Two nonpeptide tachykinin antagonists act through epitopes on corresponding segments of the NK1 and NK2 receptors

Deciphering specificity and cross-reactivity in tachykinin NK1 and NK2 receptors

The tachykinin receptors neurokinin 1 (NK1R) and neurokinin 2 (NK2R) are G protein-coupled receptors that bind preferentially to the natural peptide ligands substance P and neurokinin A, respectively, and have been targets for drug development. Despite sharing a common C-terminal sequence of Phe-X-Gly-Leu-Met-NH2 that helps direct biological function, the peptide ligands exhibit some degree of cross-reactivity toward each other's non-natural receptor. Here, we investigate the detailed structure-activity relationships of the ligand-bound receptor complexes that underlie both potent activation by the natural ligand and cross-reactivity. We find that the specificity and cross-reactivity of the peptide ligands can be explained by the interactions between the amino acids preceding the FxGLM consensus motif of the bound peptide ligand and two regions of the receptor: the β-hairpin of the extracellular loop 2 (ECL2) and a N-terminal segment leading into transmembrane helix 1. Positively charged sidechains of the ECL2 (R177 of NK1R and K180 of NK2R) are seen to play a vital role in the interaction. The N-terminal positions 1 to 3 of the peptide ligand are entirely dispensable. Mutated and chimeric receptor and ligand constructs neatly swap around ligand specificity as expected, validating the structure-activity hypotheses presented. These findings will help in developing improved agonists or antagonists for NK1R and NK2R.

Biased Gs versus Gq proteins and β-arrestin signaling in the NK1 receptor determined by interactions in the water hydrogen bond network

X-ray structures, molecular dynamics simulations, and mutational analysis have previously indicated that an extended water hydrogen bond network between trans-membranes I-III, VI, and VII constitutes an allosteric interface essential for stabilizing different active and inactive helical constellations during the seven-trans-membrane receptor activation. The neurokinin-1 receptor signals efficiently through Gq, Gs, and β-arrestin when stimulated by substance P, but it lacks any sign of constitutive activity. In the water hydrogen bond network the neurokinin-1 has a unique Glu residue instead of the highly conserved AspII:10 (2.50). Here, we find that this GluII:10 occupies the space of a putative allosteric modulating Na(+) ion and makes direct inter-helical interactions in particular with SerIII:15 (3.39) and AsnVII:16 (7.49) of the NPXXY motif. Mutational changes in the interface between GluII:10 and AsnVII:16 created receptors that selectively signaled through the following: 1) Gq only; 2) β-arrestin only; and 3) Gq and β-arrestin but not through Gs. Interestingly, increased constitutive Gs but not Gq signaling was observed by Ala substitution of four out of the six core polar residues of the network, in particular SerIII:15. Three residues were essential for all three signaling pathways, i.e. the water-gating micro-switch residues TrpVI:13 (6.48) of the CWXP motif and TyrVII:20 (7.53) of the NPXXY motif plus the totally conserved AsnI:18 (1.50) stabilizing the kink in trans-membrane VII. It is concluded that the interface between position II:10 (2.50), III:15 (3.39), and VII:16 (7.49) in the center of the water hydrogen bond network constitutes a focal point for fine-tuning seven trans-membrane receptor conformations activating different signal transduction pathways.

Are biological actions of neurokinin A in the adult brain mediated by a cross-talk between the NK1 and NK2 receptors?

Mice lacking the NK(1) receptor (NK(1)R-/- mice) and selective, high-affinity, non-peptide, NK(1), NK(2) and NK(3) receptor antagonists were used to identify the tachykinin receptor subtype(s) mediating the central responses induced by neurokinin A (NKA). The peptides, substance P (SP), NKA and senktide and the antagonists were injected intracerebroventricularly (ICV) through an implanted cannula. NKA (50 pmol) was as potent as SP (50 pmol) in inducing grooming behaviour (face washing and hind limb grooming) in wild-type mice, but both peptides failed to induce behavioural responses in NK(1)R-/- mice. In wild-type mice, the NK(1) receptor antagonist, RP 67580 (2 nmol), effectively inhibited grooming behaviour elicited by SP, but was inactive against grooming induced by NKA, which in turn was abolished after pre-treatment with the selective NK(2) receptor agonist, SR 48968 (2 nmol). Unlike NKA, the selective NK(2) receptor agonists, (β Ala(8)) NKA 4-10 and (NLeu(10)) NKA 4-10, injected ICV at doses of 50 or 100 pmol did not elicit any behavioural response in wild-type mice. The NK(3) receptor antagonist, SR 142801, inhibited behaviours induced by the NK(3) receptor agonist, senktide, but did not alter behavioural responses to either SP or NKA in wild-type mice. The present findings demonstrate that central biological actions of SP and senktide are mediated by activation of NK(1) and NK(3) receptors, respectively. Our results also indicate that NK(1) receptors are essential for generating central actions induced by NKA, which are most probably mediated by a cross-talk between the NK(1) and NK(2) receptors.

Topological analysis of the complex formed between neurokinin A and the NK2 tachykinin receptor

Neurokinin A stimulates physiological responses in the peripheral and central nervous systems upon interacting primarily with the tachykinin NK2 receptor (NK2R). In this study, the structure of NKA bound to the NK2R is characterised by use of fluorescence resonance energy transfer. Four fluorescent NKA analogues with Texas red introduced at amino acid positions 1, 4, 7 and 10 were prepared. When bound to a NK2R carrying enhanced green fluorescent protein at the N-terminus, all peptides reduce green fluorescent protein fluorescence from 10% to 50% due to energy transfer. The derived donor-acceptor distances are 46, 55, 59 and 69 A for the fluorophore linked to positions 1-10, respectively. The monotonic increase in distance clearly indicates that the peptide adopts an extended structure when bound to its receptor. The present data are used, in combination with rhodopsin structure, fluorescence studies, photoaffinity labelling and site-directed mutagenesis data to design a computer model of the NKA-NK2R complex. We propose that the N-terminus of NKA is exposed and accessible to the extracellular medium. Subsequent amino acids of the NKA peptide become progressively more buried residues up to approximately one-third of the transmembrane-spanning domain.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Molecular mechanism of 7TM receptor activation--a global toggle switch model

The multitude of chemically highly different agonists for 7TM receptors apparently do not share a common binding mode or active site but nevertheless act through induction of a common molecular activation mechanism. A global toggle switch model is proposed for this activation mechanism to reconcile the accumulated biophysical data supporting an outward rigid-body movement of the intracellular segments, as well as the recent data derived from activating metal ion sites and tethered ligands, which suggests an opposite, inward movement of the extracellular segments of the transmembrane helices. According to this model, a vertical see-saw movement of TM-VI-and to some degree TM-VII-around a pivot corresponding to the highly conserved prolines will occur during receptor activation, which may involve the outer segment of TM-V in an as yet unclear fashion. Small-molecule agonists can stabilize such a proposed active conformation, where the extracellular segments of TM-VI and -VII are bent inward toward TM-III, by acting as molecular glue deep in the main ligand-binding pocket between the helices, whereas larger agonists, peptides, and proteins can stabilize a similar active conformation by acting as Velcro at the extracellular ends of the helices and the connecting loops.

