Light and electron microscopic immunohistochemical localization of N-acetylaspartylglutamate (NAAG) in the olivocerebellar pathway of the rat

Parallel Metabolomics and Lipidomics of a PSMA/GCPII Deficient Mouse Model Reveal Alteration of NAAG Levels and Brain Lipid Composition

Glutamate carboxypeptidase II (GCPII, also known as PSMA or FOLH1) is responsible for the cleavage of N-acetyl-aspartyl-glutamate (NAAG) to N-acetyl-aspartate and glutamate in the central nervous system and facilitates the intestinal absorption of folate by processing dietary folyl-poly-γ-glutamate in the small intestine. The physiological function of GCPII in other organs like kidneys is still not known. GCPII inhibitors are neuroprotective in various conditions (e.g., ischemic brain injury) in vivo; however, their utilization as potential drug candidates has not been investigated in regard to not yet known GCPII activities. To explore the GCPII role and possible side effects of GCPII inhibitors, we performed parallel metabolomic and lipidomic analysis of the cerebrospinal fluid (CSF), urine, plasma, and brain tissue of mice with varying degrees of GCPII deficiency (fully deficient in Folh1, -/-; one allele deficient in Folh1, +/-; and wild type, +/+). Multivariate analysis of metabolites showed no significant differences between wild-type and GCPII-deficient mice (except for NAAG), although changes were observed between the sex and age. NAAG levels were statistically significantly increased in the CSF, urine, and plasma of GCPII-deficient mice. However, no difference in NAAG concentrations was found in the whole brain lysate likely because GCPII, as an extracellular enzyme, can affect only extracellular and not intracellular NAAG concentrations. Regarding the lipidome, the most pronounced genotype-linked changes were found in the brain tissue. In brains of GCPII-deficient mice, we observed statistically significant enrichment in phosphatidylcholine-based lipids and reduction of sphingolipids and phosphatidylethanolamine plasmalogens. We hypothesize that the alteration of the NAA-NAAG axis by absent GCPII activity affected myelin composition. In summary, the absence of GCPII and thus similarly its inhibition do not have detrimental effects on metabolism, with just minor changes in the brain lipidome.

Dendritic Localization and Exocytosis of NAAG in the Rat Hippocampus

While a lot is known about classical, anterograde neurotransmission, less is known about the mechanisms and molecules involved in retrograde neurotransmission. Our hypothesis is that N-acetylaspartylglutamate (NAAG), the most abundant dipeptide in the brain, may act as a retrograde transmitter in the brain. NAAG was predominantly localized in dendritic compartments of glutamatergic synapses in the intact hippocampus, where it was present in close proximity to synaptic-like vesicles. In acute hippocampal slices, NAAG was depleted from postsynaptic dendritic elements during neuronal stimulation induced by depolarizing concentrations of potassium or by exposure to glutamate receptor (GluR) agonists. The depletion was completely blocked by botulinum toxin B and strictly dependent on extracellular calcium, indicating exocytotic release. In contrast, there were low levels of NAAG and no effect by depolarization or GluR agonists in presynaptic glutamatergic terminals or GABAergic pre- and postsynaptic elements. Together these data suggest a possible role for NAAG as a retrograde signaling molecule at glutamatergic synapses via exocytotic release.

Evidence of NAAG-family tripeptide NAAG2 in the Drosophila nervous system

N-acetylaspartylglutamate (NAAG) is a common neurotransmitter in the mammalian nervous system; however, it has never been reported in the nervous system of the fruit fly, Drosophila melanogaster. Using antiserum against NAAG, we localized NAAG-like immunoreactivity to neurons in the ventral nerve cord and to type Is glutamatergic nerve terminals in larval neuromuscular junctions. Using liquid chromatography tandem mass spectrometry (LC-MS), we failed to find NAAG but found the related peptide N-acetylaspartylglutamylglutamate (NAAG₂ ) in Drosophila CNS and body wall tissue. This is the first report of any NAAG-family peptide in the nervous system of Drosophila and is also the first report of NAAG₂ being present in a much higher concentration than NAAG in the nervous system of any species. Thus, the larval fruit fly presents an interesting model for the study of the functional role of NAAG₂ of which very little is known-especially in the absence of an abundance of NAAG.

