Metabotropic glutamate receptors: synaptic transmission, modulation, and plasticity

L-AP4 (L-(+)-2-amino-4-phosphonobutyric acid) induced impairment of spatial learning in the rat is antagonized by MAP4 ((S)-2-amino-2-methyl-4-phosphonobutanoic acid)

L-AP4, an agonist at the metabotrophic glutamate receptors 4, 6, 7, 8 and 9 produced a selective spatial learning impairment in a water maze as well as in an 8-arm maze task when injected i.c.v. (5 microliters of a 80 mM solution), a dose previously reported to block consolidation of long-term potentiation in vivo. Acquisition and recall of the spatial water-maze task, as measured by escape latency and quadrant bias, respectively, were impaired, whereas swim speed was not affected. In contrast, ability to perform a non-spatial control task was not impaired; latency to reach a visible escape platform was not delayed in L-AP4-treated animals. No behavioral difference was visible in the open field. MAP4, an antagonist of mGluRs mediating L-AP4 induced reduction of transmitter release, when administered pretraining i.c.v. (5 microliters of an 80 mM solution) did not affect motor activity in the open field test but did impair learning of both spatial tasks. In addition, swim speed was increased. However, injecting L-AP4 and MAP4 in combination at equimolar concentrations had no effect on learning in both spatial tasks or on swim speed in the water maze. Neither latency in the visible-platform test nor behavior in the open field was affected. We conclude that L-AP4 sensitive metabotropic glutamate receptors play a selective role in learning and memory formation of the rat.

Presynaptic calcium current modulation by a metabotropic glutamate receptor

Metabotropic glutamate receptors (mGluRs) regulate transmitter release at mammalian central synapses. However, because of the difficulty of recording from mammalian presynaptic terminals, the mechanism underlying mGluR-mediated presynaptic inhibition is not known. Here, simultaneous recordings from a giant presynaptic terminal, the calyx of Held, and its postsynaptic target in the medial nucleus of the trapezoid body were obtained in rat brainstem slices. Agonists of mGluRs suppressed a high voltage-activated P/Q-type calcium conductance in the presynaptic terminal, thereby inhibiting transmitter release at this glutamatergic synapse. Because several forms of presynaptic modulation and plasticity are mediated by mGluRs, this identification of a target ion channel is a first step toward elucidation of their molecular mechanism.

Compartmental localization of a metabotropic glutamate receptor (mGluR7): two different active sites at a retinal synapse

The distribution of the metabotropic glutamate receptor 7 (mGluR7) was studied in the rat retina using a specific antiserum. Punctate immunofluorescence that corresponded to synaptic localization was present exclusively in the inner plexiform layer. Double-labeling experiments suggested that mGluR7 is expressed at the synaptic terminals of certain cone bipolar cells. Electron microscopy showed that mGluR7 was present both presynaptically, as an autoreceptor in cone bipolar cell ribbon synapses, and postsynaptically in amacrine cells. There are usually two postsynaptic processes at a bipolar cell ribbon synapse; however, the presynaptic aggregation of mGluR7 was restricted to one half of the active zone and therefore was opposed to only one of the postsynaptic processes. This selective localization of mGluR7 could differentially regulate the glutamate release from the ribbon synapse, thus leading to a differential activation of the postsynaptic neurons.

Immunohistochemical distribution of metabotropic glutamate receptor subtypes mGluR1b, mGluR2/3, mGluR4a and mGluR5 in human hippocampus

The metabotropic glutamate receptors (mGluRs) can be classified into three families based on amino acid sequence homology, signal transduction mechanisms and pharmacological properties. Generally, class I mGluRs mediate an excitation of neurons while activation of class II and III mGluRs results in a depression of synaptic transmission. In this study we have analyzed the expression pattern of mGluRs in human hippocampus using a panel of polyclonal antibodies specific for mGluR1b, mGluR2/3, mGluR4a, and mGluR5. Immunoreactivity for mGluR1b and mGluR5, i.e., the subtypes representing class I mGluRs, was found in all hippocampal neurons. The mGluR1b antiserum stained perikarya and proximal dendrites, whereas immunoreactivity for mGluR5 was also detectable in the distal dendritic compartments. Immunoreactivity for mGluR2/3, members of class II mGluRs, was present in all principle neurons in the dentate gyrus as well as in the CA4, CA3 and CA2 regions. Pyramidal cells of the CA1 region exhibited only weak labeling for mGluR2/3. Glial cells were also mGluR2/3-immunoreactive. The reaction obtained with an antiserum directed against mGluR4a, a member of class III mGluRs, was confined to the mossy fiber projection field in CA3 stratum lucidum. These data demonstrate differential expression of mGluR variants in the human hippocampus and may provide an important basis for future studies of mGluRs under various neuropathological conditions such as temporal lobe epilepsy, ischemia and neurodegenerative disorders.

Microvesicle-mediated exocytosis of glutamate is a novel paracrine-like chemical transduction mechanism and inhibits melatonin secretion in rat pinealocytes

Mammalian pinealocytes are neuroendocrine cells that synthesize and secrete melatonin, these processes being positively controlled by norepinephrine derived from innervating sympathetic neurons. Previously, we showed that pinealocytes contain a large number of microvesicles (MVs) that specifically accumulate L-glutamate through a vesicular glutamate transporter and contain proteins for exocytosis such as synaptobrevin 2 (VAMP2). These findings suggested that the MVs are counterparts of synaptic vesicles and are involved in paracrine-like chemical transduction in the pineal gland. Here, we show that pinealocytes actually secrete glutamate upon stimulation by KCl in the presence of Ca2+ at 37 degrees C. The ability of glutamate secretion disappeared when the cells were incubated at below 20 degrees C. Loss of the activity was also observed on successive stimulation, but it was recovered after 12 hr incubation. A low concentration of cadmium chloride or omega-conotoxin GVIA inhibited the secretion. Botulinum neurotoxin E cleaved synaptic vesicle-associated protein 25 (SNAP-25) and thus inhibited the secretion. The released L-glutamate stimulated pinealocytes themselves via glutamate receptor(s) and inhibited norepinephrine-stimulated melatonin secretion. These results strongly suggest that pinealocytes are glutaminergic paraneurons, and that the glutaminergic system regulates negatively the synthesis and secretion of melatonin. The MV-mediated paracrine-like chemical transduction seems to be a novel mechanism that regulates hormonal secretion by neuroendocrine cells.

Purification and biochemical characterization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-sensitive L-glutamate receptors of pig brain

Two preparations of glutamate receptors were purified from the synaptic junctions of pig brain by a combination of detergent solubilization, anion-exchange chromatography, wheat-germ agglutinin affinity chromatography and sedimentation through sucrose gradients. These preparations were enriched in specific L-[3H]glutamate binding activity (> 5000 pmol of glutamate binding sites/mg of protein), and the rank order of ligand affinity for binding to these preparations was: quisqualate > 6-cyano-7- nitroquinoxaline-2,3-dione > alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) > L-glutamate > kainate > > N-methyl-D-aspartate approximately L-2-amino-4-phosphonobutyrate. SDS/PAGE analysis revealed that more than 80% of the protein in either of these preparations appeared as a single protein band of 106 kDa. Two-dimensional gel electrophoresis further revealed that these 106 kDa proteins consisted of a series of acidic proteins which were recognized by antibodies against rat AMPA receptor subunits. These 106 kDa proteins were also recognized by wheatgerm agglutinin and concanavalin A; in addition, peptide N-glycosidase F treatment of these preparations decreased their size to 99 kDa. Our results suggest that the putative glutamate receptors isolated here are likely to belong to the AMPA subtype of glutamate receptors in pig brain. Using the purification procedure reported here, 5 micrograms of AMPA receptor proteins can be isolated from 250 g of pig brain tissue.