Successful virtual screening for a submicromolar antagonist of the neurokinin-1 receptor based on a ligand-supported homology model

The neurokinin-1 (NK1) receptor belongs to the family of G-protein-coupled receptors (GPCRs), which represents one of the most relevant target families in small-molecule drug design. In this paper, we describe a homology modeling of the NK1 receptor based on the high-resolution X-ray structure of rhodopsin and the successful virtual screening based on this protein model. The NK1 receptor model has been generated using our new MOBILE (modeling binding sites including ligand information explicitly) approach. Starting with preliminary homology models, it generates improved models of the protein binding pocket together with bound ligands. Ligand information is used as an integral part in the homology modeling process. For the construction of the NK1 receptor, antagonist CP-96345 was used to restrain the modeling. The quality of the obtained model was validated by probing its ability to accommodate additional known NK1 antagonists from structurally diverse classes. On the basis of the generated model and on the analysis of known NK1 antagonists, a pharmacophore model was deduced, which subsequently guided the 2D and 3D database search with UNITY. As a following step, the remaining hits were docked into the modeled binding pocket of the NK1 receptor. Finally, seven compounds were selected for biochemical testing, from which one showed affinity in the submicromolar range. Our results suggest that ligand-supported homology models of GPCRs may be used as effective platforms for structure-based drug design.

Design and synthesis of substituted N-methylbenzamide analogues derived from SR 48,968 as neurokinin-2 receptor antagonists

A series of N-methylbenzamide analogues (2-18) that is structurally derived from SR 48,968, a potent neurokinin-2 (NK(2)) receptor antagonist (pK(b)9.1), has been obtained using asymmetric synthesis. Isothiocyanato-N-methylbenzamide (10-12) and bromoacetamido-N-methylbenzamide derivatives (16-18) have been designed to serve as potential electrophilic affinity labels. Nitro-N-methylbenzamide (4-6) and acetamido-N-methylbenzamide (13-15) were designed to serve as the nonelectrophilic controls for these ligands. Functional assay results using guinea pig trachea indicate that electrophilic N-methylbenzamide analogues exhibit potent but surmountable NK(2) receptor antagonist activity. Several members of this series (2, 3, 7-9) exhibit potent NK(2) receptor antagonist potencies with pK(b) values in the range of 9.1-9.7. para-Fluoro substituted analogue 3 was found to be highly potent with a pK(b) of 9.7.

Biophysical methods to study ligand-receptor interactions of neuropeptide Y

Neuropeptide Y (NPY) is a 36 amino acids peptide amide that was isolated for the first time almost 20 years ago from porcine brain. NPY displays a multiplicity of physiological effects that are transmitted by at least six G-protein coupled receptors (GPCRs) named Y(1), Y(2), Y(3), Y(4), Y(5), and y(6). Because of the difficulty in obtaining high-resolution crystallographic structures from GPCRs that all belong to seven transmembrane helices proteins, a variety of biophysical methods have been applied in order to characterize the interaction of ligand and receptor. In this review article we present the most relevant outcomes of the studies performed in this field by our group and others. The use of photoaffinity labeling allowed the molecular characterization of the Y(2) receptor. The concerted application of molecular modeling and mutagenesis studies led to a model for the interaction of the natural agonist and nonpeptide antagonists with the Y(1) receptor. The three-dimensional (3D) structure and dynamics of micelle-bound NPY and their implications for receptor selection have been studied by NMR. The characterization of the tertiary and quaternary structure of the NPY dimer in solution at millimolar concentrations has been performed by NMR and extended to physiologically relevant concentrations by fluorescence resonance energy transfer (FRET) experiments performed with fluorescence-labeled analogues.

New insights in insurmountable antagonism

Antagonists that produce parallel rightward shifts of agonist dose-response curves with no alteration of the maximal response are traditionally classified as surmountable, while insurmountable antagonists also depress the maximal response. Although the longevity of the antagonist-receptor complex is quoted in many studies to explain insurmountable antagonism, slowly interconverting receptor conformations, allosteric binding sites, and receptor internalization have been evoked as alternative explanations. To complicate matters even further, insurmountable antagonism is not only drug-related; it may also depend on the tissue, species and experimental design. For the sake of drug development, it is important to elucidate the molecular mechanisms of insurmountable antagonism. New experimental approaches, such as intact cell studies and the use of computer-assisted simulations based on dynamic receptor models, herald the advent of better insight in the future.

The NK-2 receptor antagonist SR 48968C does not improve adenosine hyperresponsiveness and airway obstruction in allergic asthma

When stimulated, excitatory nonadrenergic noncholinergic (e-NANC) nerves locally release tachykinins like Neurokinin (NK) A and Substance P, causing neurogenic inflammation and airway obstruction via activation of specific NK-1 and NK-2 receptors. The recently developed nonpeptide NK-2 receptor antagonist SR 48968C has a high affinity for the NK-2 receptor, and is a strong and selective antagonist of NK-2 receptor mediated airway obstruction. In a placebo-controlled cross-over study, we investigated the effect of SR 48968C, administrated orally once-daily in a dosage of 100 mg during 9 days, on airway responsiveness to adenosine 5'-monophosphate (AMP) in 12 allergic asthmatic patients. Furthermore, we assessed its effect on airway obstruction, by measuring FEV1 on the first and last day of each treatment period and by peak flow registration at home throughout the study period. SR 48968C had no significant effect on PC20AMP or on FEV1 measured on day 1 and 9, and morning and evening peakflow measured at home on day 2-8. Thus, although SR 48968C was administrated in a dosage that might cause a demonstrable blocking effect on airway NK-2 receptors in asthma, it did not have a significant bronchodilatory or bronchoprotective effect against adenosine hyperresponsiveness in this study. Further studies are needed to assess the value of SR 48968C and other NK receptor antagonists in the treatment of asthma

Evidence that the proposed novel human "neurokinin-4" receptor is pharmacologically similar to the human neurokinin-3 receptor but is not of human origin

