Seizures and neuronal damage induced in the rat by activation of group I metabotropic glutamate receptors with their selective agonist 3,5-dihydroxyphenylglycine

Metabotropic Glutamate Receptors and Interacting Proteins in Epileptogenesis

Neurotransmitter and receptor systems are involved in different neurological and neuropsychological disorders such as Parkinson's disease, depression, Alzheimer’s disease and epilepsy. Recent advances in studies of signal transduction pathways or interacting proteins of neurotransmitter receptor systems suggest that different receptor systems may share the common signal transduction pathways or interacting proteins which may be better therapeutic targets for development of drugs to effectively control brain diseases. In this paper, we reviewed metabotropic glutamate receptors (mGluRs) and their related signal transduction pathways or interacting proteins in status epilepticus and temporal lobe epilepsy, and proposed some novel therapeutical drug targets for controlling epilepsy and epileptogenesis.

Neurodevelopmental Expression Profile of Dimeric and Monomeric Group 1 mGluRs: Relevance to Schizophrenia Pathogenesis and Treatment

Group 1 metabotropic glutamate receptors (mGluR1/mGluR5) play an integral role in neurodevelopment and are implicated in psychiatric disorders, such as schizophrenia. mGluR1 and mGluR5 are expressed as homodimers, which is important for their functionality and pharmacology. We examined the protein expression of dimeric and monomeric mGluR1α and mGluR5 in the prefrontal cortex (PFC) and hippocampus throughout development (juvenile/adolescence/adulthood) and in the perinatal phencyclidine (PCP) model of schizophrenia. Under control conditions, mGluR1α dimer expression increased between juvenile and adolescence (209-328%), while monomeric levels remained consistent. Dimeric mGluR5 was steadily expressed across all time points; monomeric mGluR5 was present in juveniles, dramatically declining at adolescence and adulthood (-97-99%). The mGluR regulators, Homer 1b/c and Norbin, significantly increased with age in the PFC and hippocampus. Perinatal PCP treatment significantly increased juvenile dimeric mGluR5 levels in the PFC and hippocampus (37-50%) but decreased hippocampal mGluR1α (-50-56%). Perinatal PCP treatment also reduced mGluR1α dimer levels in the PFC at adulthood (-31%). These results suggest that Group 1 mGluRs have distinct dimeric and monomeric neurodevelopmental patterns, which may impact their pharmacological profiles at specific ages. Perinatal PCP treatment disrupted the early expression of Group 1 mGluRs which may underlie neurodevelopmental alterations observed in this model.

Postsynaptic scaffold protein Homer 1a protects against traumatic brain injury via regulating group I metabotropic glutamate receptors

Traumatic brain injury (TBI) produces excessive glutamate, leading to excitotoxicity via the activation of glutamate receptors. Postsynaptic density scaffold proteins have crucial roles in mediating signal transduction from glutamate receptors to their downstream mediators. Therefore, studies on the mechanisms underlying regulation of excitotoxicity by scaffold proteins can uncover new treatments for TBI. Here, we demonstrated that the postsynaptic scaffold protein Homer 1a was neuroprotective against TBI in vitro and in vivo, and this neuroprotection was associated with its effects on group I metabotropic glutamate receptors (mGluRs). Upon further study, we found that Homer 1a mainly affected neuronal injury induced by mGluR1 activation after TBI and also influenced mGluR5 function when its activity was restored. The ability of Homer 1a to disrupt mGluR-ERK signaling contributed to its ability to regulate the functions of mGluR1 and mGluR5 after traumatic injury. Intracellular Ca(2+) and PKC were two important factors involved in the mediation of mGluR-ERK signaling by Homer 1a. These results define Homer 1a as a novel endogenous neuroprotective agent against TBI.

Mechanism based neurotoxicity of mGlu5 positive allosteric modulators--development challenges for a promising novel antipsychotic target

Previous work has suggested that activation of mGlu5 receptor augments NMDA receptor function and thereby may constitute a rational approach addressing glutamate hypofunction in schizophrenia and a target for novel antipsychotic drug development. Here, we report the in vitro activity, in vivo efficacy and safety profile of 5PAM523 (4-Fluorophenyl){(2R,5S)-5-[5-(5-fluoropyridin-2-yl)-1,2,4-oxadiazol-3-yl]-2-methylpiperidin-1-yl}methanone), a structurally novel positive allosteric modulator selective of mGlu5. In cells expressing human mGlu5 receptor, 5PAM523 potentiated threshold responses to glutamate in fluorometric calcium assays, but does not have any intrinsic agonist activity. 5PAM523 acts as an allosteric modulator as suggested by the binding studies showing that 5PAM523 did not displace the binding of the orthosteric ligand quisqualic acid, but did partially compete with the negative allosteric modulator, MPyEP. In vivo, 5PAM523 reversed amphetamine-induced locomotor activity in rats. Therefore, both the in vitro and in vivo data demonstrate that 5PAM523 acts as a selective mGlu5 PAM and exhibits anti-psychotic like activity. To study the potential for adverse effects and particularly neurotoxicity, brain histopathological exams were performed in rats treated for 4 days with 5PAM523 or vehicle. The brain exam revealed moderate to severe neuronal necrosis in the rats treated with the doses of 30 and 50 mg/kg, particularly in the auditory cortex and hippocampus. To investigate whether this neurotoxicity is mechanism specific to 5PAM523, similar safety studies were carried out with three other structurally distinct selective mGlu5 PAMs. Results revealed a comparable pattern of neuronal cell death. Finally, 5PAM523 was tested in mGlu5 knock-out (KO) and wild type (WT) mice. mGlu5 WT mice treated with 5PAM523 for 4 days at 100 mg/kg presented significant neuronal death in the auditory cortex and hippocampus. Conversely, mGlu5 KO mice did not show any neuronal loss by histopathology, suggesting that enhancement of mGlu5 function is responsible for the toxicity of 5PAM523. This study reveals for the first time that augmentation of mGlu5 function with selective allosteric modulators results in neurotoxicity.

The role of glutamate receptors in traumatic brain injury: implications for postsynaptic density in pathophysiology

Traumatic brain injury (TBI) is the major cause of death and disability, and the incidence of TBI continues to increase rapidly. In recent years, increasing attention has been paid to an important structure at the postsynaptic membrane: the postsynaptic density (PSD). Glutamate receptors, as major components of the PSD, are highly responsive to alterations in the glutamate concentration at excitatory synapses and activate intracellular signal transduction via calcium and other second messengers following TBI. PSD scaffold proteins (PSD-95, Homer, and Shank), which anchor glutamate receptors and form a network structure, also have potential effects on these downstream signaling pathways. The changes in the function and structure of these major PSD proteins are also induced by TBI, indicating that there is a more complicated mechanism associated with PSD proteins in the pathophysiological process of TBI.

Early continuous inhibition of group 1 mGlu signaling partially rescues dendritic spine abnormalities in the Fmr1 knockout mouse model for fragile X syndrome

RATIONALE

Abnormal dendritic spine morphology is a significant neuroanatomical defect in fragile X mental retardation. It has been suggested that overactive group 1 metabotropic glutamate receptor (mGlu) signaling is associated with the spine dysmorphology occurring in fragile X syndrome (FXS). Thus, group 1 mGlu became a new therapeutic target for the treatment of FXS.

OBJECTIVE

The purpose of this study was to identify the effect of inhibition of mGlu signaling in FXS.

METHODS

We observed the changes in dendritic spines after pharmacological modulation of mGlu signaling in an Fmr1 knockout (KO) mouse model.

RESULTS

The activation of group 1 mGlu resulted in elongation of dendritic spines in the cultured neurons derived from Fmr1 KO mice and wild-type (WT) mice. Antagonism of group 1 mGlu reduced the average spine length of Fmr1 KO neurons. Furthermore, systemic administration of the selective group 1 mGlu5 antagonist 2-methyl-6-phenylethynyl pyridine (MPEP) reduced the average spine length and density in the cortical neurons of Fmr1 KO mice at developmental age. For the adult mice, MPEP administration was less effective for the restoration of spine length. The percentage of immature spines showed a similar reduction in parallel to the changes of spine length. Temporary MPEP intervention with single-dose treatment did not show any effect.

CONCLUSION

These results show that MPEP administration could partially rescue the morphological deficits of dendritic spines in Fmr1 KO mice at developmental age.

