Influence of receptor number on functional responses elicited by agonists acting at the human adenosine A(1) receptor: evidence for signaling pathway-dependent changes in agonist potency and relative intrinsic activity

Clathrin-independent internalization of the human histamine H1-receptor in CHO-K1 cells

The aim of the present study was to investigate the cellular pathway involved in histamine-stimulated internalization of the human H1-receptor in CHO-K1 cells expressing N-terminal myc-tagged H1-receptor (Myc-H1) or N-terminal myc-C-terminal green fluorescent protein (Myc-GFP H1) versions of the receptor. Studies of 3H-mepyramine binding and histamine-stimulated 3H-inositol phosphate accumulation in these cells showed that the Myc-H1 and Myc-GFP H1-receptors had identical pharmacology to the wild-type H1-receptor. The Myc-H1-receptor was rapidly internalized in CHO-K1 cells following stimulation with histamine (0.1 mM). This response occurred within 15 min, and could be prevented by the quaternary H1-receptor antagonist alpha-pirdonium. Similar data were obtained with the Myc-GFP H1-receptors. Internalization of the Myc-GFP H1-receptor was maintained in the absence of extracellular calcium and was not inhibited by the CAM kinase II inhibitor KN-62 (10 microM). Phorbol dibutyrate, an activator of protein kinase C, was also able to stimulate internalization of the H1-receptor. However, inhibition or downregulation of protein kinase C (which significantly modified histamine-stimulated inositol phosphate responses) was without effect on the internalization of the H1-receptor stimulated by histamine. Hypertonic sucrose did not prevent histamine-induced internalization of the Myc-GFP H1-receptor, but was able to attenuate internalization of transferrin via clathrin-mediated endocytosis in the same cells. In contrast, preincubation of cells with filipin or nystatin, which disrupts caveolae and lipid rafts, completely inhibited the histamine-induced internalization of the Myc-GFP H1-receptor, but was without effect on the sequestration of transferrin. The H1-receptor and cholera toxin subunit B were colocalized under resting conditions at the cell surface. Immunohistochemical studies with an antibody to caveolin-1 confirmed that this protein was also localized predominantly to the plasma membrane. However, following stimulation of CHO-Myc-GFP H1 cells with histamine, there was no evidence for internalization of caveolin-1 in parallel with the H1-receptor. These data provide strong evidence that the H1-receptor is internalized via a clathrin-independent mechanism and most likely involves lipid rafts.

A comparison of the antagonist affinities for the Gi- and Gs-coupled states of the human adenosine A1-receptor

The antagonist affinity for a given receptor is traditionally considered to be constant, reflecting the chemical nature of the specific ligand-receptor interaction. However, recent observations with all three beta-adrenoceptors have cast doubt on this basic pharmacological principle. The extent to which this finding applies to other G protein-coupled receptors and their interaction with different G proteins is unknown. Therefore, we studied the influence of different agonists on antagonist affinity measurements for Gi- and Gs-coupled conformations of the adenosine A1-receptor in Chinese hamster ovary cells stably expressing the human adenosine A1-receptor and a cAMP-response element (CRE)-secreted placental alkaline phosphatase reporter gene. Gi-coupled inhibition of [3H]cAMP accumulation via the A1-receptor was observed at low concentrations of agonist; however, a small increase in [3H]cAMP accumulation was also seen at higher agonist concentrations. This biphasic response was more evident for A1-stimulated CRE-gene transcription. The inhibitory component was abolished by pretreatment with pertussis toxin, whereas the stimulatory component was augmented, suggesting that the responses were due to an A1-Gi-coupled inhibition followed by an A1-Gs-coupled stimulation. However, the antagonist affinity values measured at the Gi-coupled and Gs-coupled conformations of the receptor were the same in both functional responses and whole-cell binding. Thus, in marked contrast to the beta-adrenoceptors, the A1-receptor conforms to the long-held principle of pharmacology that antagonist affinity measurements are constant regardless of the response being measured and the competing agonist used to stimulate that response. This was true even when the receptor was shown, in the same assay, to exist in two different conformational states coupled to two different G proteins.

