[Effect of Rehmanniae Radix on depression-like behavior and hippocampal monoamine neurotransmitters of chronic unpredictable mild stress model rats]

To investigate the effect of Rehmanniae Radix on depression-like behavior and monoamine neurotransmitters of chronic unpredictable mild stress(CUMS) model rats. CUMS combined with isolated feeding was used to induce the depression model of rats. The depression-like behavior of rats was evaluated by sucrose preference test, open field test, and forced swim test. Hematoxylin-Eosin(HE) staining was used to investigate the pathological changes of neurons in the CA1 and CA3 area of hippocampus. Ultra performance liquid chromatography-tandem mass spectrometry(UPLC-MS) was used to detect the contents of 5-hydroxytryptamine(5-HT), 5-hydroxyindoleacetic acid(5-HIAA), dopamine(DA), 3,4-dihydroxyphenylacetic acid(DOPAC), homovanillic acid(HVA), norepinephrine(NE), and 3-methoxy-4-hydroxyphenyl glycol(MHPG) in rats. Western blot was used to detect the protein expressions of tryptophan hydroxylase 2(TPH2), serotonin transporter(SERT), and monoamine oxidase A(MAO-A) in the hippocampus of rats. Compared with the normal group, depressive-like behavior of rats was obvious in the model group. The arrangements of neurons in the CA1 and CA3 area of hippocampus were loose and disorderly. The levels of 5-HT, 5-HIAA, and 5-HT/5-HIAA in the hippocampal area were decreased(P<0.01). The protein expression of TPH2 was decreased(P<0.01), but those of SERT and MAO-A were increased(P<0.01). In the Rehmanniae Radix groups with 1.8 g·kg~(-1) and 7.2 g·kg~(-1), the depression-like behavior of CUMS rats and pathological changes of neurons in CA1, CA3 area of hippocampus were improved. The protein expression of TPH2(P<0.05, P<0.01) was increased, and those of SERT and MAO-A were down-regulated(P<0.05, P<0.01). The levels of 5-HT, 5-HIAA, and 5-HT/5-HIAA in hippocampus were increased(P<0.05, P<0.01). The changes in DA, DOPAC, HVA, DA/(DOPAC +HVA), NE, DHPG, and NE/DHPG were not statistically significant. The results suggested that Rehmanniae Radix improved depression-like behavior of CUMS rats, and the mechanism might be related to the regulation of synthesis, transportation, and metabolism of 5-HT neurotransmitter in the hippocampus.

Synaptic protein interaction networks encode experience by assuming stimulus-specific and brain-region-specific states

A core network of widely expressed proteins within the glutamatergic post-synapse mediates activity-dependent synaptic plasticity throughout the brain, but the specific proteomic composition of synapses differs between brain regions. Here, we address the question, how does proteomic composition affect activity-dependent protein-protein interaction networks (PINs) downstream of synaptic activity? Using quantitative multiplex co-immunoprecipitation, we compare the PIN response of in vivo or ex vivo neurons derived from different brain regions to activation by different agonists or different forms of eyeblink conditioning. We report that PINs discriminate between incoming stimuli using differential kinetics of overlapping and non-overlapping PIN parameters. Further, these "molecular logic rules" differ by brain region. We conclude that although the PIN of the glutamatergic post-synapse is expressed widely throughout the brain, its activity-dependent dynamics show remarkable stimulus-specific and brain-region-specific diversity. This diversity may help explain the challenges in developing molecule-specific drug therapies for neurological disorders.

Reelin Regulates Neuronal Excitability through Striatal-Enriched Protein Tyrosine Phosphatase (STEP61) and Calcium Permeable AMPARs in an NMDAR-Dependent Manner

Alzheimer's disease (AD) is a progressive neurodegenerative disease marked by the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles. Aβ oligomers cause synaptic dysfunction early in AD by enhancing long-term depression (LTD; a paradigm for forgetfulness) via metabotropic glutamate receptor (mGluR)-dependent regulation of striatal-enriched tyrosine phosphatase (STEP61). Reelin is a neuromodulator that signals through ApoE (apolipoprotein E) receptors to protect the synapse against Aβ toxicity (Durakoglugil et al., 2009) Reelin signaling is impaired by ApoE4, the most important genetic risk factor for AD, and Aβ-oligomers activate metabotropic glutamate receptors (Renner et al., 2010). We therefore asked whether Reelin might also affect mGluR-LTD. To this end, we induced chemical mGluR-LTD using DHPG (Dihydroxyphenylglycine), a selective mGluR5 agonist. We found that exogenous Reelin reduces the DHPG-induced increase in STEP61, prevents the dephosphorylation of GluA2, and concomitantly blocks mGluR-mediated LTD. By contrast, Reelin deficiency increased expression of Ca2+-permeable GluA2-lacking AMPA receptors along with higher STEP61 levels, resulting in occlusion of DHPG-induced LTD in hippocampal CA1 neurons. We propose a model in which Reelin modulates local protein synthesis as well as AMPA receptor subunit composition through modulation of mGluR-mediated signaling with implications for memory consolidation or neurodegeneration.SIGNIFICANCE STATEMENT Reelin is an important neuromodulator, which in the adult brain controls synaptic plasticity and protects against neurodegeneration. Amyloid-β has been shown to use mGluRs to induce synaptic depression through endocytosis of NMDA and AMPA receptors, a mechanism referred to as LTD, a paradigm of forgetfulness. Our results show that Reelin regulates the phosphatase STEP, which plays an important role in neurodegeneration, as well as the expression of calcium-permeable AMPA receptors, which play a role in memory formation. These data suggest that Reelin uses mGluR LTD pathways to regulate memory formation as well as neurodegeneration.

Temporal Quantitative Proteomics of mGluR-induced Protein Translation and Phosphorylation in Neurons

At neuronal synapses, activation of group I metabotropic glutamate receptors (mGluR1/5) triggers a form of long-term depression (mGluR-LTD) that relies on new protein synthesis and the internalization of AMPA-type glutamate receptors. Dysregulation of these processes has been implicated in the development of mental disorders such as autism spectrum disorders and therefore merit a better understanding on a molecular level. Here, to study mGluR-induced signaling pathways, we integrated quantitative phosphoproteomics with the analyses of newly synthesized proteins via bio-orthogonal amino acids (azidohomoalanine) in a pulsed labeling strategy in cultured hippocampal neurons stimulated with DHPG, a specific agonist for group I mGluRs. We identified several kinases with important roles in DHPG-induced mGluR activation, which we confirmed using small molecule kinase inhibitors. Furthermore, changes in the AMPA receptor endocytosis pathway in both protein synthesis and protein phosphorylation were identified, whereby Intersectin-1 was validated as a novel player in this pathway. This study revealed several new insights into the molecular pathways downstream of group I mGluR activation in hippocampal neurons, and provides a rich resource for further analyses.

Ribosome profiling in mouse hippocampus: plasticity-induced regulation and bidirectional control by TSC2 and FMRP

BACKGROUND

Mutations in TSC2 are the most common cause of tuberous sclerosis (TSC), a disorder with a high incidence of autism and intellectual disability. TSC2 regulates mRNA translation required for group 1 metabotropic glutamate receptor-dependent synaptic long-term depression (mGluR-LTD) and behavior, but the identity of mRNAs responsive to mGluR-LTD signaling is largely unknown.

METHODS

We utilized Tsc2+/- mice as a mouse model of TSC and prepared hippocampal slices from these animals. We induced mGluR-LTD synaptic plasticity in slices and processed the samples for RNA-seq and ribosome profiling to identify differentially expressed genes in Tsc2+/- and following mGluR-LTD synaptic plasticity.

RESULTS

Ribosome profiling reveals that in Tsc2+/- mouse hippocampal slices, the expression of several mRNAs was dysregulated: terminal oligopyrimidine (TOP)-containing mRNAs decreased, while FMRP-binding targets increased. Remarkably, we observed the opposite changes of FMRP binding targets in Fmr1-/y hippocampi. In wild-type hippocampus, induction of mGluR-LTD caused rapid changes in the steady-state levels of hundreds of mRNAs, many of which are FMRP targets. Moreover, mGluR-LTD failed to promote phosphorylation of eukaryotic elongation factor 2 (eEF2) in TSC mice, and chemically mimicking phospho-eEF2 with low cycloheximide enhances mGluR-LTD in TSC mice.

CONCLUSION

These results suggest a molecular basis for bidirectional regulation of synaptic plasticity and behavior by TSC2 and FMRP. Our study also suggests that altered mGluR-regulated translation elongation contributes to impaired synaptic plasticity in Tsc2+/- mice.

