S32006, a novel 5-HT2C receptor antagonist displaying broad-based antidepressant and anxiolytic properties in rodent models

DSP-6745, a novel 5-hydroxytryptamine modulator with rapid antidepressant, anxiolytic, antipsychotic and procognitive effects

BACKGROUND

Current treatment of major depressive disorder is facing challenges, including a low remission rate, late onset of efficacy, and worsening severity due to comorbid symptoms such as psychosis and cognitive dysfunction. Serotonin (5-HT) neurotransmission is involved in a wide variety of psychiatric diseases and its potential as a drug target continues to attract attention.

OBJECTIVES

The present study elucidates the effects of a novel 5-HT modulator, DSP-6745, on depression and its comorbid symptoms.

RESULTS

In vitro radioligand binding and functional assays showed that DSP-6745 is a potent inhibitor of 5-HT transporter and 5-HT2A, 5-HT2C, and 5-HT7 receptors. In vivo, DSP-6745 (6.4 and 19.1 mg/kg as free base, p.o.) increased the release of not only 5-HT, norepinephrine, and dopamine, but also glutamate in the medial prefrontal cortex. The results of in vivo mouse phenotypic screening by SmartCube® suggested that DSP-6745 has a behavioral signature combined with antidepressant-, anxiolytic-, and antipsychotic-like signals. A single oral dose of DSP-6745 (6.4 and 19.1 mg/kg) showed rapid antidepressant-like efficacy in the rat forced swim test, even at 24 h post-dosing, and anxiolytic activity in the rat social interaction test. Moreover, DSP-6745 (12.7 mg/kg, p.o.) led to an improvement in the apomorphine-induced prepulse inhibition deficit in rats. In the marmoset object retrieval with detour task, which is used to assess cognitive functions such as attention and behavioral inhibition, DSP-6745 (7.8 mg/kg, p.o.) enhanced cognition.

CONCLUSIONS

These data suggest that DSP-6745 is a multimodal 5-HT receptor antagonist and a 5-HT transporter inhibitor and has the potential to be a rapid acting antidepressant with efficacies in mitigating the comorbid symptoms of depression.

Serotonergic mediation of the brain-wide neurogenesis: Region-dependent and receptor-type specific roles on neurogenic cellular transformation

Brain serotonin (5-hydroxytryptamine, 5-HT) is a key molecule for the mediation of depression-related brain states, but the neural mechanisms underlying 5-HT mediation need further investigation. A possible mechanism of the therapeutic antidepressant effects is neurogenic cell production, as stimulated by 5-HT signaling. Neurogenesis, the proliferation of neural stem cells (NSCs), and cell differentiation and maturation occur across brain regions, particularly the hippocampal dentate gyrus and the subventricular zone, throughout one's lifespan. 5-HT plays a major role in the mediation of neurogenic processes, which in turn leads to the therapeutic effect on depression-related states. In this review article, we aim to identify how the neuronal 5-HT system mediates the process of neurogenesis, including cell proliferation, cell-type differentiation and maturation. First, we will provide an overview of the neurogenic cell transformation that occurs in brain regions containing or lacking NSCs. Second, we will review brain region-specific mechanisms of 5-HT-mediated neurogenesis by comparing regions localized to NSCs, i.e., the hippocampus and subventricular zone, with those not containing NSCs. Highlighting these 5-HT mechanisms that mediate neurogenic cell production processes in a brain-region-specific manner would provide unique insights into the role of 5-HT in neurogenesis and its associated effects on depression.

Antidepressant-like Effect of 1-(2-(4-(4-Ethylphenyl)-1H-1,2,3-triazol-1-yl)phenyl)ethan-1-one in Mice: Evidence of the Contribution of the Serotonergic System

Major depressive disorder (MDD) is a psychiatric disorder that affects a large portion of the population, with dysregulation of the serotonergic system, which is deeply involved in both the pathophysiology of MDD and mechanism of action of many antidepressants. Current pharmacological therapies do not meet the neurobiological needs of all depressed individuals, making the development of new antidepressants necessary. In recent decades, compounds containing triazoles have become promising due to their range of biological activities, including antidepressant activity. In this study, we evaluated the antidepressant-like effect of a hybrid containing triazole and acetophenone, 1-(2-(4-(4-ethylphenyl)-1H-1,2,3-triazol-1-yl)phenyl)ethan-1-one (ETAP) (0.5-5 mg/kg), in the forced swimming test (FST) and tail suspension test (TST) in mice, as well as the involvement of the serotonergic system in this effect. Our findings demonstrated that ETAP exhibited an antidepressant-like effect from the dose of 1 mg/kg and that this effect is modulated by 5-HT_2A/2C and 5-HT₄ receptors. We also demonstrated that this effect may be related to inhibition of monoamine oxidase A activity in the hippocampus. Additionally, we evaluated the in silico pharmacokinetic profile of ETAP, which predicted its penetration into the central nervous system. ETAP exhibited a low potential for toxicity at a high dose, making this molecule interesting for the development of a new therapeutic strategy for MDD.

Involvement of the serotonergic system in the antidepressant-like effect of 1-(phenylselanyl)-2-(p-tolyl)indolizine in mice

RATIONALE

Depression is a mental disorder that affects approximately 280 million people worldwide. In the search for new treatments for mood disorders, compounds containing selenium and indolizine derivatives show promising results.

OBJECTIVES AND METHODS

To evaluate the antidepressant-like effect of 1-(phenylselanyl)-2-(p-tolyl)indolizine (MeSeI) (0.5-50 mg/kg, intragastric-i.g.) on the tail suspension test (TST) and the forced swim test (FST) in adult male Swiss mice and to elucidate the role of the serotonergic system in this effect through pharmacological and in silico approaches, as well to evaluate acute oral toxicity at a high dose (300 mg/kg).

RESULTS

MeSeI administered 30 min before the FST and the TST reduced immobility time at doses from 1 mg/kg and at 50 mg/kg and increased the latency time for the first episode of immobility, demonstrating an antidepressant-like effect. In the open field test (OFT), MeSeI did not change the locomotor activity. The antidepressant-like effect of MeSeI (50 mg/kg, i.g.) was prevented by the pre-treatment with p-chlorophenylalanine (p-CPA), a selective tryptophan hydroxylase inhibitor (100 mg/kg, intraperitoneally-i.p. for 4 days), with ketanserin, a 5-HT_2A/2C receptor antagonist (1 mg/kg, i.p.), and with GR113808, a 5-HT₄ receptor antagonist (0.1 mg/kg, i.p.), but not with WAY100635, a selective 5-HT_1A receptor antagonist (0.1 mg/kg, subcutaneous-s.c.) and ondansetron, a 5-HT₃ receptor antagonist (1 mg/kg, i.p.). MeSeI showed a binding affinity with 5-HT_2A, 5 -HT_2C, and 5-HT₄ receptors by molecular docking. MeSeI (300 mg/kg, i.g.) demonstrated low potential to cause acute toxicity in adult female Swiss mice.

CONCLUSION

In summary, MeSeI exhibits an antidepressant-like effect mediated by the serotonergic system and could be considered for the development of new treatment strategies for depression.

Molecular Pathways of the Therapeutic Effects of Ayahuasca, a Botanical Psychedelic and Potential Rapid-Acting Antidepressant

Ayahuasca is a psychoactive brew traditionally used in indigenous and religious rituals and ceremonies in South America for its therapeutic, psychedelic, and entheogenic effects. It is usually prepared by lengthy boiling of the leaves of the bush Psychotria viridis and the mashed stalks of the vine Banisteriopsis caapi in water. The former contains the classical psychedelic N,N-dimethyltryptamine (DMT), which is thought to be the main psychoactive alkaloid present in the brew. The latter serves as a source for β-carbolines, known for their monoamine oxidase-inhibiting (MAOI) properties. Recent preliminary research has provided encouraging results investigating ayahuasca’s therapeutic potential, especially regarding its antidepressant effects. On a molecular level, pre-clinical and clinical evidence points to a complex pharmacological profile conveyed by the brew, including modulation of serotoninergic, glutamatergic, dopaminergic, and endocannabinoid systems. Its substances also interact with the vesicular monoamine transporter (VMAT), trace amine-associated receptor 1 (TAAR1), and sigma-1 receptors. Furthermore, ayahuasca’s components also seem to modulate levels of inflammatory and neurotrophic factors beneficially. On a biological level, this translates into neuroprotective and neuroplastic effects. Here we review the current knowledge regarding these molecular interactions and how they relate to the possible antidepressant effects ayahuasca seems to produce.

