Subsynaptic segregation of metabotropic and ionotropic glutamate receptors as revealed by immunogold localization

Localization of diacylglycerol lipase-alpha around postsynaptic spine suggests close proximity between production site of an endocannabinoid, 2-arachidonoyl-glycerol, and presynaptic cannabinoid CB1 receptor

2-arachidonoyl-glycerol (2-AG) is an endocannabinoid that is released from postsynaptic neurons, acts retrogradely on presynaptic cannabinoid receptor CB1, and induces short- and long-term suppression of transmitter release. To understand the mechanisms of the 2-AG-mediated retrograde modulation, we investigated subcellular localization of a major 2-AG biosynthetic enzyme, diacylglycerol lipase-alpha (DAGLalpha), by using immunofluorescence and immunoelectron microscopy in the mouse brain. In the cerebellum, DAGLalpha was predominantly expressed in Purkinje cells. DAGLalpha was detected on the dendritic surface and occasionally on the somatic surface, with a distal-to-proximal gradient from spiny branchlets toward somata. DAGLalpha was highly concentrated at the base of spine neck and also accumulated with much lower density on somatodendritic membrane around the spine neck. However, DAGLalpha was excluded from the main body of spine neck and head. In hippocampal pyramidal cells, DAGLalpha was also accumulated in spines. In contrast to the distribution in Purkinje cells, DAGLalpha was distributed in the spine head, neck, or both, whereas somatodendritic membrane was labeled very weakly. These results indicate that DAGLalpha is essentially targeted to postsynaptic spines in cerebellar and hippocampal neurons, but its fine distribution within and around spines is differently regulated between the two neurons. The preferential spine targeting should enable efficient 2-AG production on excitatory synaptic activity and its swift retrograde modulation onto nearby presynaptic terminals expressing CB1. Furthermore, different fine localization within and around spines suggests that the distance between postsynaptic 2-AG production site and presynaptic CB1 is differentially controlled depending on neuron types.

Postsynaptic inositol 1,4,5-trisphosphate signaling maintains presynaptic function of parallel fiber-Purkinje cell synapses via BDNF

The maintenance of synaptic functions is essential for neuronal information processing, but cellular mechanisms that maintain synapses in the adult brain are not well understood. Here, we report an activity-dependent maintenance mechanism of parallel fiber (PF)-Purkinje cell (PC) synapses in the cerebellum. When postsynaptic metabotropic glutamate receptor (mGluR) or inositol 1,4,5-trisphosphate (IP(3)) signaling was chronically inhibited in vivo, PF-PC synaptic strength decreased because of a decreased transmitter release probability. The same effects were observed when PF activity was inhibited in vivo by the suppression of NMDA receptor-mediated inputs to granule cells. PF-PC synaptic strength similarly decreased after the in vivo application of an antibody against brain-derived neurotrophic factor (BDNF). Furthermore, the weakening of synaptic connection caused by the blockade of mGluR-IP(3) signaling was reversed by the in vivo application of BDNF. These results indicate that a signaling cascade comprising PF activity, postsynaptic mGluR-IP(3) signaling and subsequent BDNF signaling maintains presynaptic functions in the mature cerebellum.

Cellular and subcellular localization of neurokinin-1 and neurokinin-3 receptors in primate globus pallidus

The primate globus pallidus receives massive innervations from GABAergic striatal neurons that co-release the neuropeptide substance P (SP). To expand our knowledge regarding SP interaction at pallidal level, we used single and double antigen retrieval methods to study the cellular and subcellular localization of SP and its high-affinity receptors neurokinin-1 (NK-1R) and neurokinin-3 (NK-3R) in the globus pallidus of the squirrel monkey (Saimiri sciureus). At the light microscopic level, a large number of neurons and fibers located in both the external (GPe) and internal (GPi) segments of the globus pallidus expressed NK-1R or NK-3R immunoreactivity. At the electron microscopic level, both NK-1R and NK-3R were mainly associated with intracellular sites or located at extrasynaptic positions on the plasma membrane. Presynaptic axon terminals forming symmetric and asymmetric synapses occasionally contained NK-1R and NK-3R. Neurokinin receptors were also observed in a proportion of SP-immunoreactive axon terminals, but these terminals preferentially expressed NK-3R. The pattern of distribution of NK-1R and NK-3R in GPe and GPi indicates that SP effects at pallidal level are mediated through postsynaptic receptor as well as presynaptic autoreceptors and heteroreceptors. These morphological data suggest that, either alone or in conjunction with GABA, SP could have a wide range of effects at pallidal level. This neuroactive peptide may influence in a significant manner the integration and treatment of neural information that flows through the basal ganglia.

Novel subcellular distribution pattern of A-type K+ channels on neuronal surface

Potassium channels comprise the most diverse family of ion channels. In nerve cells, their critical roles in synaptic integration and output generation have been demonstrated. Here, we provide evidence for a distribution that predicts a novel role of K+ channels in the CNS. Our experiments revealed a highly selective clustering of the Kv4.3 A-type K+ channel subunits at specialized junctions between climbing fibers and cerebellar GABAergic interneurons. High-resolution ultrastructural and immunohistochemical experiments demonstrated that these junctions are distinct from known chemical and electrical (gap junctions) synapses and also from puncta adherentia. Each cerebellar interneuron contains many such K+ channel-rich specializations, which seem to be distributed throughout the somatodendritic surface. We also show that such K+ channel-rich specializations are not only present in the cerebellum but are widespread in the rat CNS. For example, mitral cells of the main olfactory bulb establish Kv4.2 subunit-positive specializations with each other. At these specializations, both apposing membranes have a high density of K+ channels, indicating bidirectional signaling. Similar specializations with pronounced coclustering of the Kv4.2 and 4.3 subunits were observed between nerve cells in the medial nucleus of the habenula. Based on our results and on the known properties of A-type K+ channels, we propose that strategically clustered K+ channels at unique membrane specializations could mediate a novel type of communication between nerve cells.

The scaffold protein Homer1b/c links metabotropic glutamate receptor 5 to extracellular signal-regulated protein kinase cascades in neurons

Group I metabotropic glutamate receptors (mGluRs) increase cellular levels of inositol-1,4,5-triphosphate (IP3) and thereby trigger intracellular Ca2+ release. Also, group I mGluRs are organized with members of Homer scaffold proteins into multiprotein complexes involved in postreceptor signaling. In this study, we investigated the relative importance of the IP3/Ca2+ signaling and novel Homer proteins in group I mGluR-mediated activation of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) in cultured rat striatal neurons. We found that selective activation of mGluR5, but not mGluR1, increased ERK1/2 phosphorylation. Whereas the IP3/Ca2+ cascade transmits a small portion of signals from mGluR5 to ERK1/2, the member of Homer family Homer1b/c forms a central signaling pathway linking mGluR5 to ERK1/2 in a Ca2+-independent manner. This was demonstrated by the findings that the mGluR5-mediated ERK1/2 phosphorylation was mostly reduced by a cell-permeable Tat-fusion peptide that selectively disrupted the interaction of mGluR5 with the Homer1b/c and by small interfering RNAs that selectively knocked down cellular levels of Homer1b/c proteins. Furthermore, ERK1/2, when only coactivated by both IP3/Ca2+- and Homer1b/c-dependent pathways, showed the ability to phosphorylate two transcription factors, Elk-1 and cAMP response element-binding protein, and thereby facilitated c-Fos expression. Together, we have identified two coordinated signaling pathways (a conventional IP3/Ca2+ vs a novel Homer pathway) that differentially mediate the mGluR5-ERK coupling in neurons. Both the Ca2+-dependent and -independent pathways are corequired to activate ERK1/2 to a level sufficient to achieve the mGluR5-dependent synapse-to-nucleus communication imperative for the transcriptional regulation.

