N-acetylaspartylglutamate (NAAG) protects against rat striatal quinolinic acid lesions in vivo

NAAG peptidase inhibitor improves motor function and reduces cognitive dysfunction in a model of TBI with secondary hypoxia

Immediately following traumatic brain injury (TBI) and TBI with hypoxia, there is a rapid and pathophysiological increase in extracellular glutamate, subsequent neuronal damage and ultimately diminished motor and cognitive function. N-acetyl-aspartyl glutamate (NAAG), a prevalent neuropeptide in the CNS, is co-released with glutamate, binds to the presynaptic group II metabotropic glutamate receptor subtype 3 (mGluR3) and suppresses glutamate release. However, the catalytic enzyme glutamate carboxypeptidase II (GCP II) rapidly hydrolyzes NAAG into NAA and glutamate. Inhibition of the GCP II enzyme with NAAG peptidase inhibitors reduces the concentration of glutamate both by increasing the duration of NAAG activity on mGluR3 and by reducing degradation into NAA and glutamate resulting in reduced cell death in models of TBI and TBI with hypoxia. In the following study, rats were administered the NAAG peptidase inhibitor PGI-02776 (10mg/kg) 30 min following TBI combined with a hypoxic second insult. Over the two weeks following injury, PGI-02776-treated rats had significantly improved motor function as measured by increased duration on the rota-rod and a trend toward improved performance on the beam walk. Furthermore, two weeks post-injury, PGI-02776-treated animals had a significant decrease in latency to find the target platform in the Morris water maze as compared to vehicle-treated animals. These findings demonstrate that the application of NAAG peptidase inhibitors can reduce the deleterious motor and cognitive effects of TBI combined with a second hypoxic insult in the weeks following injury.

Glycine release is regulated by metabotropic glutamate receptors sensitive to mGluR2/3 ligands and activated by N-acetylaspartylglutamate (NAAG)

The presence of metabotropic glutamate receptors (mGluRs) of group II modulating glycine exocytosis from glycinergic nerve endings of mouse spinal cord was investigated. Purified synaptosomes were selectively prelabeled with [(3)H]glycine through the neuronal transporter GlyT2 and subsequently depolarized by superfusion with 12 mM KCl. The selective mGluR2/3 agonist LY379268 inhibited the K(+)-evoked overflow of [(3)H]glycine in a concentration-dependent manner (EC(50) about 0.2 nM). The effect of LY379268 was prevented by the selective mGluR2/3 antagonist LY341495 (IC(50) about 1 nM). N-acetylaspartylglutamate (NAAG) inhibited [(3)H]glycine overflow with extraordinary potency (EC(50) about 50 fmol). In contrast, glutamate was ineffective up to 0.1 nM, excluding that glutamate contamination of commercial NAAG samples is responsible for the reported activity of NAAG at mGluR3. LY341495 antagonized the NAAG inhibition of [(3)H]glycine release. The effect of a combination of maximally effective concentrations of LY379268 and NAAG exhibited no additivity. The non-hydrolysable NAAG analogue N-acetylaspartyl-β-linked glutamate (β-NAAG) antagonized NAAG and LY379268. In conclusion, our results show that glycinergic nerve endings in spinal cord are endowed with group II mGluRs mediating inhibition of glycine exocytosis. NAAG can activate these presynaptic receptors with extremely high affinity and with characteristics compatible with the reported mGluR3 pharmacology. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Delivery of peptide and protein drugs over the blood-brain barrier

Peptide and protein (P/P) drugs have been identified as showing great promises for the treatment of various neurodegenerative diseases. A major challenge in this regard, however, is the delivery of P/P drugs over the blood-brain barrier (BBB). Intense research over the last 25 years has enabled a better understanding of the cellular and molecular transport mechanisms at the BBB, and several strategies for enhanced P/P drug delivery over the BBB have been developed and tested in preclinical and clinical-experimental research. Among them, technology-based approaches (comprising functionalized nanocarriers and liposomes) and pharmacological strategies (such as the use of carrier systems and chimeric peptide technology) appear to be the most promising ones. This review combines a comprehensive overview on the current understanding of the transport mechanisms at the BBB with promising selected strategies published so far that can be applied to facilitate enhanced P/P drug delivery over the BBB.

The use of knock-out mice unravels distinct roles for mGlu2 and mGlu3 metabotropic glutamate receptors in mechanisms of neurodegeneration/neuroprotection

