Pharmacology and physiology of metabotropic glutamate receptors in mammalian central nervous system

Neuroprotective strategies in epilepsy

Dr. Carlen reviews the evidence that seizures may cause cell death and discusses possible strategies for preventing seizure-induced brain damage.

Metabotropic glutamate receptor activation and blockade: their role in long-term potentiation, learning and neurotoxicity

Metabotropic glutamate receptors represent a fairly recent addition to the family of glutamate receptors. These receptors have the distinguishing feature of being coupled to G-proteins rather than ion channels and they appear to have a variety of functional characteristics. These receptors play a vital role, for example, in the induction and maintenance of long-term potentiation, the most popular current model of the biological correlates of learning and memory. Blockade of metabotropic glutamate receptors prevents long-term potentiation induction and learning in a variety of tasks in different species. Chronic metabotropic glutamate receptor activation is also associated with neurodegeneration and selective neuronal loss when agonists of these receptors are injected in high concentrations directly into the brain. Metabotropic glutamate receptors also play a role in the normal development of the nervous system and these sites within the central nervous system offer possible routes for drug therapies; selective receptor antagonists, for example, may prove to have the very desirable feature of endowing neuroprotection during ischaemic episodes whilst allowing normal excitatory neurotransmission to occur.

Protein kinase C in synaptic plasticity: changes in the in situ phosphorylation state of identified pre- and postsynaptic substrates

1. Long-term potentiation and its counterpart long-term depression are two forms of activity dependent synaptic plasticity, in which protein kinases and protein phosphatases are essential. 2. B-50/GAP-43 and RC3/neurogranin are two defined neuronal PKC substrates with different synaptic localization. B-50/GAP-43 is a presynaptic protein and RC3/neurogranin is only found at the postsynaptic site. Measuring their phosphorylation state in hippocampal slices, allows us to simultaneously monitor changes in pre- and postsynaptic PKC mediated phosphorylation. 3. Induction of LTP in the CA1 field of the hippocampus is accompanied with an increase in the in situ phosphorylation of both B-50/GAP-43 and RC3/neurogranin, during narrow, partially overlapping, time windows. 4. Pharmacological data show that mGluR stimulation results in an increase in the in situ phosphorylation of B-50/GAP-43 and RC3/neurogranin.

Potentiation of spontaneous acetylcholine release from motor nerve terminals by glutamate in Xenopus tadpoles

Extracellular application of glutamate (100 microM) increased the spontaneous secretion of acetylcholine, as well as the amplitude and decay time of miniature endplate potentials at developing neuromuscular synapses in Xenopus tadpoles. Kainate, quisqualate and N-methyl-D-aspartate (100 microM each) increased miniature endplate potential frequency by 26-, 13- and four-fold, respectively. The rank order of efficacy at 100 microM was kainate > quisqualate > N-methyl-D-aspartate > glutamate. The effect of kainate on miniature endplate potential frequency was inhibited by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (20 microM), but not by (+/-)-2-amino-5-phosphonovalerate (20 microM). Treatment with the voltage-dependent Ca2+ channel blockers verapamil (10 microM), Cd2+ (100 microM) or omega-conotoxin (1 microM) inhibited the potentiating action of kainate on miniature endplate potential frequency. On the other hand, 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (300 microM), a glutamate metabotropic receptor agonist, inhibited the spontaneous acetylcholine release, which was antagonized by the application of 2-amino-3-phosphonopropionate (500 microM). The potentiating effect of glutamate receptor agonists on the miniature endplate potential frequency declined or disappeared in older Xenopus tadpoles. Quisqualate (100 microM) and N-methyl-D-aspartate (100 microM) but not kainate (30 microM) increased the amplitude and decay time of miniature endplate potential, whereas 1S, 3R-1-aminocyclopentane-1, 3-dicarboxylate (300 microM) only increased the decay time of miniature endplate potentials. These results suggest that there are kainate/quisqualate and N-methyl-D-aspartate receptors existing in the motor nerve terminals of younger Xenopus tadpoles and the activation of these receptors potentiates spontaneous acetylcholine release through increasing Ca2+ influx. Our data suggest that the presynaptic glutamate receptors on cholinergic terminals may be involved in feedback regulation of acetylcholine secretion at earlier embryonic stages.

