Both mGluR1 and mGluR5 mediate Ca2+ release and inward currents in hippocampal CA1 pyramidal neurons

Altered neuronal group 1 metabotropic glutamate receptor- and endoplasmic reticulum-mediated Ca2+ signaling in two rodent models of Alzheimer's disease

Calcium mobilization from the endoplasmic reticulum (ER) induced by, for example, IP3 receptor (IP3R) stimulation, and its subsequent crosstalk with extracellular Ca2+ influx mediated through voltage-gated calcium channels (VGCCs) and neuronal store-operated calcium entry (nSOCE), is essential for normal neuronal signaling and cellular homeostasis. However, several studies suggest that chronic calcium dysregulation may play a key role in the onset and/or progression of neurodegenerative conditions, particularly Alzheimer's disease (AD). Here, using early postnatal hippocampal tissue from two transgenic murine models of AD, we provide further evidence that not only are crucial calcium signaling pathways dysregulated, but also that such dysregulation occurs at very early stages of development. Utilizing epifluorescence calcium imaging, we investigated ER-, nSOCE- and VGCC-mediated calcium signaling in cultured primary hippocampal neurons from two transgenic rodent models of AD: 3xTg-AD mice (PS1M146V/APPSWE/TauP301L) and TgF344-AD rats (APPSWE/PS1ΔE9) between 2 and 9 days old. Our results reveal that, in comparison to control hippocampal neurons, those from 3xTg-AD mice possessed significantly greater basal ER calcium levels, as measured by larger responses to I-mGluR-mediated ER Ca2+ mobilization (amplitude; 4 (0-19) vs 21(12-36) a.u., non-Tg vs 3xTg-AD; median difference (95 % Cl) = 14 a.u. (11-18); p = 0.004)) but reduced nSOCE (15 (4-22) vs 8(5-11) a.u., non-Tg vs 3xTg-AD; median difference (95 % Cl) = -7 a.u. (-3- -10 a.u.); p < 0.0001). Furthermore, unlike non-Tg neurons, where depolarization enhanced the amplitude, duration and area under the curve (A.U.C.) of I-mGluR-evoked ER-mediated calcium signals when compared with basal conditions, this was not apparent in 3xTg-AD neurons. Whilst the amplitude of depolarization-enhanced I-mGluR-evoked ER-mediated calcium signals from both non-Tg F344 and TgF344-AD neurons was significantly enhanced relative to basal conditions, the A.U.C. and duration of responses were enhanced significantly upon depolarization in non-Tg F344, but not in TgF344-AD, neurons. Overall, the nature of basal I-mGluR-mediated calcium responses did not differ significantly between non-Tg F344 and TgF344-AD neurons. In summary, our results characterizing ER- and nSOCE-mediated calcium signaling in neurons demonstrate that ER Ca2+ dyshomeostasis is an early and potentially pathogenic event in familial AD.

Target cell-specific plasticity rules of NMDA receptor-mediated synaptic transmission in the hippocampus

Long-term potentiation and depression of NMDA receptor-mediated synaptic transmission (NMDAR LTP/LTD) can significantly impact synapse function and information transfer in several brain areas. However, the mechanisms that determine the direction of NMDAR plasticity are poorly understood. Here, using physiologically relevant patterns of presynaptic and postsynaptic burst activities, whole-cell patch clamp recordings, 2-photon laser calcium imaging in acute rat hippocampal slices and immunoelectron microscopy, we tested whether distinct calcium dynamics and group I metabotropic glutamate receptor (I-mGluR) subtypes control the sign of NMDAR plasticity. We found that postsynaptic calcium transients (CaTs) in response to hippocampal MF stimulation were significantly larger during the induction of NMDAR-LTP compared to NMDAR-LTD at the MF-to-CA3 pyramidal cell (MF-CA3) synapse. This difference was abolished by pharmacological blockade of mGluR5 and was significantly reduced by depletion of intracellular calcium stores, whereas blocking mGluR1 had no effect on these CaTs. In addition, we discovered that MF to hilar mossy cell (MF-MC) synapses, which share several structural and functional commonalities with MF-CA3 synapses, also undergoes NMDAR plasticity. To our surprise, however, we found that the postsynaptic distribution of I-mGluR subtypes at these two synapses differ, and the same induction protocol that induces NMDAR-LTD at MF-CA3 synapses, only triggered NMDAR-LTP at MF-MC synapses, despite a comparable calcium dynamics. Thus, postsynaptic calcium dynamics alone cannot predict the sign of NMDAR plasticity, indicating that both postsynaptic calcium rise and the relative contribution of I-mGluR subtypes likely determine the learning rules of NMDAR plasticity.

Comparative assessment of the effects of DREADDs and endogenously expressed GPCRs in hippocampal astrocytes on synaptic activity and memory

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) have proven themselves as one of the key in vivo techniques of modern neuroscience, allowing for unprecedented access to cellular manipulations in living animals. With respect to astrocyte research, DREADDs have become a popular method to examine the functional aspects of astrocyte activity, particularly G-protein coupled receptor (GPCR)-mediated intracellular calcium (Ca2+) and cyclic adenosine monophosphate (cAMP) dynamics. With this method it has become possible to directly link the physiological aspects of astrocytic function to cognitive processes such as memory. As a result, a multitude of studies have explored the impact of DREADD activation in astrocytes on synaptic activity and memory. However, the emergence of varying results prompts us to reconsider the degree to which DREADDs expressed in astrocytes accurately mimic endogenous GPCR activity. Here we compare the major downstream signaling mechanisms, synaptic, and behavioral effects of stimulating Gq-, Gs-, and Gi-DREADDs in hippocampal astrocytes of adult mice to those of endogenously expressed GPCRs.

The Neuroprotective Effects of mGlu1 Receptor Antagonists Are Mediated by an Enhancement of GABAergic Synaptic Transmission via a Presynaptic CB1 Receptor Mechanism

In this study, we investigated the cross-talk between mGlu1 and CB1 receptors in modulating GABA hippocampal output in whole-cell voltage clamp recordings in rat hippocampal acute slices, in organotypic hippocampal slices exposed to oxygen and glucose deprivation (OGD) and in gerbils subjected to global ischemia. CB1 receptor expression was studied using immunohistochemistry and the CA1 contents of anandamide (AEA) and 2-arachidonoylglycerol (2-AG) were measured by LC-MS/MS. Our results show that mGlu1 receptor antagonists enhance sIPSCs in CA1 pyramidal cells and the basal and ischemic hippocampal release of GABA in vivo in a manner that is mediated by CB1 receptor activation. In hippocampal slices exposed to OGD and in ischemic gerbils, mGlu1 receptor antagonists protected CA1 pyramidal cells against post-ischemic injury and this effect was reduced by CB1 receptor activation. OGD induced a transient increase in the hippocampal content of AEA and this effect is prevented by mGlu1 receptor antagonist. Finally, OGD induced a late disruption of CB1 receptors in the CA1 region and the effect was prevented when CA1 pyramidal cells were protected by mGlu1 antagonists. Altogether, these results suggest a cooperative interaction between mGlu1 receptors and the endocannabinoid system in the mechanisms that lead to post-ischemic neuronal death.

mGluR5 regulates REST/NRSF signaling through N-cadherin/β-catenin complex in Huntington's disease

Repressor element 1-silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) is a transcription repressor and its expression is regulated by the Wnt pathway through β-catenin. Metabotropic glutamate receptor 5 (mGluR5) signaling plays a key role in controlling neuronal gene expression. Interestingly, REST/NRSF nuclear translocation and signaling, as well as mGluR5 signaling are altered in the presence of mutant huntingtin. It remains unclear whether mGluR5 can modulate Wnt and REST/NRSF signaling under physiological conditions and whether this modulation is altered in Huntington's disease (HD). Using primary corticostriatal neurons derived from wild type mouse embryos, we find that targeting mGluR5 using the agonist, DHPG, or the negative allosteric modulator, CTEP, modulates REST/NRSF expression by regulating the assembly of N-cadherin/ β-catenin complex in a Src kinase-dependent manner. We have validated our in vitro findings in vivo using two HD mouse models. Specifically, we show that pharmacological inhibition of mGluR5 in zQ175 mice and genetic ablation of mGluR5 in BACHD mice corrected the pathological activation of Src and rescued REST/NRSF-dependent signaling. Together, our data provide evidence that mGluR5 regulates REST/NRSF expression via the Wnt pathway and highlight the contribution of impaired REST/ NRSF signaling to HD pathology.

