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

Distinctive in vitro signal transduction profile of NLX-112, a potent and efficacious serotonin 5-HT1A receptor agonist

Objectives

NLX-112 (befiradol, F13640) is a selective serotonin 5-HT1A receptor agonist. Although it has been tested in vivo, little has been reported on its in vitro signal transduction profile.

Methods

NLX-112 was tested on G-protein activation, inhibition of adenylyl cyclase, ERK1/2 phosphorylation (pERK) and receptor internalization in recombinant cell lines. NLX-112 was also tested on G-protein activation in rat hippocampal membranes. Gα subunit mRNA expression in cell lines and rat brain tissue was quantified by quantitative PCR.

Key findings

For all signalling measures, NLX-112 exhibited agonist efficacy greater than for reference compounds ((±)8-OH-DPAT or buspirone), but similar to the endogenous agonist, serotonin, and was more potent for pERK than other responses. In rat hippocampal membranes, NLX-112 stimulated ‘total G-proteins' but, unlike (±)8-OH-DPAT and buspirone, was more potent for Gαo activation. Cell lines predominantly expressed Gαi1 and Gαi2 mRNA, with low levels of Gαo, whereas in rat brain Gαo subunits showed highest mRNA expression.

Conclusions

Unlike reference compounds, NLX-112 was a highly efficacious agonist in vitro, preferentially activating pERK in cell lines and Gαo proteins in rat hippocampal membranes. However, Gα subunit mRNA levels differ markedly between rat brain and cell lines, warranting caution when extrapolating from recombinant systems to native tissues.

Analysis of mechanisms for memory enhancement using novel and potent 5-HT1A receptor ligands

In light of the involvement of serotonergic 5-HT1A receptors in the mediation of the memory of aversive events, the potent and selective 5-HT1A receptor antagonists, MC18 fumarate and VP08/34 fumarate, were tested in the passive avoidance task (PA), a rodent model of instrumental conditioning. Either alone or in combination with the prototypical agonist 8-OH-DPAT, MC18 fumarate at doses (0.1, 0.3 and 1mg/kg given 15min prior to training) exerted a dose-dependent facilitation of PA memory retention. When administered 15min prior to 8-OH-DPAT (0.3 and 1mg/kg), MC18 fumarate at a dose of 0.3mg/kg, enhanced significantly the impairment of PA retention caused by 8-OH-DPAT (1mg/kg). However, VP08/34 fumarate given at the same doses exerted no statistically effect on PA retention memory. Furthermore, VP08/34 fumarate given 15min prior to 8-OH-DPAT (0.3 and 1mg/kg) only slightly enhanced the PA impairment induced by 8-OH-DPAT. In conclusion, the profile of MC18 fumarate is intriguing since it behaves in a manner which differs from both full receptor antagonists such as NAD-299 or partial receptor agonists. The results also illustrate the importance of subtle receptor interaction and probably ligand efficacy in determining the actions of two almost identical 5-HT1A receptor ligands in cognitive function such as instrumental learning.

Drug-induced hypothermia by 5HT1A agonists provide neuroprotection in experimental stroke: new perspectives for acute patient treatment

BACKGROUND

Drug-induced hypothermia reduces brain damage in animal stroke models and is an undiscovered potential in human stroke treatment. We studied hypothermia induced by the serotonergic agonists S14671 (1-[2-(2-thenoylamino)ethyl]-4[1-(7- methoxynaphtyl)]piperazine) and ipsapirone in a rat stroke model and in man by literature meta-analysis.

METHODS

Rats had 60 minutes of middle cerebral artery occlusion (MCAO) and then 7 days of survival. Body temperatures were monitored for 22 hours. Thirty minutes after MCAO, 1 group (n = 9) received bolus of S14671 (.75 mg/kg) and continuous infusion of .06 mg/kg hour(-1) S14671 for 20 hours. Other MCAO rats (n = 7) had bolus of ipsapirone (.75 mg/kg) and continuous infusion of .25 mg/kg hour(-1) ipsapirone for 3 hours. Controls (n = 9; n = 5) received similar amounts of vehicle as bolus and continuous infusion for 20 hours/3 hours. Additional controls of the S14761 effect in MCAO were performed as previously mentioned (n = 10) but with rats kept normothermic by a heating lamp for 22 hours. Finally, a meta-analysis of ipsapirone-induced hypothermia in man was included.

