Expression of functional mGlu5 metabotropic glutamate receptors in human melanocytes

Cultured human melanocytes express mGlu5 metabotropic glutamate (mGlu) receptors, as shown by RT-PCR, immunocytochemistry, Western blot analysis, and measurement of agonist-stimulated polyphosphoinositide hydrolysis. The mGlu5 receptor agonists (S)-3, 5-dihydroxyphenylglycine and quisqualate increased [(3)H-methyl]thymidine incorporation and melanocyte proliferation in subconfluent cultures, but impaired cell viability in confluent cultures. Both effects were prevented by 2-methyl-6-(2-phenyl-1-ethynyl)-pyridine, a potent and highly selective mGlu5 receptor antagonist. Agonists of other mGlu receptor subtypes (such as the mGlu2/3 receptor agonist, 2S,2'R,3'R-2-2', 3'-dicarboxycyclopropylglycine, or the mGlu4/6/7/8 receptor agonist, L-2-amino-4-phosphonobutanoate) or selective agonists of ionotropic glutamate receptors (N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, and kainate) did not affect melanocyte proliferation or viability. The presence of a receptor for glutamate, the major excitatory neurotransmitter, in human melanocytes is intriguing. mGlu5 receptors may be involved in the control of melanocyte proliferation (and perhaps in other functions), but harbor a potential toxicity and may therefore contribute to cell damage under pathological conditions.     

In vitro and in vivo characterization of MPEP,an allosteric modulator of the metabotropic glutamate receptor subtype 5: review article

Summary.  There is a need to identify subtype-specific ligands for mGlu receptors to elucidate the potential of these receptors for the treatment of nervous system disorders. To date, most mGlu receptor antagonists are amino acid-like compounds acting as competitive antagonists at the glutamate binding site located in the large extracellular N-terminal domain. We have characterized novel subtype-selective mGlu₅ receptor antagonists which are structurally unrelated to competitive mGlu receptor ligands. Using a series of chimeric receptors and point mutations we demonstrate that these antagonists act as inverse agonists with a novel allosteric binding site in the seven-transmembrane domain. Recent studies in animal models implicate mGlu₅ receptors as a potentially important therapeutic target particularly for the treatment of pain and anxiety. Received July 2, 2001 Accepted August 6, 2001 Published online September 10, 2002     

Metabotropic glutamate receptor subtype 4 selectively modulates both glutamate and GABA transmission in the striatum: implications for Parkinson's disease treatment

Alterations of striatal synaptic transmission have been associated with several motor disorders involving the basal ganglia, such as Parkinson's disease. For this reason, we investigated the role of group-III metabotropic glutamate (mGlu) receptors in regulating synaptic transmission in the striatum by electrophysiological recordings and by using our novel orthosteric agonist (3 S )-3-[(3-amino-3-carboxypropyl(hydroxy)phosphinyl)-hydroxymethyl]-5-nitrothiophene (LSP1-3081) and l -2-amino-4-phosphonobutanoate (L-AP4). Here, we show that both drugs dose-dependently reduced glutamate- and GABA-mediated post-synaptic potentials, and increased the paired-pulse ratio. Moreover, they decreased the frequency, but not the amplitude, of glutamate and GABA spontaneous and miniature post-synaptic currents. Their inhibitory effect was abolished by ( RS )-α-cyclopropyl-4-phosphonophenylglycine and was lost in slices from mGlu4 knock-out mice. Furthermore, ( S )-3,4-dicarboxyphenylglycine did not affect glutamate and GABA transmission. Finally, intrastriatal LSP1-3081 or L-AP4 injection improved akinesia measured by the cylinder test. These results demonstrate that mGlu4 receptor selectively modulates striatal glutamate and GABA synaptic transmission, suggesting that it could represent an interesting target for selective pharmacological intervention in movement disorders involving basal ganglia circuitry.     

Synthesis and SAR of a novel metabotropic glutamate receptor 4 (mGlu4) antagonist: unexpected 'molecular switch' from a closely related mGlu4 positive allosteric modulator

Herein we report the discovery and SAR of a novel antagonist of metabotropic glutamate receptor 4 (mGlu(4)). The antagonist was discovered via a molecular switch from a closely related mGlu(4) positive allosteric modulator (PAM). This antagonist (VU0448383) displays an IC(50) value of 8.2±0.4 μM and inhibits an EC(80) glutamate response by 63.1±6.6%.     

New probes of the agonist binding site of metabotropic glutamate receptors.

