Pharmacological characterization of recombinant human 5-hydroxytryptamine1A receptors using a novel antagonist radioligand, [3H]WAY-100635

N-(4-[(18)F]-fluoropyridin-2-yl)-N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}carboxamides as analogs of WAY100635. New PET tracers of serotonin 5-HT(1A) receptors

N-(4-[(18)F]-Fluoropyridin-2-yl)-N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-carboxamides were prepared by labeling their 4-nitropyridin-2-yl precursors through nitro substitution by the (18)F anion. In vitro and in vivo tests showed that the cyclohexanecarboxamide derivative is a reversible, selective and high affinity 5-HT1A receptor antagonist (IC50 = 0.29 nM, ki = 0.18 nM) with high brain uptake, slow brain clearance and stability to defluorination when compared with conventional standards. This PET radioligand is a promising candidate for an improved in vivo quantification of 5-HT1A receptors in neuropsychiatric disorders.

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.

Comparison of high and low affinity serotonin 1A receptors by PET in vivo in nonhuman primates

Serotonin (5-HT) 1A receptors exist in high and low affinity states. Agonist ligands bind preferentially to the high affinity state receptors, providing a more functionally relevant measure than antagonist binding. We now report comparison of 5-HT(1A) binding in vivo using both [¹¹C]CUMI-101 (agonist) and [¹¹C]WAY100635 (antagonist) in nonhuman primates. PET studies show that both tracers bind to known 5-HT(1A) receptor (5-HT(1A)R)-rich regions of baboon brain. The binding (BP(F)) of [¹¹C]CUMI-101 was lower on an average of 55% across the regions of interest (ROIs) compared to [¹¹C]WAY100635. This ratio is consistent with the in vitro binding data of agonist and antagonist 5-HT(1A)R ligands previously reported.

Computational and experimental analysis of the transmembrane domain 4/5 dimerization interface of the serotonin 5-HT(1A) receptor

Experimental evidence suggests that most members of class A G-protein coupled receptors (GPCRs) can form homomers and heteromers in addition to functioning as single monomers. In particular, serotonin (5-HT) receptors were shown to homodimerize and heterodimerize with other GPCRs, although the details and the physiological role of the oligomerization has not yet been fully elucidated. Here we used computational modeling of the 5-HT(1A) receptor monomer and dimer to predict residues important for dimerization. Based on these results, we carried out rationally designed site-directed mutagenesis. The ability of the mutants to dimerize was evaluated using different FRET-based approaches. The reduced levels of acceptor photobleaching-Förster resonance energy transfer (FRET) and the lower number of monomers participating in oligomers, as assessed by lux-FRET, confirmed the decreased ability of the mutants to dimerize and the involvement of the predicted contacts (Trp175(4.64), Tyr198(5.41), Arg151(4.40), and Arg152(4.41)) at the interface. This information was reintroduced as constraints for computational protein-protein docking to obtain a high-quality dimer model. Analysis of the refined model as well as molecular dynamics simulations of wild-type (WT) and mutant dimers revealed compensating interactions in dimers composed of WT and W175A mutant. This provides an explanation for the requirement of mutations of Trp175(4.64) in both homomers for disrupting dimerization. Our iterative computational-experimental study demonstrates that transmembrane domains TM4/TM5 can form an interaction interface in 5-HT(1A) receptor dimers and indicates that specific amino acid interactions maintain this interface. The mutants and the optimized model of the dimer structure may be used in functional studies of serotonin dimers.

WAY-100635 has high selectivity for serotonin 5-HT1A versus dopamine D4 receptors

The serotonin 5-HT(1A) receptor antagonist WAY-100635 was recently reported to have potent agonist properties at dopamine D(4) receptors (Chemel et al., 2006, Psychopharmacology 188, 244-251.). Herein WAY-100635 (pK(i) at human (h) serotonin 5-HT(1A) receptors=9.51; pK(i) at dopamine hD(4.4) receptors=7.42) stimulated [(35)S]GTPgammaS incorporation in membranes of Chinese Hamster Ovary cells expressing dopamine hD(4.4) receptors with only moderate potency and modest efficacy (pEC(50)=6.63; E(max)=19% of dopamine). Moreover, in antagonism experiments, WAY-100635 had a much lower potency at dopamine hD(4.4) receptors (pK(B)=7.09), than at serotonin h5-HT(1A) receptors (pK(B)=9.47). These data demonstrate that WAY-100635 has high selectivity for serotonin h5-HT(1A)versus dopamine hD(4.4) receptors.

