The effect of SB-236057-A, a selective 5-HT1B receptor inverse agonist, on in vivo extracellular 5-HT levels in the freely-moving guinea-pig

Zhi Zi Chi decoction (Gardeniae fructus and semen Sojae Praeparatum) attenuates anxious depression via modulating microbiota-gut-brain axis in corticosterone combined with chronic restraint stress-induced mice

BACKGROUND

The microbiota-gut-brain axis plays a critical role in neuropsychiatric disorders, particularly anxious depression, and attracts more attention gradually. Zhi Zi Chi decoction (ZZCD) consisting of Gardenia jasminoides J. Ellis and Glycine max (L.) Merr, is a classic formula in clinic and widely applied in anxiety and depression treatment. However, the underlying mechanisms of regulating microbiota-gut-brain axis in the treatment of anxious depression by oral administration of ZZCD remain elusive.

MATERIALS AND METHODS

In this project, we clarified the origin and preparation methods of the Gardenia jasminoides J. Ellis and Glycine max (L.) Merr and examined the chemical ingredients of ZZCD by liquid chromatograph mass spectrometer. Then, corticosterone combined with chronic restraint stress was applied to establish an anxious depression model. After treated with ZZCD standard decoction, based on enzyme-linked immunosorbent assay (ELISA), 16S rRNA technology, high-throughput sequencing, quantitative RT-PCR and fecal microbiota transplantation (FMT), the multiple associations between nucleus accumbens and intestinal flora in anxious depression mice were determined to clarify the mechanism of ZZCD in the treatment of anxiety and depression disorder.

RESULTS

We found various substances with antidepressant and antianxiety properties in ZZCD such as rosiridin and oleanolic acid. ZZCD could alleviate depressive and anxiety behaviors in anxious depression mice via regulating the disturbance of gut microbiota. Meanwhile, the bioactive compounds of ZZCD might directly active on neurodevelopment and neuroimmune-related genes. Furthermore, the secretion of prolactin and estrogen, and interfering with mitogen-activated protein kinase (MAPK) and tumor necrosis factor (TNF) signaling pathways were mainly involved in the multi-target therapeutic effects of ZZCD against anxiety and depression.

CONCLUSIONS

These findings suggested that ZZCD exerts antidepressant effects pleiotropically through modulating the microbiota-gut-brain.

5-hydroxtryptamine receptors in systemic hypertension: an arterial focus

BACKGROUND

Serotonin (5-hydroxytryptamine [5-HT]) was named for its isolation from blood serum (sero-) and ability to contract smooth muscle (-tonin). Thus, its relationship with the cardiovascular system began with its discovery.

AIMS

This review will focus on the effects of 5-HT and its receptors in the vasculature, with a focus on their involvement in high blood pressure (hypertension). Two seemingly contradictory bodies of evidence exist that make it difficult to assign any one function to 5-HT in vascular control of blood pressure.

RESULTS

 In vitro, 5-HT is an established vasoconstrictor, the effects of which are amplified in hypertension. By contrast, 5-HT (or its precursor 5-hydroxytryptophan) lowers blood pressure when given chronically in vivo. We will discuss ideas that might help us understand these differences, discuss relatively new pharmacology parameters (e.g. biased, inverse agonism) as they pertain to 5-HT receptors, and pose questions that are vital to answer so as to understand the role played by 5-HT in control of blood pressure, especially as it pertains to vascular function.

CONCLUSIONS

Our goal is to understand if the actions of 5-HT in hypertension are physiologically and clinically relevant. The community understands 5-HT has complex cardiovascular effects, and clinical studies have proven equivocal in terms of the involvement of 5-HT. This article provides a balanced view of evidence/literature that illustrates involvement of 5-HT in hypertension as controversial. It contributes new pharmacological knowledge of 5-HT compounds, and poses timely questions as to how this field can move forward. The take home message is that the cardiovascular effects of 5-HT are markedly complex such that we have not yet answered the question of whether 5-HT is beneficial or detrimental to hypertension.

