The 5-HT1A antagonist WAY-100635 selectively potentiates the presynaptic effects of serotonergic antidepressants in rat brain

Depressive-like state sensitizes 5-HT1A and 5-HT1B auto-receptors in the dorsal raphe nucleus sub-system

Dorsal raphe (DR) and median raphe (MR) 5-HT neurons are two distinct sub-systems known to be regulated by 5-HT_1A and 5-HT_1B auto-receptors. Whether the auto-receptors in each sub-system are functionally altered in depressive-like state remains unknown. The present study is aimed to study a specific circuit (DR-ventral hippocampus and MR-dorsal hippocampus) within each sub-system to investigate changes in receptor sensitivity in the pathogenesis of depression. A mouse model of depression was developed through the social defeat paradigm, and was then treated with fluoxetine (FLX). 5-HT_1A auto-receptor in the neuronal cell body (DR or MR) and 5-HT_1B auto-receptor in the axonal terminal (ventral or dorsal hippocampus) were directly targeted by local perfusion of antagonists (5-HT_1A: WAY100635; 5-HT_1B: GR127935) through reverse microdialysis. Time courses of dialysate 5-HT measured at the axonal terminal were subsequently determined for each circuit. At baseline, 5-HT_1A and 5-HT_1B antagonists dose-dependently increased dialysate 5-HT, with sub-circuit specificity. In the depressive-like state, greater increases in dialysate 5-HT were observed only in the DR-ventral hippocampus circuit following local delivery of both antagonists, which were then fully restored following the FLX treatment. In contrast, no changes were observed in the MR-dorsal hippocampus circuit. Our results demonstrate differential changes in sensitivities of 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus and MR-dorsal hippocampus circuits. 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus circuit are sensitized in the depressive-like state. Taken together, these results suggest that the DR sub-system maybe the neural substrate mediating depressive phenotypes.

Purpurin exerted antidepressant-like effects on behavior and stress axis reactivity: evidence of serotonergic engagement

RATIONALE AND OBJECTIVES

Major depression represents a significant public health problem worldwide, and effective regimen is lacking. The present study investigated the antidepressant-like effects of purpurin, a natural anthraquinone compound from Rubia tinctorum L., and explored the underlying mechanism(s).

METHODS

Forced swim test (FST) and tail suspension test (TST) were used to assess antidepressant-like effects of purpurin in mice. Effects of purpurin on neuroendocrine responsivity were evaluated at the level of corticosterone and ACTH following acute restraint stress and intracerebroventricular injection of corticotrophin-releasing-factor (CRF). Serotonergic mechanisms underlying purpurin antidepressant effect were explored using biochemical, neurochemical, and pharmacological paradigms.

RESULTS

Chronic purpurin treatment exerted in mice dose-dependently antidepressant-like effects on behavior and stress axis reactivity (n = 9-11 per group). The purpurin-triggered antidepressant-like effects are serotonergically dependent, since purpurin-treated mice showed escalated levels of brain serotonin and suppressed monoamine oxidase (MAO) activity (n = 8-11 per group). Consistently, chemical depletion of brain serotonin by p-chlorophenylalanine (PCPA) abolished the antidepressant-like effects of purpurin on behavior and stress axis responsivity (n = 9-10 per group). Moreover, the antidepressant effect by purpurin was preferentially counteracted by 1A-selective 5-HT receptor antagonist WAY-100635, but potentiated by 1A-selective agonist 8-OH-DPAT and sub-effective dose of serotonergic antidepressant fluoxetine (n = 9-11 per group), suggesting a crucial role for 5-HT1A related serotonergic system in mediating such purpurin antidepressant effect.

CONCLUSION

We have revealed the antidepressant-like effects of purpurin on both behavior and stress axis reactivity in mice, with serotonergic system that preferentially couples with 5-HT1A receptors being critically engaged.

Can we increase speed and efficacy of antidepressant treatments? Part I: General aspects and monoamine-based strategies

Major depressive disorder (MDD) is a severe psychiatric syndrome with high prevalence and socioeconomic impact. Current antidepressant treatments are based on the blockade of serotonin (5-hydroxytryptamine, 5-HT) and/or noradrenaline transporters. These drugs show slow onset of clinical action and limited efficacy, partly due to the activation of physiological negative feed-back mechanisms operating through autoreceptors (5-HT_1A, 5-HT_1B, α₂-adrenoceptors) and postsynaptic receptors (e.g., 5-HT₃). As a result, clinically-relevant doses of reuptake inhibitors increase extracellular (active) 5-HT concentrations in the midbrain raphe nuclei but not in forebrain, as indicated by rodent microdialysis studies and by PET-scan studies in primate/human brain. The prevention of these self-inhibitory mechanisms by antagonists of the above receptors augments preclinical and clinical antidepressant effects. Hence, the mixed ß-adrenoceptor/5-HT_1A antagonist pindolol accelerated, and in some cases enhanced, the clinical action of selective serotonin reuptake inhibitors (SSRI). This strategy has been incorporated into two new multi-target antidepressant drugs, vilazodone and vortioxetine, which combine 5-HT reuptake inhibition and partial agonism at 5-HT_1A receptors. Vortioxetine shows also high affinity for other 5-HT receptors, including excitatory 5-HT₃ receptors located in cortical and hippocampal GABA interneurons. 5-HT₃ receptor blockade by vortioxetine enhances pyramidal neuron activity in prefrontal cortex as well as cortical and hippocampal 5-HT release. It is still too soon to know whether these new antidepressants will represent a real advance over existing drugs in the real world. However, their development opened the way to future antidepressant drugs based on the prevention of local and distal self-inhibitory mechanisms attenuating monoamine activity.

Generalization of serotonin and dopamine ligands to the discriminative stimulus effects of different doses of ±3,4-methylenedioxymethamphetamine

Studies that have attributed the discriminative stimulus effects of ±3,4-methylenedioxymethamphetamine (MDMA) to serotonergic mechanisms typically use a relatively low training dose of 1.5 mg/kg. The role of serotonin in the discriminative stimulus effects of higher doses of MDMA is, however, unknown. Separate groups of rats were trained to discriminate MDMA (1.5 or 3.0 mg/kg) from saline using a two-lever, food-reinforced drug-discrimination procedure. Generalization tests were carried out with a range of serotonin and dopamine ligands. Fluoxetine (0.3-3 mg/kg), clomipramine (1-10 mg/kg) and meta-chlorophenylpiperazine (0.3-2 mg/kg) dose-dependently substituted for the 1.5 mg/kg MDMA stimulus, but not the 3.0 mg/kg MDMA stimulus. 8-OH-DPAT (0.03-0.3 mg/kg) and RU-24969 (0.3-3 mg/kg) substituted for both the low-dose and the high-dose MDMA stimulus. The generalization dose-effect curve produced by 2,5-dimethoxy-4-iodoamphetamine (0.3-3 mg/kg) was shifted to the right for the 3.0 mg/kg MDMA-trained group. Amphetamine (0.25 and 0.5 mg/kg) and apomorphine (0.125 and 0.25 mg/kg) substituted for the 3.0 mg/kg, but not the 1.5 mg/kg MDMA stimulus. The results suggest some differences in the role of serotonin and dopamine in the discriminative stimulus effects of a low versus a higher dose of MDMA.

