Metabotropic glutamate receptors are required for the induction of long-term potentiation

mGluR1α expression in the hippocampus, subiculum, entorhinal cortex and superior temporal gyrus in Alzheimer's disease

Glutamate is the main excitatory neurotransmitter in the central nervous system, responsible for a plethora of cellular processes including memory formation and higher cerebral function and has been implicated in various neurological disease states. Alzheimer's disease (AD) is the leading neurodegenerative disorder worldwide and is characterized by significant cell loss and glutamatergic dysfunction. While there has been a focus on ionotropic glutamatergic receptors few studies have attempted to elucidate the pathological changes of metabotropic glutamate receptors (mGluRs) in AD. mGluRs are G-protein coupled receptors which have a wide-ranging functionality, including the regulation of neuronal injury and survival. In particular, the group I mGluRs (mGluR1 and mGluR5) are associated with ionotropic receptor activation and upregulation with resultant glutamate release in normal neuronal functioning. The mGluR subtype 1 splice variant a (mGluR1α) is the longest variant of the mGluR1 receptor, is localized to dendritic processes and is mainly plasma membrane-bound. Activation of mGluR1a has been shown to result in increased constitutive activity of ionotropic receptors, although its role in neurodegenerative and other neurological diseases is controversial, with some animal studies demonstrating potential neuroprotective properties in excito- and neurotoxic environments. In this study, the expression of mGluR1a within normal and AD human hippocampal tissue was quantified using immunohistochemistry. We found a significantly reduced expression of mGluR1α within the stratum pyramidale and radiatum of the CA1subregion, subiculum, and entorhinal cortex. This downregulation could result in potential dysregulation of the glutamatergic system with consequences on AD progression by promoting excitotoxicity, but alternatively may also be a neuroprotective mechanism to prevent mGluR1α associated excitotoxic effects. In summary, more research is required to understand the role and possible consequences of mGluR1α downregulation in the human AD hippocampus, subiculum and entorhinal cortex and its potential as a therapeutic target.

Long-term depression of excitatory transmission in the lateral septum

Neurons in the lateral septum (LS) integrate glutamatergic synaptic inputs, primarily from hippocampus, and send inhibitory projections to brain regions involved in reward and the generation of motivated behavior. Motivated learning and drugs of abuse have been shown to induce long-term changes in the strength of glutamatergic synapses in the LS, but the cellular mechanisms underlying long-term synaptic modification in the LS are poorly understood. Here, we examined synaptic transmission and long-term depression (LTD) in brain slices prepared from male and female C57BL/6 mice. No sex differences were observed in whole cell patch-clamp recordings of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R)- and N-methyl-d-aspartate receptor (NMDA-R)-mediated currents. Low-frequency stimulation of the fimbria fiber bundle (1 Hz 15 min) induced LTD of the LS field excitatory postsynaptic potential (fEPSP). Induction of LTD was blocked by the NMDA-R antagonist (d)-2-amino-5-phosphonovaleric acid (APV), but not the selective antagonist of GluN2B-containing NMDA-Rs ifenprodil. These results demonstrate the NMDA-R dependence of LTD in the LS. The LS is a sexually dimorphic structure, and sex differences in glutamatergic transmission have been reported in vivo; our results suggest sex differences observed in vivo result from network activity rather than intrinsic differences in glutamatergic transmission.NEW & NOTEWORTHY The lateral septum (LS) integrates information from hippocampus and other regions to provide context-dependent (top down or higher order) regulation of mood and motivated behavior. Learning and drugs of abuse induce long-term changes in the strength of glutamatergic projections to the LS; however, the cellular mechanisms underlying such changes are poorly understood. Here, we demonstrate there are no apparent sex differences in fast excitatory transmission and that long-term synaptic depression in the LS is NMDA-R dependent.

NMDA receptor-independent LTP in mammalian nervous system

Long-term potentiation (LTP) of synaptic transmission is a form of activity-dependent synaptic plasticity that exists at most synapses in the nervous system. In the central nervous system (CNS), LTP has been recorded at numerous synapses and is a prime candidate mechanism associating activity-dependent plasticity with learning and memory. LTP involves long-lasting increase in synaptic strength with various underlying mechanisms. In the CNS, the predominant type of LTP is believed to be dependent on activation of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR), which is highly calcium-permeable. However, various forms of NMDAR-independent LTP have been identified in diverse areas of the nervous system. The NMDAR-independent LTP may require activation of glutamate metabotropic receptors (mGluR) or ionotropic receptors other than NMDAR such as nicotinic acetylcholine receptor (α7-nAChR), serotonin 5-HT3 receptor or calcium-permeable AMPA receptor (CP-AMPAR). In this review, NMDAR-independent LTP of various areas of the central and peripheral nervous systems are discussed.

Neuropsychopathology of Autism Spectrum Disorder: Complex Interplay of Genetic, Epigenetic, and Environmental Factors

Autism spectrum disorder (ASD) is a complex heterogeneous consortium of pervasive development disorders (PDD) which ranges from atypical autism, autism, and Asperger syndrome affecting brain in the developmental stage. This debilitating neurodevelopmental disorder results in both core as well as associated symptoms. Core symptoms observed in autistic patients are lack of social interaction, pervasive, stereotyped, and restricted behavior while the associated symptoms include irritability, anxiety, aggression, and several comorbid disorders.ASD is a polygenic disorder and is multifactorial in origin. Copy number variations (CNVs) of several genes that regulate the synaptogenesis and signaling pathways are one of the major factors responsible for the pathogenesis of autism. The complex integration of various CNVs cause mutations in the genes which code for molecules involved in cell adhesion, voltage-gated ion-channels, scaffolding proteins as well as signaling pathways (PTEN and mTOR pathways). These mutated genes are responsible for affecting synaptic transmission by causing plasticity dysfunction responsible, in turn, for the expression of ASD.Epigenetic modifications affecting DNA transcription and various pre-natal and post-natal exposure to a variety of environmental factors are also precipitating factors for the occurrence of ASD. All of these together cause dysregulation of glutamatergic signaling as well as imbalance in excitatory: inhibitory pathways resulting in glial cell activation and release of inflammatory mediators responsible for the aberrant social behavior which is observed in autistic patients.In this chapter we review and provide insight into the intricate integration of various genetic, epigenetic, and environmental factors which play a major role in the pathogenesis of this disorder and the mechanistic approach behind this integration.

Signaling mechanisms mediating the regulation of synaptic plasticity and memory by estradiol

This article is part of a Special Issue "Estradiol and Cognition". Estradiol participates in the regulation of the function and plasticity of synaptic circuits in key cognitive brain regions, such as the prefrontal cortex and the hippocampus. The mechanisms elicited by estradiol are mediated by the regulation of transcriptional activity by nuclear estrogen receptors and by intracellular signaling cascades activated by estrogen receptors associated with the plasma membrane. In addition, the mechanisms include the interaction of estradiol with the signaling of other factors involved in the regulation of cognition, such as brain derived neurotrophic factor, insulin-like growth factor-1 and Wnt. Modifications in these signaling pathways by aging or by a long-lasting ovarian hormone deprivation after menopause may impair the enhancing effects of estradiol on synaptic plasticity and cognition.

