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

Group I mGlu receptors regulate the expression of the neuronal calcium sensor protein VILIP-1 in vitro and in vivo: implications for mGlu receptor-dependent hippocampal plasticity?

Metabotropic glutamate (mGlu) receptors are involved in several forms of synaptic plasticity in the rat hippocampus. Agonists which activate group I mGlu receptors induce slow-onset potentiation without prior tetanization in the hippocampal area CA1. Activation of group I mGlu receptors induces protein synthesis which may contribute to mGlu receptor-dependent forms of long-term plasticity. Calcium-binding proteins are widely considered to comprise key elements for synaptic plasticity. Therefore, we investigated whether the calcium sensor protein VILIP-1 is associated with group I mGlu receptor-mediated plasticity in the dentate gyrus (DG) in vivo.Application of either the group I and II mGlu agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) or the selective group I agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) resulted in slow-onset potentiation in the DG of adult rats. In hippocampal cell cultures both agonists elicited an enhanced expression of VILIP-1. In situ hybridization revealed strong hippocampal expression of VILIP-1 and intracerebral application of DHPG to adult rats significantly enhanced hippocampal VILIP-1 expression. The DHPG effects in both, hippocampal cultures and in vivo, were prevented by the group I mGlu receptor antagonist 4-Carboxyphenylglycine (4CPG). Calcium sensor proteins thus appear to be regulated by mGlu receptors in an activity-dependent manner. A specific role for group I mGlu receptors is evident. Furthermore, the sensor proteins may function as molecular switches for the long-term regulation of synaptic plasticity.

A role for Seven in Absentia Homolog (Siah1a) in metabotropic glutamate receptor signaling

BACKGROUND

The mammalian homologue of Seven in Absentia (Siah) can act in the ubiquitin/proteasome pathway. Recent work has shown that Siah can bind group I metabotropic glutamate receptors (mGluRs), but the functional consequences of this interaction are unknown.

RESULTS

The effects of coexpression of Siah on group I mGluR signaling were examined using heterologous expression in rat sympathetic, superior cervical ganglion neurons. Siah1a attenuated heterologously expressed group I mGluR-mediated calcium current inhibition, but was without effect on group II mGluR- or NE-mediated calcium current modulation via heterologously expressed mGluR2 or native a2 adrenergic receptors, respectively, indicating that the effect of Siah was specific for group I mGluRs. Surface expression and subcellular distribution of group I mGluRs were not detectably altered in the presence of Siah1a as assessed by immunoflourescence experiments with epitope tagged receptors and imaging of a GFP/mGluR fusion construct. In addition, an N-terminal Siah deletion construct, which cannot function in the proteolysis pathway, displayed effects similar to the wild type Siah1a. Finally, coexpression of calmodulin, which competes with Siah1a for binding to the C-terminal tail of group I mGluRs, reversed the effect of Siah1a on mGluR-mediated signaling.

CONCLUSIONS

These data supported the conclusion that the attenuation of mGluR signaling induced by Siah1a expression was likely a direct consequence of Siah/mGluR association rather than a result of targeting of the receptors to the proteosome. In addition, the data suggest that the binding of CaM and Siah may play an important role in the regulation of group I mGluR function.

Role of immediate early gene expression in cortical morphogenesis and plasticity

During the development of the central nervous system, there is a fundamental requirement for synaptic activity in transforming immature neuronal connections into organized functional circuits (Katz 1996). The molecular mechanisms underlying activity-dependent adaptive changes in neurons are believed to involve regulated cascades of gene expression. Immediate early genes (IEGs) comprise the initial cascade of gene expression responsible for initiating the process of stimulus-induced adaptive change, and were identified initially as transcription factors that were regulated in brain by excitatory synaptic activity. More recently, a class of neuronal immediate early genes has been identified that encodes growth factors, signaling molecules, extracellular matrix and adhesion proteins, and cytoskeletal proteins that are rapidly and transiently expressed in response to glutamatergic neurotransmission. This review focuses on the neuronal immediate early gene (nIEG) response, in particular, the class of "effector" immediate early gene proteins that may directly modify neuronal and synaptic function.

Associative mossy fibre LTP induced by pairing presynaptic stimulation with postsynaptic hyperpolarization of CA3 neurons in rat hippocampal slice

Whole cell recordings of excitatory postsynaptic potentials/currents (EPSPs/EPSCs) evoked by minimal stimulation of commissural-associative (CF) and mossy fibre (MF) inputs were performed in CA3 pyramidal neurons. Paired responses (at 50 ms intervals) were recorded before, during and after hyperpolarization of the postsynaptic membrane (20-30 mV for 15-35 min). Membrane hyperpolarization produced a supralinear increase of EPSPs/EPSCs amplitude in MF-inputs. Synaptic responses remained potentiated for the rest of the recording period (up to 40 min) after resetting the membrane potential to control level (221 +/- 60%, n = 15 and 219 +/- 61%, n = 11 for MF-EPSP and MF-EPSC, respectively). We shall refer to this effect as hyperpolarization-induced LTP (HI-LTP). In the absence of afferent stimulation, membrane hyperpolarization was unable to produce HI-LTP. In contrast to MF-EPSPs, the mean amplitude of CF-EPSPs did not increase significantly after hyperpolarization relative to controls (138 +/- 29%, n = 22). HI-LTP was associated with modifications of classical indices of presynaptic release: paired-pulse facilitation, failures rate, coefficient of variation of EPSP amplitudes and quantal content. The induction of HI-LTP was NMDA independent but was dependent on metabotropic glutamate receptors (mGluRs) activation and calcium release from inositol 1,4,5-triphosphate (IP3)-sensitive intracellular stores: it was prevented by mGluR antagonist, intracellular heparin and BAPTA. We conclude that while the induction of HI-LTP was postsynaptic, its expression was presynaptic.

