Metabotropic glutamate receptor 5 (mGluR5)-mediated phosphoinositide hydrolysis and NMDA-potentiating effects are blunted in the striatum of aged rats: a possible additional mechanism in striatal senescence

Activity-dependent PI4P synthesis by PI4KIIIα regulates long-term synaptic potentiation

Phosphatidylinositol 4-phosphate (PI4P) is a low abundant phospholipid with important roles in lipid transport and membrane trafficking. However, little is known of its metabolism and function in neurons. Here, we investigate its subcellular distribution and functional roles in dendrites of rodent hippocampal neurons during resting state and long-term synaptic potentiation (LTP). We show that neural activity causes dynamic reversible changes in PI4P metabolism in dendrites. Upon LTP induction, PI4KIIIα, a type III phosphatidylinositol 4-kinase, localizes to the dendritic plasma membrane (PM) in a calcium-dependent manner and causes substantial increase in the levels of PI4P. Acute inhibition of PI4KIIIα activity abolishes trafficking of the AMPA-type glutamate receptor to the PM during LTP induction, and silencing of PI4KIIIα expression in the hippocampal CA1 region causes severe impairment of LTP and long-term memory. Collectively, our results identify an essential role for PI4KIIIα-dependent PI4P synthesis in synaptic plasticity of central nervous system neurons.

Chronic Nicotine Exposure Alters Metabotropic Glutamate Receptor 5: Longitudinal PET Study and Behavioural Assessment in Rats

Using positron emission tomography (PET), a profound alteration of the metabotropic glutamate receptor 5 (mGluR5) was found in human smoking addiction and abstinence. As human PET data either reflect the impact of chronic nicotine exposure or a pre-existing vulnerability to nicotine addiction, we designed a preclinical, longitudinal study to investigate the effect of chronic nicotine exposure on mGluR5 with the novel radiotracer [18F]PSS232 using PET. Twelve male dark Agouti rats at the age of 6 weeks were assigned randomly to three groups. From day 0 to day 250 the groups received 0 mg/L, 4 mg/L, or 8 mg/L nicotine solution in the drinking water. From day 250 to 320 all groups received nicotine-free drinking water. PET scans with [18F]PSS232 were performed in all animals on days 0, 250, and 320. To assess locomotion, seven tests in square open field arenas were carried out 72 days after the last PET scan. During the first four tests, rats received 0 mg/L nicotine and for the last three tests 4 mg/L nicotine in the drinking water. After 250 days of nicotine consumption [18F]PSS232 binding was reduced in the striatum, hippocampus, thalamus, and midbrain. At day 320, after nicotine withdrawal, [18F]PSS232 binding increased. These effects were more pronounced in the 4 mg/L nicotine group. Chronic administration of nicotine through the drinking water reduced exploratory behaviour. This preliminary longitudinal PET study demonstrates that chronic nicotine administration alters behaviour and mGluR5 availability. Chronic nicotine administration leads to decreased [18F]PSS232 binding which normalizes after prolonged nicotine withdrawal.

Peptidomic analysis of zebrafish embryos exposed to polychlorinated biphenyls and their impact on eye development

Polychlorinated biphenyls (PCBs), a class of persistent organic pollutant, are closely related to abnormal eye development in children. However, little is known regarding the role of peptides in the development of PCB-induced ocular dysplasia. To characterize the nature of PCB exposure on peptides involved in the development of the ocular system, we used liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) to detect differential expression of peptides between normal and PCB-exposed zebrafish embryos. A total of 7900 peptides were analyzed, 90 of which were differentially expressed, with 29 being up-regulated and 61 down-regulated. These peptides were investigated using ingenuity pathway analysis (IPA) and gene ontology (GO) analysis to explore their role in eye development. This study identified 18 peptides associated with the development of the optic nerve and ocular system in the PCB-exposure group, as well as 10 peptides that are located in the functional domain of their precursor proteins. These peptides provide potential biomarkers for the treatment of ocular dysplasia caused by PCBs and may help us understand the mechanism of abnormal eye development caused by organic pollutants.

