GR127935 acts as a partial agonist at recombinant human 5-HT1D alpha and 5-HT1D beta receptors

Effect of sumatriptan on acetic acid-induced experimental colitis in rats: a possible role for the 5-HT1B/1D receptors

Mucosal inflammation in colitis is associated with changes in the intestinal serotonin (5-HT) level. Sumatriptan, a 5-HT_1B/1D receptor agonist, has demonstrated anti-inflammatory characteristics. The purpose of this study was to determine the effects of sumatriptan in a rat model of acute experimental colitis and to elucidate the probable participation of presynaptic 5-HT_1B/1D receptors. To induce colitis, acetic acid (4%) was injected intrarectally. Treatments were given intraperitoneally (IP) once daily over 3 consecutive days starting 1-h post-induction. Sumatriptan was given at 0.5, 1, 2, and 5 mg/kg. GR-127935, a 5-HT_1B/1D receptor antagonist, was injected (0.1 and 0.3 mg/kg) 30 min prior to the most effective dose of sumatriptan (1 mg/kg). On day 4, the colon samples were isolated. Significant enhancements of the tissue tumor necrosis factor-alpha (TNF-α), myeloperoxidase (MPO), microscopic and macroscopic damages, body weight losses, and also reductions in tissue superoxide dismutase (SOD) and 5-HT were observed in colitis rats. On the other hand, sumatriptan at doses 0.5, 1, and 2 mg/kg could diminish pathologic changes in the measured biomarkers, histopathologic damages, and body weight losses. Although GR-127935 at dose 0.3 mg/kg could markedly improve the pathologic indexes, its sub-effective dose (0.1 mg/kg) reversed the protective effect of sumatriptan (1 mg/kg). Moreover, sumatriptan (1 and 5 mg/kg) and GR-127935 (0.3 mg/kg) increased the serotonin level. Post-treatment with low-dose sumatriptan demonstrated a protective impact on this peripheral inflammatory condition. Notably, this protective effect may be mediated, at least in part, through 5-HT_1B/1D receptors, as well as anti-inflammatory and anti-oxidative characteristics.

5-HT1D receptors inhibit the monosynaptic stretch reflex by modulating C-fiber activity

The monosynaptic stretch reflex (MSR) plays an important role in feedback control of movement and posture but can also lead to unstable oscillations associated with tremor and clonus, especially when increased with spinal cord injury (SCI). To control the MSR and clonus after SCI, we examined how serotonin regulates the MSR in the sacrocaudal spinal cord of rats with and without a chronic spinal transection. In chronic spinal rats, numerous 5-HT receptor agonists, including zolmitriptan, methylergonovine, and 5-HT, inhibited the MSR with a potency highly correlated to their binding affinity to 5-HT_1D receptors and not other 5-HT receptors. Selective 5-HT_1D receptor antagonists blocked this agonist-induced inhibition, although antagonists alone had no action, indicating a lack of endogenous or constitutive receptor activity. In normal uninjured rats, the MSR was likewise inhibited by 5-HT, but at much higher doses, indicating a supersensitivity after SCI. This supersensitivity resulted from the loss of the serotonin transporter SERT with spinal transection, because normal and injured rats were equally sensitive to 5-HT after SERT was blocked or to agonists not transported by SERT (zolmitriptan). Immunolabeling revealed that the 5-HT_1D receptor was confined to superficial lamina of the dorsal horn, colocalized with CGRP-positive C-fibers, and eliminated by dorsal rhizotomy. 5-HT_1D receptor labeling was not found on large proprioceptive afferents or α-motoneurons of the MSR. Thus serotonergic inhibition of the MSR acts indirectly by modulating C-fiber activity, opening up new possibilities for modulating reflex function and clonus via pain-related pathways. NEW & NOTEWORTHY Brain stem-derived serotonin potently inhibits afferent transmission in the monosynaptic stretch reflex. We show that serotonin produces this inhibition exclusively via 5-HT_1D receptors, and yet these receptors are paradoxically mostly confined to C-fibers. This suggests that serotonin acts by gating of C-fiber activity, which in turn modulates afferent transmission to motoneurons. We also show that the classic supersensitivity to 5-HT after spinal cord injury results from a loss of SERT, and not 5-HT_1D receptor plasticity.

Development of Flashplate™ Technology to Measure [35S]GTPyS Binding to Chinese Hamster Ovary Cell Membranes Expressing the Cloned Human 5-HTIB Receptor

With the exponential rate at which proposed drugs are being produced for disease therapy, it is now essential to automate assays used to screen these compounds and increase throughput. This has been rapidly adopted for simple radioligand binding assays but is less amenable for certain functional screens. [35S]GTPγS binding represents a convenient method for screening ligands that bind to G protein-coupled receptors and, ultimately, stimulate G-protein activation. In this study we have investigated the use of 96-well FlashPlates™ (NEN DuPont, Stevenage, England) to measure [35S]GTPγS binding to human 5-HT1B receptors expressed in Chinese hamster ovary cells. The cells were added to the individual wells of the FlashPlate and incubated with [35S]GTPγS in the presence or absence of test drug and bound radioactivity measured in a 96-well spectrometer. 5-HT produced a stimulation of basal [35S]GTPγS binding, which was robust within and between experiments, with pEC50= 8.1. The 5-HT1Bpartial agonist GR127935 (2′n-methyl-4′-5-methyl-1,2,4 oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide) caused a partial stimulation (pEC5o = 8.3, intrinsic activity = 0.7), and the selective 5-HTIB receptor antagonist SB-224289 (2,3,6,7-tetrahydro-1′-methyl-5-{2′-methyl-4′-[(5-methyl-1,2,4-oxadiazole-3-yl)biphenyl-4-yl]carbonyl}furo[2,3-flindole-3-spiro-4′-piperidine oxalate) displayed inverse agonism with pEC50 = 7.6. These results are similar to those obtained using the conventional filtration method and indicate that FlashPlate technology can provide a rapid method for measuring [35S]GTPγS binding.

Functional topography of serotonergic systems supports the Deakin/Graeff hypothesis of anxiety and affective disorders

Over 20 years ago, Deakin and Graeff hypothesized about the role of different serotonergic pathways in controlling the behavioral and physiologic responses to aversive stimuli, and how compromise of these pathways could lead to specific symptoms of anxiety and affective disorders. A growing body of evidence suggests these serotonergic pathways arise from topographically organized subpopulations of serotonergic neurons located in the dorsal and median raphe nuclei. We argue that serotonergic neurons in the dorsal/caudal parts of the dorsal raphe nucleus project to forebrain limbic regions involved in stress/conflict anxiety-related processes, which may be relevant for anxiety and affective disorders. Serotonergic neurons in the "lateral wings" of the dorsal raphe nucleus provide inhibitory control over structures controlling fight-or-flight responses. Dysfunction of this pathway could be relevant for panic disorder. Finally, serotonergic neurons in the median raphe nucleus, and the developmentally and functionally-related interfascicular part of the dorsal raphe nucleus, give rise to forebrain limbic projections that are involved in tolerance and coping with aversive stimuli, which could be important for affective disorders like depression. Elucidating the mechanisms through which stress activates these topographically and functionally distinct serotonergic pathways, and how dysfunction of these pathways leads to symptoms of neuropsychiatric disorders, may lead to the development of novel approaches to both the prevention and treatment of anxiety and affective disorders.

