Characteristics of hippocampal glycine release in cell-damaging conditions in the adult and developing mouse

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.

Astrocytic Regulation of Glutamate Transmission in Schizophrenia

According to the glutamate hypothesis of schizophrenia, the abnormality of glutamate transmission induced by hypofunction of NMDA receptors (NMDARs) is causally associated with the positive and negative symptoms of schizophrenia. However, the underlying mechanisms responsible for the changes in glutamate transmission in schizophrenia are not fully understood. Astrocytes, the major regulatory glia in the brain, modulate not only glutamate metabolism but also glutamate transmission. Here we review the recent progress in understanding the role of astrocytes in schizophrenia. We focus on the astrocytic mechanisms of (i) glutamate synthesis via the glutamate-glutamine cycle, (ii) glutamate clearance by excitatory amino acid transporters (EAATs), (iii) D-serine release to activate NMDARs, and (iv) glutamatergic target engagement biomarkers. Abnormality in these processes is highly correlated with schizophrenia phenotypes. These findings will shed light upon further investigation of pathogenesis as well as improvement of biomarkers and therapies for 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.

Effect of ginkgolide B on striatal extracellular amino acids in middle cerebral artery occluded rats

ETHNOPHARMACOLOGICAL RELEVANCE

Ginkgo biloba leaves are traditionally used in China for its health-promoting properties. There is substantial experimental evidence to support the view that Ginkgo biloba extracts have neuroprotective properties under conditions such as hypoxia/ischemia. Although a number of studies have investigated that ginkgolide B, a purified terpene lactone component extracted from Ginkgo biloba leaves, is available "platelet activating factor (PAF) receptors antagonist", "antioxidant" with a variety of actions, very little has been performed to explore the effect of ginkgolide B on extracellular amino acids in experimental animal of focal cerebral ischemia/reperfusion. In this study, the effect of ginkgolide B on the striatal extracellular levels of glutamate (Glu), aspartic acid (Asp), glycine (Gly) and γ-aminobutyric acid (GABA) was evaluated in rats undergone middle cerebral artery occlusion (MCAO) for 1h followed by 23 h reperfusion.

MATERIALS AND METHODS

The Sprague-Dawley (SD) rats received intraperitoneal injections of ginkgolide B dissolved at a dose of 10 mg kg(-1)d(-1), 20 mg kg(-1)d(-1), or normal saline (NS) of same volume 3d before the middle cerebral artery occlusion model establishment. Extracellular concentrations of glutamate, aspartic acid, glycine and GABA in striatum were monitored using in vivo microdialysis and analyzed using high-performance liquid chromatography. Excitotoxic index (EI) was calculated. Twenty-four hours after MCAO, the cerebral infarct volume was detected on 2,3,5-triphenyltetrazolium chloride-stained coronal sections.

RESULTS

The result showed that administration of ginkgolide B (10 or 20 mg kg(-1)) before ischemia reduced the ischemia-induced elevation of levels of glutamate, aspartic acid and glycine, increased the elevation of extracellular GABA, decreased the excitotoxic index and diminished the volume of cerebral infarction, although a clear concentration-response relationship was not found.

CONCLUSIONS

The present work provides the first evidence that ginkgolide B protects against cerebral ischemic injury by inhibiting excitotoxicity by modulating the imbalance of excitatory amino acids versus inhibitory amino acids, which may support the traditional use of Ginkgo biloba leaves for the treatment of stroke.

Ionic dysregulations typical of ischemia provoke release of glycine and GABA by multiple mechanisms

