Excitotoxicity in neurological disorders--the glutamate paradox

Dynamic changes in N-methyl-D-aspartate receptors after closed head injury in mice: Implications for treatment of neurological and cognitive deficits

Traumatic brain injury is a leading cause of mortality and morbidity among young people. For the last couple of decades, it was believed that excess stimulation of brain receptors for the excitatory neurotransmitter glutamate was a major cause of delayed neuronal death after head injury, and several major clinical trials in severely head injured patients used blockers of the glutamate N-methyl-D-aspartate (NMDA) receptor. All of these trials failed to show efficacy. Using a mouse model of traumatic brain injury and quantitative autoradiography of the activity-dependent NMDA receptor antagonist MK801, we show that hyperactivation of glutamate NMDA receptors after injury is short-lived (<1 h) and is followed by a profound and long-lasting (> or =7 days) loss of function. Furthermore, stimulation of NMDA receptors by NMDA 24 and 48 h postinjury produced a significant attenuation of neurological deficits (blocked by coadministration of MK801) and restored cognitive performance 14 days postinjury. These results provide the underlying mechanism for the well known but heretofore unexplained short therapeutic window of glutamate antagonists after brain injury and support a pharmacological intervention with a relatively long (> or =24 h) time window easily attainable for treatment of human accidental head injury.

Adenosine A2A receptor blockade differentially influences excitotoxic mechanisms at pre- and postsynaptic sites in the rat striatum

Adenosine A(2A) receptor antagonists are being regarded as potential neuroprotective drugs, although the mechanisms underlying their effects need to be better studied. The aim of this work was to investigate further the mechanism of the neuroprotective action of A(2A) receptor antagonists in models of pre- and postsynaptic excitotoxicity. In microdialysis studies, the intrastriatal perfusion of the A(2A) receptor antagonist ZM 241385 (5 and 50 nM) significantly reduced, in an inversely dose-dependent way, the raise in glutamate outflow induced by 5 mM quinolinic acid (QA). In rat corticostriatal slices, ZM 241385 (30-100 nM) significantly reduced 4-aminopyridine (4-AP)-induced paired-pulse inhibition (PPI; an index of neurotransmitter release), whereas it worsened the depression of field potential amplitude elicited by N-methyl-D-aspartate (NMDA; 12.5 and 50 microM). The A(2A) antagonist SCH 58261 (30 nM) mimicked the effects of ZM 241385, whereas the A(2A) agonist CGS 21680 (100 nM) showed a protective influence toward 50 microM NMDA. In rat striatal neurons, 50 nM ZM 241385 did not affect the increase in [Ca(2+)](i) or the release of lactate dehydrogenase (LDH) induced by 100 and 300 microM NMDA, respectively. The ability of ZM 241385 to prevent QA-induced glutamate outflow and 4-AP-induced effects confirms that A(2A) receptor antagonists have inhibitory effects on neurotransmitter release, whereas the results obtained toward NMDA-induced effects suggest that A(2A) receptor blockade does not reduce, or even amplifies, excitotoxic mechanisms due to direct NMDA receptor stimulation. This indicates that the neuroprotective potential of A(2A) antagonists may be evident mainly in models of neurodegeneration in which presynaptic mechanisms play a major role.

Antagonist, CP-101,606, Enhances the Functional Recovery The NMDA NR2B Subunit-Selective Receptor and Reduces Brain Damage after Cortical Compression-Induced Brain Ischemia

Using a novel in vivo model for cerebral ischemia produced by short-lasting compression of a well-defined brain area of sensorimotor cortex we studied neuroprotective effects of the NMDA NR2B subunit selective antagonist, CP-101,606, in Sprague-Dawley rats. Cortical compression for 30 min produced a consistent and highly reproducible functional impairment, that is paresis of contralateral hind and fore limbs. The neurological deficit was accompanied by marked brain damage in cerebral cortex, hippocampus and thalamus as identified by Fluoro-Jade, a marker of general neuronal cell death. Using a daily performed beam walking test it was shown that untreated animals recovered from their functional impairment within 5-7 days following surgery. Intravenous administration of increasing doses (1, 5, 10, 20 mg/kg) of the NMDA NR2B subunit receptor specific antagonist, CP-101,606, dose-dependently improved the rate of functional recovery and protected against the ischemic brain damage in cerebral cortex, hippocampus, and thalamus as identified 2 days after the ischemic insult. Based upon these results, we conclude that NMDA NR2B receptor subunits represent potential targets to reduce not only the functional deficits, but also neuronal death in cortex and several midbrain regions produced by moderate, transient, cerebral ischemia.