Interneuron subtype specific activation of mGluR1/5 during epileptiform activity in hippocampus

PURPOSE

Specific inhibitory interneurons in area CA1 of the hippocampus, notably those located in stratum oriens-alveus (O/A-INs), are selectively vulnerable in patients and animal models of temporal lobe epilepsy (TLE). The excitotoxic mechanisms underlying the selective vulnerability of interneurons have not been identified but could involve group I metabotropic glutamate receptor subtypes (mGluR1/5), which have generally proconvulsive actions and activate prominent cationic currents and calcium responses specifically in O/A-INs.

METHODS

In this study, we examine the role of mGluR1/5 in interneurons during epileptiform activity using whole-cell recordings from CA1 O/A-INs and selective antagonists of mGluR1α (LY367385) and mGluR5 (MPEP) in a disinhibited rat hippocampal slice model of epileptiform activity.

RESULTS

Our data indicate more prominent epileptiform burst discharges and paroxysmal depolarizations (PDs) in O/A-INs than in interneurons located at the border of strata radiatum and lacunosum/moleculare (R/LM-INs). In addition, mGluR1 and mGluR5 significantly contributed to epileptiform responses in O/A-INs but not in R/LM-INs. Epileptiform burst discharges in O/A-INs were partly dependent on mGluR5. PDs and associated postsynaptic currents were dependent on both mGluR1α and mGluR5. These receptors contributed differently to postsynaptic currents underlying PDs, with mGluR5 contributing to the fast and slow components and mGluR1α to the slow component.

DISCUSSION

These findings support interneuron subtype-specific activation and differential contributions of mGluR1α and mGluR5 to epileptiform activity in O/A-INs, which could be important for their selective vulnerability in TLE.

Distinctions between persistent and reversible group I mGluR-induced epileptiform burst prolongation

We have previously shown that selective activation of group I metabotropic glutamate receptors (mGluRs) results in long-lasting enhancement of synchronized network activity in the hippocampal slice. Data herein suggest that activation of group I mGluRs need not result in this potentially epileptogenic effect. (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (ACPD), a nonselective mGluR agonist, elicits ictaform bursts identical in appearance to those induced by selective agonists, but ACPD-induced bursts do not persist following removal of the agent. Like the bursts induced by selective agonist, the ACPD bursts are blocked with group I mGluR antagonists and are not dependent on activation of either N-methyl-D-aspartate (NMDA) receptors or protein kinase C. However, they differ from the persistent bursts in that they do not require active protein synthesis and they are not suppressed with L-cysteine sulfinic acid, an agonist at a phospholipase D-coupled metabotropic receptor. These novel findings provide evidence that group I mGluR-induced epileptogenesis may be preventable.

Metabotropic glutamate mGlu1 receptor stimulation and blockade: therapeutic opportunities in psychiatric illness

Metabotropic glutamate mGlu(1) receptors play a modulatory role in the nervous system. They enhance cell excitability, modulate synaptic neurotransmission and are involved in synaptic plasticity. During the last 10 years, several selective metabotropic glutamate mGlu(1) receptor competitive antagonists and potentiators have been discovered. These pharmacological tools, together with early and later work in metabotropic glutamate mGlu(1) receptor mutant mice have allowed studying the role of the receptor in various aspects of psychiatric illnesses such as anxiety, depression and schizophrenia. We here review the data on selective metabotropic glutamate mGlu(1) receptor antagonists in support of their potential as anxiolytic and antidepressant treatments. We propose a rationale for the development of metabotropic glutamate mGlu(1) receptor positive allosteric modulators for the treatment of schizophrenia. Potential side effects of blockade and activation of metabotropic glutamate mGlu(1) receptors are addressed, with special focus on the differential effects of metabotropic glutamate mGlu(1) receptor antagonists in cognition models with positive reinforcement versus those that use aversive learning procedures. Further development of negative allosteric modulators and more drug-like positive allosteric modulators will be required in order to decipher the therapeutic efficacy and safety margin of these compounds in the clinic.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Effects of subtype-selective group I mGluR antagonists on synchronous activity induced by 4-aminopyridine/CGP 55845 in adult guinea pig hippocampal slices

Co-application of the convulsant 4-aminopyridine (4-AP) and the GABA(B) receptor antagonist CGP 55845 to adult guinea pig hippocampal slices elicits giant GABA-mediated postsynaptic potentials (GPSPs) and epileptiform discharges. Here we tested the effects of the group I metabotropic glutamate receptor (mGluR) subtype-selective antagonists LY 367385 (mGlu1, 100 microM), MPEP (mGlu5, 10 microM), and MTEP (mGlu5, 500 nM) on this synchronous activity. Electrophysiological field recordings were performed in the CA3 region of hippocampal slices from adult guinea pigs. The mGlu5 receptor antagonists increased GPSP rate, but the mGlu1 receptor antagonist did not. This ability of mGlu5 receptor antagonists to increase the rate of GPSPs indicates that enough endogenous glutamate is released under these conditions to activate group I mGluR; nevertheless, co-application of a mGlu1 receptor antagonist (LY 367385 or JNJ 16259685) and MPEP did not decrease pre-existing epileptiform activity. Furthermore, co-application of LY 367,385 and MPEP did not prevent the emergence of epileptiform activity. When ionotropic glutamate receptor (iGluR) antagonists were present, neither MPEP nor the group I mGluR agonist DHPG changed GPSP rate, suggesting that pyramidal cell-to-interneuron iGluR-mediated synaptic connections are involved in the rate change mechanism. In contrast to the lack of effect of group I mGluR antagonists on epileptiform activity in the 4-AP/CGP 55845 model, group I mGluR antagonists blocked the emergence of longer epileptiform events and decreased the overall amount of synchronous activity in the GABA(A) antagonist/4-AP model. In conclusion, in the 4-AP/CGP 55845 model, enough glutamate was released to activate group I mGluRs and affect GPSP rate via mGlu5 receptors; however, this group I mGluR activation was not required for the generation of the epileptiform activity.

Forebrain-specific inactivation of Gq/G11 family G proteins results in age-dependent epilepsy and impaired endocannabinoid formation

Metabotropic receptors coupled to Gq/G11 family G proteins critically contribute to nervous system functions by modulating synaptic transmission, often facilitating excitation. We investigated the role of Gq/G11 family G proteins in the regulation of neuronal excitability in mice that selectively lack the alpha-subunits of Gq and G11, G alpha q and G alpha 11, respectively, in forebrain principal neurons. Surprisingly, mutant mice exhibited increased seizure susceptibility, and the activation of neuroprotective mechanisms was impaired. We found that endocannabinoid levels were reduced under both basal and excitotoxic conditions and that increased susceptibility to kainic acid could be normalized by the enhancement of endocannabinoid levels with an endocannabinoid reuptake inhibitor, while the competitive cannabinoid type 1 receptor antagonist SR141716A did not cause further aggravation. These findings indicate that Gq/G11 family G proteins negatively regulate neuronal excitability in vivo and suggest that impaired endocannabinoid formation in the absence of Gq/G11 contributes to this phenotype.

Metabotropic glutamate receptors as a strategic target for the treatment of epilepsy

Epilepsy is a chronic neurological disorder that has many known types, including generalized epilepsies that involve cortical and subcortical structures. A proportion of patients have seizures that are resistant to traditional anti-epilepsy drugs, which mainly target ion channels or postsynaptic receptors. This resistance to conventional therapies makes it important to identify novel targets for the treatment of epilepsy. Given the involvement of the neurotransmitter glutamate in the etiology of epilepsy, targets that control glutamatergic neurotransmission are of special interest. The metabotropic glutamate receptors (mGluRs) are of a family of eight G-protein-coupled receptors that serve unique regulatory functions at synapses that use the neurotransmitter glutamate. Their distribution within the central nervous system provides a platform for both presynaptic control of glutamate release, as well as postsynaptic control of neuronal responses to glutamate. In recent years, substantial efforts have been made towards developing selective agonists and antagonists which may be useful for targeting specific receptor subtypes in an attempt to harness the therapeutic potential of these receptors. We examine the possibility of intervening at these receptors by considering the specific example of absence seizures, a form of generalized, non-convulsive seizure that involves the thalamus. Views of the etiology of absence seizures have evolved over time from the "centrencephalic" concept of a diffuse subcortical pacemaker toward the "cortical focus" theory in which cortical hyperexcitability leads the thalamus into the 3-4 Hz rhythms that are characteristic of absence seizures. Since the cortex communicates with the thalamus via a massive glutamatergic projection, ionotropic glutamate receptor (iGluR) blockade has held promise, but the global nature of iGluR intervention has precluded the clinical effectiveness of drugs that block iGluRs. In contrast, mGluRs, because they modulate iGluRs at glutamatergic synapses only under certain conditions, may quell seizure activity by selectively reducing hyperactive glutamatergic synaptic communication within the cortex and thalamus without significantly affecting normal response rates. In this article, we review the circuitry and events leading to absence seizure generation within the corticothalamic network, we present a comprehensive review of the synaptic location and function of mGluRs within the thalamus and cerebral cortex, and review the current knowledge of mGluR modulation and seizure generation. We conclude by reviewing the potential advantages of Group II mGluRs, specifically mGluR2, in the treatment of both convulsive and non-convulsive seizures.