Simultaneous blockade of 5-HT1A/B receptors and 5-HT transporters results in acute increases in extracellular 5-HT in both rats and guinea pigs: in vivo characterization of the novel 5-HT1A/B receptor antagonist/5-HT transport inhibitor SB-649915-B

RATIONALE

The delay in onset and treatment resistance of subpopulations of depressed patients to conventional serotonin reuptake inhibitors has lead to new drug development strategies to produce agents with improved antidepressant efficacy.

OBJECTIVES

We report the in vivo characterization of the novel 5-HT(1A/1B) autoreceptor antagonist/5-HT transporter inhibitor (6-[(1-{2-[(2-methyl-5-quinolinyl)oxy]ethyl}-4-piperidinyl)methyl]-2H-1,4-benzoxazin-3(4H)-one), SB-649915-B.

MATERIALS AND METHODS

Ex vivo binding was used to ascertain 5-HT(1A) receptor and serotonin transporter occupancy. 8-OH-DPAT-induced hyperlocomotion and SKF-99101-induced elevation of seizure threshold were used as markers of central blockade of 5-HT(1A) and 5-HT(1B) receptors, respectively. In vivo electrophysiology in the rat dorsal raphe and microdialysis in freely moving guinea pigs and rats were used to evaluate the functional outcome of SB-649915-B.

RESULTS

SB-649915-B (1-10 mg/kg p.o.) produced a dose-related inhibition of 5-HT(1A) receptor radioligand binding and inhibited ex vivo [(3)H]5-HT uptake in both guinea pig and rat cortex. SB-649915-B (0.1-10 mg/kg p.o.) reversed both 8-OH-DPAT-induced hyperlocomotor activity and SKF-99101-induced elevation of seizure threshold in the rat, demonstrating in vivo blockade of both 5-HT(1A) and 5-HT(1B) receptors, respectively. SB-649915-B (0.1-3 mg/kg i.v.) produced no change in raphe 5-HT neuronal cell firing per se but attenuated the inhibitory effect of 8-OH-DPAT. Acute administration of SB-649915-B resulted in increases (approximately two- to threefold) in extracellular 5-HT in the cortex of rats and the dentate gyrus and cortex of guinea pigs.

CONCLUSIONS

Based on these data, one may speculate that the 5-HT autoreceptor antagonist/5-HT transport inhibitor SB-649915-B will have therapeutic efficacy in the treatment of affective disorders with the potential for a faster onset of action compared to current selective serotonin reuptake inhibitors.

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.

Characterisation of the selective 5-HT1B receptor antagonist SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2′-methyl-4′-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride): In vivo neurochemical and behavioural evidence of anxiolytic/antidepressant activity

The 5-HT1B receptor has attracted significant interest as a potential target for the development of therapeutics for the treatment of affective disorders such as anxiety and depression. Here we present the in vivo characterisation of a novel, selective and orally bioavailable 5-HT1B receptor antagonist, SB-616234-A (1-[6-(cis-3,5-dimethylpiperazin-1-yl)-2,3-dihydro-5-methoxyindol-1-yl]-1-[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]methanone hydrochloride). SB-616234-A reversed the 5-HT1/7 receptor agonist, SKF-99101H-induced hypothermia in guinea pigs in a dose related manner with an ED50 of 2.4 mg/kg p.o. Using in vivo microdialysis in freely moving guinea pigs, SB-616234-A (3-30 mg/kg p.o.) caused a dose-related increase in extracellular 5-HT in the dentate gyrus. Evaluation of antidepressant- and anxiolytic-like effects of this 5-HT1B receptor antagonist was performed in a variety of models and species. SB-616234-A produced a decrease in immobility time in the mouse forced swim test; an effect suggestive of antidepressant activity. Furthermore, SB-616234-A produced dose-related anxiolytic effects in both rat and guinea pig maternal separation-induced vocalisation models with an ED50 of 1.0 and 3.3 mg/kg i.p., respectively (vs fluoxetine treatment ED50 = 2.2 mg/kg i.p. in both species). Also a significant reduction in posturing behaviours was observed in the human threat test in marmosets; an effect indicative of anxiolytic activity. In summary, SB-616234-A is a novel, potent and orally bioavailable 5-HT1B receptor antagonist which exhibits a neurochemical and behavioural profile that is consistent with both anxiolytic- and antidepressant-like activity in a variety of species. Taken together these data suggest that SB-616234-A may have therapeutic efficacy in the treatment of affective disorders.

