Heterologous expression of metabotropic glutamate receptors in adult rat sympathetic neurons: subtype-specific coupling to ion channels

TrkB receptor interacts with mGlu2 receptor and mediates antipsychotic-like effects of mGlu2 receptor activation in the mouse

Metabotropic glutamate receptor 2 (mGlu2) attracts particular attention as a possible target for a new class of antipsychotics. However, the signaling pathways transducing the effects of mGlu2 in the brain remain poorly characterized. Here, we addressed this issue by identifying native mGlu2 interactome in mouse prefrontal cortex. Nanobody-based affinity purification and mass spectrometry identified 149 candidate mGlu2 partners, including the neurotrophin receptor TrkB. The later interaction was confirmed both in cultured cells and prefrontal cortex. mGlu2 activation triggers phosphorylation of TrkB on Tyr816 in primary cortical neurons and prefrontal cortex. Reciprocally, TrkB stimulation enhances mGlu2-operated Gi/o protein activation. Furthermore, TrkB inhibition prevents the rescue of behavioral deficits by glutamatergic antipsychotics in phencyclidine-treated mice. Collectively, these results reveal a cross-talk between TrkB and mGlu2, which is key to the behavioral response to glutamatergic antipsychotics.

Role of group II metabotropic glutamate receptors in ketamine's antidepressant actions

Major Depressive Disorder (MDD) is a serious neuropsychiatric disorder afflicting around 16-17 % of the global population and is accompanied by recurrent episodes of low mood, hopelessness and suicidal thoughts. Current pharmacological interventions take several weeks to even months for an improvement in depressive symptoms to emerge, with a significant percentage of individuals not responding to these medications at all, thus highlighting the need for rapid and effective next-generation treatments for MDD. Pre-clinical studies in animals have demonstrated that antagonists of the metabotropic glutamate receptor subtype 2/3 (mGlu_2/3 receptor) exert rapid antidepressant-like effects, comparable to the actions of ketamine. Therefore, it is possible that mGlu₂ or mGlu₃ receptors to have a regulatory role on the unique antidepressant properties of ketamine, or that convergent intracellular mechanisms exist between mGlu_2/3 receptor signaling and ketamine's effects. Here, we provide a comprehensive and critical evaluation of the literature on these convergent processes underlying the antidepressant action of mGlu_2/3 receptor inhibitors and ketamine. Importantly, combining sub-threshold doses of mGlu_2/3 receptor inhibitors with sub-antidepressant ketamine doses induce synergistic antidepressant-relevant behavioral effects. We review the evidence supporting these combinatorial effects since sub-effective dosages of mGlu_2/3 receptor antagonists and ketamine could reduce the risk for the emergence of significant adverse events compared with taking normal dosages. Overall, deconvolution of ketamine's pharmacological targets will give critical insights to influence the development of next-generation antidepressant treatments with rapid actions.

Expression of group II metabotropic glutamate receptors in rat superior cervical ganglion

BACKGROUND

The superior cervical ganglion (SCG) plays critical roles in the regulation of blood pressure and cardiac output. Metabotropic glutamate receptors (mGluRs) in the SCG are not clearly elucidated yet. Most studies on the expression and functions of mGluRs in the SCG focused on the cultured SCG neurons, and yet little information has been reported in the SCG tissue. Chronic intermittent hypoxia (CIH), one of the major clinical features of obstructive sleep apnea (OSA) patients, is a critical pathological cause of secondary hypertension in OSA patients, but its impact on the level of mGluRs in the SCG is unknown.

OBJECTIVE

To explore the expression and localization of mGluR2/3 and the effect of CIH on mGluR2/3 level in rat SCG tissue.

METHODS

RT-PCR and immunostaining were conducted to examine the mRNA and protein expression of mGluR2/3 in rat SCG. Immunofluorescence staining was conducted to examine the distribution of mGluR2/3. Rats were divided into control and CIH group which the rats were exposed to CIH for 6 weeks. Western blots were performed to examine the level of mGluR2/3 in rat SCG.

RESULTS

mRNAs of mGluR2/3 expressed in rat SCG. mGluR2 distributed in principal neurons and small intensely fluorescent cells but not in satellite glial cells, nerve fibers, and vascular endothelial cells; mGluR3 was detected in nerve fibers rather than in the cells mentioned above. CIH exposure reduced the protein level of mGluR2/3 in rat SCG.

CONCLUSION

mGluR2/3 exists in rat SCG with diverse distribution patterns, and may be involved in CIH-induced hypertension.

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

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

Shock wave-induced permeabilization of mammalian cells

Controlled permeabilization of mammalian cell membranes is fundamental to develop gene and cell therapies based on macromolecular cargo delivery, a process that emerged against an increasing number of health afflictions, including genetic disorders, cancer and infections. Viral vectors have been successfully used for macromolecular delivery; however, they may have unpredictable side effects and have been limited to life-threatening cases. Thus, several chemical and physical methods have been explored to introduce drugs, vaccines, and nucleic acids into cells. One of the most appealing physical methods to deliver genes into cells is shock wave-induced poration. High-speed microjets of fluid, emitted due to the collapse of microbubbles after shock wave passage, represent the most significant mechanism that contributes to cell membrane poration by this technique. Herein, progress in shock wave-induced permeabilization of mammalian cells is presented. After covering the main concepts related to molecular strategies whose applications depend on safer drug delivery methods, the physics behind shock wave phenomena is described. Insights into the use of shock waves for cell membrane permeation are discussed, along with an overview of the two major biomedical applications thereof-i.e., genetic modification and anti-cancer shock wave-assisted chemotherapy. The aim of this review is to summarize 30 years of data showing underwater shock waves as a safe, noninvasive method for macromolecular delivery into mammalian cells, encouraging the development of further research, which is still required before the introduction of this promising tool into clinical practice.

Neuronal Glutamate Transporters Control Dopaminergic Signaling and Compulsive Behaviors

There is an ongoing debate on the contribution of the neuronal glutamate transporter EAAC1 to the onset of compulsive behaviors. Here, we used behavioral, electrophysiological, molecular, and viral approaches in male and female mice to identify the molecular and cellular mechanisms by which EAAC1 controls the execution of repeated motor behaviors. Our findings show that, in the striatum, a brain region implicated with movement execution, EAAC1 limits group I metabotropic glutamate receptor (mGluRI) activation, facilitates D1 dopamine receptor (D1R) expression, and ensures long-term synaptic plasticity. Blocking mGluRI in slices from mice lacking EAAC1 restores D1R expression and synaptic plasticity. Conversely, activation of intracellular signaling pathways coupled to mGluRI in D1R-containing striatal neurons of mice expressing EAAC1 leads to reduced D1R protein level and increased stereotyped movement execution. These findings identify new molecular mechanisms by which EAAC1 can shape glutamatergic and dopaminergic signals and control repeated movement execution.SIGNIFICANCE STATEMENT Genetic studies implicate Slc1a1, a gene encoding the neuronal glutamate transporter EAAC1, with obsessive-compulsive disorder (OCD). EAAC1 is abundantly expressed in the striatum, a brain region that is hyperactive in OCD. What remains unknown is how EAAC1 shapes synaptic function in the striatum. Our findings show that EAAC1 limits activation of metabotropic glutamate receptors (mGluRIs) in the striatum and, by doing so, promotes D1 dopamine receptor (D1R) expression. Targeted activation of signaling cascades coupled to mGluRIs in mice expressing EAAC1 reduces D1R expression and triggers repeated motor behaviors. These findings provide new information on the molecular basis of OCD and suggest new avenues for its treatment.