The role of glutamate in the photic regulation of the suprachiasmatic nucleus

Endogenous circadian rhythms govern most aspects of physiology and behaviour in mammals, including body temperature, autonomic and endocrine function, and sleep-wake cycles. Such rhythms are generated by the suprachiasmatic nucleus of the hypothalamus (SCN), but are synchronised to the environmental light-dark cycle by photic cues perceived by the retina and conveyed to the SCN via the retinohypothalamic tract (RHT). This review considers many lines of evidence from diverse experimental approaches indicating that the RHT employs glutamate (or a related excitatory amino acid) as a neurotransmitter. Ultrastructural studies demonstrate the presence of glutamate in presynaptic terminals within the SCN. In situ hybridisation and immunocytochemical studies reveal the presence of several NMDA (NMDAR1, NMDAR2C), non-NMDA (GluR1, GluR2, GluR4) and metabotropic (mGluR1) glutamate receptor subunits in the SCN. Messenger RNA encoding a glutamate transporter protein is also present. In behavioural tests, glutamate antagonists can block the effects of light in phase-shifting circadian rhythms. Such treatments also block the induction of c-fos within SCN cells by light, whereas a glutamate agonist (NMDA) induces c-fos expression. In hypothalamic slice preparations in vitro, electrical stimulation of the optic nerves induces release of glutamate and aspartate, and glutamate antagonists block field potentials in the SCN evoked by stimulation of the optic nerve. Circadian rhythms of electrical activity which persist in vitro are phase shifted by application of glutamate in a manner which mimics the phase shifting effects of light in vivo. This wide range of experimental findings provides strong support for the hypothesis that glutamate is the principal neurotransmitter within the RHT, and thus conveys photic cues to the circadian timing system in the SCN.

Group II and group III metabotropic glutamate receptors in the rat retina: distributions and developmental expression patterns

We have studied the distributions of group II metabotropic glutamate receptors, mGluR2 and mGluR3, and a group III metabotropic glutamate receptor, mGluR4, in the adult rat retina and during postnatal development using receptor specific anti-peptide antisera. Of the three receptors examined, mGluR3 was not expressed in the retina. MGluR2 showed a distinct stratification pattern in the inner plexiform layer (IPL). Double-labelling immunocytochemistry revealed that mGluR2 was localized in the processes of cholinergic amacrine cells. MGluR4 was found throughout the entire IPL. At the subcellular level, both mGluR2 and mGluR4 were found to be localized exclusively in processes postsynaptic to bipolar cell synapses in the IPL. During postnatal development, labelling for mGluR2 was detected at around postnatal day five. MGluR4 was already present at postnatal day one, prior to the establishment of synaptic connections in the IPL. The differential expression patterns of individual metabotropic glutamate receptors in the adult and developing rat retina suggest distinct roles for these receptors in retinal synaptic circuitry.

Metabotropic glutamate receptor subtype agonists facilitate long-term potentiation within a distinct time window in the dentate gyrus in vivo

Trans-azetidine-2,4-dicarboxylic acid (ADA) is a putative selective agonist of group 1 metabotropic glutamate receptors. It has been shown previously that application of ADA prior to a short-term potentiation-inducing high-frequency tetanus facilitates long-term potentiation in vivo. In order to examine the role of metabotropic glutamate receptors in this response, we studied the effect of ADA in the dentate gyrus of the rat when applied after high-frequency tetanus to the perforant path. A comparison was made with the effects of the metabotropic glutamate receptor group 1 agonist 3,5-dihydroxyphenylglycine. Drugs were applied via a cannula implanted in the lateral cerebral ventricle. Both population spike amplitude and field excitatory postsynaptic potential were measured. Weak tetanization produced a short-term potentiation of field excitatory postsynaptic potential and population spike which decayed to baseline values by 90 min, and was unaffected by vehicle injections. Application of ADA (20 mM/5 microliters) or 3,5-dihydroxyphenylglycine (4mM/5 microliters) 5 min after high-frequency tetanus facilitated short-term potentiation into a long-term potentiation which lasted over 24 h. (R,S)-alpha-Methyl-4-carboxyphenylglycine (200 mM/5 microliters), a metabotropic glutamate receptor antagonist, when applied prior to high-frequency tetanus and ADA or 3,5-dihydroxyphenylglycine, completely inhibited this effect. ADA applied 10,15,20 and 25 min after high-frequency tetanus also facilitated short-term potentiation into long-term potentiation, but the magnitude of long-term potentiation was smaller than than produced by ADA given 5 min after tetanus. Similar effects were seen with 3,5-dihydroxyphenylglycine applied 25 min after high-frequency tetanus. When (R,S)-alpha-methyl-4-carboxyphenylglycine was applied prior to high-frequency tetanus and ADA or 3,5-dihydroxyphenylglycine applied 30 min after high-frequency tetanus, or after short-term potentiation decay, elicited no facilitation of long-term potentiation. These results indicate that a distinct time window for the enhancement by ADA and 3,5-dihydroxyphenylglycine of short-term potentiation into long-term potentiation occurs in the dentate gyrus in vivo. This suggests that metabotropic glutamate receptor activation in long-term potentiation occurs within a finite period of time and may be mediated by group 1 metabotropic glutamate receptors. Furthermore, it suggests that metabotropic glutamate receptor modulation of N-methyl-D-aspartate receptors does not account for the role of metabotropic glutamate receptors in long-term potentiation.

Ionotropic and metabotropic glutamate receptors show unique postsynaptic, presynaptic, and glial localizations in the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is a major brain center for integration of auditory information, and excitatory amino acid neurotransmission plays a central role in the processing of this information. In this study, the distribution of glutamate receptors was examined with preembedding immunocytochemistry, using 14 antibodies to ionotropic (GluR1, GluR2/3, GluR4, GluR5-7, GluR6/7, KA2, NR1, NR2A/B, delta 1/2) and metabotropic (mGluR1 alpha, mGluR2/3, mGluR5) glutamate receptor subtypes. Each of these antibodies produced a specific immunolabeling pattern, including a variety of postsynaptic, presynaptic, and glial localizations. Some antibodies showed widespread distribution patterns, notably the antibodies to the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits, GluR2 and GluR3, and the N-methyl-D-aspartate (NMDA) receptor subunit, NR1. In contrast, antibodies to other glutamate receptor subunits produced more restricted distribution patterns, especially that to GluR1, which stained the outer neuropil of the DCN, cartwheel cells, and a small population of presumptive interneurons associated with the dorsal acoustic stria, but produced little or no staining in fusiform cells or deep DCN neurons. Staining of the postsynaptic density and membrane of the granule cell-parallel fiber/cartwheel cell spins synapse was most prevalent with delta 1/2 and mGluR1 alpha antibodies. A unique pattern of staining was found with mGluR2/3 antibody--with staining concentrated in Golgi cells and unipolar brush cells of the middle to deep DCN. Distribution of some glutamate receptors in the DCN shows similarities to that of the cerebellum, where delta 2 and mGluR1 alpha may modulate neurotransmission at parallel fiber synapses, while mGluR2 and/or mGluR3 may modulate mossy terminal function.

Activation of group III metabotropic glutamate receptors is neuroprotective in cortical cultures

(RS)-alpha-Methyl-4-phosphonophenylglycine (MPPG) and (S)-alpha-methyl-3-carboxyphenylalanine (M3CPA), two novel preferential antagonists of group III metabotropic glutamate (mGlu) receptors, antagonized the neuroprotective activity of L-2-amino-4-phosphono-butanoate (L-AP4) or L-serine-O-phosphate in mice cultured cortical cells exposed to a toxic pulse of N-methyl-D-aspartate. In contrast, MPPG did not influence the neuroprotective activity of the selective group II mGlu receptor agonist, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxy-cyclopropyl) glycine (DCG-IV). These results indicate that activation of group III mGu receptors exerts neuroprotective activity against excitotoxic neuronal death. At least one of the two major group III mGlu receptor subtypes, i.e. mGlu4 receptor, is expressed by cultured cortical neurons, as shown by immunocytochemical analysis with specific polyclonal antibodies.