The Implication of 5-HT Receptor Family Members in Aggression, Depression and Suicide: Similarity and Difference

Being different multifactorial forms of psychopathology, aggression, depression and suicidal behavior, which is considered to be violent aggression directed against the self, have principal neurobiological links: preclinical and clinical evidence associates depression, aggression and suicidal behavior with dysregulation in central serotonergic (5-HT) neurotransmission. The implication of different types of 5-HT receptors in the genetic and epigenetic mechanisms of aggression, depression and suicidality has been well recognized. In this review, we consider and compare the orchestra of 5-HT receptors involved in these severe psychopathologies. Specifically, it concentrates on the role of 5-HT_1A, 5-HT_1B, 5-HT_2A, 5-HT_2B, 5-HT_2C, 5-HT₃ and 5-HT₇ receptors in the mechanisms underlying the predisposition to aggression, depression and suicidal behavior. The review provides converging lines of evidence that: (1) depression-related 5-HT receptors include those receptors with pro-depressive properties (5-HT_2A, 5-HT₃ and 5-HT₇) as well as those providing an antidepressant effect (5-HT_1A, 5-HT_1B, 5-HT_2C subtypes). (2) Aggression-related 5-HT receptors are identical to depression-related 5-HT receptors with the exception of 5-HT₇ receptors. Activation of 5-HT_1A, 5-HT_1B, 5-HT_2A, 5-HT_2C receptors attenuate aggressiveness, whereas agonists of 5-HT₃ intensify aggressive behavior.

Agomelatine for the treatment of generalized anxiety disorder: focus on its distinctive mechanism of action

Generalized anxiety disorder (GAD), the most frequently diagnosed form of anxiety, is usually treated by cognitive-behavioural approaches or medication; in particular, benzodiazepines (acutely) and serotonin or serotonin/noradrenaline reuptake inhibitors (long term). Efficacy, compliance, and acceptability are, however, far from ideal, reinforcing interest in alternative options. Agomelatine, clinically employed in the treatment of major depression, expresses anxiolytic properties in rodents and was effective in the treatment of GAD (including severely ill patients) in several double-blind, short-term (12 weeks) and relapse-prevention (6 months) studies. At active doses, the incidence of adverse effects was no higher than for placebo. Agomelatine possesses a unique binding profile, behaving as a melatonin (MT₁/MT₂) receptor agonist and 5-HT_2C receptor antagonist, yet recognizing neither monoamine transporters nor GABA_A receptors. Extensive evidence supports a role for 5-HT_2C receptors in the induction of anxious states, and their blockade likely plays a primary role in mediating the anxiolytic actions of agomelatine, including populations in the amygdala and bed nucleus of stria terminalis, as well as the hippocampus. Recruitment of MT receptors in the suprachiasmatic nucleus, thalamic reticular nucleus, and hippocampus appears to fulfil a complimentary role. Downstream of 5-HT_2C and MT receptors, modulation of stress-sensitive glutamatergic circuits and altered release of the anxiogenic neuropeptides, corticotrophin-releasing factor, and vasopressin, may be implicated in the actions of agomelatine. To summarize, agomelatine exerts its anxiolytic actions by mechanisms clearly distinct from those of other agents currently employed for the management of GAD.

PLAIN LANGUAGE SUMMARY

How agomelatine helps in the treatment of anxiety disorders.

INTRODUCTION

• Anxiety disorders have a significant negative impact on quality of life.• The most common type of anxiety disorder, called generalized anxiety disorder (GAD), is associated with nervousness and excessive worry.• These symptoms can lead to additional symptoms like tiredness, sleeplessness, irritability, and poor attention.• GAD is generally treated through either cognitive-behavioural therapy or medication. However, widely used drugs like benzodiazepines and serotonin reuptake inhibitors have adverse effects.• Agomelatine, a well-established antidepressant drug, has shown anxiety-lowering ('anxiolytic') properties in rats and has been shown to effectively treat GAD with minimal side effects.• However, exactly how it acts on the brain to manage GAD is not yet clear.• Thus, this review aims to shed light on agomelatine's mechanism of action in treating GAD.

METHODS

• The authors reviewed studies on how agomelatine treats anxiety in animals.• They also looked at clinical studies on the effects of agomelatine in people with GAD.

RESULTS

• The study showed that agomelatine 'blocks' a receptor in nerve cells, which plays a role in causing anxiety, called the 5-HT_2C receptor.• Blocking this receptor, especially in specific brain regions such as nerve cells of the amygdala, bed nucleus of stria terminalis, and hippocampus, produced the anxiety reduction seen during agomelatine treatment.• Agomelatine also activates the melatonin (MT) receptor, which is known to keep anxiety in check, promote sleep, and maintain the sleep cycle.• Agomelatine should thus tackle sleep disturbances commonly seen in patients with GAD.• Beyond 5-HT_2C and MT receptors, signalling molecules in nerve cells that are known to be involved in anxiety disorders (called 'neurotransmitters' and 'neuropeptides') are also affected by agomelatine.

CONCLUSION

• Agomelatine's anxiolytic effects are caused by mechanisms that are distinct from those of other medications currently used to treat GAD.• This explains its therapeutic success and minimal adverse side effects.

Astroglial Serotonin Receptors as the Central Target of Classic Antidepressants

Major depressive disorder (MDD) presents multiple clinical phenotypes and has complex underlying pathological mechanisms. Existing theories cannot completely explain the pathophysiological mechanism(s) of MDD, while the pharmacology of current antidepressants is far from being fully understood. Astrocytes, the homeostatic and defensive cells of the central nervous system, contribute to shaping behaviors, and regulating mood and emotions. A detailed introduction on the role of astrocytes in depressive disorders is thus required, to which this chapter is dedicated. We also focus on the interactions between classic antidepressants and serotonin receptors, overview the role of astrocytes in the pharmacological mechanisms of various antidepressants, and present astrocytes as targets for the treatment of bipolar disorder. We provide a foundation of knowledge on the role of astrocytes in depressive disorders and astroglial 5-HT_2B receptors as targets for selective serotonin reuptake inhibitors in vivo and in vitro.

Agomelatine: An astounding sui-generis antidepressant?

Major depressive disorder (MDD) is one of the foremost causes of disability and premature death worldwide. Although the available antidepressants are effective and well tolerated, they also have many limitations. Therapeutic advances in developing a new drug's ultimate relation between MDD and chronobiology, which targets the circadian rhythm, have led to a renewed focus on psychiatric disorders. In order to provide a critical analysis about antidepressant properties of agomelatine, a detailed PubMed (Medline), Scopus (Embase), Web of Science (Web of Knowledge), Cochrane Library, Google Scholar, and PsycInfo search was performed using the following keywords: melatonin analog, agomelatine, safety, efficacy, adverse effects, pharmacokinetics, pharmacodynamics, circadian rhythm, sleep disorders, neuroplasticity, MDD, bipolar disorder, anhedonia, anxiety, generalized anxiety disorder (GAD), and mood disorders. Agomelatine is a unique melatonin analog with antidepressant properties and a large therapeutic index that improves clinical safety. It is a melatonin receptor agonist (MT1 and MT2) and a 5-HT2C receptor antagonist. The effects on melatonin receptors enable the resynchronization of irregular circadian rhythms with beneficial effects on sleep architectures. In this way, agomelatine is accredited for its unique mode of action, which helps to exert antidepressant effects and resynchronize the sleep-wake cycle. To sum up, an agomelatine has not only antidepressant properties but also has anxiolytic effects.

5-HT Receptors and the Development of New Antidepressants

Serotonin modulates several physiological and cognitive pathways throughout the human body that affect emotions, memory, sleep, and thermal regulation. The complex nature of the serotonergic system and interactions with other neurochemical systems indicate that the development of depression may be mediated by various pathomechanisms, the common denominator of which is undoubtedly the disturbed transmission in central 5-HT synapses. Therefore, the deliberate pharmacological modulation of serotonergic transmission in the brain seems to be one of the most appropriate strategies for the search for new antidepressants. As discussed in this review, the serotonergic system offers great potential for the development of new antidepressant therapies based on the combination of SERT inhibition with different pharmacological activity towards the 5-HT system. The aim of this article is to summarize the search for new antidepressants in recent years, focusing primarily on the possibility of benefiting from interactions with various 5-HT receptors in the pharmacotherapy of depression.

