Inhibition of the phospholipase C-linked metabotropic glutamate receptor by 2-amino-3-phosphonopropionate is dependent on extracellular calcium

Structural, signalling and regulatory properties of the group I metabotropic glutamate receptors: prototypic family C G-protein-coupled receptors

In 1991 a new type of G-protein-coupled receptor (GPCR) was cloned, the type 1a metabotropic glutamate (mGlu) receptor, which, despite possessing the defining seven-transmembrane topology of the GPCR superfamily, bore little resemblance to the growing number of other cloned GPCRs. Subsequent studies have shown that there are eight mammalian mGlu receptors that, together with the calcium-sensing receptor, the GABA(B) receptor (where GABA is gamma-aminobutyric acid) and a subset of pheromone, olfactory and taste receptors, make up GPCR family C. Currently available data suggest that family C GPCRs share a number of structural, biochemical and regulatory characteristics, which differ markedly from those of the other GPCR families, most notably the rhodopsin/family A GPCRs that have been most widely studied to date. This review will focus on the group I mGlu receptors (mGlu1 and mGlu5). This subgroup of receptors is widely and differentially expressed in neuronal and glial cells within the brain, and receptor activation has been implicated in the control of an array of key signalling events, including roles in the adaptative changes needed for long-term depression or potentiation of neuronal synaptic connectivity. In addition to playing critical physiological roles within the brain, the mGlu receptors are also currently the focus of considerable attention because of their potential as drug targets for the treatment of a variety of neurological and psychiatric disorders.

Glutamate regulates IP3-type and CICR stores in the avian cochlear nucleus

Neurons of the avian cochlear nucleus, nucleus magnocellularis (NM), are activated by glutamate released from auditory nerve terminals. If this stimulation is removed, the intracellular calcium ion concentration ([Ca2+]i) of NM neurons rises and rapid atrophic changes ensue. We have been investigating mechanisms that regulate [Ca2+]i in these neurons based on the hypothesis that loss of Ca2+ homeostasis causes the cascade of cellular changes that results in neuronal atrophy and death. In the present study, video-enhanced fluorometry was used to monitor changes in [Ca2+]i stimulated by agents that mobilize Ca2+ from intracellular stores and to study the modulation of these responses by glutamate. Homobromoibotenic acid (HBI) was used to stimulate inositol trisphosphate (IP3)-sensitive stores, and caffeine was used to mobilize Ca2+ from Ca2+-induced Ca2+ release (CICR) stores. We provide data indicating that Ca2+ responses attributable to IP3- and CICR-sensitive stores are inhibited by glutamate, acting via a metabotropic glutamate receptor (mGluR). We also show that activation of C-kinase by a phorbol ester will reduce HBI-stimulated calcium responses. Although the protein kinase A accumulator, Sp-cAMPs, did not have an effect on HBI-induced responses. CICR-stimulated responses were not consistently attenuated by either the phorbol ester or the Sp-cAMPs. We have previously shown that glutamate attenuates voltage-dependent changes in [Ca2+]i. Coupled with the present findings, this suggests that in these neurons mGluRs serve to limit fluctuations in intracellular Ca2+ rather than increase [Ca2+]i. This system may play a role in protecting highly active neurons from calcium toxicity resulting in apoptosis.

Metabotropic glutamate receptors regulate N-methyl-D-aspartate-mediated synaptic transmission in nucleus accumbens