A new highly selective metabotropic excitatory amino acid agonist: 2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid

The homologous series of acidic amino acids, ranging from aspartic acid (1) to 2-aminosuberic acid (5), and the corresponding series of 3-isoxazolol bioisosteres of these amino acids, ranging from (RS)-2-amino-2-(3-hydroxy-5-methylisoxazol-4-yl)acetic acid (AMAA, 6) to (RS)-2-amino-6-(3-hydroxy-5-methylisoxazol-4-yl)hexanoic acid (10), were tested as ligands for metabotropic excitatory amino acid receptors (mGlu1 alpha, mGlu2, mGlu4a, and mGlu6). Whereas AMAA (6) and (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propinoic acid (AMPA, 7) are potent and highly selective agonists at N-methyl-D-aspartic acid (NMDA) and AMPA receptors, respectively, the higher homologue of AMPA (7), (RS)-2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid (homo-AMPA, 8), is inactive at ionotropic excitatory amino acid receptors. Homo-AMPA (8), which is a 3-isoxazolol bioisostere of 2-aminoadipic acid (3), was, however, shown to be a specific and rather potent agonist at mGlu6, approximately 4 times weaker than the nonselective excitatory amino acid receptor agonist (S)-glutamic acid. 2-Aminoadipic acid (3), which shows a complex excitatory amino acid synaptic pharmacology, was an agonist at mGlu6 as well as mGlu2. AMPA (7) and the higher homologue of homo-AMPA (8), (RS)-2-amino-5-(3-hydroxy-5-methylisoxazol-4-yl)pentanoic acid (9), showed relatively weak agonist effects at mGlu6. It is concluded that homo-AMPA (8) is likely to be a useful tool for studies of the pharmacology and physiological role of mGlu6. We describe a new versatile synthesis of this homologue of AMPA and the synthesis of compound 10.

The modulation of calcium currents by the activation of mGluRs. Functional implications

Glutamatergic transmission in the central nervous system (CNS) is mediated by ionotropic, ligand-gated receptors (iGluRs), and metabotropic receptors (mGluRs). mGluRs are coupled to GTP-binding regulatory proteins (G-proteins) and modulate different second messenger pathways. Multiple effects have been described following their activation; among others, regulation of fast synaptic transmission, changes in synaptic plasticity, and modification of the threshold for seizure generation. Some of the major roles played by the activation of mGluRs might depend on the modulation of high-voltage-activated (HVA) calcium (Ca2+) currents. Some HVA Ca2+ channels (N-, P-, and Q-type channels) are signaling components at most presynaptic active zones. Their mGluR-mediated inhibition reduces synaptic transmission. The interference, by agonists at mGluRs, on L-type channels might affect the repetitive neuronal firing behavior and the integration of complex events at the somatic level. In addition, the mGluR-mediated effects on voltage-gated Ca2+ signals have been suggested to strongly influence neurotoxicity. Rather different coupling mechanisms underlie the relation between mGluRs and Ca2+ currents: Together with a fast, membrane-delimited mechanism of action, much slower responses, involving intracellular second messengers, have also been postulated. In the recent past, the relative paucity of selective agonists and antagonists for the different subclasses of mGluRs had hampered the clear definition of the roles of mGluRs in brain function. However, the recent availability of new pharmacological tools is promising to provide a better understanding of the neuronal functions related to different mGluR subtypes. The analysis of the mGluR-mediated modulation of Ca2+ conductances will probably offer new insights into the characterization of synaptic transmission and the development of neuroprotective agents.

Perisynaptic location of metabotropic glutamate receptors mGluR1 and mGluR5 on dendrites and dendritic spines in the rat hippocampus

Ionotropic and metabotropic (mGluR1a) glutamate receptors were reported to be segregated from each other within the postsynaptic membrane at individual synapses. In order to establish whether this pattern of distribution applies to the hippocampal principal cells and to other postsynaptic metabotropic glutamate receptors, the mGluR1a/b/c and mGluR4 subtypes were localized by immunocytochemistry. Principal cells in all hippocampal fields were reactive for mGluR5, the strata oriens and radiatum of the CA1 area being most strongly immunolabelled. Labelling for mGluR1b/c was strongest on some pyramids in the CA3 area, weaker on granule cells and absent on CA1 pyramids. Subpopulations of non-principal cells showed strong mGluR1 or mGluR5 immunoreactivity. Electron microscopic pre-embedding immunoperoxidase and both pre- and postembedding immunogold methods consistently revealed the extrasynaptic location of both mGluRs in the somatic and dendritic membrane of pyramidal and granule cells. The density of immunolabelling was highest on dendritic spines. At synapses, immunoparticles for both mGluR1 and mGluR5 were found always outside the postsynaptic membrane specializations. Receptors were particularly concentrated in a perisynaptic annulus around type 1 synaptic junctions, including the invaginations at 'perforated' synapses. Measurements of immunolabelling on dendritic spines showed decreasing levels of receptor as a function of distance from the edge of the synaptic specialization. We propose that glutamergic synapses with an irregular edge develop in order to increase the circumference of synaptic junctions leading to an increase in the metabotropic to ionotropic glutamate receptor ratio at glutamate release sites. The perisynaptic position of postsynaptic metabotropic glutamate receptors appears to be a general feature of glutamatergic synaptic organization and may apply to other G-protein-coupled receptors.

Activation of multiple metabotropic glutamate receptor subtypes prevents NMDA-induced excitotoxicity in rat hippocampal slices

Metabotropic glutamate receptors (mGluRs) belong to a relative large receptor family consisting of multiple members with important roles in a number of brain functions. We report here that activation of mGluRs prevents the neurotoxic effect induced by N-methyl-D-aspartate (NMDA) in slices from the rat hippocampus. Neuroprotection was elicited when slices were simultaneously exposed to both the selective mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (tACPD) and NMDA. Persisting stimulation of mGluRs after the toxic exposure did not improve the survival of pyramidal or granular cells. The neuroprotection elicited by tACPD toxic exposure did not improve the survival of pyramidal or granular cells. The neuroprotection elicited by tACPD was also evoked by its active isomer, (1S, 3R)-ACPD, and was prevented by the selective mGluR antagonist (+)-alpha-methyl-4-carboxyphenyl-glycine (500 microM), confirming that mGluR activation is involved in the mechanism of action of tACPD. The effect of 100 microM tACPD was reproduced by 100 microM quisqualate, an agonist of mGluR2 and mGluR3 subtypes. No neuroprotection was induced by L-2-amino-4-phosphonobutyrate, a selective agonist for mGluR4, mGluR6, mGluR7 and mGluR8, at 500 microM. Since the NMDA-mediated cell death in hippocampal slices is considered relevant to ischaemia-induced brain injury, these results indicate that mGluRs may be important safety devices used by neurons to decrease their sensitivity to excitotoxic stimuli and increase their chance of survival.

Effects of excitatory amino acids on phosphoinositide metabolism in frog retina

The effects of excitatory amino acid receptor agonists on the hydrolysis of phosphoinositides were examined using frog retinal membranes prelabeled in vitro with either 32PO4 or [3H]inositol. Glutamate stimulated release of [3H]inositol phosphates (IPs) from the retinas and altered the 32P-labeling pattern of phosphatidylinositol (PI) cycle intermediates. This indicates that glutamate affects not only the hydrolysis of phosphoinositides but possibly other steps involved in the PI cycle. Among glutamate analogs, kainate (KA), quisqualate (QA), and, to a lesser extent, N-methyl-D-aspartate (NMDA) mimicked the glutamate effect, whereas L-2-amino-4-phosphonobutyrate (L-AP4) was not effective in causing either the accumulation of [3H]IPs or the alteration of the 32P-labeling pattern of PI cycle intermediates. Among QA specific agonists, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), but not ibotenate (IBO) or trans-1-aminocyclopentane-1,3-dicarboxylate (ACPD) was active in stimulating IPs formation. KA effect on IPs formation may be due to indirect (polysynaptic) activation of receptor(s) other than L-AP4, IBO, or ACPD specific QA receptors. To avoid activating polysynaptic pathways, retinal synaptoneurosomes prelabeled with [3H]inositol were used to examine the hydrolysis of phosphoinositides. As in whole retinas, KA, carbachol (CARB), and NMDA stimulated the release of IPs while L-AP4 had minimal effect. Glycine (GLY) had no effect. Our results show CARB and KA to be the most effective in stimulating the production of IPs. Their effects were exerted directly through separate receptors and not through polysynaptic pathways. ACPD and IBO were the least effective in eliciting the release of IPs. Our studies provide evidence that ionotropic and not metabotropic glutamate receptors are involved in PI metabolism in the retina.

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.