Repeated administration of group I mGluR antagonists prevents seizure-induced long-term aberrations in hippocampal synaptic plasticity

Kindling induced by repeated application of the convulsant pentylenetetrazole (PTZ) is a validated model of epilepsy and epilepsy-related neuromorphological, neurophysiological and behavioural alterations. In this study, we examined whether kindling-induced long-term aberrations in hippocampal synaptic plasticity can be prevented by application of group I mGluR antagonists. Kindling resulted in a higher magnitude of long-term potentiation (LTP) induced by a strong high-frequency stimulation in the hippocampal CA1 region in vitro. When the specific mGluR1 antagonist LY 367385 (0.40 microMol) or the specific mGluR5 inhibitor MPEP (0.06 microMol) were given 30 min prior to PTZ, this kindling-induced enhancement of LTP was almost completely prevented. In addition, application of MPEP led to an impaired maintenance of population spike LTP in kindled animals. LY 367385 applied to unkindled control animals caused a reduction of the initial magnitude of population spike LTP. MPEP, in contrast, left the initial magnitude untouched but resulted in a faster decay of potentiation. A single administration of LY 367385 (200 microM) and MPEP (50 microM), respectively, directly into the bath had almost no effect. Our data suggest that the long-lasting aberrations of hippocampal synaptic plasticity induced by the repeated occurrence of generalized epileptic seizures ultimately require a concurrent operation of mGluR1 and mGluR5.

The preferential mGlu2/3 receptor antagonist, LY341495, reduces the frequency of spike-wave discharges in the WAG/Rij rat model of absence epilepsy

We examined the expression and function of group-II metabotropic glutamate (mGlu) receptors in an animal model of absence seizures using genetically epileptic WAG/Rij rats, which develop spontaneous non-convulsive seizures after 2-3 months of age. Six-month-old WAG/Rij rats showed an increased expression of mGlu2/3 receptors in the ventrolateral regions of the somatosensory cortex, ventrobasal thalamic nuclei, and hippocampus, but not in the reticular thalamic nucleus and in the corpus striatum, as assessed by immunohistochemistry and Western blotting. In contrast, mGlu2/3 receptor signalling was reduced in slices prepared from the somatosensory cortex of 6-month-old WAG/Rij rats, as assessed by the ability of the agonist, LY379268, to inhibit forskolin-stimulated cAMP formation. None of these changes was found in "pre-symptomatic" 2-month-old WAG/Rij rats. To examine whether pharmacological activation or inhibition of mGlu2/3 receptors affects absence seizures, we recorded spontaneous spike-wave discharges (SWDs) in 6-month-old WAG/Rij rats systemically injected with saline, the mGlu2/3 receptor agonist LY379268 (0.33 or 1 mg/kg, i.p.), or with the preferential mGlu2/3 receptor antagonist, LY341495 (0.33, 1 or 5 mg/kg, i.p.). Injection of 1mg/kg of LY379268 (1 mg/kg, i.p.) increased the number of SWDs during 3-7 h post-treatment, whereas injection with LY341495 reduced the number of seizures in a dose-dependent manner. It can be concluded that mGlu2/3 receptors are involved in the generation of SWDs and that an upregulation of these receptors in the somatosensory cortex might be involved in the pathogenesis of absence epilepsy.

Group I mGluR-induced epileptogenesis: distinct and overlapping roles of mGluR1 and mGluR5 and implications for antiepileptic drug design

The group I metabotropic glutamate receptor subtypes, mGluR1 and mGluR5, have both distinct and overlapping actions in epileptogenesis. Data are reviewed revealing how activation of these receptor subtypes participates in the induction and maintenance of the long-lasting epileptiform discharges elicited in the hippocampal circuit. Differences in the cellular actions and regional distributions of mGluR1 and mGluR5 provide hints regarding the potential usefulness and limitations of subtype-specific antagonists as antiepileptic agents.

Agonists and antagonists of metabotropic glutamate receptors: anticonvulsants and antiepileptogenic agents?

Anticonvulsant and neuroprotective effects of agonist and antagonist of metabotropic glutamate receptors (mGluRs) have been known for more than 10 years from multiple studies. However, it is not certain whether these candidate drugs are also antiepileptic and antiepileptogenic, as few studies included the chronic stages to determine whether spontaneous recurrent seizures could be prevented or stopped. Even in the acute stage, differences in experimental design such as timing and route of administration of candidate drugs, age, species and strain of experimental animal and experimental model make it difficult to determine the anticonvulsant and europrotective effects of each candidate drug. This paper, reviews in vivo neuropharmacological studies on agonsists and antagonists of mGluRs in different seizure and epilepsy models in last more than ten years. By combining with our neuropharmacological studies on the effect of mGluR agonists and antagonists in the mouse pilocarpine model of temporal lobe epilepsy, an ideal model for future development of mGluR agonists and antagonists as antiepileptogenic drugs will be proposed.

Global ischemia downregulates the function of metabotropic glutamate receptor subtype 5 in hippocampal CA1 pyramidal neurons

Within the hippocampus, electrophysiological and immunohistochemical studies showed that metabotropic glutamate receptor subtype 5 (mGluR5) is the major postsynaptic mGluR expressed in CA1 pyramidal neurons. To better understand the role of mGluR5 in ischemia-induced neuronal death, whole-cell patch-clamp recordings using hippocampal slices were performed to investigate the functional change of mGluR5 in CA1 pyramidal neurons following transient global ischemia. Our results indicated that 6 to 24 h after global ischemia, mGluR5-induced cationic currents and mGluR5-mediated enhancement of NMDA-evoked currents in CA1 pyramidal neurons were significantly reduced. Further TaqMan real-time quantitative RT-PCR assay showed that mGluR5 mRNA expression in hippocampal CA1 region or single CA1 pyramidal neurons was significantly downregulated following ischemic insults. The present study suggests that transient global ischemia downregulates mGluR5 function of CA1 pyramidal neurons by decreasing mGluR5 mRNA and that the resulting reduced mGluR5-mediated excitotoxicity could contribute to the survival of CA1 pyramidal neurons after ischemic insult.

Behavioral and convulsant effects of the (S) enantiomer of the group I metabotropic glutamate receptor agonist 3,5-DHPG in mice

The purpose of the present studies was to investigate the behavioral and convulsant effects produced by the group I metabotropic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine (DHPG). Administered i.c.v. to mice, (S)-3,5-DHPG produced a behavioral syndrome consisting of scratching and/or facial grooming, tremors, slow forelimb clonus, rearing, and falling that increased over the dose range of 10-400 nmol. The full syndrome, produced by 400 nmol of (S)-3,5-DHPG, was antagonized by the selective mGlu1 receptor antagonist LY456236 but not by the mGlu5 receptor antagonist MPEP or the mGlu2/3 receptor antagonist LY341495. The behaviors induced by the 400 nmol dose were not blocked by the NMDA receptor antagonist MK-801, but were attenuated by the non-NMDA receptor antagonists GYKI 52466 and NBQX, and the Ca2+ mobilization inhibitor dantrolene, but at motor-impairing doses. The scratching behaviors produced by 30 nmol of (S)-3,5-DHPG were antagonized by LY456236 but not by MPEP, LY341495 or MK-801. GYKI 52466 and dantrolene, but not NBQX, inhibited scratching at motor-impairing doses. Both 400 and 30 nmol of (S)-3,5-DHPG produced a generalized seizure as recorded by surface EEG electrodes. LY456236 blocked the seizures produced by 30 nmol but not by 400 nmol; dantrolene was ineffective in blocking seizures produced by either dose. The present findings suggest that (S)-3,5-DHPG produces an increase in excitation that is mediated by mGlu1 and non-NMDA receptors.

