Co-operative regulation of ligand binding to melanocortin receptor subtypes: Evidence for interacting binding sites

Dimerization of melanocortin 4 receptor controls puberty onset and body size polymorphism

Xiphophorus fish exhibit a clear phenotypic polymorphism in puberty onset and reproductive strategies of males. In X. nigrensis and X. multilineatus, puberty onset is genetically determined and linked to a melanocortin 4 receptor (Mc4r) polymorphism of wild-type and mutant alleles on the sex chromosomes. We hypothesized that Mc4r mutant alleles act on wild-type alleles by a dominant negative effect through receptor dimerization, leading to differential intracellular signaling and effector gene activation. Depending on signaling strength, the onset of puberty either occurs early or is delayed. Here, we show by Förster Resonance Energy Transfer (FRET) that wild-type Xiphophorus Mc4r monomers can form homodimers, but also heterodimers with mutant receptors resulting in compromised signaling which explains the reduced Mc4r signaling in large males. Thus, hetero- vs. homo- dimerization seems to be the key molecular mechanism for the polymorphism in puberty onset and body size in male fish.

Structural mechanism of calcium-mediated hormone recognition and Gβ interaction by the human melanocortin-1 receptor

Melanocortins are peptide hormones critical for the regulation of stress response, energy homeostasis, inflammation, and skin pigmentation. Their functions are mediated by five G protein-coupled receptors (MC1R-MC5R), predominately through the stimulatory G protein (Gs). MC1R, the founding member of melanocortin receptors, is mainly expressed in melanocytes and is involved in melanogenesis. Dysfunction of MC1R is associated with the development of melanoma and skin cancer. Here we present three cryo-electron microscopy structures of the MC1R-Gs complexes bound to endogenous hormone α-MSH, a marketed drug afamelanotide, and a synthetic agonist SHU9119. These structures reveal the orthosteric binding pocket for the conserved HFRW motif among melanocortins and the crucial role of calcium ion in ligand binding. They also demonstrate the basis of differential activities among different ligands. In addition, unexpected interactions between MC1R and the Gβ subunit were discovered from these structures. Together, our results elucidate a conserved mechanism of calcium-mediated ligand recognition, a specific mode of G protein coupling, and a universal activation pathway of melanocortin receptors.

Structures of active melanocortin-4 receptor-Gs-protein complexes with NDP-α-MSH and setmelanotide

The melanocortin-4 receptor (MC4R), a hypothalamic master regulator of energy homeostasis and appetite, is a class A G-protein-coupled receptor and a prime target for the pharmacological treatment of obesity. Here, we present cryo-electron microscopy structures of MC4R–Gs-protein complexes with two drugs recently approved by the FDA, the peptide agonists NDP-α-MSH and setmelanotide, with 2.9 Å and 2.6 Å resolution. Together with signaling data from structure-derived MC4R mutants, the complex structures reveal the agonist-induced origin of transmembrane helix (TM) 6-regulated receptor activation. The ligand-binding modes of NDP-α-MSH, a high-affinity linear variant of the endogenous agonist α-MSH, and setmelanotide, a cyclic anti-obesity drug with biased signaling toward Gq/11, underline the key role of TM3 in ligand-specific interactions and of calcium ion as a ligand-adaptable cofactor. The agonist-specific TM3 interplay subsequently impacts receptor–Gs-protein interfaces at intracellular loop 2, which also regulates the G-protein coupling profile of this promiscuous receptor. Finally, our structures reveal mechanistic details of MC4R activation/inhibition, and provide important insights into the regulation of the receptor signaling profile which will facilitate the development of tailored anti-obesity drugs.

cAMP Biosensor Assay Using BacMam Expression System: Studying the Downstream Signaling of LH/hCG Receptor Activation

Cyclic adenosine monophosphate (cAMP) serves as a second messenger for numerous G-protein-coupled receptors. Changes in cellular cAMP levels reflect the biological activity of various GPCR-specific agents, including protein hormones. cAMP biosensors based on detection of Förster-type resonance energy transfer (FRET) offer unique advantages including the ratiometric nature of measurement, adjustable affinity toward detected molecule, capability of monitoring kinetics of cAMP release, and compatibility with the multi-well format and fluorescence plate reader platforms. In this chapter, we introduce the optimized version of the previously reported method to achieve sufficient and reproducible level of cAMP biosensor protein expression with the means of BacMam transduction system. As a practical challenge, we address the applicability of the designed assay for screening of biological activity of human hormones, including human chorionic gonadotropin (hCG) bearing different posttranslational modifications.