Activation of metabotropic glutamate receptor type 2/3 suppresses transmission at rat hippocampal mossy fibre synapses

1. The effects of metabotropic glutamate receptor (mGluR) agonists on excitatory transmission at mossy fibre-CA3 synapses were studied in rat hippocampal slice preparations using both extracellular and whole-cell clamp recording techniques. 2. Application of a novel and potent mGluR2/mGluR3-specific agonist (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV, 0.1 microM) reversibly suppressed field excitatory postsynaptic potentials evoked by mossy fibre stimulation. DCG-IV at the same concentration did not affect other glutamatergic excitatory transmissions at the commissural/associational input to CA3 or at the Schaffer collateral/commissural input to CA1 regions. 3. This suppressing effect of DCG-IV on mossy fibre transmission was dose dependent and partly antagonized by a competitive mGluR antagonist (+)-methyl-4-carboxylphenylglycine (1 mM). 4. The field potential changes induced by pressure application of glutamate (0.1 mM) to the stratum lucidum of the CA3 region was unaffected by 0.1 microM DCG-IV. 5. In whole-cell clamp experiments, 0.1 microM DCG-IV suppressed excitatory postsynaptic currents evoked by mossy fibre stimulation without inducing detectable inward current in CA3 neurons, and paired-pulse facilitation was enhanced by DCG-IV application. 6. These results suggest that mGluR2/mGluR3 are specifically expressed at mossy fibre synapses in the hippocampal CA3 region, and activation of the receptor suppresses synaptic transmission by an action on a presynaptic site.

Interactions of excitatory amino acid antagonists with conventional antiepileptic drugs

Excitatory amino acid antagonists possess anticonvulsant properties in many experimental models of epilepsy and were shown to potentiate the protective activity of conventional antiepileptics against maximal electroshock-induced seizures in mice. Combined treatments of valproate with either D,L-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid or dizocilpine (NMDA antagonists), which provided a 50% protection against maximal electroshock, produced no side-effects, as measured in the chimney test (motor coordination) or passive avoidance task (long-term memory). Valproate alone at its ED50 against maximal electroshock, induced severe adverse effects. The NMDA antagonists, D-3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid, memantine, procyclidine, and trihexyphenidyl also potentiated the protective activity of conventional antiepileptics but these treatments were associated with considerable side-effects. The non-NMDA receptor antagonists, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline and 1-(amino-phenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine, also enhanced the anticonvulsive action of antiepileptic drugs against maximal electroshock, and these combinations generally resulted in no adverse effects. The potential clinical importance of some combinations of common antiepileptics with excitatory amino acid antagonists is postulated.

The enhancement and the inhibition of noradrenaline-induced cyclic AMP accumulation in rat brain by stimulation of metabotropic glutamate receptors

1. The actions of several metabotropic glutamate receptor and antagonists on noradrenaline (NA)-stimulated [3H]-cyclic AMP accumulation were investigated in rat cerebral cortical slices. 2. Quisqualate (QUIS), L-2-amino-3-phosphonopropionic acid (L-AP3) and glutamate (GLU) elicited concentration-dependent inhibition of (NA)-stimulated [3H]-cyclic AMP accumulation, with IC50 values of 105 +/- 29, 275 +/- 36 and 944 +/- 150 microM respectively. In contrast (Rs)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) (0.5 mM) and N-methyl-D-aspartic acid (NMDA) (0.5 mM) had no effect. 3. (2S,3S,4S)-alpha-(Carboxycyclopropyl)glycine (L-CCGI), 1-Aminocyclo-pentane-1S,3R-dicarbo-xylate (1S,3R-ACPD), ibotenate (IBO) and (RS)-4-carboxy-3-hydroxy-phenylglycine (CHPG)elicited a concentration-dependent enhancement of NA-stimulated [3H]-cyclic AMP accumulation, with EC50 values of 2.5 +/- 0.11, 42 +/- 1.3, 97.8 +/- 2.1 and 157 +/- 13.4 microM, respectively. 4. (S)-3-carboxy-4-hydroxyphenylglycine (3C4HPG) and (S)-4-carboxy-3-hydroxyphenyl-glycine (4C3HPG) produced a biphasic effect, at concentrations up to 100 and 500 microM, respectively, they significantly enhanced the action of NA (100 microM), at 1mM concentration both compounds as well as alpha-methyl-4-carboxyphenylglycine (MCPG) produced a significant inhibition of NA-stimulated cyclic AMP accumulation. 5. A putative mGluR antagonist-L-AP3, inhibited the 1S,3R-ACPD (100 microM) induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation in a biphasic manner with an IC50 of 4.5 microM for the high affinity site, which represented 65% of the total and an IC50 of 283 microM for the low affinity site. 6. beta-adrenoceptor antagonist propranolol inhibited the interaction between 1S,3R-ACPD (100 microM) and NA (100 microM) on [3H]-cyclic AMP accumulation by about 80%, with an IC50 of 0.52 +/- 0.011 microM, to the level observed after 1S,3R-ACPD alone. Prazosin, an alpha 1-adrenoceptor antagonist was more potent (IC50 of 0.091 +/- 0.012 microM) but less efficacious (60% inhibition) as an inhibitor of the interaction either between NA and 1S,3R-ACPD while yohimbine, na alpha 2-adrenoceptor antagonist (up to 1 microM) had no effect. 7. Neither the protein kinase C inhibitor - staurosporine (10 microM) nor thapsigargin (1 microM), which depletes IP3 sensitive calcium stores, inhibited significantly the 1S,3R-ACPD (100 microM)-induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation. 8. Adenosine deaminase (0.5 U/ml) abolished both the 1S,3R-ACPD (100 microM)-induced [3H]-cyclic AMP accumulation and the synergistic interaction of this compound with NA (100 microM). 9. These results indicate the existence of different subtypes of metabotropic glutamate receptors in rat brain which either inhibit or enhance the NA-stimulated [3H]-cyclic AMP accumulation. The enhancement in cerebral cortical slices is mediated via receptors which are blocked with high affinity by L-AP3 and occurs via interactions with endogenous adenosine; the inhibition is mediated by receptors sensitive to quisqualate, L-AP3 and glutamate and may represent a predominant interaction between NA and excitatory amino acids (EAA), which in cerebral cortical slices is masked by excitatory effects.

Molecular cloning, functional expression, pharmacological characterization and chromosomal localization of the human metabotropic glutamate receptor type 3

Glutamic acid is the major excitatory amino acid of the central nervous system which interacts with two receptor families, the ionotropic and metabotropic glutamate receptors. The metabotropic glutamate receptors (mGluRs) are coupled to G proteins and can be divided into three subgroups based on their sequence homology, signal transduction pathway and pharmacology. In this study, we describe the cloning of the cDNA encoding the human metabotropic glutamate receptor type 3 (HmGluR3). It was obtained by reverse transcription-polymerase chain reaction (RT-PCR) with degenerate oligonucleotides corresponding to highly conserved sequences between rat mGluRs. The receptor shows 879 amino acids with 96% amino acid sequence identity with rat mGluR3. It is strongly expressed in fetal and adult whole brain, especially in caudate nucleus and corpus callosum. The gene was identified by fluorescence in situ hybridization on chromosome 7 band q22. Activation of the human mGluR3, permanently expressed in Baby Hamster Kidney (BHK) cells, by excitatory amino acid inhibits the forskolin-stimulated accumulation of intracellular cAMP. The rank order of potency is L-glutamic acid > or = (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R)-ACPD) >> ibotenic acid > quisqualic acid. (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)-MCPG, 1 mM] is without effect on inhibition of forskolin-induced cAMP accumulation by L-glutamic acid.

Enhanced early developmental expression of the metabotropic glutamate receptor mGluR5 in rat brain: protein, mRNA splice variants, and regional distribution

Glutamate stimulates phosphatidyl inositol hydrolysis and mobilizes intracellular calcium through the mediation of metabotropic glutamate receptors (mGluRs), in particular the "Group I" receptors mGluRs, mGluR1, and mGluR5. This activity is markedly enhanced in developing brain relative to the adult. To determine whether this may be due to an increased amount of mGluR5 present in the developing brain, we examined mGluR5 expression using western blotting to measure mGluR5 protein reverse transcription polymerase chain reaction (RT-PCR) to measure mGluR5 mRNA, and immunocytochemistry to assess the regional distribution of mGluR5 morphologically. Western blotting revealed that in all brain regions examined there is more mGluR5 protein present in developing brain than in the adult. In most regions, the developmental decrease was over two-fold. Total mGluR5 mRNA also decreased with development in most regions, but to a much lesser extent than the protein, suggesting that there is considerable post-transcriptional regulation of the expression of this receptor. RT-PCR analysis also demonstrated that in most regions the mGluR5a splice variant is most abundant in the young animals but mGluR5b predominates in the adult. Light microscopic immunocytochemistry indicated that expression is widespread in developing brain, and that the developmental decrease in receptor concentration is due to both an increased growth of receptor-poor tissue regions and decreased expression within receptor-rich regions.

