The Concise Guide to PHARMACOLOGY 2015/16: G protein‐coupled receptors

The Concise Guide to PHARMACOLOGY 2015/16 provides concise overviews of the key properties of over 1750 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.13348/full. G protein‐coupled receptors are one of the eight major pharmacological targets into which the Guide is divided, with the others being: ligand‐gated ion channels, voltage‐gated ion channels, other ion channels, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The Concise Guide is published in landscape format in order to facilitate comparison of related targets. It is a condensed version of material contemporary to late 2015, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in the previous Guides to Receptors & Channels and the Concise Guide to PHARMACOLOGY 2013/14. It is produced in conjunction with NC‐IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR‐DB and GRAC and provides a permanent, citable, point‐in‐time record that will survive database updates.


Receptors with known ligands
a Numbers in brackets refer to orphan receptors for which an endogenous ligand has been proposed in at least one publication, see [396]; b [1443]; c [1309]; d [1866].
Much of our current understanding of the structure and function of GPCRs is the result of pioneering work on the visual pigment rhodopsin and on the β 2 adrenoceptor, the latter culminating in the award of the 2012 Nobel Prize in chemistry to Robert Lefkowitz Table 1 lists a number of putative GPCRs identified by NC-IUPHAR [530], for which preliminary evidence for an endogenous ligand has been published, or for which there exists a potential link to a disease, or disorder. These GPCRs have recently been reviewed in detail [396]. The GPCRs in Table 1 are all Class A, rhodopsin-like GPCRs. Class A orphan GPCRs not listed in Table 1 are putative GPCRs with as-yet unidentified endogenous ligands.  GPR1  GPR3  GPR4  GPR6  GPR12  GPR15  GPR17  GPR20   GPR22  GPR26  GPR31  GPR34  GPR35  GPR37  GPR39  GPR50   GPR63  GRP65  GPR68  GPR75  GPR84  GPR87  GPR88  GPR132   GPR149  GPR161  GPR183 LGR4 LGR5 LGR6 In addition the orphan receptors GPR18, GPR55 and GPR119 which are reported to respond to endogenous agents analogous to the endogenous cannabinoid ligands have been grouped together (GPR18, GPR55 and GPR119).

Comments
Reported to act as a co-receptor for HIV [1724]. See review [396] for discussion of pairing with chemerin.
An initial report that sphingosine 1-phosphate (S1P) was a high-affinity ligand (EC 50 value of 39nM) [815,1921] was not repeated by arrestin PathHunter[TM] assays [1785,2093]. Reported to activate adenylyl cyclase constitutively through G s and to be located intracellularly [1453]. GPR6-deficient mice showed reduced striatal cyclic AMP production in vitro and selected alterations in instrumental conditioning in vivo. [1134].
Reported to act as a co-receptor for HIV [462]. In an infection-induced colitis model, Gpr15 knockout mice were more prone to tissue damage and inflammatory cytokine expression [945].
Reported to be a dual leukotriene and uridine diphosphate receptor [344]. Another group instead proposed that GPR17 functions as a negative regulator of the CysLT 1 receptor response to leukotriene D 4 (LTD 4 ). For further discussion, see [396]. Reported to antagonize CysLT 1 receptor signalling in vivo and in vitro [1175]. See reviews [250] and [396].  [708]. GPR20 deficient mice exhibit hyperactivity characterised by increased total distance travelled in an open field test [207].
Gpr21 knockout mice were resistant to diet-induced obesity, exhibiting an increase in glucose tolerance and insulin sensitivity, as well as a modest lean phenotype [1448].
Gene disruption results in increased severity of functional decompensation following aortic banding [10]. Identified as a susceptibility locus for osteoarthritis [494,929,1935].
-Has been reported to activate adenylyl cyclase constitutively through G s [880]. Gpr26 knockout mice show increased levels of anxiety and depression-like behaviours [2117].
Rank order of potency --

Comments
Knockdown of Gpr27 reduces endogenous mouse insulin promotor activity and glucose-stimulated insulin secretion [1012].
See [396] for discussion of pairing.
resolvin D1 has been demonstrated to activate GPR32 in two publications [316,1006]. The pairing was not replicated in a recent study based on arrestin recruitment [1785]. GPR32 is a pseudogene in mice and rats. See reviews [250] and [396].
GPR33 is a pseudogene in most individuals, containing a premature stop codon within the coding sequence of the second intracellular loop [1621].
Lysophosphatidylserine has been reported to be a ligand of GPR34 in several publications, but the pairing was not replicated in a recent study based on arrestin recruitment [1785]. Fails to respond to a variety of lipid-derived agents [2093]. Gene disruption results in an enhanced immune response [1102]. Characterization of agonists at this receptor is discussed in [819] and [396].  [1436] but these results were not replicated in an arrestin assay [1785]. The phosphodiesterase inhibitor zaprinast [1863] has become widely used as a surrogate agonist to investigate GPR35 pharmacology and signalling [1863]. GPR35 is also activated by the pharmaceutical adjunct pamoic acid [2124]. See reviews [396] and [429].
Reported to associate and regulate the dopamine transporter [1207] and to be a substrate for parkin [1205]. Gene disruption results in altered striatal signalling [1206]. The peptides prosaptide and prosaposin are proposed as endogenous ligands for GPR37 and GPR37L1 [1264].
The peptides prosaptide and prosaposin are proposed as endogenous ligands for GPR37 and GPR37L1 [1264].
Zn 2+ has been reported to be a potent and efficacious agonist of human, mouse and rat GPR39 [2089]. obestatin (GHRL, Q9UBU3), a fragment from the ghrelin precursor, was reported initially as an endogenous ligand, but subsequent studies failed to reproduce these findings. GPR39 has been reported to be down-regulated in adipose tissue in obesity-related diabetes [273]. Gene disruption results in obesity and altered adipocyte metabolism [1497]. Reviewed in [396]. - Comments -GPR50 is structurally related to MT 1 and MT 2 melatonin receptors, with which it heterodimerises constitutively and specifically [1089]. Gpr50 knockout mice display abnormal thermoregulation and are much more likely than wild-type mice to enter fasting-induced torpor [111].
GPR4, GPR65, GPR68 and GPR132 are now thought to function as proton-sensing receptors detecting acidic pH [396,1704]. Reported to activate adenylyl cyclase; gene disruption leads to reduced eosinophilia in models of allergic airway disease [1000].
CCL5 (CCL5, P13501) was reported to be an agonist of GPR75 [816], but the pairing could not be repeated in an arrestin assay [1785].
-Mice with Gpr82 knockout have a lower body weight and body fat content associated with reduced food intake, decreased serum triglyceride levels, as well as higher insulin sensitivity and glucose tolerance [479].
One isoform has been implicated in the induction of CD4(+) CD25(+) regulatory T cells (Tregs) during inflammatory immune responses [696]. The extracellular N-terminal domain is reported as an intramolecular inverse agonist [1352].
-Small molecule agonists have been reported [1890,2106]. -Gpr149 knockout mice displayed increased fertility and enhanced ovulation, with increased levels of FSH receptor and cyclin D2 mRNA levels [463].
-GPR151 responded to galanin with an EC 50 value of 2 M, suggesting that the endogenous ligand shares structural features with galanin (GAL, P22466) [813].
Comments ---A C-terminal truncation (deletion) mutation in Gpr161 causes congenital cataracts and neural tube defects in the vacuolated lens (vl) mouse mutant [1226]. The mutated receptor is associated with cataract, spina bifida and white belly spot phenotypes in mice [994]. Gene disruption is associated with a failure of asymmetric embryonic development in zebrafish [1085].
Endogenous agonists --lysophosphatidylserine (pEC 50 7.1) [825] --Comments GPR171 has been shown to be activated by the endogenous peptide BigLEN {Mouse}. This receptor-peptide interaction is believed to be involved in regulating feeding and metabolism responses [621].
-See [819] which discusses characterization of agonists at this receptor.
-Rat GPR182 was first proposed as the adrenomedullin receptor [904]. However, it was later reported that rat and human GPR182 did not respond to adrenomedullin [927] and GPR182 is not currently considered to be a genuine adrenomedullin receptor [722].
See reviews [396] and [1779].  1739]. Reports that bovine adrenal medulla peptide 8-22 (PENK, P01210) was the most potent of a series of proenkephalin A-derived peptides as an agonist of MRGPRX1 in assays of calcium mobilisation and radioligand binding [1080] were replicated in an independent study using an arrestin recruitment assay [1785]. See reviews [396] and [1779].
A diverse range of substances has been reported to be agonists of MRGPRX2, with cortistatin 14 the highest potency agonist in assays of calcium mobilisation [1594], also confirmed in an independent study using an arrestin recruitment assay [1785]. See reviews [396] and [1779].
TAAR3 is thought to be a pseudogene in man though functional in rodents [396].
Pseudogene in man but functional in rodents [396].
--TAAR9 appears to be functional in most individuals but has a polymorphic premature stop codon at amino acid 61 (rs2842899) with an allele frequency of 10-30% in different populations [1944]. Overview: Whilst the taste of acid and salty foods appear to be sensed by regulation of ion channel activity, bitter, sweet and umami tastes are sensed by specialised GPCR. Two classes of taste GPCR have been identified, T1R and T2R, which are similar in sequence and structure to Class C and Class A GPCR, respectively. Activation of taste receptors appears to involve gustducin-(Gαt3) and Gα14mediated signalling, although the precise mechanisms remain obscure. Gene disruption studies suggest the involvement of PLCβ2 [2122], TRPM5 [2122] and IP3 [764] receptors in post-receptor signalling of taste receptors. Although predominantly associated with the oral cavity, taste receptors are also located elsewhere, including further down the gastrointestinal system, in the lungs and in the brain.