On long-term depression induced by activation of G-protein coupled receptors

In this mini-review I consider the mechanisms by which activation of glutamate and acetylcholine metabotropic receptors can result in the induction of long-term depression. Two regions of the CNS will receive particular attention; the CA1 region of hippocampus and the perirhinal cortex.

The mechanism of presynaptic long-term depression mediated by group I metabotropic glutamate receptors

1. Metabotropic glutamate receptors (mGluRs) are known to play a role in synaptic plasticity. In a study of rat hippocampal brain slices, we find that a brief perfusion of a group I mGluR agonist, (S)-3,5-dihydroxyphenylglycine (DHPG), induced a robust long-term depression (DHPG-LTD) in area CA1. 2. The action was accompanied by an enhancement of the paired-pulse facilitation (PPF) ratio. 3. At the same time DHPG enhanced ionophoretic responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), kainic acid (KA), and N-methyl-D-aspartate (NMDA) in CA1 pyramidal neurons. This was only partially reversed by washing. 4. These observations indicate that DHPG exerts two opposing actions, suppression of the synaptic transmission and facilitation of postsynaptic responses. However, the presynaptic action dominates, since the net effect of monosynaptic activation is a reduction of response. 5. Perfusion of DHPG reduced three calcium-dependent responses in CA3 pyramidal neurons, which are presynaptic to CA1 neurons. These are calcium spike width and amplitude, after-hyperpolarization (AHP), and spike frequency adaptation (SFA). 6. These results suggest that the DHPG-LTD results from modulation of the presynaptic calcium currents by group I mGluRs.

Distribution of GluR1 is altered in the olfactory bulb following neonatal naris occlusion

The olfactory system is well suited for studies of glutamate receptor plasticity. The sensory neurons are glutamatergic, and they turn over throughout life, and the olfactory bulb neurons that process their inputs express many of the known glutamate receptor subunits. Neonatal naris occlusion alters olfactory bulb development and the expression of certain neuroactive substances and receptors, at least in part due to loss of the sensory inputs. We therefore postulated that neonatal naris occlusion might alter glutamate receptor expression during postnatal development. Single nares of newborn mice were occluded on postnatal days 1-2, and the distribution of glutamate receptor subunits was evaluated using immunoperoxidase methods. Light microscopic examination on postnatal day 6 failed to reveal adult-like staining of neuronal cell bodies in the olfactory bulbs. By day 12, cell bodies that were immunoreactive (-IR) for the GluR1 subunit were visible in the external plexiform layer (EPL) of both sides. By day 18, many of the GluR1-IR cell bodies could be identified as cell types that had previously been reported to express homomeric GluR1 receptors. Analysis of single, mid-dorsal sections from 18-25-day-old mice showed that the medial EPL of the occluded side had a significantly lower density of these cell bodies. The GluR1 staining of the adjacent mitral cell layer (MCL) was also heavier on the occluded side, but no gross differences in staining for other glutamate receptor subunits were observed. Neonatal naris occlusion therefore appears to provide a new model for studying expression of GluR1 receptors during the development of a discrete population of olfactory bulb neurons.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

Molecular aspects of glutamate dysregulation: implications for schizophrenia and its treatment

The glutamate system is involved in many aspects of neuronal synaptic strength and function during development and throughout life. Synapse formation in early brain development, synapse maintenance, and synaptic plasticity are all influenced by the glutamate system. The number of neurons and the number of their connections are determined by the activity of the glutamate system and its receptors. Malfunctions of the glutamate system affect neuroplasticity and can cause neuronal toxicity. In schizophrenia, many glutamate-regulated processes seem to be perturbed. Abnormal neuronal development, abnormal synaptic plasticity, and neurodegeneration have been proposed to be causal or contributing factors in schizophrenia. Interestingly, it seems that the glutamate system is dysregulated and that N-methyl-D-aspartate receptors operate at reduced activity. Here we discuss how the molecular aspects of glutamate malfunction can explain some of the neuropathology observed in schizophrenia, and how the available treatment intervenes through the glutamate system.

New life in an old idea: the synaptic plasticity and memory hypothesis revisited

The notion that changes in synaptic efficacy underlie learning and memory processes is now widely accepted, although definitive proof of the synaptic plasticity and memory hypothesis is still lacking. This article reviews recent evidence relevant to the hypothesis, with particular emphasis on studies of experience-dependent plasticity in the neocortex and hippocampus. In our view, there is now compelling evidence that changes in synaptic strength occur as a consequence of certain forms of learning. A major challenge will be to determine whether such changes constitute the memory trace itself or play a less specific supporting role in the information processing that accompanies memory formation.

Bidirectional synaptic plasticity in the cerebellum-like mammalian dorsal cochlear nucleus

The dorsal cochlear nucleus integrates acoustic with multimodal sensory inputs from widespread areas of the brain. Multimodal inputs are brought to spiny dendrites of fusiform and cartwheel cells in the molecular layer by parallel fibers through synapses that are subject to long-term potentiation and long-term depression. Acoustic cues are brought to smooth dendrites of fusiform cells in the deep layer by auditory nerve fibers through synapses that do not show plasticity. Plasticity requires Ca(2+)-induced Ca(2+) release; its sensitivity to antagonists of N-methyl-d-aspartate and metabotropic glutamate receptors differs in fusiform and cartwheel cells.

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.

Metabotropic glutamate receptor expression in the rat superior cervical ganglion

To begin to determine what role metabotropic glutamate receptors (mGluRs) play in the peripheral nervous system, reverse transcriptase-polymerase chain reaction (RT-PCR) was used to probe RNA isolated from sympathetic neurons of the rat superior cervical ganglion (SCG). RT-PCR primers were designed to detect each of the eight rat mGluR transcripts. Only one, mGluR7, was detected in RNA from rat SCG, though each appeared to be present in RNA from whole rat brain. Although mGluR7 messenger RNA is apparently present in rat SCG, functional mGluR7 was not observed, as application of neither glutamate nor the group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) produced calcium current modulation in isolated SCG neurons, as would be expected. However, following mGluR7 heterologous expression, application of L-AP4 did produce moderate calcium current modulation in isolated neurons, indicating that mGluR7 can express on the surface of SCG soma, and that its activation can modulate calcium currents.