Dual metabotropic glutamate 2/3 (mGlu2/3) receptor agonists have been examined with success in the clinic with positive proof of efficacy in several tests of anxiety and schizophrenia. Moreover, a large body of evidence has accumulated that these drugs have significant neuroprotective potential. An important discussion in the field deals with dissecting effects on mGlu2 versus effects on mGlu3 receptors, which is relevant for the potential use of subtype-selective agonists or allosteric activators. We addressed this issue using mGlu2 and mGlu3 receptor knock-out mice. We used mixed cultures of cortical cells in which astrocytes and neurons were plated at different times and could therefore originate from different mice. Cultures were challenged with NMDA for the induction of excitotoxic neuronal death. The mGlu2/3 receptor agonist, (-)-2-oxa-4-aminocyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268), was equally neuroprotective in cultures containing neurons from wild-type, mGlu2-/-, or mGlu3-/- mice. Neuroprotection was instead abolished when astrocytes lacked mGlu3 receptors, unless neuronal mGlu2 receptors were also absent. The latter condition partially restored the protective activity of LY379268. Cultures in which neurons originated from mGlu2-/- mice were also intrinsically resistant to NMDA toxicity. In in vivo experiments, systemic administration of LY379268 protected striatal neurons against NMDA toxicity in wild-type and mGlu2-/- mice but not in mGlu3-/- mice. In addition, LY379268 was protective against nigrostriatal degeneration induced by low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine only in mice lacking mGlu2 receptors. We conclude that neuroprotection by mGlu2/3 receptor agonists requires the activation of astrocytic mGlu3 receptors, whereas, unexpectedly, activation of mGlu2 receptors might be harmful to neurons exposed to toxic insults.

Different transfer of nociceptive sensitivity from rats with postnatal hippocampal lesions to control rats

Hippocampal lesions in newborn rats alter the development of mechanisms involved in the processing of nociception. The hippocampal lesion was induced by the bilateral infusion, into the lateral cerebral ventricles, of 0.25 microL of saline containing either 0.25 micromol quinolinic acid (QUIN) and/or 0.25 micromol N-acetyl-L-aspartyl-L-glutamate (NAAG) on postnatal day 12. The same amount of sterile saline was injected into the sham-operated animals (group SHAM). It was expected that the QUIN- and NAAG-lesioned rats would exhibit some differences in thermal pain perception; however, we wanted to know if the control rats would exhibit, at least in part, similar changes in pain perception as their chemically lesioned siblings with which they were housed. Young adult NAAG-injured rats exhibited increased withdrawal latencies in the tail-flick and plantar tests, whereas young adult QUIN-injured animals exhibited only marginally decreased latencies. Nociceptive responses in the SHAM rats paralleled the littermates that had been neonatally treated with QUIN or NAAG, i.e. the responses in the SHAM(QUIN) group decreased, whereas the responses in the SHAM(NAAG) group increased. No significant changes in nociception were observed in intact animals, regardless of which group they were housed with. Our results show that social factors, which were originally demonstrated only for the pain behavior, may also influence basal nociceptive sensitivity in rats. We concluded that the "sham operation" may have had a long-term, nonspecific impact on nociceptive behavior by inducing behavioral mimicry of other animals.

Effects of NAAG peptidase inhibitor 2-PMPA in model chronic pain - relation to brain concentration

N-acetylated-alpha-linked-acidic peptidase (NAAG peptidase) converts N-acetyl-aspartyl-glutamate (NAAG, mGluR3 agonist) into N-acetyl-aspartate and glutamate. The NAAG peptidase inhibitor 2-PMPA (2-(phosphonomethyl)pentanedioic acid) had neuroprotective activity in an animal model of stroke and anti-allodynic activity in CCI model despite its uncertain ability to penetrate the blood-brain barrier. The NAAG concentration in brain ECF under basal conditions and its alteration in relation to the brain ECF concentration of 2-PMPA is unclear. We therefore assessed those brain concentrations after i.p. administration of 2-PMPA, using in vivo microdialysis combined with LC/MS/MS analysis. Administration of 2-PMPA (50mg/kg) produced a mean peak concentration of 2-PMPA of 29.66+/-8.1microM. This concentration is about 100,000 fold more than is needed for inhibition of NAAG peptidase, and indicates very good penetration to the brain. Application of 2-PMPA was followed by a linear increase of NAAG-concentration reaching a maximum of 2.89+/-0.42microM at the end of microdialysis. However, during the time the anti-allodynic effects of 2-PMPA were observed, the NAAG concentration in the ECF did not reach levels which are likely to have an impact on any known target. It appears therefore that the observed behavioural effects of 2-PMPA may not be mediated by NAAG nor, in turn, by mGluR3 receptors.

Differential negative coupling of type 3 metabotropic glutamate receptor to cyclic GMP levels in neurons and astrocytes

Metabotropic receptors may couple to different G proteins in different cells or perhaps even in different regions of the same cell. To date, direct studies of group II and group III metabotropic glutamate receptors' (mGluRs) relationships to second messenger cascades have reported negative coupling of these receptors to cyclic AMP (cAMP) levels in neurons, astrocytes and transfected cells. In the present study, we found that the peptide neurotransmitter N-acetylaspartylglutamate (NAAG), an mGluR3-selective agonist, decreased sodium nitroprusside (SNP)-stimulated cyclic GMP (cGMP) levels in cerebellar granule cells and cerebellar astrocytes. The mGluR3 and group II agonists FN6 and LY354740 had similar effects on cGMP levels. The mGluR3 and group II antagonists beta-NAAG and LY341495 blocked these actions. Treatment with pertussis toxin inhibited the effects of NAAG on SNP-stimulated cGMP levels in rat cerebellar astrocytes but not in cerebellar neurons. These data support the conclusion that mGluR3 is also coupled to cGMP levels and that this mGluR3-induced reduction of cGMP levels is mediated by different G proteins in cerebellar astrocytes and neurons. We previously reported that this receptor is coupled to a cAMP cascade via a pertussis toxin-sensitive G protein in cerebellar neurons, astrocytes and transfected cells. Taken together with the present data, we propose that mGluR3 is coupled to two different G proteins in granule cell neurons. These data greatly expand knowledge of the range of second messenger cascades induced by mGluR3, and have implications for clinical conditions affected by NAAG and other group II mGluR agonists.