Cross-compartmental Modulation of Dendritic Signals for Retinal Direction Selectivity

Compartmentalized signaling in dendritic subdomains is critical for the function of many central neurons. In the retina, individual dendritic sectors of a starburst amacrine cell (SAC) are preferentially activated by different directions of linear motion, indicating limited signal propagation between the sectors. However, the mechanism that regulates this propagation is poorly understood. Here, we find that metabotropic glutamate receptor 2 (mGluR2) signaling, which acts on voltage-gated calcium channels in SACs, selectively restricts cross-sector signal propagation in SACs, but does not affect local dendritic computation within individual sectors. mGluR2 signaling ensures sufficient electrotonic isolation of dendritic sectors to prevent their depolarization during non-preferred motion, yet enables controlled multicompartmental signal integration that enhances responses to preferred motion. Furthermore, mGluR2-mediated dendritic compartmentalization in SACs is important for the functional output of direction-selective ganglion cells (DSGCs). Therefore, our results directly link modulation of dendritic compartmentalization to circuit-level encoding of motion direction in the retina.

The Role of mGlu Receptors in Hippocampal Plasticity Deficits in Neurological and Psychiatric Disorders: Implications for Allosteric Modulators as Novel Therapeutic Strategies

Long-term potentiation (LTP) and long-term depression (LTD) are two distinct forms of synaptic plasticity that have been extensively characterized at the Schaffer collateral-CA1 (SCCA1) synapse and the mossy fiber (MF)-CA3 synapse within the hippocampus, and are postulated to be the molecular underpinning for several cognitive functions. Deficits in LTP and LTD have been implicated in the pathophysiology of several neurological and psychiatric disorders. Therefore, there has been a large effort focused on developing an understanding of the mechanisms underlying these forms of plasticity and novel therapeutic strategies that improve or rescue these plasticity deficits. Among many other targets, the metabotropic glutamate (mGlu) receptors show promise as novel therapeutic candidates for the treatment of these disorders. Among the eight distinct mGlu receptor subtypes (mGlu1-8), the mGlu1,2,3,5,7 subtypes are expressed throughout the hippocampus and have been shown to play important roles in the regulation of synaptic plasticity in this brain area. However, development of therapeutic agents that target these mGlu receptors has been hampered by a lack of subtype-selective compounds. Recently, discovery of allosteric modulators of mGlu receptors has provided novel ligands that are highly selective for individual mGlu receptor subtypes. The mGlu receptors modulate the multiple forms of synaptic plasticity at both SC-CA1 and MF synapses and allosteric modulators of mGlu receptors have emerged as potential therapeutic agents that may rescue plasticity deficits and improve cognitive function in patients suffering from multiple neurological and psychiatric disorders.

Single Cell Transfection through Precise Microinjection with Quantitatively Controlled Injection Volumes

Cell transfection is a technique wherein foreign genetic molecules are delivered into cells. To elucidate distinct responses during cell genetic modification, methods to achieve transfection at the single-cell level are of great value. Herein, we developed an automated micropipette-based quantitative microinjection technology that can deliver precise amounts of materials into cells. The developed microinjection system achieved precise single-cell microinjection by pre-patterning cells in an array and controlling the amount of substance delivered based on injection pressure and time. The precision of the proposed injection technique was examined by comparing the fluorescence intensities of fluorescent dye droplets with a standard concentration and water droplets with a known injection amount of the dye in oil. Injection of synthetic modified mRNA (modRNA) encoding green fluorescence proteins or a cocktail of plasmids encoding green and red fluorescence proteins into human foreskin fibroblast cells demonstrated that the resulting green fluorescence intensity or green/red fluorescence intensity ratio were well correlated with the amount of genetic material injected into the cells. Single-cell transfection via the developed microinjection technique will be of particular use in cases where cell transfection is challenging and genetically modified of selected cells are desired.

Human GPR42 is a transcribed multisite variant that exhibits copy number polymorphism and is functional when heterologously expressed

FFAR3 (GPR41) is a G-protein coupled receptor for which short-chain fatty acids serve as endogenous ligands. The receptor is found on gut enteroendocrine L-cells, pancreatic β-cells, and sympathetic neurons, and is implicated in obesity, diabetes, allergic airway disease, and altered immune function. In primates, FFAR3 is segmentally duplicated resulting in GPR42, a gene currently classified as a suspected pseudogene. In this study, we sequenced FFAR3 and GPR42 open reading frames from 56 individuals and found an unexpectedly high frequency of polymorphisms contributing to several complex haplotypes. We also identified a frequent (18.8%) structural variation that results in GPR42 copy number polymorphism. Finally, sequencing revealed that 50.6% of GPR42 haplotypes differed from FFAR3 by only a single non-synonymous substitution and that the GPR42 reference sequence matched only 4.4% of the alleles. Sequencing of cDNA from human sympathetic ganglia and colon revealed processed transcripts matching the GPR42 genotype. Expression of several GPR42 haplotypes in rat sympathetic neurons revealed diverse pharmacological phenotypes that differed in potency and efficacy. Our data suggest that GPR42 be reclassified as a functioning gene and that recognition of sequence and copy number polymorphism of the FFAR3/GPR42 complex be considered during genetic and pharmacological investigation of these receptors.

Activation of group II metabotropic glutamate receptors blocks zinc release from hippocampal mossy fibers

Background

The hippocampal CA3 area contains large amounts of vesicular zinc in the mossy fiber terminals which is released during synaptic activity, depending on presynaptic calcium. Another characteristic of these synapses is the presynaptic localization of high concentrations of group II metabotropic glutamate receptors, specifically activated by DCG-IV. Previous work has shown that DCG-IV affects only mossy fiber-evoked responses but not the signals from associational-commissural afferents, blocking mossy fiber synaptic transmission. Since zinc is released from mossy fibers even for single stimuli and it is generally assumed to be co-released with glutamate, the aim of the work was to investigate the effect of DCG-IV on mossy fiber zinc signals.

Results

Studies were performed using the membrane-permeant fluorescent zinc probe TSQ, and indicate that DCG-IV almost completely abolishes mossy fiber zinc changes as it does with synaptic transmission.

Conclusions

Zinc signaling is regulated by the activation of type II metabotropic receptors, as it has been previously shown for glutamate, further supporting the corelease of glutamate and zinc from mossy fibers.

Optical control of metabotropic glutamate receptors

G-protein coupled receptors (GPCRs), the largest family of membrane signaling proteins, respond to neurotransmitters, hormones and small environmental molecules. The neuronal function of many GPCRs has been difficult to resolve because of an inability to gate them with subtype-specificity, spatial precision, speed and reversibility. To address this, we developed an approach for opto-chemical engineering native GPCRs. We applied this to the metabotropic glutamate receptors (mGluRs) to generate light-agonized and light-antagonized “LimGluRs”. The light-agonized “LimGluR2”, on which we focused, is fast, bistable, and supports multiple rounds of on/off switching. Light gates two of the primary neuronal functions of mGluR2: suppression of excitability and inhibition of neurotransmitter release. The light-antagonized “LimGluR2block” can be used to manipulate negative feedback of synaptically released glutamate on transmitter release. We generalize the optical control to two additional family members: mGluR3 and 6. The system works in rodent brain slice and in zebrafish in vivo, where we find that mGluR2 modulates the threshold for escape behavior. These light-gated mGluRs pave the way for determining the roles of mGluRs in synaptic plasticity, memory and disease.

