Group I metabotropic glutamate receptors interfere in different ways with pentylenetetrazole seizures, kindling, and kindling-related learning deficits

Superior control of inflammatory pain by corticotropin-releasing factor receptor 1 via opioid peptides in distinct pain-relevant brain areas

Background

Under inflammatory conditions, the activation of corticotropin-releasing factor (CRF) receptor has been shown to inhibit pain through opioid peptide release from immune cells or neurons. CRF’s effects on human and animal pain modulation depend, however, on the distribution of its receptor subtypes 1 and 2 (CRF-R1 and CRF-R2) along the neuraxis of pain transmission. The objective of this study is to investigate the respective role of each CRF receptor subtype on centrally administered CRF-induced antinociception during inflammatory pain.

Methods

The present study investigated the role of intracerebroventricular (i.c.v.) CRF receptor agonists on nociception and the contribution of cerebral CRF-R1 and/or CRF-R2 subtypes in an animal model of Freund’s complete adjuvant (FCA)-induced hind paw inflammation. Methods used included behavioral experiments, immunofluorescence confocal analysis, and reverse transcriptase-polymerase chain reaction.

Results

Intracerebroventricular, but systemically inactive, doses of CRF elicited potent, dose-dependent antinociceptive effects in inflammatory pain which were significantly antagonized by i.c.v. CRF-R1-selective antagonist NBI 27914 (by approximately 60%) but less by CRF-R2-selective antagonist K41498 (by only 20%). In line with these findings, i.c.v. administration of CRF-R1 agonist stressin I produced superior control of inflammatory pain over CRF-R2 agonist urocortin-2. Intriguingly, i.c.v. opioid antagonist naloxone significantly reversed the CRF as well as CRF-R1 agonist-elicited pain inhibition. Consistent with existing evidence of high CRF concentrations in brain areas such as the thalamus, hypothalamus, locus coeruleus, and periaqueductal gray following its i.c.v. administration, double-immunofluorescence confocal microscopy demonstrated primarily CRF-R1-positive neurons that expressed opioid peptides in these pain-relevant brain areas. Finally, PCR analysis confirmed the predominant expression of the CRF-R1 over CRF-R2 in representative brain areas such as the hypothalamus.

Conclusion

Taken together, these findings suggest that CRF-R1 in opioid-peptide-containing brain areas plays an important role in the modulation of inflammatory pain and may be a useful therapeutic target for inflammatory pain control.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02498-8.

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.

2-Methyl-6-(phenylethynyl)pyridine Hydrochloride Modulates Metabotropic Glutamate 5 Receptors Endogenously Expressed in Zebrafish Brain

Due to phylogenetic proximity to the human, zebrafish has been recognized as a reliable model to study Alzheimer's disease (AD) and other central nervous system disorders. Furthermore, metabotropic glutamate receptors have been previously reported to be impaired in brain from AD patients. Metabotropic glutamate 5 (mGlu₅) receptors are G-protein coupled receptors proposed as potential targets for therapy of different neurodegenerative disorders. Thus, MPEP (2-methyl-6-(phenylethynyl)pyridine hydrochloride), a selective noncompetitive mGlu₅ receptor antagonist, has been suggested for pharmacological treatment of AD. The aim of the present work was to quantify mGlu₅ receptors in brain from zebrafish and to study the possible modulation of these receptors by MPEP treatment. To this end, radioligand binding assay and open field test were used. Results showed a slightly higher presence of mGlu₅ receptors in brain from male than in that from female zebrafish. However, a significant increase of mGlu₅ receptor in male without variation in female was observed after MPEP treatment. This gender specific response was also observed in locomotor behavior, being significantly decreased only in male zebrafish. These results confirm the presence of mGlu₅ receptors in brain from zebrafish and their gender specific modulation by selective antagonist treatment and suggest a role of these receptors on locomotor activity, which is affected in many disorders. In addition, our data point to zebrafish as a useful model to study mGlu receptor function in both healthy and pathological conditions.