Contractile effects of 5-hydroxytryptamine and 5-carboxamidotryptamine in the equine jejunum

The use of human prokinetic drugs in colic horses leads to inconsistent results. This might be related to differences in gastrointestinal receptor populations. The motor effects of 5-hydroxytryptamine (5-HT; serotonin) on the equine mid-jejunum were therefore studied. Longitudinal muscle preparations were set up for isotonic measurement. 5-HT induced tonic contractions with superimposed phasic activity; these responses were not influenced by tetrodotoxin and atropine, suggesting a non-neurogenic, non-cholinergic pathway. The 5-HT receptor antagonists GR 127935 (5-HT(1B,D)), ketanserin (5-HT(2A)), SB 204741 (5-HT(2B)), RS 102221 (5-HT(2C)), granisetron (5-HT(3)), GR 113808 (5-HT(4)) and SB 269970 (5-HT(7)) had no influence on the 5-HT-induced response; the 5-HT(1A) receptor antagonists NAN 190 (pK(b)=8.13+/-0.06) and WAY 100635 (pK(b)=8.69+/-0.07), and the 5-HT(1,2,5,6,7) receptor antagonist methysergide concentration-dependently inhibited the 5-HT-induced contractile response. The 5-HT(1,7) receptor agonist 5-carboxamidotryptamine (5-CT) induced a contractile response similar to that of 5-HT; its effect was not influenced by tetrodotoxin and atropine, and SB 269970, but antagonised by WAY 100635. 8-OHDPAT, buspiron and flesinoxan, which are active at rat and human 5-HT(1A) receptors, had no contractile influence. These results suggest that the contractile effect of 5-HT in equine jejunal longitudinal muscle is due to interaction with muscular 5-HT receptors, which cannot be characterised between the actually known classes of 5-HT receptors.

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.

WAY-100635 antagonist-induced plasticity of 5-HT receptors: regulatory differences between a stable cell line and an in vivo native system

We present evidence that the 5-hydroxytryptamine(1A) (5-HT(1A)) receptor antagonist, N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide (WAY-100635), can induce receptor internalization in a human (h)5-HT(1A) receptor Chinese hamster ovary (CHO-K1) cell system. Exposure of h5-HT(1A) CHO cells to WAY-100635 decreased the cell-surface h5-HT(1A) receptor density in a way that was both time (24-72 h) and concentration (1-100 nm) dependent.[(3)H]WAY-100635 and [(3)H]8-hydroxy-dipropylaminotetralin ([(3)H]8-OH-DPAT) saturation analyses demonstrated a significant reduction (50-60%) in total h5-HT(1A) receptor number in the WAY-100635-treated (100 nm; 72 h) compared with control cells. In WAY-100635-treated cells, the 8-OH-DPAT-mediated inhibition of forskolin (FSK)-stimulated cAMP accumulation was right-shifted and the maximal inhibitory response of 8-OH-DPAT was impaired compared with control cells. Similar results were obtained for 8-OH-DPAT-mediated Ca(2+) mobilization after WAY-100635 treatment. h5-HT(1A) receptors labeled with [(3)H]WAY-100635, as well as [(3)H]4-(2'-Methoxy)-phenyl-1-[2'-(N-2''-pyridinyl)-p-fluorobenzamido]ethyl-piperazine (MPPF), exhibited a time-dependent rate of cellular internalization that was blocked by endocytotic suppressors and was pertussis-toxin insensitive. In contrast, quantitative autoradiographic studies demonstrated that chronic treatment of rats with WAY-100635 for two weeks produced a region-specific increase in the 5-HT(1A) receptor density. In conclusion, prolonged exposure of an h5-HT(1A) cell-based system to the 5-HT(1A) antagonist, WAY-100635, induced a paradoxical internalization of cell surface receptor resulting in depressed functional activity. This suggests that an antagonist can influence 5-HT(1A) receptor recycling in vitro differently to in vivo regulatory conditions.