2-phosphonomethyl-pentanedioic acid (glutamate carboxypeptidase II inhibitor) increases threshold for electroconvulsions and enhances the antiseizure action of valproate against maximal electroshock-induced seizures in mice

This study examined the effect of 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a potent and selective inhibitor of glutamate carboxypeptidase II (GCP II), an enzyme releasing glutamate and N-acetyl-aspartate from synaptical terminals, on the electroconvulsive threshold in mice. Moreover, the influence of 2-PMPA on the anticonvulsant activities of four conventional antiepileptic drugs (carbamazepine, phenobarbital, phenytoin and valproate) was evaluated in the maximal electroshock-induced seizure test in mice. Results indicated that 2-PMPA (at a dose range of 50-200 mg/kg, i.p.) raised the electroconvulsive threshold in mice dose-dependently. Linear regression analysis of dose-response relationship between the doses of 2-PMPA and their corresponding threshold values allowed the calculation of threshold increasing dose by 20% (TID20), which was 109.2 mg/kg. Moreover, 2-PMPA administered i.p. at a constant dose of 150 mg/kg (the dose increasing the threshold for electroconvulsions) enhanced significantly the anticonvulsant action of valproate, by reducing its median effective dose (ED50) from 281.4 to 230.1 mg/kg (P<0.05). In contrast, 2-PMPA at the lower dose of 100 mg/kg (i.p.) had no impact on the antiseizure activity of valproate in the maximal electroshock-induced seizure test. Likewise, 2-PMPA at 100 and 150 mg/kg did not affect the antiseizure action of carbamazepine, phenobarbital and phenytoin against maximal electroshock-induced seizures in mice. Additionally, none of the combinations investigated between 2-PMPA (150 mg/kg, i.p.) and carbamazepine, phenobarbital, phenytoin and valproate (at their ED50 values) produced motor coordination impairment in the chimney test. Pharmacokinetic evaluation of interaction between 2-PMPA and valproate revealed that 2-PMPA at 150 mg/kg selectively increased total brain concentrations of valproate, remaining simultaneously without any effect on free plasma concentrations of valproate, indicating a pharmacokinetic nature of observed interaction in the maximal electroshock-induced seizures in mice. Based on our preclinical data, it may be concluded that 2-PMPA possesses a seizure modulating property by increasing the electroconvulsive threshold. The reduction of glutamate neurotransmission in the brain, as a consequence of inhibition of GCP II activity by 2-PMPA, was however insufficient to enhance the anticonvulsant activity of conventional antiepileptic drugs, except for valproate, whose antiseizure action against maximal electroconvulsions was potentiated by 2-PMPA. Unfortunately, the favourable interaction between 2-PMPA and valproate was associated with a pharmacokinetic increase in total brain valproate concentrations.

Mice lacking glutamate carboxypeptidase II are protected from peripheral neuropathy and ischemic brain injury

Excessive glutamate release is associated with neuronal damage. A new strategy for the treatment of neuronal injury involves inhibition of the neuropeptidase glutamate carboxypeptidase II (GCP II), also known as N-acetylated alpha-linked acidic dipeptidase. GCP II is believed to mediate the hydrolysis of N-acetyl-aspartyl-glutamate (NAAG) to glutamate and N-acetyl-aspartate, and inhibition of NAAG peptidase activity (by GCP II and other peptidases) is neuroprotective. Mice were generated in which the Folh1 gene encoding GCP II was disrupted (Folh1-/- mice). No overt behavioral differences were apparent between Folh1-/- mice and wild-type littermates, with respect to their overall performance in locomotion, coordination, pain threshold, cognition and psychiatric behavioral paradigms. Morphological analysis of peripheral nerves, however, showed significantly smaller axons (reduced myelin sheaths and axon diameters) in sciatic nerves from Folh1-/- mice. Following sciatic nerve crush, Folh1-/- mice suffered less injury and recovered faster than wild-type littermates. In a model of ischemic injury, the Folh1-/- mice exhibited a significant reduction (p < 0.05) in infarct volume compared with their wild-type littermates when subjected to middle cerebral artery occlusion, a model of stroke. These findings support the hypothesis that GCP II inhibitors may represent a novel treatment for peripheral neuropathies as well as stroke.

