Regulation of synaptic plasticity by mGluR1 studied in vivo in mGluR1 mutant mice

Sodium butyrate ameliorates the impairment of synaptic plasticity by inhibiting the neuroinflammation in 5XFAD mice

Current strategies for the treatment of Alzheimer's disease (AD) focus on the pathology in the later stages of disease progression. Early microglia abnormality and β-amyloid (Aβ) deposition trigger disease development before identical symptoms emerge, which leads to poor clinical treatment effects in the later stages. In the early stage of disease progression, microglia in brains of 5XFAD mice have been activated by Aβ plaques to secrete more pro-inflammatory cytokines. In the meantime, these cytokines up-regulate Aβ via increasing the APP processing. Sodium butyrate (NaB), as one of the short chain fatty acid (SCFA) generated by gut microbiota, is the inhibitor of histone deacetylase (HDAC), which reduces the secretion of pro-inflammatory cytokines. In our experiment, 8-week-old 5XFAD mice and their litter WT mice were treated with NaB or normal saline for 2 weeks (WT + Vehicle group, WT + NaB group, AD + Vehicle group and AD + NaB group). After treatment, behavioral tests were carried out. The novel object recognition (NOR) and Morris water maze (MWM) tests demonstrated that there was no significant difference between four groups of mice. The results of long-term potentiation (LTP) and depotentiation (DEP) illustrated that the synaptic plasticity was promoted in 5XFAD mice after treatment with NaB. Compared to the AD + Vehicle group, the dendritic spines were more abundant in other groups of mice. Furthermore, the synapse-associated proteins (PSD-95, SYP, NR2B) were reduced and the pro-inflammatory cytokines (TNF-α, IL-6, IL-1β) were increased in the AD + Vehicle group. These phenomena were reversed after treatment with NaB. Moreover, our results suggested that NaB suppressed the over-activation of microglia and the accumulation of Aβ in AD mice. Altogether, all results illustrated that HDAC inhibitor NaB could ameliorate the synaptic plasticity by reducing neuroinflammation in 5XFAD mice in the early stage of the disease.

Distribution of glutamate receptor, ionotropic, kainate 1 and neuropeptide calcitonin gene-related peptide mRNAs during formation of the embryonic and postnatal mouse molar in the maxilla

The neuropeptide calcitonin gene-related peptide (CGRP) is a well-characterized neurotransmitter. Glutamate receptor, ionotropic, kainate 1 (Grik1) has also been demonstrated to generate high-affinity kainate receptors. However, little is known about the roles of CGRP and Grik1 during the developmental formation of teeth. In this study, we endeavoured to analyse the expression and localization of CGRP and Grik1 mRNAs using in situ hybridization on the mouse maxilla during development from the embryonic stage (E18.5) to after birth (P10, P15 and P20). We found that hybridization with an anti-sense probe for CGRP clearly localized in the maxilla at E18.5 in contrast to that of P15 and P20. Hybridization with an anti-sense probe for CGRP was not detected in the dental pulp of molars in the maxilla at P10, which is in contrast to Grik1 mRNA at the same developmental stage. Hybridization with an anti-sense probe for Grik1 mRNA was detected in the basal region of the dental pulp of molars at P10 and P15. Finally, these markers were not detected in molars in the mouse maxilla at P20. The ratio of positive cells for the hybridization signals of Grik1and CGRP in the dental pulp decreased from E18.5 (p<0.001). These features in CGRP and Grik1r mRNAs may indicate roles of function during tooth development between embryonic and postnatal stages with root formation and erupted movements.

Dexmedetomidine suppresses long-term potentiation in the hippocampal CA1 field of anesthetized rats

PURPOSE

The aim of this study was to evaluate the effect of dexmedetomidine (DEX) on hippocampal synaptic activity in vivo.

