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Activation of Phospholipases C and D by the Novel Metabotropic Glutamate Receptor Agonist tADA
Neuropharmacology,Vol. 36, No. 2, pp. 261-263, 1997 Copyright01997 Elsevier ScienceLtd. All rights reserved Printed in Great Britain 0028-3908/97$17.00 + 0.00
Pergamon PII: S0028-3908(96)00174-8
Rapid Communication Activation of Phospholipases C and D by the Novel Metabotropic Glutamate Receptor Agonist tADA J. KLEIN,l* K. G. REYMANN2and G. RIEDEL3 IDepartment of Pharmacology, University of Mainz, Obere Zahlbacher Str. 67, D-55101 Mainz, Germany; 2Department of Neurophysiology, Federal Institute of Neurobiology, P.O. Box 1860, D-39008 Magdeburg, Germany and 3Department of Psychology, University of York Heslington, York YO1 5DD, U.K. (Accepted 24 October 1996)
Summary-In hippocampalslicestakenfrom8-day-oldrats,trans-azetidine-2,4-dicarboxylic acid(tADA),a novelglutamatergicagonistactingpreferentiallyat classI mGluRreceptors,activatesphosphoinositide and phosphatidylcholine hydrolysiswithwidelydifferentpotencies.Inositolphosphateformationwasmaximally increasedat 10~M tADA (EC50: 1.2PM), while phospholipaseD activationwas observedat a tADA concentrationof 1 mM.Thisis thefirstreportof a tADA-induced phospholipase D activity.@ 1997Elsevier ScienceLtd.All rightsreserved. Keywords-Metabotropicglutamatereceptors(mGluRs),trans-azetidine-2,4-dicarboxylic acid (tADA), phospholipaseC, phospholipase D, hippocampalslice.
mGluR5a-transfectedthan in mGluRla-transfected cells (Manahan-Vaughan et al., 1996). It must be noted, however, that tADA was ineffective in transected cell lines expressingthe human mGluRlb or 5a subtypes,but acted as an agonist at hmGluR2 in this study (Knopfel et al., 1995). Class I metabotropicglutamate receptor agonists lead to the formation of inositol phosphates and diacylglycerol, increase intracellular calcium and activate classical protein kinase C (cPKC) isoforms. In the adult hippocampus,proteinkinase C activationhas been linked to the phenomenonof long-termpotentiation(LTP), and metabotropic agonists including tADA were found to evoke a form of LTP in the absence of NMDA receptor stimulation (see Manahan-Vaughan et al. (1996) and references cited therein). However, recent research has demonstrated that the receptor-mediated hydrolysis of choline-containing phospholipids by phospholipase D (PLD) is an alternative source for the formation of diacylglycerol required for PKC activation. The PLDcatalysed hydrolysisof phosphatidylcholineyields phosphatidic acid, a possible activator of the atypical form PKC (. Moreover, phosphatidic acid is further hydrolyzed to diacylglycerol,and this pathway may eontribute *Towhomcorrespondenceshouldbe addressed.Fax:(0049) to the activation of the DAG-dependent, calciumindependent PKC il and e (for review, see Klein et al., (6131)176611.