Bitter
The composition and stoichiometry of bitter taste receptors is not yet established. Bitter receptors appear to separate into two groups, with very restricted ligand specificity or much broader responsiveness. For example, T2R5 responded to cycloheximide, but not 10 other bitter compounds [287], while T2R14 responded to at least eight different bitter tastants, including (-)-α-thujone and picrotoxinin [119].
Specialist database BitterDB contains additional information on bitter compounds and receptors [2023].
[ 3 H]prazosin (0.25 nM) and [ 125 I]HEAT (0.1 nM; also known as BE2254) are relatively selective radioligands. The α 1A -adrenoceptor antagonist S(+)-niguldipine also has high affinity for L-type Ca 2+ channels. The conotoxin rho-TIA acts as a negative allosteric modulator at the α 1B -adrenoceptor [1716], while the snake toxin -Da1a acts as a selective non-competitive antagonist at the α 1Aadrenoceptor [1236,1548]. Fluorescent derivatives of prazosin (Bodipy PL-prazosin-QAPB) are increasingly used to examine cellular localisation of α 1 -adrenoceptors. The vasoconstrictor effects of selective α1-adrenoceptor agonists have led to their use as nasal decongestants; antagonists are used to treat hypertension (doxazosin, prazosin) and benign prostatic hyperplasia (alfuzosin, tamsulosin). The combined α 1 -and β 2 -adrenoceptor antagonist carvedilol is widely used to treat congestive heart failure, although the contribution of α 1 -adrenoceptor blockade to the therapeutic effect is unclear. Several anti-depressants and anti-psychotic drugs possess α 1 -adrenoceptor blocking properties that are believed to contribute to side effects such as orthostatic hypotension and extrapyramidal effects.

Comments
The agonists indicated have less than two orders of magnitude selectivity [81].
Signalling is predominantly via G q/11 but α 1 -adrenoceptors also couple to G i/o , G s and G 12/13 . Several ligands activating α 1Aadrenoceptors display ligand directed signalling bias relative to noradrenaline. For example, oxymetazoline is a full agonist for extracellular acidification rate (ECAR) and a partial agonist for Ca 2+ release but does not stimulate cAMP production. Phenylephrine is biased toward ECAR versus Ca 2+ release or cAMP accumulation but not between Ca 2+ release and cAMP accumulation [495]. There are also differences between subtypes in coupling efficiency to different pathwayse.g. in some systems coupling efficiency to Ca 2+ signalling is α 1A α 1B α 1D , but for MAP kinase signalling is α 1D α 1A α 1B . In vascular smooth muscle, the potency of agonists is related to the predominant subtype, α 1D -conveying greater agonist sensitivity than α 1A -adrenoceptors [526].
Adrenoceptors, α 2 ARC-239 (pK i 8.0) and prazosin (pK i 7.5) show selectivity for α 2Band α 2C -adrenoceptors over α 2A -adrenoceptors.Oxymetazoline is a reduced efficacy agonist and is one of many α 2 -adrenoceptor agonists that are imidazolines or closely related compounds. Other binding sites for imidazolines, distinct from α 2 -adrenoceptors, and structurally distinct from the 7TM adrenoceptors, have been identified and classified as I 1 , I 2 and I 3 sites [390]; catecholamines have a low affinity, while rilmenidine and moxonidine are selective ligands for these sites, evoking hypotensive effects in vivo. I 1 -imidazoline receptors are involved in central inhibition of sympathetic tone, I 2 -imidazoline receptors are an allosteric binding site on monoamine oxidase B, and I 3 -imidazoline receptors regulate insulin secretion from pancreatic β-cells.
α 2A -adrenoceptor stimulation reduces insulin secretion from β-islets [2083], with a polymorphism in the 5'-UTR of the ADRA2A gene being associated with increased receptor expression in β-islets and heightened susceptibility to diabetes [1599].
α 2A -and α 2C -adrenoceptors form homodimers [1758]. Heterodimers between α 2A -and either the α 2c -adrenoceptor or opioid peptide receptor exhibit altered signalling and trafficking properties compared to the individual receptors [1758,1858,1956] [810]. There are 3 β-adrenoceptors in turkey (termed the tβ, tβ3c and tβ4c) that have a pharmacology that differs from the human β-adrenoceptors [82]. Numerous polymorphisms have been described for the three β-adrenoceptors; some are associated with alterations in agonistevoked signalling and trafficking, altered susceptibility to disease and/or altered responses to pharmacotherapy [1103].
All β-adrenoceptors couple to G s (activating adenylyl cyclase and elevating cAMP levels), but it is also clear that they activate other G proteins such as G i and many other G protein-independent signalling pathways, including arrestin-mediated signalling, which may in turn lead to activation of MAP kinases. Many antagonists at β 1and β 2 -adrenoceptors are agonists at β 3 -adrenoceptors (CL316243, CGP 12177 and carazolol). Many 'antagonists' that block agoniststimulated cAMP accumulation, for example carvedilol and bucindolol, are able to activate MAP kinase pathways [85,497,559,560,1649,1650] and thus display 'protean agonism'. Bupranolol appears to act as a neutral antagonist in most systems so far examined. Agonists also display biased signalling at the β 2 -adrenoceptor via G s or arrestins [443].
The X-ray crystal structures have been described of the agonist bound [1988] and antagonist bound forms of the β 1 - [1989], agonist-bound [313] and antagonist-bound forms of the β 2adrenoceptor [1561,1598], as well as a fully active agonist-bound, G s protein-coupled β 2 -adrenoceptor [1562]. Carvedilol and bucindolol bind to an extended site of the β 1 -adrenoceptor involving contacts in TM2, 3, and 7 and extracellular loop 2 that may facilitate coupling to arrestins [1989]. Compounds displaying arrestin-biased signalling at the β 2 -adrenoceptor also have a greater effect on the conformation of TM7, whereas full agonists for G s coupling promote movement of TM5 and TM6 [1127]. Recent studies using NMR spectroscopy have demonstrated significant conformational flexibility in the β 2 -adrenoceptor which is stabilized by both agonist and G proteins highlighting the dynamic nature of interactions with both ligand and downstream signalling partners [946,1199,1413]. Such flexibility will likely have consequences for our understanding of biased agonism, and for the future therapeutic exploitation of this phenomenon.  Comments: AT 1 receptors are predominantly coupled to Gq /11 , however they are also linked to arrestin recruitment and stimulate G protein-independent arrestin signalling [1156]. Most species express a single AGTR1 gene, but two related agtr1a and agtr1b receptor genes are expressed in rodents. The AT 2 receptor counteracts several of the growth responses initiated by the AT 1 receptors. The AT 2 receptor is much less abundant than the AT 1 receptor in adult tissues and is upregulated in pathological conditions. AT 1 receptor antagonists bearing substituted 4-phenylquinoline moieties have been synthesized, which bind to AT 1 receptors with nanomolar affinity and are slightly more potent than losartan in functional studies [264].
The antagonist activity of CGP42112 at the AT 2 receptor has also been reported [2147].
The AT 1 and bradykinin B2 receptors have been proposed to form a heterodimeric complex [3].
There is also evidence for an AT 4 receptor that specifically binds angiotensin IV (AGT, P01019) and is located in the brain and kidney. An additional putative endogenous ligand for the AT 4 receptor has been described (LVV-hemorphin (HBB, P68871), a globin decapeptide) [1296].  [1864]. A second family of peptides discovered independently and named Elabela [323] or Toddler, that has little sequence similarity to apelin, has been proposed as a second endogenous apelin receptor ligand [1475]. Comments: Potency order determined for heterologously expressed human apelin receptor (pD 2 values range from 9.5 to 8.6). The apelin receptor may also act as a co-receptor with CD4 for isolates of human immunodeficiency virus, with apelin blocking this function [279]. A modified apelin-13 peptide, apelin-13(F13A) was reported to block the hypotensive response to apelin in rat in vivo [1067], however, this peptide exhibits agonist activity in HEK293 cells stably expressing the recombinant apelin receptor [503].

