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

Dendrimer-2PMPA selectively blocks upregulated microglial GCPII activity and improves cognition in a mouse model of multiple sclerosis

Cognitive impairment is a common aspect of multiple sclerosis (MS) for which there are no treatments. Reduced brain N-acetylaspartylglutamate (NAAG) levels are linked to impaired cognition in various neurological diseases, including MS. NAAG levels are regulated by glutamate carboxypeptidase II (GCPII), which hydrolyzes the neuropeptide to N-acetyl-aspartate and glutamate. GCPII activity is upregulated multifold in microglia following neuroinflammation. Although several GCPII inhibitors, such as 2-PMPA, elevate brain NAAG levels and restore cognitive function in preclinical studies when given at high systemic doses or via direct brain injection, none are clinically available due to poor bioavailability and limited brain penetration. Hydroxyl-dendrimers have been successfully used to selectively deliver drugs to activated glia.

Methods: We attached 2-PMPA to hydroxyl polyamidoamine (PAMAM) dendrimers (D-2PMPA) using a click chemistry approach. Cy5-labelled-D-2PMPA was used to visualize selective glial uptake in vitro and in vivo. D-2PMPA was evaluated for anti-inflammatory effects in LPS-treated glial cultures. In experimental autoimmune encephalomyelitis (EAE)-immunized mice, D-2PMPA was dosed biweekly starting at disease onset and cognition was assessed using the Barnes maze, and GCPII activity was measured in CD11b+ hippocampal cells.

Results: D-2PMPA showed preferential uptake into microglia and robust anti-inflammatory activity, including elevations in NAAG, TGFβ, and mGluR3 in glial cultures. D-2PMPA significantly improved cognition in EAE mice, even though physical severity was unaffected. GCPII activity increased >20-fold in CD11b+ cells from EAE mice, which was significantly mitigated by D-2PMPA treatment.

Conclusions: Hydroxyl dendrimers facilitate targeted drug delivery to activated microglia. These data support further development of D-2PMPA to attenuate elevated microglial GCPII activity and treat cognitive impairment in MS.

Long reach of the NAAG family tree: An Editorial for "Evidence of NAAG-family tripeptide NAAG2 in the Drosophila nervous system" on page 38

The last common ancestor of humans and fruit flies lived about 800 million years ago, yet both of us have nervous systems that share a number of common important features, for example the use of glutamate as a neurotransmitter. We can now possibly add another common feature to the neural tissue of humans and fruit flies which is that of N-acetylaspartylglutamate (NAAG) peptides. This Editorial highlights an article by Kozik and coworkers in the current issue of the Journal of Neurochemistry, in which the authors report the discovery, in Drosophila melanogaster nervous system, of NAA-glutamylglutamate (NAAG2).

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.

Effects of pulsed magnetic stimulation on isolated crayfish heart

Cardiac muscular contraction of the neurogenic heart that could be excited by pulsed magnetic stimulation (PMS) was investigated using preparation of the isolated crayfish heart. When a figure-eight magnetic coil was set over the isolated heart, cardiac contraction induced by a single PMS was not observed. Cardiac arrest occurred immediately after repetitive PMS and persisted for dozens of seconds depending on the number of stimuli. We concluded that PMS caused neuronal modulation in the neuronal network in the cardiac ganglion.

Chemotherapy-induced peripheral neuropathy: What do we know about mechanisms?

Cisplatin, oxaliplatin, paclitaxel, vincristine and bortezomib are some of the most effective drugs successfully employed (alone or in combinations) as first-line treatment for common cancers. However they often caused severe peripheral neurotoxicity and neuropathic pain. Structural deficits in Dorsal Root Ganglia and sensory nerves caused symptoms as sensory loss, paresthesia, dysaesthesia and numbness that result in patient' suffering and also limit the life-saving therapy. Several scientists have explored the various mechanisms involved in the onset of chemotherapy-related peripheral neurotoxicity identifying molecular targets useful for the development of selected neuroprotective strategies. Dorsal Root Ganglia sensory neurons, satellite cells, Schwann cells, as well as neuronal and glial cells in the spinal cord, are the preferential sites in which chemotherapy neurotoxicity occurs. DNA damage, alterations in cellular system repairs, mitochondria changes, increased intracellular reactive oxygen species, alterations in ion channels, glutamate signalling, MAP-kinases and nociceptors ectopic activation are among the events that trigger the onset of peripheral neurotoxicity and neuropathic pain. In the present work we review the role of the main players in determining the pathogenesis of anticancer drugs-induced peripheral neuropathy.