Preclinical pharmacology and pharmacokinetics of AZD3783, a selective 5-hydroxytryptamine 1B receptor antagonist

The preclinical pharmacology and pharmacokinetic properties of (2R)-6-methoxy-8-(4-methylpiperazin-1-yl)-N-(4-morpholin-4-ylphenyl)chromane-2-carboxamide (AZD3783), a potent 5-hydroxytryptamine 1B (5-HT(1B)) receptor antagonist, were characterized as part of translational pharmacokinetic/pharmacodynamic hypothesis testing in human clinical trials. The affinity of AZD3783 to the 5-HT(1B) receptor was measured in vitro by using membrane preparations containing recombinant human or guinea pig 5-HT(1B) receptors and in native guinea pig brain tissue. In vivo antagonist potency of AZD3783 for the 5HT(1B) receptor was investigated by measuring the blockade of 5-HT(1B) agonist-induced guinea pig hypothermia. The anxiolytic-like potency was assessed using the suppression of separation-induced vocalization in guinea pig pups. The affinity of AZD3783 for human and guinea pig 5-HT(1B) receptor (K(i), 12.5 and 11.1 nM, respectively) was similar to unbound plasma EC(50) values for guinea pig receptor occupancy (11 nM) and reduction of agonist-induced hypothermia (18 nM) in guinea pig. Active doses of AZD3783 in the hypothermia assay were similar to doses that reduced separation-induced vocalization in guinea pig pups. AZD3783 demonstrated favorable pharmacokinetic properties. The predicted pharmacokinetic parameters (total plasma clearance, 6.5 ml/min/kg; steady-state volume of distribution, 6.4 l/kg) were within 2-fold of the values observed in healthy male volunteers after a single 20-mg oral dose. This investigation presents a direct link between AZD3783 in vitro affinity and in vivo receptor occupancy to preclinical disease model efficacy. Together with predicted human pharmacokinetic properties, we have provided a model for the quantitative translational pharmacology of AZD3783 that increases confidence in the optimal human receptor occupancy required for antidepressant and anxiolytic effects in patients.

Characterization of the potent 5-HT(1A/B) receptor antagonist and serotonin reuptake inhibitor SB-649915: preclinical evidence for hastened onset of antidepressant/anxiolytic efficacy

An increase in brain serotonin (5-HT) levels is thought to be a key mechanism of action responsible for generating antidepressant efficacy. It has been proven that selective serotonin reuptake inhibitors are effective antidepressants, but the delay to therapeutic onset of these agents is thought to be due to the time required for 5-HT1A, and possibly 5-HT1B, autoreceptors to desensitize. Therefore, an agent incorporating 5-HT reuptake inhibition coupled with 5-HT1A and/or 5-HT1B autoreceptor antagonism may provide a fast-acting clinical agent. The current studies review the profile of SB-649915 (6-[(1-{2-[(2-methylquinolin-5-yl)oxy]ethyl}piperidin-4-yl)methyl]-2H-1,4-benzoxazin-3(4H)-one), a novel compound with high affinity for human (h) 5-HT1A and 5-HT1B receptors (pKi values of 8.6 and 8.0, respectively) as well as the (h) 5-HT transporter (SERT) (pKi value of 9.3). SB-649915 behaved as an antagonist at both 5-HT1A and 5-HT1B receptors in vitro and in vivo, reversing 5-HT, (+)8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and SKF99101-induced functional/behavioral responses. Furthermore, it inhibited [3H]5-HT reuptake in rat cortical synaptosomes, in vitro and ex vivo. In electrophysiological studies SB-649915 had no effect on rat dorsal raphe neuronal cell firing per se, but reversed 8-OH-DPAT-induced inhibition of firing both in vitro and in vivo. In addition, in a microdialysis study, it produced an acute increase in extracellular 5-HT in forebrain structures of the rat. Finally, SB-649915 demonstrated acute anxiolytic activity in both rodent and non-human primate and reduced the latency to onset of anxiolytic behavior, compared to paroxetine, in the rat social interaction paradigm. In summary, SB-649915 is a novel, potent 5-HT1A/1B autoreceptor antagonist, and 5-HT reuptake inhibitor. This particular pharmacological profile provides a novel mechanism that could offer fast-acting antidepressant activity.

SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2'methyl-4'-(5-methyl-1,2,3-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride): a novel, potent and selective 5-HT1B receptor antagonist

SB-616234-A possesses high affinity for human 5-HT1B receptors stably expressed in Chinese hamster ovary (CHO) cells (pKi 8.3+/-0.2), and is over 100-fold selective for a range of molecular targets except h5-HT1) receptors (pKi 6.6+/-0.1). Similarly, affinity (pKi) for rat and guinea pig striatal 5-HT1B receptors is 9.2+/-0.1. In [35S]-GTPgammaS binding studies in the human recombinant cell line, SB-616234-A acted as a high affinity antagonist with a pA2 value of 8.6+/-0.2 whilst providing no evidence of agonist activity in this system. In [35S]-GTPgammaS binding studies in rat striatal membranes, SB-616234-A acted as a high affinity antagonist with an apparent pKB of 8.4+/-0.5, again whilst providing no evidence of agonist activity in this system. SB-616234-A (1 microM) potentiated electrically stimulated [3H]-5-HT release from guinea pig and rat cortical slices (S2/S1) ratios of 1.8 and 1.6, respectively). SB-616234-A (0.3-30 mg kg(-1) p.o.) caused a dose-dependent inhibition of ex vivo [3H]-GR125743 binding to rat striatal 5-HT1B receptors with an ED50 of 2.83+/-0.39 mg kg(-1) p.o. Taken together these data suggest that SB-616234-A is a potent and selective 5-HT(1B) autoreceptor antagonist that occupies central 5-HT1B receptors in vivo following oral administration.

SB-649915, a novel, potent 5-HT1A and 5-HT1B autoreceptor antagonist and 5-HT re-uptake inhibitor in native tissue

An increase in brain 5-HT levels is thought to be the key mechanism of action which results in an antidepressant response. It has been proven that selective serotonin re-uptake inhibitors are effective antidepressants but the delay to therapeutic onset of these agents is thought to be due to the time required for 5-HT1A, and possibly 5-HT1B, autoreceptor desensitisation. Therefore an agent incorporating 5-HT re-uptake inhibition coupled with 5-HT1A and/or 5-HT1B autoreceptor antagonism may provide a fast acting clinical agent. The current studies describe the in vitro profile of SB-649915 (6-[(1-{2-[(2-methylquinolin-5-yl)oxy]ethyl}piperidin-4-yl)methyl]-2H-1,4-benzoxazin-3(4H)-one), a novel compound which has high affinity for human recombinant 5-HT1A, 5-HT1B and 5-HT1D receptors (pKi values of 8.6, 8.0, 8.8, respectively) and the human recombinant 5-HT transporter (pKi value of 9.3). SB-649915 also displays high affinity for rat, guinea pig, mouse and marmoset native tissue 5-HT1A, 5-HT1B and 5-HT1D receptors and rat native tissue 5-HT transporters (pKi values>or=7.5). In functional [35S]GTPgammaS binding studies, SB-649915 (up to 1 microM) does not display intrinsic activity in HEK293 cells expressing human recombinant 5-HT1A receptors but acts as a partial agonist at human recombinant 5-HT1B and 5-HT1D receptors with intrinsic activity values of 0.3 and 0.7, respectively, as compared to the full agonist 5-HT. From Schild analysis, SB-649915 caused a concentration-dependent, rightward shift of 5-HT-induced stimulation of basal [35S]GTPgammaS binding in cells expressing human recombinant 5-HT1A or 5-HT1B receptors to yield pA2 values of 9.0 and 7.9, respectively. In electrophysiological studies in rat dorsal raphe nucleus, SB-649915 did not affect the cell firing rate up to 1 microM but attenuated (+)8-hydroxy-2-(di-n-propylamino) tetralin-induced inhibition of cell firing with an apparent pKb value of 9.5. SB-649915 (1 microM) significantly attenuated exogenous 5-HT-induced inhibition of electrically-stimulated [3H]5-HT release from guinea pig cortex. In studies designed to enhance endogenous 5-HT levels, and therefore increase tone at 5-HT1B autoreceptors, SB-649915 significantly potentiated [3H]5-HT release at 100 and 1000 nM. In LLCPK cells expressing human recombinant 5-HT transporters and in rat cortical synaptosomes, SB-649915 inhibited [3H]5-HT re-uptake with pIC50 values of 7.9 and 9.7, respectively. In summary, SB-649915 is a novel, potent 5-HT1A/1B autoreceptor antagonist and 5-HT re-uptake inhibitor in native tissue systems and represents a novel mechanism that could offer fast acting antidepressant action.

