Non-invasive optical biosensor for assaying endogenous G protein-coupled receptors in adherent cells

INTRODUCTION

Screening drugs against G protein-coupled receptors (GPCRs) - the single largest family of drug targets in the human genome - is still a major effort in pharmaceutical and biotech industries. Conventional cell-based assays generally measure a single cellular event, such as the generation of a second messenger or the relocation of a specific protein target. However, manipulation or engineering of cells is often a prerequisite for these technologies to achieve desired sensitivities. The present study is focused on the use of non-invasive and manipulation-free optical biosensors for assaying endogenous GPCRs in adherent cells.

METHODS

Resonant waveguide grating (RWG) biosensor was applied to manifest ligand-induced dynamic mass redistribution (DMR) within the bottom portion of adherent cell layer. The DMR signatures mediated through the activation of several endogenous GPCRs in cells were characterized. Endogenous receptor panning was examined at cell system level by using a panel of agonists known to activate many GPCRs, and also at family receptor level by determining the efficacies of a set of family-specific agonists.

RESULTS

Three major types of optical signatures were identified; each was correlated with the activation of a class of GPCRs, depending on the G protein with which the receptor is coupled (i.e., G(q), G(s) and G(i)). The characteristics of DMR signals, mostly the amplitude and kinetics of a DMR event, were dependent on the doses of agonists and the expression levels of endogenous receptors. All three classes of endogenous receptors were found in human epidermoid carcinoma A431 cells. Interestingly, the dose-dependent switching from one type of DMR signal to another was observed for several GPCR agonists examined. A small panel of P2Y receptor agonists exhibited distinct efficacies in three cell lines examined.

DISCUSSIONS

The RWG biosensors were applicable to study the activation of endogenous GPCRs. Like second messengers or gene expression, the DMR signals obtained could be considered as novel and quantifiable physiological responses of living cells mediated through GPCRs and used for studying receptor biology.

Fatty acid receptors as new therapeutic targets for diabetes

G-protein-coupled receptors (GPCRs) are key regulators of several physiological functions. Their roles in cellular signal transduction have made them the target for majority of all currently prescribed drugs. Additionally, there are many orphan GPCRs that provide potential novel therapeutic targets. Several GPCRs are involved in metabolic regulation and glucose homeostasis such as GLP-1 receptor, glucagon receptor, adiponectin receptor and so on. Recently, free fatty acids (FFAs) have been demonstrated as ligands for orphan GPCRs and have been proposed to play a critical role in physiological glucose homeostasis. GPR40 and GPR120 are activated by medium and long-chain FFAs, whereas GPR41 and GPR43 can be activated by short-chain FFAs. GPR40, which is preferentially expressed in pancreatic beta-cells, mediates the majority of the effects of FFAs on insulin secretion. In this review, these findings and also critical analysis of these GPCRs as novel targets for diabetes are discussed.

PIP(2)-dependent inhibition of M-type (Kv7.2/7.3) potassium channels: direct on-line assessment of PIP(2) depletion by Gq-coupled receptors in single living neurons

The open state of M(Kv7.2/7.3) potassium channels is maintained by membrane phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)). They can be closed on stimulating receptors that induce PI(4,5)P(2) hydrolysis. In sympathetic neurons, closure induced by stimulating M1-muscarinic acetylcholine receptors (mAChRs) has been attributed to depletion of PI(4,5)P(2), whereas closure by bradykinin B(2)-receptors (B2-BKRs) appears to result from formation of IP(3) and release of Ca(2+), implying that BKR stimulation does not deplete PI(4,5)P(2). We have used a fluorescently tagged PI(4,5)P(2)-binding construct, the C-domain of the protein tubby, mutated to increase sensitivity to PI(4,5)P(2) changes (tubby-R332H-cYFP), to provide an on-line read-out of PI(4,5)P(2) changes in single living sympathetic neurons after receptor stimulation. We find that the mAChR agonist, oxotremorine-M (oxo-M), produces a near-complete translocation of tubby-R332H-cYFP into the cytoplasm, whereas bradykinin (BK) produced about one third as much translocation. However, translocation by BK was increased to equal that produced by oxo-M when synthesis of PI(4,5)P(2) was inhibited by wortmannin. Further, wortmannin 'rescued' M-current inhibition by BK after Ca(2+)-dependent inhibition was reduced by thapsigargin. These results provide the first direct support for the view that BK accelerates PI(4,5)P(2) synthesis in these neurons, and show that the mechanism of BKR-induced inhibition can be switched from Ca(2+) dependent to PI(4,5)P(2) dependent when PI(4,5)P(2) synthesis is inhibited.