We recorded intracellularly from core nucleus accumbens (NAcc) neurons in brain slices to study the regulation by metabotropic glutamate receptors (mGluRs) of pharmacologically isolated N-methyl--aspartate-mediated excitatory postsynaptic currents (NMDA-EPSCs). Monosynaptic NMDA-EPSCs, evoked by local stimulation, were isolated by superfusion of the non-NMDA and gamma-aminobutyric acid-A (GABAA) receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and bicuculline (15 microM), respectively. Trans-1-aminocyclopentane-1,3-decarboxylic acid (trans-ACPD; 50 microM), a nonspecific group 1 and 2 mGluR agonist, had no effect on resting membrane potential (RMP) or input resistance of NAcc neurons. However, it consistently decreased NMDA-EPSC areas (time integrals) dose dependently (1-100 microM; EC50 = 8 microM) and reversibly. The specific group 1 mGluR agonists quisqualate (1-4 microM) and (RS)-3, 5-dihydroxyphenylglycine (DHPG; 100 microM) did not mimic the trans-ACPD effect on NMDA-EPSCs, nor did exposure of the slice to the group 1 mGluR antagonist (+)-2-amino-3-phosphonopropionic acid (-AP3, 0.4 mM) inhibit the trans-ACPD effect. The putative mGluR1 and mGluR2 antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) at 0.5 mM failed to antagonize trans-ACPD effects but at 1 mM blocked them. Both the group 2 mGluR agonist (2S,3S, 4S)-alpha-(carboxycyclopropyl)-glycine (-CCG-I, 2 microM) and the group 3 mGluR specific agonist (+)-2-amino-4-phosphonobutyric acid (-AP4, 20 microM) attenuated NMDA-EPSC areas; the effect of -AP4 was blocked by the group 3 antagonist (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4; 0.5 mM). Exogenously applied NMDA, in the presence of tetrodotoxin to prevent presynaptic effects, induced inward currents that were decreased by 20 microM -AP4 but not by 10 microM trans-ACPD. These findings suggest that NMDA receptor-mediated neurotransmission in NAcc is under dual inhibitory regulation by group 2 and 3 metabotropic receptor subtypes: -AP4-sensitive receptors located postsynaptically and those sensitive to trans-ACPD located presynaptically.

Stimulation of phospholipase C activity by norepinephrine, t-ACPD and bombesin in LA-N-2 cells

The release of [3H]inositol phosphates from myo-[3H]inositol-prelabeled LA-N-2 cells was measured in the presence of beta-adrenoceptor, metabotropic glutamate and bombesin agonists. Norepinephrine and isoproterenol increased the formation of [3H]inositol phosphates in a dose-dependent manner, with an EC50 of 100 microM for norepinephrine and an EC50 of 5 microM for isoproterenol. These stimulations were abolished by propranolol, a beta-adrenoceptor antagonist, with an IC50 in the range of 50-55 microM for both norepinephrine and isoproterenol. The stimulation of [3H]inositol phosphate appearance occurred with varying concentrations of trans-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD), a metabotropic glutamate receptor agonist. This release of [3H] inositol phosphates was blunted by its antagonist, 2-amino-3-phosphonopropionic acid (AP-3). Bombesin and neuromedin-B, a bombesin-like peptide, also increased the appearance of [3H]inositol phosphates. This was blunted by the antagonist [Tyr4, D-Phe12] bombesin. The appearance of [3H]inositol phosphates stimulated by t-ACPD was coupled through a cholera toxin-sensitive G-protein and the bombesin-stimulated appearance of [3H]inositol phosphates was coupled through a pertussis toxin-sensitive G-protein. The norepinephrine-stimulated appearance of [3H]inositol phosphates was toxin insensitive. The stimulation of the [3H]inositol phosphate appearance by these three agonists was protein kinase and Ca2+ independent.

Pharmacological characterization of metabotropic glutamate receptors potentiating NMDA responses in mouse cortical wedge preparations