mGluR5 antagonism inhibits cocaine reinforcement and relapse by elevation of extracellular glutamate in the nucleus accumbens via a CB1 receptor mechanism

Metabotropic glutamate receptor 5 (mGluR5) antagonism inhibits cocaine self-administration and reinstatement of drug-seeking behavior. However, the cellular and molecular mechanisms underlying this action are poorly understood. Here we report a presynaptic glutamate/cannabinoid mechanism that may underlie this action. Systemic or intra-nucleus accumbens (NAc) administration of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) dose-dependently reduced cocaine (and sucrose) self-administration and cocaine-induced reinstatement of drug-seeking behavior. The reduction in cocaine-taking and cocaine-seeking was associated with a reduction in cocaine-enhanced extracellular glutamate, but not cocaine-enhanced extracellular dopamine (DA) in the NAc. MPEP alone, when administered systemically or locally into the NAc, elevated extracellular glutamate, but not DA. Similarly, the cannabinoid CB1 receptor antagonist, rimonabant, elevated NAc glutamate, not DA. mGluR5s were found mainly in striatal medium-spiny neurons, not in astrocytes, and MPEP-enhanced extracellular glutamate was blocked by a NAc CB1 receptor antagonist or N-type Ca⁺⁺ channel blocker, suggesting that a retrograde endocannabinoid-signaling mechanism underlies MPEP-induced glutamate release. This interpretation was further supported by our findings that genetic deletion of CB1 receptors in CB1-knockout mice blocked both MPEP-enhanced extracellular glutamate and MPEP-induced reductions in cocaine self-administration. Together, these results indicate that the therapeutic anti-cocaine effects of mGluR5 antagonists are mediated by elevation of extracellular glutamate in the NAc via an endocannabinoid-CB1 receptor disinhibition mechanism.

Effects of short- and long-term aripiprazole treatment on Group I mGluRs in the nucleus accumbens: Comparison with haloperidol

The D2 receptor partial agonist, aripiprazole, has shown increased therapeutic efficacy for schizophrenia, autism and Tourette's syndrome compared to traditional antipsychotics such as the D2 receptor antagonist, haloperidol. Recent evidence suggests this superior profile may be associated with downstream effects on glutamatergic synapses. Group 1 metabotropic glutamate receptors (mGluRs) and their endogenous modulators, Norbin and Homer1, are regulated by D2 receptor activity, particularly within the nucleus accumbens (NAc), a target region of aripiprazole and haloperidol. This study sought to evaluate the effects of aripiprazole on Group 1 mGluRs, Norbin and Homer1 in the NAc, in comparison to haloperidol. Sprague-Dawley rats were orally administered daily doses of aripiprazole (2.25mg/kg), haloperidol (0.3mg/kg) or vehicle for 1 or 10-weeks. Immunoblot analyses revealed Group 1 mGluR protein levels were not altered following 1-week and 10-week aripiprazole or haloperidol treatment, compared to vehicle treated rodents. However, 1-week aripiprazole and haloperidol treatment significantly elevated Homer1a and Norbin protein expression, respectively. After 10 weeks of treatment, aripiprazole, but not haloperidol, significantly increased Norbin expression. These findings indicate the antipsychotics, aripiprazole and haloperidol, exert differential temporal effects on Norbin and Homer1 expression that may have consequences on synaptic glutamatergic transmission underlying their therapeutic profile.

Preclinical molecular imaging of glutamatergic and dopaminergic neuroreceptor kinetics in obsessive compulsive disorder

BACKGROUND

Molecular neuroimaging was applied in the quinpirole rat model for compulsive checking in OCD to visualize the D2- and mGluR5-receptor occupancy with Raclopride and ABP-688 microPET/CT.

METHODS

Animals (n=48) were exposed to either saline (CTRL; 1mL/kg) or quinpirole (QP; dopamine D2-agonist, 0.5mg/kg) in a single injection (RAC and ABP acute groups) or twice-weekly during 7weeks (chronic group). Animals underwent PET/CT after the 1st injection (acute) or before initial exposure and following the 10th injection in week 5 (chronic). For the latter, each injection was paired with an open field test and video tracking.