There have been proposals that the tachykinin receptor classification should be extended to include a novel receptor, the "neurokinin-4" receptor (NK-4R), which has a close homology with the human NK-3 receptor (hNK-3R). We compared the pharmacological and molecular biological characteristics of the hNK-3R and NK-4R. Binding experiments, with (125)I-[MePhe(7)]-NKB binding to HEK 293 cell membranes transiently expressing the hNK-3R (HEK 293-hNK-3R) or NK-4R (HEK 293-NK-4R), and functional studies (Ca(2+) mobilization in the same cells) revealed a similar profile of sensitivity to tachykinin agonists and antagonists for both receptors; i.e., in binding studies with the hNK-3R, MePhe(7)-NKB > NKB > senktide >> NKA = Substance P; with the NK-4R, MePhe(7)-NKB > NKB = senktide >> Substance P = NKA; and with antagonists, SB 223412 = SR 142801 > SB 222200 >> SR 48968 >> CP 99994 for both hNK-3R and NK-4R. Thus, the pharmacology of the two receptors was nearly identical. However, attempts to isolate or identify the NK-4R gene by using various molecular biological techniques were unsuccessful. Procedures, including nested polymerase chain reaction studies, that used products with restriction endonuclease sites specific for either hNK-3R or NK-4R, failed to demonstrate the presence of NK-4R in genomic DNA from human, monkey, mouse, rat, hamster, or guinea pig, and in cDNA libraries from human lung, brain, or heart, whereas the hNK-3R was detectable in the latter libraries. In view of the failure to demonstrate the presence of the putative NK-4R it is thought to be premature to extend the current tachykinin receptor classification.

Amino acid residues conferring ligand binding properties of prostaglandin I and prostaglandin D receptors. Identification by site-directed mutagenesis

Using chimeras of the mouse prostaglandin (PG) I receptor (mIP) and the mouse PGD receptor (mDP), we previously revealed that the cyclopentane ring recognition by these receptors is specified by a region from the first to third transmembrane domain of each receptor; recognition by this region of mIP is broad, accommodating the D, E, and I types of cyclopentane rings, whereas that of mDP binds the D type of PGs alone (Kobayashi, T., Kiriyama, M., Hirata, T., Hirata, M., Ushikubi, F., and Narumiya, S. (1997) J. Biol. Chem. 272, 15154-15160). In the present study, we performed a more detailed chimera analysis, and narrowed the domain for the ring recognition to a region from the first transmembrane domain to the first extracellular loop. One chimera with the replacement of the second transmembrane domain and the first extracellular loop of mDP with that of mIP bound only iloprost. The amino acid substitutions in this chimera suggest that Ser(50) in the first transmembrane domain of mIP confers the broad ligand recognition of mIP and that Lys(75) and Leu(83) in the second transmembrane domain of mDP confer the high affinity to PGD(2) and the strict specificity of ligand binding of mDP, respectively.

Molecular determinants of peptide and nonpeptide NK-2 receptor antagonists binding sites of the human tachykinin NK-2 receptor by site-directed mutagenesis

A series of 14 mutants on nine selected residues of the human tachykinin NK(2) receptor was produced and stably transfected into CHO cells to investigate the binding of the peptide MEN 11420 and the nonpeptide SR 48968 antagonists. The main interactions found for MEN 11420 were with Thr171, Tyr206, Tyr266 and Phe270. In the case of SR 48968 crucial residues were Tyr266 and Tyr289. While some overlapping of the binding sites exists, the binding modes suggested by this study appear not to allow structural correlation, and therefore general SAR, between these two antagonists.

Molecular characterization of the ligand-receptor interaction of the neuropeptide Y family

Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) belong to the NPY hormone family and activate a class of receptors called the Y-receptors, and also belong to the large superfamily of the G-protein coupled receptors. Structure-affinity and structure-activity relationship studies of peptide analogs, combined with studies based on site-directed mutagenesis and anti-receptor antibodies, have given insight into the individual characterization of each receptor subtype relative to its interaction with the ligand, as well as to its biological function. A number of selective antagonists at the Y1-receptor are available whose structures resemble that of the C-terminus of NPY. Some of these compounds, like BIBP3226, BIBO3304 and GW1229, have recently been used for in vivo investigations of the NPY-induced increase in food intake. Y2-receptor selective agonists are the analog cyclo-(28/32)-Ac-[Lys28-Glu32]-(25-36)-pNPY and the TASP molecule containing two units of the NPY segment 21-36. Now the first antagonist with nanomolar affinity for the Y2-receptor is also known, BIIE0246. So far, the native peptide PP has been shown to be the most potent ligand at the Y4-receptor. However, by the design of PP/NPY chimera, some analogs have been found that bind not only to the Y4-, but also to the Y5-receptor with subnanomolar affinities, and are as potent as NPY at the Y1-receptor. For the characterization of the Y5-receptor in vitro and in vivo, a new class of highly selective agonists is now available. This consists of analogs of NPY and of PP/NPY chimera which all contain the motif Ala31-Aib32. This motif has been shown to induce a 3(10)-helical turn in the region 28-31 of NPY and is suggested to be the key motif for high Y5-receptor selectivity. The results of feeding experiments in rats treated with the first highly specific Y5-receptor agonists support the hypothesis that this receptor plays a role in the NPY-induced stimulation of food intake. In conclusion, the selective compounds for the different Y receptor subtypes known so far are promising tools for a better understanding of the physiological properties of the hormones of the NPY family and related receptors.

Uncovering molecular mechanisms involved in activation of G protein-coupled receptors

G protein-coupled, seven-transmembrane segment receptors (GPCRs or 7TM receptors), with more than 1000 different members, comprise the largest superfamily of proteins in the body. Since the cloning of the first receptors more than a decade ago, extensive experimental work has uncovered multiple aspects of their function and challenged many traditional paradigms. However, it is only recently that we are beginning to gain insight into some of the most fundamental questions in the molecular function of this class of receptors. How can, for example, so many chemically diverse hormones, neurotransmitters, and other signaling molecules activate receptors believed to share a similar overall tertiary structure? What is the nature of the physical changes linking agonist binding to receptor activation and subsequent transduction of the signal to the associated G protein on the cytoplasmic side of the membrane and to other putative signaling pathways? The goal of the present review is to specifically address these questions as well as to depict the current awareness about GPCR structure-function relationships in general.