N-Arachidonyl glycine does not activate G protein-coupled receptor 18 signaling via canonical pathways

Recent studies propose that N-arachidonyl glycine (NAGly), a carboxylic analogue of anandamide, is an endogenous ligand of the Gα(i/o) protein-coupled receptor 18 (GPR18). However, a high-throughput β-arrestin-based screen failed to detect activation of GPR18 by NAGly (Yin et al., 2009; JBC, 18:12328). To address this inconsistency, this study investigated GPR18 coupling in a native neuronal system with endogenous signaling pathways and effectors. GPR18 was heterologously expressed in rat sympathetic neurons, and the modulation of N-type (Ca(v)2.2) calcium channels was examined. Proper expression and trafficking of receptor were confirmed by the "rim-like" fluorescence of fluorescently tagged receptor and the positive staining of external hemagglutinin-tagged GPR18-expressing cells. Application of NAGly on GPR18-expressing neurons did not inhibit calcium currents but instead potentiated currents in a voltage-dependent manner, similar to what has previously been reported (Guo et al., 2008; J Neurophysiol, 100:1147). Other proposed agonists of GPR18, including anandamide and abnormal cannabidiol, also failed to induce inhibition of calcium currents. Mutants of GPR18, designed to constitutively activate receptors, did not tonically inhibit calcium currents, indicating a lack of GPR18 activation or coupling to endogenous G proteins. Other downstream effectors of Gα(i/o)-coupled receptors, G protein-coupled inwardly rectifying potassium channels and adenylate cyclase, were not modulated by GPR18 signaling. Furthermore, GPR18 did not couple to other G proteins tested: Gα(s), Gα(z), and Gα(15). These results suggest NAGly is not an agonist for GPR18 or that GPR18 signaling involves noncanonical pathways not examined in these studies.

The interaction between mGluR1 and the calcium channel Cav₂.₁ preserves coupling in the presence of long Homer proteins

Group I metabotropic glutamate receptors (mGluR1 and 5) are G protein coupled receptors that regulate neuronal activity in a number of ways. Some of the most well studied functions of group I mGluRs, such as initiation of multiple forms of mGluR-dependent long-term depression, require receptor localization near the post-synaptic density (PSD). This localization is in turn dependent on the Homer family of scaffolding proteins which bind to a small motif on the distal C-termini of mGluR1 and 5, localize the receptors near the PSD, strengthen coupling to post-synaptic effectors and simultaneously uncouple the mGluRs from extra-synaptic effectors such as voltage dependent ion channels. Here the selectivity of this uncoupling process was examined by testing the ability of Homer-2b to uncouple mGluR1 from multiple voltage dependent calcium channels including Ca(V2.2) (N-type), Ca(V3.2) (T-type), and Ca(V2.1) (P/Q-type) expressed in rat sympathetic neurons from the superior cervical ganglion (SCG). Of these, only the mGluR1-Ca(V2.1) modulatory pathway was insensitive to Homer-2b expression. Uncoupling from this channel was achieved by co-expression of an mGluR1 C-terminal protein designed to disrupt a previously described direct interaction between these two proteins, suggesting that this interaction allows incorporation of Ca(V2.1) into the mGluR1/Homer signaling complex, thereby preserving modulation in the presence of scaffolding Homer proteins. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

The orthosteric agonist 2-chloro-5-hydroxyphenylglycine activates mGluR5 and mGluR1 with similar efficacy and potency

BACKGROUND

The efficacy, potency, and selectivity of the compound 2-Chloro-5-hydroxyphenylglycine (CHPG), a nominally selective agonist for metabotropic glutamate receptor 5 (mGluR5), were examined with select mGluRs by examining their ability to induce modulation of the native voltage dependent ion channels in isolated sympathetic neurons from the rat superior cervical ganglion (SCG). SCG neurons offer a null mGluR-background in which specific mGluR subtypes can be made to express via intranuclear cDNA injection.

RESULTS

Consistent with previous reports, CHPG strongly activated mGluR5b expressed in SCG neurons with an apparent EC50 around 60 μM. Surprisingly, CHPG also activated two mGluR1 splice variants with a similar potency as at mGluR5 when calcium current inhibition was used as an assay for receptor function. No effect of 1 mM CHPG was seen in cells expressing mGluR2 or mGluR4, suggesting that CHPG only activates group I mGluRs (mGluR1 and 5). CHPG was also able to induce modulation of M-type potassium current through mGluR1, but not as consistently as glutamate. Since this channel is modulated through a Gq-dependent pathway, these data indicate that CHPG may exhibit some biased agonist properties on mGluR1. Closer examination of the voltage-independent, Gq-mediated component of mGluR-induced calcium current modulation data confirmed that some biased agonism was evident, but the effect was weak and inconsistent.

CONCLUSIONS

These data contrast with the established literature which suggests that CHPG is a selective mGluR5 agonist. Instead, CHPG appears to act equally well as an agonist at mGluR1. While some weak biased agonism was observed with CHPG acting on mGluR1, but not mGluR5, favoring Gi/o signaling over Gq/11, this effect does not appear sufficient to fully explain the discrepancies in the literature.

Metabotropic glutamate receptors: from the workbench to the bedside

Metabotropic glutamate (mGlu) receptors were discovered in the mid 1980s and originally described as glutamate receptors coupled to polyphosphoinositide hydrolysis. Almost 6500 articles have been published since then, and subtype-selective mGlu receptor ligands are now under clinical development for the treatment of a variety of disorders such as Fragile-X syndrome, schizophrenia, Parkinson's disease and L-DOPA-induced dyskinesias, generalized anxiety disorder, chronic pain, and gastroesophageal reflux disorder. Prof. Erminio Costa was linked to the early times of the mGlu receptor history, when a few research groups challenged the general belief that glutamate could only activate ionotropic receptors and all metabolic responses to glutamate were secondary to calcium entry. This review moves from those nostalgic times to the most recent advances in the physiology and pharmacology of mGlu receptors, and highlights the role of individual mGlu receptor subtypes in the pathophysiology of human disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Mammalian cell transfection: the present and the future

Transfection is a powerful analytical tool enabling study of the function of genes and gene products in cells. The transfection methods are broadly classified into three groups; biological, chemical, and physical. These methods have advanced to make it possible to deliver nucleic acids to specific subcellular regions of cells by use of a precisely controlled laser-microcope system. The combination of point-directed transfection and mRNA transfection is a new way of studying the function of genes and gene products. However, each method has its own advantages and disadvantages so the optimum method depends on experimental design and objective.

Molecular reconstruction of mGluR5a-mediated endocannabinoid signaling cascade in single rat sympathetic neurons

Endocannabinoids (eCB) such as 2-arachidonylglycerol (2-AG) are lipid metabolites that are synthesized in a postsynaptic neurons and act upon CB(1) cannabinoid receptors (CB(1)R) in presynaptic nerve terminals. This retrograde transmission underlies several forms of short and long term synaptic plasticity within the CNS. Here, we constructed a model system based on isolated rat sympathetic neurons, in which an eCB signaling cascade could be studied in a reduced, spatially compact, and genetically malleable system. We constructed a complete eCB production/mobilization pathway by sequential addition of molecular components. Heterologous expression of four components was required for eCB production and detection: metabotropic glutamate receptor 5a (mGluR5a), Homer 2b, diacylglycerol lipase alpha, and CB(1)R. In these neurons, application of l-glutamate produced voltage-dependent modulation of N-type Ca(2+) channels mediated by activation of CB(1)R. Using both molecular dissection and pharmacological agents, we provide evidence that activation of mGluR5a results in rapid enzymatic production of 2-AG followed by activation of CB(1)R. These experiments define the critical elements required to recapitulate retrograde eCB production and signaling in a single peripheral neuron. Moreover, production/mobilization of eCB can be detected on a physiologically relevant time scale using electrophysiological techniques. The system provides a platform for testing candidate molecules underlying facilitation of eCB transport across the plasma membrane.