Metabotropicglutamate receptors (mGluRs) are a family of G-protein coupled receptors which have been implicated in a variety of physiological functions and possible pharmacological applications (for review, see Pin and Duvoisin (1995)). In the search for subtypespecific agents, trans-azetidine-2,4-dicarboxylic acid (WA) has recently been suggested as a selective agonist at group I mGluRs which include the mGluRl and 5 subtypes.Both of these subtypesare coupled to the phosphoinositide-specificphospholipase C (PLC), and increases of inositol phosphates have been described in responseto tADA in rat cerebella granule cells (Favaron et al., 1993), murine cortical neurones (Bruno et al., 1995) and hippocampal slices taken from adult rats (Manahan-Vaughan et al., 1996). Several findings suggest that tADA may act preferentiallyat the mGluR5 subtype. Thus, tADA was ineffective in mGluRltransfectedHEK293 cells (Favaron et al., 1993),whereas tADA-responsive murine cortical neurons have been found to be rich in mGluR5 but express little mGluRl protein (Bruno et al., 1995).Moreover,tADA displayeda higher potency for inosito] phosphate formation in
261
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262
prepared from the dissected hippocampi and superfused with Krebs-Henseleit buffer (KHB) at 35”C. The 1 T solution was gassed with 9570 oxygen and 570 carbon dioxide and had the following composition (mM): Na+ 143.0; K+ 7.1; Ca2+1.3; Mg2+1.2; Cl– 125.3; HPO~– 1.2; SO:– 1.2; HCOj_ 25.0 and glucose 11.7. For the measurementof inositol phosphate formation, the tissue was incubated with 40 pCi of [3H]myo-inositolin 5 ml KHB for 2 hr. Subsequently,the slices were superfused with KHB containing10 mM lithiumchloridefor 20 min. tADA was added to the superfusion medium, and the slices were superfused for a further 10 min in the ADA ADA ADA @ continuous presence of lithium. After homogenization, Ctr 1 PM 10 IJM 100 pM lnivl the aqueousphasewas transferredto a column containing BioRad AG l-X8 anion exchange resin, the individual inositol phosphateswere eluted by the stepwise addition of solutions containing increasing formiate concentrations and countedfor radioactivityin a liquidscintillation counter. For the determination of phospholipase D activity, the phospholipidswere labelled by incubation of the tissue with 40 flCi [3H]glycerolin 5 ml KHB for 2 hr. After a 30 min wash, drugs were added to the superfusionmediumin the presence of propanol (2?io)for 10 min. At the end of the incubation, the slices were homogenized in 20 volumes of chloroform/methanol (2:1), and two phases were obtained by adding four volumesof 0.1 M KC1solution.The major phospholipids were separated by TLC (solvent: upper phase of ethylacetate/isooctane/aceticacid/water, 13:2:3:10),and ADA ADA ADA @l the radioactivity associated with [3H]phosphatidylCtr o.lm?d 0.3mM Imhd ltnhl propanol (PtdPro) was determined and expressed as Fig.1.Influenceof tADAon theactivityof (A)phospholipase percentage of the radioactivitypresent in the lipid phase. C and(B)phospholipase D in hippocampalslicestakenfrom8- Data are presented as means+ SEM of 4-6 experiments. day-oldrats.Forthedetermination ofphospholipase C activity, Statistical significancewas evaluated by repeated measlicesprelabelledwith[3H]inositol weresuperfusedfor 10min sures analysis of variance (ANOVA). in the presenceof tADA,homogenizedand the radioactivity The resultsof the inositolphosphatedeterminationsare associatedwith total inositol phosphates[Z (1P’s)]was measuredand expressedas percentageof total incorporated shown in Fig. l(A). In the immature slices used in the radioactivity.For the determinationof phospholipaseD present experiments, individual inositol phosphates are activity,slicesprelabelledwith [3H]glycerol were superfused formed in the approximate ratio (IP3:IP2:IP1) of 8:4:1; for 10min in the presenceof 2% propanol,tADA andlor tADA treatmentdid not change this pattern, so the data in glutamate,homogenizedand the radioactivityassociatedwith Fig. l(A) are expressed as total inositol phosphate [3H]phosphatidylpropanol IPtdPro]was measured and ex- formation. On the background of a relatively high basal pressedas percentageof the total incorporatedradioactivity phospholipaseC activity in immature slices, the addition in the lipid phase. The data are means+ SEM of ~ of tADA led to a concentration-dependentincrease of determinations.Statistical significancewas evaluated by total inositol phosphate formation (Fig. l(A)). In these ANOVA(paireddata).