Identification of residues involved in neurotensin binding and modeling of the agonist binding site in neurotensin receptor 1

The neurotensin receptor 1 (NTR1) subtype belongs to the family of G protein-coupled receptors and mediates most of the known effects of the neuropeptide including modulation of central dopaminergic transmission. This suggested that nonpeptide agonist mimetics acting at the NTR1 might be helpful in the treatment of Parkinson's disease and schizophrenia. Here, we attempted to define the molecular interactions between neurotensin-(8-13), the pharmacophore of neurotensin, and the rat NTR1. Mutagenesis of the NTR1 identified residues that interact with neurotensin. Structure-activity studies with neurotensin-(8-13) analogs identified the peptide residues that interact with the mutated amino acids in the receptor. By taking these data into account, computer-assisted modeling techniques were used to build a tridimensional model of the neurotensin-(8-13)-binding site in which the N-terminal tetrapeptide of neurotensin-(8-13) fits in the third extracellular loop and the C-terminal dipeptide binds to residues at the junction between the extracellular and transmembrane domains of the receptor. Interestingly, the agonist binding site lies on top of the previously described NTR1-binding site for the nonpeptide neurotensin antagonist SR 48692. Our data provide a basis for understanding at the molecular level the agonist and antagonist binding modes and may help design nonpeptide agonist mimetics of the NTR1.

Contribution of NK(2) tachykinin receptors to propulsion in the rabbit distal colon

The role of the tachykinin neurokinin (NK)(2) receptors on rabbit distal colon propulsion was investigated by using two selective NK(2)-receptor antagonists, MEN-10627 and SR-48968. Experiments on colonic circular muscle strips showed that contractile responses to [beta-Ala(8)]NKA-(4-10) (1 nM-1 microM), a selective NK(2)-receptor agonist, were competitively antagonized by MEN-10627 (1-100 nM), whereas SR-48968 (0.1-10 nM) caused an insurmountable antagonism, thus confirming the difference in the mode of action of the two compounds. Colonic propulsion was elicited by distending a mobile rubber balloon with 0.3 ml (submaximal stimulus) or 1.0 ml (maximal stimulus) of water. The velocity of anal displacement of the balloon (mm/s) was considered the main propulsion parameter. At low concentrations (1.0-100 nM and 0.1-10 nM, respectively), MEN-10627 and SR-48968 facilitated the velocity of propulsion, whereas at high concentrations (100 nM and 1 microM, respectively) they decelerated propulsion. The excitatory and inhibitory effects of both antagonists were observed only with submaximal stimulus. We focused on the hypothesis that the facilitatory effect on propulsion may result from blockade of neuronal NK(2) receptors and the inhibitory effect from suppression of the excitatory transmission mediated by NK(2) receptors on smooth muscle cells. In the presence of N(G)-nitro-L-arginine (300 microM), a nitric oxide synthase inhibitor, MEN-10627, at a concentration (10 nM) that was found to accelerate propulsion in control experiments inhibited the velocity of propulsion. In the presence of threshold (1-10 nM) or full (1 microM) concentration of atropine, which inhibited to a great extent the velocity of propulsion, the inhibitory effect of MEN-10627 (1 microM) was markedly increased. In conclusion, in the rabbit distal colon NK(2) receptors may decelerate propulsion by activating a nitric oxide-dependent neuronal mechanism and may accelerate it by a postjunctional synergistic interaction with cholinergic muscarinic receptors.

The neurokinin-1 and neurokinin-2 receptor binding sites of MDL103,392 differ

Several small molecule non-peptide antagonists of the NK-1 and NK-2 receptors have been developed. Mutational analysis of the receptor protein sequence has led to the conclusion that the binding site for these non-peptide antagonists lies within the bundle created by transmembrane domains IV-VII of the receptor and differs from the binding sites of peptide agonists and antagonists. The current investigation uses site-directed mutagenesis of the NK-1 and NK-2 receptors to elucidate the amino acids that are important for binding and functional activity of the first potent dual NK-1/NK-2 antagonist MDL103,392. The amino acids found to be important for MDL103,392 binding to the NK-1 receptor are Gln-165, His-197, Leu-203, Ile-204, Phe-264, His-265 and Tyr-272. The amino acids found to be important for MDL103,392 binding to the NK-2 receptor are Gln-166, His-198, Tyr-266 and Tyr-289. While residues in transmembrane (TM) domains IV and V are important in both receptors (Gln-165/166 and His-197/198), residues in TM V and VI are more important for the NK-1 receptor and residues in TM VII play a more important role in the NK-2 receptor. These data are the first report of the analysis of the binding site of a dual tachykinin receptor antagonist and indicate that a single compound (MDL103,392) binds to each receptor in a different manner despite there being a high degree of homology in the transmembrane bundles. In addition, this is the first report in which a model for the binding of a non-peptide antagonist to the NK-2 receptor is proposed.

Defects of tyrosine289phenylalanine mutation on binding and functional properties of the human tachykinin NK2 receptor stably expressed in chinese hamster ovary cells

A point mutation was made at position 289 in the transmembrane segment 7 of the human tachykinin NK2 receptor to yield a tyrosine/phenylalanine (Tyr/Phe) substitution. Chinese hamster ovary cells stably transfected with the wild-type or Tyr289Phe mutant NK2 receptor both bound neurokinin A (NKA) and the synthetic NK2 receptor-selective agonists, GR 64349 and [betaAla8]NKA(4-10), with high and even affinities. Neurokinin B (NKB) and substance P (SP) also displayed sizeable binding affinities, albeit with lower affinity as compared to NKA. In a functional assay (production of inositol-1,4,5-trisphosphate, IP3), NKA, GR 64349, and [betaAla8]INKA(4-10) stimulated IP3 accumulation via the wild-type and mutant receptors with similar potencies. On the other hand, NKB and SP exhibited a dramatic reduction in their agonist efficacies at the mutant receptor, NKB acting as a partial agonist (maximum effect = 50% of the response to NKA) and SP being totally inactive. The results obtained with phenoxybenzamine inactivation experiments indicated that a large and similar receptor reserve existed for both the wild-type and the mutant receptor. SP, which displayed sizeable binding affinity for the mutant receptor but did not stimulate IP3 accumulation, antagonized the agonist effect of NKA. The antagonist action of SP at the mutant NK2 receptor cannot be ascribed to receptor internalization. The Tyr/Phe replacement at position 289 markedly reduced the binding affinity and antagonist potency of the non-peptide ligand, SR 48968, without affecting the binding affinity and antagonist potency of the bicyclic peptide antagonist MEN 11420. The results indicate that the hydroxyl radical function of Tyr289 in transmembrane segment 7 of the human NK2 receptor is, directly or indirectly, involved in stimulus transduction when the NK2 receptor is occupied by NKB or SP, but not when using NKA or NK2 receptor-selective agonists.