Protective Effect of Quinine on Chemical Kindling and Passive Avoidance Test in Rats

Background

In humans, convulsive diseases such as temporal lobe epilepsy are usually accompanied by learning and memory impairments. In recent years, the role of gap junction channels as an important target of antiepileptic drugs has been studied and discussed. Quinine, as a gap junction blocker of connexin 36, can abolish ictal epileptiform activity in brain slices.

Objectives

The role of quinine in memory retrieval in pentylenetetrazole (PTZ)-kindled rats was examined using a step-through passive avoidance task.

Methods

Forty rats were used in this experimental study in groups of 10 animals. Quinine (15, 30, and 60 mg/kg, i.p.) and PTZ (35 mg/kg, i.p.) were injected into the rats before the start of the learning test. Then, retention tests were conducted after the treatments ended.

Results

Quinine could attenuate seizure severity at doses of 15, 30 and 60 mg/kg compared with the control at the beginning of the kindling experiment by lowering the mean seizure stages (P < 0.01, P < 0.001, P < 0.001). Quinine at doses of 15 and 30 mg/kg could significantly increase memory retrieval compared with the control in the retention test 24 and 48 hours after training (P < 0.05). Quinine at a dose of 60 mg/kg increased latency to enter the dark chamber 24 and 48 hours after training (P < 0.001). The results of the retention test one and two weeks after training of quinine were not significant (P > 0.05).

Conclusions

Quinine may decrease the severity of seizure and improve the memory retrieval of animals by inhibiting the gap junction channel. However, further studies are needed to evaluate the molecular mechanism underlying the effects of quinine.

Plasticity of Hippocampal Excitatory-Inhibitory Balance: Missing the Synaptic Control in the Epileptic Brain

Synaptic plasticity is the capacity generated by experience to modify the neural function and, thereby, adapt our behaviour. Long-term plasticity of glutamatergic and GABAergic transmission occurs in a concerted manner, finely adjusting the excitatory-inhibitory (E/I) balance. Imbalances of E/I function are related to several neurological diseases including epilepsy. Several evidences have demonstrated that astrocytes are able to control the synaptic plasticity, with astrocytes being active partners in synaptic physiology and E/I balance. Here, we revise molecular evidences showing the epileptic stage as an abnormal form of long-term brain plasticity and propose the possible participation of astrocytes to the abnormal increase of glutamatergic and decrease of GABAergic neurotransmission in epileptic networks.

Effects of quercetin on oxidative stress and memory retrieval in kindled rats

Flavonoids are a class of polyphenolic compounds present in fruits and vegetables. Several studies have demonstrated a relationship between the consumption of flavonoid-rich diets and the prevention of human diseases including neurodegenerative disorders. Thus, we assessed the effect of quercetin (3,3',4',5,7-pentahydroxyflavone) on oxidative stress and memory retrieval using a step-through passive avoidance task in kindled rats. Quercetin (25, 50, and 100 mg/kg) was administered intraperitoneally (i.p.) before pentylenetetrazole (PTZ) every other day prior to the training. Retention tests were performed to assess memory in rats. Compared to control, pretreatment with 50 mg/kg of quercetin could attenuate seizure severity from the beginning of the kindling experiment by lowering the mean seizure stages. Moreover, quercetin 50 mg/kg significantly increased the step-through latency of the passive avoidance response compared to the control in the retention test. Malondialdehyde (MDA) levels were significantly increased in the quercetin groups compared to the PTZ group in the hippocampus and cerebral cortex following PTZ kindling. In the quercetin groups, higher sulfhydryl (SH) contents were not observed compared to the PTZ group. These results indicate that quercetin at a specific dose results in decreased seizure severity during kindling and performance improvement in a passive avoidance task in kindled rats. All doses of quercetin led to increased oxidative stress in the hippocampi and cerebral cortices of kindled rats.