Differential expression of NMDA and AMPA/KA receptor subunits in the inferior olive of postnatal rats

We have employed immunohistochemistry to determine the expression patterns of receptor subunits of N-methyl-d-aspartate (NMDA-NR1 and NR2A/B) and alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid/kainic acid (AMPA/KA-GluR1, GluR2, GluR2/3, GluR4, and GluR5/6/7) in the inferior olive of postnatal rats up to adulthood. Immunoreactivity for distinct receptor subunits was predominantly localized in the soma and dendrites of neurons. Semi-quantification showed that the overall immunoreactivity in the inferior olive of adults was intense for GluR1, moderate for NR1 and NR2A/B, and low for GluR2, GluR2/3, GluR4, and GluR5/6/7. At P7, GluR1 was restricted to the dorsomedial cell column, subnucleus beta, principal nucleus and ventrolateral protrusion while the other subunits were found in all subnuclei of the inferior olive. The immunoreactivities for all glutamate receptor subunits ranged from low to moderate. As the rats matured, the immunoreactivity of GluR4 decreased after the second postnatal week, while those of the other subunits showed a general trend of increase, reaching adult level during the third postnatal week. Double immunofluorescence revealed that all NR1-containing neurons exhibited NR2A/B immunoreactivity, indicating that native NMDA receptors comprise of hetero-oligomeric combinations of NR1 and NR2A/B. Furthermore, co-localization of NMDA and AMPA/KA receptor subunits was demonstrated in individual neurons of the inferior olive. All NR1-containing neurons exhibited GluR1 immunoreactivity, and all NR2A/B-containing neurons showed GluR5/6/7 immunoreactivity. Our data suggest that NMDA and AMPA/KA receptors are involved in glutamate-mediated neurotransmission, contributing to synaptic plasticity and reorganization of circuitry in the inferior olive during postnatal development.

Postnatal development and synapse elimination of climbing fiber to Purkinje cell projection in the cerebellum

Cerebellar climbing fiber (CF) to Purkinje cell (PC) synapses in rodents provides a good model to study mechanisms underlying postnatal development of synaptic functions and elimination of redundant synapses in the central nervous system. At birth, each PC is innervated by multiple CFs. Then, single CF input is selected, matured and strengthened, while surplus CFs are eliminated. By the end of the third postnatal week, most PCs become innervated by single CFs. This up-date article aims to provide an overview of recent studies on the mechanisms of this process.

A novel Ca2+-independent signaling pathway to extracellular signal-regulated protein kinase by coactivation of NMDA receptors and metabotropic glutamate receptor 5 in neurons

The specification and organization of glutamatergic synaptic transmission require the coordinated interaction among glutamate receptors and their synaptic adaptor proteins closely assembled in the postsynaptic density (PSD). Here we investigated the interaction between NMDA receptors and metabotropic glutamate receptor 5 (mGluR5) in the integral regulation of extracellular signal-regulated protein kinase (ERK) and gene expression in cultured rat striatal neurons. We found that coapplication of NMDA and the mGluR5 agonist (S)-3,5-dihydroxyphenylglycine synergistically increased ERK phosphorylation. Interestingly, the synergistic increase in ERK phosphorylation was dependent on the cross talk between NMDA receptor-associated synaptic adaptor protein PSD-95 and the mGluR5-linked adaptor protein Homer1b/c but not on the conventional Ca2+ signaling derived from NMDA receptors (Ca2+ influx) and mGluR5 (intracellular Ca2+ release). This was demonstrated by the findings that the synergistic phosphorylation of ERK induced by coactivation of NMDA receptors and mGluR5 was blocked by either a Tat peptide that disrupts NMDA receptor/PSD-95 binding or small interfering RNAs that selectively reduce cellular levels of Homer1b/c. Furthermore, ERK activated through this PSD-95/Homer1b/c-dependent and Ca2+-independent pathway was able to phosphorylate the two key transcription factors Elk-1 and cAMP response element-binding protein, which further leads to facilitation of c-Fos expression. Together, we have identified a novel Ca2+-independent signaling pathway to ERK by the synergistic interaction of NMDA receptors and mGluR5 via their adaptor proteins in the PSD of neurons, which underlies a synapse-to-nucleus communication important for the transcriptional regulation.

Pharmacology of the metabotropic glutamate receptor mediated current at the climbing fiber to Purkinje cell synapse

Different forms of synaptic plasticity in the cerebellum are mediated by metabotropic glutamate receptors (mGluRs). At parallel fiber (PF) to Purkinje cell (PC) synapses activation of mGluR gives rise to a well known slow synaptic current inhibited by antagonists of mGluR1. The distribution of mGluR types in the climbing fiber (CF) to PC synapses is not well known. However, a mGluR1alpha-mediated all-or-none postsynaptic current was also demonstrated at the CF-PC synapse (Dzubay and Otis, Neuron 36, 1159, 2002). Using whole cell patch-clamp recording from PCs in rat cerebellar slices with AMPA receptors blocked and glutamate uptake impaired we demonstrate a more complex pharmacology of a current obtained by single or train CF stimulation. The mGluR1 specific antagonist CPCCOEt in a group of cells suppressed this response while in a similar number of other cells it induced a potentiating effect. The antagonists of mGluR groups II and III (LY341495 and MSOP, respectively) predominantly suppressed the current. The ambiguous effect of CPCCOEt was checked by measuring the paired-pulse depression of the CF EPSC, which was not changed with the antagonist in normal as well as in low (0.5 mM) external Ca(2+) (used to prevent saturation of AMPARs), thus excluding a presynaptic effect. However, CPCCOEt induced a rise in the amplitude (by approximately 50%) as well as a prolongation (p<0.05) of the decay time of CF EPSCs at normal 2 mM Ca(2+), i.e. under conditions of AMPAR saturation, thus indicating an effect of postsynaptic origin. In 0.5 mM Ca(2+) the decay of CF EPSCs was longer but it was also significantly prolonged (p?0.01) by CPCCOEt. However, the CF EPSC amplitude was not significantly affected indicating an underlying Ca(2+)-dependent mechanism. Thus, the pharmacology of the PC mGluR-mediated response points to a dual postsynaptic role of mGluR1 giving rise to a slow postsynaptic current but also regulating other presumably mGluR-dependent currents via second messenger molecules and Ca(2+). The additional electrophysiological role of mGluR II & III types was also indicated. Such a complex regulatory mechanism may have an important role in the mGluR-dependent forms of homosynaptic plasticity and motor learning at the CF-PC synapse.

Cross talk between metabotropic and ionotropic glutamate receptor-mediated signaling in parallel fiber-induced inositol 1,4,5-trisphosphate production in cerebellar Purkinje cells

In many excitatory glutamatergic synapses, both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are closely distributed on the postsynaptic membrane. However, the functional significance of the close distribution of the two types of glutamate receptors has not been fully clarified. In this study, we examined the functional interaction between iGluR and mGluR at parallel fiber (PF)--> Purkinje cell synapses in the generation of inositol 1,4,5-trisphosphate (IP3), a key second messenger that regulates many important cellular functions. We visualized local IP3 dynamics in Purkinje cells using the green fluorescent protein-tagged pleckstrin homology domain (GFP-PHD) as a fluorescent IP3 probe. Purkinje cells were transduced with Sindbis virus encoding GFP-PHD and imaged with a two-photon laser scanning microscope. Translocation of GFP-PHD from the plasma membrane to the cytoplasm attributable to an increase in IP3 concentration was observed on PF stimulation in fine dendrites of Purkinje cells. Surprisingly, this PF-induced IP3 production was blocked not only by the group I mGluR antagonist but also by the AMPA receptor (AMPAR) antagonist. The PF-induced IP3 production was blocked by either the inhibition of G-protein activation by GDP-betaS or intracellular Ca2+ buffering by BAPTA. These results show that IP3 production is mediated cooperatively by group I mGluR and AMPAR through G-protein activation and Ca2+ influx at PF--> Purkinje cell synapses, identifying the robust cross talk between iGluR and mGluR for the generation of IP3 signals.