The Good and Bad Sides of NAAG

Why has such a small peptide been the source of controversy in neuroscience over the last 5 decades? Is N-acetyl-aspartyl-glutamate (NAAG) a neurotransmitter? Is NAAG located in neuronal tissue or in astrocytes? Is NAAG involved in neuropsychiatric and neurodegenerative disorders? Is NAAG therapeutically beneficial in the treatment of stroke or in initiating cascades of events leading to psychosis? After many years of intense research there is no clear consensus within the scientific community on how NAAG behaves in the brain. One of the major controversies about NAAG is its physiological action at N-methyl-d-aspartate (NMDA) receptors. While some researchers strongly argue that NAAG acts as a weak agonist at NMDA receptors, others have suggested that NAAG could behave as a potent antagonist. Published data from our laboratory demonstrate that the effect of NAAG on NMDA receptors could be influenced by a number of factors including the subcellular localization and subunit composition of NMDA receptors, as well as protons. In this chapter, we will summarize the knowledge of the literature on NAAG, however, we will place emphasis on our recently published data. More specifically, we have reported interesting findings on the effects of NAAG on NMDA receptors at synaptic and extrasynaptic sites using a pharmacological paradigm to distinguish the two populations of NMDA receptors. Additionally, we have evaluated the role of NAAG on GluN2A- and GluN2B-containing NMDA receptors using a HEK293 cell recombinant system. Finally, we have studied the effects of NAAG on GluN2A- and GluN2B-containing NMDA receptors in different extracellular pH conditions. We believe that our findings could potentially resolve some aspects of the debate regarding the role of NAAG at NMDA receptors.

Nitric oxide and cerebrovascular regulation

The cerebrovascular regulation involves highly complex mechanisms to assure that the brain is perfused at all times. These mechanisms depend on all components of the neurovascular units: neurons, glia, and vascular cells. All these cell types can produce nitric oxide (NO), a powerful vasodilator through different NO synthases. Many studies underlined the key role of NO in the maintenance of resting cerebral blood flow (CBF) as well as in the mechanisms that control cerebrovascular tone: autoregulation and neurovascular coupling. However, although the role of NO in the control of CBF has been largely investigated, the complexity of the NO system and the lack of specific NO synthase inhibitors led to still unresolved questions such as the origin of NO and the pathways by which it controls the vascular tone. In this chapter, the role of NO in the regulation of CBF is critically reviewed and discussed in the context of the neurovascular unit and the general principles of cerebrovascular regulation.

Immunohistological and electrophysiological evidence that N-acetylaspartylglutamate is a co-transmitter at the vertebrate neuromuscular junction

Immunohistochemical studies previously revealed the presence of the peptide transmitter N-acetylaspartylglutamate (NAAG) in spinal motor neurons, axons and presumptive neuromuscular junctions (NMJs). At synapses in the central nervous system, NAAG has been shown to activate the type 3 metabotropic glutamate receptor (mGluR3) and is inactivated by an extracellular peptidase, glutamate carboxypeptidase II. The present study tested the hypothesis that NAAG meets the criteria for classification as a co-transmitter at the vertebrate NMJ. Confocal microscopy confirmed the presence of NAAG immunoreactivity and extended the resolution of the peptide's location in the lizard (Anolis carolinensis) NMJ. NAAG was localised to a presynaptic region immediately adjacent to postsynaptic acetylcholine receptors. NAAG was depleted by potassium-induced depolarisation and by electrical stimulation of motor axons. The NAAG receptor, mGluR3, was localised to the presynaptic terminal consistent with NAAG's demonstrated role as a regulator of synaptic release at central synapses. In contrast, glutamate receptors, type 2 metabotropic glutamate receptor (mGluR2) and N-methyl-d-aspartate, were closely associated with acetylcholine receptors in the postsynaptic membrane. Glutamate carboxypeptidase II, the NAAG-inactivating enzyme, was identified exclusively in perisynaptic glial cells. This localisation was confirmed by the loss of immunoreactivity when these cells were selectively eliminated. Finally, electrophysiological studies showed that exogenous NAAG inhibited evoked neurotransmitter release by activating a group II metabotropic glutamate receptor (mGluR2 or mGluR3). Collectively, these data support the conclusion that NAAG is a co-transmitter at the vertebrate NMJ.

