Group I metabotropic glutamate receptors activate a calcium-sensitive transient receptor potential-like conductance in rat hippocampus

NMDAR-mediated activation of pannexin1 channels contributes to the detonator properties of hippocampal mossy fiber synapses

Pannexins are large-pore ion channels expressed throughout the mammalian brain that participate in various neuropathologies; however, their physiological roles remain obscure. Here, we report that pannexin1 channels (Panx1) can be synaptically activated under physiological recording conditions in rodent acute hippocampal slices. Specifically, NMDA receptor (NMDAR)-mediated responses at the mossy fiber to CA3 pyramidal cell synapse were followed by a slow postsynaptic inward current that could activate CA3 pyramidal cells but was absent in Panx1 knockout mice. Immunoelectron microscopy revealed that Panx1 was localized near the postsynaptic density. Further, Panx1-mediated currents were potentiated by metabotropic receptors and bidirectionally modulated by burst-timing-dependent plasticity of NMDAR-mediated transmission. Lastly, Panx1 channels were preferentially recruited when NMDAR activation enters a supralinear regime, resulting in temporally delayed burst-firing. Thus, Panx1 can contribute to synaptic amplification and broadening the temporal associativity window for co-activated pyramidal cells, thereby supporting the auto-associative functions of the CA3 region.

Subunit composition, molecular environment, and activation of native TRPC channels encoded by their interactomes

In the mammalian brain TRPC channels, a family of Ca²⁺-permeable cation channels, are involved in a variety of processes from neuronal growth and synapse formation to transmitter release, synaptic transmission and plasticity. The molecular appearance and operation of native TRPC channels, however, remained poorly understood. Here, we used high-resolution proteomics to show that TRPC channels in the rodent brain are macro-molecular complexes of more than 1 MDa in size that result from the co-assembly of the tetrameric channel core with an ensemble of interacting proteins (interactome). The core(s) of TRPC1-, C4-, and C5-containing channels are mostly heteromers with defined stoichiometries for each subtype, whereas TRPC3, C6, and C7 preferentially form homomers. In addition, TRPC1/C4/C5 channels may co-assemble with the metabotropic glutamate receptor mGluR1, thus guaranteeing both specificity and reliability of channel activation via the phospholipase-Ca²⁺ pathway. Our results unveil the subunit composition of native TRPC channels and resolve the molecular details underlying their activation.

Group 1 metabotropic glutamate receptor 5 is involved in synaptically-induced Ca2+-spikes and cell death in cultured rat hippocampal neurons

Group 1 metabotropic glutamate receptors (mGluRs) can positively affect postsynaptic neuronal excitability and epileptogenesis. The objective of the present study was to determine whether group 1 mGluRs might be involved in synaptically-induced intracellular free Ca²⁺ concentration ([Ca²⁺]_i) spikes and neuronal cell death induced by 0.1 mM Mg²⁺ and 10 µM glycine in cultured rat hippocampal neurons from embryonic day 17 fetal Sprague–Dawley rats using imaging methods for Ca²⁺ and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays for cell survival. Reduction of extracellular Mg²⁺ concentration ([Mg²⁺]_o) to 0.1 mM induced repetitive [Ca²⁺]_i spikes within 30 sec at day 11.5. The mGluR5 antagonist 6-Methyl-2-(phenylethynyl) pyridine (MPEP) almost completely inhibited the [Ca²⁺]_i spikes, but the mGluR1 antagonist LY367385 did not. The group 1 mGluRs agonist, 3,5-dihydroxyphenylglycine (DHPG), significantly increased the [Ca²⁺]_i spikes. The phospholipase C inhibitor U73122 significantly inhibited the [Ca²⁺]_i spikes in the absence or presence of DHPG. The IP₃ receptor antagonist 2-aminoethoxydiphenyl borate or the ryanodine receptor antagonist 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate also significantly inhibited the [Ca²⁺]_i spikes in the absence or presence of DHPG. The TRPC channel inhibitors SKF96365 and flufenamic acid significantly inhibited the [Ca²⁺]_i spikes in the absence or presence of DHPG. The mGluR5 antagonist MPEP significantly increased the neuronal cell survival, but mGluR1 antagonist LY367385 did not. These results suggest a possibility that mGluR5 is involved in synaptically-induced [Ca²⁺]_i spikes and neuronal cell death in cultured rat hippocampal neurons by releasing Ca²⁺ from IP₃ and ryanodine-sensitive intracellular stores and activating TRPC channels.

Membrane trafficking and positioning of mGluRs at presynaptic and postsynaptic sites of excitatory synapses

The plethora of functions of glutamate in the brain are mediated by the complementary actions of ionotropic and metabotropic glutamate receptors (mGluRs). The ionotropic glutamate receptors carry most of the fast excitatory transmission, while mGluRs modulate transmission on longer timescales by triggering multiple intracellular signaling pathways. As such, mGluRs mediate critical aspects of synaptic transmission and plasticity. Interestingly, at synapses, mGluRs operate at both sides of the cleft, and thus bidirectionally exert the effects of glutamate. At postsynaptic sites, group I mGluRs act to modulate excitability and plasticity. At presynaptic sites, group II and III mGluRs act as auto-receptors, modulating release properties in an activity-dependent manner. Thus, synaptic mGluRs are essential signal integrators that functionally couple presynaptic and postsynaptic mechanisms of transmission and plasticity. Understanding how these receptors reach the membrane and are positioned relative to the presynaptic glutamate release site are therefore important aspects of synapse biology. In this review, we will discuss the currently known mechanisms underlying the trafficking and positioning of mGluRs at and around synapses, and how these mechanisms contribute to synaptic functioning. We will highlight outstanding questions and present an outlook on how recent technological developments will move this exciting research field forward.

mGlu1 potentiation enhances prelimbic somatostatin interneuron activity to rescue schizophrenia-like physiological and cognitive deficits

Evidence for prefrontal cortical (PFC) GABAergic dysfunction is one of the most consistent findings in schizophrenia and may contribute to cognitive deficits. Recent studies suggest that the mGlu1 subtype of metabotropic glutamate receptor regulates cortical inhibition; however, understanding the mechanisms through which mGlu1 positive allosteric modulators (PAMs) regulate PFC microcircuit function and cognition is essential for advancing these potential therapeutics toward the clinic. We report a series of electrophysiology, optogenetic, pharmacological magnetic resonance imaging, and animal behavior studies demonstrating that activation of mGlu1 receptors increases inhibitory transmission in the prelimbic PFC by selective excitation of somatostatin-expressing interneurons (SST-INs). An mGlu1 PAM reverses cortical hyperactivity and concomitant cognitive deficits induced by N-methyl-d-aspartate (NMDA) receptor antagonists. Using in vivo optogenetics, we show that prelimbic SST-INs are necessary for mGlu1 PAM efficacy. Collectively, these findings suggest that mGlu1 PAMs could reverse cortical GABAergic deficits and exhibit efficacy in treating cognitive dysfunction in schizophrenia.

Insights on the Functional Interaction between Group 1 Metabotropic Glutamate Receptors (mGluRI) and ErbB Receptors

It is well-appreciated that phosphorylation is an essential post-translational mechanism of regulation for several proteins, including group 1 metabotropic glutamate receptors (mGluRI), mGluR1, and mGluR5 subtypes. While contributions of various serine/threonine protein kinases on mGluRI modulation have been recognized, the functional role of tyrosine kinases (TKs) is less acknowledged. Here, while describing current evidence supporting that mGluRI are targets of TKs, we mainly focus on the modulatory roles of the ErbB tyrosine kinases receptors—activated by the neurotrophic factors neuregulins (NRGs)—on mGluRI function. Available evidence suggests that mGluRI activity is tightly dependent on ErbB signaling, and that ErbB’s modulation profoundly influences mGluRI-dependent effects on neurotransmission, neuronal excitability, synaptic plasticity, and learning and memory processes.