Glutamate release by neurons evokes a purinergic inhibitory mechanism of osmotic glial cell swelling in the rat retina: activation by neuropeptide Y

Glial cell swelling is a central cause of ischemic edema in the brain and retina; however, the regulation of glial cell volume by endogenous factors in situ is largely unknown. In slices of the postischemic retina of the rat, the somata of glial (Müller) cells swell upon hypotonic stress that is not observed in slices of control retinas. We describe an endogenous signaling pathway that leads to inhibition of the osmotic glial cell swelling, and that is evoked by the release of glutamate from retinal neurons upon application of neuropeptide Y. Glutamate activates metabotropic glutamate receptors on swollen glial cells, which evokes a Ca2+ -independent purinergic signaling cascade that involves release of ATP, P2Y1 receptor activation, and transporter-mediated release of adenosine. Activation of A1 receptors causes the inhibition of osmotic glial cell swelling, by a protein kinase A-dependent activation of K+ and Cl- channels. It is proposed that the glutamate-evoked purinergic receptor signaling of glial cells is crucially involved in the cell volume homeostasis of the retina, and that this mechanism may contribute to the protective effect of adenosine in the ischemic tissue.

Comparison of the relative efficacy and potency of mu-opioid agonists to activate Galpha(i/o) proteins containing a pertussis toxin-insensitive mutation

Pertussis toxin (PTX)-insensitive mutants of Galpha(i/o) proteins expressed in C6mu cells were used to examine the hypothesis that there are agonist-specific conformational states of the mu-opioid receptor with coupling preferences to different Galpha(i/o) subtypes, as measured by the degree of stimulation of [(35)S]guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) binding. Binding of [(35)S]GTPgammaS to endogenous Galpha(i/o) proteins stimulated by the full mu-opioid agonist [d-Ala(2),MePhe(4),Gly(5)-ol]enkephalin (DAMGO) was completely blocked by overnight treatment with 100 ng/ml PTX. Treatment for 4 h with lower concentrations led to a PTX-dependent reduction in the maximal effect of DAMGO but no alteration in the potency of DAMGO or morphine nor in the relative maximal effect (relative efficacy) of the partial agonists morphine and buprenorphine compared with the full agonist DAMGO. Using PTX-insensitive Galpha mutants in which the PTX-sensitive cysteine was replaced with isoleucine, the potency for a series of mu-opioid agonists was highest in cells expressing Galpha(i3) and Galpha(o) and lowest with Galpha(i1) and Galpha(i2), with no significant change in the order of potency, namely, etorphine >> endomorphin-1 = DAMGO = endomorphin-2 = fentanyl = morphine >> meperidine. The order of agonist relative efficacy, etorphine = DAMGO = endomorphin-1 = endomorphin-2 = fentanyl > or = morphine > or = meperidine > buprenorphine > or = nalbuphine, was also the same across all of the PTX-insensitive Galpha(i/o) subtypes. Highest relative efficacy to stimulate [(35)S]GTPgammaS binding was seen with Galpha(i3). Consequently, reported observations of agonist-directed trafficking at mu-opioid receptors most likely involve non-PTX-sensitive Galpha protein mechanisms.

The cannabinoid agonist WIN55,212-2 increases intracellular calcium via CB1 receptor coupling to Gq/11 G proteins

Central nervous system responses to cannabis are primarily mediated by CB(1) receptors, which couple preferentially to G(i/o) G proteins. Here, we used calcium photometry to monitor the effect of CB(1) activation on intracellular calcium concentration. Perfusion with 5 microM CB(1) aminoalkylindole agonist, WIN55,212-2 (WIN), increased intracellular calcium by several hundred nanomolar in human embryonic kidney 293 cells stably expressing CB(1) and in cultured hippocampal neurons. The increase was blocked by coincubation with the CB(1) antagonist, SR141716A, and was absent in nontransfected human embryonic kidney 293 cells. The calcium rise was WIN-specific, being essentially absent in cells treated with other classes of cannabinoid agonists, including Delta(9)-tetrahydrocannabinol, HU-210, CP55,940, 2-arachidonoylglycerol, methanandamide, and cannabidiol. The increase in calcium elicited by WIN was independent of G(i/o), because it was present in pertussis toxin-treated cells. Indeed, pertussis toxin pretreatment enhanced the potency and efficacy of WIN to increase intracellular calcium. The calcium increases appeared to be mediated by G(q) G proteins and phospholipase C, because they were markedly attenuated in cells expressing dominant-negative G(q) or treated with the phospholipase C inhibitors U73122 and ET-18-OCH(3) and were accompanied by an increase in inositol phosphates. The calcium increase was blocked by the sarco/endoplasmic reticulum Ca(2+) pump inhibitor thapsigargin, the inositol trisphosphate receptor inhibitor xestospongin D, and the ryanodine receptor inhibitors dantrolene and 1,1'-diheptyl-4,4'-bipyridinium dibromide, but not by removal of extracellular calcium, showing that WIN releases calcium from intracellular stores. In summary, these results suggest that WIN stabilizes CB(1) receptors in a conformation that enables G(q) signaling, thus shifting the G protein specificity of the receptor.