METHODS

The adult rats used for this study received a intraperitoneal bolus injection of 3, 10, 30, or 100 μg/kg of DEX or an equivalent volume of saline. Electrophysiological recording of the hippocampal CA1 region was initiated 20 min after drug administration. The results are expressed as the percentages of the population spike amplitude measured just before high-frequency stimulation (HFS). The electrophysiological data were analyzed with an area under the curve (AUC) of 10-60 min after HFS. Moreover, to investigate the sedative dose of DEX in rats, we recorded the duration of loss of spontaneous movement after the administration of each dose of DEX.

RESULTS

Intraperitoneal administration of DEX at doses of 30 and 100 μg/kg induced a range of sedative effects. The AUC measurements were significantly lower in the 30 and 100 μg/kg groups than in those injected with vehicle (vehicle: 8.81 ± 0.49, n = 7; DEX 30 µg/kg: 6.02 ± 0.99, n = 6; DEX 100 µg/kg: 5.10 ± 0.43, n = 5; P < 0.05).

CONCLUSION

The results of our in vivo study reveal that sedative doses of DEX impaired the induction of hippocampal long-term potentiation (LTP). These findings may signify a causal link between DEX-induced sedative action and hippocampal LTP suppression, providing a better understanding of the mechanisms underlying the DEX-induced sedative and/or amnestic effect.

Pharmacological reversal of synaptic plasticity deficits in the mouse model of fragile X syndrome by group II mGluR antagonist or lithium treatment

Fragile X syndrome is the leading single gene cause of intellectual disabilities. Treatment of a Drosophila model of Fragile X syndrome with metabotropic glutamate receptor (mGluR) antagonists or lithium rescues social and cognitive impairments. A hallmark feature of the Fragile X mouse model is enhanced mGluR-dependent long-term depression (LTD) at Schaffer collateral to CA1 pyramidal synapses of the hippocampus. Here we examine the effects of chronic treatment of Fragile X mice in vivo with lithium or a group II mGluR antagonist on mGluR-LTD at CA1 synapses. We find that long-term lithium treatment initiated during development (5-6 weeks of age) and continued throughout the lifetime of the Fragile X mice until 9-11 months of age restores normal mGluR-LTD. Additionally, chronic short-term treatment beginning in adult Fragile X mice (8 weeks of age) with either lithium or an mGluR antagonist is also able to restore normal mGluR-LTD. Translating the findings of successful pharmacologic intervention from the Drosophila model into the mouse model of Fragile X syndrome is an important advance, in that this identifies and validates these targets as potential therapeutic interventions for the treatment of individuals afflicted with Fragile X syndrome.

Effect of the class I metabotropic glutamate receptor antagonist AIDA on certain behaviours in rats with experimental chronic hyperammonemia

PURPOSE

This study examines possible interactions between behavioral effects and mGluR1 (class I metabotropic glutamate receptor) by injecting AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] in rats with experimental chronic hyperammonemia (chHA).

MATERIAL/METHODS

The effects of mGluR1 antagonist on some behaviors were tested in control groups of rats and in rats with chHA. Experimental chHA was induced by intraperitoneal injection of ammonium acetate (12 mmol/kg) for five consecutive days. We used the following behavioural tests: the open field test, the passive avoidance test and the elevated "plus" maze.

RESULTS

In control rats AIDA administered intracerebroventricularly (i.c.v.) at the dose 100 nmol decreased the number of crossings and bar approaches in the open field test and impaired acquisition and recall in the passive avoidance situation. ChHA significantly inhibited locomotor and exploratory activity and profoundly impaired acquisition and recall processes in the passive avoidance test and significantly increased acute stress responses. AIDA increased locomotor activity in chHA rats (especially number of crossed fields and rearings) and produced anxiety enhancement in rats with chHA. AIDA used in rats with chHA significantly improved acquisition and retrieval processes.

CONCLUSIONS

The obtained results suggest that AIDA, the antagonist of mGluR1, had beneficial effects on learning and memory in rats with experimental chronic hyperammonemia.

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.