Transcriptional corepressor SIN3A regulates hippocampal synaptic plasticity via Homer1/mGluR5 signaling

Long-term memory depends on the control of activity-dependent neuronal gene expression, which is regulated by epigenetic modifications. The epigenetic modification of histones is orchestrated by the opposing activities of 2 classes of regulatory complexes: permissive coactivators and silencing corepressors. Much work has focused on coactivator complexes, but little is known about the corepressor complexes that suppress the expression of plasticity-related genes. Here, we define a critical role for the corepressor SIN3A in memory and synaptic plasticity, showing that postnatal neuronal deletion of Sin3a enhances hippocampal long-term potentiation and long-term contextual fear memory. SIN3A regulates the expression of genes encoding proteins in the postsynaptic density. Loss of SIN3A increases expression of the synaptic scaffold Homer1, alters the metabotropic glutamate receptor 1α (mGluR1α) and mGluR5 dependence of long-term potentiation, and increases activation of ERK in the hippocampus after learning. Our studies define a critical role for corepressors in modulating neural plasticity and memory consolidation and reveal that Homer1/mGluR signaling pathways may be central molecular mechanisms for memory enhancement.

Multiple Lines of Evidence Indicate That Gliotransmission Does Not Occur under Physiological Conditions

A major controversy persists within the field of glial biology concerning whether or not, under physiological conditions, neuronal activity leads to Ca²⁺-dependent release of neurotransmitters from astrocytes, a phenomenon known as gliotransmission. Our perspective is that, while we and others can apply techniques to cause gliotransmission, there is considerable evidence gathered using astrocyte-specific and more physiological approaches which suggests that gliotransmission is a pharmacological phenomenon rather than a physiological process. Approaches providing evidence against gliotransmission include stimulation of Gq-GPCRs expressed only in astrocytes, as well as removal of the primary proposed source of astrocyte Ca²⁺ responsible for gliotransmission. These approaches contrast with those supportive of gliotransmission, which include mechanical stimulation, strong astrocytic depolarization using whole-cell patch-clamp or optogenetics, uncaging Ca²⁺ or IP3, chelating Ca²⁺ using BAPTA, and nonspecific bath application of agonists to receptors expressed by a multitude of cell types. These techniques are not subtle and therefore are not supportive of recent suggestions that gliotransmission requires very specific and delicate temporal and spatial requirements. Other evidence, including lack of propagating Ca²⁺ waves between astrocytes in healthy tissue, lack of expression of vesicular release machinery, and the demise of the d-serine gliotransmission hypothesis, provides additional evidence against gliotransmission. Overall, the data suggest that Ca²⁺-dependent release of neurotransmitters is the province of neurons, not astrocytes, in the intact brain under physiological conditions.Dual Perspectives Companion Paper: Gliotransmission: Beyond Black-and-White, by Iaroslav Savtchouk and Andrea Volterra.

Gliotransmission and adenosinergic modulation: insights from mammalian spinal motor networks

Astrocytes are proposed to converse with neurons at tripartite synapses, detecting neurotransmitter release and responding with release of gliotransmitters, which in turn modulate synaptic strength and neuronal excitability. However, a paucity of evidence from behavioral studies calls into question the importance of gliotransmission for the operation of the nervous system in healthy animals. Central pattern generator (CPG) networks in the spinal cord and brain stem coordinate the activation of muscles during stereotyped activities such as locomotion, inspiration, and mastication and may therefore provide tractable models in which to assess the contribution of gliotransmission to behaviorally relevant neural activity. We review evidence for gliotransmission within spinal locomotor networks, including studies indicating that adenosine derived from astrocytes regulates the speed of locomotor activity via metamodulation of dopamine signaling.

Glutamate Delta-1 Receptor Regulates Metabotropic Glutamate Receptor 5 Signaling in the Hippocampus

The delta family of ionotropic glutamate receptors consists of glutamate delta-1 (GluD1) and glutamate delta-2 receptors. We have previously shown that GluD1 knockout mice exhibit features of developmental delay, including impaired spine pruning and switch in the N-methyl-D-aspartate receptor subunit, which are relevant to autism and other neurodevelopmental disorders. Here, we identified a novel role of GluD1 in regulating metabotropic glutamate receptor 5 (mGlu5) signaling in the hippocampus. Immunohistochemical analysis demonstrated colocalization of mGlu5 with GluD1 punctas in the hippocampus. Additionally, GluD1 protein coimmunoprecipitated with mGlu5 in the hippocampal membrane fraction, as well as when overexpressed in human embryonic kidney 293 cells, demonstrating that GluD1 and mGlu5 may cooperate in a signaling complex. The interaction of mGlu5 with scaffold protein effector Homer, which regulates mechanistic target of rapamycin (mTOR) signaling, was abnormal both under basal conditions and in response to mGlu1/5 agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) in GluD1 knockout mice. The basal levels of phosphorylated mTOR and protein kinase B, the signaling proteins downstream of mGlu5 activation, were higher in GluD1 knockout mice, and no further increase was induced by DHPG. We also observed higher basal protein translation and an absence of DHPG-induced increase in GluD1 knockout mice. In accordance with a role of mGlu5-mediated mTOR signaling in synaptic plasticity, DHPG-induced internalization of surface α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunits was impaired in the GluD1 knockout mice. These results demonstrate that GluD1 interacts with mGlu5, and loss of GluD1 impairs normal mGlu5 signaling potentially by dysregulating coupling to its effector. These studies identify a novel role of the enigmatic GluD1 subunit in hippocampal function.

mGluR-mediated calcium signalling in the thalamic reticular nucleus

The thalamic reticular nucleus (TRN) plays a major role in modulating the transfer of information from the thalamus to the cortex. GABAergic inhibition via the TRN is differentially regulated by metabotropic glutamate receptors (mGluRs) and the effect of mGluRs on the membrane potential, on ion channels, and on the plasticity of electrical coupling of TRN neurons has been studied previously. Although mGluRs are generally known to trigger Ca(2+) transients, mGluR-mediated Ca(2+)-transients in TRN neurons have not yet been investigated. In this study, we show that mGluRs can trigger Ca(2+)-transients in TRN neurons, that these transients depend on intracellular Ca(2+)-stores, and are mediated by IP3 receptors. Ca(2+) transients caused by the group I mGluR agonist DHPG elicit a current that is sensitive to flufenamic acid and has a reversal potential around -40mV. Our results add mGluR-mediated Ca(2+)-signalling in the TRN to the state-dependent modulators of the thalamocortical system.

Metabotropic glutamate receptor-mediated cyclic ADP ribose signalling

Group I metabotropic glutamate receptors (I-mGluRs) modulate numerous cellular functions such as specific membrane currents and neurotransmitter release linked to their ability to mobilize calcium from intracellular calcium stores. As such, most I-mGluR research to date has focused on the coupling of these receptors to phospholipase C (PLC)-dependent and inositol (1,4,5) trisphosphate (IP3)-mediated calcium release via activation of IP3 receptors located upon the sarco/endoplasmic reticulum. However, there are now numerous examples of PLC- and IP3-independent I-mGluR-evoked signals, which may instead be mediated by activation of ryanodine receptors (RyRs). A prime candidate for mediating this coupling between I-mGluR activation and RyR opening is cyclic ADP ribose (cADPR) and, indeed, several of these PLC-/IP3-independent I-mGluR-evoked calcium signals have now been shown to be mediated wholly or partly by cADPR-evoked activation of RyRs. The contribution of cADPR signalling to I-mGluR-mediated responses is relatively complex, dependent as it is on factors such as cell type, excitation state of the cell and location of I-mGluRs on the cell. However, these factors notwithstanding, I-mGluR-mediated cADPR signalling remains poorly characterized, with several key aspects yet to be fully elucidated such as (1) the range of stimuli which evoke cADPR production, (2) the specific molecular mechanism(s) coupling cADPR to RyR activation and (3) the contribution of cADPR-mediated responses to downstream outputs such as synaptic plasticity. Furthermore, it is possible that the cADPR pathway may play a role in diseases underpinned by dysregulated calcium homoeostasis such as Alzheimer's disease (AD).

