Ca2+T are negatively modulated is yet poorly understood. In this work, we evaluated GlyR expression and its role on calcium signaling modulation in rat brain astrocytes. We first proved that GlyR, predominantly subunits α2 and β, was expressed in brain astrocytes and its localization was confirmed in the cytoplasm and astrocytic processes by immunohistochemistry assays. Calcium imaging experiments in cultured astrocytes showed that glycine (500 μM), a GlyR agonist, caused a concentration-dependent reduction in ATP-induced Ca2+T, an effect abolished by the GlyR antagonist, strychnine (0.8 μM), as well as by nocodazole (1 μM), known to impair GlyR anchorage to the plasma membrane. This effect was mimicked by activation of GABAAR, another Cl--permeable channel. In summary, we demonstrated that GlyR activation in astrocytes mediates an inhibitory effect upon ATP induced Ca2+T, which most probably involves changes in membrane permeability to Cl- and requires GlyR anchorage at the plasma membrane. GlyR in astrocytes may thus be part of a mechanism to modulate astrocyte-to-neuron communication.

Cerebral Cortical Circuitry Formation Requires Functional Glycine Receptors

The development of the cerebral cortex is a complex process that requires the generation, migration, and differentiation of neurons. Interfering with any of these steps can impair the establishment of connectivity and, hence, function of the adult brain. Neurotransmitter receptors have emerged as critical players to regulate these biological steps during brain maturation. Among them, α2 subunit-containing glycine receptors (GlyRs) regulate cortical neurogenesis and the present work demonstrates the long-term consequences of their genetic disruption on neuronal connectivity in the postnatal cerebral cortex. Our data indicate that somatosensory cortical neurons of Glra2 knockout mice (Glra2KO) have more dendritic branches with an overall increase in total spine number. These morphological defects correlate with a disruption of the excitation/inhibition balance, thereby increasing network excitability and enhancing susceptibility to epileptic seizures after pentylenetetrazol tail infusion. Taken together, our findings show that the loss of embryonic GlyRα2 ultimately impairs the formation of cortical circuits in the mature brain.

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.

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.

Organization, control and function of extrasynaptic NMDA receptors

N-methyl D-aspartate receptors (NMDARs) exist in different forms owing to multiple combinations of subunits that can assemble into a functional receptor. In addition, they are located not only at synapses but also at extrasynaptic sites. There has been intense speculation over the past decade about whether specific NMDAR subtypes and/or locations are responsible for inducing synaptic plasticity and excitotoxicity. Here, we review the latest findings on the organization, subunit composition and endogenous control of NMDARs at extrasynaptic sites and consider their putative functions. Because astrocytes are capable of controlling NMDARs through the release of gliotransmitters, we also discuss the role of the glial environment in regulating the activity of these receptors.

Glycine receptor-mediated inhibition of medial prefrontal cortical pyramidal cells

Using whole-cell patch clamp recording on medial prefrontal cortical slices of rats aged 17-33 postnatal days, we demonstrated the glycine-induced strychnine-sensitive outward currents. The amplitude of the peak current increased with the concentrations of glycine with an EC50 of 74.7 μM. Application of 1μM strychnine alone to cells caused a slight inward current without blocking the sIPSCs, indicating that GlyRs in the mPFC are activated by an endogenous ligand that can be released tonically. Glycine reversibly depressed firing rate in cells from both layer 6 and layer 3, with significantly greater inhibition on the former than the latter (EC50 12.9 vs 85.6 μM). Glycine hyperpolarized membrane potential in cells of both layer 6 and layer 3 depending on its concentrations, with an IC50 of 99.1 and 207.2 μM, respectively. We propose that GlyRs participate in a novel inhibitory mechanism in mPFC, modulating neuronal activity. This finding further supports an important role of GlyR in cortical function and dysfunction.

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.