Metabotropic glutamate receptors: electrical and chemical signaling properties

Over the last two decades, glutamate has been established as the main excitatory neurotransmitter in the mammalian brain. Glutamate released from synapses activates ion channel-forming receptors at postsynaptic cells and consequently mediates fast postsynaptic potentials. These receptors are termed ionotropic glutamate receptors (iGluRs). The subsequent discovery of metabotropic glutamate receptors (mGluRs) revealed that glutamate can also mediate slow synaptic potentials, modulate ion channels, and directly couple to GTP binding proteins. In contrast to the iGluRs, the mGluRs possess seven transmembrane domains and a large intracellular C-terminus that involves interactions with a variety of other intracellular signaling systems. Eight functionally distinct mGluR subtypes are known to be localized to specific neuron types at presynaptic and/or postsynaptic membranes. Their physiological functions involve the generation of slow excitatory and inhibitory synaptic potentials, modulation of synaptic transmission, synaptic integration, and plasticity. The classical role of glutamate as a fast excitatory synaptic transmitter was largely extended by mGluRs acting as a neuromodulator and even as an activator of inhibitory mechanisms at certain synapses.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.

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

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

Metabotropic glutamate receptors provide intrinsic modulation of the lamprey locomotor network

Spinal networks generate the motor pattern underlying locomotion. These are subject to modulatory systems that influence their operation and thereby result in a flexible network organization. In this review, we have summarized the mechanisms by which the different metabotropic glutamate receptor subtypes fine-tune the cellular and synaptic properties and thus underlie intrinsic modulation of the activity of the locomotor network in the lamprey.

Plasticity-associated molecular and structural events in the injured brain

Injury to the brain appears to create a fertile ground for functional and structural plasticity that is, at least partly, responsible for functional recovery. Increases in dendritic arborization, spine density, and synaptogenesis in both peri-injury and intact cortical areas are the potential morphological strategies that enable the brain to reorganize its neuronal circuits. These injury-initiated alterations are time-dependent and frequently proceed in interaction with behavior-related signals. A complex concert of a variety of genes/proteins is required to tightly control these changes. Two broad categories of molecules appear to be involved. First, regulatory molecules or effector molecules with regulatory function, such as immediate early genes/transcription factors, kinase network proteins, growth factors, and neurotransmitter receptors, and second, structural proteins, such as adhesion molecules and compounds of synapses, growth cones, and cytoskeleton. A better understanding of the processes contributing to postinjury plasticity may be an advantage for developing new and more effective therapeutic approaches. This knowledge might also shed light on other forms of brain plasticity, such as those involved in learning processes or ontogeny.

Different metabotropic glutamate receptors play opposite roles in synaptic plasticity of the rat medial vestibular nuclei

In the medial vestibular nuclei (MVN) of rat brainstem slices, the role of group II and III metabotropic glutamate receptors (mGluRs) and of the subtypes of group I mGluRs: mGluR1, mGluR5, was investigated in basal synaptic transmission and in the induction and maintenance of long-term potentiation (LTP). We used selective antagonists and agonists for mGluRs and we analysed the field potentials evoked by vestibular afferent stimulation before and after high-frequency stimulation (HFS) to induce LTP. The group II and III mGluR antagonist, (R,S)-alpha-2-methyl-4sulphonophenylglycine (MSPG), induced LTP per se and caused a reduction of the paired-pulse facilitation (PPF) ratio indicating an enhancement of glutamate release. This suggests that group II and III mGluRs are activated under basal conditions to limit glutamate release. Both the group II and III mGluR selective antagonists, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoate (LY341495) and (R,S)-alpha-methylserine-O-phosphate (MSOP), induced LTP, and the selective agonists, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed the field potentials and prevented HFS-LTP, with a prevailing contribution of group II mGluRs over that of group III mGluRs. The mGluR1 antagonist, 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) prevented the full development and maintenance of HFS-LTP. By contrast, the mGluR5 antagonist, 2-methyl-6-phenylethynylpyridine (MPEP) induced LTP per se, which was impeded by CPCCOEt, and it had no effect on LTP once induced by HFS. The PPF analysis showed an enhancement of glutamate release during MPEP potentiation. The group I mGluR agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced LTP per se, which was blocked by CPCCOEt. By contrast the mGluR5 agonist, (R,S)-2-chloro-5-hydroxypheylglycine (CHPG) prevented LTP elicited by HFS and DHPG as well. In conclusion vestibular LTP is inhibited by group II and III mGluRs during the early induction phase while it is facilitated by mGluR1 for achieving its full expression and consolidation. An additional inhibitory control is exerted by mGluR5 at the level of this facilitatory phase.

Evaluation of Affymetrix Gene Chip sensitivity in rat hippocampal tissue using SAGE analysis. Serial Analysis of Gene Expression

DNA microarrays are a powerful tool for monitoring thousands of transcript levels simultaneously. However, the use of DNA microarrays in studying the central nervous system faces several challenges. These include the detection of low-abundance transcripts in highly complex tissue as well as estimating relatively low-magnitude changes in transcript levels in response to experimental manipulation. Many transcripts important to brain function have low expression levels or are expressed in relatively few cells, making them difficult to detect in the complex background of brain tissue. The aim of the present study is to evaluate the sensitivity of Gene Chip detection of transcripts in brain by using results from serial analysis of gene expression (SAGE) studies. The results of this comparison indicate that Affymetrix Gene Chips, like SAGE, only reliably detect medium- to high-abundance transcripts and that detection of low-abundance transcripts, many of which have great relevance to biological function in brain, is inconsistent. Specifically, we estimate that Gene Chips reliably detect no more than 30% of the hippocampal transcriptome when using a gross hippocampal dissection as the source tissue. This report provides the first broad evaluation of Affymetrix Gene Chip sensitivity relevant to studying the brain.