The metabotropic glutamate receptors, mGluR2 and mGluR3, show unique postsynaptic, presynaptic and glial localizations

Glutamate neurotransmission involves numerous ionotropic and metabotropic glutamate receptor types in postsynaptic, presynaptic and glial locations. Distribution of the metabotropic glutamate receptors mGluR2 and mGluR3 was studied with an affinity-purified, characterized polyclonal antibody made from a C-terminus peptide. This antibody, mGluR2/3, recognized both mGluR2 and mGluR3, but did not cross-react with any other type of metabotropic glutamate receptor except for a very slight recognition of mGluR5. Light microscope distribution of the antibody binding sites in the nervous system matched the combined distributions of messenger RNA for mGluR2 and mGluR3. For example, dense staining seen in the accessory olfactory bulb and cerebellar Golgi cells matched high levels of mGluR2 messenger RNA in these structures, while moderately dense staining in the reticular nucleus of the thalamus and light to moderate staining in glia throughout the brain matched significant levels of mGluR3 messenger RNA in these structures. In the rostral olfactory structures, the densest stained neurons belonged to presumptive "necklace olfactory glomeruli." In the hippocampus, staining was densest in the neuropil of the stratum lucidum/pyramidale, stratum lacunosum/moleculare, hilus and middle third of the molecular layer of the dentate gyrus. Ultrastructural studies of the cerebral cortex, hippocampus and caudate-putamen revealed significant staining in postsynaptic and presynaptic structures and glial wrappings of presumptive excitatory synapses; frequently, this staining was concentrated in discrete patches at or near active zones. In the hippocampus, presynaptic staining appeared to be concentrated in terminals of two populations of presumptive glutamatergic axons: mossy fibers originating from granule cells and perforant path fibers originating from the entorhinal cortex. These data suggest that populations of mGluR2 and/or mGluR3 receptors are localized differentially in synapses, i.e. those in and near the postsynaptic and presynaptic membranes and in glial wrappings of synapses, in several regions of the brain. In addition, we provide immunocytochemical evidence of mGluR2 or mGluR3 receptors in presynaptic terminals of glutamatergic synapses. Thus, mGluR2 and mGluR3 are found in various combinations of presynaptic, postsynaptic and glial localizations that may reflect differential modulation of excitatory amino acid transmission.

Structure-activity relationships of new agonists and antagonists of different metabotropic glutamate receptor subtypes

1. We investigated the agonist and antagonist activities of 22 new phenylglycine and phenylalanine derivatives for metabotropic glutamate receptors (mGluRs) by examining their effects on the signal transduction of mGluR1, mGluR2 and mGluR6 subtypes expressed in Chinese hamster ovary cells. This analysis revealed several structural characteristics that govern receptor subtype specificity of the agonist and antagonist activities of phenylglycine derivatives. 2. Hydroxyphenylglycine derivatives possessed either an agonist activity on mGluR1/mGluR6 or an antagonist activity on mGluR1. 3. Carboxyphenylglycine derivatives showed an agonist activity on mGluR2 but an antagonist activity on mGluR1. 4. alpha-Methylation or alpha-ethylation of the carboxyphenylglycine derivatives converts the agonist property for mGluR2 to an antagonist property, thus producing antagonists at both mGluR1 and mGluR2. 5. Structurally-corresponding phenylalanine derivatives showed little or no agonist or antagonist activity on any subtypes of the receptors. 6. This investigation demonstrates that the nature and positions of side chains and ring substituents incorporated into the phenylglycine structure are critical in determining the agonist and antagonist activities of members of this group of compounds on different subtypes of the mGluR family. 7. We also tested two alpha-methyl derivatives of mGluR agonists. (2S, 1'S, 2'S)-2-(2-Carboxycyclopropyl)glycine (L-CCG-I) is a potent agonist for mGluR2 but alpha-methylation of this compound changes its activity to that of an mGluR2-selective antagonist. In contrast, alpha-methylation of L-2-amino-4-phosphonobutyrate (L-AP4) results in retention of an agonist activity on mGluR6. Thus, alpha-methylation produces different effects, depending on the chemical structures of lead compounds and/or on the subtype of mGluR tested.

Differences in agonist and antagonist activities for two indices of metabotropic glutamate receptor-stimulated phosphoinositide turnover

1. The abilities of the four diastereoisomers of 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) to stimulate, and the metabotropic glutamate receptor (mGluR) antagonist (+/-)-alpha-methylcarboxyphenylglycine (MCPG) to inhibit, phosphoinositide turnover in neonatal rat cerebral cortex have been studied. Two indices of phosphoinositide cycle activity were assessed; inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass accumulation, and total inositol phosphate [3H]-InsPx accumulation (in the presence of Li+) in myo-[3H]-inositol prelabelled slices. 2. The diastereoisomers of ACPD stimulated each response with a rank order of potency of 1S, 3R > 1R, 3R > 1S, 3S >> 1R, 3S. The response to 1R, 3R-ACPD was largely prevented by pre-addition of the NMDA-receptor antagonist, MK-801, or omission of extracellular Ca2+, suggesting that this isomer acts indirectly on phosphoinositide responses through activation of NMDA-type ionotropic glutamate receptors. In contrast, the responses to 1S, 3R- and 1S, 3S-ACPD were unaffected by prior addition of MK-801, but were blocked by MCPG. 3. The concentration of 1S, 3R-ACPD required to half-maximally stimulate the Ins(1,4,5)P3 response (-log EC50 (M), -4.09 +/- 0.10) was significantly higher than that required to exert a similar effect on [3H]-InsPx accumulation (-log EC50 (M), -4.87 +/- 0.07; P < 0.01; n = 4). A similar marked 8-9 fold discrepancy between these two values was observed for the 1S, 3S isomer, which elicited similar maximal responses to those caused by 1S, 3R-ACPD. 4. Significant differences were also observed with respect to the ability of (+/-)-MCPG (1 mM) to cause a rightward shift in the concentration-response relationships for 1S, 3R-ACPD-stimulated Ins(1,4,5)P3 (5.59 +/- 0.24 fold shift) and [3H]-InsPx (3.04 +/- 0.34 fold shift; P < 0.01; n = 4) responses, giving rise to Kd values of 218 and 490 microM for (+/-)-MCPG antagonism of the respective responses. 5. The potency difference between the 1S, 3R-ACPD-stimulated Ins(1,4,5)P3 and [3H]-InsPx responses was reduced when experiments were performed in nominally calcium-free medium ([Ca2+]e = 2 - 5 microM) and EC50 values were almost identical when extracellular calcium was reduced further by EGTA addition ([Ca2+]e < or = 100 nM). Similarly, the Kd value for (+/-)-MCPG antagonism of the 1S, 3R-ACPD-stimulated [3H]-InsPx response decreased under [Ca2+]e-free conditions, approaching those obtained for the 1S, 3R-ACPD-stimulated Ins(1,4,5)P3 response in the presence of normal [Ca2+]e. 6. These data suggest that estimates of the activities of mGluR agonists and antagonists, derived by measuring phosphoinositide turnover, can differ significantly depending on whether Ins(1,4,5)P3 mass or [3H]-InsPx responses are measured. In particular, the possibility that the mGluR-mediated [3H]-InsPx response may not simply reflect direct receptor/G protein/phosphoinositidase C (PIC) activation, but may also be the consequence of stimulation of a facilitatory Ca2+-influx pathway is discussed.