Activation of glycine receptors modulates spontaneous epileptiform activity in the immature rat hippocampus

While the expression of glycine receptors in the immature hippocampus has been shown, no information about the role of glycine receptors in controlling the excitability in the immature CNS is available. Therefore, we examined the effect of glycinergic agonists and antagonists in the CA3 region of an intact corticohippocampal preparation of the immature (postnatal days 4-7) rat using field potential recordings. Bath application of 100 μM taurine or 10 μM glycine enhanced the occurrence of recurrent epileptiform activity induced by 20 μM 4-aminopyridine in low Mg(2+) solution. This proconvulsive effect was prevented by 3 μM strychnine or after incubation with the loop diuretic bumetanide (10 μM), suggesting that it required glycine receptors and an active NKCC1-dependent Cl(-) accumulation. Application of higher doses of taurine (≥ 1 mM) or glycine (100 μM) attenuated recurrent epileptiform discharges. The anticonvulsive effect of taurine was also observed in the presence of the GABAA receptor antagonist gabazine and was attenuated by strychnine, suggesting that it was partially mediated by glycine receptors. Bath application of the glycinergic antagonist strychnine (0.3 μM) induced epileptiform discharges. We conclude from these results that in the immature hippocampus, activation of glycine receptors can mediate both pro- and anticonvulsive effects, but that a persistent activation of glycine receptors is required to suppress epileptiform activity. In summary, our study elucidated the important role of glycine receptors in the control of neuronal excitability in the immature hippocampus.

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.

Typical toxic components in traditional Chinese medicine

INTRODUCTION

Traditional Chinese medicine (TCM) is widely used around the world. However, with its wide use has been the identification of a number of toxicological issues that have severely restricted its use in clinical treatment. The identification of these toxic substances within TCM has become somewhat of a hot topic in recent years.

AREAS COVERED

This article reviews literature published on professional authoritative journals in the last 10 years on the toxic constituents and toxicology of TCM, including chemical structures, absorption and metabolism. The literature search for this article was based, but not limited to, toxic constituents including: alkaloids, glycosides, toxic proteins, polypeptide, amino acids, phenols or organic acids, terpenes and lactones. The authors discuss the toxic substances referring to their toxicity on organs, tissues and systems.

EXPERT OPINION

More and more toxic constituents from different TCMs have been identified, in addition to information on how they act in the body at a molecular level. However, the toxicology of TCMs is very complex, and although some progress has been made, a lot work is still needed in order to put an end to toxic incidents.

Glycine receptors support excitatory neurotransmitter release in developing mouse visual cortex

Glycine receptors (GlyRs) are found in most areas of the brain, and their dysfunction can cause severe neurological disorders. While traditionally thought of as inhibitory receptors, presynaptic-acting GlyRs (preGlyRs) can also facilitate glutamate release under certain circumstances, although the underlying molecular mechanisms are unknown. In the current study, we sought to better understand the role of GlyRs in the facilitation of excitatory neurotransmitter release in mouse visual cortex. Using whole-cell recordings, we found that preGlyRs facilitate glutamate release in developing, but not adult, visual cortex. The glycinergic enhancement of neurotransmitter release in early development depends on the high intracellular to extracellular Cl(-) gradient maintained by the Na(+)-K(+)-2Cl(-) cotransporter and requires Ca(2+) entry through voltage-gated Ca(2+) channels. The glycine transporter 1, localized to glial cells, regulates extracellular glycine concentration and the activation of these preGlyRs. Our findings demonstrate a developmentally regulated mechanism for controlling excitatory neurotransmitter release in the neocortex.

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.

Age-related changes of glycine receptor at the rat hippocampus: from the embryo to the adult

Glycinergic inhibitory transmission has been described in spinal cord, but rather disregarded in the brain. The spatial-temporal characterization of glycine receptors (GlyR) in the hippocampus over development is herein reported. GlyR expression increases from late embryonic stage (E18) to 7 days postnatal (P7) and decreases from P7 on. Quantitative real-time PCR showed that GlyR subunit expression changes over neuronal maturation with a preponderance of α2 and α3, over α1 and β. In immature stages, GlyR delineate the cell body of neurons at the Dentate Gyrus and Cornus Ammonis 1 and 3 (CA1/CA3) and are composed of α2 and α3 subunits. At P7, synaptic GlyRα2β can already be observed in the dendritic areas of Dentate Gyrus and of CA1/CA3. In the mature hippocampus, synaptic GlyR decrease and, although a few synaptic GlyRα1β can still be detected in the dendritic layers, extrasynaptic α2/α3-containing GlyR and somatic localized GlyRα3 are the most abundant. Our results point towards an important function of a slow tonic activation of extrasynaptic GlyR, over a fast phasic activation of synaptic GlyRα1β. We clearly show that GlyR are widely expressed in hippocampus and that their subcellular localization and subunit composition change over development.