Fragile X: leading the way for targeted treatments in autism

Two different mutations in the FMR1 gene may lead to autism. The full mutation, with >200 CGG repeats in the 5' end of FMR1, leads to hypermethylation and transcriptional silencing of FMR1, resulting in absence or deficiency of the protein product, FMRP. Deficiency of FMRP in the brain causes fragile X syndrome (FXS). Autism occurs in approximately 30% of those with FXS, and pervasive developmental disorders-not otherwise specified occur in an additional 30%. FMRP is an RNA binding protein that modulates receptor-mediated dendritic translation; deficiency leads to dysregulation of many proteins important for synaptic plasticity. Group I metabotropic glutamate receptor (mGluR1/5) activated translation is upregulated in FXS, and new targeted treatments that act on this system include mGluR5 antagonists and GABA agonists, which may reverse the cognitive and behavioral deficits in FXS. Matrix metalloproteinase 9 (MMP-9) is one of the proteins elevated in FXS, and minocycline reduces excess MMP-9 activity in the Fmr1 knockout mouse model of FXS. Both minocycline and mGluR5 antagonists are currently being evaluated in patients with FXS through controlled treatment trials. The premutation (55-200 CGG repeats) may also contribute to the mechanism of autism in approximately 10% of males and 2-3% of females. Premutations with <150 repeats exert cellular effects through a different molecular mechanism, one that involves elevated levels of FMR1 mRNA, CGG-mediated toxicity to neurons, early cell death, and fragile X-associated tremor/ataxia syndrome. In those with large premutations (150-200), lowered levels of FMRP also occur.

mTOR signaling: at the crossroads of plasticity, memory and disease

Mammalian target of rapamycin (mTOR) is a protein kinase involved in translation control and long-lasting synaptic plasticity. mTOR functions as the central component of two multi-protein signaling complexes, mTORC1 and mTORC2, which can be distinguished from each other based on their unique compositions and substrates. Although the majority of evidence linking mTOR function to synaptic plasticity comes from studies utilizing rapamycin, studies in genetically modified mice also suggest that mTOR couples receptors to the translation machinery for establishing long-lasting synaptic changes that are the basis for higher order brain function, including long-term memory. Finally, perturbation of the mTOR signaling cascade appears to be a common pathophysiological feature of human neurological disorders, including mental retardation syndromes and autism spectrum disorders.

Suppression of two major Fragile X Syndrome mouse model phenotypes by the mGluR5 antagonist MPEP

Fragile X Syndrome is the most common form of inherited mental retardation worldwide. A Fragile X mouse model, fmr1(tm1Cgr), with a disruption in the X-linked Fmr1 gene, has three substantial deficits observed in several strains: (1) sensitivity to audiogenic seizures (AGS), (2) tendency to spend significantly more time in the center of an open field, and (3) enlarged testes. Alterations in metabotropic glutamate receptor group I signaling were previously identified in the fmr1(tm1Cgr) mouse. In this study, we examined the effect of MPEP, an antagonist of the group I metabotropic glutamate receptor mGluR5, on audiogenic seizures and open field activity of fmr1(tm1Cgr) mice. Genetic analysis revealed synergistic reactions between fmr1(tm1Cgr) and inbred AGS alleles. In addition, AGS sensitivity due to the fmr1(tm1Cgr) allele was restricted during development. Examination of phenotypes combining mGluR5 inhibition and Fmr1 mutation indicated that absence of FMRP may affect mGluR5 signaling through indirect as well as direct pathways. All strains of fmr1(tm1Cgr) mice tested (FVB/NJ, C57BL/6J, and an F1 hybrid of the two) had a more excitable AGS pathway than wild-type, and consequently required more MPEP to achieve seizure suppression. At high doses of mGluR5 antagonists, a Fragile X specific tolerance (loss of drug activity) was observed. The tolerance effect could be overcome by a further increase in drug dose. In open field tests, MPEP reduced fmr1(tm1Cgr) center field behavior to one indistinguishable from wild-type. Therefore, mGluR5 antagonists were able to rescue two of the major phenotypes of the FX mouse. Modulation of mGluR5 signaling may allow amelioration of symptoms of Fragile X Syndrome.

Regulation of Homer and group I metabotropic glutamate receptors by nicotine

The present study focuses on the nicotine-induced modulation of mRNA and protein expression of a number of genes involved in glutamatergic synaptic transmission in rat brain over different time periods of exposure. A subchronic (3 days) but not the chronic (7 or 14 days) administration of nicotine resulted in the up-regulation of Homer2a/b mRNA in the amygdala while in the ventral tegmental area (VTA) no change in expression of either Homer2a/b or Homer1b/c was observed. Although the increase in Homer2a/b mRNA was not translated into the protein level in the amygdala, a slight but significant up-regulation of Homer1b/c protein was observed in the same region at day 3. Both Homer forms were up-regulated at the protein level in the VTA at day 3. In the nucleus accumbens, 14 days of nicotine treatment up-regulated mRNA of Homer2b/c by 68.2% (P < 0.05), while the short form Homer1a gene was down-regulated by 65.0% at day 3 (P < 0.05). In regard to other components of the glutamatergic signalling, we identified an acute and intermittent increase in the mRNA and protein levels of mGluR1 and mGluR5 in the amygdala. In the VTA, however, the effects of nicotine on mGluR mRNA expression were long-lasting but rather specific to mGluR1. Nevertheless, mGluR1 protein levels in the VTA area were up-regulated only at day 3, as in the amygdala. These data provide further evidence for the involvement of nicotine in the glutamatergic neuronal synaptic activity in vivo, suggesting a role for the newly identified Homer proteins in this paradigm.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Spinal axonal injury transiently elevates the level of metabotropic glutamate receptor 5, but not 1, in cord-projection central neurons

In investigating the effect of spinal injury on cord-projection central neurons, we found that rat rubrospinal neurons retained glutamatergic afferents and, in general, ionotropic glutamate receptor expression following spinal axotomy. Since glutamate also acts on second-messenger-coupled metabotropic receptors, the expression of group I metabotropic glutamate receptors, mGluR1 and mGluR5, was examined following similar treatment. mGluR1 expression began to decline in the perikarya 2 days postlesion and a day later in the neuropil. The decline slowed down by the fifth day and recovered in both the perikarya and neuropil 1 week postlesion. However, expression in both the perikarya and neuropil declined again and persisted up to 2 years postlesion. Similarly, the mGluR5 displayed an early transient decrease and returned to normal levels by 7 days post-lesion. However, rather than progressing to a secondary decline, the expression of mGluR5 increased to levels dramatically higher than those of control nuclei at 2-4 weeks postlesion, subsiding again by 8 weeks, and remaining low up to 2 years postinjury. Although mGluR5 has been shown to save cultured neurons from excitotoxic cell death, its elevated expression in the present model corresponds in time to an increased input/output relationship and excitability of the injured neurons as well as a period of maximal somatic shrinkage and cell loss. In addition to the cell bodies and dendrites, axon-like profiles also contain mGluR1. Their decrease following rubrospinal axotomy suggests that axonal injury may also compromise the presynaptic regulation of afferent activities onto injured cord-projection central neurons.

Metabotropic glutamate receptors mediate expression of LTP in slices of rat visual cortex

Long-term-potentiation (LTP) can be induced by application of a standard theta-burst stimulation protocol in slice preparations of the neocortex. This type of LTP is known to be dependent on the activation of NMDA receptors. The present study used specific experimental conditions to evoke a non-NMDA receptor mediated type of LTP. By use of weak theta-burst stimulation (wTBS) we describe a non-NMDA receptor dependent LTP in rat visual cortex in vitro, which is sensitive to an antagonist of metabotropic glutamate receptors (mGluR). In slices of the visual cortex we stimulated ascending inputs in cortical layer IV and recorded extracellular field potentials (FPs) from cortical layers II/III. In disinhibited slices (with 1 microm picrotoxin), a wTBS induced LTP to 138% of control. The expression of this potentiation was insensitive to the NMDA-receptor antagonist, D-AP5, but could be abolished by application of the mGluR antagonist MCPG. These data suggest an NMDA-independent mechanism for LTP induction in the visual cortex which can be observed in layer II/III neurons.

