Group I metabotropic glutamate receptor-mediated calcium signalling and immediate early gene expression in cultured rat striatal neurons

Pannexin 1 role in the trigeminal ganglion in infraorbital nerve injury-induced mechanical allodynia

OBJECTIVES

The detailed pathological mechanism of orofacial neuropathic pain remains unknown. We aimed to examine the pannexin 1 (Panx1) signaling in the trigeminal ganglion (TG) involvement in infraorbital nerve injury (IONI)-induced orofacial neuropathic pain.

MATERIALS AND METHODS

Mechanical head-withdrawal threshold (MHWT) was measured in IONI-treated rats receiving intra-TG Panx1 inhibitor or metabotropic glutamate receptor 5 (mGluR5) antagonist administration and MHWTs in naive rats receiving intra-TG mGluR5 agonist administration post-IONI. Glutamate and Panx1 in the TG were measured post-IONI. Panx1, mGluR5, and glutamine synthetase expression in TG were immunohistochemically identified, and changes in the number of mGluR5-P2X₃ -expressed TG neurons were examined.

RESULTS

MHWT was significantly decreased post-IONI, and this decrease was reversed by Panx1 inhibition or mGluR5 antagonism. mGluR5 agonism induced a decrease in the MHWT. IONI increased extracellular glutamate in TG. Panx1 was expressed in satellite glial cells and TG neurons, and intra-TG mGluR5 antagonism decreased the number of mGluR5 and P2X₃ positive TG neurons post-IONI.

CONCLUSIONS

IONI facilitates glutamate release via Panx1 that activates mGluR5 which was expressed in the nociceptive TG neurons innervating the orofacial region. In turn, P2X₃ receptor-expressed TG neurons is enhanced via mGluR5 signaling, resulting in orofacial neuropathic pain.

NMDA receptor modulation of the pedunculopontine tegmental nucleus underlies the motivational drive for feeding induced by midbrain dopaminergic neurons

The mesolimbic system, particularly the somatodendritic ventral tegmental area (VTA), is responsible for the positive reinforcing aspects of various homeostatic stimuli. In turn, the pedunculopontine tegmental nucleus (PPN) is anatomically and functionally connected with the VTA and substantia nigra (SN). In the present study, we investigated the role of glutamate receptors in the PPN in motivated behaviors by using a model of feeding induced by electrical stimulation of the VTA in male Wistar rats (n = 80). We found that injection of 2.5/5 µg dizocilpine (MK-801; NMDA receptor antagonist) to the PPN significantly reduced the feeding response induced by unilateral VTA-stimulation. This reaction was significantly impaired after local injection of MK-801 into the PPN in the ipsilateral rather than the contralateral hemisphere. After NMDA injection (2/3 µg) to the PPN we did not observe behavioral changes, only a trend of a lengthening/shortening of the latency to a feeding reaction at the highest dose of NMDA (3 µg). Immunohistochemical TH+/c-Fos+ analysis revealed a decrease in the number of TH+ cells in the midbrain (VTA-SN) in all experimental groups and altered activity of c-Fos+ neurons in selected brain structures depending on drug type (MK-801/NMDA) and injection site (ipsi-/contralateral hemisphere). Additionally, the pattern of TH+/c-Fos+ expression showed lateralization of feeding circuit functional connectivity. We conclude that the level of NMDA receptor arousal in the PPN regulates the activity of the midbrain dopaminergic cells, and the PPN-VTA circuit may be important in the regulation of motivational aspects of food intake.

The Beneficial Effect of Acute Exercise on Motor Memory Consolidation is Modulated by Dopaminergic Gene Profile

When aerobic exercise is performed following skilled motor practice, it can enhance motor memory consolidation. Previous studies have suggested that dopamine may play a role in motor memory consolidation, but whether it is involved in the exercise effects on consolidation is unknown. Hence, we aimed to investigate the influence of dopaminergic pathways on the exercise-induced modulation of motor memory consolidation. We compared the effect of acute exercise on motor memory consolidation between the genotypes that are known to affect dopaminergic transmission and learning. By combining cluster analyses and fitting linear models with and without included polymorphisms, we provide preliminary evidence that exercise benefits the carriers of alleles that are associated with low synaptic dopamine content. In line with previous reports, our findings implicate dopamine as a modulator of the exercise-induced effects on motor memory consolidation, and suggest exercise as a potential clinical tool to counteract low endogenous dopamine bioavailability. Further experiments are needed to establish causal relations.

Blockade of metabotropic glutamate receptor 5 protects against DNA damage in a rotenone-induced Parkinson's disease model

Glutamate excitotoxicity contributes to the development of Parkinson's disease (PD) and pharmacological blockade of metabotropic glutamate receptor 5 (mGluR5) has beneficial anti-akinetic effects in animal models of PD; however, the mechanism by which these antagonists alleviate PD symptoms is largely unknown. In our study, the effects of mGluR5 inhibition on DNA damage were investigated in a rotenone-induced model of PD. We first found that the selective mGluR5 antagonist, 2-methyl-6- (phenylethynyl) pyridine, prevented rotenone-induced DNA damage in MN9D dopaminergic neurons through a mechanism involving the downregulation of intracellular calcium release which was associated with a reduction in endoplasmic reticulum stress and reactive oxygen species (ROS)-related mitochondrial dysfunction. Interestingly, the ROS-related mitochondrial dysfunction was accompanied by an increase in expression of the antioxidant protein, Trx2. Treatment of cells with the calcium chelating agent 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or the ROS scavenger N-acetyl-L-cysteine, also reduced rotenone-induced DNA damage, while transfection of a dominant-negative form of Trx2 increased it. In addition, mGluR5 inhibition altered the expression profiles of proteins involved in DNA repair activity. Specifically, the expression of phosphorylated ERK (p-ERK) and CREB, as well as APE1 and Rad51 were elevated after rotenone stimulation and were subsequently downregulated following blockade of mGluR5. These findings were confirmed in vivo in a rotenone-induced rat model of PD. Inhibition of mGluR5 protected against neurotoxicity by mitigating oxidative stress-related DNA damage associated with 8-hydroxy-2'-deoxyguanosine production and also reduced p-ERK activity and Trx2 expression. These findings provide a novel link between mGluR5 and DNA damage in a model of PD, and reveal a potential mechanism by which mGluR5 mediates DNA damage in neurodegenerative diseases.