Synthesis and Structure-Activity Relationship Analysis of 5-HT₇ Receptor Antagonists: Piperazin-1-yl Substituted Unfused Heterobiaryls

A series of piperazin-1-yl substituted unfused heterobiaryls was synthesized as ligands of the 5-HT₇ receptors. The goal of this project was to elucidate the structural features that affect the 5-HT₇ binding affinity of this class of compounds represented by the model ligand 4-(3-furyl)-2-(4-methylpiperazin-1-yl)pyrimidine (2). The SAR studies included systematical structural changes of the pyrimidine core moiety in 2 to quinazoline, pyridine and benzene, changes of the 3-furyl group to other heteroaryl substituents, the presence of various analogs of the 4-methylpiperazin-1-yl group, as well as additional substitutions at positions 5 and 6 of the pyrimidine. Substitution of position 6 of the pyrimidine in the model ligand with an alkyl group results in a substantial increase of the binding affinity (note a change in position numbers due to the nomenclature rules). It was also demonstrated that 4-(3-furyl) moiety is crucial for the 5-HT₇ binding affinity of the substituted pyrimidines, although, the pyrimidine core can be replaced with a pyridine ring without a dramatic loss of the binding affinity. The selected ethylpyrimidine (12) and butylpyrimidine (13) analogs of high 5-HT₇ binding affinity showed antagonistic properties in cAMP functional test and varied selectivity profile-compound 12 can be regarded as a dual 5-HT₇/5-HT2AR ligand, and 13 as a multi-receptor (5-HT₇, 5-HT2A, 5-HT₆ and D₂) agent.

Study on anti-inflammatory and immunomodulatory effects of clomipramine in carrageenan- and lipopolysaccharide-induced rat models of inflammation

The aim of the present study was to evaluate the anti-inflammatory effect of clomipramine in carrageenan- and lipopolysaccharide-induced (LPS-induced) models of inflammation by investigating the changes in serum levels of the pro-inflammatory cytokine TNF-α and the anti-inflammatory cytokines IL-10 and TGF-β after single and repeated administration of the drug.

In order to study the effect of single and repeated doses of clomipramine on carrageenan-induced paw oedema, male Wistar rats were divided in five groups (n = 8): control, positive control group and three experimental groups treated with 5, 10 and 20 mg/kg bw clomipramine, respectively. The effect of single and repeated doses of clomipramine on serum cytokine levels was studied as animals were divided in four groups: two control groups treated with saline and two experimental groups treated with clomipramine 20 mg/kg bw. Carrageenan and LPS were injected immediately after clomipramine or saline injection. Serum cytokine concentrations were tested by enzyme immunoassay.

Following acute administration only the highest dose that was used inhibited the carrageenan-induced inflammation. Oedema inhibition was observed with 5, 10 and 20 mg/kg bw clomipramine after repeated administration. Single and repeated administration of clomipramine at a dose of 20 mg/kg bw did not significantly change the serum levels of TGF-1β, IL-10 and TNF-α when compared to the controls in carrageenan-induced inflammation. Following LPS-induced inflammation clomipramine significantly increased the serum levels of TGF-1β after repeated administration and decreased TNF-α in rats after single-dose and repeated pretreatment with 20 mg/kg bw clomipramine. A significant increase in the levels of IL-10 in relation to this inflammatory model was observed only in single dose treated animals.

Clomipramine possesses an anti-inflammatory effect in the carrageenan-induced model of exudative inflammation. In LPS-induced inflammation, clomipramine showed an immunomodulatory effect, decreasing TNF-α and increasing TGF-1β after repeated administration, and increasing IL-10 after a single dose.

Mechanism of action of the bimodal antidepressant vilazodone: evidence for serotonin1A-receptor-mediated auto-augmentation of extracellular serotonin output

RATIONALE

The recently approved antidepressant vilazodone, a serotonin (5-HT)1A receptor partial agonist/selective 5-HT reuptake inhibitor offers new possibilities to study the underlying mechanisms of depression pharmacotherapy and of 5-HT augmenting antidepressants.

OBJECTIVE

The role of the 5-HT1A receptor with respect to the regulation of 5-HT output in the mechanism of action of vilazodone.

METHOD

We measured 5-HT levels in two subregions of the rat prefrontal cortex by microdialysis, and 5-hydroxytryptophan (5-HTP) accumulation and tissue 5-HT concentrations ex vivo.

RESULTS

Vilazodone-induced maximal 5-HT levels were similar in the medial and the lateral cortex and were up to sixfold higher than those induced by paroxetine, citalopram, or fluoxetine tested in parallel. Depolarization/autoreceptor-insensitive 5-HT release by vilazodone could be excluded. The citalopram (1 μM, locally infused)-induced increase of 5-HT was further increased by vilazodone (1 mg/kg i.p.), but not by citalopram (10 mg/kg i.p.). Unlike fluoxetine, vilazodone-induced extracellular 5-HT output was not potentiated by cotreatment with the 5-HT1A receptor blocker N-[2-(4-{2-methoxyphenyl}-1-piperazinyl)-ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635). In contrast to fluoxetine, vilazodone exhibited intrinsic 5-HT1A agonist activity: it reduced, similar to (±)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT), 5-HTP accumulation in striatum and n. raphe of reserpinized rats. Hence, vilazodone's agonistic actions must be 5-HT1A receptor-related since endogenous 5-HT is lacking in the reserpine-depleted animal.

CONCLUSIONS

In spite of high intrinsic 5-HT1A activity in reserpinized rats, the net effect of vilazodone at release-regulating 5-HT1A autoreceptors must be inhibitory, leading to markedly increased 5-HT output. Another possibility is that vilazodone rapidly desensitizes autoinhibitory 5-HT1A receptors by an unknown mechanism.

Serotonergic receptor mechanisms underlying antidepressant-like action in the progesterone withdrawal model of hormonally induced depression in rats

Hormonally induced mood disorders such as premenstrual dysphoric disorder (PMDD) are characterized by a range of physical and affective symptoms including anxiety, irritability, anhedonia, social withdrawal and depression. Studies demonstrated rodent models of progesterone withdrawal (PWD) have a high level of constructive and descriptive validity to model hormonally-induced mood disorders in women. Here we evaluate the effects of several classes of antidepressants in PWD female Long-Evans rats using the forced swim test (FST) as a measure of antidepressant activity. The study included fluoxetine, duloxetine, amitriptyline and an investigational multimodal antidepressant, vortioxetine (5-HT(3), 5-HT(7) and 5-HT(1D) receptor antagonist; 5-HT(1B) receptor partial agonist; 5-HT(1A) receptor agonist; inhibitor of the serotonin transporter (SERT)). After 14 days of administration, amitriptyline and vortioxetine significantly reduced immobility in the FST whereas fluoxetine and duloxetine were ineffective. After 3 injections over 48 h, neither fluoxetine nor duloxetine reduced immobility, whereas amitriptyline and vortioxetine significantly reduced FST immobility during PWD. When administered acutely during PWD, the 5-HT(1A) receptor agonist, flesinoxan, significantly reduced immobility, whereas the 5-HT(1A) receptor antagonist, WAY-100635, increased immobility. The 5-HT(3) receptor antagonist, ondansetron, significantly reduced immobility, whereas the 5-HT(3) receptor agonist, SR-57227, increased immobility. The 5-HT(7) receptor antagonist, SB-269970, was inactive, although the 5-HT(7) receptor agonist, AS-19, significantly increased PWD-induced immobility. None of the compounds investigated (ondansetron, flesinoxan and SB-269970) improved the effect of fluoxetine during PWD. These data indicate that modulation of specific 5-HT receptor subtypes is critical for manipulating FST immobility in this model of hormone-induced depression.