Activation of inhibitory pathways suppresses the induction of long-term potentiation in neurons of the rat lateral septal nucleus

Long-term potentiation of the hippocampal-septal pathway was examined by intracellular recording techniques. High frequency stimulation (two 100-Hz 1-s trains with a 20-s interval between them) of the hippocampal CA3 area resulted in a transient depolarization in rat lateral septal nucleus neurons. High frequency stimulation was followed by a facilitation of fast and slow inhibitory postsynaptic potentials, lasting for more than 2 h, but not by a long-lasting increase in the excitatory postsynaptic potential in the normal solution. Long-term potentiation (>2 h) of the excitatory postsynaptic potential did not appear in 74% of neurons tested, even when the fast inhibitory postsynaptic potential was blocked by bicuculline (30 microM), a GABA(A) receptor antagonist. High frequency stimulation produced long-term potentiation of the excitatory postsynaptic potential in the Mg(2+)-free solution containing bicuculline. When the fast and slow inhibitory postsynaptic potentials were blocked by GABA(A) and GABA(B) receptor antagonists (bicuculline and CGP 55845A respectively), high frequency stimulation produced a large and sustained depolarization followed by long-term potentiation of the excitatory postsynaptic potential. However, the excitatory postsynaptic potential was not enhanced by administration of these drugs after termination of high frequency stimulation. Pretreatment with 2-amino-5-phosphonopentanoate, a NMDA receptor antagonist, resulted in loss of long-term potentiation in both sets of experiments. Paired-pulse stimulation of the hippocampal CA3 region with interstimulus intervals between 200 and 800 ms depressed the second excitatory postsynaptic potential in the presence of bicuculline. CGP 35348, a GABA(B) receptor antagonist, reversed the depression of excitatory postsynaptic potentials to facilitation. These data suggest that high frequency stimulation of hippocampal CA3 neurons enhances the efficacy of GABAergic inhibitory circuits which, in turn, depress the ability of lateral septal nucleus neurons to express long-term potentiation.

Synaptic plasticity in the medial vestibular nuclei: role of glutamate receptors and retrograde messengers in rat brainstem slices

The analysis of cellular-molecular events mediating synaptic plasticity within vestibular nuclei is an attempt to explain the mechanisms underlying vestibular plasticity phenomena. The present review is meant to illustrate the main results, obtained in vitro, on the mechanisms underlying long-term changes in synaptic strength within the medial vestibular nuclei. The synaptic plasticity phenomena taking place at the level of vestibular nuclei could be useful for adapting and consolidating the efficacy of vestibular neuron responsiveness to environmental requirements, as during visuo-vestibular recalibration and vestibular compensation. Following a general introduction on the most salient features of vestibular compensation and visuo-vestibular adaptation, which are two plastic events involving neuronal circuitry within the medial vestibular nuclei, the second and third sections describe the results from rat brainstem slice studies, demonstrating the possibility to induce long-term potentiation and depression in the medial vestibular nuclei, following high frequency stimulation of the primary vestibular afferents. In particular the mechanisms sustaining the induction and expression of vestibular long-term potentiation and depression, such as the role of various glutamate receptors and retrograde messengers have been described. The relevant role of the interaction between the platelet-activating factor, acting as a retrograde messenger, and the presynaptic metabotropic glutamate receptors, in determining the full expression of vestibular long-term potentiation is also underlined. In addition, the mechanisms involved in vestibular long-term potentiation have been compared with those leading to long-term potentiation in the hippocampus to emphasize the most significant differences emerging from vestibular studies. The fourth part, describes recent results demonstrating the essential role of nitric oxide, another retrograde messenger, in the induction of vestibular potentiation. Finally the fifth part suggests the possible functional significance of different action times of the two retrograde messengers and metabotropic glutamate receptors, which are involved in mediating the presynaptic mechanism sustaining vestibular long-term potentiation.

PLC-beta1, activated via mGluRs, mediates activity-dependent differentiation in cerebral cortex

During development of the cerebral cortex, the invasion of thalamic axons and subsequent differentiation of cortical neurons are tightly coordinated. Here we provide evidence that glutamate neurotransmission triggers a critical signaling mechanism involving the activation of phospholipase C-beta1 (PLC-beta1) by metabotropic glutamate receptors (mGluRs). Homozygous null mutation of either PLC-beta1 or mGluR5 dramatically disrupts the cytoarchitectural differentiation of 'barrels' in the mouse somatosensory cortex, despite segregation in the pattern of thalamic innervation. Furthermore, group 1 mGluR-stimulated phosphoinositide hydrolysis is dramatically reduced in PLC-beta1-/- mice during barrel development. Our data indicate that PLC-beta1 activation via mGluR5 is critical for the coordinated development of the neocortex, and that presynaptic and postsynaptic components of cortical differentiation can be genetically dissociated.

Dual modulation of excitatory synaptic transmission by agonists at group I metabotropic glutamate receptors in the rat spinal dorsal horn

The effects of group I metabotropic glutamate (mGlu) receptors on excitatory transmission in the rat dorsal horn, but mostly substantia gelatinosa, neurons were investigated using conventional intracellular recording in slices. The broad spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S, 3R-ACPD), the group I mGlu receptor selective agonist (S)-3, 5-dihydroxyphenylglycine (DHPG), and the selective mGlu subtype 5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), all induce long-lasting depression of A primary afferent fibers-mediated monosynaptic excitatory postsynaptic potential (EPSP), and long-lasting potentiation of polysynaptic EPSP, and EPSP in cells receiving C-afferent fiber input. The DHPG potentiation of polysynaptic EPSP was partially or fully reversed by (S)-4-carboxyphenylglycine (S-4CPG), the mGlu subtype 1 preferring antagonist. 2-Methyl-6-(phenylethynyl)-pyridine, the potent and selective mGlu subtype 5 antagonist, partially reversed the CHPG potentiation of polysynaptic EPSP. The effects of DHPG on monosynaptic and polysynaptic EPSPs were reduced, or abolished, by the N-methyl-D-aspartate (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP5). A clear and pronounced facilitation of the expression of DHPG- and CHPG-induced enhancement of polysynaptic EPSP, and EPSP evoked at C-fiber strength, was seen in the absence of gamma-aminobutyric acid subtype A receptor- and glycine-mediated synaptic inhibition. Besides dual modulation of excitatory synaptic transmission, DHPG induces depression of inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons. In addition, group I mGlu receptor agonists produced a direct persistent excitatory postsynaptic effect consisting of a slow membrane depolarization, an increase in input resistance, and an intense neuronal discharge. Cyclothiazide and (S)-4-CPG, the mGlu receptor subtype 1 preferring antagonists, significantly attenuated the DHPG-induced depolarization. These results demonstrate that the pharmacological activation of group I metabotropic glutamate receptors induces long-term depression (LTD) and long-term potentiation (LTP) of synaptic transmission in the spinal dorsal horn. These types of long-term synaptic plasticity may play a functional role in the generation of post-injury hypersensitivity (LTP) or antinociception (LTD).