Molecular characterization of N-acetylaspartylglutamate synthetase

The dipeptide N-acetylaspartyl-glutamate (NAAG) is an abundant neuropeptide in the mammalian brain. Despite this fact, its physiological role is poorly understood. NAAG is synthesized by a NAAG synthetase catalyzing the ATP-dependent condensation of N-acetylaspartate and glutamate. In vitro NAAG synthetase activity has not been described, and the enzyme has not been purified. Using a bioinformatics approach we identified a putative dipeptide synthetase specifically expressed in the nervous system. Expression of the gene, which we named NAAGS (for NAAG synthetase) was sufficient to induce NAAG synthesis in primary astrocytes or CHO-K1 and HEK-293T cells when they coexpressed the NAA transporter NaDC3. Furthermore, coexpression of NAAGS and the recently identified N-acetylaspartate (NAA) synthase, Nat8l, in CHO-K1 or HEK-293T cells was sufficient to enable these cells to synthesize NAAG. Identity of the reaction product of NAAGS was confirmed by HPLC and electrospray ionization tandem mass spectrometry (ESI-MS). High expression levels of NAAGS were restricted to the brain, spinal cord, and testis. Taken together our results strongly suggest that the identified gene encodes a NAAG synthetase. Its identification will enable further studies to examine the role of this abundant neuropeptide in the vertebrate nervous system.

Neuron and gliocyte death induced by photodynamic treatment: signal processes and neuron-glial interactions

The mechanisms of photodynamic (PD) damage to neurons and gliocytes are discussed. The spike reactions of neurons are described, with stimulation at high concentrations of photosensitizer and inhibition at low concentrations, accompanying necrosis. Glial cells developed both necrosis and apoptosis. Local laser inactivation of neurons increased light-induced apoptosis of gliocytes, i.e., neurons maintained gliocyte survival. Inter-and intracellular signaling plays an important role in the photolesioning of these cells. Studies using inhibitors and activators of signal proteins demonstrated the involvement of the Ca(2+)-dependent, adenylate cyclase, and tyrosine kinase pathways in the responses of neurons and gliocytes to PD treatment. Pharmacological modulation may alter the selectivity of PD neuron and gliocyte damage and the efficacy of PD treatment.

N-Acetylaspartate in the CNS: from neurodiagnostics to neurobiology

The brain is unique among organs in many respects, including its mechanisms of lipid synthesis and energy production. The nervous system-specific metabolite N-acetylaspartate (NAA), which is synthesized from aspartate and acetyl-coenzyme A in neurons, appears to be a key link in these distinct biochemical features of CNS metabolism. During early postnatal central nervous system (CNS) development, the expression of lipogenic enzymes in oligodendrocytes, including the NAA-degrading enzyme aspartoacylase (ASPA), is increased along with increased NAA production in neurons. NAA is transported from neurons to the cytoplasm of oligodendrocytes, where ASPA cleaves the acetate moiety for use in fatty acid and steroid synthesis. The fatty acids and steroids produced then go on to be used as building blocks for myelin lipid synthesis. Mutations in the gene for ASPA result in the fatal leukodystrophy Canavan disease, for which there is currently no effective treatment. Once postnatal myelination is completed, NAA may continue to be involved in myelin lipid turnover in adults, but it also appears to adopt other roles, including a bioenergetic role in neuronal mitochondria. NAA and ATP metabolism appear to be linked indirectly, whereby acetylation of aspartate may facilitate its removal from neuronal mitochondria, thus favoring conversion of glutamate to alpha ketoglutarate which can enter the tricarboxylic acid cycle for energy production. In its role as a mechanism for enhancing mitochondrial energy production from glutamate, NAA is in a key position to act as a magnetic resonance spectroscopy marker for neuronal health, viability and number. Evidence suggests that NAA is a direct precursor for the enzymatic synthesis of the neuron specific dipeptide N-acetylaspartylglutamate, the most concentrated neuropeptide in the human brain. Other proposed roles for NAA include neuronal osmoregulation and axon-glial signaling. We propose that NAA may also be involved in brain nitrogen balance. Further research will be required to more fully understand the biochemical functions served by NAA in CNS development and activity, and additional functions are likely to be discovered.