Antidepressant activity: contribution of brain microdialysis in knock-out mice to the understanding of BDNF/5-HT transporter/5-HT autoreceptor interactions

Why antidepressants vary in terms of efficacy is currently unclear. Despite the leadership of selective serotonin reuptake inhibitors (SSRIs) in the treatment of depression, the precise neurobiological mechanisms involved in their therapeutic action are poorly understood. A better knowledge of molecular interactions between monoaminergic system, pre- and post-synaptic partners, brain neuronal circuits and regions involved may help to overcome limitations of current treatments and identify new therapeutic targets. Intracerebral in vivo microdialysis (ICM) already provided important information about the brain mechanism of action of antidepressants first in anesthetized rats in the early 1990s, and since then in conscious wild-type or knock-out mice. The principle of ICM is based on the balance between release of neurotransmitters (e.g., monoamines) and reuptake by selective transporters [e.g., serotonin transporter for serotonin 5-hydroxytryptamine (5-HT)]. Complementary to electrophysiology, this technique reflects pre-synaptic monoamines release and intrasynaptic events corresponding to ≈80% of whole brain tissue content. The inhibitory role of serotonergic autoreceptors infers that they limit somatodendritic and nerve terminal 5-HT release. It has been proposed that activation of 5-HT_1A and 5-HT_1B receptor sub-types limits the antidepressant-like activity of SSRIs. This hypothesis is based partially on results obtained in ICM experiments performed in naïve, non-stressed rodents. The present review will first remind the principle and methodology of ICM performed in mice. The crucial need of developing animal models that display anxiety and depression-like behaviors, neurochemical and brain morphological phenotypes reminiscent of these mood disorders in humans, will be underlined. Recently developed genetic mouse models have been generated to independently manipulate 5-HT_1A auto and heteroreceptors and ICM helped to clarify the role of the pre-synaptic component, i.e., by measuring extracellular levels of neurotransmitters in serotonergic nerve terminal regions and raphe nuclei. Finally, we will summarize main advantages of using ICM in mice through recent examples obtained in knock-outs (drug infusion through the ICM probe allows the search of a correlation between changes in extracellular neurotransmitter levels and antidepressant-like activity) or alternatives (infusion of a small-interfering RNA suppressing receptor functions in the mouse brain). We will also focus this review on post-synaptic components such as brain-derived neurotrophic factor in adult hippocampus that plays a crucial role in the neurogenic and anxiolytic/antidepressant-like activity of chronic SSRI treatment. Limitations of ICM will also be considered.

5-HT depletion, but not 5-HT1A antagonist, prevents the anxiolytic-like effect of citalopram in rat contextual conditioned fear stress model

OBJECTIVE

Selective serotonin reuptake inhibitors (SSRIs) have been widely used in the treatment of most anxiety disorders. In this study, to clarify the mechanism of the anxiolytic effect, we investigated the mechanism underlying the effect of the SSRI citalopram on rat contextual conditioned fear stress (CFS), an animal model of anxiety.

METHODS

Rats individually received footshocks in a shock chamber. More than 1 day later, they were given citalopram and/or dl-p-chlorophenylalanine (PCPA), various subtype-selective serotonin (5-HT) receptor antagonists: the 5-HT1A receptor antagonist WAY 100635, the 5-HT2A receptor antagonist MDL 100907, the 5-HT2C receptor antagonist SB 242084, the 5-HT3 receptor antagonist tropisetron, the 5-HT4 receptor antagonist GR 125487, the 5-HT6 receptor antagonist SB 258585 or the 5-HT7 receptor antagonist SB 269970. After drug administration, freezing behaviour, which was used as an index of anxiety, was analysed in the same shock chamber without shocks.

RESULTS

Citalopram dose dependently reduced conditioned freezing behaviour. The anxiolytic-like effect of citalopram was prevented completely by pretreatment with the 5-HT-depleting agent PCPA, but not by the 5-HT1A receptor antagonist WAY 100635. Furthermore, none of the subtype-selective 5-HT receptor antagonists significantly affected conditioned freezing or affected the anxiolytic-like effect of citalopram.

CONCLUSION

The anxiolytic-like effect of citalopram in contextual CFS model depends on 5-HT availability. In addition, contextual CFS model is suggested to be completely different from conventional anxiety models in neural mechanism or manners of serotonergic involvement. However, further studies are needed to identify the pharmacological mechanisms responsible for the anxiolytic-like effect of citalopram.

Evidence for the involvement of 5-HT1A receptor in the acute antidepressant-like effect of creatine in mice

Creatine was previously shown to produce an antidepressant-like effect in the tail suspension test through a modulation of the dopaminergic system. In this study, the mechanisms underlying its antidepressant-like effect were further evaluated by investigating the involvement of the serotonergic system in its effect. The anti-immobility effect of creatine (1mg/kg) was prevented by the pretreatment of mice with p-chlorophenylalanine methyl ester (PCPA; 100mg/kg, i.p., for 4 consecutive days, an inhibitor of serotonin (5-HT) synthesis). Creatine (0.01 mg/kg, sub-effective dose) in combination with sub-effective doses of WAY100635 (0.1mg/kg, s.c., a 5-HT1A receptor antagonist), 8-OH-DPAT (0.1mg/kg, i.p., a 5-HT1A receptor agonist) or selective serotonin reuptake inhibitors fluoxetine (5mg/kg, p.o.), paroxetine (0.1mg/kg, p.o.), citalopram (0.1mg/kg, p.o.) and sertraline (3mg/kg, p.o.) reduced the immobility time in the tail suspension test as compared with either drug alone. These results indicate that the antidepressant-like effect of creatine is likely mediated by an interaction with 5-HT1A receptors. Of note, the present results also indicate that creatine improves the effectiveness of the selective serotonin reuptake inhibitors, a finding that may have therapeutic implications for the treatment of depressive disorders.

Evidence for the involvement of the serotonergic 5-HT(1A) receptors in the antidepressant-like effect caused by hesperidin in mice

The present study investigated a possible antidepressant-like activity of hesperidin using two predictive tests for antidepressant effect in mice: the forced swimming test (FST) and the tail suspension test (TST). Results demonstrated that hesperidin (0.1, 0.3 and 1 mg/kg, intraperitoneal, i.p.) decreased the immobility time in the FST and TST without affecting the locomotor activity in the open field test. The antidepressant-like effect of hesperidin (0.3 mg/kg) on the TST was prevented by the pretreatment of mice with p-chlorophenylalanine methyl ester (pCPA; 100 mg/kg, i.p., an inhibitor of serotonin synthesis) and WAY100635 (0.1 mg/kg, subcutaneous, s.c., a selective 5-HT(1A) receptor antagonist). Pretreatment of mice with prazosin (1 mg/kg, i.p., an α(1)-adrenoceptor antagonist), yohimbine (1 mg/kg, i.p., an α(2)-adrenoceptor antagonist), propranolol (2 mg/kg, i.p., a β-adrenoceptor antagonist), AMPT (100 mg/kg, i.p., an inhibitor of tyrosine hydroxylase), SCH23390 (0.05 mg/kg, s.c., a dopamine D(1) receptor antagonist), sulpiride (50 mg/kg, i.p., a dopamine D(2) receptor antagonist), ketanserin (1mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist) or MDL72222 (1 mg/kg, i.p., a 5-HT(3) receptor antagonist) did not block the antidepressant-like effect of hesperidin (0.3 mg/kg, i.p.) in the TST. Administration of hesperidin (0.01 mg/kg, i.p.) and fluoxetine (1 mg/kg), at subeffective doses, produced an antidepressant-like effect in the TST. The antidepressant-like effect caused by hesperidin in mice in the TST was dependent on an interaction with the serotonergic 5-HT(1A) receptors. Taken together, these results suggest that hesperidin possesses antidepressant-like property and may be of interest source for therapeutic agent for the treatment of depressive disorders.