Phenotypic characterization of mice heterozygous for a null mutation of glutamate carboxypeptidase II

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Disturbed glutamate signaling resulting in hypofunction of N-methyl-D-aspartate receptors (NMDAR) has been implicated in the pathophysiology of schizophrenia. Glutamate Carboxypeptidase II (GCP II) hydrolyzes N-acetyl-alpha L-aspartyl-L-glutamate (NAAG) into glutamate and N-acetyl-aspartate. NAAG is a neuropeptide that is an NMDAR antagonist as well as an agonist for the metabotropic glutamate receptor-3 (mGluR3), which inhibits glutamate release. The aggregate effect of NAAG is thus to attenuate NMDAR activation. To manipulate the expression of GCP II, LoxP sites were inserted flanking exons 1 and 2, which were excised by crossing with a Cre-expressing mouse. The mice heterozygous for this deletion showed a 50% reduction in the expression level of protein and functional activity of GCP II in brain samples. Heterozygous mutant crosses did not yield any homozygous null animals at birth or as embryos (N > 200 live births and fetuses). These data are consistent with the previous report that GCP II homozygous mutant mice generated by removing exons 9 and 10 of GCP II gene were embryonically lethal and confirm our hypothesis that GCP II plays an essential role early in embryonic development. Heterozygous mice, however, developed normally to adulthood and exhibited increased locomotor activity, reduced social interaction, and a subtle cognitive deficit in working memory.

Endogenous N-acetylaspartylglutamate reduced NMDA receptor-dependent current neurotransmission in the CA1 area of the hippocampus

N-Acetylaspartylglutamate (NAAG) is a neuropeptide found in high concentrations in the brain. Using whole-cell recordings of CA1 pyramidal neurons in acute hippocampal slices, we found that either (i) the application of exogenous NAAG or (ii) an increase of endogenous extracellular NAAG, caused by the inhibition of its catabolic enzyme glutamate carboxypeptidase II (GCP II), resulted in a significant reduction in the amplitude of the isolated NMDA receptor (NMDAR) component of the evoked excitatory postsynaptic current (EPSC). Conversely, reduction of endogenous extracellular NAAG caused by either (i) perfusion with a soluble form of pure human GCP II or (ii) affinity purified antibodies against NAAG, enhanced the amplitude of the isolated NMDAR current. Bath application of GCP II inhibitor induced a progressive loss of spontaneous NMDAR miniatures. Furthermore, NAAG blocked the induction of long-term potentiation at Schaffer collateral axons-CA1 pyramidal neuron synapses. All together, these results suggest that NAAG acts as an endogenous modulator of NMDARs in the CA1 area of the hippocampus.

Selective vulnerability of hippocampal NAAGergic neurons in experimental temporal lobe epilepsy

The dipeptide N-acetylaspartylglutamate (NAAG) has been recently implicated in numerous neurological disorders. NAAG binds and stimulates group II metabotropic glutamate receptors producing a down-modulation of synaptic glutamate release. In the present immunohistochemical study, we compare the distribution of NAAG-containing (NAAGergic) neurons between the hippocampus of control and chronic epileptic rats obtained with the pilocarpine model of temporal lobe epilepsy. In the hippocampal formation, NAAGergic neurons comprise a subpopulation of GABAergic neurons. Examination by light microscopy revealed a significant reduction of NAAG-immunoreactive neurons in CA3 stratum oriens (35.8%) and CA1 stratum oriens (78.87%), stratum pyramidale (40%), and stratum radiatum (56.6%). Similar loss of NAAGergic neurons was observed in the subiculum characterized by 71.82% and 77.53% reduction in the stratum oriens and radiatum, respectively, when compared with controls. NAAGergic neurons in CA2 and dentate gyrus were apparently resistant to seizure-related cell loss but appeared more complex and exhibited numerous NAAG-positive puncta. Our findings indicate a selective vulnerability of NAAGergic neurons in temporal lobe epilepsy.