Cooperation between mglu receptors: a depressing mechanism?

Recent findings from the perirhinal cortex have shed new light on the ways in which metabotropic glutamate receptors could be involved in synaptic plasticity, and in particular in long-term depression (LTD) of synaptic transmission. Importantly, these findings have also led to a greater understanding of mechanisms that could regulate mglu-receptor signalling and the ways in which mglu receptors interact with one another.

The group I metabotropic glutamate receptor mGluR5 is required for fear memory formation and long-term potentiation in the lateral amygdala

The group I metabotropic glutamate receptor subtype mGluR5 has been shown to play a key role in the modulation of synaptic plasticity. The present experiments examined the function of mGluR5 in the circuitry underlying Pavlovian fear conditioning using neuroanatomical, electrophysiological, and behavioral techniques. First, we show using immunocytochemical and tract-tracing methods that mGluR5 is localized to dendritic shafts and spines in the lateral nucleus of the amygdala (LA) and is postsynaptic to auditory thalamic inputs. In electrophysiological experiments, we show that long-term potentiation at thalamic input synapses to the LA is impaired by bath application of a specific mGluR5 antagonist, 2-methyl-6-(phenyle-thynyl)-pyridine (MPEP), in vitro. Finally, we show that intra-amygdala administration of MPEP dose-dependently impairs the acquisition, but not expression or consolidation, of auditory and contextual fear conditioning. Collectively, the results of this study indicate that mGluR5 in the LA plays a crucial role in fear conditioning and in plasticity at synapses involved in fear conditioning.

Calcineurin plays different roles in group II metabotropic glutamate receptor- and NMDA receptor-dependent long-term depression

We investigated metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) in hippocampal CA1 pyramidal neurons of 6- to 8-d-old [postnatal days 6-8 (P6-P8)] and 21- to 25-d-old (P21-P25) rats. In P6-P8 rats, induction of LTD depended on the activity of group II mGluRs. In P21-P25 rats, however, this LTD disappeared, and instead, NMDA receptor (NMDAR)-dependent LTD appeared. A bath containing a specific calcineurin (CaN) inhibitor restored the group II mGluR-dependent LTD in the neurons of the P21-P25 rats. Although postsynaptic injection of CaN inhibitors suppressed NMDAR-dependent LTD, it did not affect induction of group II mGluR-dependent LTD. These results demonstrate that CaN plays different roles in the induction of two forms of LTD: presynaptic CaN inhibits group II mGluR-dependent LTD, whereas postsynaptic CaN facilitates NMDAR-dependent LTD. These findings are the first demonstration in vitro of group II mGluR-dependent LTD that is negatively regulated by CaN via an age-dependent mechanism.

The requirement of presynaptic metabotropic glutamate receptors for the maintenance of locomotion

Spinal circuits known as central pattern generators maintain vertebrate locomotion. In the lamprey, the contralaterally alternating ventral root activity that defines this behavior is driven by ipsilateral glutamatergic excitation (Buchanan and Grillner, 1987) coupled with crossed glycinergic inhibition (Buchanan, 1982; Alford and Williams, 1989). These mechanisms are distributed throughout the spinal cord. Glutamatergic excitatory synapses activate AMPA and NMDA receptors known to be necessary for the maintenance of the locomotor rhythm. Less is known of the role and location of metabotropic glutamate receptors (mGluRs), although group I mGluRs enhance transmitter release at giant synapses in the lamprey spinal cord, whereas group II/III receptors may inhibit release. In this study we show that group I mGluR antagonists block fictive locomotion, a neural correlate of locomotion, by acting at the presynaptic terminal. Under physiological conditions, synaptically released glutamate activates presynaptic group I mGluRs (autoreceptors) during the repetitive activation of glutamatergic terminals. The resulting rise in [Ca2+]i caused by the release from presynaptic intracellular stores is coincident with an enhancement of synaptic transmission. Thus, blocking mGluRs reduces glutamate release during the repetitive activity that is characteristic of locomotion, leading to the arrest of locomotor activity. We found the effects of group I mGluRs on locomotion to be inconsistent with a postsynaptic effect on the central pattern generator. Consequently, the activation of metabotropic glutamate autoreceptors is necessary to maintain rhythmic motor output. Our results demonstrate the role of presynaptic mGluRs in the physiological control of movement for the first time.

Extracellular calcium controls the dynamic range of neuronal metabotropic glutamate receptor responses

The metabotropicglutamate receptors (mGluRs) are neurotransmitter receptors important for synaptic plasticity in the brain. Here we report that native mGluR-mediated neuronal responses to glutamate are profoundly modulated by extracellular calcium (Ca2+(o)). In mouse cerebellar Purkinje cells (PCs), Ca2+(o) drastically broadened the effective dose range for glutamate analogs in which native mGluR1-mediated cation current and intracellular Ca2+ mobilization were evoked. This effect has not been observed for recombinant mGluRs expressed in the heterologous cell systems. Ca2+(o) also drastically augmented these native mGluR-mediated responses to the glutamate analog. These Ca2+(o) effects were observed in both the wild-type mice and the mutant mice expressing mGluR1 specifically in their PCs, suggesting that the native mGluR1 in the PCs but not those in other cell types are the key mediators of the effects. These findings demonstrate that Ca2+(o) plays an important role in regulating native mGluR-mediated neuronal responses.

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.