Transient depression of excitatory synapses on interneurons contributes to epileptiform bursts during gamma oscillations in the mouse hippocampal slice

Persistent gamma frequency (30-70 Hz) network oscillations occur in hippocampal slices under conditions of metabotropic glutamate receptor (mGluR) activation. Excessive mGluR activation generated a bistable pattern of network activity during which epochs of gamma oscillations of increasing amplitude were terminated by synchronized bursts and very fast oscillations (>70 Hz). We provide experimental evidence that, during this behavior, pyramidal cell-to-interneuron synaptic depression takes place, occurring spontaneously during the gamma rhythm and associated with the onset of epileptiform bursts. We further provide evidence that excitatory postsynaptic potentials (EPSPs) in pyramidal cells are potentiated during the interburst gamma oscillation. When these two types of synaptic plasticity are incorporated, phenomenologically, into a network model previously shown to account for many features of persistent gamma oscillations, we find that epochs of gamma do indeed alternate with epochs of very fast oscillations and epileptiform bursts. Thus the same neuronal network can generate either gamma oscillations or epileptiform bursts, in a manner depending on the degree of network drive and network-induced fluctuations in synaptic efficacies.

Anticonvulsant effects of LY456236, a selective mGlu1 receptor antagonist

Several lines of evidence suggest that mGlu1 metabotropic glutamate receptors may be involved in seizure disorders such as epilepsy. For example, the mGlu1 agonist DHPG produces limbic seizures and group I antagonists such as 4C3HPG and 4CPG are anticonvulsant when administered intracerebrally. The purpose of the present experiments was to characterize the anticonvulsant effects of the selective mGlu1 receptor antagonist LY456236 in mice and rats. In male and female DBA/2 mice, LY456236 produced a dose-related inhibition of sound-induced clonic-tonic seizures. In male CF1 mice, LY456236 produced a dose-related inhibition of tonic extensor seizures in the threshold electroshock model, and limbic seizures in the 6-Hz focal seizure model. However, this antagonist did not inhibit clonic seizures produced by pentylenetetrazol. In amygdala-kindled male Sprague-Dawley rats, LY456236 produced dose-related decreases in behavioral and electrographic seizures at threshold stimulus intensity. In addition, LY456236 produced a dose-related increase in the stimulus intensity required to produce generalized seizures. Taken together, the present results support the conclusion that mGlu1 receptor antagonists such as LY456236 may have clinical utility in the treatment of epilepsy and other seizure disorders.

The role of group I metabotropic glutamate receptors in neuronal excitotoxicity in Alzheimer's disease

Neurodegenerative diseases such as Huntington's disease, ischemia, and Alzheimer's disease (AD) are major causes of death. Recently, metabotropic glutamate receptors (mGluRs), a group of seven-transmembrane-domain proteins that couple to G-proteins, have become of interest for studies of pathogenesis. Group I mGluRs control the levels of second messengers such as inositol 1,4,5-triphosphate (IP3), Ca2+ ions and cAMP. They elicit the release of arachidonic acid via intracellular Ca2+ mobilization from intracellular stores such as mitochondria and endoplasmic reticulum. This facilitates the release of glutamate and could trigger the formation of neurofibrillary tangles, a pathological hallmark of AD. mGluRs regulate neuronal injury and survival, possibly through a series of downstream protein kinase and cysteine protease signaling pathways that affect mitochondrially mediated programmed cell death. They may also play a role in glutamate-induced neuronal death by facilitating Ca(II) mobilization. Hence, mGluRs have become a target for neuroprotective drug development. They represent a pharmacological path to a relatively subtle amelioration of neurotoxicity because they serve a modulatory rather than a direct role in excitatory glutamatergic transmission.

Contribution of GABA(B) receptor-mediated inhibition to the expression and termination of group I mGluR-induced ictaform bursts

In guinea pig hippocampal slices, the GABA(B) receptor antagonist CGP 35348 increased the length of picrotoxin-induced interictal bursts by only 22%, yet it increased the length of group I mGluR-induced ictaform bursts by 85%. These data suggest that (1) suppression of GABAergic inhibition is insufficient to account for group I mGluR agonist-induced ictaform bursts, and (2) although GABA(B) plays a minor role in terminating interictal bursts, it is a major contributor to the termination of mGluR-induced ictaform bursts.

Effects of the group I metabotropic glutamate receptor agonist, DHPG, and injection stress on striatal cell signaling in food-restricted and ad libitum fed rats

Background

Chronic food restriction augments the rewarding effect of centrally administered psychostimulant drugs and this effect may involve a previously documented upregulation of D-1 dopamine receptor-mediated MAP kinase signaling in nucleus accumbens (NAc) and caudate-putamen (CPu). Psychostimulants are known to induce striatal glutamate release, and group I metabotropic glutamate receptors (mGluR) have been implicated in the cellular and behavioral responses to amphetamine. The purpose of the present study was to evaluate whether chronic food restriction increases striatal MAP kinase signaling in response to the group I mGluR agonist, DHPG.

Results

Western immunoblotting was used to demonstrate that intracerebroventricular (i.c.v.) injection of DHPG (500 nmol) produces greater activation of ERK1/2 and CREB in CPu and NAc of food-restricted as compared to ad libitum fed rats. Fos-immunostaining induced by DHPG was also stronger in CPu and NAc core of food-restricted relative to ad libitum fed rats. However, i.c.v. injection of saline-vehicle produced greater activation of ERK1/2 and CREB in CPu and NAc of food-restricted relative to ad libitum fed rats, and this difference was not seen when subjects received no i.c.v. injection prior to sacrifice. In addition, although DHPG activated Akt, there was no difference in Akt activation between feeding groups. To probe whether the augmented ERK1/2 and CREB activation in vehicle-injected food-restricted rats are mediated by one or more GluR types, effects of an NMDA antagonist (MK-801, 100 nmol), AMPA antagonist (DNQX, 10 nmol), and group I mGluR antagonist (AIDA, 100 nmol) were compared to saline-vehicle. Antagonist injections did not diminish activation of ERK1/2 or CREB.

Conclusions

These results indicate that a group I mGluR agonist induces phosphorylation of Akt, ERK1/2 and CREB in both CPu and NAc. However, group I mGluR-mediated signaling may not be upregulated in food-restricted rats. Rather, a physiological response to "i.c.v. injection stress" is augmented by food restriction and appears to summate with effects of the group I mGluR agonist in activating ERK1/2 and CREB. While the augmented cellular response of food-restricted rats to i.c.v. injection treatment represents additional evidence of enhanced CNS responsiveness in these subjects, the functional significance and underlying mechanism(s) of this effect remain to be elucidated.

Hippocampal mGluR1 and consolidation of contextual fear conditioning

The effects of post-training intra-hippocampal injections of group I mGluR agonists and antagonists, were examined in the contextual fear test, in rats. It was found that (S)-3,5-dihydroxyphenylglycine (DHPG) (a mGluR1-5 agonist) decreased, and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (a mGluR1 antagonist) increased fear conditioning (a freezing reaction), examined 24h after conditioning session. (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) (a mGluR5 agonist), and 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) (a mGluR5 antagonist) did not cause any effect. In the immunocytochemical study, the post-conditioning administration of AIDA decreased the c-Fos induction in the dentate gyrus and CA1 layer of the hippocampus proper, 2h after exposure of animals to the aversive context, and 24h after conditioning session. It is suggested that overactivation of glutamatergic transmission in the critical for memory trace formation structure and period of time, may result in an attenuation of memory consolidation. On the other hand, reduction of an exaggerated glutamatergic tone can facilitate learning and memory processes. The immunocytochemical study and factor analysis of experimental data revealed that hippocampal mGlu1 receptors significantly influence the memory consolidation in a way dependant on the level of glutamatergic activity. Furthermore, they indicate that changes of glutamatergic activity within brain limbic structures can affect the threshold for the induction of the long-term neuronal plasticity, involved in some forms of learning and memory.

Plasticity Mechanisms Underlying mGluR-Induced Epileptogenesis

Transient application of group I metabotropic glutamate receptor (mGluR) agonists to hippocampal slices produces ictal-like discharges that persist for hours after the removal of the agonist. This effect of group I mGluR stimulation--converting a 'normal' hippocampal slice into an 'epileptic-like' one--may represent a form of epileptogenesis. Because this epileptogenic process can be induced in vitro and it occurs within hours, it has been possible to examine the cellular and transduction processes underlying the generation and long-term maintenance of ictal-like bursts. ImGluR(V), a voltage-dependent depolarizing current activated by group I mGluR agonists, appears to play an important role in the expression of the ictal-like bursts. Long-term activation of ImGluR(V) following mGluR stimulation is a possible plastic change that enables the long-term maintenance of ictal discharges. Induction of ImGluR(V) may represent a cellular event underlying the mGluR-induced epileptogenesis.