The paradigm of Huntington's disease: therapeutic opportunities in neurodegeneration

Despite a relatively small number of affected patients, Huntington's disease (HD) has been a historically important disease, embodying many of the major themes in modern neuroscience, including molecular genetics, selective neuronal vulnerability, excitotoxicity, mitochondrial dysfunction, apoptosis, and transcriptional dysregulation. The discovery of the HD gene in 1993 opened the door to the mechanisms of HD pathogenesis. Multiple pathologic mechanisms have been discovered, each one serving as a potential therapeutic target. HD thus continues to serve as a paradigmatic disorder, with basic bench research generating clinically relevant insights and stimulating the development of therapeutic human trials.

Morphology and ultrastructure of rat hippocampal formation after i.c.v. administration of N-acetyl-L-aspartyl-L-glutamate

N-Acetyl-L-aspartyl-L-glutamate (NAAG) is one of the most abundant neuroactive compounds in the mammalian CNS. Our recent observations have suggested that NAAG administered into rat cerebral ventricles can cause neuronal death by apparently excitotoxic mechanisms that can be antagonized by the N-methyl-D-aspartate-receptor blockers and by ligands of metabotropic glutamate receptor of Group II. Therefore, the principal aim of the present study has been to use quantitative morphology, electron microscopy and terminal deoxynucleotidyl transferase-mediated biotin dUTP nick-end labeling to study a dose- and time-dependence as well as regional distribution of neurodegeneration in hippocampi of rats after the intraventricular infusion of 0.25 micromol NAAG/ventricle and of equimolar doses of L-glutamate (L-GLU) and N-acetyl-L-aspartate (NAA), breakdown products of NAAG. The degenerative changes were observed after the infusion of 0.25 and 1.25 micromol of NAAG/ventricle, but not when a dose of 0.05 micromol of NAAG/ventricle was injected into each lateral cerebral ventricle. With a dose of 0.25 micromol of NAAG/ventricle the number of degenerated neurons reached a maximum on the fourth day after the infusion. The neuronal damage following bilateral administration of 0.25 micromol of NAAG/lateral cerebral ventricle exhibited features of a delayed neuronal degeneration, expressed mainly in the layer of dentate granule neurons. The degeneration was characterized on the basis of ultrastructural appearance and DNA-fragmentation. The morphological changes caused by L-glutamate and NAA were much smaller than those observed after the administration of NAAG and displayed a different pattern of regional distribution. The present findings suggest that NAAG can cause a loss of hippocampal neurons in vivo, apparently resulting from the neurotoxicity of NAAG itself.

NAAG fails to antagonize synaptic and extrasynaptic NMDA receptors in cerebellar granule neurons

The peptide transmitter N-acetylaspartylglutamate (NAAG) selectively activates the group II metabotropic glutamate receptors. Several reports also suggest that this peptide acts as a partial agonist at N-methyl-D-aspartate (NMDA) receptors but its putative antagonist effects have not been directly tested. To do this, we used whole cell recordings from cerebellar granule cells (CGC) in culture that allow the highest possible resolution of NMDA channel activation. When CGC were activated with equimolar concentrations of NMDA and NAAG, the peptide failed to alter the peak current elicited by NMDA. Very high concentrations of NAAG (100-200 microM) did not significantly reduce the current elicited by 10 microM NMDA or 0.1 microM glutamate, while 400 microM NAAG produced only a very small (less than 15%) reduction in these whole cell currents. Similarly, NAAG (400 microM) failed to significantly alter the average decay time constant or the peak amplitude of NMDA receptor-mediated miniature excitatory post-synaptic currents (mEPSCs). We conclude that high concentrations of the peptide do not exert physiologically relevant antagonist actions on synaptic NMDA receptor activation following vesicular release of glutamate. As an agonist, purified NAAG was found to be at least 10,000-fold less potent than glutamate in increasing "background" current via NMDA receptors on CGC. Inasmuch as it is difficult to confirm that NAAG preparations are completely free from contamination with glutamate at the 0.01% level, the peptide itself appears unlikely to have a direct agonist activity at the NMDA receptor subtypes found in CGC. Recent reports indicate that enhancing the activity of endogenous NAAG may be an important therapeutic approach to excitotoxicity and chronic pain perception. These effects are likely mediated by group II mGluRs, not NMDA receptors.