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.

Long-lasting changes of calcium oscillations in astrocytes. A new form of glutamate-mediated plasticity

Long-term changes of synaptic strength in the central nervous system are mediated by an increase of cytosolic calcium concentration ([Ca2+]i) following activation of excitatory neurotransmitter receptors. These phenomena, which represent a possible cellular basis for learning and memory processes in eukaryotes, are believed to be restricted to neurons. Here we provide evidence for a long-term change of the response elicited by the excitatory neurotransmitter glutamate in a non-neuronal cell population of the central nervous system, i.e. visual cortical astrocytes in culture. Stimulation with glutamate induces in astrocytes a regular pattern of [Ca2+]i oscillations. A second stimulation, after an interval ranging from 2 to 60 min, induces an oscillatory response characterized by an increased frequency. Induction of this change in the astrocyte response is abolished by a specific inhibitor of the nitric oxide synthase and recovers upon exogenous nitric oxide generation or addition of a permeant cGMP analogue. Local brief pulses of glutamate to individual astrocytes, at a rate of 1 Hz, also elicit [Ca2+]i oscillations whose frequency increases following a second series of pulses. The long-lasting modification in the [Ca2+]i oscillatory response induced by glutamate in astrocytes demonstrates that in the central nervous system cellular memory is not a unique feature of neurons.

Excitatory amino acid receptors in central cardiovascular regulation

Excitatory amino acid neurotransmitters and their receptors are the principal mediators of fast synaptic transmission within the central nervous system. Accumulating evidence suggests that synaptic activation of excitatory amino acid receptors in the nucleus of the tractus solitarius, rostral and caudal ventrolateral medulla, and in the spinal cord play a key role in neural transmission of cardiovascular information in the central nervous system. Pharmacological blockade of excitatory amino acid receptors at these sites eliminates a variety of centrally-mediated cardiovascular responses. These include baroreceptor reflexes and increases in arterial pressure produced by stimulation of various brain regions as well as peripheral afferent nerves. These observations indicate that synaptic activation of EAA receptors at specific sites within the brainstem and in the spinal cord play an important role in central cardiovascular regulation.

Role of metabotropic glutamate receptors in ventrolateral medulla of hypertensive rats

Evidence is accumulating for the role of metabotropic, as well as ionotropic, glutamate receptors in cardiovascular regulation. We sought to determine whether stimulation of metabotropic glutamate receptors in the rostral ventrolateral medulla would evoke enhanced cardiovascular responses in spontaneously hypertensive rats (SHR). Thus, we microinjected (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD], a selective agonist of metabotropic glutamate receptors, into the rostral ventrolateral medulla of urethane-anesthetized adult SHR and age-matched Wistar-Kyoto (WKY) rats. Microinjection of (1S,3R)-ACPD (1 nmol/50 nL) produced increases in mean arterial pressure and splanchnic sympathetic nerve activity in SHR (+41 +/- 6 mm Hg and +34 +/- 4%, respectively) that were significantly greater than those observed in WKY rats (+18 +/- 3 mm Hg and +22 +/- 3%, P < .005 and P < .05, respectively). The pressor responses evoked by microinjection of L-glutamate (2 nmol), N-methyl-D-aspartate (20 pmol), or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (5 pmol) were also significantly (P < .001) augmented in SHR (+55 +/- 3, +61 +/- 7, and +53 +/- 5 mm Hg, respectively, in SHR versus +31 +/- 1, +30 +/- 3, and +28 +/- 2 mm Hg in WKY rats). Results indicate that stimulation of metabotropic, as well as ionotropic, glutamate receptors in the rostral ventrolateral medulla evokes enhanced cardiovascular responses in SHR, which may contribute to hypertension in this model.