TRPC1 Regulates the Activity of a Voltage-Dependent Nonselective Cation Current in Hippocampal CA1 Neurons

The cation channel subunit TRPC1 is strongly expressed in central neurons including neurons in the CA1 region of the hippocampus where it forms complexes with TRPC4 and TRPC5. To investigate the functional role of TRPC1 in these neurons and in channel function, we compared current responses to group I metabotropic glutamate receptor (mGluR I) activation and looked for major differences in dendritic morphology in neurons from TRPC1^+/+ and TRPC1^-/- mice. mGluR I stimulation resulted in the activation of a voltage-dependent nonselective cation current in both genotypes. Deletion of TRPC1 resulted in a modification of the shape of the current-voltage relationship, leading to an inward current increase. In current clamp recordings, the percentage of neurons that responded to depolarization in the presence of an mGluR I agonist with a plateau potential was increased in TRPC1^-/- mice. There was also a small increase in the minor population of CA1 neurons that have more than one apical dendrite in TRPC1^-/- mice. We conclude that TRPC1 has an inhibitory effect on receptor-operated nonselective cation channels in hippocampal CA1 neurons probably as a result of heterotetramer formation with other TRPC isoforms, and that TRPC1 deletion has only minor effects on dendritic morphology.

Single Neuron Imaging Reveals Metabotropic Glutamate Receptor-Mediated Bursting and Delay in Calcium Oscillation in Hippocampal Neurons

Measurement of cytosolic calcium activity in single neuron and quantification of response to drug treatment over a longer period remain challenging. Especially, analysis of amplitude, frequency, and delay in calcium response in G-protein coupled receptors (GPCR) targeting drug-treated neurons is complicated due to inherent heterogeneity. To address this, we have utilized k- means clustering to identify various sub-populations. Here we focus on measuring drug dose response in the single neuron by means of high-resolution confocal imaging. Metabotropic glutamate receptor was activated using 3,5-dihydroxyphenylglycine (DHPG). The result reveals an unusual delay in calcium response for higher dose particularly in responding neurons. The proposed methodology can be used for selection of drug dose and optimization of the time window for neurodegenerative diseases.

Molecular interactions between 3,4-dihydroxyphenylglycol and pectin and antioxidant capacity of this complex in vitro

This study explored the interaction of pectin with 3,4-dihydroxyphenylglycol (DHPG), a potent phenolic antioxidant naturally found in olive fruit, via encapsulation into pectinate beads. MALDI TOF-TOF analysis supported the formation of complexes between DHPG and pectin. A combination of covalent bonds (ester bonds) and non-covalent interactions, mostly hydrogen bonding, were suggested as the cause of DHPG-pectin complex formation. Free radical scavenging assays confirmed that DHPG maintained its antioxidant activity after complexation and after a digestion simulated in vitro with gastric and intestinal fluids. Therefore, DHPG-pectin beads could reach the large intestine and contribute to a healthy antioxidant environment.

Association of social defeat stress-induced anhedonia-like symptoms with mGluR1-dependent decrease in membrane-bound AMPA-GluR1 in the mouse ventral midbrain

Anhedonia is a core symptom of social defeat stress (SDS)-induced depression associated with the reward system. We previously reported that decreased membrane-bound AMPA-GluR1 in the reward system is associated with lipopolysaccharide-induced anhedonia-like symptoms. Since group I metabotropic glutamate receptor (mGluR) activation reduces the surface density of GluR1, we examined whether group I mGluR-dependent decrease in membrane-bound GluR1 in the reward system is involved in SDS-induced anhedonia-like symptoms. Mice exposed to SDS for 4 consecutive days had markedly decreased membrane-bound GluR1 and GluR2 in the prefrontal cortex (PFC) and membrane-bound GluR1 in the ventral midbrain (VM) along with lower sucrose preference (SP). Intra-PFC injection of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG; 100 μmol) demonstrated decrease in membrane-bound GluR1 and GluR2 in the PFC 2 and 24 h and membrane-bound GluR1 in the VM 24 h after injection. Moreover, intra-PFC injection of DHPG decreased SP only in the second 24-h (24-48 h) period. Conversely, intra-VM injection of DHPG decreased SP in both the first and second 24-h period and decreased membrane-bound GluR1 in the VM 2 and 24 h after injection. Pre-treatment with the mGluR1 antagonist JNJ16259685 (30 mg/kg, subcutaneous) prevented SDS-decreased SP and membrane-bound GluR1 in the VM. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10 mg/kg, subcutaneous) prevented SDS-induced decrease in membrane-bound GluR1 and GluR2 in the PFC, whereas MPEP did not affect SDS-induced decrease in SP and membrane-bound GluR1 in the VM. These results suggest that mGluR1-mediated decrease in membrane-bound GluR1 in VM is involved in SDS-induced anhedonia-like symptoms.

mGlu1 receptor canonical signaling pathway contributes to the opening of the orphan GluD2 receptor

The orphan Glutamate receptor Delta2 (GluD2) intrinsic ion channel activity is indirectly triggered by glutamate through stimulation of the metabotropic glutamate receptor 1 (mGlu1), in cerebellar Purkinje cells. However, the mechanisms of GluD2 ion channel opening are entirely unknown. In this work, we investigated the signaling pathways underlying the mGlu1-induced GluD2 current, performing whole-cell voltage-clamp recordings from mGlu1 and GluD2 transfected HEK293 cells. We show that the activation of GluD2 channels via DHPG-induced mGlu1 stimulation is Gαq-dependent. Moreover, inhibition of the downstream components of the mGlu1 canonical signaling pathway PLC and PKC with U73122 and GF109203X, respectively, strongly reduced the DHPG-induced GluD2 current. These results were further confirmed on endogenous receptors at the Parallel Fiber - Purkinje Cell cerebellar synapse, indicating that the opening of the GluD2 channel by mGlu1 receptor mobilizes the canonical Gq-PLC-PKC pathway. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Naturally occurring hydroxytyrosol derivatives: hydroxytyrosyl acetate and 3,4-dihydroxyphenylglycol modulate inflammatory response in murine peritoneal macrophages. Potential utility as new dietary supplements

This work evaluated the effects of extra virgin olive oil (EVOO) phenols, hydroxytyrosyl acetate (2) and 3,4-dihydroxyphenylglycol (3), as well as two new acyl derivatives of 3, 4-(1,2-di(butanoyloxy)ethyl)benzene-1,2-diol (7) and 4-(1,2-di(lauroyloxy)ethyl)benzene-1,2-diol (8), on LPS-stimulated murine peritoneal macrophages in comparison with hydroxytyrosol (HTy, 1). Compounds 2, 3, 7, and 8 showed a strong reactive oxygen species (ROS)-scavenging activity, reducing significantly nitrite levels with a significant decrease on iNOS expression [2 (50 μM, 0.44 ± 0.03; 100 μM, 0.44 ± 0.01; p < 0.01); 3 (50 μM, 0.37 ± 0.03; 100 μM, 0.37 ± 0.01; p < 0.001); 7 (50 μM, 0.45 ± 0.06; p < 0.01)] . However, only 2 and 3 down-regulated COX-2 expression [2 (50 μM, 0.72 ± 0.04, p < 0.05; 100 μM, 0.54 ± 0.06, p < 0.01); 3 (50 μM, 0.56 ± 0.05, p < 0.05; 100 μM, 0.37 ± 0.04; p < 0.001)] and prevented IKBα degradation [2 (100 μM, 1.63 ± 0.14, p < 0.01); 3 (100 μM, 1.82 ± 0.09; p < 0.01)] ; the diacylated compounds 7 and 8 showed worse anti-inflammatory activity than the parent 3. In conclusion, 2 and 3 phenolic derivatives could play an important role in the anti-inflammatory effect of EVOO. The implication of this study for the nutrition and general health of the population rests in the possible use of natural HTy derivatives with better hydrophilic/lipophilic balance, thus improving its pharmacodynamic and pharmacokinetic profiles, as new dietary supplements in foods.

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.