Sampling glutamate and GABA with microdialysis: suggestions on how to get the dialysis membrane closer to the synapse

Microdialysis is currently optimized to sample the extrasynaptic pool. As such, the technique has facilitated discovery of ischemia-induced excitotoxic glutamate overflow (Benveniste H, Drejer J, Schousboe A, Diemer NH, 1987, Regional cerebral glucose phosphorylation and blood flow after insertion of a microdialysis fiber through the dorsal hippocampus in the rat. J. Neurochem., 49, 729-734) and adenosinergic sleep drive (Porkka-Heiskanen T, Strecker RE, Thakkar M, Bjorkum AA, Greene RW, McCarley RW, 1997, Adenosine: a mediator of the sleep-inducing effects of prolonged wakefulness. Science, 276 (5316), 1265-1268); and is proving essential for clinical monitoring of glutamate and cellular metabolites in stroke and head trauma (Sarrafzadeh AS, Sakowitz OW, Kiening KL, Benndorf G, Lanksch WR, Unterberg AW. Bedside microdialysis: a tool to monitor cerebral metabolism in subarachnoid hemorrhage patients? Crit. Care Med. 2002, 30 (5): 1062-1070). Study of the origin of extrasynaptic glutamate sampled with microdialysis has advanced understanding of extrasynaptic signal processing (Baker DA, Xi ZX, Shen H, Swanson CJ, Kalivas PW. The origin and neuronal function of in vivo nonsynaptic glutamate. J. Neurosci. 2002, 22 (20): 9134-9141; Baker DA, McFarland K, Lake RW, Shen H, Tang XC, Toda S, Kalivas PW, 2003, Neuroadaptations in cystine-glutamate exchange underlie cocaine relapse. Nat. Neurosci., 6, 743-749) in the CNS. Microdialysis studies furthermore demonstrate that synaptic pools of some neurotransmitters spill into the extrasynaptic space. For this reason, microdialysis has provided a window into the synaptic pool that has significantly advanced understanding of neurotransmitter control of behavior (Tanda G, Pontieri FE, Di Chiara G, 1997, Cannabinoid and heroin activation of mesolimbic dopamine transmission by a common mu1 opioid receptor mechanism. Science, 276, 2048-2050). Nonetheless, ability to sample synaptic pools of neurotransmitters is limited. Here we summarize evidence that microdialysis often fails to sample synaptic pools of neurotransmitters, such as glutamate and GABA because of rapid clearance and limited diffusion of these neurotransmitters from the synapse. Moreover, we consider means to move the dialysis membrane closer to the synapse to facilitate sampling of the synaptic pool of these neurotransmitters by minimizing tissue trauma, decreasing probe size and increasing temporal resolution.

Preliminary evidence of attenuation of the disruptive effects of the NMDA glutamate receptor antagonist, ketamine, on working memory by pretreatment with the group II metabotropic glutamate receptor agonist, LY354740, in healthy human subjects

RATIONALE

Some of the behavioral consequences of deficits in N-methyl-D-aspartate (NMDA) glutamate receptor function are thought to arise from the disinhibition of cortical glutamatergic circuitry.

OBJECTIVE

This study evaluated whether pretreatment with a drug that reduces glutamatergic activation, the group II metabotropic glutamate receptor (mGluR) agonist, LY354740, reduced the cognitive effects of the NMDA glutamate receptor antagonist, ketamine, in healthy human subjects.

METHODS

Nineteen healthy human subjects completed 3 test days during which LY354740 (matched placebo, 100 mg, 400 mg) was administered under double-blind conditions 4 h prior to the single-blind intravenous administration of saline and 5.7 h prior to ketamine administration (bolus of 0.26 mg/kg over 1 min, infusion of 0.65 mg/kg per hour for 100 min). Thus on each test day each subject received a single dose of LY354740 (or its matched placebo) and both saline and ketamine infusions.

RESULTS

Ketamine impaired attention, working memory, and delayed recall. It also produced positive and negative symptoms, perceptual changes, and dysphoric mood. LY354740 did not have a significant effect on working memory on the placebo day; however, it produced a significant dose-related improvement in working memory during ketamine infusion.

CONCLUSIONS

These data provide preliminary and suggestive evidence that LY354740 or other group II mGluR agonists might play a role in treating working memory impairment related to deficits in NMDA receptor function.

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

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

Signaling complex formation of phospholipase Cβ4 with metabotropic glutamate receptor type 1α and 1,4,5-trisphosphate receptor at the perisynapse and endoplasmic reticulum in the mouse brain

Upon activation of cell surface receptors coupled to the Gq subclass of G proteins, phospholipase C (PLC) beta hydrolyses membrane phospholipid to yield a pair of second messengers, inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol. PLCbeta4 has been characterized as the isoform enriched in cerebellar Purkinje cells (PCs) and the retina and involved in motor and visual functions. Here we examined cellular and subcellular distributions of PLCbeta4 in adult mouse brains. Immunohistochemistry showed that high levels of PLCbeta4 were detected in the somatodendritic domain of neuronal populations expressing the metabotropic glutamate receptor (mGluR) type 1alpha, including olfactory periglomerular cells, neurons in the bed nucleus anterior commissure, thalamus, substantia nigra, inferior olive, and unipolar brush cells and PCs in the cerebellum. Low to moderate levels were detected in many other mGluR1alpha-positive neurons and in a few mGluR1alpha-negative neurons. In PCs, immunogold electron microscopy localized PLCbeta4 to the perisynapse, at which mGluR1alpha is concentrated, and to the smooth endoplasmic reticulum in dendrites and spines, an intracellular Ca2+ store gated by IP3 receptors. In the cerebellum, immunoblot demonstrated its concentrated distribution in the post-synaptic density and microsomal fractions, where mGluR1alpha and type 1 IP3 receptor were also greatly enriched. Furthermore, PLCbeta4 formed coimmunoprecipitable complexes with mGluR1alpha, type 1 IP3 receptor and Homer 1. These results suggest that PLCbeta4 is preferentially localized in the perisynapse and smooth endoplasmic reticulum as a component of the physically linked phosphoinositide signaling complex. This close molecular relationship might provide PLCbeta4 with a high-fidelity effector function to mediate various neuronal responses under physiological and pathophysiological conditions.

Activation of group I metabotropic glutamate receptors enhances efficacy of glutamatergic inputs to neonatal rat hypoglossal motoneurons in vitro

Group I metabotropic glutamate receptors (mGluRs) are the main class of metabotropic receptors expressed in the hypoglossus nucleus. Their role in glutamatergic transmission was investigated using patch-clamp recording from motoneurons in a neonatal rat brainstem slice preparation. After pharmacological block of gamma-aminobutyric acid and glycine-mediated inhibition, under voltage-clamp, the selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) induced a motoneuron inward current by depressing a leak conductance, and strongly facilitated spontaneous glutamatergic synaptic currents. This effect was blocked by 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) and unaffected by 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), indicating a role for subtype 1 mGluRs. The frequency but not the amplitude of miniature glutamatergic currents was also enhanced by DHPG. Currents elicited by puffer application of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the presence of tetrodotoxin were also unchanged, suggesting that DHPG facilitated release of glutamate. Glutamatergic currents evoked by electrically stimulating the dorsomedullary reticular column premotoneurons were, however, depressed by DHPG in a CPCCOEt-sensitive fashion. Neither CPCCOEt nor MPEP per se changed glutamatergic transmission. Under current-clamp, even if DHPG depressed excitatory postsynaptic potentials, motoneuron spike threshold and time to peak were reduced so that facilitation of synaptic potential/spike coupling became apparent. We propose a wiring diagram to account for the differential action by DHPG on spontaneous and evoked transmission, based on the discrete distribution of subtype 1 mGluRs on glutamatergic afferents. Although under standard recording conditions there was insufficient ambient glutamate to activate mGluRs, such receptors were a powerful target to upregulate excitatory synaptic transmission and enhance signalling by hypoglossal motoneurons to tongue muscles.

Calcium dependence of native metabotropic glutamate receptor signaling in central neurons

Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors that are distributed throughout the brain and play important roles in regulation of synaptic efficacy. Some studies report that mGluRs heterologously expressed in nonneuronal cells are sensitive not only to glutamate but also to extracellular Ca2+ (Ca2+o). We studied the Ca2+o-sensitivity of native mGluRs in mammalian central neurons. In cerebellar Purkinje cells that naturally express type-1 mGluR (mGluR1), physiological levels of Ca2+o (around 2 mM) activate mGluR1-mediated intracellular Ca2+ mobilization. The activation of the native mGluR1 response to Ca2+o appears to be slower than that to glutamate. Ca2+o (2 mM) also augments glutamate analog-evoked, native mGluR1-mediated inward cation current and intracellular Ca2+o mobilization. Detailed analysis of this effect suggests that Ca2+o modulates the glutamate responsiveness of native and heterologously expressed mGluR1s in different manners. These findings suggest that Ca2+o may enhance the basal level and glutamate responsiveness of neuronal mGluR signaling in vivo.