Pharmacological mechanisms involved in the vasodilator effects of extracts from Echinodorus grandiflorus

We investigated the vascular effects of a crude aqueous extract (AEEG) of Echinodorus grandiflorus (Alismataceae) using the in vitro experimental models of the rabbit isolated aorta and perfused kidney. Echinodorus grandiflorus, a native semi-aquatic plant widely distributed in Brazil, has been extensively used in Brazilian folk medicine for the treatment of high blood pressure and inflammatory diseases. The bolus injection of AEEG (0.1-10 mg) into the rabbit renal circulation pre-contracted with norepinephrine induced marked and dose-dependent vasodilator responses (maximum of 37+/-4%; n=6; P<0.001), which was similar to that induced by injection of 10 mmol acetylcholine (41+/-3%). Moreover, AEEG elicited a significant and concentration-dependent relaxation in the endothelium-intact, but not endothelium-denuded aortic rings, reaching the maximum of 81+/-5% (n=7, P<0.001). Inhibition of the nitric oxide-cGMP pathway with L-NAME (100 microM) or Methylene Blue (20 microM) reduced maximum relaxation induced by AEEG from 81+/-5% to 46+/-3 and 45+/-3%, respectively (n=7, P<0.001). A similar reduction was obtained with the incubation of the aortic rings with the selective PAF receptor antagonist WEB 2086 (10 microM) (from 81+/-5% to 55+/-3%; n=7; P<0.01). Conversely, blockade of muscarinic receptors with atropine (10 microM) did not affect the vasodilator effects of AEEG, while inhibition of the enzyme cyclooxigenase not only did not block, but rather potentiated vasodilation induced by AEEG (n=7, P<0.001). Finally, blockade of Ca(2+)- and ATP-activated K(+) channels using the specific blockers charydbotoxin (100 nM) and glibenclamide (3 microM), respectively, did not modify aortic relaxation induced by AEEG. We conclude that water-soluble extracts from leaves of Echinodorus grandiflorus elicit an endothelium-dependent, nitric oxide and PAF receptor-mediated vasodilation in rabbit aortic rings, which does not appear to involve the generation of vasodilating prostaglandins or the activation of K(+) channels. This potent vasodilator effect of the extracts was confirmed in the isolated rabbit renal circulation.

Small-molecule vasopressin-2 receptor antagonist identified by a g-protein coupled receptor "pathway" screen

G-protein-coupled receptors (GPCRs) such as the vasopressin-2 receptor (V(2)R) are an important class of drug targets. We developed an efficient screen for GPCR-induced cAMP elevation using as read-out cAMP activation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. Fischer rat thyroid cells expressing CFTR and a halide-sensing yellow fluorescent protein (H148Q/I152L) were transfected with V(2)R. Increased cell Cl(-) conductance after agonist-induced cAMP elevation was assayed using a plate reader from cell fluorescence after solution I(-) addition. The Z' factor for the assay was approximately 0.7 with the V(2)R agonist [deamino-Cys1, Val4, d-Arg8]-vasopressin (1 nM) as positive control. Primary screening of 50,000 small molecules yielded a novel, 5-aryl-4-benzoyl-3-hydroxy-1-(2-arylethyl)-2H-pyrrol-2-one class of V(2)R antagonists that are unrelated structurally to known V(2)R antagonists. The most potent compound, V(2)R(inh)-02, which was identified by screening 35 structural analogs, competitively inhibited V(2)R-induced cAMP elevation with K(i) value of approximately 70 nM and fully displaced radiolabeled vasopressin in binding experiments. V(2)R(inh)-02 did not inhibit forskolin or beta(2)-adrenergic receptor-induced cAMP production and was more than 50 times more potent for V(2)R than for V(1a)R. The favorable in vitro properties of the pyrrol-2-one antagonists suggests their potential usefulness in aquaretic applications. The CFTR-linked cAMP assay developed here is applicable for efficient, high-throughput identification of modulators of cAMP-coupled GPCRs.

Angiotensin-(1-7) potentiates responses to bradykinin but does not change responses to angiotensin I