Self-organized synaptic plasticity contributes to the shaping of gamma and beta oscillations in vitro

gamma (30-70 Hz) followed by beta (10-30 Hz) oscillations are evoked in humans by sensory stimuli and may be involved in working memory. Phenomenologically similar gamma-->beta oscillations can be evoked in hippocampal slices by strong two-site tetanic stimulation. Weaker stimulation leads only to two-site synchronized gamma. In vitro oscillations have memory-like features: (1) EPSPs increase during gamma-->beta; (2) after a strong one-site stimulus, two-site stimulation produces desynchronized gamma; and (3) a single synchronized gamma-->beta epoch allows a subsequent weak stimulus to induce synchronized gamma-->beta. Features 2 and 3 last >50 min and so are unlikely to be caused by presynaptic effects. A previous model replicated the gamma-->beta transition when it was assumed that K(+) conductance(s) increases and there is an ad hoc increase in pyramidal EPSCs. Here, we have refined the model, so that both pyramidal-->pyramidal and pyramidal-->interneuron synapses are modifiable. This model, in a self-organized way, replicates the gamma-->beta transition, along with features 1 and 2 above. Feature 3 is replicated if learning rates, or the time course of K(+) current block, are graded with stimulus intensity. Synaptic plasticity allows simulated oscillations to synchronize between sites separated by axon conduction delays over 10 msec. Our data suggest that one function of gamma oscillations is to permit synaptic plasticity, which is then expressed in the form of beta oscillations. We propose that the period of gamma oscillations, approximately 25 msec, is "designed" to match the time course of [Ca(2+)](i) fluctuations in dendrites, thus facilitating learning.

A characterisation of long-term depression induced by metabotropic glutamate receptor activation in the rat hippocampus in vitro

1. In the CA1 region of hippocampal slices prepared from juvenile (12- to 18-day-old) rats, activation of group I metabotropic L-glutamate (mGlu) receptors by the specific agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) induces a form of long-term depression (LTD) of excitatory synaptic transmission. 2. We have used a variety of electrophysiological techniques applied to CA1 neurones in hippocampal slices and from pyramidal cells in dissociated hippocampal cultures to investigate the Ca2+ dependence and locus of expression of DHPG-induced LTD. 3. In patch-clamp experiments from hippocampal slices, bath application of DHPG induced a depression of synaptically evoked responses that persisted for the duration of the recording (up to 2 h after commencing washout of DHPG) in 27 of 29 neurones investigated. 4. DHPG-induced LTD was associated with an increase in both the paired-pulse facilitation ratio and the coefficient of variation of EPSCs. 5. Using dendritic recording, there was a decrease in EPSC success rate (number of trials that elicited a detectable response) but no change in potency (mean EPSC amplitude excluding failures) associated with DHPG-induced LTD. 6. In experiments using dissociated hippocampal cultures, application of DHPG elicited a persistent decrease in the frequency of tetrodotoxin-resistant miniature EPSCs but no change in the amplitude of such events. 7. DHPG-induced LTD was not blocked by intracellular application of the calcium chelator BAPTA. It was also unaffected when intracellular calcium stores were depleted by perfusion with thapsigargin. Furthermore, when synaptic transmission was blocked by perfusing with Ca2+-free medium, DHPG application reliably induced LTD. 8. These data suggest that DHPG-induced LTD is Ca2+ independent and is expressed presynaptically.

Synaptic secretion of BDNF after high-frequency stimulation of glutamatergic synapses

The protein brain-derived neurotrophic factor (BDNF) has been postulated to be a retrograde or paracrine synaptic messenger in long-term potentiation and other forms of activity-dependent synaptic plasticity. Although crucial for this concept, direct evidence for the activity-dependent synaptic release of BDNF is lacking. Here we investigate secretion of BDNF labelled with green fluorescent protein (BDNF-GFP) by monitoring the changes in fluorescence intensity of dendritic BDNF-GFP vesicles at glutamatergic synaptic junctions of living hippocampal neurons. We show that high-frequency activation of glutamatergic synapses triggers the release of BDNF-GFP from synaptically localized secretory granules. This release depends on activation of postsynaptic ionotropic glutamate receptors and on postsynaptic Ca(2+) influx. Release of BDNF-GFP is also observed from extrasynaptic dendritic vesicle clusters, suggesting that a possible spatial restriction of BDNF release to specific synaptic sites can only occur if the postsynaptic depolarization remains local. These results support the concept of BDNF being a synaptic messenger of activity-dependent synaptic plasticity, which is released from postsynaptic neurons.

Long-term depression: a cascade of induction and expression mechanisms

The aims of this paper are to provide a comprehensive and up to date review of the mechanisms of induction and expression of long-term depression (LTD) of synaptic transmission. The review will focus largely on homosynaptic LTD and other forms of LTD will be considered only where appropriate for a fuller understanding of LTD mechanisms. We shall concentrate on what are felt to be some of the most interesting recent findings concerning LTD in the central nervous system. Wherever possible we shall try to consider some of the disparities in results and possible reasons for these. Finally, we shall briefly consider some of the possible functional consequences of LTD for normal physiological function.

Long-term depression: a cellular basis for learning?

Long-term depression (LTD) comprises a persistent activity-dependent reduction in synaptic efficacy which typically occurs following repeated low frequency afferent stimulation. Hippocampal LTD has been a subject of particular interest due to the established role of the hippocampus in certain forms of information storage and retrieval. Recently, it was reported that LTD in the CA1 region may be associated with novelty acquisition in rats. CA1 LTD expression may also be increased in stressful conditions. This suggests a more complex role for this form of plasticity than the oft-cited hypothesis that it simply serves to prevent synapse saturation, by means, for example, of enabling reversal of long-term potentiation (LTP). One possibility is that LTD may be directly involved in the creation of a memory trace. Alternatively, LTD may prime a synapse in readiness for the expression of LTP, thereby contributing indirectly to information storage. There is increasing evidence that LTD is not mechanistically the reverse of LTP. Although some common processes exist, molecular, biochemical, electrophysiological and pharmacological studies all point to several quite distinct induction and maintenance mechanisms for this form of synaptic plasticity. Taken together these findings suggest that hippocampal LTD must be considered in a new light. This review focuses on the interpretation of novel and established information with regard to LTD in the hippocampal CA1 region in terms of its possible role as a cellular basis for learning and memory.