Molecular characterization and localization of human metabotropic glutamate receptor type 4

Oligonucleotides of consensus sequences from rat metabotropic glutamate receptor (mGluR) genes were synthesized and used to amplify human DNA by the polymerase chain reaction (PCR). Five unique human sequences homologous to these rat receptor genes were isolated including mGluR4. A human cerebellum cDNA library was screened using this amplified mGluR4 sequence as a probe and yielded clones which between them contained the complete coding sequence for human mGluR4. The coding sequence is very similar to the equivalent rat gene (90% DNA sequence identity and 97% predicted protein sequence identity). The mGluR4 cDNA was transfected in Chinese hamster ovary (CHO) cells and stable clonal cell lines were isolated. Stimulation of the expressed receptor by L-2-amino-4-phosphonobutyrate (L-AP4), L-glutamate or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) resulted in a reduction of forskolin-stimulated cyclic AMP (cAMP) with EC50 values of 0.2, 13 and 90 microM respectively. Quisqualate had little effect at concentrations up to 1 mM. In Northern blots mGluR4 mRNA appears to be brain-specific, and shows a distinct distribution (excluding the cerebellum), being expressed in the thalamus, hypothalamus and caudate nucleus. In situ hybridization studies on human brain sections confirmed this general pattern of distribution. The strongest mGluR4 mRNA signal was found in the cerebellar granule cells consistent with the reported distribution of mGluR4 in the rat brain. The major difference from the rat brain is the presence in the human brain of mGluR4 mRNA in the caudate nucleus and putamen.

A metabotropic glutamate receptor agonist DCG-IV suppresses synaptic transmission at mossy fiber pathway of the guinea pig hippocampus

The effects of specific metabotropic glutamate receptor (mGluR) agonists on field excitatory postsynaptic potentials (fEPSPs) at mossy fiber-CA3 synapses were examined in guinea pig hippocampal slice preparations. Application of a novel and potent group II-selective mGluR agonist (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV; 0.1 microM) reversibly reduced the fEPSPs. Both the group III-selective agonist DL-2-amino-4-phosphonobutyric acid (AP4; 50 microM) and the broad-spectrum agonist 1S,3R-1-aminocyclopentane-1, 3-dicarboxylic acid (1S,3R-ACPD; 5 microM) also reversibly suppressed the fEPSPs. These results suggest that multiple mGluR subtypes (belonging to groups II and III) are expressed at mossy fiber synapses of the guinea pig hippocampus and activation of the receptors reduces the synaptic excitation, although we cannot exclude the possibility that guinea pig mossy fiber-CA3 synapses express a single class of mGluRs with unique pharmacological profiles.

Role of endocrine cell microvesicles in intercellular chemical transduction

Microvesicles (MVs) in endocrine cells are morphologically similar to neuronal synaptic vesicles. MVs were shown to contain proteins involved in neurotransmitter storage such as vacuolar H(+)-ATPase and neurotransmitter transporters, and ones in vesicular trafficking such as synaptobrevins and N-ethylmaleimide-sensitive fusion protein. Isolated MVs accumulate cell-specific neurotransmitters in an energy-dependent manner. Upon stimulation, the MVs may fuse with the plasma membrane and secrete the internal neurotransmitters. Thus, endocrine cells possess an MV-mediated secretion system as an intercellular signal transducing system.

Activation of hamster suprachiasmatic neurons in vitro via metabotropic glutamate receptors

The neurophysiological effects of ionophoretic application of the metabotropic glutamate receptor agonist (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid on the firing rate of single suprachiasmatic nucleus cells was studied in a hypothalamic slice preparation. In addition, the effects of the phenylglycine derivative (RS)-alpha-methyl-4-carboxyphenylglycine, a selective metabotropic glutamate receptor antagonist, on responses evoked by (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid and N-methyl-D-aspartate were assessed. (1S,3R)-1-Amino-1,3-cyclopentanedicarboxylic acid elicited current-dependent increases in neuronal activity in 65% of all suprachiasmatic nucleus cells studied, while N-methyl-D-aspartate activated 93% of the same cells. Cells in the ventrolateral suprachiasmatic nucleus were more sensitive to (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid application (82% of cells activated) than those in the dorsomedial suprachiasmatic nucleus (28% of cells activated). In addition, responses evoked by (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid in the ventrolateral suprachiasmatic nucleus had a rapid onset and a prolonged recovery from agonist application, whereas responses elicited in the dorsomedial suprachiasmatic nucleus were slower in onset and recovered more quickly from agonist application. Co-application of (RS)-alpha-methyl-4-carboxyphenylglycine selectivity attenuated (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid-evoked responses but had no effect on N-methyl-D-aspartate-evoked activity in the same cells. These results indicate that (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid application activates single cells in the hamster suprachiasmatic nucleus and these responses are selectively attenuated by (RS)-alpha-methyl-4-carboxyphenylglycine. Cells in the ventrolateral suprachiasmatic nucleus are more sensitive to (1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid application than those in the dorsomedial suprachiasmatic nucleus. These results are the first demonstration that metabotropic glutamate receptors can modulate spontaneous neuronal activity within the suprachiasmatic nucleus, the predominant mammalian circadian pacemaker.

Differentiation of group 2 and group 3 metabotropic glutamate receptor cAMP responses in the rat hippocampus

The effects of group 2- versus group 3-selective metabotropic glutamate (mGlu) receptor agonists were examined against forskolin (10 microM)-, vasoactive intestinal peptide (VIP; 1 microM)- and 5'-N-ethylcarboxamidoadenosine (NECA; 10 microM)-stimulated cAMP accumulations in adult rat hippocampal slices (in the presence of adenosine deaminase). Group 2 mGlu receptor-selective ((1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid (1S,3R-ACPD) and (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (L-CCG I)) and group 3 mGlu receptor-selective (L-2-amino-4-phosphonobutyric acid (L-AP4) and L-serine-O-phosphate) agonists greatly inhibited forskolin-stimulated cAMP formation ( > 80% at maximally effective concentrations). In contrast, stimulation of cAMP by VIP or NECA was inhibited by group 3, but not by group 2, mGlu receptor agonists. In fact, group 2 mGlu receptor agonists greatly potentiated cAMP accumulation evoked by NECA. Both the inhibitory effects of 1S,3R-ACPD on forskolin-stimulated cAMP and the potentiating effects on NECA-stimulated cAMP accumulation were reversed by the competitive group 1/2 mGlu receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG). However, (+)-MCPG had no effects on L-AP4 inhibition of cAMP. Thus, the effects of group 2 versus group 3 mGlu receptor agonists on cAMP coupling can be pharmacologically as well as functionally differentiated in the rat hippocampus.

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.

A metabotropic glutamate receptor agonist regulates neurotrophin messenger RNA in rat forebrain

We have examined the role of metabotropic glutamate receptor activation in regulating neurotrophin messenger RNA levels in the brain with the use of the selective agonist (1S,3R)-1-aminocy-clopentane-1,3-dicarboxylic acid. Intracerebroventricular injection of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid into adult adult rats resulted in increased expression of nerve growth factor and brain-derived neurotrophic factor messenger RNA in the hippocampal and pyriform cortex and decreased levels of neurotrophin-3 messenger RNA in the hippocampal dentate gyrus granule cell layer. C-fos messenger RNA levels were also increased throughout hippocampal and cortical subfields following (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid administration. (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid-induced changes in messenger RNA levels occurred without behavioral seizures, yet these changes were similar in magnitude and time course to early changes in neurotrophin and c-fos messenger RNA levels observed following recurrent limbic seizures. In contrast quisqualate, a potent agonist of metabotropic as well as ionotropic kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, was only capable of inducing increased expression of brain-derived neurotrophic factor messenger RNA at doses which produced recurrent motor seizures, and both effects were completely inhibited by the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. Neurotrophin messenger RNA changes induced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid were also partially susceptible to 6-cyano-7-nitroquinoxaline-2,3-dione antagonism, as well as the specific N-methyl-D-aspartate receptor antagonist (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)-cyclohepten-5,10- iminedizoleipine. These results suggest that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-sensitive metabotropic glutamate receptors can dramatically increase the expression of neurotrophin and c-fos messenger RNAs in rat forebrain without producing significant behavioral trauma and that these influences may involve ionotropic glutamate receptors in certain brain regions.

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.

Intrinsic neuromodulation: altering neuronal circuits from within

There are two sources of neuromodulation for neuronal circuits: extrinsic inputs and intrinsic components of the circuits themselves. Extrinsic neuromodulation is known to be pervasive in nervous systems, but intrinsic neuromodulation is less recognized, despite the fact that it has now been demonstrated in sensory and neuromuscular circuits and in central pattern generators. By its nature, intrinsic neuromodulation produces local changes in neuronal computation, whereas extrinsic neuromodulation can cause global changes, often affecting many circuits simultaneously. Studies in a number of systems are defining the different properties of these two forms of neuromodulation.