N-acetylaspartylglutamate and β-NAAG protect against injury induced by NMDA and hypoxia in primary spinal cord cultures

The acidic dipeptide N-acetylaspartylglutamate (NAAG) is the most prevalent peptide in the central nervous system. NAAG is a low potency agonist at the NMDA receptor, and hydrolysis of NAAG yields the more potent excitatory amino acid neurotransmitter glutamate. beta-NAAG is a competitive inhibitor of the NAAG hydrolyzing enzyme N-acetylated alpha-linked acidic dipeptidase (NAAG peptidase activity) or glutamate carboxypeptidase II, and may also act as a NAAG-mimetic at some of the sites of NAAG pharmacological activity. Since NAAG has been shown to have neuroprotective characteristics in a number of experimental preparations, it is the purpose of the present study to specifically evaluate the possible efficacy of NAAG and beta-NAAG against NMDA- and hypoxia-induced injury to spinal cord mixed neuronal and glial cell cultures. NAAG (500-1000 microM) protected against NMDA- or hypoxia-induced injuries to spinal cord cultures, and the nonhydrolyzable analog beta-NAAG (250-1000 microM) completely eliminated the loss of viability caused by either insult. Both peptides also attenuated NMDA-induced increases in intraneuronal Ca(2+). Nonspecific mGluR antagonists, pertussis toxin, a stable cAMP analog, and manipulation of NAAG peptidase activity did not by themselves alter cell damage and did not influence the neuroprotective effects of NAAG. NAAG was not protective against kainate- or AMPA-induced cellular injury, while beta-NAAG was partially neuroprotective against both insults. At 2 mM, NAAG and beta-NAAG reduced neuronal survival and increased intraneuronal Ca(2+); these effects were only marginally attenuated by dizocilpine and APV. The results indicate that NAAG and beta-NAAG protect against excitotoxic and hypoxic injury to spinal cord neurons, and do so predominantly by interactions with NMDA and not mGluR receptors.

DCG-IV but not other group-II metabotropic receptor agonists induces microglial BDNF mRNA expression in the rat striatum. Correlation with neuronal injury

We have previously described a neuroprotective action of (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl)glycine (DCG-IV), an agonist for group-II metabotropic receptors, on dopaminergic nerve terminals against the degeneration induced by 1-methyl-4-phenylpyridinium (MPP+). This effect was accompanied by an up-regulation of brain-derived neurotrophic factor (BDNF) mRNA expression in the rat striatum. We have now analyzed the phenotypic nature of the BDNF mRNA-expressing cells in response to intrastriatal injection of DCG-IV. Dual in situ hybridization and immunohistochemistry revealed that microglial cells but not astrocytes were responsible for this induction. Subsequent analysis demonstrated that this effect was accompanied by striking loss of striatal glutamic acid decarboxylase (GAD) mRNA and massive appearance of internucleosomal DNA fragmentation, a hallmark of apoptosis. A dose-response study demonstrated that doses of DCG-IV as low as 5 nmol was very toxic in terms GAD mRNA and apoptosis. 0.5 nmol of DCG-IV did not induce toxicity at all in terms of GAD mRNA and apoptosis. Activation of group-II metabotropic receptors in striatum with N-Acetyl-Asp-Glu (NAAG; a mGlu3 agonist) and (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (a mGlu2 and mGlu3 agonist) did not induce neither loss of GAD mRNA nor appearance of apoptosis (doses up to 20 nmol). In additional experiments, NAAG, in contrast to DCG-IV, failed to protect the striatal dopaminergic system against the degeneration induced by MPP+ as studied by microdialysis. Finally, we studied the mechanism by which DCG-IV is highly toxic. For that, selective antagonists of either metabotropic--(R,S)-alpha-methyl-4-carboxyphenylglycine and LY 341495--or ionotropic (N-methyl-D-aspartate, NMDA)--DL-2-amino-5-phosphonovaleric acid (AP-5) glutamate receptors --were co-administered with DCG-IV. Only AP-5 highly protected the striatum against the degeneration induced by DCG-IV. Since DCG-IV also activates the NMDA receptor at concentrations higher than 3 microM, it is conceivable that a intrastriatal concentration equal or higher than 3 microM after a single striatal injection of 5-20 nmol of DCG-IV. Our findings suggest that much caution must be exerted when testing the numerous neuroprotective effects ascribed to group-II metabotropic receptor activation, in particular when using DCG-IV. We conclude that the neuroprotectant capability of a given compound on a specific system does not exclude the possibility of inducing toxicity on a different one.

Neuroprotective effects of N-acetylaspartylglutamate in a neonatal rat model of hypoxia-ischemia