The (2S,4R)- and (2S,4S)-4-hydroxyglutamates activate cloned mGlu(1a), mGlu(2), and mGlu(8a) receptors with different potencies. Best results were obtained with the (2S,4S) isomer being almost as potent as glutamate on mGlu(1a)R and mGlu(8a)R. Data are interpreted on the basis of the binding site model and X-ray structure.     

Combined administration of PHCCC,a positive allosteric modulator of mGlu4 receptors and ACPT-I,mGlu III receptor agonist evokes antidepressant-like effects in rats

Summary. Numerous pharmacological data indicate involvement of glutamate, the major excitatory neurotransmitter in the brain, in the pathophysiology of several neuropsychiatric disorders. It was shown in the preclinical studies that compounds which can reduce the excess of glutamate release (for example group III metabotropic receptors agonists) possess potential therapeutic properties. Thus we focused our interests on (−)-N-phenyl-7-(hydroxyimino) cyclopropa[b]chromen-1a-carboxamide (PHCCC), which is a positive allosteric modulator of mGlu4 receptor. We examined the potential antidepressant-like activity of PHCCC after injection into the brain ventricles alone, or together with (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid (ACPT-I), a nonselective group III mGlu receptor agonist, using the forced swimming test (FST) in rats. We found that ACPT-I induced a dose dependent antidepressant-like effect in FST, which was blocked by an antagonist of group III mGlu receptors (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG). PHCCC injected intracerebroventricular was not effective, however when the compound was administered together with non-effective dose of ACPT-I, a profound antidepressant-like activity in FST was demonstrated. This effect was reversed by CPPG, group III mGlu receptors antagonist. Results of our studies indicate that a combined administration positive allosteric modulation of mGlu4 receptor and agonists of group III mGlu receptors may be a promising target in the future treatment of depressive disorder.     

Neuroprotective and symptomatic effects of targeting group III mGlu receptors in neurodegenerative disease

Neurodegenerative disorders possess common pathological mechanisms, such as protein aggregation, inflammation, oxidative stress (OS) and excitotoxicity, raising the possibility of shared therapeutic targets. As a result of the selective cellular and regional expression of group III metabotropic glutamate (mGlu) receptors, drugs targeting such receptors have demonstrated both neuroprotective properties and symptomatic improvements in several models of neurodegeneration. In recent years, the discovery and development of subtype‐selective ligands for the group III mGlu receptors has gained pace, allowing further research into the functions of these receptors and revealing their roles in health and disease. Activation of this class of receptors results in neuroprotection, with a variety of underlying mechanisms implicated. Group III mGlu receptor stimulation prevents excitotoxicity by inhibiting glutamate release from neurons and microglia and increasing glutamate uptake by astrocytes. It also attenuates the neuroinflammatory response by reducing glial reactivity and encourages neurotrophic phenotypes. This article will review the current literature with regard to the neuroprotective and symptomatic effects of group III mGlu receptor activation and discuss their promise as therapeutic targets in neurodegenerative disease.

     

In vivo modulation of extracellular hippocampal glutamate and GABA levels and limbic seizures by group I and II metabotropic glutamate receptor ligands

The effects of several metabotropic receptor (mGluR) ligands on baseline hippocampal glutamate and GABA overflow in conscious rats and the modulation of limbic seizure activity by these ligands were investigated. Intrahippocampal mGluR group I agonist perfusion via a microdialysis probe [1 mm (R,S)-3,5-dihydroxyphenylglycine] induced seizures and concomitant augmentations in amino acid dialysate levels. The mGlu1a receptor antagonist LY367385 (1 mm) decreased baseline glutamate but not GABA concentrations, suggesting that mGlu1a receptors, which regulate hippocampal glutamate levels, are tonically activated by endogenous glutamate. This decrease in glutamate may contribute to the reported LY367385-mediated anticonvulsant effect. The mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (50 mg/kg) also clearly abolished pilocarpine-induced seizures. Agonist-mediated actions at mGlu2/3 receptors by LY379268 (100 microm, 10 mg/kg intraperitoneally) decreased basal hippocampal GABA but not glutamate levels. This may partly explain the increased excitation following systemic LY379268 administration and the lack of complete anticonvulsant protection within our epilepsy model with the mGlu2/3 receptor agonist. Group II selective mGluR receptor blockade with LY341495 (1-10 microm) did not alter the rats' behaviour or hippocampal amino acid levels. These data provide a neurochemical basis for the full anticonvulsant effects of mGlu1a and mGlu5 antagonists and the partial effects observed with mGlu2/3 agonists in vivo.     