Tonic GABAergic control of mouse dentate granule cells during postnatal development

The dentate gyrus is the main hippocampal input structure receiving strong excitatory cortical afferents via the perforant path. Therefore, inhibition at this 'hippocampal gate' is important, particularly during postnatal development, when the hippocampal network is prone to seizures. The present study describes the development of tonic GABAergic inhibition in mouse dentate gyrus. A prominent tonic GABAergic component was already present at early postnatal stages (postnatal day 3), in contrast to the slowly developing phasic postsynaptic GABAergic currents. Tonic currents were mediated by GABA(A) receptors containing α(5)- and δ-subunits, which are sensitive to low ambient GABA concentrations. The extracellular GABA level was determined by synaptic GABA release and GABA uptake via the GABA transporter 1. The contribution of these main regulatory components was surprisingly stable during postnatal granule cell maturation. Throughout postnatal development, tonic GABAergic signals were inhibitory. They increased the action potential threshold of granule cells and reduced network excitability, starting as early as postnatal day 3. Thus, tonic inhibition is already functional at early developmental stages and plays a key role in regulating the excitation/inhibition balance of both the adult and the maturing dentate gyrus.

Glycinergic inhibition in the hippocampus

Glycine and GABA are the two main inhibitory neurotransmitters in the central nervous system (CNS). While GABA receptors in the hippocampus have been studied in great detail, the role of glycine receptors (GlyRs) in the hippocampus is less understood. Here we examine recent evidence suggesting that GlyRs are present and active throughout the hippocampus. Extracellular glycine levels are controlled through a combination of release and transport mechanisms, both of which, along with the GlyRs themselves, can be modulated by a number of factors. We discuss the role of GlyRs in suppressing excitation by decreasing postsynaptic membrane resistance in the hippocampus, as well as the contribution of GlyRs to both short- and long-term plasticity.

Clozapine inhibits strychnine-sensitive glycine receptors in rat hippocampal neurons

Seizure is one kind of severe side effects during clozapine (Clo) treatment. However, the mechanism of seizure associated with Clo therapy is not completely clear. Strychnine-sensitive glycine receptors (GlyRs) play an important role in regulating the excitability in the hippocampus. In the present study, we investigated the effect of Clo on GlyRs in cultured hippocampal neurons of rats. Clo reversibly inhibited the glycine-induced chloride currents (I(Gly)) in a concentration-dependent manner. The half-maximal effect concentration (EC(50)) for glycine alone was 25.6+/-0.7 microM with the Hill coefficient 1.5+/-0.1; in the presence of Clo, the EC(50) and the Hill coefficient were 28.9+/-6.3 microM and 1.2+/-0.3 respectively, which were not significantly affected by Clo. In addition, Clo inhibition of I(Gly) was not influenced by blocking D(1) and D(2) dopamine receptors with haloperidol (Hal). Taken together, these results suggest that Clo is a non-competitive antagonist of GlyR independent of its activation of dopamine receptors, which may contribute to seizure associated with Clo therapy.

Glycine receptor in rat hippocampal and spinal cord neurons as a molecular target for rapid actions of 17-beta-estradiol

Glycine receptors (GlyRs) play important roles in regulating hippocampal neural network activity and spinal nociception. Here we show that, in cultured rat hippocampal (HIP) and spinal dorsal horn (SDH) neurons, 17-β-estradiol (E₂) rapidly and reversibly reduced the peak amplitude of whole-cell glycine-activated currents (I_Gly). In outside-out membrane patches from HIP neurons devoid of nuclei, E₂ similarly inhibited I_Gly, suggesting a non-genomic characteristic. Moreover, the E₂ effect on I_Gly persisted in the presence of the calcium chelator BAPTA, the protein kinase inhibitor staurosporine, the classical ER (i.e. ERα and ERβ) antagonist tamoxifen, or the G-protein modulators, favoring a direct action of E₂ on GlyRs. In HEK293 cells expressing various combinations of GlyR subunits, E₂ only affected the I_Gly in cells expressing α2, α2β or α3β subunits, suggesting that either α2-containing or α3β-GlyRs mediate the E₂ effect observed in neurons. Furthermore, E₂ inhibited the GlyR-mediated tonic current in pyramidal neurons of HIP CA1 region, where abundant GlyR α2 subunit is expressed. We suggest that the neuronal GlyR is a novel molecular target of E₂ which directly inhibits the function of GlyRs in the HIP and SDH regions. This finding may shed new light on premenstrual dysphoric disorder and the gender differences in pain sensation at the CNS level.