(2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl) glycine positively modulates metabotropic glutamate receptors coupled to polyphosphoinositide hydrolysis in rat hippocampal slices

In rat hippocampal slices, the novel metabotropic glutamate receptor (mGluR) ligand, (1S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) enhanced the stimulation of polyphosphoinositide (PPI) hydrolysis elicited by quisqualate or by submaximal concentrations of ibotenate or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). The enhancing effect of DCG-IV was (i) specific for mGluR agonists, (ii) restricted to hippocampal slice preparation, (iii) reversible, and (iv) not subject to homologous desensitization, in addition, DCG-IV did not interact with L-2-amino-4-phosphonobutanoate (AP4), a noncompetitive antagonist of mGluRs coupled to PPI hydrolysis in brain slices [32]. The action of DCG-IV on quisqualate-stimulated PPI hydrolysis was insensitive to antagonists of ionotropic glutamate receptors and did not appear to be a consequence of a reduction in the intracellular levels of cAMP [14]. When the stimulation of PPI hydrolysis was measured as a function of the incubation time, DCG-IV potentiated quisqualate-stimulated PPI hydrolysis after 60 min of incubation, when quisqualate had already reached its maximal effect. Knowing that activation of protein kinase C (PKC) limits the extent of mGluR agonist-stimulated PPI hydrolysis over time, we have studied the enhancing effect of DCG-IV in the presence of the PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA). As expected [9], TPA reduced quisqualate-stimulated PPI hydrolysis in control slices, but was inactive in slices incubated in the presence of DCG-IV. Taken collectively, these results suggest that DCG-IV positively modulates the activity of mGluRs coupled to PPI hydrolysis through a mechanism, which involves PKC-mediated phosphorylation processes.

Metabotropic glutamate receptor activation decreases epileptiform activity in rat neocortex

Intracellular and extracellular recordings were obtained from layers II-III of slices of adult rat neocortex maintained in vitro. Excitatory postsynaptic potentials (EPSPs) and epileptiform discharges (paroxysmal depolarizing shifts, PDSs) were evoked in the presence of bicuculline methiodide. Responses were monitored before, during, and after bath application of the putative metabotropic glutamate receptor agonists, 1S, 3R-ACPD and L-AP4. Peak EPSP amplitude and both PDS amplitude and duration were reduced during 1S,3R-ACPD application. Area measurements indicate that PDSs were reduced to 51.7 +/- 19.9% of control. Stimulus threshold for evoking a PDS was increased in the presence of 1S,3R-ACPD or L-AP4. Pretreatment of slices with 1S.3R-ACPD did not prevent the generation of epileptiform events when bicuculline subsequently was applied. These results indicate that mGluR activation by 1S,3R-ACPD has significant suppressive effects on evoked epileptiform activity in the adult rat neocortex in vitro.

Glutamate agonists and [3H]GABA release from rat hippocampal slices: involvement of metabotropic glutamate receptors in the quisqualate-evoked release

The effects of glutamate agonists and their selective antagonists on the Ca(2+)-dependent and independent releases of [3H]GABA from rat coronal hippocampal slices were studied in a superfusion system. The Ca(2+)-dependent release evoked by glutamate, kainate and N-methyl-D-aspartate (NMDA) gradually declined with time despite the continuous presence of the agonists. Quisqualate (QA) caused a sustained release which exhibited no tendency to decline within the 20-min period of stimulation. This release was enhanced in Ca(2+)-free medium. The release evoked by QA in Ca(2+)-containing medium was significantly inhibited by (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohept-5,10-imine hydrogen maleate (MK-801) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), showing that QA activates NMDA receptors directly or indirectly through (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. The inhibition of MK-801 was slightly diminished and that of CNQX totally abolished in Ca(2+)-free medium. Verapamil inhibited the QA-activated release in both Ca(2+)-containing and Ca(2+)-free media. The effect of QA but not that of AMPA was blocked in Ca(2+)-free medium by L(+)-2-amino-3-phosphonopropionate (L-AP3), a selective antagonist of the metabotropic glutamate receptor. It is suggested that the sustained release of GABA is also mediated partly by activation of metabotropic receptors and mobilization of Ca2+ form intracellular stores.