Identification of a potent and selective 5-HT1B receptor antagonist

An SAR study around the mixed 5-HT1ABD receptor antagonist SB-272183 found that introduction of cis-2,6-dimethyl substitution onto the piperazine ring was a key structural change, which imparted a combination of both excellent selectivity over the 5-HT1A and 5-HT1D receptors and low intrinsic activity. This led to the identification of the selective 5-HT1B receptor antagonist SB-616234.

Discovery of the First Potent, Selective 5-Hydroxytryptamine1D Receptor Antagonist

A series of 5-(piperidinylethyloxy)quinoline 5-hydroxytryptamine(1D) (5-HT(1D)) receptor antagonists have been discovered from elaboration of the series of dual 5-hydroxytryptamine(1)-selective serotonin reuptake inhibitors (5HT(1)-SSRIs) reported previously. This is the first report of highly potent, selective antagonists for the 5-HT(1D) receptor, which represents an extremely useful set of pharmacological tools for further understanding the roles of the 5-HT(1) receptor subtypes.

Blockade of serotonin 5-HT1B and 5-HT2A receptors suppresses the induction of locomotor activity by 5-HT reuptake inhibitors, citalopram and fluvoxamine, in NMRI mice exposed to a novel environment: a comparison to other 5-HT receptor subtypes

RATIONALE

Though 5-HT plays an important role in the modulation of motor function, which is perturbed in depressive states, little is known concerning the influence of serotonin reuptake inhibitors (SSRIs) on locomotor activity (LA). Recently, we demonstrated that SSRIs, such as citalopram, enhance LA in mice exposed to a novel environment.

OBJECTIVES

This study examined the role of multiple classes of 5-HT receptor in citalopram-induced LA.

METHODS

The most selective antagonists currently available were used.

RESULTS

Citalopram-induced LA was dose-dependently attenuated by the 5-HT1B/1D receptor antagonists, S18127, GR125,743 and GR127,935, and by the selective 5-HT1B antagonist, SB224,289, but unaffected by the selective 5-HT1A antagonist, WAY100,635. The selective antagonists at 5-HT2A receptors, MDL100,907 and SR46,349 also dose-dependently attenuated induction of locomotion by citalopram, whereas the 5-HT2B antagonist, SB204,741, and the 5-HT2B/2C antagonist, SB206,553 were ineffective. Further, the selective 5-HT2C antagonist, SB242,084, potentiated the response to citalopram. Selective antagonists at 5-HT3 (ondansetron), 5-HT4 (GR125,487), 5-HT6 (SB271,046) and 5-HT7 (SB269,970) receptors did not significantly modify the action of citalopram. Underpinning these findings, SB224,289, GR125,743, MDL100,907 and SR46,349 likewise attenuated induction of locomotion by a further SSRI, fluvoxamine.

CONCLUSIONS

The locomotor response to SSRIs of mice exposed to a novel environment is mediated via 5-HT1B and 5-HT2A receptors. In view of the importance of motor function to the etiology and treatment of depression, the significance of these observations to the clinical actions of SSRIs will be of interest to elucidate.

Suppression by eletriptan of the activation of trigeminovascular sensory neurons by glyceryl trinitrate

The effect of intracarotid arterial infusions of glyceryl trinitrate (GTN), a substance known to precipitate vascular headache, on the spontaneous activity of trigeminal neurons with craniovascular input was studied in cats. Cats were anaesthetised with alpha-chloralose, immobilised and artificially ventilated. The superior sagittal sinus (SSS) was isolated and stimulated electrically. Facial receptive fields (RF) were also stimulated. Single neurons were recorded from the trigeminal nucleus caudalis with a metal microelectrode equipped with six glass barrels for microiontophoresis. Infusions of GTN were administered via a catheter inserted retrogradely into the common carotid artery through the lingual artery. Infusions of GTN (mean rate 19+/-7, range 5-100 microg kg(-1) min(-1), in a volume of 2 ml min(-1)) increased the spontaneous discharge rate of second-order neurons which received dural and facial sensory input to 429+/-80% of control. Iontophoretic application of the 5-HT(1B/1D) receptor agonist eletriptan (50 nA) at the peak of the response decreased the discharge rate of neurons towards pre-GTN control levels. In the presence of continuous iontophoretic application of the 5-HT(1B/1D) receptor antagonist GR127935, the decrease in discharge rate caused by eletriptan was antagonised. We conclude (1) that GTN activates craniovascular sensory pathways at a site at, or peripheral to, the second-order neuron and that such an action may account for at least the acute-onset headache induced by GTN and (2) that the antimigraine agent eletriptan is able to selectively suppress noxious sensory information from the dura, induced by GTN, via an action at 5-HT(1B/1D) receptors.

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.

SB-236057-A: a selective 5-HT1B receptor inverse agonist

5-HT1B autoreceptors are involved in the control of extracellular 5-HT levels from both the terminal and cell body regions of serotonergic neurons. In this manuscript we review the pharmacological and pharmacokinetic data available for the selective and potent 5-HT1B receptor inverse agonist, SB-236057-A (1'-ethyl-5-(2'-methyl-4'-(5-methyl-1,3,4-oxadiazolyl-2-yl)biphenyl-4-carbonyl)-2,3,6,7-tetrahydrospiro (furo[2,3-f]indole-3,4'-piperidine) hydrochloride). SB 236057-A has been shown to have high affinity for human 5-HT1B receptors (pK(i) = 8.2) and displays 80 or more fold selectivity for the human 5-HT1B receptor over other 5-HT receptors and a range of additional receptors, ion channels and enzymes. In functional studies at human 5-HT1B receptors SB-236057-A displayed inverse agonism (pA(2) = 8.9) using [(35)S]GTPgammaS binding, and silent antagonism (pA(2) = 9.2) using cAMP accumulation. SB-236057-A also acted as an antagonist at the 5-HT terminal autoreceptor as measured by [3H]5-HT release from electrically stimulated guinea pig and human cortical slices. In the guinea pig, pharmacokinetic analysis demonstrated that SB-236057-A was bioavailable and according to in vivo pharmacodynamic assays it enters brain and has a long duration of action. Importantly no side effect liability was evident at relevant doses from anxiogenic, cardiovascular, sedative or migraine viewpoints. In vivo microdialysis studies demonstrated that SB-236057-A is an antagonist in the guinea pig cortex but has no effect on extracellular 5-HT levels per se. In contrast, SB-236057-A increased extracellular 5-HT levels in the guinea pig dentate gyrus. This increase in 5-HT release was comparable to that observed after 14 days of paroxetine administration. SB-236057-A has been a useful tool in confirming that, in either guinea pigs or humans, the terminal 5-HT autoreceptor is of the 5-HT1B subtype. It appears that acute 5-HT1B receptor blockade, by virtue of increased 5-HT release in the dentate gyrus, may provide a rapidly acting antidepressant.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

SB-272183, a selective 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptor antagonist in native tissue