Extracellular recordings of rodents in vivo: their contribution to integrative neuroscience

The prevalent theory in learning and memory processes is that they are underlain by short and long-term changes in synaptic weight, which continuously modulates neural networks during acquisition and recall. This synaptic plasticity has been revealed by recording extracellular field potentials. The enhancement of synaptic transmission was primarily noted in the hippocampus and was named long-term potentiation (LTP). The opposite mechanism, long-term depression (LTD), a reduction of synaptic transmission, was first discovered in the cerebellum. Since then, the LTP-model has been studied mainly using in vitro and acute anesthetized in vivo preparations. This approach has led to remarkable progress in the comprehension of intracellular molecular processes during LTP and LTD. In this review, we focus mainly on what we can learn about molecular events using extracellular field potential recordings with a more ecological model, i.e., studies using the freely behaving animal, with animals that are genetically modified or not, in several behavioral paradigms aimed at gaining insight into some of the conflicting results obtained with in vitro and in vivo preparations.

MGluR5 mediates the interaction between late-LTP, network activity, and learning

Hippocampal synaptic plasticity and learning are strongly regulated by metabotropic glutamate receptors (mGluRs) and particularly by mGluR5. Here, we investigated the mechanisms underlying mGluR5-modulation of these phenomena. Prolonged pharmacological blockade of mGluR5 with MPEP produced a profound impairment of spatial memory. Effects were associated with 1) a reduction of mGluR1a-expression in the dentate gyrus; 2) impaired dentate gyrus LTP; 3) enhanced CA1-LTP and 4) suppressed theta (5–10 Hz) and gamma (30–100 Hz) oscillations in the dentate gyrus. Allosteric potentiation of mGluR1 after mGluR5 blockade significantly ameliorated dentate gyrus LTP, as well as suppression of gamma oscillatory activity. CA3-lesioning prevented MPEP effects on CA1-LTP, suggesting that plasticity levels in CA1 are driven by mGluR5-dependent synaptic and network activity in the dentate gyrus. These data support the hypothesis that prolonged mGluR5-inactivation causes altered hippocampal LTP levels and network activity, which is mediated in part by impaired mGluR1-expression in the dentate gyrus. The consequence is impairment of long-term learning.

Bidirectional synaptic plasticity in the dentate gyrus of the awake freely behaving mouse

There is significant interest in in vivo synaptic plasticity in mice due to the many relevant genetic mutants now available. Nevertheless, use of in vivo models remains limited. To date long-term potentiation (LTP) has been studied infrequently, and long-term depression (LTD) has not been characterized in the mouse in vivo. Herein we describe protocols and improved methodologies we developed to record hippocampal synaptic plasticity reliably from the dentate gyrus of the awake freely behaving mouse. Seven days prior to recording, we implanted microelectrodes encapsulated within a lightweight, low profile head stage assembly. On the day of recording, we induced either LTP or LTD in the awake freely behaving animal, and monitored subsequent changes in population spike amplitude for at least 24h. Using this protocol we attained 80% success in inducing and maintaining either LTP or LTD. Recording from a chronic implant using this improved methodology is best suited to reveal naturally occurring brain activity and avoids both acute effects of local electrode insertion and drifts in neuronal excitability associated with anesthesia. Ultimately a reliable freely behaving mouse model of bi-directional synaptic plasticity is invaluable for full characterization of genetic models of disease states and manipulations of the mechanisms implicated in learning and memory.

