Glycine-induced CA1 excitotoxicity in the rat hippocampal slice

Effects of time-of-day on the concentration of defined excitatory and inhibitory amino acids in the cerebrospinal fluid of rats: a microdialysis study

Amino acid neurotransmitters are responsible for many physiological and pathological processes, and their cerebral concentrations respond to external influences such as the light-dark cycle and to the synthesis, release, and recapture rhythms and form part of the biochemical relationships derived from excitatory-inhibitory (E/I), glutamine-glutamate sum (GLX), glutamatergic processing (glutamine-glutamate ratio) and excitotoxic indexes. The changes in these variables during a 24-h period (1 day) are important because they allow organisms to adapt to external stimuli and form part of physiological processes. Under pathological conditions, the damage produced by acute events may depend on diurnal variations. Therefore, it is important to analyze the extracellular levels of amino acids as well as the above-mentioned indexes over a 24-h period. We focused on determining the cerebrospinal fluid levels of different amino acid neurotransmitters, and the E/I, GLX, glutamatergic processing and excitotoxic indexes, determined by microdialysis over a 24-h cycle. Our results showed significant changes during the 24-h light/dark cycle. Specifically, we found increments in the levels of glutamate (325%), GABA (550%), glutamine (300%), glycine (194%), alanine (304%) and the GLX index (263%) throughout the day, and the maximum levels of glutamate, glutamine, glycine, and alanine were obtained during the last period of the light period. In conclusion, the concentration of some amino acid neurotransmitters and the GLX index show variations depending on the light-dark cycle.

Glutamate-induced depression of EPSP-spike coupling in rat hippocampal CA1 neurons and modulation by adenosine receptors

The presence of high concentrations of glutamate in the extracellular fluid following brain trauma or ischaemia may contribute substantially to subsequent impairments of neuronal function. In this study, glutamate was applied to hippocampal slices for several minutes, producing over-depolarization, which was reflected in an initial loss of evoked population potential size in the CA1 region. Orthodromic population spikes recovered only partially over the following 60 min, whereas antidromic spikes and excitatory postsynaptic potentials (EPSPs) showed greater recovery, implying a change in EPSP-spike coupling (E-S coupling), which was confirmed by intracellular recording from CA1 pyramidal cells. The recovery of EPSPs was enhanced further by dizocilpine, suggesting that the long-lasting glutamate-induced change in E-S coupling involves NMDA receptors. This was supported by experiments showing that when isolated NMDA-receptor-mediated EPSPs were studied in isolation, there was only partial recovery following glutamate, unlike the composite EPSPs. The recovery of orthodromic population spikes and NMDA-receptor-mediated EPSPs following glutamate was enhanced by the adenosine A1 receptor blocker DPCPX, the A2A receptor antagonist SCH58261 or adenosine deaminase, associated with a loss of restoration to normal of the glutamate-induced E-S depression. The results indicate that the long-lasting depression of neuronal excitability following recovery from glutamate is associated with a depression of E-S coupling. This effect is partly dependent on activation of NMDA receptors, which modify adenosine release or the sensitivity of adenosine receptors. The results may have implications for the use of A1 and A2A receptor ligands as cognitive enhancers or neuroprotectants.

Adenosine preconditions against ouabain but not against glutamate on CA1-evoked potentials in rat hippocampal slices

Hypoxic and ischaemic brain damage are believed to involve excessive release of glutamate, and recent work shows that glutamate-induced damage in brain slices can be reduced by preconditioning with hypoxia or glutamate itself. Because adenosine is a powerful preconditioning agent, we have investigated whether adenosine could precondition against glutamate in vitro. In rat hippocampal slices, glutamate depolarization reduced the amplitudes of antidromic- and orthodromic-evoked potentials, with only partial recovery. Applying adenosine before these insults failed to increase that recovery. Ouabain also produced depolarization with partial reversibility, but adenosine pretreatment increased the extent of recovery. The preconditioning effect of adenosine on ouabain responses was prevented by blocking receptors for N-methyl-D-aspartate (NMDA), but not receptors for kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and was blocked by inhibiting nitric oxide synthase. Preconditioning was also abolished by the ATP-dependent potassium channel blockers, glibenclamide (cytoplasmic) or 5-hydroxydecanoate (mitochondrial). We conclude that adenosine does not precondition against glutamate in hippocampal slices, but that it does precondition against ouabain with a pharmacology similar to studies in vivo. Ischaemic neuronal damage is a complex of many factors, and because adenosine can precondition against ischaemic neuronal damage, its failure to protect against glutamate highlights limitations of using glutamate alone as a model for ischaemia. Because damage following ischaemia, trauma or excitotoxicity also involves reduced Na(+),K(+)-ATPase activity, and adenosine can precondition against ouabain, we propose that ouabain-induced damage represents an additional or alternative model for the contribution to cell damage of Na(+),K(+)-ATPase loss, this being more relevant to the mechanisms of preconditioning.