Age-related changes in the expression of axonal and glial group I metabotropic glutamate receptor in the rat substantia nigra pars reticulata

Neuronal systems undergo many significant changes during the course of brain development. To characterize the developmental changes in the substantia nigra pars reticulata (SNr) associated with the expression of group I metabotropic glutamate receptors (mGluRs), we used the immunoperoxidase and immunogold methods at the electron microscope level to determine whether the subcellular and subsynaptic patterns of distribution of mGluR1a and mGluR5 differ between young (P14-P18) and adult (>2 months) rats. The SNr of young rats contained a significantly higher density of labeled unmyelinated axons for both receptor subtypes. In addition, mGluR5-immunoreactive glial processes were very abundant in young rats but absent in the adults. On the other hand, the relative proportion of immunoreactive dendrites was the same for both age groups. Analysis of immunogold-labeled rat SNr revealed similar proportions of plasma membrane-bound mGluR1a and mGluR5 in adult (59.8 and 19.4%, respectively) and young (60.6 and 18.4%, respectively) rats. The pattern of subsynaptic localization of mGluR1a also remained the same between young and adults. However, the proportion of extrasynaptic mGluR5 decreased, whereas proportions of gold particles associated with symmetric synapses increased in adults. The results of this study demonstrate significant differences in the expression of group I mGluRs in the SNr of young and adult rats. These findings support a role for group I mGluRs during development and emphasize the importance of using brain tissue from age-matched subjects when attempting to correlate functional data from young rat brain slices with immunocytochemical localization of group I mGluRs.

Subcellular and subsynaptic localization of group I metabotropic glutamate receptors in the monkey subthalamic nucleus

Both subtypes of group I metabotropic glutamate receptor, mGluR1 and mGluR5, are expressed postsynaptically in neurons of the subthalamic nucleus (STN), and their activation induces different physiological responses. To test whether these effects could be explained by a differential localization of the two group I mGluRs, we analyzed the subcellular and subsynaptic distribution of mGluR1a and mGluR5 in the monkey STN. Double-immunofluorescence and light microscopic analyses revealed that both group I mGluR subtypes were strongly coexpressed in the neuropil and neuronal perikarya. Astrocytic perikarya exhibited intense mGluR1a, but no detectable mGluR5, immunoreactivity. At the electron microscopic level, immunoperoxidase labeling for both mGluR1a and mGluR5 was localized mainly in dendrites. A significant proportion of the total pool of mGluR1a-immunoreactive elements was accounted for by glial cell processes, whereas glial cell labeling was much less frequently encountered in sections immunostained for mGluR5. Preembedding immunogold labeling in STN dendrites revealed that 60-70% of the gold labeling for both mGluR subtypes was intracellular, whereas 30-40% was apposed to the plasma membrane. Of the plasma membrane-apposed particles, more than 90% were extrasynaptic; fewer than 10% were associated with symmetric or asymmetric synapses. Most of the synapse-associated labeling was found at the edges of both asymmetric and symmetric postsynaptic specializations. Some extrasynaptic gold particles were aggregated on parts of the plasma membrane tightly apposed by glial processes. These findings demonstrate that mGluR1a and mGluR5 exhibit a similar pattern of subsynaptic localization in monkey STN neurons, with both receptor subtypes exhibiting substantial extrasynaptic and perisynaptic localization.

Excitation of cerebellar interneurons by group I metabotropic glutamate receptors

Cerebellar basket and stellate neurons (BSNs) provide feed-forward inhibition to Purkinje neurons (PNs) and thereby play a principal role in determining the output of the cerebellar cortex. During low-frequency transmission, glutamate released at parallel fiber synapses excites BSNs by binding to AMPA receptors; high-frequency transmission also recruits N-methyl-d-aspartate (NMDA) receptors. We find that, in addition to these ligand-gated receptors, a G-protein-coupled glutamate receptor subtype participates in exciting BSNs. Stimulation of metabotropic glutamate receptor 1alpha (mGluR1alpha) with the mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) leads to an increase in spontaneous firing of BSNs and indirectly to an increase in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded in PNs. Under conditions in which ligand-gated glutamate receptors are blocked, parallel fiber stimulation generates a slow excitatory postsynaptic current (EPSC) in BSNs that is inhibited by mGluR1alpha-selective antagonists. This slow EPSC is capable of increasing BSN spiking and indirectly increasing sIPSCs frequency in PNs. Our findings reinforce the idea that distinct subtypes of glutamate receptors are activated in response to different patterns of activity at excitatory synapses. The results also raise the possibility that mGluR1alpha-dependent forms of synaptic plasticity may occur at excitatory inputs to BSNs.

Loss of GLUR2 alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor subunit differentially affects remaining synaptic glutamate receptors in cerebellum and cochlear nuclei

The alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) type of ionotropic glutamate receptor is the major mediator of fast neurotransmission in the brain and spinal cord. Most AMPA receptors are impermeable to calcium because they contain the GluR2 subunit. However, some AMPA receptors lack GluR2 and pass calcium which can mediate synaptic plasticity and, in excess, neurotoxicity. Previously, we showed a decrease in the density of synaptic AMPA receptors in the hippocampus of mice lacking GluR2. In this study, using these GluR2-lacking mice, we examined other areas of the brain that differ in the amount of GluR2 normally present. Like hippocampal spines, cerebellar Purkinje spines normally express AMPA receptors with high GluR2 and showed a decrease in synaptic AMPA receptors in mutant mice. In contrast, neurons that normally express AMPA receptors with little or no GluR2, such as in the anteroventral cochlear nucleus, showed no decrease in AMPA receptors and even showed an increase in one AMPA receptor subunit. These two different patterns may relate to preadaptations to prevent calcium neurotoxicity; such mechanisms might be absent in Purkinje and hippocampal spines so that these neurons must decrease their total expression of synaptic AMPA receptors (calcium permeable in mutant mice) to prevent calcium neurotoxicity. In addition, we found that another glutamate receptor, GluRdelta2, which is abundant only in parallel fibre synapses on Purkinje cells and in the dorsal cochlear nucleus, is up-regulated at these synapses in mutant mice; this probably reflects some change in GluRdelta2 targeting to these synapses.

Group II and III metabotropic glutamate receptors and the control of the nucleus reticularis thalami input to rat thalamocortical neurones in vitro

Intracellular recordings were made from neurones in the thalamic reticular nucleus (TRN) and ventro-basal (VB) thalamus in slices of rat midbrain in vitro. Electrical stimulation of the medial lemniscus or TRN resulted in the generation of complex synaptic potentials containing disynaptic inhibitory post-synaptic potentials (IPSPs) in VB thalamocortical neurones. Analysis of the excitatory synaptic responses in TRN neurones indicates they can produce burst output response irrespective of the level of sub-threshold membrane potential. This suggests that network-evoked IPSPs in VB thalamocortical neurones occur following a burst of TRN action potentials. Using ionotropic glutamate receptor antagonists, the activation of these disynaptic events was blocked, and the monosynaptic IPSPs that resulted from the direct activation of the TRN could be isolated. The selective Group II agonists LY354740 (1-10 microM) and N-acetyl-aspartyl-glutamate (NAAG; 100-500 microM) both caused a reversible depression of these monosynaptic TRN IPSPs without any effect on membrane potential or input resistance. Likewise, the specific Group III agonist L-2-amino-4-phosphonobutanoate (10-500 microM), but not (RS)-4-phosphonophenylglycine (1 and 30 microM) also caused a reversible depression of these IPSPs, again without any effect on membrane potential or input resistance.Thus, the IPSPs recorded in VB thalamocortical neurones, evoked by TRN activation, can be depressed by the activation of either Group II or III metabotropic glutamate receptors. This is consistent with the location of these receptor types on the presynaptic terminals of TRN axons in the VB thalamus. This raises the possibility that, during periods of intense excitatory activity, glutamate release could influence the release of GABA from TRN axon terminals in the thalamus. In addition, as NAAG is located in the axons and terminals arising from the TRN, there is the possibility that this dipeptide is also released by these terminals to control the release of GABA during periods of high activity in the TRN.