Energy deprivation during ischemia causes dysregulations of ions, particularly sodium, potassium and calcium. Under these conditions, release of neurotransmitters is often enhanced and can occur by multiple mechanisms. The aim of this work was to characterize the modes of exit of glycine and GABA from nerve endings exposed to stimuli known to reproduce some of the ionic changes typical of ischemic conditions. Their approach was chosen instead of application of ischemic conditions because the release evoked during ischemia is mechanistically too heterogeneous. Mouse hippocampus and spinal cord synaptosomes, pre-labeled with [(3)H]glycine or [(3)H]GABA, were exposed in superfusion to 50 mM KCl or to 10 microM veratridine. The evoked overflows differed greatly between the two transmitters and between the two regions examined. Significant portions of the K(+)- and the veratridine-evoked overflows occurred by classical exocytosis. Carrier-mediated release of GABA, but not of glycine, was evoked by high K(+); GABA and, less so, glycine were released through transporter reversal by veratridine. External calcium-dependent overflows were only in part sensitive to omega-conotoxins; significant portions occurred following reversal of the plasmalemmal Na(+)/Ca(2+) exchanger. Finally, a relevant contribution to the overall transmitter overflows came from cytosolic calcium originating through the mitochondrial Na(+)/Ca(2+) exchanger. To conclude, ionic dysregulations typical of ischemia cause neurotransmitter release by heterogeneous mechanisms that differ depending on the transmitters and the CNS regions examined.

Chronological distribution of Rip immunoreactivity in the gerbil hippocampus during normal aging

Age-dependent studies on oligodendrocytes, which are the myelinating cells in the central nervous system, have been relatively less investigated. We examined age-dependent changes in Rip immunoreactivity and its protein level in the gerbil hippocampus during normal aging using immunohistochemistry and Western blot analysis with Rip antibody, an oligodendrocyte marker. Rip immunoreactivity and its protein level in the hippocampal CA1 region significantly increased at postnatal month 3 (PM 3). Thereafter, they decreased in the hippocampal CA1 region with age. At PM 24, Rip immunoreactive processes in the hippocampal CA1 region markedly decreased in the stratum radiatum. In the hippocampal CA2/3 region and dentate gyrus, the pattern of changes in Rip immunoreactivity and its protein level was similar to those in the hippocampal CA1 region; however, no significant changes were found in the CA2/3 region and dentate gyrus at various age stages. These results indicate that Rip immunoreactivity and protein level in the hippocampal CA1 region decreases significantly at PM 24 compared to the CA2/3 region and dentate gyrus.

Characteristics and Regulation of Glycine Transport in Bergmann Glia

In the vertebrate CNS, glycine acts as an inhibitory neurotransmitter and as the obligatory coagonist of glutamate at N-methyl-D-aspartate receptors. These roles depend on extracellular glycine levels, regulated by Na+/Cl--dependent transporters GLYT1, present mainly in glial cells, and GLYT2, predominantly neuronal. In Bergmann glia, GLYT1 mediates both, glycine uptake and efflux, which, in turn, influences excitatory neurotransmission at Purkinje cell synapses. The biochemical properties of GLYTs and their regulation by signaling pathways in these cells are largely unknown. We characterized Gly uptake in confluent primary cultures of Bergmann glia from chick cerebellum. Transport was found to be energy- and Na+-dependent, and was resolved into a high (Km=25 microM) and a low affinity (Km=1.1 mM) components identified as GLYT1 and transport System A, respectively. Results show that high affinity transport by GLYT1 is regulated by calcium from intracellular stores, calmodulin, and myosin light chain kinase through an actin cytoskeleton-mediated action.

Intrahippocampal administration of the branched-chain α-hydroxy acids accumulating in maple syrup urine disease compromises rat performance in aversive and non-aversive behavioral tasks

Maple syrup urine disease (MSUD) is an inherited metabolic disease predominantly characterized by neurological dysfunction. Although a variable degree of psychomotor/delay/mental retardation is found in a considerable number of MSUD patients, the mechanisms underlying the neuropathology of this disorder are yet not defined. The present study investigated the effect of acute intrahippocampal administration of the branched-chain alpha-hydroxy acids (BCHA) accumulating in MSUD on rat behavior in non-aversive (open field) and aversive (inhibitory avoidance) tasks. Cannulated 60-day-old male Wistar rats received bilateral intrahippocampal injection of alpha-hydroxyisocaproic acid (HIC, 1.5 micromol), alpha-hydroxyisovaleric acid (HIV, 2.5 micromol), alpha-hydroxy-beta-methyl-n-valeric acid (HMV, 1.5 micromol), or NaCl (2.5 micromol)(controls) immediately after or 10 min before training. Testing session was performed 24 h later. Administration of the hydroxy acids immediately after training caused no effect on the cognitive performance of the rats. In contrast, HIV and HMV administered 10 min before training provoked a habituation deficit in the open field task. Motor activity, assessed by crossing responses, was the same for the groups infused with BCHA and NaCl. The effect of MK-801, succinate, creatine, and the antioxidants ascorbic acid plus alpha-tocopherol on the behavioral alterations provoked by HIV in the open field task revealed that only the energetic substrates (succinate and creatine) prevented these effects, reflecting a possible compromise of brain energy production by HIV. We also observed that rats pretreated with HIC, HIV, or HMV did not increase their latency in the testing session in the step-down inhibitory avoidance task, revealing an impairment of retrieval (memory retention or acquisition) in this task. Furthermore, no differences between controls and rats receiving BCHA were detected in the latency to leave the platform in the training test, suggesting similar motor activity of all groups. The data indicate that the alpha-hydroxy acids accumulating in MSUD impair cognition and may be implicated in the neuropathology and psychomotor delay/mental retardation observed in the affected patients.