Laminar and temporal heterogeneity of NMDA/metabotropic glutamate receptor binding in posterior cingulate cortex

Both N-methyl-D-aspartate (NMDA) and quisqualate/AMPA-insensitive metabotropic glutamate (mGlu) receptors mediate plasticity induction in neocortex, but their interlaminar distribution in cortical microcircuits is largely unknown. We used (+)(3)H-MK801 and (3)H-glutamate binding plus saturating concentrations of NMDA, AMPA, and quisqualate to autoradiographically map NMDA and mGlu receptor sites by lamina in posterior cingulate cortex in adult male rats. Specific binding at NMDA receptor sites in laminae II/III and VI was significantly reduced in comparison to other laminae. Brains prepared from rats killed during dark phase of a 12h/12h light/dark cycle showed a mean 129% increase in overall (+)(3)H-MK801 binding versus light phase brains but retained reduced binding densities in laminae II/III and VI. In contrast to NMDA findings, specific binding at mGlu sites was consistently elevated during light phase in both laminae II/III and VI. Specific (3)H-glutamate binding in dark-phase brains showed an overall 147% increase versus light phase binding but did not retain significant interlaminar heterogeneity. Interpreted in accordance with our physiologically derived models of hippocampo-cortical microcircuitry, these results suggest that spatial and temporal variations in glutamate receptor distribution may play an important role in intracingulate neural processing of afferent input from hippocampus.

Inhibition of hippocampal kindling by metabotropic glutamate receptor antisense oligonucleotides

Recent work has shown that metabotropic glutamate receptors (mGluRs) increase in response to seizure activity and can contribute significantly to the expression and progression of partial seizures. Using the kindling model of temporal lobe seizures, we evaluated the ability of local hippocampal injections of mGluR1 antisense or mGluR3 antisense oligonucleotides to suppress receptor expression and alter hippocampal kindling. Daily antisense injections in the hippocampus resulted in a significant decrease in mGluR1 or mGluR2/3 immunoreactivity. Rats injected with mGluR3 antisense showed a brief suppression of afterdischarge duration when compared to matched rats injected with a nonsense-oligonucleotide. Rats injected with a mGluR1 antisense oligonucleotide had a dramatic suppression of the rate of seizure progression with no significant effect on afterdischarge duration. Suppression of mGluR1 synthesis by local antisense inhibition may provide a new therapeutic approach for the control of epileptogenesis.

Cocaine and kindling alter the sensitivity of group II and III metabotropic glutamate receptors in the central amygdala

G-protein-coupled metabotropic glutamate receptors (mGluRs) are being implicated in various forms of neuroplasticity and CNS disorders. This study examined whether the sensitivities of mGluR agonists are modulated in a distinct fashion in different models of synaptic plasticity, specifically, kindling and chronic cocaine treatment. The influence of kindling and chronic cocaine exposure in vivo was examined in vitro on the modulation of synaptic transmission by group II and III metabotropic glutamate receptors using whole cell voltage-clamp recordings of central amygdala (CeA) neurons. Synaptic transmission was evoked by electrical stimulation of the basolateral amygdala (BLA) and ventral amygdaloid pathway (VAP) afferents in brain slices from control rats and from rats treated with cocaine or exposed to three to five stage-five kindled seizures. This study shows that after chemical stimulation with chronic cocaine exposure or after electrical stimulation with kindling the receptor sensitivities for mGluR agonists are altered in opposite ways. In slices from control rats, group II agonists, (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (LCCG1) and (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740), depressed neurotransmission more potently at the BLA-CeA than at the VAP-CeA synapse while group III agonist, L(+)-2-amino-4-phosphonobutyrate (LAP4), depressed neurotransmission more potently at the VAP-CeA synapse than at the BLA-CeA. These agonist actions were not seen (were absent) in amygdala neurons from chronic cocaine-treated animals. In contrast, after kindling, concentration response relationships for LCCG1 and LAP4 were shifted to the left, suggesting that sensitivity to these agonists is increased. Except at high concentrations, LCCG1, LY354740, and LAP4 neither induced membrane currents nor changed current-voltage relationships. Loss of mGluR inhibition with chronic cocaine treatment may contribute to counter-adaptive changes including anxiety and depression in cocaine withdrawal. Drugs that restore the inhibitory effects of group II and III mGluRs may be novel tools in the treatment of cocaine dependence. The enhanced sensitivity to group II and III mGluR agonists in kindling is similar to that recorded at the lateral to BLA synapse in the amygdala where they reduce epileptiform bursting. These findings suggest that drugs modifying mGluRs may prove useful in the treatment of cocaine withdrawal or epilepsy.

Modulation of synaptic transmission in the rat ventral septal area by the pharmacological activation of metabotropic glutamate receptors

The ventral septal area (VSA) is considered to be critically involved in the control of the height and duration of fever. The major excitatory input to this region of the brain is glutamatergic, and the aim of this study was to investigate possible modulation of this synapse by metabotropic glutamate (mGlu) receptors. Whole-cell patch recordings were made from individual VSA neurons voltage-clamped at -60 mV. Activation of either group I or group II mGlu receptors (by bath application of 3,5-dihydroxyphenylglycine (DHPG) or (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV), respectively) produced a long-lasting depression of synaptic transmission which in both cases was insensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (D-AP5). In contrast, application of (S)-2-amino-4-phosphonobutyric acid (L-AP4), a group III mGlu receptor agonist, had a biphasic effect on synaptic transmission in the VSA, first eliciting a transient depression of transmission during drug application, followed by a marked and sustained potentiation of synaptic transmission upon drug washout. The response elicited by L-AP4 was dependent on NMDA receptor activation, as in the presence of D-AP5 the potentiation was replaced by an underlying long-term depression (LTD) of transmission. These data provide the first evidence that metabotropic glutamate receptor agonists can induce both NMDA receptor-dependent and -independent modulation of synaptic transmission in the VSA.

Role of metabotropic glutamate receptor subclasses in modulation of adenylyl cyclase activity by a nootropic NS-105

The involvement of metabotropic glutamate (mGlu) receptors in the modulatory actions of a novel cognition enhancer, (+)-5-oxo-D-prolinepiperidinamide monohydrate (NS-105), on adenylyl cyclase activity in rat cerebrocortical membranes and primary neuronal cultures was investigated using selective antagonists and antisense oligodeoxynucleotides for mGlu receptor subclasses. In rat cerebrocortical membranes, the inhibitory action of NS-105 (0.1 microM) on forskolin-stimulated cAMP formation was blocked by a group II mGlu receptor antagonist, (+/-)-alpha-ethylglutamic acid, and by a group III antagonist, (+)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP-4), but not by a group I antagonist, (+/-)-1-aminoindan-1,5-dicarboxylic acid (AIDA), whereas the facilitation of cAMP formation by NS-105 (1 microM) in pertussis toxin-pretreated membranes was abolished by AIDA but not by (+/-)-alpha-ethylglutamic acid or MAP-4. In primary cultured neurons of mouse cerebral cortex, the inhibitory action of NS-105 on adenylyl cyclase activity disappeared after treatment with antisense oligodeoxynucleotides for group II (mGlu(2) and mGlu(3) receptors) and group III (mGlu(4) and mGlu(7) receptors) but not group I (mGlu(5) receptor) mGlu receptor subclasses. These findings suggest that the inhibitory action of NS-105 on adenylyl cyclase activity is mediated through group II and group III mGlu receptor subclasses while the facilitatory action is dependent on the group I mGlu receptor subclass.

Localization of phospholipase Cbeta isozymes in the mouse cerebellum

To elucidate the role of phospholipase Cbeta (PLCbeta) isozymes in the cerebellum, the distributions of PLCbeta3 and PLCbeta4 were examined in wild-type and PLCbeta4-deficient mutant mice using immunohistochemistry, and the functions were evaluated by measurement of type 1 metabotropic glutamate receptor (mGluR1)-mediated inward current and Ca(2+) mobilization. In wild-type mice, PLCbeta4 was distributed equally in both rostral and caudal cerebellum, while PLCbeta3 was enriched in the caudal versus the rostral cerebellum. In PLCbeta4-deficient mice, there was no measurable inward current or intracellular Ca(2+) elevation in the rostral cerebellum, whereas small responses were observed in the caudal cerebellum. In wild-type mice, the inward current was observed only following the release of caged GTPgammaS, not caged IP(3). These results suggest that the signal transduction machinery, including receptors, G-proteins, PLCbeta3, PLCbeta4, and effectors, form a functional unit, and the deletion of PLCbeta4 alters this unit, markedly changing signal transduction efficacy.