Pharmacological characterization of cGMP regulation by the biarylpropylsulfonamide class of positive, allosteric modulators of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

The biarylpropylsulfonamide class of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) potentiators represented by N-2-(4-(4-cyanophenol)phenol)propyl-2-propanesulfonamide (LY404187) and (R)-4'-[1-fluoro-1-methyl-2-(propane-2-sulfonylamino)-ethyl]-biphenyl-4-carboxylic acid methylamide (LY503430) are positive, allosteric AMPA receptor activators, which enhance AMPA receptor-mediated neurotransmission by reducing desensitization of the ion channel. Although these compounds have efficacy in in vivo rodent models of cognition, depression, and Parkinson's disease, little is known about biochemical pathways activated by these agents. Given the well established regulation of the nitric oxide/cGMP pathway by excitatory neurotransmission, the current study characterized AMPA receptor potentiator-mediated cGMP response in mouse cerebellum. Acute treatment by both LY404187 and LY503430 [2.0, 5.0, or 10 mg/kg subcutaneously (s.c.)] elevated basal cerebellar cGMP levels in a dose-dependent manner. Pretreatment with the noncompetitive, allosteric AMPA receptor-selective antagonist 7H-1,3-dioxolo[4,5-h][2,3]benzodiazepine-7-carboxamide, 5-(4-aminophenyl)-8,9-dihydro-N,8-dimethyl-monohydrochloride-(9CI) (GYKI 53655) [3.0 mg/kg intraperitoneally (i.p.)], completely blocked the effect of LY404187, demonstrating that activation of AMPA receptors induces cGMP levels. Interestingly, pretreatment with the N-methyl-d-aspartate (NMDA) open channel blocker dizocilpine (0.3 and 1.0 mg/kg i.p.) also abolished the AMPA receptor potentiator-mediated cGMP accumulation, indicating that activation of AMPA receptors leads to NMDA receptor-mediated transmission involved in cGMP regulation. Pharmacological augmentation of the endogenous glutamate tone via the alkaloid harmaline (20-60 mg/kg i.p.) synergized with AMPA potentiator activity and provided further direct evidence of in vivo allosteric activation of AMPA receptors by LY404187. The synergism between harmaline and LY404187 was specific, since cGMP accumulation induced by foot-shock stress was not augmented by the AMPA receptor potentiator. Taken together, these data indicate that the cGMP system may play an important role in pharmacological efficacy of the biarylpropylsulfonamide class of AMPA receptor potentiators.

A journey across the wall of varicose veins: what physicians do not often see with the naked eye

OBJECTIVE

To examine varicose veins (VVs) from inside out in order to help surgeons and general practitioners better understand the pathogenesis of the disease and improve their management.

MATERIAL AND METHODS

A comprehensive examination of the wall of VVs was performed using transmission electron microscopy. The ultrastructural morphology of the collagen, elastin and smooth muscle content of the wall was analyzed in a sample of 10 patients (4 male and 6 female) and 10 matched controls aged between 37 and 50 years.

RESULTS

Analysis of the tunica media revealed that the smooth muscle cells were significantly separated from each other by a marked increase in amorphous and fibrous tissue in which many of the collagen and elastin fibers lost their normal structural arrangement. The cells contained a large number of membrane-bound intracellular vesicles and cytoplasmic vacuoles. The collagen fibers were smaller and thinner than what is commonly seen in normal veins, and they were widely separated from each other. A light electron-lucent center was observed in the middle of the fibers. Similar changes were also seen in the intima and were associated with irregular plaque-like intimal thickening.

CONCLUSION

Our study revealed a significant separation among smooth muscle cells in the wall of VVs, and the presence of an abnormal amorphous extracellular matrix and intracytoplasmic vacuoles could reflect 'unusual' possible secretory and phagocytic roles of smooth muscle cells. This could provide an important explanation for the abnormal contractile function of these cells in VVs.

Antinociceptive effects of N-acetylaspartylglutamate (NAAG) peptidase inhibitors ZJ-11, ZJ-17 and ZJ-43 in the rat formalin test and in the rat neuropathic pain model