RESULTS

The QP animals displayed a strong increase in visiting frequency (checking) in the chronic group (+699.29%) compared to the control animals. Acute administration of the drug caused significant (p<0.01) decreases in D2R occupancy in the CP (-42.03%±4.01%). Chronical exposure resulted in significantly stronger decreases in the CP (-52.29%±3.79%). Furthermore significant increases in mGluR5 occupancy were found in the CP (10.36%±4.09%), anterior cingulate cortex (13.26%±4.01%), amygdala (24.36%±6.86%), entorhinal cortex (18.49%±5.14%) and nucleus accumbens (13.8%±4.87%) of the chronic group, not present after acute exposure.

CONCLUSIONS

Compared to acute exposure, sensitisation to QP as a model for OCD differs both on a dopaminergic and glutamateric level, indicating involvement of processes such as receptor internalization and changes in extracellular availability of both neurotransmitters.

Metabotropic Glutamate Receptors and Interacting Proteins in Epileptogenesis

Neurotransmitter and receptor systems are involved in different neurological and neuropsychological disorders such as Parkinson's disease, depression, Alzheimer’s disease and epilepsy. Recent advances in studies of signal transduction pathways or interacting proteins of neurotransmitter receptor systems suggest that different receptor systems may share the common signal transduction pathways or interacting proteins which may be better therapeutic targets for development of drugs to effectively control brain diseases. In this paper, we reviewed metabotropic glutamate receptors (mGluRs) and their related signal transduction pathways or interacting proteins in status epilepticus and temporal lobe epilepsy, and proposed some novel therapeutical drug targets for controlling epilepsy and epileptogenesis.

Characterization of brain mGluR5 binding in a pilot study of late-life major depressive disorder using positron emission tomography and [¹¹C]ABP688

The metabotropic glutamate receptor subtype 5 (mGluR5) has been implicated in the pathophysiology of mood and anxiety disorders and is a potential treatment target in major depressive disorder (MDD). This study compared brain mGluR5 binding in elderly patients suffering from MDD with that in elderly healthy volunteers using positron emission tomography (PET) and [(11)C]ABP688. Twenty elderly (mean age: 63.0 ± 6.3) subjects with MDD and twenty-two healthy volunteers in the same age range (mean age: 66.4 ± 7.3) were examined with PET after a single bolus injection of [(11)C]ABP688, with many receiving arterial sampling. PET images were analyzed on a region of interest and a voxel level to compare mGluR5 binding in the brain between the two groups. Differences in [(11)C]ABP688 binding between patients with early- and late-onset depression were also assessed. In contrast to a previously published report in a younger cohort, no significant difference in [(11)C]ABP688 binding was observed between elderly subjects with MDD and healthy volunteers. [(11)C]ABP688 binding was also similar between subgroups with early- or late-onset depression. We believe this is the first study to examine mGluR5 expression in depression in the elderly. Although future work is required, results suggest potential differences in the pathophysiology of elderly depression versus depression earlier in life.

NMDA receptor dysfunction contributes to impaired brain-derived neurotrophic factor-induced facilitation of hippocampal synaptic transmission in a Tau transgenic model