Transient receptor potential channels and their role in modulating radial glial-neuronal interaction: a signaling pathway involving mGluR5

The guidance of developing neurons to the right position in the central nervous system is of central importance in brain development. Canonical transient receptor potential (TRPC) channels are thought to mediate turning responses of growth cones to guidance cues through fine control of calcium transients. Proliferating and 1- to 5-day-differentiated neural progenitor cells (NPCs) showed expression of Trpc1 and Trpc3 mRNA, while Trpc4-7 was not clearly detected. Time-lapse imaging showed that the motility pattern of neuronal cells was phasic with bursts of rapid movement (>60 μm/h), changes in direction, and intermittent slow phases or stallings (<40 μm/h), which frequently occurred in close contact with radial glial processes. Genetic interference with the TRPC3 and TRPC1 channel enhanced the motility of NPCs (burst frequency/stalling frequency). TRPC3-deficient cells or cells treated with the TRPC3 blocker pyr3 infrequently changed direction and seldom contacted radial glial processes. TRPC channels are also activated by group I metabotropic glutamate receptors (mGluR1 and mGluR5). As shown here, pyr3 blocked the calcium response mediated through mGluR5 in radial glial processes. Furthermore, 2-methyl-6-(phenylethynyl)pyridine, a blocker of mGluR5, affected the motility pattern in a similar way as TRPC3/6 double knockout or pyr3. The results suggest that radial glial cells exert attractant signals to migrating neuronal cells, which alter their motility pattern. Our results suggest that mGluR5 acting through TRPC3 is of central importance in radial glial-mediated neuronal guidance.

New tools for investigating astrocyte-to-neuron communication

Gray matter protoplasmic astrocytes extend very thin processes and establish close contacts with synapses. It has been suggested that the release of neuroactive gliotransmitters at the tripartite synapse contributes to information processing. However, the concept of calcium (Ca²⁺)-dependent gliotransmitter release from astrocytes, and the release mechanisms are being debated. Studying astrocytes in their natural environment is challenging because: (i) astrocytes are electrically silent; (ii) astrocytes and neurons express an overlapping repertoire of transmembrane receptors; (iii) the size of astrocyte processes in contact with synapses are below the resolution of confocal and two-photon microscopes (iv) bulk-loading techniques using fluorescent Ca²⁺ indicators lack cellular specificity. In this review, we will discuss some limitations of conventional methodologies and highlight the interest of novel tools and approaches for studying gliotransmission. Genetically encoded Ca²⁺ indicators (GECIs), light-gated channels, and exogenous receptors are being developed to selectively read out and stimulate astrocyte activity. Our review discusses emerging perspectives on: (i) the complexity of astrocyte Ca²⁺ signaling revealed by GECIs; (ii) new pharmacogenetic and optogenetic approaches to activate specific Ca²⁺ signaling pathways in astrocytes; (iii) classical and new techniques to monitor vesicle fusion in cultured astrocytes; (iv) possible strategies to express specifically reporter genes in astrocytes.

Effect of glutamate receptor antagonists on migrating neural progenitor cells

Neurotransmitters such as glutamate are potential regulators of neurogenesis. Interference with defined glutamate receptor subtypes affects proliferation, migration and differentiation of neural progenitor cells. The cellular targets for the actions of different glutamate receptor ligands are less well known. In this study we have combined calcium imaging, measurement of membrane potential, time-lapse imaging and immunocytochemistry to obtain a spatial overview of migrating mouse embryonic neural progenitor cell-derived cells responding to glutamate receptor agonists and antagonists. Responses via metabotropic glutamate receptor 5 correlated with radial glial cells and dominated in the inner migration zones close to the neurosphere. Block of metabotropic glutamate receptor 5 resulted in shorter radial glial processes, a transient increase in neuron-like cells emerging from the neurosphere and increased motility of neuron-like cells. α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors are present on the majority of migrating neuronal cells, which with time accumulate at the outer edge of the migration zone. Blocking these receptors leads to an enhanced extension of radial glial processes and a reduced motility of neuron-like cells. Our results indicate that functional glutamate receptors have profound effects on the motility of neural progenitor cells. The main target for metabotropic glutamate receptor 5 appears to be radial glial cells while AMPA/kainate receptors are mainly expressed in newborn neuronal cells and regulate the migratory progress of these cells. The results suggest that both metabotropic glutamate receptor 5 and AMPA/kainate receptors are of importance for the guidance of migrating embryonic progenitor cells.

Bidirectional scaling of astrocytic metabotropic glutamate receptor signaling following long-term changes in neuronal firing rates

Very little is known about the ability of astrocytic receptors to exhibit plasticity as a result of changes in neuronal activity. Here we provide evidence for bidirectional scaling of astrocytic group I metabotropic glutamate receptor signaling in acute mouse hippocampal slices following long-term changes in neuronal firing rates. Plasticity of astrocytic mGluRs was measured by recording spontaneous and evoked Ca²⁺ elevations in both astrocytic somata and processes. An exogenous astrocytic Gq G protein-coupled receptor was resistant to scaling, suggesting that the alterations in astrocyte Ca²⁺ signaling result from changes in activity of the surface mGluRs rather than a change in intracellular G protein signaling molecules. These findings suggest that astrocytes actively detect shifts in neuronal firing rates and adjust their receptor signaling accordingly. This type of long-term plasticity in astrocytes resembles neuronal homeostatic plasticity and might be important to ensure an optimal or expected level of input from neurons.

Dendritic Signaling in Inhibitory Interneurons: Local Tuning via Group I Metabotropic Glutamate Receptors

Communication between neurons is achieved by rapid signal transduction via highly specialized structural elements known as synaptic contacts. In addition, numerous extrasynaptic mechanisms provide a flexible platform for the local regulation of synaptic signals. For example, peri- and extra-synaptic signaling through the group I metabotropic glutamate receptors (mGluRs) can be involved in the highly compartmentalized regulation of dendritic ion conductances, the induction of input-specific synaptic plasticity, and the local release of retrograde messengers. Therefore, extrasynaptic mechanisms appear to play a key role in the local tuning of dendritic computations. Here, we review recent findings on the role of group I mGluRs in the dendritic signaling of inhibitory interneurons. We propose that group I mGluRs provide a dual-mode signaling device that integrates different patterns of neural activity. By implementing distinct forms of intrinsic and synaptic regulation, group I mGluRs may be responsible for the local fine-tuning of dendritic function.

Cyclic ADP ribose-dependent Ca2+ release by group I metabotropic glutamate receptors in acutely dissociated rat hippocampal neurons

Group I metabotropic glutamate receptors (group I mGluRs; mGluR1 and mGluR5) exert diverse effects on neuronal and synaptic functions, many of which are regulated by intracellular Ca²⁺. In this study, we characterized the cellular mechanisms underlying Ca²⁺ mobilization induced by (RS)-3,5-dihydroxyphenylglycine (DHPG; a specific group I mGluR agonist) in the somata of acutely dissociated rat hippocampal neurons using microfluorometry. We found that DHPG activates mGluR5 to mobilize intracellular Ca²⁺ from ryanodine-sensitive stores via cyclic adenosine diphosphate ribose (cADPR), while the PLC/IP₃ signaling pathway was not involved in Ca²⁺ mobilization. The application of glutamate, which depolarized the membrane potential by 28.5±4.9 mV (n = 4), led to transient Ca²⁺ mobilization by mGluR5 and Ca²⁺ influx through L-type Ca²⁺ channels. We found no evidence that mGluR5-mediated Ca²⁺ release and Ca²⁺ influx through L-type Ca²⁺ channels interact to generate supralinear Ca²⁺ transients. Our study provides novel insights into the mechanisms of intracellular Ca²⁺ mobilization by mGluR5 in the somata of hippocampal neurons.

