Pharmacological characterization of the quisqualate receptor coupled to phospholipase C (Qp) in striatal neurons

The GluN2B subunit of N-methy-D-asparate receptor regulates the radial migration of cortical neurons in vivo

The formation of layered structure of the mammalian neocortex requires a fine organized migration of post-mitotic neurons during early development. However, whether the radial migration is regulated by NMDA receptor and specific subunits remains contradictory and unknown. Here, we reported that in the developing rat cortex, migration of presumptive layer II/III neurons to their deserved destination was regulated by NMDA receptors with GluN2B but not GluN2A subunit. Using in utero electroporation of small interference RNA (siRNA) of distinct NMDA receptor subunits, we found that knockdown GluN1 and GluN2B subunits dramatically delayed the neuronal migration to proper layer II/III, while improperly stayed at lower layers or even the germinal regions, without changing the cell fate. In contrast, knockdown of GluN2A subunit did not impair the neuronal migration. Additionally, the ecotopic neurons by GluN2B RNAi developed to well dendritic differentiation, while the ones by GluN1 RNAi still kept morphology of migrating neurons. Therefore, GluN2B subunit of NMDA receptor plays an essential role in regulating proper neuronal migration and cortical lamination.

Glutamate potentiates lipopolysaccharide-stimulated interleukin-10 release from neonatal rat spinal cord astrocytes

Interleukin-10 (IL-10) has important anti-inflammatory effects and can be protective in inflammatory conditions, such as chronic pain and infection. Exploring factors that modulate IL-10 levels may provide insight into pathomechanisms of inflammatory conditions and may provide a method of neuroprotection during these conditions. Lipopolysaccharide (LPS) stimulation of astrocytes is a source of IL-10; hence, it is of interest to investigate factors that modulate this process. Glutamate is present in increased concentrations in inflammatory conditions, and astrocytes also express glutamate receptors. The present study, therefore, investigated whether glutamate modulates LPS stimulation of IL-10 release from neonatal spinal cord astrocytes. Enzyme-linked immunosorbent assays (ELISAs) were used to quantify IL-10 release from cultured neonatal spinal cord astrocytes, and reverse transcriptase-polymerase chain reaction (RT-PCR) was used to measure IL-10 mRNA expression. Glutamate (1 mM) significantly increased LPS (1 μg/ml)-stimulated IL-10 release from astrocytes by 166% and significantly upregulated IL-10 mRNA levels. Glutamate synergistically signaled through metabotropic glutamate receptor subgroups and the phospholipase C signaling pathway. Spinal cord astrocytes may, therefore, play a larger anti-inflammatory role than first thought in situations where glutamate and a high concentration of Toll-like receptor 4 (TLR4) agonists are present.

Transmitter release from Rana pipiens vestibular hair cells via mGluRs: a role for intracellular Ca(++) release

The response of the semicircular canal (SCC) to the group I mGluR-selective agonist dihydroxyphenylglycine (DHPG; 300 microM) - facilitation of afferent discharge rate - was dose-dependently reduced by the phospholipase C inhibitor U-73122 (1-100 microM; IC(50): 22 microM), the smooth endoplasmic reticulum Ca(++) ATPase inhibitor thapsigargin (100 nM-3 microM; IC(50): 500 nM), and xestospongin C (100 pM-1 microM; IC(50): 11 nM), an inositol trisphosphate receptor (IP(3)R) antagonist. Ryanodine, a modulator of Ca(++)-induced Ca(++) release, biphasically facilitated, then suppressed this response (1 nM-1 mM; approximate IC(50): 50 microM). 5 mM caffeine increased the amplitude (34.6+/-13.4%) and duration (453+/-169.8%; n=4) of the response of the SCC to DHPG, while 50 mM caffeine eliminated this response (n=2). The protein kinase C inhibitor bisindolylmaleimide I-HCl (10-100 microM; n=3) and the cyclic-ADP ribose antagonist 8-Br-cyclic-ADP ribose (1-10 microM; n=3) had no effect on the response of the SCC to DHPG. These data suggest that the increase in transmitter release following activation of group I mGluRs on vestibular hair cells is associated with intracellular Ca(++) release from both IP(3)-sensitive and ryanodine/caffeine-sensitive intracellular Ca(++) stores. Such positive feedback on transmitter release may serve to enhance the contrast between the spontaneous and stimulus-evoked modes of hair cell transmitter release, thereby optimizing signal discrimination at the synapse between hair cells and vestibular afferent fibers.

A Nitric Oxide Synthase Activity Selectively Stimulated by NMDA Receptors Depends on Protein Kinase C Activation in Mouse Striatal Neurons

In mouse striatal neurons in primary culture, the maximal increase in intracellular cyclic guanosine monophosphate level evoked by N-methyl-d-aspartic acid (NMDA) receptor activation was twice that induced by kainate, KCl and ionomycin. Quisqualate was almost inactive. All responses were mediated by nitric oxide (NO) production since they were blocked by haemoglobin (a NO scavenger) and by l-NG-monomethylarginine and l-NG-nitroarginine, the effects of both arginine analogues being reversed by an excess of l-arginine. Several results indicate that NMDA receptors stimulate a specific NO synthase activity. This specifically NMDA-activated NO synthase was blocked by nanomolar concentrations of l-NG-nitroarginine, whereas the responses evoked by other agents, including kainate, KCl and ionomycin, were only blocked by micromolar concentrations of this NO synthase inhibitor. The NMDA response could not be totally reproduced by an increase in cytosolic calcium (Ca2+) alone. In contrast, in the presence of staurosporine, an inhibitor of protein kinases C (PKC), as well as after desensitization of PKC induced by long-term treatment with the phorbol ester, phorbol-12, 13-dibutyrate, NMDA-stimulated NO production was selectively reduced, reaching the level evoked by kainate or Ca2+ increase. In conclusion, our results suggest that in striatal neurons, NMDA selectively stimulates a NO synthase activity which is inhibited by low concentrations of l-NG-nitro-arginine, through a mechanism involving PKC.

Both metabotropic glutamate I and II receptors mediate augmentation of dopamine release from the striatum in methamphetamine-sensitized rats

The role of metabotropic glutamate receptor (mGluR) on dopamine overflow from the striatum was studied in methamphetamine (MAP)-sensitized rats. The increase of dopamine release by MAP was significantly inhibited by perfusion of a mGluR antagonist R,S-alpha-methyl-4-carboxyphenylglycine. The perfused mGluR agonist [S,3R-1-aminocyclopentane-1,3-dicarboxylic acid enhanced the dopamine level. The enhancement was significantly attenuated by co-perfusion of a mGluR group I antagonist (S)-4-carboxy-3-hydroxyphenylglycine or a mGluR group II antagonist R,S-a-methyl-4-tetrazolylphenylglycine. These suggest that both mGluR group I and II mediate augmentation of dopamine release in MAP-sensitized rats.

Group I metabotropic glutamate receptors in spinal cord injury: roles in neuroprotection and the development of chronic central pain

Spinal cord injury (SCI) initiates a cascade of biochemical events that leads to an increase in extracellular excitatory amino acid (EAA) concentrations, which results in glutamate receptor-mediated excitotoxic events. An important division of these glutamate receptors is the metabotropic glutamate receptor (mGluR) class, which is divided into three groups. Of these three groups, group I (mGluR1 and mGluR5) activation can initiate a number of intracellular pathways that lead to increased extracellular EAA concentrations. To evaluate subtypes of group I mGluRs in SCI, we administered AIDA (group I antagonist), LY 367385 (mGluR1 specific antagonist), or MPEP (mGluR5 specific antagonist) by interspinal injection to adult male Sprague-Dawley rats (175-200 g) immediately following injury at T10 with an NYU impactor (12.5-mm drop, 10-g rod, 2 mm in diameter). AIDA- and LY 367385-treated subjects had improved locomotor scores and demonstrated an attenuation in the development of mechanical allodynia as measured by von Frey stimulation of the forelimbs; however, LY 367385 potentiated the development of thermal hyperalgesia. MPEP had no effect on locomotor recovery or mechanical allodynia, but attenuated the development of thermal hyperalgesia. AIDA and LY 367385 treatment resulted in a significant increase in tissue sparing compared to the vehicle-treated group at 4 weeks following SCI. These results suggest that mGluRs play an important role in EAA toxicity and have different acute pathophysiological roles following spinal cord injury.