Metabotropic and ionotropic excitatory amino acid receptor agonists induce different behavioral effects in mice

Intracerebroventricular (i.c.v.) infusion in mice of the selective metabotropic excitatory amino acid receptor agonist 1S,3R-1- aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD) (0.6-575 nmol/min) dose dependently induced face washing and scratching. In contrast, the subtype-specific ionotropic excitatory amino acid receptor agonists N-methyl-D-aspartate (NMDA), kainate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) (0.3-3.0 nmol/min) dose dependently induced clonic convulsions. I.c.v. infusion of the non-selective metabotropic receptor agonists ibotenate (6 nmol/min) or quisqualate (30 nmol/min) induced clonic convulsions. However, when ionotropic receptors were blocked with (+)-5-methyl-10,11-dihydro-5H-dibenzo-(a,d)cyclohepten-5,10-imine maleate (MK-801, dizoclipine) (3 nmol/min) or 2,3-dihydroxy-6-nitro-7- sulfamoyl-benzo(f)-quinoxaline (NBQX) (9 nmol/min), respectively, face washing and scratching behavior emerged. Neither MK-801 or NBQX (ED50 value > 100 nmol/min), nor the putative metabotropic receptor antagonist L-amino-3-phosphoro-propionic acid (L-AP3) (> 176 nmol/min); nor the dopamine receptor antagonists SCH 23390 (> 74 nmol/min), metoclopramide (> 89 nmol/min) and haloperidol (> 27 nmol/min) antagonized 1S,3R-ACPD-induced scratching (144 nmol/min). These results suggest that the behavioral consequences of i.c.v. infusion of 1S,3R-ACPD in mice reflect a selective activation of metabotropic receptors that differs from the behavioral changes observed with i.c.v. infusion of ionotropic receptor agonists.

Intracerebral 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) produces limbic seizures that are not blocked by ionotropic glutamate receptor antagonists

The functional role of metabotropic glutamate receptor (mGluR) activation was investigated following intracerebral administration of 1S,3R-ACPD in mice. Injections of 1S,3R-ACPD (50-800 nmol in 5 microliters) into the thalamus produced a dose-dependent increase in limbic seizures. These effects were stereoselective since 1R,3S-ACPD, did not elicit seizure activity. Pharmacologically, limbic seizures were attenuated by the mGluR partial agonist/antagonist L-2-amino-3-phosphonopropionate (L-AP3) and dantrolene, an inhibitor of intracellular calcium mobilization, but not by D-AP3 or ionotropic glutamate receptor antagonists (MK-801 or GYKI-52466). Thus, activation of mGluRs by 1S,3R-ACPD in mice, induces limbic seizures that may involve the mobilization of intracellular calcium stores.

Nitric oxide and hippocampal synaptic plasticity

The dependence of NMDA receptor-dependent LTP on postsynaptic depolarization and increases in postsynaptic calcium, coupled with evidence supporting presynaptically mediated increases in transmitter release accompanying LTP, suggest that a retrograde transsynaptic messenger participates in the synaptic enhancement. Although many questions remain unanswered, the available evidence suggests a role for NO as such a messenger in certain LTP paradigms. It is unclear, however, whether NO contributes to LTP under differing experimental conditions and whether other messengers, acting in concert with or independent of NO, contribute to a retrograde signalling system. Furthermore, the conditions under which NMDA receptor activation, postsynaptic calcium increases and NO contribute to synaptic enhancement, synaptic depression and excitotoxic neuronal injury need to be clarified. Furthermore, efforts aimed at clarifying the molecular targets of NO must remain a priority of this line of research.