Structural Complexity and Plasticity of Signaling Regulation at the Melanocortin-4 Receptor

The melanocortin-4 receptor (MC4R) is a class A G protein-coupled receptor (GPCR), essential for regulation of appetite and metabolism. Pathogenic inactivating MC4R mutations are the most frequent cause of monogenic obesity, a growing medical and socioeconomic problem worldwide. The MC4R mediates either ligand-independent or ligand-dependent signaling. Agonists such as α-melanocyte-stimulating hormone (α-MSH) induce anorexigenic effects, in contrast to the endogenous inverse agonist agouti-related peptide (AgRP), which causes orexigenic effects by suppressing high basal signaling activity. Agonist action triggers the binding of different subtypes of G proteins and arrestins, leading to concomitant induction of diverse intracellular signaling cascades. An increasing number of experimental studies have unraveled molecular properties and mechanisms of MC4R signal transduction related to physiological and pathophysiological aspects. In addition, the MC4R crystal structure was recently determined at 2.75 Å resolution in an inactive state bound with a peptide antagonist. Underpinned by structural homology models of MC4R complexes simulating a presumably active-state conformation compared to the structure of the inactive state, we here briefly summarize the current understanding and key players involved in the MC4R switching process between different activity states. Finally, these perspectives highlight the complexity and plasticity in MC4R signaling regulation and identify gaps in our current knowledge.

Determination of the melanocortin-4 receptor structure identifies Ca2+ as a cofactor for ligand binding

The melanocortin-4 receptor (MC4R) is involved in energy homeostasis and is an important drug target for syndromic obesity. We report the structure of the antagonist SHU9119-bound human MC4R at 2.8-angstrom resolution. Ca2+ is identified as a cofactor that is complexed with residues from both the receptor and peptide ligand. Extracellular Ca2+ increases the affinity and potency of the endogenous agonist α-melanocyte-stimulating hormone at the MC4R by 37- and 600-fold, respectively. The ability of the MC4R crystallized construct to couple to ion channel Kir7.1, while lacking cyclic adenosine monophosphate stimulation, highlights a heterotrimeric GTP-binding protein (G protein)-independent mechanism for this signaling modality. MC4R is revealed as a structurally divergent G protein-coupled receptor (GPCR), with more similarity to lipidic GPCRs than to the homologous peptidic GPCRs.

The constitutive activity of melanocortin-4 receptors in cAMP pathway is allosterically modulated by zinc and copper ions

Melanocortin-4 receptors (MC₄ R) are unique among G-protein-coupled receptors (GPCRs) as they have endogenous ligands that can exhibit inverse agonistic properties in the case of elevated basal activity. It is known that the constitutive activity of GPCRs strongly affects the ligand-dependent physiological responses, but little is known about these regulatory mechanisms. Since several metal ions have been shown to be important modulators of the signal transduction of GPCRs, we hypothesized that metal ions regulate the basal activity of MC₄ Rs. Implementation of a fluorescence anisotropy assay and novel redshifted fluorescent peptides enabled kinetic characterization of ligand binding to MC₄ R expressed on budded baculoviruses. We show that Ca²⁺ is required for high-affinity ligand binding, but Zn²⁺ and Cu²⁺ in the presence of Ca²⁺ behave as negative allosteric modulators of ligand binding to MC₄ R. FRET-based cAMP biosensor was used to measure the activation of MC₄ R stably expressed in CHO-K1 cells. At low micromolar concentrations, Zn²⁺ caused MC₄ R-dependent activation of the cAMP pathway, whereas Cu²⁺ reduced the activity of MC₄ R even below the basal level. These findings indicate that at physiologically relevant concentrations can Zn²⁺ and Cu²⁺ function as MC₄ R agonists or inverse agonists, respectively. This means that depending on the level of constitutive activity induced by Zn²⁺ ions, the pharmacological effect of orthosteric ligands of MC₄ R can be switched from a partial to an inverse agonist. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. More information about the Open Science badges can be found at https://cos.io/our-services/open-science-badges/.