Differential roles of hippocampal metabotropic glutamate receptors 1 and 5 in inhibitory avoidance learning

Group I metabotropic glutamate receptors (mGlu1 and 5) have been implicated in synaptic plasticity and learning and memory. However, much of our understanding of how these receptors in different brain regions contribute to distinct memory stages in different learning tasks remains incomplete. The present study investigated the effects of the mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and mGlu1 receptor antagonist, (S)-(+)-alpha-amino-4-carboxy-2-methylbenzene-acetic acid (LY 367385) in the dorsal hippocampus on the consolidation and extinction of memory for inhibitory avoidance learning. Male, Sprague-Dawley rats were trained in a single-trial step-down inhibitory avoidance task. MPEP, LY 367385 or saline were infused bilaterally into the CA1 region immediately after training or immediately after the first retention test which was given 24h after training. Rats receiving MPEP (1.5 or 5.0 microg/side) or LY 367385 (0.7 or 2.0 microg/side) infusion exhibited a dose-dependent decrease in retention when tested 24h later. MPEP was ineffective while LY 367385 significantly attenuated extinction when injected after the first retention test using an extinction procedure. These findings indicate a selective participation of hippocampal group I mGlu receptors in memory processing in this task.

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.

Gene targeting of presynaptic proteins in synaptic plasticity and memory: across the great divide

The past few decades have seen an explosion in our understanding of the molecular basis of learning and memory. The majority of these studies in mammals focused on post-synaptic signal transduction cascades involved in post-synaptic long-lasting plasticity. Until recently, relatively little work examined the role of presynaptic proteins in learning and memory in complex systems. The synaptic cleft figuratively represents a "great divide" between our knowledge of post- versus presynaptic involvement in learning and memory. While great strides have been made in our understanding of presynaptic proteins, we know very little of how presynaptically expressed forms of short- and long-term plasticity participate in information processing and storage. The paucity of cognitive behavioral research in the area of presynaptic proteins, however, is in stark contrast to the plethora of information concerning presynaptic protein involvement in neurotransmitter release, in modulation of release, and in both short- and long-term forms of presynaptic plasticity. It is now of great interest to begin to link the extensive literature on presynaptic proteins and presynaptic plasticity to cognitive behavior. In the future there is great promise with these approaches for identifying new targets in the treatment of cognitive disorders. This review article briefly surveys current knowledge on the role of presynaptic proteins in learning and memory in mammals and suggests future directions in learning and memory research on the presynaptic rim of the "great divide."

Disruption of prepulse inhibition in mice lacking mGluR1

Sensorimotor gating, measured by prepulse inhibition of the startle response (PPI), is a cross-species form of information processing that is deficient in patients with schizophrenia and is widely used as a model to study the neurobiology of this disorder. The eight known metabotropic glutamate receptors (mGluRs) are divided into three groups on the basis of sequence homology and pharmacological properties. Group I consists of mGluR5 and mGluR1, both of which are coupled positively to phospholipase C. Mice lacking mGluR5 exhibit a deficit in PPI. Like mGluR5, mGluR1 is located in regions that are involved in the modulation of PPI. To test the hypothesis that mGluR1 is involved in the modulation of PPI we assessed PPI in mGluR1 knockout (KO) mice. Littermate mGluR1 wild-type and KO mice were tested at multiple ages in a standard PPI paradigm containing a 65 dB background, 120 dB pulses and prepulses of 69, 73 and 77 dB. At all ages tested, mGluR1 KO mice exhibited a significant PPI deficit. The PPI deficit of the mGluR1 KO mice was not further exaggerated by administration of the N-methyl-d-aspartate antagonist phencyclidine nor was it reversed by administration of the dopamine antagonist raclopride (3.0 mg/kg). The PPI deficit of the mGluR1 KO mice was, however, ameliorated by administration of the mood stabilizer lamotrigine (27 mg/kg base equivalent weight), though increases in PPI were also seen with lamotrigine in the wild-type mice. Thus, both group I metabotropic glutamate receptors are involved in the regulation of PPI in mice.