Regulation of N-acetylaspartate and N-acetylaspartylglutamate biosynthesis by protein kinase activators

The neuronal dipeptide N-acetylaspartylglutamate (NAAG) is thought to be synthesized enzymatically from N-acetylaspartate (NAA) and glutamate. We used radiolabeled precursors to examine NAA and NAAG biosynthesis in SH-SY5Y human neuroblastoma cells stimulated with activators of protein kinase A (dbcAMP; N6,2'-O-dibutyryl cAMP) and protein kinase C (PMA; phorbol-12-myristate-13-acetate). Differentiation over the course of several days with dbcAMP resulted in increased endogenous NAA levels and NAAG synthesis from l-[(3)H]glutamine, whereas PMA-induced differentiation reduced both. Exogenously applied NAA caused dose dependent increases in intracellular NAA levels, and NAAG biosynthesis from l-[(3)H]glutamine, suggesting precursor-product and mass-action relationships between NAA and NAAG. Incorporation of l-[(3)H]aspartate into NAA and NAAG occurred sequentially, appearing in NAA by 1 h, but not in NAAG until between 6 and 24 h. Synthesis of NAAG from l-[(3)H]aspartate was increased by dbcAMP and decreased by PMA at 24 h. The effects of PMA on l-[(3)H]aspartate incorporation into NAA were temporally biphasic. Using short incubation times (1 and 6 h), PMA increased l-[(3)H]aspartate incorporation into NAA, but with longer incubation (24 h), incorporation was significantly reduced. These results suggest that, while the neuronal production of NAA and NAAG are biochemically related, significant differences exist in the regulatory mechanisms controlling their biosynthesis.

PDT-induced death of sensory neurons and glial cells in the isolated crayfish stretch receptor after proteolytic treatment

To study the involvement of neuroglial interactions in photodynamic damage of crayfish stretch receptor, which consists of only two neurons surrounded by satellite glial cells (SGCs), we attempted to proteolytically uncouple neurons and glia and then compare the responses of these cells to photosensitization when intercellular communications were intact or impaired. After incubation of isolated stretch receptors with pronase or collagenase they were photosensitized with Photosens, a mixture of sulfonated alumophthalocyanines AlPcS(n) (n = 2, 3, and 4; mean n = 3.1). In the next 6 hr the preparations were double fluorochromed with propidium iodide and Hoechst-33342 to visualize necrotic and apoptotic cells. Proteolytic treatment shortened bioelectric neuron response and precipitated its functional inactivation; however, it did not significantly impair neuron morphology and did not induce its necrosis either in the darkness or under photosensitization. Photodynamic treatment induced necrosis of neurons and SGC and apoptosis of glial cells. Pronase but not collagenase increased percent of necrotic and apoptotic SGCs in the darkness and thus reduced the number of glial cells around the neuron; however, both pronase and collagenase prevented photodynamically induced apoptosis of glial cells. The involvement of neuron-to-glia signaling interactions in this phenomenon is suggested.

Regulation of glutamate carboxypeptidase II hydrolysis of N ‐acetylaspartylglutamate (NAAG) in crayfish nervous tissue is mediated by glial glutamate and acetylcholine receptors

Glutamate carboxypeptidase II (GCPII), a glial ectoenzyme, is responsible for N-acetylaspartylglutamate (NAAG) hydrolysis. Its regulation in crayfish nervous tissue was investigated by examining uptake of [3H]glutamate derived from N-acetylaspartyl-[3H]glutamate ([3H]NAAG) to measure GCPII activity. Electrical stimulation (100 Hz, 10 min) during 30 min incubation with [3H]NAAG increased tissue [3H]glutamate tenfold. This was prevented by 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a GCPII inhibitor, suggesting that stimulation increased the hydrolysis of [3H]NAAG and metabolic recycling of [3H]glutamate. Antagonists of glial group II metabotropic glutamate receptors (mGLURII), NMDA receptors and acetylcholine (ACh) receptors that mediate axon-glia signaling in crayfish nerve fibers decreased the effect of stimulation by 58-83%, suggesting that glial receptor activation leads to stimulation of GCPII activity. In combination, they reduced [3H]NAAG hydrolysis during stimulation to unstimulated control levels. Agonist stimulation of mGLURII mimicked the effect of electrical stimulation, and was prevented by antagonists of GCPII or mGLURII. Raising extracellular K+ to three times the normal level stimulated [3H]NAAG release and GCPII activity. These effects were also blocked by antagonists of GCPII and mGLUR(II). No receptor antagonist or agonist tested or 2-PMPA affected uptake of [3H]glutamate. We conclude that NAAG released from stimulated nerve fibers activates its own hydrolysis via stimulation of GCPII activity mediated through glial mGLURII, NMDA and ACh receptors.