Deficient serotonin neurotransmission and depression-like serotonin biomarker alterations in tryptophan hydroxylase 2 (Tph2) loss-of-function mice

Probably the foremost hypothesis of depression is the 5-hydroxytryptamine (5-HT, serotonin) deficiency hypothesis. Accordingly, anomalies in putative 5-HT biomarkers have repeatedly been reported in depression patients. However, whether such anomalies in fact reflect deficient central 5-HT neurotransmission remains unresolved. We employed a naturalistic model of 5-HT deficiency, the tryptophan hydroxylase 2 (Tph2) R439H knockin mouse, to address this question. We report that Tph2 knockin mice have reduced basal and stimulated levels of extracellular 5-HT (5-HT(Ext)). Interestingly, cerebrospinal fluid (CSF) 5-hydroxyindoleacetic acid (5-HIAA) and fenfluramine-induced plasma prolactin levels are markedly diminished in the Tph2 knockin mice. These data seemingly confirm that low CSF 5-HIAA and fenfluramine-induced plasma prolactin reflects chronic, endogenous central nervous system (CNS) 5-HT deficiency. Moreover, 5-HT(1A) receptor agonist-induced hypothermia is blunted and frontal cortex 5-HT(2A) receptors are increased in the Tph2 knockin mice. These data likewise parallel core findings in depression, but are usually attributed to anomalies in the respective receptors rather than resulting from CNS 5-HT deficiency. Further, 5-HT(2A) receptor function is enhanced in the Tph2 knockin mice. In contrast, 5-HT(1A) receptor levels and G-protein coupling is normal in Tph2 knockin mice, indicating that the blunted hypothermic response relates directly to the low 5-HT(Ext). Thus, we show that not only low CSF 5-HIAA and a blunted fenfluramine-induced prolactin response, but also blunted 5-HT(1A) agonist-induced hypothermia and increased 5-HT(2A) receptor levels are bona fide biomarkers of chronic, endogenous 5-HT deficiency. Potentially, some of these biomarkers could identify patients likely to have 5-HT deficiency. This could have clinical research utility or even guide pharmacotherapy.

In vitro antioxidant activity and in vivo antidepressant-like effect of α-(phenylselanyl) acetophenone in mice

In this study, the antioxidant and antidepressant-like effects of α-(phenylselanyl) acetophenone (PSAP), an organoselenium compound, were investigated. To assess the in vitro antioxidant properties, PSAP was evaluated in four test systems (DPPH, ABTS, FRAP and inhibition of lipid peroxidation). PSAP (100-500 μM) showed potent antioxidant activity and protected against lipid peroxidation. Additionally, we investigated whether PSAP, when administered in mice (100, 200 and 400mg/kg, per oral, p.o.), could cause acute toxicity. Our results demonstrated that PSAP did not cause the death of any animal, significantly reduce body weight or cause any oxidative tissue stress following treatment. This study also evaluated the effect of PSAP (0.1-10 mg/kg, p.o) on mice in a forced swim test (FST) and tail suspension test (TST), assays that are predictive of depressant activity and motor activity in the open-field. PSAP (5-10 mg/kg) significantly reduced immobility time in the FST and TST without affecting motor activity. In addition, the antidepressant-like effect caused by PSAP (5m/kg, p.o) in mice during the TST was dependent on an interaction with the serotonergic system (5-HT(1A) receptors), but not with the noradrenergic, dopaminergic or adenosinergic system. Together, these results suggest that PSAP possesses antioxidant and antidepressant-like properties and may be of interest as a therapeutic agent for the treatment of depressive disorders.

Potential serotonin 5-HT(1A) and dopamine D(4) receptor modulation of the discriminative stimulus effects of amphetamine in rats

Activation of the dopaminergic system underlies the behavioral effects of (+)-amphetamine, and plays a major role in its discriminative stimulus properties. Although serotonin receptors modulate dopamine levels in the brain, and 5-HT(1A) and 5-HT(2) receptor agonists do not mimic (+)-amphetamine, pretreatment with 5-HT(2A/2C) agonists significantly potentiates the (+)-amphetamine cue. Further, 5-HT(2) antagonists do not modify the discriminative stimulus effect of (+)-amphetamine, but 5-HT(1A) antagonists have never been tested in (+)-amphetamine-trained rats. This study sought to characterize the effects of the 5-HT(1A) antagonist WAY 100635 on (+)-amphetamine-induced discriminative stimulus effects. Male Sprague-Dawley rats were trained in a two-lever, fixed ratio 50, food-reinforced task with (+)-amphetamine sulfate (1.0 mg/kg, i.p., 30 min pretreatment time) as the discriminative stimulus. Substitution and combination tests with WAY 100635 were then performed. WAY 100635 did not produce substitution in amphetamine-trained rats, but significantly increased behavioral disruption. In combination tests 0.4 and 5.4 mg/kg doses of WAY 100635 potentiated the amphetamine cue. We suggest that low doses of WAY 100635 potentiated the (+)-amphetamine cue by blockade of 5-HT(1A) receptors, but stimulation of the dopamine D(4) receptor by higher doses of WAY 100635 may be responsible for potentiation of amphetamine-induced behavioral sensitization. The high percentage of behavioral disruption in substitution tests might suggest that rats trained to discriminate (+)-amphetamine from saline show behavioral sensitization that is not detectable by the drug discrimination assay but may be expressed as hyperactivity and stereotypic behavior that disrupts operant behavior.

Mechanisms of antidepressant action: an integrated dopaminergic perspective

The molecular mechanisms that cause and maintain the major depressive disorder (MDD) are currently unknown. Consistently, antidepressant treatments are characterized by insufficient success rates. This causes high social costs and severe personal sufferings. In the present review we analyze some of the paradigms that are used to explain MDD, particularly from the perspective of the dopaminergic (DA) system. DA has been more classically associated with psychosis and substance abuse disorders, even though a role of DA in MDD has been proposed as well and some antidepressants with DA profile exist. In the present work, we review some of the molecular mechanisms that underpin MDD from the perspective of the dopaminergic system, in the hope of unifying some of the major theories of MDD - the monoaminergic, inflammatory, epigenetics, neurotrophin and anti-apoptotic theories. Several shared components of these theories are highlighted, partially accounted by the functions of the DA system (see supplementary video).

Role of olfactory bulb serotonin in olfactory learning in the greater short-nosed fruit bat, Cynopterus sphinx (Chiroptera: Pteropodidae)

The role of olfactory bulb (OB) serotonin [5-hydroxytryptamine (5-HT)] in olfactory learning and memory was tested in the greater short-nosed fruit bat, Cynopterus sphinx (family Pteropodidae). Graded concentrations (25, 40, and 60microg) of 5,7-dihydroxytryptamine (5,7-DHT) or saline were injected into the OB of bats one day before training to the novel odor. In a behavioral test, 5,7-DHT (60microg) injected bats made significantly fewer feeding attempts and bouts when compared to saline-injected bats during learning and in the memory test. Subsequent biochemical analysis showed that 5-HT level was effectively depleted in the OB of 5,7-DHT injected bats. To test odor-induced 5-HT mediated changes in 5-HT receptors and second messenger cascade in the OB, we examined the expression of 5-HT receptors and mitogen-activated protein kinase (MAPK)/Erk cascade after training to the novel odor. We found that odor stimulation up-regulated the expression of 5-HT(1A) receptor, Erk1 and Creb1 mRNA, and phosphorylation of ERK1 and CREB1. Odor stimulation failed to induce expression in 5-HT-depleted bats, which is similar to control bats and significantly low compared to saline-treated bats. Together these data revealed that the level of 5-HT in the OB may regulate olfactory learning and memory in C. sphinx through Erk and CREB.