Hypobaric Preconditioning Modifies Group I mGluRs Signaling in Brain Cortex

The study assessed involvement of Ca(2+) signaling mediated by the metabotropic glutamate receptors mGluR1/5 in brain tolerance induced by hypoxic preconditioning. Acute slices of rat piriform cortex were tested 1 day after exposure of adult rats to mild hypobaric hypoxia for 2 h at a pressure of 480 hPa once a day for three consecutive days. We detected 44.1 ± 11.6 % suppression of in vitro anoxia-induced increases of intracellular Ca(2+) levels and a fivefold increase in Ca(2+) transients evoked by selective mGluR1/5 agonist, DHPG. Western blot analysis of cortical homogenates demonstrated a 11 ± 4 % decrease in mGluR1 immunoreactivity (IR), and in the nuclei-enriched fraction a 12 ± 3 % increase in IR of phospholipase Cβ1 (PLCβ1), which is a major mediator of mGluR1/5 signaling. Immunocytochemical analysis of the cortex revealed increase in the mGluR1/5 and PLCβ1 IR in perikarya, and a decrease in IR of the neuronal inositol trisphosphate receptors (IP3Rs). We suggest that enhanced expression of mGluR5 and PLCβ1 and potentiation of Ca(2+) signaling may represent pro-survival upregulation of Ca(2+)-dependent genomic processes, while decrease in mGluR1 and IP3R IR may be attributed to a feedback mechanism preventing excessive intracellular Ca(2+) release.

Metabotropic glutamate receptor-1 as a novel target for the antiangiogenic treatment of breast cancer

Metabotropic glutamate receptors (mGluRs) are normally expressed in the central nervous system, where they mediate neuronal excitability and neurotransmitter release. Certain cancers, including melanoma and gliomas, express various mGluR subtypes that have been implicated as playing a role in disease progression. Recently, we detected metabotropic glutamate receptor-1 (gene: GRM1; protein: mGluR1) in breast cancer and found that it plays a role in the regulation of cell proliferation and tumor growth. In addition to cancer cells, brain endothelial cells express mGluR1. In light of these studies, and because angiogenesis is both a prognostic indicator in cancer correlating with a poorer prognosis and a potential therapeutic target, we explored a potential role for mGluR1 in mediating endothelial cell (EC) proliferation and tumor-induced angiogenesis. GRM1 and mGluR1 were detected in various types of human ECs and, using mGluR1-specific inhibitors or shRNA silencing, we demonstrated that EC growth and Matrigel tube formation are dependent on mGluR1 signaling. In addition, loss of mGluR1 activity leads to reduced angiogenesis in a murine Matrigel sponge implant model as well as a murine tumor model. These results suggest a role for mGluR1 in breast cancer as a pro-angiogenic factor as well as a mediator of tumor progression. They also suggest mGluR1 as a potential new molecular target for the anti-angiogenic therapy of breast cancer.

Both neurons and astrocytes exhibited tetrodotoxin-resistant metabotropic glutamate receptor-dependent spontaneous slow Ca2+ oscillations in striatum

The striatum plays an important role in linking cortical activity to basal ganglia outputs. Group I metabotropic glutamate receptors (mGluRs) are densely expressed in the medium spiny projection neurons and may be a therapeutic target for Parkinson's disease. The group I mGluRs are known to modulate the intracellular Ca²⁺ signaling. To characterize Ca²⁺ signaling in striatal cells, spontaneous cytoplasmic Ca²⁺ transients were examined in acute slice preparations from transgenic mice expressing green fluorescent protein (GFP) in the astrocytes. In both the GFP-negative cells (putative-neurons) and astrocytes of the striatum, spontaneous slow and long-lasting intracellular Ca²⁺ transients (referred to as slow Ca²⁺ oscillations), which lasted up to approximately 200 s, were found. Neither the inhibition of action potentials nor ionotropic glutamate receptors blocked the slow Ca²⁺ oscillation. Depletion of the intracellular Ca²⁺ store and the blockade of inositol 1,4,5-trisphosphate receptors greatly reduced the transient rate of the slow Ca²⁺ oscillation, and the application of an antagonist against mGluR5 also blocked the slow Ca²⁺ oscillation in both putative-neurons and astrocytes. Thus, the mGluR5-inositol 1,4,5-trisphosphate signal cascade is the primary contributor to the slow Ca²⁺ oscillation in both putative-neurons and astrocytes. The slow Ca²⁺ oscillation features multicellular synchrony, and both putative-neurons and astrocytes participate in the synchronous activity. Therefore, the mGluR5-dependent slow Ca²⁺ oscillation may involve in the neuron-glia interaction in the striatum.