Involvement of TrkB- and p75(NTR)-signaling pathways in two contrasting forms of long-lasting synaptic plasticity

The repetition of experience is often necessary to establish long-lasting memory. However, the cellular mechanisms underlying this repetition-dependent consolidation of memory remain unclear. We previously observed in organotypic slice cultures of the rodent hippocampus that repeated inductions of long-term potentiation (LTP) led to a slowly developing long-lasting synaptic enhancement coupled with synaptogenesis. We also reported that repeated inductions of long-term depression (LTD) produced a long-lasting synaptic suppression coupled with synapse elimination. We proposed these phenomena as useful in vitro models for analyzing repetition-dependent consolidation. Here, we hypothesized that the enhancement and suppression are mediated by the brain-derived neurotrophic factor (BDNF)-TrkB signaling pathway and the proBDNF-p75(NTR) pathway, respectively. When we masked the respective pathways, reversals of the enhancement and suppression resulted. These results suggest the alternative activation of the p75(NTR) pathway by BDNF under TrkB-masking conditions and of the TrkB pathway by proBDNF under p75(NTR)-masking conditions, thus supporting the aforementioned hypothesis.

Tet1 is critical for neuronal activity-regulated gene expression and memory extinction

The ten-eleven translocation (Tet) family of methylcytosine dioxygenases catalyze oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) and promote DNA demethylation. Despite the abundance of 5hmC and Tet proteins in the brain, little is known about the functions of the neuronal Tet enzymes. Here, we analyzed Tet1 knockout mice (Tet1KO) and found downregulation of multiple neuronal activity-regulated genes, including Npas4, c-Fos, and Arc. Furthermore, Tet1KO animals exhibited abnormal hippocampal long-term depression and impaired memory extinction. Analysis of the key regulatory gene, Npas4, indicated that its promoter region, containing multiple CpG dinucleotides, is hypermethylated in both naive Tet1KO mice and after extinction training. Such hypermethylation may account for the diminished expression of Npas4 itself and its downstream targets, impairing transcriptional programs underlying cognitive processes. In summary, we show that neuronal Tet1 regulates normal DNA methylation levels, expression of activity-regulated genes, synaptic plasticity, and memory extinction.

Social deprivation enhances VTA synaptic plasticity and drug-induced contextual learning

Drug addiction is driven, in part, by powerful drug-related memories. Deficits in social life, particularly during adolescence, increase addiction vulnerability. Social isolation in rodents has been used extensively to model the effects of deficient social experience, yet its impact on learning and memory processes underlying addiction remains elusive. Here, we show that social isolation of rats during a critical period of adolescence (postnatal days 21-42) enhances long-term potentiation of NMDA receptor (NMDAR)-mediated glutamatergic transmission in the ventral tegmental area (VTA). This enhancement, which is caused by an increase in metabotropic glutamate receptor-dependent Ca(2+) signaling, cannot be reversed by subsequent resocialization. Notably, memories of amphetamine- and ethanol-paired contextual stimuli are acquired faster and, once acquired, amphetamine-associated contextual memory is more resistant to extinction in socially isolated rats. We propose that NMDAR plasticity in the VTA may represent a neural substrate by which early life deficits in social experience increase addiction vulnerability.

N-acetylcysteine protects against apoptosis through modulation of group I metabotropic glutamate receptor activity

The activation of group I metabotropic glutamate receptor (group I mGlus) has been shown to produce neuroprotective or neurotoxic effects. In this study, we investigated the effects of N-acetylcysteine (NAC), a precursor of the antioxidant glutathione, on group I mGlus activation in apoptosis of glial C6 and MN9D cell lines, and a rat model of Parkinson's disease (PD). We demonstrated that NAC protected against apoptosis through modulation of group I mGlus activity. In glial C6 cells, NAC promoted phosphorylation of ERK induced by (s)-3,5- dihydroxy-phenylglycine (DHPG), an agonist of group I mGlus. NAC enhanced the group I mGlus-mediated protection from staurosporine (STS)-induced apoptosis following DHPG treatment. Moreover, in rotenone-treated MN9D cells and PD rat model, NAC protected against group I mGlus-induced toxicity by compromising the decrease in phosphorylation of ERK, phosphorylation or expression level of TH. Furthermore, the results showed that NAC prohibited the level of ROS and oxidation of cellular GSH/GSSG (E_h) accompanied by activated group I mGlus in the experimental models. Our results suggest that NAC might act as a regulator of group I mGlus-mediated activities in both neuroprotection and neurotoxicity via reducing the oxidative stress, eventually to protect cell survival. The study also suggests that NAC might be a potential therapeutics targeting for group I mGlus activation in the treatment of PD.

Pain-related deactivation of medial prefrontal cortical neurons involves mGluR1 and GABA(A) receptors

Pain-related hyperactivity in the amygdala leads to deactivation of the medial prefrontal cortex (mPFC) and decision-making deficits. The mechanisms of pain-related inhibition of the mPFC are not yet known. Here, we used extracellular single-unit recordings of prelimbic mPFC neurons to determine the role of GABA(A) receptors and metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5, in pain-related activity changes of mPFC neurons. Background and evoked activity of mPFC neurons decreased after arthritis induction. To determine pain-related changes, the same neuron was recorded continuously before and after induction of arthritis in one knee joint by intra-articular injection of kaolin/carrageenan. Stereotaxic administration of a GABA(A) receptor antagonist {[R-(R*,S*)]-5-(6,8-dihydro-8-oxofuro[3,4-e]-1,3-benzodioxol-6-yl)-5,6,7,8-tetrahydro-6,6-dimethyl-1,3-dioxolo[4,5-g]isoquinolinium iodide (bicuculline)} into the mPFC by microdialysis reversed pain-related inhibition, whereas offsite injections into the adjacent anterior cingulate cortex had no or opposite effects on prelimbic mPFC neurons. A selective mGluR1/5 agonist [(S)-3,5-dihydroxyphenylglycine (DHPG)] inhibited background and evoked activity under normal conditions through a GABAergic mechanism, because the inhibitory effect was blocked with bicuculline. In the arthritis pain state, DHPG, alone or in the presence of bicuculline, had no effect. Consistent with the involvement of mGluR1 in pain-related inhibition of the mPFC, a selective mGluR1 antagonist [(S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid] reversed the pain-related decrease of background and evoked activity of mPFC neurons in arthritis, whereas a selective mGluR5 antagonist [2-methyl-6-(phenylethynyl)pyridine hydrochloride] had no effect. The mGluR antagonists had no effect under normal conditions. We interpret our data to suggest that pain-related inhibition of mPFC neurons in the arthritis model depends on mGluR1-mediated endogenous activation of GABA(A) receptors. Exogenous activation of mGluR1/5 produces GABAergic inhibition under normal conditions. Restoring normal activity in the mPFC may be a therapeutic strategy to improve cognitive deficits associated with persistent pain.

Activation of group I mGlu receptors contributes to facilitation of NMDA receptor membrane current in spinal dorsal horn neurons after hind paw inflammation in rats

The interaction between the group I metabotropic glutamate (mGlu) receptors and N-methyl-D-aspartate (NMDA) receptors plays a critical role in spinal hyperexcitability and hyperalgesia. The cellular mechanisms underlying this interaction remain unknown. Utilizing an ex vivo spinal slice preparation from young adult rats, we investigated the group I mGlu receptor modulation of NMDA receptor-mediated current in superficial dorsal horn neurons by patch clamp recording after complete Freund's adjuvant (CFA)-induced hind paw inflammation. We show that NMDA receptor-mediated dorsal root stimulation-evoked EPSC (eEPSC) and NMDA-induced current was enhanced in the inflamed rats, compared to naïve rats and this effect was attenuated by AIDA (1 mM), a group I mGlu receptor antagonist. There were also increases in the frequency and amplitude of miniature excitatory postsynaptic currents in the presence of tetrodotoxin, suggesting enhanced presynaptic glutamate release probability and postsynaptic membrane responsiveness in inflamed rats. DHPG (10 μM), a selective group I mGlu receptor agonist, further facilitated NMDA receptor-mediated eEPSC and NMDA-induced current in inflamed rats. The DHPG-produced facilitation of NMDA-induced current was blocked by intracellular dialysis of GDP-beta-S (1 mM), a G protein antagonist, and BAPTA (15 mM), an intracellular calcium chelating agent; and by pretreatment with U73,122 (10 μM), a PLC inhibitor, or 2-APB (100 μM), an IP₃-receptor antagonist. These findings support the hypothesis that signal transduction coupling between group I mGlu receptors and NMDA receptors underlies the activation of NMDA receptors in spinal hyperexcitability and hyperalgesia.

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.