Cyclobutane quisqualic acid analogues as selective mGluR5a metabotropic glutamic acid receptor ligands

The conformationally constrained cyclobutane analogues of quisqualic acid (Z)- and (E)-1-amino-3-[2'-(3',5'-dioxo-1',2', 4'-oxadiazolidinyl)]cyclobutane-1-carboxylic acid, compounds 2 and 3, respectively, were synthesized. Both 2 and 3 stimulated phosphoinositide (PI) hydrolysis in the hippocampus with EC50 values of 18 +/- 6 and 53 +/- 19 microM, respectively. Neither analogue stimulated PI hydrolysis in the cerebellum. The effects of 2 and 3 were also examined in BHK cells which expressed either mGluR1a or mGluR5a receptors. Compounds 2 and 3 stimulated PI hydrolysis in cells expressing mGluR5a but not in those cells expressing mGluR1a. The EC50 value for 2 was 11 +/- 4 microM, while that for 3 was 49 +/- 25 microM. Both 2 and 3 did not show any significant effect on cells expressing the mGluR2 and mGluR4a receptors. In addition, neither compound blocked [3H]glutamic acid uptake into synaptosomal membranes, and neither compound was able to produce the QUIS effect as does quisqualic acid. This pharmacological profile indicates that 2 and 3 are selective ligands for the mGluR5a metabotropic glutamic acid receptor.

Modulation of long-term synaptic plasticity at excitatory striatal synapses

Modulation of long-term plasticity by both the intrinsic activation of metabotropic glutamate receptors and dopamine released from the nigrostriatal pathway was investigated at excitatory striatal synapses. Intracellular recordings demonstrated that tetanic stimulation at an intensity equal to that used for synaptic sampling produced, on average, a slight long-term depression of excitatory postsynaptic potentials. The long-term response pattern was variable, however, with some cells showing potentiation and others no plasticity. Block of metabotropic glutamate receptors with 3-aminophosphonovaleric acid changed the pattern of responses, increasing the percentage of cells showing long-term potentiation. Similarly, 6-hydroxydopamine lesions to the substantia nigra changed the pattern of response to tetanic stimulation, increasing the expression of long-term potentiation. These data indicate that metabotropic glutamate receptor and dopamine receptor activation may function to regulate the expression of activity-dependent plasticity at corticostriatial synapses. Paired-pulse stimulation revealed that post-tetanic plasticity was negatively correlated with changes in paired-pulse plasticity in the control and 6-hydroxydopamine-lesioned groups, suggesting that the expression of long-term plasticity has a presynaptic component at corticostriatal synapses.

Metabotropic glutamate receptors: electrophysiological properties and role in plasticity

Electrophysiological research on mGluRs is now very extensive, and it is clear that activation of mGluRs results in a large number of diverse cellular actions. Studies of mGluRs and on ionic channels has clearly demonstrated that mGluR activation has a widespread and potent inhibitory action on both voltage-gated Ca2+ channels and K+ channels. Inhibition of N-type Ca2+ channels, and inhibition of Ca(++)-dependent K+ current, IAHP, and IM being particularly prominent. Potentiation of activation of both Ca2+ and K+ channels has also been observed, although less prominently than inhibition, but mGluR-mediated activation of non-selective cationic channels is widespread. In a small number of studies, generation of an mGluR-mediated slow excitatory postsynaptic potential has been demonstrated as a consequence of the effect of mGluR activation on ion channels, such as activation of a non-selective cationic channels. Although certain mGluR-modulation of channels is a consequence of direct G-protein-linked action, for example, inhibition of Ca2+ channels, many other effects occur as a result of activation of intracellular messenger pathways, but at present, little progress has been made on the identification of the messengers. The field of study of the involvement of mGluRs in synaptic plasticity is very large. Evidence for the involvement of mGluRs in one form of LTD induction in the cerebellum and hippocampus is now particularly impressive. However, the role of mGluRs in LTP induction continues to be a source of dispute, and resolution of the question of the exact involvement of mGluRs in the induction of LTP will have to await the production of more selective ligands and of selective gene knockouts.

Kindled seizures increase metabotropic glutamate receptor expression and function in the rat supraoptic nucleus

The spread of experimentally kindled seizures in rats results in sustained increases in plasma vasopressin (VP) and VP mRNA in the supraoptic nucleus (SON). These increases provide an excellent example of the pathological plasticity that can develop in normal cells exposed to recurrent seizure activity. To test whether this plasticity might be due in part to changes in metabotropic glutamate receptors (mGluRs), we examined mGluR mRNA expression in the SON 1 month after stage 5 amygdala kindling. Three mGluR subtypes were detected by in situ hybridization in the SON in the following relative levels: mGluR3 > mGluR1 > mGluR7. Both mGluR1 and mGluR3 mRNAs were significantly increased in the SON (+28-61%) and cortex (+27-42%) after kindling. Immunoreactivity for mGluR1 but not mGluR2/3 was significantly increased in vivo in the SON. Receptor protein expression and intracellular calcium accumulation in response to the mGluR agonist, 1S,3R ACPD, were evaluated after in vitro "kindling" of neuroendocrine cells by Mg2+ deprivation. Increased immunoreactivity for mGluR1 and mGluR2/3 was seen in all cultures 3 days after a brief exposure to Mg2+-free medium. 1S,3R 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) induced rapid peak responses and gradual accumulations of intracellular Ca2+ in neurons. Both responses were increased in the "kindled" cells. Increases in the expression of functional mGluR1 and perhaps mGluR3 receptors may contribute to the development of long-lasting plastic changes associated with seizure activity.

Effects of metabotropic glutamate receptor block on the synaptic transmission and plasticity in the rat medial vestibular nuclei

In rat brainstem slices, we investigated the possible role of metabotropic glutamate receptors in modulating the synaptic transmission within the medial vestibular nuclei, under basal and plasticity inducing conditions. We analysed the effect of the metabotropic glutamate receptor antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine on the amplitude of the field potentials and latency of unitary potentials evoked in the ventral portion of the medial vestibular nuclei by primary vestibular afferent stimulation, and on the induction and maintenance of long-term potentiation, after high-frequency stimulation. Two effects were observed, consisting of a slight increase of the field potentials and reduction of unit latency during the drug infusion, and a further long-lasting development of these modifications after the drug wash-out. The long-term effect depended on N-methyl-D-aspartate receptor activation, as D,L-2-amino-5-phosphonopentanoic acid prevented its development. We suggest that (R,S)-alpha-methyl-4carboxyphenylglycine enhances the vestibular responses and induces N-methyl-D-aspartate-dependent long-term potentiation by increasing glutamate release, through the block of presynaptic metabotropic glutamate receptors which actively inhibit it. The block of these receptors was indirectly supported by the fact that the agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid reduced the vestibular responses and blocked the induction of long-term potentiation by high-frequency stimulation. The simultaneous block of metabotropic glutamate receptors facilitating synaptic plasticity, impedes the full expression of the long-term effect throughout the (R,S)-alpha-methyl-4-carboxyphenylglycine infusion. The involvement of such a facilitatory mechanism in the potentiation is supported by its reversible reduction following a second (R,S)-alpha-methyl-4-carboxyphenylglycine infusion. The drug also reduced the expression of potentiation induced by high-frequency stimulation. Conversely the electrical long-term potentiation was still induced, but it was occluded by the previous drug potentiation. We conclude that metabotropic glutamate receptors play a dual functional role in the medial vestibular nuclei, consisting in the inhibition of glutamate release under basal conditions, and the facilitation of N-methyl-D-aspartate-dependent plasticity phenomena.