Leite L, da S. Tavares-Henriques M, Lopes LAF, Oliveira LGR, Silva-Filho SE, da Silveira APS, Cuman RKN, de S. Silva-Comar FM, Comar JF, do A. Brasileiro L, dos Santos JN, de Freitas WR, Leão KV, da Silva JG, Klein RC, Klein MHF, da S. Ramos BH, Fernandes CKC, de L. Ribas DG, Oesterreich SA. Modulation of the Serotonergic Receptosome in the Treatment of Anxiety and Depression: A Narrative Review of the Experimental Evidence

Serotonin (5-HT) receptors are found throughout central and peripheral nervous systems, mainly in brain regions involved in the neurobiology of anxiety and depression. 5-HT receptors are currently promising targets for discovering new drugs for treating disorders ranging from migraine to neuropsychiatric upsets, such as anxiety and depression. It is well described in the current literature that the brain expresses seven types of 5-HT receptors comprising eighteen distinct subtypes. In this article, we comprehensively reviewed 5-HT1-7 receptors. Of the eighteen 5-HT receptors known today, thirteen are G protein-coupled receptors (GPCRs) and represent targets for approximately 40% of drugs used in humans. Signaling pathways related to these receptors play a crucial role in neurodevelopment and can be modulated to develop effective therapies to treat anxiety and depression. This review presents the experimental evidence of the modulation of the “serotonergic receptosome” in the treatment of anxiety and depression, as well as demonstrating state-of-the-art research related to phytochemicals and these disorders. In addition, detailed aspects of the pharmacological mechanism of action of all currently known 5-HT receptor families were reviewed. From this review, it will be possible to direct the rational design of drugs towards new therapies that involve signaling via 5-HT receptors.

Synergistic antidepressant- and anxiolytic-like effects of harmaline along with cinanserin in acute restraint stress-treated mice

RATIONALE

Acute restraint stress (ARS) is an experimental paradigm used for the induction of rodent models of stress-produced neuropsychiatric disorders, such as depression and anxiety. β-carbolines and serotonin (5-HT) systems are involved in the modulation of depression and anxiety behaviors.

OBJECTIVE

This study was designed to examine the effects of intracerebroventricular (i.c.v.) injection of cinanserin (5-HT2 receptor antagonist) on harmaline-induced responses on depression- and anxiety-like behaviors in the ARS mice.

METHODS

For i.c.v. infusion, guide cannula was surgically implanted in the left lateral ventricle of mice. The ARS model was conducted via movement restraint at a period of 4 h. Depression- and anxiety-related behaviors were evaluated by forced swim test (FST) and elevated plus maze (EPM), respectively.

RESULTS

The results displayed that the ARS mice showed depressive- and anxiety-like responses. I.p. administration of different doses of harmaline (0.31, 0.625 and 1.25 mg/kg) or i.c.v. microinjection of cinanserin (1, 2.5, and 5 μg/mouse) blocked depression- and anxiogenic-like behaviors in the ARS mice. Furthermore, co-administration of harmaline (1.25 mg/kg; i.p.) and cinanserin (5 μg/mouse; i.c.v.) prevented the depression- and anxiogenic-like effects in the ARS mice. We found a synergistic antidepressant- and anxiolytic-like effects of harmaline and cinanserin in the ARS mice.

CONCLUSIONS

These results propose an interaction between harmaline and cinanserin to prevent depressive- and anxiogenic-like behaviors in the ARS mice.

A Subset of Purposeless Oral Movements Triggered by Dopaminergic Agonists Is Modulated by 5-HT2C Receptors in Rats: Implication of the Subthalamic Nucleus

Dopaminergic medication for Parkinson’s disease is associated with troubling dystonia and dyskinesia and, in rodents, dopaminergic agonists likewise induce a variety of orofacial motor responses, certain of which are mimicked by serotonin2C (5-HT_2C) receptor agonists. However, the neural substrates underlying these communalities and their interrelationship remain unclear. In Sprague-Dawley rats, the dopaminergic agonist, apomorphine (0.03–0.3 mg/kg) and the preferential D2/3 receptor agonist quinpirole (0.2–0.5 mg/kg), induced purposeless oral movements (chewing, jaw tremor, tongue darting). The 5-HT_2C receptor antagonist 5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxyl]-5-pyridyl]carbamoyl]-6-trifluoromethylindone (SB 243213) (1 mg/kg) reduced the oral responses elicited by specific doses of both agonists (0.1 mg/kg apomorphine; 0.5 mg/kg quinpirole). After having confirmed that the oral bouts induced by quinpirole 0.5 mg/kg were blocked by another 5-HT_2C antagonist (6-chloro-5-methyl-1-[6-(2-methylpiridin-3-yloxy)pyridine-3-yl carbamoyl] indoline (SB 242084), 1 mg/kg), we mapped the changes in neuronal activity in numerous sub-territories of the basal ganglia using c-Fos expression. We found a marked increase of c-Fos expression in the subthalamic nucleus (STN) in combining quinpirole (0.5 mg/kg) with either SB 243213 or SB 242084. In a parallel set of electrophysiological experiments, the same combination of SB 243213/quinpirole produced an irregular pattern of discharge and an increase in the firing rate of STN neurons. Finally, it was shown that upon the electrical stimulation of the anterior cingulate cortex, quinpirole (0.5 mg/kg) increased the response of substantia nigra pars reticulata neurons corresponding to activation of the “hyperdirect” (cortico-subthalamonigral) pathway. This effect of quinpirole was abolished by the two 5-HT_2C antagonists. Collectively, these results suggest that induction of orofacial motor responses by D2/3 receptor stimulation involves 5-HT_2C receptor-mediated activation of the STN by recruitment of the hyperdirect (cortico-subthalamonigral) pathway.

Effect of 5-HT2C receptor agonist and antagonist on chronic unpredictable stress (CUS) - Mediated anxiety and depression in adolescent Wistar albino rat: Implicating serotonin and mitochondrial ETC-I function in serotonergic neurotransmission

Anxiety and depression are among the major neuropsychiatric disorders worldwide, and yet the etiologies of these disorders remain unclear to date. Chronic unpredictable stress (CUS) procedure mimics several behavioral characteristics such as anxiety and depression in rodents. Using this animal model, we have attempted to understand the serotonergic system in the hippocampus and prefrontal cortex, while using the 5-HT_2CR agonist and antagonist in evaluating 5-HT_2C receptor neurotransmission. A decrease in serotonin (5-HT) level, tryptophan hydroxylase-2 activity and, 5-HT_2CR receptor protein down-regulation in the CUS exposed group, explains the involvement of 5-HT and 5-HT_2CR neurotransmission in the genesis of anxiety and depression. Besides, the oxidative stress - attenuated electrolyte imbalance via decrease ATPase pump activity, and compromised oxidative phosphorylation via decrease ETC-I activity are some of the underlying factors affecting neuronal cell survival and serotonergic neurotransmission. To complement our finding, altered behavioral performance scored in the open field test, elevated plus maze test, and the forced swim test, when exposed to CUS is indicative or consistent with anxiety, depression, emotional and locomotor status of the animals. Keeping these findings in mind, treatment with 5-HT_2CR agonist (1-Methylpsilocin at 0.7 mg/kg), and 5-HT_2CR antagonist (RS-102221 hydrochloride at 1 mg/kg) displayed varying results. One prominent finding was the anxiolytic ability of the 5-HT_2CR agonist and the anti-depressive ability of the 5-HT_2CR antagonist on the 7th-day treatment. Though the exact mechanism of action is not clear, their ability to equilibrate brain redox status, restoring Ca²⁺ level via Ca²⁺ATPase pump activity, and sustaining the mitochondrial bioenergetics can all be accounted for facilitating neurogenesis and the serotonergic system.