Interaction of DHPG-LTD and synaptic-LTD at senescent CA3-CA1 hippocampal synapses

The susceptibility, but not the magnitude, of long-term depression (LTD) induced by hippocampal CA3-CA1 synaptic activity (synaptic-LTD) increases with advanced age. In contrast, the magnitude of LTD induced by pharmacological activation of CA3-CA1 group I metabotropic glutamate receptors (mGluRs) increases during aging. This study examined the signaling pathways involved in induction of LTD and the interaction between paired-pulse low frequency stimulation-induced synaptic-LTD and group I mGluR selective agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG, 100 µM)-induced DHPG-LTD in hippocampal slices obtained from aged (22-24 months) male Fischer 344 rats. Prior induction of synaptic-LTD did not affect induction of DHPG-LTD; however, prior induction of the DHPG-LTD occluded synaptic-LTD suggesting that expression of DHPG-LTD may incorporate synaptic-LTD mechanisms. Application of individual antagonist for the group I mGluR (AIDA), the N-methyl-d-aspartate receptor (NMDAR) (AP-5), or L-type voltage-dependent Ca(2+) channel (VDCC) (nifedipine) failed to block synaptic-LTD and any two antagonists severely impaired synaptic-LTD induction, indicating that activation of any two mechanisms is sufficient to induce synaptic-LTD in aged animals. For DHPG-LTD, AIDA blocked DHPG-LTD and individually applied NMDAR or VDCC attenuated but did not block DHPG-LTD, indicating that the magnitude of DHPG-LTD depends on all three mechanisms.

Modulation of neurological deficits and expression of glutamate receptors during experimental autoimmune encephalomyelitis after treatment with selected antagonists of glutamate receptors

The aim of our investigation was to characterize the role of group I mGluRs and NMDA receptors in pathomechanisms of experimental autoimmune encephalomyelitis (EAE), the rodent model of MS. We tested the effects of LY 367385 (S-2-methyl-4-carboxyphenylglycine, a competitive antagonist of mGluR1), MPEP (2-methyl-6-(phenylethynyl)-pyridine, an antagonist of mGluR5), and the uncompetitive NMDA receptor antagonists amantadine and memantine on modulation of neurological deficits observed in rats with EAE. The neurological symptoms of EAE started at 10-11 days post-injection (d.p.i.) and peaked after 12-13 d.p.i. The protein levels of mGluRs and NMDA did not increase in early phases of EAE (4 d.p.i.), but starting from 8 d.p.i. to 25 d.p.i., we observed a significant elevation of mGluR1 and mGluR5 protein expression by about 20% and NMDA protein expression by about 10% over the control at 25 d.p.i. The changes in protein levels were accompanied by changes in mRNA expression of group I mGluRs and NMDARs. During the late disease phase (20–25 d.p.i.), the mRNA expression levels reached 300% of control values. In contrast, treatment with individual receptor antagonists resulted in a reduction of mRNA levels relative to untreated animals.

ω-3 fatty acid eicosapentaenoic acid attenuates MPP+-induced neurodegeneration in fully differentiated human SH-SY5Y and primary mesencephalic cells

Eicosapentaenoic acid (EPA), a neuroactive omega-3 fatty acid, has been demonstrated to exert neuroprotective effects in experimental models of Parkinson's disease (PD), but the cellular mechanisms of protection are unknown. Here, we studied the effects of EPA in fully differentiated human SH-SY5Y cells and primary mesencephalic neurons treated with MPP(+) . In both in-vitro models of PD, EPA attenuated an MPP(+) -induced reduction in cell viability. EPA also prevented the presence of electron-dense cytoplasmic inclusions in SH-SY5Y cells. Then, possible mechanisms of the neuroprotection were studied. In primary neurons, EPA attenuated an MPP(+) -induced increase in Tyrosine-related kinase B (TrkB) receptors. In SH-SY5Y cells, EPA down-regulated reactive oxygen species and nitric oxide. This antioxidant effect of EPA may have been mediated by its inhibition of neuronal NADPH oxidase and cyclo-oxygenase-2 (COX-2), as MPP(+) increased the expression of these enzymes. Furthermore, EPA prevented an increase in cytosolic phospholipase A2 (cPLA2), an enzyme linked with COX-2 in the potentially pro-inflammatory arachidonic acid cascade. Lastly, EPA attenuated an increase in the bax:bcl-2 ratio, and cytochrome c release. However, EPA did not prevent mitochondrial enlargement or a decrease in mitochondrial membrane potential. This study demonstrated cellular mechanisms by which EPA provided neuroprotective effects in experimental PD.

Group I metabotropic glutamate receptor-mediated gene transcription and implications for synaptic plasticity and diseases

Stimulation of group I metabotropic glutamate receptors (mGluRs) initiates a wide variety of signaling pathways. Group I mGluR activation can regulate gene expression at both translational and transcriptional levels, and induces translation or transcription-dependent synaptic plastic changes in neurons. The group I mGluR-mediated translation-dependent neural plasticity has been well reviewed. In this review, we will highlight group I mGluR-induced gene transcription and its role in synaptic plasticity. The signaling pathways (PKA, CaMKs, and MAPKs) which have been shown to link group I mGluRs to gene transcription, the relevant transcription factors (CREB and NF-κB), and target proteins (FMRP and ARC) will be documented. The significance and future direction for characterizing group I mGluR-mediated gene transcription in fragile X syndrome, schizophrenia, drug addiction, and other neurological disorders will also be discussed.