Glycinergic tonic inhibition of hippocampal neurons with depolarizing GABAergic transmission elicits histopathological signs of temporal lobe epilepsy

An increasing number of epilepsy patients are afflicted with drug-resistant temporal lobe epilepsy (TLE) and require alternative therapeutic approaches. High-affinity glycine receptors (haGlyRs) are functionally adapted to tonic inhibition due to their response to hippocampal ambient glycine, and their synthesis is activity-dependent. Therefore, in our study, we scanned TLE hippocampectomies for expression of haGlyRs and characterized the effects mediated by these receptors using primary hippocampal neurons. Increased haGlyR expression occurred in TLE hippocampi obtained from patients with a severe course of disease. Furthermore, in TLE patients, haGlyR and potassium chloride cotransporter 2 (KCC2) expressions were inversely regulated. To examine this potential causal relationship with respect to TLE histopathology, we established a hippocampal cell culture system utilising tonic inhibition mediated by haGlyRs in response to hippocam-pal ambient glycine and in the context of a high Cl equilibrium potential, as is the case in TLE hippocampal neurons. We showed that hypoactive neurons increase their ratio between glutamatergic and GABAergic synapses, reduce their dendrite length and finally undergo excitotoxicity. Pharmacological dissection of the underlying processes revealed ionotropic glutamate and TrkB receptors as critical mediators between neuronal hypoactivity and the emergence of these TLE-characteristic histopathological signs. Moreover, our results indicate a beneficial role for KCC2, because decreasing the Cl- equilibrium potential by KCC2 expression also rescued hypoactive hippocampal neurons. Thus, our data support a causal relationship between increased haGlyR expression and the emergence of histopathological TLE-characteristic signs, and they establish a pathophysiological role for neuronal hypoactivity in the context of a high Cl- equilibrium potential.

Alpha2 subunit specificity of cyclothiazide inhibition on glycine receptors

In the mammalian cortex, alpha2 subunit-containing glycine receptors (GlyRs) mediate tonic inhibition, but the precise functional role of this type of GlyRs is difficult to establish because of the lack of subtype-selective antagonist. In this study, we found that cyclothiazide (CTZ), an epileptogenic agent, potently inhibited GlyR-mediated current (I(Gly)) in cultured rat hippocampal neurons. The inhibition was glycine concentration-dependent, suggesting a competitive mechanism. Note that GlyRs containing the alpha2 but not alpha1 or alpha3 subunits, when being heterologously expressed in human embryonic kidney 293T cells, were inhibited by CTZ, indicating subunit specificity of CTZ action. In addition, the degree of CTZ inhibition on I(Gly) in rat spinal neurons declined with time in culture, in parallel with a decline of alpha2 subunit expression, which is known to occur during spinal cord development. Furthermore, site-directed mutagenesis indicates that a single-amino acid threonine at position 59 near the N terminus of the alpha2 subunit confers the specificity of CTZ action. Thus, CTZ is a potent and selective inhibitor of alpha2-GlyRs, and threonine at position 59 plays a critical role in the susceptibility of GlyR to CTZ inhibition.