Homer binds a novel proline-rich motif and links group 1 metabotropic glutamate receptors with IP3 receptors

Group I metabotropic glutamate receptors (mGluRs) activate PI turnover and thereby trigger intracellular calcium release. Previously, we demonstrated that mGluRs form natural complexes with members of a family of Homer-related synaptic proteins. Here, we present evidence that Homer proteins form a physical tether linking mGluRs with the inositol trisphosphate receptors (IP3R). A novel proline-rich "Homer ligand" (PPXXFr) is identified in group 1 mGluRs and IP3R, and these receptors coimmunoprecipitate as a complex with Homer from brain. Expression of the IEG form of Homer, which lacks the ability to cross-link, modulates mGluR-induced intracellular calcium release. These studies identify a novel mechanism in calcium signaling and provide evidence that an IEG, whose expression is driven by synaptic activity, can directly modify a specific synaptic function.

Metabotropic glutamate receptor activation in vivo induces intraneuronal amyloid immunoreactivity in guinea pig hippocampus

Stimulation of metabotropic glutamate receptors in vitro has been shown to accelerate the breakdown of amyloid precursor protein (APP) to form increased production of non-amyloidogenic secreted APP (sAPP). The mechanism whereby this occurs is not entirely clear but it is presumed to be linked to generation of diacylglycerol and activation of protein kinase C because other neurotransmitter receptors such as m1 and m3 muscarinic receptors, known to be coupled to this second messenger cascade, likewise increase sAPP production. Although it is presumed that a reciprocal relationship exists between the formation of amyloid beta protein (Abeta) and the production of sAPP, recent evidence suggests alternative processing can occur. Given the fact that much of the observations on APP metabolism have been made in vitro we sought to investigate the effect of metabotropic receptor activation on Abeta in vivo in a species known to contain the same amino acid sequence of Abeta as found in humans. Intrahippocampal injection of the mGluR agonist 1S,3R-ACPD in guinea pigs produced neurodegeneration of CA1 hippocampal pyramidal neurons at 12 h postinjection. Immunocytochemistry of sections from ACPD injected animals using selective antibodies to Abeta revealed the presence of punctate intraneuronal granules in pyramidal neurons of the hippocampus. These structures appeared to be localized within the nucleus and were particularly prominent in neurons within the region of neurodegeneration. Immunoreactivity was not observed in vehicle injected controls nor in sections from ACPD injected animals stained with preadsorbed antiserum. Abeta immunodetection was correlated with the onset of neurodegeneration since animals evaluated at 1 h and 4 h postinjection lacked both Abeta immunoreactivity as well as neurodegeneration. Evaluation of animals injected with NMDA revealed neurodegeneration but no Abeta immunoreactivity suggesting Abeta formation did not appear to be due to non-selective excitotoxicity. Staining of sections with antibodies directed to various regions of APP demonstrated increased C-terminal APP immunoreactivity in pyramidal neurons in the vicinity of degeneration. These data support recent in vitro studies illustrating that Abeta can be found intracellularly within neurons.

Changes in subcellular localization of metabotropic glutamate receptor subtypes during postnatal development of mouse thalamus

High resolution immunoelectron microscopy was used to study subcellular localization patterns of three metabotropic glutamate receptor subtypes (mGluR1alpha, mGluR5, and mGluR2/3) during postnatal development of mouse ventral posterior (VP) thalamic nucleus. Immunoreactivity for all three mGluRs was detected from birth (postnatal day 0, P0), but mGluR1alpha showed dramatic changes in localization with age. In the first postnatal week, mGluR1alpha immunoreactivity was mainly found in proximal dendrites and somata and not usually associated with synaptic contacts. From the second postnatal week, it became concentrated in distal dendrites and preferentially associated with corticothalamic (RS) synapses. mGluR5 immunoreactivity was weaker than mGluR1alpha immunoreactivity at all postnatal ages and showed a similar change in subcellular distribution to that of mGluR1alpha. It was also localized in astrocytic processes. mGluR2/3 immunoreactivity was mainly localized in astrocytic processes surrounding neuronal somata and synapses and this pattern was consistently maintained through all postnatal ages. A small number of presynaptic axon terminals were labeled for mGluR2/3 immunoreactivity and formed asymmetrical synapses. This study demonstrates that Group I mGluR proteins (mGluR1alpha and mGluR5) become redistributed in association with the development of corticothalamic function as demonstrated physiologically, whereas Group II mGluR proteins (mGluR2/3) are mainly associated with neuroglia.

Developmental and sensory-dependent changes of phosphoinositide-linked metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) can modulate synaptic transmission, and there is evidence that phosphoinositide (PI)-linked mGluRs may be involved in sensory-dependent plasticity during the development of cat visual cortex. Consequently, we asked the questions: Where are the PI-linked mGluRs (mGluR1alpha and mGluR5) in the visual cortex? Does the quantity and distribution of these receptors change in the cat visual cortex during postnatal development, and are these features sensory-dependent? We found that the quantity of mGluR1alpha decreases with age, whereas the laminar distribution of mGluR1alpha remains the same. Quantity of mGluR5 also decreases, but the laminar distribution of mGluR5 changes during development. The pattern and timing of the mGluR5 change in distribution follow the development of geniculocortical afferents. Immunostaining indicates that reduction of receptor occurs mainly in layers V-VI for mGluR1alpha and outside layer IV for mGluR5. Dark-rearing postpones the laminar change of mGluR5 and produces an increased level of mGluR5 between postnatal 1.5-6 weeks of age but has no significant effect on the mGluR1alpha distribution or the mGluR1alpha quantity. These results suggest that mGluR1alpha and mGluR5 are involved in different aspects of cortical development. The mGluR5 is more likely to be involved in sensory-dependent events than mGluR1alpha. The lack of developmental correlation between mGluR quantities and the critical period for ocular dominance plasticity also suggests that other factors besides mGluR quantities are important for ocular dominance plasticity.

Action of a metabotropic glutamate receptor agonist in rat lateral septum: induction of a sodium-dependent inward aftercurrent

The mechanism by which (1S,3R)-ACPD, a metabotropic glutamate receptor agonist, induces burst firing in lateral septal neurons of the rat was investigated in coronal brainstem slices. Membrane currents were characterized in voltage clamp using whole-cell recordings. In the presence of (1S,3R)-ACPD, following depolarizing voltage jumps, repolarization towards the holding potential generated an inward aftercurrent. It could have a plateau-like phase and decayed exponentially. This (1S,3R)-ACPD-dependent inward aftercurrent was accompanied by an increase in cell conductance and was reduced following partial replacement of extracellular sodium by N-methyl-D-glucamine. It was unaffected by TEA or barium, and persisted in Cs-loaded neurons or following partial replacement of extracellular chloride by isethionate. This suggests that it was mainly carried by sodium. Loading neurons with the calcium chelator, BAPTA, or blocking transmembrane calcium currents, suppressed the (1S,3R)-ACPD-dependent aftercurrent. By contrast, partial replacement of extracellular sodium by lithium did not affect it. Thus, this current was dependent upon calcium influx but was not due to a sodium/calcium exchanger. It was probably mediated by G protein activation. Indeed, in neurons loaded with GTP-gamma-S, following depolarizing voltage jumps, repolarization towards the holding potential revealed an inward aftercurrent having properties similar to those of the (1S,3R)-ACPD-dependent current. We suggest that (1S,3R)-ACPD induced calcium-activated non-selective channels. In the presence of this agonist, a depolarization-evoked calcium influx could thus evoke a cationic inward current. This current probably promotes the burst firing observed in lateral septal neurons in current clamp.