“p c 0.05;**P<0.01. experiments,tADA displayeda remarkablyhigh potency. Maximal PLC activationwas observed at 10PM tADA, and the EC50 value was calculated as 1.2 PM. In mature 1995). In our previous work, we had reported that slices, concentrationsof >100 PM tADA were required glutamate, acting via metabotropic receptors, was cap- for PLC activation(Manahan-Vaughanet al., 1996).The able of activating PLD in hippocampal slices, and this maximum effect of tADA (167 t 23Y0of control at activation was most pronounced in immature tissue 100 PM) was higher than that observed in mature slices (Holler et al., 1993). We therefore compared the effects reported previously (133 ~ 270; Manahan-Vaughan et of the novel mGluR agonist tADA on phospholipaseC al., 1996). For comparison, the endogenous agonist and D activitiesin hippocampalslices taken from 8-day- glutamate (1 mM) increased inositol phosphates in immature slices to 192.6 t 13% (Fig. l(A)). old rats. In separate experiments, tADA was also found to Sprague–Dawleyrats (7–9 days of age) were killed by activate phospholipase D activity as determined by the decapitation,hippocampal slices (400 ~m) were quickly A
20
* —-
*
RapidCommunication transphosphatidylation assay (Fig. l(B)). Maximum stimulation of PLD activity of 153 & 12% (p< 0.05; IV= 6) was observed at 1 mM tADA, an effect that was comparable to the extent of PLC stimulation.However, concentrations below 1 mM tADA (100 and 300 PM) were ineffective. Glutamate (1 mM), the endogenous agonist, evoked a stimulation of PLD activity to 213 f 6.4% of controls (Fig. l(B)); this effect was not significantly altered in the presence of 1 mM tADA (196 + 7.7%; not illustrated). The present data demonstratethat tADA activatesboth phospholipaseC and D in immature hippocampalslices, but with widely differring potencies. As discussed previously, tADA was suggested to be a mGluR agonist with preference at mGluR5 receptors. Ah activation of phospholipase C by mGluR5 in the immature hippocampus seems plausible because of the high potency of tADA in our experiments (Fig. l(A)). Moreover, mGluR5 receptors are known to be expressed early during hippocampalontogenesis,while mGluRl receptor expression slowly increases postnatally (Catania et al., 1994). However, the activation of PLD by tADA apparently occurs via a metabotropic receptor subtype which is distinct from the subtype coupled to phospholipase C (or adenylyl cyclase). This conclusion is supportedby the findingthat widely different concentrations of tADA were required for PLC and PLD activation in this study.The data shown in Fig. 1 give evidencefor a 100-fold difference of tADA potency at PLC and PLD activation. This difference of potencies is a unique finding with tADA; glutamate and trarzs-1-aminocyclopentane-1,3-dicarboxylicacid (ACPD), for comparison, activate PLC and PLD with very similar potencies in newborn rat hippocampus(Holler et al., 1993;Winder et al., 1993).The involvementof a novel mGluR subtypein PLD activation has, in fact, been proposed recently in a study which investigated the potencies of various classical agonists in adult rat hippocampal slices (Pellegrini-Giampietroet al., 1996),and in a studywhich investigated PLD activation by L-cysteine sulfinic acid (Boss et al., 1994). In conclusion, metabotropic receptor activation by tADA leads to the activation of both PLC and PLD, implyingthat PLD activationmaybe involvedin some of the effects of tADA in recent electrophysiologicaland behavioral experiments. Based on the different potencies of tADA in activating PLC and PLD pathways, tADA may be a useful tool to discriminate between cellular responses coupled to PLC or PLD activation, respectively.
263
thankM.CampaniniandU. Kreisfor excellenttechnicalassistance.Theexperimentsreportedherein weresupportedby theDeutscheForschungsgemeinschaft.
Acknowledgements—We
REFERENCES BossV., NuttK. M. andCormP. J. (1994)L-Cysteinesulfinic acid as an endogenousagonistof a novel metabotropic receptorcoupledto stimulationof phospholipase D activity. Molec. Pharmacol. 45: 1177–1182.
Bruno V., CopaniA., KnopfelT., Kuhn R., CasabonaG., Del~bani P., CondorelliD. F. and NicolettiF. (1995) Activationof metabotropicglutamatereceptorscoupledto inositolphospholipidhydrolysisamplifiesNMDA-induced neuronal degeneration in cultured cortical cells. Neuropharmacology 34: 1089–1098.