Angiotensin-(1-7) (Ang-(1-7)), a bioactive peptide in the renin-angiotensin system, has counterregulatory actions to angiotensin II (Ang II). However, the mechanism by which Ang-(1-7) enhances vasodepressor responses to bradykinin (BK) is not well understood. In the present study, the effects of Ang-(1-7) on responses to BK, BK analogs, angiotensin I (Ang I), and Ang II were investigated in the anesthetized rat. The infusion of Ang-(1-7) (55 pmol/min i.v.) enhanced decreases in systemic arterial pressure in response to i.v. injections of BK and the BK analogs [Hyp3, Tyr(Me)8]-bradykinin (HT-BK) and [Phe8psi (CH2-NH) Arg9]-bradykinin (PA-BK) without altering pressor responses to Ang I or II, or depressor responses to acetylcholine and sodium nitroprusside. The angiotensin-converting enzyme (ACE) inhibitor enalaprilat enhanced responses to BK and the BK analog HT-BK without altering responses to PA-BK and inhibited responses to Ang I. The potentiating effects of Ang-(1-7) and enalaprilat on responses to BK were not attenuated by the Ang-(1-7) receptor antagonist A-779. Ang-(1-7)- and ACE inhibitor-potentiated responses to BK were attenuated by the BK B2 receptor antagonist Hoe 140. The cyclooxygenase inhibitor sodium meclofenamate had no significant effect on responses to BK or Ang-(1-7)-potentiated BK responses. These results suggest that Ang-(1-7) potentiates responses to BK by a selective B2 receptor mechanism that is independent of an effect on Ang-(1-7) receptors, ACE, or cyclooxygenase product formation. These data suggest that ACE inhibitor-potentiated responses to BK are not mediated by an A-779-sensitive mechanism and are consistent with the hypothesis that enalaprilat-induced BK potentiation is due to decreased BK inactivation.

A2A adenosine-receptor-mediated facilitation of noradrenaline release in rat tail artery involves protein kinase C activation and betagamma subunits formed after alpha2-adrenoceptor activation

This work aimed to investigate the molecular mechanisms involved in the interaction of alpha2-adrenoceptors and adenosine A2A-receptor-mediated facilitation of noradrenaline release in rat tail artery, namely the type of G-protein involved in this effect and the step or steps where the signalling cascades triggered by alpha2-adrenoceptors and A2A-receptors interact. The selective adenosine A2A-receptor agonist 2-p-(2-carboxy ethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 100 nM) enhanced tritium overflow evoked by trains of 100 pulses at 5 Hz. This effect was abolished by the selective adenosine A2A-receptor antagonist 5-amino-7-(2-phenyl ethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo [1,5-c]pyrimidine (SCH 58261; 20 nM) and by yohimbine (1 microM). CGS 21680-mediated effects were also abolished by drugs that disrupted G(i/o)-protein coupling with receptors, PTX (2 microg/ml) or NEM (40 microM), by the anti-G(salpha) peptide (2 microg/ml) anti-G(betagamma) peptide (10 microg/ml) indicating coupling of A2A-receptors to G(salpha) and suggesting a crucial role for G(betagamma) subunits in the A(2A)-receptor-mediated enhancement of tritium overflow. Furthermore, phorbol 12-myristate 13-acetate (PMA; 1 microM) or forskolin (1 microM), direct activators of protein kinase C and of adenylyl cyclase, respectively, also enhanced tritium overflow. In addition, PMA-mediated effects were not observed in the presence of either yohimbine or PTX. Results indicate that facilitatory adenosine A2A-receptors couple to G(salpha) subunits which is essential, but not sufficient, for the release facilitation to occur, requiring the involvement of G(i/o)-protein coupling (it disappears after disruption of G(i/o)-protein coupling, PTX or NEM) and/or G(betagamma) subunits (anti-G(betagamma)). We propose a mechanism for the interaction in study suggesting group 2 AC isoforms as a plausible candidate for the interaction site, as these isoforms can integrate inputs from G(salpha) subunits (released after adenosine A2A-receptor activation; prime-activation), G(betagamma) subunits (released after activation of G(i/o)-protein coupled receptors) which can directly synergistically stimulate the prime-activated AC or indirectly via G(betagamma) activation of the PLC-PKC pathway.

Preclinical and clinical experiences with the role of somatostatin receptors in the treatment of pituitary adenomas

The patho-physiological role of somatostatin receptor subtypes (sst) in neuro endocrine diseases has gained enhanced scientific interest in the past few years. The development of novel somatotropin-release inhibiting factor analogs, both sst-specific and universal ligands, seem promising as a tool to further increase fundamental insights in sst function. Eventually, this research should result in novel medical therapeutic opportunities in patients suffering from neuro-endocrine diseases. In the present review, the functional role of sst in all types of pituitary adenomas, based on recent preclinical and clinical studies, is being discussed.