Excitotoxic neuronal damage and neuropsychiatric disorders

Excitatory amino acids (EAA) serve important physiological functions in the vertebrate CNS, including participation in fast excitatory synaptic transmission, modulation of synaptic plasticity and regulation of neuronal morphology during development. However, paradoxically they also harbor neurotoxic (excitotoxic) potential, which, if unleashed, can cause widespread degeneration of CNS neurons. Accumulating evidence suggests a role for excitotoxins in a variety of human neuropsychiatric disorders. This paper reviews the classes of EAA receptors in the CNS, the mechanisms underlying EAA-mediated neuronal damage and the role of EAA in specific human disorders.

A postsynaptic G-protein in hippocampal long-term potentiation

The effects of guanosine triphosphate (GTP)-binding protein (G-protein) blockade on hippocampal LTP at stratum radiatum-CA1 synapses was studied. Bath application of 20 mM lithium chloride (LiCl) inhibited long-term potentiation (LTP) of extracellularly-recorded excitatory postsynaptic potentials (EPSPs). Inclusion of 100 mM LiCl in intracellular recording electrodes was shown to block postsynaptic G-proteins by bath-application of baclofen, an agonist at the G-protein linked gamma-aminobutyric acid (GABAB) receptor. Under normal conditions, GABAB receptor activation causes a hyperpolarization postsynaptically, and a decrease in neurotransmitter release presynaptically. With LiCl in the recording electrodes, the postsynaptically-mediated hyperpolarization was blocked, while the presynaptically-mediated depression of EPSPs was unaffected. With postsynaptic G-proteins blocked in this manner, LTP at these synapses was inhibited. These studies provide evidence for the involvement of a postsynaptic G-protein in LTP of stratum radiatum-CA1 synapses.

Glutamate metabotropic receptor activation in neonatal rat cerebral cortex by sulphur-containing excitatory amino acids

The sulphur-containing acidic amino acids (SAAs) display neuroexcitatory actions similar to those of L-glutamate and are widely regarded as bona fide transmitter candidates. In this study, L-cysteine sulphinic acid, L-cysteic acid, DL-homocysteic acid and L-homocysteine sulphinic acid were investigated for their ability to stimulate phosphatidylinositol hydrolysis in rat pup cerebrocortical slices and compared with L-glutamate and the selective agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD). Each of the SAAs possessed agonist activity at metabotropic glutamate receptors (mGluRs) and, although they exhibited lower efficacy than L-glutamate, they were more potent (EC50s of 401-487 microM compared with 791 microM for L-glutamate). These data are consistent with the possibility that SAAs may have a physiological role as endogenous activators of metabotropic (and presumably ionotropic) excitatory amino acid receptors.

Metabotropic glutamate receptors in the ventrolateral medulla of rats

We investigated the hypothesis that stimulation of metabotropic excitatory amino acid receptors in the ventrolateral medulla evokes cardiovascular responses. Thus, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD], a selective agonist of metabotropic excitatory amino acid receptors, was microinjected into the rostral or caudal ventrolateral medulla of halothane-anesthetized Sprague-Dawley rats. Microinjections of (1S,3R)-ACPD (100 pmol-1 nmol) into the rostral ventrolateral medulla produced dose-dependent increases in mean arterial pressure (+20 +/- 4 mm Hg by 100 pmol and +35 +/- 2 mm Hg by 1 nmol, p < 0.01 versus artificial cerebrospinal fluid) and integrated splanchnic sympathetic nerve activity (+17 +/- 3% and +46 +/- 4%, respectively, p < 0.01), whereas (1S,3+)-ACPD microinjected into the caudal ventrolateral medulla decreased mean arterial pressure (-28 +/- 2 mm Hg by 100 pmol and -48 +/- 6 mm Hg by 1 nmol, p < 0.01 versus artificial cerebrospinal fluid) and splanchnic sympathetic nerve activity (-24 +/- 4% and -49 +/- 5%, p < 0.01). The blockade of ionotropic excitatory amino acid receptors by the combined injection of 2-amino-7-phosphonoheptanoic acid (200 pmol) and 6,7-dinitroquinoxaline-2,3-dione (200 pmol), which effectively blocked the responses elicited by either N-methyl-D-aspartate (20 pmol) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (5 pmol), failed to affect the responses evoked by either (1S,3R)-ACPD (100 pmol) or L-glutamate (2 nmol) microinjected in the rostral and caudal ventrolateral medulla. These results suggest that metabotropic receptors are present and mediate cardiovascular responses evoked by L-glutamate injections into the rostral and caudal ventrolateral medulla.