CataniaM. V., Landwehrmeyer G. B., TestaC. M., Standaert D. G.,PenneyJ. B.Jr andYoungA. B. (1994)Metabotropic glutamate receptors are differentiallyregulated during development. Neuroscience 61: 481-495. FavaronM.,ManevR. M.,CandeoP.,ArbanR., GabelliniN., KozikowskiA. P. andManevH. (1993)Trans-azetidine-2,4dicarboxylicacidactivatesneuronalmetabotropicreceptors. NeuroReport 4: 967–970.
Holler T., Cappel E., Klein J. and L6ffelholzK. (1993) Glutamateactivatesphospholipase D in hippocampalslices of newbornandadultrats.J. Neurochem. 61: 1569-1572. KleinJ., ChalifaV., LiscovitchM. and L6ffelholzK. (1995) Role of phospholipaseD activationin nervous system physiologyand pathophysiology. J. Neurochem. 65: 1445– 1455.
KnopfelT., SakakiJ., Flor P., BaumannP., SacaanA. I., VelicelebiG., KuhnR. andAllgeierH. (1995)Profilingof trans-azetidine-2,4-dicarboxylic acid at the humanmetabotropicglutamatereceptorshmGluRlb,2, 4a and5a. Eur. J. Pharmacol. 288: 389–392.
Manahan-VaughanD., Reiser M., Pin J.-P., Wilsch V., Bockaert J., Reymann K. G. and Riedel G. (1996) Physiologicaland pharmacologicalprofileof trans-azetidine-2,4-dicarboxylic acid:metabotropicglutamatereceptor agonismandeffectsonlong-termpotentiation. Neuroscience 72: 999-1008.
Pellegrini-Giampietro D. E., TorregrossaS. A. andMoroniF. (1996) Pharmacologicalcharacterizationof metabotropic glutamatereceptorscoupledto phospholipaseD in the rat hippocampus. Br. J. Pharmacol. 118: 1035–1043. PinJ.-P.and DuvoisinR. (1995)The metabotropicglutamate receptors:structureand functions.Neuropharmacology 34: 1–26.
WinderD. G.,SmithT. andCormP.J. (1993)Pharmacological differentiationof metabotropicglutamatereceptorscoupled topotentiationofcyclicadenosinemonophosphate responses and phosphoinositide hydrolysis.J. Pharmacol. Exp. Ther. 266: 518-525.
Pergamon PII: S0028-3908(96)00174-8
Rapid Communication Activation of Phospholipases C and D by the Novel Metabotropic Glutamate Receptor Agonist tADA J. KLEIN,l* K. G. REYMANN2and G. RIEDEL3 IDepartment of Pharmacology, University of Mainz, Obere Zahlbacher Str. 67, D-55101 Mainz, Germany; 2Department of Neurophysiology, Federal Institute of Neurobiology, P.O. Box 1860, D-39008 Magdeburg, Germany and 3Department of Psychology, University of York Heslington, York YO1 5DD, U.K. (Accepted 24 October 1996)
Summary-In hippocampalslicestakenfrom8-day-oldrats,trans-azetidine-2,4-dicarboxylic acid(tADA),a novelglutamatergicagonistactingpreferentiallyat classI mGluRreceptors,activatesphosphoinositide and phosphatidylcholine hydrolysiswithwidelydifferentpotencies.Inositolphosphateformationwasmaximally increasedat 10~M tADA (EC50: 1.2PM), while phospholipaseD activationwas observedat a tADA concentrationof 1 mM.Thisis thefirstreportof a tADA-induced phospholipase D activity.@ 1997Elsevier ScienceLtd.All rightsreserved. Keywords-Metabotropicglutamatereceptors(mGluRs),trans-azetidine-2,4-dicarboxylic acid (tADA), phospholipaseC, phospholipase D, hippocampalslice.