Glutamatergic pathway targeting in melanoma: single-agent and combinatorial therapies

PURPOSE

Melanoma is a heterogeneous disease where monotherapies are likely to fail due to variations in genomic signatures. B-RAF inhibitors have been clinically inadequate but response might be augmented with combination therapies targeting multiple signaling pathways. We investigate the preclinical efficacy of combining the multikinase inhibitor sorafenib or the mutated B-RAF inhibitor PLX4720 with riluzole, an inhibitor of glutamate release that antagonizes metabotropic glutamate receptor 1 (GRM1) signaling in melanoma cells.

EXPERIMENTAL DESIGN

Melanoma cell lines that express GRM1 and either wild-type B-RAF or mutated B-RAF were treated with riluzole, sorafenib, PLX4720, or the combination of riluzole either with sorafenib or with PLX4720. Extracellular glutamate levels were determined by glutamate release assays. MTT assays and cell-cycle analysis show effects of the compounds on proliferation, viability, and cell-cycle profiles. Western immunoblotting and immunohistochemical staining showed apoptotic markers. Consequences on mitogen-activated protein kinase pathway were assessed by Western immunoblotting. Xenograft tumor models were used to determine the efficacy of the compounds in vivo.

RESULTS

The combination of riluzole with sorafenib exhibited enhanced antitumor activities in GRM1-expressing melanoma cells harboring either wild-type or mutated B-RAF. The combination of riluzole with PLX4720 showed lessened efficacy compared with the combination of riluzole and sorafenib in suppressing the growth of GRM1-expressing cells harboring the B-RAF(V600E) mutation.

CONCLUSIONS

The combination of riluzole with sorafenib seems potent in suppressing tumor proliferation in vitro and in vivo in GRM1-expressing melanoma cells regardless of B-RAF genotype and may be a viable therapeutic clinical combination.

Dopaminergic projections from midbrain to primary motor cortex mediate motor skill learning

The primary motor cortex (M1) of the rat contains dopaminergic terminals. The origin of this dopaminergic projection and its functional role for movement are obscure. Other areas of cortex receive dopaminergic projections from the ventral tegmental area (VTA) of the midbrain, and these projections are involved in learning phenomena. We therefore hypothesized that M1 receives a dopaminergic projection from VTA and that this projection mediates the learning of a motor skill by inducing cellular plasticity events in M1. Retrograde tracing from M1 of Long-Evans rats in conjunction with tyrosine hydroxylase immunohistochemistry identified dopaminergic cell bodies in VTA. Electrical stimulation of VTA induced expression of the immediate-early gene c-fos in M1, which was blocked by intracortical injections of D(1) and D(2) antagonists. Destroying VTA dopaminergic neurons prevented the improvements in forelimb reaching seen in controls during daily training. Learning recovered on administration of levodopa into the M1 of VTA-lesioned animals. Lesioning VTA did not affect performance of an already learned skill, hence, left movement execution intact. These findings provide evidence that dopaminergic terminals in M1 originate in VTA, contribute to M1 plasticity, and are necessary for successful motor skill learning. Because VTA dopaminergic neurons are known to signal rewards, the VTA-to-M1 projection is a candidate for relaying reward information that could directly support the encoding of a motor skill within M1.

mGluR5 positive modulators both potentiate activation and restore inhibition in NMDA receptors by PKC dependent pathway

BACKGROUND

In order to understand the interaction between the metabotropic glutamate subtype 5 (mGluR5) and N-methyl-D-aspartate (NMDA) receptors, the influence of mGluR5 positive modulators in the inhibition of NMDA receptors by the noncompetitive antagonist ketamine, the competitive antagonist D-APV and the selective NR2B inhibitor ifenprodil was investigated.

METHODS

This study used the multi-electrode dish (MED) system to observe field potentials in hippocampal slices of mice.

RESULTS

Data showed that the mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), as well as the positive allosteric modulators 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl) benzamide (CDPPB) and 3,3'-difluorobenzaldazine (DFB) alone did not alter the basal field potentials, but enhanced the amplitude of field potentials induced by NMDA. The inhibitory action of ketamine on NMDA-induced response was reversed by CHPG, DFB, and CDPPB, whereas the blockade of NMDA receptor by D-APV was restored by CHPG and CDPPB, but not by DFB. Alternatively, activation of NMDA receptors prior to the application of mGluR5 modulators, CHPG was able to enhance NMDA-induced field potentials and reverse the suppressive effect of ketamine and D-APV, but not ifenprodil. In addition, chelerythrine chloride (CTC), a protein kinase C (PKC) inhibitor, blocked the regulation of mGluR5 positive modulators in enhancing NMDA receptor activation and recovering NMDA receptor inhibition. The PKC activator (PMA) mimicked the effects of mGluR5 positive modulators on enhancing NMDA receptor activation and reversing NMDA antagonist-evoked NMDA receptor suppression.

CONCLUSION

Our results demonstrate that the PKC-dependent pathway may be involved in the positive modulation of mGluR5 resulting in potentiating NMDA receptor activation and reversing NMDA receptor suppression induced by NMDA antagonists.