RESULTS

Infarct volumes were reduced by 50% in hypothermic rats versus controls (P < .05). S14671 rats kept normothermic did not show infarct reduction (P > .05). The body temperature after stroke was reduced 1.0-3.0°C compared with controls for 20 hours with S14671 treatment and for 6 hours with ipsapirone treatment. In humans, ipsapirone reduced temperature in average with .55 °C ranging between .1-1.4 °C.

CONCLUSIONS

5-hydroxytryptamine receptor 1A (5HT(1A)) agonists significantly reduce infarct volumes in MCAO rats primarily because of the hypothermic drug effect. 5HT(1A) agonists may be introduced to reduce body temperatures rapidly and prepare patients for further therapeutic hypothermia.

Similar anxiolytic effects of agonists targeting serotonin 5-HT1A or cannabinoid CB receptors on zebrafish behavior in novel environments

The discovery that selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine are present and bioaccumulate in aquatic ecosystems have spurred studies of fish serotonin transporters (SERTs) and changes in SSRI-sensitive behaviors as adverse outcomes relevant for risk assessment. Many SSRIs also act at serotonin 5-HT1A receptors. Since capitalizing on this action may improve treatments of clinical depression and other psychiatric disorders, novel multimodal drugs that agonize 5-HT1A and block SERT were introduced. In mammals both 5-HT1A and CB agonists, such as buspirone and WIN55,212-2, reduce anxious behaviors. Immunological and behavioral evidence suggests that 5-HT1A-like receptors may function similarly in zebrafish (Danio rerio), yet their pharmacological properties are not well characterized. Herein we compared the density of [(3)H] 8-hydroxy-2-di-n-propylamino tetralin (8-OH-DPAT) binding to 5-HT1A-like sites in the zebrafish brain, to that of similarly Gαi/o-coupled cannabinoid receptors. [(3)H] 8-OH-DPAT specific binding was 176±8, 275±32, and 230±36fmol/mg protein in the hypothalamus, optic tectum, and telencephalon. [(3)H] WIN55,212-2 binding density was higher in those same brain regions at 6±0.3, 5.5±0.4 and 7.3±0.3pm/mg protein. The aquatic light-dark plus maze was used to examine behavioral effects of 5-HT1A and CB receptor agonists on zebrafish novelty-based anxiety. With acute exposure to the 5-HT1A partial-agonist buspirone (50mg/L), or dietary exposure to WIN55,212-2 (7μg/week) zebrafish spent more time in and/or entered white arms more often than controls (p<0.05). Acute exposure to WIN55,212-2 at 0.5-50mg/L reduced mobility. These behavioral findings suggest that azipirones, like cannabinoid agonists, have anxiolytic and/or sedative properties on fish in novel environments. These observations highlight the need to consider potential ecological risks of azapirones and multimodal antidepressants in the future.

Characterization of 5-HT₁A receptors and their complexes with G-proteins in budded baculovirus particles using fluorescence anisotropy of Bodipy-FL-NAN-190

Bodipy-FL-NAN-190 was found to be well suited for characterization of ligand binding to 5-HT1A receptors expressed in budded baculovirus particles, as binding is accompanied by large increases in fluorescence intensity and anisotropy. This ligand appears to bind rapidly (t1/2,ass<1 min), reversibly (t1/2,diss∼6 min) and has high affinity (Kd=0.30 ± 0.13 nM). This fluorescence anisotropy assay based on Bodipy-FL-NAN-190 binding to baculovirus particles was also a suitable assay system for the pharmacological characterization of non-labelled serotonergic ligands, as well as being sensitive to the presence of G-proteins and guanine nucleotides. Coexpression of αi subunits of human G-proteins in baculovirus particles resulted in the appearance of significantly greater proportion of nucleotide sensitive high affinity agonist binding sites. There were no significant differences between αi1 and αi3 subtypes, while ligand binding in the presence of αi2 had higher sensitivity to GDP and Mn(2+).