Glutamate metabotropic receptors as targets for drug therapy in epilepsy

Metabotropic glutamate (mGlu) receptors have multiple actions on neuronal excitability through G-protein-linked modifications of enzymes and ion channels. They act presynaptically to modify glutamatergic and gamma-aminobutyric acid (GABA)-ergic transmission and can contribute to long-term changes in synaptic function. The recent identification of subtype-selective agonists and antagonists has permitted evaluation of mGlu receptors as potential targets in the treatment of epilepsy. Agonists acting on group I mGlu receptors (mGlu1 and mGlu5) are convulsant. Antagonists acting on mGlu1 or mGlu5 receptors are anticonvulsant against 3,5-dihydroxyphenylglycine (DHPG)-induced seizures and in mouse models of generalized motor seizures and absence seizures. The competitive, phenylglycine mGlu1/5 receptor antagonists generally require intracerebroventricular administration for potent anticonvulsant efficacy but noncompetitive antagonists, e.g., (3aS,6aS)-6a-naphthalen-2-ylmethyl-5-methyliden-hexahydrocyclopenta[c]furan-1-on (BAY36-7620), 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), and 2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893) block generalized seizures with systemic administration. Agonists acting on group II mGlu receptors (mGlu2, mGlu3) to reduce glutamate release are anticonvulsant, e.g., 2R,4R-aminopyrrolidine-2,4-dicarboxylate [(2R,4R)-APDC], (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740), and (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268). The classical agonists acting on group III mGlu receptors such as L-(+)-2-amino-4-phosphonobutyric acid, and L-serine-O-phosphate are acutely proconvulsant with some anticonvulsant activity. The more recently identified agonists (R,S)-4-phosphonophenylglycine [(R,S)-PPG] and (S)-3,4-dicarboxyphenylglycine [(S)-3,4-DCPG] and (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid [ACPT-1] are all anticonvulsant without proconvulsant effects. Studies in animal models of kindling reveal some efficacy of mGlu receptor ligands against fully kindled limbic seizures. In genetic mouse models, mGlu1/5 antagonists and mGlu2/3 agonists are effective against absence seizures. Thus, antagonists at group I mGlu receptors and agonists at groups II and III mGlu receptors are potential antiepileptic agents, but their clinical usefulness will depend on their acute and chronic side effects. Potential also exists for combining mGlu receptor ligands with other glutamatergic and non-glutamatergic agents to produce an enhanced anticonvulsant effect. This review also discusses what is known about mGlu receptor expression and function in rodent epilepsy models and human epileptic conditions.

In vivo receptor occupancy of mGlu5 receptor antagonists using the novel radioligand [3H]3-methoxy-5-(pyridin-2-ylethynyl)pyridine)

In vivo receptor occupancy of mGlu5 receptor antagonists was quantified in rat and mouse brain using the mGlu5 receptor selective antagonist [3H]3-methoxy-5-(pyridin-2-ylethynyl)pyridine) ([3H]methoxy-PEPy). Administration of [3H]methoxy-PEPy (50 microCi/kg i.v.) to mGlu5 receptor-deficient mice revealed binding at background levels in forebrain, whereas wild-type mice exhibited 14-fold higher binding in forebrain relative to cerebellum. Systemic administration of the mGlu5 receptor antagonists 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) reduced the binding of [3H]methoxy-PEPy in rats and mice, reflecting mGlu5 receptor occupancy by these compounds. MPEP (10 mg/kg i.p.) and MTEP (3 mg/kg i.p.) maintained >75% receptor occupancy for 2 h in rats, while in mice MPEP and MTEP achieved >75% occupancy for only 30 and 15 min, respectively. Compound levels in plasma were substantially lower in mice suggesting species differences in receptor occupancy result from differences in absorption or metabolism of the compounds. These findings demonstrate that [3H]methoxy-PEPy is useful for determining the occupancy of mGlu5 receptors in the brain.

Lesion of substantia nigra pars compacta by the GluR5 agonist ATPA

Dopamine (DA) released by substantia nigra pars compacta (SNc) neurons is a key regulator of motor activity. A deficiency in the striatum DA content due to SNc degeneration is a characteristic of Parkinson's disease. The involvement of excitotoxic mechanisms in this pathology has been suggested. The kainate receptor subunit GluR5 has been identified in a few basal ganglia but it is strongly expressed in SNc. Here we examine whether (RS)-2-amino-3-(3-hydroxy-5-tbutylisoxazol-4-yl) propanoic acid (ATPA), a selective agonist of GluR5, induces damage in dopaminergic (DAergic) neurons. ATPA (13 nmol) was administered to rat SNc. Immediately after recovery from surgery, the rats displayed ipsilateral turning. This behavior disappeared in subsequent days. The administration of the D1/D2 agonist, apomorphine (1 mg/kg, s.c.) 1 and 2 weeks after ATPA-infusion also induced ipsilateral turning. Histological studies-performed 21 days after ATPA-infusion-showed a lesion of the lateral and central part of the SNc, where a significant loss (36%) of DAergic cells was detected by tyrosine hydroxylase immunohistochemistry. The lesion was restricted to the SNc, since no damage or glial reaction was observed in the substantia nigra pars reticulata as assessed by Nissl staining, tomato lectin staining for microglial cells and GFAP immunohistochemistry for astrocytes.

IN CONCLUSION

(1). ATPA-infusion induces neuronal damage in the SNc in the rat and (2). the behavioral effects of unilateral infusion of ATPA are consistent with DAergic alterations in basal ganglia.

Role of synaptic metabotropic glutamate receptors in epileptiform discharges in hippocampal slices

Application of group I metabotropic glutamate receptor (mGluR) agonists elicits seizure discharges in vivo and prolonged ictal-like activity in in vitro brain slices. In this study we examined 1) if group I mGluRs are activated by synaptically released glutamate during epileptiform discharges induced by convulsants in hippocampal slices and, if so, 2) whether the synaptically activated mGluRs contribute to the pattern of the epileptiform discharges. The GABA(A) receptor antagonist bicuculline (50 microM) was applied to induce short synchronized bursts of approximately 250 ms in mouse hippocampal slices. Addition of 4-aminopyridine (4-AP; 100 microM) prolonged these bursts to 0.7-2 s. The mGluR1 antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY 367385; 25-100 microM) and the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10-50 microM), applied separately, significantly reduced the duration of the synchronized discharges. The effects of these antagonists were additive when applied together, suggesting that mGluR1 and mGluR5 exert independent actions on the epileptiform bursts. In phospholipase C beta1 (PLCbeta1) knockout mice, bicuculline and 4-AP elicited prolonged synchronized discharges of comparable duration as those observed in slices from wild-type littermates. Furthermore, mGluR1 and mGluR5 antagonists reduced the duration of the epileptiform discharges to the same extent as they did in the wild-type preparations. The results suggest that mGluR1 and mGluR5 are activated synaptically during prolonged epileptiform discharges induced by bicuculline and 4-AP. Synaptic activation of these receptors extended the duration of synchronized discharges. In addition, the data indicate that the synaptic effects of the group I mGluRs on the duration of epileptiform discharges were mediated by a PLCbeta1-independent mechanism.

Complex interactions between mGluR1 and mGluR5 shape neuronal network activity in the rat hippocampus

Group I metabotropic glutamate receptors (mGluRs) cause increased neuronal excitability that can lead to epileptogenesis and neurodegeneration. Here we have examined how individual members of this subgroup of mGluRs affect synchronised hippocampal synaptic activity under normal and disinhibited conditions similar to those that occur during certain epileptic states. We demonstrate that activation of both mGluR1 and mGluR5 are important in increasing neuronal synaptic excitability by increasing synchrony between cells and driving correlated network activity in circuits that contain, or are devoid of, GABA(A) receptor-mediated synaptic inputs. The precise patterning of activity that occurs is complex and depends upon: (1) the existing pattern of ongoing network activity prior to mGluR activation; and (2) the relative extent of activation of each mGluR subtype. However, mGluR5 appears to be the principal mGluR subtype that initiates bursting activity irrespective of the inhibitory synaptic tone within the neuronal network.