Activation of group I metabotropic glutamate receptors reduces neuronal apoptosis but increases necrotic cell death in vitro

Glutamate released during acute CNS insults acts at metabotropic glutamate receptors (mGluR), including group I mGluR. Blockade of group I mGluR during in vitro neuronal trauma provides neuroprotection, whereas activation exacerbates such injury. However, the effects of group I mGluR agonists or antagonists have been primarily studied in in vitro models characterized by necrotic cell death. We examined the role of group I mGluR in the modulation of neuronal injury induced during oxygen-glucose deprivation (OGD), a well-studied model of necrosis, and by application of two well established pro-apoptotic agents: staurosporine and etoposide. Inhibition of group I mGluR attenuated necrosis induced by OGD, whereas selective activation of group I mGluR exacerbated such injury. In contrast, activation of group I mGluR, including selective activation of mGluR5, significantly attenuated apoptotic cell death induced by both staurosporine and etoposide. This effect was completely reversed by co-application of a group I mGluR antagonist. Thus, group I mGluR appear to exhibit opposite effects on necrotic and apoptotic neuronal cell death. Our findings suggest that activation of mGluR1 exacerbates neuronal necrosis whereas both mGluR1 and mGluR5 play a role in attenuation of neuronal apoptosis.

Regulation of the nigrostriatal pathway by metabotropic glutamate receptors during development

Dopamine neurons in the substantia nigra heavily innervate the striatum, making it the nucleus with the highest levels of dopamine in the adult brain. The present study analyzes the time course and the density of striatal innervation by nigral dopamine neurons and characterizes the role of the neurotransmitter glutamate during the development of the nigrostriatal pathway. For this purpose, organotypic cultures containing the cortex, the striatum, and the substantia nigra (triple cultures) were prepared from rat brains at postnatal day (PND) 0-2 and were cultured for up to 60 d in vitro (DIV). Dopamine fibers and neurons were labeled using tyrosine hydroxylase (TH) immunohistochemistry. Striatal TH-ir fiber density was quantitatively analyzed using confocal laser scanning microscopy (CLSM). In long-term triple cultures (44 +/- 3 DIV), the striatal dopamine fiber density was high and was weakly correlated with the number of nigral dopamine neurons. The high striatal dopamine fiber density mainly resulted from an enhanced ingrowth and ramification of dopamine fibers from nigral neurons during 8-17 DIV. The metabotropic glutamate receptor (mGluR) antagonist L(+)-2-amino-3-phosphonopropionic acid (L-AP-3) selectively inhibited this dopaminergic innervation of the striatum, whereas ionotropic GluR antagonists had no effect. The L-AP-3-mediated inhibition was prevented by the mGluR agonist 1S, 3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). The inhibition of the striatal dopaminergic innervation by L-AP-3 was further confirmed by anterograde tracing of the nigrostriatal projection with Phaseolus vulgaris leucoagglutinin. These results indicate that glutamate, by acting on group I mGluRs, plays an important "trophic" role for the development of the nigrostriatal dopamine pathway.

Carbachol-induced hydrolysis of phospholipids in hippocampal slices may be mediated in part by subsequent activation of metabotropic glutamate receptors

We observed that AP-3, an antagonist of metabotropic glutamate receptors, reduced carbachol-induced hydrolysis of phospholipids in hippocampal slices. This inhibition could be explained in different ways, e.g.: 1) AP-3 acts also as antagonist of muscarinic receptors mediating the hydrolysis of phospholipids induced by carbachol, 2) Carbachol induces the release of glutamate which, by activating metabotropic glutamate receptors, leads to additional hydrolysis of phospholipids. The aim of this work was to test these possibilities. It is shown that AP-3 reduces carbachol-induced hydrolysis of phospholipids in hippocampal slices but not in cerebellar neurons at 10-14 days of culture, when these cells are not able to induce hydrolysis of phospholipids following activation of metabotropic glutamate receptors. It is also shown that carbachol induces a release of [3H]aspartate in hippocampal slices. The results reported suggest that the hydrolysis of phospholipids induced by carbachol in hippocampal slices would have two components. One part would be due to direct activation by carbachol of muscarinic receptors associated to activation of phospholipase C. This part would not be inhibited by AP-3. The second part would be due to subsequent release of glutamate and activation of metabotropic glutamate receptors. This part would be inhibited by AP-3.

Effect of metabotropic glutamate receptor activation on receptor-mediated cyclic AMP responses in primary cultures of rat striatal neurones

Co-activation of group I metabotropic glutamate (mGlu) receptors and adenosine receptors resulted in an augmented cyclic AMP response in primary cultures of rat striatal neurones. L-glutamate and the selective group I agonist, (S)-dihydroxyphenylglycine (S-DHPG) evoked concentration-dependent potentiations of cyclic AMP accumulation stimulated by the adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA), with EC50 values of 3.41+/-0. 39 and 5.69+/-1.64 microM, respectively, and maximal augmentations of approximately 350% at concentrations of 100 microM. The S-DHPG potentiation was inhibited by group I mGlu receptor antagonists and a protein kinase C inhibitor, Ro 31-8220, implicating products of PI hydrolysis in this effect. Furthermore, L-glutamate and S-DHPG stimulated PI hydrolysis in striatal neuronal cultures with similar EC50 values to those observed for the augmentation of NECA cyclic AMP responses (5.19+/-1.18 and 3.78+/-1.42 microM, respectively). In situ hybridization and immunofluorescence techniques indicate that group I mGlu receptor-evoked potentiations are likely to be mediated via mGlu5 receptors, which are expressed at high levels in these cultures. In contrast to cross-chopped slices of neonatal rat striatum, of equivalent age, the group II mGlu receptor agonist, (2S, 2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) was without effect on NECA- or forskolin-stimulated cyclic AMP responses in primary striatal neuronal cultures. This lack of effect might be due to a low level of expression of group II mGlu receptors in cultured striatal neurones.

Excitotoxic mechanisms of neurodegeneration in transmissible spongiform encephalopathies

Endogenous excitatory amino acids (EAAs) such as glutamic or aspartic acids have been proposed to mediate the brain damage to EAA receptor-rich brain sites that is caused by a variety of external toxic agents (glutamic acid, domoic acid, kainic acid, ibogaine, trimethyltin (TMT), 3-nitropropionic acid (3-NPA)), as well as from such naturally-occurring age-related neurodegenerative diseases as Alzheimer's disease, Huntington's chorea, and Parkinson's disease. Sites often damaged include the hypothalamus (glutamate), the hippocampal and neocortical pyramidal neurons (domoic acid), the cerebellar Purkinje neurons (ibogaine) and the corpus striatum (3-NPA, amphetamine). The excitotoxic damage occurs to neuronal cell bodies and their dendrites, resulting in a characteristics appearance of pyknotic neurons surrounded by their vacuolated, swollen dendrites. Axons passing through the region that lack EAA receptors are completely spared. However, astrocytes with swollen perikarya and nuclei (Alzheimer's type II "reactive" astrocytes) are often observed in the vicinity of the lesions. Animal and human "Prion Diseases" or "Transmissible Spongiform Encephalopathies" (TSEs) result (after a period of months to years) in a neurodegenerative picture characterized by pyknotic neurons surrounded by vacuoles with numerous reactive astrocytes in the vicinity of the damage. In addition, amyloid deposits composed of a protease-resistant protein (PrPSc) characteristic of the particular host species with the disease are found near the degenerating neurons. By using different strains of the scrapies TSE agent to inoculate hamsters and mice, reproducible models of hypothalamic, hippocampal, or cerebellar damage resulting in the appropriate functional deficits may be obtained. Because of the close similarity in the appearance, localization, and functional consequences from TSE neuropathology compared to some of the well-known EAA syndromes, we propose that excitotoxic mechanisms may play a role in the pathogenesis of TSE neurodegenerative diseases. The similarity in pathogenesis of the neurodegenerative processes in excitotoxicity compared to TSE diseases also implies that neuroprotective strategies against excitotoxicity may also be effective against TSEs.