Neuroprotective activity of antazoline against neuronal damage induced by limbic status epilepticus

Imidazoline drugs exert neuroprotective effects in cerebral ischaemia models. They also have effects against mouse cerebellar and striatal neuronal death induced by N-methyl-D-aspartate (NMDA) through the blockade of NMDA currents. Here, we investigated the effects of antazoline on NMDA toxicity and current in rat hippocampal neuronal cultures, and on an in vivo model of status epilepticus. In hippocampal cultures, antazoline (30 microM) decreased NMDA-mediated neurotoxicity and also blocked the NMDA current with voltage-dependent and fast-reversible action (inhibition by 85+/-3% at -60 mV). Status epilepticus was induced by injecting pilocarpine (200 nmol) directly into the right pyriform cortex of male adult rats. The rats then received immediately three consecutive i.p. injections at 30-min intervals of either PBS (control group) or antazoline at 10 mg/kg (low-dose group) or at 45 mg/kg (high-dose group). During the 6-h recording, status epilepticus lasted more than 200 min in all groups. In the high-dose group only, seizures completely ceased 1 h after the third injection of antazoline, then started again 1 h later. Rats were killed 1 week later, and Cresyl Violet-stained sections of their brain were analysed for damage quantification. On the ipsilateral side to the pilocarpine injection, pyriform cortex and hippocampal CA1 and CA3 areas were significantly protected in both antazoline-treated groups, whilst prepyriform and entorhinal cortices were only in the high-dose group. On the contralateral side to the pilocarpine injection, only the hippocampal CA3 area was significantly protected in the low-dose group, but all investigated structures were in the high-dose group. In conclusion, antazoline is a potent neuroprotective drug in different models of neuronal primary culture, as previously shown in striatal and cerebellar granule neurons [Neuropharmacology 39 (2000) 2244], and here in hippocampal neurons. Antazoline is also neuroprotective in vivo in the intra-pyriform pilocarpine-induced status epilepticus model.

Glutamatergic systems in Alzheimer's disease

Glutamate is the major transmitter of the brain and is involved in all aspects of cognitive function since it is the transmitter of cortical and hippocampal pyramidal neurones. Furthermore, glutamate and glutamate receptors are involved in long-term potentiation, a process believed to underlie learning and memory. Histological studies indicate loss of pyramidal neurones and their synapses in Alzheimer's disease (AD), this together with biochemical evidence suggests presynaptic (and postsynaptic) glutamatergic hypoactivity. This represents a 'double blow' as the activity of glutamatergic neurones is heavily influenced by the cholinergic system, which is also dysfunctional in AD. The clinical relevance of these changes is emphasised because glutamatergic and cholinergic dysfunction are strong correlates of cognitive decline in AD. The mechanism by which glutamatergic (and cholinergic) cells die is likely to be a combination of necrosis and apoptosis caused by a range of factors which include tangle formation and the effects of too much and too little glutamatergic neurotransmission.

Continuous monitoring of intracerebral glutamate levels in awake monkeys using microdialysis and enzyme fluorometric detection

A technique for continuous on-line detection of glutamate using brain microdialysis in awake primates is described. The method is based on an enzymatic assay using fluorescence detection of glutamate. The time resolution of the continuous fluorescent readout compares favorably with that of most published studies, which have used standard high-pressure liquid chromatography detection methods for glutamate. Exposure of the system to other amino acids (GABA, aspartate, glutamine, ascorbate, taurine, valine, alanine, and D-glutamate) revealed that this method is highly specific for L-glutamate. In vitro, the system detects reliably glutamate levels as low as 0.5 micromol/l. In vivo testing in the striatum of Rhesus macaques showed that glutamate levels were enhanced after reverse microdialysis with a glutamate uptake blocker. Stimulation with high potassium increased substantially the levels of glutamate, an effect that was calcium-dependent. Glutamate levels were also increased when the microdialysis solution contained the blocker of voltage-gated potassium channels, 4-aminopyridine. This technique effectively detects short-term changes in glutamate levels evoked by physiologic or pharmacologic manipulations in the primate brain.