1. Mouse cortical wedge preparations were used in order to study the effects of metabotropic glutamate receptor (mGluR) agonists and antagonists on the depolarization induced by N-methyl-D-aspartate (NMDA) or by (S)-alpha-amino-4-bromo-3-hydroxy-5-isoxazolepropionic acid (AMPA). 2. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (30-300 microM) significantly potentiated the depolarizations induced by NMDA, leaving unchanged those mediated by AMPA. This potentiation developed slowly and lasted for up to 60 min provided that the slices were continuously perfused with the mGluR agonist. 3. Concentration-response curves to NMDA in the absence and in the presence of 1S,3R-ACPD (100 microM) indicated that the potentiation was due to increased affinity of the NMDA receptor complex for its agonist. The maximal responses to NMDA were not potentiated. 4. Selective agonists of group 1 mGluR such as quisqualate (Quis) (30 microM) or (RS)-3,5-dihydroxyphenylglycine (DHPG) (300 microM) did not potentiate NMDA responses. Similarly, selective agonists of group 2 mGluRs, such as (2S,3S,4S)-alpha-carboxycyclopropyl-glycine (L-CCG-I) (3-30 microM), and of group 3, such as L-2-amino-4-phosphonobutyric acid (L-AP4) (100 microM) were inactive in our test. A number of other putative mGluR agents having partial agonist activity on mGluRs in brain slices and in expression systems, such as 1R,3S-ACPD (500 microM), DL-2-amino-3-phosphonopropionic acid (DL-AP3) (300 microM) and (S)-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG; 500 microM), when placed in the experimental protocol we used, did not change NMDA responses. 5. Available mGluR antagonists, such as DL-AP3 (1 mM), (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) (500 microM), S-4-carboxyphenylglycine (4CPG; 500 microM) and S-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG; 500 microM), did not reduce 1S,3R-ACPD potentiation of NMDA responses. 6. It is concluded that the potentiation of NMDA currents induced by the mGluR agonist 1S,3R-ACPD, in mouse cortical wedges, has a pharmacological profile which is different from that of the three mGluR groups so far described in expression systems.

Modulation of adenosine-induced cAMP accumulation via metabotropic glutamate receptors in chick optic tectum

Changes on cyclic adenosine monophosphate (cAMP) levels in response to adenosine and glutamate and the subtype of glutamate receptors involved in this interaction were studied in slices of optic tectum from 3-day-old chicks. cAMP accumulation mediated by adenosine (100 microM) was abolished by 8-phenyltheophylline (15 microM). Glutamate and the glutamatergic agonists kainate or trans-D, L-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) did not evoke cAMP accumulation. Glutamate blocked the adenosine response in a dose-dependent manner. At 100 microM, glutamate did not inhibit the effect of adenosine. The 1 mM and 10 mM doses of glutamate inhibited adenosine-induced cAMP accumulation by 55% and 100%, respectively. When glutamatergic antagonists were used, this inhibitory effect was not affected by 200 microM 6,7-dihydroxy-2,3,dinitroquinoxaline (DNQX), an ionotropic antagonist, and was partially antagonized by 1 mM (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)M-CPG], a metabotropic antagonist, while 1 mM L-2-amino-3-phosphonopropionate (L-AP3) alone, another metabotropic antagonist, presented the same inhibitory effect of glutamate. Kainate (10 mM) and trans-ACPD (100 microM and 1 mM) partially blocked the adenosine response. This study indicates the involvement of metabotropic glutamate receptors in adenylate cyclase inhibition induced by glutamate and its agonists trans-ACPD and kainate.

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.

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.

Glutamate-stimulated phosphatidylinositol metabolism in the avian cochlear nucleus

This study examined the ability of the excitatory amino acid glutamate and its analogs to stimulate phosphatidylinositol metabolism in isolated cochlear nucleus tissue from young chicks. In the presence of lithium chloride, glutamate and (+/-)-1-aminocyclopentyl-trans-1,3-dicarboxylate (ACPD) stimulated the formation of inositol phosphates to levels significantly above unstimulated control levels. Unexpectedly, quisqualate did not stimulate inositol phosphates formation. The N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV), the ionotropic kainate/quisqualate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the putative metabotropic glutamate receptor antagonist 2-amino-3-phosphonopropionate (AP3) had no effect on the glutamate stimulated formation of inositol phosphates. We conclude that a metabotropic glutamate receptor is present on cochlear nucleus neurons of posthatch chicks and is able to stimulate formation of inositol phosphates.