Lorcaserin Alters Serotonin and Noradrenaline Tissue Content and Their Interaction With Dopamine in the Rat Brain

Lorcaserin is a preferential serotonin2C receptor (5-HT_2CR) agonist effective to treat obesity that has also recently been proposed to treat addiction and epilepsy. Central dopamine (DA) mechanisms are likely involved in the lorcaserin mechanism of action, but other monoamines 5-HT and noradrenaline (NA) contents or their interaction with DA might account for its effects. Here we showed that lorcaserin at 3, but not 0.3 mg/kg enhanced 5-HT content in the insular cortex, the core of the nucleus accumbens, and ventral hypothalamus. Without affecting the metabolite 5-hydroxy indole acetic acid, lorcaserin reduced the indirect index of 5-HT turnover in the hippocampus, substantia nigra, and habenula. Lorcaserin at 3 mg/kg increased NA content in the orbitofrontal cortex, the central amygdala (also at 0.3 mg/kg), the ventral hypothalamus, and the shell of the nucleus accumbens. A correlative analysis of the tissue contents between pairs of brain regions revealed that 0.3 mg/kg lorcaserin enhanced the number of correlations for 5-HT, its metabolism, and NA to a lower extent. The correlation profiles were very different between saline, 0.3 and 3 mg/kg lorcaserin. Lorcaserin enhanced the correlations established between NA or 5-HT at 0.3 and 3 mg/kg and reduced the number of correlations established between the index of the turnover for DA and 5-HT. These results show that lorcaserin modulates the biochemistry of NA and 5-HT systems in a subset of brain regions. Qualitatively, they reveal, oppositely to the DA changes, that lorcaserin at 0.3, but not 3 mg/kg, enhanced the number of correlations of 5-HT content between brain regions.

Serotonin receptors in depression and anxiety: Insights from animal studies

Serotonin regulates many physiological processes including sleep, appetite, and mood. Thus, serotonergic system is an important target in the treatment of psychiatric disorders, such as major depression and anxiety. This natural neurotransmitter interacts with 7 families of its receptors (5-HT1-7), which cause a variety of pharmacological effects. Using genetically modified animals and selective or preferential agonists and antagonist, numerous studies demonstrated the involvement of almost all serotonin receptor subtypes in antidepressant- or anxiolytic-like effects. In this review, based on animal studies, we discuss the possible involvement of serotonin receptor subtypes in depression and anxiety.

Asenapine reduces anxiety-related behaviours in rat conditioned fear stress model

OBJECTIVE

Asenapine is an atypical antipsychotic that is currently available for the treatment of schizophrenia and bipolar I disorder. Although the atypical antipsychotics clozapine and olanzapine are effective for depression and anxiety in schizophrenia, as demonstrated by animal model studies, this has not been clarified for asenapine. Therefore, we compared the effects of asenapine in the conditioned fear stress model with those of clozapine and olanzapine.

METHOD

Rats were individually fear conditioned using electrical foot shock in a Skinner box. Approximately 24 h later, individual animals were returned to the same Skinner box (without electrical shock) and their freezing behaviour was observed for 5 min. Animals were treated with asenapine, clozapine, olanzapine, the 5-HT1A receptor partial agonist buspirone, or the 5-HT2C receptor antagonist SB242084 at 30 min before freezing behaviour assessment. The 5-HT1A receptor antagonist WAY100635 or the 5-HT2C receptor agonist Ro60-0175 was also used concomitantly with asenapine. The effects of asenapine, clozapine, and olanzapine on serotonin release in the rat hippocampus were also measured using in vivo microdialysis.

RESULTS

Asenapine reduced freezing behaviour, while neither clozapine nor olanzapine reduced freezing behaviour. Buspirone and SB242084 also reduced freezing behaviour. The effect of asenapine in reducing freezing behaviour was not altered by the concomitant administration of WAY100635 or Ro60-0175. Both asenapine and clozapine, but not olanzapine, increased serotonin release in the rat hippocampus.

CONCLUSION

Asenapine may have superior therapeutic effect on anxiety symptoms than other agents, although the underlying mechanism of its anxiolytic activity remains unknown.

Marine cyanobacteria-derived serotonin receptor 2C active fraction induces psychoactive behavioral effects in mice

CONTEXT

Marine cyanobacteria offer a robust resource for natural products drug discovery due to the secondary metabolites they produce.

OBJECTIVE

To identify novel cyanobacterial compounds that exhibit CNS psychoactive effects.

MATERIALS AND METHODS

Cyanobacteria were collected from Las Perlas Archipelago, Panama and subjected to dichloromethane/methanol extraction and fractionation by column chromatography before being screened for affinity against a panel of CNS targets. A 50:50 ethyl acetate:methanol fraction of one cyanobacterial extract (2064H) was subjected to HPLC and the major peak was isolated (2064H3). At a dose of 20 μg per animal, 2064H and 2064H3 were tested in mice using behavioral assays that included the forced swim, open field and formalin tests.

RESULTS

2064H was shown to bind to the serotonin 2C (5-HT_2C) receptor, a known target for depression and pain treatment. 2064H showed 59.6% inhibition of binding of [³H]-mesulergine with an IC₅₀ value of 179 ng/mL and did not show inhibition of binding greater than 45% with any other receptors tested. Both 2064H and 2064H3 decreased immobility time in the first minute of the tail suspension test. 2064H increased time, distance and number of entries in the center region in the first half of the open field test. 2064H increased overall nocifensive behaviors in the formalin test.

DISCUSSION AND CONCLUSION

Overall, manipulating the 5-HT_2C receptor with these receptor-specific ligands derived from cyanobacteria altered pain, depression and anxiety-like behaviors, illustrating the importance of this receptor in affective behaviors. These results demonstrate the potential of cyanobacteria as a source for CNS active compounds.

The frontal cortex as a network hub controlling mood and cognition: Probing its neurochemical substrates for improved therapy of psychiatric and neurological disorders

The highly-interconnected and neurochemically-rich frontal cortex plays a crucial role in the regulation of mood and cognition, domains disrupted in depression and other central nervous system disorders, and it is an important site of action for their therapeutic control. For improving our understanding of the function and dysfunction of the frontal cortex, and for identifying improved treatments, quantification of extracellular pools of neuromodulators by microdialysis in freely-moving rodents has proven indispensable. This approach has revealed a complex mesh of autoreceptor and heteroceptor interactions amongst monoaminergic pathways, and led from selective 5-HT reuptake inhibitors to novel classes of multi-target drugs for treating depression like the mixed α₂-adrenoceptor/5-HT reuptake inhibitor, S35966, and the clinically-launched vortioxetine and vilazodone. Moreover, integration of non-monoaminergic actions resulted in the discovery and development of the innovative melatonin receptor agonist/5-HT_2C receptor antagonist, Agomelatine. Melatonin levels, like those of corticosterone and the "social hormone", oxytocin, can now be quantified by microdialysis over the full 24 h daily cycle. Further, the introduction of procedures for measuring extracellular histamine and acetylcholine has provided insights into strategies for improving cognition by, for example, blockade of 5-HT₆ and/or dopamine D₃ receptors. The challenge of concurrently determining extracellular levels of GABA, glutamate, d-serine, glycine, kynurenate and other amino acids, and of clarifying their interactions with monoamines, has also been resolved. This has proven important for characterizing the actions of glycine reuptake inhibitors that indirectly augment transmission at N-methyl-d-aspartate receptors, and of "glutamatergic antidepressants" like ketamine, mGluR5 antagonists and positive modulators of AMPA receptors (including S47445). Most recently, quantification of the neurotoxic proteins Aβ42 and Tau has extended microdialysis studies to the pathogenesis of neurodegenerative disorders, and another frontier currently being broached is microRNAs. The present article discusses the above themes, focusses on recent advances, highlights opportunities for clinical "translation", and suggests avenues for further progress.

Mouse psychosocial stress reduces motivation and cognitive function in operant reward tests: A model for reward pathology with effects of agomelatine

A major domain of depression is decreased motivation for reward. Translational automated tests can be applied in humans and animals to study operant reward behaviour, aetio-pathophysiology underlying deficits therein, and effects of antidepressant treatment. Three inter-related experiments were conducted to investigate depression-relevant effects of chronic psychosocial stress on operant behaviour in mice. (A) Non-manipulated mice were trained on a complex reversal learning (CRL) test with sucrose reinforcement; relative to vehicle (VEH), acute antidepressant agomelatine (AGO, 25mg/kg p.o.) increased reversals. (B) Mice underwent chronic social defeat (CSD) or control handling (CON) on days 1-15, and were administered AGO or VEH on days 10-22. In a progressive ratio schedule motivation test for sucrose on day 15, CSD mice made fewer responses; AGO tended to reverse this effect. In a CRL test on day 22, CSD mice completed fewer reversals; AGO tended to increase reversals in CSD mice associated with an adaptive increase in perseveration. (C) Mice with continuous operant access to water and saccharin solution in the home cage were exposed to CSD or CON; CSD mice made fewer responses for saccharin and water and drank less saccharin in the active period, and drank more water in the inactive period. In a separate CSD cohort, repeated AGO was without effect on these home cage operant and consummatory changes. Overall, this study demonstrates that psychosocial stress in mice leads to depression-relevant decreases in motivation and cognition in operant reward tests; partial reversal of these deficits by AGO provides evidence for predictive validity.