Low sensitivity of retina to AMPA-induced calcification

Glutamate is involved in most CNS neurodegenerative diseases. In particular, retinal diseases such as retinal ischemia, retinitis pigmentosa, and diabetic retinopathy are associated with an excessive synaptic concentration of this neurotransmitter. To gain more insight into retinal excitotoxicity, we carried out a dose-response study in adult rats using alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), a glutamate analogue. AMPA intraocular injections (between 0.27 and 10.8 nmol) caused no morphologic modification, but a 10.8 + 21 nmol double injection in a 10-day interval produced a lesion characterized by discrete neuronal loss, astroglial and microglial reactions, and calcium precipitation. Abundant calcium deposits similar to those present in rat and human brain excitotoxicity or hypoxia-ischemia neurodegeneration were detected by alizarin red staining within the retinal surface and the optic nerve. Glial reactivity, associated normally with astrocytes in the nerve fiber, was assessed in Müller cells. GABA immunoreactivity was detected not only in neuronal elements but also in Müller cells. In contrast to the high vulnerability of the brain to excitotoxin microinjection, AMPA-induced retinal neurodegeneration may provide a useful model of low central nervous system sensitivity to excitotoxicity.

Changes in dialysate concentrations of glutamate and GABA in the brain: an index of volume transmission mediated actions?

Brain microdialysis has become a frequently used method to study the extracellular concentrations of neurotransmitters in specific areas of the brain. For years, and this is still the case today, dialysate concentrations and hence extracellular concentrations of neurotransmitters have been interpreted as a direct index of the neuronal release of these specific neurotransmitter systems. Although this seems to be the case for neurotransmitters such as dopamine, serotonin and acetylcholine, the extracellular concentrations of glutamate and GABA do not provide a reliable index of their synaptic exocytotic release. However, many microdialysis studies show changes in extracellular concentrations of glutamate and GABA under specific pharmacological and behavioural stimuli that could be interpreted as a consequence of the activation of specific neurochemical circuits. Despite this, we still do not know the origin and physiological significance of these changes of glutamate and GABA in the extracellular space. Here we propose that the changes in dialysate concentrations of these two neurotransmitters found under specific treatments could be an expression of the activity of the neurone-astrocyte unit in specific circuits of the brain. It is further proposed that dialysate changes of glutamate and GABA could be used as an index of volume transmission mediated actions of these two neurotransmitters in the brain. This hypothesis is based firstly on the assumption that the activity of neurones is functionally linked to the activity of astrocytes, which can release glutamate and GABA to the extracellular space; secondly, on the existence of extrasynaptic glutamate and GABA receptors with functional properties different from those of GABA receptors located at the synapse; and thirdly, on the experimental evidence reporting specific electrophysiological and neurochemical effects of glutamate and GABA when their levels are increased in the extracellular space. According to this concept, glutamate and GABA, once released into the extracellular compartment, could diffuse and have long-lasting effects modulating glutamatergic and/or GABAergic neurone-astrocytic networks and their interactions with other neurotransmitter neurone networks in the same areas of the brain.

Dextromethorphan protects dopaminergic neurons against inflammation-mediated degeneration through inhibition of microglial activation

Inflammation in the brain has increasingly been recognized to play an important role in the pathogenesis of several neurodegenerative disorders, including Parkinson's disease and Alzheimer's disease. Inflammation-mediated neurodegeneration involves activation of the brain's resident immune cells, the microglia, which produce proinflammatory and neurotoxic factors, including cytokines, reactive oxygen intermediates, nitric oxide, and eicosanoids that impact on neurons to induce neurodegeneration. Hence, identification of compounds that prevent microglial activation may be highly desirable in the search for therapeutic agents for inflammation-mediated neurodegenerative diseases. In this study, we report that dextromethorphan (DM), an ingredient widely used in antitussive remedies, reduced the inflammation-mediated degeneration of dopaminergic neurons through inhibition of microglial activation. Pretreatment (30 min) of rat mesencephalic neuron-glia cultures with DM (1-10 micro M) reduced, in a dose-dependent manner, the microglia-mediated degeneration of dopaminergic neurons induced by lipopolysaccharide (LPS, 10 ng/ml). Significant neuroprotection by DM was also evident when DM was applied to cultures up to 60 min after the addition of LPS. The neuroprotective effect of DM was attributed to inhibition of LPS-stimulated microglial activation because DM significantly inhibited the LPS-induced production of tumor necrosis factor-alpha, nitric oxide, and superoxide free radicals. This conclusion was further supported by the finding that DM failed to prevent 1-methyl-4-phenylpyridinium- or beta-amyloid peptide (1-42)-induced dopaminergic neurotoxicity in neuron-enriched cultures. In addition, because LPS did not produce any significant increase in the release of excitatory amino acids from neuron-glia cultures and N-methyl-D-aspartate antagonist dizocilpine maleate failed to afford significant neuroprotection, it is unlikely that the neuroprotective effect of DM is mediated through N-methyl-D-aspartate receptors. These results suggest that DM may be a promising therapeutic agent for the treatment of Parkinson's disease.