The characteristics and pharmacology of olfactory cortical LTP induced by theta-burst high frequency stimulation and 1S,3R-ACPD

Long-term potentiation (LTP) of monosynaptic excitations in the olfactory cortex slice by theta burst high frequency stimulation (theta-HFS) and application of the metabotropic glutamate receptor-selective agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) has been studied. Theta-HFS optimally induced LTP when given 4 times at intervals of 20-30 min. The degree of LTP was significantly potentiated by the inclusion of picrotoxin in the perfusion medium but induction was prevented by D-2-amino-5-phosphonopentanoate (25 microM), L-2-amino-3-phosphonopropionate (125 microM), 5-(isoquinolinyl-sulphonyl)-2-methylpiperazine (50 microM), sangivamycin (25 microM) and thapsigargin (1 microM). Of the drugs tested, only D-2-amino-5-phosphonopentanoate failed to depotentiate established LTP. Application of 1S,3R-ACPD (100 microM) repeated 4 times at intervals of 20-30 min also optimally induced an LTP which was significantly less in unstimulated preparations and showed the same pattern of sensitivity to the drugs tested as LTP induced by theta-HFS. It is concluded that the induction of LTP by theta-HFS and 1S,3R-ACPD requires activation of both N-methyl-D-aspartate and metabotropic glutamate receptors and that a protein kinase is essential for the induction and maintenance of LTP. The likely mechanisms of induction and maintenance of olfactory cortical and hippocampal LTP are contrasted.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) induces burst firing via an inositol-1,4,5-triphosphate-independent pathway at rat dorsolateral septal nucleus

We have previously reported that a L-2-amino-3-phosphonopropionate (L-AP3)-sensitive metabotropic glutamate receptor was required for the induction of long-term potentiation (LTP) in rat dorsolateral septal nucleus neurons. (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a selective agonist for metabotropic glutamate receptors, also causes burst firing of dorsolateral septal nucleus (DLSN) neurons. In this study, we investigated whether this response was mediated by a phospholipase C-(PLC) coupled metabotropic glutamate receptor. The threshold concentration of 1S,3R-ACPD for the induction of burst firing was about 5 microM, while 10 microM 1S,3R-ACPD produced a maximal effect. L-AP3 (50 microM) reduced the burst firing induced by 1S,3R-ACPD (5 microM). Although 1S,3R-ACPD stimulated the formation of inositol-1,4,5-triphosphate [Ins(1,4,5)P3] suggesting the presence of PLC-coupled metabotropic glutamate receptors, it was only effective in a higher (30-100 microM) concentration range. In addition, the 1S,3R-ACPD-stimulated formation of Ins(1,4,5)P3 level was not affected by L-AP3. These observations suggest that the 1S,3R-ACPD induced burst firing is not mediated by PLC-coupled metabotropic glutamate receptors.

Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[3H]-aspartate output in rat striatum

1. The effects of several agonists of the metabotropic glutamate receptor (mGluR) were studied in adult rat striatal slices by measuring (i) KCl (30 mM)-induced output of previously taken up D-[3H]-aspartate (Asp), (ii) forskolin (30 microM)-induced adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation and (iii) phophoinositide (PI) hydrolysis. 2. K(+)-induced efflux of D-[3H]-Asp was inhibited by the following mGluR agonists: (1S,3S,4S)-(carboxycyclopropyl)glycine (L-CCG-I), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and quisqualic acid (Quis). 2-Amino-4-phosphonobutyrate (L-AP4) was inactive up to 300 microM. The maximal inhibition of D-[3H]-Asp output was 60 +/- 8%. The EC50s of mGluR agonists were: 0.5 microM for L-CCG-I, 100 microM for 1S,3R-ACPD and 100 microM for Quis. 3. Forskolin-induced cyclic AMP accumulation was also inhibited by mGluR agonists. The maximal inhibition was 50 +/- 4% and was obtained at a concentration of 10 microM for L-CCG-I and 100 microM for 1S,3R-ACPD. The EC50s for this inhibition were: 0.9 microM for L-CCG-I and 20 microM for 1S,3R-ACPD. Quis (300 microM) inhibited cyclic AMP accumulation by approximately 20%. L-AP4 slightly potentiated cyclic AMP accumulation. 4. PI hydrolysis was stimulated by mGluR agonists. The most potent compound was Quis (100 microM), which increased inositol phosphate formation up to 2.2 fold over control values. Its EC50 was 15 microM. L-CCG-I and 1S,3R-ACPD increased inositol phosphate formation by approximately 1.8 fold and their EC50 values were 30 and 25 microM, respectively. L-AP4 did not affect PI hydrolysis. 5. In conclusion, mGluR agonists that reduce D-[3H]-Asp output have a pharmacological profile similar to that of mGluR agonists inhibiting cyclic AMP accumulation. L-CCG-I appears to be a relatively selective agonist for the mGluR receptor which inhibits D-[3H]-Asp efflux and cyclic AMP accumulation,while Quis appears to act preferentially on the mGluR receptor linked to the metabolism of PIs.