Group i metabotropic glutamate receptors in spiral ganglion neurons contribute to excitatory neurotransmissions in the cochlea

Evidence has accumulated over the years supporting glutamate as the primary neurotransmitter used by hair cells in afferent cochlear neurotransmission. Besides acting on ionotropic glutamate receptors, glutamate also activates second messenger systems via G-protein-coupled metabotropic glutamate receptors (mGluRs) to modulate neuronal excitability. However, it is unclear whether mGluRs participate in cochlear neurotransmission. We present evidence directly supporting a functional role for group I metabotropic glutamate receptors (mGluRIs) in spiral ganglion (SG) neurons. The presence of mGluRI and downstream G-protein subunits was demonstrated by molecular biology and immunolabeling methods. Direct activation of mGluRIs in cultured SG neurons resulted in transient increases of intracellular Ca(++) concentration and transient inward currents that gave rise to firings of multiple action potentials. These responses showed mGluRI pharmacological specificity and quickly desensitized. We next examined changes in cochlear function after noise exposure as a result of pharmacologically manipulating cochlear glutamate neurotransmission. These in vivo tests showed that blocking non-N-methyl-D-aspartic acid glutamate receptors was sufficient to eliminate compound action potentials of the auditory nerve, and pharmacologically inhibiting mGluRIs in the cochlea did not significantly affect the hearing threshold. In contrast, blocking mGluRIs lowered the amplitude of compound action potentials at louder sound levels and reduced the noise-induced temporary threshold shift. Our results suggest that although mGluRIs did not initiate fast excitatory cochlear neurotransmission, their activation contributed to the growth of excitatory responses of the cochlea. As a result, the cochlea was more resistant to noise-induced temporary hearing losses without the activation of mGluRIs in SG neurons.

Active surface transport of metabotropic glutamate receptors through binding to microtubules and actin flow

Receptors for neurotransmitters are concentrated and stabilized at given sites such as synapses through interactions with scaffolding proteins and cytoskeletal elements. The transport of receptors first involves directed vesicular trafficking of intracellularly stored receptors followed by their targeting to the plasma membrane. Once expressed at the cell surface, receptors are thought to reach their final location by random Brownian diffusion in the plasma membrane plane. Here, we investigate whether the metabotropic glutamate receptor mGluR5 can also be transported actively on the cell surface. We used single particle tracking to follow mGluR5 movement in real time at the surface of neuronal growth cones or fibroblast lamellipodia, both of which bear a particularly active cytoskeleton. We found that after a certain lag time mGluR5 undergoes directed rearward transport, which depends on actin flow. On actin depolymerization, directed movement was suppressed, but receptors still bound to a rigid structure. By contrast, receptor transport and immobilization was fully suppressed by microtubule depolymerization but favored by microtubule stabilization. Furthermore, mGluR5 could be immunoprecipitated with tubulin from rat brains, confirming the ability of mGluR5 to bind to microtubules. We propose that mGluR5 can be transported on the cell surface through actin-mediated retrograde transport of microtubules. This process may play a role in receptor targeting and organization during synapse formation or during glutamate-mediated growth cone chemotaxis.

Group I Metabotropic Glutamate Receptors Bind to Protein Phosphatase 1C

The modulation of neurotransmitter receptors by kinases and phosphatases represents a key mechanism in controlling synaptic signal transduction. However, molecular determinants involved in the specific targeting and interactions of these enzymes are largely unknown. Here, we identified both catalytic gamma-isoforms of protein phosphatase 1C (PP1gamma1 and PP1gamma2) as binding partners of the group I metabotropic glutamate receptors type 1a, 5a, and 5b in yeast cells and pull-down assays, using recombinant and native protein preparations. The tissue distribution of interacting proteins was compared, and protein phosphatase 1C was detected in dendrites of retinal bipolar cells expressing the respective interacting glutamate receptors. We mapped interacting domains within binding partners and identified five amino acids in the intracellular C termini of the metabotropic glutamate receptors type 1a, 5a, 5b, and 7b being both necessary and sufficient to bind protein phosphatase 1C. Furthermore, we show a dose-dependent competition of these C termini in binding the enzyme. Based on our data, we investigated the structure of the identified amino acids bound to protein phosphatase 1C by homology-based molecular modeling. In summary, these results provide a molecular description of the interaction between protein phosphatase 1C and metabotropic glutamate receptors and thereby increase our understanding of glutamatergic signal transduction.

NCAM180 and glutamate receptor subtypes in potentiated spine synapses: an immunogold electron microscopic study

Activity-dependent changes in expression and localization of the largest major isoform of the neural cell adhesion molecule NCAM180 and three subtypes of glutamate receptors predominantly expressed in the outer part of the molecular layer of the dentate gyrus of adult rats-the NMDA receptor NR2A, the AMPA receptor GluR2/3, and the metabotropic glutamate receptor mGluR1 - were investigated using postembedding immunogold labeling, and electron microscopy. In synaptic membranes of nonstimulated spine synapses, NCAM180 and NR2A accumulated in the center of the postsynaptic density, whereas GluR2/3 and mGluR1 were distributed evenly. Twenty-four hours following induction of long-term potentiation in vivo, NCAM180 and NR2A accumulated at the edges of postsynaptic densities, whereas GluR2/3 was localized more centrally. Also, the distribution of gold particles per synapse significantly changed for NCAM180, NR2A, and mGluR1. Thus, changes in synaptic strength are associated with concomitant changes in the expression and distribution of NCAM180 and glutamate receptors, particularly of the NR2A subtype.

Activation of the TRPC1 cation channel by metabotropic glutamate receptor mGluR1

Group I metabotropic glutamate receptors (consisting of mGluR1 and mGluR5) are G-protein-coupled neurotransmitter receptors that are found in the perisynaptic region of the postsynaptic membrane. These receptors are not activated by single synaptic volleys but rather require bursts of activity. They are implicated in many forms of neural plasticity including hippocampal long-term potentiation and depression, cerebellar long-term depression, associative learning, and cocaine addiction. When activated, group I mGluRs engage two G-protein-dependent signalling mechanisms: stimulation of phospholipase C and activation of an unidentified, mixed-cation excitatory postsynaptic conductance (EPSC), displaying slow activation, in the plasma membrane. Here we report that the mGluR1-evoked slow EPSC is mediated by the TRPC1 cation channel. TRPC1 is expressed in perisynaptic regions of the cerebellar parallel fibre-Purkinje cell synapse and is physically associated with mGluR1. Manipulations that interfere with TRPC1 block the mGluR1-evoked slow EPSC in Purkinje cells; however, fast transmission mediated by AMPA-type glutamate receptors remains unaffected. Furthermore, co-expression of mGluR1 and TRPC1 in a heterologous system reconstituted a mGluR1-evoked conductance that closely resembles the slow EPSC in Purkinje cells.

Novel treatment of excitotoxicity: targeted disruption of intracellular signalling from glutamate receptors

Glutamate signalling plays key physiological roles in excitatory neurotransmission and CNS plasticity, but also mediates excitotoxicity, the process responsible for triggering neurodegeneration through glutamate receptor overactivation. Excitotoxicity is thought to be a key neurotoxic mechanism in neurological disorders, including brain ischemia, CNS trauma and epilepsy. However, treating excitotoxicity using glutamate receptor antagonists has not proven clinically viable, necessitating more sophisticated approaches. Increasing knowledge of the composition of the postsynaptic density at glutamatergic synapses has allowed us to extend our understanding of the molecular mechanisms of excitotoxicity and to dissect out the distinct signalling pathways responsible for excitotoxic damage. Key molecules in these pathways are physically linked to the cytoplasmic face of glutamate receptors by scaffolding proteins that exhibit binding specificity for some receptors over others. This imparts specificity to physiological and pathological glutamatergic signalling. Recently, we have capitalized on this knowledge and, using targeted peptides to selectively disrupt intracellular interactions linked to glutamate receptors, have blocked excitotoxic signalling in neurones. This therapeutic approach circumvents the negative consequences of blocking glutamate receptors, and may be a practical strategy for treating neurological disorders that involve excitotoxicity.

Postsynaptic modulation of AMPA- and NMDA-receptor currents by Group III metabotropic glutamate receptors in rat nucleus accumbens

Whole cell patch clamp recordings from rat nucleus accumbens neurons were made in order to study the effect of metabotropic glutamate receptors and dopamine on postsynaptic glutamate receptor mediated currents. AMPA- and NMDA-R currents were evoked by flash photolysis of caged glutamate, while spike-dependent release of neurotransmitters was prevented by adding tetrodotoxin and bicuculline to the bath solution. Spontaneous potentiation of NMDA- but not AMPA-R current was observed in the early phase of stimulation, followed by depotentiation and subsequent stabilization. The Group III metabotropic glutamate receptor antagonist MAP4 induced a transient potentiation of both AMPA- and NMDA-R current amplitudes, without affecting rise times and decay time constants. In contrast, the Group I-II metabotropic glutamate receptor antagonist MCPG and the neurotransmitter dopamine did not exert significant effects on either AMPA- or NMDA-R currents. These data suggest that at least one of the Group III subtypes is located postsynaptically in the nucleus accumbens and is able to dampen the activity of ionotropic glutamatergic receptors. In contrast, our results do not support a modulation of postsynaptic AMPA- and NMDA-R currents by Group I/II metabotropic glutamate receptors or dopamine. Modulation of both AMPA- and NMDA-R currents in the nucleus accumbens is likely to play a major role in setting the cellular excitability in response to behaviourally relevant limbic inputs, and in regulating the plasticity of these responses.