Receptor and mechanism that mediate endothelin- and big endothelin-1-induced phosphoinositide hydrolysis in the rat spinal cord

In rat spinal cord slices, endothelin-1 and endothelin-3 enhanced [3H]inositol phosphate production between 1 nM and 10 microM (endothelin-1 > endothelin-3) while sarafotoxin 6c and the endothelin ETB receptor agonist IRL-1620 (Suc-[Glu9,Ala11,15]endothelin-1-(8-21)) were almost ineffective. BQ-123 (cyclo(D-Trp,D-Asp,L-Pro,D-Val,L-Leu), a selective endothelin ETA receptor antagonist, reduced the endothelin-1- and endothelin-3-induced [3H]inositol phosphate production, with similar inhibition constants (IC50: 16.7 +/- 3.4 and 8.0 +/- 1.6 microM, respectively). The inhibition of endothelin-1 was enhanced when BQ-123 was preincubated for 30 min instead of 15 min. BQ-788 (N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D -1-methoxy- carbonyltryptophanyl-D-Nle), a selective ETB receptor antagonist, did not modify the endothelin-1-induced [3H]inositol phosphate production. Big endothelin-1 (1 nM to 1 microM) was slightly less potent than endothelin-1 in enhancing [3H]inositol phosphate production. This response was sensitive to phosphoramidon and [Phe22]big endothelin-1-(19-37), two inhibitors of endothelin-converting enzyme. Pretreatment of slices with pertussis toxin, indomethacin or PN 200-110 ((-)-isradipine, a dual inhibitor of L- and R-type Ca2+ channels) did not alter the response to 1 microM endothelin-1 while this response was abolished by tetrodotoxin. Finally, endothelin-1 enhanced [3H]inositol phosphate production with an identical EC50 (2.1 nM) in spinal cord slices of Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) although the maximal response was reduced in SHR. These data indicate that endothelins stimulated [3H]inositol phosphate production in the rat spinal cord through the activation of an endothelin ETA receptor that trigger the release of an unidentified neurotransmitter. This effect does not appear to be associated to activation of a Gi/G(o)-type of G-protein, dihydropyridine-sensitive L-type Ca2+ channels or to the production of prostaglandins. Furthermore, the findings support the presence of a phosphoramidon-sensitive endothelin-converting enzyme in the spinal cord.

Role of Ca2+ in the action of adrenocorticotropin in cultured human adrenal glomerulosa cells

The present report details the role of Ca2+ in the early events of ACTH action in human adrenal glomerulosa cells. Threshold stimulations of both aldosterone and cAMP production were obtained with a concentration of 10 pM ACTH, an ED50 of 0.1 nM, and maximal aldosterone stimulation (5.5-fold increase over control) at 10 nM ACTH. ACTH also induced a sustained increase of intracellular calcium ([Ca2+]i) with maximal stimulation of 1.6 +/- 0.1-fold over control values. This increase does not involve mobilization of calcium from intracellular pools since no response was observed in Ca2+-free medium or in the presence of nifedipine, suggesting the involvement of Ca2+ influx by L-type Ca2+ channels. This was confirmed by patch clamp studies that demonstrated that ACTH stimulates L-type Ca2+ channels. Moreover, the Ca2+ ion is not required for ACTH binding to its receptor, but is essential for sustained cAMP production and aldosterone secretion after ACTH stimulation. These results indicate that, in human adrenal glomerulosa cells, a positive feedback loop between adenylyl cyclase-protein kinase A-Ca2+ channels ensures a slow but sustained [Ca2+]i increase that is responsible for sustained cAMP production and aldosterone secretion.

Stimulatory effects of the putative metabotropic glutamate receptor antagonist L-AP3 on phosphoinositide turnover in neonatal rat cerebral cortex

1. The effects of the metabotropic glutamate receptor (mGluR) antagonist, L-2-amino-3-phosphonopropionate (L-AP3) on phosphoinositide turnover in neonatal rat cerebral cortex slices has been investigated. 2. At concentrations of < or = 300 microM, L-AP3 inhibited total [3H]-inositol phosphate ([3H]-InsPx) and Ins(1,4,5)P3 mass responses stimulated by the selective mGluR agonist, 1-amino-cyclopentane-1S, 3R-dicarboxylic acid (1S, 3R-ACPD). Comparison with the competitive mGluR antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine ((+/-)-MCPG) clearly demonstrated that L-AP3 caused inhibition by a mechanism that was not competitive, as L-AP3 decreased the maximal response to 1S, 3R-ACPD (by approximately 40% at 300 microM L-AP3) without significantly affecting the concentration of 1S, 3R-ACPD required to cause half-maximal stimulation of the [3H]-InsPx response. 3. In contrast, at a higher concentration L-AP3 (1 mM) caused a large increase in [3H]-InsPx accumulation which was similar in magnitude in both the absence and presence of 1S, 3R-ACPD (300 microM). D-AP3 (1 mM) had no stimulatory effect alone and did not affect the response evoked by 1S, 3R-ACPD. L-AP3 (1 mM) also caused a large increase in Ins(1,4,5)P3 accumulation. The magnitude of the response (4-5 fold increase over basal) approached that evoked by a maximally effective concentration of 1S, 3R-ACPD, but differed substantially in the time-course of the response. The stimulatory effects of 1S, 3R-ACPD and L-AP3 on Ins(1,4,5)P3 accumulation were also similarly affected by decreases in extracellular calcium concentration. 4. Detailed analysis of the inositol phospholipid labelling pattern and the inositol (poly)phosphate isomeric species generated following addition of L-AP3 was also performed. In the continued presence of myo-[3H]-inositol, L-AP3 (1 mM) stimulated a significant increase in phosphatidylinositol labelling, but not that of the polyphosphoinositides, and the inositol (poly)phosphate profile suggested that substantial Ins(1,4,5)P3 metabolism occurs via both 5-phosphatase and 3-kinase routes. 5. A significant stimulatory effect of L-AP3 (1 mM) on [3H]-InsPx accumulation was also observed in neonatal rat hippocampus, and cerebral cortex and hippocampus slices prepared from adult rat brain. 6. These data demonstrate that whilst L-AP3 antagonizes mGluR-mediated phosphoinositide responses at concentrations of < or = 300 microM, higher concentrations substantially stimulate this response. The ability of (+/-)-MCPG (1 mM) to attenuate significantly L-AP3-stimulated [3H]-InsPx accumulation, suggests that both the inhibitory and stimulatory effects of L-AP3 may be mediated by mGluRs.

Regulation of N-methyl-D-aspartate-induced toxicity in the neostriatum: a role for metabotropic glutamate receptors?

Glutamate release activates multiple receptors that interact with each other and thus determine the response of the cell. Exploring these interactions is critical to developing an understanding of the functional consequences of synaptic transmission. Activation of metabotropic glutamate receptors (mGluRs) inhibits N-methyl-D-aspartate (NMDA)-evoked responses measured electrophysiologically in neostriatal slices. The present study examines the functional consequences of this regulation using infrared differential interference contrast videomicroscopy to measure and characterize glutamate receptor-induced cell swelling in a neostriatal brain slice preparation. This swelling is, in many cases, a prelude to necrotic cell death and the dye trypan blue was used to confirm that swelling can result in the death of neostriatal cells. Activation of mGluRs by the agonist 1-aminocyclopentane-1,3-dicarboxylic acid (tACPD) inhibited NMDA but not amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-induced swelling. This regulation was cell-type specific as tACPD did not alter NMDA-induced swelling in pyramidal cells of the hippocampus. Importantly, these findings could be extended to in vivo preparations. Pretreatment with tACPD limited the size of lesions and associated behavioral deficits induced by intrastriatal administration of the NMDA receptor agonist quinolinic acid.