BDNF isoforms: a round trip ticket between neurogenesis and serotonin?

The brain-derived neurotrophic factor, BDNF, was discovered more than 30 years ago and, like other members of the neurotrophin family, this neuropeptide is synthetized as a proneurotrophin, the pro-BDNF, which is further cleaved to yield mature BDNF. The myriad of actions of these two BDNF isoforms in the central nervous system is constantly increasing and requires the development of sophisticated tools and animal models to refine our understanding. This review is focused on BDNF isoforms, their participation in the process of neurogenesis taking place in the hippocampus of adult mammals, and the modulation of their expression by serotonergic agents. Interestingly, around this triumvirate of BDNF, serotonin, and neurogenesis, a series of recent research has emerged with apparently counterintuitive results. This calls for an exhaustive analysis of the data published so far and encourages thorough work in the quest for new hypotheses in the field. BDNF is synthetized as a pre-proneurotrophin. After removal of the pre-region, proBDNF can be cleaved by intracellular or extracellular proteases. Mature BDNF can bind TrkB receptors, promoting their homodimerization and intracellular phosphorylation. Phosphorylated-TrkB can activate three different signaling pathways. Whereas G-protein-coupled receptors can transactivate TrkB receptors, truncated forms can inhibit mBDNF signaling. Pro-BDNF binds p75(NTR) by its mature domain, whereas the pro-region binds co-receptors.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

Altering the course of schizophrenia: progress and perspectives

Despite a lack of recent progress in the treatment of schizophrenia, our understanding of its genetic and environmental causes has considerably improved, and their relationship to aberrant patterns of neurodevelopment has become clearer. This raises the possibility that 'disease-modifying' strategies could alter the course to - and of - this debilitating disorder, rather than simply alleviating symptoms. A promising window for course-altering intervention is around the time of the first episode of psychosis, especially in young people at risk of transition to schizophrenia. Indeed, studies performed in both individuals at risk of developing schizophrenia and rodent models for schizophrenia suggest that pre-diagnostic pharmacotherapy and psychosocial or cognitive-behavioural interventions can delay or moderate the emergence of psychosis. Of particular interest are 'hybrid' strategies that both relieve presenting symptoms and reduce the risk of transition to schizophrenia or another psychiatric disorder. This Review aims to provide a broad-based consideration of the challenges and opportunities inherent in efforts to alter the course of schizophrenia.

The effects of the dopamine stabilizer (-)-OSU6162 on aggressive and sexual behavior in rodents

The dopamine stabilizer (-)-OSU61612 dampens locomotion in rodents rendered hyperactive by exposure to a novel environment or treatment with amphetamine, but stimulates locomotion in habituated animals displaying low motor activity, tentatively exerting this profile by selectively blocking extrasynaptic D2 receptors. The major aim of the present study was to explore the possible usefulness of (-)-OSU61612 as an anti-aggressive drug. To this end, the effect of (-)-OSU61612 on isolation-induced aggression in male mice and estrous cycle-dependent aggression in female rats were studied using the resident intruder test; in addition, the possible influence of (-)-OSU61612 on sexual behavior in male mice and on elevated plus maze (EPM) performance in male rats were assessed. (-)-OSU61612 at doses influencing neither locomotion nor sexual activity reduced aggression in male mice. The effect was observed also in serotonin-depleted animals and is hence probably not caused by the antagonism of serotonin receptors displayed by the drug; refuting the possibility that it is due to 5-HT1B activation, it was also not counteracted by isamoltane. (-)-OSU61612 did not display the profile of an anxiogenic or anxiolytic drug in the EPM but caused a general reduction in activity that is well in line with the previous finding that it reduces exploratory behavior of non-habituated animals. In line with the observations in males, (-)-OSU61612 reduced estrus cycle-related aggression in female Wistar rats, a tentative animal model of premenstrual dysphoria. By stabilizing dopaminergic transmission, (-)-OSU61612 may prove useful as a well-tolerated treatment of various forms of aggression and irritability.

Quetiapine and its metabolite norquetiapine: translation from in vitro pharmacology to in vivo efficacy in rodent models

BACKGROUND AND PURPOSE

Quetiapine has a range of clinical activity distinct from other atypical antipsychotic drugs, demonstrating efficacy as monotherapy in bipolar depression, major depressive disorder and generalized anxiety disorder. The neuropharmacological mechanisms underlying this clinical profile are not completely understood; however, the major active metabolite, norquetiapine, has been shown to have a distinct in vitro pharmacological profile consistent with a broad therapeutic range and may contribute to the clinical profile of quetiapine.

EXPERIMENTAL APPROACH

We evaluated quetiapine and norquetiapine, using in vitro binding and functional assays of targets known to be associated with antidepressant and anxiolytic drug actions and compared these activities with a representative range of established antipsychotics and antidepressants. To determine how the in vitro pharmacological properties translate into in vivo activity, we used preclinical animal models with translational relevance to established antidepressant-like and anxiolytic-like drug action.

KEY RESULTS

Norquetiapine had equivalent activity to established antidepressants at the noradrenaline transporter (NET), while quetiapine was inactive. Norquetiapine was active in the mouse forced swimming and rat learned helplessness tests. In in vivo receptor occupancy studies, norquetiapine had significant occupancy at NET at behaviourally relevant doses. Both quetiapine and norquetiapine were agonists at 5-HT1A receptors, and the anxiolytic-like activity of norquetiapine in rat punished responding was blocked by the 5-HT1A antagonist, WAY100635.

CONCLUSIONS AND IMPLICATIONS

Quetiapine and norquetiapine have multiple in vitro pharmacological actions, and results from preclinical studies suggest that activity at NET and 5-HT1A receptors contributes to the antidepressant and anxiolytic effects in patients treated with quetiapine.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Learning from the past and looking to the future: Emerging perspectives for improving the treatment of psychiatric disorders

Modern neuropsychopharmacology commenced in the 1950s with the serendipitous discovery of first-generation antipsychotics and antidepressants which were therapeutically effective yet had marked adverse effects. Today, a broader palette of safer and better-tolerated agents is available for helping people that suffer from schizophrenia, depression and other psychiatric disorders, while complementary approaches like psychotherapy also have important roles to play in their treatment, both alone and in association with medication. Nonetheless, despite considerable efforts, current management is still only partially effective, and highly-prevalent psychiatric disorders of the brain continue to represent a huge personal and socio-economic burden. The lack of success in discovering more effective pharmacotherapy has contributed, together with many other factors, to a relative disengagement by pharmaceutical firms from neuropsychiatry. Nonetheless, interest remains high, and partnerships are proliferating with academic centres which are increasingly integrating drug discovery and translational research into their traditional activities. This is, then, a time of transition and an opportune moment to thoroughly survey the field. Accordingly, the present paper, first, chronicles the discovery and development of psychotropic agents, focusing in particular on their mechanisms of action and therapeutic utility, and how problems faced were eventually overcome. Second, it discusses the lessons learned from past successes and failures, and how they are being applied to promote future progress. Third, it comprehensively surveys emerging strategies that are (1), improving our understanding of the diagnosis and classification of psychiatric disorders; (2), deepening knowledge of their underlying risk factors and pathophysiological substrates; (3), refining cellular and animal models for discovery and validation of novel therapeutic agents; (4), improving the design and outcome of clinical trials; (5), moving towards reliable biomarkers of patient subpopulations and medication efficacy and (6), promoting collaborative approaches to innovation by uniting key partners from the regulators, industry and academia to patients. Notwithstanding the challenges ahead, the many changes and ideas articulated herein provide new hope and something of a framework for progress towards the improved prevention and relief of psychiatric and other CNS disorders, an urgent mission for our Century.

Evidence for a role of 5-HT2C receptors in the motor aspects of performance, but not the efficacy of food reinforcers, in a progressive ratio schedule

RATIONALE

5-Hydroxytryptamine2C (5-HT2C) receptor agonists reduce the breakpoint in progressive ratio schedules of reinforcement, an effect that has been attributed to a decrease of the efficacy of positive reinforcers. However, a reduction of the breakpoint may also reflect motor impairment. Mathematical models can help to differentiate between these processes.