Glycine-induced neurotoxicity in organotypic hippocampal slice cultures

The role of the neutral amino acid glycine in excitotoxic neuronal injury is unclear. Glycine coactivates glutamate N-methyl-D-aspartate (NMDA) receptors by binding to a distinct recognition site on the NR1 subunit. Purely excitatory glycine receptors composed of NR1 and NR3/NR4 NMDA receptor subunits have recently been described, raising the possibility of excitotoxic effects mediated by glycine alone. We have previously shown that exposure to high concentrations of glycine causes extensive neurotoxicity in organotypic hippocampal slice cultures by activation of NMDA receptors. In the present study, we investigated further properties of in vitro glycine-mediated toxicity. Agonists of the glycine recognition site of NMDA receptors (D-serine and D-alanine) did not have any toxic effect in hippocampal cultures, whereas competitive blockade of the glycine site by 7-chlorokynurenic acid was neuroprotective. Stimulation (taurine, beta-alanine) or inhibition (strychnine) of the inhibitory strychnine-sensitive glycine receptors did not produce any neurotoxicity. The toxic effects of high-dose glycine were comparable in extent to those produced by the excitatory amino acid glutamate in our model. When combined with sublethal hypoxia/hypoglycemia, the threshold of glycine toxicity was decreased to less than 1 mM, which corresponds to the range of concentrations of excitatory amino acids measured during in vivo cerebral ischemia. Taken together, these results further support the assumption of an active role of glycine in excitotoxic neuronal injury.

Intrahippocampal administration of the alpha-keto acids accumulating in maple syrup urine disease provokes learning deficits in rats

Learning disability is a common feature of patients affected by maple syrup urine disease (MSUD). However, the pathomechanisms underlying learning deficit in this disorder are poorly known. In the present study, we investigated the effect of acute administration of the alpha-keto acids accumulating in MSUD into the hippocampus on the behavior of rats in the open field and in the inhibitory avoidance tasks. Adult male Wistar rats received intrahippocampal injections of alpha-ketoisocaproic acid (KIC, 8 micromol), alpha-ketoisovaleric acid (KIV, 5 micromol), alpha-keto-beta-methylvaleric acid (KMV, 5 micromol), or NaCl (8 micromol) (controls) immediately after or 10 min before training. Testing session was performed 24 h later. Posttraining administration of the keto acids had no effect on learning in the open-field task. In contrast, pretraining administration of KIV and KMV impaired habituation in the open field. Similarly, pretraining administration of KIC, KIV, and KMV affected rat performance in the inhibitory avoidance task, suggesting disruption of acquisition. The results indicate that the alpha-keto acids accumulating in MSUD induce learning deficits in aversive and nonaversive tasks. We therefore suggest that these findings may be related to the psychomotor delay/mental retardation observed in MSUD, and may indicate the contribution of increased brain concentrations of these organic acids to the pathophysiology of the neurological dysfunction of MSUD patients.