Characterization of [3H]LUF5834: A novel non-ribose high-affinity agonist radioligand for the adenosine A1 receptor

The adenosine A(1) receptor is a promising therapeutic target for neurological disorders such as cognition deficits and is involved in cardiovascular preconditioning. Classically adenosine receptor agonists were all derivatives of adenosine, and thought to require a D-ribose moiety. More recently, however, the discovery of non-adenosine agonists for the human adenosine A(1) receptor (hA(1)R) has challenged this dogma (Beukers et al., 2004). In this study we characterize the tritiated form of one of these compounds, [(3)H]LUF5834, as the first non-ribose partial agonist radioligand with nanomolar affinity for the hA(1)R. Due to its partial agonist efficacy, [(3)H]LUF5834 labeled both G protein-coupled and uncoupled receptors with a similar high affinity. Using [(3)H]LUF5834 we performed competition binding experiments to characterize a range of A(1)R ligands varying in efficacy from the full agonist CPA to the inverse agonist DPCPX. Surprisingly, in the control condition both agonists and inverse agonists displayed biphasic isotherms. With the addition of 1mM GTP the high affinity isotherm of agonists or the low affinity isotherm of inverse agonists was lost revealing the mechanism of action of such inverse agonists at the A(1)R. Consequently, [(3)H]LUF5834 represents a novel high affinity radioligand for the A(1)R and may prove a useful tool to provide estimates of inverse agonist efficacy at this receptor.

Evaluation of [C]S14506 and [F]S14506 in rat and monkey as agonist PET radioligands for brain 5-HT(1A) receptors

In vitro and ex vivo measurements have shown that the binding of the selective high-affinity agonist, S14506 (1-[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxy-naphthyl)piperazine), to 5-HT(1A) receptors, is similar in affinity (K(d) = 0.79 nM) and extent (B(max)) to that of the antagonist, WAY 100635. We aimed to test whether S14506, labeled with a positron-emitter, might serve as a radioligand for imaging brain 5-HT(1A) receptors in vivo with positron emission tomography (PET). Here we evaluated [(11)C]S14506 and [(18)F]S14506 in rat and rhesus monkey in vivo. After intravenous administration of [(11)C]S14506 into rat, radioactivity entered brain, reaching 210% SUV at 2 min. Radioactivity uptake into brain was higher (~ 350% SUV) in rats pre-treated with the P-glycoprotein (P-gp) inhibitor, cyclosporin A. In rhesus monkey, peak brain uptake of radioactivity after administration of [(11)C]S14506 or [(18)F]S14506 was also moderate and for [(11)C]S14506 increased from ~ 170% SUV after 7 min, to 240% SUV in a monkey pre-treated with the P-gp inhibitor, tariquidar. The ratios of radioactivity in 5-HT(1A) receptor-rich regions, such as cingulate or hippocampus to that in receptor-poor cerebellum reached between 1.35 and 1.5 at 60 min for both [(11)C]S14506 and [(18)F]S14506. [(11)C]S14506 gave one major polar radiometabolite in monkey plasma, and [(18)F]S14506 gave three and two more polar radiometabolites in rat and monkey plasma, respectively. The rat radiometabolites of [(18)F]S14506 did not accumulate in brain. [(18)F]S14506 was not radiodefluorinated in monkey. Thus, despite high-affinity and lack of troublesome brain radiometabolites, both [(11)C]S14506 and [(18)F]S14506 were ineffective for imaging rat or monkey brain 5-HT(1A) receptors in vivo, even under P-gp inhibited conditions. Explanations for the failure of these radioligands are offered.

Interactions of Magnolia and Ziziphus extracts with selected central nervous system receptors

ETHNOPHARMACOLOGICAL RELEVANCE

Magnolia officinalis Rehder and Wilson [Magnoliaceae] bark and Ziziphus spinosa (Buhge) Hu ex. Chen. [Fam. Rhamnaceae] seed have a history of use in traditional Asian medicine for mild anxiety, nervousness and sleep-related problems.

AIM OF THE STUDY

To identify pharmacological targets, extracts of Magnolia officinalis (ME), Ziziphus spinosa (ZE), and a proprietary fixed combination (MZE) were tested for affinity with central nervous system receptors associated with relaxation and sleep.

METHODS

In vitro radioligand binding and cellular functional assays were conducted on: adenosine A(1), dopamine (transporter, D(1), D(2S), D(3), D(4.4) and D(5)), serotonin (transporter, 5-HT(1A), 5-HT(1B), 5-HT(4e), 5-HT(6) and 5-HT(7)) and the GABA benzodiazepine receptor.