Further Reading
Chandrasekaran  Comments: The triterpenoid natural product betulinic acid has also been reported to inhibit inflammatory signalling through the NF B pathway [1842]. Disruption of GPBA expression is reported to protect from cholesterol gallstone formation [1951]. A new series of 5-phenoxy-1,3-dimethyl-1H-pyrazole-4-carboxamides have been reported as highly potent agonists [1138].

Further Reading
Duboc H et al.

Overview:
Bombesin receptors (nomenclature recommended by the NC-IUPHAR Subcommittee on bombesin receptors, [857]) are activated by the endogenous ligands gastrin-releasing peptide (GRP, P07492) (GRP), neuromedin B (NMB, P08949) (NMB) and GRP-(18-27) (GRP, P07492) (previously named neuromedin C). Bombesin is a tetradecapeptide, originally derived from amphibians, and is an agonist at BB 1 and BB 2 receptors. These receptors couple primarily to the G q/11 family of G proteins (but see also [857]). Each of these receptors is widely distributed in the CNS and peripheral tissues [625,857,1556,1642]. Activation of BB 1 and BB 2 receptors causes a wide range of physiological actions, including the stimulation of normal and neoplastic tissue growth, smooth-muscle contraction, appetite and feeding behavior, secretion and many central nervous system effects [857,858,859,1185,1317,1556]. A physiological role for the BB 3 receptor has yet to be fully defined although recently studies using receptor knockout mice and newly described agonists/antagonists suggest an important role in glucose and insulin regulation, metabolic homeostasis, feeding and other CNS behaviors and growth of normal/neoplastic tissues [625,1186,1430].  There are species differences in peptide se-quences, particularly for the CTs. CTR-stimulating peptide {Pig} (CRSP) is another member of the family with selectivity for the CTR but it is not expressed in humans [907]. Olcegepant (also known as BIBN4096BS, pKi 10.5) and telcagepant (also known as MK0974, pKi 9) are the most selective antagonists available, having a high selectivity for CGRP receptors, with a particular preference for those of primate origin.
CLR by itself binds no known endogenous ligand, but in the presence of RAMPs it gives receptors for CGRP, adrenomedullin and adrenomedullin 2/intermedin. Comments: It is important to note that a complication with the interpretation of pharmacological studies with AMY receptors in transfected cells is that most of this work has likely used a mixed population of receptors, encompassing RAMP-coupled CTR as well as CTR alone. This means that although in binding assays human calcitonin (CALCA, P01258) has low affinity for 125 I-AMY binding sites, cells transfected with CTR and RAMPs can display potent CT functional responses. Transfection of human CTR with any RAMP can generate receptors with a high affinity for both salmon CT and AMY and varying affinity for different antagonists [337,718,719]. The major human CTR splice variant (hCT (a) , which does not contain an insert) with RAMP1 (i.e. the AMY 1(a) receptor) has a high affinity for CGRP, unlike hCT (a) -RAMP3 (i.e. AMY 3(a) receptor) [337,718]. However, the AMY receptor phenotype is RAMP-type, splice variant and cell-line-dependent [1886]. In particular, CGRP is a more potent agonist than amylin (IAPP, P10997) at increasing cAMP at the delta 47 hCT(a) receptor, when transfected with RAMP1 (to give the corresponding AMY1(a) receptor) in Cos 7 cells [1543].
The ligands described represent the best available but their selectivity is limited. For example, adrenomedullin has appreciable affinity for CGRP receptors. CGRP can show significant cross-reactivity at AMY receptors and AM 2 receptors. Adrenomedullin 2/intermedin also has high affinity for the AM 2 receptor [779]. CGRP-(8-37) acts as an antagonist of CGRP (pK i 8) and inhibits some AM and AMY responses (pK i 6-7). It is weak at CT receptors. Salmon CT-(8-32) is an antagonist at both AMY and CT receptors. AC187, a salmon CT analogue, is also an antagonist at AMY and CT receptors. Human AM-  has some selectivity towards AM receptors, but with modest potency (pK i 7), limiting its use [720]. AM-(22-52) is slightly more effective at AM 1 than AM 2 receptors but this difference is not sufficient for this peptide to be a useful discriminator of the AM receptor subtypes. Olcegepant shows the greatest selectivity between receptors but still has significant affinity for AMY 1 receptors [1973].
Ligand responsiveness at CT and AMY receptors can be affected by receptor splice variation and can depend on the pathway being measured. Particularly for AMY receptors, relative potency can vary with the type and level of RAMP present and can be influenced by other factors such as G proteins [1324,1886].
G s is a prominent route for effector coupling for CLR and CTR but other pathways (e.g. Ca 2+ , ERK, Akt), and G proteins can be activated [1972]. There is evidence that CGRP-RCP (a 148 amino-acid hydrophilic protein, ASL (P04424) is important for the coupling of CLR to adenylyl cyclase [498].
[ 125 I]-Salmon CT is the most common radioligand for CT receptors but it has high affinity for AMY receptors and is also poorly reversible.
[ 125 I]-Tyr 0 -CGRP is widely used as a radioligand for CGRP receptors.
Some early literature distinguished between CGRP 1 and CGRP 2 receptors. It is now clear that the complex of CALCRL and RAMP1 represents the CGRP 1 subtype and is now known simply as the CGRP receptor [721]. The CGRP 2 receptor is now considered to have arisen from the actions of CGRP at AM 2 and AMY receptors. This term should not be used [721].

Calcium-sensing receptors G protein-coupled receptors Calcium-sensing receptors
Overview: The calcium-sensing receptor (CaS, provisional nomenclature as recommended by NC-IUPHAR [530]) responds to extracellular calcium and magnesium in the millimolar range and to gadolinium and some polycations in the micromolar range [229]. The sensitivity of CaS to primary agonists can be increased by aromatic L-amino acids [362] and also by elevated extracellular pH [1544] or decreased extracellular ionic strength [1545]. This receptor bears no sequence or structural relation to the plant calcium receptor, also called CaS.