Prevention of neurotoxicity in hypoxic cortical neurons by the noble gas xenon

Hypoxia-induced neuronal damage and glutamate release were investigated in a N(2)- or in xenon-atmosphere for embryonic rat cortical neurons; cellular damage and glutamate over-release were observed in N(2)-treated cells whereas xenon protected the cells from the hypoxic insult. The protective effect of xenon was strongly reduced by pre-incubating neurons with the calcium-chelator BAPTA-AM indicating a role for calcium in this process. The results demonstrate (a) the neuroprotective properties of xenon, suggest (b) a relationship between the prevention of neurotransmitter release in a hypoxic situation and neuroprotection and present (c) evidence that such neuroprotection may be based on yet other xenon-dependent mechanisms.

Neonatal monosodium glutamate treatment modifies glutamic acid decarboxylase activity during rat brain postnatal development

Monosodium glutamate (MSG) produces neurodegeneration in several brain regions when it is administered to neonatal rats. From an early embryonic age to adulthood, GABA neurons appear to have functional glutamatergic receptors, which could convert them in an important target for excitotoxic neurodegeneration. Changes in the activity of the GABA synthesizing enzyme, glutamic acid decarboxylase (GAD), have been shown after different neuronal insults. Therefore, this work evaluates the effect of neonatal MSG treatment on GAD activity and kinetics in the cerebral cortex, striatum, hippocampus and cerebellum of the rat brain during postnatal development. Neonatal MSG treatment decreased GAD activity in the cerebral cortex at 21 and 60 postnatal days (PD), mainly due to a reduction in the enzyme affinity (K(m)). In striatum, the GAD activity and the enzyme maximum velocity (V(max)) were increased at PD 60 after neonatal MSG treatment. Finally, in the hippocampus and cerebellum, the GAD activity and V(max) were increased, but the K(m) was found to be lower in the experimental group. The results could be related to compensatory mechanisms from the surviving GABAergic neurons, and suggest a putative adjustment in the GAD isoform expression throughout the development of the postnatal brain, since this enzyme is regulated by the synaptic activity under physiological and/or pathophysiological conditions.

In vivo excitotoxicity induced by ouabain, a Na+/K+-ATPase inhibitor

The susceptibility of immature rat brain to neurotoxicity of N-methyl-D-aspartate (NMDA) has provided a widely used paradigm to study excitotoxicity relevant to acute neurodegenerative diseases such as cerebral ischemia. In this study, excitotoxicity was induced via injection of ouabain (1 mM/0.5 microL), a Na+/K+ -ATPase-inhibitor, into neonatal rat brain and compared with NMDA injection. The aim of the study was to induce excitotoxicity secondary to cellular membrane depolarization, thereby more closely mimicking the pathophysiologic processes of ischemia-induced brain injury where NMDA-receptor overstimulation by glutamate follows, not precedes, membrane depolarization. Na+/K+ -ATPase-inhibition caused an acute, 40% +/- 8% decrease of the apparent diffusion coefficient (ADC) of water, as measured using diffusion-weighted magnetic resonance imaging (MRI), and resulted in infarctlike lesions as measured using T2-weighted MRI and histology up to 2 weeks later. Localized one- and two-dimensional 1H-magnetic resonance spectroscopy (MRS) demonstrated that the early excitotoxic diffusion changes were not accompanied by an overall metabolic disturbance. Furthermore, 31P-MRS demonstrated that energy depletion is not a prerequisite for ADC decrease or excitotoxic cell death. Treatment with the NMDA-antagonist MK-801 (1 mg/kg) attenuated the volume of tissue exhibiting a decreased ADC (P < 0.005), demonstrating that the ouabain-induced injury is indeed excitotoxic in nature. The authors argue that, compared with NMDA-injection, ouabain-induced excitotoxicity elicits more appropriate glutamate-receptor overstimulation and is better suited to detect relevant neuroprotection in that it is more sensitive to attenuation of synaptic glutamate levels.