RESULTS

Interactions were demonstrated with the adenosine A(1) receptor, dopamine transporter and dopamine D(5) receptor (antagonist activity), serotonin receptors (5-HT(1B) and 5-HT(6) antagonist activity) and the GABA benzodiazepine receptor at a concentration of 100 microg/ml or lower. ME had an affinity with adenosine A(1) (K(i) of 9.2+/-1.1 microg/ml) and potentiated the GABA activated chloride current at the benzodiazepine subunits of the GABA receptor (maximum effect at 50 microg/ml). ME had a modest antagonist action with 5-HT(6) and ZE with the 5-HT(1B) receptor.

CONCLUSION

The interactions in the receptor binding models are consistent with the traditional anxiolytic and sleep-inducing activities of Magnolia officinalis bark and Ziziphus spinosa seed.

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.

In vitro and in vivo characterization of F-97013-GD, a partial 5-HT1A agonist with antipsychotic- and antiparkinsonian-like properties

In order to better define the role of 5-HT(1A) receptors in the modulation of extrapyramidal motor functions, we investigated the effect of 5-HT(1A) agonists on tacrine-induced tremulous jaw movements (TJM) in rats, a putative model of parkinsonian tremor. Acute injection of 5-HT(1A) agonists 8-OH-DPAT and buspirone dose-dependently counteracted the tacrine-induced oral movements (ED(50)=0.04 and 1.0mg/kg, respectively), an effect reversed by the selective 5-HT(1A) antagonist WAY 100,635. In contrast to classical antipsychotics, the atypical antipsychotics risperidone (ED(50)=0.3mg/kg) and clozapine (ED(50)=1.5mg/kg) blocked the oral movements induced by the cholinomimetic agent at or below the doses required for suppression of conditioned avoidance response. The compound F-97013-GD (6-methyl-2-[4-(naphtylpiperazin-1-yl)butyl]-3-(2H)-pyridazinone), a putative antipsychotic drug that in functional in vitro and in vivo assays behaved as a mixed dopamine D(2)-antagonist and 5-HT(1A)-partial agonist, also displayed a potent antitremorgenic effect in this paradigm (ED(50)=0.5mg/kg). Interestingly, pretreatment with WAY 100,635 blocked the inhibitory effect of F-97013-GD but not that of clozapine. The 5-HT depleting agent para-chlorophenylalanine (PCPA) partially attenuated tacrine-induced TJM but did not block the suppressive effect of 5-HT(1A) agonists. In addition, only high doses of F-97013-GD induced catalepsy in rodents and, like 8-OH-DPAT and clozapine, the compound reversed the haloperidol-induced catalepsy in rats. These results show that 5-HT(1A) receptors play a role in the regulation of tacrine-induced TJM and suggest that their activation by novel antipsychotics may not only reduce the extrapyramidal side effects EPS liability, but also be effective in the treatment of parkinsonian tremor.

Differential ion current activation by human 5-HT(1A) receptors in Xenopus oocytes: evidence for agonist-directed trafficking of receptor signalling

The subject of the present study was the functional and pharmacological characterization of human 5-HT(1A) receptor regulation of ion channels in Xenopus oocytes. Activation of the heterologously expressed human 5-HT(1A) receptor induced two distinct currents in Xenopus oocytes, consisting of a smooth inward current (I(smooth)) and an oscillatory calcium-activated chloride current, I(Cl(Ca)). 5-HT(1A) receptor coupling to both ionic responses as well as to co-expressed inward rectifier potassium (GIRK) channels was pharmacologically characterized using 5-HT(1A) receptor agonists. The relative order of efficacy for activation of GIRK current was 5-HT approximately F 13714 approximately L 694,247 approximately LY 228,729>flesinoxan approximately (+/-)8-OH-DPAT. In contrast, flesinoxan and (+/-)8-OH-DPAT typically failed to activate I(Cl(Ca)). The other ligands behaved as full or partial agonists, exhibiting an efficacy rank order of 5-HT approximately L 694,247>F 13714 approximately LY 228,729. The pharmacological profile of I(smooth) activation was completely distinct: flesinoxan and F 13714 were inactive and rather exhibited an inhibition of this current. I(smooth) was activated by the other agonists with an efficacy order of L 694,247>5-HT approximately LY 228,729>(+/-)8-OH-DPAT. Moreover, activation of I(smooth) was not affected by application of pertussis toxin or the non-hydrolyzable GDP-analogue, guanosine-5'-O-(2-thio)-diphosphate (GDP betaS), suggesting a GTP binding protein-independent pathway. Together, these results suggest the existence of distinct and agonist-specific signalling states of this receptor.