OBJECTIVE

The effects of the 5-HT2C receptor agonist Ro-600175 ((αS)-6-chloro-5-fluoro-α-methyl-1H-indole-1-ethanamine) and the non-selective 5-HT receptor agonist 1-(m-chlorophenyl)piperazine (mCPP) on rats' performance on a progressive ratio schedule maintained by food pellet reinforcers were assessed using a model derived from Killeen's Behav Brain Sci 17:105-172, 1994 general theory of schedule-controlled behaviour, 'mathematical principles of reinforcement'.

METHOD

Rats were trained under the progressive ratio schedule, and running and overall response rates in successive ratios were analysed using the model. The effects of the agonists on estimates of the model's parameters, and the sensitivity of these effects to selective antagonists, were examined.

RESULTS

Ro-600175 and mCPP reduced the breakpoint. Neither agonist significantly affected a (the parameter expressing incentive value), but both agonists increased δ (the parameter expressing minimum response time). The effects of both agonists could be attenuated by the selective 5-HT2C receptor antagonist SB-242084 (6-chloro-5-methyl-N-{6-[(2-methylpyridin-3-yl)oxy]pyridin-3-yl}indoline-1-carboxamide). The effect of mCPP was not altered by isamoltane, a selective 5-HT1B receptor antagonist, or MDL-100907 ((±)2,3-dimethoxyphenyl-1-(2-(4-piperidine)methanol)), a selective 5-HT2A receptor antagonist.

CONCLUSIONS

The results are consistent with the hypothesis that the effect of the 5-HT2C receptor agonists on progressive ratio schedule performance is mediated by an impairment of motor capacity rather than by a reduction of the incentive value of the food reinforcer.

GABAergic control of depression-related brain states

The GABAergic deficit hypothesis of major depressive disorders (MDDs) posits that reduced γ-aminobutyric acid (GABA) concentration in brain, impaired function of GABAergic interneurons, altered expression and function of GABA(A) receptors, and changes in GABAergic transmission dictated by altered chloride homeostasis can contribute to the etiology of MDD. Conversely, the hypothesis posits that the efficacy of currently used antidepressants is determined by their ability to enhance GABAergic neurotransmission. We here provide an update for corresponding evidence from studies of patients and preclinical animal models of depression. In addition, we propose an explanation for the continued lack of genetic evidence that explains the considerable heritability of MDD. Lastly, we discuss how alterations in GABAergic transmission are integral to other hypotheses of MDD that emphasize (i) the role of monoaminergic deficits, (ii) stress-based etiologies, (iii) neurotrophic deficits, and (iv) the neurotoxic and neural circuit-impairing consequences of chronic excesses of glutamate. We propose that altered GABAergic transmission serves as a common denominator of MDD that can account for all these other hypotheses and that plays a causal and common role in diverse mechanistic etiologies of depressive brain states and in the mechanism of action of current antidepressant drug therapies.

Adult neurogenesis and mental illness

Several lines of evidence suggest that adult neurogenesis, the production of new neurons in adulthood, may play a role in psychiatric disorders, including depression, anxiety, and schizophrenia. Medications and other treatments for mental disorders often promote the proliferation of new neurons; the time course for maturation and integration of new neurons in circuitry parallels the delayed efficacy of psychiatric therapies; adverse and beneficial experiences similarly affect development of mental illness and neurogenesis; and ablation of new neurons in adulthood alters the behavioral impact of drugs in animal models. At present, the links between adult neurogenesis and depression seem stronger than those suggesting a relationship between new neurons and anxiety or schizophrenia. Yet, even in the case of depression there is currently no direct evidence for a causative role. This article reviews the data relating adult neurogenesis to mental illness and discusses where research needs to head in the future.

A Review of Biomarkers in Mood and Psychotic Disorders: A Dissection of Clinical vs. Preclinical Correlates

Despite significant research efforts aimed at understanding the neurobiological underpinnings of mood (depression, bipolar disorder) and psychotic disorders, the diagnosis and evaluation of treatment of these disorders are still based solely on relatively subjective assessment of symptoms as well as psychometric evaluations. Therefore, biological markers aimed at improving the current classification of psychotic and mood-related disorders, and that will enable patients to be stratified on a biological basis into more homogeneous clinically distinct subgroups, are urgently needed. The attainment of this goal can be facilitated by identifying biomarkers that accurately reflect pathophysiologic processes in these disorders. This review postulates that the field of psychotic and mood disorder research has advanced sufficiently to develop biochemical hypotheses of the etiopathology of the particular illness and to target the same for more effective disease modifying therapy. This implies that a "one-size fits all" paradigm in the treatment of psychotic and mood disorders is not a viable approach, but that a customized regime based on individual biological abnormalities would pave the way forward to more effective treatment. In reviewing the clinical and preclinical literature, this paper discusses the most highly regarded pathophysiologic processes in mood and psychotic disorders, thereby providing a scaffold for the selection of suitable biomarkers for future studies in this field, to develope biomarker panels, as well as to improve diagnosis and to customize treatment regimens for better therapeutic outcomes.

Antidepressant-like effects of a novel 5-HT3 receptor antagonist 6z in acute and chronic murine models of depression

AIM

To investigate the antidepressant-like effects of a novel 5-HT3 receptor antagonist N-(benzo[d]thiazol-2-yl)-3-methoxyquinoxalin-2-carboxamide (6z) in acute and chronic murine models of depression.

METHODS

5-HT3 receptor antagonism was examined in guinea pig ileum in vitro. A tail suspension test (TST) was used as acute depression model to evaluate the antidepressant-like behavior in mice treated with 6z (0.5-2 mg/kg, ip). In chronic depression model, mice were exposed to a 4-week chronic unpredictable stress (CUS) protocol, and treated with 6z (0.5-2 mg·kg(-1)·d(-1), po) or a positive drug fluoxetine (10 mg·kg(-1)·d(-1), po) in the last 2 weeks, followed by behavioral and biochemical assessments.

RESULTS

The 5-HT3 receptor antagonism of 6z (pA2=7.4) in guinea pig ileum was more potent than that of a standard 5-HT3 receptor antagonist ondansetron (pA2=6.9). In acute depression model, 6z administration significantly decreased the immobility duration. In chronic depression model, 6z administration reversed CUS-induced depressive-like behavior, as evidenced by increased immobility duration in the forced swim test and sucrose preference in the sucrose preference test. Furthermore, chronic administration of 6z prevented CUS-induced brain oxidative stress, with significant reduction of pro-oxidant markers and elevation of antioxidant enzyme activity. Moreover, chronic administration of 6z attenuated CUS-induced hypothalamic-pituitary-adrenal axis hyperactivity, as shown by reduced plasma corticosterone levels. Similar results were observed in the fluoxetine-treated group.

CONCLUSION

6z is a novel 5-HT3 receptor antagonist with potential antidepressant-like activities, which may be related to modulating hypothalamic-pituitary-adrenal axis and attenuating brain oxidative damage.

New insights on the antidepressant discontinuation syndrome

OBJECTIVE

Antidepressants are at best 50–55% effective. Non-compliance and the antidepressant discontinuation syndrome (ADS) are causally related yet poorly appreciated. While ADS is associated with most antidepressants, agomelatine seems to be devoid of such risk. We review the neurobiology and clinical consequences of antidepressant non-compliance and the ADS. Agomelatine is presented as a counterpoint to learn more on how ADS risk is determined by pharmacokinetics and pharmacology.

DESIGN

The relevant literature is reviewed through a MEDLINE search via PubMed, focusing on agomelatine and clinical and preclinical research on ADS.

RESULTS

Altered serotonergic dysfunction appears central to ADS so that how an antidepressant targets serotonin will determine its relative risk for inducing ADS and thereby affect later treatment outcome. Low ADS risk with agomelatine versus other antidepressants can be ascribed to its unique pharmacokinetic characteristics as well as its distinctive actions on serotonin, including melatonergic, monoaminergic and glutamatergic-nitrergic systems.

CONCLUSIONS

This review raises awareness of the long-term negative aspects of non-compliance and inappropriate antidepressant discontinuation, and suggests possible approaches to “design-out” a risk for ADS. It reveals intuitive and rational ideas for antidepressant drug design, and provides new thoughts on antidepressant pharmacology, ADS risk and how these affect long-term outcome.