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.

Chronic treatment with fluvoxamine desensitizes 5-HT2C receptor-mediated hypolocomotion in rats

The effectiveness of fluvoxamine, a selective serotonin re-uptake inhibitor (SSRI), in the treatment of anxiety disorders, such as obsessive-compulsive, panic and social anxiety disorders, has been confirmed in clinical studies. The hypersensitivity of 5-HT2C receptors has been reported in subjects with these disorders, and SSRIs have been suggested to have therapeutic effects in such cases through the desensitization of the 5-HT2C receptor function. In the present study, we investigated whether chronic administration of fluvoxamine desensitizes 5-HT2C receptors using a putative in vivo rat model of 5-HT2C receptor function. Acute treatment with fluvoxamine or another SSRI, paroxetine, reduced spontaneous locomotion, as observed with the administration of m-chlorophenylpiperazine (mCPP). This effect of fluvoxamine was reversed by treatment with a selective 5-HT2C receptor antagonist, SB 242084. On the other hand, chronic treatment with fluvoxamine or paroxetine inhibited mCPP-induced hypolocomotion, while they had no effects in control rats. In addition, chronic treatment with these drugs had no effects on the mCPP concentration in the rat brain. These results suggest that 5-HT2C receptors are desensitized by chronic treatment with fluvoxamine, as well as paroxetine. Thus, the clinical efficacy of fluvoxamine on anxiety disorders might involve the normalization of the 5-HT2C receptor function.

Effects of acute or chronic administration of anti-migraine drugs sumatriptan and zolmitriptan on serotonin synthesis in the rat brain

Triptans are 5-HT1 receptor agonists used as anti-migraine drugs. They act primarily on meningeal blood vessels and on trigeminovascular afferents, but they may also exert central effects. We studied the regional effects of acute and chronic treatment with sumatriptan or zolmitriptan on the rate of serotonin (5-HT) synthesis in the rat brain, using the alpha-14C-methyl-L-tryptophan quantitative autoradiographic method. Sumatriptan at low (300 microg/kg, s.c.) and high (1 mg/kg) doses, as well as zolmitriptan (100 microg/kg), acutely decreased (15-40%, P < 0.05-0.001) 5-HT synthetic rate in many brain regions, including the dorsal raphe nucleus. Chronically, sumatriptan (21 days, approximately 300 microg/kg per day via osmotic minipumps) induced significant increases in the 5-HT synthesis rate in many projection areas but had no effect in the dorsal raphe nucleus. The acute effects on 5-HT synthesis rate would be compatible with activation of 5-HT1 autoreceptors that inhibit serotonin release. In contrast, the increased 5-HT synthesis rate observed after chronic sumatriptan might possibly result from a down-regulation/desensitization of 5-HT1 receptors and/or unmasking of excitatory triptan-sensitive 5-HT receptors. Overall, the present findings indicate that not only zolmitriptan but also sumatriptan affect brain serotonergic neurotransmission.

Differential autoreceptor control of extracellular 5-HT in guinea pig and rat: species and regional differences

RATIONALE

Central 5-hydroxytryptamine (5-HT) release is regulated by inhibitory 5-HT autoreceptors, including 5-HT(1A) and 5-HT(1B) receptors.