Prolonged postinhibitory rebound firing in the cerebellar nuclei mediated by group I metabotropic glutamate receptor potentiation of L-type calcium currents

Neurons in the cerebellar nuclei fire at accelerated rates for prolonged periods after trains of synaptic inhibition that interrupt spontaneous firing. Both in vitro and in vivo, however, this prolonged rebound firing is favored by strong stimulation of afferents, suggesting that neurotransmitters other than GABA may contribute to the increased firing rates. Here, we tested whether metabotropic glutamate receptors modulate excitability of nuclear cells in cerebellar slices from mouse. In current clamp, the prolonged rebound firing rate after high-frequency synaptic stimulation was reduced by a variety of group I mGluR antagonists, including CPCCOEt [7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester], JNJ16259685 (3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone) plus MPEP, or 3-MATIDA (α-amino-5-carboxy-3-methyl-2-thiopheneacetic acid) plus MPEP, as long as both mGluR1 and mGluR5 were blocked. This mGluR-dependent acceleration of firing was reduced but still evident when IPSPs were prevented by GABA(A) receptor antagonists. In voltage clamp, voltage ramps revealed a non-inactivating, low-voltage-activated, nimodipine-sensitive current that was enhanced by the selective group I mGluR agonist s-DHPG [(S)-3,5-dihydroxyphenylglycine]. This putative L-type current also increased when mGluRs were activated by trains of evoked synaptic currents instead of direct application of agonist. In current clamp, blocking L-type Ca channels with the specific blocker nifedipine greatly reduced prolonged poststimulus firing and occluded the effect of adding group I mGluR antagonists. Thus, potentiation of a low-voltage-activated L-type current by synaptically released glutamate accounted nearly fully for the mGluR-dependent acceleration of firing. Together, these data suggest that prolonged rebound firing in the cerebellar nuclei in vivo is most likely to occur when GABA(A) and mGluRs are simultaneously activated by concurrent excitation and inhibition.

A post-burst after depolarization is mediated by group i metabotropic glutamate receptor-dependent upregulation of Ca(v)2.3 R-type calcium channels in CA1 pyramidal neurons

The excitability of hippocampal pyramidal neurons is regulated by activation of metabotropic glutamate receptors, an effect that is mediated by modulation of R-type calcium channels.

Activation of group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5) regulates neural activity in a variety of ways. In CA1 pyramidal neurons, activation of group I mGluRs eliminates the post-burst afterhyperpolarization (AHP) and produces an afterdepolarization (ADP) in its place. Here we show that upregulation of Ca_v2.3 R-type calcium channels is responsible for a component of the ADP lasting several hundred milliseconds. This medium-duration ADP is rapidly and reversibly induced by activation of mGluR5 and requires activation of phospholipase C (PLC) and release of calcium from internal stores. Effects of mGluR activation on subthreshold membrane potential changes are negligible but are large following action potential firing. Furthermore, the medium ADP exhibits a biphasic activity dependence consisting of short-term facilitation and longer-term inhibition. These findings suggest that mGluRs may dramatically alter the firing of CA1 pyramidal neurons via a complex, activity-dependent modulation of Ca_v2.3 R-type channels that are activated during spiking at physiologically relevant rates and patterns.

The hippocampus is an essential structure in the brain for the formation of new declarative memories. Understanding the cellular basis of memory formation, storage, and recall in the hippocampus requires a knowledge of the properties of the relevant neurons and how they are modulated by activity in the neural circuit. For many years, we have known that various chemical neurotransmitters can modulate the electrical excitability of neurons in the hippocampus. Here, we report new experiments to reveal how the chemical neurotransmitter glutamate increases neuronal excitability. The effect we study is the conversion of the afterhyperpolarization (a cellular consequence of firing an action potential) to an afterdepolarization. We identified the metabotropic glutamate receptors involved in this conversion (receptors called mGluR1 and mGluR5) as well as the final target of modulation (R-type calcium channels composed of Ca_v2.3 subunits), which cause the neurons to exhibit altered excitability in the presence of glutamate. We also determined some of the intermediate steps between activation of the glutamate receptors and modulation of the calcium channels responsible for the change in excitability, offering further mechanistic insight into how synaptic transmission can regulate cellular and network activity.

A novel non-CB1/TRPV1 endocannabinoid-mediated mechanism depresses excitatory synapses on hippocampal CA1 interneurons

Endocannabinoids (eCBs) mediate various forms of synaptic plasticity at excitatory and inhibitory synapses in the brain. The eCB anandamide binds to several receptors including the transient receptor potential vanilloid 1 (TRPV1) and cannabinoid receptor 1 (CB1). We recently identified that TRPV1 is required for long-term depression at excitatory synapses on CA1 hippocampal stratum radiatum interneurons. Here we performed whole-cell patch clamp recordings from CA1 stratum radiatum interneurons in rat brain slices to investigate the effect of the eCB anandamide on excitatory synapses as well as the involvement of Group I metabotropic glutamate receptors (mGluRs), which have been reported to produce eCBs endogenously. Application of the nonhydrolysable anandamide analog R-methanandamide depressed excitatory transmission to CA1 stratum radiatum interneurons by ∼50%. The Group I mGluR agonist DHPG also depressed excitatory glutamatergic transmission onto interneurons to a similar degree, and this depression was blocked by the mGluR5 antagonist MPEP (10 μM) but not by the mGluR1 antagonist CPCCOEt (50 μM). Interestingly, however, neither DHPG-mediated nor R-methanandamide-mediated depression was blocked by the TRPV1 antagonist capsazepine (10 μM), the CB1 antagonist AM-251 (2 μM) or a combination of both, suggesting the presence of a novel eCB receptor or anandamide target at excitatory hippocampal synapses. DHPG also occluded R-methanandamide depression, suggesting the possibility that the two drugs elicit synaptic depression via a shared signaling mechanism. Collectively, this study illustrates a novel CB1/TRPV1-independent eCB pathway present in the hippocampus that mediates depression at excitatory synapses on CA1 stratum radiatum interneurons.

Involvement of the metabotropic glutamate receptor mGluR5 in NMDA receptor-dependent, learning-facilitated long-term depression in CA1 synapses

Learning-facilitated synaptic plasticity describes the ability of hippocampal synapses to respond with persistent synaptic plasticity to the coupling of weak afferent stimulation, which is subthreshold for the induction of plasticity, with a spatial learning experience. The metabotropic glutamate receptor subtype 5 (mGluR5) is critically involved in enabling the persistency of multiple forms of hippocampal synaptic plasticity. We compared the effects of pharmacological allosteric antagonism of mGluR5 in learning-facilitated plasticity with plasticity that had been induced solely by patterned afferent stimulation of the Schaffer collateral pathway to the CA1 stratum radiatum of adult freely behaving rats. Intracerebroventricular injection of the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) had no effect on basal synaptic transmission but significantly prevented both long-term depression (LTD) elicited by electrical stimulation and LTD facilitated by novel object-place configuration learning. NMDA receptor antagonism also prevented learning-facilitated LTD. Habituation to the objects was prevented by MPEP application. Whereas reexposure to the object-place configuration (after 7 days) failed to facilitate LTD in control animals, those who had been treated previously with MPEP expressed LTD, suggesting that inhibition of learning contributed to the initial prevention of LTD. These data support a pivotal role for mGluR5 in both hippocampal LTD and the acquisition of object-place configurations.