The Glucocorticoid Triamcinolone Acetonide Inhibits Osmotic Swelling of Retinal Glial Cells via Stimulation of Endogenous Adenosine Signaling

The glucocorticoid triamcinolone acetonide is clinically used for the treatment of macular edema. However, the edema-resolving mechanisms of triamcinolone are incompletely understood. Since cell swelling is a central cause of cytotoxic edema in the brain and retina, we determined the effects of triamcinolone acetonide on the swelling of retinal ganglion and Müller glial cells in acutely isolated retinas from rats and guinea pigs in situ. Triamcinolone acetonide (100 microM) had no effect on the swelling of ganglion cells that was evoked in isolated whole mounts of the guinea pig retina by acute application of glutamate (1 mM) or high K+ (50 mM). However, triamcinolone reversed the osmotic swelling of Müller glial cells in retinas of the rat that was observed under various experimental conditions: in retinas isolated at 3 days after transient retinal ischemia, in retinas of eyes with lipopolysaccharide-induced ocular inflammation, and in control retinas in the presence of Ba2+ (1 mM), H2O2 (200 microM), arachidonic acid (10 microM), or prostaglandin E2 (30 nM). The inhibiting effect of triamcinolone on osmotic glial cell swelling was mediated by stimulation of transporter-mediated release of endogenous adenosine and subsequent A1 receptor activation, resulting in an elevation of the intracellular cAMP level and activation of the protein kinase A, and, finally, in an opening of extrusion pathways for K+ and Cl- ions. The inhibitory effect on the cytotoxic swelling of glial cells may contribute to the fast edema-resolving effect of vitreal triamcinolone observed in human patients.

G protein-dependent pharmacology of histamine H3 receptor ligands: evidence for heterogeneous active state receptor conformations

Previously reported pharmacological studies using the imidazole-containing histamine H3 receptor ligands GT-2331 (Cipralisant) and proxyfan resulted in a range of classifications (antagonist, agonist, and protean) for these compounds. We examined the role that the signaling system, with particular emphasis on the type of G protein, had on the pharmacology observed for H3 ligands. Ligands were assessed using assays measuring neurotransmitter release, cAMP, and guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding. Whereas clobenpropit and ciproxifan were consistently antagonists, GT-2331, proxyfan, and imetit exhibited differential activity. Although GT-2331 and proxyfan exhibited little agonist activity in neurotransmitter release assays, both demonstrated full agonism relative to (R)-alpha-methylhistamine in cAMP assays. In [35S]GTPgammaS binding assays, GT-2331 and proxyfan demonstrated partial agonism. Imetit showed full agonism in most assays, but it was slightly less efficacious in a neurotransmitter release assay and in [35S]GTPgammaS binding at the human H3 receptor. To further examine these ligands, we coexpressed G alpha16 or chimeric G alpha q/i5 in human embryonic kidney cells expressing the human H3 receptor and assayed intracellular calcium and cAMP levels. GT-2331, proxyfan, and imetit demonstrated full agonism in all assays of cAMP activity. However, in cells expressing G alpha16, they exhibited minimal agonism in calcium mobilization assays, whereas imetit showed partial agonism. When G alpha q/i5 was used, the activity of both GT-2331 and proxyfan increased, whereas imetit became a full agonist. These results demonstrate that GT-2331 and proxyfan's differential pharmacology at the H3 receptor depends on the type of G protein used and provide indirect evidence for differential ligand-bound active states that mediate signaling by the H3 receptor.

The origins of diversity and specificity in g protein-coupled receptor signaling

The modulation of transmembrane signaling by G protein-coupled receptors (GPCRs) constitutes the single most important therapeutic target in medicine. Drugs acting on GPCRs have traditionally been classified as agonists, partial agonists, or antagonists based on a two-state model of receptor function embodied in the ternary complex model. Over the past decade, however, many lines of investigation have shown that GPCR signaling exhibits greater diversity and "texture" than previously appreciated. Signal diversity arises from numerous factors, among which are the ability of receptors to adopt multiple "active" states with different effector-coupling profiles; the formation of receptor dimers that exhibit unique pharmacology, signaling, and trafficking; the dissociation of receptor "activation" from desensitization and internalization; and the discovery that non-G protein effectors mediate some aspects of GPCR signaling. At the same time, clustering of GPCRs with their downstream effectors in membrane microdomains and interactions between receptors and a plethora of multidomain scaffolding proteins and accessory/chaperone molecules confer signal preorganization, efficiency, and specificity. In this context, the concept of agonist-selective trafficking of receptor signaling, which recognizes that a bound ligand may select between a menu of active receptor conformations and induce only a subset of the possible response profile, presents the opportunity to develop drugs that change the quality as well as the quantity of efficacy. As a more comprehensive understanding of the complexity of GPCR signaling is developed, the rational design of ligands possessing increased specific efficacy and attenuated side effects may become the standard mode of drug development.