Characterization of metabotropic glutamate receptor-mediated facilitation of N-methyl-D-aspartate depolarization of neocortical neurones

1. Facilitation of the N-methyl-D-aspartate (NMDA) receptor-mediated depolarization of cortical neurones induced by metabotropic glutamate receptor (mGluR) agonists in the presence of tetrodotoxin has been examined by use of grease-gap recording. 2. Quisqualate (1-2 microM) and 10 to 100 microM 1S,3R-I-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) facilitated the NMDA-, but not the kainate-induced depolarization with an EC50 of 16 microM for 1S,3R-ACPD. The facilitation induced by quisqualate was reduced, but not blocked, by 4 microM 6-cyano-7-nitroquinoxaline-2,3-dione. 3. D,L-2-Amino-3-phosphonopropionic acid and D,L-2-amino-4-phosphonobutyric acid antagonized the 1S,3R-ACPD facilitation in a non-competitive manner with IC50 values of 0.24 microM and 4.4 microM respectively. 4. Homologous desensitization of the 1S,3R-ACPD induced facilitation was not observed. The facilitation was not altered by 10 nM staurosporine or 3 microM phorbol diacetate. 5. Substitution of 20 microM 8-bromo-cyclic adenosine monophosphate, 20 microM 8-bromo-cyclic guanosine monophosphate, or 10 microM arachidonic acid for 1S,3R-ACPD did not induce facilitation of the NMDA response. However, the 1S,3R-ACPD facilitation was potentiated by 10 mM myo-inositol and exhibited heterologous desensitization following exposure to 100 microM 5-hydroxytryptamine. 6. The 1S,3R-ACPD-induced facilitation persisted in both 10 microM nifedipine and nominally Ca(2+)-free medium and was only gradually eliminated following addition of 100 microM bis-(-o-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid in Ca(2+)-free medium. Facilitation of the NMDA response induced by carbachol, but not phenylephrine, was also observed in nominally Ca(2+)-free medium. Perfusing 50 microM bis-(-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid aminoethoxy eliminated the 1S,3R-ACPD facilitation. 7. These experiments have shown that mGluR agonists selectively facilitate the NMDA depolarization of cortical wedges, most likely by activating one or more mGluR subtypes that couple to phospholipase C. We conclude the facilitation results from a Ca(2+)-sensitive mechanism dependent on activation of phospholipase C and release of internal Ca2+. The facilitation is not contingent on activation of protein kinase C or entry of Ca2+ through nifedipine-sensitive Ca2+ channels.

Functions of ionotropic and metabotropic glutamate receptors in sensory transmission in the mammalian thalamus

The thalamic relay nuclei play a pivotal role in gating and processing sensory information en route to the cerebral cortex. The major ascending sensory afferents and the descending cortico-fugal afferents to the thalamus almost certainly use the excitatory amino acid L-glutamate as their transmitter. This paper reviews the nature of this transmission in terms of the receptor types which may be used (NMDA, AMPA, kainate and metabotropic glutamate receptors), their electrophysiological and pharmacological properties, and their differential location in the thalamus on neurones, terminals and glial elements. Whilst AMPA receptors, probably of more than one variety, are likely to mediate fast transmission in the thalamus, the contributions of NMDA receptors and metabotropic glutamate receptors to sensory responses under different stimulus conditions may be more varied. This is discussed in the context of the possible functional significance of the interplay of L-glutamate-gated currents with intrinsic membrane currents of thalamic neurones. The interaction of L-glutamate transmission with other modulators (acetylcholine, noradrenaline, serotonin, glycine, D-serine, nitric oxide, arginine, redox agents) is considered.

Dual effects of fluoroaluminate on activation of calcium influx and inhibition of agonist-induced calcium mobilization in rat glomerulosa cells

Results presented in this study demonstrate that, in rat glomerulosa cells, fluoroaluminate (AlF4-) alone stimulates both cAMP accumulation (maximal stimulation 10-fold, ED50, 24 mM) and total inositol phosphate accumulation (maximal stimulation 12-fold, ED50 14 mM). Despite a transient accumulation of Ins(1,4,5)P3 after AlF4- stimulation, no rapid and transient intracellular calcium mobilization was observed. In contrast to angiotensin II (Ang II) or vasopressin (AVP), AlF4- induces only a slow and sustained increase in intracellular Ca2+. We demonstrate that this increase results from a Ca2+ influx mediated by cAMP-protein kinase A (PKA) pathway since preincubation with H-89, a potent PKA inhibitor, inhibits this influx. Moreover, a short preincubation (15 min at 37 degrees C) of cells with AlF4- or ACTH prevents the initial release of Ca2+ from intracellular stores induced by Ang II, but does not affect the amount of InsPs accumulated under Ang II stimulation. This rapid inhibition of Ang II action is mediated by ACTH- or AlF4(-)-stimulated cAMP production since pretreatment with H-89 leads to a complete reversal. cAMP most likely acts at the level of Ins(1,4,5)P3 receptors since an increase in intracellular cAMP blunts the calcium response induced by addition of exogenous Ins(1,4,5)P3 to permeabilized cells. These results point out that, in rat glomerulosa cells, activation of the cAMP pathway can induce a rapid desensitization of the phospholipase C pathway by acting downstream of inositol phosphate accumulation.

Triiodo-L-thyronine enhances TRH-induced TSH release from perifused rat pituitaries and intracellular Ca2+ levels from dispersed pituitary cells

There is now increasing evidence that Ca2+ serves as the first messenger for the prompt and non-genomic effects of 3,5,3' triiodo-L-thyronine (T3) in several tissues. We have previously shown that the first phase of thyroid stimulating hormone (TSH) release in response to thyrotropin-releasing hormone (TRH) can be potentiated by messengers of hypothalamic origin, by a Ca(2+)-dependent phenomenon involving the activation of dihydropyridine-sensitive Ca2+ channels. By perifusing rat pituitary fragments, we have investigated whether T3 would modify TSH release when the hormone is applied for a short time (i.e. 30 min) before a 6 min pulse of physiological concentration of TRH, thus excluding the genomic effect of T3. We show that: (1) increasing concentrations of T3 (100 nM-10 microM) in the perifused medium potentiates the TRH-induced TSH release in a dose-dependent manner; (2) the T3 potentiation is not reproduced by diiodothyronine and T3 does not potentiate the increase if TSH release induced by a depolarizing concentration of KCl; (3) the protein synthesis inhibitor cycloheximide, does not significantly modify the effect of T3; (4) addition of Co2+, nifedipine, verapamil, or omega-conotoxin in the medium, at a concentration which does not modify the TSH response to TRH, reverses the T3 potentiation of that response. We also tested whether T3 would change intracellular concentrations of Ca2+, by measuring [Ca2+]i with fura-2 imaging on primary cultures of dispersed pituitary cells, either in basal conditions or after stimulation by TRH or/and T3. Both substances induced a fast increase of [Ca2+]i, with a peak at 15 s, followed by a subsequent progressive decay with TRH and a rapid return with T3. Our data suggest that T3 enhances TRH-induced TSH release by a protein synthesis-independent and Ca(2+)-dependent phenomenon, probably due to an increase in Ca2+ entry through the activation of dihydropyridine- and omega-conotoxin-sensitive Ca2+ channels. They also show that T3 may acutely enhance [Ca2+]i in pituitary cells. These findings support the idea of the occurrence of a prompt and stimulatory role of T3 at the plasma membrane level in normal rat pituitary gland.

The metabotropic glutamate receptors: structure and functions

Glutamate is the main excitatory neurotransmitter in the brain. For many years it has been considered to act only on ligand-gated receptor channels--termed NMDA, AMPA and kainate receptors--involved in the fast excitatory synaptic transmission. Recently, glutamate has been shown to regulate ion channels and enzymes producing second messengers via specific receptors coupled to G-proteins. The existence of these receptors, called metabotropic glutamate receptors, is changing our views on the functioning of fast excitatory synapses.