SH-SY5Y neuroblastoma cells: a model system for studying biosynthesis of NAAG

N-Acetylaspartylglutamate (NAAG) is a neuropeptide that is thought to modulate neurotransmitter release through pre-synaptic mGluR3 receptors. Despite years of research into NAAG biochemistry, almost nothing is known about NAAG biosynthesis. To date, NAAG biosynthesis has only been demonstrated conclusively in explanted animal neural tissues, including frog retina, rat dorsal root ganglia and crayfish nerve cord, but not in human cells or tissues. We show here that a human neuroblastoma cell line, SH-SY5Y, provides a good model system for the study of NAAG biosynthesis. Radiolabled NAAG synthesis occurred using both L-[3H]glutamic acid and L-[3H]glutamine as precursors, with glutamine being the preferred substrate. Differentiation of SH-SY5Y cells with retinoic acid resulted in decreased radiolabel incorporation into NAAG, whereas differentiation with nerve growth factor did not affect radiolabel incorporation.

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.

Properties of glutaminase of crayfish CNS: implications for axon-glia signaling

Glutaminase of crayfish axons is believed to participate in recycling of axon-glia signaling agent(s). We measured the activity and properties of glutaminase in crude homogenates of crayfish CNS, using ion exchange chromatography to separate radiolabeled product from substrate. Crayfish glutaminase activity is cytoplasmic and/or weakly bound to membranes and dependent on time, tissue protein, and glutamine concentration. It resembles the kidney-type phosphate-activated glutaminase of mammals in being stimulated by inorganic phosphate and alkaline pH and inhibited by the product glutamate and by the glutamine analog 6-diazo-5-oxo-L-norleucine. During incubation of crayfish CNS fibers in Na(+)-free saline containing radiolabeled glutamine, there is an increased formation of radiolabeled glutamate in axoplasm that is temporally associated with an increase in axonal pH from about 7.1 to about 8.0. Both the formation of glutamate and the change in pH are reduced by 6-diazo-5-oxo-L-norleucine. Our results suggest that crayfish glutaminase activity is regulated by cellular changes in pH and glutamate concentration. Such changes could impact availability of the axon-glia signaling agents glutamate and N-acetylaspartylglutamate.

New insights into neuron-glia communication

Two-way communication between neurons and nonneural cells called glia is essential for axonal conduction, synaptic transmission, and information processing and thus is required for normal functioning of the nervous system during development and throughout adult life. The signals between neurons and glia include ion fluxes, neurotransmitters, cell adhesion molecules, and specialized signaling molecules released from synaptic and nonsynaptic regions of the neuron. In contrast to the serial flow of information along chains of neurons, glia communicate with other glial cells through intracellular waves of calcium and via intercellular diffusion of chemical messengers. By releasing neurotransmitters and other extracellular signaling molecules, glia can affect neuronal excitability and synaptic transmission and perhaps coordinate activity across networks of neurons.

Glutamine uptake and metabolism to N-acetylaspartylglutamate (NAAG) by crayfish axons and glia

We have proposed that N-acetylaspartylglutamate (NAAG) or its hydrolytic product glutamate, is a chemical signaling agent between axons and periaxonal glia at non-synaptic sites in crayfish nerves, and that glutamine is a probable precursor for replenishing the releasable pool of NAAG. We report here, that crayfish central nerve fibers synthesize NAAG from exogenous glutamine. Cellular accumulation of radiolabel during in vitro incubation of desheathed cephalothoracic nerve bundles with [3H]glutamine was 74% Na(+)-independent. The Na(+)-independent transport was temperature-sensitive, linear with time for at least 4 h, saturable between 2.5 and 10 mM L-glutamine, and blocked by neutral amino acids and analogs that inhibit mammalian glutamine transport. Radiolabeled glutamine was taken up and metabolized by both axons and glia to glutamate and NAAG, and a significant fraction of these products effluxed from the cells. Both the metabolism and release of radiolabeled glutamine was influenced by extracellular Na(+). The uptake and conversion of glutamine to glutamate and NAAG by axons provides a possible mechanism for recycling and formation of the axon-to-glia signaling agent(s).