What's for lunch at the conformational cafeteria?

In this issue of Molecular Pharmacology, Mukhopadhyay and Howlett present evidence for ligand-selective conformations of the CB1 cannabinoid receptor with differential coupling to G proteins. Ligand-directed signaling to different cellular effector pathways extends drug selectivity beyond that afforded by differential affinity for different receptor subtypes. The challenge for pharmacologists of the future will be not only to identify ligand-selective receptor conformations but also to develop an understanding of the relationships between those conformations, cell function, and ultimately therapeutics. As we learn more about ligand-selective receptor conformations, it should be possible to develop response-selective drugs that maximize therapeutic efficacy and minimize unwanted effects.

A1 and A2A adenosine receptor modulation of alpha 1-adrenoceptor-mediated contractility in human cultured prostatic stromal cells

1. This study investigated the possibility that adenosine receptors modulate the alpha(1)-adrenoceptor-mediated contractility of human cultured prostatic stromal cells (HCPSC). 2. The nonselective adenosine receptor agonist, 5'-N-ethylcarboxamido-adenosine (NECA; 10 nm-10 microm), and the A(1) adenosine receptor selective agonist, cyclopentyladenosine (CPA; 10 nm-10 microm), elicited significant contractions in HCPSC, with maximum contractile responses of 18+/-3% and 17+/-2% reduction in initial cell length, respectively. 3. In the presence of a threshold concentration of phenylephrine (PE) (100 nm), CPA (1 nm-10 microm) caused contractions, with an EC(50) of 124+/-12 nm and maximum contractile response of 37+/-4%. The A(1) adenosine receptor-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX 100 nm) blocked this effect. In the presence of DPCPX (100 nm), NECA (1 nm-10 microm) inhibited contractions elicited by a submaximal concentration of PE (10 microm), with an IC(50) of 48+/-2 nm. The A(2A) adenosine receptor-selective antagonist 4-(2-[7-amino-2-[furyl][1,2,4]triazolo[2,3-alpha][1,3,5,]triazin-5-yl amino]ethyl)phenol (Zm241385 100 nm) blocked this effect. 4. In BCECF-AM (10 microm)-loaded cells, both CPA (100 pM-1 microm) and NECA (100 pm-10 microm) elicited concentration-dependent decreases in intracellular pH (pH(i)), with EC(50) values of 3.1+/-0.3 and 6.0+/-0.3 nm, respectively. The response to NECA was blocked by Zm241385 (100 nm; apparent pK(B) of 9.4+/-0.4), but not by DPCPX (100 nm). The maximum response to CPA was blocked by DPCPX (100 nm), and unaffected by Zm241385 (100 nm). 5. NECA (10 nm-10 microm) alone did not increase [(3)H]-cAMP in HCPSC. In the presence of DPCPX (100 nm), NECA (10 nm-10 microm) caused a concentration dependent increase in [(3)H]-cAMP, with an EC(50) of 1.2+/-0.1 microm. This response was inhibited by Zm241385 (100 nm). CPA (10 nm-10 microm) had no effect on cAMP, in the presence or absence of forskolin (1 microm). 6. These findings are consistent with a role for adenosine receptors in the modulation of adrenoceptor-mediated contractility in human prostate-derived cells.

Coupling of the human A1 adenosine receptor to different heterotrimeric G proteins: evidence for agonist-specific G protein activation