Pharmacology of metabotropic glutamate receptor inhibition of cyclic AMP formation in the adult rat hippocampus

Phosphoinositide-linked metabotropic glutamate receptors have been well characterized in a variety of CNS tissues. In this study the pharmacology of metabotropic glutamate receptors negatively coupled to cAMP formation was investigated in cross-chopped slices of the adult rat hippocampus. Excitatory amino acid agonists and antagonists were examined for effects on forskolin (30 microM)-simulated cAMP formation. The selective metabotropic glutamate agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and various nonselective metabotropic/ionotropic agonists were found to inhibit forskolin-stimulated cAMP formation. Inhibition of cAMP formation was observed using 1S,3R-ACPD (57% at 100 microM), quisqualate (92% at 500 microM), ibotenate (44% at 500 microM), L-glutamate (41% at 1000 microM), and L-aspartate (59% at 1000 microM). Inhibition of forskolin-stimulated cAMP formation induced by these agonists was observed even in the presence of the ionotropic antagonists MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione. Up to 500 microM of the ionotropic agonists N-methyl-D-aspartate, AMPA, and kainate did not inhibit forskolin-stimulated cAMP formation. L-2-amino-3-phosphonopropionate (500 microM) greatly inhibited the stimulatory effect of 1S,3R-ACPD on phosphoinositide hydrolysis, even in the presence of forskolin. However when measuring cAMP formation, L-2-amino-3-phosphonoproprionate (500 microM) mimicked the effect of 1S,3R-ACPD, producing 64% inhibition of forskolin-stimulated cAMP. These studies show that in the adult rat hippocampus metabotropic glutamate receptors that are negatively linked to cAMP formation have a pharmacology that is distinct from ionotropic glutamate receptors and phosphoinositide-linked metabotropic glutamate receptors.

Chronic sabeluzole treatment of cultured rat cerebellar granule cells reduces N-methyl-D-aspartate-induced inward current

The effects of sabeluzole, a drug that protects rat hippocampal neurones from glutamate- and N-methyl-D-aspartate (NMDA)-induced toxicity, were investigated in rat cerebellar granule cells in vitro with the whole-cell voltage clamp technique. Acute exposure of 0.1 microM sabeluzole for 20 min prior to experiments did not significantly affect glutamate receptor-mediated inward currents. Conversely, exposure of cultured granule cells to sabeluzole for 7 days reduced the NMDA-induced inward current and did not affect the non-NMDA responses evoked by kainic acid. The results suggest that chronic treatment with sabeluzole selectively reduces the functional NMDA response.