The peptide neurotransmitter N-acetylaspartylglutamate (NAAG) acts as an agonist at group II metabotropic glutamate receptors (mGluRs). NAAG is inactivated by extracellular peptidase activity yielding glutamate and N-acetylaspartate. We recently developed a series of potent NAAG peptidase inhibitors, including ZJ-11, ZJ-17 and ZJ-43. In the present study, we examined the effects of intrathecally administered ZJ-11 and ZJ-17 and intravenously administered ZJ-11 and ZJ-43 in the rat formalin test (an inflammatory pain model) and in the rat partial sciatic nerve ligation model (a neuropathic pain model). Intrathecal injection of ZJ-11 or ZJ-17 or intravenous injection of ZJ-11 or ZJ-43 suppressed both phases of the agitation behaviour induced by paw formalin injection. Intrathecal and intravenous injection of ZJ-11 suppressed the expression of Fos-like immunoreactivity, induced by paw formalin injection, in laminae I-II in segments L4-L5 of the spinal cord, suggesting an action on sensory spinal transmission. Partial sciatic nerve ligation induced significant mechanical allodynia 7 days after the nerve injury. Intrathecal injection of ZJ-11 or ZJ-17 or intravenous administration of ZJ-11 or ZJ-43 attenuated the level of mechanical allodynia induced by this nerve ligation. These effects of intrathecally or intravenously administered ZJ compounds in both the formalin test and the partial sciatic nerve ligation model were completely antagonized by pretreatment with LY-341495, a highly selective group II mGluR antagonist. Thus, elevation of extracellular NAAG, induced by the inhibition of NAAG peptidase, activates group II mGluRs and produces an analgesic effect in neuropathic and inflammatory and pain models. In contrast, peptidase inhibition did not affect the threshold for withdrawal from a noxious mechanical stimulus or from an acute thermal stimulus in the hotplate test.

GCP II (NAALADase) inhibition suppresses mossy fiber-CA3 synaptic neurotransmission by a presynaptic mechanism

We tested the hypothesis that endogenous N-acetylaspartylglutamate (NAAG) presynaptically inhibits glutamate release at mossy fiber-CA3 synapses. For this purpose, we made use of 2-(3-mercaptopropyl)pentanedioic acid (2-MPPA), an inhibitor of glutamate carboxypeptidase II [GCP II; also known as N-acetylated alpha-linked acidic dipeptidase (NAALADase)], the enzyme that hydrolyzes NAAG into N-acetylaspartate and glutamate. Application of 2-MPPA (1-20 microM) had no effect on intrinsic membrane properties of CA3 pyramidal neurons recorded in vitro in whole cell current- or voltage-clamp mode. Bath application of 10 microM 2-MPPA suppressed evoked excitatory postsynaptic current (EPSC) amplitudes. Attenuation of EPSC amplitudes was accompanied by a significant increase in paired-pulse facilitation (50-ms interpulse intervals), suggesting that a presynaptic mechanism is involved. The group II metabotropic glutamate receptor (mGluR) antagonist 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-y l) propanoic acid (LY341495) prevented the 2-MPPA-dependent suppression of EPSC amplitudes. 2-MPPA reduced the frequencies of TTX-insensitive miniature EPSCs (mEPSC), without affecting their amplitudes, further supporting a presynaptic action for GCP II inhibition. 2-MPPA-induced reduction of mEPSC frequencies was prevented by LY341495, reinforcing the role of presynaptic group II mGluR. Because GCP II inhibition is thought to increase NAAG levels, these results suggest that NAAG suppresses synaptic transmission at mossy fiber-CA3 synapses through presynaptic activation of group II mGluRs.

Glutamate carboxypeptidase II gene expression in the human frontal and temporal lobe in schizophrenia

There is decreased activity of glutamate carboxypeptidase II (GCP II) in the dorsolateral prefrontal cortex (DLPFC) and hippocampus of patients with schizophrenia. GCP II hydrolzses N-acetyl-alpha L-aspartyl-L-glutamate (NAAG), a peptide in the mammalian brain that binds to the N-methyl D-aspartate (NMDA) receptor and a group II metabotropic glutamate receptor, both of which have been implicated in the pathophysiology of schizophrenia. We examined the expression of GCP II mRNA in the DLPFC, entorhinal cortex (ERC), and hippocampus in postmortem samples from patients with schizophrenia and normal controls using in situ hybridization followed by silver grain detection. GCP II mRNA was detected in glial cells. Glial-rich regions, specifically the DLPFC and ERC white matter and the molecular and polymorphic layers in the hippocampus, express high levels of GCP II mRNA. Given the earlier finding of decreased GCP II activity in brains of subjects with schizophrenia, we expected to find lower GCP II mRNA levels in schizophrenia. Contrary to this expectation, we found a significantly higher expression of GCP II mRNA in one of the brain areas examined, the hippocampal CA3 polymorphic region. This may reflect a compensatory increase to correct for the decreased activity of GCP II activity. Our findings support the notion that the hydrolysis of NAAG is disrupted in schizophrenia and that specific anatomical regions may show discrete abnormalities in GCP II synthesis.