Therapeutic promise and principles: metabotropic glutamate receptors

For a number of disease entities, oxidative stress becomes a significant factor in the etiology and progression of cell dysfunction and injury. Therapeutic strategies that can identify novel signal transduction pathways to ameliorate the toxic effects of oxidative stress may lead to new avenues of treatment for a spectrum of disorders that include diabetes, Alzheimer's disease, Parkinson's disease and immune system dysfunction. In this respect, metabotropic glutamate receptors (mGluRs) may offer exciting prospects for several disorders since these receptors can limit or prevent apoptotic cell injury as well as impact upon cellular development and function. Yet the role of mGluRs is complex in nature and may require specific mGluR modulation for a particular disease entity to maximize clinical efficacy and limit potential disability. Here we discuss the potential clinical translation of mGluRs and highlight the role of novel signal transduction pathways in the metabotropic glutamate system that may be vital for the clinical utility of mGluRs.

Developmental alterations of DHPG-induced long-term depression of corticostriatal synaptic transmission: switch from NMDA receptor-dependent towards CB1 receptor-dependent plasticity

In animal models of early Parkinson's disease (PD), motor deficits are accompanied by excessive striatal glutamate release. Blockade of group I metabotropic glutamate receptors (mGluRs), endocannabinoid degradation and nitric oxide (NO) synthesis combats PD symptoms. Activation of group I mGluRs with the specific agonist 3,5-dihydroxyphenylglycine (DHPG) induces long-term depression of corticostriatal transmission (LTD(DHPG)) in the adult mouse striatum requiring NO synthesis downstream to cannabinoid CB1 receptor (CB1R) activation suggesting a dual role for LTD(DHPG): neuroprotective by down-regulation of glutamatergic transmission and, under certain circumstances, neurotoxic by release of NO. We report now that LTD(DHPG) undergoes a developmental switch from N-methyl-D-aspartate (NMDA)-receptor-dependent/CB1R-independent to NMDA receptor-independent/CB1R-dependent plasticity with NO playing an essential role for LTD(DHPG) at all developmental stages. The gain in function of CB1R is explained by their developmental up-regulation evaluated with real-time reverse transcription-polymerase chain reaction. These findings are relevant for the pathophysiology and therapy of PD as they link the activation of group I mGluRs, endocannabinoid release, and striatal NO production.

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

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

Regulation of extracellular signal-regulated kinase phosphorylation in cultured rat striatal neurons

Recent studies demonstrate that activation of Ca(2+)-permeable N-methyl-D-aspartate (NMDA) receptors upregulates phosphorylation of mitogen-activated protein kinases (MAPKs) in heterologous cells and neurons. In cultured rat striatal neurons, the present work systematically evaluated the role of a number of protein kinases in forming a signaling cascade transducing NMDA receptor signals to MAPKs. It was found that a brief NMDA application consistently induced rapid and transient phosphorylation of the extracellular signal-regulated kinase 1/2 (ERK1/2), a best characterized subclass of MAPKs. This ERK1/2 phosphorylation was resistant to the inhibition of protein kinase C, p38 MAPK, cyclin-dependent kinase 5, receptor tyrosine kinase (epidermal growth factor receptors), or non-receptor tyrosine kinases (including Src) by their selective inhibitors. However, the increase in ERK1/2 phosphorylation was partially blocked by a protein kinase A (PKA) inhibitor. The inhibitors for Ca(2+)/calmodulin-dependent protein kinase (CaMK) or phosphatidylinositol 3-kinase (PI3-kinase) completely blocked the NMDA-stimulated ERK1/2 phosphorylation. In an attempt to characterize the sequential role of CaMK and PI3-kinase, we found that NMDA increased PI3-kinase phosphorylation on Tyr(508), which kinetically corresponded to the ERK1/2 phosphorylation and was blocked by the CaMK inhibitor. These results indicate that the protein kinases are differentially involved in linking NMDA receptors to ERK1/2 in striatal neurons.

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

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

L-type calcium channel blockade on haloperidol-induced c-Fos expression in the striatum

Haloperidol-induced c-Fos expression in the lateral part of the neostriatum has been correlated with motor side effects while c-Fos induction in the medial part of the neostriatum and the nucleus accumbens is thought to be associated with the therapeutic effects of the drug. Induction of c-Fos in the striatum by haloperidol involves dopamine D(2) (DA D(2)) receptor antagonism and is dependent on activation of N-methyl-d-aspartate (NMDA) receptors and L-type Ca(2+) channels. In the current study, pretreatment with L-type Ca(2+) channel blockers suppressed haloperidol-induced c-Fos throughout the neostriatum and the nucleus accumbens at 2 h postinjection. However, elevated c-Fos protein expression was observed only in the lateral part of the neostriatum at 5 h postinjection of haloperidol following pretreatment of L-type Ca(2+) channel blocker compared with rats pretreated with vehicle alone. In addition, pretreatment prolonged the duration of haloperidol-induced catalepsy in rats. Infusions of L-type Ca(2+) channel blockers directly into the neostriatum mimicked similar patterns of changes in haloperidol-induced c-Fos expression. Prolonged expression of c-Fos was not observed following coadministration of nifedipine and a dopamine D(1) (DA D(1)) receptor agonist, SKF 81297, but could be mimicked by the DA D(2/3) receptor antagonist raclopride, suggesting that the phenomenon is likely related to DA D(2) receptor antagonism. Moreover, the expression levels of haloperidol-induced zif 268 and haloperidol-induced phosphorylated CREB and phosphorylated Elk-1 were also substantially elevated for a prolonged period of time in the lateral, but not the medial part of the neostriatum, following blockade of L-type Ca(2+) channels. Collectively, the results suggest that coadministration of L-type Ca(2+) channel blockers affects haloperidol signaling in the lateral part of the neostriatum and may exacerbate the development of acute motor side effects.