Excitatory roles of protein kinase C in striatal cholinergic interneurons

Protein kinase C (PKC) plays critical roles in neuronal activity and is widely expressed in striatal neurons. However, it is not clear how PKC activation regulates the excitability of striatal cholinergic interneurons. In the present study, we found that PKC activation significantly inhibited A-type potassium current (I(A)), but had no effect on delayed rectifier potassium currents. Consistently, application of PKC activator caused an increase of firing in response to depolarizing currents in cholinergic interneurons, which was persistent in the presence of both excitatory and inhibitory neurotransmission blockers. These excitatory effects of PKC could be partially mimicked and occluded by blockade of I(A) with potassium channel blocker 4-aminopyridine. In addition, immunostaining demonstrated that PKCalpha, but not PKCgamma and PKCepsilon, was expressed in cholinergic interneurons. Furthermore, activation of group I metabotropic glutamate receptors (mGluRs) led to an inhibition of I(A) through a PKC-dependent pathway. These data indicate that activation of PKC, most likely PKCalpha, increases the neuronal excitability of striatal cholinergic interneurons by down-regulating I(A). Group I mGluR-mediated I(A) inhibition might be important for the glutamatergic regulation of cholinergic tone in the neostriatum.

Protein kinase Calpha mediates a novel form of plasticity in the accessory olfactory bulb

Modification of synapses in the accessory olfactory bulb (AOB) is believed to underlie pheromonal memory that enables mate recognition in mice. The memory, which is acquired with single-trial learning, forms only with coincident noradrenergic and glutamatergic inputs to the AOB. The mechanisms by which glutamate and norepinephrine (NE) alter the AOB synapses are not well understood. Here we present results that not only reconcile the earlier, seemingly contradictory, observations on the role of glutamate and NE in changing the AOB synapses, but also reveal novel mechanisms of plasticity. Our studies suggest that initially, glutamate acting at Group II metabotropic receptors and NE acting at alpha(2)-adrenergic receptors inhibit N-type and R-type Ca(2+) channels in mitral cells via a G-protein. The N-type and R-type Ca(2+) channel inhibition is reversed by activation of alpha(1)-adrenergic receptors and protein kinase Calpha (PKCalpha). Based on these results, we propose a hypothetical model for a new kind of synaptic plasticity in the AOB that accounts for the previous behavioral data on pheromonal memory. According to this model, initial inhibition of the Ca(2+) channels suppresses the GABAergic inhibitory feedback to mitral cells, causing disinhibition and Ca(2+) influx. NE also activates phospholipase C (PLC) through alpha(1)-adrenergic receptors generating inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Calcium and DAG together activate PKCalpha which switches the disinhibition to increased inhibition of mitral cells. Thus, PKCalpha is likely to be a coincidence detector integrating glutamate and NE input in the AOB and bridging the short-term signaling to long-term structural changes resulting in enhanced inhibition of mitral cells that is thought to underlie memory formation.

Metabotropic glutamate 1 receptor: current concepts and perspectives

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

Endogenous homer proteins regulate metabotropic glutamate receptor signaling in neurons

Group I metabotropic glutamate receptors (mGluR1 and mGluR5) are important neuronal mediators of postsynaptic signaling that influence synaptic strength, plasticity, and other factors. Regulation of group I mGluR localization and function by Homer proteins appears to be a viable means for neurons to fine-tune these processes. The presence of different Homer isoforms can act as a switch to reprioritize mGluR1 and mGluR5 signaling at the point of IP(3) receptor activation by promoting or reducing activation of specific downstream effectors. Furthermore, these Homer-dependent effects on mGluR signaling may mechanistically underlie many of the long-term changes in neuronal function associated with changes in Homer protein expression described in the recent literature. However, most studies focusing on mGluR regulation by Homer proteins used relatively long-term overexpression. Thus, a definitive demonstration of mGluR1/5 signal regulation by natively expressed Homer proteins has been elusive. I examined the ability of endogenous Homer 1a to alter mGluR signaling in rat sympathetic neurons and hippocampal autapses using pituitary adenylate cyclase activating peptide (PACAP) to induce native Homer 1a expression. In sympathetic neurons, both Homer 1a overexpression and PACAP treatment reversed the decrease in mGluR1-mediated calcium current modulation associated with Homer 2b expression. In hippocampal autapses, PACAP treatment uncoupled postsynaptic mGluR5 from EPSC inhibition, similar to the effect of Homer 1a overexpression. In both cases, RNA silencing of Homer 1a but not control RNA interference treatment prevented the PACAP effect, suggesting that it resulted specifically from native Homer 1a expression.

The mGlu(4) receptor allosteric modulator N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide acts as a direct agonist at mGlu(6) receptors

Effects of the mGlu(4) receptor allosteric modulator N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC) were tested on closely related mGlu6 receptors. Modulation of native calcium currents in isolated sympathetic neurons from rat superior cervical ganglion by expressed mGlu(4) and mGlu(6) receptor was used to assay receptor activity. Glutamate concentration-response curves with and without PHCCC confirmed that the drug is an allosteric modulator of mGlu(4) receptor, without direct agonist activity. Conversely, PHCCC directly activates the mGlu(6) receptor and does not enhance activity of glutamate. Therefore, PHCCC is a direct mGlu(6) receptor agonist, but lacks allosteric modulatory properties.

Metabotropic glutamate receptor ligands as possible anxiolytic and antidepressant drugs

Depression and anxiety represent a major problem. However, the current treatment of both groups of diseases is not satisfactory. As the glutamatergic system may play an important role in pathophysiology of both depression and anxiety, we decided to discuss the recent data on possible anxiolytic and/or antidepressant effects of metabotropic glutamate (mGlu) receptor ligands. Preclinical data indicated that antagonists of group I mGlu receptors, particularly antagonists of mGlu5 receptors, produced both anxiolytic-like and antidepressant-like effects. Clinical data also demonstrated that mGlu5 receptor antagonist, fenobam, was an active anxiolytic drug. The anxiolytic effects exerted by mGlu5 receptor antagonists are profound, comparable with or stronger than those of benzodiazepines. However, the problem with the psychotomimetic activity of mGlu5 receptor antagonists and their possible influence on memory has to be further investigated. Among all mGlu receptor ligands, group II mGlu receptor agonists seem to be the drugs with the most promising therapeutic potential and a good safety profile. Animal studies showed anxiolytic-like effects of group II mGlu receptor agonists. Currently, group II mGlu receptor agonists are in phase III clinical trials for potential treatment of anxiety disorders. On the other hand, data has been accumulated, indicating that antagonists of group II mGlu receptors have an antidepressant potential. Group III mGlu receptor ligands represent the least investigated group of mGlu receptors. However, preclinical data also indicates that ligands of these receptors, both agonists and antagonists, may have an anxiolytic-like and antidepressant-like potential.

G protein coupling profile of mGluR6 and expression of Gα proteins in retinal ON bipolar cells

Metabotropic glutamate receptor 6 (mGluR6) is a group III, pertussis toxin (PTX)-sensitive G protein coupled mGluR that plays a specialized role in the retina. Retinal ON bipolar cells, which receive direct glutamatergic input from photoreceptor cells, express mGluR6 as their primary postsynaptic glutamate receptor. Activation of mGluR6 in these cells initiates an intracellular signaling cascade ultimately leading to inhibition of a cation channel and cell hyperpolarization. The primary mediator of this pathwayin vivois Gαo, but the potential roles of other G proteins from the Gαi/ofamily in the regulation of this or other signaling pathways in ON bipolar cells are unclear. To determine which specific G proteins from the Gαi/ofamily are able to couple to mGluR6, a Gα reconstitution system was employed using PTX-insensitive Gα mutants expressed with mGluR6 in PTX-treated sympathetic neurons from the rat superior cervical ganglion (SCG). The efficiency of coupling to mGluR6 was Goa> Gob, Gi1> Gi2, Gi3, whereas no coupling was observed with Gαz, nor with the retinal Gα proteins, rod (GNAT2) or cone (GNAT1) transducin (GαTr-R, GαTr-C). Finally, the expression of Gα proteins determined to couple with mGluR6 was examined in rat ON bipolar cells using single cell RT-PCR. Co-expression of mGluR6 message was used to distinguish ON from OFF bipolar cells. Expression of Gαowas detected in every ON bipolar cell examined. Message for Gαi1, which coupled moderately to mGluR6, was not detected in ON bipolar cells, nor was Gαi3, which coupled to mGluR6 in only a few cells but on average did not exhibit statistically significant coupling. Finally, though Gαi2was detectable in ON bipolar cells, its coupling to mGluR6 in the SCG system was not significant. Together, these data indicate that signaling through mGluR6 in mammalian ON bipolar cells is highly focused, apparently acting through a single Gα protein subtype.