While the spatiotemporal development of Tau pathology has been correlated with occurrence of cognitive deficits in Alzheimer's patients, mechanisms underlying these deficits remain unclear. Both brain-derived neurotrophic factor (BDNF) and its tyrosine kinase receptor TrkB play a critical role in hippocampus-dependent synaptic plasticity and memory. When applied on hippocampal slices, BDNF is able to enhance AMPA receptor-dependent hippocampal basal synaptic transmission through a mechanism involving TrkB and N-methyl-d-Aspartate receptors (NMDAR). Using THY-Tau22 transgenic mice, we demonstrated that hippocampal Tau pathology is associated with loss of synaptic enhancement normally induced by exogenous BDNF. This defective response was concomitant to significant memory impairments. We show here that loss of BDNF response was due to impaired NMDAR function. Indeed, we observed a significant reduction of NMDA-induced field excitatory postsynaptic potential depression in the hippocampus of Tau mice together with a reduced phosphorylation of NR2B at the Y1472, known to be critical for NMDAR function. Interestingly, we found that both NR2B and Src, one of the NR2B main kinases, interact with Tau and are mislocalized to the insoluble protein fraction rich in pathological Tau species. Defective response to BDNF was thus likely related to abnormal interaction of Src and NR2B with Tau in THY-Tau22 animals. These are the first data demonstrating a relationship between Tau pathology and synaptic effects of BDNF and supporting a contribution of defective BDNF response and impaired NMDAR function to the cognitive deficits associated with Tauopathies.

mGlu5R promotes glutamate AMPA receptor phosphorylation via activation of PKA/DARPP-32 signaling in striatopallidal medium spiny neurons

Group I metabotropic glutamate receptors (mGluRs), which comprise mGlu1Rs and mGlu5Rs, are enriched in striatal medium spiny neurons (MSNs), where they modulate glutamatergic transmission. Here, we have examined the effect of group I mGluRs on the regulation of the state of phosphorylation of the GluA1 subunit of the AMPA glutamate receptor. We found that incubation of mouse striatal slices with the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) promotes GluA1 phosphorylation at the cAMP-dependent protein kinase (PKA) site, Ser845. This effect is prevented by 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), a selective mGlu5R antagonist. The increase in GluA1 phosphorylation produced by DHPG is also prevented by blockade of adenosine A2A receptors (A2ARs), which are known to promote cAMP signaling specifically in striatopallidal MSNs, as well as by enzymatic degradation of endogenous adenosine, achieved with adenosine deaminase. The ability of DHPG to increase PKA-dependent phosphorylation of GluA1 depends on concomitant activation of the dopamine- and cAMP-regulated phosphoprotein of 32kDa (DARPP-32). Thus, inactivation of the PKA phosphorylation site of DARPP-32 abolishes the effect of DHPG. Moreover, cell-specific knock out of DARPP-32 in striatopallidal, but not in striatonigral, MSNs prevents the increase in Ser845 phosphorylation induced by DHPG. These results indicate that activation of mGlu5Rs promotes PKA/DARPP-32-dependent phosphorylation of downstream target proteins in striatopallidal MSNs and that this effect is exerted via potentiation of tonic A2AR transmission. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

mGluR5-PLCbeta4-PKCbeta2/PKCgamma pathways in hippocampal CA1 pyramidal neurons in pilocarpine model of status epilepticus in mGluR5+/+ mice

While it is generally accepted that phospholipase C (PLC) and protein kinase C (PKC) are down-stream proteins involved in metabotropic glutamate receptor 5 (mGluR5)-related signal transduction, we still do not know which subtype of PLC or PKC is specifically regulated after mGluR5 activation. In the present study in mGluR5 wild-type (mGluR5+/+) mice, we showed induced PKCbeta2 or PKCgamma expression at the border between the stratum oriens and alveus (O/A border) at 2h during pilocarpine induced status epilepticus (SE), and in the stratum pyramidale in CA1 area at 1 day after pilocarpine induced SE; at 1 day, induced expression of PLCbeta4 in the stratum pyramidale of CA1 area was observed. Furthermore, double labeling revealed the co-localization of induced PKCbeta2 or PKCgamma with mGluR5 or with induced PLCbeta4 in the stratum pyramidale of CA1 area. These induced expression, however, were not found in mGluR5 mutant (mGluR5-/-) mice. It suggests that induced PLCbeta4-PKCbeta2/PKCgamma at 1 day after pilocarpine induced SE in pyramidal neurons or PKCbeta2 or PKCgamma in interneurons at O/A border at 2h during pilocarpine induced SE may be specifically linked to the activation of mGluR5. When compared to mGluR5+/+ mice, significant shorter latency (from pilocarpine injection to the occurrence of status epilepticus) and maintenance period (from beginning to the end of status epilepticus) for status epilepticus in mGluR5-/- mice were also demonstrated. It is possible that mGluR5 may play a negative role in initiation of status epilepticus by interacting with muscarinic acetylcholine receptor in mGluR5+/+ mice.