Cloned human 5-HT1A receptor pharmacology determined using agonist binding and measurement of cAMP accumulation

Twenty agonists and nine antagonists were evaluated for their ability to compete for [3H]-8-hydroxy-2-(di-n-propylamino)tetralin ([3H]-8-OH-DPAT) binding to the cloned human serotonin-1A (ch-5-HT1A) receptor expressed in Chinese hamster ovary cells and for their ability to alter adenylyl cyclase activity in the same cells. The most potent full agonists of high affinity included N,N-dipropyl-5-carboxamidotryptamine (pEC50=9.6 +/- 0.1), MDL 73005EF (pEC50=9.3 +/- 0.2), 5-methyl-urapidil (pEC50=9.2 +/- 0.1), 5-carboxamidotryptamine (pEC50=9.1 +/- 0.2), R(+)-8-OH-DPAT (pEC50=8.6 +/- 0.1) and BMY-7378 (pEC50=8.6 +/- 0.1). WB-4101 (pEC50=8.3 +/- 0.2; IA=79%), clozapine (pEC50=8.1 +/- 0.3; IA=29%), (buspirone (pEC50=7.6 +/- 0.2; IA=79%), quipazine (pEC50 <5; IA=45%) and R-DOI (pEC50 < 5; IA=31%) were weaker agonists with partial agonist properties. The most potent antagonists were WAY-100,635 (pKi=10.2 +/- 0.1), methiothepin (pKi=8.8 +/- 0.2), spiperone (pKi=8.7 +/- 0.2) and NAN-190 (pKi=8.5 +/- 0.2). The receptor affinities and functional potencies were well correlated (r=0.88; P <0.0001). Our binding data correlated well with the pharmacology of endogenous 5-HT1A receptors in the rabbit iris-ciliary body (r=0.91; P <0.001) and rat hippocampus (r=0.93, P <0.0001). Our functional cAMP data correlated well with other cAMP accumulation data (r=0.8, P <0.01 vs calf hippocampus) but less so with [35S]-GTPgammaS binding to the ch-5-HT(1A) receptor as a functional activity read-out (r=0.58, P <0.05). The present study provides a detailed pharmacological characterization of the ch-5-HT1A receptor using binding and functional assays.

A competitive interaction model predicts the effect of WAY-100,635 on the time course of R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin-induced hypothermia

The objective of this investigation was to characterize quantitatively the pharmacodynamic interaction between N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide (WAY-100,635) and R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT) in vivo. The 8-OH-DPAT-induced change in body temperature was used as a pharmacodynamic endpoint. Four groups of rats each received 1 mg/kg 8-OH-DPAT in 5 min during computer-controlled infusions of physiological saline or WAY-100,635, targeted at steady-state concentrations of 20, 85, and 170 ng/ml. Body temperature was monitored continuously with a telemetric system, and frequent blood samples were obtained to determine the pharmacokinetics of both drugs. Large differences in pharmacokinetics were observed between WAY-100,635 and R-8-OH-DPAT, reflected in values of the terminal elimination half-life of 33 and 143 min, respectively. Infusion of WAY-100,635 had no influence on the pharmacokinetics of R-8-OH-DPAT. With regard to the pharmacodynamics, clear antagonism of the R-8-OH-DPAT-induced hypothermia was observed. The complex pharmacological effect versus time profiles of R-8-OH-DPAT were analyzed on the basis of an indirect physiological response model with set point control coupled to a competitive interaction model for an agonist and antagonist acting at a common receptor. This model converged, yielding precise estimates of the pharmacodynamic parameters of both WAY-100,635 and R-8-OH-DPAT, which were independent of the infusion rate of WAY-100,635. The estimated in vivo binding constant of WAY-100,635 was 0.98 ng/ml (2.3 nM), which is very similar to the reported value from in vitro receptor binding assays. The findings of this investigation show that, in contrast to earlier reports in the literature, WAY 100,635 behaves as a pure competitive antagonist at the 5-hydroxytryptamine(1A) receptor in vivo.