A novel compound, SB-272183 (5-Chloro-2, 3-dihydro-6-[4-methylpiperazin-1-yl]-1[4-pyridin-4-yl]napth-1-ylaminocarbonyl]-1H-indole), has been shown to have high affinity for human 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors with pK(i) values of 8.0, 8.1 and 8.7 respectively and is at least 30 fold selective over a range of other receptors. [(35)S]-GTPgammaS binding studies showed that SB-272183 acts as a partial agonist at human recombinant 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors with intrinsic activities of 0.4, 0.4 and 0.8 respectively, compared to 5-HT. SB-272183 inhibited 5-HT-induced stimulation of [(35)S]-GTPgammaS binding at human 5-HT(1A) and 5-HT(1B) receptors to give pA(2) values of 8.2 and 8.5 respectively. However, from [(35)S]-GTPgammaS autoradiographic studies in rat and human dorsal raphe nucleus, SB-272183 did not display intrinsic activity up to 10 microM but did block 5-HT-induced stimulation of [(35)S]-GTPgammaS binding. From electrophysiological studies in rat raphe slices in vitro, SB-272183 did not effect cell firing rate up to 1 microM but was able to attenuate (+)8-OH-DPAT-induced inhibition of cell firing to give an apparent pK(b) of 7.1. SB-272183 potentiated electrically-stimulated [(3)H]-5-HT release from rat and guinea-pig cortical slices at 100 and 1000 nM, similar to results previously obtained with the 5-HT(1B) and 5-HT(1D) receptor antagonist, GR127935. Fast cyclic voltammetry studies in rat dorsal raphe nucleus showed that SB-272183 could block sumatriptan-induced inhibition of 5-HT efflux, with an apparent pK(b) of 7.2, but did not effect basal efflux up to 1 microM. These studies show that, in vitro, SB-272183 acts as an antagonist at native tissue 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors.

The ability of WAY100,635 to potentiate the neurochemical and functional actions of fluoxetine is enhanced by co-administration of SB224,289, but not BRL15572

The present study employed a combined neurochemical and behavioural approach to address the question of whether blockade of (presynaptic) 5-HT(1B) or 5-HT(1D) receptors enhances the facilitatory influence of 5-HT(1A) autoreceptor antagonism upon the actions of selective serotonin re-uptake inhibitors (SSRI). In the presence of the selective 5-HT(1A) antagonist, WAY100,635, the fluoxetine-induced increase in dialysate levels of 5-HT in the frontal cortex (FCX) of freely-moving rats was significantly potentiated. The selective 5-HT(1B) antagonist, SB224,289, likewise potentiated the increase in 5-HT levels evoked by fluoxetine. Further, administered together, WAY100,635 and SB224,289, at least additively, potentiated the influence of fluoxetine upon 5-HT levels. This effect was selective inasmuch as, either alone or together, WAY100,635 and SB224,289 did not modify the influence of fluoxetine upon FCX levels of dopamine (DA) or noradrenaline (NA) quantified in the same dialysis samples. Co-administration of SB224,289 also enhanced the ability of WAY100,635 to potentiate the induction of head-twitches (HTW) by fluoxetine. This response reflects activation of 5-HT(2A) sites in FCX and was abolished by the selective 5-HT(2A) antagonist, MDL100,907. In contrast to SB224,289, the 5-HT(1D) antagonist, BRL15572, failed to enhance the facilitatory influence of WAY100,635 upon the neurochemical or behavioural actions of fluoxetine. In conclusion, co-joint blockade of 5-HT(1B) - but not 5-HT(1D) - with 5-HT(1A) autoreceptors markedly potentiates the neurochemical and functional actions of the SSRI, fluoxetine.

Serotonin autoreceptor function and antidepressant drug action

This article briefly summarizes, within the context of a brief review of the relevant literature, the outcome of our recent rat microdialysis studies on (1) the relative importance of serotonin (5-HT)1A versus 5-HT1B autoreceptors in the mechanism of action of 5-HT reuptake blocking agents, including putative regional differences in this regard, and (2) autoreceptor responsiveness following chronic SSRI administration. First, our data are consistent with the primacy of 5-HT1A autoreceptors in restraining the elevation of 5-HT levels induced by SSRIs, whereas nerve terminal 5-HT1B autoreceptors appear to have an accessory role in this regard. Second, there is an important interplay between cell body and nerve terminal 5-HT autoreceptors, and recent findings suggest that this interplay may potentially be exploited to obtain regionally preferential effects on 5-HT neurotransmission in the central nervous system, even upon systemic drug administration. In particular, emerging data suggest that somatodendritic 5-HT1A autoreceptor- and nerve terminal 5-HT1B autoreceptor-mediated feedback may be relatively more important in the control of 5-HT output in dorsal raphe-frontal cortex and median raphe-dorsal hippocampus systems, respectively. Third, 5-HT autoreceptors evidently retain the capability to limit the 5-HT transmission-promoting effect of SSRIs after chronic treatment. Thus, although the responsiveness of these sites is probably somewhat reduced, residual autoreceptor capacity still remains an effective restraint on large increases in extracellular 5-HT, even after prolonged treatment. If a further increase in extracellular 5-HT is crucial to the remission of depression in patients responding only partially to prolonged administration of antidepressants, then sustained adjunctive treatment with autoreceptor-blocking drugs may consequently prove useful in the long term.

Effect of 5-HT(1B) receptor ligands on self-administration of ethanol in an operant procedure in rats

Recent evidence suggests that 5-HT(1B) receptor activation modifies ethanol's reinforcing, intoxicating and discriminative stimulus effects. The present study further explored the role played by 5-HT(1A/1B) receptors by examining their influence on oral ethanol self-administration. Male Wistar rats were trained on an FR 4 schedule to obtain a reinforcer of 0.1 12% w/v ethanol solution. Once responding was stable, the effect of the 5-HT(1A/1B) agonist RU24969 alone and in combination with the 5-HT(1B) antagonist GR127935 or the 5-HT(1A) antagonists (+) WAY100135 and (+) WAY100635 was assessed. The effect of RU24969 on ethanol's pharmacokinetic profile and on operant oral saline self-administration was also examined to assess if alterations in oral ethanol self-administration were due to nonspecific effects on level pressing. For comparison, we examined the effect of another 5-HT(1A/1B) agonist, CGS12066B, on oral ethanol self-administration. Both RU24969 (0.1 to 1 mg/kg) and CGS12066B (0.1 to 1 mg/kg) significantly suppressed oral ethanol self-administration. Administration of GR127935 (1 mg/kg), significantly reversed the effects elicited by RU24969, whereas neither WAY100635 (1 mg/kg) nor (+)WAY100135 (1 mg/kg) had any effect. The effects of lower doses of RU24969 on oral ethanol self-administration were selective as oral saline self-administration and blood ethanol levels were not altered by these doses. These data demonstrate that 5-HT(1B) receptor activation suppresses oral ethanol self-administration. These studies provide further evidence that 5-HT(1B) receptors play a modulatory role in ethanol's behavioral effects.