Emerging therapeutic approaches multi-targeting receptor tyrosine kinases and g protein-coupled receptors in cardiovascular disease

Advances in molecular biology and functional genomics have demonstrated that the "one gene-one phenotype-one drug" paradigm, that has dominated pharmaceutical industry and clinical pharmacology thinking, is too simplistic for management of complex polygenic traits. The traditional highly specific drugs with unique target have proven their clinical usefulness. However, they do not always display the required efficacy versus side-effect profile, in major part because polygenic traits are determined by redundant mechanisms. Simultaneously modulating multiple targets may enhance therapeutic efficacy in the treatment of a range of disorders. Multi-targeting can be achieved by the combination of different drugs having specific single target activity. This approach introduces potential problems with pharmacokinetic interactions, toxicity and patient compliance. High efficacy can be achieved, alternatively, by administering selectively non-selective drugs with complex pharmacological profiles directed towards various molecular targets and affording pleiotropic actions. Dual- or multiple-ligands can be discovered accidentally, but can also be rationally designed according to validated medicinal chemical approaches. The merits of multiple-target versus single-target approaches for cardiovascular disease traits are assessed in the present review. The main aim is to make evident the molecular biological basis of the possibility for targeting multiple sites and the subsequently emerging strategies for interventions with superior clinical value by harnessing receptor tyrosine kinases (RTKs) such as VEGFR, PDGFR, bFGFR, as well as G protein-coupled receptors (GPCRs). The premises for lead discovery in this new area and the challenges of medicinal chemistry behind the rational design of multitasked ligands are also discussed.

G protein-coupled receptors in major psychiatric disorders

Although the molecular mechanisms underlying psychiatric illnesses such as depression, bipolar disorder and schizophrenia remain incompletely understood, there is increasing clinical, pharmacologic, and genetic evidence that G protein-coupled receptors (GPCRs) play critical roles in these disorders and their treatments. This perspectives paper reviews and synthesizes the available data. Dysfunction of multiple neurotransmitter and neuropeptide GPCRs in frontal cortex and limbic-related regions, such as the hippocampus, hypothalamus and brainstem, likely underlies the complex clinical picture that includes cognitive, perceptual, affective and motoric symptoms. The future development of novel agents targeting GPCR signaling cascades remains an exciting prospect for patients refractory to existing therapeutics.

Differential regional effects of methadone maintenance compared to heroin dependence on mu-opioid receptor desensitization in rat brain

Methadone maintenance therapy has been the mainstay of treatment for heroin addiction since the 1970s. Recent studies indicate that methadone is of greater relative intrinsic efficacy than the active metabolites of heroin at mu-opioid receptors and that the extent of mu-opioid receptor desensitization is dependent upon agonist efficacy. Regional differences have been found for mu-opioid receptor desensitization with chronic heroin self-administration, and a similar paradigm was employed to compare regional differences between the effects of heroin and methadone. Rats were trained to self-administer heroin i.v., and the dose available was increased incrementally to a terminal value of 6 mg/kg for each infusion. Half of these rats were allowed to continue to self-administer heroin, while dependence was maintained in the others by hourly infusions of 3 mg/kg of methadone. A separate group of animals was kept on a low dose of heroin. Activation of G-proteins by the high efficacy agonist DAMGO was decreased to a greater extent in animals treated chronically with methadone compared with those allowed to self-administer heroin in amygdala, periaqueductal gray, and subicular nucleus. Activation of G-proteins by the partial agonist endomorphin was decreased in striatum, thalamus, and amygdala in rats from all drug treatment groups, but to a greater extent in the striatum in methadone treated rats compared with the heroin groups. Elucidating the mechanisms by which methadone induces differential desensitization of mu-opioid receptors across brain regions compared with heroin could provide insights to improve the pharmacotherapy of heroin addiction.

Designing active template molecules by combining computational de novo design and human chemist's expertise

We used a new software tool for de novo design, the "Molecule Evoluator", to generate a number of small molecules. Explicit constraints were a relatively low molecular weight and otherwise limited functionality, for example, low numbers of hydrogen bond donors and acceptors, one or two aromatic rings, and a small number of rotatable bonds. In this way, we obtained a collection of scaffold- or templatelike molecules rather than fully "decorated" ones. We asked medicinal chemists to evaluate the suggested molecules for ease of synthesis and overall appeal, allowing them to make structural changes to the molecules for these reasons. On the basis of their recommendations, we synthesized eight molecules with an unprecedented (not patented) yet simple structure, which were subsequently tested in a screen of 83 drug targets, mostly G protein-coupled receptors. Four compounds showed affinity for biogenic amine targets (receptor, ion channel, and transport protein), reflecting the training of the medicinal chemists involved. Apparently the generation of leadlike solutions helped the medicinal chemists to select good starting points for future lead optimization, away from existing compound libraries.