Characterization of [(3)H]Quisqualate binding to recombinant rat metabotropic glutamate 1a and 5a receptors and to rat and human brain sections

We have investigated the binding properties of [(3)H]quisqualate to rat metabotropic glutamate (mGlu) 1a and 5a receptors and to rat and human brain sections. Saturation isotherms gave K:(D) values of 27 +/- 4 and 81 +/- 22 nM: for mGlu1a and mGlu5a receptors, respectively. Several compounds inhibited the binding to mGlu1a and mGlu5a receptors concentration-dependently. (S:)-4-Carboxyphenylglycine, (S:)-4-carboxy-3-hydroxyphenylglycine, and (R,S)-1-aminoindan-1,5-dicarboxylic acid, which completely inhibited [(3)H]quisqualate binding to the mGlu5a receptor, were inactive in a functional assay using this receptor. The distribution and abundance of binding sites in rat and human brain sections were studied by quantitative receptor radioautography and image analysis. Using 10 nM: [(3)H]quisqualate, a high density of binding was detected in various brain regions with the following rank order of increasing levels: medulla, thalamus, olfactory bulb, cerebral cortex, spinal cord dorsal horn, olfactory tubercle, dentate gyrus molecular layer, CA1-3 oriens layer of hippocampus, striatum, and cerebellar molecular layer. The ionotropic component of this binding could be inhibited by 30 microM: kainate, revealing the distribution of mGlu1+5 receptors. The latter were almost completely inhibited by the group I agonist (S:)-3,5-dihydroxyphenylglycine. The binding profile correlated well with the cellular sites of synthesis and regional expression of the respective group I receptor proteins revealed by in situ hybridization histochemistry and immunohistochemistry, respectively.     

Localization and function of group III metabotropic glutamate receptors in rat pancreatic islets

Pancreatic islets contain ionotropic glutamate receptors that can modulate hormone secretion. The purpose of this study was to determine whether islets express functional group III metabotropic glutamate (mGlu) receptors. RT-PCR analysis showed that rat islets express the mGlu8 receptor subtype. mGlu8 receptor immunoreactivity was primarily displayed by glucagon-secreting alpha-cells and intrapancreatic neurons. By demonstrating the immunoreactivities of both glutamate and the vesicular glutamate transporter 2 (VGLUT2) in these cells, we established that alpha-cells express a glutamatergic phenotype. VGLUT2 was concentrated in the secretory granules of islet cells, suggesting that glutamate might play a role in the regulation of glucagon processing. The expression of mGlu8 by glutamatergic cells also suggests that mGlu8 may function as an autoreceptor to regulate glutamate release. Pancreatic group III mGlu receptors are functional because mGlu8 receptor agonists inhibited glucagon release and forskolin-induced accumulation of cAMP in isolated islets, and (R,S)-cyclopropyl-4-phosphonophenylglycine, a group III mGlu receptor antagonist, reduced these effects. Because excess glucagon secretion causes postprandial hyperglycemia in patients with type 2 diabetes, group III mGlu receptor agonists could be of value in the treatment of these patients.     

Activation of group II mGlu receptors inhibits voltage-gated Ca2+ currents in myenteric neurons

The enteric nervous system (ENS) contains functional ionotropic and group I metabotropic glutamate (mGlu) receptors. In this study, we determined whether enteric neurons express group II mGlu receptors and the effects of mGlu receptor activation on voltage-gated Ca(2+) currents in these cells. (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC), a group II mGlu receptor agonist, reversibly suppressed the Ba(2+) current in myenteric neurons isolated from the guinea pig ileum. Significant inhibition was also produced by L-glutamate and the group II mGlu receptor agonists, (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) and (2S,1'S,2'S)-2-(2-carboxycyclopropyl)glycine (L-CCG-I), with a rank order potency of 2R,4R-APDC > DCG-IV > L-glutamate > L-CCG-I, and was reduced by the group II mGlu receptor antagonist LY-341495. Pretreatment of neurons with pertussis toxin (PTX) reduced the action of mGlu receptor agonists, suggesting participation of G(i)/G(o) proteins. Finally, omega-conotoxin GVIA blocked current suppression by DCG-IV, suggesting modulation of N-type calcium channels. mGlu2/3 receptor immunoreactivity was displayed by neurons in culture and in the submucosal and myenteric plexus of the ileum. A subset of these cells displayed a glutamatergic phenotype as shown by the expression of vesicular glutamate transporter 2. These results provide the first evidence for functional group II mGlu receptors in the ENS and show that these receptors are PTX sensitive and negatively coupled to N-type calcium channels. Inhibition of N-type calcium channels produced by activation of group II mGlu receptors may modulate enteric neurotransmission.     