Primary innervation of the avian and mammalian cochlear nucleus

The auditory nerve of birds and mammals exhibits differences and similarities, but given the millions of years since the two classes diverged from a common ancestor, the similarities are much more impressive than the differences. The avian nerve is simpler than that of mammals, but share many fundamental features including principles of development, structure, and physiological properties. Moreover, the available evidence shows that the human auditory nerve follows this same general organizational plan. Equally impressive are reports that homologous genes in worms, flies, and mice exert the same heredity influences in man. The clear implication is that animal studies will produce knowledge that has a direct bearing on the human condition.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.

Mechanisms underlying cerebellar motor deficits due to mGluR1-autoantibodies

Patients with Hodgkin's disease can develop paraneoplastic cerebellar ataxia because of the generation of autoantibodies against mGluR1 (mGluR1-Abs). Yet, the pathophysiological mechanisms underlying their motor coordination deficits remain to be elucidated. Here, we show that application of IgG purified from the patients' serum to cerebellar slices of mice acutely reduces the basal activity of Purkinje cells, whereas application to the flocculus of mice in vivo evokes acute disturbances in the performance of their compensatory eye movements. In addition, the mGluR1-Abs block induction of long-term depression in cultured mouse Purkinje cells, whereas the cerebellar motor learning behavior of the patients is affected in that they show impaired adaptation of their saccadic eye movements. Finally, postmortem analysis of the cerebellum of a paraneoplastic cerebellar ataxia patient showed that the number of Purkinje cells was significantly reduced by approximately two thirds compared with three controls. We conclude that autoantibodies against mGluR1 can cause cerebellar motor coordination deficits caused by a combination of rapid effects on both acute and plastic responses of Purkinje cells and chronic degenerative effects.

Interactions between ionotropic and metabotropic glutamate receptors regulate cAMP response element-binding protein phosphorylation in cultured striatal neurons

The striatum is a key structure of basal ganglia controlling extrapyramidal motor activity and processing addictive plasticity of abused substances. Glutamatergic transmission that is enriched in the striatum regulates a variety of striatal neuronal activities via selective activation of ionotropic and metabotropic glutamate receptors (mGluRs). In this study, the interaction between N-methyl-D-aspartate (NMDA) receptors and group I mGluRs (mGluR1 and mGluR5 subtypes) in activating a phosphorylation cascade to a transcription factor cAMP response element-binding protein (CREB) was investigated in primary cultures of E18 or postnatal day 1 striatal neurons. We found that activation of NMDA receptors with NMDA rapidly and concentration-dependently increased the number of neurons expressing phosphorylated CREB (pCREB) as revealed by immunocytochemistry. The increased pCREB expression by NMDA was sensitive to an NMDA antagonist MK801. Co-incubation of a subthreshold dose of a group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) that itself did not alter basal pCREB expression augmented NMDA-induced CREB phosphorylation. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride blocked the DHPG augmentation of NMDA-induced CREB phosphorylation, while the mGluR1 antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester did not. Interestingly, the protein kinase C inhibitors chelerythrine and Gö6983 also prevented DHPG from enhancing CREB phosphorylation induced by NMDA. Whereas a low dose of the protein kinase C activator phorbol 12-myristate 13-acetate mimicked the DHPG potentiation. These results indicate a facilitatory regulation of an NMDA cascade to CREB phosphorylation by concurrent glutamatergic tone on mGluR5, which is probably processed via an intracellular signaling pathway involving protein kinase C.

Glutamate receptor functions in sensory relay in the thalamus

It is known that glutamate is a major excitatory transmitter of sensory and cortical afferents to the thalamus. These actions are mediated via several distinct receptors with postsynaptic excitatory effects predominantly mediated by ionotropic receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate varieties (NMDA). However, there are also other kinds of glutamate receptor present in the thalamus, notably the metabotropic and kainate types, and these may have more complex or subtle roles in sensory transmission. This paper describes recent electrophysiological experiments done in vitro and in vivo which aim to determine how the metabotropic and kainate receptor types can influence transmission through the sensory thalamic relay. A particular focus will be how such mechanisms might operate under physiological conditions.

Climbing fiber activation of metabotropic glutamate receptors on cerebellar purkinje neurons

In the cerebellum, metabotropic glutamate receptors (mGluRs) are required for distinct forms of synaptic plasticity expressed at parallel fiber (PF) and climbing fiber (CF) synapses. At PF synapses, mGluR activation generates a slow synaptic current and triggers intracellular calcium release; at CF synapses, mGluR activation has not been observed. This has led some investigators to propose that mGluR-dependent changes in CF synaptic strength are induced heterosynaptically. Here we describe an mGluR-mediated response to CF stimulation consisting of two parallel signaling pathways: one leading to a slow synaptic conductance and the other leading to internal calcium release. This additional target for glutamate broadens the signaling capabilities of CF synapses and raises the possibility that changes in CF strength are homosynaptically triggered.

The making of a complex spike: ionic composition and plasticity

Climbing fiber (CF) activation evokes a large all-or-nothing electrical response in Purkinje cells (PCs), the complex spike. It has been suggested that the role of CFs (and thus complex spikes) is that of a "teacher" in simple learning paradigms such as associative eyeblink conditioning. An alternative hypothesis describes the olivocerebellar system as part of a timing device and denies a role of the CF input in learning. To date, neither of these hypotheses nor others can definitively be verified or discounted. Similarly, the complex spike evades a clear understanding when it comes to the cellular events underlying complex spike generation. What is known, however, is that complex spikes are associated with large dendritic calcium signals that are required for the induction of long-term depression (LTD) at the parallel fiber (PF)-PC synapse. PF-LTD is a form of long-term synaptic plasticity that has been suggested to underlie certain forms of cerebellar motor learning. In contrast to the PF input, the CF input has been considered invariant. Our recent discovery of LTD at the CF input shows that complex spikes are less static than previously assumed. In addition to depression of CF-evoked excitatory postsynaptic currents, long-lasting, selective reduction of slow complex spike components could be observed after brief CF tetanization. To understand the functional implications of CF-LTD, it is crucial to know the types of currents constituting the specific complex spike components. Here we review the "anatomy" of the complex spike as well as our observations of activity-dependent complex spike waveform modifications. In addition, we discuss which properties CF-LTD might add to the circuitry of the cerebellar cortex.

Synaptic localization of nitric oxide synthase and soluble guanylyl cyclase in the hippocampus

Functional evidence suggests that nitric oxide released from CA1 pyramidal cells can act as a retrograde messenger to mediate hippocampal long-term potentiation, but the failure to find neuronal nitric oxide synthase (NOS-I) in the dendritic spines of these cells has cast doubt on this suggestion. We hypothesized that NOS-I may be in spines but in a form inaccessible to antibody when using standard histological fixation procedures. Supporting this hypothesis, we found that after a weak fixation protocol shown previously to enhance staining of synaptic proteins, CA1 pyramidal cells exhibit clear immunoreactivity for NOS-I. Confocal microscopy revealed that numerous dendritic spines in the stratum radiatum contained the NR2 subunit of the NMDA receptor and the adaptor protein postsynaptic density-95, and a subset of these spines also contained NOS-I. Quantitative studies showed that only approximately 8% of synaptic puncta (identified by synaptophysin staining) were associated with NOS-I, and approximately 9% contained the beta subunit of soluble guanylyl cyclase (sGC), a major target of NO. However, the majority of NOS-I-positive synaptic puncta was associated with sGC and vice versa. Postembedding immunogold electron microscopy showed that NOS-I concentrates just inside the postsynaptic plasma membrane of asymmetric axospinous synapses in the stratum radiatum of CA1, whereas sGCbeta concentrates just inside the presynaptic membrane. Together, these findings support the possibility that NO may act as a retrograde messenger to help mediate homosynaptic plasticity in a subpopulation of synapses in the stratum radiatum of CA1.