Group I mGluR5 metabotropic glutamate receptors regulate proliferation of neuronal progenitors in specific forebrain developmental domains

Major classical neurotransmitters including GABA and glutamate play novel morphogenic roles during development of the mammalian CNS. During forebrain neurogenesis, glutamate regulates neuroblast proliferation in different germinal domains using receptor subtype-specific mechanisms. For example, ionotropic N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors mediate distinct proliferative effects in ventral or dorsal forebrain germinal domains, and regulate the correct number of neurons that populate the striatum or cerebral cortex. Recent work suggests metabotropic receptors may also mediate glutamate's proliferative effects. Group I mGluR5 receptor subtypes are highly expressed in forebrain germinal zones. Using in vitro and in vivo methods, we demonstrate mGluR5 receptor activation plays an important role in neuroblast proliferation in the ventral telencephalon, and helps determine the complement of striatum projection neurons. mGluR5 receptor-mediated effects on striatal neuronal progenitors are restricted mainly to early cycling populations in the ventricular zone, with little effect on secondary proliferative populations in the subventricular zone. In contrast to proliferative effects in the ventral telencephalon, mGluR5 receptors do not modulate proliferation of dorsal telencephalon-derived cortical neuroblasts. Heterogeneous domain-specific proliferative effects of glutamate-mediated by specific receptor subtypes provide an important developmental mechanism allowing generation of the correct complement of neuronal subtypes that populate the mammalian forebrain.

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.

Activator protein-1 responsive to the group II metabotropic glutamate receptor subtype in association with intracellular calcium in cultured rat cortical neurons

Activation of ionotropic glutamate (Glu) receptors, such as N-methyl-d-aspartate receptors, is shown to modulate the gene transcription mediated by the transcription factor activator protein-1 (AP1) composed of Fos and Jun family proteins in the brain, while little attention has been paid to the modulation of AP1 expression by metabotropic Glu receptors (mGluRs). In cultured rat cortical neurons, where constitutive expression was seen with all groups I, II and III mGluR subtypes, a significant and selective increase was seen in the DNA binding activity of AP1 120 min after the brief exposure to the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) for 5 min. In cultured rat cortical astrocytes, by contrast, a significant increase was induced by a group I mGluR agonist, but not by either a group II or III mGluR agonist. The increase by DCG-IV was significantly prevented by a group II mGluR antagonist as well as by either an intracellular Ca(2+) chelator or a voltage-sensitive Ca(2+) channel blocker, but not by an intracellular Ca(2+) store inhibitor. Moreover, DCG-IV significantly prevented the increase of cAMP formation by forskolin in cultured neurons. Western blot analysis revealed differential expression profiles of Fos family members in neurons briefly exposed to DCG-IV and NMDA. Prior or simultaneous exposure to DCG-IV led to significant protection against neuronal cell death by NMDA. These results suggest that activation of the group II mGluR subtype would modulate the gene expression mediated by AP1 through increased intracellular Ca(2+) levels in cultured rat cortical neurons.

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

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

The dynamic range and domain-specific signals of intracellular calcium in photoreceptors

Vertebrate photoreceptors consist of strictly delimited subcellular domains: the outer segment, ellipsoid, cell body and synaptic terminal, each hosting crucial cellular functions, including phototransduction, oxidative metabolism, gene expression and transmitter release. We used optical imaging to explore the spatiotemporal dynamics of Ca(2+) signaling in non-outer segment regions of rods and cones. Sustained depolarization, designed to emulate photoreceptor activation in the darkness, evoked a standing Ca(2+) gradient in tiger salamander photoreceptors with spatially-averaged intracellular Ca(2+) concentration within synaptic terminals of approximately 2 microM and lower (approximately 750 nM) intracellular calcium concentration in the ellipsoid. Measurements from axotomized cell bodies and isolated ellipsoids showed that Ca(2+) enters the two compartments via both local L-type Ca(2+) channels and diffusion. The results from optical imaging studies were supported by immunostaining analysis. L-type voltage-operated Ca(2+) channels and plasma membrane Ca(2+) ATPases were highly expressed in synaptic terminals with progressively lower expression levels in the cell body and ellipsoid. These results show photoreceptor Ca(2+) homeostasis is controlled in a region-specific manner by direct Ca(2+) entry and diffusion as well as Ca(2+) extrusion. Moreover, quantitative measurement of intracellular calcium concentration levels in different photoreceptor compartments indicates that the dynamic range of Ca(2+) signaling in photoreceptors is approximately 40-fold, from approximately 50 nM in the light to approximately 2 microM in darkness.

The effect of mGlu5 receptor positive allosteric modulators on signaling molecules in brain slices

Positive allosteric modulators of metabotropic glutamate receptor subtype 5 (mGlu5) have promising therapeutic potential. The effects of selective mGlu5 receptor positive allosteric modulators on signaling molecules in brain slices have not been previously reported. The current study demonstrated that the selective mGlu5 receptor positive allosteric modulator, N-{4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2yl)-methyl]phenyl}-2-hydrobenzamide (CPPHA) potentiated the response to a subthreshold concentration of 3,5-dihydroxy-phenylglycine (DHPG) on extracellular signal-regulated protein kinase (ERK) and cyclic-AMP responsive element-binding protein (CREB) activity, as well as N-methyl d-aspartate (NMDA) receptor subunit NR1 phosphorylation in cortical and hippocampal slices. These results suggest that allosteric modulators of mGlu5 receptor could have physiologically significant effects by potentiating the actions of glutamate.