Developing a Biased Unmatched Bivalent Ligand (BUmBL) Design Strategy to Target the GPCR Homodimer Allosteric Signaling (cAMP over β-Arrestin 2 Recruitment) Within the Melanocortin Receptors

Understanding the functional relevance of G protein-coupled receptor (GPCR) homodimerization has been limited by the insufficient tools to assess asymmetric signaling occurring within dimers comprised of the same receptor type. We present unmatched bivalent ligands (UmBLs) to study the asymmetric function of melanocortin homodimers. UmBLs contain one agonist and one antagonist pharmacophore designed to target a melanocortin homodimer such that one receptor is occupied by an agonist and the other receptor by an antagonist pharmacophore. First-in-class biased UmBLs (BUmBLs) targeting the human melanocortin-4 receptor (hMC4R) were discovered. The BUmBLs displayed biased agonism by potently stimulating cAMP signaling (EC₅₀ ∼ 2-6 nM) but minimally activating the β-arrestin recruitment pathway (≤55% maximum signal at 10 μM). To our knowledge, we report the first single-compound strategy to pharmacologically target melanocortin receptor allosteric signaling that occurs between homodimers that can be applied straightforwardly in vitro and in vivo to other GPCR systems.

Bench-top to clinical therapies: A review of melanocortin ligands from 1954 to 2016

The discovery of the endogenous melanocortin agonists in the 1950s have resulted in sixty years of melanocortin ligand research. Early efforts involved truncations or select modifications of the naturally occurring agonists leading to the development of many potent and selective ligands. With the identification and cloning of the five known melanocortin receptors, many ligands were improved upon through bench-top in vitro assays. Optimization of select properties resulted in ligands adopted as clinical candidates. A summary of every melanocortin ligand is outside the scope of this review. Instead, this review will focus on the following topics: classic melanocortin ligands, selective ligands, small molecule (non-peptide) ligands, ligands with sex-specific effects, bivalent and multivalent ligands, and ligands advanced to clinical trials. Each topic area will be summarized with current references to update the melanocortin field on recent progress. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

An in Vitro and in Vivo Investigation of Bivalent Ligands That Display Preferential Binding and Functional Activity for Different Melanocortin Receptor Homodimers

Pharmacological probes for the melanocortin receptors have been utilized for studying various disease states including cancer, sexual function disorders, Alzheimer's disease, social disorders, cachexia, and obesity. This study focused on the design and synthesis of bivalent ligands to target melanocortin receptor homodimers. Lead ligands increased binding affinity by 14- to 25-fold and increased cAMP signaling potency by 3- to 5-fold compared to their monovalent counterparts. Unexpectedly, different bivalent ligands showed preferences for particular melanocortin receptor subtypes depending on the linker that connected the binding scaffolds, suggesting structural differences between the various dimer subtypes. Homobivalent compound 12 possessed a functional profile that was unique from its monovalent counterpart providing evidence of the discrete effects of bivalent ligands. Lead compound 7 significantly decreased feeding in mice after intracerebroventricular administration. To the best of our knowledge, this is the first report of a melanocortin bivalent ligand's in vivo physiological effects.

Radiopharmacological characterization of ⁶⁴Cu-labeled α-MSH analogs for potential use in imaging of malignant melanoma

The melanocortin-1 receptor (MC1R) plays an important role in melanoma growth, angiogenesis and metastasis, and is overexpressed in melanoma cells. α-Melanocyte stimulating hormone (α-MSH) and derivatives are known to bind with high affinity at this receptor that provides the potential for selective targeting of melanoma. In this study, one linear α-MSH-derived peptide Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH2 (NAP-NS1) without linker and with εAhx-β-Ala linker, and a cyclic α-MSH derivative, [Lys-Glu-His-D-Phe-Arg-Trp-Glu]-Arg-Pro-Val-NH2 (NAP-NS2) with εAhx-β-Ala linker were conjugated with p-SCN-Bn-NOTA and labeled with (64)Cu. Radiochemical and radiopharmacological investigations were performed with regard to transchelation, stability, lipophilicity and in vitro binding assays as well as biodistribution in healthy rats. No transchelation reactions, but high metabolic stability and water solubility were demonstrated. The linear derivatives showed higher affinity than the cyclic one. [(64)Cu]Cu-NOTA-εAhx-β-Ala-NAP-NS1 ([(64)Cu]Cu-2) displayed rapid cellular association and dissociation in murine B16F10 cell homogenate. All [(64)Cu]Cu-labeled conjugates exhibited affinities in the low nanomolar range in B16F10. [(64)Cu]Cu-2 showed also high affinity in human MeWo and TXM13 cell homogenate. In vivo studies suggested that [(64)Cu]Cu-2 was stable, with about 85 % of intact peptide in rat plasma at 2 h p.i. Biodistribution confirmed the renal pathway as the major elimination route. The uptake of [(64)Cu]Cu-2 in the kidney was 5.9 % ID/g at 5 min p.i. and decreased to 2.0 % ID/g at 60 min p.i. Due to the prospective radiochemical and radiopharmacological properties of the linear α-MSH derivative [(64)Cu]Cu-2, this conjugate is a promising candidate for tracer development in human melanoma imaging.