The present study investigates the effect of varying ligand structure on the ability of agonists to activate guanine nucleotide-binding proteins of the Gi, Gs and Gq families via the A(1) adenosine receptor. In CHO cells expressing this receptor, inhibition or potentiation of forskolin-stimulated cAMP accumulation was used as an end point to measure the activation of Gi and, in Pertussis toxin (PTX)-treated cells, Gs, respectively. Stimulation of inositol phosphate accumulation in PTX-treated cells was used as an index of Gq activation. CPA (N(6)-cyclopentyladenosine), NECA (5'-N-ethyl-carboxyamidoadenosine) and eight analogues of these ligands presented a range of guanine nucleotide-binding protein (G-protein)-activating profiles. Some ligands could only activate Gi (e.g. 2'deoxyCPA), some primarily Gi and Gs (and only weakly Gq) (e.g. 3'deoxyCPA), highlighting the importance of the ribose hydroxyls in agonist activation of multiple G proteins. CHA (N(6)-cyclohexyladenosine) activated Gi, Gs and Gq, but was more efficacious than CPA in activating Gs. The NECA analogues 5'-N-cyclopropyl-carboxamidoadenosine, 5'-N-cyclobutyl-carboxamidoadenosine and 5'-N-cyclopentyl-carboxamidoadenosine (CPeCA) also activated all three G proteins, although their ability to activate Gs and Gq (relative to CPA) was reduced with increasing substituent size, such that CPeCA produced only a small stimulation (at 100 microM) at Gq, but was a full agonist, relative to CPA, at Gi and Gs. This study suggests that the A(1) adenosine receptor can adopt agonist-specific conformations, arising from small changes in ligand structure, which lead to the differential activation of Gi, Gs and Gq.

Functional coupling of the nociceptin/orphanin FQ receptor in dog brain membranes

Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ receptor (NOP) which is yet to be functionally characterized in dog brain. Ligand binding data reports low NOP density (29 fmol mg(-1) protein) in dog. In this study using dog brain membranes, we have examined the effects of N/OFQ on [leucyl-(3)H]N/OFQ(1-17)OH ([leucyl-(3)H]N/OFQ) binding in the presence and absence of 120 mM NaCl and 100 microM GTPgammaS. Data from standard [(35)S]GTPgammaS binding and immunoprecipitation (G(alphai1-3)) assays are also presented, along with data from a limited number of control experiments with human NOP expressed in Chinese hamster ovary (CHO(hNOP)) cells. N/OFQ displaced [leucyl-(3)H]N/OFQ binding with pK(i) and slope values of 9.62+/-0.07 and 0.38+/-0.05, respectively. Addition of NaCl/GTPgammaS produced a steepening (slope 0.95+/-0.06, n=3) of the curve. N/OFQ stimulated [(35)S]GTPgammaS binding with pEC(50) and E(max) values of 8.21+/-0.17 and 1.17+/-0.01, respectively (in CHO(hNOP), pEC(50) and E(max) values were 8.47+/-0.01 and 7.01+/-0.63). N/OFQ stimulated [(35)S]GTPgammaS binding in dog and CHO(hNOP) cell membranes could be immunoprecipitated with an anti-G(alphai1-3) antibody, indicating coupling to a pertussis toxin (PTx)-sensitive G-protein. N/OFQ actions were competitively antagonized by the selective NOP antagonists, 100 nM J-113397, 1 microM [Nphe(1)]N/OFQ(1-13)NH(2) and 1 microM [Phe(1)Psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)NH(2) (partial agonist) yielding pK(B) values of 8.58+/-0.21, 7.06+/-0.59 and 7.32+/-0.41, respectively (in CHO(hNOP), a pK(B) for J-113397 of 8.33+/-0.02 was obtained). Despite relatively low receptor density, we were able to detect functional activity of native dog NOP, with pharmacology consistent with reports for other species.

HEME OXYGENASE-1 GENE EXPRESSION IN PERICENTRAL HEPATOCYTES THROUGH β1-ADRENOCEPTOR STIMULATION

Induction of heme oxygenase (HO)-1 may confer hepatocellular protection, e.g., in reperfusion injury. Previous reports suggest that intracellular cAMP up-regulates HO-1. The aim of the present study was to assess the role of adrenoceptor agonists as a means to induce HO-1 and to assess molecular mechanisms of HO-1 gene expression by adrenoceptor agonists. Induction of HO-1 in primary cultures of hepatocytes and in rat liver in vivo was assessed by Northern blot, Western blot, and immunohistochemistry. The beta-receptor agonists (+/-)isoproterenol and (-)isoproterenol induced HO-1 in primary cultures of hepatocytes but not the inactive enantiomer (+)isoproterenol. No induction of HO-1 was observed after alpha1, alpha2, beta2, or beta 3 agonists. beta1-Receptor agonists dobutamine and xamoterol induced HO-1 dose dependently, whereas the beta1-receptor antagonist metoprolol attenuated HO-1 induction by beta1-receptor agonists. Furthermore, 8 Br-cAMP and forskolin induced HO-1. Inhibition of protein kinase A (PKA) abolished induction by dobutamine and 8 Br-cAMP. Parallel changes were observed for the transcription factor AP-1. In vivo infusion of dobutamine for 6 h induced HO-1 in rat livers. Immunohistochemical detection of HO-1 revealed a pericentral expression pattern of HO-1 in hepatocytes, i.e., the area at risk for ischemia/reperfusion injury. These results suggest induction of HO-1 by beta1-adrenoceptor agonists via the PKA pathway in hepatocytes, reflecting a potential means for "pharmacological preconditioning."