mGluR5a-transfectedthan in mGluRla-transfected cells (Manahan-Vaughan et al., 1996). It must be noted, however, that tADA was ineffective in transected cell lines expressingthe human mGluRlb or 5a subtypes,but acted as an agonist at hmGluR2 in this study (Knopfel et al., 1995). Class I metabotropicglutamate receptor agonists lead to the formation of inositol phosphates and diacylglycerol, increase intracellular calcium and activate classical protein kinase C (cPKC) isoforms. In the adult hippocampus,proteinkinase C activationhas been linked to the phenomenonof long-termpotentiation(LTP), and metabotropic agonists including tADA were found to evoke a form of LTP in the absence of NMDA receptor stimulation (see Manahan-Vaughan et al. (1996) and references cited therein). However, recent research has demonstrated that the receptor-mediated hydrolysis of choline-containing phospholipids by phospholipase D (PLD) is an alternative source for the formation of diacylglycerol required for PKC activation. The PLDcatalysed hydrolysisof phosphatidylcholineyields phosphatidic acid, a possible activator of the atypical form PKC (. Moreover, phosphatidic acid is further hydrolyzed to diacylglycerol,and this pathway may eontribute *Towhomcorrespondenceshouldbe addressed.Fax:(0049) to the activation of the DAG-dependent, calciumindependent PKC il and e (for review, see Klein et al., (6131)176611.
Metabotropicglutamate receptors (mGluRs) are a family of G-protein coupled receptors which have been implicated in a variety of physiological functions and possible pharmacological applications (for review, see Pin and Duvoisin (1995)). In the search for subtypespecific agents, trans-azetidine-2,4-dicarboxylic acid (WA) has recently been suggested as a selective agonist at group I mGluRs which include the mGluRl and 5 subtypes.Both of these subtypesare coupled to the phosphoinositide-specificphospholipase C (PLC), and increases of inositol phosphates have been described in responseto tADA in rat cerebella granule cells (Favaron et al., 1993), murine cortical neurones (Bruno et al., 1995) and hippocampal slices taken from adult rats (Manahan-Vaughan et al., 1996). Several findings suggest that tADA may act preferentiallyat the mGluR5 subtype. Thus, tADA was ineffective in mGluRltransfectedHEK293 cells (Favaron et al., 1993),whereas tADA-responsive murine cortical neurons have been found to be rich in mGluR5 but express little mGluRl protein (Bruno et al., 1995).Moreover,tADA displayeda higher potency for inosito] phosphate formation in
261
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prepared from the dissected hippocampi and superfused with Krebs-Henseleit buffer (KHB) at 35”C. The 1 T solution was gassed with 9570 oxygen and 570 carbon dioxide and had the following composition (mM): Na+ 143.0; K+ 7.1; Ca2+1.3; Mg2+1.2; Cl– 125.3; HPO~– 1.2; SO:– 1.2; HCOj_ 25.0 and glucose 11.7. For the measurementof inositol phosphate formation, the tissue was incubated with 40 pCi of [3H]myo-inositolin 5 ml KHB for 2 hr. Subsequently,the slices were superfused with KHB containing10 mM lithiumchloridefor 20 min. tADA was added to the superfusion medium, and the slices were superfused for a further 10 min in the ADA ADA ADA @ continuous presence of lithium. After homogenization, Ctr 1 PM 10 IJM 100 pM lnivl the aqueousphasewas transferredto a column containing BioRad AG l-X8 anion exchange resin, the individual inositol phosphateswere eluted by the stepwise addition of solutions containing increasing formiate concentrations and countedfor radioactivityin a liquidscintillation counter. For the determination of phospholipase D activity, the phospholipidswere labelled by incubation of the tissue with 40 flCi [3H]glycerolin 5 ml KHB for 2 hr. After a 30 min wash, drugs were added to the superfusionmediumin the presence of propanol (2?io)for 10 min. At the end of the incubation, the slices were homogenized in 20 volumes of chloroform/methanol (2:1), and two phases were obtained by adding four volumesof 0.1 M KC1solution.The major phospholipids were separated by TLC (solvent: upper phase of ethylacetate/isooctane/aceticacid/water, 13:2:3:10),and ADA ADA ADA @l the radioactivity associated with [3H]phosphatidylCtr o.lm?d 0.3mM Imhd ltnhl propanol (PtdPro) was determined and expressed as Fig.1.Influenceof tADAon theactivityof (A)phospholipase percentage of the radioactivitypresent in the lipid phase. C and(B)phospholipase D in hippocampalslicestakenfrom8- Data are presented as means+ SEM of 4-6 experiments. day-oldrats.Forthedetermination ofphospholipase C activity, Statistical significancewas evaluated by repeated measlicesprelabelledwith[3H]inositol weresuperfusedfor 10min sures analysis of variance (ANOVA). in the presenceof tADA,homogenizedand the radioactivity The resultsof