Gene expression profile in rat hippocampus with and without memory deficit

The cholinergic neuronal system, through its projections to the hippocampus, plays an important role in learning and memory. The aim of the study was to identify genes and networks in rat hippocampus with and without memory deficit. Genome-scale screening was used to analyze gene expression changes in rats submitted or not to intraparenchymal injection of 192 IgG-saporin and trained in spatial/object novelty tasks. Results showed learning processes were associated with significant expression of genes that could be grouped into several clusters of similar expression profiles and that are involved in biological functions, namely lipid metabolism, signal transduction, protein metabolism and modification, and transcription regulation. Memory loss following hippocampal cholinergic deafferentation was associated with significant expression of genes that did not show similar cluster organization. Only one cluster of genes could be identified; it included genes that would be involved in tissue remodeling. More important, most of the genes significantly altered in lesioned rats were down-regulated.

Hippocampal short- and long-term plasticity are not modulated by astrocyte Ca2+ signaling

The concept that astrocytes release neuroactive molecules (gliotransmitters) to affect synaptic transmission has been a paradigm shift in neuroscience research over the past decade. This concept suggests that astrocytes, together with pre- and postsynaptic neuronal elements, make up a functional synapse. Astrocyte release of gliotransmitters (for example, glutamate and adenosine triphosphate) is generally accepted to be a Ca2+-dependent process. We used two mouse lines to either selectively increase or obliterate astrocytic Gq G protein-coupled receptor Ca2+ signaling to further test the hypothesis that astrocytes release gliotransmitters in a Ca2+-dependent manner to affect synaptic transmission. Neither increasing nor obliterating astrocytic Ca2+ fluxes affects spontaneous and evoked excitatory synaptic transmission or synaptic plasticity. Our findings suggest that, at least in the hippocampus, the mechanisms of gliotransmission need to be reconsidered.

Activity-dependent tuning of inhibitory neurotransmission based on GABAAR diffusion dynamics

An activity-dependent change in synaptic efficacy is a central tenet in learning, memory, and pathological states of neuronal excitability. The lateral diffusion dynamics of neurotransmitter receptors are one of the important parameters regulating synaptic efficacy. We report here that neuronal activity modifies diffusion properties of type-A GABA receptors (GABA(A)R) in cultured hippocampal neurons: enhanced excitatory synaptic activity decreases the cluster size of GABA(A)Rs and reduces GABAergic mIPSC. Single-particle tracking of the GABA(A)R gamma2 subunit labeled with quantum dots reveals that the diffusion coefficient and the synaptic confinement domain size of GABA(A)R increases in parallel with neuronal activity, depending on Ca(2+) influx and calcineurin activity. These results indicate that GABA(A)R diffusion dynamics are directly linked to rapid and plastic modifications of inhibitory synaptic transmission in response to changes in intracellular Ca(2+) concentration. This transient activity-dependent reduction of inhibition would favor the onset of LTP during conditioning.

Ca2+/calmodulin-dependent protein kinase IV links group I metabotropic glutamate receptors to fragile X mental retardation protein in cingulate cortex

Fragile X syndrome is caused by a lack of fragile X mental retardation protein (FMRP) due to silencing of the FMR1 gene. The metabotropic glutamate receptors (mGluRs) in the central nervous system contribute to higher brain functions including learning/memory, persistent pain, and mental disorders. Our recent study has shown that activation of Group I mGluR up-regulated FMRP in anterior cingulate cortex (ACC), a key region for brain cognitive and executive functions; Ca(2+) signaling pathways could be involved in the regulation of FMRP by Group I mGluRs. In this study we demonstrate that stimulating Group I mGluRs activates Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) in ACC neurons. In ACC neurons of adult mice overexpressing CaMKIV, the up-regulation of FMRP by stimulating Group I mGluR is enhanced. The enhancement occurs at the transcriptional level as the Fmr1 mRNA level was further elevated compared with wild-type mice. Using pharmacological approaches, we found that inhibition of CaMKIV could attenuate the up-regulation of FMRP by Group I mGluRs. CaMKIV contribute to the regulation of FMRP by Group I mGluRs probably through cyclic AMP-responsive element binding protein (CREB) activation, as manipulation of CaMKIV could simultaneously cause the change of CREB phosphorylation induced by Group I mGluR activation. Our study has provided strong evidence for CaMKIV as a molecular link between Group I mGluRs and FMRP in ACC neurons and may help us to elucidate the pathogenesis of fragile X syndrome.

Sorting out astrocyte physiology from pharmacology

A number of exciting findings have been made in astrocytes during the past 15 years that have led many researchers to redefine how the brain works. Astrocytes are now widely regarded as cells that propagate Ca(2+) over long distances in response to stimulation, and, similar to neurons, release transmitters (called gliotransmitters) in a Ca(2+)-dependent manner to modulate a host of important brain functions. Although these discoveries have been very exciting, it is essential to place them in the proper context of the approaches used to obtain them to determine their relevance to brain physiology. This review revisits the key observations made in astrocytes that greatly impact how they are thought to regulate brain function, including the existence of widespread propagating intercellular Ca(2+) waves, data suggesting that astrocytes signal to neurons through Ca(2+)-dependent release of glutamate, and evidence for the presence of vesicular machinery for the regulated exocytosis of gliotransmitters.

Cellular plasticity for group I mGluR-mediated epileptogenesis

Stimulation of group I metabotropic glutamate receptors (mGluRs) by the agonist (S)-dihydroxyphenylglycine in the hippocampus transforms normal neuronal activity into prolonged epileptiform discharges. The conversion is long lasting in that epileptiform discharges persist after washout of the inducing agonist and serves as a model of epileptogenesis. The group I mGluR model of epileptogenesis took on special significance because epilepsy associated with fragile X syndrome (FXS) may be caused by excessive group I mGluR signaling. At present, the plasticity mechanism underlying the group I mGluR-mediated epileptogenesis is unknown. I(mGluR(V)), a voltage-gated cationic current activated by group I mGluR agonists in CA3 pyramidal cells in the hippocampus, is a possible candidate. I(mGluR(V)) activation is associated with group I mGluR agonist-elicited epileptiform discharges. For I(mGluR(V)) to play a role in epileptogenesis, long-term activation of the current must occur after group I mGluR agonist exposure or synaptic stimulation. We observed that I(mGluR(V)), once induced by group I mGluR agonist stimulation in CA3 pyramidal cells, remained undiminished for hours after agonist washout. In slices prepared from FXS model mice, repeated stimulation of recurrent CA3 pyramidal cell synapses, effective in eliciting mGluR-mediated epileptiform discharges, also induced long-lasting I(mGluR(V)) in CA3 pyramidal cells. Similar to group I mGluR-mediated prolonged epileptiform discharges, persistent I(mGluR(V)) was no longer observed in preparations pretreated with inhibitors of tyrosine kinase, of extracellular signal-regulated kinase 1/2, or of mRNA protein synthesis. The results indicate that I(mGluR(V)) is an intrinsic plasticity mechanism associated with group I mGluR-mediated epileptogenesis.