Phenylglycine derivatives as new pharmacological tools for investigating the role of metabotropic glutamate receptors in the central nervous system

The possible roles of G-protein coupled metabotropic glutamate receptors in central nervous function are currently the focus of intensive investigation. The complexity of effects produced by agonists at these receptors probably reflects the activity of a range of sub-types. The metabotropic glutamate receptors first described are linked to phospholipase C, mediating phosphoinositide hydrolysis and release of Ca2+ from intracellular stores. A substance generally considered to be a selective agonist for these receptors is (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD). This substance not only stimulates phosphoinositide hydrolysis, but also inhibits cyclic AMP formation. A family of metabotropic glutamate receptors, incorporating both phospholipase C- and adenylcyclase-linked sub-types has been cloned. Various effects of metabotropic glutamate receptor agonists on membrane ion fluxes and synaptic events have been reported, including neuronal depolarization and/or excitation, hyperpolarization, inhibition of Ca(2+)-dependent and voltage-gated K+ currents, potentiation of N-methyl-D-aspartate-induced responses, depression of synaptic excitation and either induction or augmentation of long-term potentiation. To clarify the role of metabotropic glutamate receptors in central nervous activity and to aid the characterization of the various receptor types that may be involved, a range of highly selective agonists and antagonists is required. To date, currently available antagonists such as L-2-amino-3-phosphonopropionate and L-aspartic acid-beta-hydroxamate appear to be unselective and insufficiently potent. We report here the actions of three phenylglycine derivatives, the particular agonist and/or antagonist properties of which may help to elucidate the roles of metabotropic glutamate receptors in central nervous activity.(ABSTRACT TRUNCATED AT 250 WORDS)

In rats, the metabotropic glutamate receptor-triggered hippocampal neuronal damage is strain-dependent

The effect of intrahippocampal (i.h.) and intraocular (i.o.) administration of the selective metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) was studied in different rat strains. A massive hippocampal damage was observed in CD/SD and Fischer 344 but not in SD/Rij and Brown Norway rats 7 days following the i.h. injection of 1S,3R-ACPD, while no retinal damage was observed following its i.o. administration. Moreover, 1S,3R-ACPD reduced the N-methyl-D-aspartate (NMDA) toxicity in the retina of both CD/SD and SD/Rij rats. Regardless of its toxic action on hippocampal neurons the i.h. injection of 1S,3R-ACPD caused an acute stimulation of motor activity in both CD/SD and SD/Rij rats. This effect was blocked by the intracerebroventricular (i.c.v.) administration of the putative mGluR antagonist L-2-amino-3-phosphono-propionic acid (L-AP3). It is suggested that the differential expression of mGluR subtypes might determine their role in brain pathology.

Metabotropic glutamate receptors in brain function and pathology

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

Metabotropic excitatory amino acid receptor activation stimulates phospholipase D in hippocampal slices

Metabotropic excitatory amino acid (EAA) receptors are coupled to effector systems through G proteins. Because various G protein-coupled receptors stimulate the hydrolysis of phosphatidylcholine by phospholipase D (PLD), we examined the possibility that metabotropic EAA receptors exist that are coupled to the activation of PLD. We found that the selective metabotropic glutamate receptor (mGluR) agonists 1S,3R-amino-1,3-cyclopentanedicarboxylic acid (ACPD) and 1S,3S-ACPD, but not the inactive isomer, 1R,3S-ACPD, induce a concentration-dependent increase in PLD activity in hippocampal slices. Selective ionotropic glutamate receptor (iGluR) antagonists did not block 1S,3R-ACPD-induced PLD stimulation. Furthermore, although selective iGluR agonists did not activate this response, the nonselective mGluR-iGluR agonists, ibotenate and quisqualate, caused significant increases in PLD activity (all in the presence of iGluR antagonists). L-2-Amino-3-phosphonopropionic acid, which blocks the mGluR that is coupled to phosphoinositide hydrolysis in various brain regions, activates PLD to the same extent as the active isomers of ACPD. These data suggest that metabotropic EAA receptors exist in hippocampus that are coupled to PLD activation and are pharmacologically distinct from phosphoinositide hydrolysis-coupled mGluRs.