Nomenclature
CaS receptor GPRC6 receptor Amino-acid rank order of potency L-phenylalanine, L-tryptophan, L-histidine L-alanine L-serine, L-proline, L-glutamic acid L-aspartic acid (not L-lysine, L-arginine, L-leucine and L-isoleucine) [ -Comments 2-benzylpyrrolidine derivatives of NPS 2143 are also negative allosteric modulators of the calcium sensing receptor [2087]. etelcalcetide is a novel peptide agonist of the receptor [1975]. GPRC6 is a related G q -coupled receptor which responds to basic amino acids [2004].  [1756,1783]). Anandamide is also an agonist at vanilloid receptors (TRPV1) and PPARs [1418,2135]. There is evidence for an allosteric site on the CB 1 receptor [1532]. All of the compounds listed as antagonists behave as inverse agonists in some bioassay systems [1494]. Moreover, GPR18, GPR55 and GPR119, although showing little structural similarity to CB 1 and CB 2 receptors, respond to endogenous agents that are structurally similar to the endogenous cannabinoid ligands [1494]. ) comprise a large subfamily of 7TM proteins that bind one or more chemokines, a large family of small cytokines typically possessing chemotactic activity for leukocytes. Chemokine re-ceptors can be divided by function into two main groups: G proteincoupled chemokine receptors, which mediate leukocyte trafficking, and "Atypical chemokine receptors", which may signal through non-G protein-coupled mechanisms and act as chemokine scavengers to downregulate inflammation or shape chemokine gradients [78].
Chemokines in turn can be divided by structure into four subclasses by the number and arrangement of conserved cysteines. CC (also known as β-chemokines; n= 28), CXC (also known as α-chemokines; n= 17) and CX3C (n= 1) chemokines all have four conserved cysteines, with zero, one and three amino acids separating the first two cysteines respectively. C chemokines (n= 2) have only the second and fourth cysteines found in other chemokines. Chemokines can also be classified by function into homeostatic and inflammatory subgroups. Most chemokine receptors are able to bind multiple high-affinity chemokine ligands, but the ligands for a given receptor are almost always restricted to the same structural subclass. Most chemokines bind to more than one receptor subtype. Receptors for inflammatory chemokines are typically highly promiscuous with regard to ligand specificity, and may lack a selective endogenous ligand. G protein-coupled chemokine receptors are named acccording to the class of chemokines bound, whereas ACKR is the root acronym for atypical chemokine receptors [79]. Listed are those human agonists with EC 50 values 50 nM in either Ca 2+ flux or chemotaxis assays at human recombinant G protein-coupled chemokine receptors expressed in mammalian cell lines. There can be substantial cross-species differences in the sequences of both chemokines and chemokine receptors, and in the pharmacology and biology of chemokine recep-tors. Endogenous and microbial non-chemokine ligands have also been identified for chemokine receptors. Many chemokine receptors function as HIV co-receptors, but CCR5 is the only one demonstrated to play an essential role in HIV/AIDS pathogenesis. The tables include both standard chemokine receptor names [2101] and the most commonly used aliases. Numerical data quoted are typically pK i or pIC 50 values from radioligand binding to heterologously expressed receptors.   There are only two distinct subtypes of CCK receptors, CCK 1 and CCK 2 receptors [992,1986], with some alternatively spliced forms most often identified in neoplastic cells. The CCK receptor subtypes are distinguished by their peptide selectivity, with the CCK 1 receptor requiring the carboxyl-terminal heptapeptide-amide that includes a sulfated tyrosine for high affinity and potency, while the CCK 2 receptor requires only the carboxyl-terminal tetrapeptide shared by both CCK and gastrin peptides. These receptors have characteristic and distinct distributions, with both present in both the central nervous system and peripheral tissues.  Comments: While a cancer-specific CCK receptor has been postulated to exist, which also might be responsive to incompletely processed forms of CCK (Gly-extended forms), this has never been isolated. An alternatively spliced form of the CCK 2 receptor in which intron 4 is retained, adding 69 amino acids to the intracellular loop 3 (ICL3) region, has been described to be present particularly in certain neoplasms where mRNA mis-splicing has been commonly observed [1764], but it is not clear that this receptor splice form plays a special role in carcinogenesis. Another alternative splicing event for the CCK 2 receptor was reported [1782], with alternative donor sites in exon 4 resulting in long (452 amino acids) and short (447 amino acids) forms of the receptor differing by five residues in ICL3, however, no clear functional differences have been observed. with the low density lipoprotein receptors LRP5 (O75197) and LRP6 (O75581), lead to the inhibition of a constitutively active destruction complex, which results in the accumulation of β-catenin and subsequently its translocation to the nucleus. β-Catenin, in turn, modifies gene transcription by interacting with TCF/LEF transcription factors. β-Catenin-independent FZD signalling is far more complex with regard to the diversity of the activated pathways. WNT/FZD signalling can lead to the activation of pertussis toxin-sensitive heterotrimeric G proteins [939], the elevation of intracellular calcium [1757], activation of cGMP-specific PDE6 [17] and elevation of cAMP as well as RAC-1, JNK, Rho and Rho kinase signalling [695]. Fur-thermore, the phosphoprotein Disheveled constitutes a key player in WNT/FZD signalling. As with other GPCRs, members of the Frizzled family are functionally dependent on the arrestin scaffolding protein for internalization [306], as well as for β-catenin-dependent [235] and -independent [236,940] signalling. The pattern of cell signalling is complicated by the presence of additional ligands, which can enhance or inhibit FZD signalling (secreted Frizzled-related proteins (sFRP), Wnt-inhibitory factor (WIF1, Q9Y5W5) (WIF), sclerostin (SOST, Q9BQB4) or Dickkopf (DKK)), as well as modulatory (co)receptors with Ryk, ROR1, ROR2 and Kremen, which may also function as independent signalling proteins.  [1979] Selective antagonists vismodegib (pK i 7.8) [1979] Comments: There is limited knowledge about WNT/FZD specificity and which molecular entities determine the signalling outcome of a specific WNT/FZD pair. Understanding of the coupling to G proteins is incomplete (see [423]). There is also a scarcity of information on basic pharmacological characteristics of FZDs, such as binding constants, ligand specificity or concentration-response relationships [937].  Comments: SB290157 has also been reported to have agonist properties at the C3a receptor [1218]. The putative chemoattractant receptor termed C5a 2 (also known as GPR77, C5L2) binds [ 125 I]C5a

Extracellular proteins that interact with WNTs or LRPs
with no clear signalling function, but has a putative role opposing inflammatory responses [257,568,585]. Binding to this site may be displaced with the rank order C5a des-Arg (C5) C5a (C5, P01031) [257,1440] while there is controversy over the ability of C3a (C3, P01024) and C3a des Arg (C3, P01024) to compete [778,894,895,1440]. C5a 2 appears to lack G protein signalling and has been termed a decoy receptor [1684]. However, C5a 2 does recruit arrestin after ligand binding, which might provide a signaling pathway for this receptor [89,1937], and forms heteromers with C5a 1 . C5a, but not C5a-des Arg, induces upregulation of heteromer formation between complement C5a receptors C5aR and C5L2 [380]. There are also reports of pro-inflammatory activity of C5a 2 , mediated by HMGB1, but the signaling pathway that underlies this is currently unclear (reviewed in [1095]).

Further Reading
Hajishengallis G. (2010)    Comments: A CRF binding protein has been identified (CRHBP, P24387) to which both corticotrophin-releasing hormone (CRH, P06850) and urocortin 1 (UCN, P55089) bind with high affinities, which has been suggested to bind and inactivate circulating corticotrophin-releasing hormone (CRH, P06850) [1489].    Comments: Splice variants of the ET A receptor have been identified in rat pituitary cells; one of these, ET A R-C13, appeared to show loss of function with comparable plasma membrane expression to wild type receptor [713]. Subtypes of the ET B receptor have been proposed, although gene disruption studies in mice suggest that only a single gene product exists [1295]. Comments: Antagonists at the nuclear estrogen receptor, such as fulvestrant and tamoxifen [515], as well as the flavonoid 'phytoestrogens' genistein and quercetin [1177], are agonists at GPER receptors.
The agonist activity of the lipid mediators described has been questioned [697,1513], which may derive from batch-to-batch differences, partial agonism or biased agonism. Recent results from Cooray et al. (2013) [365] have addressed this issue and the role of homodimers and heterodimers in the intracellular signaling .
-Comments: Note that the data for FPR2/ALX are also reproduced on the leukotriene receptor page.   [1802]. GW1100 is also an oxytocin receptor antagonist [217]. TUG-770 and GW9508 are both approximately 100 fold selective for FFA1 over FFA4 [217,332]. AMG-837 and the related analogue AM6331 have been suggested to have an allosteric mechanism of action at FFA1, with respect to the orthosteric fatty acid binding site [1110, 2064].
-Beta-hydroxybutyrate has been reported to antagonise FFA3 responses to short chain fatty acids [951]. A range of FFA3 selective molecules with agonist and antagonist properties, but which bind at sites distinct from the short chain fatty acid binding site, have recently been described [799].
-Comments: Short (361 amino acids) and long (377 amino acids) splice variants of human FFA4 have been reported [1318], which differ by a 16 amino acid insertion in intracellular loop 3, and exhibit differences in intracellular signalling properties in recombinant sys-tems [1996]. The long FFA4 splice variant has not been identified in other primates or rodents to date [757,1318].
GPR42 was originally described as a pseudogene within the family (ENSFM00250000002583), but the recent discovery of several poly-morphisms suggests that some versions of GPR42 may be functional [1101]. GPR84 is a structurally-unrelated G protein-coupled receptor which has been found to respond to medium chain fatty acids [1981].