[(3)H]-SB-269970--A selective antagonist radioligand for 5-HT(7) receptors

Binding of the 5-HT(7) receptor antagonist radioligand [(3)H]-SB-269970 to human 5-HT(7(a)) receptors expressed in HEK293 cell membranes (h5-HT(7(a))/293) and to guinea-pig cerebral cortex membranes, was characterized and compared with [(3)H]-5-CT binding. [(3)H]-SB-269970 (1 nM) showed full association with h5-HT(7(a))/293 membranes after 40 min. Specific binding at equilibrium represented >90% of total binding and was fully reversible by methiothepin (10 microM), full dissociation occurring by 100 min. The association (k(+1)) and dissociation (k(-1)) rate constants were 0.05 nM(-1)min(-1) and 0.05 min(-1) respectively, giving a K(D) (k(-1)/k(+1)) of 1.0 nM. [(3)H]-SB-269970 bound saturably and apparently monophasically to both h5-HT(7(a))/293 and guinea-pig cortex membranes, with K(D) values of 1.25+/-0.05 and 1.7+/-0.3 nM respectively. The B(max) for [(3)H]-SB-269970 to both h5-HT(7(a))/293 and guinea-pig cortex membranes (5780+/-380 and 125+/-8.2 fmoles mg protein(-1) respectively) was similar to that for [(3)H]-5-CT (6190+/-940 and 143+/-19 fmoles mg protein(-1) respectively). These data suggest that, in each tissue, both radioligands labelled the same population of receptors, which appear to be present in an agonist high affinity state. The profile of compound inhibition of [(3)H]-SB-269970 binding to h5-HT(7(a))/293 and guineapig cortex membranes correlated well (corr. coeff. 0.98) with those for [(3)H]-5-CT binding and were consistent with the profiles reported previously for the human 5-HT(7(a)) and guinea-pig cortex 5-HT(7) receptors using [(3)H]-5-CT. Hill slopes for inhibition of [(3)H]-SB-269970 and [(3)H]-5-CT binding were close to 1, consistent with binding to a single receptor population in both tissues. [(3)H]-SB-269970 represents the first selective 5-HT(7) antagonist radioligand, which should aid further characterization of 5-HT(7) receptors in recombinant and native tissues and help establish their role in brain function.

A G protein-coupled receptor for UDP-glucose

Uridine 5'-diphosphoglucose (UDP-glucose) has a well established biochemical role as a glycosyl donor in the enzymatic biosynthesis of carbohydrates. It is less well known that UDP-glucose may possess pharmacological activity, suggesting that a receptor for this molecule may exist. Here, we show that UDP-glucose, and some closely related molecules, potently activate the orphan G protein-coupled receptor KIAA0001 heterologously expressed in yeast or mammalian cells. Nucleotides known to activate P2Y receptors were inactive, indicating the distinctly novel pharmacology of this receptor. The receptor is expressed in a wide variety of human tissues, including many regions of the brain. These data suggest that some sugar-nucleotides may serve important physiological roles as extracellular signaling molecules in addition to their familiar role in intermediary metabolism.

5-HT reuptake inhibitors with 5-HT(1B/1D) antagonistic activity: a new approach toward efficient antidepressants

As part of our research program toward new, potential antidepressants, a series of unsymmetrical ureas has been prepared and evaluated as 5-HT reuptake inhibitors with 5-HT(1B/1D) antagonistic activities. The design of these compounds was based on coupling of various indole derivatives, previously shown to inhibit 5-HT reuptake, to three different aniline moieties, which are part of known 5-HT(1B/1D) ligands. Binding experiments in rat frontal cortex using [(125)I]iodocyanopindolol, in calf striatum using [(3)H]5-HT, and in rat hippocampus using [(3)H]8-OH-DPAT as radioligands, respectively, revealed significantly higher affinity at the 5-HT(1B) receptor as compared to the affinities for the 5-HT(1A) and 5-HT(1D) receptors for a number of compounds, among them 4-(5-fluoro-1H-indol-3-yl)piperidine-1-carboxylic acid [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]amide (5), the corresponding 4-fluoro-1H-indol-3-yl analogue 21a, and the corresponding 6-fluoro-1H-indol-3-yl analogue 21b. Conformational restriction of the aniline moiety in 5 only slightly enhanced the 5-HT(1B) affinity, whereas introduction of an aniline moiety with higher conformational flexibility resulted in a less potent 5-HT(1B) receptor ligand as compared to 5. The functional 5-HT(1B/1D) antagonistic activity was investigated using the rabbit saphenous vein model as well as the [(3)H]5-HT release from guinea pig cortical slices. All new compounds tested in the rabbit saphenous vein model were shown to antagonize the sumatriptan-evoked contractile responses with pA(2) values ranging from 7.3 to 8.7. These observations were consistent with the results of the cortical slice model, in which the ureas were found to block the sumatriptan-induced inhibition of potassium-evoked [(3)H]5-HT release. The 5-HT reuptake inhibition of the ureas determined in rat brain synaptosomes was found to be either increased or decreased as compared to the uncoupled indole derivatives indicating that the reuptake inhibition shown by the ureas is not only due to the indole part but also affected by the aniline moiety of the molecule. Among this series of compounds described the ureas 5, 21a, and 21b seem to be the most interesting candidates showing both 5-HT reuptake inhibition and 5-HT(1B/1D) antagonism in vitro. This dual pharmacological profile should in theory lead to a pronounced enhancement in serotonergic neurotransmission and consequently to a more efficient treatment of depression.

New selective and potent 5-HT(1B/1D) antagonists: chemistry and pharmacological evaluation of N-piperazinylphenyl biphenylcarboxamides and biphenylsulfonamides

A series of new analogues of N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl] 2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-carboxamide (1; GR127935) as potent and selective 5-HT(1B/1D) antagonists were synthesized and evaluated pharmacologically. Their receptor binding profiles were comparable to that of 1. The 1,3,4-oxadiazole isomer 2 and the 4'-aminocarbonyl and 4'-amidinyl analogues (9 and 10) of 1 had higher affinities at the rat 5-HT(1B) receptor (IC(50) = 0.93, 1. 3, and 0.5 nM, respectively) and calf 5-HT(1D) receptor (IC(50) = 37, 10, and 3 nM, respectively) than did 1 (1.6 and 52 nM for rat 5-HT(1B) and calf 5-HT(1D) receptors, respectively). In the functional in vitro testing of 5-HT(1B/1D) antagonistic properties, 2, 9, 10, 11b (O-demethylated derivative of 2), 13a (O-methylsulfonyl analogue of 2), and 16 (which differs from 2 with a sulfonamide linker) showed more pronounced effects in the K(+)-induced 5-HT release in the cortex of guinea pig than did 1 and 3 (SB224289). Compounds 2, 9, and 10 were equally potent as 1 in rabbit saphenous vein model (pA(2) > 9). A biochemical study of 2 with in vivo microdialysis in the rat brain showed that it is capable of augmenting citalopram (a selective serotonin reuptake inhibitor, SSRI) induced 5-HT release in rat ventral hippocampus, while preventing the decrease in acetylcholine release elicited by citalopram administration. The molecular structure of 2 was determined by single-crystal X-ray analysis. The log P and log D values of these compounds were calculated. This study contributes to the SAR study of N-piperazinylphenyl biphenylcarboxamides as selective and potent 5-HT(1B/1D) antagonists.

On the role of 5-HT1B/1D receptors in modulating transmission in a spinal reflex pathway in the decerebrated rabbit

1. In decerebrated rabbits, the selective 5-HT1B/1D receptor antagonist GR 127,935 had no significant effects on reflexes evoked in medial gastrocnemius motoneurones by electrical stimulation of the sural nerve, or on arterial blood pressure or heart rate when given by the intrathecal (up to 543 nmol cumulative) or intravenous (up to 1.8 micromol cumulative) routes. 2. In decerebrated, spinalized rabbits, intrathecal GR 127,935 in doses of up to 543 nmol, had no effect on the sural-gastrocnemius reflex. Furthermore, this drug failed to alter enhancement of the sural-gastrocnemius reflex induced by 8-hydroxy-2-(di-n-propyl)aminotetralin (8-OH-DPAT), given at 300 nmol kg-1 i.v. 3. In decerebrated, spinalized rabbits, the selective 5-HT1B/1D receptor agonists L-694,247 (cumulative doses of 2 - 243 nmol kg-1 i.v.) and L-741,604 (cumulative doses of 3 - 307 nmol kg-1 i.v.), each caused the sural-gastrocnemius reflex to increase to 140% of pre-drug levels, and arterial blood pressure to rise by about 10 mmHg. Subsequent administration of GR 127,935 at 0.9 - 1.8 micromol kg-1 reversed the pressor effect of the agonists but not the increase in reflexes. The 5-HT1A receptor antagonist WAY-100,635 (185 nmol kg-1 i.v.) also failed to reverse the increase in reflexes, but the 5-HT1B/1D/5-HT2/5-HT7 ligand ritanserin (1.6 micromol kg-1 i.v.) restored reflexes to pre-drug control values after L-741,604 (it was not tested against L-694,247). 4. These data indicate that 5-HT1B/1D receptors do not significantly modulate transmission in the sural-gastrocnemius reflex pathway, and that the enhancement of reflexes by 8-OH-DPAT and L-741,604 is probably mediated by 5-HT7 receptors.