Extracellular-signal regulated kinase 1-dependent metabotropic glutamate receptor 5-induced long-term depression in the bed nucleus of the stria terminalis is disrupted by cocaine administration

The bed nucleus of the stria terminalis (BNST) is a key component of the CNS stress and reward circuit. Synaptic plasticity in this region could in part underlie the persistent behavioral alterations in generalized anxiety and addiction. Group I metabotropic glutamate receptors (mGluRs) have been implicated in stress, addiction, and synaptic plasticity, but their roles in the BNST are unknown. We find that activation of group I mGluRs in the dorsal BNST induces depression of excitatory synaptic transmission through two distinct mechanisms. First, a combined activation of group I mGluRs (mGluR1 and mGluR5) induces a transient depression that is cannabinoid 1 receptor dependent. Second, as with endocannabinoid-independent group I mGluR long-term depression (LTD) in the adult hippocampus, we find that activation of mGluR5 induces an extracellular signal-regulated kinase (ERK)-dependent LTD. Surprisingly, our data demonstrate that this LTD requires the ERK1 rather than ERK2 isoform, establishing a key role for this isoform in the CNS. Finally, we find that this LTD is dramatically reduced after multiple exposures but not a single exposure to cocaine, suggesting a role for this form of plasticity in the actions of psychostimulants on anxiety and reward circuitries and their emergent control of animal behavior.

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

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

Dopamine modulation of state-dependent endocannabinoid release and long-term depression in the striatum

Endocannabinoids are important mediators of short- and long-term synaptic plasticity, but the mechanisms of endocannabinoid release have not been studied extensively outside the hippocampus and cerebellum. Here, we examined the mechanisms of endocannabinoid-mediated long-term depression (eCB-LTD) in the dorsal striatum, a brain region critical for motor control and reinforcement learning. Unlike other cell types, strong depolarization of medium spiny neurons was not sufficient to yield detectable endocannabinoid release. However, when paired with postsynaptic depolarization sufficient to activate L-type calcium channels, activation of postsynaptic metabotropic glutamate receptors (mGluRs), either by high-frequency tetanic stimulation or an agonist, induced eCB-LTD. Pairing bursts of afferent stimulation with brief subthreshold membrane depolarizations that mimicked down-state to up-state transitions also induced eCB-LTD, which not only required activation of mGluRs and L-type calcium channels but also was bidirectionally modulated by dopamine D2 receptors. Consistent with network models, these results demonstrate that dopamine regulates the induction of a Hebbian form of long-term synaptic plasticity in the striatum. However, this gating of plasticity by dopamine is accomplished via an unexpected mechanism involving the regulation of mGluR-dependent endocannabinoid release.

Driving cellular plasticity and survival through the signal transduction pathways of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) share a common molecular morphology with other G protein-linked receptors, but there expression throughout the mammalian nervous system places these receptors as essential mediators not only for the initial development of an organism, but also for the vital determination of a cell's fate during many disorders in the nervous system that include amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, Multiple Sclerosis, epilepsy, trauma, and stroke. Given the ubiquitous distribution of these receptors, the mGluR system impacts upon neuronal, vascular, and glial cell function and is activated by a wide variety of stimuli that includes neurotransmitters, peptides, hormones, growth factors, ions, lipids, and light. Employing signal transduction pathways that can modulate both excitatory and inhibitory responses, the mGluR system drives a spectrum of cellular pathways that involve protein kinases, endonucleases, cellular acidity, energy metabolism, mitochondrial membrane potential, caspases, and specific mitogen-activated protein kinases. Ultimately these pathways can converge to regulate genomic DNA degradation, membrane phosphatidylserine (PS) residue exposure, and inflammatory microglial activation. As we continue to push the envelope for our understanding of this complex and critical family of metabotropic receptors, we should be able to reap enormous benefits for both clinical disease as well as our understanding of basic biology in the nervous system.

Metabotropic glutamate receptors in the basal ganglia motor circuit

In recent years there have been tremendous advances in our understanding of the circuitry of the basal ganglia and our ability to predict the behavioural effects of specific cellular changes in this circuit on voluntary movement. These advances, combined with a new understanding of the rich distribution and diverse physiological roles of metabotropic glutamate receptors in the basal ganglia, indicate that these receptors might have a key role in motor control and raise the exciting possibility that they might provide therapeutic targets for the treatment of Parkinson's disease and related disorders.

Signal transduction pathways of group I metabotropic glutamate receptor-induced long-term depression at sensory spinal synapses

Activation of spinal group I metabotropic glutamate receptors (mGluRs) may have antinociceptive or pro-nociceptive effects in different pain models. Pharmacological activation of group I mGluRs leads to long-term depression (LTD) of synaptic strength between Adelta-fibers and neurons in lamina II of spinal dorsal horn of the rat. Here, we studied the signal transduction pathways involved. Synaptic strength between Adelta-fibers and lamina II neurons was assessed by perforated whole-cell patch-clamp recordings in a spinal cord-dorsal root slice preparation of young rats. Bath application of the specific group I mGluR agonist (S)-3,5-dihydroxyphenylglycine [(S)-3,5-DHPG] produced an LTD of Adelta-fiber-evoked responses. LTD induction by (S)-3,5-DHPG was prevented, when intracellular Ca(2+) stores were depleted by thapsigargin or cyclopiazonic acid (CPA). Preincubation with ryanodine to inhibit Ca(2+)-induced Ca(2+) release had no effect on LTD-induction by (S)-3,5-DHPG. In contrast, pretreatment with 2-aminoethoxydiphenyl borate (2-APB), an inhibitor of inositol-1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) stores prevented LTD induction. Preincubation with the specific protein kinase C (PKC) inhibitors bisindolylmaleimide I (BIM) or chelerythrine, respectively, had no effect. Inhibition of L-type VDCCs by verapamil or nifedipine prevented LTD-induction by (S)-3,5-DHPG. The presently identified signal transduction cascade may be relevant to the long-term depression of sensory information in the spinal cord, including nociception.