Involvement of metabotropic and ionotropic glutamate receptors in inositol polyphosphate formation in carp retinal slices

The contribution of ionotropic and metabotropic glutamate receptors to inositol polyphosphate accumulation in carp retinal slices was investigated using myo-[2-3H]inositol prelabelling. In the presence of the glutamate agonists quisqualate, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and trans-(+/-)-1-amino-1,3-cyclopentane-dicarboxylic acid (t-ACPD), formation of [3H]inositol phosphate was significantly increased in a dose-dependent manner, with EC50 values of 350 nM, 1.5 microM and 10 microM respectively. The complete AMPA-induced response and a large component of the quisqualate-induced response were inhibited in a competitive manner when the ionotropic antagonist 6-cyano-7-nitroquinoxalin- 2,3-dione (CNQX) was present. Furthermore, the remaining level of quisqualate-induced [3H]inositol phosphate formation closely matched that produced by ACPD alone, and coincubation of AMPA and ACPD showed additive effects, suggesting that the quisqualate-induced response resulted from coactivation of metabotropic and ionotropic glutamate receptors. The ionotropic component was partially reduced in the presence of cobalt, suggesting indirect effects resulting from synaptic interactions. We could exclude indirect effects through depolarization-induced release of other neurotransmitters. Only serotonin (EC50 1 microM) and carbachol (at a concentration of 1 mM) stimulated [3H]inositol phosphate formation, but their antagonists did not affect the quisqualate response and coactivation with quisqualate and serotonin or carbachol resulted in additive effects. The ionotropic component was completely suppressed when Ca2+ was omitted from the medium and cobalt was present. This makes it likely that the ionotropic component resulted from Ca2+ entry through AMPA-gated channels and subsequent Ca(2+)-dependent activation of phospholipase C.

Metabotropic glutamate receptors modulate N-methyl-D-aspartate receptor function in neostriatal neurons

The functional roles played by metabotropic glutamate receptors in the neostriatum is just beginning to be examined. One possibility, raised by previous studies, is that metabotropic glutamate receptors act to modulate responses mediated by ionotropic glutamate receptors. In the present study, we examined this possibility in a neostriatal brain slice preparation using intracellular recording and iontophoretic techniques. We found that the iontophoretic application of the metabotropic glutamate receptor agonist 1-amino-cyclopentane-1,3-dicarboxylic acid markedly attenuated both the amplitude and duration of excitatory responses induced by the iontophoretic application of N-methyl-D-aspartate. These inhibitory effects were stereo-selective and relatively long-lasting. The metabotropic glutamate receptor antagonist 2-amino-3-phosphonopropionic acid applied either iontophoretically or in the bath prevented the inhibitory effects of 1-amino-cyclopentane-1,3-dicarboxylic acid. The inhibitory action of 1-amino-cyclopentane-1,3-dicarboxylic acid was specific to N-methyl-D-aspartate, as 1-amino-cyclopentane-1,3-dicarboxylic acid had no consistent action on the responses evoked by the iontophoretic application of glutamate, amino-3-hydroxy-5-methyl-4-isoazolepropionic acid or quisqualate. Bath application of 1-amino-cyclopentane-1,3-dicarboxylic acid inhibited the evoked depolarizing postsynaptic potentials recorded in neostriatal cells. Thus, activation of metabotropic glutamate receptors may play an important role in modulating N-methyl-D-aspartate receptor function in neostriatal neurons.

Requirement of metabotropic glutamate receptors for the generation of inflammation-evoked hyperexcitability in rat spinal cord neurons

In the central nervous system the transmitter L-glutamate activates both ionotropic receptors coupled to cation channels and metabotropic receptors coupled to G-proteins. The role of metabotropic receptors in the processing of mechanosensory and nociceptive information was studied in a subset of spinal cord neurons with afferent input from the knee joint in anaesthetized rats using electrophysiological methods. The ionophoretic administration of L-2-amino-3-phosphonopropionic acid (L-AP3), an antagonist at the metabotropic receptor, had no effect on the responses to innocuous and noxious pressure applied to the normal knee joint, although the antagonist prevented the potentiation of these responses evoked by the ionophoretic administration of a specific agonist at the metabotropic receptor, trans-(+/-)-1-amino-(1S,3R)-cyclopentane-dicarboxylic acid (t-ACPD). By contrast, in neurons that were rendered hyperexcitable by acute inflammation in the knee joint L-AP3 reduced the responses to pressure applied to the knee. When L-Ap3 was applied during induction of inflammation and throughout the subsequent 1.5 h the spinal neurons did not develop hyperexcitability over this time period. L-AP3 did not impair the activation of ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA receptors by the specific agonists. We conclude that spinal metabotropic glutamate receptors are not involved in the mediation of responses to innocuous and noxious mechanical stimuli applied under normal conditions. They are required, however, for the generation of inflammation-evoked hyperexcitability of spinal cord neurons, a form of functional plasticity underlying the painfulness in pathophysiological conditions such as inflammation.

Metabotropic glutamate receptors are involved in the control of breathing at the medulla oblongata level of anaesthetized rats

The goal of the present study was to identify sites in the medulla oblongata where metabotropic glutamate receptors are involved in regulating respiration. Unilateral microinjections (50 nl) of L-glutamate (L-glu) (10-25-50 mM) into the nucleus tractus solitarii (NTS) of anaesthetized rats elicited apnea (8.6 +/- 0.3 sec; 21.3 +/- 3.6 sec; 66.3 +/- 16.5 sec respectively; N = 6) and arterial hypotension (7.3 +/- 2.4 mmHg; 10.1 +/- 2.3 mmHg; 35.3 +/- 7.5 mmHg respectively; N = 6). Similarly, in other rats 1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD) (1-5-10 mM), a selective agonist of metabotrophic glutamate receptors, also induced apnea (22.4 +/- 2.5 sec; 32.5 +/- sec; 92.5 +/- 1.4 sec respectively; N = 6) and arterial hypotension (12.7 +/- 2.2 mmHg; 19.6 +/- 4.3 mmHg; 26.5 +/- 1.5 mmHg respectively; N = 6). Paired experiments showed that unilateral microinjections of L-glu (50 mM) and ACPD (1 mM) into the nucleus retroambigualis (NRA) of anaesthetized rats elicited apnea (20.2 +/- 2.6 sec and 33.8 +/- 3.2 sec respectively; N = 6) and arterial hypotension (15.7 +/- 3.7 mmHg and 22.5 +/- 4.5 mmHg respectively; N = 6). The ACPD effects on apnea and hypotension in NTS and NRA were not prevented by a 3 min pretreatment with L-AP3 (30 mM), a putative antagonist of metabotropic glutamate receptors (19.5 +/- 1.4 sec; 12.3 +/- 3.2 mmHg and 30.6 +/- 2.9 sec; 23.4 +/- 3.8 mmHg respectively; N = 6). These data suggest that metabotropic glutamate receptors are involved in NTS and NRA regulation of cardiorespiratory functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acids (EAAs) stimulate phosphatidylinositol turnover in adult rat striatal slices: interaction between NMDA and EAA metabotropic receptors

The effect of excitatory amino acids (EAAs) on phosphatidylinositol (PI) turnover in adult rat striatal slices was investigated. Quisqualic acid (QA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainic acid (KA), ibotenic acid (IBO) and N-methyl-D-aspartic acid (NMDA) maximally increased inositol phosphate (IP) formation at 10 microM while trans-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) was maximally effective at 100 microM. The NMDA channel blocker dizolcipine (MK-801) counteracted the effect of NMDA 10 microM and IBO 10 microM while it potentiated that of IBO 100 microM and IBO 1000 microM. Conversely, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) prevented the effect of AMPA and KA and reduced that of QA (all at 10 microM). Lowering extracellular Ca2+ concentrations ([Ca2+]0) differentially affected the PI response to EAAs. The ACPD 30 microM effect was unchanged at low [Ca2+]0 (but abolished when EGTA 2 mM was added), while that of ACPD 100 microM was halved in 0.1 mM and almost abolished in a nominally free Ca2+ medium. NMDA 10 microM and AMPA 10 microM were ineffective at low [Ca2+]0 while NMDA 100 microM, ineffective in a 1.2 mM Ca2+ medium, strongly stimulated IP formation in 0.1 mM Ca2+ but not in a nominally free Ca2+ medium. The effect of NMDA on EAA metabotropic receptor agonist stimulated PI turnover was also studied. NMDA 10 microM potentiated the effect of ACPD 30 microM. This positive cooperation persisted at low [Ca2+]0 but not in the presence of EGTA. Conversely, NMDA 100 microM prevented the effect of ACPD 100 microM. This negative interference was reversed when Ca2+ was omitted from the medium. This study shows that in the adult rat striatum both EAA metabotropic and ionotropic receptor activation increases IP formation. A positive and negative interaction between NMDA and metabotropic receptor activation was also found to regulate PI turnover. The role of [Ca2+]0 in subserving the PI response to EAAs was made evident.