Efficacy of antipsychotic agents at human 5-HT(1A) receptors determined by [3H]WAY100,635 binding affinity ratios: relationship to efficacy for G-protein activation

5-HT(1A) receptors are implicated in the aetiology of schizophrenia. Herein, the influence of 15 antipsychotics on the binding of the selective 'neutral' antagonist, [3H]WAY100,635 ([3H]N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-cyclo-hexanecarboxamide), was examined at human 5-HT(1A) receptors expressed in Chinese Hamster Ovary cells. In competition binding experiments, 5-HT displayed biphasic isotherms which were shifted to the right in the presence of the G-protein uncoupling agent, GTPgammaS (100 microM). In analogy, the isotherms of ziprasidone, quetiapine and S16924 (((R-2-[1-[2-(2,3-dihydro-benzo[1,4]dioxin-5-yloxy)-ethyl]-pyrrolidin-3yl]-1-(4-fluoro-phenyl)-ethanone), were displaced to the right by GTPgammaS, consistent with agonist actions. Binding of several other antipsychotics, such as ocaperidone, olanzapine and risperidone, was little influenced by GTPgammaS. Isotherms of the neuroleptics, haloperidol, chlorpromazine and thioridazine were shifted to the left in the presence of GTPgammaS, suggesting inverse agonist properties. For most ligands, the magnitude of affinity changes induced by GTPgammaS (alteration in pK(i) values) correlated well with their previously determined efficacies in [35S]GTPgammaS binding studies [Eur. J. Pharmacol. 355 (1998) 245]. In contrast, the affinity of the 'atypical' antipsychotic agent, clozapine, which is a known partial agonist at 5-HT(1A) receptors, was less influenced by GTPgammaS. When the ratio of high-/low-affinity values was plotted against efficacy, hyperbolic isotherms were obtained, consistent with a modified ternary complex model which assumes that receptors can adopt active conformations in the absence of agonist. In conclusion, modulation of [3H]-WAY100,635 binding by GTPgammaS differentiated agonist vs. inverse agonist properties of antipsychotics at 5-HT(1A) receptors. These may contribute to differing profiles of antipsychotic activity.

Estrogen modulates 5-HT(1A) agonist inhibition of lordosis behavior but not binding of [(3)H]-8-OH-DPAT

Previous studies showed that repeated estrogen treatment reduces the ability of the 5-HT(1A) receptor agonist, 8-hydroxy-2(di-n-propylamino) tetralin (8-OH-DPAT), to inhibit lordosis behavior of female rats. The present study evaluated the effects of repeated estrogen treatment on lordosis behavior and 5-HT(1A) receptor binding and coupling to G protein in the hypothalamus-preoptic area using the agonist ligand [3H]-8-OH-DPAT, which binds selectively to G-protein-coupled 5-HT(1A) receptors. Rats were injected twice with 25 or 50 microg of estradiol benzoate (EB) 7 days apart followed by 500 microg of progesterone (P) 48 h after the second EB injection. Controls received a single injection of 25 or 50 microg EB followed 48 h later by 500 microg of P. Four hours after P, 0.15 mg/kg 8-OH-DPAT was injected, and lordosis behavior examined for 30 min. Rats treated twice with EB showed significantly less 8-OH-DPAT inhibition of lordosis behavior than rats receiving a single EB injection. For receptor binding, rats received EB without P treatment. None of the estrogen treatments reduced [3H]-8-OH-DPAT binding density or affinity in the hypothalamus-preoptic area or hippocampus. These studies suggest that estrogen modulates 5-HT(1A) agonist potency without a measurable change in 5-HT(1A) receptor density or coupling to G protein.

Differential modulation by GTPgammaS of agonist and inverse agonist binding to h5-HT(1A) receptors revealed by [3H]-WAY100,635