Deletion of the glutamate carboxypeptidase II gene in mice reveals a second enzyme activity that hydrolyzes N-acetylaspartylglutamate

Glutamate carboxypeptidase II (GCPII, EC 3.14.17.21) is a membrane-bound enzyme found on the extracellular face ofglia. The gene for this enzyme is designated FOLH1 in humans and Folh1 in mice. This enzyme has been proposed to be responsible for inactivation of the neurotransmitter N-acetylaspartylglutamate (NAAG) following synaptic release. Mice harboring a disruption of the gene for GCPII/Folh1 were generated by inserting into the genome a targeting cassette in which the intron-exon boundary sequences of exons 1 and 2 were removed and stop codons were inserted in exons 1 and 2. Messenger RNA for GCPII was not detected by northern blotting or RT-PCR analysis of RNA from the brains of -/- mutant mice nor was GCPII protein detected on western blots of this tissue. These GCPII null mutant mice developed normally to adulthood and exhibited a normal range of neurologic responses and behaviors including mating, open field activity and retention of position in rotorod tests. No significant differences were observed among responses of wild type, heterozygous mutant and homozygous mutant mice on tail flick and hot plate latency tests. Glutamate, NAAG and mRNA for metabotropic glutamate receptor type 3 levels were not significantly altered in response to the deletion of glutamate carboxypeptidase II. A novel membrane-bound NAAG peptidase activity was discovered in brain, spinal cord and kidney of the GCPII knock out mice. The kinetic values for brain NAAG peptidase activity in the wild type and GCPII nullmutant were Vmax = 45 and 3 pmol/mg/min and Km = 2650 nm and 2494 nm, respectively. With the exception of magnesium and copper, this novel peptidase activity had a similar requirement for metal ions as GCPII. Two potent inhibitors of GCPII, 4,4'-phosphinicobis-(butane-1,3 dicarboxilic acid) (FN6) and 2-(phosphonomethyl)pentanedioic acid (2-PMPA) inhibited the residual activity. The IC50 value for 2-PMPA was about 1 nm for wild-type brain membrane NAAG peptidase activity consistent with its activity against cloned ratand human GCPII, and 88 nm for the activity in brain membranes of the null mutants.

Synthesis and release of N-acetylaspartylglutamate (NAAG) by crayfish nerve fibers: implications for axon-glia signaling

Early physiological and pharmacological studies of crayfish and squid giant nerve fibers suggested that glutamate released from the axon during action potential generation initiates metabolic and electrical responses of periaxonal glia. However, more recent investigations in our laboratories suggest that N-acetylaspartylglutamate (NAAG) may be the released agent active at the glial cell membrane. The investigation described in this paper focused on NAAG metabolism and release, and its contribution to the appearance of glutamate extracellularly. Axoplasm and periaxonal glial cell cytoplasm collected from medial giant nerve fibers (MGNFs) incubated with radiolabeled L-glutamate contained radiolabeled glutamate, glutamine, NAAG, aspartate, and GABA. Total radiolabel release was not altered by electrical stimulation of nerve cord loaded with [(14)C]glutamate by bath application or loaded with [(14)C]glutamate, [(3)H]-D-aspartate or [(3)H]NAAG by axonal injection. However, when radiolabeled glutamate was used for bath loading, radiolabel distribution among glutamate and its metabolic products in the superfusate was changed by stimulation. NAAG was the largest fraction, accounting for approximately 50% of the total recovered radiolabel in control conditions. The stimulated increase in radioactive NAAG in the superfusate coincided with its virtual clearance from the medial giant axon (MGA). A small, stimulation-induced increase in radiolabeled glutamate in the superfusate was detected only when a glutamate uptake inhibitor was present. The increase in [(3)H]glutamate in the superfusion solution of nerve incubated with [(3)H]NAAG was reduced when beta-NAAG, a competitive glutamate carboxypeptidase II (GCP II) inhibitor, was present.Overall, these results suggest that glutamate is metabolized to NAAG in the giant axon and its periaxonal glia and that, upon stimulation, NAAG is released from the axon and converted in part to glutamate by GCP II. A quisqualate- and beta-NAAG-sensitive GCP II activity was detected in nerve cord homogenates. These results, together with those in the accompanying paper demonstrating that NAAG can activate a glial electrophysiological response comparable to that initiated by glutamate, implicate NAAG as a probable mediator of interactions between the MGA and its periaxonal glia.