How TRPC Channels Modulate Hippocampal Function

Transient receptor potential canonical (TRPC) proteins constitute a group of receptor-operated calcium-permeable nonselective cationic membrane channels of the TRP superfamily. They are largely expressed in the hippocampus and are able to modulate neuronal functions. Accordingly, they have been involved in different hippocampal functions such as learning processes and different types of memories, as well as hippocampal dysfunctions such as seizures. This review covers the mechanisms of activation of these channels, how these channels can modulate neuronal excitability, in particular the after-burst hyperpolarization, and in the persistent activity, how they control synaptic plasticity including pre- and postsynaptic processes and how they can interfere with cell survival and neurogenesis.

The involvement of TRP channels in memory formation and task retrieval in a passive avoidance task in one-day old chicks

An increase in the intracellular Ca2+ level in neurons is one of the main steps in the memory formation cascade. The increase results from extracellular Ca2+ influx by activation of ionotropic glutamate receptors and release from intracellular stores by the stimulation of IP3 receptors (IP3Rs) via group I metabotropic glutamate receptors (mGluR1/5). Recent data indicate an additional mechanism resulting in Ca2+ influx into neurons, triggered by intracellular signals that are directly connected to the activation of group I mGluRs. This influx occurs through transient receptor potential (TRP) channels, which are permeable to Na+, K+ and, mainly, Ca2+. These channels are activated by increases in intracellular Ca2+, diacylglycerol (DAC) and inositol 1,4,5-triphosphate (IP3) level resulting from a group I mGluR activation. The aim of the present study was to investigate the participation of TRP channels, especially from TRPC and TRPV groups, in memory consolidation and reconsolidation and memory retrieval processes in a passive avoidance task in one-day old chicks. TRP channels were blocked by the injection of the unspecific channel modulators SKF 96365 (2.5 µl 30 µM/hemisphere) and 2-APB (2.5 µl 10 µM/hemisphere) directly into the intermediate medial mesopallium (IMM) region of the chick brain immediately after initial training or after a reminder. The inhibition of specific TRP channels (TRPV1, TRPV3 or TRPC3) was achieved by the application of selective antibodies. Our results demonstrate that the inhibition of TRP channels by the application of both modulators disrupted memory consolidation, resulting in permanent task amnesia. The inhibition of the TRPV1, TRPC3 and TRPV3 channels by specific antibodies resulted in similar amnesia. Moreover, the inhibition of TRP channels by SKF 96365 and 2-APB at different time points after initial training or after the reminder also resulted in amnesia, indicating the role of TRP channels in memory retrieval. The inhibition of calcium influx through these channels resulted in permanent memory disruption, which suggests that the calcium signal generated by TRP channels is crucial for memory formation and retrieval processes. For the first time, the important role of TRPV3 channels in memory formation was demonstrated.

Role of the TRPC1 Channel in Hippocampal Long-Term Depression and in Spatial Memory Extinction

Group I metabotropic glutamate receptors (mGluR) are involved in various forms of synaptic plasticity that are believed to underlie declarative memory. We previously showed that mGluR5 specifically activates channels containing TRPC1, an isoform of the canonical family of Transient Receptor Potential channels highly expressed in the CA1-3 regions of the hippocampus. Using a tamoxifen-inducible conditional knockout model, we show here that the acute deletion of the Trpc1 gene alters the extinction of spatial reference memory. mGluR-induced long-term depression, which is partially responsible for memory extinction, was impaired in these mice. Similar results were obtained in vitro and in vivo by inhibiting the channel by its most specific inhibitor, Pico145. Among the numerous known postsynaptic pathways activated by type I mGluR, we observed that the deletion of Trpc1 impaired the activation of ERK1/2 and the subsequent expression of Arc, an immediate early gene that plays a key role in AMPA receptors endocytosis and subsequent long-term depression.

Metabotropic Glutamate Receptors at the Aged Mossy Fiber - CA3 Synapse of the Hippocampus

Metabotropic glutamate receptors (mGluRs) are a group of G-protein-coupled receptors that exert a broad array of modulatory actions at excitatory synapses of the central nervous system. In the hippocampus, the selective activation of the different mGluRs modulates the intrinsic excitability, the strength of synaptic transmission, and induces multiple forms of long-term plasticity. Despite the relevance of mGluRs in the normal function of the hippocampus, we know very little about the changes that mGluRs functionality undergoes during the non-pathological aging. Here, we review data concerning the physiological actions of mGluRs, with particular emphasis on hippocampal area CA3. Later, we examine changes in the expression and functionality of mGluRs during the aging process. We complement this review with original data showing an array of electrophysiological modifications observed in the synaptic transmission and intrinsic excitability of aged CA3 pyramidal cells in response to the pharmacological stimulation of the different mGluRs.

Synergy of Glutamatergic and Cholinergic Modulation Induces Plateau Potentials in Hippocampal OLM Interneurons

Oriens-lacunosum moleculare (OLM) cells are hippocampal inhibitory interneurons that are implicated in the regulation of information flow in the CA1 circuit, inhibiting cortical inputs to distal pyramidal cell dendrites, whilst disinhibiting CA3 inputs to pyramidal cells. OLM cells express metabotropic cholinergic (mAChR) and glutamatergic (mGluR) receptors, so modulation of these cells via these receptors may contribute to switching between functional modes of the hippocampus. Using a transgenic mouse line to identify OLM cells, we found that both mAChR and mGluR activation caused the cells to exhibit long-lasting depolarizing plateau potentials following evoked spike trains. Both mAChR- and mGluR-induced plateau potentials were eliminated by blocking transient receptor potential (TRP) channels, and were dependent on intracellular calcium concentration and calcium entry. Pharmacological tests indicated that Group I mGluRs are responsible for the glutamatergic induction of plateaus. There was also a pronounced synergy between the cholinergic and glutamatergic modulation, plateau potentials being generated by agonists applied together at concentrations too low to elicit any change when applied individually. This synergy could enable OLM cells to function as coincidence detectors of different neuromodulatory systems, leading to their enhanced and prolonged activation and a functional change in information flow within the hippocampus.

Glutamatergic Signaling in the Central Nervous System: Ionotropic and Metabotropic Receptors in Concert

Glutamate serves as both the mammalian brain's primary excitatory neurotransmitter and as a key neuromodulator to control synapse and circuit function over a wide range of spatial and temporal scales. This functional diversity is decoded by two receptor families: ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). The challenges posed by the complexity and physiological importance of each of these subtypes has limited our appreciation and understanding of how these receptors work in concert. In this review, by comparing both receptor families with a focus on their crosstalk, we argue for a more holistic understanding of neural glutamate signaling.

Hippocampal mGluR1-dependent long-term potentiation requires NAADP-mediated acidic store Ca2+ signaling

Acidic organelles, such as endosomes and lysosomes, store Ca²⁺ that is released in response to intracellular increases in the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). In neurons, NAADP and Ca²⁺ signaling contribute to synaptic plasticity, a process of activity-dependent long-term potentiation (LTP) [or, alternatively, long-term depression (LTD)] of synaptic strength and neuronal transmission that is critical for neuronal function and memory formation. We explored the function of and mechanisms regulating acidic Ca²⁺ store signaling in murine hippocampal neurons. We found that metabotropic glutamate receptor 1 (mGluR1) was coupled to NAADP signaling that elicited Ca²⁺ release from acidic stores. In turn, this released Ca²⁺-mediated mGluR1-dependent LTP by transiently inhibiting SK-type K⁺ channels, possibly through the activation of protein phosphatase 2A. Genetically removing two-pore channels (TPCs), which are endolysosomal-specific ion channels, switched the polarity of plasticity from LTP to LTD, indicating the importance of specific receptor store coupling and providing mechanistic insight into how mGluR1 can produce both synaptic potentiation and synaptic depression.