Novel cognitive improving and neuroprotective activities of Polygala tenuifolia Willdenow extract, BT-11

We carried out this study to search a new active constituent that had cognitive enhancing activity and low side effects from natural source. We found that the extract of dried root of Polygala tenuifolia Willdenow (BT-11, 10 mg/kg, i.p.) could significantly reverse scopolamine-induced cognitive impairments in rat, using a passive avoidance and a water maze test. We also investigated the effects of BT-11 on neurotoxicity induced by glutamate (Glu) and toxic metabolites of amyloid precursor protein (APP) such as amyloid beta protein (A beta) and C-terminal fragment of APP (CT) in primary cultured neurons of rat. The pretreatment of BT-11 (0.5, 3, and 5 micro g/ml) significantly reduced cell death induced by Glu (1 mM), A beta (10 micro M) and CT105 (10 micro M) in a dose-dependent manner. In addition, BT-11 inhibited acetylcholinesterase (AChE) activity in a dose-dependent and non-competitive manner (IC(50) value; 263.7 micro g/ml). Our novel findings suggest the possibility that this extract may have some protective effects against neuronal death and cognitive impairments in Alzheimer's disease (AD), or other neurodegenerative diseases related to excitotoxicity and central cholinergic dysfunction.

In vivo microdialysis and gas-chromatography/mass-spectrometry for 13C-enrichment measurement of extracellular glutamate in rat brain

Extracellular glutamate (GLU(ECF)) was collected by microdialysis from the corticostriatal region of awake rats, at the basal level and after elevation by perfusion of GLU uptake inhibitor, L-trans-pyrrolidine-2,4-dicarboxylic acid. Concurrently, [2,5-(13)C]glucose was infused intravenously to 13C-enrich brain GLU predominantly at C5. The 13C enrichment of GLU(ECF) was measured, after tert-butyldimethylsilylation, by gas-chromatography/mass-spectrometry. Excellent signal-to-noise ratios of the analyte signals at three selected ion-pairs were achieved at approximately 20 pmol. The fractional 13C enrichment of basal dialysate GLU C5, collected during 0.75-1.25 h of [2,5-(13)C]glucose infusion, was 0.263+/-0.01, very close to the enrichment of whole-brain (predominantly intracellular) GLU C5 measured in parallel NMR study. The result strongly suggests that the dialysate GLU consists predominantly of neurotransmitter GLU, which was 13C-enriched in, and released from, neurons by exocytosis and had diffused to the dialysis probe; the label is undiluted by 12C-GLU(ECF) present before the enrichment. Hence, our result supports the view, proposed on the basis of Ca(2+)- and tetrodotoxin-sensitivity of dialysate GLU, that basal dialysate GLU in awake non-stimulated brain mainly represents neurotransmitter GLU. Isotope labeling provides a novel method for determining the extent to which dialysate GLU reflects synaptic GLU(ECF), and for measuring its turnover under physiological or pathological conditions.

Neonatal exposure to monosodium L-glutamate induces loss of neurons and cytoarchitectural alterations in hippocampal CA1 pyramidal neurons of adult rats

Glutamatergic post-synaptic receptors are closely related to the known excitotoxic effects of high doses of L-glutamate. Several behavioral abnormalities, glial reaction, and an increase of expression of the NMDA receptor sub-units have been observed in the rat hippocampus after early monosodium glutamate exposure. Thus, a quantitative morphological study was carried out to determine the effects of early exposure to monosodium glutamate on post-synaptic structures that mediate glutamate excitatory neurotransmission in the hippocampal CA1 field. Four milligrams per gram body weight of monosodium glutamate was subcutaneously injected into neonatal Wistar rats, at 1, 3, 5, and 7 days. Cell loss and several cytoarchitectonic parameters were evaluated in pyramidal cells from the hippocampal CA1 field in the treated rats at 60 days of age. An untreated group of rats were used as controls. Cell number in the hippocampus of experimental rats was 11.5% less than that in control animals. In addition, both dendritic arborization and dendritic spine density were adversely affected, and thin and mushroom-shaped spines became proportionally more numerous, while the opposite occurred to stubby spines. These results strongly suggest the occurrence of cell death and also show some cytoarchitectural modifications in the surviving neurons. These could lead to functional alterations in the hippocampal integrative activity, due to an early cytoexcitotoxic effect of monosodium glutamate.