Altered mRNA transport, docking, and protein translation in neurons lacking fragile X mental retardation protein

Fragile X syndrome is caused by the absence of functional fragile X mental retardation protein (FMRP), an RNA binding protein. The molecular mechanism of aberrant protein synthesis in fmr1 KO mice is closely associated with the role of FMRP in mRNA transport, delivery, and local protein synthesis. We show that GFP-labeled Fmr1 and CaMKIIalpha mRNAs undergo decelerated motion at 0-40 min after group I mGluR stimulation, and later recover at 40-60 min. Then we investigate targeting of mRNAs associated with FMRP after neuronal stimulation. We find that FMRP is synthesized closely adjacent to stimulated mGluR5 receptors. Moreover, in WT neurons, CaMKIIalpha mRNA can be delivered and translated in dendritic spines within 10 min in response to group I mGluR stimulation, whereas KO neurons fail to show this response. These data suggest that FMRP can mediate spatial mRNA delivery for local protein synthesis in response to synaptic stimulation.

Excess phosphoinositide 3-kinase subunit synthesis and activity as a novel therapeutic target in fragile X syndrome

Fragile X syndrome (FXS) is an inherited neurologic disease caused by loss of fragile X mental retardation protein (FMRP), which is hypothesized to mediate negative regulation of mRNA translation at synapses. A prominent feature of FXS animal models is exaggerated signaling through group 1 metabotropic glutamate receptors (gp1 mGluRs), and therapeutic strategies to treat FXS are targeted mainly at gp1 mGluRs. Recent studies, however, indicate that a variety of receptor-mediated signal transduction pathways are dysregulated in FXS, suggesting that FMRP acts on a common downstream signaling molecule. Here, we show that deficiency of FMRP results in excess activity of phosphoinositide 3-kinase (PI3K), a downstream signaling molecule of many cell surface receptors. In Fmr1 knock-out neurons, excess synaptic PI3K activity can be reduced by perturbation of gp1 mGluR-mediated signaling. Remarkably, increased PI3K activity was also observed in FMRP-deficient non-neuronal cells in the absence of gp1 mGluRs. Here, we show that FMRP regulates the synthesis and synaptic localization of p110beta, the catalytic subunit of PI3K. In wild type, gp1 mGluR activation induces p110beta translation, p110beta protein expression, and PI3K activity. In contrast, both p110beta protein synthesis and PI3K activity are elevated and insensitive to gp1 mGluR stimulation in Fmr1 knock-out. This suggests that dysregulated PI3K signaling may underlie the synaptic impairments in FXS. In support of this hypothesis, we show that PI3K antagonists rescue three FXS-associated phenotypes: dysregulated synaptic protein synthesis, excess AMPA receptor internalization, and increased spine density. Targeting excessive PI3K activity might thus be a potent therapeutic strategy for FXS.

Persistent synapse loss induced by repetitive LTD in developing rat hippocampal neurons

Synaptic pruning is a physiological event that eliminates excessive or inappropriate synapses to form proper synaptic connections during development of neurons. Appropriate synaptic pruning is required for normal neural development. However, the mechanism of synaptic pruning is not fully understood. Strength of synaptic activity under competitive circumstances is thought to act as a selective force for synaptic pruning. Long-term depression (LTD) is a synaptic plasticity showing persistent decreased synaptic efficacy, which is accompanied by morphological changes of dendritic spines including transient retraction. Repetitive induction of LTD has been shown to cause persistent loss of synapses in mature neurons. Here, we show that multiple, but not single, induction of LTD caused a persistent reduction in the number of dendritic synapses in cultured rat developing hippocampal neurons. When LTD was induced in 14 days in vitro cultures by application of (RS)-3,5-dihydroxyphenylglycine (DHPG), a group I metabotropic glutamate receptor (mGluR) agonist, and repeated three times with a one day interval, there was a significant decrease in the number of dendritic synapses. This effect continued up to at least two weeks after the triple LTD induction. The persistent reduction in synapse number occurred in the proximal dendrites, but not the distal dendrites, and was prevented by simultaneous application of the group I/II mGluR antagonist (S)-a-methyl-4-carboxyphenylglycine (MCPG). In conclusion, we found that repetitive LTD induction in developing neurons elicits synaptic pruning and contributes to activity-dependent regulation of synapse number in rat hippocampal neurons.

Shift in induction mechanisms underlies an age-dependent increase in DHPG-induced synaptic depression at CA3 CA1 synapses

Several forms of log-term synaptic plasticity have been identified and the mechanisms for induction and expression of synaptic modifications change over development and maturation. The present study examines age-related changes in the induction of group I metabotropic receptor selective agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced long-term synaptic depression (DHPG-LTD) at CA3-CA1 synapses. The results demonstrate that the magnitude of DHPG-LTD is enhanced in male aged Fischer 344 rats compared with young adults. The role of mGluR1 in the induction of DHPG-LTD was increased with advanced age and, in contrast to young adults, induction involved a significant contribution of NMDA receptors and L-type Ca(2+) channels. Moreover, the protein tyrosine phosphatase inhibitor sodium orthovanadate significantly attenuated DHPG-LTD only in young adults. The expression of DHPG-LTD in aged animals was dependent on protein synthesis and the enhanced expression was associated with an increase in paired-pulse facilitation. The results provide evidence that DHPG-LTD is one of the few forms of synaptic plasticity that increases with advanced age and suggest that DHPG-LTD may contribute to age-related changes in hippocampal function.

Modulation of a spinal locomotor network by metabotropic glutamate receptors

We have explored the potential involvement of the three main classes of metabotropic glutamate receptor in the modulation of a spinal locomotor network using tadpoles of the anuran amphibian Xenopus laevis. Selective activation of group I receptors in Xenopus embryos and young larvae using the general group I agonist DHPG [(S)-3,5-dihyroxyphenylglycine] significantly increased the frequency of swimming and the number of spontaneously occurring swimming episodes, as monitored by extracellular recordings from ventral roots. Group I receptor activation was without significant effect on the duration or amplitude of motor bursts, the duration of swimming episodes, or the head-to-tail delay in the propagation of swimming activity. Activation of either group II or group III receptors, however, following bath applications of the specific agonists APDC [(2R,4R)-aminopyrrolidine-2,4-dicarboxylic acid] and L-AP4 (L-2-amino-4-phosphonobutanoate), respectively, produced a net inhibitory effect on many of the parameters of fictive swimming at both developmental stages, including a reduction in swimming frequency and episode duration, along with a significant reduction in motor burst amplitude and duration in larval animals only. Applications of selective antagonists provide evidence for activation of all three groups during swimming. The group II and III antagonists EGLU (1-ethyl-2-benzimidazolinone) and MAP4 [(S)-2-amino-2-methyl-4-phosphonobutanoate], respectively, increased, while group I antagonists, CPCCOEt and MPEP, decreased swim frequency. Our findings thus provide evidence for the presence and endogenous activation of three classes of metabotropic glutamate receptor which may function as an intrinsic modulatory control system during fictive swimming in Xenopus tadpoles.

Membrane estrogen receptor-alpha interactions with metabotropic glutamate receptor 1a modulate female sexual receptivity in rats

In rats, female sexual behavior is regulated by a well defined limbic-hypothalamic circuit that integrates sensory and hormonal information. Estradiol activation of this circuit results in mu-opioid receptor (MOR) internalization in the medial preoptic nucleus, an important step for full expression of sexual receptivity. Estradiol acts through both membrane and intracellular receptors to influence neuronal activity and behavior, yet the mechanism(s) and physiological significance of estradiol-mediated membrane responses in vivo have remained elusive. Recent in vitro evidence found that stimulation of membrane-associated estrogen receptor-alpha (ER alpha) led to activation of metabotropic glutamate receptor 1a (mGluR1a). Furthermore, mGluR1a signaling was responsible for the observed downstream effects of estradiol. Here we present data that show that ER alpha and mGluR1a directly interact to mediate a rapid estradiol-induced activation of MOR in the medial preoptic nucleus, leading to female sexual receptivity. In addition, blockade of mGluR1a in the arcuate nucleus of the hypothalamus resulted in a significant attenuation of estradiol-induced MOR internalization, leading to diminished female sexual behavior. These results link membrane-initiated estradiol actions to neural events modulating behavior, demonstrating the physiological importance of ER alpha-to-mGluR1a signaling.