Fluorophore-assisted light inactivation produces both targeted and collateral effects on N-type calcium channel modulation in rat sympathetic neurons

Fluorophore-assisted light inactivation (FALI) is a method to inactivate specific proteins on a time scale of seconds to minutes using either diffuse or coherent light. Here we examine a novel FALI modality that utilizes a fluorescein-conjugated polypeptide, alpha-bungarotoxin (BTX) and a 13 amino acid BTX-binding site engineered into the N-terminus of metabotropic glutamate receptor 8a (mGluR8a), a class C G-protein-coupled receptor (GPCR). The tagged mGluR8a was expressed in rat sympathetic neurons and labelled with fluorescein-conjugated BTX (FL-BTX). The efficacy of FALI was evaluated by monitoring mGluR8a-mediated inhibition of calcium currents (I(Ca)) using whole-cell voltage-clamp techniques. Following either wide-field or laser illumination of FL-BTX-labelled neurons, mGluR8a-mediated I(Ca) inhibition was greatly attenuated whereas holding current and basal I(Ca), measures of non-specific effects, were minimally affected. Sodium azide, a collision quencher of singlet oxygen, reduced the magnitude of FALI-mediated effects supporting a role for reactive oxygen species in the process. Although these results were consistent with an acute inactivation of mGluR8a, the intended target, two findings confounded this interpretation. First, effects on a natively expressed signalling pathway, alpha(2)-adrenergic receptor-mediated I(Ca) modulation, were observed following illumination of neurons expressing FL-BTX-labelled sodium channel beta2 subunits or ionotropic 5-HT(3) receptors, proteins with no overt relationship to GPCR signalling pathways. Second, GPCR-independent I(Ca) modulation induced with intracellular guanylyl imidophosphate was also attenuated by FALI. These data challenge the assumption that the fluorophore-tagged protein is the sole target of FALI and provide evidence that collateral damage to proximal proteins occurs following fluorophore illumination.

Cellular plasticity cascades: targets for the development of novel therapeutics for bipolar disorder

For a number of patients with bipolar disorder, current pharmacotherapy is generally insufficient. Despite adequate treatment, patients continue to have recurrent mood episodes, residual symptoms, functional impairment, psychosocial disability, and significant medical and psychiatric comorbidity. Drug development for bipolar disorder may occur through one of two approaches: the first is by understanding the therapeutically relevant biochemical targets of currently effective medications. Two promising direct targets of lithium and valproate are glycogen synthase kinase-3 and histone deacetylase. The second path results from our understanding that severe mood disorders, although not classical neurodegenerative disorders, are associated with regional impairments of structural plasticity and cellular resilience. This suggests that effective treatments will need to provide both trophic and neurochemical support, which serves to enhance and maintain normal synaptic connectivity, thereby allowing the chemical signal to reinstate the optimal functioning of critical circuits necessary for normal affective functioning. For many refractory patients, drugs mimicking "traditional" strategies, which directly or indirectly alter monoaminergic levels, may be of limited benefit. Newer "plasticity enhancing" strategies that may have utility in the treatment of mood disorders include inhibitors of glutamate release, N-methyl-D-aspartate antagonists, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid potentiators, cyclic adenosine monophosphate phosphodiesterase inhibitors, and glucocorticoid receptor antagonists.

Activation of receptors negatively coupled to adenylate cyclase is required for induction of long-term synaptic depression at Schaffer collateral-CA1 synapses

Chemical LTD (CLTD) of synaptic transmission is triggered by simultaneously increasing presynaptic [cGMP] while inhibiting PKA. Here, we supply evidence that class II, but not III, metabotropic glutamate receptors (mGluRs), and A1 adenosine receptors, both negatively coupled to adenylate cyclase, play physiologic roles in providing PKA inhibition necessary to promote the induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices. Simultaneous activation of group II mGluRs with the selective agonist (2S,2'R,3'R)-2-(2',3'-dicarboxy-cyclopropyl) glycine (DCGIV; 5 microM), while raising [cGMP] with the type V phosphodiesterase inhibitor, zaprinast (20 microM), resulted in a long-lasting depression of synaptic strength. When zaprinast (20 microM) was combined with a cell-permeant PKA inhibitor H-89 (10 microM), the need for mGluR IIs was bypassed. DCGIV, when combined with a "submaximal" low frequency stimulation (1 Hz/400 s), produced a saturating LTD. The mGluR II selective antagonist, (2S)-alpha-ethylglutamic acid (EGLU; 5 microM), blocked induction of LTD by prolonged low frequency stimulation (1 Hz/900 s). In contrast, the mGluR III selective receptor blocker, (RS)-a-Cyclopropyl-[3- 3H]-4-phosphonophenylglycine (CPPG; 10 microM), did not impair LTD. The selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 100 nM), also blocked induction of LTD, while the adenosine A1 receptor agonist N6-cyclohexyl adenosine (CHA; 50 nM) significantly enhanced the magnitude of LTD induced by submaximal LFS and, when paired with zaprinast (20 microM), was sufficient to elicit CLTD. Inhibition of PKA with H-89 rescued the expression of LTD in the presence of either EGLU or DPCPX, confirming the hypothesis that both group II mGluRs and A1 adenosine receptors enhance the induction of LTD by inhibiting adenylate cyclase and reducing PKA activity.

Cannabinoids and dopamine receptors' action on calcium current in rat neurons

OBJECTIVE

To study the effects of cannabinoid, glutamate, and dopamine agonists and antagonists on the calcium current rat sympathetic neurons.

METHODS

Calcium current was recorded using the whole-cell variant of the patch-clamp technique. After expression in neuronal membranes of the cannabinoid CB1, glutamate mGluR2, or dopamine D1 receptor (by microinjection of the levant receptor's cDNA into the neuron's nucleus) agonists' and antagonists' effects were observed.

RESULTS

Applications of agonists of the expressed receptor (0.1-10 microM) decreased the calcium current. The calcium current was increased after application of cannabinoid antagonists (AM251 and AM630); these compounds thus act as inverse agonists in this preparation. Glutamate and dopamine antagonists had no effects on the calcium current by themselves. Combined application of cannabinoids and dopamine, but not glutamate, agonists produced a decrement in the calcium current that was bigger than either of the effects seen when one agonist was applied alone.

CONCLUSIONS

These results suggest that cannabinoid with dopamine receptors have an interactive inhibitory effect on the calcium current in this preparation, indicating that within the nervous system, receptor interactions may be important in the regulation of ion-channel functions.