Metabotropic glutamate 5 receptor (mGluR5) antagonists decrease nicotine seeking, but do not affect the reinforcement enhancing effects of nicotine

Nicotine self-administration models typically evaluate the effects of smoking cessation aides on 'primary reinforcement' engendered by nicotine. However, the more recently described reinforcement enhancing effects of the drug are not always included in experimental analyses of potential therapeutics. We evaluated the effects of pretreatment with noncompetitive antagonists of the metabotropic glutamate 5 receptor (mGluR5) on each reinforcement-related effect of nicotine using a model in which a reinforcing visual stimulus (VS) and nicotine infusions were concurrently available. Five groups (2-lever, VS-only, NIC+VS, NIC-only, or SAL-only) were instrumented for self-administration. The 2-lever group could earn a nicotine infusion (0.06 mg/kg per infusion free base) for meeting the schedule on one lever (eg right), or VS for meeting the schedule on the other lever (eg left). The VS-only group could earn VS or saline under similar contingencies. Remaining rats could press one lever to earn both reinforcers (NIC+VS), nicotine infusions (NIC-only), or saline infusions (SAL-only); the other lever was 'inactive'. Responding on the VS lever in the 2-lever group was greater than that of the VS-only group, reflecting the reinforcement-enhancing effect of nicotine. Pretreatment with 2-methyl-6-(phenylethynyl)-pyridine (MPEP) or 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) decreased nicotine intake as well as the enhanced responding for the concurrently available VS. In follow-up studies, replacing nicotine via experimenter-administered infusions sustained the drugs reinforcement enhancing effect; neither MPEP nor MTEP decreased the enhancing effects of nicotine. These findings are consistent with other studies suggesting that mGlu5 receptors mediate nicotine seeking, but do not alter the reinforcement enhancing effects of nicotine.

Distinct roles of group I mGlu receptors in striatal function

In the recent past, evidence accumulated in favour of a central role of group I metabotropic glutamate (mGlu) receptors, mGlu1 and mGlu5, in the modulation of cell excitability both of striatal medium spiny projection neurons (MSNs) and interneuronal population. Electrophysiological and pharmacological studies have clearly shown that activation of mGlu1 and mGlu5 receptors exerts distinct actions, depending on the neuronal subtype involved. MGlu5 receptor activation mediates the potentiation of NMDA responses in MSNs, and underlies the retrograde inhibitory signaling by endocannabinoids at corticostriatal synapses. Conversely, both group I mGlu receptors are involved in long-term synaptic plasticity of MSNs. Likewise, either mGlu1 or mGlu5 receptors are engaged in shaping the excitability of large cholinergic interneurons, playing different roles in the membrane responses. Differently, although GABAergic parvalbumin-positive, fast-spiking interneurons have been shown to express both group I receptors, only mGlu1 receptor seems to mediate membrane and synaptic responses. This review provides a brief survey of the cellular and synaptic actions of group I mGlu receptors, and discusses the potential relevance of these findings in neostriatal function and motor control.

Chronic treatment with the mGlu5R antagonist MPEP reduces the functional effects of the mGlu5R agonist CHPG in the striatum of 6-hydroxydopamine-lesioned rats: Possible relevance to the effects of mGlu5R blockade in Parkinson's disease