Serotonin 2C receptor antagonists induce fast-onset antidepressant effects

Current antidepressants must be administered for several weeks to produce therapeutic effects. We show that selective serotonin 2C (5-HT2C) antagonists exert antidepressant actions with a faster-onset (5 days) than that of current antidepressants (14 days) in mice. Subchronic (5 days) treatment with 5-HT2C antagonists induced antidepressant behavioral effects in the chronic forced swim test (cFST), chronic mild stress (CMS) paradigm and olfactory bulbectomy paradigm. This treatment regimen also induced classical markers of antidepressant action: activation of cAMP response element-binding protein (CREB) and induction of brain-derived neurotrophic factor (BDNF) in the medial prefrontal cortex (mPFC). None of these effects were induced by subchronic treatment with citalopram, a prototypical selective serotonin reuptake inhibitor (SSRI). Local infusion of 5-HT2C antagonists into the ventral tegmental area was sufficient to induce BDNF in the mPFC, and dopamine D1 receptor antagonist treatment blocked the antidepressant behavioral effects of 5-HT2C antagonists. 5-HT2C antagonists also activated mammalian target of rapamycin (mTOR) and eukaryotic elongation factor 2 (eEF2) in the mPFC, effects recently linked to rapid antidepressant action. Furthermore, 5-HT2C antagonists reversed CMS-induced atrophy of mPFC pyramidal neurons. Subchronic SSRI treatment, which does not induce antidepressant behavioral effects, also activated mTOR and eEF2 and reversed CMS-induced neuronal atrophy, indicating that these effects are not sufficient for antidepressant onset. Our findings reveal that 5-HT2C antagonists are putative fast-onset antidepressants, which act through enhancement of mesocortical dopaminergic signaling.

Agomelatine: mechanism of action and pharmacological profile in relation to antidepressant properties

Agomelatine behaves both as a potent agonist at melatonin MT1 and MT2 receptors and as a neutral antagonist at 5-HT2C receptors. Accumulating evidence in a broad range of experimental procedures supports the notion that the psychotropic effects of agomelatine are due to the synergy between its melatonergic and 5-hydroxytryptaminergic effects. The recent demonstration of the existence of heteromeric complexes of MT1 and MT2 with 5-HT2C receptors at the cellular level may explain how these two properties of agomelatine translate into a synergistic action that, for example, leads to increases in hippocampal proliferation, maturation and survival through modulation of multiple cellular pathways (increase in trophic factors, synaptic remodelling, glutamate signalling) and key targets (early genes, kinases). The present review focuses on the pharmacological properties of this novel antidepressant. Its mechanism of action, strikingly different from that of conventional classes of antidepressants, opens perspectives towards a better understanding of the physiopathological bases underlying depression.

On 'polypharmacy' and multi-target agents, complementary strategies for improving the treatment of depression: a comparative appraisal

Major depression is a heterogeneous disorder, both in terms of symptoms, ranging from anhedonia to cognitive impairment, and in terms of pathogenesis, with many interacting genetic, epigenetic, developmental and environmental causes. Accordingly, it seems unlikely that depressive states could be fully controlled by a drug possessing one discrete mechanism of action and, in the wake of disappointing results with several classes of highly selective agent, multi-modal treatment concepts are attracting attention. As concerns pharmacotherapy, there are essentially two core strategies. First, multi-target antidepressants that act via two or more complementary mechanisms and, second, polypharmacy, which refers to co-administration of two distinct drugs, usually in separate pills. Both multi-target agents and polypharmacy ideally couple a therapeutically unexploited action to a clinically established mechanism in order to enhance efficacy, moderate side-effects, accelerate onset of action and treat a broader range of symptoms. The melatonin MT1/MT2 agonist and 5-HT(2C) antagonist, agomelatine, which is effective in the short- and long-term treatment of depression, exemplifies the former approach, while evidence-based polypharmacy is illustrated by the adjunctive use of second-generation antipsychotics with serotonin reuptake inhibitors for treatment of resistant depression. Histone acetylation and methylation, ghrelin signalling, inflammatory modulators, metabotropic glutamate-7 receptors and trace amine-associated-1 receptors comprise attractive substrates for new multi-target and polypharmaceutical strategies. The present article outlines the rationale underpinning multi-modal approaches for treating depression, and critically compares and contrasts the pros and cons of established and potentially novel multi-target vs. polypharmaceutical treatments. On balance, the former appear the most promising for the elaboration, development and clinical implementation of innovative concepts for the more effective management of depression.

Negative symptoms of schizophrenia: clinical characteristics, pathophysiological substrates, experimental models and prospects for improved treatment

Schizophrenia is a complex and multifactorial disorder generally diagnosed in young adults at the time of the first psychotic episode of delusions and hallucinations. These positive symptoms can be controlled in most patients by currently-available antipsychotics. Conversely, they are poorly effective against concomitant neurocognitive dysfunction, deficits in social cognition and negative symptoms (NS), which strongly contribute to poor functional outcome. The precise notion of NS has evolved over the past century, with recent studies - underpinned by novel rating methods - suggesting two major sub-domains: "decreased emotional expression", incorporating blunted affect and poverty of speech, and "avolition", which embraces amotivation, asociality and "anhedonia" (inability to anticipate pleasure). Recent studies implicate a dysfunction of frontocortico-temporal networks in the aetiology of NS, together with a disruption of cortico-striatal circuits, though other structures are also involved, like the insular and parietal cortices, amygdala and thalamus. At the cellular level, a disruption of GABAergic-glutamatergic balance, dopaminergic signalling and, possibly, oxytocinergic and cannibinoidergic transmission may be involved. Several agents are currently under clinical investigation for the potentially improved control of NS, including oxytocin itself, N-Methyl-d-Aspartate receptor modulators and minocycline. Further, magnetic-electrical "stimulation" strategies to recruit cortical circuits and "cognitive-behavioural-psychosocial" therapies likewise hold promise. To acquire novel insights into the causes and treatment of NS, experimental study is crucial, and opportunities are emerging for improved genetic, pharmacological and developmental modelling, together with more refined readouts related to deficits in reward, sociality and "expression". The present article comprises an integrative overview of the above issues as a platform for this Special Issue of European Neuropsychopharmacology in which five clinical and five preclinical articles treat individual themes in greater detail. This Volume provides, then, a framework for progress in the understanding - and ultimately control - of the debilitating NS of schizophrenia.

Protective effects of phosphodiesterase 2 inhibitor on depression- and anxiety-like behaviors: involvement of antioxidant and anti-apoptotic mechanisms

Stress occurs in everyday life, but the relationship between stress and the onset or development of depression/anxiety remains unknown. Increasing evidence suggests that the impairment of antioxidant defense and the neuronal cell death are important in the process of emotional disorders. Chronic stress impairs the homeostasis of antioxidants/oxidation, which results in the aberrant stimulation of the cell cycle proteins where cGMP-PKG signaling is thought to have an inhibitory role. Phosphodiesterase 2 (PDE2) is linked to cGMP-PKG signaling and highly expressed in the limbic brain regions including hippocampus and amygdala, which may play important roles in the treatment of depression and anxiety. To address the possible effects of PDE2 inhibitors on depression-/anxiety-like behaviors and the underlying mechanisms, Bay 60-7550 (0.75, 1.5 and 3 mg/kg, i.p.) was administered 30 min before chronic stress. The results suggested that Bay 60-7550 not only restored the behavioral changes but also regulated Cu/Zn superoxide dismutase (SOD) levels differentially in hippocampus and amygdala, which were increased in the hippocampus while decreased in the amygdala. It was also significant that Bay 60-7550 regulated the abnormalities of pro- and anti-apoptotic components, such as Bax, Caspase 3 and Bcl-2, and the indicator of PKG signaling characterized by pVASP(ser239), in these two brain regions. The results suggested that Bay 60-7550 is able to alleviate oxidative stress and mediate part of the apoptotic machinery in neuronal cells possibly through SOD-cGMP/PKG-anti-apoptosis signaling and that inhibition of PDE2 may represent a novel therapeutic target for psychiatric disorders, such as depression and anxiety.