Further Reading
Mancini AD et al.  [194,1507]) are formed from the heterodimerization of two similar 7TM subunits termed GABA B1 and GABA B2 [194,478,1506,1507,1926]. GABA B receptors are widespread in the CNS and regulate both pre-and postsynaptic activity. The GABA B1 subunit, when expressed alone, binds both antagonists and agonists, but the affinity of the latter is generally 10-100-fold less than for the native receptor. The GABA B1 subunit when expressed alone is not transported to the cell membrane and is non-functional. However, Richer et al.. (2008) report that GABA B1 alone can control ERK/MAPK pathway activity [1585]. Co-expression of GABA B1 and GABA B2 subunits allows transport of GABA B1 to the cell surface and generates a functional receptor that can couple to signal transduction pathways such as high-voltage-activated Ca 2+ channels (Ca v 2.1, Ca v 2.2), or inwardly rectifying potassium channels (Kir3) [144,194,195]. The GABA B2 subunit also determines the rate of internalisation of the dimeric GABA B receptor [693]. The GABA B1 subunit har-bours the GABA (orthosteric)-binding site within an extracellular domain (ECD) venus flytrap module (VTM), whereas the GABA B2 subunit mediates G protein-coupled signalling [194,591,592,1506]. The two subunits interact by direct allosteric coupling [1313], such that GABA B2 increases the affinity of GABA B1 for agonists and reciprocally GABA B1 facilitates the coupling of GABA B2 to G proteins [591,1013,1506]. GABA B1 and GABA B2 subunits assemble in a 1:1 stoichiometry by means of a coiled-coil interaction between αhelices within their carboxy-termini that masks an endoplasmic reticulum retention motif (RXRR) within the GABA B1 subunit but other domains of the proteins also contribute to their heteromerization [144,243,1506]. Recent evidence indicates that higher order assemblies of GABA B receptor comprising dimers of heterodimers occur in recombinant expression systems and in vivo and that such complexes exhibit negative functional cooperativity between heterodimers [361,1505]. Adding further complexity, KCTD (potassium channel tetramerization proteins) 8, 12, 12b and 16 associate as tetramers with the carboxy terminus of the GABA B2 subunit to impart altered signalling kinetics and agonist potency to the receptor complex [102, 1680,1914] and reviewed by [1508]. Four isoforms of the human GABA B1 subunit have been cloned. The predominant GABA B1(a) and GABA B1(b) isoforms, which are most prevalent in neonatal and adult brain tissue respectively, differ in their ECD sequences as a result of the use of alternative transcription initiation sites. GABA B1(a) -containing heterodimers localise to distal axons and mediate inhibition of glutamate release in the CA3-CA1 terminals, and GABA release onto the layer 5 pyramidal neurons, whereas GABA B1(b) -containing receptors occur within dendritic spines and mediate slow postsynaptic inhibition [1541,1955]. Isoforms generated by alternative splicing are GABA B1(c) that differs in the ECD, and GABA B1(e) , which is a truncated protein that can heterodimerize with the GABA B2 subunit but does not constitute a functional receptor. Only the 1a and 1b variants are identified as components of native receptors [194]. Additional GABA B1 subunit isoforms have been described in rodents and humans [1065] and reviewed by [144].  1932]. A similar phenotype has been found for GABA B2 / BALB/c mice [582].

Comments:
The glucagon receptor has been reported to interact with receptor activity modifying proteins (RAMPs), specifically RAMP2, in heterologous expression systems [333], although the physiological significance of this has yet to be established.

Comments
Animal follitropins are less potent than the human hormone as agonists at the human FSH receptor. Gainand loss-of-function mutations of the FSH receptor are associated with human reproductive disorders [19,109,650,1900]. The rat FSH receptor also stimulates phosphoinositide turnover through an unidentified G protein [1547].
Loss-of-function mutations of the LH receptor are associated with Leydig cell hypoplasia and gain-of-function mutations are associated with male-limited gonadotropin-independent precocious puberty (e.g. [1044,1720]) and Leydig cell tumours [1126].
Autoimmune antibodies that act as agonists of the TSH receptor are found in patients with Graves' disease (e.g. [1558]). Mutants of the TSH receptor exhibiting constitutive activity underlie hyperfunctioning thyroid adenomas [1464] and congenital hyperthyroidism [993]. TSH receptor loss-of-function mutations are associated with TSH resistance [1824].

Further Reading
Chiamolera MI et al.

Gonadotrophin-releasing hormone receptors G protein-coupled receptors
Gonadotrophin-releasing hormone receptors Overview: GnRH 1 and GnRH 2 receptors (provisonal nomenclature [530], also called Type I and Type II GnRH receptor, respectively [1284]) have been cloned from numerous species, most of which express two or three types of GnRH receptor [1283,1284,1741]. GnRH I (GNRH1, P01148) (p-Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) is a hypothalamic decapeptide also known as luteinizing hormone-releasing hormone, gonadoliberin, luliberin, gonadorelin or simply as GnRH. It is a member of a family of similar peptides found in many species [1283,1284,1741] including GnRH II (GNRH2, O43555) (pGlu-His-Trp-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH 2 (which is also known as chicken GnRH-II). Receptors for three forms of GnRH exist in some species but only GnRH I and GnRH II and their cognate receptors have been found in mammals [1283,1284,1741]. GnRH 1 receptors are expressed primarily by pituitary gonadotrophs, and mediate central control of mammalian reproduction. They are selectively activated by GnRH I and all lack the COOH-terminal tails found in other GPCRs. GnRH 2 receptors do have COOH-terminal tails and (where tested) are selective for GnRH II over GnRH I. GnRH 2 receptors are expressed by some primates but are thought not to be expressed by humans because the human GN-RHR2 gene contains a frame shift and an internal stop codon [1325]. An alternative phylogenetic classification divides GnRH receptors into three classes and includes both GnRH I-selective mammalian and GnRH II-selective non-mammalian GnRH receptors as GnRH 1 receptors [1284]. A more recent phylogenetic classification groups vertebrate GnRH receptors into five subfamilies [2028] and highlights examples of gene loss through evolution, with humans notably retaining only one ancient gene. Although thousands of peptide analogues of GnRH I have been synthesized and several (agonists and antagonists) are used therapeutically [934], the potency of most of these peptides at GnRH 2 receptors is unknown.  [1284].
Comments: GnRH 1 and GnRH 2 receptors couple primarily to G q/11 [653] but coupling to G s and G i is evident in some systems [1009,1024]. GnRH  receptor and deficiency of GnRH I (GNRH1, P01148) are associated with hypogonadotropic hypogonadism although some 'loss of function' mutations may actually prevent trafficking of 'functional' GnRH 1 receptors to the cell surface, as evidenced by recovery of function by nonpeptide antagonists [1061]. Human GnRH 1 receptors appear to be poorly expressed at the cell surface because of failure to meet structural quality control criteria for endoplasmic reticu-lum exit [517,1061]. This may increase susceptibility to point mutations that further impair trafficking and also increase effects of nonpeptide antagonists on GnRH 1 receptor trafficking to the plasma membrane [517,1061]. GnRH receptor signalling may be dependent upon receptor oligomerisation [363,1007].

GPR18, GPR55 and GPR119
G protein-coupled receptors GPR18, GPR55 and GPR119 Overview: GPR18, GPR55 and GPR119 (provisional nomenclature), although showing little structural similarity to CB 1 and CB 2 cannabinoid receptors, respond to endogenous agents analogous to the endogenous cannabinoid ligands, as well as some natural/synthetic cannabinoid receptor ligands [1494]. Although there are multiple reports to indicate that GPR18, GPR55 and GPR119 can be activated in vitro by N-arachidonoylglycine, lysophosphatidylinositol and N-oleoylethanolamide, respectively, there is a lack of evidence for activation by these lipid messengers in vivo. As such, therefore, these receptors retain their orphan status.