Neuroprotective effects of N-acetylaspartylglutamate (NAAG), the precursor of glutamate and a selective agonist at the Group II metabotropic glutamate (mGlu) receptor, against hypoxic-ischemic brain injury were examined in a neonatal rat model of cerebral hypoxia-ischemia. The neonatal hypoxia-ischemia procedure (unilateral carotid artery ligation followed by exposure to an 8% oxygen hypoxic condition for 1.5 h) was performed in 7-day-old rat pups. Following unilateral carotid artery ligation, NAAG (0.5 to 20 mg/kg, i.p.) was administered before or after the hypoxic exposure. Brain injury was examined 1-week later by weight reduction in the ipsilateral brain and by neuron density in the hippocampal CA1 area. In the saline-treated rat, neonatal hypoxia-ischemia resulted in severe brain injury as indicated by a 24% reduction in the ipsilateral brain weight. Low doses of NAAG (2-10 mg/kg, but not 0.5 mg/kg), administered before or even if 1 h after the hypoxic exposure, greatly reduced hypoxia-ischemia-induced brain injury (3.8-14.2% reduction in the ipsilateral brain weight). A high dose of NAAG (20 mg/kg) was ineffective. While L(+)-2-Amino-4-phosphonobutyric acid (L-AP4) and trans-[1S,3R]-1-Amino-cyclopentane-1, 3-dicarboxylic acid (t-ACPD) were unable to provide protection against hypoxic-ischemic brain injury, 2-(phosphonomethyl) pentanedioic acid (2-PMPA), an inhibitor of N-acetylated alpha-linked acidic dipeptidase (NAALADase), which hydrolyzes endogenous NAAG into N-acetyl-aspartate and glutamate, significantly reduced neonatal hypoxia-ischemia-induced brain injury. (alphaS)-alpha-Amino-alpha-[(1S, 2S)-2-carboxycyclopropyl]-9H-xanthine-9-propanoic acid (LY341495), a selective antagonist at the mGlu2/3 receptor, prevented the neuroprotective effect of NAAG. Neuron density data measured in the hippocampal CA1 area confirmed that ipsilateral brain weight reduction was a valid measure for hypoxic-ischemic brain injury. Neonatal hypoxia-ischemia stimulated an elevation of cyclic AMP (cAMP) concentration in the saline-treated rat brain. NAAG, L-AP4 and t-ACPD all significantly decreased hypoxia-ischemia-induced elevation of cAMP. LY341495 blocked the effect of NAAG, but not of L-AP4 or t-ACPD, on hypoxia-ischemia-stimulated cAMP elevation. The overall results suggest that the neuroprotective effect of NAAG is largely associated with activation of mGlu2/3 receptor.

N-acetylaspartylglutamate (NAAG) is the probable mediator of axon-to-glia signaling in the crayfish medial giant nerve fiber

Glial cell hyperpolarization previously has been reported to be induced by high frequency stimulation or glutamate. We now report that it also is produced by the glutamate-containing dipeptide N-acetylaspartylglutamate (NAAG), by its non-hydrolyzable analog beta-NAAG, and by NAAG in the presence of 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a potent inhibitor of the NAAG degradative enzyme glutamate carboxypeptidase II. The results indicate that NAAG mimics the effect of nerve fiber stimulation on the glia. Although glutamate has a similar effect, the other presumed product of NAAG hydrolysis, N-acetylaspartate, is without effect on glial cell membrane potential, as is aspartylglutamate (in the presence of 2-PMPA). The hyperpolarization induced by stimulation, glutamate, NAAG, beta-NAAG, or NAAG plus 2-PMPA is completely blocked by the Group II metabotropic glutamate receptor antagonist (S)-alpha-ethylglutamate but is not altered by antagonists of Group I or III metabotropic glutamate receptors. The N-methyl-D-aspartate receptor antagonist MK801 reduces but does not eliminate the hyperpolarization generated by glutamate, NAAG or stimulation. These results, in combination with those of the preceding paper, are consistent with the premise that NAAG could be the primary axon-to-glia signaling agent. When the unstimulated nerve fiber is treated with cysteate, a glutamate reuptake blocker, there is a small hyperpolarization of the glial cell that can be substantially reduced by pretreatment with 2-PMPA before addition of cysteate. A similar effect of cysteate is seen during a 50 Hz/5 s stimulation. From these results we suggest that glutamate derived from NAAG hydrolysis appears in the periaxonal space under the conditions of these experiments and may contribute to the glial hyperpolarization.

Neuroprotection afforded by NAAG and NAALADase inhibition requires glial cells and metabotropic glutamate receptor activation

N-acetylated-alpha-linked-acidic-dipeptidase (NAALADase or glutamate carboxypeptidase II) cleaves the neuropeptide N-acetyl-aspartyl-glutamate (NAAG) to glutamate and N-acetyl-aspartate (NAA). Previously, NAAG and 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a potent and selective NAALADase inhibitor, were found to be neuroprotective in neuronal/glial co-cultures and in animals following transient middle cerebral artery occlusion. In this report, we examined the involvement of glial cells and metabotropic glutamate (mGlu) receptors in neuroprotection mediated by NAAG and 2-PMPA in an in vitro model of metabolic inhibition. Neuroprotection of neuronal/glial co-cultures by both NAAG and 2-PMPA, against metabolic inhibition, was significantly higher than neuroprotection in the absence of glia. Similarly, (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV), a selective group II mGlu receptor agonist, was less neuroprotective in the absence of glia. Selective group II mGlu receptor antagonists and (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), a non-selective mGlu receptor antagonist, reduced the protection afforded by both NAAG and 2-PMPA when using neuronal/glial co-cultures. In contrast, groups I and III mGlu receptor antagonists did not affect NAAG or 2-PMPA neuroprotection. These results underscore the critical involvement of glia and group II mGlu receptors in NAAG and 2-PMPA-mediated neuroprotection.