Therapeutic potential of metabotropic glutamate receptor modulators

Glutamate is the main excitatory neurotransmitter in the central nervous system (CNS) and is a major player in complex brain functions. Glutamatergic transmission is primarily mediated by ionotropic glutamate receptors, which include NMDA, AMPA and kainate receptors. However, glutamate exerts modulatory actions through a family of metabotropic G-protein-coupled glutamate receptors (mGluRs). Dysfunctions of glutamatergic neurotransmission have been implicated in the etiology of several diseases. Therefore, pharmacological modulation of ionotropic glutamate receptors has been widely investigated as a potential therapeutic strategy for the treatment of several disorders associated with glutamatergic dysfunction. However, blockade of ionotropic glutamate receptors might be accompanied by severe side effects due to their vital role in many important physiological functions. A different strategy aimed at pharmacologically interfering with mGluR function has recently gained interest. Many subtype selective agonists and antagonists have been identified and widely used in preclinical studies as an attempt to elucidate the role of specific mGluRs subtypes in glutamatergic transmission. These studies have allowed linkage between specific subtypes and various physiological functions and more importantly to pathological states. This article reviews the currently available knowledge regarding the therapeutic potential of targeting mGluRs in the treatment of several CNS disorders, including schizophrenia, addiction, major depressive disorder and anxiety, Fragile X Syndrome, Parkinson’s disease, Alzheimer’s disease and pain.

Relative contributions of peripheral versus supraspinal or spinal opioid receptors to the antinociception of systemic opioids

The contribution of supraspinal, spinal or peripheral mu-opioid receptors (MORs) to the overall antinociception of systemic centrally penetrating versus peripherally restricted opioids has not been thoroughly investigated. Therefore, we examined paw pressure thresholds in Wistar rats with complete Freund's adjuvant hindpaw inflammation following different doses of intraplantar (i.pl.) as well as intravenous (i.v.) fentanyl (6.25-50 μg/kg), morphine (1-7.5 mg/kg) or loperamide (1-7.5 mg/kg). Antagonism of the i.v. mu-opioid agonists by intracerebroventricular (i.c.v.), intrathecal (i.t.) or i.pl. naloxone-methiodide (NLXM) revealed the relative contributions of supraspinal, spinal and peripheral MOR to the overall antinociceptive effects. In parallel, the MOR density at these three levels of pain transmission was assessed by radioligand binding. Antinociceptive effects of i.v. fentanyl and morphine, but not of the peripherally restricted loperamide were two- to threefold greater and longer lasting compared with their i.pl. administration. I.c.v. but not i.pl. NLXM significantly antagonized fentanyl's and morphine's antinociception by 70-80%, whereas i.t. NLXM reduced it by 20-30%. In contrast, antinociception of i.v. loperamide was abolished by i.pl. but not by i.c.v. or i.t. NLXM. In parallel, a respective 32- and sixfold higher MOR density in supraspinal and spinal versus peripheral sensory neurons was detected. In conclusion, in comparison with supraspinal and spinal opioid receptors, peripheral opioid receptors do not significantly contribute to the antinociception of systemic fentanyl and morphine during inflammatory pain. Antinociception of their i.v. administration was superior over both i.v and i.pl. loperamide, acting exclusively via peripheral MOR. These findings may guide the future development of novel peripherally restricted opioids.

The effects of rutin on the development of pentylenetetrazole kindling and memory retrieval in rats

Individuals with epilepsy often complain about memory deficits. Various synthetic derivatives of natural flavonoids are known to have neuroactive properties. Rutin is a flavonoid that is an important dietary constituent of foods and plant-based beverages. The aim of the present study was to investigate the effects of rutin on memory retrieval in pentylenetetrazole (PTZ)-kindled rats using a step-through passive avoidance task. We administered rutin and PTZ intraperitoneally every other day prior to the start of training. Two retention tests were subsequently performed to assess memory in these rats. The results suggest that pretreatment with rutin at 50 and 100mg/kg can attenuate seizure severity during the kindling procedure. Furthermore, rutin administration significantly increased the step-through latency in the passive avoidance paradigm. Taken together, these results indicate that rutin has a potential role in enhancing memory retrieval in kindled rats.