Neonatal exposure to D,L-2-amino-3-phosphonopropionate (D,L-AP3) produces lesions in the eye and optic nerve of adult rats

Metabotropic glutamate receptors are a recently described receptor class with emerging importance in synaptic plasticity and brain development. Activation of metabotropic glutamate receptors results in several cellular secondary messenger events that are especially important during postnatal development. This study characterized the effects of D,L-2-amino-3-phosphonopropionate (D,L-AP3), an aspartic acid analog with agonist and antagonist activity at the metabotropic receptor, on the postnatal development of the rat eye and optic nerve. Sprague-Dawley rat pups were treated daily (i.p.) with saline or 500 mg/kg D,L-AP3 on postnatal days (PND) 4-10 or 10-14. After making clinical and ophthalmoscopic examinations, rats were necropsied between 65 and 70 days of age and light microscopic evaluations were made of eyes and optic nerves. Between postnatal days 10-20, all treated rats exhibited motor tremors, circling, and head tilt. Ophthalmoscopic lesions were more severe in rats treated on days 4-10 than days 10-14 and included decreased retinal vasculature, cataracts, and retinal dysplasia, hypoplasia, and detachment. All rats treated on days 4-10 had severe optic nerve atrophy/hypoplasia grossly and severe retinal atrophy, retinal detachment, and cataracts histologically. Seven of eight rats treated on days 10-14 had qualitatively similar but less severe lesions. Overall, rats treated with D,L-AP3 on PND 4-10 had earlier and more severe retinal and optic nerve lesions when compared to rats treated on PND 10-14. These data characterize the morphologic effects in adult rats exposed to D,L-AP3 as neonates and suggest a possible role for the metabotropic receptor in the postnatal development of retina and optic nerve.

Multiple subtypes of excitatory amino acid receptors coupled to the hydrolysis of phosphoinositides in rat brain

The excitatory amino acid (EAA) analogues quisqualate, ibotenate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (trans-ACPD) activate the metabotropic EAA receptors that are coupled to the hydrolysis of phosphoinositides (PI). Previous studies of hippocampal cross sections demonstrated that PI hydrolysis stimulated by these agonists can be inhibited by either L-aspartate-beta-hydroxamate (L-A beta HA) or DL-2-amino-3-phosphonopropionate (DL-AP3). The goal of the present studies was to determine if all metabotropic EAA receptors are sensitive to L-A beta HA and DL-AP3. Two approaches were used. In the first, using cerebellar cross sections, the effects of these agonists and inhibitors were examined. The EC50 values (the concentrations required to evoke half-maximal stimulation) of quisqualate, ibotenate, and trans-ACPD in cerebellum were similar to the EC50 values that we observed previously in hippocampus, but neither L-A beta HA nor DL-AP3 blocked PI hydrolysis. The EC50 values were 0.65 +/- 0.17 microM for quisqualate, 12.8 +/- 2.5 microM for ibotenate, and 18.1 +/- 3.1 microM for trans-ACPD. All data were best fit to theoretical curves that had Hill slopes of 1. In the second approach, another EAA analogue, D-aspartate, was identified as an agonist that stimulates PI hydrolysis. The EC50 for PI hydrolysis stimulated by D-aspartate was 470 +/- 90 microM in hippocampus. Neither L-A beta HA nor DL-AP3 blocked PI hydrolysis stimulated by D-aspartate in hippocampus.(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.