Coupling of metabotropic glutamate receptor 8 to N-type Ca2+ channels in rat sympathetic neurons

Group III metabotropic glutamate receptors (mGluRs; mGluR4, 6, 7, and 8) couple to the Galpha(i/o)-containing G protein heterotrimers and act as autoreceptors to regulate glutamate release, probably by inhibiting voltage-gated Ca(2+) channels. Although most mGluRs have been functionally expressed in a variety of systems, few studies have demonstrated robust coupling of mGluR8 to downstream effectors. We therefore tested whether activation of mGluR8 inhibited Ca(2+) channels. Both L-glutamate (L-Glu) and l-2-amino-4-phosphonobutyric acid (L-AP4), a selective agonist for group III mGluRs, inhibited N-type Ca(2+) current in rat superior cervical ganglion neurons previously injected with a cDNA encoding mGluR8a/b. L-AP4 was approximately 100-fold more potent (IC(50) = 0.1 microM) than L-Glu ( approximately 10 microM), but it had efficacy similar to that of L-Glu ( approximately 50% maximal inhibition). The potency and efficacy of L-AP4 and L-Glu were similar for both splice variants. Agonist-induced inhibition was abolished by pretreatment with (R,S)-alpha-cyclopropyl-4-phosphonophenylglycine, a selective group III mGluR antagonist, and pertussis toxin. Deletion of either a calmodulin (CaM) binding motif in the C terminus or the entire C terminus of mGluR8 did not affect mGluR8-mediated response. Our studies indicate that both mGluR8a and 8b are capable of inhibiting N-type Ca(2+) channel, suggesting a role as presynaptic autoreceptors to regulate neuronal excitability. The studies also imply that the potential CaM binding domain is not required for the mGluR8-mediated Ca(2+) channel inhibition and the C terminus of mGluR8a is dispensable for receptor coupling to N-type Ca(2+) channels.

Dual signaling by mGluR5a results in bi-directional modulation of N-type Ca2+ channels

We have studied how N-type Ca2+ channels are modulated by the metabotropic glutamate receptor 5a (mGluR5a) in Xenopus oocytes. Stimulation of the receptor with glutamate initiated two parallel responses, a rapid inhibition followed by an upregulation of the Ca2+ current. Although a subsequent stimulation did not upregulate the Ca2+ current, it did still produce a reduction in the amplitude of the current. The upregulation of Ca2+ channels was prevented by the protein kinases inhibitor staurosporine and it was mimicked by the activation of PKC with phorbol esters. In contrast, the inhibition of the Ca2+ current was insensitive to staurosporine. These results show that mGluR5a exerts a bi-directional influence on Ca2+ channels, which may explain how group I mGluRs facilitate and inhibit glutamate release at central synapses.

The behavioral profile of the potent and selective mGlu5 receptor antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) in rodent models of anxiety

Previous reports have demonstrated the anxiolytic effect of the potent and systemically active metabotropic glutamate subtype 5 (mGlu5) receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) in rodents. Here, we present evidence for the anxiolytic activity of a novel mGlu5 receptor antagonist, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), in rats and compare its profile to the benzodiazepine receptor agonist diazepam. MTEP occupied mGlu5 receptors in a dose-dependent manner with essentially full receptor occupancy at the highest dose tested (10 mg/kg, i.p.). At doses appropriate for mGlu5 receptor-mediated effects, MTEP significantly reduced fear-potentiated startle and increased punished responding in a modified Geller-Seifter conflict model consistent with an anxiolytic-like profile. In both models, the magnitude of the anxiolytic-like response was similar to that seen with diazepam. In contrast, MTEP decreased unpunished responding to a lesser extent than diazepam and had no effect on rotarod performance when administered either alone or in combination with ethanol. Repeated dosing with MTEP in this model eliminated the increase in punished responding observed with acute dosing. The present results suggest that mGlu5 receptor antagonists lack the side effects seen with benzodiazepines, such as sedation and ethanol interaction, and provide insight into a possible role for mGlu5 receptor antagonists in the modulation of mood disorders.

Activation of metabotropic glutamate receptor 1 dimers requires glutamate binding in both subunits

Group I metabotropic glutamate receptors (mGluRs) form stable, disulfide-linked homodimers. Lack of a verifiably monomeric mGluR1 mutant has led to difficulty in assessing the role of dimerization in the molecular mechanism of mGluR1 activation. The related GABA(B) receptor exhibits striking intradimer cross talk (ligand binding at one subunit effectively produces G protein activation at the other), but it is unclear whether group I mGluRs exhibit analogous cross talk. Signaling of heterologously expressed mGluR1 was examined in isolated rat sympathetic neurons by measuring glutamate-mediated inhibition of native calcium currents. To examine mGluR1 activity when only one dimer subunit has access to glutamate ligand, wildtype mGluR1 was coexpressed with mGluR1 Y74A, a mutant with impaired glutamate binding, and the activity of the heterodimer (mutant/wild type) was examined. The mGluR1 Y74A mutant alone had a dose-response curve that was shifted by about 2 orders of magnitude. The half-maximal dose of glutamate shifted from 1.3 (wild-type mGluR1) to about 450 (mGluR1 Y74A) microM. However, the maximal effect was similar. Wild-type mGluR1 was expressed with excess Y74A mGluR1 to generate a receptor population consisting largely of mutant homodimers and mutant/wild-type heterodimers but without detectable wild-type homodimers. Under these conditions, no glutamate-mediated calcium current inhibition was observed below approximately 300 microM glutamate, although wild-type mGluR1 protein was detectable with immunofluorescence. These data suggest that mutant/wild-type heterodimeric receptors are inactive at ligand concentrations favoring glutamate association with receptor dimers at only one subunit.

Neurotransmitter modulation of neuronal calcium channels

There are many different calcium channels expressed in the mammalian nervous system, but N-type and P/Q-type calcium channels appear to dominate the presynaptic terminals of central and peripheral neurons. The neurotransmitter-induced modulation of these channels can result in alteration of synaptic transmission. This review highlights the mechanisms by which neurotransmitters affect the activity of N-type and P/Q-type calcium channels. The inhibition of these channels by voltage-dependent and voltage-independent mechanisms is emphasized because of the wealth of information available on the intracellular mediators and on the effect of these pathways on the single-channel gating.

Emerging experimental therapeutics for bipolar disorder: clues from the molecular pathophysiology

Bipolar affective disorder (manic-depressive illness) is a common, severe, chronic, and often life-threatening illness, associated with significant comorbidity. The recognition of the significant morbidity and mortality of patients with bipolar disorder, as well as the growing appreciation that a high percentage of patients respond poorly to existing treatments, has made the task of discovering new therapeutic agents, that are both efficacious and have few side effects increasingly more important. Most recent agents introduced into the pharmacopeia for the treatment of bipolar disorder have been anticonvulsants and atypical antipsychotics. We propose that novel treatments developed specifically for bipolar disorder will arise from (1) understanding more precisely the molecular mechanisms of treatments that are clearly efficacious or (2) developing medications based on the knowledge obtained of the underlying pathophysiology of bipolar disorder. Knowledge with regard to the underlying pathophysiology of bipolar disorder is increasing at a rapid pace, including alterations in intracellular signaling cascades as well as impairments of cellular plasticity and resilience in critical neuronal circuits. We propose that therapeutics designed to enhance cellular plasticity and resilience and that counter maladaptive stress-responsive systems may have considerable utility for the treatment of bipolar disorder. Therapeutic strategies designed to address cellular resilience and plasticity include the regulation of neurotrophic pathways, glucocorticoid signaling, phosphodiesterase activity, and glutamatergic throughput and mitochondrial function. While the task of developing novel medications for bipolar disorder is truly daunting, these and similar approaches will ultimately lead to better medications for the millions who suffer from this devastating illness.