Effects of mGluR5 and mGluR1 antagonists on anxiety-like behavior and learning in developing rats

Antagonists of group I metabotropic receptors exhibit anxiolytic action in adult rats. In immature animals we demonstrated anticonvulsant action of MPEP and AIDA, antagonists of group 5 and group 1, respectively. However, there are no developmental data on anxiolytic-like and learning actions of both compounds. This study investigated whether the anticonvulsant dose range of MPEP and AIDA affects anxiety-like behavior and learning ability in immature rats. Animals at 12, 18 and 25 postnatal (P) days received MPEP in doses of 10, 20 or 40 mg/kg i.p., AIDA in doses of 10 or 20 mg/kg i.p. In P18 and P25 rats anxiety-like behavior and locomotor activity were tested in the light-dark box and open-field test at 15 (1st session) and 60 (2nd session) minutes after drug administration. Learning ability of P12, P18, and P25 animals was examined in the homing response test 15 min after drug administration. Both antagonists exhibited anxiolytic-like action in the 1st session, effects in the 2nd session were less marked. In the open-field test both antagonists increased locomotion only in P18 animals. Age-dependent changes were found in the homing response test, the return latency being longer only in P12 animals. While MPEP in doses of 20- and 40-mg/kg in P12 and 40-mg/kg in P18 rats prolonged the homing response, AIDA did not affect the homing behavior. Both MPEP and AIDA exert anxiolytic-like effect also in immature rats. Except for the youngest animals no changes in learning ability in the homing response test were found.

Limbic epileptogenesis in a mouse model of fragile X syndrome

Fragile X syndrome (FXS), caused by silencing of the Fmr1 gene, is the most common form of inherited mental retardation. Epilepsy is reported to occur in 20-25% of individuals with FXS. However, no overall increased excitability has been reported in Fmr1 knockout (KO) mice, except for increased sensitivity to auditory stimulation. Here, we report that kindling increased the expressions of Fmr1 mRNA and protein in the forebrain of wild-type (WT) mice. Kindling development was dramatically accelerated in Fmr1 KO mice, and Fmr1 KO mice also displayed prolonged electrographic seizures during kindling and more severe mossy fiber sprouting after kindling. The accelerated rate of kindling was partially repressed by inhibiting N-methyl-D-aspartic acid receptor (NMDAR) with MK-801 or mGluR5 receptor with 2-methyl-6-(phenylethynyl)-pyridine (MPEP). The rate of kindling development in WT was not effected by MPEP, however, suggesting that FMRP normally suppresses epileptogenic signaling downstream of metabolic glutamate receptors. Our findings reveal that FMRP plays a critical role in suppressing limbic epileptogenesis and predict that the enhanced susceptibility of patients with FXS to epilepsy is a direct consequence of the loss of an important homeostatic factor that mitigates vulnerability to excessive neuronal excitation.

Inhibition of inflammatory pain by CRF at peripheral, spinal and supraspinal sites: involvement of areas coexpressing CRF receptors and opioid peptides

There is conflicting evidence on the antinociceptive effects of corticotropin-releasing factor (CRF) along the neuraxis of pain transmission and the responsible anatomical sites of CRF's action at the level of the brain, spinal cord and periphery. In an animal model of tonic pain, that is, Freunds complete adjuvant (FCA) hindpaw inflammation, we systematically investigated CRF's ability to modulate inflammatory pain at those three levels of pain transmission by algesiometry following the intracerebroventricular, intrathecal, and intraplantar application of low, systemically inactive doses of CRF. At each level, CRF elicits potent antinociceptive effects, which are dose dependent and antagonized by local, but not systemic CRF receptor antagonist alpha-helical CRF indicating CRF receptor specificity. Consistently, we have identified by immunohistochemistry multiple brain areas, inhibitory interneurons within the dorsal horn of the spinal cord as well as immune cells within subcutaneous tissue--but not peripheral sensory neurons--that coexpress both CRF receptors and opioid peptides. In line with these anatomical findings, local administration of CRF together with the opioid receptor antagonist naloxone dose-dependently reversed CRF's antinociceptive effects at each of these three levels of pain transmission. Therefore, local application of low, systemically inactive doses of CRF at the level of the brain, spinal cord and periphery inhibits tonic inflammatory pain most likely through an activation of CRF receptors on cells that coexpress opioid peptides which results in opioid-mediated pain inhibition. Future studies have to delineate whether endogenous CRF at these three levels contributes to the body's response to cope with the stressful stimulus pain in an opioid-mediated manner.