Comments
The pairing of N-arachidonoylglycine with GPR18 was not replicated in two studies based on arrestin assays [1785,2093]. See [396] for discussion.
See reviews [396] and [1732]. In addition to those shown above, further small molecule agonists have been reported [687]. Comments: GPR18 failed to respond to a variety of lipidderived agents in an in vitro screen [2093], but has been reported to be activated by ½ 9 -tetrahydrocannabinol [1246]. GPR55 responds to AM251 and rimonabant at micromolar concentrations, compared to their nanomolar affinity as CB 1 re-ceptor antagonists/inverse agonists [1494]. It has been reported that lysophosphatidylinositol acts at other sites in addition to GPR55 [2075]. N-Arachidonoylserine has been suggested to act as a low efficacy agonist/antagonist at GPR18 in vitro [1244]. It has also been suggested oleoyl-lysophosphatidylcholine acts, at least in part, through GPR119 [1400]. Although PSN375963 and PSN632408 produce GPR119-dependent responses in heterologous expression systems, comparison with N-oleoylethanolamidemediated responses suggests additional mechanisms of action [1400].

Hydroxycarboxylic acid receptors G protein-coupled receptors Hydroxycarboxylic acid receptors
Overview: The hydroxycarboxylic acid family of receptors (ENSFM00500000271913, nomenclature as agreed by the NC-IUPHAR Subcommittee on Hydroxycarboxylic acid receptors [396,1424]) respond to organic acids, including the endogenous hydroxy carboxylic acids 3-hydroxy butyric acid and L-lactic acid, as well as the lipid lowering agents nicotinic acid (niacin), acipimox and acifran [1774,1913,2036]. These receptors were provisionally described as nicotinic acid receptors, although nicotinic acid shows submicromolar potency at HCA 2 receptors only and is unlikely to be the natural ligand [1913,2036].   Comments The agonist activity of the lipid mediators described has been questioned [697,1513], which may derive from batch-to-batch differences, partial agonism or biased agonism. Recent results from Cooray et al. (2013) [365] have addressed this issue and the role of homodimers and heterodimers in the intracellular signaling . -

Comments:
The FPR2/ALX receptor (nomenclature as agreed by the NC-IUPHAR subcommittee on Leukotriene and Lipoxin Receptors [250]) is activated by the endogenous lipidderived, anti-inflammatory ligands lipoxin A 4 (LXA 4 ) and 15-epi-LXA 4 (aspirin triggered lipoxin A4, ATL). The FPR2/ALX receptor also interacts with endogenous peptide and protein ligands, such as MHC binding peptide [315] as well as annexin I (ANXA1, P04083) (ANXA1) and its N-terminal peptides [365,1491]. In addition, a soluble hydrolytic product of protease action on the urokinase-type plasminogen activator receptor has been reported to activate the FPR2/ALX receptor [1572]. Furthermore, FPR2/ALX has been suggested to act as a receptor mediating the proinflammatory actions of the acute-phase reactant, serum amyloid A [1772,1809]. A receptor selective for LXB 4 has been suggested from functional studies [53,1168,1596]. Note that the data for FPR2/ALX are also reproduced on the Formylpeptide receptor pages.  [556]). In native systems, analysis of binding data is complicated by metabolism and high levels of nonspecific binding, and therefore the relation-ship between recombinant and endogenously expressed receptors is unclear. Targeted deletion of LPA receptors has clarified signalling pathways and identified physiological and pathophysiological roles. Independent validation by multiple groups has been reported in the peer-reviewed literature for all six LPA receptors described in the tables, including further validation using a distinct read-out via a novel TGFα "shedding" assay [825]. LPA has also been described as an agonist at other orphan GPCRs (PSP24, GPR87 and GPR35), as well as at the nuclear hormone PPARγ receptors [1247,1743], although the physiological significance of these observations remain unclear.  [735] and VPC32179 [729] have antagonist activity at LPA 1 and LPA 3 receptors. There is growing evidence for in vivo efficacy of these chemical antagonists in several disorders, including fetal hydrocephalus [2107], lung fibrosis [1429], and systemic sclerosis [1429].

G protein-coupled receptors Lysophospholipid (S1P) receptors
Overview: Sphingosine 1-phosphate (S1P) receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Lysophospholipid receptors [936]) are activated by the endogenous lipid sphingosine 1-phosphate (S1P) and with lower apparent affinity, sphingosylphosphorylcholine (SPC). Originally cloned as orphan members of the endothelial differentiation gene (edg) family, deorphanisation as lysophospholipid receptors for S1P was based on sequence homology to LPA receptors. Current gene names have been codified as S1PR1, etc. to reflect the receptor function of these proteins. Most cellular phenomena ascribed to S1P can be explained by receptor-mediated mechanisms; S1P has also been described to act at intracellular sites [1841], and awaits precise definition. Previously-proposed SPC (or lysophophosphatidylcholine) receptors-G2A, TDAG8, OGR1 and GPR4 -continue to lack confirmation of these roles [396]. The relationship between recombinant and endogenously expressed receptors is unclear. Radioligand binding has been conducted in heterologous expression systems using [ 32 P]S1P (e.g [1438]). In native systems, analysis of binding data is complicated by metabolism and high levels of nonspecific binding. Targeted deletion of several S1P receptors and key enzymes involved in S1P biosynthesis or degradation has clarified signalling pathways and physiological roles. A crystal structure of an S1P 1 -T4 fusion protein has been described [698].
The S1P receptor modulator, fingolimod (FTY720, Gilenya), has received world-wide approval as the first oral therapy for relapsing forms of multiple sclerosis. This drug has a novel mechanism of action involving modulation of S1P receptors in both the immune and nervous systems [325,356,654], although the precise nature of its interaction requires clarification.
In addition to orthosteric ligands that directly interact with the glutamate recognition site directly, allosteric modulators have been described. Negative allosteric modulators are listed separately. The positive allosteric modulators most often act as 'potentiators' of an orthosteric agonist response, without significantly activating the receptor in the absence of agonist.
Although mGlu receptors have been thought to only form homodimers, recent studies revealed the possible formation of heterodimers between either group-I receptors, or within and between group-II and -III receptors [441]. Although well characterized in transfected cells, co-localization and specific pharmacological properties also suggest the existence of such heterodimers in the brain [2094].   across all known subtypes of mGlu receptors. Potential differences linked to the species (e.g. human versus rat or mouse) of the receptors and the receptor splice variants are generally not known. The influence of receptor expression level on pharmacology and selectivity has not been controlled for in most studies, particularly those involving functional assays of receptor coupling.
(S)-(+)-CBPG is an antagonist at mGlu 1 , but is an agonist (albeit of reduced efficacy) at mGlu 5 receptors. DCG-IV also exhibits agonist activity at NMDA glutamate receptors [1931], and is an antagonist at all group-III mGluRs with an IC 50 of 30 M. A potential novel metabotropic glutamate receptor coupled to phospho-inositide turnover has been observed in rat brain; it is activated by 4-methylhomoibotenic acid (ineffective as an agonist at recombinant Group I metabotropic glutamate receptors), but resistant to LY341495 [341]. There are also reports of a distinct metabotropic glutamate receptor coupled to phospholipase D in rat brain, which does not readily fit into the current classification [964,1482] A related class C receptor composed of two distinct subunits, T1R1 + T1R3 is also activated by glutamate and is responsible for umami taste detection.
All selective antagonists at metabotropic glutamate receptors are competitive.  Comments: In laboratory rodents, the gene encoding the motilin percursor appears to be absent, while the receptor appears to be a pseudogene [725,1637]. Functions of motilin (MLN, P12872) are not usually detected in rodents, although brain and other responses to motilin and the macrolide alemcinal have been reported and the mechanism of these actions are obscure [1249,1396]. Marked dif-ferences in ligand affinities for the motilin receptor in dogs and humans may be explained by significant differences in receptor structure [1638]. Note that for the complex macrolide structures, selectivity of action has often not been rigorously examined and other actions are possible (e.g. P2X inhibition by erythromycin; [2123]). Small molecule motilin receptor agonists are now described [1093,1639,2013]. The motilin receptor does not appear to have constitutive activity [774]. Although not proven, the existence of biased agonism at the receptor has been suggested [1225, 1292,1636]. A truncated 5-transmembrane structure has been identified but this is without activity when transfected into a host cell [507].   [1287]. In humans, NmU-25 appears to be the sole product of a precursor gene (NMU, P48645) showing a broad tissue distribution, but which is expressed at highest lev-els in the upper gastrointestinal tract, CNS, bone marrow and fetal liver. Much shorter versions of NmU are found in some species, but not in human, and are derived at least in some instances from the proteolytic cleavage of the longer NmU. Despite species differences in NmU structure, the C-terminal region (particularly the C-terminal pentapeptide) is highly conserved and contains biological activity. Neuromedin S (neuromedin S-33 (NMS, Q5H8A3)) has also been identified as an endogenous agonist [1326]. NmS-33 is, as its name suggests, a 33 amino-acid product of a precursor protein derived from a single gene and contains an amidated C-terminal heptapeptide identical to NmU. NmS-33 appears to activate NMU receptors with equivalent potency to NmU-25. Comments: An orphan receptor GPR83 (Q9NYM4) shows sequence similarities with NPFF1, NPFF2, PrRP and QRFP receptors. The antagonist RF9 is selective for NPFF receptors, but does not distinguish between the NPFF1 and NPFF2 subtypes (pK i 7.1 and 7.2, respectively, [1745]).