Quantitative analysis of the formation and diffusion of A1-adenosine receptor-antagonist complexes in single living cells

The A1-adenosine receptor (A1-AR) is a G protein-coupled receptor that mediates many of the physiological effects of adenosine in the brain, heart, kidney, and adipocytes. Currently, ligand interactions with the A1-AR can be quantified on large cell populations only by using radioligand binding. To increase the resolution of these measurements, we have designed and characterized a previously undescribed fluorescent antagonist for the A1-AR, XAC-BY630, based on xanthine amine congener (XAC). This compound has been used to quantify ligand-receptor binding at a single cell level using fluorescence correlation spectroscopy (FCS). XAC-BY630 was a competitive antagonist of A1-AR-mediated inhibition of cAMP accumulation [log10 of the affinity constant (pKb) = 6.7)] and stimulation of inositol phosphate accumulation (pKb = 6.5). Specific binding of XAC-BY630 to cell surface A1-AR could also be visualized in living Chinese hamster ovary (CHO)-A1 cells by using confocal microscopy. FCS analysis of XAC-BY630 binding to the membrane of CHO-A1 cells revealed three components with diffusion times (tauD) of 62 micros (tauD1, free ligand), 17 ms (tauD2, A1-AR-ligand), and 320 ms (tauD3). Confirmation that tauD2 resulted from diffusion of ligand-receptor complexes came from the similar diffusion time observed for the fluorescent A1-AR-Topaz fusion protein (15 ms). Quantification of tauD2 showed that the number of receptor-ligand complexes increased with increasing free ligand concentration and was decreased by the selective A1-AR antagonist, 8-cyclopentyl-1,3-dipropylxanthine. The combination of FCS with XAC-BY630 will be a powerful tool for the characterization of ligand-A1-AR interactions in single living cells in health and disease.

Partial agonist behaviour depends upon the level of nociceptin/orphanin FQ receptor expression: studies using the ecdysone-inducible mammalian expression system

(1) Partial agonism is primarily dependent upon receptor density and coupling efficiency. As these parameters are tissue/model dependent, intrinsic activity in different tissues can vary. We have utilised the ecdysone-inducible expression system containing the human nociceptin/orphanin FQ (N/OFQ) peptide receptor (hNOP) expressed in Chinese hamster ovary cells (CHOINDhNOP) to examine the activity of a range of partial agonists in receptor binding, GTPgamma35S binding and inhibition of adenylyl cyclase studies. (2) Incubation of CHOINDhNOP cells with ponasterone A (PON) induced hNOP expression ([leucyl-3H]N/OFQ binding) of 24, 68, 191 and 1101 fmol mg-1 protein at 1, 2, 5 and 10 microm PON, respectively. At 191 fmol mg-1, protein hNOP pharmacology was identical to that reported for other traditional expression systems. (3) pEC50 values for GTPgamma35S binding ranged from 7.23 to 7.72 (2-10 microm PON) for the partial agonist [Phe1psi(CH2-NH)Gly2]N/OFQ(1-13)-NH2 ([F/G]N/OFQ(1-13)-NH2) and 8.12-8.60 (1-10 microm PON) for N/OFQ(1-13)-NH2 and Emax values (stimulation factor relative to basal) ranged from 1.51 to 3.21 (2-10 microm PON) for [F/G]N/OFQ(1-13)-NH2 and 1.28-6.95 (1-10 microm) for N/OFQ(1-13)-NH2. Intrinsic activity of [F/G]N/OFQ(1-13)-NH2 relative to N/OFQ(1-13)-NH2 was 0.3-0.5. [F/G]N/OFQ(1-13)-NH2 did not stimulate GTPgamma35S binding at 1 microm PON, but competitively antagonised the effects of N/OFQ(1-13)-NH2 with a pKB=7.62. (4) pEC50 values for cAMP inhibition ranged from 8.26 to 8.32 (2-10 microm PON) for [F/G]N/OFQ(1-13)-NH2 and 9.42-10.35 for N/OFQ(1-13)-NH2 and Emax values (% inhibition) ranged from 19.6 to 83.2 for [F/G]N/OFQ(1-13)-NH2 and 40.9-86.0 for N/OFQ(1-13)-NH2. The intrinsic activity of [F/G]N/OFQ(1-13)-NH2 relative to N/OFQ(1-13)-NH2 was 0.48-0.97. (5) In the same cellular environment with receptor density as the only variable, we show that the profile of [F/G]N/OFQ(1-13)-NH2 can be manipulated to encompass full and partial agonism along with antagonism.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