Rapid desensitization of the metabotropic glutamate receptor that facilitates glutamate release in rat cerebrocortical nerve terminals

The metabotropic autoreceptor of glutamatergic nerve terminals from the cerebral cortex of adult rats has been characterized. Receptor activation involves a rapid and transient increase in diacylglycerol, which is sensitive to L-2-amino-3-phosphonopropionate (L-AP3) and L-2-amino-4-phosphonobutanoic acid (L-AP4) and is partially blocked by pertussis toxin. Protein kinase C (PKC) has a negative feedback control in this transduction pathway because the activation of the kinase, either by phorbol esters or by the endogenous diacylglycerol produced by the receptor, results in a reversible receptor desensitization, with loss of the ability to further facilitate glutamate release. It is concluded that the facilitatory metabotropic receptor located at the glutamatergic nerve endings belongs to the subclass coupled to phosphoinositide hydrolysis and that the rapid and use-dependent desensitization of the facilitatory pathway may underlie a mechanism to prevent its permanent activation and thereby to avoid neurotoxicity.

Metabotropic glutamate receptors: an original family of G protein-coupled receptors

In 1985, we discovered a new glutamate receptor which was coupled to phospholipase C via a G protein and which was later termed metabotropic glutamate receptor (mGluR). In this review, both the diversity of mGluRs and the cellular events they control are discussed, as well as their roles in physiological regulation and brain function.

Excitatory amino acid-induced phosphoinositide hydrolysis in Müller glia

The presence of excitatory amino acid (EAA) receptors coupled to phosphoinositide metabolism in primary cultures of Müller (glial) cells from the chick retina was established. The order of potency of analogues for stimulating [3H]inositol phosphate (IP) accumulation was quisqualate (QA) > L-glutamate (L-Glu) = kainate (KA) > N-methyl-D-aspartate (NMDA) > L-aspartate (L-Asp) with EC50 in the range of 1-100 microM. 1-Aminocyclopentane-1,3-dicarboxylate (trans-ACPD), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), 2-amino-3-phosphonopropionate (AP3), and 2-amino-4-phosphonobutyrate (AP4) showed no effect either on basal concentration or on stimulated accumulation of [3H]IPs. The effect of EAA was potently inhibited by the ionotropic NMDA receptor antagonists 2-amino-5-phosphonopentanoate (AP5), 3-[(RS)-2-carboxy-piperazin-4-yl)]-propyl-1-phosphonate (CPP), and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-10- imine (MK-801); L-Glu antagonists at non-NMDA receptors, the quinoxalines NBQX and DNQX, inhibited weakly the response to L-Glu, KA, and NMDA, and more potently that to QA. The translocation of protein kinase C was also stimulated by EAA with the same pharmacological profile, and was partially inhibited by kynurenate (KYN). L-Glu and KA induced 45Ca2+ influx, which was decreased by KYN and CNQX. EAA-induced [3H]IPs accumulation was decreased by verapamil but not by nifedipine, and slightly diminished by dantrolene. Results demonstrate that EAA-induced phosphoinositide hydrolysis in Müller cells shows pharmacological differences with that in astrocytes and neuronal cells and could be triggered by a different mechanism.

Alterations of inositol phosphate turnover in striatum of aged rats

Age-related inositol phosphate turnover in the rat central nervous system was investigated. Higher phospholipase-C activity and drastically higher (almost 2.5-fold) inositol 1,4,5-trisphosphate concentration in the corpus striatum (caudate-putamen) of extremely old (approximately 40 months) female Wistar rats in comparison to the young adult (approximately 3.5 months) rats were observed. Dopamine seems to slightly inhibit total inositol phosphate formation and this effect was antagonized by (-)-sulpiride.

A pharmacological characterization of the mGluR1 alpha subtype of the metabotropic glutamate receptor expressed in a cloned baby hamster kidney cell line

The pharmacological specificity of the mGluR1 alpha subtype of the metabotropic glutamate receptor (mGluR) was examined in a cloned baby hamster kidney cell line (BHK-ts13) measuring [3H]glutamate binding and inositol phosphate (PI) hydrolysis. PI-hydrolysis was maximally stimulated by quisqualate (1112 +/- 105% of basal), glutamate (1061 +/- 70% of basal), ibotenate (1097 +/- 115% of basal) and beta-N-methylamino-L-alanine (BMAA) (1010 +/- 104% of basal). In contrast, the maximal stimulation of PI-hydrolysis by (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (t-ACPD) was only 673 +/- 78% of the basal level. The relative order of potency was quisqualate > glutamate > ibotenate > t-ACPD > BMAA. Agonist-stimulated PI-hydrolysis was attenuated (25 +/- 4% inhibition) by L-2-amino-3-phosphonopropionic acid and partially blocked (44 +/- 7%) by pertussis toxin treatment. Saturation binding studies with [3H]glutamate on membranes prepared from BHK-ts13 cells expressing the mGluR1 alpha subtype showed that glutamate binds to a single affinity state of this receptor with a limited capacity (Kd = 296 nM, Bmax = 0.8 pmol/mg protein). In competition experiments, [3H]glutamate was displaced by quisqualate, glutamate, ibotenate, t-ACPD and BMAA with a rank order of potency similar to that found for stimulation of PI-hydrolysis.

Attenuation of excitatory amino acid toxicity by metabotropic glutamate receptor agonists and aniracetam in primary cultures of cerebellar granule cells

Activation of glutamate ionotropic receptors represents the primary event in the neurotoxicity process triggered by excitatory amino acids. We demonstrate here that the concentration-dependent stimulation of metabotropic glutamate receptor (mGluR) by the selective agonist trans-1-aminocyclopentane-1,3-dicarboxylate or by quisqualate counteracts both glutamate- and kainate-induced neurotoxicity in primary cultures of rat cerebellar granule cells. The mGluR-evoked responses are potentiated by aniracetam, which per se also elicits neuroprotection. Aniracetam concentration-dependently counteracted glutamate-, kainate-, or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced cell death and greatly facilitated neuroprotective response achieved by different concentrations of both quisqualate and trans-1-aminocyclopentane-1,3-dicarboxylate. In addition, aniracetam potentiated the mGluR-coupled stimulation of phospholipase C, as revealed by the measurement of 3H-inositol phosphate formation. Thus, mGluRs could be a suitable target for novel pharmacological strategies pointing to the treatment of neurodegenerative diseases.

A comparison of two alternatively spliced forms of a metabotropic glutamate receptor coupled to phosphoinositide turnover

A comparison of the pharmacological and physiological properties of the metabotropic glutamate 1 alpha and 1 beta receptors (mGluR1 alpha and mGluR1 beta) expressed in baby hamster kidney (BHK 570) cells was performed. The mGluR1 beta receptor is an alternatively spliced form of mGluR1 alpha with a modified carboxy terminus. Immunoblots of membranes from the two cell lines probed with receptor-specific antipeptide antibodies showed that mGluR1 alpha migrated with an M(r) = 154,000, whereas mGluR1 beta migrated with an M(r) = 96,000. Immunofluorescence imaging of receptors expressed in BHK 570 cells revealed that the mGluR1 alpha receptor was localized to patches along the plasmalemma and on intracellular membranes surrounding the nucleus, whereas mGluR1 beta was distributed diffusely throughout the cell. Agonist activation of the mGluR1 alpha and the mGluR1 beta receptors stimulated phosphoinositide hydrolysis. At both receptors, glutamate, quisqualate, and ibotenate were full agonists, whereas trans-(+)-1-aminocyclopentane-1,3-dicarboxylate appeared to act as a partial agonist. The stimulation of phosphoinositide hydrolysis by mGluR1 alpha showed pertussis toxin-sensitive and insensitive components, whereas the mGluR1 beta response displayed only the toxin-insensitive component. The mGluR1 alpha and mGluR1 beta receptors also increased intracellular calcium levels by inducing release from intracellular stores. These results indicate that the different carboxy terminal sequences of the two receptors directly influences G protein coupling and subcellular deposition of the receptor polypeptides and suggest that the two receptors may subserve different roles in the nervous system.