Activation of TRPC1 Channel by Metabotropic Glutamate Receptor mGluR5 Modulates Synaptic Plasticity and Spatial Working Memory

Group I metabotropic glutamate receptors, in particular mGluR5, have been implicated in various forms of synaptic plasticity that are believed to underlie declarative memory. We observed that mGluR5 specifically activated a channel containing TRPC1, an isoform of the canonical family of transient receptor potential (TRPC) channels highly expressed in CA1-3 regions of the hippocampus. TRPC1 is able to form tetrameric complexes with TRPC4 and/or TRPC5 isoforms. TRPC1/4/5 complexes have recently been involved in the efficiency of synaptic transmission in the hippocampus. We therefore used a mouse model devoid of TRPC1 expression to investigate the involvement of mGluR5-TRPC1 pathway in synaptic plasticity and memory formation. Trpc1^-/- mice showed alterations in spatial working memory and fear conditioning. Activation of mGluR increased synaptic excitability in neurons from WT but not from Trpc1^-/- mice. LTP triggered by a theta burst could not maintain over time in brain slices from Trpc1^-/- mice. mGluR-induced LTD was also impaired in these mice. Finally, acute inhibition of TRPC1 by Pico145 on isolated neurons or on brain slices mimicked the genetic depletion of Trpc1 and inhibited mGluR-induced entry of cations and subsequent effects on synaptic plasticity, excluding developmental or compensatory mechanisms in Trpc1^-/- mice. In summary, our results indicate that TRPC1 plays a role in synaptic plasticity and spatial working memory processes.

TRPC1 Channels Are Expressed in Pyramidal Neurons and in a Subset of Somatostatin Interneurons in the Rat Neocortex

Disturbances in calcium homeostasis due to canonical transient receptor potential (TRPC) and/or store-operated calcium (SOC) channels can play a key role in a large number of brain disorders. TRPC channels are plasma membrane cation channels included in the transient receptor potential (TRP) superfamily. The most widely distributed member of the TRPC subfamily in the brain is TRPC1, which is frequently linked to group I metabotropic glutamate receptors (mGluRs) and to the components of SOC channels. Proposing TRPC/SOC channels as a therapeutic target in neurological diseases previously requires a detailed knowledge of the distribution of such molecules in the brain. The aim of our study was to analyze the neuroanatomical distribution of TRPC1 in the rat neocortex. By double- and triple-labeling and confocal microscopy, we tested the presence of TRPC1 by using a series of specific neurochemical markers. TRPC1 was abundant in SMI 32-positive pyramidal neurons, and in some glutamic acid decarboxylase 67 (GAD67) interneurons, but was lacking in glial fibrillary acidic protein (GFAP)-positive glial cells. In neurons it colocalized with postsynaptic marker MAP2 in cell bodies and apical dendritic trunks and it was virtually absent in synaptophysin-immunoreactive terminals. By using a panel of antibodies to classify interneurons, we identified the GABAergic interneurons that contained TRPC1. TRPC1 was lacking in basket and chandelier parvalbumin (PVALB) cells, and a very low percentage of calretinin (CALR) or calbindin (CALB) interneurons expressed TRPC1. Moreover, 63% of somatostatin (SST) expressing-cells and 37% of reelin-positive cells expressed TRPC1. All the SST/TRPC1 double-labeled cells, many of which were presumptive Martinotti cells (MC), were positive for reelin. The presence of TRPC1 in the somata and apical dendritic trunks of neocortical pyramidal cells suggests a role for this channel in sensory processing and synaptic plasticity. Conversely in SST/reelin interneurons, TRPC1 could modulate GABAergic transmission, which is responsible for shaping the coordinated activity of the pyramidal cells in the cortical network. In future studies, it would be relevant to investigate whether TRPC1 could be involved in the expression or processing of reelin in SST inhibitory interneurons.

Control of neuronal excitability by Group I metabotropic glutamate receptors

Metabotropic glutamate (mGlu) receptors couple through G proteins to regulate a large number of cell functions. Eight mGlu receptor isoforms have been cloned and classified into three Groups based on sequence, signal transduction mechanisms and pharmacology. This review will focus on Group I mGlu receptors, comprising the isoforms mGlu1 and mGlu5. Activation of these receptors initiates both G protein-dependent and -independent signal transduction pathways. The G-protein-dependent pathway involves mainly Gαq, which can activate PLCβ, leading initially to the formation of IP3 and diacylglycerol. IP3 can release Ca2+ from cellular stores resulting in activation of Ca2+-dependent ion channels. Intracellular Ca2+, together with diacylglycerol, activates PKC, which has many protein targets, including ion channels. Thus, activation of the G-protein-dependent pathway affects cellular excitability though several different effectors. In parallel, G protein-independent pathways lead to activation of non-selective cationic currents and metabotropic synaptic currents and potentials. Here, we provide a survey of the membrane transport proteins responsible for these electrical effects of Group I metabotropic glutamate receptors.

Differential Contribution of Ca2+-Dependent Mechanisms to Hyperexcitability in Layer V Neurons of the Medial Entorhinal Cortex

Temporal lobe epilepsy is characterized by recurrent seizures in one or both temporal lobes of the brain; some in vitro models show that epileptiform discharges initiate in entorhinal layer V neurons and then spread into other areas of the temporal lobe. We previously found that, in the presence of GABA_A receptor antagonists, stimulation of afferent fibers, terminating both at proximal and distal dendritic locations, initiated hyperexcitable bursts in layer V medial entorhinal neurons. We investigated the differential contribution of Ca²⁺-dependent mechanisms to the plateaus underlying these bursts at proximal and distal synapses. We found that the NMDA glutamatergic antagonist D,L-2-amino-5-phosphonovaleric acid (APV; 50 μM) reduced both the area and duration of the bursts at both proximal and distal synapses by about half. The L-type Ca²⁺ channel blocker nimodipine (10 μM) and the R- and T-type Ca²⁺ channel blocker NiCl₂ (200 μM) decreased the area of the bursts to a lesser extent; none of these effects appeared to be location-dependent. Remarkably, the perfusion of flufenamic acid (FFA; 100 μM), to block Ca²⁺-activated non-selective cation currents (I_CAN) mediated by transient receptor potential (TRP) channels, had a location-dependent effect, by abolishing burst firing and switching the suprathreshold response to a single action potential (AP) for proximal stimulation, but only minimally affecting the bursts evoked by distal stimulation. A similar outcome was found when FFA was pressure-applied locally around the proximal dendrite of the recorded neurons and in the presence of a selective blocker of melastatin TRP (TRPM) channels, 9-phenanthrol (100 μM), whereas a selective blocker of canonical TRP (TRPC) channels, SKF 96365, did not affect the bursts. These results indicate that different mechanisms might contribute to the initiation of hyperexcitability in layer V neurons at proximal and distal synapses and could shed light on the initiation of epileptiform activity in the entorhinal cortex.