Glutamate in neurologic diseases

Excitotoxicity has been implicated as a mechanism of neuronal death in acute and chronic neurologic diseases. Cerebral ischemia, head and spinal cord injury, and prolonged seizure activity are associated with excessive release of glutamate into the extracellular space and subsequent neurotoxicity. Accumulating evidence suggests that impairment of intracellular energy metabolism increases neuronal vulnerability to glutamate which, even when present at physiologic concentrations, can damage neurons. This mechanism of slow excitotoxicity may be involved in neuronal death in chronic neurodegenerative diseases such as the mitochondrial encephalomyopathies, Huntington's disease, spinocerebellar degeneration syndromes, and motor neuron diseases. If so, glutamate antagonists in combination with agents that selectively inhibit the multiple steps downstream of the excitotoxic cascade or help improve intracellular energy metabolism may slow the neurodegenerative process and offer a therapeutic approach to treat these disorders.

LTP induction dependent on activation of Ni2+-sensitive voltage-gated calcium channels, but not NMDA receptors, in the rat dentate gyrus in vitro

LTP induction dependent on activation of Ni2+-sensitive voltage-gated calcium channels, but not NMDA receptors, in the rat dentate gyrus in vitro. J. Neurophysiol. 78: 2574-2581, 1997. A N-methyl--aspartate receptor (NMDAR)-independent long-term potentiation (LTP) has been investigated in the dentate gyrus of the hippocampus in vitro in the presence of the NMDAR antagonist, -2-amino-phosphonopentanoate (50-100 mu M), at a concentration that completely blocked NMDAR-mediated excitatory postsynaptic currents (EPSCs). LTP of patch-clamped EPSCs was induced by pairing low-frequency evoked EPSCs (1 Hz) with depolarizing voltage pulses designed to predominately open low-voltage-activated (LVA) Ca2+ channels. Voltage pulses alone induced only a short-term potentiation. The LTP was blocked by intracellular application of the rapid Ca2+ chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, demonstrating that a rise in intracellular Ca2+ is required for the NMDAR-independent LTP induction. The NMDAR-independent LTP induction also was blocked by Ni2+ at a low extracellular concentration (50 mu M), which is known to strongly block LVA Ca2+ channels. However, Ni2+ did not inhibit the NMDAR-dependent LTP induced by high-frequency stimulation (HFS). The NMDAR-independent LTP induction was not blocked by high concentrations of the L-type Ca2+ channel blocker nifedipine (10 mu M). The NMDAR-independent LTP was inhibited by the metabotropic glutamate receptor ligand (+)-alpha-methyl-4-carboxyphenylglycine. These experiments demonstrate the presence of a NMDAR-independent LTP induced by Ca2+ influx via Ni2+-sensitive, nifedipine-insensitive voltage-gated Ca2+ channels, probably LVA Ca2+ channels. Induction of the NMDAR-independent LTP was inhibited by prior induction of HFS-induced NMDAR-dependent LTP, demonstrating that although the NMDAR-dependent and NMDAR-independent LTP use a different Ca2+ channel for Ca2+ influx, they share a common intracellular pathway.

Glutamate metabotropic receptor agonists depress excitatory and inhibitory transmission on rat mesencephalic principal neurons

Intracellular and whole-cell patch-clamp recordings were used to evaluate the actions of different metabotropic glutamate receptor (mGluR) agonists on the synaptic inputs evoked on principal cells of the rat mesencephalon. Bath application of the group III mGluR agonists L-2-amino-4-phosphonobutyric acid (L-AP4) and L-serine-O-phosphonobutanoate (L-SOP) did not change the holding current of the cells held at resting potential (-60 mV) but produced a dose-dependent inhibition of the amplitude of the excitatory and inhibitory events. L-AP4 and L-SOP were more effective at inhibiting the excitatory postsynaptic currents (EPSCs) than the GABA(A) and GABA(B) inhibitory postsynaptic currents (IPSCs). The suppressing effects of L-AP4 and L-SOP were antagonized by (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP-4) but not by +/- -alpha-methyl-4-carboxyphenylglycine (MCPG). Moreover, the group II agonist (2S,1'S,2'S)-(carboxycyclopropyl)glycine (L-CCG1) and the group I agonist (RS)-3,5-dihydrophenylglycine (3,5-DHPG) depressed in a dose-related manner the EPSC, the GABA(A) IPSC and the GABA(B) IPSC. The suppressing effect of the two mGluRs agonists was partially antagonized by MCPG but not by MAP-4. In addition, both L-CCG1 and 3,5-DHPG caused an inward shift of the holding current. To characterize the site of action of the metabotropic receptor agonists, experiments were performed to examine the amplitude and ratio of EPSC and GABA(A) IPSC pairs. The increase of the s2/s1 ratio caused by the agonists suggests that the location of the inhibitory mGluRs was presynaptic. These results indicate that the activation of presynaptic mGluRs controls the release of excitatory and inhibitory transmitters on presumed dopaminergic cells within the ventral mesencephalon.

The L-AP4 receptor

1. The L-2-amino-4-phosphonobutyric acid (L-AP4) receptor was originally discovered by the ability of L-AP4 to depress synaptic transmission in hippocampal glutamatergic pathways and in the retina. 2. The molecular identity of the L-AP4 receptor is not yet resolved; however, with the molecular cloning of subtypes of metabotropic glutamate receptors (mGluRs), high affinity targets for L-AP4 have been identified. 3. As the information on the pharmacology of the mGluRs and the electrophysiological and biochemical studies on L-AP4 receptor physiology becomes elaborated it seems evident that the L-AP4 receptor is not a single molecular target but may involve multiple receptor subtypes.

Long-lasting enhancement of synaptic activity in dissociated cerebral neurons induced by brief exposure to Mg2+-free conditions

The long-lasting enhancement of periodic clusters of spontaneous excitatory postsynaptic currents (SEPSCs) was examined in dissociated chick cerebral neurons that had been transiently exposed to Mg2+-free solution for 15 min. Since the enhancement was diminished by blockade of synaptic transmission, it clearly depended on synaptic activities. A specific antagonist of N-methyl-D-aspartate receptors (NMDARs) also inhibited the potentiations. Furthermore, the presence of inhibitors of protein and RNA synthesis in the Mg2+-free solution blocked the potentiation. In the potentiated neurons, the frequency of miniature excitatory postsynaptic currents (mEPSPs) increased. In addition, a diffusible molecule(s) that promoted the potentiation appeared to be involved in this phenomenon, since the conditioned medium of Mg2+-free treated neurons enhanced synaptic activity in other dish.

Mice lacking metabotropic glutamate receptor 5 show impaired learning and reduced CA1 long-term potentiation (LTP) but normal CA3 LTP

Class I metabotropic glutamate receptors (mGluRs) have been postulated to play a role in synaptic plasticity. To test the involvement of one member of this class, we have recently generated mutant mice that express no mGluR5 but normal levels of other glutamate receptors. The CNS revealed normal development of gross anatomical features. To examine synaptic functions we measured evoked field EPSPs in the hippocampal slice. Measures of presynaptic function, such as paired pulse facilitation in mutant CA1 neurons, were normal. The response of mutant CA1 neurons to low concentrations of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) was missing, which suggests that mGluR5 may be the primary high affinity ACPD receptor in these neurons. Long-term potentiation (LTP) in mGluR5 mutants was significantly reduced in the NMDA receptor (NMDAR)-dependent pathways such as the CA1 region and dentate gyrus of the hippocampus, whereas LTP remained intact in the mossy fiber synapses on the CA3 region, an NMDAR-independent pathway. Some of the difference in CA1 LTP could lie at the level of expression, because the reduction of LTP in the mutants was no longer observed 20 min after tetanus in the presence of 2-amino-5-phosphonopentanoate. We propose that mGluR5 plays a key regulatory role in NMDAR-dependent LTP. These mutant mice were also impaired in the acquisition and use of spatial information in both the Morris water maze and contextual information in the fear-conditioning test. This is consistent with the hypothesis that LTP in the CA1 region may underlie spatial learning and memory.