the inositolphosphatedeterminationsare associatedwith total inositol phosphates[Z (1P’s)]was measuredand expressedas percentageof total incorporated shown in Fig. l(A). In the immature slices used in the radioactivity.For the determinationof phospholipaseD present experiments, individual inositol phosphates are activity,slicesprelabelledwith [3H]glycerol were superfused formed in the approximate ratio (IP3:IP2:IP1) of 8:4:1; for 10min in the presenceof 2% propanol,tADA andlor tADA treatmentdid not change this pattern, so the data in glutamate,homogenizedand the radioactivityassociatedwith Fig. l(A) are expressed as total inositol phosphate [3H]phosphatidylpropanol IPtdPro]was measured and ex- formation. On the background of a relatively high basal pressedas percentageof the total incorporatedradioactivity phospholipaseC activity in immature slices, the addition in the lipid phase. The data are means+ SEM of ~ of tADA led to a concentration-dependentincrease of determinations.Statistical significancewas evaluated by total inositol phosphate formation (Fig. l(A)). In these ANOVA(paireddata).“p c 0.05;**P<0.01. experiments,tADA displayeda remarkablyhigh potency. Maximal PLC activationwas observed at 10PM tADA, and the EC50 value was calculated as 1.2 PM. In mature 1995). In our previous work, we had reported that slices, concentrationsof >100 PM tADA were required glutamate, acting via metabotropic receptors, was cap- for PLC activation(Manahan-Vaughanet al., 1996).The able of activating PLD in hippocampal slices, and this maximum effect of tADA (167 t 23Y0of control at activation was most pronounced in immature tissue 100 PM) was higher than that observed in mature slices (Holler et al., 1993). We therefore compared the effects reported previously (133 ~ 270; Manahan-Vaughan et of the novel mGluR agonist tADA on phospholipaseC al., 1996). For comparison, the endogenous agonist and D activitiesin hippocampalslices taken from 8-day- glutamate (1 mM) increased inositol phosphates in immature slices to 192.6 t 13% (Fig. l(A)). old rats. In separate experiments, tADA was also found to Sprague–Dawleyrats (7–9 days of age) were killed by activate phospholipase D activity as determined by the decapitation,hippocampal slices (400 ~m) were quickly A
20
* —-
*
RapidCommunication transphosphatidylation assay (Fig. l(B)). Maximum stimulation of PLD activity of 153 & 12% (p< 0.05; IV= 6) was observed at 1 mM tADA, an effect that was comparable to the extent of PLC stimulation.However, concentrations below 1 mM tADA (100 and 300 PM) were ineffective. Glutamate (1 mM), the endogenous agonist, evoked a stimulation of PLD activity to 213 f 6.4% of controls (Fig. l(B)); this effect was not significantly altered in the presence of 1 mM tADA (196 + 7.7%; not illustrated). The present data demonstratethat tADA activatesboth phospholipaseC and D in immature hippocampalslices, but with widely differring potencies. As discussed previously, tADA was suggested to be a mGluR agonist with preference at mGluR5 receptors. Ah activation of phospholipase C by mGluR5 in the immature hippocampus seems plausible because of the high potency of tADA in our experiments (Fig. l(A)). Moreover, mGluR5 receptors are known to be expressed early during hippocampalontogenesis,while mGluRl receptor expression slowly increases postnatally (Catania et al., 1994). However, the activation of PLD by tADA apparently occurs via a metabotropic receptor subtype which is distinct from the subtype coupled to phospholipase C (or adenylyl cyclase). This conclusion is supportedby the findingthat widely different concentrations of tADA were required for PLC and PLD activation in this study.The data shown in Fig. 1 give evidencefor a 100-fold difference of tADA potency at PLC and PLD activation. This difference of potencies is a unique finding with tADA; glutamate and trarzs-1-aminocyclopentane-1,3-dicarboxylicacid (ACPD), for comparison, activate PLC and PLD with very similar potencies in newborn rat hippocampus(Holler et al., 1993;Winder et al., 1993).The involvementof a novel mGluR subtypein PLD activation has, in fact, been proposed recently in a study which investigated the potencies of various classical agonists in adult rat hippocampal slices (Pellegrini-Giampietroet al., 1996),and in a studywhich investigated PLD activation by L-cysteine sulfinic acid (Boss et al., 1994). In conclusion, metabotropic receptor activation by tADA leads to the activation of both PLC and PLD, implyingthat PLD activationmaybe involvedin some of the effects of tADA in recent electrophysiologicaland behavioral experiments. Based on the different potencies of tADA in activating PLC and PLD pathways, tADA may be a useful tool to discriminate between cellular responses coupled to PLC or PLD activation, respectively.