Neural activity: sculptor of 'barrels' in the neocortex

A major portion of the primary somatosensory cortex of rodents is characterized by the discrete and patterned distribution of thalamocortical axons and layer IV granule cells ('barrels'), which correspond to the spatial distribution of whiskers and sinus hairs on the snout. In recent years several mutant mouse models began unveiling the cellular and molecular mechanisms by which these patterns emerge presynaptically and are reflected postsynaptically. Neural activity plays a crucial role in conferring presynaptic patterns to postsynaptic cells via neurotransmitter receptor-mediated intracellular signals. Here we review recent evidence that is finally opening the doors to understanding the cellular and molecular mechanisms of pattern formation in the neocortex.

mGlu1 receptors mediate a post-tetanic depression at parallel fibre-Purkinje cell synapses in rat cerebellum

Metabotropic glutamate (mGlu) receptors are located pre- and postsynaptically at central synapses. Activation of the receptors by exogenous agonists usually results in a reversible depression of fast glutamatergic neurotransmission. Evidence that synaptically released glutamate has such an action, however, is scarce. Sharp microelectrode recordings were used to investigate the modulatory role of mGlu receptors at a well-studied glutamatergic synapse, the one between parallel fibres and Purkinje cells in rat cerebellar slices. Brief, tetanic stimulation of the parallel fibres caused a depression of subsequent fast EPSPs. This post-tetanic depression (PTD) reached its maximum 4.5 s after the tetanus. Measured at this point, PTD was frequency-dependent; 10 stimuli at 20 Hz produced no significant depression, whereas, at 100 Hz the same number of stimuli was maximally effective (approximately 50% depression). The nonselective mGlu antagonist, (S)-alpha-methyl-4-carboxyphenylglycine 1 mm or the GABAB antagonist, CGP35348 (1 mm), both decreased the magnitude of the PTD. In the presence of CGP35348 the mGlu1 antagonist, 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester (300 microm), inhibited PTD further. A group II/III mGlu antagonist had no effect. These observations indicate that synaptically activated mGlu1 receptors not only generate a slow EPSP and induce Ca2+ mobilization in Purkinje cells, as reported previously, but also produce a transient depression of fast synaptic transmission. This short-term plasticity may be important for shaping the output of cerebellar circuits and/or it could provide a substrate for long-term depression when additional mechanisms are superimposed.

Synaptic modification by correlated activity: Hebb's postulate revisited

Correlated spiking of pre- and postsynaptic neurons can result in strengthening or weakening of synapses, depending on the temporal order of spiking. Recent findings indicate that there are narrow and cell type-specific temporal windows for such synaptic modification and that the generally accepted input- (or synapse-) specific rule for modification appears not to be strictly adhered to. Spike timing-dependent modifications, together with selective spread of synaptic changes, provide a set of cellular mechanisms that are likely to be important for the development and functioning of neural networks. When an axon of cell A is near enough to excite cell B or repeatedly or consistently takes part in firing it, some growth or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased.

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.

Gene targeting reveals a role for the glutamate receptors mGluR5 and GluR2 in learning and memory

This work suggests that class I mGluRs are involved in long-term potentiation (LTP) at CA1 synapses within the hippocampus. Our data support a pathway linking class I-mGluRs with PKC and src to enhance the open probability of the NMDAR channel. This leads to LTP of the NMDAR, but not the AMPAR. We are currently analyzing double mGluR1 X mGluR5 knockouts with Collingridge for a loss of the LTP induction switch [Nature 368 (1994) 740.]. This induction of LTP of the NMDAR is necessary for "spatial" learning and memory to occur, since mice lacking the mGluR5 are deficient in the Morris water maze and context-dependent fear conditioning. We postulate that AMPARs may provide negative feedback inhibition to the NMDAR. Hence, in null mutants lacking the AMPAR subtype, GluR2, LTP in the CA1 region of hippocampal slices was markedly enhanced (twofold) and non-saturating, whereas neuronal excitability and paired-pulse facilitation were normal. The ninefold increase in Ca(2+) permeability, in response to kainate application, suggests one possible mechanism for enhanced LTP. Enhanced LTP could result from enhanced AMPAR channel conductance or increased recruiting of previously silent synapses. Since the GluR2 null mutants showed reduced exploration and impaired motor coordination, we could make no conclusion about its role in learning and memory. Future work will be directed to inducible deletion of GluR2 only in CA1 after development is complete. These results support the correlation between LTP and learning and memory.

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.

G(alpha)q-deficient mice lack metabotropic glutamate receptor-dependent long-term depression but show normal long-term potentiation in the hippocampal CA1 region

Long-term potentiation (LTP) and depression (LTD) are potential cellular mechanisms involved in learning and memory. Group I metabotropic glutamate receptors (mGluR), which are linked to heterotrimeric G-proteins of the G(q) family (G(q) and G(11)), have been reported to facilitate both hippocampal LTP and LTD. To evaluate their functional role in synaptic plasticity, we studied LTD and LTP in the CA1 region of the hippocampus from wild-type, Galpha(q)(-/-), and Galpha(11)(-/-) mice. Basic parameters of the synaptic transmission were not altered in Galpha(q)(-/-) and Galpha(11)(-/-) mice. Moreover, these mice showed normal LTP in response to a strong tetanus and to a weak tetanus. However, LTD induced either by a group I mGluRs agonist or by paired-pulse low-frequency stimulation (PP-LFS) was absent in Galpha(q)(-/-) mice. Moreover, PP-LFS caused potentiation of the synaptic transmission in these mice that was not affected by the NMDAR antagonist AP-5. These results show that G(q) plays a crucial role in the mGluR-dependent LTD, whereas hippocampal LTP is not affected by the lack of a single member of the G(q) family.