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

Functional glycine receptors (GlyRs) are enriched in the hippocampus, but their role in hippocampal function remains unclear. Since the concentration of ambient glycine is determined by the presence of powerful glycine transporter (GlyT), we blocked the reuptake of glycine in hippocampal slices to examine the role of GlyRs. Antagonists of GlyT type 1 (GlyT1) but not that of GlyT type 2 (GlyT2) induced excitatory postsynaptic potential (EPSP)-spike depression, which was reversed by the specific GlyR antagonist strychnine. Moreover, endogenously elevating the glycine concentration with the GlyT1 antagonists facilitated NMDA receptor-dependent long-term potentiation induction, and elicited a strychnine-sensitive chloride current. In addition, impairment of glial function with fluoroacetate blocked the effect of GlyT1 antagonists on the EPSP-spike curve. Furthermore, pretreatment with sarcosine was effective in controlling pentylenetetrazol-induced seizures. These results indicate an essential role of GlyTs in fine-tuning tonic activation of GlyRs and suggest a potential role of GlyR-dependent EPSP-spike depression in hippocampal network stability.

Regulation of excitation by glycine receptors

Our knowledge of glycine receptor (GlyR) regulation of excitation has advanced significantly in recent years. GlyRs are widespread in the CNS, are heterogeneous, and undergo developmental changes. Activation of GlyRs of immature neurons induces outflow of Cl( - ), membrane depolarization, neuronal excitation, calcium influx, and transmitter release, in contrast to the inhibitory effects these receptors have in mature neurons. Thus, GlyRs are important for neuronal excitability in both the developing and the mature CNS. This chapter is an overview of selective studies on the newly discovered roles of GlyRs in regulating neuronal excitation, and inhibition, particularly in the upper brain areas.

A substance in broad beans (Vicia faba) is protective against experimentally induced convulsions in mice

Vicia faba (VF, broad beans) constitutes a major food item for the River Nile populations. Contrary to tropical Africa, the prevalence rate of epilepsy (0.9-1 per 1000) among schoolchildren of Khartoum Province, Sudan, is lower compared with the rates in Europe and North America. To explore whether broad beans contain any anticonvulsant that can explain this observation, Balb/c mice were either treated with VF extract (0.01 mL/g) or kept as a control. Various doses of strychnine and picrotoxin were used to explore the effect of VF extract on strychnine-sensitive glycine receptors and GABA(A) receptors. Diazepam (DIZ) was used as anticonvulsant. Thin-layer chromatography was run for the extract against phenobarbital, DIZ, and/or glycine. VF extract demonstrated a clear protective effect against strychnine-induced convulsions and death in Balb/c mice. Diazepam (20 mg/kg, intraperitoneally), administered 20 minutes prior to strychnine (0.112 mg/kg, intraperitoneally), increased the survival rate to 66.7% and, when given with VF extract (0.01 mL/g), to 100%. Various doses of DIZ protected against picrotoxin-induced convulsions (40 mg/kg, intraperitoneally) and deaths. Pretreatment of mice with VF extract was not protective. On chromatography, VF extract separated in a manner similar to the glycine spot, and revealed one peak coinciding with, but not identical to that of glycine. The extract of broad beans (VF) protects against convulsions probably through the inhibitory glycine receptors, and may contain a substance that is intimately related to glycine. Further research is needed to substantiate this by directly assessing the binding of the VF extract to the glycine receptors or a change in receptor physiology.

Zinc Enhances the Inhibitory Effects of Strychnine-Sensitive Glycine Receptors in Mouse Hippocampal Neurons

Although extracellular Zn2+ is an endogenous biphasic modulator of strychnine-sensitive glycine receptors (GlyRs), the physiological significance of this modulation remains poorly understood. Zn2+ modulation of GlyR may be especially important in the hippocampus where presynaptic Zn2+ is abundant. Using cultured embryonic mouse hippocampal neurons, we examined whether 1 μM Zn2+, a potentiating concentration, enhances the inhibitory effects of GlyRs activated by sustained glycine applications. Sustained 20 μM glycine (EC25) applications alone did not decrease the number of action potentials evoked by depolarizing steps, but they did in 1 μM Zn2+. At least part of this effect resulted from Zn2+ enhancing the GlyR-induced decrease in input resistance. Sustained 20 μM glycine applications alone did not alter neuronal bursting, a form of hyperexcitability induced by omitting extracellular Mg2+. However, sustained 20 μM glycine applications depressed neuronal bursting in 1 μM Zn2+. Zn2+ did not enhance the inhibitory effects of sustained 60 μM glycine (EC70) applications in these paradigms. These results suggest that tonic GlyR activation could decrease neuronal excitability. To test this possibility, we examined the effect of the GlyR antagonist strychnine and the Zn2+ chelator tricine on action potential firing by CA1 pyramidal neurons in mouse hippocampal slices. Co-applying strychnine and tricine slightly but significantly increased the number of action potentials fired during a depolarizing current step and decreased the rheobase for action potential firing. Thus Zn2+ may modulate neuronal excitability normally and in pathological conditions such as seizures by potentiating GlyRs tonically activated by low agonist concentrations.