Glycine-induced long-term synaptic potentiation is mediated by the glycine transporter GLYT1

The negative symptoms of schizophrenia are reverted by treatment with glycine or other agonists of the glycine-B site which facilitate NMDA receptor function. On the other hand, there are experimental observations showing that exogenous application of glycine (0.5-10mM) results in a long-lasting potentiation of glutamatergic synaptic transmission (LTP-GLY). The characterization of the mechanisms underlying LTP-GLY could be useful to develop new therapies for schizophrenia. The main goal of this work is to deepen the understanding of this potentiation phenomenon. The present study demonstrates in rat hippocampal slices that superfusion of glycine 1mM during 30 min produces a potentiation of excitatory postsynaptic potentials in CA3-CA1 pathway lasting at least 1h. Glycine application does not modify neither presynaptic fiber volley nor paired-pulse facilitation of synaptic potentials. This LTP-GLY is independent of both strychnine-sensitive glycine receptors and nifedipine-sensitive calcium channels. Interestingly, LTP-GLY is not inhibited but strengthened by NMDA receptors antagonists such as AP-5 or MK-801. In contrast, LTP-GLY is partially or totally blocked with the antagonists of glycine transporter GLYT1, sarcosine or ALX-5407, respectively. These results indicate that LTP-GLY requires the activation of GLYT1, a glycine transporter co-localized and associated to NMDA receptors. In addition, the fact that NMDA receptor inhibition increases LTP-GLY magnitude, opens the possibility that these receptors could have a negative control on GLYT1 activity.

Glycine and neuroprotective effect of hypothermia in hypoxic-ischemic brain damage

The beneficial effects of hypothermia have long been known in non-traditional medicine but it is only in the past few decades that studies on the neuroprotective effects of hypothermia in hypoxic-ischemic brain injury have begun. Different mechanisms have been put forward to explain hypothermic neuroprotection including reduction of the excessive release of the excitatory amino acid neurotransmitter, glutamate. Recent experiments have questioned the key role of this neurotoxin in hypoxic-ischemic neuropathogenesis. In contrast, a mediatory role for another neurotransmitter, glycine in the neuroprotective effects of hypothermia has become more attractive, along with an indication of its role in the pathogenesis of ischemic neuronal damage. Thus, on the basis of reviewing relevant literature the hypothesis of a glycine-related mechanism of hypothermic neuroprotection in ischemia-induced neuronal injury has been put forward.

Net efflux of cysteine, glutathione and related metabolites from rat hippocampal slices during oxygen/glucose deprivation: dependence on gamma-glutamyl transpeptidase

Extracellular metabolism of the protective substance glutathione (gamma-glutamyl-cysteinyl-glycine) may generate cysteine, glycine, several gamma-glutamyl-containing dipeptides and possibly free glutamate, all of which could participate in neurotoxicity. In the present study, we have examined how blockage of gamma-glutamyl transpeptidase, the key enzyme in glutathione degradation, influences the extracellular concentrations of glutathione, cysteine and related metabolites during anoxia/aglycemia of rat hippocampal slices. The net efflux, i.e., the increase in extracellular concentration due to changes in release and/or uptake, of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate, gamma-glutamyl-glutamine, glutathione, gamma-glutamyl-cysteine and glutamate increased as a result of anoxia/aglycemia. These increases in net efflux of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate and gamma-glutamyl-glutamine were reduced or blocked by acivicin, an inhibitor of gamma-glutamyl transpeptidase. In contrast, acivicin caused an increase in both basal and anoxia/aglycemia-induced net efflux of glutathione whereas the basal and anoxia/aglycemia-induced efflux of glutamate was unchanged by acivicin treatment. The effect of acivicin on the efflux of gamma-glutamyl-cysteine was similar to that of glutathione although less pronounced. Addition of beta-mercaptoethanol to the incubation medium during and after 30 min of anoxia/aglycemia decreased the net efflux of cysteine sulfinate specifically, indicating that the increase in cysteine sulfinate during anoxia/aglycemia may be partly derived from the spontaneous oxidation of cysteine. The results suggest that gamma-glutamyl transpeptidase may be involved in the regulation of the extracellular concentrations of cysteine, several gamma-glutamyl-containing dipeptides and glutathione but not glutamate during ischemia.

Electrophysiological effects of felbamate

Felbamate is a broad spectrum antiepileptic drug recently introduced into clinical practice for controlling seizures in patients affected by Lennox-Gastaut epilepsy, complex partial seizures or otherwise intractable epilepsies. However, the cellular mechanisms by which the drug exerts its anticonvulsant actions are not fully understood. The aim of the present article is to outline the possible mechanisms of action of felbamate as suggested by findings obtained with electrophysiological approaches.