Synaptically activated calcium responses in dendrites of hippocampal oriens-alveus interneurons

Activation of metabotropic glutamate receptors (mGluRs) by agonists increases intracellular calcium levels ([Ca(2+)](i)) in interneurons of stratum oriens/alveus (OA) of the hippocampus. We examined the mechanisms that contribute to dendritic Ca(2+) increases in these interneurons during agonist activation of mGluRs and during synaptically evoked burst discharges, using simultaneous whole cell recordings and confocal Ca(2+) imaging in rat hippocampal slices. First, we found that the group I/II mGluR agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 100 microM) increased dendritic [Ca(2+)](i) and depolarized OA interneurons. Dendritic Ca(2+) responses were correlated with membrane depolarizations, but Ca(2+) responses induced by ACPD were larger in amplitude than those elicited by equivalent somatic depolarization. Next, we used linescans to measure changes in dendritic [Ca(2+)](i) during synaptically evoked burst discharges and somatically elicited repetitive firing in disinhibited slices. Dendritic Ca(2+) signals and electrophysiological responses were stable over repeated trials. Peak Ca(2+) responses were linearly related to number and frequency of action potentials in burst discharges for both synaptic and somatic stimulation, but the slope of the relationship was steeper for responses evoked somatically. Synaptically evoked [Ca(2+)](i) rises and excitatory postsynaptic potentials were abolished by antagonists of ionotropic glutamate receptors. The group I/II mGluR antagonist S-alpha-methyl-4-carboxyphenylglycine (500 microM) produced a significant partial reduction of synaptically evoked dendritic Ca(2+) responses. The mGluR antagonist did not affect synaptically evoked burst discharges and did not reduce either Ca(2+) responses or burst discharges evoked somatically. Therefore ionotropic glutamate receptors appear necessary for synaptically evoked dendritic Ca(2+) responses, and group I/II mGluRs may contribute partially to these responses. Dendritic [Ca(2+)](i) rises mediated by both ionotropic and metabotropic glutamate receptors may be important for synaptic plasticity and the selective vulnerability to excitotoxicity of OA interneurons.

Age-dependent consequences of seizures: relationship to seizure frequency, brain damage, and circuitry reorganization

Seizures in the developing brain pose a challenge to the clinician. In addition to the acute effects of the seizure, there are questions regarding the impact of severe or recurrent seizures on the developing brain. Whether provoked seizures cause brain damage, synaptic reorganization, or epilepsy is of paramount importance to patients and physicians. Such questions are especially relevant in the decision to treat or not treat febrile seizures, a common occurrence in childhood. These clinical questions have been addressed using clinical and animal research. The largest prospective studies do not find a causal connection between febrile seizures and later temporal lobe epilepsy. The immature brain seems relatively resistant to the seizure-induced neuronal loss and new synapse formation seen in the mature brain. Laboratory investigations using a developmental rat model corresponding to human febrile seizures find that even though structural changes do not result from hyperthermic seizures, synaptic function may be chronically altered. The increased understanding of the cellular and synaptic mechanisms of seizure-induced damage may benefit patients and clinicians in the form of improved therapies to attenuate damage and changes induced by seizures and to prevent the development of epilepsy.

Dendritic and axonal targeting of type 5 metabotropic glutamate receptor is regulated by homer1 proteins and neuronal excitation

The physiological actions of neurotransmitter receptors are intimately linked to their proper neuronal compartment localization. Here we studied the effect of the metabotropic glutamate receptor (mGluR)-interacting proteins, Homer1a, b, and c, in the targeting of mGluR5 in neurons. We found that mGluR5 was exclusively localized in cell bodies when transfected alone in cultured cerebellar granule cells. In contrast, mGluR5 was found also in dendrites when coexpressed with Homer1b or Homer1c, and in both dendrites and axons when cotransfected with Homer1a. In dendrites, cotransfected mGluR5 and Homer1b/c formed clusters that colocalized with the synaptic marker synaptophysin. Interestingly when transfected alone, the Homer proteins were also translocated to neurites but did not form such clusters. Depolarization of the neurons with a mixture of ionotropic glutamate receptor agonists, NMDA and kainate, or potassium channel blockers, tetraethylammonium and 4-aminopyridine, induced transient expression of endogenous Homer1a and persistent neuritic localization of transfected mGluR5 even long after degradation of Homer1a. These results suggest that Homer1a/b/c proteins are involved in the targeting of mGluR5 to dendritic synaptic sites and/or axons and that this effect can be regulated by neuronal activity. Because the activity-dependent effect of endogenous Homer1a was also long-lasting, the axonal targeting of mGluR5 by this protein is likely to play an important role in synaptic plasticity.

Role of Ca2+ stores in metabotropic L-glutamate receptor-mediated supralinear Ca2+ signaling in rat hippocampal neurons

The role of metabotropic l-glutamate (mGlu) receptors in supralinear Ca(2+) signaling was investigated in cultured hippocampal cells using Ca(2+) imaging techniques and whole-cell voltage-clamp recording. In neurons, but not glia, global supralinear Ca(2+) release from intracellular stores was observed when the mGlu receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) was combined with elevated extracellular K(+) levels (10.8 mm), moderate depolarization (15-30 mV), or NMDA (3 micrometer). There was a delay (2-8 min) before the stores were fully charged, and the enhancement persisted for a short period (up to 10 min) after removal of the store-loading stimulus. Studies with the mGlu receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine demonstrated that these effects were mediated by activation of the mGlu(5) receptor subtype. The L-type voltage-gated Ca(2+) channel antagonist nifedipine (10 micrometer) substantially reduced responses to DHPG obtained in the presence of elevated extracellular K(+) but not NMDA. This suggests that the Ca(2+) that is required to load the stores can enter either through L-type voltage-gated Ca(2+) channels or directly through NMDA receptors. The findings that both depolarization and NMDA receptor activation can facilitate mGlu receptor Ca(2+) signaling adds considerable flexibility to the processes that underlie activity-dependent changes in synaptic strength. In particular, a temporal separation between the store-loading stimulus and the activation of mGlu receptors could be used as a recency detector in neurons.