Amino acids are still as exciting as ever

Glutamate is probably the most important excitatory transmitter in the vertebrate central nervous system. Its multiple functional roles in the brain and spinal cord make therapeutic manipulation of these systems fraught with difficulties. There has, however, been recent progress in pharmacological manipulations of NMDA receptor subtypes and non-NMDA receptors, and understanding of the roles of NAAG, that promise rapid advances in pain control.

Regional distribution and pharmacological characteristics of [3H]N-acetyl-aspartyl-glutamate (NAAG) binding sites in rat brain

Autoradiographical studies revealed that 10 nM [3H]N-acetyl-aspartyl-glutamate (NAAG) labelled grey matter structures, particularly in the hippocamus, cerebral neocortex, striatum, septal nuclei and the cerebellar cortex. The binding was inhibited by (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG IV), an agonist at group II metabotropic glutamate receptors (mGluR II). (RS)-alpha-Methyl-4-tetrazolylphenylglycine (MTPG), (RS)-alpha-cyclopropyl-4-phosphonoglycine (CPPG) and (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE), all antagonists at mGluR II and mGluR III, also inhibited [3H]NAAG binding. Other inhibitors were (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD), a broad-spectrum mGluR agonist with preference for groups I and II and the mGluR I agonists/mGluR II antagonists (S)-3-carboxy-4-hydroxyphenylglycine (3,4-CHPG) and (S)-4-carboxy-3-hydroxyphenylglycine (4,3-CHPG). Neither the mGluR I specific agonist (S)-dihydroxyphenylglycine nor any of the ionotropic glutamate receptor ligands such as kainate, AMPA and MK-801 had strong effects (except for the competitive NMDA antagonist CGS 19755, which produced 20-40% inhibition at 100 microM) suggesting that, at low nM concentrations, [3H]NAAG binds predominantly to metabotropic glutamate receptors, particularly those of the mGluR II type. Several studies have indicated that NAAG can interact with mGluR II and the present study supports this notion by demonstrating that sites capable of binding NAAG at low concentrations and displaying pharmacological characteristics of mGluR II exist in the central nervous tissue. Furthermore, the results show that autoradiography of [3H]NAAG binding can be used to quantify the distribution of such sites in distinct brain regions and study their pharmacology at the same time.

N-acetylaspartylglutamate protects against transient focal cerebral ischemia in rats

The inhibition of N-acetylated alpha-linked acidic dipeptidase (NAALADase: glutamate carboxypeptidase II) has been previously shown to protect against ischemic injury presumably through mechanisms of decreasing glutamate and increasing N-acetylaspartylglutamate (NAAG). Preventing excessive glutamate release is known to be neuroprotective. However, the role of increased NAAG is not clear. We used a middle cerebral artery occlusion model in rats to investigate the neuroprotective effect of NAAG via its action as a metabotropic glutamate (mGlu) receptor agonist. Rats received intracerebral injections of NAAG (1, 2, or 4 micromol), or a co-injection of NAAG (2 micromol) and the non-selective mGlu receptor antagonist, (R,S)-alpha-methyl-4-carboxyphenylglycine, (MCPG, 2 micromol). Immediately after the treatment, the animals received 2 h of middle cerebral artery occlusion followed by 22 h of reperfusion. Treatment with 1 or 2 micromol of NAAG significantly reduced total infarct volume. Treatment with MCPG partially attenuated the neuroprotective effect of NAAG, indicating that the protective effect of NAAG against ischemic injury may be in part mediated via activation of mGlu receptors.

Neuroprotection produced by the NAALADase inhibitor 2-PMPA in rat cerebellar neurons

The present study examined the neuroprotective actions of the N-acetylated-alpha-linked-acidic dipeptidase (NAALADase) inhibitor 2-(phosphonomethyl)pentanedioic acid (2-PMPA) in four in vitro models of neurotoxicity. Using neuron-enriched primary cultures derived from rat embryo (E15) cerebellum, 2-PMPA afforded 100% neuroprotection from injuries induced by hypoxia (EC(50)=8.4 microM). In contrast, against glutamate or N-methyl-D-aspartate (NMDA) injury, 2-PMPA was less potent and its efficacy limited to a maximum of 46% and 16%, respectively. 2-PMPA was not effective against veratridine-induced injury. Also, the less potent analog of 2-PMPA, 2-[phosphonomethyl]succinic acid (2-PMSA), was ineffective. Unlike 2-PMPA, the endogenous NAALADase substrate and mGlu(3) receptor agonist N-acetyl-aspartyl-glutamate (NAAG) was neuroprotective against all four injury mechanisms and compared to 2-PMPA, exhibited a different "phosphate effect" on neuroprotection. These results confirm the superior efficacy of 2-PMPA to protect against injury caused by cellular anoxia, and are discussed relative to upstream modulation of hyperglutamatergic activity vs. downstream modulation of metabotropic receptors as possible targets for ischemia/stroke therapy.