In vitro characterization of tachykinin NK2-receptors modulating motor responses of human colonic muscle strips

1. Human in vitro preparations of transverse or distal colonic circular smooth muscle were potently and dose-dependently contracted by neurokinin A (EC50, 4.9 nM), the tachykinin NK2-receptor selective agonist [beta-Ala8]neurokinin A (4-10) ([beta-Ala8]NKA (4-10)) (EC50, 5.0 nM), neurokinin B (EC50, 5.3 nM) and substance P (EC50, 160 nM), but not by the tachykinin NK1-receptor selective agonist [Sar9Met(O2)11] substance P, or the NK3-receptor selective agonists, senktide and [MePhe7] neurokinin B. No regional differences between transverse and distal colon were observed in response to [beta-Ala8]NKA (4-10). 2. Atropine (1 microM) and tetrodotoxin (1 microM) did not significantly inhibit responses to [beta-Ala8]NKA (4-10), neurokinin A, substance P or neurokinin B. 3. The newly developed non-peptide antagonists for tachykinin NK2-receptors SR 48968, SR 144190 and its N-demethyl (SR 144743) and N,N-demethyl (SR 144782) metabolites, were used to challenge agonist responses, as appropriate. SR 144190 and the metabolites all potently and competitively antagonized the response to [beta-Ala8]NKA (4-10), with similar potency (Schild plot pA2 values 9.4, 9.4 and 9.3, slope = 1). SR 48968 antagonism was not competitive: the Schild plot slope was biphasic with a high (X intercept approximately 9.3) and a low (X intercept 8.4, slope 1.6) affinity site. Co-incubation of SR 48968 (10, 100 nM) and SR 144782 (10 nM) produced additive effects; in this experimental condition, SR 48968 apparent affinity (pKB) was 8.2. In addition, SR 144782 (0.1 microM) antagonized responses to neurokinin A, substance P and neurokinin B, with pKB consistent with its affinity for tachykinin NK2-receptors. The potent and selective NK1 and NK3-receptor antagonists, SR 140333 and SR 142801 (both 0.1 microM), failed to inhibit contractions induced by SP or NKB. 4. In conclusion, the in vitro mechanical responses of circular smooth muscle preparations from human colon are strongly consistent with the presence of non-neuronal tachykinin NK2-receptors, but not tachykinin NK1- or NK3-receptors. Our findings with SR 48968 suggest the existence of two tachykinin NK2-receptor subtypes, that it seems to distinguish, unlike SR 144190 and its metabolites. However, the precise nature of SR 48968 allotopic antagonism remains to be elucidated, since allosteric effects at the tachykinin NK2-receptor might well account for the complexity of the observed interaction.

Mutagenesis and modeling of the neurotensin receptor NTR1. Identification of residues that are critical for binding SR 48692, a nonpeptide neurotensin antagonist

The two neurotensin receptor subtypes known to date, NTR1 and NTR2, belong to the family of G-protein-coupled receptors with seven putative transmembrane domains (TM). SR 48692, a nonpeptide neurotensin antagonist, is selective for the NTR1. In the present study we attempted, through mutagenesis and computer-assisted modeling, to identify residues in the rat NTR1 that are involved in antagonist binding and to provide a tentative molecular model of the SR 48692 binding site. The seven putative TMs of the NTR1 were defined by sequence comparison and alignment of bovine rhodopsin and G-protein-coupled receptors. Thirty-five amino acid residues within or flanking the TMs were mutated to alanine. Additional mutations were performed for basic residues. The wild type and mutant receptors were expressed in COS M6 cells and tested for their ability to bind 125I-NT and [3H]SR 48692. A tridimensional model of the SR 48692 binding site was constructed using frog rhodopsin as a template. SR 48692 was docked into the receptor, taking into account the mutagenesis data for orienting the antagonist. The model shows that the antagonist binding pocket lies near the extracellular side of the transmembrane helices within the first two helical turns. The data identify one residue in TM 4, three in TM 6, and four in TM 7 that are involved in SR 48692 binding. Two of these residues, Arg327 in TM 6 and Tyr351 in TM 7, play a key role in antagonist/receptor interactions. The former appears to form an ionic link with the carboxylic group of SR 48692, as further supported by structure-activity studies using SR 48692 analogs. The data also show that the agonist and antagonist binding sites in the rNTR1 are different and help formulate hypotheses as to the structural basis for the selectivity of SR 48692 toward the NTR1 and NTR2.

Two related neurokinin-1 receptor antagonists have overlapping but different binding sites

The neuropeptide substance P binds to the G protein-coupled neurokinin-1 (NK-1) receptor and elicits cellular responses thought to be involved in pain, neurogenic inflammation, vasodilatation, and plasma exudation. Several small molecule nonpeptide antagonists of the substance P/NK-1 receptor interaction have been developed. Mutational analysis of the receptor protein sequence has led to the conclusion that the binding site for these nonpeptide antagonists lies within the bundle created by transmembrane domains IV-VII of the receptor. This current investigation employs site directed mutagenesis of the NK-1 receptor to compare the binding site of CP-96,345 with that of a related compound CP-99,994. The data demonstrate that while both compounds appear to bind within the transmembrane domain bundle, the contribution of individual amino acid residues to the binding of each compound differs.

Identification of domains conferring ligand binding specificity to the prostanoid receptor. Studies on chimeric prostacyclin/prostaglandin D receptors

To identify domains conferring ligand binding specificity to prostanoid receptors, we constructed a series of chimeric receptors by successively replacing the regions from the carboxyl-terminal tail of mouse prostacyclin (prostaglandin I (PGI)) receptor (mIP) with the corresponding regions of the mouse PGD receptor (mDP). The mIP receptor expressed in COS 7 cells bound [3H]iloprost, a PGI2 analog, and [3H]PGE1 with Kd values of 13 and 27 nM, respectively. This receptor did not bind [3H]PGD2, [3H]PGE2, and [3H]PGF2alpha. The mDP receptor bound only [3H]PGD2 with a Kd value of 43 nM. The chimeric IPN-VII/DPC receptor with replacement of the carboxyl tail of the mIP receptor with that of the mDP receptor showed 12-16-fold higher affinities for [3H]iloprost and [3H]PGE1 than the mIP receptor. The region extending from the sixth transmembrane domain to the carboxyl terminus of the mIP receptor was next replaced with the corresponding region of the mDP receptor. This chimeric IPN-V/DPVI-C receptor acquired the ability to bind [3H]PGD2 and [3H]PGE2 without decreasing the affinities of the mIP receptor to [3H]iloprost and [3H]PGE1. These binding characteristics did not change when the fourth and fifth transmembrane domains of the mIP receptor were further replaced with the corresponding regions of the mDP receptor. However, when the first extracellular to second intracellular loop of the mIP receptor containing the third transmembrane domain was further replaced with those of the mDP receptor, the affinities for [3H]PGE1, [3H]PGE2, and [3H]iloprost were markedly decreased, whereas that for [3H]PGD2 was increased by about 2-fold. [3H]PGF2alpha showed no affinity for the mIP, mDP, and all the chimeric receptors. These results suggest that the sixth to seventh transmembrane domain of the mIP receptor confers the specificity of this receptor to bind selectively to PGE1 and not to PGE2 and that the third transmembrane domain of the mDP receptor confers the selective binding of PGD2 to this receptor.