Further Reading
Moulédous L et al. (2010) Opioid-modulating properties of the neuropeptide FF system. Comments: Polymorphisms in the NPS receptor have been suggested to be associated with asthma [1953] and irritable bowel syndrome [386].   [554,1725]. C-terminally extended forms of the peptides (neuropeptide W-30 (NPW, Q8N729) and neuropeptide B-29 (NPB, Q8NG41)) also activate NPBW1 [211]. Unique to both forms of neuropeptide B is the N-terminal bromination of the first tryptophan residue, and it is from this post-translational modification that the nomenclature NPB is derived. These peptides were first identified from bovine hypothalamus and therefore are classed as neuropeptides. Endogenous variants of the peptides without the N-terminal bromination, des-Br-neuropeptide B-23 (NPB, Q8NG41) and des-Br-neuropeptide B-29 (NPB, Q8NG41), were not found to be major components of bovine hypothalamic tissue extracts. The NPBW2 receptor is activated by the short and C-terminal extended forms of neuropeptide W and neuropeptide B [211]. Comments: Potency measurements were conducted with heterologously-expressed receptors with a range of 0.14-0.57 nM (NPBW1) and 0.98-21 nM (NPBW2). ). The y6 receptor is a functional gene product in mouse, absent in rat, but contains a frame-shift mutation in primates producing a truncated non-functional gene [642]. Many of the agonists exhibit differing degrees of selectivity dependent on the species examined. For example, the potency of PP is greater at the rat Y 4 receptor than at the human receptor [485]. In addition, many agonists lack selectiv-ity for individual subtypes, but can exhibit comparable potency against pairs of NPY receptor subtypes, or have not been examined for activity at all subtypes. [ [191] and later in rat [1492].
Endomorphin-1 and endomorphin-2 have been identified as highly selective, putative endogenous agonists for the -opioid receptor. At present, however, the mechanisms for endomorphin synthesis in vivo have not been established, and there is no gene identified that encodes for either. Thus, the status of these peptides as endogenous ligands remains unproven.
Two areas of increasing importance in defining opioid receptor function are the presence of functionally relevant single nucleotide polymorphisms in human -receptors [1423] and the identification of bi-ased signalling by opioid receptor ligands, in particular, compounds previously characterized as antagonists [231]. Pathway bias for agonists makes general rank orders of potency and efficacy somewhat obsolete, so these do not appear in the table. As ever, the mechanisms underlying the acute and long term regulation of opiod receptor function are the subject of intense investigation and debate.
The richness of opioid receptor pharmacology has been enhanced with the recent discovery of allosteric modulators of MOPr and DOPr, notably the positive allosteric modulators and silent allosteric "antagonists" outlined in [240,241]. Negative allosteric modulation of opioid receptors has been previously suggested [908], whether all compounds are acting at a similar site remains to be established.

Comments:
The primary coupling of orexin receptors to G q/11 proteins is rather speculative and based on the strong activation of phospholipase C. Coupling of both receptors to G i/o and G s has also been reported [1019,1555]; for most cellular responses observed, the G protein pathway is unknown. The rank order of endogenous agonist potency may depend on the cellular signal transduction machinery.   2115], MRS2279 (pK i 7.9) [1970], MRS2179 (pK i 7-7.1) [197,1970], 2,2'-pyridylisatogen tosylate (pK i 6.8) [570] AR-C118925XX (pIC 50 1848]. NF157 also has antagonist activity at P2X 1 receptors [1923]. Uridine diphosphate has been reported to be an antagonist at the P2Y 14 receptor [548]. [ 35 S]ATPαS has been used to label P2Y receptors in rat synaptosomal membranes [1682,1683]. An orphan GPCR suggested to be a 'P2Y 15 ' receptor [823] appears not to be a genuine nucleotide receptor [2], but rather responds to dicarboxylic acids [728]. Further P2Y-like receptors have been cloned from non-mammalian sources; a clone from chick brain, termed a p2y 3 receptor (ENSGALG00000017327), couples to the G q/11 family of G proteins and shows the rank order of potency adenosine diphosphate uridine triphosphate ATP = uridine diphosphate [1998]. In addition, human sources have yielded a clone with a preliminary identification of p2y5 (LPAR6, P43657) and contradictory evidence of responses to ATP [954,1999]. This protein is now classified as LPA 6 , a receptor for lysophosphatidic acid (LPA) [1467,2079]. The clone termed p2y9 (LPAR4, Q99677) is also a receptor for lysophosphatidic acid, LPA 4 [1406]. The clone p2y7 (NOP9, Q86U38), originally suggested to be a P2Y receptor [22], has been shown to encode a leukotriene receptor [2095]. A P2Y receptor that was initially termed a p2y8 receptor (P79928) has been cloned from Xenopus laevis; it shows the rank order of potency ADPβS ATP = uridine triphosphate = guanosine-5'-triphosphate = CTP = TTP = ITP ATPγS and elic-its a Ca 2+ -dependent Clcurrent in Xenopus oocytes [169]. The p2y10 clone (P2RY10, O00398) lacks functional data. Diadenosine polyphosphates also have effects on as yet uncloned P2Y-like receptors with the rank order of potency of Ap 4 A Ap5a Ap3a, coupling via G q/11 [270]. P2Y-like receptors have recently been described on mitochondria [126]. CysLT1 and CysLT2 leukotriene receptors respond to nanomolar concentrations of uridine diphosphate, although they are activated principally by leukotrienes LTC 4 and LTD 4 [1257,1258]. Human GPR17 (13304) and rat GPR17, which are structurally related to CysLT and P2Y receptors, are also activated by leukotrienes [1542] as well as uridine diphosphate and UDP-glucose [344,540]. Activity at the rat GPR17 is inhibited by submicromolar concentrations of MRS2179 and cangrelor [344].