A living cell-based biosensor utilizing G-protein coupled receptors: principles and detection methods

This study explores the feasibility of using a bullfrog fibroblast cell line (FT cells) expressing G protein coupled receptors (GPCRs) as the basis for a living cell-based biosensor. We have fabricated gold microelectrode arrays on a silicon dioxide substrate that supports long term, robust growth of the cells at room temperature and under ambient atmospheric conditions. Activation of an endogenous GPCR to ATP was monitored with an optical method that detects rises in intracellular calcium and with an electrochemical method that monitors the increased secretion of pre-loaded norepinephrine on a MEMS device. FT cells were also transfected to express reporter genes driven by several different promoters, raising the possibility that they could be modified genetically to express novel GPCRs as well. The ability to harness GPCRs for BioMEMS applications by using cells that are easy to grow on MEMS devices and to modify genetically opens the way for a new generation of devices based on these naturally selective and highly sensitive chemoreceptors.

“Phenotypic” pharmacology: The influence of cellular environment on G protein-coupled receptor antagonist and inverse agonist pharmacology

A central dogma of G protein-coupled receptor (GPCR) pharmacology has been the concept that unlike agonists, antagonist ligands display equivalent affinities for a given receptor, regardless of the cellular environment in which the affinity is assayed. Indeed, the widespread use of antagonist pharmacology in the classification of receptor expression profiles in vivo has relied upon this 'antagonist assumption'. However, emerging evidence suggests that the same gene-product may exhibit different antagonist pharmacological profiles, depending upon the cellular context in which it is expressed-so-called 'phenotypic' profiles. In this commentary, we review the evidence relating to some specific examples, focusing on adrenergic and muscarinic acetylcholine receptor systems, where GPCR antagonist/inverse agonist pharmacology has been demonstrated to be cell- or tissue-dependent, before going on to examine some of the ways in which the cellular environment might modulate receptor pharmacology. In the majority of cases, the cellular factors responsible for generating phenotypic profiles are unknown, but there is substantial evidence that factors, including post-transcriptional modifications, receptor oligomerization and constitutive receptor activity, can influence GPCR pharmacology and these concepts are discussed in relation to antagonist phenotypic profiles. A better molecular understanding of the impact of cell background on GPCR antagonist pharmacology is likely to provide previously unrealized opportunities to achieve greater specificity in new drug discovery candidates.

The histamine H4 receptor: A novel modulator of inflammatory and immune disorders

All 4 known histamine receptors (H(1)R, H(2)R, H(3)R and H(4)R) have been used or proposed as therapeutic targets for varied diseases. This article reviews the recent progress in understanding the function of the recently described histamine receptor H(4)R in a variety of immune responses and the potential therapeutic value of H(4)R antagonists. The H(4)R is expressed primarily on cells involved in inflammation and immune response. It has effects on chemotaxis, as well as cytokine and chemokine production of mast cells, eosinophils, dendritic cells, and T cells. H(4)R antagonists, JNJ 7777120 and JNJ 10191584 (also known as VUF 6002) have been developed with excellent affinity and selectivity towards human and rodent H(4)R. These antagonists also demonstrate efficacy as anti-inflammatory agents in vivo. H(4)R antagonists have shown promising activity in down-regulating immune responses in a range of animal disease models including acute inflammation, hapten-mediated colitis, and allergic airway inflammation. Due to its distribution on immune cells and its proven role in inflammatory functions, the H(4)R appears to be a therapeutic target for the treatment of a variety of immune disorders.

2-Arachidonylglyceryl ether and abnormal cannabidiol-induced vascular smooth muscle relaxation in rabbit pulmonary arteries via receptor-pertussis toxin sensitive G proteins-ERK1/2 signaling

The receptor(s) used by cannabinoids to relax vascular smooth muscle is unknown. Here, we investigated the effects of 2-arachidonylglyceryl ether (2-AG ether), a metabolically stable endocannabinoid, and abnormal cannabidiol (abn-CBD) on relaxation of permeabilized pulmonary arterial strips monitored with force, and on extracellular signal-regulated mitogen-activated protein kinases (ERK1/2) phosphorylation in permeabilized vascular smooth muscle cells using immunoblotting. We found that 2-AG ether and abn-CBD caused relaxation and increased phosphorylation of ERK1/2. 2-AG ether effects were completely abolished by N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251), and N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A), and partially blocked by (-)-1.3-dimethoxy-2-(3-3,4-trans-p-menthadien-(1,8)-yl)-orcinol (O-1918). In contrast, abn-CBD effects were completely abolished by O-1918, and only partially blocked by AM251, and SR141716A. Both 2-AG ether and abn-CBD effects were partially blocked by pertussis toxin, an inhibitor of Gi/o proteins. PD98059, an inhibitor of mitogen activated protein kinase kinase (MEK), completely abolished the relaxation, but only partially blocked the increased phosphorylation of ERK1/2 by 2-AG ether. In contrast, abn-CBD-induced relaxation was partially blocked and the increased phosphorylation of ERK1/2 was abolished by PD98059. These findings suggest that 2-AG ether and abn-CBD-induced vascular smooth muscle relaxation are mediated by the cannabinoid CB1 receptor, and the abn-CBD receptor, respectively, and are modulated by cross-talk between the receptors. These responses occur mainly by coupling to pertussis toxin sensitive G proteins, but also, in part independent of these G proteins, which have been classically thought to initiate MEK/ERK1/2 signaling to relax vascular smooth muscle.