Long-term depression of cortico-striatal synaptic transmission by DHPG depends on endocannabinoid release and nitric oxide synthesis

In models of early stage Parkinson's disease (PD), motor deficits are accompanied by excessive activation of striatal glutamate receptors. Metabotropic glutamate group I receptors (mGluR I) play an important but not well-understood role in PD progression. In mouse brain slices, bath application of the mGluR I agonist (RS)-DHPG (3,5-dihydroxyphenylglycine, 100 microm for 20 min) caused a long-term depression of corticostriatal transmission (LTD(DHPG)), which was reversed by three mGluR I antagonists: LY 367385, CPCCOEt and MPEP. LTD(DHPG) required nitric oxide (NO) synthesis as it was blocked by the broad-spectrum NO synthase (NOS) inhibitor Nomega-nitro-l-arginine (NL-Arg) and impaired under blockade of neuronal NOS and in endothelial NOS-deficient mice. Release of endocannabinoids (eCB) was critically involved in this form of striatal plasticity givem that the CB1 receptor antagonist AM251 prevented LTD(DHPG), while the CB1 agonist ACEA elicited LTD. The NO synthesis necessary for LTD(DHPG) induction occurred downstream of CB1 activation as ACEA-evoked LTD was also abolished by NL-Arg. These findings are relevant for the pathophysiology of PD, as they link the overactivation of group I mGluRs and striatal NO production.

Loss of metabotropic glutamate receptor-dependent long-term depression via downregulation of mGluR5 after status epilepticus

Synaptic plasticity is thought to be a key mechanism of information storage in the CNS. Different forms of synaptic long-term potentiation have been shown to be impaired in neurological disorders. Here, we show that metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), but not NMDA receptor-dependent LTD at Schaffer collateral-CA1 synapses, is profoundly impaired after status epilepticus. Brief application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (100 microM; 5 min) induced mGluR LTD in control, but not in pilocarpine-treated rats. Experiments in the presence of selective inhibitors of either mGluR5 [2-methyl-6-(phenylethynyl)-pyridine] or mGluR1 [7-(hydroxyimino)cyclopropachromen-carboxylate ethyl ester and (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid] demonstrate that loss of mGluR LTD is most likely attributable to a loss of mGluR5 function. Quantitative real-time reverse transcription PCR revealed a specific downregulation of mGluR5 mRNA, but not of mGluR1 mRNA in the CA1 region. Furthermore, we detected a strong reduction in mGluR5 protein expression by immunofluorescence and quantitative immunoblotting. Additionally, the scaffolding protein Homer that mediates coupling of mGluR5 to downstream signaling cascades was downregulated. Thus, we conclude that the reduction of mGluR LTD after pilocarpine-induced status epilepticus is the result of the subtype-specific downregulation of mGluR5 and associated downstream signaling components.

Rapid synthesis and synaptic insertion of GluR2 for mGluR-LTD in the ventral tegmental area

The activation of metabotropic glutamate receptors (mGluRs) leads to long-term depression (mGluR-LTD) at many synapses of the brain. The induction of mGluR-LTD is well characterized, whereas the mechanisms underlying its expression remain largely elusive. mGluR-LTD in the ventral tegmental area (VTA) efficiently reverses cocaine-induced strengthening of excitatory inputs onto dopamine neurons. We show that mGluR-LTD is expressed by an exchange of GluR2-lacking AMPA receptors for GluR2-containing receptors with a lower single-channel conductance. The synaptic insertion of GluR2 depends on de novo protein synthesis via rapid messenger RNA translation of GluR2. Regulated synthesis of GluR2 in the VTA is therefore required to reverse cocaine-induced synaptic plasticity.

D1- and D2-like dopamine receptors regulate signaling properties of group I metabotropic glutamate receptors in the rat globus pallidus

Dopamine is essential to the proper functioning of basal ganglia (BG) because loss of dopaminergic input profoundly alters the activity of these nuclei. Experimental evidence suggests that multiple aspects of glutamatergic neurotransmission in the BG are altered with the loss of dopaminergic input. Using whole-cell patch-clamp recording in rat brain slices, we examined whether activity of dopamine receptors is necessary to maintain signaling properties of group I metabotropic glutamate receptor subtypes, mGluR1 and 5, in the rat globus pallidus (GP), one of the nuclei in the BG circuit. Dopaminergic depletion due to systemic treatment with reserpine caused a change in the signaling properties of group I mGluRs, where mGluR1 lost the ability to depolarize GP neurons, while mGluR5 gained such ability. Bath-application of dopamine or D1- and D2-like dopamine receptor agonists to slices from reserpinized rats partly reversed these effects and caused mGluR1 to gain back its ability to depolarize GP neurons. On the other hand, stimulation of either D1-like or D2-like dopamine receptors was sufficient to abolish the activating properties of mGluR5 acquired following reserpine treatment. Interestingly, inhibition of protein kinase A activity alone was sufficient to largely reverse plasticity in function of group I mGluRs that was induced by reserpine treatment. Our data reveal that specific roles of group I mGluRs in the GP depend on the activity of D1-like and D2-like dopamine receptors, further corroborating the importance of dopamine in maintaining proper glutamatergic neurotransmission in the BG.

Acute stress facilitates hippocampal CA1 metabotropic glutamate receptor-dependent long-term depression

Acute stress affects NMDA receptor (NMDAR)-dependent synaptic plasticity in the CA1 region of the hippocampus, with long-term potentiation and long-term depression (LTD) being, respectively, diminished and facilitated by acute exposure to stress. Here, we examined whether this facilitatory effect of stress on NMDAR-dependent LTD extends to metabotropic glutamate receptor (mGluR)-dependent LTD at Schaffer collateral-CA1 synapses. Application of a low dose (50 microM) of the selective group 1 mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) promoted LTD in slices from stressed, but not from control, rats. Pretreatment of stressed rats with the glucocorticoid receptor (GR) antagonist RU38486 prevented the facilitation of DHPG-induced LTD (DHPG-LTD), indicating the involvement of corticosterone secretion and, in turn, stimulation of GRs. Finally, pretreatment of slices with an mGluR1, but not an mGluR5, antagonist blunted the sensitizing effect of stress on DHPG-LTD. These results indicate that acute stress, through corticosterone stimulation of GRs, facilitates the expression of mGluR1-dependent DHPG-LTD in the hippocampal CA1 region.

Retrograde endocannabinoid inhibition of goldfish retinal cones is mediated by 2-arachidonoyl glycerol

A functional role for retinal endocannabinoids has not been determined. We characterized retrograde suppression of membrane currents of goldfish cones in a retinal slice. Whole-cell recordings were obtained from cone inner segments under voltage clamp. I(K(V)) was elicited by a depolarizing pulse to +54 mV from a holding potential of -70 mV. A fifty-millisecond puff of saline with 70 mM KCl or Group I mGluR agonist DHPG was applied through a pipette directly at a mixed rod/cone (Mb) bipolar cell body. The amplitude of I(K(V)) decreased 25% compared to the pre-puff control. Retrograde suppression of I(K(V)) was blocked by CB1 receptor antagonist, SR141716A. The FAAH inhibitor URB597 had no effect on the suppression of I(K(V)), whereas nimesulide, a COX-2 inhibitor, prolonged the effects of the K+ puff 10-fold. Orlistat, a blocker of 2-AG synthesis, blocked the effect of the K+ puff. Group I mGluR activation of Gq/11 was demonstrated in that a puff with DHPG decreased I(K(V)) of cones by 32%, an effect blocked by SR141716A. The effect of DHPG was not blocked by the mGluR5 antagonist MPEP, indicating involvement of mGluR1. The suppressive effect of the K+ puff vanished in a Ca2+-free, 2 mM Co2+ saline. TMB-8 or ryanodine, blocked the effect of DHPG, but not that of the K+ puff, showing that calcium influx or release from intracellular stores could mediate retrograde release. We suggest that retrograde suppression of cone I(K(V)) is mediated by Ca2+-dependent release of 2-AG from Mb bipolar cell dendrites by separate mechanisms: (1) voltage-dependent, mimicked by the K+ puff, that may be activated by the depolarizing ON response to light; (2) voltage-independent, occurring under ambient illumination, mediated by tonic mGluR1 activation. The negative feedback of this latter mechanism could regulate tonic glutamate release from cones within narrow limits, regardless of ambient illumination.