Metabotropic Glutamate Receptors and Epileptogenesis

Activation of metabotropic glutamate receptors (mGluRs) often produces long-lasting effects on the excitability of cortical neurons. For example, mGluR stimulation induces long-term potentiation or depression of excitatory synaptic transmission in the hippocampus. Similarly, the effects of mGluRs on cortical epileptiform activities also are enduring. A transient application of group I mGluR agonists to hippocampal slices produces ictal-like discharges that persist for hours after the removal of the applied agonist. This action of group I mGluRs-transforming "normal" hippocampal slice into an "epileptic-like" one-may represent a form of epileptogenesis. The advent of such a model, in which epileptogenesis can be reliably induced in an in vitro preparation and the process is complete within hours, may facilitate the exploration of cellular mechanisms underlying epileptogenesis.

Metabotropic Glutamate Receptors and Epileptogenesis

Activation of metabotropic glutamate receptors (mGluRs) often produces long-lasting effects on the excitability of cortical neurons. For example, mGluR stimulation induces long-term potentiation or depression of excitatory synaptic transmission in the hippocampus. Similarly, the effects of mGluRs on cortical epileptiform activities also are enduring. A transient application of group I mGluR agonists to hippocampal slices produces ictal-like discharges that persist for hours after the removal of the applied agonist. This action of group I mGluRs—transforming “normal” hippocampal slice into an “epileptic-like” one—may represent a form of epileptogenesis. The advent of such a model, in which epileptogenesis can be reliably induced in an in vitro preparation and the process is complete within hours, may facilitate the exploration of cellular mechanisms underlying epileptogenesis.

Differential roles for mGluR1 and mGluR5 in the persistent prolongation of epileptiform bursts

Transient activation of group I metabotropic glutamate receptors (mGluRs) with the selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) produces persistent prolongation of epileptiform bursts in guinea pig hippocampal slices, the maintenance of which can be reversibly suppressed with group I mGluR antagonists. To determine the relative roles of mGluR1 and mGluR5 in these group I mGluR-dependent induction and maintenance processes, subtype-selective antagonists were utilized. In the presence of picrotoxin, DHPG (50 microM, 20-45 min) converted interictal bursts into 1- to 3-s discharges that persisted for hours following washout of the mGluR agonist. 2-methyl-6-(phenylethynyl)-pyridine (MPEP, an mGluR5 antagonist; 25 microM) and (+)-2-methyl-4-carboxyphenylglycine (LY367385, an mGluR1 antagonist; 20-25 microM) each significantly suppressed the ongoing expression of the mGluR-induced prolonged bursts. However, LY367385 was more effective, reducing the burst prolongation by nearly 90%; MPEP only produced a 64% reduction in burst prolongation. Nevertheless, MPEP was more effective at preventing the induction of the burst prolongation; all 10 slices tested failed to express prolonged bursts both during and after co-application of DHPG with MPEP. Co-application of DHPG with LY367385, in contrast, resulted in significant burst prolongation (in 68% of slices tested) that was revealed on washout of the two agents. These results suggest that while both receptor subtypes participate in both the induction and maintenance of mGluR-mediated burst prolongation, mGluR1 activation plays a greater role in sustaining the expression of prolonged bursts, whereas mGluR5 activation may be a more critical contributor to the induction process underlying this type of epileptogenesis.

(S)-3,5-DHPG: a review

3,5-dihydroxyphenylglycine (3,5-DHPG) was the first agonist shown to be group I metabotropic glutamate receptor selective with its agonist effects residing exclusively in the S-isomer. Some results suggest that (S)-3,5-DHPG may be a partial agonist of mGluR1a and mGluR5a in neurons and astrocytes. It has been reported that (S)-3,5-DHPG can, under certain conditions, interact with NMDA receptors. (S)-3,5-DHPG exerts different effects on second messengers in adult and neonatal tissues. It stimulates phosphoinositide hydrolysis in a dose-dependent manner in both the adult and neonate hippocampus, inhibits stimulated cAMP levels in the adult and enhances the cAMP in the neonate. It is an effective antagonist of mGluRs linked to phospholipase D (PLD) in the adult and an agonist in the neonate brain or astrocyte cultures. (S)-3,5-DHPG induces elevation of [Ca2+]i and regulates multiple subtypes of Ca2+ channels. This agonist of group I mGluRs may modulate neurotransmitters release, reflecting the diversity of mechanisms involved. Depending on the dose, (S)-3,5-DHPG enhances or decreases excitatory postsynaptic potentials (EPSPs) and under appropriate conditions it can induce long-term depression (LTD) and long-term potentiation (LTP). Some studies suggested a therapeutic role for (S)-3,5-DHPG in neuronal injury, regulation of intestinal motility and secretion, learning and memory processes and in cardiovascular system. (S)-3,5-DHPG may be useful as a cognitive enhancing agent in memory impairment associated with ischemia or hypoxia. Recent investigations suggested possible beneficial effects of (S)-3,5-DHPG in Alzheimer's disease.

Polyamine metabolism and glutamate receptor agonists-mediated excitotoxicity in the rat brain

Putrescine (PUT) increases have been seen in a range of models of neuropathological disturbances. The present study was designed to compare the ability of various types of glutamate receptor agonist to promote excitotoxic brain damage and to examine whether a PUT increase is a general marker of excitotoxic brain damage. To that end, we evaluated features of brain damage associated with the excitotoxicity induced by both ionotropic glutamate receptor (iGluR) and metabotropic glutamate receptor (mGluR) agonists in the conscious rat and the changes produced in the regulation of polyamine metabolism. Intracerebroventricular infusion of N-methyl-D-aspartate (NMDA; 80 nmol), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; 15 nmol), kainic acid (KA; 2.3 nmol), (R,S)-3,5-dihydroxyphenylglycine (3,5-DHPG; 1.5 micromol), and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 2 micromol) produced similar seizure incidences (76-84%) in the rat. The convulsant episodes appeared sooner after iGluR (13-22 min) than after mGluR agonists (50-179 min). Histological analysis of the hippocampus 24 hr after seizures indicated several degrees of excitotoxic injury after equiconvulsive doses of the iGluR and mGluR agonists assayed. The agonists can be placed in the following order, according to the degree of damage they produce: AMPA > 3,5-DHPG approximately KA > NMDA > 1S,3R-ACPD. In the frontal cortex, moderate to low levels of damage were observed after all GluR agonists. Both iGluR- and mGluR-induced seizures produced an overshoot in the hippocampal and cortical PUT concentration, whereas spermidine and spermine levels were similar to control. Moreover, a concurrence of increased PUT levels and brain damage was observed, indicating that PUT is a general marker of excitotoxic brain damage.

Neuroprotective and behavioral effects of the selective metabotropic glutamate mGlu(1) receptor antagonist BAY 36-7620

This study characterized the neuroprotective and behavioral effects of (3aS,6aS)-6a-naphtalen-2-ylmethyl-5-methyliden-hexahydro-cyclopenta[c]furan-1-on (BAY 36-7620), a novel, selective and systemically active metabotropic glutamate (mGlu)(1) receptor antagonist. In the rat, neuroprotective effects were obtained in the acute subdural hematoma model (efficacy of 40-50% at 0.01 and 0.03 mg/kg/h, i.v. infusion during the 4 h following surgery); whereas in the middle cerebral artery occlusion model, a trend for a neuroprotective effect was obtained after triple i.v. bolus application of 0.03-3 mg/kg, given immediately, 2 and 4 h after occlusion. Hypothermic effects were mild and only obtained at doses which were considerably higher than those at which maximal neuroprotective efficacy was obtained, indicating that the neuroprotective effects are not a consequence of hypothermia. BAY 36-7620 protected against pentylenetetrazole-induced convulsions in the mouse (MED: 10 mg/kg, i.v.). As assessed in rats, BAY 36-7620 was devoid of the typical side-effects of the ionotropic glutamate (iGlu) receptor antagonists phencyclidine and (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-imine (MK-801). Thus, BAY 36-7620 did not disrupt sensorimotor gating, induce phencyclidine-like discriminative effects or stereotypical behavior, or facilitate intracranial self-stimulation behavior. Although behavioral stereotypies and disruption of sensorimotor gating induced by amphetamine or apomorphine were not affected by BAY 36-7620, the compound attenuated some behavioral effects of iGlu receptor antagonists, such as excessive grooming or licking, and their facilitation of intracranial self-stimulation behavior. It is concluded that mGlu(1) receptor antagonism results in neuroprotective and anticonvulsive effects in the absence of the typical side-effects resulting from antagonism of iGlu receptors.