Metabotropic glutamate receptors in neurodegeneration/neuroprotection: Still a hot topic?

Moving from early studies, we here review the most recent evidence linking metabotropic glutamate (mGlu) receptors to processes of neurodegeneration/neuroprotection. The use of knockout mice and subtype-selective drugs has increased our knowledge of the precise role played by individual mGlu receptor subtypes in these processes. Activation of mGlu1 and mGlu5 receptors may either amplify or reduce neuronal damage depending on the context and the nature of the toxic insults. In contrast, mGlu1 and mGlu5 receptors antagonists are consistently protective in in vitro and in vivo models of neuronal death. A series of studies suggest that mGlu1 receptor antagonists or negative allosteric modulators (NAMs) are promising candidates for the treatment of ischemic brain damage, whereas mGlu5 receptor NAMs, which have been clinically developed for the treatment of Parkinson's disease (PD) and l-DOPA-induced dyskinesias, protect nigro-striatal dopaminergic neurons against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in mice and monkeys. Activation of glial mGlu3 receptors promotes the formation of various neurotrophic factors, such as transforming growth factor-β (TGF-β), glial-derived neurotrophic factor (GDNF), nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF). Hence, selective mGlu3 receptor agonists or positive allosteric modulators (PAMs) (not yet available) are potentially helpful in the treatment of chronic neurodegenerative disorders such as PD, Alzheimer's disease (AD), and amyotrophic lateral sclerosis. Selective mGlu2 receptor PAMs should be used with caution in AD patients because these drugs are shown to amplify β-amyloid neurotoxicity. Finally, mGlu4 receptor agonists/PAMs share with mGlu5 receptor NAMs the ability to improve motor symptoms associated with PD and attenuate nigro-striatal degeneration at the same time. No data are yet available on the role of mGlu7 and mGlu8 receptors in neurodegeneration/neuroprotection.     

Modulation of glutamate release from parallel fibers by mGlu4 and pre-synaptic GABA(A) receptors

The regulation of pre-synaptic glutamate release is important in the maintenance and fidelity of excitatory transmission in the nervous system. In this study, we report a novel interaction between a ligand-gated ion channel and a G-protein coupled receptor which regulates glutamate release from parallel fiber axon terminals. Immunocytochemical analysis revealed that GABA(A) receptors and the high affinity group III metabotropic glutamate receptor subtype 4 (mGlu4) are co-localized on glutamatergic parallel fiber axon terminals in the cerebellum. GABA(A) and mGlu4 receptors were also found to co-immunoprecipitate from cerebellar membranes. Independently, these two receptors have opposing roles on glutamate release: pre-synaptic GABA(A) receptors promote, while mGlu4 receptors inhibit, glutamate release. However, coincident activation of GABA(A) receptors with muscimol and mGlu4 with the agonist (2S)-S-2-amino-4-phosphonobutanoic acid , increased glutamate release from [(3) H]glutamate-loaded cerebellar synaptosomes above that observed with muscimol alone. Further support for an interaction between GABA(A) and mGlu4 receptors was obtained in the mGlu4 knockout mouse which displayed reduced binding of the GABA(A) ligand [(35) S]tert-butylbicyclophosphorothionate, and decreased expression of the α1, α6, β2 GABA(A) receptor subunits in the cerebellum. Taken together, our data suggest a new role for mGlu4 whereby simultaneous activation with GABA(A) receptors acts to amplify glutamate release at parallel fiber-Purkinje cell synapses.     

4-(1-Phenyl-1H-pyrazol-4-yl)quinolines as novel, selective and brain penetrant metabotropic glutamate receptor 4 positive allosteric modulators

4-(1-Phenyl-1H-pyrazol-4-yl)quinoline (1) was identified by screening the Lundbeck compound collection, and characterized as having mGlu4 receptor positive allosteric modulator properties. Compound 1 is selective over other mGlu receptors and a panel of GPCRs, ion channels and enzymes, but has suboptimal lipophilicity and high plasma and brain non-specific binding. In view of the challenges at the hit-to-lead stage previously reported in the development of mGlu4 receptor positive allosteric modulators (PAMs), a thorough structure-mGlu4 PAM activity relationship study was conducted to interrogate the chemical tractability of this chemotype. The central pyrazole ring tolerates the addition of one or two methyl groups. The C-7 position of the quinoline ring provides a site tolerant to hydrophilic substituents, enabling the design of diverse analogs with good in vitro mGlu4 PAM potency and efficacy, as well as improved microsomal turnover in vitro, compared to 1. In spite of the excellent ligand efficiency of 1 (LE=0.43), optimization of in vitro potency for this series reached a plateau around EC(50)=200 nM.     