Mechanisms of homomeric alpha1 glycine receptor endocytosis

Little is known regarding the mechanism(s) by which glycine receptors are endocytosed. Here we examined the endocytosis of homomeric alpha1 glycine receptors expressed in HEK 293 cells using immunofluorescence/confocal microscopy and whole-cell patch-clamp recordings. Our studies demonstrate that constitutive endocytosis of glycine receptors is blocked by the dominant negative dynamin construct K44A and that intracellular dialysis with peptide P4, a dynamin/amphiphysin-disrupting peptide, increased whole-cell glycine-gated chloride currents. To examine whether receptor endocytosis could be regulated by PKC, experiments with the PKC activator PMA (phorbol 12-myristate 13-acetate) were performed. PMA, but not its inactive analogue PMM (phorbol 12-monomyristate), stimulated receptor endocytosis and inhibited glycine-gated chloride currents. Similar to constitutive endocytosis, PKC-stimulated endocytosis was blocked by dynamin K44A. Mutation of a putative AP2 adaptin dileucine motif (L314A, L315A) present in the receptor cytoplasmic loop blocked PMA-stimulated receptor endocytosis and also prevented PMA inhibition of glycine receptor currents. In patch-clamp experiments, intracellular dialysis of a 12-amino acid peptide corresponding to the region of the receptor containing the dileucine motif prevented PKC modulation of wild-type glycine receptors. Unlike PKC modulation of the receptor, constitutive endocytosis was not affected by mutation of this dileucine motif. These results demonstrate that PKC activation stimulates glycine receptor endocytosis, that both constitutive endocytosis and PKC-stimulated endocytosis are dynamin-dependent, and that PKC-stimulated endocytosis, but not constitutive endocytosis, occurs via the dileucine motif (L314A, L315A) within the cytoplasmic loop of the receptor.

Quercetin subunit specifically reduces GlyR-mediated current in rat hippocampal neurons

Quercetin is a substance of low molecular weight found in vascular plants with a wide range of biological activities including antioxidative and anti-inflammatory activities. In the present study, the effects of quercetin on native glycine receptors (GlyRs) in cultured rat hippocampal neurons were investigated using a whole-cell patch-clamp technique. Quercetin reversibly and concentration-dependently depressed glycine-induced current (I(Gly)), with an IC50 of 10.7+/-0.24 microM and a Hill coefficient of 1.08+/-0.12. Quercetin depressed maximum I(Gly) and significantly changed the EC50 for glycine and the Hill coefficient. Kinetic analysis indicated that quercetin accelerated the rates of desensitization. Interestingly, after the end of glycine with quercetin coapplication, a transient rebound occurred. The quercetin effects also displayed voltage-dependence, being greater at positive membrane potentials. These effects suggested that quercetin may act as an open channel blocker. Furthermore, in the sequential application protocol, quercetin inhibited the peak amplitude of I(Gly) to a macroscopic degree while slowing GlyR desensitization. These effects implied that quercetin has a depressant effect independent of GlyR channel's opening, which maybe caused by an allosteric mechanism. Strikingly, quercetin inhibited the amplitude of recombinant-induced current mediated by alpha2-, alpha2beta-, alpha3- and alpha3beta-GlyRs but had no effects on alpha1- and alpha1beta-GlyRs that were expressed in HEK293T cells. We also investigated the effects of quercetin on I(Gly) in spinal neurons during development in vitro. The extent of blockade by quercetin on I(Gly) was slighter in spinal neurons than in hippocampal neurons in a development-dependent manner. Taken together, our results suggest that quercetin has possible effects in information processing within a neuronal network by inhibition of I(Gly) and may be useful as a pharmacological probe for identifying the subunit types of GlyRs.