Mechanism of NMDA receptor contribution to axon-to-glia signaling in the crayfish medial giant nerve fiber

Electrical stimulation of crayfish giant axons at high frequency activates group II metabotropic and NMDA glutamate receptors on adjacent glial cells via release of N-acetylaspartylglutamate and glutamate formed upon its hydrolysis. This produces a transient depolarization followed by a prolonged hyperpolarization of glial cells that involves nicotinic acetylcholine receptor activation. The hyperpolarization is nearly completely blocked by antagonists of metabotropic glutamate receptors but only slightly reduced by inhibition of NMDA receptors. We report that the NMDA-induced hyperpolarization of glial cells is reduced by decreased calcium in the solution bathing the giant nerve fiber, while removal of sodium ions or block of voltage-dependent calcium channels completely prevents the glial response to NMDA. Inhibition of nicotinic acetylcholine receptors or removal of extracellular Cl(-) converts the glial response from a hyperpolarization to a depolarization that is sensitive to NMDA receptor antagonist. We propose that NMDA receptor activation by glutamate, formed from extracellular N-acetylaspartylglutamate during nerve stimulation, contributes to glial hyperpolarization by increasing intracellular Ca(2+) via opening of voltage-sensitive Ca(2+) channels. Based on our previous work, we propose further that the added Ca(2+) supplements that produced by N-acetylaspartylglutamate and glutamate acting on group II metabotropic glutamate receptors to cause an increased release of acetylcholine and a larger hyperpolarization.

Mechanisms for clearance of released N-acetylaspartylglutamate in crayfish nerve fibers: implications for axon-glia signaling

Crayfish nerve fibers incubated with radiolabeled glutamate or glutamine accumulate these substrates and synthesize radioactive N-acetylaspartylglutamate (NAAG). Upon stimulation of the medial giant nerve fiber, NAAG is the primary radioactive metabolite released. Since NAAG activates a glial hyperpolarization comparable to that initiated by glutamate or axonal stimulation through the same receptor, we have proposed that it is the likely mediator of interactions between the medial giant axon and its periaxonal glia. This manuscript reports investigations of possible mechanisms for termination of NAAG-signaling activity. N-acetylaspartyl-[(3)H]glutamate was not accumulated from the bath saline by unstimulated crayfish giant axons or their associated glia during a 30-min incubation. Stimulation of the central nerve cord at 50 Hz during the last minute of the incubation dramatically increased the levels of radiolabeled glutamate, NAAG, and glutamine in the medial giant axon and its associated glia. These results indicate that stimulation-sensitive peptide hydrolysis and metabolic recycling of the radiolabeled glutamate occurred. There was a beta-NAAG-, quisqualate- and 2-(phosphonomethyl)-pentanedioic acid-inhibitable glutamate carboxypeptidase II activity in the membrane fraction of central nerve fibers, but not in axonal or glial cytoplasmic fractions. Inactivation of this enzyme by 2-(phosphonomethyl)-pentanedioic acid or inhibition of N-methyl-D-aspartate (NMDA) receptors by MK801 reduced the glial hyperpolarization activated by high-frequency stimulation. These results indicate that axon-to-glia signaling is terminated by NAAG hydrolysis and that the glutamate formed contributes to the glial electrical response in part via activation of NMDA receptors. Both NAAG release and an increase in glutamate carboxypeptidase II activity appear to be induced by nerve stimulation.

Effect of 2-(phosphono-methyl)-pentanedioic acid on allodynia and afferent ectopic discharges in a rat model of neuropathic pain

Increased glutamate availability in the spinal cord and primary afferent nerves plays an important role in acute and chronic pain. Afferent ectopic discharges from the site of nerve injury constitute a source of abnormal sensory input to the spinal dorsal horn. The ectopic afferent activity is largely responsible for the development of hypersensitivity of dorsal horn neurons and neuropathic pain. Inhibition of glutamate carboxypeptidase II (GCP II) reduces glutamate release generated from N-acetyl-aspartyl-glutamate in nerve tissues and may have an analgesic effect on neuropathic pain. In the present study, we determined the effect of a GCP II inhibitor, 2-(phosphono-methyl)-pentanedioic acid (2-PMPA), on allodynia and ectopic afferent discharges in an animal model of neuropathic pain. Neuropathic pain was induced by partial ligation of the left sciatic nerve in rats. Tactile allodynia was assessed using von Frey filaments applied to the plantar surface of the injured hindpaw. Single-unit activity of ectopic discharges was recorded from the sciatic nerve proximal to the site of ligation. Intravenous injection of 50 to 100 mg/kg 2-PMPA significantly reduced allodynia in a dose-dependent manner. Furthermore, 2-PMPA dose-dependently attenuated the ectopic discharge activity of injured sciatic afferent nerves. At a dose of 100 mg/kg, 2-PMPA significantly inhibited the ectopic activity from 14.7 +/- 2.1 to 4.4 +/- 0.5 impulses/s without altering the conduction velocity of afferent nerves. Therefore, these data suggest that the antiallodynic effect of 2-PMPA may be mediated, at least in part, by inhibition of ectopic afferent discharges at the site of nerve injury.

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.