Gliotransmission modulates baseline mechanical nociception

Group II metabotropic glutamate receptors (mGluRs) couple to the inhibitory G-protein Gi. The group II mGluRs include two subtypes, mGlu2 and mGlu3, and their pharmacological activation produces analgesic effects in inflammatory and neuropathic pain states. However, the specific contribution of each one of the two subtypes has not been clarified due to the lack of selective orthosteric ligands that can discriminate between mGlu2 and mGlu3 subtypes. In this study we used mGlu2 or mGlu3 knock-out mice to dissect the specific role for these two receptors in the endogenous control of inflammatory pain and their specific contribution to the analgesic activity of mixed mGlu2/3 receptor agonists. Our results showed that mGlu2^-/- mice display a significantly greater pain response compared to their wild type littermates. Interestingly the increased pain sensitivity in mGlu2^-/- mice occurred only in the second phase of the formalin test. No differences were observed in the first phase. In contrast, mGlu3^-/- mice did not significantly differ from their wild type littermates in either phase of the formalin test. When systemically injected, a single administration of the mGlu2/3 agonist, LY379268 (3 mg/kg, ip), showed a significant reduction of both phases in wild-type mice and in mGlu3^-/- but not in mGlu2^-/- mice. However tolerance to the analgesic effect of LY379268 (3 mg/kg, ip) in mGlu3^-/- mice developed following 5 consecutive days of injection. Taken together, these results demonstrate that: (i) mGlu2 receptors play a predominant role over mGlu3 receptors in the control of inflammatory pain in mice; (ii) the analgesic activity of mixed mGlu2/3 agonists is entirely mediated by the activation of the mGlu2 subtype and (iii) the development of tolerance to the analgesic effect of mGlu2/3 agonists develops despite the lack of mGlu3 receptors.     

Activation of group II metabotropic glutamate receptors underlies microglial reactivity and neurotoxicity following stimulation with chromogranin A,a peptide up-regulated in Alzheimer's disease

Regulation of microglial reactivity and neurotoxicity is critical for neuroprotection in neurodegenerative diseases. Here we report that microglia possess functional group II metabotropic glutamate receptors, expressing mRNA and receptor protein for mGlu2 and mGlu3, negatively coupled to adenylate cyclase. Two different agonists of these receptors were able to induce a neurotoxic microglial phenotype which was attenuated by a specific antagonist. Chromogranin A, a secretory peptide expressed in amyloid plaques in Alzheimer's disease, activates microglia to a reactive neurotoxic phenotype. Chromogranin A-induced microglial activation and subsequent neurotoxicity may also involve an underlying stimulation of group II metabotropic glutamate receptors since their inhibition reduced chromogranin A-induced microglial reactivity and neurotoxicity. These results show that selective inhibition of microglial group II metabotropic glutamate receptors has a positive impact on neuronal survival, and may prove a therapeutic target in Alzheimer's disease.     

Excitatory Amino Acid Receptor Potency and Subclass Specificity of Sulfur-Containing Amino Acids

The sulfur-containing amino acids, L- and D-cysteate, L-cysteine, L- and D-cysteine sulfinate, L- and D-cysteine-S-sulfate, L-cystine, L- and D-homocysteate, L- and D-homocysteine sulfinate, L-homocysteine, L-serine-O-sulfate, and taurine were tested in two excitatory amino acid receptor functional assays and in receptor binding assays designed to label specifically the AA1/N-methyl-D-aspartate (NMDA), AA2/quisqualate, and AA3/kainate receptor recognition sites, as well as a CaCl2-dependent L-2-amino-4-phosphonobutanoate site, and a putative glutamate uptake site. Agonist efficacies were determined by chick retinal excitotoxicity and stimulated sodium efflux from rat brain slices. D-Homocysteine sulfinate, L-homocysteate, and L-serine-O-sulfate had affinities most selective for the NMDA binding site, whereas the binding affinities of D-cysteate, D-cysteine sulfinate, D-homocysteate, and L-homocysteine sulfinate were less selective. However, the correlation of agonist activity sensitive to blockade by D-2-amino-7-phosphonoheptanoate or D-2-amino-5-phosphonopentanoate in the functional assays with affinity in the NMDA binding assay (r = 0.87, p less than 0.005 and r = 0.98, p less than 0.005 for excitotoxicity and sodium efflux, respectively) allows characterization of these sulfur-containing amino acids as acting at NMDA subclass receptors. L-Homocysteate, which has been found in the brain, and L-serine-O-sulfate are selective agonists and could serve as endogenous neurotransmitters at the NMDA receptor.     