SB-236057, a selective 5-HT1B receptor inverse agonist, blocks the 5-HT human terminal autoreceptor

A novel compound, SB-236057 (1'-ethyl-5-(2'-methyl-4'-(5-methyl-1,3,4-oxadiazol-2-yl)biphenyl- 4-carbonyl)-2,3,6,7-tetrahydrospiro[furo[2,3-f]indole-3,4'-piperid ine]) has been shown to have high affinity for human 5-hydroxytryptamine1B (5-HT1B) receptors (pKi = 8.2) and displays over 75 or more-fold selectivity for the human 5-HT1B receptor over other 5-HT receptors, including the human 5-HT1D receptor, and a range of other receptors, ion channels and enzymes. In functional studies using [35S]GTPgammaS binding, SB-236057 displayed negative intrinsic activity (pEC50 = 8.0) at human 5-HT1B receptors stably expressed in Chinese Hamster Ovary (CHO) cells and caused a rightward shift of agonist concentration response curves consistent with competitive antagonism (pA2 = 8.9). SB-236057 potentiated [3H]5-HT release from electrically stimulated guinea pig or human cortical slices. SB-236057 also abolished the inhibitory effect of exogenously superfused 5-HT on electrically-stimulated release from slices of the guinea pig cortex. These studies using SB-236057 confirm that, in both the guinea pig and human cerebral cortex, the terminal 5-HT autoreceptor is of the 5-HT1B subtype.

Pharmacological diversity between native human 5-HT1B and 5-HT1D receptors sited on different neurons and involved in different functions

The releases of [3H]5-hydroxytryptamine ([3H]5-HT) and of endogenous glutamic acid and their modulation through presynaptic h5-HT1B autoreceptors and h5-HT1D heteroreceptors have been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery. The inhibition by 5-HT of the K+ (15 mM)-evoked overflow of [3H]5-HT was antagonized by the 5-HT1B/5-HT1D receptor ligand GR 127935, which was ineffective on its own; this drug was previously found to behave as a full agonist at the h5-HT1D heteroreceptor regulating glutamate release. The recently proposed selective h5-HT1B receptor ligand SB-224289 also prevented the effect of 5-HT at the autoreceptor, being inactive on its own; in contrast, SB-224289, at 1 microM, was unable to interact with the h5-HT1D heteroreceptor. The inhibitory effect of 5-HT on the K+-evoked overflow of glutamate was antagonized by the h5-HT1D receptor ligand BRL-15572; added in the absence of 5-HT the compound was without effect. BRL-15572 (1 microM) was unable to modify the effect of 5-HT at the autoreceptor regulating [3H]5-HT release. The selective 5-HT1A receptor antagonist (+)-WAY 100135, previously found to be an agonist at the h5-HT1D heteroreceptor regulating glutamate release, could not interact with the h5-HT1B autoreceptor when added at 1 microM. It is concluded that native h5-HT1B and h5-HT1D receptors exhibit a hitherto unexpected pharmacological diversity.

Functional effects of the muscarinic receptor agonist, xanomeline, at 5-HT1 and 5-HT2 receptors

Xanomeline [3(3-hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-me thylpyridine] has been reported to act as a functionally selective muscarinic partial agonist with potential use in the treatment of Alzheimer's disease. This study examined the functional activity of xanomeline at 5-HT1 and 5-HT2 receptors in native tissue and/or human cloned receptors. Xanomeline had affinity for muscarinic receptors in rat cortical membranes where the ratio of the displacement affinity of [3H]-Quinuclidinyl benzilate vs that of [3H]-Oxotremorine-M was 16, indicative of partial agonist activity. Radioligand binding studies on human cloned receptors confirmed that xanomeline had substantial affinity for M1, M2, M3, M4, M5 receptors and also for 5-HT1 and 5-HT2 receptor subtypes. Carbachol and xanomeline stimulated basal [35S]-GTPgammaS binding in rat cortical membranes with micromolar affinity. The response to carbachol was attenuated by himbacine and pirenzepine with pA2 of 8.2, 6.9 respectively consistent with the response being mediated, predominantly, via M2 and M4 receptors. Xanomeline-induced stimulation of [35S]-GTPgammaS binding was inhibited by himbacine with an apparent pKb of 6.3, was not attenuated by pirenzepine up to 3 microM and was inhibited by the selective 5-HT1A antagonist WAY100635 with an apparent pKb of 9.4. These data suggest the agonist effect of xanomeline in this tissue is, in part, via 5-HT1A receptors. Similar studies on human cloned receptors confirmed that xanomeline is an agonist at human cloned 5-HT1A and 5-HT1B receptors. In studies using the fluorescent cytoplasmic Ca2+ indicator FLUO-3AM, xanomeline induced an increase in cytoplasmic Ca2+ concentration in SH-SY5Y cells expressing recombinant human 5-HT2C receptors. Atropine antagonized this response, consistent with mediation via endogenously-expressed muscarinic receptors. In the presence of atropine, xanomeline antagonized 5-HT-induced cytoplasmic changes in Ca2+ concentration in cells expressing h5-HT2A, h5-HT2B and h5-HT2c receptors with potencies similar to its affinity at these receptors. These studies indicate that xanomeline is a potent agonist at 5-HT1A and 5-HT1B receptors and an antagonist at 5-HT2 receptor subtypes.

SB-224289--a novel selective (human) 5-HT1B receptor antagonist with negative intrinsic activity

1. Human 5-HT1B (h5-HT1B) and human 5-HT1D (h5-HT1D) receptors show remarkably similar pharmacology with few compounds discriminating the receptors. We report here on a novel compound, SB-224289 (1'-Methyl-5-[[2'-methyl-4'-(5-methyl- 1,2,4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydro spiro [furo [2,3-f]indole-3,4'-piperidine] oxalate), which has high affinity for h5-HT1B receptors (pK1=8.16+/-0.06) and displays over 75 fold selectivity for the h5-HT1B receptor over all other 5-HT receptors including the h5-HT1D receptor and all other receptors tested thus far. 2. Functional activity of SB-224289 was measured in a [15S]GTPgammaS binding assay on recombinant h5-HT1B and h5-HT1D receptors expressed in Chinese Hamster Ovary (CHO) cells. SB-224289 displayed negative intrinsic activity at both receptors with higher potency at h5-HT1B receptors. SB-224289 caused a rightward shift of agonist concentration response curves consistent with competitive antagonism and generated affinities comparable with those obtained from competition radioligand receptor binding studies. 3. SB-224289 potentiated [3H]5-HT release from electrically stimulated guinea-pig cerebral cortical slices to the same extent as as the non-selective 5-HT1 antagonist methiothepin. SB-224289 also fully reversed the inhibitory effect of exogenously superfused 5-HT on electrically stimulated release. 4. Using SB-224289 as a tool compound, we confirm that in guinea-pig cerebral cortex the terminal 5-HT autoreceptor is of the 5-HT1B subtype.