N-Acetylaspartylglutamate: the most abundant peptide neurotransmitter in the mammalian central nervous system

In the progress of science, as in life, timing is important. The acidic dipeptide, N-acetylaspartylglutamate (NAAG), was discovered in the mammalian nervous system in 1965, but initially was not considered to be a neurotransmitter candidate. In the mid-1980s, a few laboratories revisited the question of NAAG's role in the nervous system and pursued hypotheses regarding its function that ranged from a precursor for the transmitter pool of glutamate to a direct role as a peptide transmitter. Since that time, NAAG has been tested against nearly all of the established criteria for identification of a neurotransmitter. It successfully meets each of these tests, including a concentrated presence in neurons and synaptic vesicles, release from axon endings in a calcium-dependent manner following initiation of action potentials, and extracellular hydrolysis by membrane-bound peptidase activity. NAAG is the most prevalent and widely distributed neuropeptide in the mammalian nervous system. NAAG activates NMDA receptors with a low potency that may vary among receptor subtypes, and it is a highly selective agonist at the type 3 metabotropic glutamate receptor (mGluR3). Acting through this receptor, NAAG reduces cyclic AMP levels, decreases voltage-dependent calcium conductance, suppresses excitotoxicity, influences long-term potentiation and depression, regulates GABA(A) receptor subunit expression, and inhibits synaptic release of GABA from cortical neurons. Cloning of peptidase activities against NAAG provides opportunities to study the cellular and molecular mechanisms by which synaptic NAAG peptidase activity is controlled. Given the codistribution of this peptide with a spectrum of traditional transmitters and its ability to activate mGluR3, we speculate that one role for NAAG following synaptic release is the activation of metabotropic autoreceptors that inhibit subsequent transmitter release. A second role is the production of extracellular glutamate following NAAG hydrolysis.

Glutamate as a neurotransmitter in the brain: review of physiology and pathology

Glutamate is the principal excitatory neurotransmitter in brain. Our knowledge of the glutamatergic synapse has advanced enormously in the last 10 years, primarily through application of molecular biological techniques to the study of glutamate receptors and transporters. There are three families of ionotropic receptors with intrinsic cation permeable channels [N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate]. There are three groups of metabotropic, G protein-coupled glutamate receptors (mGluR) that modify neuronal and glial excitability through G protein subunits acting on membrane ion channels and second messengers such as diacylglycerol and cAMP. There are also two glial glutamate transporters and three neuronal transporters in the brain. Glutamate is the most abundant amino acid in the diet. There is no evidence for brain damage in humans resulting from dietary glutamate. A kainate analog, domoate, is sometimes ingested accidentally in blue mussels; this potent toxin causes limbic seizures, which can lead to hippocampal and related pathology and amnesia. Endogenous glutamate, by activating NMDA, AMPA or mGluR1 receptors, may contribute to the brain damage occurring acutely after status epilepticus, cerebral ischemia or traumatic brain injury. It may also contribute to chronic neurodegeneration in such disorders as amyotrophic lateral sclerosis and Huntington's chorea. In animal models of cerebral ischemia and traumatic brain injury, NMDA and AMPA receptor antagonists protect against acute brain damage and delayed behavioral deficits. Such compounds are undergoing testing in humans, but therapeutic efficacy has yet to be established. Other clinical conditions that may respond to drugs acting on glutamatergic transmission include epilepsy, amnesia, anxiety, hyperalgesia and psychosis.

The IGF-I amino-terminal tripeptide glycine-proline-glutamate (GPE) is neuroprotective to striatum in the quinolinic acid lesion animal model of Huntington's disease

Huntington's disease is an incurable genetic neurological disorder characterized by the relatively selective degeneration of the striatum. Lesioning of the striatum in rodents using the excitatory amino acid agonist, quinolinic acid (QA), effectively mimics the human neuropathology seen in Huntington's disease. Using this animal model of Huntington's disease, we investigated the ability of the insulin-like growth factor-I (IGF-I) amino-terminal tripeptide glycine-proline-glutamate (GPE) to protect striatal neurons from degeneration. Adult rats received a single unilateral intrastriatal injection of QA (100 nmol) and then daily injection of either vehicle or GPE (0.3 microgram/microliter/day) into the striatum for 7 days. QA at this dose resulted in a partial lesioning of the striatum after 7 days to approximately 50% of cells of unlesioned levels in vehicle-treated animals. The major striatal neuronal phenotype, GABAergic projection neurons, were identified by immunocytochemical labeling of either glutamate decarboxylase 67 (GAD(67)) or the calcium binding protein calbindin in alternate sections. Treatment with GPE for 7 days reversed the loss in projection neurons when assessed by counts of calbindin-stained cells; however, these rescued cells did not regain immunologically detectable levels of GAD(67). GPE also significantly reversed the phenotypic degeneration of cholinergic interneurons identified by immunolabeling for choline acetyltransferase (ChAT) and NADPH diaphorase interneurons identified histochemically. GPE treatment failed to rescue the calcium binding protein interneuron populations of parvalbumin and calretinin neurons. These findings reveal that exogenous administration of GPE selectively prevents excitotoxin induced phenotypic degeneration of striatal projection neurons and cholinergic and NADPH diaphorase interneurons in an animal model of Huntington's disease.