Bergmann glial GlyT1 mediates glycine uptake and release in mouse cerebellar slices

Glycine is an inhibitory neurotransmitter and is critical for NMDA receptor activation. These roles are dependent on extracellular glycine levels, which are regulated by Na(+)/Cl(-)-dependent glycine transporters (GlyTs) in neurones and glia. The glial GlyT subtype GlyT1 is well located to activate NMDA receptors. However, glial GlyTs have not been studied in an intact system thus far. Whole-cell patch-clamp recordings were obtained from Bergmann glia in mice cerebellar slices to determine whether these glia express functional GlyT1 that can mediate both glycine uptake and efflux. In the presence of a glycine receptor blocker, glycine and a substrate agonist for GlyT1, sarcosine, induced voltage-dependent inward currents that were abolished by removing external Na(+), identifying them as transport currents. Inhibitors of glycine transport through GlyT1 (sarcosine and (N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS)) reduced glycine currents by approximately 85%, consistent with positive immunostaining for GlyT1 in Bergmann glia while inhibitors of glycine transport through GlyT2 (4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (ORG 25543) and amoxapine) or through systems A and ASC did not affect glycine transport currents. Following internal glycine perfusion during the recording, outward currents progressively developed at -50 mV and external glycine-induced uptake currents were reduced. Using paired recordings of a Bergmann glial cell and a granule cell in the whole cell and outside-out modes, respectively, depolarizations of Bergmann glia to +20 mV induced a 73% increase in the open probability of glycine receptor channels in membrane patches of granule cells. This increase was prevented when NFPS was included in the bath solution. Overall, these results demonstrate for the first time that Bergmann glia express functional GlyT1 that can work in reverse at near-physiological ionic and internal glycine conditions in brain slices. These glial GlyTs can probably mediate glycine efflux under conditions of metabolic impairments like ischaemia.

Slow death in the leopard frogRana pipiens: neurotransmitters and anoxia tolerance

While frogs such as Rana temporaria are known to withstand 4-5 h anoxia at room temperature, little is known about the neurological adaptations that permit this. Previous research has shown that changes in neuroactive compounds such as glutamate and dopamine in anoxia-sensitive (mammalian)brains follow a strikingly different pattern than is observed in truly anoxia-tolerant vertebrates such as the freshwater turtle. The present study measured changes in the levels of whole brain and extracellular amino acids,and extracellular dopamine, in the normoxic and 3-4 h anoxic frog Rana pipiens, in order to determine whether their neurotransmitter responses resemble the anoxia-vulnerable or anoxia-tolerant response. Increases in whole brain serine, glycine, alanine and GABA levels were similar to those seen in anoxia-tolerant species, although the levels of glutamine, taurine and glutamate did not increase as occurs in true facultative anaerobes. Extracellular levels of aspartate, taurine and GABA also increased significantly, while glutamate levels decreased. The maintenance of low extracellular glutamate was the most significant difference between the frog and the anoxic/ischemic mammalian brain, although aspartate did increase 215%over a 4 h period of anoxia. A 12-fold increase in extracellular dopamine levels during anoxia was the biggest contrast between anoxia-tolerant vertebrates and R. pipiens. The frog could thus be an interesting model in which to examine the mechanisms of dopamine failure in early anoxia,which occurs rapidly in the mammal but over a period of hours in the `slow death' of the anoxic frog brain.

Protection of cortical neurons against oxygen-glucose deprivation and N-methyl-D-aspartate by DIDS and SITS

The Cl(-) channel blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) or 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) dose-dependently protected against oxygen-glucose deprivation in cultured rat cortical neurons. DIDS or SITS attenuated oxygen-glucose deprivation-induced increases in extracellular glutamate concentrations and intracellular Ca(2+). DIDS or SITS provided moderate protection against N-methyl-D-aspartate (NMDA) toxicity and decreased NMDA receptor-mediated increases in intracellular Ca(2+). Whole-cell NMDA receptor currents were attenuated 39+/-2% and 21+/-3% by 1 mM DIDS and SITS, respectively. Other Cl(-) channel blockers as equipotent as DIDS and SITS did not decrease oxygen-glucose deprivation- or NMDA-mediated neuronal Ca(2+) influx or toxicity. Neurotoxicity by exogenous glutamate was not prevented by SITS and was exacerbated by DIDS. Reductions in oxygen-glucose deprivation-induced increases in intracellular Ca(2+) levels underlie neuroprotection by DIDS and SITS. This was a reflection of lower extracellular [glutamate], direct inhibition of Ca(2+) influx through postsynaptic NMDA receptors, and possibly through other protective properties associated with DIDS and SITS.