Mechanisms of Group I mGluR-Dependent Long-Term Depression of NMDA Receptor–Mediated Transmission at Schaffer Collateral–CA1 Synapses

The mechanisms underlying group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) of N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic currents (EPSCsNMDAR) are poorly understood. Here we investigated the effects of ( R,S)-3,5-dihydroxyphenylglycine (DHPG), a selective agonist of group I mGluRs, on the EPSCsNMDAR in area CA1 of acute hippocampal slices from 6- to 8-wk Sprague-Dawley rats. DHPG acutely and persistently depressed the isolated EPSCNMDAR and transiently slowed its decay rate. Combined antagonism of mGluR1 and mGluR5 blocked the effects of DHPG. Strong calcium buffering with intracellular BAPTA did not reduce the acute depression or LTD, making the involvement of elevated postsynaptic calcium unlikely. The acute depression and LTD were not mediated by activation of tyrosine kinases or phosphatases, nor were they dependent on protein synthesis. However, the LTD was prevented by the intracellular actin-stabilizer jasplakinolide, raising the possibility that it was associated with a lateral movement of NMDARs. Supporting this hypothesis, when the effective spatial spread of synaptically released glutamate was increased using the glutamate transporter inhibitor TBOA, the resultant EPSCNMDAR did not undergo LTD in response to DHPG. Importantly, isolation of the extrasynaptic EPSCNMDAR by blockade of synaptic NMDARs with MK-801 showed that this was not due to a potentiation of the preexisting extrasynaptic component. These findings indicate that LTD of NMDAR-mediated synaptic transmission occurs via lateral movement of receptors away from the synapse.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Involvement of endocannabinoid signaling in the neuroprotective effects of subtype 1 metabotropic glutamate receptor antagonists in models of cerebral ischemia

Experimental evidence indicates that metabotropic glutamate (mGlu) receptors of the mGlu1 and mGlu5 subtypes play a differential role in models of cerebral ischemia and that only mGlu1 receptors are implicated in the pathways leading to postischemic neuronal injury. The localization of mGlu1 receptors in GABA-containing interneurons rather than in hippocampal CA1 pyramidal cells that are vulnerable to ischemia has prompted experimental studies that have demonstrated mGlu1 receptor antagonist agents attenuate postischemic injury by enhancing GABA-mediated neurotransmission, thus providing a new viewpoint on the neuroprotective mechanism of these pharmacological agents. In view of the recent discovery of a functional interaction between group I mGlu receptors and the cannabinoid system in the modulation of synaptic transmission, we propose a novel mechanism that predicts that the neuroprotective effects of mGlu1 receptor antagonists on CA1 pyramidal cells are mediated by a mechanism that overcomes the "synaptic circuit break" operated by endocannabinoids on GABAergic transmission.

Activation of group I metabotropic glutamate receptors induces long-term depression in the hippocampal CA1 region of adult rats in vitro

Previous studies have implicated that long-term depression (LTD) was developmentally regulated since LTD can be readily induced by low frequency stimulation (LFS) in acute hippocampal slices prepared from juvenile but not adult animals. Here, we have examined the LTD induced by LFS (1Hz, 900 pulses) paired with a certain pattern at the Schaffer collateral-CAl synapse in adult hippocampal slices. We found that, in the 90-day-old rat hippocampus, LTD could be induced reliably by LFS paired with stronger stimulus intensity than that used during baseline recording. However, this synaptic depression could be completely abolished by application of metabotropic glutamate receptor (mGluR) antagonist (S)-amethyl-4-carboxyphenylglycine (MCPG) which had no effect on that induced by the same protocol in the 16-day-old rat hippocampus. Furthermore, preincubation with group I mGluR antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and (S)-2-methyl-4-carboxyphenylglycine (LY367385), also completely prevented the LFS-induced LTD. In contrast, group II mGluR antagonist (2S)-a-ethylglutamic acid (EGLU), N-methyl-d-aspartate (NMDA) receptor antagonist APV and voltage-gated calcium channel antagonist nimodipine had no effect on the LFS-induced LTD. Taken together, these observations suggest that LFS paired with strong stimulus strength can efficiently induce group I mGluR-dependent LTD in the adult hippocampal CA1 region, proving insight into the functional significance of hippocampal mGluR-mediated LTD in learning and memory.

Roles of calcium-stimulated adenylyl cyclase and calmodulin-dependent protein kinase IV in the regulation of FMRP by group I metabotropic glutamate receptors

The fragile X syndrome is caused by the lack of fragile X mental retardation protein (FMRP) attributable to silencing of the FMR1 gene. The metabotropic glutamate receptors (mGluRs) in the CNS contribute to different brain functions, including learning/memory, mental disorders, drug addiction, and persistent pain. Most of the previous studies have been focused on downstream targets of FMRP in hippocampal neurons, and fewer studies have been reported for the second-messenger signaling pathways between group I mGluRs and FMRP. Furthermore, no molecular study has been performed in the anterior cingulate cortex (ACC), a key region involved in high brain cognitive and executive functions. In this study, we demonstrate that activation of group I mGluR upregulated FMRP in ACC neurons of adult mice through the Ca(2+)-dependent signaling pathways. Using genetic approaches, we found that Ca(2+)/calmodulin-stimulated adenylyl cyclase 1 (AC1) and calcium/calmodulin-dependent kinase IV (CaMKIV) contribute to the upregulation of FMRP induced by stimulating group I mGluRs. The upregulation of FMRP occurs at the transcriptional level. The cAMP-dependent protein kinase is activated by stimulating group I mGluRs through AC1 in ACC neurons. Both AC1 and CaMKIV contribute to the regulation of FMRP by group I mGluRs probably through cAMP response element-binding protein activation. Our study has provided the first evidence for a molecular link between group I mGluRs and FMRP in ACC neurons and may help us to understand the pathogenesis of fragile X syndrome.

Metabotropic glutamate receptor 1 (mGluR1) and 5 (mGluR5) regulate late phases of LTP and LTD in the hippocampal CA1 region in vitro

The group I metabotropic glutamate receptors, mGluR1 and mGluR5, exhibit differences in their regulation of synaptic plasticity, suggesting that these receptors may subserve separate functional roles in information storage. In addition, although effects in vivo are consistently described, conflicting reports of the involvement of mGluRs in hippocampal synaptic plasticity in vitro exist. We therefore addressed the involvement of mGluR1 and mGluR5 in long-term potentiation (LTP) and long-term depression (LTD) in the hippocampal CA1 region of adult male rats in vitro. The mGluR1 antagonist (S)-(+)-α-amino-4-carboxy-2-methylbenzene-acetic acid (LY367385) impaired both induction and late phases of both LTP and LTD, when applied before high-frequency tetanization (HFT; 100 Hz) or low-frequency stimulation (LFS; 1 Hz), respectively. Application after either HFT or LFS had no effect. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP), when given before HFT, inhibited both the induction and late phases of LTP. When given after HFT, late LTP was inhibited. MPEP, given prior to LFS, impaired LTD induction, although stable LTD was still expressed. Application after LFS significantly impaired late phases of LTD. Activation of protein synthesis may comprise a key mechanism underlying the group I mGluR contribution to synaptic plasticity. The mGluR5 agonist (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG) converted short-term depression into LTD. Effects were prevented by application of the protein synthesis inhibitor anisomycin, suggesting that protein synthesis is triggered by group I mGluR activation to enable persistency of synaptic plasticity. Taken together, these data support the notion that both mGluR1 and mGluR5 are critically involved in bidirectional synaptic plasticity in the CA1 region and may enable functional differences in information encoding through LTP and LTD.

Rapid direct excitation and long-lasting enhancement of NMDA response by group I metabotropic glutamate receptor activation of hypothalamic melanin-concentrating hormone neurons

The effect of group I metabotropic glutamate receptor (mGluR1 and mGluR5) activation on identified melanin-concentrating hormone (MCH) neurons was studied using patch-clamp recording in hypothalamic slices from green fluorescent protein-expressing transgenic mice. S-3,5-dihydroxyphenylglycine (DHPG), a selective group I mGluR agonist, depolarized MCH cells and increased spike frequency. The mGluR-mediated depolarization was not blocked with tetrodotoxin but was significantly reduced by replacement of extracellular Na+ with Tris, by Ni2+ or the Na+/Ca2+ exchanger blocker KB-R7943, or with BAPTA in the pipette, consistent with a mechanism based on activation of the Na+/Ca2+ exchanger. DHPG also decreased potassium currents. DHPG-induced depolarization was reduced by either mGluR1 or mGluR5 antagonists, suggesting involvement of both receptor subtypes. DHPG-induced depolarization desensitized; blockade of mGluR1 prevented the desensitization. Group I mGluR activation enhanced NMDA-evoked currents; this enhancement was remarkably long lasting and could be blocked by protein kinase A or C blockers. DHPG potentiated electrically evoked NMDA receptor-mediated postsynaptic currents, and mGluR5 antagonists blocked this action. Group I mGluRs increased spontaneous EPSCs in MCH neurons, possibly by stimulation of nearby mGluR-expressing hypocretin neurons. We found no tonic activation of mGluRs. However, electrical stimulation produced a slow inward current, which could be blocked by group I mGluR antagonists, suggesting high, but not low, levels of synaptically released glutamate activated mGluRs. Together, group I mGluRs increase MCH neuron activity by multiple presynaptic and postsynaptic mechanisms, suggesting mGluRs may therefore play a role in hypothalamic signaling relating to MCH neuron modulation of food intake and energy metabolism.