Glycine receptor activation regulates short-term plasticity in CA1 area of hippocampal slices of rats

Functional glycine receptors (GlyRs) are enriched in the hippocampus, but their roles in synaptic transmission are unclear. In this study, we examined the effect of GlyR activation on paired-pulse stimulation of the whole-cell postsynaptic currents (PSCs) in the Schaffer-CA1 synapses in rat hippocampal slices. Bath application of glycine reduced the amplitude of PSCs, accompanied by an increase in holding current and resting conductance. Moreover, glycine application increased the paired-pulse ratio (PPR) of PSCs significantly, an effect largely abolished by the GlyR specific antagonist strychnine. Interestingly, glycine application had no significant effect on either the amplitude or the PPR of excitatory postsynaptic currents (EPSCs). Our findings suggest that GlyR activation regulates hippocampal short-term plasticity by altering GABAergic neurotransmission.

Glycine receptors in CNS neurons as a target for nonretrograde action of cannabinoids

At many central synapses, endocannabinoids released by postsynaptic cells act retrogradely on presynaptic G-protein-coupled cannabinoid receptors to inhibit neurotransmitter release. Here, we demonstrate that cannabinoids may directly affect the functioning of inhibitory glycine receptor (GlyR) channels. In isolated hippocampal pyramidal and Purkinje cerebellar neurons, endogenous cannabinoids anandamide and 2-arachidonylglycerol, applied at physiological concentrations, inhibited the amplitude and altered the kinetics of rise time, desensitization, and deactivation of the glycine-activated current (I(Gly)) in a concentration-dependent manner. These effects of cannabinoids were observed in the presence of cannabinoid CB1/CB3, vanilloid receptor 1 antagonists, and the G-protein inhibitor GDPbetaS, suggesting a direct action of cannabinoids on GlyRs. The effect of cannabinoids on I(Gly) desensitization was strongly voltage dependent. We also demonstrate that, in the presence of a GABA(A) receptor antagonist, GlyRs may contribute to the generation of seizure-like activity induced by short bursts (seven stimuli) of high-frequency stimulation of inputs to hippocampal CA1 region, because this activity was diminished by selective GlyR antagonists (strychnine and ginkgolides B and J). The GlyR-mediated rhythmic activity was also reduced by cannabinoids (anandamide) in the presence of a CB1 receptor antagonist. These results suggest that the direct inhibition of GlyRs by endocannabinoids can modulate the hippocampal network activity.

Glycine-gated chloride channels depress synaptic transmission in rat hippocampus

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus is beginning to be appreciated. We have reported previously that CA1 pyramidal cells and GABAergic interneurons recorded in 3- to 4-wk-old rat hippocampal slices express functional GlyRs, dispelling previous misconceptions that GlyR expression ceases in early development. However, the effect of GlyR activation on cell excitability and synaptic circuits in hippocampus has not been fully explored. Using whole cell current-clamp recordings, we show that activation of strychnine-sensitive GlyRs through exogenous glycine application causes a significant decrease in input resistance and prevents somatically generated action potentials in both CA1 pyramidal cells and interneurons. Furthermore, GlyR activation depresses the synaptic network by reducing suprathreshold excitatory postsynaptic potentials (EPSPs) to subthreshold events in both cell types. Blockade of postsynaptic GlyRs with the chloride channel blocker 4, 4'-diisothiocyanatostilbene-2-2'-disulfonic acid (DIDS) or altering the chloride ion driving force in recorded cells attenuates the synaptic depression, strongly indicating that a postsynaptic mechanism is responsible. Increasing the local glycine concentration by blocking reuptake causes a strychnine-sensitive synaptic depression in interneuron recordings, suggesting that alterations in extracellular glycine will impact excitability in hippocampal circuits. Finally, using immunohistochemical methods, we show that glycine and the glycine transporter GlyT2 are co-localized selectively in GABAergic interneurons, indicating that interneurons contain both inhibitory neurotransmitters. Thus we report a novel mechanism whereby activation of postsynaptic GlyRs can function to depress activity in the synaptic network in hippocampus. Moreover, the co-localization of glycine and GABA in hippocampal interneurons, similar to spinal cord, brain stem, and cerebellum, suggests that this property is likely to be a general characteristic of inhibitory interneurons throughout the CNS.