Exposure to ebselen changes glutamate uptake and release by rat brain synaptosomes

We investigated effects of Ebselen, diphenyl diselenide (PhSe)2 and diphenyl ditelluride (PhTe)2 on [3H]glutamate uptake and release by brain synaptosomes. Ebselen after acute exposure inhibited K+-stimulated [3H]glutamate release by brain synaptosomes. (PhSe)2 and (PhTe)2 did not change [3H]glutamate release by brain synaptosomes. Ebselen, (PhSe)2 and (PhTe)2 had no significantly effects on [3H]glutamate uptake after acute exposure. In vitro, Ebselen (100 microM) inhibited [3H]glutamate release and uptake. (PhSe)2 had no significant effect, while (PhTe)2 (100 microM) inhibited [3H]glutamate uptake by brain synaptosomes. In vitro, (PhSe)2, (PhTe)2 and Ebselen caused a significant inhibition of [3H]glutamate uptake by brain synaptic vesicles in vitro. The results demonstrated that organochalcogenides have a rather complex effect on glutamate homeostasis depending on the compound and the schedule of exposition. We propose that the neuroprotective action of Ebselen can be related, in addition to its glutathione peroxidase-like and antilipoperoxidative activity, to a direct interaction with the glutamatergic system by reducing K+-evoked glutamate release.

Blockade of striatal adenosine A2A receptor reduces, through a presynaptic mechanism, quinolinic acid-induced excitotoxicity: possible relevance to neuroprotective interventions in neurodegenerative diseases of the striatum

The aim of the present study was to evaluate whether, and by means of which mechanisms, the adenosine A2A receptor antagonist SCH 58261 [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine] exerted neuroprotective effects in a rat model of Huntington's disease. In a first set of experiments, SCH 58261 (0.01 and 1 mg/kg) was administered intraperitoneally to Wistar rats 20 min before the bilateral striatal injection of quinolinic acid (QA) (300 nmol/1 microl). SCH 58261 (0.01 but not 1 mg/kg, i.p.) did reduce significantly the effects of QA on motor activity, electroencephalographic changes, and striatal gliosis. Because QA acts by both increasing glutamate outflow and directly stimulating NMDA receptors, a second set of experiments was performed to evaluate whether SCH 58261 acted by preventing the presynaptic and/or the postsynaptic effects of QA. In microdialysis experiments in naive rats, striatal perfusion with QA (5 mm) enhanced glutamate levels by approximately 500%. Such an effect of QA was completely antagonized by pretreatment with SCH 58261 (0.01 but not 1 mg/kg, i.p.). In primary striatal cultures, bath application of QA (900 microm) significantly increased intracellular calcium levels, an effect prevented by the NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]. In this model, bath application of SCH 58261 (15-200 nm) tended to potentiate QA-induced calcium increase. We conclude the following: (1) the adenosine A2A receptor antagonist SCH 58261 has neuroprotective effects, although only at low doses, in an excitotoxic rat model of HD, and (2) the inhibition of QA-evoked glutamate outflow seems to be the major mechanism underlying the neuroprotective effects of SCH 58261.

Molecular pharmacology of the Na+-dependent transport of acidic amino acids in the mammalian central nervous system