Perinatal exposure to polychlorinated biphenyls alters excitatory synaptic transmission and short-term plasticity in the hippocampus of the adult rat

Developmental exposure to polychlorinated biphenyls (PCBs) has been associated with cognitive deficits in humans and laboratory animals. Previous work has demonstrated a reduced capacity to support long-term potentiation (LTP) in animals exposed to a PCB mixture, Aroclor 1254 (A1254) via the dam in utero and throughout the preweaning period [Brain Res. 850;1999:87-95; Toxicol. Sci. 57;2000:102-11]. Assessment of normalized input/output (I/O) functions collected prior to LTP induction failed to reveal consistent differences in baseline synaptic transmission between control and PCB-exposed groups. The present study was designed to systematically evaluate excitatory and inhibitory synaptic transmission using a more extensive I/O analysis and paired pulse functions to assess short-term plasticity. Pregnant Long-Evans rats were administered either corn oil (control) or 6 mg/kg per day of A1254 by gavage from gestational day (GD) 6 until pups were weaned on postnatal day (PND) 21. In adult male offspring (5-11 months of age), field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia. Detailed I/O functions were assessed by averaging the responses evoked in the dentate gyrus to stimulus pulses delivered to the perforant path in an extensive ascending intensity series. Population spike (PS) and postsynaptic potential (PSP) amplitudes recorded in the dentate gyrus were significantly enhanced in PCB-exposed animals relative to controls at midrange intensities. No group differences were observed in EPSP slope amplitudes. Short-term plasticity was assessed by delivering pairs of stimulus pulses at interpulse intervals (IPIs) ranging from 10 to 70 ms. In the dentate gyrus this range of intervals activates both inhibitory and excitatory mechanisms leading to a pattern of depression at brief intervals (<30 ms) followed by facilitation as the interval between pulses is extended. Paired pulse depression was decreased at an intermediate IPI (30 ms) with submaximal stimulus intensities. These data augment previous work demonstrating persistent changes in hippocampal plasticity as a result of exposure to PCBs during development. Furthermore, as increases in field potential amplitudes were observed, these findings support previous conclusions that A1254-induced LTP deficits are not readily attributable to reductions in synaptic excitability. Thus, in addition to impairment in use-dependent synaptic plasticity reported previously, the present report reveals that basic components of information processing within the hippocampus are permanently altered as a result of perinatal exposure to PCBs.

Corticostriatal LTP requires combined mGluR1 and mGluR5 activation

Metabotropic glutamate receptors (mGluRs) have been demonstrated to play a role in synaptic plasticity. It has been recently shown that mGluR1 is involved in corticostriatal long-term depression, by means of pharmacological approach and by using mGluR1-knockout mice. Here, we report that both mGluR1 and mGluR5 are involved in corticostriatal long-term potentiation (LTP). In particular, the mGluR1 antagonist LY 367385, as well as the mGluR5 antagonist MPEP, reduce LTP amplitude. Moreover, blockade of both mGluR1 and mGluR5 by LY 367385 and MPEP co-administration fully suppresses LTP. Accordingly, group II and group III mGluRs antagonists fail to affect LTP induction. Interestingly, LTP amplitude is also significantly reduced in both mGluR1- and mGluR5-knockout mice. The differential function of mGluR1 and mGluR5 in corticostriatal synaptic plasticity may play a role in the modulation of the motor activity mediated by the basal ganglia, thus providing a substrate for the pharmacological treatment of motor disorders involving the striatum.

mGluR1 in cerebellar Purkinje cells is required for normal association of temporally contiguous stimuli in classical conditioning