cAMP assay for GPCR ligand characterization: application of BacMam expression system

Cyclic adenosine monophosphate (cAMP) is a second messenger of many G-protein-coupled receptors. The change in cellular cAMP level has widely been used to estimate the biological activity of various GPCR-specific agents. Förster resonance energy transfer (FRET) biosensors have been around for almost 10 years and became increasingly popular for cAMP detection. Ratiometric sensitized emission assay of a FRET biosensor can easily be implemented on fluorescence plate reader platforms. For such assays a considerable amount of cells expressing the desired biosensor is needed. A method to achieve sufficient and reproducible level of cAMP biosensor protein expression with the means of BacMam transduction system is the subject of this chapter.

Calcium affects OX1 orexin (hypocretin) receptor responses by modifying both orexin binding and the signal transduction machinery

BACKGROUND AND PURPOSE

One of the major responses upon orexin receptor activation is Ca(2+) influx, and this influx seems to amplify the other responses mediated by orexin receptors. However, the reduction in Ca(2+) , often used to assess the importance of Ca(2+) influx, might affect other properties, like ligand-receptor interactions, as suggested for some GPCR systems. Hence, we investigated the role of the ligand-receptor interaction and Ca(2+) signal cascades in the apparent Ca(2+) requirement of orexin-A signalling.

EXPERIMENTAL APPROACH

Receptor binding was assessed in CHO cells expressing human OX1 receptors with [(125) I]-orexin-A by conventional ligand binding as well as scintillation proximity assays. PLC activity was determined by chromatography.

KEY RESULTS

Both orexin receptor binding and PLC activation were strongly dependent on the extracellular Ca(2+) concentration. The relationship between Ca(2+) concentration and receptor binding was the same as that for PLC activation. However, when Ca(2+) entry was reduced by depolarizing the cells or by inhibiting the receptor-operated Ca(2+) channels, orexin-A-stimulated PLC activity was much more strongly inhibited than orexin-A binding.

CONCLUSIONS AND IMPLICATIONS

Ca(2+) plays a dual role in orexin signalling by being a prerequisite for both ligand-receptor interaction and amplifying orexin signals via Ca(2+) influx. Some previous results obtained utilizing Ca(2+) chelators have to be re-evaluated based on the results of the current study. From a drug discovery perspective, further experiments need to identify the target for Ca(2+) in orexin-A-OX1 receptor interaction and its mechanism of action.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 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.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. 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 the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

The melanocortin 4 receptor: oligomer formation, interaction sites and functional significance

This study involves the structural and functional properties of the recombinant melanocortin 4 receptor (MC(4)R) expressed in the HEK-293 cell line. Using co-immuno-purification approaches, the receptor appears to be an oligomer, which can be crosslinked through disulphide bonds involving a native cysteine residue (84) to give a dimeric species. This position is located near the cytosolic region of transmembrane segment 2 and it is suggested that this is an interacting interface between MC(4)R monomers. Using co-expression of the native protein and a C84A mutant, it appears that the receptor also forms higher order oligomers via alternative interfaces. Interestingly, disulphide crosslink formation does not occur if the receptor is uncoupled from its G-protein, even though the oligomeric state is preserved. This suggests that the conformational changes, which occur on activation, affect the TM2 interface. The pharmacology of the agonist, NDP-MSH, indicates that the MC(4)R retains high affinity for the ligand in the absence of the G-protein but occupancy for the ligand is increased. The data can be interpreted to suggest that the G-protein exerts a negative allosteric effect on the receptor. Co-expression of one receptor lacking the ability to signal with another, which cannot bind the agonist, restored ligand-dependent activation of the G-protein to situations in which neither receptor on its own could activate the G-protein. Such transactivation suggests meaningful cross talk between the receptor subunits in the oligomeric complex. These studies demonstrate further unique features of the MC(4)R.