263
thankM.CampaniniandU. Kreisfor excellenttechnicalassistance.Theexperimentsreportedherein weresupportedby theDeutscheForschungsgemeinschaft.
Acknowledgements—We
REFERENCES BossV., NuttK. M. andCormP. J. (1994)L-Cysteinesulfinic acid as an endogenousagonistof a novel metabotropic receptorcoupledto stimulationof phospholipase D activity. Molec. Pharmacol. 45: 1177–1182.
Bruno V., CopaniA., KnopfelT., Kuhn R., CasabonaG., Del~bani P., CondorelliD. F. and NicolettiF. (1995) Activationof metabotropicglutamatereceptorscoupledto inositolphospholipidhydrolysisamplifiesNMDA-induced neuronal degeneration in cultured cortical cells. Neuropharmacology 34: 1089–1098.
CataniaM. V., Landwehrmeyer G. B., TestaC. M., Standaert D. G.,PenneyJ. B.Jr andYoungA. B. (1994)Metabotropic glutamate receptors are differentiallyregulated during development. Neuroscience 61: 481-495. FavaronM.,ManevR. M.,CandeoP.,ArbanR., GabelliniN., KozikowskiA. P. andManevH. (1993)Trans-azetidine-2,4dicarboxylicacidactivatesneuronalmetabotropicreceptors. NeuroReport 4: 967–970.
Holler T., Cappel E., Klein J. and L6ffelholzK. (1993) Glutamateactivatesphospholipase D in hippocampalslices of newbornandadultrats.J. Neurochem. 61: 1569-1572. KleinJ., ChalifaV., LiscovitchM. and L6ffelholzK. (1995) Role of phospholipaseD activationin nervous system physiologyand pathophysiology. J. Neurochem. 65: 1445– 1455.
KnopfelT., SakakiJ., Flor P., BaumannP., SacaanA. I., VelicelebiG., KuhnR. andAllgeierH. (1995)Profilingof trans-azetidine-2,4-dicarboxylic acid at the humanmetabotropicglutamatereceptorshmGluRlb,2, 4a and5a. Eur. J. Pharmacol. 288: 389–392.
Manahan-VaughanD., Reiser M., Pin J.-P., Wilsch V., Bockaert J., Reymann K. G. and Riedel G. (1996) Physiologicaland pharmacologicalprofileof trans-azetidine-2,4-dicarboxylic acid:metabotropicglutamatereceptor agonismandeffectsonlong-termpotentiation. Neuroscience 72: 999-1008.
Pellegrini-Giampietro D. E., TorregrossaS. A. andMoroniF. (1996) Pharmacologicalcharacterizationof metabotropic glutamatereceptorscoupledto phospholipaseD in the rat hippocampus. Br. J. Pharmacol. 118: 1035–1043. PinJ.-P.and DuvoisinR. (1995)The metabotropicglutamate receptors:structureand functions.Neuropharmacology 34: 1–26.
WinderD. G.,SmithT. andCormP.J. (1993)Pharmacological differentiationof metabotropicglutamatereceptorscoupled topotentiationofcyclicadenosinemonophosphate responses and phosphoinositide hydrolysis.J. Pharmacol. Exp. Ther. 266: 518-525.