A Monte Carlo model reveals independent signaling at central glutamatergic synapses

We have developed a biophysically realistic model of receptor activation at an idealized central glutamatergic synapse that uses Monte Carlo techniques to simulate the stochastic nature of transmission following release of a single synaptic vesicle. For the a synapse with 80 AMPA and 20 NMDA receptors, a single quantum, with 3000 glutamate molecules, opened approximately 3 NMDARs and 20 AMPARs. The number of open receptors varied directly with the total number of receptors, and the fraction of open receptors did not depend on the ratio of co-localized AMPARs and NMDARs. Variability decreased with increases in either total receptor number or quantal size, and differences between the variability of AMPAR and NMDAR responses were due solely to unequal numbers of receptors at the synapse. Despite NMDARs having a much higher affinity for glutamate than AMPARs, quantal release resulted in similar occupancy levels in both receptor types. Receptor activation increased with number of transmitter molecules released or total receptor number, whereas occupancy levels were only dependent on quantal size. Tortuous diffusion spaces reduced the extent of spillover and the activation of extrasynaptic receptors. These results support the conclusion that signaling is spatially independent within and between central glutamatergic synapses.

Group I metabotropic glutamate receptor signaling via Galpha q/Galpha 11 secures the induction of long-term potentiation in the hippocampal area CA1

Heterotromeric G-proteins of the Gq family are thought to transduce signals from group I metabotropic glutamate receptors (mGluRs) in central neurons. We investigated roles of this cascade in hippocampal long-term potentiation (LTP) by using null-mutant mice lacking the alpha subunit of Gq (Galphaq) or G11 (Galpha11). We found no obvious abnormalities in the morphology, layer structure, expression of NMDA receptors, and basic parameters of excitatory synaptic transmission in the hippocampus of Galphaq mutant mice. We used theta burst stimulation (TBS) (3-10 burst trains at 5 Hz; each train consisted of five stimuli at 100 Hz) to induce LTP at Schaffer collateral to CA1 pyramidal cell synapses. Conventional TBS with 10 burst trains induced robust LTP in wild-type, Galphaq mutant, and Galpha11 mutant mice. Weak TBS with three burst trains consistently induced LTP in wild-type mice. In contrast, the same weak TBS was insufficient to induce LTP in Galphaq and Galpha11 mutant mice. In wild-type mice, the LTP by weak TBS was abolished by inhibiting group I mGluR or protein kinase C (PKC) but not by blocking muscarinic acetylcholine receptors. Prior activation of group I mGluR by an agonist significantly enhanced the LTP by weak TBS in wild-type mice. However, this priming effect was absent in Galphaq mutant mice. These results indicate that the signaling from group I mGluR to PKC involving Galphaq/Galpha11 does not constitute the main pathway for LTP, but it secures LTP induction by lowering its threshold in the hippocampal area CA1.

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.

Metabotropic glutamate 1 (mGlu1) receptor antagonists enhance GABAergic neurotransmission: a mechanism for the attenuation of post-ischemic injury and epileptiform activity?

Selective antagonists of mGlu1 metabotropic glutamate receptors attenuate neuronal death in models of cerebral ischemia. Because GABAergic mechanisms have recently been proposed to contribute to these neuroprotective effects, we examined the effects of selective mGlu1 antagonists characterized in our laboratory on GABAergic transmission in three different models of neuropathology. In rat organotypic hippocampal slices exposed to oxygen-glucose deprivation, the mGlu1 antagonists AIDA, CBPG and 3-MATIDA reduced CA1 pyramidal cell loss when added to the medium during the insult and the subsequent recovery period. This effect was mimicked by the GABA(A) and GABA(B) agonists muscimol and baclofen and partially prevented by the antagonists bicuculline and CGP 55845. In gerbils subjected to global ischemia, protection of CA1 pyramidal cells by transdialytic perfusion of AIDA and CBPG was associated with a significant increase in the basal and ischemic output of GABA and minor changes in the output of glutamate. In a mouse cortical wedge model, both muscimol and 3-MATIDA reduced the frequency of spontaneous bursts induced by 4-aminopyridine and this reduction was prevented by co-perfusion with bicuculline. Taken together, our results suggest that the release of GABA, and the subsequent activation of GABA receptors, may contribute to the attenuation of post-ischemic neuronal damage and epileptiform activity induced by mGlu1 receptor antagonists.

Homer as both a scaffold and transduction molecule

Increasing evidence shows that scaffold proteins not only control membrane assembly of receptors and channels, but also modulate intracellular signaling by assembled receptors. The Homer family of proteins act as scaffolds to bind clusters of proteins and glutamate receptors at postsynaptic sites. We review results of cloning and gene expression of this protein family, and summarize roles in glutamate receptor function and intracellular signaling in neurons. Homer proteins trigger the localization of metabotropic glutamate receptor subtype 5 (mGlu5 receptor) to the postsynaptic plasma membrane. They can also alter the kinetics and peak amplitude of the intracellular Ca2+ responses of mGlu1 and mGlu5 receptors. Homer proteins can either prevent or promote spontaneous activation of these receptors, depending on the type of Homer protein isoform expressed.

The metabotropic glutamate receptor mGluR7b binds to the catalytic gamma-subunit of protein phosphatase 1

Correct targeting of enzymes represents an important biological mechanism to control post-translational modifications of neurotransmitter receptors. The metabotropic glutamate receptor type 7 (mGluR7) exists in two splice variants (mGluR7a and mGluR7b), defined by different C-termini that are phosphorylated by protein kinase C (PKC). Recently, the search for mGluR7a binding partners yielded several proteins that interacted with its C-terminus. Here, a yeast two-hybrid screen using the mGluR7b C-terminus identified both variants of the catalytic gamma-subunit of protein phosphatase 1 (PP1gamma1 and PP1gamma2) as binding partners. The minimal interacting region of PP1gamma1/2 contained the core domain and was homologous to a region of PP1alpha that is needed for functional expression. Although this core domain is highly conserved within the protein phosphatase family, PP1alpha1 and PP1beta did not interact with mGluR7b. Binding between PP1gamma1 and mGluR7b might be regulated by alternative splicing, as the variant-specific distal part of the mGluR7b C-terminus mediated the interaction. Within this domain, amino acids involved in the binding to PP1gamma1 were mapped and biochemical assays using recombinant and native proteins verified the proposed interaction. Finally, the expression pattern of PP1gamma1, PP1gamma2 and mGluR7b was analysed in various CNS regions. In summary, these results suggest a regulation of mGluR7b by PP1gamma.

Activation of metabotropic glutamate receptors by repetitive stimulation in auditory cortex

To determine whether metabotropic glutamate receptors (mGluRs) contribute to the responses of neurons to repetitive stimulation in the rat auditory cortex in vitro, five stimulus pulses were delivered at 2-100 Hz which elicited five depolarizing synaptic responses, f-EPSPs: f-EPSPs(1-5). Stimulus pulses 2-5 delivered at low frequencies (2-10 Hz) elicited f-EPSPs(2-5) that were about 15% smaller than the response elicited by the first pulse (f-EPSP(1)). In the presence of the nonspecific mGluR agonist, ACPD, the amplitude of all f-EPSPs was 40% smaller than predrug responses. APV, CNQX, or bicuculline (antagonists of NMDA-, AMPA/kainate-, and GABA(A)-receptors, respectively) did not change this effect of ACPD. The mGluR antagonist, MCPG, had no effect on f-EPSPs but did reduce the effect of ACPD. High-frequency stimulation (50-100 Hz) elicited f-EPSPs that were smaller with each successive stimulus. In ACPD, f-EPSP(1) was 40% smaller than predrug, but f-EPSPs(3-5) were not changed compared to pre-ACPD f-EPSPs(3-5), indicating that ACPD occludes the effect of repetitive stimulation. MCPG increased f-EPSP(5) by 15%, indicating that a portion of the reduction of f-EPSPs during high-frequency stimulation is mediated by mGluRs. MCPG also partially blocked the effect of ACPD. In CNQX, ACPD only decreased EPSPs, but APV or bicuculline did not change the effect of ACPD. These results suggest that the successive reduction of f-EPSPs during a high-frequency train is partially a result of mGluR activation.