Transduction of group I mGluR-mediated synaptic plasticity by β-arrestin2 signalling

Conventional signalling by the group I metabotropic glutamate receptors, mGluR1 and mGluR5, occurs through G-protein coupling, but evidence suggests they might also utilize other, non-canonical effector pathways. Here we test whether group I mGluRs require β-arrestin signalling during specific forms of plasticity at hippocampal excitatory synapses. We find that genetic ablation of β-arrestin2, but not β-arrestin1, results in deficits in plasticity mediated by mGlu1 receptors in CA3 pyramidal neurons and by mGlu5 receptors in CA1 pyramidal neurons. Pharmacological studies additionally support roles for Src kinases and MAPK/ERK downstream of β-arrestin2 in CA3 neurons. mGluR1 modulation of intrinsic conductances is otherwise preserved in β-arrestin2^−/− mice with the exception of a rebound depolarization, and non-mGluR-mediated long-term potentiation is unaltered. These results reveal a signalling pathway engaged by group I mGluRs to effect changes in synaptic and cell intrinsic physiology dependent upon β-arrestin rather than G proteins. Pharmacological manipulation of mGluRs with effector-biased ligands could lead to novel therapies to treat neurological disease.

mGluRs are known to undergo non-canonical signalling regulation, although the underlying mechanisms are unclear. Here, the authors identify a role for β-arrestin2, but not β-arrestin1, in group I mGluR-mediated plasticity at hippocampal synapses.

mGluR-LTD at Excitatory and Inhibitory Synapses in the Lateral Habenula Tunes Neuronal Output

Excitatory and inhibitory transmission onto lateral habenula (LHb) neurons is instrumental for the expression of positive and negative motivational states. However, insights into the molecular mechanisms modulating synaptic transmission and the repercussions for neuronal activity within the LHb remain elusive. Here, we report that, in mice, activation of group I metabotropic glutamate receptors triggers long-term depression at excitatory (eLTD) and inhibitory (iLTD) synapses in the LHb. mGluR-eLTD and iLTD rely on mGluR1 and PKC signaling. However, mGluR-dependent adaptations of excitatory and inhibitory synaptic transmission differ in their expression mechanisms. mGluR-eLTD occurs via an endocannabinoid receptor-dependent decrease in glutamate release. Conversely, mGluR-iLTD occurs postsynaptically through PKC-dependent reduction of β2-containing GABAA-R function. Finally, mGluR-dependent plasticity of excitation or inhibition decides the direction of neuronal firing, providing a synaptic mechanism to bidirectionally control LHb output. We propose mGluR-LTD as a cellular substrate that underlies LHb-dependent encoding of opposing motivational states.

Calmodulin activity regulates group I metabotropic glutamate receptor-mediated signal transduction and synaptic depression

Group I metabotropic glutamate receptors (mGluR), including mGluR1 and mGluR 5 (mGluR1/5), are coupled to Gq and modulate activity-dependent synaptic plasticity. Direct activation of mGluR1/5 causes protein translation-dependent long-term depression (LTD). Although it has been established that intracellular Ca(2+) and the Gq-regulated signaling molecules are required for mGluR1/5 LTD, whether and how Ca(2+) regulates Gq signaling and upregulation of protein expression remain unknown. Through pharmacological inhibition, we tested the function of the Ca(2+) sensor calmodulin (CaM) in intracellular signaling triggered by the activation of mGluR1/5. CaM inhibitor N-[4-aminobutyl]-5-chloro-2-naphthalenesulfonamide hydrochloride (W13) suppressed the mGluR1/5-stimulated activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and p70-S6 kinase 1 (S6K1) in hippocampal neurons. W13 also blocked the mGluR1/5 agonist-induced synaptic depression in hippocampal slices and in anesthetized mice. Consistent with the function of CaM, inhibiting the downstream targets Ca(2+) /CaM-dependent protein kinases (CaMK) blocked ERK1/2 and S6K1 activation. Furthermore, disruption of the CaM-CaMK-ERK1/2 signaling cascade suppressed the mGluR1/5-stimulated upregulation of Arc expression. Altogether, our data suggest CaM as a new Gq signaling component for coupling Ca(2+) and protein upregulation and regulating mGluR1/5-mediated synaptic modification.

Slow synaptic transmission mediated by TRPV1 channels in CA3 interneurons of the hippocampus

Metabotropic glutamate receptors (mGluRs) modulate various neuronal functions in the central nervous system. Many studies reported that mGluRs have linkages to neuronal disorders such as schizophrenia and autism related disorders, indicating that mGluRs are involved in critical functions of the neuronal circuits. To study this possibility further, we recorded mGluR-induced synaptic responses in the interneurons of the CA3 stratum radiatum using rat hippocampal organotypic slice cultures. Electrical stimulation in the CA3 pyramidal cell layer evoked a slow inward current in the interneurons at a holding potential of -70mV in the presence of antagonists for AMPA/kainate receptors, NMDA receptors, GABAA receptors and GABAB receptors. The slow inward current was blocked in the absence of extracellular calcium, suggesting that this was a synaptic response. The slow excitatory postsynaptic current (EPSC) reversed near 0mV, reflecting an increase in a non-selective cationic conductance. The slow EPSC is mediated by group I mGluRs, as it was blocked by AP3, a group I mGluR antagonist. Neither a calcium chelator BAPTA nor a phospholipase C (PLC) inhibitor U73122 affected the slow EPSC. La(3+), a general TRP channel blocker or capsazepine, a selective TRPV1 channel antagonist significantly suppressed the slow EPSC. DHPG, a selective group I mGluRs agonist induced an inward current, which was suppressed by capsazepine. These results indicate that in the interneurons of the hippocampal CA3 stratum radiatum group I mGluRs activate TRPV1 channels independently of PLC and intracellular Ca(2+), resulting in the slow EPSC in the interneurons.

Utilizing GCaMP transgenic mice to monitor endogenous Gq/11-coupled receptors

The family of GCaMPs are engineered proteins that contain Ca²⁺ binding motifs within a circularly permutated variant of the Aequorea Victoria green fluorescent protein (cp-GFP). The rapidly advancing field of utilizing GCaMP reporter constructs represents a major step forward in our ability to monitor intracellular Ca²⁺ dynamics. With the use of these genetically encoded Ca²⁺ sensors, investigators have studied activation of endogenous G_q types of G protein-coupled receptors (GPCRs) and subsequent rises in intracellular calcium. Escalations in intracellular Ca²⁺ from GPCR activation can be faithfully monitored in space and time as an increase in fluorescent emission from these proteins. Further, transgenic mice are now commercially available that express GCaMPs in a Cre recombinase dependent fashion. These GCaMP reporter mice can be bred to distinct Cre recombinase driver mice to direct expression of this sensor in unique populations of cells. Concerning the central nervous system (CNS), sources of calcium influx, including those arising from G_q activation can be observed in targeted cell types like neurons or astrocytes. This powerful genetic method allows simultaneous monitoring of the activity of dozens of cells upon activation of endogenous G_q-coupled GPCRs. Therefore, in combination with pharmacological tools, this strategy of monitoring GPCR activation is amenable to analysis of orthosteric and allosteric ligands of G_q-coupled receptors in their endogenous environments.

Chronic stress induces anxiety via an amygdalar intracellular cascade that impairs endocannabinoid signaling

Collapse of endocannabinoid (eCB) signaling in the amygdala contributes to stress-induced anxiety, but the mechanisms of this effect remain unclear. eCB production is tied to the function of the glutamate receptor mGluR5, itself dependent on tyrosine phosphorylation. Herein, we identify a novel pathway linking eCB regulation of anxiety through phosphorylation of mGluR5. Mice lacking LMO4, an endogenous inhibitor of the tyrosine phosphatase PTP1B, display reduced mGluR5 phosphorylation, eCB signaling, and profound anxiety that is reversed by genetic or pharmacological suppression of amygdalar PTP1B. Chronically stressed mice exhibited elevated plasma corticosterone, decreased LMO4 palmitoylation, elevated PTP1B activity, reduced amygdalar eCB levels, and anxiety behaviors that were restored by PTP1B inhibition or by glucocorticoid receptor antagonism. Consistently, corticosterone decreased palmitoylation of LMO4 and its inhibition of PTP1B in neuronal cells. Collectively, these data reveal a stress-responsive corticosterone-LMO4-PTP1B-mGluR5 cascade that impairs amygdalar eCB signaling and contributes to the development of anxiety.