OBJECTIVES

The purpose of this study was to use combinations of selective autoreceptor antagonists to elucidate the role of these receptors in controlling extracellular 5-HT in terminal areas. METHODS. Microdialysis was carried out in awake rats and guinea pigs to measure extracellular 5-HT in the frontal cortex and dentate gyrus. Using the selective 5-HT(1A) receptor antagonist, WAY-100635, and the selective 5-HT(1B) receptor antagonist, SB-224289, we have compared the roles of 5-HT(1A) and 5-HT(1B) autoreceptors in controlling extracellular 5-HT.

RESULTS

SB-224289 (4 mg/kg i.p.) alone produced a significant 50% increase in extracellular 5-HT in the dentate gyrus of guinea pigs, but not in the frontal cortex of the same animals. Co-administration of WAY-100635 (0.3 mg/kg s.c.), did not change the SB-224289-induced increase in dentate gyrus 5-HT but did produce a significant augmentation (60% increase) of guinea pig frontal cortex 5-HT. In contrast, neither autoreceptor antagonist, alone or in combination, affected extracellular 5-HT in the frontal cortex or dentate gyrus of rats.

CONCLUSIONS

These data indicate that there is a species difference in the autoreceptor control of 5-HT release. Furthermore, in the guinea pig there is a divergence between dorsal and median raphe innervated brain regions. On the basis that antagonism of 5-HT(1A) and 5-HT(1B) receptors produced an immediate increase in extracellular 5-HT in multiple brain regions in the guinea pig, it is suggested that this might be a novel mechanism for achieving antidepressant efficacy.

Therapeutic potential of serotonin antagonists in depressive disorders

Although the precise neurochemical imbalances in depression are still unknown, a role for the neurotransmitter 5-hydroxytryptamine (serotonin) has been implicated since the identification of the first effective antidepressants, imipramine and iproniazid. This led to the development of the selective serotonin re-uptake inhibitors which are widely used in the treatment of depression and depressive disorders, including generalised anxiety disorder, social phobia, obsessive compulsive disorder etc. Studies involving chronic administration in rats led to the hypothesis that the different classes of antidepressant treatment produce a common neuroadaptive change, namely an enhancement of serotonin neurotransmission, albeit via different pre and postsynaptic mechanisms. From this, it was suggested that serotonin antagonists should induce similar neuroadaptive changes, either directly or through a potentiation of other antidepressant agents. Here, the profiles of novel serotonin antagonists currently in preclinical development are reviewed and their therapeutic potential is assessed.

Molecular, pharmacological and functional diversity of 5-HT receptors

Serotonin (5-hydroxytryptamine, 5-HT) is probably unique among the monoamines in that its effects are subserved by as many as 13 distinct heptahelical, G-protein-coupled receptors (GPCRs) and one (presumably a family of) ligand-gated ion channel(s). These receptors are divided into seven distinct classes (5-HT(1) to 5-HT(7)) largely on the basis of their structural and operational characteristics. Whilst this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity continues to emerge. The challenge for modern 5-HT research has therefore been to define more precisely the properties of the systems that make this incredible diversity possible. Much progress in this regard has been made during the last decade with the realisation that serotonin is possibly the least conservative monoamine transmitter and the cloning of its many receptors. Coupled with the actions of an extremely avid and efficient reuptake system, this array of receptor subtypes provides almost limitless signalling capabilities to the extent that one might even question the need for other transmitter systems. However, the complexity of the system appears endless, since posttranslational modifications, such as alternate splicing and RNA editing, increase the number of proteins, oligomerisation and heteromerisation increase the number of complexes, and multiple G-protein suggest receptor trafficking, allowing phenotypic switching and crosstalk within and possibly between receptor families. Whether all these possibilities are used in vivo under physiological or pathological conditions remains to be firmly established, but in essence, such variety will keep the 5-HT community busy for quite some time. Those who may have predicted that molecular biology would largely simplify the life of pharmacologists have missed the point for 5-HT research in particular and, most probably, for many other transmitters. This chapter is an attempt to summarise very briefly 5-HT receptor diversity. The reward for unravelling this complex array of serotonin receptor--effector systems may be substantial, the ultimate prize being the development of important new drugs in a range of disease areas.