Phosphorylation of critical serine residues in Gem separates cytoskeletal reorganization from down-regulation of calcium channel activity

Gem is a small GTP-binding protein that has a ras-like core and extended chains at each terminus. The primary structure of Gem and other RGK family members (Rad, Rem, and Rem2) predicts a GTPase deficiency, leading to the question of how Gem functional activity is regulated. Two functions for Gem have been demonstrated, including inhibition of voltage-gated calcium channel activity and inhibition of Rho kinase-mediated cytoskeletal reorganization, such as stress fiber formation and neurite retraction. These functions for Gem have been ascribed to its interaction with the calcium channel beta subunit and Rho kinase beta, respectively. We show here that these functions are separable and regulated by distinct structural modifications to Gem. Phosphorylation of serines 261 and 289, located in the C-terminal extension, is required for Gem-mediated cytoskeletal reorganization, while GTP and possibly calmodulin binding are required for calcium channel inhibition. In addition to regulating cytoskeletal reorganization, phosphorylation of serine 289 in conjunction with serine 23 results in bidentate 14-3-3 binding, leading to increased Gem protein half-life. Evidence presented shows that phosphorylation of serine 261 is mediated via a cdc42/protein kinase Czeta-dependent pathway. These data demonstrate that phosphorylation of serines 261 and 289, outside the GTP-binding region of Gem, controls its inhibition of Rho kinase beta and associated changes in the cytoskeleton.

Gating of the polycystin ion channel signaling complex in neurons and kidney cells

Mutations in either polycystin-2 (PC2) or polycystin-1 (PC1) proteins cause severe, potentially lethal, kidney disorders and multiple extrarenal (including brain) disease phenotypes. PC2, a member of the transient receptor potential channel superfamily, and PC1, an orphan membrane receptor of largely unknown function, are thought to be part of a common signaling pathway. Here, we show that in rat sympathetic neurons and kidney cells, coassembly of full-length PC1 with PC2 forms a plasmalemmal ion channel signaling complex in which PC1 stimulation simultaneously activates PC2 ion channels and Gi/o-proteins. PC2 activation occurs through a structural rearrangement of PC1, independent of G-protein activation. Thus, PC1 acts as a prototypical membrane receptor that concordantly regulates PC2 channels and G-proteins, a bimodal mechanism that may account for the multifunctional roles of polycystin proteins in fundamental cellular processes of various cell types.

Expression and functional properties of group I metabotropic glutamate receptors in bovine chromaffin cells

We demonstrate the presence and functional properties of Group I metabotropic glutamate receptors (mGluRs) expressed in chromaffin cells. Immunocytochemical techniques revealed that two mGluR subtypes (mGluR1alpha and mGluR5) are expressed in chromaffin cells, located in both the cytoplasmic membrane and the cytosol surrounding the nucleus. These mGluRs are functionally active on catecholamine (CA) secretion in chromaffin cells because both (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) and the specific agonist of Group I mGluRs, (S)-3,5-dihydroxyphenylglycine (DHPG), were able to stimulate the release of CAs (adrenaline and noradrenaline) in a dose-response manner. These effects were specifically reversed by L-(+)-2-amino-3-phosphonopropionic acid (L-AP3), a selective antagonist of the Group I metabotropic glutamate receptors. t-ACPD induced an increase in CA secretion in both the presence and absence of extracellular calcium, the former effect being accompanied by cell membrane depolarization. Noradrenaline (NA) release was higher in the presence of extracellular calcium than in its absence, whereas adrenaline release was of the same order under both conditions. These results indicate that different subtypes of Group I mGluRs are present in noradrenergic and adrenergic cells. Fluorescence imaging techniques in single cells showed different t-ACPD-induced increases in intracellular calcium in different chromaffin cells: in chromaffin cells, 67% expressed functional metabotropic glutamate receptors and with nicotinic receptors, whereas the remaining 33% expressed only nicotinic receptors. In the absence of external calcium, only about 25% of cells responded to t-ACPD-increased intracellular calcium by increasing inositol 1,4,5-trisphosphate (IP(3)) concentration and subsequent calcium mobilization from intracellular stores, whereas the remaining 75% increased intracellular calcium by promoting Ca(2+) influx from the extracellular medium through L- and N- but not P/Q voltage-dependent calcium channels.

Subtype-specific coupling with ADP-ribosyl cyclase of metabotropic glutamate receptors in retina, cervical superior ganglion and NG108-15 cells

Cyclic ADP-ribose (cADP-ribose) is a putative second messenger or modulator. However, the role of cADP-ribose in the downstream signals of the metabotropic glutamate receptors (mGluRs) is unclear. Here, we show that glutamate stimulates ADP-ribosyl cyclase activity in rat or mouse crude membranes of retina via group III mGluRs or in superior cervical ganglion via group I mGluRs. The retina of mGluR6-deficient mice showed no increase in the ADP-ribosyl cyclase level in response to glutamate. GTP enhanced the initial rate of basal and glutamate-stimulated cyclase activity. GTP-gamma-S also stimulated basal activity. To determine whether the coupling mode of mGluRs to ADP-ribosyl cyclase is a feature common to individual cloned mGluRs, we expressed each mGluR subtype in NG108-15 neuroblastoma x glioma hybrid cells. The glutamate-induced stimulation of the cyclase occurs preferentially in NG108-15 cells over-expressing mGluRs1, 3, 5, and 6. Cells expressing mGluR2 or mGluRs4 and 7 exhibit inhibition or no coupling, respectively. Glutamate-induced activation or inhibition of the cyclase activity was eliminated after pre-treatment with cholera or pertussis toxin, respectively. Thus, the subtype-specific coupling of mGluRs to ADP-ribosyl cyclase via G proteins suggests that some glutamate-evoked neuronal functions are mediated by cADP-ribose.

A role for Seven in Absentia Homolog (Siah1a) in metabotropic glutamate receptor signaling

BACKGROUND

The mammalian homologue of Seven in Absentia (Siah) can act in the ubiquitin/proteasome pathway. Recent work has shown that Siah can bind group I metabotropic glutamate receptors (mGluRs), but the functional consequences of this interaction are unknown.

RESULTS

The effects of coexpression of Siah on group I mGluR signaling were examined using heterologous expression in rat sympathetic, superior cervical ganglion neurons. Siah1a attenuated heterologously expressed group I mGluR-mediated calcium current inhibition, but was without effect on group II mGluR- or NE-mediated calcium current modulation via heterologously expressed mGluR2 or native a2 adrenergic receptors, respectively, indicating that the effect of Siah was specific for group I mGluRs. Surface expression and subcellular distribution of group I mGluRs were not detectably altered in the presence of Siah1a as assessed by immunoflourescence experiments with epitope tagged receptors and imaging of a GFP/mGluR fusion construct. In addition, an N-terminal Siah deletion construct, which cannot function in the proteolysis pathway, displayed effects similar to the wild type Siah1a. Finally, coexpression of calmodulin, which competes with Siah1a for binding to the C-terminal tail of group I mGluRs, reversed the effect of Siah1a on mGluR-mediated signaling.

CONCLUSIONS

These data supported the conclusion that the attenuation of mGluR signaling induced by Siah1a expression was likely a direct consequence of Siah/mGluR association rather than a result of targeting of the receptors to the proteosome. In addition, the data suggest that the binding of CaM and Siah may play an important role in the regulation of group I mGluR function.