TRPC1 and metabotropic glutamate receptor expression in rat auditory midbrain neurons

Canonical transient receptor potential (TRPC) channels are plasma membrane cation channels included in the TRP superfamily. TRPC1 is expressed widely in the central nervous system and is linked to group I metabotropic glutamate receptors (mGluRs). In the auditory brainstem, TRPC1 expression has never been described, although group I mGluRs are present. In the central nucleus of the inferior colliculus (CIC), activation of group I mGluRs induces an extracellular Ca(2+) influx after store depletion. Therefore, this study examines whether TRPC1 is expressed in this region to establish a correlation with mGluRs. By quantitative reverse transcription-polymerase chain reaction and Western blotting, this study assesses the presence of TRPC1 along with both group I mGluR subtypes mGluR1 and mGluR5 in the rat inferior colliculus (IC). All these molecules present a robust expression in the IC. By confocal double immunofluorescence, this study also demonstrates that TRPC1 colocalizes with parvalbumin, a CIC neuronal marker, in many cells. Conversely, TRPC1 was lacking in glial fibrillary acidic protein-positive glial cells. All the glutamate acid decarboxylase 67 (GAD67)-immunoreactive neurons and many GAD67-negative neurons were positive to TRPC1, which indicates the presence of TRPC1 in γ-aminobutyric acid (GABA)-ergic and non-GABAeregic neurons. With regard to subcellular distribution, TRPC1 was absent in synaptophysin-immunoreactive axonic terminals but colocalized with postsynaptic marker microtubule-associated protein 2 in cell bodies and dendrites. TRPC1 totally overlapped group I mGluRs, which supports the involvement of TRPC1 in the mGluR pathway and, likely, in auditory signal processing at the midbrain level. .

mGluR1-mediated excitation of cerebellar GABAergic interneurons requires both G protein-dependent and Src-ERK1/2-dependent signaling pathways

Stimulation of type I metabotropic glutamate receptors (mGluR1/5) in several neuronal types induces slow excitatory responses through activation of transient receptor potential canonical (TRPC) channels. GABAergic cerebellar molecular layer interneurons (MLIs) modulate firing patterns of Purkinje cells (PCs), which play a key role in cerebellar information processing. MLIs express mGluR1, and activation of mGluR1 induces an inward current, but its precise intracellular signaling pathways are unknown. We found that mGluR1 activation facilitated spontaneous firing of mouse cerebellar MLIs through an inward current mediated by TRPC1 channels. This mGluR1-mediated inward current depends on both G protein-dependent and -independent pathways. The nonselective protein tyrosine kinase inhibitors genistein and AG490 as well as the selective extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitors PD98059 and SL327 suppressed the mGluR1-mediated current responses. Following G protein blockade, the residual mGluR1-mediated inward current was significantly reduced by the selective Src tyrosine kinase inhibitor PP2. In contrast to cerebellar PCs, GABA_B receptor activation in MLIs did not alter the mGluR1-mediated inward current, suggesting that there is no cross-talk between mGluR1 and GABA_B receptors in MLIs. Thus, activation of mGluR1 facilitates firing of MLIs through the TRPC1-mediated inward current, which depends on not only G protein-dependent but also Src–ERK1/2-dependent signaling pathways, and consequently depresses the excitability of cerebellar PCs.

The role of canonical transient receptor potential channels in seizure and excitotoxicity

Canonical transient receptor potential (TRPC) channels are a family of polymodal cation channels with some degree of Ca²⁺ permeability. Although initially thought to be channels mediating store-operated Ca²⁺ influx, TRPC channels can be activated by stimulation of Gq-coupled G-protein coupled receptors, or by an increase in intracellular free Ca²⁺ concentration. Thus, activation of TRPC channels could be a common downstream event of many signaling pathways that contribute to seizure and excitotoxicity, such as N-methyl-D-aspartate (NMDA) receptor-mediated Ca²⁺ influx, or metabotropic glutamate receptor activation. Recent studies with genetic ablation of various TRPC family members have demonstrated that TRPC channels, in particular heteromeric TRPC1/4 channels and homomeric TRPC5 channels, play a critical role in both pilocarpine-induced acute seizures and neuronal cell death. However, exact underlying mechanisms remain to be fully elucidated, and selective TRPC modulators and antibodies with better specificity are urgently needed for future research.

New insights into the mechanism of neurolathyrism: L-β-ODAP triggers [Ca2+]i accumulation and cell death in primary motor neurons through transient receptor potential channels and metabotropic glutamate receptors

Neurolathyrism is a motor neuron (MN) disease caused by β-N-oxalyl-L-α,β-diaminopropionic acid (L-β-ODAP), an AMPA receptor agonist. L-β-ODAP caused a prolonged rise of intracellular Ca(2+) ([Ca(2+)]i) in rat spinal cord MNs, and the [Ca(2+)]i accumulation was inversely proportional to the MN's life span. The [Ca(2+)]i rise induced by L-β-ODAP or (S)-AMPA was antagonized completely by NBQX, an AMPA-receptor blocker. However, blocking the L-type Ca(2+) channel with nifedipine significantly lowered [Ca(2+)]i induced by (S)-AMPA, but not that by L-β-ODAP. Tetrodotoxin completely extinguished the [Ca(2+)]i rise induced by (S)-AMPA or kainic acid, whereas that induced by L-β-ODAP was only attenuated by 65.6±6% indicating the prominent involvement of voltage-independent Ca(2+) entry. The tetrodotoxin-resistant [Ca(2+)]i induced by L-β-ODAP was blocked by 2-APB, Gd(3+), La(3+), 1-(β-[3-(4-methoxy-phenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SKF-96365) and flufenamic acid, which all are blockers of the transient receptor potential (TRP) channels. Blockers of group I metabotropic glutamate receptors (mGluR I), 7-(hydroxyiminocyclopropan[b]chromen-1α-carboxylate ethyl ester (CPCCPEt) and 2-methyl-6-(phenylethynyl)-pyridine (MPEP) also lowered the [Ca(2+)]i rise by L-β-ODAP. MN cell death induced by L-β-ODAP was prolonged significantly with SKF-96365 as well as NBQX. The results show the involvement of TRPs and mGluR I in L-β-ODAP-induced MN toxicity through prolonged [Ca(2+)]i mobilization, a unique characteristic of this neurotoxin.

Dendritic generation of mGluR-mediated slow afterdepolarization in layer 5 neurons of prefrontal cortex

Many prefrontal cortex (PFC)-dependent tasks require individual neurons to fire persistently in response to brief stimuli. Persistent activity is proposed to involve changes in intrinsic properties, resulting in an increased sensitivity to inputs. The dendrite is particularly relevant to this hypothesis because it receives the majority of synaptic inputs and is enriched for conductances implicated in persistent firing. We provide evidence that dendritic conductances contribute to persistent activity-related changes in intrinsic properties. The effects of Group 1 metabotropic glutamate receptor (mGluR) activation on persistent activity-related properties were tested in two classes of rat L5 neurons with distinct membrane properties: those projecting to the pons (CPn) and those projecting across the commissure to the contralateral cortex (COM). mGluR activation produced long-term changes in the subthreshold properties of CPn, but not COM neurons. These changes were indicative of a decrease in hyperpolarization-activated cation nonselective current (I(h)) at the soma and dendrite. mGluR activation also transiently increased the amplitude of the postburst slow afterdepolarization potential (sADP) at the soma of both neuron types. Interestingly, the sADP occurred along the extent of the apical dendrite in CPn and COM neurons. Simultaneous somatic/dendritic recordings revealed that the dendritic sADP does not result solely from passive propagation of the somatic sADP. Focal mGluR activation in L5, near the soma or at the border of L1/L2, near the tuft, generates a local sADP. This dendritic depolarization may act synergistically with synaptic input to regulate mnemonic activity in PFC.