Selective stimulation of astrocyte calcium in situ does not affect neuronal excitatory synaptic activity

Astrocytes are considered the third component of the synapse, responding to neurotransmitter release from synaptic terminals and releasing gliotransmitters--including glutamate--in a Ca(2+)-dependent manner to affect neuronal synaptic activity. Many studies reporting astrocyte-driven neuronal activity have evoked astrocyte Ca(2+) increases by application of endogenous ligands that directly activate neuronal receptors, making astrocyte contribution to neuronal effect(s) difficult to determine. We have made transgenic mice that express a Gq-coupled receptor only in astrocytes to evoke astrocyte Ca(2+) increases using an agonist that does not bind endogenous receptors in brain. By recording from CA1 pyramidal cells in acute hippocampal slices from these mice, we demonstrate that widespread Ca(2+) elevations in 80%-90% of stratum radiatum astrocytes do not increase neuronal Ca(2+), produce neuronal slow inward currents, or affect excitatory synaptic activity. Our findings call into question the developing consensus that Ca(2+)-dependent glutamate release by astrocytes directly affects neuronal synaptic activity in situ.

TRPC3 channels are necessary for brain-derived neurotrophic factor to activate a nonselective cationic current and to induce dendritic spine formation

Brain-derived neurotrophic factor (BDNF) exerts prominent effects on hippocampal neurons, but the mechanisms that initiate its actions are poorly understood. We report here that BDNF evokes a slowly developing and sustained nonselective cationic current (I(BDNF)) in CA1 pyramidal neurons. These responses require phospholipase C, IP3 receptors, Ca2+ stores, and Ca2+ influx, suggesting the involvement of transient receptor potential canonical subfamily (TRPC) channels. Indeed, I(BDNF) is absent after small interfering RNA-mediated TRPC3 knockdown. The sustained kinetics of I(BDNF) appears to depend on phosphatidylinositol 3-kinase-mediated TRPC3 membrane insertion, as shown by surface biotinylation assays. Slowly emerging membrane currents after theta burst stimulation are sensitive to the scavenger TrkB-IgG and TRPC inhibitors, suggesting I(BDNF) activation by evoked released of endogenous, native BDNF. Last, TRPC3 channels are necessary for BDNF to increase dendritic spine density. Thus, TRPC channels emerge as novel mediators of BDNF-mediated dendritic remodeling through the activation of a slowly developing and sustained membrane depolarization.

MGluR-mediated calcium waves that invade the soma regulate firing in layer V medial prefrontal cortical pyramidal neurons

Factors that influence the activity of prefrontal cortex (PFC) pyramidal neurons are likely to play an important role in working memory function. One such factor may be the release of Ca2+ from intracellular stores. Here we investigate the hypothesis that metabotropic glutamate receptors (mGluRs)-mediated waves of internally released Ca2+ can regulate the intrinsic excitability and firing patterns of PFC pyramidal neurons. Synaptic or focal pharmacological activation of mGluRs triggered Ca2+ waves in the dendrites and somata of layer V medial PFC pyramidal neurons. These Ca2+ waves often evoked a transient SK-mediated hyperpolarization followed by a prolonged depolarization that respectively decreased and increased neuronal excitability. Generation of the hyperpolarization depended on whether the Ca2+ wave invaded or came near to the soma. The depolarization also depended on the extent of Ca2+ wave propagation. We tested factors that influence the propagation of Ca2+ waves into the soma. Stimulating more synapses, increasing inositol trisphosphate concentration near the soma, and priming with physiological trains of action potentials all enhanced the amplitude and likelihood of evoking somatic Ca2+ waves. These results suggest that mGluR-mediated Ca2+ waves may regulate firing patterns of PFC pyramidal neurons engaged by working memory, particularly under conditions that favor the propagation of Ca2+ waves into the soma.

TBOA-sensitive uptake limits glutamate penetration into brain slices to a few micrometers

Removal of neurotransmitter from the extracellular space is crucial for normal functioning of the central nervous system. In this study, we have used high-affinity metabotropic glutamate receptors (mGluRs) expressed by hippocampal CA1 pyramidal cells to test how far bath-applied glutamate penetrates into slice tissue before being removed by uptake mechanisms. Activation of group I mGluRs by 100 microM DHPG produced an inward current of -48+/-10pA (I(mGluR)), which was blocked by application of group I mGluR antagonists. In contrast, bath application of 100 microM glutamate in the presence of a ionotropic glutamate receptor antagonist and TTX did not activate I(mGluR) in CA1 cells patch-clamped at a depth of approximately 30 microm. Similarly, sole inhibition of glutamate transporters by the broad-spectrum glutamate transporter antagonist TBOA did not induce I(mGluR) under the same conditions. Only if glutamate was co-applied with TBOA an I(mGluR) of -39+/-8pA was recorded which was also blocked by group I antagonists. The data suggest that TBOA-sensitive uptake mechanisms are able to maintain a steep concentration gradient of glutamate to such a degree that a CA1 neuron at a depth of 30 microm is exposed to low extracellular glutamate levels that are not sufficient to induce a detectable activation of group I mGluRs (< 2 microM).

Transient receptor potential channels as novel effectors of brain-derived neurotrophic factor signaling: potential implications for Rett syndrome

In addition to their prominent role as survival signals for neurons in the developing nervous system, neurotrophins have established their significance in the adult brain as well, where their modulation of synaptic transmission and plasticity may participate in associative learning and memory. These crucial activities are primarily the result of neurotrophin regulation of intracellular Ca(2+) homeostasis and, ultimately, changes in gene expression. Outlined in the following review is a synopsis of neurotrophin signaling with a particular focus upon brain-derived neurotrophic factor (BDNF) and its role in hippocampal synaptic plasticity and neuronal Ca(2+) homeostasis. Neurotrophin signaling through tropomyosin-related kinase (Trk) and pan-neurotrophin receptor 75 kD (p75(NTR)) receptors are also discussed, reviewing recent results that indicate signaling through these two receptor modalities leads to opposing cellular outcomes. We also provide an intriguing look into the transient receptor potential channel (TRPC) family of ion channels as distinctive targets of BDNF signaling; these channels are critical for capacitative Ca(2+) entry, which, in due course, mediates changes in neuronal structure including dendritic spine density. Finally, we expand these topics into an exploration of mental retardation (MR), in particular Rett Syndrome (RTT), where dendritic spine abnormalities may underlie cognitive impairments. We propose that understanding the role of neurotrophins in synapse formation, plasticity, and maintenance will make fundamental contributions to the development of therapeutic strategies to improve cognitive function in developmental disorders associated with MR.

Priming of long-term potentiation mediated by ryanodine receptor activation in rat hippocampal slices

Administration of the Group 1 metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) facilitates ("primes") subsequent long-term potentiation (LTP) through a phospholipase C signaling cascade that may involve release of Ca2+ from the endoplasmic reticulum (ER). We investigated the intracellular calcium pathways involved in this priming effect, recording field potentials from area CA1 of rat hippocampal slices before and after high-frequency stimulation. The priming of LTP by DHPG was prevented by co-administration of cyclopiazonic acid, which depletes ER Ca2+ stores. The priming effect was also blocked by the ryanodine receptor (RYR) antagonist ryanodine (RYA, 100 microM). In contrast, a low dose of RYA (10 microM) which opens the RYR channel, by itself primed LTP. In addition to RYR activation, entry of extracellular calcium through store-operated channels appears necessary for priming, since diverse treatments known to impede store-operated channel activity completely blocked both RYA and DHPG priming effects. Thus, RYR activation plays a critical role in the priming of LTP by Group 1 mGluRs, and this effect is coupled to the entry of extracellular calcium, probably through store-operated calcium channels.