Ghrelin administration into tegmental areas stimulates locomotor activity and increases extracellular concentration of dopamine in the nucleus accumbens

Ghrelin stimulates appetite, increases food intake and causes adiposity by mechanisms that include direct actions on the brain. Previously, we showed that intracerebroventricular administration of ghrelin has stimulatory and dopamine-enhancing properties. These effects of ghrelin are mediated via central nicotine receptors, suggesting that ghrelin can activate the acetylcholine-dopamine reward link. This reward link consists of cholinergic input from the laterodorsal tegmental area (LDTg) to the mesolimbic dopamine system that originates in the ventral tegmental area (VTA) and projects to the nucleus accumbens. Given that growth hormone secretagogue receptors (GHSR-1A) are expressed in the VTA and LDTg, brain areas involved in reward, the present series of experiments were undertaken to examine the hypothesis that these regions may mediate the stimulatory and dopamine-enhancing effects of ghrelin, by means of locomotor activity and in vivo microdialysis in freely moving mice. We found that local administration of ghrelin into the VTA (1 microg in 1 microl) induced an increase in locomotor activity and in the extracellular concentration of accumbal dopamine. In addition, local administration of ghrelin into the LDTg (1 microg in 1 microl) caused a locomotor stimulation and an increase in the extracellular levels of accumbal dopamine. Taken together, this indicates that ghrelin might, via activation of GHSR-1A in the VTA and LDTg, stimulate the acetylcholine-dopamine reward link, implicating that ghrelin is a part of the neurochemical overlap between the reward systems and those that regulate energy balance.

Cycloheximide: no ordinary bitter stimulus

Cycloheximide (CyX), a toxic antibiotic with a unique chemical structure generated by the actinomycete, Streptomyces griseus, has emerged as a primary focus of studies on mammalian bitter taste. Rats and mice avoid it at concentrations well below the thresholds for most bitter stimuli and T2R G-protein-coupled receptors specific for CyX with appropriate sensitivity are identified for those species. Like mouse and rat, golden hamsters, Mesocricetus auratus, also detected and rejected micromolar levels of CyX, although 1mM CyX failed to activate the hamster chorda tympani nerve. Hamsters showed an initial tolerance for 500microM CyX, but after that, avoidance of CyX dramatically increased, plasticity not reported for rat or mouse. As the hamster lineage branches well before division of the mouse-rat lineage in evolutionary time, differences between hamster and mouse-rat reactions to CyX are not surprising. Furthermore, unlike hamster LiCl-induced learned aversions, the induced CyX aversion neither specifically nor robustly generalized to other non-ionic bitter stimuli; and unlike adverse reactions to other chemosensory stimuli, aversions to CyX were not mollified by adding a sweetener. Thus, CyX is unlike other bitter stimuli. The gene for the high-affinity CyX receptor is a member of a cluster of five orthologous T2R genes that are likely rodent-specific; this "CyX clade" is found in the mouse, rat and probably hamster, but not in the human or rabbit genome. The rodent CyX-T2R interaction may be one of multiple lineage-specific stimulus-receptor interactions reflecting a response to a particular environmental toxin. The combination of T2R multiplicity, species divergence and gene duplication results in diverse ligands for multiple species-specific T2R receptors, which confounds definition of 'bitter' stimuli across species.

Integrins regulate opioid receptor signaling in trigeminal ganglion neurons

The binding of integrins to the extracellular matrix results in focal organization of the cytoskeleton and the genesis of intracellular signals that regulate vital neuronal functions. Recent evidence suggests that integrins modulate G-protein-coupled receptor (GPCR) signaling in hippocampal neurons. In this study we evaluated the hypothesis that integrins regulate the mu opioid receptor in rat trigeminal ganglion neurons. For these studies, primary cultures of adult rat trigeminal ganglion neurons were used to demonstrate the colocalization of beta1 and beta3 integrins with mu opioid receptor in caveolin-1-rich membrane fractions, and at focal adhesions sites generated by integrin ligand binding. Furthermore, we show that the mu opioid receptor agonist, DAMGO ([D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin), inhibits cyclic AMP (cAMP) accumulation in response to prostaglandin E2 (PGE(2)) stimulation in bradykinin-primed, but not unprimed, cultured trigeminal ganglia neurons. Application of soluble GRGDS (Gly-Arg-Gly-Asp-Ser) peptides that bind specific integrins (i.e. RGD-binding integrins) completely abolished the DAMGO effect in bradykinin-primed trigeminal ganglia neurons, but did not alter bradykinin-mediated hydrolysis of phosphatidylinositol. Likewise, monospecific anti-beta1 and anti-beta3 integrin subunit antibodies blocked this DAMGO effect in bradykinin-primed trigeminal ganglia neurons. Indeed, application of anti-beta1 integrin subunit actually reversed DAMGO signaling, resulting in increased cAMP accumulation in these cells. This suggests that the relative amounts of specific activated integrins at focal adhesions may govern signaling by the mu opioid receptor, perhaps by altering interactions with G proteins (e.g. Galphai vs. Galphas). Collectively, these data provide the first evidence that specific integrins regulate opioid receptor signaling in sensory neurons.