Human 5-HT1F receptor-stimulated [35S]GTPgammaS binding: correlation with inhibition of guinea pig dural plasma protein extravasation

To determine the potency and efficacy of 5-HT1F receptor ligands, a [35S]GTPgammaS binding assay was developed and optimized for the human 5-HT1F receptor. Compounds which are known to be effective in the abortive treatment of migraine were tested for efficacy and potency in this assay. Naratriptan, sumatriptan, zolmitriptan, and rizatriptan all had agonist activity. The 5-HT1F receptor ligand LY334370 (4-fluoro-N-[3-(1-methyl-4-piperidinyl)-1H-indol-5-yl]-benzamide) was the most potent compound tested with an EC50 of 2.13 +/- 0.15 nM. LY302148 (5-fluoro-3-[1-[2-(1-methyl-1H-pyrazol-4-yl)ethyl]-4-piperidinyl]-1H-ind ole), methysergide, LY306258 (3-dimethylamino-2,3,4,9-tetrahydro-1H-carbazol-6-ol), dihydroergotamine (DHE), L-694,247 and CP-122,288 were also investigated for potency and efficacy. There was a statistically significant correlation between the pEC50 for the stimulation of [35S]GTPgammaS binding and the pID50 for the inhibition of trigeminal nerve-stimulated dural plasma protein extravasation in the guinea pig. In the course of these studies, it was found that the purportedly selective 5-HT1D receptor antagonist GR127935 inhibited 5-HT1F receptor-stimulated [35S]GTPgammaS binding with a Ki of 39.6 +/- 9.5 nM. These studies demonstrate that 5-HT1F receptor-mediated stimulation of [35S]GTPgammaS binding in a clonal cell system is a reproducible, high throughput assay that is predictive of an in vivo model of 5-HT1F receptor activation.

The selective 5-HT1B receptor inverse agonist 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2, 4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydro- spiro[furo[2,3-f]indole-3,4'-piperidine] (SB-224289) potently blocks terminal 5-HT autoreceptor function both in vitro and in vivo

5-HT1 receptors are members of the G-protein-coupled receptor superfamily and are negatively linked to adenylyl cyclase activity. The human 5-HT1B and 5-HT1D receptors (previously known as 5-HT1Dbeta and 5-HT1Dalpha, respectively), although encoded by two distinct genes, are structurally very similar. Pharmacologically, these two receptors have been differentiated using nonselective chemical tools such as ketanserin and ritanserin, but the absence of truly selective agents has meant that the precise function of the 5-HT1B and 5-HT1D receptors has not been defined. In this paper we describe how, using computational chemistry models as a guide, the nonselective 5-HT1B/5-HT1D receptor antagonist 4 was structurally modified to produce the selective 5-HT1B receptor inverse agonist 5, 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2, 4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6, 7-tetrahydrospiro[furo[2,3-f]indole-3,4'-piperidine] (SB-224289). This compound is a potent antagonist of terminal 5-HT autoreceptor function both in vitro and in vivo.

Cataleptogenic effect of subtype selective 5-HT receptor antagonists in the rat

5-HT receptor antagonists with selectivity for 5-HT1A WAY-100635 (N-[2-[-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide), 5-HT1B GR 127935 (N-[methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'(5-methyl-1,2, 4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide x HCl), 5-HT2C SB 200646A (N-(1-methyl-5-indolyl)-N'-(3-pyridyl)urea x HCl) and 5-HT2A (ketanserin, fananserin and MDL 100,151 ((+/-)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipe ridinemethanol) receptors were tested for cataleptogenic responses in rats. WAY-100635 (0.1-3 mg/kg, s.c.), ketanserin (0.1-3 mg/kg, s.c.), MDL 100,151 (0.3-3 mg/kg, s.c.) and fananserin (RP 62203; 3 mg/kg, s.c.) induced a significant catalepsy. GR 127935 (1 mg/kg, s.c.), SB 200646A (without effect per se at 10 mg/kg, s.c.) and MDL 100,151 (0.3 mg/kg, s.c.) did not inhibit the cataleptic response to the dopamine D2 receptor antagonist, loxapine (0.3 mg/kg, s.c.). Catalepsy induced by MDL 100,151 (3 mg/kg) was blocked by co-treatment with clozapine, but not by SB 200646A (both at 10 mg/kg, s.c.). Although clozapine displays significant affinity to 5-HT1A, 5-HT1B, 5-HT2A and 5-HT2C receptors, the present results suggest that blockade of these receptors is not responsible for clozapine's anticataleptic activity.

Glutamate release in human cerebral cortex and its modulation by 5-hydroxytryptamine acting at h 5-HT1D receptors

1. The release of glutamic acid and its modulation by 5-hydroxytryptamine (5-HT) in the human brain has been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery to reach deeply sited tumours. 2. The Ca2+-dependent K+ (15 mM)-evoked overflow of glutamate was inhibited by 5-HT in a concentration-dependent manner (EC50 = 2.9 nM; maximal effect approximately 50%). The inhibition caused by 5-HT was antagonized by the 5-HT1/5-HT2 receptor antagonist methiothepin. The 5-HT1B/5-HT1D receptor agonist sumatriptan mimicked 5-HT (EC50 = 6.4 nM; maximal effect approximately 50%); the effect of sumatriptan was also methiothepin-sensitive. Selective 5-HT1A receptor antagonists could not prevent the inhibition of glutamate release by 5-HT. 3. The 5-HT1B/5-HT1D receptor ligand GR 127935 and the 5-HT2C/5-HT1B/5-HT1D receptor ligand metergoline were unable to prevent the 5-HT effect; instead they inhibited glutamate release, their effects being abolished by methiothepin. Some 5-HT1A receptor antagonists also displayed intrinsic agonist activity. 4. The effect of sumatriptan was prevented by ketanserin, a drug known to display much higher affinity for recombinant h 5-HT1D than for h 5-HT1B receptors. 5. We propose that neocortical glutamatergic nerve terminals in human brain cortex possess release-inhibiting presynaptic heteroreceptors that appear to belong to the h 5-HT1D subtype.

The effect of FMRFamide analogs on [35S]GTP-gamma-S stimulation in squid optic lobes

Pharmacological study of Phe-Met-Leu-Phe-amide (FMRFa) receptors is hindered by the lack of selective ligands. The classification of these selective ligands is further hampered by the limited availability of functional assays. In this study, we evaluated several synthetic FMRFa analogs for agonist and antagonist activity by measuring their abilities to produce [35-S]-GTP-gamma-S stimulation or to inhibit FMRFa-induced [35S]-GTP-gamma-S binding in squid optic lobes. Analogs included acetyl-Phe-norLeu-Arg-Phe-amide (acFnLRFa), desamino-Tyr-Phe-Leu-Arg-amide (daYFLRa), desamino Tyr-Phe-norLeu-Arg-Phe-amide (daYFnLRFa), desamino Tyr-Phe-norLeu-Arg-[TIC]-amide (daYFnLR[TIC]a), desamino Tyr-Trp-norLeu-Arg-amide (daYWnLRa), (D)-Tyr-Phe-norLeu-Arg-Phe-amide (D)-YFnLRFa), Phe-Leu-Arg-Phe-amide (FLRFa), and the D-amino acid analogs of FMRFa (D-FMRFa, F-(D)-MRFa and FM-(D)-RFa). For agonist studies, full dose-response curves were generated and analyzed for potency and efficacy (maximal percent effect). FMRFamide as well as analogs ac-FnLRFa, daYFnLRFa, daYFnLR[TIC]a, D-YFnLRFa, FLRFa, and (D)-FMRFa stimulated [35S]-GTP-gamma-S binding. Analogs daYWnLRa, daYFLRa, F-(D)-MRFa, and FM-(D)-RFa failed to stimulate either [35S]-GTP-gamma-S binding or to inhibit FMRFa-induced [35S]-GTP-gamma-S binding. The rank order of potency was daYFnLRFa > or = daYFnLRF[TIC]a > acFnLRFa > (D)YFnLRFa > FLRFa > or = FMRFa >> (D)-FMRFa. The order of efficacy was daYFnLRFa = acFnLRFa = (D)-YFnLRFa > FLRFa = FMRFa > or = (D)-FMRFa > or = daYFnLRF[TIC]a. Peptide analog daYFnLR[TIC]a was less efficacious (59% maximal stimulation) than analogs daYFnLRFa, acFnLRFa, and (D)-YFnLRFa (113-146% maximal stimulation). A maximal concentration of daYFnLR[TIC]a (10 microM) reduced daYFnLRFa, acFnLRFa, and (D)-YFnLRFa induced [35S]-GTP-gamma-S stimulation, indicating that daYFnLR[TIC]a is a partial agonist at the receptor stimulated by the FMRFamide analogs. Analysis of the structural requirements needed for promoting [35S]-GTP-gamma-S binding show that elongation (i.e., daYFnLRFa, D-YFnLRFa) or modification of Phe1 (ac-FnLRFa) leads to increased efficacy and potency. Moreover, elimination of the C-terminal Phe (daYWnLRa, daYFLRa,) leads to a loss of biological activity. However, substitution with L-1,2,3,4 tetrahydroisoquinoline-3-carboxylic acid, a rigid analog of the C-terminal Phe (daYFnLR[TIC]a), leads to decreased efficacy but not loss of potency. The data suggest that immobilization or modification of the C-terminal Phe may produce highly selective and potent FMRFamide antagonists. These results agree with published receptor radioligand studies and indicate that the [35S]GTP-gamma-S assay may be useful in classifying novel FMRFamide-selective ligands.