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

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

Stimulation of group I metabotropic glutamate receptors evokes calcium signals and c-jun and c-fos gene expression in human T cells

To study if the activation of group I mGlu receptors in human T cells modifies intracellular Ca2+ concentration ([Ca2+](i)) and cell function, we measured [Ca2+](i) on cell suspensions (spectrofluorimetric method) or single cell (digital Ca2+ imaging system) using fura-2 as indicator. Early-inducible gene (c-jun and c-fos) expression was studied by reverse transcriptase-polymerase chain reaction assay as representative of Ca(2+)-sensitive gene expression. (1S,3R)-ACPD (100 microM), the selective mGlu receptor agonist, evoked a significant increase (34.1+/-4.9%) of [Ca2+](i), pharmacologically characterized as mediated by group I mGlu receptors, since both (S)-3,5-DHPG (100 microM), a selective group I mGlu receptor agonist and CHPG (1mM), the specific mGlu5 receptor agonist, reproduced the effects, that were abolished by AIDA (1mM), a selective group I mGlu receptor antagonist. (S)-3,5-DHPG-induced a rapid [Ca2+](i) rise (initial phase) followed by a slow decrease (second phase) to the baseline. Both extracellular Ca2+ and Ca2+ released from intracellular stores contribute to the [Ca2+](i) increase which depend on PLC activation. In a Ca(2+)-free buffer, the second phase rapidly return to the baseline; LaCl3 (1-10 microM), an inhibitor of extracellular Ca2+ influx, significantly reduced the second phase only; thapsigargin (1microM), by discharging intracellular Ca2+ stores, U 73122 (10 microM) and D609 (300 microM), by inhibiting PLC activity, prevented both phases. In our system, PTX pre-treatment increased (S)-3,5-DHPG effects, demonstrating that PXT-sensitive G(i/o) proteins are involved. Finally, specific stimulation of these receptors in Jurkat cells upregulates c-jun and c-fos gene expression, thus activating multiple downstream signalling regulating important T cell functions.

Regulation of MAPK/ERK phosphorylation via ionotropic glutamate receptors in cultured rat striatal neurons

Extracellular signals may regulate mitogen-activated protein kinase (MAPK) cascades through a receptor-mediated mechanism. As a signaling superhighway to the nucleus, active Ras-MAPK cascades phosphorylate transcription factors and facilitate gene expression. In cultured rat striatal neurons, the present work systemically examined the linkage between glutamate receptors and the extracellular signal-regulated kinase 1/2 (ERK1/2) subclass of MAPK. We found that glutamate induced a rapid and transient phosphorylation of ERK1/2. Similar responses of ERK1/2 phosphorylation were also induced by the ligands selective for each of three subtypes of ionotropic receptors (NMDA, AMPA and kainate), although not by the subgroup-selective agonists for three subgroups of metabotropic glutamate receptors after 8-9 days in culture. The ERK1/2 phosphorylation induced by all ionotropic receptor agents was dose-, time- and Ca(2+) influx-dependent and occurred in neurons, but not glia. The NMDA-, AMPA- and kainate-induced ERK1/2 phosphorylation was blocked only by the antagonists selective for respective subtypes. The ERK1/2 phosphorylation induced by these agents was also sensitive to the MAPK kinase 1 (MEK1) inhibitor PD98059 and the MEK1/2 inhibitor U0126. In a further attempt to evaluate the role of active ERK1/2 in activating a downstream transcription factor cAMP response element-binding protein (CREB), NMDA, AMPA, and kainate were found to increase CREB phosphorylation. The NMDA- and AMPA/kainate-induced CREB phosphorylation was completely and partially blocked by U0126, respectively. These results revealed a positive linkage between ionotropic glutamate receptors and MEK-sensitive ERK1/2 phosphorylation in striatal neurons. The active ERK1/2 cascade activates the downstream transcription factor CREB to participate in the regulation of gene expression.

Changes in hippocampal gene expression after neuroprotective activation of group I metabotropic glutamate receptors

Stimulation of group I metabotropic glutamate receptors (mGluRs) has been shown to protect against N-methyl-D-aspartate receptor-mediated cell death, but the underlying cellular mechanism is unknown. Using cDNA microarrays we have now compared gene expressions in organotypic hippocampal slice cultures after neuroprotective activation of group I mGluRs with (S)-3,5-dihydroxyphenylglycine (DHPG; 10 microM, 2 h) with untreated control cultures. Total RNA was extracted from the cultures immediately after the neuroprotective treatment, reverse transcribed to cDNA with incorporation of [32]P-dCTP, and then hybridized to the arrays. Of a total of 1128 genes on the Neuroarray, 33 genes displayed significant changes in expression after DHPG-treatment (six up- and 27 downregulated). These genes have been associated with regulation of synaptic excitation, inflammation, cell adhesion, cell death, and transcription. The small GTPase RAB5B associated with endocytosis emerged as a primary candidate gene for neuroprotection, and its expression was confirmed by Western blot analysis and real time polymerase chain reaction. By providing insight into genes involved in neuroprotection these data may help to identify novel therapeutic targets.

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