Metabotropic glutamate receptor subtypes as targets for neuroprotective drugs

Metabotropic glutamate (mGlu) receptors have been considered as potential targets for neuroprotective drugs, but the lack of specific drugs has limited the development of neuroprotective strategies in experimental models of acute or chronic central nervous system (CNS) disorders. The advent of potent and centrally available subtype-selective ligands has overcome this limitation, leading to an extensive investigation of the role of mGlu receptor subtypes in neurodegeneration during the last 2 years. Examples of these drugs are the noncompetitive mGlu1 receptor antagonists, CPCCOEt and BAY-36-7620; the noncompetitive mGlu5 receptor antagonists, 2-methyl-6-(phenylethynyl)pyridine, SIB-1893, and SIB-1757; and the potent mGlu2/3 receptor agonists, LY354740 and LY379268. Pharmacologic blockade of mGlu1 or mGlu5 receptors or pharmacologic activation of mGlu2/3 or mGlu4/7/8 receptors produces neuroprotection in a variety of in vitro or in vivo models. MGlu1 receptor antagonists are promising drugs for the treatment of brain ischemia or for the prophylaxis of neuronal damage induced by synaptic hyperactivity. MGlu5 receptor antagonists may limit neuronal damage induced by a hyperactivity of N-methyl-d-aspartate (NMDA) receptors, because mGlu5 and NMDA receptors are physically and functionally connected in neuronal membranes. A series of observations suggest a potential application of mGlu5 receptor antagonists in chronic neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer disease. MGlu2/3 receptor agonists inhibit glutamate release, but also promote the synthesis and release of neurotrophic factors in astrocytes. These drugs may therefore have a broad application as neuroprotective agents in a variety of CNS disorders. Finally, mGlu4/7/8 receptor agonists potently inhibit glutamate release and have a potential application in seizure disorders. The advantage of all these drugs with respect to NMDA or AMPA receptor agonists derives from the evidence that mGlu receptors do not "mediate," but rather "modulate" excitatory synaptic transmission. Therefore, it can be expected that mGlu receptor ligands are devoid of the undesirable effects resulting from the inhibition of excitatory synaptic transmission, such as sedation or an impairment of learning and memory.

Expression of metabotropic glutamate receptor 1alpha in the hippocampus of rat pilocarpine model of status epilepticus

The expression of metabotropic glutamate receptor 1alpha was studied in the rat hippocampus after pilocarpine-induced status epilepticus by Western blot and immunocytochemistry at both light and electron microscopic levels. At 1 day after pilocarpine-induced status epilepticus, there was marked decrease in metabotropic glutamate receptor 1alpha immunoreactivity at the border between stratum oriens and alveus in CA1 and CA3, and in the hilus of dentate gyrus. Between 3 and 31 days after pilocarpine-induced status epilepticus, metabotropic glutamate receptor 1alpha-immunoreactive dendrites and cell bodies in the border between stratum oriens and alveus gradually reappeared. Upregulation of metabotropic glutamate receptor 1alpha, however, was observed in the stratum oriens of CA1 at day 1, but returned to baseline by day 7. By electron microscopy, the metabotropic glutamate receptor 1alpha-immunoreactive product was demonstrated only in the post-synaptic elements in the border between the stratum oriens and alveus of CA1 and the hilus of the dentate gyrus in both control and experimental rats. At 1 day after pilocarpine-induced status epilepticus, metabotropic glutamate receptor 1alpha-immunoreactive degenerating neurons were identified in the border between stratum oriens and alveus of CA1 and the hilus of the dentate gyrus. At 7 and 31 days, many degenerating axons were also found. Present results suggest that excitoneurotoxicity mediated through post-synaptic metabotropic glutamate receptor 1alpha may be involved in degeneration and death of interneurons in the hilus of dentate gyrus, and the border between stratum oriens and alveus of CA1 in the early stage after pilocarpine-induced status epilepticus.

Attenuation of seizures induced by homocysteic acid in immature rats by metabotropic glutamate group II and group III receptor agonists

Previous studies demonstrated that selected agonists for metabotropic glutamate group II and group III receptors can provide protection against seizures in adult animals. The present study has examined the potential effect of some of these compounds on seizures induced in immature rats by intracerebroventricular infusion of DL-homocysteic acid (DL-HCA, 600 nmol/side). Rat pups were sacrificed during generalised clonic-tonic seizures, 50--60 min after infusion. Comparable time intervals were used for sacrificing the pups which had received the protective drugs. The anticonvulsant effect was evaluated according to the suppression of behavioural manifestations of seizures and the protection of energy metabolite changes which normally accompany these seizures (large decreases of glucose and glycogen, and approximately 7- to 10-fold accumulation of lactate). Partial protection was exhibited by group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV, 0.6 nmol) and this effect was abolished after pretreatment with an antagonist for group II mGluRs (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG, 100 nmol). In high doses (5--100 nmol), however, DCG IV evoked seizures which were prevented by AP7, suggesting that the convulsant effect was mediated by interaction with NMDA receptors. A pronounced anticonvulsant effect against DL-HCA-induced seizures was achieved with low doses of a highly selective group II mGluR agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC, 0.6 nmol), group II agonist and group I mGluR antagonist (S)-4-carboxy-3-hydroxyphenylglycine ((S)-4-C3HPG, 0.6 nmol) and group III mGluR agonist (RS)-1-amino-3-(phosphonomethylene) cyclobutane-carboxylic acid (32 nmol). Generalised clonic--tonic seizures were completely suppressed and the metabolic changes were markedly ameliorated, there being only a 1.5-, 2- and 2.5-fold rise of lactate, respectively. Higher doses of (S)-4-C3HPG (1--100 nmol) were, however, less anticonvulsant than low doses. The present results have confirmed that mGluRs may be considered a potential target for treatment of epilepsy.

Seizure-like activity in the disinhibited CA1 minislice of adult guinea-pigs

Spontaneous activity was monitored during pharmacological blockade of GABA(A) receptor function in the CA1 minislice (CA3 was cut off). Synaptic inhibition was blocked by competitive GABA(A) antagonists bicuculline-methiodide (Bic) or GABAZINE (GBZ) and the chloride channel blocker picrotoxin (PTX). Extra- and intracellular recordings using sharp electrodes were carried out in stratum radiatum and pyramidale. At low antagonist concentrations (Bic, GBZ: 1-10 microM; PTX: < 100 microM), synchronized bursts (< 500 ms in duration, interictal activity) were seen as described previously. However, in the presence of high concentrations (Bic, GBZ: 50-100 microM; PTX: 100-200 microM), seizure-like, ictal events (duration 4-17 s) were observed in 67 of 88 slices. No other experimental measures to increase excitability were applied: cation concentrations ([Ca2+]o = 2 mM, [Mg2+]o = 1.7 mM, [K+]o = 3 mM) and recording temperature (30-32 degrees C) were standard and GABA(B)-mediated inhibition was intact. In whole-slice recordings prominent interictal activity, but fewer ictal events were observed. A reduced ictal activity was also observed when interictal-like responses were evoked by afferent stimulation. Ictal activity was reversibly blocked by antagonists of excitatory transmission, CNQX (40 microM) or D-AP5 (50 microM). Disinhibition-induced ictal development did not rely on group I mGluR activation as it was not prevented in the presence of group I mGluR antagonists (AIDA or 4CPG). (RS)-3,5-DHPG prevented the induction and reversed the tertiary component of the ictal event through a group I mGluR-independent mechanism.

Modulation of calcium channels by group I and group II metabotropic glutamate receptors in dentate gyrus neurons from patients with temporal lobe epilepsy

PURPOSE

Metabotropic glutamate receptors (mGluRs) might be promising new drug targets for the treatment of epilepsy because the expression of certain mGluRs is regulated in epilepsy and because activation of mGluRs results in distinctive anti- and proconvulsant effects. Therefore, we examined how mGluR activation modulates high-voltage-activated (HVA) Ca2+ channels.

METHODS

Whole-cell patch-clamp recordings were obtained from granule cells and interneuron-like cells acutely isolated from the dentate gyrus of patients with pharmacoresistent temporal lobe epilepsy.

RESULTS

Agonists selective for either group I or group II mGluRs rapidly and reversibly reduced HVA currents in most dentate gyrus cells. These modulatory effects were inhibited by the respective group I and group II mGluR antagonists. The specific Ca2+ channel antagonists nifedipine and omega-conotoxin GVIA potently occluded the effects of group I and II mGluR agonists, respectively, indicating that group I mGluRs acted on L-type channels and group II mGluRs affected N-type channels. About two thirds of the responsive neurons were sensitive either to group I or group II mGluRs, whereas a minority of cells showed effects to agonists of both groups, indicating a variable mGluR expression pattern.