beta-NAAG rescues LTP from blockade by NAAG in rat dentate gyrus via the type 3 metabotropic glutamate receptor

N-Acetylaspartylglutamate (NAAG) is an agonist at the type 3 metabotropic glutamate receptor (mGluR3), which is coupled to a Gi/o protein. When activated, the mGluR3 receptor inhibits adenylyl cyclase and reduces the cAMP-mediated second-messenger cascade. Long-term potentiation (LTP) in the medial perforant path (MPP) of the hippocampal dentate gyrus requires increases in cAMP. The presence of mGluR3 receptors and NAAG in neurons of the dentate gyrus suggests that this peptide transmitter may inhibit LTP in the dentate gyrus. High-frequency stimulation (100 Hz; 2 s) of the MPP resulted in LTP of extracellularly recorded excitatory postsynaptic potentials at the MPP-granule cell synapse of rat hippocampal slices. Perfusion of the slice with NAAG (50 and 200 microM) blocked LTP. Neither 50 nor 200 microM NAAG produced N-methyl-D-aspartate receptor currents in the granule cells of the acute hippocampal slice. The group II mGluR antagonist ethyl glutamate (100 microM) and a structural analogue of NAAG, beta-NAAG (100 microM), prevented the blockade of LTP by NAAG. Paired-pulse depression of the excitatory postsynaptic potential at 20- and 80-ms interpulse intervals (IPI) was not affected by NAAG or beta-NAAG. beta-NAAG did not affect inositol trisphosphate production stimulated by the agonist glutamate in cells expressing the group I mGluR1alpha or mGluR5. beta-NAAG blocked the decrease in forskolin-stimulated cAMP by the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) but not the group III mGluR agonist L(+)-2-amino-4-phosphonobutyric acid in cerebellar granule cells. In cells transfected with mGluR3, but not mGluR2, beta-NAAG blocked forskolin-stimulated cAMP responses to glutamate, NAAG, the nonspecific group I, II agonist trans-ACPD, and the group II agonist DCG-IV. We conclude that beta-NAAG is a selective mGluR antagonist capable of differentiating between mGluR2 and mGluR3 subtypes and that the mGluR3 receptor functions to regulate activity-dependent synaptic potentiation in the hippocampus.

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: 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.

Quantitative immunoelectron microscopic colocalization of GABA and enkephalin in the ventrocaudal periaqueductal gray of the rat

In the present ultrastructural study in the ventrocaudal periaqueductal gray (PAG) of the rat, the relationship and the association between GABAergic and enkephalinergic neuronal elements were investigated using postembedding colocalization immunogold electron microscopic technique in order to establish the precise relationship between these two important neurotransmitters in this part of the brain stem. The GABA-like neuronal elements were immunoreacted with 20 nm gold particles and the enkephalin (ENK)-like immunoreactive neurons were labeled with 10 nm gold particles. Double labeling of sections with ENK and GABA produced colocalization in 23.3% and 1.2% of axon terminals and dendrites, respectively. Most of the double-labeled terminals contained more GABA-like than ENK-like immunolabeling. Approximately 19.4% of the labeled axon terminals and 8.5% of the labeled dendrites contained only GABA-like immunoreactivity, while 24% of the immunolabeled dendrites were immunoreactive with only ENK-like immunoreactivity. The synapses between the two kinds of immunolabeled neuronal profiles appear to be both asymmetrical and symmetrical. GABA-like immunolabeled terminals contained small, clear, pleomorphic or round vesicles and were found to make synapses with ENK-like immunolabeled and nonimmunolabeled dendrites, whereas most of the ENK-like immunolabeled axon terminals contained dense-cored vesicles. Approximately half of the axon terminals (51%) and dendrites (56%) in the ventrolateral PAG were not labeled for either GABA or for ENK immunoreactivity. The results are discussed in terms of GABAergic inhibition of antinociceptive mechanisms in the ventrolateral PAG and of the activation of these mechanisms by ENK neurotransmitter.