Modulation of amino acid-gated ion channels by protein phosphorylation

The major excitatory and inhibitory amino acid receptors in the mammalian central nervous system are considered to be glutamate, gamma-aminobutyric acid type A (GABAA), and glycine receptors. These receptors are widely acknowledged to participated in fast synaptic neurotransmission, which ultimately is responsible for the control of neuronal excitability. In addition to these receptors being regulated by endogenous factors, including the natural neurotransmitters, they also form target substrates for phosphorylation by a number of protein kinases, including serine/threonine and tyrosine kinases. The process of phosphorylation involves the transfer of a phosphate group(s) from adenosine triphosphate to one or more serine, threonine, or tyrosine residues, which are invariably found in an intracellular location within the receptor Phosphorylation is an important means of receptor regulation since it represents a covalent modification of the receptor structure, which can have important implications for ion channel function. This chapter reviews the current molecular and biochemical evidence regarding the sites of phosphorylation for both native neuronal and recombinant glutamate, GABAA and glycine receptors, and also reviews the functional electrophysiological implications of phosphorylation for receptor function.

Activity of 2,3-benzodiazepines at native rat and recombinant human glutamate receptors in vitro: stereospecificity and selectivity profiles

The activity and selectivity of the glutamate receptor antagonists belonging to the 2,3-benzodiazepine class of compounds have been examined at recombinant human non-NMDA glutamate receptors expressed in HEK293 cells and on native rat NMDA and non-NMDA receptors in vitro. The racemic 2,3-benzodiazepines GYKI52466, LY293606 (GYKI53405) and LY300168 (GYKI53655) inhibited AMPA (10 microM)-mediated responses in recombinant human GluR1 receptors expressed in HEK293 cells with approximate IC50 values of 18 microM, 24 microM and 6 microM, respectively and AMPA (10 microM) responses in recombinant human GluR4 expressing HEK293 cells with approximate IC50 values of 22 microM, 28 microM and 5 microM, respectively. GYKI 52466, LY293606 and LY300168 were non-competitive antagonists of AMPA receptor-mediated responses in acutely isolated rat cerebellar Purkinje neurons with approximate IC50 values of 10 microM, 8 microM and 1.5 microM, respectively. The activity of racemic compounds LY293606 and LY300168 was established to reside in the (-) isomer of each compound. At a concentration of 100 microM, GYKI52466, LY293606 and LY300168 produced < 30% inhibition of kainate-activated currents evoked in HEK293 cells expressing either human homomeric GluR5 or GluR6 receptors or heteromeric GluR6+KA2 kainate receptors. The activity of the 2,3-benzodiazepines at 100 microM was weak at kainate receptors, but was stereoselective. Similar levels of inhibition were observed for kainate-induced currents in dorsal root ganglion neurons. Intact tissue preparations were also used to examine the stereoselective actions of the 2,3-benzodiazepines. In the cortical wedge preparation, the active isomer of LY300168, LY303070, produced a non-competitive antagonism of AMPA-evoked depolarizations with smaller changes in depolarizations induced by kainate and no effect on NMDA-dependent depolarizations. LY303070 was also effective in preventing 30 microM AMPA-induced depolarizations in isolated spinal cord dorsal roots with an approximate IC50 value of 1 microM. Synaptic transmission in the hemisected spinal cord preparation was stereoselectively antagonized by the active isomers of LY300168 and LY293606. In summary, these results indicate that 2,3-benzodiazepines are potent, selective and stereospecific antagonists of the AMPA subtype of the non-NMDA glutamate receptor.

Glutamate depresses release by activating non-conventional glutamate receptors at crayfish nerve terminals

The present study shows that release of glutamate from crayfish nerve terminals is inhibited at low depolarizing current pulses by glutamate, N-methyl-D-aspartate (NMDA) and quisqualate. These agonists elicit inhibitory effects at concentrations as low as 10(-8) M (quisqualate) and 10(-7) M (glutamate and NMDA). The NMDA-mediated inhibition is blocked by (+/-)-2-amino-5-phosphonovaleric acid (APV). The quisqualate-mediated inhibition is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Both CNQX and APV are needed to block glutamate-mediated inhibition. The inhibition of release is not accompanied by a detectable change in presynaptic membrane conductance at the secondary branch. Using fura-2, Ca2+ accumulation during repetitive stimulation (100 Hz) was monitored in single release boutons. Inhibition of release, elicited by 10(-4) M glutamate, was not associated with a reduction in the accumulation of Ca2+. We show that the glutamate released from a single or a few release boutons during normal activity acts similarly to glutamate added externally, i.e. it inhibits its own release.

DCG-IV selectively attenuates rapidly triggered NMDA-induced neurotoxicity in cortical neurons

Molecular cloning has revealed the existence of at least eight subtypes of metabotropic glutamate receptors (mGluRs). We examined the effect of (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV), a selective agonist of the mGluR 2/3 subtype, on excitotoxicity in mouse cortical cell cultures. Addition of DCG-IV to the exposure medium partially attenuated the rapidly triggered excitotoxic death induced by a 5 min exposure to 200 microM NMDA. This neuroprotective effect was reversed by coapplication of alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of mGluRs, by pertussis toxin pretreatment and also by preincubation with dibutyryl cAMP, a stable analogue of cAMP. These results suggest that the activation of mGluR 2/3 is neuroprotective in our system. However, DCG-IV did not attenuate the slowly triggered neuronal death induced by 24 h exposure to low concentrations of NMDA, alpha-amino-1,3-cyclopentanedicarboxylic acid (AMPA) or kainate. The failure of DCG-IV to block slowly triggered NMDA neurotoxicity is likely due to weak NMDA agonist activity, as demonstrated in whole-cell recording.

Metabotropic glutamate group II receptors are responsible for the depression of synaptic transmission induced by ACPD in the dentate gyrus

The functional role of metabotropic glutamate (mGlu) receptors in the rat dentate gyrus was investigated. By using extracellular recording techniques in slices, it was found that the depression induced by the mGlu receptor agonist (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylate (ACPD) was mediated through the mGlu group II receptors. The mGlu receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) (500 mu M), active at group I and group II subtype receptors, was effective in antagonizing the ACPD (30 mu M) - induced depression of the excitatory field potentials. An antagonist selective for group I, (S)-4-carboxyphenylglycine (4CPG), did not block the effects induced by ACPD, but by itself produced a dose-dependent depression of the field potentials. This ACPD-like effect shown at high concentrations of 4CPG (300 mu M) is explained by its group II receptor agonistic properties and was blocked by bath application of MCPG (500 mu M). A selective agonist of group I, (S)-3-hydroxyphenylglycine (3-HPG), did not cause any depression of synaptic transmission. However, the selective mGlu group II receptor agonist, (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I), induced a marked dose-dependent depression and its action was blocked by MCPG (500 mu M). Furthermore, the selective mGlu group III receptor antagonist, alpha-methyl-L-2-amino-4-phosphonobutyrate (MAP4) (500 mu M), was not able to antagonize the depression induced by ACPD (30 mu M), but was effective in blocking the action induced by the selective mGlu group III agonist, L-2-amino-4-phosphonobutyrate (L-AP4) (100 mu M). These results indicate that mGlu group II receptors, but not groups I or III, are involved in the depression of synaptic transmission in the dentate area of the hippocampus induced by ACPD.

Presynaptic localization of a metabotropic glutamate receptor, mGluR7, in the primary afferent neurons: an immunohistochemical study in the rat

An antibody which recognizes specifically a metabotropic glutamate receptor, mGluR7, was produced by using a trpE fusion protein containing a C-terminal sequence of rat mGluR7. Neuropil in laminae I and II of the dorsal horn of the rat, as well as many neuronal cell bodies in the dorsal root ganglion, showed mGluR7-like immunoreactivity; the immunoreactivity in neuropil was seen in axon terminals, which were filled with round synaptic vesicles and constituted axodendritic and axosomatic asymmetric synapses. The mGluR7-like immunoreactivity in laminae I and II in the dorsal horn was reduced after dorsal rhizotomy. The results indicate that some axon terminals of the primary afferent fibers to laminae I and II of the dorsal horn are provided with mGluR7.