1. The interaction of serotonergic ligands at human (h) 5-HT(1A) receptors expressed in Chinese hamster ovary cells was examined with the selective 'neutral' 5-HT(1A) antagonist [(3)H]-WAY100,635. Its binding was saturable (K(D)=0.056 nM) with a B(max) (3.65 pmol mg(-1)) significantly higher than that of two other selective 5-HT(1A) radioligands: the partial agonist, [(3)H]-S15535 (2.77 pmol mg(-1)) and the agonist, [(3)H]-8-OH-DPAT (2.02 pmol mg(-1)). 2. The influence of GTPgammaS (100 microM) on the binding affinity of 15 serotonergic agonists, partial agonists, antagonists and inverse agonists was investigated in competition binding experiments with [(3)H]-WAY100,635. 3. Agonists, including 5-HT, 8-OH-DPAT and buspirone, displayed biphasic isotherms which shifted to the right in the presence of GTPgammaS. In contrast, isotherms of the inverse agonists, methiothepin, (+)butaclamol and spiperone, were shifted to the left in the presence of GTPgammaS. Unlabelled WAY100,635 was the only ligand that was unaffected by GTPgammaS, consistent with 'neutral' antagonist properties. 4. The magnitude of affinity changes induced by GTPgammaS for 13 ligands was highly correlated (r = 0.98) with their efficacy (positive and negative) previously determined by [(35)S]GTPgammaS binding. 5. In contrast, the napthylpiperazine derivative and high efficacy agonist, S14506, displayed only a modest GTPgammaS shift, in accordance with previous indications of 'atypical' binding properties of this ligand. A further full agonist, S14671, which is chemically closely-related to S14506, also displayed a minimal GTPgammaS shift, underpinning this observation. 6. In conclusion, [(3)H]-WAY100,635 constitutes a useful neutral antagonist radioligand for the characterization of drug actions at h5-HT(1A) receptors. GTPgammaS-induced affinity changes of agonist and inverse agonist competition isotherms generally correlate well with ligand efficacy, with the notable exception of two chemically-similar agents, S14506 and S14671, which are efficacious agonists, yet relatively insensitive to h5-HT(1A) receptor/G-protein coupling changes.

The 5-HT1A receptor and its ligands: structure and function

An overview is presented on progress made in research on 5-HT1A receptors and their ligands since their discovery in 1983. Molecular biology has offered new tools, for example cloned 5-HT1A receptors, their mutants and chimeras to study structure and function. Many compounds, belonging to different chemical classes, display high affinity and selectivity for 5-HT1A receptors. The majority of these compounds are agonists or partial agonists, full antagonists are still scarce. Agonists and partial agonists are active in various animal models of anxiety and depression. Partial receptor agonists have been proven to be effective in general anxiety disorder and depression in man. Potential therapeutic applications for 5-HT1A receptor antagonists are evaluated, for example, in cognition disorders.

Characterization of 5-HT1A receptor functional coupling in cells expressing the human 5-HT1A receptor as assessed with the cytosensor microphysiometer

The functional activity of a series of 5-HT1A receptor ligands has been evaluated in a cell line expressing the human 5-HT1A receptor (h5-HT1A x CHO) using the agonist-stimulated increase in extracellular acidification rate, measured with the microphysiometer, as a functional assay. Both 5-CT and 8-OH-DPAT were potent agonists in stimulating an increase in extracellular acidification rate in h5-HT1A x CHO cells with estimated EC50 values of 1.2 and 7.8 nM, respectively. Additionally, these two 5-HT1A receptor agonists elicited a similar maximum response. Concentration-dependent agonist activity was also observed in the presence of buspirone, ipsapirone, BMY7378, NAN-190 and WAY100135, and each of these compounds behaved as partial 5-HT1A receptor agonists. The selective 5-HT1A receptor antagonist WAY100635 produced a potent (IC50, 2.3 nM) and complete block of the 8-OH-DPAT-stimulated response. An evaluation of the inhibitory activity of a series of 5-HT1A receptor antagonists produced the following rank order of potency; WAY100635 > LY206130 (IC50, 7.1 nM) > WAY100135 (30.8 nM) > pindolol (76.2 nM) > (-)UH-301 (92.8 nM). Parallel studies on the inhibition of forskolin-stimulated adenylyl cyclase activity in hS-HT1A x CHO cells revealed that agonist potencies were generally similar between the two functional assays and were in good agreement with the estimated 5-HT1A receptor binding affinities. However, the relative efficacies determined for the partial agonists in the cAMP assay were substantially greater than those observed with the microphysiometer. Finally, antagonists were considerably weaker in the cAMP assay compared with the microphysiometer. The evaluation of 5-HT1A ligands using the microphysiometer, which represents a very distinct indice of 5-HT1A receptor function compared with the cAMP assay, results in a different profile of functional activity.