The sixth transmembrane domains of the human B1 and B2 bradykinin receptors are structurally compatible and involved in discriminating between subtype-selective agonists

In order to investigate the molecular basis for the ability of the human B1 and B2 bradykinin (BK) receptor subtypes to discriminate between subtype-selective ligands, we constructed chimeric proteins in which the sixth transmembrane domains (TM-VI) of these receptors were exchanged. The pharmacological profiles of the constructs were analyzed by radioligand binding in particulate preparations of transiently transfected HEK293 cells using the agonist [3H]des-Arg10-kallidin and the antagonist [3H]NPC17731. The ability of these constructs to transmit an intracellular signal was measured in transiently transfected A10 cells, a vascular smooth muscle cell line, by single cell Ca2+ imaging. Substitution of B1 TM-VI into the B2 receptor (B2(B1VI)) dramatically reduced the affinity of the B2-selective agonist BK, whereas the affinity of the B2-selective antagonist NPC17731 was unaltered. High affinity BK binding was fully regained when two residues, Tyr259 and Ala263, near the extracellular surface of TM-VI in B2(B1VI), were replaced with the corresponding residues in the wild-type B2 receptor, which are Phe259 and Thr263. The construct B1(B2VI), produced by substitution of B2 TM-VI into the B1 receptor, did not support high affinity binding of the B1-selective agonist des-Arg10-kallidin. In contrast to BK and des-Arg10-kallidin, the binding of the less subtype-selective agonist kallidin showed little sensitivity to TM-VI exchange. These results show that TM-VI in the human B1 and B2 BK receptor subtypes, although only 36% identical, are structurally compatible. Furthermore, this domain contributes significantly to the ability of these receptors to discriminate between the subtype-selective agonists BK and des-Arg10-kallidin.

The use of photolabelled peptides to localize the substance-P-binding site in the human neurokinin-1 tachykinin receptor

The amino acid p-benzoyl-L-phenylalanine, (p-Bz)Phe, has been incorporated into substance P (SP), Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2, to localize the agonist-binding domains of the human neurokinin-1 (NK-1) receptor overexpressed in a transfected mammalian cell line. The NK-1-specific agonist [Pro9]SP was modified at position 8 by (p-Bz)Phe and acylated at the N-terminus by a biotinyl sulfone reporter via a 5-aminopentanoyl spacer. After photolysis, the biotinyl sulfone moiety allowed easy and efficient removal of biotinylated fragments from the complex incubation mixture with streptavidin-coated beads. Direct elution from the beads with the matrix used for matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS), which was facilitated by saturation of streptavidin sites with biotin, and subsequent MALDI-TOF mass spectrometry analysis allowed identification of the NK-1 fragments obtained after photolysis and proteolytic digestion. Trypsin digestion and combined trypsin/Staphylococcus aureus V8 protease enzymatic cleavage established that the site of covalent attachment of the photolabelled SP resides in the second extracellular loop Thr173-Arg177. Cyanogen bromide cleavage shows that the probe is covalently attached to the methyl group of a methionine residue from human NK-1. These experiments identified Met174 as the modified residue.

Construction of a high affinity zinc switch in the kappa-opioid receptor

Very limited structural information is available concerning the superfamily of G-protein-coupled receptors with their seven-transmembrane segments. Recently a non-peptide antagonist site was structurally and functionally replaced by a metal ion site in the tachykinin NK-1 receptor. Here, this Zn(II) site is transferred to the kappa-opioid receptor by substituting two residues at the outer portion of transmembrane V (TM-V), Asp223 and Lys227, and one residue at the top of TM-VI, Ala298, with histidyl residues. The histidyl residues had no direct effect on the binding of either the non-peptide antagonist [3H]diprenorphine or the non-peptide agonist, [3H]CI977, just as these mutations/substitutions did not affect the apparent affinity of a series of other peptide and non-peptide ligands when tested in competition binding experiments. However, zinc ions in a dose-dependent manner prevented binding of both agonist and antagonist ligands with an apparent affinity for the metal ion, which gradually was built up to 10(-6) M. This represents an increase in affinity for the metal ion of about 1000-fold as compared with the wild-type kappa receptor and is specific for Zn(II) as the affinity for e.g. Cu(II) was almost unaffected. The direct transfer of this high affinity metal ion switch between two only distantly related receptors indicates a common overall arrangement of the seven-helix bundle among receptors of the rhodopsin family.

The conformation of substance P in lipid environments

NMR and CD studies have been used to analyze the model membrane-bound structure of the neuropeptide substance P (RPKPQQFFGLM-NH2, SP), which has previously been proposed as the NK1 receptor active form. Conformations were determined for the SP in the presence of aqueous solutions of zwitterionic dodecylphosphocholine (DPC) and anionic sodium dodecylsulfate (SDS) micelles. The two structures are similar, although fast exchange between free and bound forms was observed for SP with DPC micelles, and predominantly bound characteristics were found for SP in SDS. The addition of 150-200 mM NaCl had no observable effect on the bound conformation in either case. Thus, the structure of SP at a micelle surface is determined largely by hydrophobic forces, and the electrostatic interactions determine the amount of SP that is bound.

Identification of residues involved in ligand binding to the neurokinin-2 receptor

Several residues of the human neurokinin-2 receptor have been identified to be critical for the binding of peptide agonists and non-peptide antagonists. Amino acid substitutions in the first and second extracellular segments and the second transmembrane segment led to substantial reduction in peptide affinity without affecting the affinity of antagonist SR48968. These effects are identical to those observed for homologous residues in the neurokinin-1 receptor, suggesting that these three regions are involved in high-affinity peptide binding to both receptor subtypes. On the other hand, some conserved residues in the fourth to seventh transmembrane segments are required for peptide binding to only one receptor subtype but not both. The conserved nature and location of these receptor residues suggest that the distance between bound peptide and helices 4-7 varies depending on the receptor subtype. It is likely that the conformational compatibility between a ligand and a given receptor determines the magnitude of binding affinity, and thus receptor subtype selectivity. While many single-residue substitutions did not affect the binding affinity of the antagonist SR48968, two double mutants in the sixth and seventh transmembrane segments were found to reduce its affinity substantially. Therefore, receptor residues participate cooperatively in the binding of SR48968. These results demonstrate the usefulness of combining single-residue substitutions in studying and confirming the role of receptor residues in ligand binding. Finally, the overlapping nature of agonist and antagonist binding sites is consistent with the observation that substitutions of some residues modify the binding affinities of both peptide agonists and non-peptide antagonists.