Assessing the neuronal serotonergic target-based antidepressant stratagem: impact of in vivo interaction studies and knockout models

Depression remains a challenge in the field of affective neuroscience, despite a steady research progress. Six out of nine basic antidepressant mechanisms rely on serotonin neurotransmitter system. Preclinical studies have demonstrated the significance of serotonin receptors (5-HT_1-3,6,7), its signal transduction pathways and classical down stream targets (including neurotrophins, neurokinins, other peptides and their receptors) in antidepressant drug action. Serotonergic control of depression embraces the recent molecular requirements such as influence on proliferation, neurogenesis, plasticity, synaptic (re)modeling and transmission in the central nervous system. The present progress report analyses the credibility of each protein as therapeutically relevant target of depression. In vivo interaction studies and knockout models which identified these targets are foreseen to unearth new ligands and help them transform to drug candidates. The importance of the antidepressant assay selection at the preclinical level using salient animal models/assay systems is discussed. Such test batteries would definitely provide antidepressants with faster onset, efficacy in resistant (and co-morbid) types and with least adverse effects. Apart from the selective ligands, only those molecules which bring an overall harmony, by virtue of their affinities to various receptor subtypes, could qualify as effective antidepressants. Synchronised modulation of various serotonergic sub-pathways is the basis for a unique and balanced antidepressant profile, as that of fluoxetine (most exploited antidepressant) and such a profile may be considered as a template for the upcoming antidepressants. In conclusion, 5-HT based multi-targeted antidepressant drug discovery supported by in vivo interaction studies and knockout models is advocated as a strategy to provide classic molecules for clinical trials.

Functional interactions between 5-HT2A and presynaptic 5-HT1A receptor-based responses in mice genetically deficient in the serotonin 5-HT transporter (SERT)

BACKGROUND AND PURPOSE

Despite decreased presynaptic 5-HT(1A) and altered 5-HT(2A) receptor function in genetically-deficient serotonin (5-HT) transporter (SERT) mice, the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100635) still induced head twitches in these mice, a well-established 5-HT(2A) receptor-mediated response.

EXPERIMENTAL APPROACH

Interactions between 5-HT(1A) and 5-HT(2A) receptors were assessed using the head-twitch response following 5-HT(1A) and 5-HT(2A) receptor agonists and antagonists in SERT wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The role of brain 5-HT availability in WAY 100635 induced head twitches was also examined.

KEY RESULTS

WAY 100635 induced head twitches in a SERT gene-dose dependent manner, inducing 5-fold more head twitches in SERT -/- versus SERT +/+ mice. In SERT -/- mice, inhibition of 5-HT synthesis with p-chlorophenylalanine (PCPA) markedly depleted tissue 5-HT in all five brain areas examined and abolished WAY 100635 induced head twitches. Further, the selective 5-HT reuptake inhibitor fluvoxamine increased WAY 100635 induced head twitches in SERT +/+ and +/- mice. Head twitches following the 5-HT(2A) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI) were robust in SERT +/+ and +/- mice but much reduced in SERT -/- mice. DOI-induced head twitches were decreased by the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in SERT +/+ and +/- mice. All drug-induced head twitches were blocked by the 5-HT(2A) receptor antagonist a-Phenyl-1-(2-phenylethyl)-4-piperidinemethanol (MDL 11,939).

CONCLUSIONS AND IMPLICATIONS

These data show that indirect activation of 5-HT(2A) receptors via blockade of presynaptic 5-HT(1A) receptors potentiated head-twitch responses, suggesting functional interactions between these receptors, interactions affected by altered 5-HT availability. Our findings strongly support the correlation of WAY 100635 induced head twitches with increased 5-HT availability, induced genetically or pharmacologically.

On the mechanism of the antidepressant-like action of group II mGlu receptor antagonist, MGS0039

RATIONALE

Several studies have suggested that modulation of the glutamatergic system could be a new, efficient way to achieve antidepressant activity. Behavioral data showed that group II mGlu receptor antagonists (i.e., (1R, 2R, 3R, 5R, 6R)-2-amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039) and (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xan th-9-yl) propanoic acid (LY341495)) elicited antidepressant activity in several animal models of depression in rats and/or mice. Although the antidepressant-like activity of MGS0039 and LY341495 is well documented, the mechanism of the antidepressant action of these compounds is still not clear.

OBJECTIVES

The aim of the present study was to specify the role of the serotonergic system in the mechanism of the antidepressant-like activity of group II mGlu receptor ligands by using the tail suspension test (TST) in mice; the role of AMPA receptors was also investigated. Furthermore, the possible antidepressant-like action of MGS0039 using the olfactory bulbectomy (OB) model of depression in rats was investigated.

RESULTS

The results of the TST studies showed that antidepressant-like action of group II mGlu receptor antagonists does not depend on serotonergic system activation. However, the AMPA receptor seems to play a key role in the antidepressant-like action of these compounds. Moreover, we have shown that repeated administration of MGS0039 attenuated OB-related deficits, confirming antidepressant-like activity of the tested compound.

CONCLUSIONS

The results suggest that the blockade of group II mGlu receptors may be effective in the treatment of depression. Moreover, we have found that the mechanism of action of group II mGlu receptor antagonists differs from that of typical antidepressants, such as SSRIs.

Differential influence of selective 5-HT5A vs 5-HT1A, 5-HT1B, or 5-HT2C receptor blockade upon light-induced phase shifts in circadian activity rhythms: interaction studies with citalopram

Though serotonergic mechanisms modulate circadian rhythms, roles of individual serotonin (5-HT) receptors remain uncertain since data are lacking for antagonists. Herein, both the 5-HT(5A) receptor antagonist, A843277 (10 mg/kg), and the 5-HT(1B) antagonist, SB224289 (1 mg/kg), inhibited light-induced phase advances in hamster circadian wheel-running rhythms. Conversely, though 5-HT(1A) and 5-HT(7) receptors are likewise implicated in circadian scheduling, their blockade by WAY100635 (0.5 mg/kg) and SB269970 (1 mg/kg), respectively, was ineffective. Since actions of 5-HT reuptake inhibitors are modified by antagonists, we evaluated their influence on suppression of phase advances by citalopram (10 mg/kg). Its action was potentiated by WAY100635 and the 5-HT(2C) antagonist, SB242084 (1 mg/kg), but not by A842377, SB224289, SB269970, and antagonists at 5-HT(2A) (MDL100907) and 5-HT(6) (SB399885) receptors. In conclusion, this is the first in vivo evidence for an influence of 5-HT(5A) receptors upon circadian rhythms, but no single class of 5-HT receptor mediates their control by citalopram.

The effect of clomipramine on wake/sleep and orexinergic expression in rats

We have previously found that neonatal treatment with clomipramine (CLI) induced a decrease in brain orexins during the juvenile period and that these changes were reversed at adulthood. This study investigated the effect of CLI on the orexinergic component and sleep/wake states. Two groups of adult male rats were conducted for 48-h polysomnographic recording. One group of rats was treated with CLI (20 mg/kg every 12 h), and a second group was treated with equivolume of saline (SAL) simultaneously after the first 24 h of polysomnographic recording. Rats were killed 2 h after the third dose of treatment. Brain tissues were collected for radioimmunoassay quantification of orexins and real-time PCR analysis of prepro-orexin and orexin receptor mRNA. The CLI group had significantly shorter rapid eye movement (REM) sleep and longer REM latency compared with both the baseline day and the SAL group and had significantly less active wake and more quiet wake. Compared with the control rats, the CLI rats had significantly higher mRNA expression of prepro-orexin in the hypothalamus and the frontal cortex, but not in the hippocampus. The CLI rats also had significantly less orexin B in the hypothalamus than the control rats. These results suggest that suppression of active wake and orexin B by CLI may be a factor responsible for CLI-induced depression and that the increase of prepro-orexin mRNA may be a sign of increased brain orexins found in this model.