CONCLUSIONS

Group I and group II mGluRs are expressed in human dentate gyrus neurons and modulate L- and N-type HVA channels, respectively. The data shed light on the possible cellular sequelae of the mGluR1 upregulation observed in human epileptic dentate gyrus as well as on possible mGluR-mediated anticonvulsant mechanisms.

Coincident activation of mGluRs and mAChRs imposes theta frequency patterning on synchronised network activity in the hippocampal CA3 region

Activation of metabotropic glutamate receptors (mGluRs) with the broad spectrum mGluR agonist 1S,3R ACPD (10-50 microM) induced spontaneous field potentials at low frequencies ('burst-mode' activity; <1 Hz) in the CA3 region of rat hippocampal slices. At higher concentrations (100-400 microM) ACPD switched this form of activity to a second, more complex pattern of activity in which intermittent episodes of theta frequency oscillations predominated ('theta-mode' activity; 4-14 Hz). Both patterns of activity were evoked by selective activation of group I mGluRs and, in particular, could be induced by activation of mGluR5 alone using the subtype selective agonist CHPG (0.5-5 mM). In contrast, activation of group II mGluRs (DCG IV; 100 microM) produced only burst-mode behaviour whilst activation of group III mGluRs (L-AP4; 100 microM) did not result in synchronised network activity. Concurrent extra- and intracellular recordings demonstrated that this mGluR-induced theta-mode activity represented the synchronous firing of CA3 pyramidal cells and that it shared a similar temporal signature to that generated by activation of muscarinic acetylcholine receptors (mAChRs). Furthermore, application of mGluR and mAChR agonists at concentrations sufficient to produce only burst-mode activity when applied individually, produced theta-mode activity when co-applied. These data suggest that the level of activation of different mGluRs and mAChRs crucially determine the pattern of rhythmical network activity generated in the hippocampal CA3 network. These results also indicate that individual receptor subtypes (i.e. mGluR5) can initiate patterns of coherent network activity but that interactions between the cholinergic and glutamatergic transmitter systems may also be important factors in governing the temporal patterning of hippocampal network activity.

Group II metabotropic glutamate receptors are a common target of N-anisoyl-GABA and 1S,3R-ACPD in enhancing ACh release in the prefrontal cortex of freely moving SHRSP

Aniracetam is a therapeutically useful cognition enhancer for treating various neuropsychiatric symptoms occurring after cerebral infarction. We recently reported that local perfusion of its major metabolites N-anisoyl-GABA and p-anisic acid, but not aniracetam itself, enhanced acetylcholine (ACh) release with a delayed onset in cerebral regions of stroke-prone spontaneously hypertensive rats (SHRSP). In this study, we examined the possible involvement of metabotropic and ionotropic glutamate (mGlu and AMPA) receptors in the N-anisoyl-GABA-induced ACh release using brain in vivo microdialysis. Basal ACh release in SHRSP was commonly lower in the nucleus reticularis thalami, dorsal hippocampus and prefrontal cortex than that in age-matched Wistar Kyoto rats. The delayed ACh release in the prefrontal cortex of SHRSP was completely blocked by MCPG, a group I and II mGlu receptor antagonist, and MCCG, a group II-selective mGlu receptor antagonist. In contrast, it was largely unaffected by AIDA, a group I-selective mGlu receptor antagonist, or by YM90K, an AMPA receptor antagonist. 1S,3R-ACPD, a preferential group II mGlu receptor agonist, enhanced ACh release with a similar latency and the effect was antagonized by MCCG, whereas AMPA induced a prompt ACh release. These results indicate that N-anisoyl-GABA and 1S,3R-ACPD share a common mechanism mediated by group II mGlu receptors in enhancing ACh release. The findings suggest a possible mechanism for aniracetam's clinical efficacy in stroke patients with cholinergic deficits.

Differential expression of glutamate receptors by the dopaminergic neurons of the primate striatum

Neostriatal neurons are targets of glutamatergic input from the cortex and thalamus. Glutamate receptors are abundantly, but differentially, expressed by the striatal neurons. We previously described the presence of dopaminergic cells intrinsic to the primate striatum that increase in number following MPTP treatment. In this study we have used double-label immunocytochemistry to analyze the expression of the glutamate receptor subunits GluR1, GluR2/3, NR1, mGluR1, and mGluR5 in the dopaminergic cells of the striatum. Our results show that 75% of these cells express GluR1 while 25% of them express NR1. They do not express GluR2/3 or the group 1 metabotropic receptors. Our results suggest that this potentially important population of cells expresses only calcium-permeable ionotropic glutamate receptors. We speculate that glutamate may play a role in regulating the number of these dopaminergic neurons after MPTP treatment and may also influence their ability to release dopamine.

Group-I metabotropic glutamate receptors: hypotheses to explain their dual role in neurotoxicity and neuroprotection

The role of group-I metabotropic glutamate receptors (mGlu1 and 5) in neurodegeneration is still controversial. While antagonists of these receptors are consistently neuroprotective, agonists have been found to either amplify or attenuate excitotoxic neuronal death. At least three variables affect responses to agonists: (i) the presence of the NR2C subunit in the NMDA receptor complex; (ii) the existence of an activity-dependent functional switch of group-I mGlu receptors, similar to that described for the regulation of glutamate release; and (iii) the presence of astrocytes expressing mGlu5 receptors. Thus, a number of factors, including the heteromeric composition of NMDA receptors, the exposure time to drugs or to ambient glutamate, and the function of astrocytes clearing extracellular glutamate and producing neurotoxic or neuroprotective factors, must be taken into account when examining the role of group-I mGlu receptors in neurodegeneration/neuroprotection.

Group I metabotropic glutamate receptors: implications for brain diseases

Glutamate is the major excitatory neurotransmitter in the brain and plays a unique role in a variety of central nervous system (CNS) functions. The discovery of the metabotropic receptors (mGluRs), a family of G-protein coupled receptors than can be activated by glutamate, has led to an impressive number of studies in recent years aimed at understanding their biochemical, physiological and pharmacological characteristics. The eight mGluRs now known are divided into three groups according to their sequence homology, signal transduction mechanisms, and agonist selectivity. Group I mGluRs include mGluR1 and mGluR5, which are linked to the activation of phospholipase C; Groups II and III include all others and are negatively coupled to adenylyl cyclases. The availability in recent years of agents selective for Group I mGluRs has made possible the study of the physiological roles of these receptors in the CNS. In addition to mediating glutamatergic neurotransmission, Group I mGluRs can modulate other neurotransmitter receptors, including GABA and the ionotropic glutamate receptors. Group I mGluRs are involved in many CNS functions and may participate in a variety of disorders such as pain, epilepsy, ischemia, and chronic neurodegenerative diseases. This class of receptor may provide important pharmacological therapeutic targets and elucidating its functions will be relevant to develop new treatments for neurological and psychiatric disorders in which glutamatergic neurotransmission is abnormally regulated. In this review anatomical, physiological and pharmacological results are presented with a special emphasis on the role of Group I mGluRs in functional and pathological processes.

Group I mGluR-mediated silent induction of long-lasting epileptiform discharges

Picrotoxin, an antagonist of GABA(A) receptor-mediated activity, elicited 320- to 475-ms synchronized bursts from the CA3 region of the guinea pig hippocampal slice. The addition of the selective group I metabotropic glutamate receptor (mGluR) agonist (S)-3, 5-dihydroxyphenylglycine (DHPG, 50 microM; 20- to 45-min application) gradually increased the burst duration to 1-4 s; this effect persisted 2-3 h after agonist removal. To determine whether the induction of this long-lasting effect required ongoing synchronized activity during mGluR activation, DHPG application in a second set of experiments took place in the presence of CNQX and (R, S)-CPP, antagonists of AMPA/kainate and NMDA receptors, respectively. In these experiments, synchronized bursting was silenced during the mGluR agonist application, yet after wash out of the DHPG and the ionotropic glutamate receptor (iGluR) blockers, epileptiform discharges 1-10 s in duration appeared and persisted at least 2 h after wash out of the mGluR agonist. The potentiated bursts were reversibly shortened by application of 500-1,000 microM (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) or (S)-4-carboxyphenylglycine (4CPG), agents with group I mGluR antagonist activity. These data suggest that transient activation of group I mGluRs, even during silencing of synchronized epileptiform activity, may have an epileptogenic effect, converting brief interictal-length discharges into persistent seizure-length events. The induction process is iGluR independent, and the maintenance is largely mediated by the action of endogenous glutamate on group I mGluRs, suggesting that autopotentiation of the group I mGluR-mediated response may underlie the epileptogenesis seen here.