Potentiation of synaptic transmission in the rat dentate gyrus in vitro by (S)-3,5-dihydroxyphenylglycine ((S)-DHPG)

The direct activation of metabotropic glutamate receptors (mGluRs) by 1S,3R-aminocyclopentane dicarboxylate (1S,3R-ACPD), has previously been shown to induce a relatively fast (maximum at 10 min) and slow (90 min) onset long-term potentiation (LTP) of synaptic transmission in the hippocampus. Here we report the first evidence for a relatively fast onset LTP of synaptic transmission in the immature male rat (50-100 g) dentate gyrus in vitro by application of the mGluR type I agonist, (S)-3,5-dihydroxyphenylglycine ((S)-DHPG; 20 microM). Bath application of (S)-DHPG caused a transient depression of the field excitatory postsynaptic potential (EPSP) slope, followed after washout by a relatively rapidly developing potentiation of synaptic transmission to a maximum increase at 12-15 min (161.1 +/- 11.4% compared to controls at 15 min; n = 8). This effect was not observed following perfusion with the enantiomer (R,S)-DHPG at the same dose. The (S)-DHPG potentiation occluded tetanically induced LTP and vice versa. The potentiation was antagonised by the non-specific mGluR antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine ((R,S)-MCPG) at high doses (500-1000 microM) but was unaffected in the presence of the N-methyl-D-aspartate (NMDA) receptor blocker, D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 50 microM). Our results demonstrate a robust NMDA-independent LTP induced by (S)-DHPG that occludes tetanically induced LTP.

Molecular cloning of a diacylglycerol kinase isozyme predominantly expressed in rat retina

We have cloned and characterized a new diacylglycerol kinase (DGK) isozyme which is expressed in the retina and the brain of rat. The cDNA contains an open reading frame of 567 amino acid residues with a predicted protein of 64 kDa and shows very high homology to human DGK epsilon. The new DGK isozyme contains two distinctive zinc-finger structures and a putative catalytic domain. This DGK expressed predominantly in the inner and outer nuclear layers of retina. This expression pattern is different from those of the previously cloned DGKs including the human DGK epsilon, suggesting that this DGK isozyme has potential importance in visual functions as was the case in Drosophila retinal cells.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Involvement of metabotropic glutamate receptors in Gi- and Gs-dependent modulation of adenylate cyclase activity induced by a novel cognition enhancer NS-105 in rat brain

The effect of a novel cognition enhancer [(+)-5-oxo-D-prolinepiperidinamide monohydrate] (NS-105) on cAMP formation was investigated in both slices and membranes of the rat cerebral cortex. NS-105 (10(-8)-10(-6) M) inhibited forskolin-stimulated cAMP formation in membranes, however, the compound significantly enhanced the cAMP formation in pertussis toxin-pre-treated membranes, an action that was abolished by cholera toxin. In contrast, in digitonin-permeabilized membranes, NS-105 had no influence on Mn2+-stimulated cAMP formation. Both of the inhibitory and facilitatory actions of NS-105 on cAMP formation were mimicked by a metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and an adrenergic alpha2 agonist UK-14,304, and blocked by a mGluR antagonist 2-amino-3-phosphonopropanoate but not by an alpha2 antagonist yohimbine. In cortical slices, NS-105 (10(-8)-10(-7) M) inhibited forskolin-stimulated cAMP accumulation but enhanced isoproterenol-stimulated cAMP accumulation, as did by a GABA(B) agonist (-)baclofen. On the other hand, (-)baclofen, while it significantly inhibited cAMP accumulation in slices, did no longer inhibit cAMP accumulation, when treated with NS-105 (10(-8)-10(-5) M). Similarly, (-)baclofen-induced inhibition of the cAMP accumulation was reversed by 1S,3R-ACPD and UK-14,304. NS-105 (10(-6)) increased [35S]GTPgammaS binding in the intact but not digitonin-permeabilized cortical membranes, as produced by UK-14,304, although the compound (10(-9)-10(-3) M) had no influence on various neurotransmitter receptor bindings, including alpha2 receptors. These results suggest that NS-105 modulates adenylate cyclase activity by stimulating mGluRs which might coupled to both Gi/Go and Gs.

A protein factor that inhibits ATP-dependent glutamate and gamma-aminobutyric acid accumulation into synaptic vesicles: purification and initial characterization

Glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, is transported into and stored in synaptic vesicles. We have purified to apparent homogeneity a protein from brain cytosol that inhibits glutamate and gamma-aminobutyric acid uptake into synaptic vesicles and have termed this protein "inhibitory protein factor" (IPF). IPF refers to three distinct proteins with relative molecular weights of 138,000 (IPF alpha), 135,000 (IPF beta), and 132,000 (IPF gamma), respectively. Gel filtration and sedimentation data suggest that all three proteins share an elongated structure, identical Stokes radius (60 A), and identical sedimentation coefficient (4.3 S). Using these values and a partial specific volume of 0.716 ml/g, we determined the native molecular weight for IPF alpha to be 103,000. Partial sequence analysis shows that IPF alpha is derived from alpha fodrin, a protein implicated in several diverse cellular activities. IPF alpha inhibits ATP-dependent glutamate uptake into purified synaptic vesicles with an IC50 of approximately 26 nM, while showing no ability to inhibit ATP-independent uptake at concentrations up to 100 nM. Moreover, IPF alpha inhibited neither norepinephrine uptake into chromaffin vesicles nor Na+-dependent glutamate uptake into synaptosomes. However, IPF alpha inhibited uptake of gamma-aminobutyric acid into synaptic vesicles derived from spinal cord, suggesting that inhibition may not be limited to glutamatergic systems. We propose that IPF could be a novel component of a presynaptic regulatory system. Such a system might modulate neurotransmitter accumulation into synaptic vesicles and thus regulate the overall efficacy of neurotransmission.

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

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

Activation of a metabotropic excitatory amino acid receptor potentiates spike-driven calcium increases in neurons of the dorsolateral septum

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), an agonist for metabotropic glutamate receptors (mGluRs), causes depolarization and burst firing in rat dorsolateral septal nucleus (DLSN) neurons and results in long-term potentiation (LTP) at DLSN synapses. In the present study, we investigated whether these actions of 1S,3R-ACPD are attributable to the release of calcium from an inositol triphosphate-sensitive store after activation of mGluRs coupled to phospholipase C. Our data demonstrated that the ACPD-induced depolarization was associated with a small but significant decrease, not an increase, in [Ca2+]i; however, changes of [Ca2+]i, during ACPD-induced bursting were up to seven times larger than those produced by regular firing. Depletion of internal calcium stores by thapsigargin or ryanodine had a small to insignificant effect on the maximum changes of [Ca2+]i, associated with ACPD-induced bursting. Thus, elevation of [Ca2+]i, during firing by 1S,3R-ACPD is likely attributable to enhancement of calcium influx through voltage-gated channels and not to calcium release from internal stores. ACPD-induced burst firing elevated somatic and dendritic calcium levels up to 3 and 6 microM, respectively. Such an increase may be the underlying mechanism for ACPD-induced LTP as well as ACPD-induced acute cell death in rat DLSN.