Activation of group I metabotropic glutamate receptors on main olfactory bulb granule cells and periglomerular cells enhances synaptic inhibition of mitral cells

Granule and periglomerular cells in the main olfactory bulb express group I metabotropic glutamate receptors (mGluRs). The group I mGluR agonist 3,4-dihydroxyphenylglycine (DHPG) increases GABAergic spontaneous IPSCs (sIPSCs) in mitral cells, yet the presynaptic mechanism(s) involved and source(s) of the IPSCs are unknown. We investigated the actions of DHPG on sIPSCs and TTX-insensitive miniature IPSCs (mIPSCs) recorded in mitral and external tufted cells in rat olfactory bulb slices. DHPG, acting at mGluR1 and mGluR5, increased the rate but not amplitude of sIPSCs and mIPSCs in both cell types. The increase in mIPSCs depended on voltage-gated Ca2+ channels but persisted when ionotropic glutamate receptors and sodium spikes were blocked. Focal DHPG puffs onto granule cells or bath application after glomerular layer (GL) excision failed to increase mIPSCs in mitral cells. Additionally, GL excision reduced sIPSCs in mitral cells by 50%, suggesting that periglomerular cells exert strong tonic GABAergic inhibition of mitral cells. In contrast, GL DHPG puffs readily increased mIPSCs. These findings indicate that DHPG-evoked GABA release from granule cells requires spikes, whereas in the GL, DHPG facilitates periglomerular cell GABA release via both spike-dependent and spike-independent presynaptic mechanisms. We speculate that mGluRs amplify spike-driven lateral inhibition through the mitral-to-granule cell circuit, whereas GL mGluRs may play a more important role in amplifying intraglomerular inhibition after subthreshold input.

Recurrent dendrodendritic inhibition of accessory olfactory bulb mitral cells requires activation of group I metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) modulate neural excitability and network tone in many brain regions. Expression of mGluRs is particularly high in the accessory olfactory bulb (AOB), a CNS structure critical for detecting chemicals that identify kin and conspecifics. Because of its relative simplicity and its direct projection to the hypothalamus, the AOB provides a model system for studying how mGluRs affect the flow of encoded sensory information to downstream areas. We investigated the role of group I mGluRs in synaptic processing in AOB slices and found that under control conditions, recurrent inhibition of principal neurons (mitral cells) was completely eliminated by the mGluR1 antagonist LY367385 [(S)-(+)-alpha-amino-4-carboxy-2 methylbenzeneacetic acid]. In addition, the group I mGluR agonist DHPG [(S)-3,5-dihydroxyphenylglycine; 20 microM] induced a dramatic increase in the rate of spontaneous IPSCs. This increase was dependent on voltage-gated calcium channels but persisted even after blockade of ionotropic glutamatergic transmission and sodium channels. Together, these results indicate that mGluR1 plays a critical role in controlling information flow through the AOB and suggest that mGluR1 may be an important locus for experience-dependent changes in synaptic function.

Decrease in PIP(2) channel interactions is the final common mechanism involved in PKC- and arachidonic acid-mediated inhibitions of GABA(B)-activated K+ current

We showed in our previous study that in hippocampal CA1 neurons the stimulation of muscarinic receptors inhibited the GIRK current (I(GIRK)) via a PLC/PKC pathway, whereas group I metabotropic glutamate receptors (mGluR) inhibited I(GIRK) via a PLA(2)/arachidonic acid pathway. In this study, we present evidence that receptor-mediated signalling pathways activated by the two G(q)-coupled receptors (G(q)PCRs) converge on the inhibition of GIRK channel-PIP(2) interaction. I(GIRK) was activated in acutely isolated hippocampal CA1 neurons by repetitive application of baclofen, a GABA(B) receptor agonist, with a 2-3 min interval. When both CCh and DHPG were pretreated before the second I(GIRK) activation, the magnitude of the second I(GIRK) was 52.2 +/- 2.5% of the first I(GIRK), which was not significantly different from the magnitude of inhibition by CCh or DHPG alone. This result shows that the effects of muscarinic receptor and group I mGluR stimulation on I(GIRK) are not additive but occlusive, suggesting that each pathway may converge to a common mechanism that finally regulates I(GIRK). To test the involvement of PIP(2) in this mechanism, the effect of CCh and DHPG on I(GIRK) was tested in cells loaded with exogenous PIP(2). The inhibition of I(GIRK) by CCh or DHPG was almost completely abolished in PIP(2)-loaded cells. We confirmed that the inhibition of I(GIRK) by direct application of phorbol ester or arachidonic acid was also completely reversed in PIP(2)-loaded cells. These results indicate that the decrease in PIP(2)-channel interactions is the final common mechanism responsible for G(q)PCR-induced inhibitions of I(GIRK) mediated by PKC and arachidonic acid.

Endocannabinoids limit metabotropic glutamate 5 receptor-mediated synaptic inhibition of striatal principal neurons

Synaptic transmission in the striatum is regulated by metabotropic glutamate (mGlu) receptors through pre- and postsynaptic mechanisms. We investigated the involvement of mGlu 1 and 5 receptors in the control of both excitatory and inhibitory transmission in the striatum. The mGlu 1 and 5 receptor agonist 3,5-DHPG failed to affect glutamate transmission, while it caused a biphasic effect on GABA transmission, characterized by early increase and late decrease in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from striatal principal neurons. Both mGlu 1 and 5 receptors were involved in the early response to 3,5-DHPG, through membrane depolarization of striatal GABAergic interneurons and action potential generation. The 3,5-DHPG-mediated late depression of inhibitory inputs to striatal principal neurons was conversely secondary to mGlu 5 receptor activation and subsequent endocannabinoid release. In conclusion, we have identified an mGlu-dependent mechanism of GABA transmission regulation of potential relevance for physiological neuronal activity.

Differential activation of protein kinases in the dorsal horn in vitro of normal and inflamed rats by group I metabotropic glutamate receptor subtypes

Group I metabotropic glutamate receptors (mGluRs) contribute to spinal sensitization and synaptic plasticity but the underlying mechanisms are unknown. Here, group I mGluR modulation of evoked monosynaptic excitatory postsynaptic currents (EPSCs) in substantia gelatinosa (SG) neurones in vitro was investigated in juvenile rats. In addition, the role of group I mGluRs in dorsal horn neuronal Fos expression was determined in tetrodotoxin (TTX)-treated in vitro spinal cords of naïve rats and those with Complete Freund's Adjuvant (CFA) peripheral inflammation. In the majority of SG neurones, (S)-3,5-dihydroxyphenylglycine (DHPG) reduced EPSCs and this effect was inhibited by the mGluR(5) antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP). Data for paired-pulse and spontaneous miniature excitatory postsynaptic currents (mEPSCs) suggest mGluR(5) acts presynaptically to reduce transmitter release. DHPG-induced reduction of EPSC amplitude operated via PKC, but not ERK, signalling cascade. In the dorsal horn of naïve but not CFA rats, DHPG increased Fos expression and this was reduced by MPEP and both PKC and ERK inhibitors. In the CFA group, basal Fos expression was reduced by MPEP and the kinase inhibitors. These data infer a role for mGluR(5) in acute modulation of nociceptive synaptic efficacy within the dorsal horn and postsynaptic activation of transcription factors such as Fos that are implicated in activity-dependent neuroplastic adaptation. These actions are achieved by differential activation of PKC- and ERK-dependent transduction pathways.

JNK1 contributes to metabotropic glutamate receptor-dependent long-term depression and short-term synaptic plasticity in the mice area hippocampal CA1

Several recent reports implicate an important role played by c-Jun N-terminal kinases (JNKs) in long-term potentiation (LTP). However, little is known about how the isoforms of JNKs participate in synaptic plasticity. Here we showed that short-term synaptic plasticity was impaired in the hippocampal area CA1 of JNK1-deficient (JNK1-/-) mice; these mice showed normal LTP in response to a strong tetanus and no alteration of N-methyl-D-aspartate receptor-dependent long-term depression (LTD) in the hippocampus. However, LTD induced either by group I metabotropic glutamate receptors (mGluRs) agonist dihydroxyphenylglycine or by paired-pulse low-frequency stimulation was absent in both the JNK1-/- slices and in JNK inhibitor anthrax [1, 9-cd] pyrazol-6(2H)-1 (SP600125)-pretreated slices. Induction of mGluR-dependent LTD resulted in an increase in phosphorylation of JNK1 substrates, including p-c-Jun and p-ATF2 in wild-type (WT) mice, and these increases failed to occur in the JNK1-/- or SP600125-pretreated mice. These results demonstrated that JNK1 played a crucial role in the short-term synaptic plasticity and mGluR-dependent LTD, whereas hippocampus LTP was not affected by JNK1 deficiency.