Two-methyl-6-phenylethynyl-pyridine (MPEP), a metabotropic glutamate receptor 5 antagonist, with low doses of MK801 and diazepam: A novel approach for controlling status epilepticus

By intravenous administration of group I metabotropic glutamate receptor antagonists at 1 or 2h during pilocarpine induced status epilepticus (PISE), we showed that mGluR1 antagonists AIDA or LY367385 (at dosages ranging from 25 to 200mg/kg), mGluR5 antagonists SIB1757 (at dosages ranging from 25 to 200mg/kg), SIB1893 (from 25 to 100mg/kg), MPEP (from 25 to 100mg/kg) injected at 1 or 2h during PISE were ineffective in controlling status epilepticus (SE). However, when administered at 1h during PISE, MPEP at 200mg/kg, combination of MPEP (200mg/kg) with MK801 (0.1mg/kg) or with MK801 (0.1mg/kg) and diazepam (0.5mg/kg), combination of SIB1893 (200mg/kg) with MK801 (0.1mg/kg) could effectively control behavioral SE, and were neuroprotective. In particular, the combination of MPEP with MK801 and diazepam could stop both behavioral SE and electrical SE (under EEG monitoring) within a few minutes after the administration. HPLC study showed that a high level of MPEP was maintained in the blood and its metabolism rate was slow in experimental mice with PISE. We therefore concluded that the combination of MPEP (200mg/kg) with MK801 (0.1mg/kg) and diazepam (0.5mg/kg) could effectively stop SE and its subsequent neuronal loss in the hippocampus when administered 1h during PISE. It may provide a new approach to effectively control intractable SE.

Pentylenetetrazol-kindling in mice overexpressing heat shock protein 70

Kindling induced by the convulsant pentylenetetrazol (PTZ) is an accepted model of primary generalized epilepsy. Because seizures represent a strong distressing stimulus, stress-induced proteins such as heat shock proteins might counteract the pathology of increased neuronal excitation. Therefore, the aim of the present study was to determine whether PTZ kindling outcome parameters are influenced by heat shock protein 70 (Hsp70) overexpression in Hsp70 transgenic mice as compared to the respective wild-type mice. Kindling was performed by nine intraperitoneal injections of PTZ (ED(16) for induction of clonic-tonic seizures, every 48 h); control animals received saline instead of PTZ. Seven days after the final injection, all mice received a PTZ challenge dose. Outcome parameters included evaluation of seizure stages and overall survival rates. In addition, histopathological findings such as cell number in hippocampal subfields CA1 and CA3 were determined. The onset of the highest convulsion stage was delayed in Hsp70 transgenic mice as compared to wild-type mice, and overall survival during kindling was improved in Hsp70 transgenic mice as compared to wild-type mice. In addition, a challenge dose after termination of kindling produced less severe seizures in Hsp70 transgenic mice than in wild-type mice. PTZ kindling did not result in significant subsequent neuronal cell loss in CA1 or CA3 neither in wild-type mice nor in the Hsp70 transgenic mice. The results of the present experiments clearly demonstrate that overexpression of Hsp70 exerts protective effects regarding seizure severity and overall survival during PTZ kindling. In addition, the decreased seizure severity in Hsp70 transgenic mice after a challenge dose suggests an interference of Hsp70 with the developmental component of kindling.