RNA editing produces glycine receptor alpha3(P185L), resulting in high agonist potency

The function of supramedullary glycine receptors (GlyRs) is still unclear. Using Wistar rat collicular slices, we demonstrate GlyR-mediated inhibition of spike discharge elicited by low glycine (10 microM). Searching for the molecular basis of this phenomenon, we identified a new GlyR isoform. GlyR alpha3(P185L), a result of cytidine 554 deamination, confers high glycine sensitivity (EC50 approximately 5 microM) to neurons and thereby promotes the generation of sustained chloride conductances associated with tonic inhibition. The level of GlyR alpha3-C554U RNA editing is sensitive to experimentally induced brain lesion, inhibition of cytidine deamination by zebularine and inhibition of mRNA transcription by actinomycin D, but not to blockade of protein synthesis by cycloheximide. Conditional regulation of GlyR alpha3(P185L) is thus likely to be part of a post-transcriptional adaptive mechanism in neurons with enhanced excitability.

Developmental regulation of beta-carboline-induced inhibition of glycine-evoked responses depends on glycine receptor beta subunit expression

In this work, we show that beta-carbolines, which are known negative allosteric modulators of GABA(A) receptors, inhibit glycine-induced currents of embryonic mouse spinal cord and hippocampal neurons. In both cell types, beta-carboline-induced inhibition of glycine receptor (GlyR)-mediated responses decreases with time in culture. Single-channel recordings show that the major conductance levels of GlyR unitary currents shifts from high levels (> or = 50 pS) in 2 to 3 days in vitro (DIV) neurons to low levels (<50 pS) in 11 to 14 DIV neurons, assessing the replacement of functional homomeric GlyR by heteromeric GlyR. In cultured spinal cord neurons, the disappearance of beta-carboline inhibition of glycine responses and high conductance levels is almost complete in mature neurons, whereas a weaker decrease in beta-carboline-evoked glycine response inhibition and high conductance level proportion is observed in hippocampal neurons. To confirm the hypothesis that the decreased sensitivity of GlyR to beta-carbolines depends on beta subunit expression, Chinese hamster ovary cells were permanently transfected either with GlyR alpha2 subunit alone or in combination with GlyR beta subunit. Single-channel recordings revealed that the major conductance levels shifted from high levels (> or = 50 pS) in GlyR-alpha2-transfected cells to low levels (<50 pS) in GlyR-alpha2+beta-containing cells. Consistently, both picrotoxin- and beta-carboline-induced inhibition of glycine-gated currents were significantly decreased in GlyR-alpha2+beta-transfected cells compared with GlyR-alpha2-containing cells. In summary, we demonstrate that the incorporation of beta subunits in GlyRs confers resistance not only to picrotoxin but also to beta-carboline-induced inhibition. Furthermore, we also provide evidence that hippocampal neurons undergo in vitro a partial maturation process of their GlyR-mediated responses.

Morphologically identified glycinergic synapses in the hippocampus

Inhibitory transmission in the hippocampus is predominantly GABAergic, but electrophysiological data evidenced strychnine-sensitive glycine-induced currents. However, synaptic currents have not been reported. Here, we describe, for the first time, the presence of GlyR clusters in several areas of the hippocampus as well as in cultured hippocampal neurons. In contrast with spinal cord, hippocampal GlyRs contain alpha2 but no alpha1 subunit. Optical and electron microscopy indicates that GlyRs can be synaptic as well as extrasynaptic. Synaptic GlyRs were apposed to glycinergic boutons characterized by the expression of the vesicular and the plasma membrane transporters of glycine (VIAAT and GlyT2, respectively). Double labeling with calcium-binding proteins showed that GlyT2 could be detected in boutons innervating both excitatory cells (soma and dendrites) and interneurons. Finally, GlyR clusters could be detected at synaptic sites with the GABAA receptor gamma2 subunit and gephyrin, suggesting that mixed GABA/glycine synapses might exist in the hippocampus.