Metabotropic glutamate receptors in the lateral superior olive activate TRP-like channels: age- and experience-dependent regulation

The lateral superior olive (LSO) is the primary auditory nucleus for processing of interaural sound level differences, which is one of the major cues for sound localization. During development, survival and maturation of LSO neurons critically depend on synaptic activity and intracellular calcium signaling. Before hearing onset, glutamatergic synaptic inputs from the cochlear nucleus (CN) to the LSO activate group I metabotropic glutamate receptors (mGluRs), which leads to calcium release from intracellular stores and large calcium influx from the extracellular milieu. Here, we investigated the nature of the mGluR-activated membrane channel that mediates the influx of extracellular calcium. Using Fura-2 calcium imaging in brain stem slices of neonatal and juvenile mice, we found that this calcium channel is blocked by Ni(2+), La(3+), and 2-aminoethoxydiphenylborane (2-APB), known antagonists of transient receptor potential (TRP) channels. During postnatal development, the contribution of extracellular calcium influx to mGluR-mediated Ca(2+) responses gradually decreased and was almost abolished by the end of the third postnatal week. Over this period, the contribution of Ca(2+) release from internal stores remained unchanged. The developmental decrease of TRP-like channel-mediated calcium influx was significantly less in congenitally deaf waltzer mice, suggesting that early auditory experience is necessary for the normal age-dependent downregulation of functional TRP channels.

Mechanisms of glutamate receptor induced proliferation of astrocytes

Astrocytes express mainly metabotropic glutamate receptor 3 and metabotropic glutamate receptor 5 receptor subtypes, which show opposing effects on cellular proliferation upon activation. In this study, we investigated the mechanisms by which activation of these receptors modulates astrocyte proliferation. Activation of metabotropic glutamate receptor 5 with (S)-3,5-dihydroxyphenylglycine increased phospholipase D activity in astrocytes as well as astrocyte proliferation. The 3,5-dihydroxyphenylglycine-induced proliferation was inhibited in the presence of the metabotropic glutamate receptor 5 antagonist (2-methyl-6-(phenylethynyl)pyridine), the protein kinase C inhibitor GF109203X, brefeldin A and 1-butanol. Activation of metabotropic glutamate receptor 3 with (2'S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine-IV (DCG-IV) inhibited astrocyte proliferation without affecting metabotropic glutamate receptor 5-mediated phospholipase D activity. Metabotropic glutamate receptor 3 activation, however, only partially inhibited metabotropic glutamate receptor 5-mediated proliferation. In conclusion, metabotropic glutamate receptor 5 stimulates astrocyte proliferation via a protein kinase C-phospholipase D-phosphatidic acid-dependent pathway, whereas metabotropic glutamate receptor 3-mediated inhibition of astrocyte proliferation does not involve phospholipase D, and is independent of metabotropic glutamate receptor 5-mediated effects.

Involvement of Na+-Ca2+ exchanger on metabotropic glutamate receptor 1-mediated [Ca2+]i transients in rat cerebellar Purkinje neurons

Cerebellar Purkinje neurons have intracellular regulatory systems including Ca2+-binding proteins, intracellular Ca2+ stores, Ca2+-ATPase and Na+-Ca2+ exchanger (NCX) that keep intracellular Ca2+ concentration ([Ca2+]i) in physiological range. Among these, NCX interacts with AMPA receptors, activation of which induces cerebellar synaptic plasticity. And the activation of metabotropic glutamate receptor 1 (mGluR1) is also involved in the induction of cerebellar long-term depression. The interaction of NCX with mGluR1 is not known yet. Thus, in this study, the functional relationship between NCX and mGluR1 in modulating the [Ca2+]i in rat Purkinje neurons was investigated. The interaction between NCX and mGluR1 in Purkinje neurons was studied by measuring intracellular Ca2+ transients induced by an agonist of group I mGluRs, 3,5-dihydroxyphenylglycine (DHPG). The DHPG-induced Ca2+ transient was significantly reduced by treatments of NCX inhibitors, bepridil and KB-R7943. When cells were pretreated with antisense oligodeoxynucleotides of NCX, the DHPG-induced Ca2+ transient was also inhibited. These results suggest that NCX modulates the activity of mGluR1 in cerebellar Purkinje neurons. Therefore, NCX appears to play an important role in the physiological function of cerebellar Purkinje neurons such as synaptic plasticity.

Axon-glia communication evokes calcium signaling in olfactory ensheathing cells of the developing olfactory bulb

Olfactory ensheathing cells (OECs) accompany receptor axons in the olfactory nerve and promote axonal growth into the central nervous system. The mechanisms underlying the communication between axons and OECs, however, have not been studied in detail yet. We investigated the effect of activity-dependent neuronal transmitter release on Ca(2+) signaling of OECs in acute mouse olfactory bulb slices using confocal Ca(2+) imaging. TTX-sensitive axonal activity upon electrical nerve stimulation triggers a rise in cytosolic Ca(2+) in OECs, which can be mimicked by application of DHPG, an agonist of metabotropic glutamate receptors (mGluRs). Both stimulation- and DHPG-induced Ca(2+) transients in OECs were abolished by depletion of intracellular Ca(2+) stores with cyclopiazonic acid (CPA). The mGluR(1)-specific antagonist CPCCOEt completely inhibited DHPG-evoked Ca(2+) transients, but reduced stimulation-induced Ca(2+) transients only partly, suggesting the involvement of another neurotransmitter. Application of ATP evoked CPA-sensitive Ca(2+) transients in OECs, which were inhibited by the P2Y(1)-specific antagonist MRS2179. Co-application of CPCCOEt and MRS2179 almost completely blocked the stimulation-induced Ca(2+) transients, indicating that they were mediated by mGluR(1) and P2Y(1) receptors. Our results show that OECs are able to respond to olfactory nerve activity with an increase in cytosolic Ca(2+) due to glutamate and ATP release.

Peripheral metabotropic glutamate receptor 5 mediates mechanical hypersensitivity in craniofacial muscle via protein kinase C dependent mechanisms

We previously demonstrated that peripherally located N-methyl-D-aspartic acid (NMDA) receptors contribute to acute muscle nociception and the development of chronic muscular hyperalgesia. In the present study, we investigated the potential role of peripheral group I metabotropic glutamate receptors (mGluRs 1/5) in the development of muscular hypersensitivity to mechanical stimulation, and attempted to elucidate intracellular signaling mechanisms associated with the mGluR activation in male Sprague-Dawley rats. First, our Western blot analyses revealed that mGluR 5 protein, but not mGluR 1 protein, is reliably detected in trigeminal ganglia and the masseter nerve. Subsequent behavioral studies demonstrated that the group I mGluR agonist, R,S-3,5-dihydroxyphenylglycol (DHPG), significantly decreased the mechanical threshold to noxious stimulation of the masseter, and that the DHPG-induced mechanical hypersensitivity can be effectively prevented by pretreatment of the masseter with 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), a selective mGluR 5 antagonist, but not by 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), a selective mGluR 1 antagonist. Moreover, the DHPG-induced mechanical hypersensitivity was significantly blocked by inhibiting either the alpha or epsilon isoform of protein kinase C (PKC). Collectively, these data provide evidence that peripherally located mGluR 5 may play an important role in the development of masseter hypersensitivity, and that PKC activation is required for the modulatory effect of peripheral mGluR 5 in the craniofacial muscle tissue. Thus, selective targeting of peripheral mGluR 5 and PKCalpha, as well as PKCepsilon, might serve as an effective therapeutic strategy in the management of chronic muscle pain conditions, such as temporomandibular disorders.

Beta rhythms (15-20 Hz) generated by nonreciprocal communication in hippocampus

Generation of gamma rhythms in reciprocally connected areas of cortex produces synchronous neuronal firing, although little is known about the consequences of gamma rhythms when generated in nonreciprocally connected regions. This nonreciprocity exists in hippocampus, where gamma rhythms are generated in area CA3 in vitro and in vivo and nonreciprocally projected to area CA1 by the Schaffer collateral pathway. Here we demonstrate how this CA3 gamma rhythm generates two different patterns of local CA1 oscillation dependent on the degree of output from area CA1. 1) In conditions where activity projected to area CA1 produces only very low principal cell recruitment the local population rhythm mimics the gamma rhythm projected from CA3. This activity is generated predominantly by recruitment of CA1 basket cells in a manner dependent on phasic, feedforward excitation of this interneuron subclass. Interneurons in stratum oriens, not receiving CA3 feedforward input, fired at theta frequencies. 2) In the presence of serotonin CA1 principal cell recruitment was appreciably enhanced, resulting in dual activation of CA1 basket cells through both feedforward and feedback excitations. Feedback excitation to CA1 stratum oriens interneurons was also enhanced. The resulting change in interneuron network dynamics generated a beta-frequency CA1 rhythm (as a near-subharmonic of the gamma rhythm projected from CA3). These findings demonstrate that in nonreciprocally connected networks, the frequency of population rhythms in target areas serves to code for degree of principal cell recruitment by afferent input.