Predicting therapeutic value in the lead optimization phase of drug discovery

Recombinant and natural cellular assays for human G-protein-coupled receptors are used to optimize initial lead molecules obtained from screening. Although the activity of these molecules can be assessed on human genotype receptors, there is increasing evidence that cells impose a phenotypic selectivity to molecules in various cellular backgrounds. This opens the possibility of dissimulations between activity seen in lead optimization assays and the intended therapeutic value in humans. This review discusses the mechanisms by which cells can impose phenotypic selectivity on molecules and approaches to reduce this practical problem for drug discovery.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Differential activation of Gq/11 and Gi(3) proteins at 5-hydroxytryptamine(2C) receptors revealed by antibody capture assays: influence of receptor reserve and relationship to agonist-directed trafficking

As determined by a guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding assay, which does not distinguish G protein subtypes, 5-hydroxytryptamine (5-HT) and 2(S)- 1-(6-chloro-5-fluoro-1H-indol-1-yl)-2-propanamine fumarate (Ro600175) behaved as full agonists at human 5-HT(2C) (h5-HT(2C)) receptors (VSV isoform) stably expressed in Chinese hamster ovary (CHO) cells, whereas 1-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), d-lysergic acid diethylamide (LSD), and lisuride exhibited partial agonist properties. After treatment with pertussis toxin to uncouple 5-HT(2C) receptors from Gi/Go but not Gq/11, DOI and LSD were as efficacious as 5-HT and Ro600175 in stimulating [(35)S]GTPgammaS binding, whereas lisuride still exhibited low efficacy (40%). Correspondingly, in a scintillation proximity assay employing specific antibodies against Gq/11, 5-HT, Ro600175, DOI, and LSD behaved as high-efficacy agonists, whereas lisuride showed efficacy of 36%. In contrast, when employing a specific antibody recognizing Gi(3), DOI and LSD were less efficacious (80 and 30%, respectively) than 5-HT and Ro600175, and lisuride was inactive. Agonist actions were specifically mediated by h5-HT(2C) receptors inasmuch as the selective 5-HT(2C) antagonist SB242,084 blocked [(35)S]GTPgammaS binding at both Gq/11 and Gi(3). Agonist potency for stimulation of Gi(3) was ~6- to 8-fold less than for Gq/11, indicating that the latter was preferentially engaged by h5-HT(2C) receptors. Inactivation of h5-HT(2C) receptors with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline did not modify the efficacy of 5-HT, Ro600175, and DOI at Gq/11, whereas their efficacies were substantially reduced at Gi(3), indicating a greater receptor reserve for the former. Finally, the preferential activation of Gq/11 versus Gi(3) by DOI, LSD, and lisuride was diminished in the presence of lower receptor number. In conclusion, h5-HT(2C) receptors couple to both Gq/11 and Gi(3) in CHO cells, and efficacy for G protein subtype activation is both ligand- and receptor reserve-dependent.

Rapid desensitization of the serotonin(2C) receptor system: effector pathway and agonist dependence

The serotonin(2C) (5-HT(2C)) receptor couples to multiple effector mechanisms, including phospholipase A(2)-mediated arachidonic acid (AA) release and phospholipase C-mediated production of inositol phosphates (IP). Agonist relative efficacy differs depending upon which response (AA release or IP accumulation) is measured. In this study, we investigated the characteristics and agonist dependence of rapid desensitization of 5-HT(2C) receptor-mediated AA release and IP accumulation measured simultaneously from the same cell population. Pretreatment with 5-HT reduced the ability of a maximal concentration of 5-HT to elicit AA release and IP accumulation by about 60%; however, the AA response desensitized more rapidly (t(1/2) = 1.3 min) than the IP response (t(1/2) = 6.9 min). In addition, desensitization of the IP response was more sensitive (occurred at lower receptor occupancy levels) than the AA response. Moreover, in response to submaximal 5-HT concentrations, after an initial transient desensitization, the AA response was enhanced by up to approximately 250%. After maximal desensitization, both responses recovered, but recovery of the AA response was complete and faster than that for IP. Desensitization of both responses was also agonist-dependent, and the capacity of agonists to elicit desensitization was not related to their efficacy to activate signaling. These data suggest that desensitization of the 5-HT(2C) receptor system is both agonist- and effector pathway-dependent and underscore the need to study multiple cellular responses to multiple agonists to understand receptor-mediated signaling systems.