Excitatory amino acid receptor-stimulated phosphoinositide turnover in primary cerebrocortical cultures

1. Characterization of excitatory amino acid-induced accumulation of [3H]-phosphoinositides was carried out in primary cerebrocortical cultures isolated from foetal rats. 2. All of the excitatory amino acid receptor agonists examined caused concentration-dependent enhancement of phosphoinositide (PI) formation. The most potent excitatory amino acid receptor agonists were quisqualate, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD), ibotenate and glutamate with mean EC50 values of 0.9 +/- 0.4 microM, 15 +/- 5 microM, 15 +/- 3 microM and 41 +/- 8 microM respectively. 3. The selective ionotropic receptor antagonists kynurenic acid (1 mM), 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX, 10 microM) and (+/-)-4-(3-phosphonopropyl)-2 piperazinecarboxylic acid (CPP, 100 microM), failed to block responses to quisqualate, (1S,3R)-ACPD or glutamate. D,L-2-Amino-3-phosphonopropionate (D,L-AP3) did not block 1S,3R-ACPD or quisqualate-induced PI turnover, but had an additive effect with quisqualate or (1S,3R)-ACPD. 4. Exposure of cultures to agonists in the absence of added extracellular calcium reduced the maximal quisqualate response by approximately 45%, revealing a two-component concentration-response curve. Concentration-response curves to ibotenate and glutamate became flattened by omission of extracellular calcium, whereas (1S,3R)-ACPD-stimulated PI turnover was unaffected. 5. Pretreatment of cultures with pertussis toxin markedly inhibited PI responses evoked by (1S,3R)-ACPD. 6. These results suggest that excitatory amino acid-stimulated PI turnover in cerebrocortical cultures is independent of ionotropic receptor activation and is mediated via specific G-protein-linked metabotropic receptors. The partial dependence of the responses to quisqualate, ibotenate and glutamate on the presence of extracellular calcium suggests that the effects of these agonists may be mediated by more than one receptor subtype.

Characterization of the metabotropic glutamate receptor in mouse cerebellar granule cells: lack of effect of 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)-quinoxaline (NBQX)

The ability of excitatory amino acids to stimulate phosphoinositide hydrolysis in mouse cerebellar granule cells was characterized. Quisqualic acid (EC50 = 2 microM), ibotenic acid (EC50 = 15 microM), kainic acid (EC50 = 30 microM), glutamate (EC50 = 51 microM) and (1S,3R)-1-amino-cyclo-pentane-1,3-dicarboxylic acid (t-ACPD) (EC50 = 175 microM) dose-dependently stimulated phosphoinositide hydrolysis. The stimulation of phosphoinositide hydrolysis was dose-dependently blocked by 2-amino-3-phosphonopropionic acid (L-AP3) and pertussis toxin, but was unaffected by other excitatory amino acid agonists or antagonists. These data suggest that the pharmacology of excitatory amino acid-stimulated phosphoinositide hydrolysis in the mouse cerebellar granule cells is mediated through the G protein coupled metabotropic glutamate receptor. The overall pharmacology of the metabotropic receptor present in mouse cerebellar granule cells differs from that of previously reported tissue preparations such as rat cerebellar granule cells. In addition, the effect of the alpha-amino-3-hydroxyl-5-methyl-1-isoxazole-4-propionic acid (AMPA) receptor antagonist, 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)quinoxaline (NBQX), on excitatory amino acid-stimulated phosphoinositide hydrolysis was also examined. NBQX was without effect on either basal phosphoinositide hydrolysis or excitatory amino acid-stimulated phosphoinositide hydrolysis, suggesting that the neuroprotective effect of NBQX is not mediated through the metabotropic glutamate receptor.

Glutamate receptor agonists enhance the expression of BDNF mRNA in cultured cerebellar granule cells

The influence of glutamate and its analogues on the expression of BDNF mRNA was studied in cultured cerebellar granule cells. Four-hour exposure of the neurons to the glutamate receptor agonists, quisqualate, kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA), increased levels of BDNF mRNA. Glutamate in combination with antagonists of the ionotropic glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), D-2-amino-5-phosphonovalerate (AP-5) and/or (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine hydrogen maleate (MK-801), also increased levels of BDNF mRNA. However, the addition of glutamate itself to the cultures produced severe neuronal death and failed to increase the mRNA level. The onset of the increase in BDNF mRNA by kainate and NMDA lagged behind that by quisqualate. These results indicate that the non-ionotropic glutamate receptor might be involved in the induction of BDNF mRNA. Quisqualate is known to be a potent agonist of both the AMPA/kainate receptor and the metabotropic glutamate receptor. The specific antagonists of the AMPA/kainate receptor, CNQX and 6,7-dinitroquinoxaline-2,3-dione (DNQX) failed to block the increase of BDNF mRNA by quisqualate. Moreover, the desensitization of the metabotropic glutamate receptor by phorbol ester abolished the increase of BDNF mRNA by quisqualate. These results suggest that stimulation of the metabotropic glutamate receptor may be the most predominant component to increase BDNF mRNA in cerebellar granule cell culture.

Pharmacological characterization of metabotropic glutamate receptors in cultured cerebellar granule cells

A detailed pharmacological characterization of metabotropic glutamate receptors (mGluR) was performed in primary cultures of cerebellar granule cells at 6 days in vitro (DIV). The rank order of agonists induced polyphosphoinositide (PPI) hydrolysis (after correcting for the ionotropic component in the response) was as follows: in terms of efficiency, Glu > quisqualate (quis) = ibotenate (ibo) > (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) > beta-methyl-amino-L-alanine (BMAA) and in terms of potency, quis > ACPD > Glu > ibo = BMAA. Ionotropic excitatory amino acid (EAA) receptor agonists, such as alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) were relatively inactive (in the presence of Mg2+). Quis and ACPD-induced PPI hydrolysis was unaffected by ionotropic Glu receptor antagonists, but was inhibited, in part by L-2-amino-3-phosphonopropionate (AP3). In contrast, Glu-or ibo- induced PPI hydrolysis was reduced, in part, by both AP3 and NMDA receptor antagonists. Characteristic interactions involving different transmitter receptors were noted. PPI hydrolysis evoked by quis and 1S,3R-ACPD was not additive. In contrast, PPI hydrolysis stimulated by quis/ACPD and carbamylcholine was additive (indicating different receptors/transduction pathways). In the presence of Mg2+, the metabotropic response to quis/AMPA and NMDA was synergistic (this being consistent with AMPA receptor-induced depolarization activating NMDA receptor). On the other hand, in Mg(2+)-free buffer the effects of quis and NMDA, at concentrations causing maximal PPI hydrolysis, were additive (indicating that PPI hydrolysis was effected by two different mechanisms).(ABSTRACT TRUNCATED AT 250 WORDS)

(R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors mediate a calcium-dependent inhibition of the metabotropic glutamate receptor-stimulated formation of inositol 1,4,5-trisphosphate

L-glutamate (3-1,000 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 10-1,000 microM), a selective agonist for the metabotropic glutamate receptor, stimulated the formation of inositol 1,4,5-trisphosphate in a concentration-dependent manner. L-Glutamate was half as efficacious as 1S,3R-ACPD. N-methyl-D-aspartate (NMDA; 1 nM to 1 mM) did not significantly influence the response to a maximally effective concentration of 1S,3R-ACPD (100 microM). On the other hand, coapplication of (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 1-300 nM) produced a concentration- and time-dependent inhibition of the 1S,3R-ACPD effect, with a maximal inhibition (97%) at 100 nM. Ten micromolar 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the AMPA receptor, blocked the inhibitory effect of AMPA. Reduced extracellular calcium concentration, as well as 10 microM nimodipine, an L-type calcium channel antagonist, inhibited the AMPA influence on the 1S,3R-ACPD response. W-7, a calcium/calmodulin antagonist, prevented the inhibition by AMPA, whereas H-7, an inhibitor of protein kinase C, had no effect. These data suggest that activation of AMPA receptors has an inhibitory influence on inositol 1,4,5-trisphosphate formation mediated by stimulation of the metabotropic glutamate receptor. The mechanism of action involves calcium influx through L-type type calcium channels and possible activation of calcium/calmodulin-dependent enzymes.