This study was designed to test whether chronic treatment with the metabotropic glutamate receptor 5 (mGlu5R) antagonist MPEP showed antiparkinsonian effects in rats unilaterally lesioned with 6-hydroxydopamine (6-OHDA) (a "classic" model of Parkinson's disease, PD), and to evaluate whether chronic MPEP influenced the functional properties and/or the expression of striatal mGlu5Rs. Wistar rats were lesioned with 6-OHDA and then treated with MPEP (3 mg/kg/day, i.p.) or its vehicle over 2 weeks. Chronic MPEP did not induce measurable antiparkinsonian effects, since no differences were found between MPEP- and vehicle-treated animals in the pattern of L-DOPA-induced contralateral rotations. In corticostriatal slices taken from animals chronically treated with MPEP, the functional effects of the mGlu5R agonist CHPG were significantly reduced in the lesioned vs. the intact side, while no changes were found in slices taken from vehicle-treated rats. The binding of [3H]MPEP to striatal membranes showed that neither the maximal number of binding sites (Bmax) nor the dissociation constant (Kd) were changed by the lesion and/or by chronic MPEP. While chronic MPEP did not potentiate L-DOPA-induced turning in a classical model of PD, its ability to reduce mGlu5R-associated signal could help to explain the neuroprotective/antiparkinsonian effects observed in other models of PD.

Mutual regulation between metabotropic glutamate type 1alpha receptor and caveolin proteins: from traffick to constitutive activity

In this paper, a molecular and functional interaction between metabotropic glutamate receptor type 1alpha (mGlu1alpha receptor) and caveolin-1 or caveolin-2beta is described. An overlapping pattern of staining for mGlu1alpha receptor with caveolin-1 and caveolin-2 by confocal laser microscopy in transiently transfected HEK-293 cells is observed. The presence of mGlu1alpha receptor in caveolin-enriched membrane fractions was demonstrated by flotation gradient analysis in the absence of detergents and the interaction between mGlu1alpha receptor with caveolin-1 and with caveolin-2beta was demonstrated by coimmunoprecipitation experiments. In HEK-293 cells, caveolin-2beta accumulates surrounding lipid droplets when single expressed but coexpression with mGlu1alpha receptor changed dramatically the subcellular localization of caveolin-2beta, directing it from lipid droplets to the cell surface. At the membrane level, the interaction between caveolin-1 and mGlu1alpha receptor could abrogate the constitutive activity exhibited by mGlu1alpha receptor. Overall, these results show that mGlu1alpha receptor interacts with caveolins and that this interaction is physiologically relevant for receptor function. Interestingly, we provide evidence that caveolin-1 is not just acting as a scaffolding protein for the mGlu1alpha receptor but that also regulates mGlu1alpha receptor constitutive activity.

Neuroprotective effects of the mGlu5R antagonist MPEP towards quinolinic acid-induced striatal toxicity: involvement of pre- and post-synaptic mechanisms and lack of direct NMDA blocking activity

The aim of this work was to investigate the potential neuroprotective effects of the metabotropic glutamate receptor 5 (mGlu5R) antagonist 2-Methyl-6-(phenylethynyl)-pyridine (MPEP) towards quinolinic acid (QA)-induced striatal excitoxicity. Intrastriatal MPEP (5 nmol/0.5 micro L) significantly attenuated the body weight loss, the electroencephalographic alterations, the impairment in spatial memory and the striatal damage induced by bilateral striatal injection of QA (210 nmol/0.7 micro L). In a second set of experiments, we aimed to elucidate the mechanisms underlying the neuroprotective effects of MPEP. In microdialysis studies in naive rats MPEP (80-250 micro m through the dialysis probe) significantly reduced the increase in glutamate levels induced by 5 mm QA. In primary cultures of striatal neurons MPEP (50 micro m) reduced the toxicity induced by direct application of glutamate [measured as release of lactate dehydrogenase [LDH]). Finally, we found that 50 micro m MPEP was unable to directly block NMDA-induced effects (namely field potential reduction in corticostriatal slices, as well as LDH release and intracellular calcium increase in striatal neurons). We conclude that: (i) MPEP has neuroprotective effects towards QA-induced striatal excitotoxicity; (ii) both pre- and post-synaptic mechanisms are involved; (iii) the neuroprotective effects of MPEP do not appear to involve a direct blockade of NMDA receptors.

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.