Cholinergic receptor activation supports persistent firing in layer III neurons in the medial entorhinal cortex

Medial temporal lobe (MTL) areas are crucial for memory tasks such as spatial working memory and temporal association memory, which require an active maintenance of memory for a short period of time (a few hundred milliseconds to tens of seconds). Recent work has shown that the projection from layer III neurons in the medial entorhinal cortex (MEC) to hippocampal region CA1, the temporoammonic (TA) pathway, might be specially important for these memory tasks. In addition, lesions to the entorhinal cortex disrupt persistent firing in CA1 which is believed to support active maintenance of memory. Injection of cholinergic antagonists and group I mGlu receptor antagonists to the MEC impairs spatial working memory and temporal association memory. Consistent with this, we have shown that group I mGlu receptor activation supports persistent firing in principal cells of the MEC layer III in vitro (Yoshida et al. [39]). However, it still remains unknown whether cholinergic receptor activation also supports persistent firing in MEC layer III neurons. In this paper, we tested this in MEC layer III cells using both ruptured and perforated whole-cell recordings in vitro. We report that the majority of cells we recorded from in MEC layer III show persistent firing during perfusion of the cholinergic agonist carbachol (2-10μM). In addition, repeated stimulation gradually suppressed persistent firing. We further discuss the possible role of persistent firing in memory function in general.

Intracellular calcium level is an important factor influencing ion channel modulations by PLC-coupled metabotropic receptors in hippocampal neurons

Signaling pathways involving phospholipase C (PLC) are involved in various neural functions. Understanding how these pathways are regulated will lead to a better understanding of their roles in neural functions. Previous studies demonstrated that receptor-driven PLCβ activation depends on intracellular Ca(2+) concentration ([Ca(2+)]i), suggesting the possibility that PLCβ-dependent cellular responses are basically Ca(2+) dependent. To test this possibility, we examined whether modulations of ion channels driven by PLC-coupled metabotropic receptors are sensitive to [Ca(2+)]i using cultured hippocampal neurons. Muscarinic activation triggered an inward current at -100 mV (the equilibrium potential for K(+)) in a subpopulation of neurons. This current response was suppressed by pirenzepine (an M1-preferring antagonist), PLC inhibitor, non-selective cation channel blocker, and lowering [Ca(2+)]i. Using the neurons showing no response at -100 mV, effects of muscarinic activation on K(+) channels were examined at -40 mV. Muscarinic activation induced a transient decrease of the holding outward current. This current response was mimicked and occluded by XE991, an M-current K(+) channel blocker, suppressed by pirenzepine, PLC inhibitor and lowering [Ca(2+)]i, and enhanced by elevating [Ca(2+)]i. Similar results were obtained when group I metabotropic glutamate receptors were activated instead of muscarinic receptors. These results clearly show that ion channel modulations driven by PLC-coupled metabotropic receptors are dependent on [Ca(2+)]i, supporting the hypothesis that cellular responses induced by receptor-driven PLCβ activation are basically Ca(2+) dependent.

DAG-sensitive and Ca(2+) permeable TRPC6 channels are expressed in dentate granule cells and interneurons in the hippocampal formation

Members of the transient receptor potential (TRP) cation channel family play important roles in several neuronal functions. To understand the precise role of these channels in information processing, their presence on neuronal elements must be revealed. In this study, we investigated the localization of TRPC6 channels in the adult hippocampal formation. Immunostainings with a specific antibody, which was validated in Trpc6 knockout mice, showed that in the dentate gyrus, TRPC6 channels are strongly expressed in granule cells. Immunogold staining revealing the subcellular localization of TRPC6 channels clarified that these proteins were predominantly present on the membrane surface of the dendritic shafts of dentate granule cells, and also in their axons, often associated with intracellular membrane cisternae. In addition, TRPC6 channels could be observed in the dendrites of some interneurons. Double immunofluorescent staining showed that TRPC6 channels were present in the dendrites of hilar interneurons and hippocampal interneurons with horizontal dendrites in the stratum oriens expressing mGlu1a receptors, whereas parvalbumin immunoreactivity was revealed in TRPC6-expressing dendrites with radial appearance in the stratum radiatum. Electron microscopy showed that the immunogold particles depicting TRPC6 channels were located on the surface membranes of the interneuron dendrites. Our results suggest that TRPC6 channels are in a key position to alter the information entry into the trisynaptic loop of the hippocampal formation from the entorhinal cortex, and to control the function of both feed-forward and feed-back inhibitory circuits in this brain region. © 2012 Wiley Periodicals, Inc.

Canonical transient receptor channel 5 (TRPC5) and TRPC1/4 contribute to seizure and excitotoxicity by distinct cellular mechanisms

Seizures are the manifestation of highly synchronized burst firing of a large population of cortical neurons. Epileptiform bursts with an underlying plateau potential in neurons are a cellular correlate of seizures. Emerging evidence suggests that the plateau potential is mediated by neuronal canonical transient receptor potential (TRPC) channels composed of members of the TRPC1/4/5 subgroup. We previously showed that TRPC1/4 double-knockout (DKO) mice lack epileptiform bursting in lateral septal neurons and exhibit reduced seizure-induced neuronal cell death, but surprisingly have unaltered pilocarpine-induced seizures. Here, we report that TRPC5 knockout (KO) mice exhibit both significantly reduced seizures and minimal seizure-induced neuronal cell death in the hippocampus. Interestingly, epileptiform bursting induced by agonists for metabotropic glutamate receptors in the hippocampal CA1 area is unaltered in TRPC5 KO mice, but is abolished in TRPC1 KO and TRPC1/4 DKO mice. In contrast, long-term potentiation is greatly reduced in TRPC5 KO mice, but is normal in TRPC1 KO and TRPC1/4 DKO mice. The distinct changes from these knockouts suggest that TRPC5 and TRPC1/4 contribute to seizure and excitotoxicity by distinct cellular mechanisms. Furthermore, the reduced seizure and excitotoxicity and normal spatial learning exhibited in TRPC5 KO mice suggest that TRPC5 is a promising novel molecular target for new therapy.

Expression of transient receptor potential (TRP) channel mRNAs in the mouse olfactory bulb

Transient receptor potential (TRP) channels are a large family of cation channels. The 28 TRP channel subtypes in rodent are divided into 6 subfamilies: TRPC1-7, TRPV1-6, TRPM1-8, TRPP2/3/5, TRPML1-3 and TRPA1. TRP channels are involved in peripheral olfactory transduction. Several TRPC channels are expressed in unidentified neurons in the main olfactory bulb (OB), but the expression of most TRP channels in the OB has not been investigated. The present study employed RT-PCR as an initial survey of the expression of TRP channel mRNAs in the mouse OB and in 3 cell types: external tufted, mitral and granule cells. All TRP channel mRNAs except TRPV5 were detected in OB tissue. Single cell RT-PCR revealed that external tufted, mitral and granule cell populations expressed in aggregate 14 TRP channel mRNAs encompassing members of all 6 subfamilies. These different OB neuron populations expressed 7-12 channel mRNAs. Common channel expression was more similar among external tufted and mitral cells than among these cells and granule cells. These results indicate that a large number of TRP channel subtypes are expressed in OB neurons, providing the molecular bases for these channels to regulate OB neuron activity and central olfactory processing.

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.

Calcium-activated sustained firing responses distinguish accessory from main olfactory bulb mitral cells

Many mammals rely on pheromones for mediating social interactions. Recent studies indicate that both the main olfactory system (MOS) and accessory olfactory system (AOS) detect and process pheromonal stimuli, yet the functional difference between these two chemosensory systems remains unclear. We hypothesized that the main functional distinction between the MOS and AOS is the type of sensory information processing performed by each system. Here we compared the electrophysiological responses of mitral cells recorded from the accessory olfactory bulb (AOB) and main olfactory bulb (MOB) in acute mouse brain slices to various stimuli and found them markedly different. The response of MOB mitral cells to brief (0.1 ms, 1-100 V) stimulation of their sensory afferents remained transient regardless of stimulus strength, whereas sufficiently strong stimuli evoked sustained firing in AOB mitral cells lasting up to several minutes. Using EPSC-like current injections (10-100 pA, 10 ms rise time constant, 5 s decay time constant) in the presence of various synaptic blockers (picrotoxin, CGP55845, APV, DNQX, E4CPG, and MSPG), we demonstrated that this difference is attributable to distinct intrinsic properties of the two neuronal populations. The AOB sustained responses were found to be mediated by calcium-activated nonselective cationic current induced by transient intense firing. This current was found to be at least partially mediated by TRPM4 channels activated by calcium influx. We hypothesize that the sustained activity of the AOS induces a new sensory state in the animal, reflecting its social context.