Controversies on the role of 5-HT(2C) receptors in the mechanisms of action of antidepressant drugs

Evidence from the various sources indicates alterations in 5-HT2C receptor functions in anxiety, depression and suicide, and other stress-related disorders treated with antidepressant drugs. Although the notion of a 5-HT2C receptor desensitization following antidepressant treatments is rather well anchored in the literature, this concept is mainly based on in vitro assays and/or behavioral assays (hypolocomotion, hyperthermia) that have poor relevance to anxio-depressive disorders. Our objective herein is to provide a comprehensive overview of the studies that have assessed the effects of antidepressant drugs on 5-HT2C receptors. Relevant molecular (second messengers, editing), neurochemical (receptor binding and mRNA levels), physiological (5-HT2C receptor-induced hyperthermia and hormone release), behavioral (5-HT2C receptor-induced changes in feeding, anxiety, defense and motor activity) data are summarized and discussed. Setting the record straight about drug-induced changes in 5-HT2C receptor function in specific brain regions should help to determine which pharmacotherapeutic strategy is best for affective and anxiety disorders.

Agomelatine in the tree shrew model of depression: effects on stress-induced nocturnal hyperthermia and hormonal status

The antidepressive drug agomelatine combines the properties of an agonist of melatonergic receptors 1 and 2 with an antagonist of the 5-HT2C receptor. We analyzed the effects of agomelatine in psychosocially stressed male tree shrews, an established preclinical model of depression. Tree shrews experienced daily social stress for a period of 5 weeks and were concomitantly treated with different drugs daily for 4 weeks. The effects of agomelatine (40 mg/kg/day) were compared with those of the agonist melatonin (40 mg/kg/day), the inverse 5-HT2C antagonist S32006 (10mg/kg/day), and the SSRI fluoxetine (15 mg/kg/day). Nocturnal core body temperature (CBT) was recorded by telemetry, and urinary norepinephrine and cortisol concentrations were measured. Chronic social stress induced nocturnal hyperthermia. Agomelatine normalized the CBT in the fourth week of the treatment (T4), whereas the other drugs did not significantly counteract the stress-induced hyperthermia. Agomelatine also reversed the stress-induced reduction in locomotor activity. Norepinephrine concentration was elevated by the stress indicating sympathetic hyperactivity, and was normalized in the stressed animals treated with agomelatine or fluoxetine but not in those treated with melatonin or S32006. Cortisol concentration was elevated by stress but returned to basal levels by T4 in all animals, irrespective of the treatment. These observations show that agomelatine has positive effects to counteract stress-induced physiological processes and to restore the normal rhythm of nocturnal CBT. The data underpin the antidepressant properties of agomelatine and are consistent with a distinctive profile compared to its constituent pharmacological components and other conventional agents.

Sexual side effects of serotonergic antidepressants: mediated by inhibition of serotonin on central dopamine release?

Antidepressant-induced sexual dysfunction adversely affects the quality of life of antidepressant users and reduces compliance with treatment. Animal models provide an instructive approach for examining potential sexual side effects of novel drugs. This review discusses the stability and reproducibility of our standardized test procedure that assesses the acute, subchronic and chronic effects of psychoactive compounds in a 30 minute mating test. In addition, we present an overview of the effects of several different (putative) antidepressants on male rat sexual behavior, as tested in our standardized test procedure. By comparing the effects of these mechanistically distinct antidepressants (paroxetine, venlafaxine, bupropion, buspirone, DOV 216,303 and S32006), this review discusses the putative mechanism underlying sexual side effects of antidepressants and their normalization. This review shows that sexual behavior is mainly inhibited by antidepressants that increase serotonin neurotransmission via blockade of serotonin transporters, while those that mainly increase the levels of dopamine and noradrenaline are devoid of sexual side effects. Those sexual disturbances cannot be normalized by simultaneously increasing noradrenaline neurotransmission, but are normalized by increasing both noradrenaline and dopamine neurotransmission. Therefore, it is hypothesized that the sexual side effects of selective serotonin reuptake inhibitors may be mediated by their inhibitory effects on dopamine signaling in sex brain circuits. Clinical development of novel antidepressants should therefore focus on compounds that simultaneously increase both serotonin and dopamine signaling.

Captodiamine, a putative antidepressant, enhances hypothalamic BDNF expression in vivo by synergistic 5-HT2c receptor antagonism and sigma-1 receptor agonism

The putative antidepressant captodiamine is a 5-HT2c receptor antagonist and agonist at sigma-1 and D3 dopamine receptors, exerts an anti-immobility action in the forced swim paradigm, and enhances dopamine turnover in the frontal cortex. Captodiamine has also been found to ameliorate stress-induced anhedonia, reduce the associated elevations of hypothalamic corticotrophin-releasing factor (CRF) and restore the reductions in hypothalamic BDNF expression. Here we demonstrate chronic administration of captodiamine to have no significant effect on hypothalamic CRF expression through sigma-1 receptor agonism; however, both sigma-1 receptor agonism or 5-HT2c receptor antagonism were necessary to enhance BDNF expression. Regulation of BDNF expression by captodiamine was associated with increased phosphorylation of transcription factor CREB and mediated through sigma-1 receptor agonism but blocked by 5-HT2c receptor antagonism. The existence of two separate signalling pathways was confirmed by immunolocalisation of each receptor to distinct cell populations in the paraventricular nucleus of the hypothalamus. Increased BDNF induced by captodiamine was also associated with enhanced expression of synapsin, but not PSD-95, suggesting induction of long-term structural plasticity between hypothalamic synapses. These unique features of captodiamine may contribute to its ability to ameliorate stress-induced anhedonia as the hypothalamus plays a prominent role in regulating HPA axis activity.

Serotonin2C Receptors and the Motor Control of Oral Activity

Data from many experiments has shown that serotonin2C (5-HT2C) receptor plays a role in the control of orofacial activity in rodents. Purposeless oral movements can be elicited either by agonists or inverse agonists implying a tight control exerted by the receptor upon oral activity. The effects of agonists has been related to an action of these drugs in the subthalamic nucleus and the striatum, the two input structures for cortical efferents to the basal ganglia, a group of subcortical structures involved in the control of motor behaviors. The oral effects of agonists are dramatically enhanced in case of chronic blockade of central dopaminergic transmission induced by neuroleptics or massive destruction of dopamine neurons. The mechanisms involved in the hypersensitized oral responses to 5-HT2C agonists are not clear and deserve additional studies. Indeed, while the oral behavior triggered by 5-HT2C drugs would barely correspond to the dyskinesia observed in humans, the clinical data have consistently postulated that 5-HT2C receptors could be involved in these aberrant motor manifestations.

5-HT2CR blockade in the amygdala conveys analgesic efficacy to SSRIs in a rat model of arthritis pain

BACKGROUND

Pain, including arthritic pain, has a negative affective component and is often associated with anxiety and depression. However, selective serotonin reuptake inhibitor antidepressants (SSRIs) show limited effectiveness in pain. The amygdala plays a key role in the emotional-affective component of pain, pain modulation and affective disorders. Neuroplasticity in the basolateral and central amygdala (BLA and CeA, respectively) correlate positively with pain behaviors. Evidence suggests that serotonin receptor subtype 5-HT2CR in the amygdala contributes critically to anxiogenic behavior and anxiety disorders. In this study, we tested the hypothesis that 5-HT2CR in the amygdala accounts for the limited effectiveness of SSRIs in reducing pain behaviors and that 5-HT2CR blockade in the amygdala renders SSRIs effective.

RESULTS

Nocifensive reflexes, vocalizations and anxiety-like behavior were measured in adult male Sprague-Dawley rats. Behavioral experiments were done in sham controls and in rats with arthritis induced by kaolin/carrageenan injections into one knee joint. Rats received a systemic (i.p.) administration of an SSRI (fluvoxamine, 30 mg/kg) or vehicle (sterile saline) and stereotaxic application of a selective 5-HT2CR antagonist (SB242084, 10 μM) or vehicle (ACSF) into BLA or CeA by microdialysis. Compared to shams, arthritic rats showed decreased hindlimb withdrawal thresholds (increased reflexes), increased duration of audible and ultrasonic vocalizations, and decreased open-arm choices in the elevated plus maze test suggesting anxiety-like behavior. Fluvoxamine (i.p.) or SB242084 (intra-BLA) alone had no significant effect, but their combination inhibited the pain-related increase of vocalizations and anxiety-like behavior without affecting spinal reflexes. SB242084 applied into the CeA in combination with systemic fluvoxamine had no effect on vocalizations and spinal reflexes.

CONCLUSIONS

The data suggest that 5-HT2CR in the amygdala, especially in the BLA, limits the effectiveness of SSRIs to inhibit pain-related emotional-affective behaviors.