Mouse beta 3a- and beta 3b-adrenoceptors expressed in Chinese hamster ovary cells display identical pharmacology but utilize distinct signalling pathways

1. This study characterizes the mouse beta(3a)-adrenoceptor (AR) and the splice variant of the beta(3)-AR (beta(3b)-AR) expressed in Chinese hamster ovary cells (CHO-K1). 2. Stable clones with high (approximately 1200), medium (approximately 500) or low receptor expression (approximately 100 fmol mg protein(-1)) were determined by saturation binding with [(125)I]-(-)-cyanopindolol. Competition binding studies showed no significant differences in affinity of beta-AR ligands for either receptor. 3. Several functional responses of each receptor were measured, namely extracellular acidification rate (EAR; cytosensor microphysiometer), cyclic AMP accumulation, and Erk1/2 phosphorylation. The beta(3)-AR agonists BRL37344, CL316243, GR265162X, L755507, SB251023, the non-conventional partial beta-AR agonist CGP12177 and the beta-AR agonist (-)-isoprenaline caused concentration-dependent increases in EAR in cells expressing either splice variant. CL316243 caused concentration-dependent increases in cyclic AMP accumulation and Erk1/2 phosphorylation in cells expressing either receptor. 4. PTX treatment increased maximum EAR and cyclic AMP responses to CL316243 in cells expressing the beta(3b)-AR but not in cells expressing the beta(3a)-AR at all levels of receptor expression. 5. CL316243 increased Erk1/2 phosphorylation with pEC(50) values and maximum responses that were not significantly different in cells expressing either splice variant. Erk1/2 phosphorylation was insensitive to PTX or H89 (PKA inhibitor) but was inhibited by LY294002 (PI3K gamma inhibitor), PP2 (c-Src inhibitor), genistein (tyrosine kinase inhibitor) and PD98059 (MEK inhibitor). 6. The adenylate cyclase activators forskolin or cholera toxin failed to increase Erk1/2 levels although both treatments markedly increased cyclic AMP accumulation in both beta(3a)- or beta(3b)-AR transfected cells. 7. These results suggest that in CHO-K1 cells, the beta(3b)-AR, can couple to both G(s) and G(i) to stimulate and inhibit cyclic AMP production respectively, while the beta(3a)-AR, couples solely to G(s) to increase cyclic AMP levels. However, the increase in Erk1/2 phosphorylation following receptor activation is not dependent upon coupling of the receptors to G(i) or the generation of cyclic AMP.

RNA-editing of the 5-HT(2C) receptor alters agonist-receptor-effector coupling specificity

1. The serotonin(2C) (5-HT(2C)) receptor couples to both phospholipase C (PLC)-inositol phosphate (IP) and phospholipase A(2) (PLA(2))-arachidonic acid (AA) signalling cascades. Agonists can differentially activate these effectors (i.e. agonist-directed trafficking of receptor stimulus) perhaps due to agonist-specific receptor conformations which differentially couple to/activate transducer molecules (e.g. G proteins). Since editing of RNA transcripts of the human 5-HT(2C) receptor leads to substitution of amino acids at positions 156, 158 and 160 of the putative second intracellular loop, a region important for G protein coupling, we examined the capacity of agonists to activate both the PLC-IP and PLA(2)-AA pathways in CHO cells stably expressing two major, fully RNA-edited isoforms (5-HT(2C-VSV), 5-HT(2C-VGV)) of the h5-HT(2C) receptor. 2. 5-HT increased AA release and IP accumulation in both 5-HT(2C-VSV) and 5-HT(2C-VGV) expressing cells. As expected, the potency of 5-HT for both RNA-edited isoforms for both responses was 10 fold lower relative to that of the non-edited receptor (5-HT(2C-INI)) when receptors were expressed at similar levels. 3. Consistent with our previous report, the efficacy order of two 5-HT receptor agonists (TFMPP and bufotenin) was reversed for AA release and IP accumulation at the non-edited receptor thus demonstrating agonist trafficking of receptor stimulus. However, with the RNA-edited receptor isoforms there was no difference in the relative efficacies of TFMPP or bufotenin for AA release and IP accumulation suggesting that the capacity for 5-HT(2C) agonists to traffic receptor stimulus is lost as a result of RNA editing. 4. These results suggest an important role for the second intracellular loop in transmitting agonist-specific information to signalling molecules.