Comparison of hippocampal G protein activation by 5-HT(1A) receptor agonists and the atypical antipsychotics clozapine and S16924

This study employed [(35)S]guanosine 5'- O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding to compare the actions of antipsychotic agents known to stimulate cloned, human 5-HT(1A) receptors with those of reference agonists at postsynaptic 5-HT(1A) receptors. In rat hippocampal membranes, the following order of efficacy was observed (maximum efficacy, E(max), values relative to 5-HT=100): (+)8-OH-DPAT (85), flesinoxan (62), eltoprazine (60), S14506 (59), S16924 (48), buspirone (41), S15535 (22), clozapine (22), ziprasidone (21), pindolol (7), p-MPPI (0), WAY100,635 (0), spiperone (0). Despite differences in species and tissue source, the efficacy and potency (pEC(50)) of agonists (with the exception of clozapine) correlated well with those determined previously at human 5-HT(1A) receptors expressed in Chinese hamster ovary (CHO) cells. In contrast, clozapine was more potent at hippocampal membranes. The selective antagonists p-MPPI and WAY100,635 abolished stimulation of binding by (+)8-OH-DPAT, clozapine and S16924 (p-MPPI), indicating that these actions were mediated specifically by 5-HT(1A) receptors. Clozapine and S16924 also attenuated 5-HT- and (+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding, consistent with partial agonist properties. In [(35)S]GTPgammaS autoradiographic studies, 5-HT-induced stimulation, mediated through 5-HT(1A) receptors, was more potent in the septum (pEC(50) approximately 6.5) than in the dentate gyrus of the hippocampus (pEC(50) approximately 5) suggesting potential differences in coupling efficiency or G protein expression. Though clozapine (30 and 100 microM) did not enhance [(35)S]GTPgammaS labelling in any structure, S16924 (10 micro M) modestly increased [(35)S]GTPgammaS labelling in the dentate gyrus. On the other hand, both these antipsychotic agents attenuated 5-HT (10 microM)-stimulated [(35)S]GTPgammaS binding in the dentate gyrus and septum. In conclusion, clozapine, S16924 and ziprasidone act as partial agonists for G protein activation at postsynaptic 5-HT(1A) receptors in the hippocampus. These data support a role of postsynaptic 5-HT(1A) receptors in the functional profiles of certain antipsychotic agents.

Antibody capture assay reveals bell-shaped concentration-response isotherms for h5-HT(1A) receptor-mediated Galpha(i3) activation: conformational selection by high-efficacy agonists, and relationship to trafficking of receptor signaling

Although serotonin 5-HT(1A) receptors couple to several Gi/o G-protein subtypes, little is known concerning their differential activation patterns. In this study, in membranes of Chinese hamster ovary cells expressing h5-hydroxytryptamine(1A) receptors (CHO-h5-HT(1A)), isotherms of 5-HT-stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding were biphasic, suggesting coupling to multiple G-protein subtypes. The high potency component was abolished by preincubation with an antibody recognizing Galpha(i3) subunits and was resistant to induction of [(35)S]GTPgammaS dissociation by unlabeled GTPgammaS, thus yielding a bell-shaped concentration-response isotherm. To directly investigate Galpha(i3) activation, we adopted an antibody-capture/scintillation proximity assay. 5-HT and other high-efficacy agonists yielded bell-shaped [(35)S]GTPgammaS binding isotherms, with peaks at nanomolar concentrations. As drug concentrations increased, Galpha(i3) stimulation progressively returned to basal values. In contrast, the partial agonists (-)-pindolol and 4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) displayed sigmoidal stimulation isotherms, whereas spiperone and other inverse agonists sigmoidally inhibited [(35)S]GTPgammaS binding. Agonist-induced stimulation and inverse agonist-induced inhibition of Galpha(i3) activation were i) abolished by pretreatment of CHO-h5-HT(1A) cells with pertussis toxin; ii) reversed by the selective 5-HT(1A) antagonist (N-[2-[4-(2-methoxy-phenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-cyclohexane-carboxamide) fumarate (WAY100,635), and iii) absent in nontransfected CHO cell membranes. 5-HT isotherms could be modified by altering sodium concentration; only stimulatory actions were observed at 300mM NaCl, whereas only inhibitory actions were seen at 10 mM NaCl. Furthermore, bell-shaped isotherms were not detected at short incubation times, suggesting time-dependent changes in receptor/Galpha(i3) coupling. Taken together, these data show that low but not high concentrations of high-efficacy 5-HT(1A) agonists direct receptor signaling to Galpha(i3). In contrast, partial agonists favor h5-HT(1A) receptor signaling to Galpha(i3) over a wide concentration range, whereas inverse agonists inhibit constitutive Galpha(i3) activation.