Glutamate and quisqualate regulate expression of metabotropic glutamate receptor mRNA in cultured cerebellar granule cells

Metabotropic glutamate receptor (type 1; mGluR1) is expressed predominantly in the hippocampus and the cerebellum. Using cultured cerebellar granule cells, we investigated the regulation of the mGluR1 mRNA expression. Levels of mGluR1 mRNA were decreased to less than half by high potassium stimulation and by glutamate and quisqualate. Although these glutamate receptor agonists tested are also known to cause neuronal cell death in culture, the effect of cell death cannot explain the observed reduction in mGluR1 mRNA because of the following reasons: (a) antagonists of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors inhibited cell death, but not the reduction of the level of mGluR1 mRNA; (b) mGluR1 mRNA returned to its initial level 48 h after the agonist application; and (c) the mRNA level of one of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate receptors (GluR1) was not altered by these conditions. Therefore, we conclude that the glutamate or quisqualate stimulation can specifically inhibit the expression of mGluR1 mRNA. The dose response of quisqualate for the reduction in mGluR1 mRNA is consistent with that for inositol phosphate formation stimulated through the cloned mGluR1. The mRNA reduction did not require extracellular calcium. Desensitization of mGluR1 with phorbol ester abolished the mRNA reduction. These results suggest that the reduction in mGluR1 mRNA is mediated by the activation of the metabotropic receptor itself.

Metabotropic glutamate receptors in brain function and pathology

Metabotropic glutamate receptors (mGluRs) are a novel family of recently cloned G protein-coupled receptors. These receptors are heterogeneous and coupled to multiple second messenger systems that include increases in phosphoinositide hydrolysis, activation of phospholipase D, decreases in cAMP formation, increases in cAMP formation, and changes in ion channel function. Using the selective mGluR agonist 1-aminocyclopentane-1,3-dicarboxylic acid (1s,3R-ACPD), considerable progress has been made towards understanding the role of this glutamate receptor class in the central nervous system. This article reviews the molecular aspects and pharmacology of mGluRs, and recent studies elucidating their role in brain function and pathology.

Characterization of a metabotropic glutamate receptor: direct negative coupling to adenylyl cyclase and involvement of a pertussis toxin-sensitive G protein

We have characterized a G-protein-coupled glutamate receptor in primary cultures of striatal neurons. Glutamate, quisqualate, or trans-1-aminocyclopentane-1,3-dicarboxylate inhibited by 30-40% either forskolin-stimulated cAMP production in intact cells or forskolin plus vasoactive intestinal peptide-activated adenylyl cyclase assayed in neuronal membrane preparations. These inhibitory effects were suppressed after treatment of striatal neurons with Bordetella pertussis toxin, suggesting the involvement of a heterotrimeric guanine nucleotide-binding protein (G protein) of the G(i)/G(o) subtype. The pharmacological profile of this glutamate receptor negatively coupled to adenylyl cyclase was different from that of the metabotropic Qp glutamate receptor coupled to phospholipase C in striatal neurons and from that of the recently cloned "mGluR2" glutamate receptor, which is negatively coupled to adenylyl cyclase when expressed in non-neuronal cells.

L-aspartate-beta-hydroxamate exhibits mixed agonist/antagonist activity at the glutamate metabotropic receptor in rat neonatal cerebrocortial slices

L-aspartate-beta-hydroxamate, a glutamate uptake inhibitor, was investigated for activity at a glutamate metabotropic receptor (mGluR) in neonatal rat cerebral cortical slices. Stimulation of phosphatidylinositol hydrolysis by 100 microM (1S,3R)-ACPD was inhibited only very weakly, to a maximal extent of 28%, L-aspartate-beta-hydroxamate did however exhibit agonist activity (EC50 = 760 microM) and, although much less potent than (1S,3R)-ACPD (EC50 = 20 microM), its efficacy was approximately 70% of the latter. These results indicate that, at least in this preparation, offspartate-beta-hydroxamate is of little value as an antagonist at the mGluR receptor.

Metabotropic glutamate receptors are required for the induction of long-term potentiation

Recent observations have led to the suggestion that the metabotropic glutamate receptor may play a role in the induction or maintenance of long-term potentiation (LTP). However, experimental evidence supporting a role for this receptor in the induction of LTP is still inconclusive and controversial. Here we report that, in rat dorsolateral septal nucleus (DLSN) neurons, which have the highest density of metabotropic receptors and show functional responses, the induction of LTP is not blocked by the NMDA receptor antagonist 2-amino-5-phosphonovalerate, but is blocked by two putative metabotropic glutamate receptor antagonists, L-2-amino-3-phosphonopropionic acid and L-2-amino-4-phosphonobutyrate. Furthermore, superfusion of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, a selective metabotropic glutamate agonist, resulted in a long-lasting potentiation of synaptic transmission similar to that induced by tetanic stimuli. Our results demonstrated that activation of postsynaptic metabotropic receptors is both necessary and sufficient for the induction of LTP in the DLSN, and we suggest that such a mechanism may be important at other CNS synapses.

Chronic lithium treatment impairs phosphatidylinositol hydrolysis in membranes from rat brain regions

Membranes prepared from rat brain regions were used to measure the receptor-coupled and/or guanine nucleotide-binding protein (G protein)-mediated hydrolysis of exogenous [3H]phosphatidylinositol ([3H]PI). Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and NaF (in the presence of AlCl3) caused concentration-dependent stimulations of [3H]PI hydrolysis, supporting the conclusion that G proteins mediating [3H]PI hydrolysis can be activated in this preparation. Neither of these responses was altered by in vitro incubation with 8 mM LiCl, but both were reduced in hippocampal, striatal, and cortical membranes from rats that had been treated with lithium for 4 weeks compared with controls. Two cholinergic agonists, carbachol and pilocarpine, induced no hydrolysis of [3H]PI unless GTP gamma S was also present, in which case each equally stimulated [3H]PI hydrolysis above that obtained with GTP gamma S alone. In the presence of GTP gamma S several excitatory amino acid agonists stimulated [3H]PI hydrolysis to an extent similar to that of carbachol. After chronic lithium treatment, [3H]PI hydrolysis stimulated by carbachol was significantly attenuated, but the response to quisqualate was unaffected. Therefore, lithium added in vitro does not have an effect on cholinergic receptor- or G protein-mediated [3H]PI hydrolysis, but each of these is reduced by chronic lithium treatment. Because exogenous [3H]PI was provided as the substrate, it is evident that the inhibitory effect of chronic lithium treatment cannot be due to substrate depletion. Impaired function of G proteins appears to be the most likely mechanism accounting for attenuated [3H]PI hydrolysis after chronic administration of lithium.

Nitric oxide-induced blockade of NMDA receptors

We studied the effects of nitric oxide (NO)-producing agents on N-methyl-D-aspartate (NMDA) receptor activation in cultured neurons. 3-Morpholino-sydnonimine (SIN-1) blocked both NMDA-induced currents and the associated increase in intracellular Ca2+. The actions of SIN-1 were reversible and suppressed by hemoglobin. A degraded SIN-1 solution that did not release NO was unable to block NMDA receptors. This showed that the SIN-1 effects were due to NO and not to another breakdown product. Similar results were obtained with 1-nitrosopyrrolidine (an NO-containing drug) and with NO released from NaNO2. Pretreatment with hemoglobin potentiated NMDA-induced effects, demonstrating that endogenous NO modulates NMDA receptors. Since NMDA receptor activation induces NO synthesis, these results suggest a feedback inhibition of NMDA receptors by NO under physiological condition.