Signal transduction and functional selectivity of F15599, a preferential post-synaptic 5-HT1A receptor agonist

BACKGROUND AND PURPOSE

Activation of post-synaptic 5-HT(1A) receptors may provide enhanced therapy against depression. We describe the signal transduction profile of F15599, a novel 5-HT(1A) receptor agonist.

EXPERIMENTAL APPROACH

F15599 was compared with a chemical congener, F13714, and with (+)8-OH-DPAT in models of signal transduction in vitro and ex vivo.

KEY RESULTS

F15599 was highly selective for 5-HT(1A) receptors in binding experiments and in [(35)S]-GTPgammaS autoradiography of rat brain, where F15599 increased labelling in regions expressing 5-HT(1A) receptors. In cell lines expressing h5-HT(1A) receptors, F15599 more potently stimulated extracellular signal-regulated kinase (ERK1/2) phosphorylation, compared with G-protein activation, internalization of h5-HT(1A) receptors or inhibition of cAMP accumulation. F13714, (+)8-OH-DPAT and 5-HT displayed a different rank order of potency for these responses. F15599 stimulated [(35)S]-GTPgammaS binding more potently in frontal cortex than raphe. F15599, unlike 5-HT, more potently and efficaciously stimulated G(alphai) than G(alphao) activation. In rat prefrontal cortex (a region expressing post-synaptic 5-HT(1A) receptors), F15599 potently activated ERK1/2 phosphorylation and strongly induced c-fos mRNA expression. In contrast, in raphe regions (expressing pre-synaptic 5-HT(1A) receptors) F15599 only weakly or did not induce c-fos mRNA expression. Finally, despite its more modest affinity in vitro, F15599 bound to 5-HT(1A) receptors in vivo almost as potently as F13714.

CONCLUSIONS AND IMPLICATIONS

F15599 showed a distinctive activation profiles for 5-HT(1A) receptor-mediated signalling pathways, unlike those of reference agonists and consistent with functional selectivity at 5-HT(1A) receptors. In rat, F15599 potently activated signalling in prefrontal cortex, a feature likely to underlie its beneficial effects in models of depression and cognition.

Effect of co-administration of the selective 5-HT1A receptor antagonist WAY 100,635 and selective 5-HT1B/1D receptor antagonist GR 127,935 on anxiolytic effect of citalopram in conditioned fear stress in the rat

This study investigated the effect of co-administration of the selective 5-HT1A receptor antagonist WAY 100,635 and selective 5-HT(1B/1D) receptor antagonist GR 127,935 with a subactive dose of citalopram [selective serotonin (5-HT) reuptake inhibitor (SSRI)] on the expression of conditioned freezing, an index of fear. In the present study, acute administration of citalopram (s.c.) reduced freezing significantly at high doses (10, 30 and 100 mg/kg), while showing no significant effect at low doses (1 and 3 mg/kg). Co-administration of WAY 100,635 (0.15 mg/kg) with citalopram (3 mg/kg) reduced freezing markedly and significantly, as compared with either drug alone. However, the addition of GR 127,935 (4 mg/kg) did not potentiate the effects of citalopram (3 mg/kg) on freezing and did not enhance the effect of WAY 100,635 (0.15 mg/kg) with citalopram (3 mg/kg). Co-administration of WAY 100,635 (0.15 mg/kg) or GR 127,935 (4 mg/kg) gave no effect on high-dose citalopram (30 mg/kg)-induced inhibition of freezing behavior. These results suggest that co-administration of WAY 100,635 (0.15 mg/kg) strengthens the anxiolytic effect of citalopram (3 mg/kg) by facilitating central 5-HT neurotransmission. Since GR 127,935 (4 mg/kg) failed to accelerate the inhibition of freezing induced by citalopram (3 mg/kg) with WAY 100,635 (0.15 mg/kg) or citalopram (3 mg/kg) alone, it is suggested that blocking 5-HT1A receptors is more effective in facilitating the anxiolytic effect of citalopram than blocking 5-HT1B/1D receptors.

Folic acid administration produces an antidepressant-like effect in mice: evidence for the involvement of the serotonergic and noradrenergic systems

Clinical studies have shown that folic acid plays a role in the pathophysiology of depression. However, very few studies have investigated its effect in behavioral models of depression. Hence, this study tested its effect in the forced swimming test (FST) and the tail suspension test (TST), two models predictive of antidepressant activity, in mice. Folic acid administered by oral route (p.o.) produced a reduction in the immobility time in the FST (50-100mg/kg) and in the TST (10-50mg/kg). The administration of folic acid by i.c.v. route also reduced the immobility time in the FST (10nmol/site) and in the TST (1-10nmol/site). Both folic acid administered by oral and i.c.v. route produced no psychostimulant effect, which indicates that its antidepressant-like effect is specific. Pretreatment of mice with p-chlorophenylalanine methyl ester (PCPA; 100mg/kg, i.p., an inhibitor of serotonin (5-HT) synthesis, for 4 consecutive days), ketanserin (5mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), prazosin (1mg/kg, i.p., an alpha(1)-adrenoceptor antagonist) or yohimbine (1mg/kg, i.p., an alpha(2)-adrenoceptor antagonist) prevented the anti-immobility effect of folic acid (50mg/kg, p.o.) in the FST. Moreover, the pretreatment of mice with WAY100635 (0.1mg/kg, s.c., a selective 5-HT(1A) receptor antagonist) blocked the decrease in immobility time in the FST elicited by folic acid (50mg/kg, p.o.), but produced a synergistic effect with a subeffective dose of folic acid (10mg/kg, p.o.). In addition, a subeffective dose of folic acid (10mg/kg, p.o.) produced a synergistic antidepressant-like effect with fluoxetine (10mg/kg, p.o.) in the FST. Overall, the results firstly indicate that folic acid produced an antidepressant-like effect in FST and in TST and that this effect appears to be mediated by an interaction with the serotonergic (5-HT(1A) and 5-HT(2A/2C) receptors) and noradrenergic (alpha(1)- and alpha(2)-adrenoceptors) systems.

Consequences of changes in BDNF levels on serotonin neurotransmission, 5-HT transporter expression and function: studies in adult mice hippocampus

In vivo intracerebral microdialysis is an important neurochemical technique that has been applied extensively in genetic and pharmacological studies aimed at investigating the relationship between neurotransmitters. Among the main interests of microdialysis application is the infusion of drugs through the microdialysis probe (reverse dialysis) in awake, freely moving animals. As an example of the relevance of intracerebral microdialysis, this review will focus on our recent neurochemical results showing the impact of Brain-Derived Neurotrophic Factor (BDNF) on serotonergic neurotransmission in basal and stimulated conditions. Indeed, although the elevation of 5-HT outflow induced by chronic administration of selective serotonin reuptake inhibitors (SSRIs) causes an increase in BDNF protein levels and expression (mRNA) in the hippocampus of rodents, the reciprocal interaction has not been demonstrated yet. Thus, the neurochemical sight of this question will be addressed here by examining the consequences of either a constitutive decrease or increase in brain BDNF protein levels on hippocampal extracellular levels of 5-HT in conscious mice.