BacMam system for FRET-based cAMP sensor expression in studies of melanocortin MC1 receptor activation

Cyclic adenosine monophosphate (cAMP) is a second messenger of many G-protein-coupled receptors (GPCRs) and a useful readout molecule to estimate the biological activity of various GPCR-specific agents. Here we report the development and use of a Förster resonance energy transfer (FRET) biosensor for cAMP (Epac2-camps) combined with a baculovirus-based BacMam transduction system. The constructed BacMam-Epac2-camps viral transduction system is a simple and robust tool for ligand screening at the second-messenger level in a variety of mammalian cell lines. The level of biosensor protein expression can easily be adjusted in a dose-dependent manner depending on the multiplicity of viral infection. For setting up the assay, we used a B16F10 murine melanoma cell line with endogenous expression of melanocortin-1 receptor (MC(1)R). The receptor activation was characterized by a set of MC(1)R full and partial agonists. Bivalent ions Ca(2+) as well as Mg(2+) modulated ligand potencies, whereas the effect was ligand and ion specific. Results obtained for MC(1)R indicate that the BacMam-Epac2-camps system may also be applicable for studying the activation of other GPCRs and may be implemented in routine analysis as well as in high-throughput screening.

Millimolar Mn2+ influences agonist binding to 5-HT1A receptors by inhibiting guanosine nucleotide binding to receptor-coupled G-proteins

Manganese is an essential trace element but its overexposure causes poisoning (called manganism) that shares several symptoms with Parkinson's disease, but with a mechanism that is still not well understood: in addition to involvement of the dopaminergic system, both serotonergic and peptiergic systems have been implicated. In the present report we have studied the influence of Mn(2+) on 5-HT(1A) receptor signaling complexes in rat brain and found that Mn(2+) in millimolar concentration caused an increase of high-affinity agonist binding to rat hippocampal membranes in comparison with experiments in the presence of Mg(2+), but not in rat cortical membranes and in Sf9 cell membranes expressing 5-HT(1A) receptors and G(i1) heterotrimers. Activation of G proteins with 30μM GTPγS turned all 5-HT(1A) receptors in these preparations into a low-affinity state for agonist binding in the presence of 1mM Mg(2+), but not in the presence of 1mM Mn(2+) in rat hippocampal membranes. However, if 1μM GTPγS was used for G protein activation, a substantial amount of high affinity agonist binding was detected in the presence of Mn(2+) also in cortical membranes and Sf9 cells, but not with Mg(2+) or EDTA. Comparison of the abilities of GDP and GTPγS to modulate high affinity agonist binding to 5-HT(1A) receptors indicated that both nucleotides were almost 10-fold less potent in the presence of MnCl(2) compared to MgCl(2). This means that by inhibiting guanosine nucleotide binding to G proteins in complex with 5-HT(1A) receptors, Mn(2+) acts as an enhancer for agonist binding and signal transduction. As the influence of Mn(2+) resembles the hypersensitivity of dopaminergic system in Parkinsonial models, it can be proposed that at least some symptoms of manganism are connected with a change of signal transduction complex caused by manganese-nucleotide complexes.

Targeting melanocortin receptors: an approach to treat weight disorders and sexual dysfunction

The melanocortin system has multifaceted roles in the control of body weight homeostasis, sexual behaviour and autonomic functions, and so targeting this pathway has immense promise for drug discovery across multiple therapeutic areas. In this Review, we first outline the physiological roles of the melanocortin system, then discuss the potential of targeting melanocortin receptors by using MC3 and MC4 agonists for treating weight disorders and sexual dysfunction, and MC4 antagonists to treat anorectic and cachectic conditions. Given the complexity of the melanocortin system, we also highlight the challenges and opportunities for future drug discovery in this area.