Receptor activation and homer differentially control the lateral mobility of metabotropic glutamate receptor 5 in the neuronal membrane

Glutamate receptors are clustered at the membrane through interactions with intracellular scaffolding proteins and cytoskeletal elements but can also be found in intracellular compartments or dispersed in the membrane. This distribution results from an equilibrium between the different pools of receptors whose dynamic is poorly known. The group I metabotropic glutamate receptor 5 (mGluR5) is concentrated in an annulus around the postsynaptic density but also found in large amounts in the extrasynaptic membrane. To analyze the dynamic of stabilization of mGluR5, we used single-particle tracking, force measurements, and fluorescence recovery to measure the mobility of mGluR5. We found that receptor activation increases receptor diffusion, whereas the scaffolding protein Homer favors confinement of receptor movements within clusters of Homer-mGluR5. However, this stabilization is reversible, because even in the presence of Homer, receptors still enter and exit from clusters at fast rates. Furthermore, clusters themselves are highly dynamic both in their movements and in their composition, which can vary within tens of seconds. Thus, exchange of receptors between dispersed and clustered states is fast and regulated during physiological processes. These properties may explain certain fast changes in receptor composition observed at postsynaptic densities.

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.

Cooperative endocannabinoid production by neuronal depolarization and group I metabotropic glutamate receptor activation

Endocannabinoids are retrograde messengers that are released from central neurons by depolarization-induced elevation of intracellular Ca2+ concentration [Ca2+]I or by activation of a group I metabotropic glutamate receptor (mGluR). We studied the interaction between these two pathways for endocannabinoid production in rat hippocampal neurons. We made a paired whole-cell recording from cultured hippocampal neurons with inhibitory synaptic connections. Activation of group I mGluRs, mainly mGluR5, by the specific agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), suppressed inhibitory postsynaptic currents (IPSCs) in about half of the neuron pairs. A cannabinoid agonist, WIN55,212-2, suppressed IPSCs in all DHPG-sensitive pairs but not in most of DHPG-insensitive pairs. The effects of both DHPG and WIN55,212-2 were abolished by the cannabinoid antagonists, AM281 and SR141716A, indicating that activation of group I mGluR releases endocannabinoids and suppress inhibitory neurotransmitter release through activation of presynaptic cannabinoid receptors. Depolarization of the postsynaptic neurons caused a transient suppression of IPSCs, a phemomenon termed depolarization-induced suppression of inhibition (DSI) that was also abolished by cannabinoid antagonists. Importantly, DSI was enhanced significantly when group I mGluRs were activated simultaneously by DHPG. This enhancement was much more prominent than expected from the simple summation of depolarization-induced and group I mGluR-induced endocannabinoid release. DHPG caused no change in depolarization-induced Ca2+ transients, indicating that the enhanced DSI by DHPG was not due to the augmentation of Ca2+ influx. Enhancement of DSI by DHPG was also observed in hippocampal slices. These results suggest that two pathways work in a cooperative manner to release endocannabinoids via a common intracellular cascade.

Roles of glutamate receptor delta 2 subunit (GluRdelta 2) and metabotropic glutamate receptor subtype 1 (mGluR1) in climbing fiber synapse elimination during postnatal cerebellar development

Climbing fiber (CF) synapse formation onto cerebellar Purkinje cells (PCs) is critically dependent on the synaptogenesis from parallel fibers (PFs), the other input to PCs. Previous studies revealed that deletion of the glutamate receptor delta2 subunit (GluRdelta2) gene results in persistent multiple CF innervation of PCs with impaired PF synaptogenesis, whereas mutation of the metabotropic glutamate receptor subtype 1 (mGluR1) gene causes multiple CF innervation with normal PF synaptogenesis. We demonstrate that atypical CF-mediated EPSCs (CF-EPSCs) with slow rise times and small amplitudes coexisted with typical CF-EPSCs with fast rise times and large amplitudes in PCs from GluRdelta2 mutant cerebellar slices. CF-EPSCs in mGluR1 mutant and wild-type PCs had fast rise times. Atypical slow CF responses of GluRdelta2 mutant PCs were associated with voltage-dependent Ca(2+) signals that were confined to PC distal dendrites. In the wild-type and mGluR1 mutant PCs, CF-induced Ca(2+) signals involved both proximal and distal dendrites. Morphologically, CFs of GluRdelta2 mutant mice extended to the superficial regions of the molecular layer, whereas those of wild-type and mGluR1 mutant mice did not innervate the superficial one-fifth of the molecular layer. It is therefore likely that surplus CFs of GluRdelta2 mutant mice form ectopic synapses onto distal dendrites, whereas those of wild-type and mGluR1 mutant mice innervate proximal dendrites. These findings suggest that GluRdelta2 is required for consolidating PF synapses and restricting CF synapses to the proximal dendrites, whereas the mGluR1-signaling pathway does not affect PF synaptogenesis but is involved in eliminating surplus CF synapses at the proximal dendrites.

Developmental changes in the localisation of the mGluR1alpha subtype of metabotropic glutamate receptors in Purkinje cells

The regulation of neurotransmitter receptors during synapse formation has been studied extensively at the neuromuscular junction, but little is known about the development of excitatory neurotransmitter receptors during synaptogenesis in central synapses. In this study we show qualitatively and quantitatively that a receptor undergoes changes in localisation on the surface of rat Purkinje cells during development in association with its excitatory synapses. The presence of mGluR1alpha at parallel and climbing fibre synapses on developing Purkinje cells was studied using high-resolution immunoelectron microscopy. Immunoreactivity for mGluR1alpha was detected from embryonic day 18 in Purkinje cells, and showed dramatic changes in its localisation with age. At early postnatal ages (P0 and P3), mGluR1alpha was found both in somata and stem dendrites but was not usually associated with synaptic contacts. At P7, mGluR1alpha became concentrated in somatic spines associated with climbing fibres and in the growing dendritic arborisation even before innervation by parallel fibres. During the second and third postnatal week, when spines and parallel fibre synapses were generated, mGluR1alpha became progressively concentrated in the molecular layer, particularly in the synaptic specialisations. As a result, during the fourth postnatal week, the pattern and level of mGluR1alpha expression became similar to the adult and mGluR1alpha appeared in high density in perisynaptic sites. Our results indicate that mGluR1alpha is present in the developing Purkinje cells prior to their innervation by climbing and parallel fibres and demonstrate that this receptor undergoes a dynamic and specific regulation during postnatal development in association with the establishment of synaptic inputs to Purkinje cell.

Augmented motor activity and reduced striatal preprodynorphin mRNA induction in response to acute amphetamine administration in metabotropic glutamate receptor 1 knockout mice

Metabotropic glutamate receptor 1 (mGluR1) is a G-protein-coupled receptor and is expressed in the medium spiny projection neurons of mouse striatum. To define the role of mGluR1 in actions of psychostimulant, we compared both motor behavior and striatal neuropeptide mRNA expression between mGluR1 mutant and wild-type control mice after a single injection of amphetamine. We found that acute amphetamine injection increased motor activity in both mutant and control mice in a dose-dependent manner (1, 4, and 12 mg/kg, i.p.). However, the overall motor responses of mGluR1 -/- mice to all three doses of amphetamine were significantly greater than those of wild-type +/+ mice. Amphetamine also induced a dose-dependent elevation of preprodynorphin mRNA in the dorsal and ventral striatum of mutant and wild-type mice as revealed by quantitative in situ hybridization. In contrast to behavioral responses, the induction of dynorphin mRNA in both the dorsal and ventral striatum of mutant mice was significantly less than that of wild-type mice in response to the two higher doses of amphetamine. In addition, amphetamine elevated basal levels of substance P mRNA in the dorsal and ventral striatum of mGluR1 mutant mice to a similar level as that of wild-type mice. There were no differences in basal levels and distribution patterns of the two mRNAs between the two genotypes of mice treated with saline. These results demonstrate a clear augmented behavioral response of mGluR1 knockout mice to acute amphetamine exposure that is closely correlated with reduced dynorphin mRNA induction in the same mice. It appears that an intact mGluR1 is specifically critical for full dynorphin induction, and impaired mobilization of inhibitory dynorphin system as a result of lacking mGluR1 may contribute to an augmentation of motor stimulation in response to acute administration of psychostimulant.