SB-649915-B, a novel 5-HT1A/B autoreceptor antagonist and serotonin reuptake inhibitor, is anxiolytic and displays fast onset activity in the rat high light social interaction test

Preclinically, the combination of an SSRI and 5-HT autoreceptor antagonist has been shown to reduce the time to onset of anxiolytic activity compared to an SSRI alone. In accordance with this, clinical data suggest the coadministration of an SSRI and (+/-) pindolol can decrease the time to onset of anxiolytic/antidepressant activity. Thus, the dual-acting novel SSRI and 5-HT(1A/B) receptor antagonist, SB-649915-B, has been assessed in acute and chronic preclinical models of anxiolysis. SB-649915-B (0.1-1.0 mg/kg, i.p.) significantly reduced ultrasonic vocalization in male rat pups separated from their mothers (ED(50) of 0.17 mg/kg). In the marmoset human threat test SB-649915-B (3.0 and 10 mg/kg, s.c.) significantly reduced the number of postures with no effect on locomotion. In the rat high light social interaction (SI), SB-649915-B (1.0-7.5 mg/kg, t.i.d.) and paroxetine (3.0 mg/kg, once daily) were orally administered for 4, 7, and 21 days. Ex vivo inhibition of [(3)H]5-HT uptake was also measured following SI. SB-649915-B and paroxetine had no effect on SI after 4 days. In contrast to paroxetine, SB-649915-B (1.0 and 3.0 mg/kg, p.o., t.i.d.) significantly (p<0.05) increased SI time with no effect on locomotion, indicative of an anxiolytic-like profile on day 7. Anxiolysis was maintained after chronic (21 days) administration by which time paroxetine also increased SI significantly. 5-HT uptake was inhibited by SB-649915-B at all time points to a similar magnitude as that seen with paroxetine. In conclusion, SB-649915-B is acutely anxiolytic and reduces the latency to onset of anxiolytic behavior compared to paroxetine in the SI model.

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.

3,4-Dihydro-2H-benzoxazinones as dual-acting 5-HT1A receptor antagonists and serotonin reuptake inhibitors

Investigation of halogen substitution in lead compound 1 has led to the identification of analogues which combine high affinity for 5-HT(1A) receptors and potent serotonin reuptake inhibitory activity. Several compounds show an improved selectivity over 5-HT(1B) and 5-HT(1D) receptors and a superior pharmacokinetic profile in the rat.

Antagonist-induced increase in 5-HT1A-receptor expression in adult rat hippocampus and cortex

Many receptor antagonists function as reverse agonists on the signaling transduction pathway, but little is known about the action of these drugs on the regulation of receptor expression. Serotonin 1A (5-HT1A) receptor expression in 5-HT and serum-free fetal hippocampal cultures is increased in the presence of a specific 5-HT1A-receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635). To study the plasticity of postsynaptic 5-HT1A receptors in the presence of antagonist in vivo, adult Sprague Dawley rats were injected i.p. either once or twice daily with a dose of WAY 100635 (3 mg/kg) over a period of 3 days. The 5-HT1A receptor expression was detected by immunocytochemistry and light microscopy, and the receptor immunoreactivity (IR) in hippocampus subregions was quantitatively assessed by using a comparative computer-assisted morphometric analysis. Following the daily injections of WAY 100635, a significant increase in 5-HT1A receptor labeling in hippocampal neurons was recorded. This marked increase in 5-HT1A receptor expression, which occurred within 4 h after a single injection of WAY 100635, is evident on the somata membrane and dendritic processes of hippocampal and cortex layer V neurons. By contrast, no increase in 5-HT1A receptor-IR was observed after multiple daily injections at a low dose (1 mg/kg) of WAY 100635. Our study shows that a single or multiple daily injections of WAY 100635 can result in an increase in 5-HT1A receptor-IR. This increase in labeling is consistent with an enhanced expression of the receptor protein. The action of this "inverse agonist" may have clinical importance in disorders such as depression, epilepsy, and Alzheimer's disease in which 5-HT1A receptor levels are deficient.

Role of serotonin type 1A receptors in fluvoxamine-induced inhibition of marble-burying behavior in mice

This study examined the roles of presynaptic and postsynaptic serotonin type 1A receptors in fluvoxamine-induced inhibition of marble-burying behavior in mice. The effect of fluvoxamine was attenuated by the serotonin type 1A receptor antagonist WAY100635 at 1 mg/kg, while it was enhanced by the antagonist at 0.1 mg/kg. Fluvoxamine (30 mg/kg) and WAY100635 (0.1 and 1 mg/kg) did not affect spontaneous locomotor activity. These results suggest that the effect of fluvoxamine is mediated by postsynaptic serotonin type 1A receptors and it is enhanced by an inhibition of presynaptic serotonin type 1A receptors.

Co-administration of fluoxetine and WAY100635 improves short-term memory function

The aim of this study was to determine whether the action of the antidepressant fluoxetine or the anxiolytic buspirone could be modified by specific 5-hydroxytriptamine (5-HT(1A)) receptor blockade in a short-term memory paradigm. Male Wistar rats were trained to perform the putative short-term memory task, delayed non-matching to position. WAY100635, a selective 5-HT(1A) receptor antagonist (0.15 mg/kg), was administered 15 min before either the selective serotonin reuptake inhibitor fluoxetine (3 mg/kg), or the partial 5-HT(1A) receptor agonist and dopamine D2 receptor antagonist, buspirone (0.3 mg/kg). 8-Hydroxy-di-n-propylamino tetralin (8-OH-DPAT), a full 5-HT(1A) receptor agonist (0.3 mg/kg), was also included in the study as a positive control. WAY100635 alone had no effect on any behavioural parameter measured (response accuracy, delay lever press activity and trial completion). 8-OH-DPAT impaired response accuracy in a delay-dependent manner, an effect reversed by WAY100635. Fluoxetine also impaired response accuracy delay-dependently. WAY100635 pretreatment not only reversed this deficit but improved response accuracy, in the presence of a significant deficit in trial completion. At the dose used, buspirone showed no significant differences compared to the control group. The data suggest that fluoxetine impairs short-term memory function by the indirect activation of 5-HT(1A) receptors, but that its co-administration with WAY100635 improves short-term memory function.

Antidepressant-like effect of coadministration of sulpiride and fluvoxamine in mice

We have recently reported that coadministration of sulpiride, an antipsychotic drug, and fluvoxamine, a selective serotonin (5-HT) reuptake inhibitor, selectively increases in vivo dopamine release in the prefrontal cortex. This study examined the effects of coadministration of these drugs on duration of immobility in the tail suspension test using mice. Neither sulpiride (3 or 10 mg/kg) nor fluvoxamine (10 or 20 mg/kg) alone affected immobility time, whereas coadministration significantly reduced immobility time. WAY 100635, a 5-HT(1A) receptor antagonist, did not affect the effects of sulpiride and fluvoxamine coadministration, but reduced immobility time in combination with fluvoxamine (20 mg/kg). A high dose of fluvoxamine alone (60 mg/kg) also reduced immobility time. These results suggest that the antidepressant-like effects of fluvoxamine in combination with sulpiride or WAY 100635 in the tail suspension test are mediated by the activation of dopamine or 5-HT systems, respectively.

Serotonin 5-HT2C receptor-mediated phosphoinositide hydrolysis in rat choroid plexus after fluoxetine and citalopram treatments

Selective serotonin reuptake inhibitors (SSRIs) bind directly to various neurotransmitter receptors. The clinical effects of SSRIs appear gradually during weeks of treatment, suggesting a role for adaptive changes in neurotransmitter receptors. Most clinically used antidepressants, e.g. fluoxetine, bind to 5-HT2C receptors. When administered chronically, many antidepressants elicit adaptive regulation of 5-HT2C receptors. The present study was conducted in order to determine the effects of acute and chronic fluoxetine and citalopram treatments on the density and function of 5-HT2C receptors in the rat choroid plexus. Acute and chronic treatments followed by phosphoinositide (PI) hydrolysis assays and quantitative receptor autoradiography were performed. Acute (single-dose) treatment with neither drug significantly affected basal or 5-HT-stimulated PI hydrolysis, but acute citalopram (20 mg/kg) treatment increased both agonist and antagonist binding to 5-HT(2C) receptors. Chronic (14 days) citalopram treatment (20 mg/kg) increased the maximal PI hydrolysis response by 40%, but fluoxetine lacked this effect. The present data suggest that sensitisation of 5-HT2C receptor-mediated intracellular signal transduction may play a role in the effects of citalopram. In contrast, fluoxetine treatment does not functionally sensitise 5-HT2C receptors. Thus, functional 5-HT2C receptor sensitisation is not a common effect of antidepressants, but the differential effects may explain some of the pharmacodynamic differences seen with these drugs, especially upon repeated administration.