Further Reading
Abbracchio Comments: ramatroban is an antagonist at both DP 2 and TP receptors. Whilst cicaprost is selective for IP receptors, it does exhibit moderate agonist potency at EP 4 receptors [7]. Apart from IP receptors, iloprost also binds to other prostanoid receptors such as EP 1 receptors. The TP receptor exists in α and β isoforms due to alternative splicing of the cytoplasmic tail [1566]. The IP receptor agonist treprostinil binds also to human EP 2 and DP 1 receptors with high affinity (pK i 8.44 and 8.36, respectively).
The EP 1 agonist 17-phenyl--trinor-PGE 2 also shows agonist activity at EP 3 receptors. Butaprost and SC46275 may require deesterification within tissues to attain full agonist potency. There is evidence for subtypes of FP [1105], IP [1851,2037] and TP [1005] receptors. mRNA for the EP 1 and EP 3 receptors undergo alternative splicing to produce two [1441] and at least six variants, respectively, which can interfere with signalling [1441] or generate complex patterns of G-protein (G i/o , G q/11 , G s and G 12,13 ) coupling (e.g. [997,1370]   Comments europium-labelled relaxin is a fluorescent ligand for this receptor (K d =0.5nM) [1707].
Comments: Relaxin has recently successfully completed a Phase III clinical trial for the treatment of acute heart failure. 48 hr infusion of relaxin reduced dyspnoea and 180 day mortality [1262]. Small molecule agonists active at RXFP1 receptors have been developed [1718,2060], and one of these (ML290) is an allosteric agonist at RXFP1 [2060]. The antifibrotic actions of relaxin are dependent on the angiotensin receptor AT 2 , are absent in AT 2 knockout mice, and are associated with heterodimer formation between RXFP1 and AT 2 [330]. . RXFP1 signalling involves lipid rafts, residues in the C-terminus of the receptor and activation of phosphatidylinositol-3kinase [682]. More recent studies provide evidence that RXFP1 is pre-assembled in signalosomes with other signalling proteins including Gα s , Gβγ and adenylyl cyclase 2 that display constitutive activity and are exquisitely sensitive to sub-picomolar concentrations of relaxin [679]. The cyclic AMP signalling pattern is highly dependent on the cell type in which RXFP1 is expressed [680].
The receptor expression profiles suggested that RXFP3 was a neuropeptide receptor and RXFP4 a gut hormone receptor. Studies in rats and mice (including wildtype, and relaxin-3 and RXFP3 genedeletion strains [671,782,1759,1971] have revealed putative roles for the relaxin-3/RXFP3 system in the modulation of feeding [564,566,714,1706,1760], anxiety [1618,2114], and reward and motivated, goal-directed behaviours [782,1619,1971], particularly in relation to the integration of stress and corticotrophin-releasing factor signalling [1162], with implications for the therapeutic treatment of clinical anxiety, depression, eating disorders and addiction (see [565,1761] for review). Relaxin-3 (RLN3, Q8WXF3) acts as an agonist at both RXFP3 and RXFP4 whereas INSL5 (INSL5, Q9Y5Q6) is an agonist at RXFP4 and a weak antagonist at RXFP3. Unlike RXFP1 and RXFP2 both RXFP3 and RXFP4 are encoded by a single exon and therefore no splice variants exist. The rat RXFP3 sequence has two potential start codons that encode RXFP3L and RXFP3S with the longer variant having an additional 7 amino-acids at the N-terminus. It is not known which variant is expressed. Rat and dog RXFP4 sequences are pseudogenes [2027]. Recent studies suggest that INSL5 is an incretin secreted from enteroendocrine L cells and that the INSL5/RXFP4 system has roles in controlling food intake and glucose homeostasis [652]. RXFP3 couples to G i/o and inhibits adenylyl cyclase [1119,2144], and also causes Erk1/2 phosphorylation [2144]. Relatively little is known about RXFP4 signalling but like RXFP3 it couples to inhibitory G i/o G-proteins [1120]. Recent studies suggest that relaxin (RLN2, P04090) also interacts with RXFP3 to cause a pattern of activation of signalling pathways that are a subset of those activated by relaxin-3 (RLN3, Q8WXF3). The two patterns of signaling observed in several cell types expressing RXFP3 are strong inhibition of forskolin-stimulated cyclic AMP accumulation, ERK1/2 activation and nuclear factor NF -B reporter gene activation with relaxin-3 (RLN3, Q8WXF3), and weaker activity with relaxin (RLN2, P04090), porcine relaxin, or insulin-like peptide 3 (INSL3 (INSL3, P51460)) and a strong stimulation of activator protein (AP)-1 reporter genes with relaxin (RLN2, P04090), and weaker activation with relaxin-3 (RLN3, Q8WXF3) or porcine relaxin [2144]. Thus at RXFP3, relaxin (RLN2, P04090) is a biased ligand compared to the cognate ligand relaxin-3 (RLN3, Q8WXF3). Two pharmacologically distinct ligand binding sites were also identified on RXFP3-expressing cells using [ 125 I]

Somatostatin receptors G protein-coupled receptors Somatostatin receptors
Overview: Somatostatin (somatotropin release inhibiting factor) is an abundant neuropeptide, which acts on five subtypes of somatostatin receptor (sst 1 -sst 5 ; nomenclature as agreed by the NC-IUPHAR Subcommittee on Somatostatin Receptors [790]). Activation of these receptors produces a wide range of physiological effects throughout the body including the inhibition of secretion of many hormones. The relationship of the cloned receptors to endogenously expressed receptors is not yet well established in some cases. Endogenous ligands for these receptors are somatostatin-14 (SRIF-14 (SST, P61278)) and somatostatin-28 (SRIF-28 (SST, P61278)). Cortistatin-14 {Mouse, Rat} has also been suggested to be an endogenous ligand for somatostatin receptors [404]. . A novel peptide somatostatin analogue, somatoprim, has affinity for sst 2 , sst 4 and sst 5 receptors and is a potent inhibitor of GH secretion [1514,1726].   Overview: Trace amine-associated receptors were initially discovered as a result of a search for novel 5-HT receptors [185], where 15 mammalian orthologues were identified and divided into two families. The TA 1 receptor (nomenclature as agreed by the NC-IUPHAR Subcommittee for the Trace amine recep-tor [1181]) has been shown to have affinity for the endogenous trace amines tyramine, β-phenylethylamine and octopamine in addition to the classical amine dopamine [185]. Emerging evidence suggests that TA 1 is a modulator of monoaminergic activity in the brain [2062] with TA 1 and dopamine D 2 receptors shown to form constitutive heterodimers when co-expressed [492]. In addition to trace amines, receptors can be activated by amphetamine-like psychostimulants, and endogenous thyronamines such as thyronamine and 3-iodothyronamine. Antagonists EPPTB (Inverse agonist) (pIC 50 5.1) [199] Labelled ligands [ 3 H]tyramine (Agonist) (pK d 7.7) [185] Comments: In addition to TA 1 , analysis has shown that in man there are up to 5 functional TAAR genes (TAAR2, 5,6,8,9). See [185] for detailed discussion. The product of the gene TAAR2 (also known as GPR58) appears to respond to β-phenylethylamine tyramine and to couple through G s [185].

Ben-Shlomo
TAAR3, in some individuals, and TAAR4 are pseudogenes in man, although functional in rodents. The signalling characteristics and pharmacology of TAA 5 (PNR, Putative Neurotransmitter Receptor: TAAR5, O14804), TAA 6 (Trace amine receptor 4, TaR-4: TAAR6, 96RI8), TAA 8 (Trace amine receptor 5, GPR102: TAAR8, Q969N4 ) and TAA 9 (trace amine associated receptor 9: TAAR9, 96RI9) are lacking. The thy-ronamines, endogenous derivatives of thyroid hormone, have been shown to have affinity for rodent cloned trace amine receptors, including TA 1 [1657]. An antagonist EPPTB has recently been described that has a pK i of 9.1 at the mouse TA 1 but less than 5.3 for human TA 1 [1792]. Overview: The urotensin-II (U-II) receptor (UT, nomenclature as agreed by the NC-IUPHAR Subcommittee on the Urotensin receptor [439,530,1952]) is activated by the endogenous dodecapeptide urotensin-II (UTS2, O95399), originally isolated from the urophysis, the endocrine organ of the caudal neurosecretory system of teleost fish [134]. Several structural forms of U-II exist in fish and amphibians. The Goby orthologue was used to identify U-II as the cognate ligand for the predicted receptor encoded by the rat gene gpr14 [375,1130,1327,1410]. Human urotensin-II (UTS2, O95399), an 11-amino-acid peptide [375], retains the cyclohexapeptide sequence of goby U-II that is thought to be important in ligand binding [219,957]. This sequence is also conserved in the deduced amino-acid sequence of rat urotensin-II {Rat} (14 aminoacids) and mouse urotensin-II {Mouse} (14 amino-acids), although the N-terminal is more divergent from the human sequence [374]. A second endogenous ligand for UT has been discovered in rat [1816]. This is the urotensin II-related peptide (UTS2B, Q765I0), an octapeptide that is derived from a different gene, but shares the C-terminal sequence (CFWKYCV) common to U-II from other species. Identical sequences to rat urotensin II-related peptide (UTS2B, Q765I0) are predicted for the mature mouse and human peptides.  ) with comparable affinity [393]. PG 99-465 [1320] has been used as a selective VPAC 2 receptor antagonist in a number of phys-iological studies, but has been reported to have significant activity at VPAC 1 and PAC 1 receptors [422]. The selective PAC 1 receptor agonist maxadilan, was extracted from the salivary glands of sand flies (Lutzomyia longipalpis) and has no sequence homology to VIP (VIP, P01282) or the PACAP peptides [1330]. Two deletion variants of maxadilan, M65 [1918] and Max.d.4 [1331] have been reported to be PAC 1 receptor antagonists, but these peptides have not been extensively characterised.