In metabotropic glutamate receptor-subtype 1 (mGluR1)-null (mGluR1-/-) mice, cerebellar long-term depression (LTD) and several forms of memory are impaired. However, because mGluR1 is expressed in various brain regions in wild-type mice, it has been difficult to identify which type of memory depends on mGluR1 expressed in a given brain region. Furthermore, severe ataxia in mGluR1-/- mice complicated interpretation of the data from non-cerebellum-dependent tasks. We have generated mGluR1-rescue mice, which express mGluR1 only in Purkinje cells (PCs) of their cerebellum, by introducing the mGluR1alpha transgene into mGluR1-/- mice under the control of a PC-specific promoter. The mGluR1-rescue mouse has normal LTD and displays no apparent ataxia. Therefore, this mouse is the first animal model in which effects of mGluR1 deficiency outside PCs can be studied without cerebellar dysfunction. We used three eyeblink conditioning paradigms with different temporal specificities between conditioned stimulus (CS) and unconditioned stimulus (US). Delay conditioning, in which CS and US coterminate, was impaired in mGluR1-/- mice but normal in mGluR1-rescue mice. However, both strains of mice displayed severe impairment in trace conditionings, in which a stimulus-free interval of 250 or 500 ms intervened between CS and US. We also examined social transmission of food-preference and novel-object-recognition memory tests. In these tasks, mGluR1-rescue mice showed normal short-term but impaired long-term memory. We conclude that mGluR1 in PCs is indispensable for normal learning of association of temporally contiguous stimuli in associative conditioning. In contrast, mGluR1 in other cell types is required for associating discontiguous stimuli and long-term memory formation in nonspatial hippocampus-dependent learning.

Dynamic structural changes of synaptic contacts in the visual system of insects

The visual system of insects provides an excellent model to study processes of transduction and transmission of photic information, synaptogenesis, synaptic plasticity, and wiring between photoreceptors and their visual interneurons in the optic lobe. This review describes synaptic contacts between photoreceptors and other neurons in the visual system of insects, especially in the fly's first optic neuropile (the lamina), and summarizes changes observed in the synapses of visual cells that have been reported both in phylogeny and ontogeny, and also examples of synaptic plasticity in adult insects that have been evoked by intrinsic and extrinsic factors. Plasticity observed in synapses of the insect's visual system seems to exemplify not only synaptic contacts in insects but, given that similar examples of plasticity have been found in other animal groups, may also be a general phenomenon in the nervous system.

Selective involvement of mGlu1 receptors in corticostriatal LTD

Although metabotropic glutamate receptors (mGluRs) have been proposed to play a role in corticostriatal long-term depression (LTD), the specific receptor subtype required for this form of synaptic plasticity has not been characterized yet. Thus, we utilized a corticostriatal brain slice preparation and intracellular recordings from striatal spiny neurons to address this issue. We observed that both AIDA (100 microM) and LY 367385 (30 microM), two blockers of mGluR1s, were able to fully prevent the induction of this form of synaptic plasticity, whereas MPEP (30 microM), a selective antagonist of the mGluR5 subtype, did not significantly affect the amplitude and time-course of corticostriatal LTD. Both AIDA and LY 367385 were ineffective on LTD when applied after its induction. The critical role of mGluR1s in the formation of corticostriatal LTD was confirmed in experiments performed on mice lacking mGluR1s. In these mice, in fact, a significant reduction of the LTD amplitude was observed in comparison to the normal LTD measured in their wild-type counterparts. We found that neither acute pharmacological blockade of mGluR1s nor the genetic disruption of these receptors affected the presynaptic modulation of corticostriatal excitatory postsynapic potentials (EPSPs) exerted by DCG-IV and L-SOP, selective agonists of group II and III mGluRs, respectively. Our data show that the induction of corticostriatal LTD requires the activation of mGluR1 but not mGluR5. mGluR1-mediated control of this form of synaptic plasticity may play a role in the modulatory effect exerted by mGluRs in the basal ganglia-related motor activity.

Glutamate and epilepsy

Epileptic syndromes have very diverse primary causes, which may be genetic, developmental or acquired. In rodent models, altering glutamate receptor or glutamate transporter expression by knockout or knockdown procedures can induce or suppress epileptic seizures. Regardless of the primary cause, synaptically released glutamate acting on ionotropic and metabotropic receptors appears to play a major role in the initiation and spread of seizure activity. In rodent models of acquired epilepsy and in human temporal lobe epilepsy, there is evidence for enhanced functional efficacy of ionotropic N-methyl-D-aspartate (NMDA) and metabotropic (Group I) receptors. In animal models of epilepsy, antagonists acting at NMDA receptors or at Group I metabotropic receptors have potent anticonvulsant actions.