Functional interaction between losartan and central tachykinin NK3 receptors in the conscious rat

1. The cardiovascular and behavioural effects elicted by the intracerebroventricular (i.c.v.) injection of substance P (SP), neurokinin A (NKA), [MePhe7]neurokinin B ([MePhe7]NKB) or angiotensin II (AII) in the conscious rat were assessed before and 5 min after i.c.v. pretreatment with antagonists selective for angiotensin AT1 (losartan and its active metabolite EXP 3174), angiotensin AT2 (PD 123,319) or tachykinin NK3 (R 486) receptors. 2. I.c.v. administration of 25 pmol AII evoked an increase in mean arterial blood pressure (MAP) and water intake behaviour, accompanied by a transient bradycardia, whereas 25 pmol [MePhe7]NKB caused a transient increase in MAP and heart rate (HR) concurrently with marked wet dog shake behaviour. At the same dose, SP and NKA were more potent than [MePhe7]NKB in increasing MAP and HR, but did not produce water intake or wet dog shake behaviours. 3. Losartan (650 pmol, i.c.v.) reduced significantly the cardiovascular and behavioural responses to AII or [MePhe7]NKB, but not to SP or NKA. While 65 pmol losartan was inactive, 260 pmol inhibited selectively the central effects of AII. Whereas EXP 3174 (6.5 nmol) blocked both AII and [MePhe7]NKB-mediated responses, the dose of 650 pmol blocked only the responses to AII. 4. The central responses to AII and [MePhe7]NKB were not affected by PD 123,319 (650 pmol). On the other hand, the [MePhe7]NKB-induced central effects were significantly reduced by R 486 (650 pmol). The NK3-selective antagonist had no effect against AII. 5. This study provides functional evidence, to support earlier binding data, that losartan (and to some extent its active metabolite EXP 3174) interact with the tachykinin NK3 receptor in rat brain. However,the cardiovascular and behavioural responses induced by central tachykinin agonists (SP, NKA and[MePhe7]NKB) and All are mediated by unrelated mechanisms.

Conversion of antagonist-binding site to metal-ion site in the tachykinin NK-1 receptor

Mutational analysis of the tachykinin NK-1 (refs 1-7), NK-2 (ref. 8) and angiotensin AT-1 (refs 9, 10) receptors indicates that non-peptide antagonists act through residues located between the seven transmembrane segments, whereas natural peptide agonists bind mainly to residues scattered in the exterior part of the receptor. The presumed contact points for the prototype NK-1 antagonist CP96,345 cluster on opposing faces of the outer portions of transmembrane helices V and VI (refs 1-5). Here we show that systematic introduction of histidyl residues at this antagonist-binding site in the human NK-1 receptor gradually converts it into a high-affinity metal-ion-binding site without affecting agonist binding. In a double mutant with histidine residues substituted at the top of transmembrane segments V and VI, respectively, Zn2+ inhibits binding of radiolabelled agonist peptide and efficiently blocks phosphoinositol turnover induced by substance P. We propose that Zn2+ and CP96,345 act as 'allosteric competitive' antagonists by stabilizing inactive conformations of the mutant and the wild-type receptor respectively. Introduction of metal-ion-binding sites could be used as a general tool in the structural and functional characterization of helix-helix interactions in G-protein-coupled receptors, as well as in other membrane proteins.

Non-peptide angiotensin agonist. Functional and molecular interaction with the AT1 receptor

Non-peptide ligands for peptide receptors for the G-protein-coupled type are generally antagonists, except in the opiate system. Recently, it was observed that a subset of biphenylimidazole derivatives surprisingly possessed angiotensin-like activity in vivo. In COS-7 cells transfected with the rat AT1 receptor a prototype of these compounds, L-162,313 stimulated phosphoinositide hydrolysis with an EC50 of 33 +/- 11 nM. The maximal response to the compound was 50% of that of angiotensin II in COS-7 cells but only 3% in stably transfected Chinese hamster ovary cells. The agonistic effect of L-162,313 was blocked by the AT1-specific antagonist L-158,809 and was not observed in untransfected cells. In Chinese hamster ovary cells, L-162,313 also acted as an insurmountable antagonist of the angiotensin stimulated phosphoinositide hydrolysis. In contrast to previously tested non-peptide ligands, L-162,313 bound with reasonably high affinity to the Xenopus laevis AT1 receptor. In the human receptor, the binding of L-162,313 was found to be unaffected by point mutations in transmembrane segments III and VII, which impaired the binding of biphenylimidazole antagonists. Substitutions in the extracellular domains of the human and rat receptor, which impaired the binding of angiotensin II, did not affect the binding of L-162,313. It is concluded that a subset of biphenylimidazole compounds can act as high affinity partial agonists on the AT1 receptor. These compounds have molecular interactions with the receptor which appear to differ both from that of the structurally similar non-peptide antagonists and from that of their functional counterpart, the peptide agonist.

Mapping peptide-binding domains of the substance P (NK-1) receptor from P388D1 cells with photolabile agonists

The tachykinin substance P (SP) is a peptide transmitter of primary afferents. Its actions on both central and peripheral targets are mediated by a G-protein-coupled receptor of known primary structure. To identify contact sites between the undecapeptide SP and its receptor, we prepared radiolabeled photoreactive analogs of SP (H-RPKPQQFFGLM-NH2) by replacing amino acids in the peptide with p-benzoyl-L-phenylalanine (BPA). SP, BPA3-SP, and BPA8-SP bind with high affinity (Kd < 3 nM) to SP receptors on the murine cell line P388D1, triggering intracellular calcium responses. Both binding and calcium responses are blocked by the specific SP receptor antagonist CP-96345. On photolysis, radioiodinated BPA3-SP, and BPA8-SP covalently label a heterogeneously glycosylated protein of about 75 kDa; labeling is abolished by excess unlabeled SP or CP-96345. The labeled receptors were digested with V8 protease and/or trypsin, and the resulting fragments were analyzed by electrophoresis, high pressure liquid chromatography, and chemical or enzymatic modification. BPA3-SP and BPA8-SP photo-incorporate into different regions of the murine SP receptor. The results establish that the third and the eighth positions of SP, respectively, interact with the NH2-terminal extracellular tail (residues 1-21) and second extracellular loop (residues 173-183) of the SP receptor. A model for the agonist peptide-binding sites of the SP receptor is proposed based on photoaffinity labeling and mutagenesis studies.

100 years lock-and-key concept: are peptide keys shaped and guided to their receptors by the target cell membrane?

The ideas developed in the Membrane Compartments Theory which allow a quantitative prediction of receptor preference are discussed in terms of our present knowledge of opioid and neurokinin receptor structure. Furthermore, conformations of regulatory peptides interacting with artificial lipid membranes are compared with those of chemically constrained molecules that react selectively with different receptor classes. Striking similarities in the topochemistry of molecules with similar activity were observed. The membrane-induced topomers were almost congruent with the artificial topomers that are selectively recognized by the same receptors.