Differentiation between partial agonists and neutral 5-HT1B antagonists by chemical modulation of 3-[3-(N,N-dimethylamino)propyl]-4-hydroxy- N-[4-(pyridin-4-yl)phenyl]benzamide (GR-55562)

The synthesis and binding affinity at cloned h5-HT1D, h5-HT1D, and h5-HT1A receptors of 3-[3-(N,N-dimethylamino)propyl]-4-hydroxy- N-[4-(pyridin-4-yl)phenyl]benzamide (2, GR-55562) and four O-methylated analogs are described. The functional activity of these compounds was determined at the h5-HT1B receptor using a [35S]GTP gamma S binding assay. The four analogs have been prepared in order to evaluate the influence of the alkylamino side chain conformation on binding and intrinsic activity. Whereas 2 and its derivatives display a similar binding affinity profile, major differences arise from analysis of the intrinsic activity data at h5-HT1B receptors. The O-methylated analog of 2, 3-[3-(N,N-dimethylamino)propyl]-4-methoxy- N-[4-(pyridin-4-yl)phenyl]benzamide (3a), and the (1Z)-3-(N,N-dimethylamino)prop-1-enyl derivative (3c) act as neutral and potent antagonists (in a similar way to 2), while the 3-(N,N-dimethylamino)-prop-1-ynyl (3b) and (1E)-3-(N,N-dimethylamino)prop-1-enyl (3d) analogs display nonnegligible agonist activity. Molecular modeling studies show that, when the common triaryl portions of the molecules are overlapped, the partial agonists and the neutral antagonists differ by a near-orthogonal orientation of the NH+ projection to the hydrogen-bond receptor site.

Stimulation of 5-HT1B receptors causes hypothermia in the guinea pig

The selective, brain penetrant, 5-HT(1B/D) (formerly 5-HT(1D beta/alpha)) receptor agonist SKF-99101H (3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate) (30 mg/kg i.p.) causes a dose related fall in rectal temperature in guinea pigs which previous studies have shown to be blocked by the non-selective 5-HT(1B/D) receptor antagonist GR-127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1'biphenyl]-4-carboxamide oxalate). The present study shows that the hypothermic response to SKF-99101H is dose-dependently blocked by SB-224289G (1'-methyl-5-(2'-methyl-4'-[(5-methyl-1,2,4-oxadiazol-3-yl)bipheny l-4-yl]carbonyl)-2,3,6,7-tetrahydrospiro[furo[2,3-f]indole-3,4'-pi peridone] hemioxalate) (0.3-10.0 mg/kg p.o.) (ED50 3.62 mg/kg), which is the first compound to be described which is more than 60 fold selective for the 5-HT1B receptor over the 5-HT1D receptor. SB-216641A (N-[3-(2-dimethylamino) ethoxy-4-methoxy-phenyl] 2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-car boxamide hydrochloride) (0.6-20.0 mg/kg i.p.), which is somewhat less selective (30 fold) for the 5-HT1B receptor over the 5-HT1D receptor had a similar effect (ED50 4.43 mg/kg). The brain penetrant 5-HT1D selective receptor antagonist, BRL-15572 (4-(3-chlorophenyl)-alpha-(diphenylmethyl)-1-piperazineethanol+ ++ dihydrochloride) (0.3-100.0 mg/kg i.p.) was inactive. When administered alone neither BRL-15572 (0.1-10 mg/kg i.p.) nor SB-224289G (2.2-22 mg/kg p.o.) had an effect on body temperature. These data demonstrate that 5-HT1B (formerly 5-HT(1D beta)) and not 5-HT1D (formerly 5-HT(1D alpha)) receptors mediate the hypothermic response to SKF-99101H (30 mg/kg i.p.) in guinea pigs. The compounds described are useful pharmacological tools for distinguishing responses to 5-HT1B and 5-HT1D receptors.

(+)-WAY 100135, a partial agonist, at native and recombinant 5-HT1B/1D receptors

1. We have studied the effects of the purportedly selective 5-HT1A receptor antagonist (+)-WAY 100135 on electrically stimulated 5-hydroxytryptamine (5-HT) efflux in the ventrolateral geniculate nucleus (vLGN), and its affinity at human 5-HT1B and 5-HT1D receptors stably expressed in Chinese hamster ovary (CHO) cells. 2. On short 'pseudo single pulse' stimulations (20 pulses at 100 Hz, 190 ms train duration), (+)-WAY 100135 (1.0 microM) decreased 5-HT efflux in the vLGN to 68 +/- 8% of pre-drug values (P < 0.01). This decrease could be blocked by the 5-HT1D/1B receptor antagonist GR 127935 (50 nM). Conversely, when long stimulations (20 pulses at 20 Hz, 950 ms train) were used, (+)-WAY 100135 had no effect on 5-HT efflux (84 +/- 8% of pre-drug values) although both methiothepin (200 nM) and GR 127935 (50 nM) caused significant increases (to 175 +/- 18 and 130 +/- 10% of pre-drug values, respectively). 3. Paroxetine (100 nM), the selective 5-HT reuptake inhibitor, increased stimulated 5-HT efflux and reuptake half-life (to 145 +/- 18% and 649 +/- 121%, respectively) on pseudo single pulse stimulations. When (+)-WAY 100135 was added in combination with the uptake blocker, the effect of paroxetine on stimulated 5-HT efflux was potentiated to 282 +/- 48% (P < 0.01) without further effect on the 5-HT reuptake half-life. 4. The affinity and intrinsic activity of (+)-WAY 100135 were determined at recombinant human 5-HT1B and 5-HT1D receptors expressed in CHO cells, by use of radioligand binding and [35S]-GTP gamma S binding (+)-WAY 100135 was a partial agonist at human 5-HT1B and 5-HT1D receptors with moderately high affinity for 5-HT1D receptors (pEC50 = 7.61). 5. In conclusion, (+)-WAY 100135 was found to be not a selective 5-HT1A autoreceptor antagonist but may act as a partial agonist at the 5-HT1B/1D receptor, displaying agonist or antagonist properties depending on the stimulation protocol used and the resultant 5-HT 'tone' at the receptor.