Effects of metabotropic glutamate receptor agonists on excitatory and inhibitory transmissions in cultured rat hippocampal neurons

Effects of the metabotropic glutamate receptor agonist, trans-ACPD (tACPD), on the excitatory and the inhibitory transmissions were examined with cultured rat hippocampal neurons, by recording monosynaptic excitatory (EPSCs) and inhibitory postsynaptic currents (IPSCs) evoked by electrical stimulation of single presynaptic neurons. EPSCs were suppressed reversibly by application of 100 microM tACPD in almost all the tested neuron pairs with excitatory connections. Effects of tACPD on IPSCs were variable. Application of tACPD did not affect IPSCs in 15 pairs, suppressed them in eight pairs and enhanced them in the remaining eight pairs out of 31 neuron pairs with inhibitory connections. The study demonstrated that in the hippocampus the activation of metabotropic glutamate receptors modulated the inhibitory transmission differently at different synapses, whereas it always suppressed the excitatory transmission.

Metabotropic glutamate receptors inhibiting excitatory synapses in the CA1 area of rat hippocampus

In the CA1 region of hippocampal slices prepared from young adult rats, we studied the ability of several specific agonists of metabotropic glutamate receptors (mGluRs) to depress excitatory synaptic transmission at the CA3-CA1 pyramidal cell synapses. Three groups of mGluRs have been described: group 1 (mGluR1 and 5) receptors are positively coupled to phospholipase C whereas group 2 (mGluR2 and 3) and group 3 (mGluR4, 6, 7 and 8) receptors are negatively coupled to adenylate cyclase. We found that the broad-spectrum agonist (1S,3R)-1-aminocyclopentyl-1,3-dicarboxylate and the group 1-specific agonist (R,S)-dihydroxyphenylglycine both reversibly inhibited evoked field excitatory postsynaptic potentials, indicating the involvement of group 1 mGluRs. (R,S)-3,5-dihydroxyphenylglycine presumably inhibited transmission via a presynaptic mechanism, as whole-cell voltage-clamp recordings revealed that inhibition of the synaptic transmission was always accompanied with an increase in paired-pulse facilitation. Treatment with a specific blocker of mGluR1 receptors, the phenylglycine derivative (S)-4-carboxyphenylglycine, was without effect on the (1S,3R)-1-amino-cyclopentyl-1,3-dicarboxylate-induced depression of the field excitatory postsynaptic potentials, strongly suggesting that mGluR5 receptors are responsible for the (1S,3R)-1-aminocyclopentyl-1,3-dicarboxylate effect. Two selective agonists of group 2 mGluRs, (2S,1's,2's)-2-(2'-carboxycyclopropyl)glycine and 4-carboxy-3-hydroxyphenylglycine, were totally ineffective in blocking CA3-CA1-evoked synaptic transmission, excluding the involvement of mGluR2/3 subtypes at this developmental stage.

Chemical synaptic transmission in the cochlea

The last two decades have witnessed major progress in the understanding of cochlear mechanical functioning, and in the emergence of cochlear neurochemistry and neuropharmacology. Recent models describe active processes within the cochlea that amplify and sharpen the mechanical response to sound. Although it is widely accepted that outer hair cells (OHCs) contribute to these processes, the nature of the medial efferent influence on cochlear mechanics needs further clarification. Acetylcholine (ACh) is the major transmitter released onto OHCs during the stimulation of these efferents. The inhibitory influence of this system is mediated by post- and presynaptic nicontinic and muscarinic receptors and the role of other neuroactive substances [gamma-aminobutyric acid (GABA), calcitonin gene-related peptide (CGRP), adenosine 5'-triphosphate (ATP) or nitric oxide (NO)] remains to be determined. The inner hair cells (IHCs) that transduce the mechanical displacements into neural activity, release glutamate on receptor-activated channels of AMPA, kainate, and NMDA types. This synapse is in turn controlled and/or regulated by the lateral efferents containing a cocktail of neuroactive substances (ACh, GABA, dopamine, enkephalins, dynorphin, CGRP). This glutamatergic nature of the IHCs is responsible for the acute destruction of the nerve endings and subsequently for neuronal death, damage usually described in various cochlear diseases (noise-induced hearing losses, neural presbycusis and certain forms of sudden deafness or peripheral tinnitus). These pathologies also include a regrowth of new dendritic processes by surviving neurons up to IHCs. Understanding the subtle molecular mechanisms which underly the control of neuronal excitability, synaptic plasticity and neuronal death in cochlear function and disease is a very important issue for the development of future therapies.

Metabotropic glutamate receptor mGluR5 subcellular distribution and developmental expression in hypothalamus

The metabotropic glutamate receptor mGluR5 is a G-protein coupled receptor that plays a key role in release of Ca2+ from internal stores via inositol triphosphate mobilization. Western and Northern blot analyses revealed a greatly enhanced expression of mGluR5 in rats during early stages of hypothalamic development compared with the adult. This enhanced developmental expression provides an explanation for the dramatic physiological response of developing neurons to metabotropic glutamate receptor activation and supports the argument that metabotropic glutamate receptors may play an important role in hypothalamic development. During development, expression of the mGluR5 gene was reduced, not only in the hypothalamus but also in other regions of the brain. A differential decrease in mGluR5 protein was found in different brain regions with Western blot analysis. The hypothalamus showed a sixfold decrease in mGluR5 with development, whereas the cortex showed only a threefold decrease. Immunocytochemistry with an affinity-purified antibody against a peptide deduced from the cloned mGluR5 gene revealed selective expression in some regions in the adult hypothalamus. In the adult and developing (postnatal day 10) brain, immunoreactive neurons were found in the suprachiasmatic nucleus, preoptic area, lateral hypothalamus, and mammillary region, areas where the related metabotropic glutamate receptor mGluR1 is also found. In contrast, the ventromedial nucleus, an area critically involved in the regulation of food intake and metabolic balances, showed strong mGluR5 immunoreactivity but no mGluR1 immunoreactivity. Little or no mGluR5 staining was found in the neurosecretory neurons of the paraventricular, supraoptic, and arcuate nuclei. Ultrastructurally, mGluR5 was associated with the cytoplasmic face of the plasmalemma on hypothalamic dendrites, dendritic spines, and neuronal perikarya in the adult. The strongest immunoreactivity was found in patches on the membrane, sometimes associated with the postsynaptic side of synapses and sometimes associated with nonsynaptic dendritic or perikaryal membrane. Intense immunostaining was found on some astrocyte processes surrounding synaptic complexes containing asymmetrical synapses. These astrocytes would be in an ideal position to receive excitatory signals from glutamatergic axons. Unlike the punctate appearance of immunolabeling on neuronal membranes, astrocytes showed continuous staining along the plasma membrane.

Excitatory amino acid-, except 1S,3R-ACPD, induced transient high stimulation of phosphoinositide metabolism during hippocampal neuron development

Rat hippocampal neurons in culture extended their neurites until day 5 in vitro (DIV). Then, the mean neuritic length slightly decreased. Excitatory amino acid (EAA)-elicited inositol phosphate (IP) formation increased from 0.5 to 2 DIV, reached a plateau between 2 and 4-5 DIV, and then gradually decreased until 10 DIV. This decrease was likely not due to neuronal death. This developmental pattern was observed for N-methyl-D-aspartate, kainate, glutamate, ibotenate and quisqualate (QA). Interestingly, the 1S,3R-aminocyclopentane dicarboxylate (1S,3R-ACPD) response slightly increased during neuronal culture development. At 3 DIV, the ionotropic antagonists 6,7-dinitro-quinoxalin-2,3-dion and D-2-amino-5-phosphonopentanoate efficiently blocked N-methyl-D-aspartate and kainate-elicited IP formation, and partially inhibited glutamate and ibotenate responses. QA and 1S,3R-ACPD responses were not affected, suggesting a metabotropic action for these two compounds. Furthermore, QA and 1S,3R-ACPD potencies significantly increased between 3 and 10 DIV. The transient high activity periods induced by EAA, except for 1S,3R-ACPD, are not observed for norepinephrine, carbachol and potassium chloride responses. Taken together, these data suggest that: (i) QA and 1S,3R-ACPD can act on two different glutamate metabotropic receptors subtypes during development; and (ii) the EAA-induced transient peaks of IP stimulation, which are specific with respect to other neuroactive substances profiles, could be involved in the development of hippocampal neurons. Indeed, these transient high activities take place when the neuritic length regularly increases in vitro.