Signalling routes and developmental regulation of group I metabotropic glutamate receptors in rat auditory midbrain neurons

Group I metabotropic glutamate receptors (mGluRs) are linked to intracellular Ca(2+) signalling and play important roles related to synaptic plasticity and development. In neurons from the central nucleus of the inferior colliculus (CIC), the activation of these receptors evokes large [Ca(2+) ](i) responses. By using optical imaging of the fluorescent Ca(2+) -sensitive dye Fura-2, we have explored which [Ca(2+) ](i) routes are triggered by group I mGluR activation in young CIC neurons and whether mGluR-induced [Ca(2+) ](i) responses are regulated during postnatal development. In addition, real-time quantitative RT-PCR was used to study the developmental expression of both group I mGluR subtypes, mGluR1 and mGluR5. Application of DHPG, a specific agonist of group I mGluRs, was used on CIC slices from young rats to elicit [Ca(2+) ](i) responses. A majority of responses consisted of an initial thapsigargin-sensitive Ca(2+) peak, related to store depletion, followed by a plateau phase, sensitive to the store-operated Ca(2+) entry blocker 2-APB. During postnatal development, from P6 to P17, DHPG-induced [Ca(2+) ](i) responses changed. The largest Ca(2+) responses were reached at P6, whereas lower peak and plateau responses were found after hearing onset, at P13-P14 and P17. qRT-PCR analysis also revealed important differences in the expression of both mGluR1 and mGluR5 subtypes during development, with the highest levels of both subtypes at P7 and a developmental decrease of both transcripts. Our results suggest both intra- and extracellular routes for [Ca(2+) ](i) increases linked to group I mGluRs in CIC neurons and a regulation of group I mGluR activity and expression during auditory development.

Identification of mRNA for endocannabinoid biosynthetic enzymes within hippocampal pyramidal cells and CA1 stratum radiatum interneuron subtypes using quantitative real-time polymerase chain reaction

The hippocampus is required for short-term memory and contains both excitatory pyramidal cells and inhibitory interneurons. These cells exhibit various forms of synaptic plasticity, the mechanism underlying learning and memory. More recently, endocannabinoids were identified to be involved in synaptic plasticity. Our goal was to describe the distribution of endocannabinoid biosynthetic enzymes within CA1 stratum radiatum interneurons and CA3/CA1 pyramidal cells. We extracted mRNA from single interneurons and pyramidal cells and used real-time quantitative polymerase chain reaction (RT-PCR) to detect the presence of 12-lipoxygenase, N-acyl-phosphatidylethanolamine-specific phospholipase D, diacylglycerol lipase α, and type I metabotropic glutamate receptors, all known to be involved in endocannabinoid production and plasticity. We observed that the expression of endocannabinoid biosynthetic enzyme mRNA does occur within interneurons and that it is coexpressed with type I metabotropic glutamate receptors, suggesting interneurons have the potential to produce endocannabinoids. We also identified that CA3 and CA1 pyramidal cells express endocannabinoid biosynthetic enzyme mRNA. Our data provide the first molecular biological evidence for putative endocannabinoid production in interneurons, suggesting their potential ability to regulate endocannabinoid-mediated processes, such as synaptic plasticity.

Functional coupling of the metabotropic glutamate receptor, InsP3 receptor and L-type Ca2+ channel in mouse CA1 pyramidal cells

Activity-dependent regulation of calcium dynamics in neuronal cells can play significant roles in the modulation of many cellular processes such as intracellular signalling, neuronal activity and synaptic plasticity. Among many calcium influx pathways into neurons, the voltage-dependent calcium channel (VDCC) is the major source of calcium influx, but its modulation by synaptic activity has still been under debate. While the metabotropic glutamate receptor (mGluR) is supposed to modulate L-type VDCCs (L-VDCCs), its reported actions include both facilitation and suppression, probably reflecting the uncertainty of both the molecular targets of the mGluR agonists and the source of the recorded calcium signal in previous reports. In this study, using subtype-specific knockout mice, we have shown that mGluR5 induces facilitation of the depolarization-evoked calcium current. This facilitation was not accompanied by the change in single-channel properties of the VDCC itself; instead, it required the activation of calcium-induced calcium release (CICR) that was triggered by VDCC opening, suggesting that the opening of CICR-coupled cation channels was essential for the facilitation. This facilitation was blocked or reduced by the inhibitors of both L-VDCCs and InsP3 receptors (InsP3Rs). Furthermore, L-VDCCs and mGluR5 were shown to form a complex by coimmunoprecipitation, suggesting that the specific functional coupling between mGluR5, InsP3Rs and L-VDCCs played a pivotal role in the calcium-current facilitation. Finally, we showed that mGluR5 enhanced VDCC-dependent long-term potentiation (LTP) of synaptic transmission. Our study has identified a novel mechanism of the interaction between the mGluR and calcium signalling, and suggested a contribution of mGluR5 to synaptic plasticity.

Dendritic calcium mechanisms and long-term potentiation in cortical inhibitory interneurons

Calcium (Ca(2+) ) is a major second messenger in the regulation of different forms of synaptic and intrinsic plasticity. Tightly organized in space and time, postsynaptic Ca(2+) transients trigger the activation of many distinct Ca(2+) signaling cascades, providing a means for a highly specific signal transduction and plasticity induction. High-resolution two-photon microscopy combined with highly sensitive synthetic Ca(2+) indicators in brain slices allowed for the quantification and analysis of postsynaptic Ca(2+) dynamics in great detail. Much of our current knowledge about postsynaptic Ca(2+) mechanisms is derived from studying Ca(2+) transients in the dendrites and spines of pyramidal neurons. However, postsynaptic Ca(2+) dynamics differ considerably among different cell types. In particular, distinct rules of postsynaptic Ca(2+) signaling and, accordingly, of Ca(2+) -dependent plasticity operate in GABAergic interneurons. Here, I review recent progress in understanding the complex organization of postsynaptic Ca(2+) signaling and its relevance to several forms of long-term potentiation at excitatory synapses in cortical GABAergic interneurons.

Heteromeric canonical transient receptor potential 1 and 4 channels play a critical role in epileptiform burst firing and seizure-induced neurodegeneration

Canonical transient receptor potential channels (TRPCs) are receptor-operated cation channels that are activated in response to phospholipase C signaling. Although TRPC1 is ubiquitously expressed in the brain, TRPC4 expression is the most restrictive, with the highest expression level limited to the lateral septum. The subunit composition of neuronal TRPC channels remains uncertain because of conflicting data from recombinant expression systems. Here we report that the large depolarizing plateau potential that underlies the epileptiform burst firing induced by metabotropic glutamate receptor agonists in lateral septal neurons was completely abolished in TRPC1/4 double-knockout mice, and was abolished in 74% of lateral septal neurons in TRPC1 knockout mice. Furthermore, neuronal cell death in the lateral septum and the cornu ammonis 1 region of hippocampus after pilocarpine-induced severe seizures was significantly ameliorated in TRPC1/4 double-knockout mice. Our data suggest that both TRPC1 and TRPC4 are essential for an intrinsic membrane conductance mediating the plateau potential in lateral septal neurons, possibly as heteromeric channels. Moreover, excitotoxic neuronal cell death, an underlying process for many neurological diseases, is not mediated merely by ionotropic glutamate receptors but also by heteromeric TRPC channels activated by metabotropic glutamate receptors. TRPC channels could be an unsuspected but critical molecular target for clinical intervention for excitotoxicity.