Specific labelling of serotonin 5-HT(1B) receptors in rat frontal cortex with the novel, phenylpiperazine derivative, [3H]GR125,743. A pharmacological characterization

Although several tritiated agonists have been used for radiolabelling serotonin (5-hydroxytryptamine, 5-HT)(1B) receptors in rats, data with a selective, radiolabelled antagonist have not been presented. Inasmuch as [3H]GR125,743 specifically labels cloned, human and native guinea pig 5-HT(1B) receptors and has been employed for characterization of cerebral 5-HT(1B) receptor in the latter species [Eur. J. Pharmacol. 327 (1997) 247.], the present study evaluated its utility for characterization of native, cerebral 5-HT(1B) sites in the rat. In homogenates of frontal cortex, [3H]GR125,743 (0.8 nM) showed rapid association (t(1/2)=3.4 min), >90% specific binding and high affinity (K(d)=0.6 nM) for a homogeneous population of receptors with a density (B(max)) of 160 fmol/mg protein. In competition binding studies, affinities were determined for 15 chemically diverse 5-HT(1B) agonists, including 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1H-indole-3-yl]ethylamine (L694,247; pK(i), 10.4), 5-carboxamidotryptamine (5-CT; 9.7), 3-[3-(2-dimethylamino-ethyl)-1H-indol-6-yl]-N-(4-methoxybenzyl)acrylamide (GR46,611; 9.6), 5-methoxy-3-(1,2,5,6-tetrahydro-4-pyridinyl)-1H-indole (RU24,969; 9.5), dihydroergotamine (DHE; 8.6), 5-H-pyrrolo[3,2-b]pyridin-5-one,1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl (CP93,129; 8.4), anpirtoline (7.9), sumatriptan (7.4), 1-[2-(3-fluorophenyl)ethyl]-4-[3-[5-(1,2,4-triazol-4-yl)-1H-indol-3-yl]propyl]piperazine (L775,606; 6.4) and (minus sign)-1(S)-[2-[4-(4-methoxyphenyl)piperazin-1-yl]ethyl]-N-methyl-3,4-dihydro-1H-2-benzopyran-6-carboxamide (PNU109,291; <5.0). Similarly, affinities were established for 13 chemically diverse antagonists, including N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-pyridyl)benzamide (GR125,743; pK(i), 9.1), (-)cyanopindolol (9.0), (-)-tertatolol (8.2), N-(4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiozol-3-yl)biphenyl-4-carboxamide (GR127,935; 8.2), N-[3-(1,4-benzodioxan-5-yl)piperidin-4-yl]N-(indan-2yl)amine (S18127; 7.9), metergoline (7.8), (-)-pindolol (7.6), 1'-methyl-5-[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-biphenyl-4-ylcarbonyl]-2,3,6,7-tetrahydro-5H-spiro[furo[2,3-f]indole-3,4'-piperidine] (SB224,289; 7.5) and ketanserin (<5.0). These rank orders of affinity correspond to the binding profile of 5-HT(1B) rather than 5-HT(1D) receptors. The low affinities of L775,066 and PNU109,291 versus L694,247 should be noted, as well as the low affinity of ketanserin as compared to SB224,289. Finally, in line with species differences, the affinities of several ligands including CP93,129, RU24,969, (-)-pindolol and (-)-propanolol in rat 5-HT(1B) sites were markedly different to guinea pig 5-HT(1B) sites labelled with [3H]GR125,743. In conclusion, [3H]GR125,743 is an appropriate tool for the radiolabelling of native, rat 5-HT(1B) receptors and permitted determination of the affinities of an extensive series of ligands at these sites.