The Na+-dependent transport of L-glutamate (GluT) has been identified in brain tissue more than thirty years ago. Neurochemical studies, performed in various experimental models during 1970's, defined the basic rules for the selection or synthesis of GluT-specific substrates and inhibitors. The protein molecules (transporters) that mediate the translocation of the substrates across the plasma membrane have been cloned and studied during the last ten years. The sites on the transporters that bind the substrates favour glutamate-like or aspartate-like molecules with one positively charged and two negatively charged ionised groups. Substituents at C3 and C4 are often tolerated but substitutions at C2 or alterations of the ionisable groups usually impede the binding. The substrate binding sites display an "anomalous" selectivity towards stereoisomers. These structural requirements are shared by all Na+-dependent glutamate transporters thus making the design of transporter-selective ligands a challenging task. Moreover, the molecular mechanisms of the transport have not yet been adequately elucidated. Data from a wide variety of experimental studies strongly indicate that Na+-dependent GluT regulates the functioning of the glutamatergic excitatory synapses-the most important rapid inter-neuronal signalling system in the mammalian brain. Altered structural and/or functional properties of the Na+-dependent glutamate transporters have been implicated in the damage to the brain tissue following cerebral ischaemia and in the progressive loss of neurons in conditions such as Alzheimer dementia and amyotrophic lateral sclerosis. Furthermore, it seems that fine-tuning of glutamatergic neurotransmission by regulating the Na+-dependent GluT could be useful in the therapy of schizophrenia.

Systemic administration of kainic acid in adult rat stimulates expression of the chemokine receptor CCR5 in the forebrain

As chemokines and their receptors are primarily expressed by glial cells in brain parenchyma, a model of glial cell proliferation may be useful to study the regulation of their expression in the brain. The well-established kainic acid seizure model was used in this study, focusing on the expression of the CCR5 chemokine receptor. Adult Sprague-Dawley rats were injected intraperitoneally with kainic acid (12 mg/kg), and in situ hybridization of CCR5 mRNA was performed at 12 h, 1, 3, or 7 days, posttreatment. Autoradiographic films and wet photographic emulsions demonstrated the very low expression of CCR5 mRNA in normal brain parenchyma, as well as in the microvasculature and ventricular/choroid plexus systems. After kainic acid treatment, brain CCR5 mRNA expression increased progressively from 12 h to 7 days, especially in the olfactory system, amygdaloid complex, thalamus, hippocampal formation, septum, and neocortex. This increase paralleled that of activated microglial cells as shown, using the microglial marker, OX-42. Moreover, CCR5 mRNA ISH combined with neuron-specific enolase immunocytochemistry showed that, in addition to its glial expression, CCR5 mRNA is expressed in neurons in the normal brain and, to a lesser extent, after kainate treatment due to neuronal losses. Finally, CCR5 protein is detected by immunocytochemistry in neurodegenerative areas in numerous glial cells, as well as in neurons, as clearly shown in the hippocampal formation. In summary, the chemokine receptor CCR5 is expressed by neuronal and non-neuronal cell types in the normal brain and is upregulated in both cell types after an insult.

Effect of dithiol chelating agents on [3H]MK-801 and [3H]glutamate binding to synaptic plasma membranes

2,3-Dimercaptopropanol (BAL- British Anti-Lewesite) is a dithiol chelating agent used for the treatment of heavy metal poisoning, however, BAL can produce neurotoxic effects in a variety of situations. Based on the low therapeutic efficiency of BAL other dithiols were developed and DMSA (meso-2,3-dimercaptosuccinic acid) and DMPS (2,3-dimercaptopropane-1-sulfonic acid) are becoming used for treatments of humans exposed to heavy metals. In the present investigation the effect of dithiols in the glutamatergic system was examined. The results showed that BAL inhibited [3H]MK-801 and [3H]glutamate binding in a concentration-dependent manner. At 100 microM BAL and DMSA caused a significantly inhibition of [3H]MK-801 binding to brain membranes (p < 0.05 by Duncan's multiple range test). BAL at 100 microM caused an inhibition of 40% on [3H]glutamate binding. DMPS and DMSA had no significant effect on [3H]glutamate binding. Dithiotreitol (DTT), abolished the inhibitory effect of BAL on [3H]MK-801 binding. The protection exerted by DTT suggests that BAL inhibit [3H]MK-801 binding by interacting with cysteinyl residues that are important for redox modulation of receptor responses. ZnCl2 inhibited [3H]glutamate and [3H]MK-801 binding to brain synaptic membrane; nevertheless, the inhibitory effect was slight more accentuated for [3H]MK-801 than [3H]glutamate binding (p < 0.05). The inhibition caused by 10 microM ZnCl2 on [3H]MK-801 binding was attenuated by BAL. The findings present in this study may provide the evidence that BAL affect the glutamatergic system and these effects can contributed to explain, at least in part, why BAL, in contrast to DMPS and DMSA is neurotoxic.