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.

The role of 5-HT1B receptors in the regulation of serotonin cell firing and release in the rat brain

The release of 5-HT in terminal areas of the rodent brain is regulated by 5-HT1B receptors. Here we examined the role of 5-HT1B receptors in the control of 5-HT output and firing in the dorsal raphe nucleus (DR), median raphe nucleus (MnR) and forebrain of the rat in vivo. The local perfusion (30-300 microM) of the selective 5-HT1B receptor agonist CP-93,129 to freely moving rats decreased 5-HT release in the DR and more markedly in the MnR. Likewise, 300 microM CP-93,129 reduced 5-HT output in substantia nigra pars reticulata, ventral pallidum, lateral habenula and the suprachiasmatic nucleus. The effect of CP-93,129 was prevented by SB-224289, but not by WAY-100635, selective 5-HT1B and 5-HT1A receptor antagonists, respectively. SB-224289 did not alter dialysate 5-HT in any raphe nuclei. The intravenous administration of the brain-penetrant selective 5-HT1B receptor agonist CP-94,253 (0.5-2.0 mg/kg) to anesthetized rats decreased dialysate 5-HT in dorsal hippocampus and globus pallidus, increased it in MnR and left it unaltered in the DR and medial prefrontal cortex. SB-224289, at a dose known to block 5-HT1B autoreceptor-mediated effects (5 mg/kg), did not prevent the effect of CP-94,253 on MnR 5-HT. The intravenous administration of CP-94,253 (0.05-1.6 mg/kg) to anesthetized rats increased the firing rate of MnR, but not DR-5-HT neurons. The local perfusion of CP-94,253 in the MnR showed a biphasic effect, with 5-HT reductions at 0.3-3 microM and increase at 300 microM. These results suggest that 5-HT cell firing and release in midbrain raphe nuclei (particularly in the MnR) are under control of 5-HT1B receptors. The activation of 5-HT1B autoreceptors (possibly located on 5-HT nerve endings and/or varicosities within DR and MnR) reduces 5-HT release. The effects of higher concentrations of 5-HT1B receptor agonists seem more compatible with the activation of 5-HT1B heteroreceptors on inhibitory neurons.

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.

Interaction of serotonin autoreceptor antagonists in the rat dorsal raphe nucleus: an in vitro fast cyclic voltammetry study

5-HT1A, 5-HT1B and 5-HT1D receptors are known to function as 5-HT autoreceptors in the rat dorsal raphe nucleus (DRN), modulating local 5-HT efflux. However, there are no studies on the simultaneous blockade of these receptors in the DRN. We investigated the effect of 5-HT1B and 5-HT1D receptor antagonists on 5-HT efflux in rat DRN, alone and in the presence of 5-HT1A receptor antagonists, using the technique of fast cyclic voltammetry. The 5-HT1A receptor antagonist, WAY 100635, and the 5-HT1B receptor antagonist, SB-224289, had no effect on 5-HT efflux while the 5-HT1B/1D receptor antagonist, GR 127935, produced a small decrease in 5-HT efflux. In contrast, the 5-HT1D receptor antagonist, BRL 15572, produced a significant increase in 5-HT efflux. Co-perfusion of WAY 100635 and SB-224289 significantly increased 5-HT efflux. In addition, WAY 100635 reversed the small inhibition of 5-HT efflux observed with GR 127935 but had no effect on the BRL 15572-induced increase. Antagonism of all three 5-HT autoreceptors with SB-224289, BRL 15572 and WAY 100635 significantly increased 5-HT efflux. These data confirm that 5-HT efflux within the DRN is under the control of 5-HT1A, 5-HT1B and 5-HT1D autoreceptors and elevation of 5-HT efflux was greatest following antagonism of 5-HT1A and 5-HT1B receptors.