Long-term potentiation in the presence of NMDA receptor antagonist arylalkylamine spider toxins

The role of the NMDA receptor (NMDAR) in long-term potentiation (LTP) is now well established. All potent NMDAR antagonists known to date inhibit the induction of LTP at the Schaffer collateral-CA1 pyramidal cell synapse in rat hippocampus, regardless of their site and mechanism of action. Arylalkylamine toxins are noncompetitive NMDAR antagonists in the mammalian central nervous system (CNS). The synthetic toxins argiotoxin-636 (Arg-636), Joro spider toxin (JSTX-3), alpha-agatoxin-489 and -505 (Agel-489 and Agel-505) and philanthotoxin-433 (delta-PhTX) were found in the present study to have no effect on the induction of LTP in the Schaffer collateral-CA1 pyramidal cell pathway in rat hippocampal slices maintained in vitro. Arylalkylamine toxins represent a class of potent NMDAR antagonists that fail to affect hippocampal LTP, and thus provide novel structural leads for the development of NMDAR antagonists that do not impair cognition.

G-protein-independent signaling by G-protein-coupled receptors

Two classes of receptors transduce neurotransmitter signals: ionotropic receptors and heptahelical metabotropic receptors. Whereas the ionotropic receptors are structurally associated with a membrane channel, a mediating mechanism is necessary to functionally link metabotropic receptors with their respective effectors. According to the accepted paradigm, the first step in the metabotropic transduction process requires the activation of heterotrimeric G-proteins. An increasing number of observations, however, point to a novel mechanism through which neurotransmitters can initiate biochemical signals and modulate neuronal excitability. According to this mechanism metabotropic receptors induce responses by activating transduction systems that do not involve G-proteins.

Hypoxic modulation of L-type Ca(2+) channels in inspiratory brainstem neurones: intracellular signalling pathways and metabotropic glutamate receptors

Brief hypoxia (2 min) enhances the activity of L-type Ca(2+) (Ca(L)) channels. The effect is due to glutamate release and concomitant stimulation of metabotropic glutamate receptors of the mGLUR1/5 type [22] [S.L. Mironov, D.W. Richter, L-type Ca(2+) channels in inspiratory neurones and their modulation by hypoxia, J. Physiol. 512 (1998) 75-87.]. Besides increasing single channel activity, hypoxia induces a negative shift of the activation curve and slows down the inactivation of the Ca(L) current. In the present study we investigated these effects further, aiming to reveal intracellular signalling pathways that mediate the coupling between mGLURs and Ca(L) channels. Channel activity was recorded in cell-attached patches from inspiratory brainstem neurones of neonatal mice (P6-11). Ca(L) channels were inhibited by the mGluR2/3 agonists. mGluR1/5 agonists accelerated and mGluR2/3 agonists suppressed the respiratory output, and correspondingly modified the hypoxic response of the respiratory center. Ca(L) channels were also modulated by protein kinase C, but this did not prevent the hypoxic modification of channel activity. G-protein activators enhanced and G-protein inhibitors suppressed the Ca(L) channel activity, and in the presence of these agents the effects of hypoxia were abolished. Ryanodine but not thapsigargin inhibited the channel activity and occluded the hypoxic potentiation. Only G-protein-specific agents and ryanodine prevented the slowing down of inactivation induced by hypoxia. Our data indicate that coupling between mGluR1/5 and Ca(L) channels is mediated by pathways that utilize G-proteins and ryanodine receptors. Glutamate release and concomitant activation of Ca(L) channels are responsible for accelerating of respiratory rhythm during early hypoxia.

DHPG-induced LTD in area CA1 of juvenile rat hippocampus; characterisation and sensitivity to novel mGlu receptor antagonists

We have used extracellular microelectrode recording to characterise a form of long-term depression (LTD) of synaptic transmission that can be induced by metabotropic glutamate (mGlu) receptor activation in the CA1 region of the young (12-18 day old) rat hippocampus. Activation of group I mGlu receptors by the specific agonist 3,5-dihydroxyphenylglyine (DHPG) induced LTD of field excitatory postsynaptic potentials (fEPSPs). The mGlu5 selective agonist 2-chloro-5-hydroxyphenylglycine was also capable of inducing LTD. In contrast, the group II specific agonist DCG-IV had no effect on synaptic transmission, whilst the group III receptor agonist (S)-2-amino-4-phosphonobutyrate elicited a depression that reversed fully upon agonist washout. DHPG-induced LTD could still be generated after prior saturation of electrically-induced NMDA receptor-dependent LTD. DHPG-induced LTD was reversed by tetanic stimulation comprising 100 shocks delivered at 100 Hz. A novel mGlu receptor antagonist, (RS)-2-amino-2-(3-cis and trans-carboxycyclobutyl-3-(9-thioxanthyl)propionic acid) (LY393053) that potently inhibits mGlu1 and mGlu5 receptors, prevented the induction of DHPG-induced LTD. Like other mGlu receptor antagonists, LY393053 also reversed pre-established DHPG-induced LTD. In contrast, a potent mGlu1 selective antagonist (S)-2-methyl-4-carboxyphenylglycine (LY367385) did not prevent the induction of DHPG-induced LTD. In conclusion, DHPG, probably via activation of mGlu5 receptors, is able to induce a robust form of LTD in the CA1 region of the young rat hippocampus that is mechanistically distinct from NMDA receptor-dependent homosynaptic LTD.