A TRPC5-regulated calcium signaling pathway controls dendrite patterning in the mammalian brain

Transient receptor potential (TRP) channels have been implicated as sensors of diverse stimuli in mature neurons. However, developmental roles for TRP channels in the establishment of neuronal connectivity remain largely unexplored. Here, we identify an essential function for TRPC5, a member of the canonical TRP subfamily, in the regulation of dendrite patterning in the mammalian brain. Strikingly, TRPC5 knockout mice harbor long, highly branched granule neuron dendrites with impaired dendritic claw differentiation in the cerebellar cortex. In vivo RNAi analyses suggest that TRPC5 regulates dendrite morphogenesis in the cerebellar cortex in a cell-autonomous manner. Correlating with impaired dendrite patterning in the cerebellar cortex, behavioral analyses reveal that TRPC5 knockout mice have deficits in gait and motor coordination. Finally, we uncover the molecular basis of TRPC5's function in dendrite patterning. We identify the major protein kinase calcium/calmodulin-dependent kinase II β (CaMKIIβ) as a critical effector of TRPC5 function in neurons. Remarkably, TRPC5 forms a complex specifically with CaMKIIβ, but not the closely related kinase CaMKIIα, and thereby induces the CaMKIIβ-dependent phosphorylation of the ubiquitin ligase Cdc20-APC at the centrosome. Accordingly, centrosomal CaMKIIβ signaling mediates the ability of TRPC5 to regulate dendrite morphogenesis in neurons. Our findings define a novel function for TRPC5 that couples calcium signaling to a ubiquitin ligase pathway at the centrosome and thereby orchestrates dendrite patterning and connectivity in the brain.

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.

Low-frequency summation of synaptically activated transient receptor potential channel-mediated depolarizations

Neurons sum their input by spatial and temporal integration. Temporally, presynaptic firing rates are converted to dendritic membrane depolarizations by postsynaptic receptors and ion channels. In several regions of the brain, including higher association areas, the majority of firing rates are low. For rates below 20 Hz, the ionotropic receptors α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor and N-methyl-d-aspartate (NMDA) receptor will not produce effective temporal summation. We hypothesized that depolarization mediated by transient receptor potential (TRP) channels activated by metabotropic glutamate receptors would be more effective, owing to their slow kinetics. On the basis of voltage-clamp and current-clamp recordings from a rat slice preparation, we constructed a computational model of the TRP channel and its intracellular activation pathway, including the metabotropic glutamate receptor. We show that synaptic input frequencies down to 3-4 Hz and inputs consisting of as few as three to five pulses can be effectively summed. We further show that the time constant of integration increases with increasing stimulation frequency and duration. We suggest that the temporal summation characteristics of TRP channels may be important at distal dendritic arbors, where spatial summation is limited by the number of concurrently active synapses. It may be particularly important in regions characterized by low and irregular rates.

Calcium/calmodulin-dependent protein kinase II mediates group I metabotropic glutamate receptor-dependent protein synthesis and long-term depression in rat hippocampus

Activation of Group I metabotropic glutamate receptors (mGluRs) in rat hippocampus induces a form of long-term depression (LTD) that is dependent on protein synthesis. However, the intracellular mechanisms leading to the initiation of protein synthesis and expression of LTD after mGluR activation are only partially understood. We investigated the role of several pathways linked to mGluR activation, translation initiation, and induction of LTD. We found that Group I mGluR-dependent protein synthesis and associated LTD, as induced by the agonist (RS)-3,5-dihydrophenylglycine (DHPG) or paired-pulse synaptic stimulation, was dependent on activation of calcium/calmodulin-dependent protein kinase IIα (CaMKII). DHPG induced a transient increase in the level of phospho-CaMKII (phospho-CaMKII(T286)) in synaptoneurosomes prepared from whole hippocampus and in CA1 minislices. In synaptoneurosomes, DHPG also induced an increase in phosphorylation of eIF4E, and an increase in protein synthesis that was abolished by translation inhibitors and the CaMKII inhibitors 1-[N,O-bis(5-isoquinolinesulphonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN62) and 2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)amino-N-(4-chloro-cinnamyl)-N-methylbenzylamine (KN93). In field recordings from CA1, both the translation inhibitor cycloheximide and KN62 significantly reduced DHPG-induced LTD. Combined application did not further reduce the LTD, suggesting a common mechanism. In whole-cell recordings, a third CaMKII inhibitor, AIP (autocamtide-2-related inhibitory peptide), significantly reduced the DHPG-induced LTD of synaptic currents. Inhibition of the classical pathway mediating many Group I mGluR effects by blocking PKC (protein kinase C) or PLC (phospholipase C) did not impair DHPG-induced protein synthesis or LTD. Collectively, these findings demonstrate an important role for CaMKII in mediating the initiation of protein synthesis that then supports the postsynaptic expression of DHPG-induced LTD.

Metabotropic glutamate receptors regulate hippocampal CA1 pyramidal neuron excitability via Ca²⁺ wave-dependent activation of SK and TRPC channels

Group I metabotropic glutamate receptors (mGluRs) play an essential role in cognitive function. Their activation results in a wide array of cellular and molecular responses that are mediated by multiple signalling cascades. In this study, we focused on Group I mGluR activation of IP3R-mediated intracellular Ca2+ waves and their role in activating Ca2+-dependent ion channels in CA1 pyramidal neurons. Using whole-cell patch-clamp recordings and high-speed Ca2+ fluorescence imaging in acute hippocampal brain slices, we show that synaptic and pharmacological stimulation of mGluRs triggers intracellular Ca2+ waves and a biphasic electrical response composed of a transient Ca2+-dependent SK channel-mediated hyperpolarization and a TRPC-mediated sustained depolarization. The generation and magnitude of the SK channel-mediated hyperpolarization depended solely on the rise in intracellular Ca2+ concentration ([Ca2+]i), whereas the TRPC channel-mediated depolarization required both a small rise in [Ca2+]i and mGluR activation. Furthermore, the TRPC-mediated current was suppressed by forskolin-induced rises in cAMP. We also show that SK- and TRPC-mediated currents robustly modulate pyramidal neuron excitability by decreasing and increasing their firing frequency, respectively. These findings provide additional evidence that mGluR-mediated synaptic transmission makes an important contribution to regulating the output of hippocampal neurons through intracellular Ca2+ wave activation of SK and TRPC channels. cAMP provides an additional level of regulation by modulating TRPC-mediated sustained depolarization that we propose to be important for stabilizing periods of sustained firing.

TRP channels and neural persistent activity

One of the integrative properties of the nervous system is its capability to, by transient motor commands or brief sensory stimuli, evoke persistent neuronal changes, mainly as a sustained, tonic action potential firing. This neural activity, named persistent activity, is found in a good number of brain regions and is thought to be a neural substrate for short-term storage and accumulation of sensory or motor information [1]. Examples of this persistent neural activity have been reported in prefrontal [2] and entorhinal [3] cortices, as part of the neural mechanisms involved in short-term working memory [4]. Interestingly, the general organization of the motor systems assumes the presence of bursts of short-lasting motor commands encoding movement characteristics such as velocity, duration, and amplitude, followed by a maintained tonic firing encoding the position at which the moving appendage should be maintained [5, 6]. Generation of qualitatively similar sustained discharges have also been found in spinal and supraspinal regions in relation to pain processing [7, 8]. Thus, persistent neural activity seems to be necessary for both behavioral (positions of fixation) and cognitive (working memory) processes. Persistent firing mechanisms have been proposed to involve the participation of a non-specific cationic current (CAN current) mainly mediated by activation of TRPC channels. Because the function and generation of persistent activity is still poorly understood, here we aimed to review and discuss the putative role of TRP-like channels on its generation and/or maintenance.