Subtype-selective reconstitution of synaptic transmission in sympathetic ganglion neurons by expression of exogenous calcium channels

Fast cholinergic neurotransmission between superior cervical ganglion neurons (SCGNs) in cell culture is initiated by N-type Ca(2+) currents through Ca(v)2.2 channels. To test the ability of different Ca(2+)-channel subtypes to initiate synaptic transmission in these cells, SCGNs were injected with cDNAs encoding Ca(v)1.2 channels, which conduct L-type currents, Ca(v)2.1 channels, which conduct P/Q-type Ca(2+) currents, and Ca(v)2.3 channels, which conduct R-type Ca(2+) currents. Exogenously expressed Ca(v)2.1 channels were localized in nerve terminals, as assessed by immunocytochemistry with subtype-specific antibodies, and these channels effectively initiated synaptic transmission. Injection with cDNA encoding Ca(v)2.3 channels yielded a lower level of presynaptic labeling and synaptic transmission, whereas injection with cDNA encoding Ca(v)1.2 channels resulted in no presynaptic labeling and no synaptic transmission. Our results show that exogenously expressed Ca(2+) channels can mediate synaptic transmission in SCGNs and that the specificity of reconstitution of neurotransmission (Ca(v)2.1 > Ca(v)2.3 >> Ca(v)1.2) follows the same order as in neurons in vivo. The specificity of reconstitution of neurotransmission parallels the specificity of trafficking of these Ca(v) channels to nerve terminals.

Coupling of the nucleotide P2Y4 receptor to neuronal ion channels

1. G protein-linked P2Y nucleotide receptors are known commonly to stimulate the phosphoinositide signalling pathway. However, we have previously demonstrated that the cloned P2Y(2), P2Y(6) and P2Y(1) receptors couple to neuronal N-type Ca(2+) channels and to M-type K(+) channels. Here we investigate the coupling of recombinant, neuronally expressed rat- and human P2Y(4) receptors (rP2Y(4), hP2Y(4)) to those channels. 2. Rat sympathetic neurones were nuclear-injected with a P2Y(4) cDNA plasmid. A subsequent activation of rP2Y(4) or hP2Y(4) by UTP (100 micro M) in whole-cell (ruptured-patch) mode produced only about 12% inhibition of the N-type Ca(2+) current (I(Ca(N))). Surprisingly, in perforated patch mode, UTP produced much more inhibition of I(Ca(N)) (maximally 51%), with an IC(50) value of 273 nM. This inhibition was voltage-dependent and was blocked by co-expression of the betagamma-binding transducin Galpha-subunit. Pertussis toxin (PTX) pretreatment also suppressed I(Ca(N)) inhibition. 3. UTP inhibited the M-current, recorded in perforated patch mode, by (maximally) 52%, with IC(50) values of 21 nM for rP2Y(4) and 28 nM for hP2Y(4). This inhibition was not affected by PTX pretreatment. 4. With rP2Y(4), ATP inhibited the M-current (IC(50) 524 nM, 26 times weaker than UTP), whereas ATP had no agonist activity at hP2Y(4). This suggests a difference in agonist binding site between rP2Y(4) and hP2Y(4). 5. We conclude that, in contrast to other nucleotide receptors studied, the P2Y(4) receptor couples much more effectively to M-type K(+) channels than to Ca(2+) channels. Coupling to the Ca(2+) channels involves the betagamma-subunits of G(i/o)-proteins and requires a diffusible intracellular component that is lost in ruptured-patch recording.

Specificity of metabotropic glutamate receptor 2 coupling to G proteins

Metabotropic glutamate receptor 2 (mGluR2) is a class 3 G protein-coupled receptor and an important mediator of synaptic activity in the central nervous system. Previous work demonstrated that mGluR2 couples to pertussis toxin (PTX)-sensitive G proteins. However, the specificity of mGluR2 coupling to individual members of the G(i/o) family is not known. Using heterologously expressed mGluR2 in rat sympathetic neurons from the superior cervical ganglion (SCG), the mGluR2/G protein coupling profile was characterized by reconstituting coupling in PTX-treated cells expressing PTX-insensitive mutant Galpha proteins and Gbetagamma. By employing this method, it was demonstrated that mGluR2 coupled strongly with Galphaob, Galphai1, Galphai2, and Galphai3, although coupling to Galphaoa was less efficient. In addition, mGluR2 did not seem to couple to the most divergent member of the G(i/o) family, Galphaz, although Galphaz coupled strongly to the endogenous alpha2 adrenergic receptor. To determine which Galpha proteins may be natively expressed in SCG neurons, the presence of mRNA for various Galpha proteins was tested using reverse transcription-polymerase chain reaction. Strong bands were detected for all members of the G(i/o) family (Galphao, Galphai1, Galphai2, Galphai3, Galphaz) as well as for Galpha11 and Galphas. A weak signal was detected for Galphaq and no Galpha15 mRNA was detected.

Techniques for gene transfer into neurons

To illuminate the function of the thousands of genes that make up the complexity of the nervous system, it is critical to be able to introduce and express DNA in neurons. Over the past two decades, many gene transfer methods have been developed, including viral vectors, liposomes and electroporation. Although the perfect gene transfer technique for every application has not yet been developed, recent technical advances have facilitated the ease of neuronal gene transfer and have increased the accessibility of these techniques to all laboratories. In order to select a transfection method for any particular experiment, the specific advantages and disadvantages of each technique must be considered.

Group I mGluRs modulate the pattern of non-synaptic epileptiform activity in the hippocampus

The hippocampus is well known for its susceptibility to epileptic seizures, in part because of its neuronal architecture that facilitates synchronization. Although synaptic networks are important for the genesis and spread of epileptiform activity, synchronization of neuronal activity can occur when action potential-dependent chemical synaptic transmission is absent. In particular, it is possible to induce epileptiform activity by perfusing hippocampal slices with a low-Ca(2+)/high-K(+) mediums. Using extracellular recording in area CA1 we have characterized the effects of metabotropic glutamate receptor (mGluR) activation on this non-synaptic bursting activity. Under control conditions, bursting occurred at intervals of 14-86 s with each burst comprising a long (up to 44 s) negative-going field potential of 2 to 13 mV superimposed upon which was sustained firing of population spikes. Activation of group I mGluRs by (S)-3,5-dihydroxyphenylglycine (DHPG) (25 microM) caused a dramatic increase in burst frequency (up to five-fold), which was accompanied by a decrease in the duration and amplitude of bursts. The selective mGluR(1) antagonist 2-methyl-4-carboxyphenylglycine (LY367385) and the selective mGluR(5) antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) both restricted the increase in burst frequency induced by DHPG. However, only LY367385 inhibited the decrease in burst duration and amplitude. Combined application of both antagonists prevented all DHPG-induced changes in bursting activity. These data provide evidence for a role of both mGluR(1) and mGluR(5) subtypes in changing the frequency of non-synaptic bursting, with mGluR(1) alone causing alterations in burst duration and amplitude. These effects are likely to contribute to the group I mGluR-induced changes in synaptic epileptic activity that are already well documented.

Eliminating membrane depolarization caused by the Alzheimer peptide Abeta(1-42, aggr.)

A high-throughput screen found compounds that eliminate the dramatic membrane depolarization caused by the aggregated Alzheimer Abeta1-42 peptide, which activates mGluR1 receptors. The library was composed of known biologically active compounds; the cell-based assay measured the changes of membrane potential with a slow-acting voltage-sensitive dye. We found 10 potentially useful compounds, some of which reduce the Abeta-induced membrane depolarization up to 96%. Interestingly, the active compounds include specific tyrosine kinase inhibitors and inhibitors of certain chloride channels. We deduce that mGluR1 receptors, activated by Abeta1-42 or otherwise, can control the membrane potential via downstream activation of certain tyrosine kinases and certain ion channels. Dopaminergic and serotonergic agonists that emerged from the screen presumably compensate for the Abeta-induced membrane depolarization by themselves causing a hyperpolarization. The hit compounds, whose pharmacokinetics are known, show promise for the restoration of cognitive function in the treatment of early and mid-stage Alzheimer's disease.