Interaction of NPY compounds with the rat glucocorticoid-induced receptor (GIR) reveals similarity to the NPY-Y2 receptor

The rat glucocorticoid-induced receptor (rGIR) is an orphan G protein-coupled receptor awaiting pharmacological characterization. Among known receptors, rGIR exhibits highest sequence similarity to the neuropeptide Y (NPY)-Y(2) receptor (38-40%). The pharmacological profile of rGIR was investigated using (125)I-PYY(3-36), a Y(2)-preferring radioligand and several NPY analogs. rGIR displayed a similar displacement profile as reported for the Y(2) receptor, in that the Y(2)-selective C terminus fragments of NPY and PYY (NPY(3-36) and PYY(3-36)) showed high affinity binding and activation of rGIR (low nanomolar range). The rank order potency for displacement was NPY(3-36)>PYY(3-36)=NPY>NPY(13-36)>Ac, Leu NPY(24-36)>[D-Trp(32)]-NPY>Leu(31), Pro(34)-NPY=hPP. NPY and Y(2)-selective agonists NPY(3-36) and PYY(3-36) led to significant activation of (35)S-GTPgammaS binding to rGIR transfected cells. BIIE0246, a specific Y(2) antagonist, displaced (125)I-PYY(3-36) binding to rGIR with high affinity (95nM). Activation of (35)S-GTPgammaS binding by Y(2)-selective agonist in rGIR transfected cells was also completely abolished by BIIE0246. Our data report, for the first time, an interaction of NPY ligands with rGIR expressed in vitro, and indicate similarities between GIR and the NPY-Y(2) receptor.

Biochemical anti-opioid action of NPFF2 receptors in rat spinal cord

Neuropeptide FF (NPFF) agonists counteract the cellular opioid actions. We demonstrate for the first time a biochemical anti-opioid effect of NPFF receptors in the rat spinal cord by using the [(35)S]GTPgammaS binding assay in autoradiography. The mu agonist DAMGO as well as the potent and selective NPFF(2) agonist dNPA, stimulated [(35)S]GTPgammaS binding at different optimal GDP concentrations. dNPA decreased the effects induced by DAMGO alone; the maximal of G-protein coupling was decreased but not the potency of opioid agonist. We conclude that NPFF(2) receptors are coupled to G-protein in the rat spinal cord and could exert a molecular anti-opioid effect.

Allosteric Modulation of G Protein–Coupled Receptors

The past decade has witnessed a significant growth in the identification of allosteric modulators of G protein-coupled receptors (GPCRs), i.e., ligands that interact with binding sites that are topographically distinct from the orthosteric site recognized by the receptor's endogenous agonist. Because of their ability to modulate receptor conformations in the presence of orthosteric ligand, allosteric modulators can "fine-tune" classical pharmacological responses. This is advantageous in terms of a potential for engendering greater GPCR subtype-selectivity, but represents a significant challenge for detecting and validating allosteric behaviors. Although allosteric sites need not have evolved to accommodate endogenous ligands, there are a number of examples of where such modulators have been shown to contribute to physiological or pathophysiological processes. Studies are also beginning to unravel the structural basis of allosteric modulation of GPCRs. It remains to be determined whether such modulation represents interactions within monomers versus across dimers.