Group I metabotropic glutamate receptor-induced Ca(2+)-gradients in rat superficial spinal dorsal horn neurons

Here, we investigated changes in the free cytosolic Ca(2+) concentration ([Ca(2+)](i)), induced by the pharmacological activation of metabotropic glutamate receptors (mGluRs), in nociceptive neurons of the superficial spinal dorsal horn. Microfluorometric Ca(2+) measurements with fura-2 in a lumbar spinal cord slice preparation from young rats were used. Bath application of the specific group I mGluR agonist (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG) resulted in a distinct increase of [Ca(2+)](i) in most of the neurons in superficial dorsal horn. In contrast, activation of groups II or III mGluRs by DCG-IV or l-AP4, respectively, failed to evoke any significant change in [Ca(2+)](i). The effect of (S)-3,5-DHPG was mediated by both group I subtypes mGluR1 and mGluR5, since combined pre-treatment with the subtype antagonists (S)-4-CPG and MPEP was necessary to abolish the [Ca(2+)](i) increase. Depleting intracellular Ca(2+) stores with CPA or inhibiting IP(3)-receptors with 2-APB, respectively, reduced the (S)-3,5-DHPG-evoked [Ca(2+)](i) increase significantly. Inhibition of voltage-dependent L-type Ca(2+) channels (VDCCs) by verapamil or nicardipine reduced the (S)-3,5-DHPG-induced [Ca(2+)](i) rise likewise. Thus, in rat spinal cord, (S)-3,5-DHPG enhances Ca(2+) signalling in superficial dorsal horn neurons, mediated by the release of Ca(2+) from IP(3)-sensitive intracellular stores and by an influx through L-type VDCCs. This may be relevant to the processing of nociceptive information in the spinal cord.

Nicotinic receptor-mediated enhancement of long-term potentiation involves activation of metabotropic glutamate receptors and ryanodine-sensitive calcium stores in the dentate gyrus

Little is known about the mechanisms underlying the enhancement of long-term potentiation (LTP) by nicotine. In the present study, the mechanisms of nicotinic enhancement of LTP were investigated in the rat dentate gyrus in vitro. Acute application of nicotine enhanced LTP induction, an action requiring activation of alpha7 nicotinic acetylcholine receptors (nAChRs), as it was blocked by the nAChR antagonist methyl-lycaconitine, mimicked by the acetylcholine receptor agonist choline and absent in mutant mice null for alpha7 nAChR. Nicotinic enhancement of LTP was both dependent on N-methyl-D-aspartate receptor activation, as no LTP was induced in the presence of nicotine and an N-methyl-D-aspartate receptor antagonist, and expressed post-synaptically, as no change in paired-pulse ratio accompanied nicotinic enhancement of LTP. The nicotinic-enhanced component of LTP, unlike control LTP, was dependent on activation of metabotropic glutamate receptors (mGluRs), being inhibited by the group I/II antagonist LY341495 and the mGluR5 antagonist MPEP, and also dependent on influx of Ca via L-type Ca channels and release from ryanodine (RyR)-sensitive intracellular stores, being prevented by nifedipine and RyR, respectively. It is suggested that nicotinic activation of the Ca-permeable alpha7 nAChRs fills RyR Ca stores and release of Ca from such stores by high-frequency stimulation via Ca-induced Ca release and activation of mGluRs induces an additional component of LTP which summates with control LTP. Chronic application of nicotine in vivo also enhanced LTP induction in slices and was dependent on activation of mGluRs and Ca release from RyR-sensitive intracellular stores, although acutely applied nicotine was not required for such enhanced LTP.

Developmental dependence, the role of the kinases p38 MAPK and PKC, and the involvement of tumor necrosis factor-R1 in the induction of mGlu-5 LTD in the dentate gyrus

The mechanisms of mGluR-LTD were studied in the dentate gyrus in vitro. The most effective protocol for inducing mGluR-LTD in 6-8 week animals was brief high frequency stimulation (HFS) applied in the presence of the NMDAR antagonist AP5. Evidence for HFS inducing LTD via activation of perisynaptically located mGluRs was established, as an inhibitor of glutamate transporter potentiated HFS-LTD. HFS-LTD was mainly mediated by activation of mGluR5, although a partial involvement of mGluR1 was found. (RS)-3,5-Dihydroxyphenylglycine (DHPG) also induced LTD, but in an age dependent manner, being large in 2 week animals but absent in 6-8 week animals. DHPG-LTD in the dentate gyrus also had a much slower rise time than that in CA1, and unlike CA1, the expression/maintenance of mGluR-LTD was not inhibited by mGluR antagonists. The use of pharmacological inhibitors showed that the induction of HFS-LTD was partially dependent upon activation of L-type Ca channels, release of Ca from ryanodine receptor-sensitive intracellular Ca stores, and the kinases p38 mitogen-activated protein kinase (MAPK), protein kinase C (PKC), but not c-Jun N-terminal kinase or COX-2. Evidence for the involvement of tumor necrosis factor-receptor 1 (TNF-R1) in the induction of mGluR-LTD was presented in the present study, with both HFS-mGluR-LTD and DHPG-LTD being absent in mutant mice null for TNF-R1.

Long-term depression at the mossy fiber-deep cerebellar nucleus synapse

Several lines of evidence have indicated that the deep cerebellar nuclei (DCN) are a site of memory storage for certain forms of motor learning, most notably associative eyelid conditioning. In particular, these experiments, together with network models, have implicated the excitatory glutamatergic synapse between mossy fibers and DCN neurons in this memory trace. However, to date, evidence for persistent use-dependent change in the strength of this synapse has been almost entirely absent. Here, we report that high-frequency burst stimulation of mossy fibers, either alone or paired with postsynaptic depolarization, gives rise to long-term depression (LTD) of the mossy fiber-DCN synapse. This form of LTD is not associated with changes in the paired-pulse ratio and is blocked by loading with a postsynaptic Ca2+ chelator but not by bath application of an NMDA receptor antagonist. Mossy fiber-DCN LTD requires activation of a group I metabotropic glutamate receptor (mGluR) and protein translation. Unlike mGluR/translation-dependent LTD in other brain regions, this form of LTD requires mGluR1 and is mGluR5 independent.

Low-frequency stimulation induces a new form of LTP, metabotropic glutamate (mGlu5) receptor- and PKA-dependent, in the CA1 area of the rat hippocampus

Low frequency-induced short-term synaptic plasticity was investigated in hippocampal slices with 60-electrode recording array. Remarkably, the application of low-frequency stimulation (1 Hz) for a short duration (3-5 min) resulted in the induction of a slow-onset long-term potentiation (LTP) in the immediate vicinity of the stimulated electrode. This phenomenon was observed exclusively in the CA1 subfield, neither in the CA3 area nor in the dentate gyrus. The induction of this slow-onset LTP required neither N-methyl-D-aspartate (NMDA) nor non-NMDA ionotropic receptor activation but was strongly dependent on metabotropic glutamate mGlu(5) receptor stimulation and [Ca(2+)]i increase. In addition, this form of synaptic plasticity was associated with an increase in cAMP concentration and required protein kinase A activation. Paired-pulse facilitation ratio and presynaptic fiber volley amplitude were unaffected when this LTP was triggered, suggesting the involvement of postsynaptic modifications. Although mitogen activated protein kinase pathway was stimulated after the application of low frequency, the induction and maintenance of this slow-onset LTP were not dependent on the activation of this intracellular pathway. The direct activation of adenylyl cyclase with forskolin also induced a synaptic enhancement displaying similar features. This new form of LTP could represent the mnesic engram of mild and repetitive stimulation involved in latent learning.