Determination of the excitatory potencies of fluoroquinolones in the central nervous system by an in vitro model

Fluoroquinolones have been reported to induce central nervous system side effects, including seizures and psychiatric events. Although relatively rare in patients up to now, the proconvulsant activity depends on the chemical structure and might be a critical endpoint of some new representatives of this valuable class of antimicrobials. The electrophysiological determination of field potentials in the CA1 region of the rat hippocampus slice allowed an assessment of the excitatory potential of fluoroquinolones and might be predictive for their neurotoxic potency in vivo. An optimization of this method and its extension to other fluoroquinolones resulted in a defined rank order. Well-known already-marketed quinolones as well as some fluoroquinolones under evaluation and development were used. The dose range tested was between 0.5 and 4 mumol/liter, which was comparable to the therapeutic concentration in the brain. All tested compounds increased the population spike amplitude in a concentration-dependent manner, and the resulting excitatory potency was highly dependent on the chemical structure, with compounds ranging from least to most excitatory as follows: ofloxacin, ciprofloxacin, nalidixic acid, moxifloxacin, BAY x 8843, [corrected] fleroxacin, lomefloxacin, enoxacin, clinafloxacin (much more excitatory than enoxacin), tosufloxacin, trovafloxacin, BAY 15-7828, and BAY x 9181 (much more excitatory than BAY 15-7828). The proposed hippocampus slice model not only is suitable for giving valuable alerts as to convulsive potential during candidate selection but also enables mechanistic investigations. These investigations pointed to the N-methyl-D-aspartate receptor as the probable target of the fluoroquinolone effects.

Glycine causes increased excitability and neurotoxicity by activation of NMDA receptors in the hippocampus

Glycine is an inhibitory neurotransmitter in the spinal cord and also acts as a permissive cofactor required for activation of the N-methyl-D-aspartate (NMDA) receptor. We have found that high concentrations of glycine (10 mM) cause marked hyperexcitability and neurotoxicity in organotypic hippocampal slice cultures. The hyperexcitability, measured using intracellular recording in CA1 pyramidal neurons was completely blocked by the NMDA receptor antagonist MK-801 (10 microM), but not by the AMPA receptor antagonist DNQX (100 microM). The neurotoxicity caused by glycine occurred in all regions of hippocampal cultures but was most marked in area CA1. There was significant CA1 neuronal damage in cultures exposed to 10 mM glycine for 30 min or longer (P < 0.01) or those exposed to 4 mM glycine for 24 h compared to control cultures (P < 0.01). The NMDA antagonists MK-801 (10 microM) and APV (100 microM) significantly reduced glycine-induced neuronal damage in all hippocampal subfields (P < 0.01). The AMPA antagonists CNQX, DNQX, and NBQX (100 microM) had no effect on glycine-induced neuronal damage. High concentrations of glycine therefore appear to enhance the excitability of hippocampal slices in an NMDA receptor-dependent manner. The neurotoxic actions of glycine are also blocked by NMDA receptor antagonists.

Traumatic neuroprotection with inhibitors of nitric oxide and ADP-ribosylation

N-Methyl-D-aspartate (NMDA) receptor activation is known to contribute to neuronal damage from head trauma. Additionally, NMDA neurotoxicity occurs in part through the generation of nitric oxide (NO), and injury from NO has been shown to be mediated by ADP-ribosylation. Therefore, we investigated whether inhibitors of NO and ADP-ribosylation would protect against acute CA1 traumatic neuronal injury in hippocampal slices subjected to fluid percussion. Treatment with the nitric oxide synthase (NOS) inhibitor, methyl-L-arginine 170 microM for 35 min after trauma injury, improved CA1 antidromic population spike (PS) recovery to 91 +/- 2%, compared to unmediated slices which recovered to only a mean of 20 +/- 4%, 90 min after trauma. Similarly, hemoglobin 50 microM, which directly binds NO, protected against traumatic neuronal injury and yielded a mean CA1 PS recovery of 92 +/- 1%. Treatment with inhibitors of poly-ADP-ribosylation was also strongly protective, with the vitamin nicotinamide 10 mM and 3-aminobenzamide 1 mM yielding PS recoveries of 98 +/- 2% and 90 +/- 3%, respectively. Protection was also seen with inhibitors of mono-ADP-ribosylation, including novobiocin 500 microM and meta-iodobenzylguanidine 20 microM which yielded recoveries of 89 +/- 6% and 96 +/- 26%. Novobiocin also protected against direct application of NO and NMDA. These findings suggest that NO and ADP-ribosylation are mediators of acute traumatic neuronal injury.

Felbamate neuroprotection against CA1 traumatic neuronal injury

Traumatic brain injury is a leading cause of disability and death. Since the anticonvulsant felbamate provides hypoxic neuroprotection, we investigated whether felbamate would provide protection against traumatic neuronal injury as well. Traumatic injury to CA1 neurons in hippocampal slices was induced by fluid percussion, and CA1 evoked response was monitored. Pre-treatment with felbamate was strongly protective against neuronal injury, and permitted CA1 antidromic population spike recovery to a mean 94 +/- 1% (S.E.M.) of initial amplitude, compared to unmedicated slices which regained only 15 +/- 6%. The felbamate EC50 for this protection was 136 mg/1, and significant protection was found at felbamate concentrations similar to those reported in felbamate monotherapy for seizures. Significant protection was also detected when felbamate was initiated 15 min after trauma. Slices given brief post-trauma felbamate treatment could demonstrate long-term potentiation when assessed 8 h after trauma. These studies indicate that felbamate is neuroprotective against CA1 traumatic neuronal injury.