Pentylenetetrazole Kindling Affects Sleep in Rats

PURPOSE

The aim of the study was to define sleep disturbances in pentylenetetrazole (PTZ)-kindled rats and to explore the effects of the nootropic drug piracetam (Pir; 100 mg/kg) and the noncompetitive N-methyl-D-aspartate (NMDA)-antagonist MK-801 (0.3 mg/kg), which normalized learning performance in PTZ-kindled rats, on altered sleep parameters.

METHODS

This is the first report showing a significant reduction in paradoxical sleep (PS) as a consequence of PTZ kindling. A correlation analysis revealed a significant correlation between seizure severity and PS deficit.

RESULTS

Pir did not interfere with seizure severity, and the substance did not ameliorate the PS deficit. However, the substance disconnected the correlation between seizure severity and PS deficit. MK-801, which reduced the severity of kindled seizures, counteracted the PS deficit efficaciously.

CONCLUSIONS

The results suggest that seizure severity and alterations in sleep architecture are two factors in the comprehensive network underlying learning impairments associated with epilepsy. Considering the results obtained in the experiments with Pir, reduction of seizure severity does not guarantee the reduction of impairments in the domain of learning.

Allosteric enhancement of metabotropic glutamate receptor 5 function promotes spatial memory

Group I metabotropic glutamate receptors (mGluRs) have been implicated in learning and memory formation. Recent findings indicate an important function of the group I mGluR subtype 5. Here, we used the Y-maze spatial alternation task and examined whether enhancement of intrinsic mGluR5 activity immediately after learning, i.e. during a critical period for memory consolidation, would have any consequences on long-term memory retention in rats. Intracerebroventricular application of the allosteric mGluR5 potentiator DFB (3,3'-difluorobenzaldazine) resulted in a marked improvement in spatial alternation retention when it was tested 24 h after training. The promnesic effect increased with the difficulty of the task and was apparently due to a substantial enhancement of consolidation. The applied dose of DFB did not cause behavioral changes in the open field, and was devoid of structural side-effects as evaluated by immunohistochemical examination. Our results suggest an important function of post-training mGluR5 activation in some types of hippocampus-dependent spatial learning.

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.

mGlu1 and mGlu5 receptor antagonists lack anticonvulsant efficacy in rodent models of difficult-to-treat partial epilepsy

Modulation of metabotropic glutamate (mGlu) receptors represents an interesting new approach for the treatment of a range of neurological and psychiatric disorders. Several lines of evidence suggest that functional blockade of group I (mGlu1 and mGlu5) receptors may be beneficial for treatment of epileptic seizures. This study was conducted to investigate whether mGlu1 or mGlu5 receptor antagonists have the potential to block partial or secondarily generalized seizures as occurring in partial epilepsy, the most common and difficult-to-treat type of epilepsy in patients. For this purpose, we systemically administered novel highly selective and brain penetrable group I mGlu receptor antagonists, i.e., the mGlu1 receptor antagonist EMQMCM [3-ethyl-2-methyl-quinolin-6-yl-(4-methoxy-cyclohexyl)-methanone methanesulfonate] and the mGlu5 receptor antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine), at doses appropriate for mGlu1 or mGlu5 receptor-mediated effects in rodent models of partial seizures. Two models were used: the 6-Hz electroshock model of partial seizures in mice and the amygdala-kindling model in rats. Clinically established antiepileptic drugs were included in the experiments for comparison. Antiepileptic drugs exerted significant anticonvulsant effects in both models, while EMQMCM and MTEP were ineffective in this regard, although both compounds were administered up to doses associated with essentially full receptor occupancy and with typical mGlu receptor-mediated effects in rodent models of anxiety or pain. Brain microdialysis for determining extracellular levels of MTEP following i.p. administration in rats substantiated that effective brain concentrations were reached at times of our experiments in seizure models. The present results do not support a significant anticonvulsant potential of group I mGlu receptor antagonists in rodent models of difficult-to-treat partial epilepsy.