N-acetylaspartylglutamate activates cyclic AMP-coupled metabotropic glutamate receptors in cerebellar astrocytes

N-Acetylaspartylglutamate (NAAG) is the most prevalent peptide in the mammalian nervous system. NAAG meets the traditional criteria of a neurotransmitter, including localization in synaptic vesicles, depolarization-induced release, low potency activation of some N-methyl-D-aspartate receptors, and highly selective activation of a cAMP-coupled metabotropic glutamate receptor (mGluR) with potency approaching that of glutamate. The peptide is present in cultured cortical glia in high concentration and is hydrolyzed by cell surface peptidase activity. In the present work, we tested the hypothesis that NAAG selectively activates a subclass of metabotropic receptors on cultured rat cerebellar glia, primarily astrocytes. These glial cells express mRNA for mGluR subtypes 1, 3, 4, 5, 6, 7, and 8. We were not able to detect message for mGluR2 in these cells using the reverse transcriptase-polymerase chain reaction. Cerebellar glia responded to NAAG, glutamate, and trans-ACPD with a decrease in forskolin-stimulated cAMP formation. AP4, an agonist of the group III receptors mGluR4, mGluR6, mGluR7, and mGluR8, had little or no effect on stimulated cAMP levels. Treatment with low micromolar NAAG significantly increased uptake of radioactive thymidine by cultured astrocytes through activation of metabotropic glutamate receptors. Antagonists of group II mGluRs prevented the decrease in cAMP and the increase in uptake of thymidine by NAAG. Cultured cerebellar astrocytes expressed 20 pmol NAAG per mg protein, a value that is at least 30-fold lower than that expressed by mixed glial cultures prepared from mouse cortex. We conclude that cerebellar astrocytes respond to NAAG via the mGluR3 receptor and that the peptide may selectively activate this receptor subtype in astrocytes following release from neurons or glia.

Activation of cerebellar climbing fibers increases cerebellar blood flow: role of glutamate receptors, nitric oxide, and cGMP

BACKGROUND

The mechanisms regulating the cerebellar microcirculation during neural activity are poorly understood. One of the major neural inputs to the cerebellar cortex is the climbing fiber (CF), a pathway that uses excitatory amino acids, including glutamate, as a transmitter. We studied whether CF activation increases cerebellar blood flow (BFcrb) and, if so, we investigated the role of glutamate receptors, nitric oxide (NO) and cGMP, in the response.

METHODS

The CF were activated by harmaline administration (40 mg/kg, i.p.) in halothane-anesthetized rats with a cranial window placed over the cerebellar vermis. BFcrb was monitored by a laser-Doppler probe, and arterial pressure and blood gases were controlled.

RESULTS

With Ringer superfusion, harmaline produced sustained increases in BFcrb that peaked 20 minutes after administration (+115 +/- 13%; n=6; P<.05). The increases in BFcrb were substantially reduced by superfusion with tetrodotoxin (10 micromol/L; -91 +/- 5%; n=5; P<.05 from Ringer). The response was also attenuated by the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor inhibitor 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxaline (100 micromol/L; -70 +/- 6%; P<.05; n=5), but not by the N-methyl-D-aspartate receptor blocker 2-amino-5-phosphonopentanoic acid (500 micromol/L; P>.05; n=5). The response was attenuated by the nonselective NO synthase (NOS) inhibitor nitro-L-arginine (1 mmol/L; -73 +/- 5%; n=6) or by 7-NI (50 mg, i.p.; -71 +/- 5%; n=5), a relatively selective neuronal NOS inhibitor. The soluble guanylyl cyclase inhibitor 1H-1,2,4oxadiazolo[4,3-a]quinoxalin-1-one (100 micromol/L) attenuated the response to harmaline (-73 +/- 5; P<.05; n=6) but not to superfusion with adenosine (P>.05; n=5) or 8-bromo-cGMP (P>.05; n=5).

CONCLUSIONS

Activation of the CF system increases BFcrb. The response depends on activation of glutamate receptors and is in large part mediated by NO via stimulation of soluble guanylyl cyclase. Glutamate receptors NO and cGMP are important factors in the mechanisms of functional hyperemia in cerebellar cortex.