Modeling of poly(ADP-ribose)polymerase (PARP) inhibitors. Docking of ligands and quantitative structure-activity relationship analysis

Poly(ADP-ribose)polymerase-1 (PARP-1) is a nuclear enzyme that has recently emerged as an important player in the mechanisms leading to postischemic neuronal death, and PARP inhibitors have been proposed as potential neuroprotective agents. With the aim of clarifying the structural basis responsible for PARP inhibition, we carried out a computational study on 46 inhibitors available through the literature. Our computational approach is composed of three parts. In the first one, representative PARP inhibitors have been docked into the crystallographic structure of the catalytic domain of PARP by using the Autodock 2.4 program. The docking studies thus carried out have provided an alignment scheme that has been instrumental for superimposing all the remaining inhibitors. Upon the basis of this alignment scheme, a quantitative structure-activity relationship (QSAR) analysis has been carried out after electrostatic and steric interaction energies have been computed with the RECEPTOR program. The QSAR analysis yielded a predictive model able to explain much of the variance of the 46-compound data set. The inspection of the QSAR coefficients revealed that the major driving force for potent inhibition is given by the extension of the contact surface between enzyme and inhibitors while electrostatic energy and hydrogen bonding capability play a minor role. Finally, the projection of the QSAR coefficients back onto the X-ray structure of the catalytic domain of PARP provides insights into the role played by specific amino acid residues. This information will be useful to address the design of new selective and potent PARP inhibitors.

T-cell-based immunity counteracts the potential toxicity of glutamate in the central nervous system

Injuries to the central nervous system (CNS) evoke self-destructive processes, which eventually lead to a much greater loss of tissue than that caused by the trauma itself. The agents of self-destruction include physiological compounds, such as glutamate, which are essential for the proper functioning of the CNS, but become cytotoxic when their normal concentrations are exceeded. The CNS is equipped with buffering mechanisms that are specific for each compound. Here we show, using Balb/c mice (a strain resistant to induction of experimental autoimmune encephalomyelitis), that after intravitreal injection of any concentration of glutamate (a neurotransmitter that becomes toxic when in excess) or ammonium-ferrous sulfate hexahydrate (which increases the formation of toxic oxygen species), the loss of retinal ganglion cells in mice devoid of mature T cells (nude mice) is significantly greater than in matched wild-type controls. We further show that this outcome can be partially reversed by supplying the T cell-defective mice with splenocytes derived from the wild-type mice. The results suggest that potentially toxic physiological compounds, when present in excessive amounts, can recruit and activate a T-cell-dependent self-protective immune mechanism. This may represent a prototype mechanism for the physiological regulation of potentially destructive CNS events by T-cell-mediated immune activity, when the local buffering mechanisms cannot adequately cope with them.

SCH 58261 (an adenosine A(2A) receptor antagonist) reduces, only at low doses, K(+)-evoked glutamate release in the striatum

The aim of the present work was to determine whether systemic administration of the adenosine A(2A) receptor antagonist, SCH 58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4,triazolo[1,5-c]pyrimidine), could modulate striatal glutamate outflow in the rat. Microdialysis experiments were performed in male Wistar rats implanted with microdialysis probes in the striatum. Pretreatment (15 min before) with SCH 58261 (0.01 and 0.1, but not 1 mg/kg intraperitoneally) significantly prevented K(+)-stimulated glutamate release. These results suggest that SCH 58261 could possess neuroprotective effects in the low dose range, while, at higher doses, the occurrence of additional mechanisms may limit the neuroprotective potential of this drug.

Neonatal exposure to monosodium glutamate induces cell death and dendritic hypotrophy in rat prefrontocortical pyramidal neurons

Monosodium glutamate was administered subcutaneously to male neonate rats, and the effects on cell number and cytoarchitecture of third-layer pyramidal neurons from the prefrontal cerebral cortex were studied in the adult. Monosodium glutamate treatment (4 mg/g of body weight, on post-natal days 1, 3, 5 and 7) resulted in fewer neurons, and shorter and less ramified dendritic processes, than those observed in control animals. Both density and proportional shapes of dendritic spines were not modified. We propose a dual effect of neonatal exposure to glutamate: an excitotoxic effect leading to cell death, and; a secondary neuroprotective effect, arising from the proliferation of glial cells and their subsequent uptake of glutamate, that favors the survival of the remaining neurons, and leads to a further hypotrophic effect on their dendritic processes.