Lysine 182 of endothelin B receptor modulates agonist selectivity and antagonist affinity: evidence for the overlap of peptide and non-peptide ligand binding sites

The potent vasoactive peptide hormone endothelin (ET) binds to receptors which belong to the G-protein coupled receptor family. The availability of non-peptide antagonists for ET receptors allows investigation of the relationship among the binding sites for peptide and non-peptide ligands. In this study, a lysine residue, conserved within transmembrane domain 3 (TM3) of the ETA and ETB receptor subtypes, is implicated in agonist and antagonist binding by its analogous position within TM3 to a binding site aspartate residue conserved within bioactive amine receptors. Replacement of this lysine within hETB by arginine, alanine, methionine, aspartate, or glutamate results in hETB variants with unaltered affinities for agonist peptide ET-1 but which have affinities for peptide agonists ET-2, ET-3, sarafotoxin 6C, and TRL 1736 which are between 1-3 orders of magnitude lower than their corresponding wild-type hETB values. Significantly, the affinities of non-peptide antagonists, (+/-)-SB 209670 and its analogs as well as Ro 46-2005, are abrogated. The results suggest that an interaction of K182 of hETB with the indan 2-carboxyl of (+/-)-SB 209670 may contribute to the high-affinity binding of the diarylindan antagonists. The results indicate that TM3 of hETB is a region of overlap among the binding sites of non-peptide antagonists and the affected peptide agonists.

Differentiation between binding sites for angiotensin II and nonpeptide antagonists on the angiotensin II type 1 receptors

To characterize binding sites for nonpeptide angiotensin antagonists on the human angiotensin II receptor type 1 (AT1 receptor) we have systematically exchanged segments of the human receptor with corresponding segments from a homologous Xenopus laevis receptor, which does not bind the nonpeptide compounds. Substitution of transmembrane segment VII of the human AT1 receptor dramatically reduced the binding affinity of all of the 11 nonpeptide antagonists tested (55- to > 2000-fold) with no effect on the binding of angiotensin. The affinity for the nonpeptide compounds decreased additionally one order of magnitude when transmembrane segment VI and the connecting extracellular loop 3 from the Xenopus receptor were also introduced into the human AT1 receptor. Exchanges of smaller segments and single residues in transmembrane segments VI and VII and extracellular loop 3 revealed that the binding of nonpeptide antagonists was dependent on nonconserved residues located deep within the transmembrane segments VI and VII, in particular Asn295 in transmembrane segment VII. Surprisingly, all exchanges in transmembrane segment VII, including the Asn295 to Ser substitution, had a more pronounced effect on the binding of the competitive antagonists relative to the insurmountable antagonists. It is concluded that the binding mode for peptide and nonpeptide ligands on the AT1 receptor is rather different and that competitive and insurmountable antagonists presumably bind to overlapping but distinct sites located in transmembrane segments VI and VII.

Comparison of tachykinin NK1 and NK2 receptors in the circular muscle of the guinea-pig ileum and proximal colon

1. The aim of this study was the pharmacological characterization of tachykinin NK1 and NK2 receptors mediating contraction in the circular muscle of the guinea-pig ileum and proximal colon. The action of substance P (SP), neurokinin A (NKA) and of the synthetic agonists [Sar9]SP sulphone, [Glp6,Pro9]SP(6-11) (septide) and [beta Ala8]NKA(4-10) was investigated. The affinities of various peptide and nonpeptide antagonists for the NK1 and NK2 receptor was estimated by use of receptor selective agonists. 2. The natural agonists, SP and NKA, produced concentration-dependent contraction in both preparations. EC50 values were 100 pM and 5 nM for SP, 1.2 nM and 19 nM for NKA in the ileum and colon, respectively. The action of SP and NKA was not significantly modified by peptidase inhibitors (bestatin, captopril and thiorphan, 1 microM each). 3. Synthetic NK1 and NK2 receptor agonists produced concentration-dependent contraction of the circular muscle of the ileum and proximal colon. EC50 values were 83 pM, 36 pM and 10 nM in the ileum, 8 nM, 0.7 nM and 12 nM in the colon for [Sar9]SP sulphone, septide and [beta Ala8]NKA-(4-10), respectively. The pseudopeptide derivative of NKA(4-10), MDL 28,564 behaved as a full or near-to-full agonist in both preparations, its EC50s being 474 nM and 55 nM in the ileum and colon, respectively. 4. Nifedipine (1 microM) abolished the response to septide and [Sar9]SP sulphone in the ileum and produced a rightward shift and large depression of the response in the colon. The response to [beta Ala8]NKA(4-10) was abolished in the ileum and largely unaffected in the colon. 5. The NK1 receptor antagonists, (+/-)-CP 96,34, FK 888 and GR 82,334 competitively antagonized the response to septide and [Sar9]SP sulphone in both preparations without affecting that to [beta Ala8]NKA(4-10). In general, the NK1 receptor antagonists were significantly more potent toward septide than [Sar9]SP sulphone in both preparations. 6. The NK2 receptor antagonists, GR 94,800 and SR 48,968 selectively antagonized the response to [beta Ala8]NKA(4-10) without affecting that to [Sar9]SP sulphone or septide in the ileum and colon. SR 48,968 produced noncompetitive antagonism of the response to the NK2 receptor agonist in the ileum and competitive antagonism in the colon. 7. MEN 10,376 and the cyclic pseudopeptide MEN 10,573 antagonized in a competitive manner the response to [beta Ala8]NKA(4-10) in the ileum and colon. While MEN 10,573 was equipotent in both preparations, MEN 10,376 was significantly more potent in the colon than in the ileum. MEN 10,376was also effective against septide in both preparations, without affecting the response to [Sar9] SP sulphone. MEN 10,573 antagonized the response to [Sar9]SP sulphone and septide in both preparations,pKB values against septide being intermediate, and significantly different from, those measured against[Beta Ala 8]NKA(4-10) and [Sa9]lSP sulphone.8. These findings show that tachykinin NK1 and NK2 receptors mediate contraction of the circular muscle of the guinea-pig ileum and colon. In both preparations NK1 receptor antagonists display higher apparent affinity when tested against septide than [Sar9]SP sulphone. These findings are compatible with the proposed existence of NK1 receptor subtypes in guinea-pig, although alternative explanations (e.g.agonist binding to different epitopes of the same receptor protein) cannot be excluded at present.Furthermore, an intraspecies heterogeneity of the NK2 receptor in the circular muscle of the guinea-pig ileum and colon is suggested.