Improvement in the selectivity and metabolic stability of the serotonin 5-HT(1A) ligand, S 15535: a series of cis- and trans-2-(arylcycloalkylamine) 1-indanols

S 15535 (1) displays a distinctive profile of agonist and antagonist (weak partial agonist) activity at pre- and postsynaptic 5-HT(1A) receptors, respectively. It has proven to be active in several models predictive of anxiolytic, antidepressant, and procognitive properties. In an attempt to increase its selectivity and metabolic stability, and guided by the results of human metabolic studies, we prepared a series of cis- and trans-2-(arylcycloalkylamine) 1-indanols. Irrespective of the nature of the arylcycloalkylamine moiety or the presence of substituents on the indanol ring, trans isomers invariably showed the highest affinity for human, recombinant h5-HT(1A) receptors. Among them, compounds 39, 42, 45, 49, 52, 53, 54, 57, 61, 64, 67, and 70 displayed similar or higher affinity than the parent compound 1 (pK(i) > or = 9.1). Lack of selectivity toward alpha1-adrenoceptors has been frequently encountered with 5-HT(1A) ligands. While S 15535 itself presents reasonable selectivity (158-fold) in this respect, trans piperazine derivatives 4-trans,35, 39, 41, 47, 64, 68, 69, 70, 71 displayed even more pronounced selectivity vs alpha1-adrenoceptors, with the nitro derivative 70 being highly selective (1259-fold). However, among the set of trans piperidines prepared, only 64, which also bears a nitro on the indanol ring, displayed selectivity greater than the parent compound 1. All trans derivatives behaved as partial agonists at h5-HT(1A) receptors, as determined by their submaximal stimulation of [35S]GTPgammaS binding to a level comparable to that observed with S 15535. In metabolic stability studies in vitro using human microsomes and hepatocytes, only trans piperazines and, in particular, 35, 39, 41, 68, 69, and 70, showed an improvement relative to 1, whereas trans piperidines did not. Compounds 35, 39, 41, and 70, which combined both improved selectivity and metabolic stability, and which retained the distinctive pharmacological characteristics of S 15535, were evaluated in animal models of anxiety. Of these, 35, which showed the highest oral bioavailability in vivo in rats, was resolved into its two isomers 36 and 37. The eutomer 37 displayed 47% oral bioavailability in the rat and was potently active (0.1-0.5 mg/kg, s.c.) in the rat ultrasonic vocalization and social interaction models, predictive of anxiolytic activity. In conclusion, 2-(arylcycloalkylamine) 1-indanols represent a novel class of potent 5-HT(1A) ligands in which the presence of the hydroxyl group in the benzylic position enhances selectivity, while substituents on the phenyl ring of the indanol moiety improve both selectivity and metabolic stability.

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.

Agonist properties of pindolol at h5-HT1A receptors coupled to mitogen-activated protein kinase

At h5-HT1A receptors, stably transfected into Chinese Hamster Ovary Cells (CHO-h5-HT1A), the selective 5-HT1A receptor agonist, (+)8-hydroxy-dipropyl-amino-tetralin, ((+)8-OH-DPAT), transiently activated mitogen-activated protein kinase (MAPK) with a pEC50 of 8.5. The arylalkylamine, (-)-pindolol, also behaved as an agonist with a maximal effect of 57% relative to (+)8-OH-DPAT (100%), and with a pEC50 of 7.2. The selective 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo-hexane carboxamide (WAY100,635), blocked (+)8-OH-DPAT- and (-)-pindolol-induced MAPK activation with pK(B)s of 9.7 and 9.9, respectively, whereas the selective 5-HT(1B) receptor antagonist, 1'-Methyl-5-[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-ylcarbonyl]-2,3,6,7-tetrahydro-5H-spiro[furo[2,3-f]indole-3,4'-piperidine] (SB224,289) was inactive. Pertussis toxin blocked the actions of (+)8-OH-DPAT and (-)-pindolol demonstrating implication of G(i)/G(o) proteins. Thus, stimulation of MAPK provides an intracellular marker and signal for expression of the agonist actions of (-)-pindolol at h5-HT1A receptors.