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.

Excitatory amino acid neurotransmission

In recent years great progress has been made in understanding the function of ionotropic and metabotropic glutamate receptors; their pharmacology and potential therapeutic applications. It should be stressed that there are already N-methyl-D-aspartate (NMDA) antagonists in clinical use, such as memantine, which proves the feasibility of their therapeutic potential. It seems unlikely that competitive NMDA receptor antagonists and high-affinity channel blockers will find therapeutic use due to limiting side-effects, whereas agents acting at the glycineB site, NMDA receptor subtype-selective agents and moderate-affinity channel blockers are far more promising. This is supported by the fact that there are several glycineB antagonists, NMDA moderate-affinity channel blockers and NR2B-selective agents under development. Positive and negative modulators of AMPA receptors such as the AMPAkines and 2,3-benzodiazepines also show more promise than e.g. competitive antagonists. Great progress has also been made in the field of metabotropic glutamate receptors since the discovery of novel, allosteric modulatory sites for these receptors. Selective agents acting at these transmembrane sites have been developed that are more drug-like and have a much better access to the central nervous system than their competitive counterparts. The chapter will critically review preclinical and scarce clinical experience in the development of new ionotropic and metabotropic glutamate receptor modulators according to the following scheme: rational, preclinical findings in animal models and finally clinical experience, where available.

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.

Bursting of prefrontal cortex neurons in awake rats is regulated by metabotropic glutamate 5 (mGlu5) receptors: rate-dependent influence and interaction with NMDA receptors

Metabotropic glutamate 5 (mGlu5) receptors have been recently implicated in prefrontal cortex (PFC)-dependent executive functions because inhibition of mGlu5 receptors impairs working memory and worsens cognitive-impairing effects of NMDA receptor antagonists. To better understand the mechanisms by which mGlu5 receptors influence PFC function, we examined the effects of selective mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP), given alone or in combination with the NMDA receptor antagonist MK801, on ensemble single unit activity in the medial PFC (mPFC) of behaving rats. MPEP decreased the spontaneous burst activity of the majority of mPFC neurons. This inhibition was selective for the most active cells because greater decreases were observed in neurons with higher baseline firing rates. MPEP augmented the effects of MK801 on burst activity, variability of spike firing and random spike activity. These findings demonstrate that in awake animals mGlu5 receptors regulate the function of PFC neurons by two related mechanisms: (i) rate-dependent excitatory influence on spontaneous burst activity; and (ii) potentiation of NMDA receptor mediated effects on firing rate and burst activity. These mechanisms support the idea that modulation of mGlu5 receptors may provide a pharmacological strategy for fine-tuning the temporal pattern of firing of PFC neurons.

Anticonvulsant action of an antagonist of metabotropic glutamate receptors mGluR5 MPEP in immature rats

Antagonists of type I of metabotropic glutamate receptors exhibit anticonvulsant action in adult as well as immature rodents. To know the anticonvulsant profile of a specific mGluR5 antagonist MPEP in developing rats, two models of epileptic seizures were used. MPEP (10, 20 or 40 mg/kg i.p.) suppressed in a dose-dependent manner epileptic afterdischarges induced by electrical stimulation of sensorimotor cortical area in three age groups (12, 18 and 25 days old). The anticonvulsant action was more expressed in the youngest group than in older animals so that in 25-day-old rats an additional dose of 80 mg/kg was used. In contrast to this marked anticonvulsant action, MPEP at a dose of 40 mg/kg i.p. in 18-day-old rat pups and at doses of 40 and 80 mg/kg in 25-day-old rat pups did not affect episodes of spike-and-wave rhythm elicited by low doses of pentetrazol. Our results delineate the profile of the anticonvulsant action of MPEP and confirm the higher efficacy of this antagonist at early developmental stages in comparison with prepubertal animals.

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.