Studies towards the next generation of antidepressants. Part 4: derivatives of 4-(5-fluoro-1H-indol-3-yl)cyclohexylamine with affinity for the serotonin transporter and the 5-HT1A receptor

Derivatives of the serotonin reuptake inhibitor 4-(5-fluoro-1H-indol-3-yl)cyclohexylamine, in which serotonin 1A (5-HT(1A)) receptor pharmacophoric elements are incorporated, are reported. Analogs exhibiting affinity for both the serotonin transporter and the 5-HT(1A) receptor are described. Compounds containing 1-(4-indolyl)piperazine and 2-(1H-indol-4-yloxy)ethylamine are promising leads for further SAR studies.

3,4-Dihydro-2H-benzoxazinones are 5-HT1A receptor antagonists with potent 5-HT reuptake inhibitory activity

Starting from a high throughput screening hit, a series of 3,4-dihydro-2H-benzoxazinones has been identified with both high affinity for the 5-HT(1A) receptor and potent 5-HT reuptake inhibitory activity. The 5-(2-methyl)quinolinyloxy derivative combined high 5-HT(1A/1B/1D) receptor affinities with low intrinsic activity and potent inhibition of the 5-HT reuptake site (pK(i)8.2). This compound also had good oral bioavailability and brain penetration in the rat.

Sulpiride in combination with fluvoxamine increases in vivo dopamine release selectively in rat prefrontal cortex

Coadministration of atypical antipsychotics and selective serotonin reuptake inhibitors (SSRIs) enhances the release of monoamines such as dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the prefrontal cortex. To clarify the role of DA-D2/3 receptors in the combination effect, we examined the effects of coadministration of the selective DA-D2/3 antagonist sulpiride and the SSRI fluvoxamine on amine neurotransmitter release in rat prefrontal cortex. Sulpiride (10 mg/kg, i.p.) and fluvoxamine (10 mg/kg, i.p.) alone did not affect extracellular DA levels, while their coadministration caused a significant increase in DA levels. Sulpiride alone did not affect extracellular levels of 5-HT and NE in the prefrontal cortex, while fluvoxamine alone caused a marked increase in 5-HT levels and a slight increase in NE levels. Sulpiride did not affect the fluvoxamine-induced increases in extracellular levels of 5-HT and NE. The DA-D2/3 antagonist haloperidol (0.1 mg/kg) in combination with fluvoxamine also caused a selective increase in extracellular DA levels in the cortex. Coadministration of sulpiride and fluvoxamine did not affect extracellular DA levels in the striatum. Combination of systemic sulpiride and local fluvoxamine did not increase the DA levels, but that of systemic fluvoxamine with local sulpiride increased. The combination effect in increasing prefrontal DA levels was antagonized systemically, but not locally, by the 5-HT1A antagonist WAY100635 at a low dose. These findings suggest that the combination of prefrontal DA-D2/3 receptor blockade and 5-HT1A receptor activation in regions other than the cortex plays an important role in sulpiride and fluvoxamine-induced increase in prefrontal DA release.

Evidence for serotonin receptor subtypes involvement in agmatine antidepressant like-effect in the mouse forced swimming test

This study investigated the involvement of 5-HT(1) and 5-HT(2) receptors in the antidepressant-like effect of agmatine in the mouse forced swimming test (FST). Pretreatment with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, intraperitoneally (i.p.), an inhibitor of serotonin synthesis, for 4 consecutive days), methysergide (5 mg/kg, i.p., a serotonin (5-HT) antagonist), pindolol (32 mg/kg, i.p., a 5-HT(1A/1B) receptor/beta-adrenoceptor antagonist), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg, subcutaneously (s.c.), a selective 5-HT(1A) receptor antagonist), 1-(2-methoxyphenyl)-4[-(2-phthalimido)butyl]piperazine) (NAN-190; 0.5 mg/kg, i.p., a 5-HT(1A) receptor antagonist), 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane; 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) antagonist) or ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with propranolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist), prevented the effect of agmatine (10 mg/kg, i.p.) in the FST. A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5 mg/kg, i.p.), WAY 100635 (0.03 mg/kg, s.c.), (+)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT; 0.01 mg/kg, i.p., a 5-HT(1A) receptor agonist), R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI; 1 mg/kg, i.p., a preferential 5-HT(2A) receptor agonist), or fluoxetine (10 mg/kg, i.p., a selective serotonin reuptake inhibitor, SSRI) but not with isamoltane (2.5 mg/kg, i.p.), ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist) or ketanserin (5 mg/kg, i.p.). Taken together, the results firstly demonstrate that agmatine antidepressant-like effects in the FST seem to be mediated, at least in part, by an interaction with 5-HT(1A/1B) and 5-HT(2) receptors.

Studies toward the discovery of the next generation of antidepressants. 3. Dual 5-HT1A and serotonin transporter affinity within a class of N-aryloxyethylindolylalkylamines

N-aryloxylethylindolealkylamines (5) having dual 5-HT transporter and 5-HT(1A) affinity are described. These compounds represent truncated analogues of our previously reported piperidinyl derivatives (3). Compounds in this investigation were found to have more similar affinities and functional activities for the 5-HT(1A) receptor and 5-HT transporter. Though 5-HT(1A) antagonism is not consistently observed throughout series 5, several molecular features were found to be essential to obtain high and balanced activities. The proper placement of a heteroatom in the aryl ring and the length of the linkage used to tether the indole moiety had significant influence on 5-HT(1A) and 5-HT transporter affinities. Introduction of a halogen into the aryl ring usually lowered intrinsic activity and in some cases led to full 5-HT(1A) antagonists. Compounds 33 and 34 were observed to be full 5-HT(1A) antagonists with K(i) values of approximately 30 nM for the 5-HT(1A) receptor and K(i) values of 5 and 0.5 nM for the 5-HT transporter, respectively. Unfortunately, similar to our previous series (3), compounds in this report also had high affinity for the alpha(1) receptor.

Regulation by 5-HT1A receptors of the in vivo release of 5-HT and DA in mouse frontal cortex

This study examines the effects of serotonin (5-HT)1A receptor ligands on the in vivo release of 5-HT and dopamine (DA) in the prefrontal cortex of mice. Oral MKC-242 and 8-OH-DPAT, selective 5-HT1A receptor agonists, decreased cortical 5-HT release at low and high doses, while the receptor agonists increased cortical DA release only at a high dose. Local application of the selective 5-HT1A receptor antagonist, WAY100635, via a dialysis probe, antagonized oral MKC-242-induced increase in cortical DA release, but did not affect the decrease in cortical 5-HT release. Local application of 8-OH-DPAT at 100 and 300 nM via a dialysis probe increased cortical DA release, but did not affect cortical 5-HT release. The effects of oral MKC-242 and 8-OH-DPAT on 5-HT release were blocked by low and high doses of WAY100635, while blocking the agonist-induced increase in DA release required a high dose of WAY100635. These results suggest that 5-HT release and DA release in the frontal cortex of mice are regulated by pre- and postsynaptic 5-HT1A receptors, respectively, and that the presynaptic 5-HT1A receptor-mediated response is more sensitive to inhibition by WAY100635 than the postsynaptic 5-HT1A receptor-mediated response in mice.