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.

Roles of metabotropic glutamate receptors in LTP and LTD in the hippocampus

Metabotropic L-glutamate receptors are involved in various forms of synaptic plasticity in the hippocampus. The use of a new antagonist (LY341495) that blocks all known metabotropic L-glutamate receptors in the brain, together with subtype-selective antagonists, has identified multiple roles both for cloned and novel metabotropic L-glutamate receptors in hippocampal long-term potentiation and long-term depression.

Glutamate receptors in epilepsy

Glutamatergic synapses play a critical role in all epileptic phenomena. Broadly enhanced activation of post-synaptic glutamate receptors (ionotropic and metabotropic) is proconvulsant. Antagonists of NMDA receptors and AMPA receptors are powerful anticonvulsants in many animal models of epilepsy. A clinical application of pure specific glutamate antagonists has not yet been established. Many different alterations in glutamate receptors or transporters can potentially contribute to epileptogenesis. Several genetic alterations have been shown to be epileptogenic in animal models but no specific mutation relating to glutamatergic function has yet been linked to a human epilepsy syndrome. There is clear evidence for altered NMDA receptor function in acquired epilepsy in animal models and in man. Changes in metabotropic receptor function may also play a key role in epileptogenesis.

Changes in mRNA levels for group I metabotropic glutamate receptors following in utero hypoxia-ischemia

The expression of group I metabotropic glutamate receptors (mGluR1 and mGluR5) and inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) mRNA was studied by in situ hybridization in the developing rat hippocampus after in utero hypoxia-ischemia. In utero hypoxia-ischemia was induced by clamping the uterine blood vessels of near-term fetuses for 10 min. Fetuses were delivered surgically, resuscitated and raised by foster mothers until postnatal day 7 and 14. Results indicated a temporal delay in the expression of mGluR1 mRNA in the dentate gyrus of the ischemic animals. The mGluR1 mRNA level was significantly lower in the ischemic animals at postnatal day 7, but reached a similar level as that of controls at postnatal day 14. In utero hypoxia-ischemia did not change the temporal-spatial expression pattern of either mGluR5 or IP3R1 mRNA in the hippocampus. Between postnatal day 7 and 14, mGluR5 mRNA showed a high and relatively constant expression, whereas IP3R1 mRNA levels were increased in all regions examined. The differences in the expressions of group I mGluRs indicate that these receptors may have different functions during hippocampal development and may play different roles in excitotoxicity.

Correlation between cannabinoid mediated effects on paired pulse depression and induction of long term potentiation in the rat hippocampal slice

Cannabinoids cause an increase in synaptic transmission via gamma-aminobutyric acid (GABA) receptors and this may be the mechanism by which activation of CB1 receptors blocks the induction of long-term potentiation (LTP). To test this hypothesis, we used paired pulse depression (PPD) of CA1 population spike responses recorded in the rat hippocampal slice as an index of GABA-ergic feedback inhibition, to establish whether the effects of a stereoselective CB1 receptor agonist on GABA-ergic transmission and LTP were correlated. The active isomer, WIN55212-2, blocked the induction of LTP and suppressed PPD over the concentration range 250 nM-5 microM, whereas the inactive isomer, WIN55212-3, was inactive at 5 microM. The effects of 5 microM WIN55212-2 on both LTP and PPD were completely blocked by the CB1 receptor antagonist SR141716A (5 microM). The results show that the effects are correlated in that both suppression of PPD and blockade of induction of LTP are probably mediated by CBI receptors. However, the suppression in PPD suggests that WIN55212-2 caused a decrease in GABA-ergic feedback transmission which would be expected to facilitate, rather than block, the induction of LTP. We therefore conclude that the blockade of LTP by cannabinoids is not via upregulation of GABA-ergic synaptic transmission.