Local anesthetics

Local anesthetics are used broadly to prevent or reverse acute pain and treat symptoms of chronic pain. This chapter, on the analgesic aspects of local anesthetics, reviews their broad actions that affect many different molecular targets and disrupt their functions in pain processing. Application of local anesthetics to peripheral nerve primarily results in the blockade of propagating action potentials, through their inhibition of voltage-gated sodium channels. Such inhibition results from drug binding at a site in the channel's inner pore, accessible from the cytoplasmic opening. Binding of drug molecules to these channels depends on their conformation, with the drugs generally having a higher affinity for the open and inactivated channel states that are induced by membrane depolarization. As a result, the effective potency of these drugs for blocking impulses increases during high-frequency repetitive firing and also under slow depolarization, such as occurs at a region of nerve injury, which is often the locus for generation of abnormal, pain-related ectopic impulses. At distal and central terminals the inhibition of voltage-gated calcium channels by local anesthetics will suppress neurogenic inflammation and the release of neurotransmitters. Actions on receptors that contribute to nociceptive transduction, such as TRPV1 and the bradykinin B2 receptor, provide an independent mode of analgesia. In the spinal cord, where local anesthetics are present during epidural or intrathecal anesthesia, inhibition of inotropic receptors, such as those for glutamate, by local anesthetics further interferes with neuronal transmission. Activation of spinal cord mitogen-activated protein (MAP) kinases, which are essential for the hyperalgesia following injury or incision and occur in both neurons and glia, is inhibited by spinal local anesthetics. Many G protein-coupled receptors are susceptible to local anesthetics, with particular sensitivity of those coupled via the Gq alpha-subunit. Local anesthetics are also infused intravenously to yield plasma concentrations far below those that block normal action potentials, yet that are frequently effective at reversing neuropathic pain. Thus, local anesthetics modify a variety of neuronal membrane channels and receptors, leading to what is probably a synergistic mixture of analgesic mechanisms to achieve effective clinical analgesia.

Membrane interactive alpha-helices in GPCRs as a novel drug target

G-Protein Coupled Receptors (GPCRs) are one of the most important targets for pharmaceutical drug design. Over the past 30 years, mounting evidence has suggested the existence of homo and hetero dimers or higher-order complexes (oligomers) that are involved in signal transduction and some diseases. The number of reports describing GPCR oligomerization has increased, and in 2003, the organization of mouse rhodopsin into two-dimensional arrays of dimers was determined by an atomic force microscopic analysis. The analysis of the mouse rhodopsin complex has enabled us to discuss the oligomerization based on structural data. Although many unsolved problems still remains, the idea that GPCRs directly interact to form oligomers has been gradually accepted. One of the recent findings in the GPCR investigations is the clarification of the mechanisms of GPCR oligomerization at a molecular level. Most of these studies have suggested the importance of transmembrane alpha-helices for GPCR oligomerization. In this review, we will first summarize the importance of GPCR oligomerization and the functions of GPCRs. Then, we will explain the involvement of transmembrane alpha-helices in the oligomerization and a drug design strategy that targets these regions for GPCR oligomerization. Considering the current drug design methods, which are based on the modification of the protein-protein interactions of soluble regions of proteins, a "peptide mimic approach" that targets the transmembrane alpha-helices constituting the interfaces would be promising in drug discovery for GPCR oligomerization. For that purpose, we must know the positions of the interfaces. However, problems specific to membrane proteins have made it difficult to identify the positions of the interfaces experimentally. Therefore, information about the interfaces predicted by bioinformatics approaches is valuable. At the end of this review, several bioinformatics approaches toward interface prediction for oligomerization are introduced. The benefits and the pitfalls of these approaches are also discussed.

Pharmacological characterization of mouse GPRC6A, an L-alpha-amino-acid receptor modulated by divalent cations

BACKGROUND AND PURPOSE

GPRC6A is a novel member of family C of G protein-coupled receptors with so far unknown function. We have recently described both human and mouse GPRC6A as receptors for L-alpha-amino acids. To date, functional characterization of wild-type GPRC6A has been impaired by the lack of activity in quantitative functional assays. The aim of this study was thus to develop such an assay and extend the pharmacological characterization of GPRC6A.

EXPERIMENTAL APPROACH

We have engineered a novel cell-based inositol phosphate turnover assay for wild-type mouse GPRC6A based on transient co-expression with the promiscuous Galpha(qG66D) protein, known to increase receptor signalling sensitivity. This assay allowed for measurements of L-alpha-amino acid potencies. Furthermore, in combination with an assay measuring inward currents at Ca(2+)-activated chloride channels in Xenopus oocytes, the divalent cation-sensing ability of the receptor was examined.

KEY RESULTS

Using our novel assay, we demonstrate that the basic L-alpha-amino acids ornithine, lysine, and arginine are the most potent agonists at wild-type mouse GPRC6A. Using two different assay systems, we show that divalent cations do not activate the G(q) signalling pathway of mouse GPRC6A per se but positively modulate the amino-acid response.

CONCLUSIONS AND IMPLICATIONS

This is the first reported assay for a wild-type GPRC6A successfully applied for quantitative pharmacological characterization of amino acid and divalent cation responses at mouse GPRC6A. The assay enables further search for GPRC6A ligands such as allosteric modulators, which may provide essential information about the physiological function of GPRC6A.