Alpha 2A-adrenoceptor-specific stimulation of [35S]GTP gamma S binding to membrane preparations of rat frontal cortex

Functional activation of alpha 2A adrenergic receptors in the crude membranes from rat frontal cortex was studied by a [35S]-guanosine 5'-O-(gamma-thiotriphosphate) ([35S]GTP gamma S) binding assay. alpha 2A agonists UK14304 and guanfacine decreased the ability of GDP to compete with [35S]GTP gamma S binding to the membranes and 0.1 mM GDP was found to be optimal for the following functional experiments. However, even after careful optimization of experimental conditions the specificity of ligands for rat alpha 2 adrenoceptors were not sufficient, as agonists as well as antagonists became activators of other signal transduction systems before achieving their maximal effect in the alpha 2A-adrenergic system. Only using compromising concentration of agonist (up to 1 microM UK14304) and antagonist (up to 1 microM RS79948) to inhibit agonist's effect, allowed us to filtrate out alpha 2A specific effect for characterization of signal transduction in rat frontal cortex membranes for the comparison efficacies of this system for different animals from behavioral experiments.

Kinetic evidence for tandemly arranged ligand binding sites in melanocortin 4 receptor complexes

The melanocortin 4 receptor (MC(4)R) binding of the peptide analogue of melanocyte stimulating hormone, [(125)I]NDP-MSH, and the low molecular weight radionucleid 1-(D-1,2,3,4-tetrahydroisoquinoline-3-carboxy-D-4-(125)iodophenylalanyl)-4-cyclohexyl-4-[(1,2,4-triazol-1-yl)methyl]piperidine trifluoroacetate ([(125)I]THIQ) were compared. Kinetic analysis indicated heterogeneity in the binding of both radioligands, the binding apparently proceeding to two tandemly arranged interconnected mutually dependent binding sites. Steric considerations and BRET analysis of Rluc and GFP tagged receptors proposed that these sites are located on different subunits of receptor dimers, which form receptor complexes. According to the minimal model proposed, ligand binding proceeds consecutively to the two binding sites of the dimer. After binding of the first ligand conformational transformations of the complex occur, which is followed by binding of the second ligand. When both receptor units have bound [(125)I]NDP-MSH, the radioligand can be released only from one unit. The [(125)I]NDP-MSH bound to the remaining unit stays practically irreversibly bound due to a very slow retransformation rate of the transformed complex. The considerably faster binding of [(125)I]THIQ did not allow accurate kinetic differentiation of the two binding sites. However, addition of NDP-MSH as well as a fragment of the human agouti protein, hAGRP(83-132) to the preformed [(125)I]THIQ-MC(4)R complex drastically retarded the release of [(125)I]THIQ from the complex, blocking conformational transformations in the complex by binding into the second binding site. The consecutive binding of ligands to the MC(4)R dimers has substantial impact on the apparent ligand potencies, when determined in competition with the two different radioligands applied herein; the apparent potencies of the same ligand differing up to three orders of magnitude when assayed in competition with [(125)I]NDP-MSH or [(125)I]THIQ.

Characteristics of Binding of [3H]WAY100635 to Rat Hippocampal Membranes

Kinetic analysis of binding of [(3)H][N-[2-[4-(2-[O-methyl-(3)H]methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexane carboxamide ([(3)H]WAY100635) to 5-HT(1A) receptors in rat hippocampal membranes has revealed complex regulation mechanism for this radioligand. Saturation binding experiments revealed that [(3)H]WAY100635 binds to a single class of receptors with very high apparent affinity (K (D) = 87 +/- 4 pM, B (max) = 15.1 +/- 0.2 fmol/mg protein). The binding was almost irreversible, as the dissociation rate constant obtained k (off) = (7.8 +/- 1.1) x 10(-3) min(-1), means that equilibrium with this radioligand cannot be achieved before 7.5 h incubation at 25 degrees C. Systematic association kinetic studies of [(3)H]WAY100635 binding revealed sharp reaction acceleration at higher radioligand concentration, proposing mechanism of positive cooperativity. The affinities of antagonists determined from competition with [(3)H]WAY100635 did not coincide with their abilities to inhibit 5-HT-dependent activation of [(35)S]GTPgammaS binding probably due to the ligand's kinetic peculiarities. Thus, [(3)H]WAY100635 appears to be an excellent tool for determining receptor binding sites, but its applicability in equilibrium studies is strongly limited.