Multiple metabotropic glutamate receptors regulate hippocampal function

Selective activation of metabotropic glutamate receptors with trans-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) stimulates phosphoinositide hydrolysis and elicits three major physiological responses in area CA1 of the hippocampus. These include direct excitation of pyramidal cells, blockade of synaptic inhibition, and decreased transmission at the Schaffer collateral-CA1 pyramidal cell synapse. Physiological effects of trans-ACPD are thought to be mediated by activation of phosphoinositide hydrolysis. However, it is now clear that multiple metabotropic glutamate receptor subtypes exist, some of which are not coupled to phosphoinositide hydrolysis. Thus, we performed a series of studies aimed at determining whether the physiological effects of trans-ACPD in the hippocampus are mediated by activation of the predominant phosphoinositide hydrolysis-linked glutamate-receptor. We report that L-2-amino-3-phosphonopropionic acid (L-AP3), an antagonist of trans-ACPD-stimulated phosphoinositide hydrolysis, does not inhibit the physiological effects of trans-ACPD in area CA1 at concentrations that maximally inhibit trans-ACPD-stimulated phosphoinositide hydrolysis in this region. Furthermore, 1S,3S-ACPD activates the phosphoinositide hydrolysis-linked glutamate receptor but does not reduce evoked field excitatory postsynaptic potentials (EPSPs) in area CA1. However, we report that the physiological effects of 1R,3S- and 1S,3R-ACPD are consistent with the hypothesis that these effects are mediated by activation of a metabotropic glutamate receptor. Thus, our data are consistent with the hypothesis that the physiological effects of trans-ACPD in area CA1 of the hippocampus are mediated by metabotropic glutamate receptors that are distinct from the AP3-sensitive phosphoinositide hydrolysis-linked glutamate receptor.

Trans-ACPD-induced phosphoinositide hydrolysis and modulation of hippocampal pyramidal cell excitability do not undergo parallel developmental regulation

The selective metabotropic glutamate receptor agonist, trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD), stimulates phosphoinositide hydrolysis and elicits a number of electrophysiological responses in the hippocampus. If these effects are mediated by the same receptor subtype, they should undergo parallel developmental regulation. Therefore, we compared the phosphoinositide hydrolysis response and the electrophysiological responses to trans-ACPD at two different developmental stages. Trans-ACPD-stimulated phosphoinositide hydrolysis was significantly greater in hippocampal slices from immature (6-11-day-old) rats than from adults. In contrast, trans-ACPD elicited decreases in spike frequency adaptation and in the amplitude of the slow afterhyperpolarization in roughly equal percentages of immature and adult CA1 pyramidal cells. Similar results were obtained using the putative endogenous agonist, glutamate. These data support the hypothesis that certain electrophysiological effects of trans-ACPD are mediated by a metabotropic glutamate receptor that is distinct from the phosphoinositide hydrolysis-linked glutamate receptor.

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

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

Activation of metabotropic glutamate receptors in the hippocampus increases cyclic AMP accumulation

The selective metabotropic glutamate receptor agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) stimulates phosphoinositide hydrolysis and elicits several physiological responses in rat hippocampal slices. However, recent studies suggest that the physiological effects of trans-ACPD in the hippocampus are mediated by activation of a receptor that is distinct from the phosphoinositide hydrolysis-linked receptor. Previous experiments indicate that cyclic AMP mimics many of the physiological effects of trans-ACPD in hippocampal slices. Furthermore, recent cloning and biochemistry experiments indicate that multiple metabotropic glutamate receptor subtypes exist, some of which are coupled to yet unidentified effector systems. Thus, we performed a series of experiments to test the hypothesis that ACPD increases cyclic AMP levels in hippocampal slices. We report that 1S,3R- and 1S,3S-ACPD (but not 1R,3S-ACPD) induce a concentration-dependent increase in cyclic AMP accumulation in hippocampal slices. This effect was blocked by the metabotropic glutamate receptor antagonist L-2-amino-3-phosphonoproprionic acid but not by selective antagonists of ionotropic glutamate receptors. Furthermore, our results suggest that 1S,3R-ACPD-stimulated increases in cyclic AMP accumulation are not secondary to increases in cell firing or to activation of phosphoinositide hydrolysis.