Selective activation of metabotropic glutamate receptor 7 induces inhibition of cellular proliferation and promotes astrocyte differentiation of ventral mesencephalon human neural stem/progenitor cells

Expression of group III metabotropic glutamate receptors (mGluR) was established by RT-PCR and immunocytochemistry on a cultured clonal human neural stem/progenitor cell (hNSPC) line derived from fetal ventral mesencephalon (VM). Selective activation of these receptors by the group III mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) prevented increases in cAMP levels following forskolin stimulation, suggesting these receptors are coupled to their canonical G-protein coupled signal transduction pathway. Tonic exposure of undifferentiated cultures to l-AP4 resulted in a decrease in cellular metabolism and proliferation in the absence of toxicity, as measured by MTT and LDH assays, in a dose-dependent manner. This was confirmed by a reduction in BrdU incorporation into nuclear DNA, suggestive of an anti-proliferative effect of l-AP4. This effect was rescued by co-addition of the broad-spectrum group III mGluR competitive antagonist (RS)-a-cyclopropyl-4-phosphonophenylglycine (CPPG), demonstrating a receptor-mediated mechanism, but not mimicked by application of the cell permeable cAMP analogue dibutyrl cAMP (db-cAMP). The potency of these effects of l-AP4 indicates that this is an mGlu7 subtype-mediated effect. Tonic exposure of undifferentiated cultures to the mGlu7 selective allosteric agonist N,N′-bis(diphenylmethyl)-1,2-ethanediamine dihydrochloride (AMN082), but not the mGlu4 selective allosteric agonist (±)-cis-2-(3,5-dicholorphenylcarbamoyl)cyclohexanecarboxylic acid (VU0155041), or the mGlu8 selective agonist (S)-3,4-dicarboxyphenylglycine ((S)-3,4-DCPG) resulted in an identical anti-proliferative effect to l-AP4, confirming the involvement of the mGlu7 subtype. In differentiating cultures, tonic exposure to l-AP4 or AMN082 resulted in a significant shift towards an astrocyte cell fate. The mGlu7 receptor therefore provides a new opportunity to influence the proliferation and differentiation of ventral mesencephalon-derived hNSPC.     

The inhibition of pineal arylalkylamine n-acetyltransferase by glutamic acid and its analogues

Previous studies from this laboratory have identified in bovine pineal gland a glutamate receptor site with a dissociation equilibrium constant (K_D) value of 0.534 μM and a receptor density (B_max) value of 4.84 pmol/mg protein. This pH- and temperature-dependent binding site showed stereospecificity, was activated by Ca²⁺ and displayed affinity for both glutamate receptor agonists and antagonists. The role of this glutamate receptor site was investigated by studying the effects of select glutamate receptor agonists and antagonists and of γ-aminobutyric acid on the basal- and on the norepinephrine-stimulated activity of arylalkylamine N-acetyltransferase in rat pineal glands that were incubated in Dulbecco's Modified Eagle Medium at 37°C for 20 min in an atmosphere of 5% CO₂/95% O₂. l-Glutamate, l-aspartate and glutamate receptor agonists such as γ-amino-3-hydroxy-5-methylisoxazole-4-propinonic acid and quisqualate were all also potent inhibitors of norepinephrine-induced stimulation of N-acetyltransferase. On the other hand, the known glutamate receptor antagonists such as d-glutamylaminomethylsulphonic acid and γ-d-glutamyltaurine stimulated the basal activity of N-acetyltransferase.Evidence of a high concentration of glutamic acid, the presence of glutamate receptors and the inhibition by glutamate receptor agonists of pineal N-acetyltransferase compel one to speculate that, in addition to its well-known metabolic roles, glutamate may modulate in an unknown fashion the activity of melatonin synthesizing enzyme, and the functions of mammalian pineal glands.