Receptor-specific inhibition of GABAB-activated K+ currents by muscarinic and metabotropic glutamate receptors in immature rat hippocampus

It has been shown that the activation of G(q)-coupled receptors (G(q)PCRs) in cardiac myocytes inhibits the G protein-gated inwardly rectifying K(+) current (I(GIRK)) via receptor-specific depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)). In this study, we investigated the mechanism of the receptor-mediated regulation of I(GIRK) in acutely isolated hippocampal CA1 neurons by the muscarinic receptor agonist, carbachol (CCh), and the group I metabotropic glutamate receptor (mGluR) agonist, 3,5-dihydroxyphenylglycine (DHPG). I(GIRK) was activated by the GABA(B) receptor agonist, baclofen. When baclofen was repetitively applied at intervals of 2-3 min, the amplitude of the second I(GIRK) was 92.3 +/- 1.7% of the first I(GIRK) in control. Pretreatment of neurons with CCh or DHPG prior to the second application of baclofen caused a reduction in the amplitude of the second I(GIRK) to 54.8 +/- 1.3% and 51.4 +/- 0.6%, respectively. In PLCbeta1 knockout mice, the effect of CCh on I(GIRK) was significantly reduced, whereas the effect of DHPG remained unchanged. The CCh-mediated inhibition of I(GIRK) was almost completely abolished by PKC inhibitors and pipette solutions containing BAPTA. The DHPG-mediated inhibition of I(GIRK) was attenuated by the inhibition of phospholipase A(2) (PLA(2)), or the sequestration of arachidonic acid. We confirmed that DHPG eliminated the inhibition of I(GIRK) by arachidonic acid. These results indicate that muscarinic inhibition of I(GIRK) is mediated by the PLC/PKC signalling pathway, while group I mGluR inhibition of I(GIRK) occurs via the PLA(2)-dependent production of arachidonic acid. These results present a novel receptor-specific mechanism for crosstalk between G(q)PCRs and GABA(B) receptors.

The fragile X mental retardation protein-RNP granules show an mGluR-dependent localization in the post-synaptic spines

The localization of RNA/mRNA in dendrites plays a role in both local and temporal regulation of protein synthesis, which is required for certain forms of synaptic plasticity. A key molecule in these processes is the fragile X mental retardation protein (FMRP). Using in situ hybridization coupled to immunofluorescence confocal microscopy, we find that the FMRP-RNP particle contains alphaCaMKII and BC1 RNAs as well as Staufen and CPEB proteins. Furthermore, following mGluR activation, the FMRP-mRNP complex moves into spines as shown by co-localization with the PSD-95 and Shank proteins. This study shows, for the first time, that the translationally inactive FMRP-mRNP complex relocates into neuronal spines after stimulation and that de novo protein synthesis mainly contributes to the pool of FMRP at synapses.

Group I metabotropic glutamate receptors are differentially expressed by two populations of olfactory bulb granule cells

In the main olfactory bulb, several populations of granule cells (GCs) can be distinguished based on the soma location either superficially, interspersed with mitral cells within the mitral cell layer (MCL), or deeper, within the GC layer (GCL). Little is known about the physiological properties of superficial GCs (sGCs) versus deep GCs (dGCs). Here, we used patch-clamp recording methods to explore the role of Group I metabotropic glutamate receptors (mGluRs) in regulating the activity of GCs in slices from wildtype and mGluR-/- mutant mice. In wildtype mice, bath application of the selective Group I mGluR agonist DHPG depolarized and increased the firing rate of both GC subtypes. In the presence of blockers of fast synaptic transmission (APV, CNQX, gabazine), DHPG directly depolarized both GC subtypes, although the two GC subtypes responded differentially to DHPG in mGluR1-/- and mGluR5-/- mice. DHPG depolarized sGCs in slices from mGluR5-/- mice, although it had no effect on sGCs in slices from mGluR1-/- mice. By contrast, DHPG depolarized dGCs in slices from mGluR1-/- mice but had no effect on dGCs in slices from mGluR5-/- mice. Previous studies showed that mitral cells express mGluR1 but not mGluR5. The present results therefore suggest that sGCs are more similar to mitral cells than dGCs in terms of mGluR expression.

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.

Regional interaction of endoplasmic reticulum Ca2+ signals between soma and dendrites through rapid luminal Ca2+ diffusion

The endoplasmic reticulum (ER) Ca2+ store plays a key role in integration and conveyance of Ca2+ signals in highly polarized neurons. The interconnected ER network in neurons generates Ca2+ signals in local domains, but the regional interaction is unclear. Here, we show that continuous or repetitive applications of caffeine produced robust Ca2+ release from the ER Ca2+ store in dendritic areas without severe store depletion, but that similar stimuli applied to soma caused rapid store depletion in acutely isolated midbrain dopamine neurons. Partial emptying of the ER Ca2+ store within a dendrite caused a similar level of store depletion in unstimulated dendrites, as well as in soma. Photobleaching and local stimulation experiments revealed that Ca2+ and the dye trapped within the ER diffused rapidly from the soma to dendrites up to 90 microm, which we could resolve, suggesting that the ER network acts as a functional tunnel for rapid Ca2+ transport. These data imply that the ER in soma acts as a Ca2+ reservoir supplying Ca2+ to the dendritic store, and that the dendritic store, hence, is able to respond to Ca2+-mobilizing input signals endurably.

Stimulation of group I mGlu receptors in the ventrotegmental area enhances extracellular dopamine in the rat medial prefrontal cortex

Group I mGlu receptors have been implicated in the control of brain dopamine release. However, the receptor subtype involved and the precise site of action have not been determined. In this study we show that (R,S)3,5-dihydroxyphenylglycine (DHPG; 6 and 60 nmol ICV), a selective group I mGlu receptor agonist, raised extracellular dopamine respectively by 176% and 243% of basal values in the medial prefrontal cortex as assessed by in vivo microdialysis in conscious rats. (R,S)2-chloro-5-hydroxyphenylglycine (60 nmol ICV), a selective mGlu5 receptor agonist, raised extracellular dopamine by 396% of basal values. Intra-VTA DHPG (0.6-6 nmol) mimicked ICV injection whereas intracortical infusion (1-1000 micromol/L) had no effect. DHPG-induced rise of extracellular dopamine was reversed by tetrodotoxin and by the selective mGlu1 and mGlu5 receptor antagonists 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (CPCCOEt) and 2-methyl-6-(phenylethynyl)pyridine (MPEP) either ICV or into the ventrotegmental area (VTA), suggesting that neuronal release and both mGlu1 and mGlu5 receptors were involved. These results support the existence of functional mGlu1 and mGlu5 receptors in the VTA regulating the release of dopamine in the medial prefrontal cortex.

Abnormal Ca2+ dynamics in transgenic mice with neuron-specific mitochondrial DNA defects

Maintenance of mitochondrial DNA (mtDNA) depends on nuclear-encoded proteins such as mtDNA polymerase (POLG), whose mutations are involved in the diseases caused by mtDNA defects including mutation and deletion. The defects in mtDNA and in intracellular Ca2+ ([Ca2+]i) homeostasis have been reported in bipolar disorder (BD). To understand the relevance of the mtDNA defects to BD, we studied transgenic (Tg) mice in which mutant POLG (mutPOLG) was expressed specifically in neurons. mtDNA defects were accumulated in the brains of mutPOLG Tg mice in an age-dependent manner and the mutant mice showed BD-like behavior. However, the molecular and cellular basis for the abnormalities has not been clarified. In this study, we investigated Ca2+ regulation by isolated mitochondria and [Ca2+]i dynamics in the neurons of mutPOLG Tg mice. Mitochondria from the mutant mice sequestered Ca2+ more rapidly, whereas Ca2+ retention capacity and membrane potential, a driving force of Ca2+ uptake, of mitochondria were unaffected. To elucidate the molecular mechanism of the altered Ca2+ uptake, we performed DNA microarray analysis and found that the expression of cyclophilin D (CyP-D), a component of the permeability transition pore, was downregulated in the brains of mutPOLG Tg mice. Cyclosporin A, an inhibitor of CyP-D, mimicked the enhanced Ca2+ uptake in mutant mice. Furthermore, G-protein-coupled receptor-mediated [Ca2+]i increase was attenuated in hippocampal neurons of the mutant mice. These findings suggest that mtDNA defects lead to enhancement of Ca2+ uptake rate via CyP-D downregulation and alter [Ca2+]i dynamics, which may be involved in the pathogenesis of BD.