Entrainment of rat circadian rhythms by the melatonin agonist S-20098 requires intact suprachiasmatic nuclei but not the pineal

Hypotensive effects of melatonin in rats: Focus on the model, measurement, application, and main mechanisms

The hypotensive effects of melatonin are based on a negative correlation between melatonin levels and blood pressure in humans. However, there is a positive correlation in nocturnal animals that are often used as experimental models in cardiovascular research, and the hypotensive effects and mechanism of melatonin action are often investigated in rats and mice. In rats, the hypotensive effects of melatonin have been studied in normotensive and spontaneously or experimentally induced hypertensive strains. In experimental animals, blood pressure is often measured indirectly during the light (passive) phase of the day by tail-cuff plethysmography, which has limitations regarding data quality and animal well-being compared to telemetry. Melatonin is administered to rats in drinking water, subcutaneously, intraperitoneally, or microinjected into specific brain areas at different times. Experimental data show that the hypotensive effects of melatonin depend on the experimental animal model, blood pressure measurement technique, and the route, time and duration of melatonin administration. The hypotensive effects of melatonin may be mediated through specific membrane G-coupled receptors located in the heart and arteries. Due to melatonin's lipophilic nature, its potential hypotensive effects can interfere with various regulatory mechanisms, such as nitric oxide and reactive oxygen species production and activation of the autonomic nervous and circadian systems. Based on the research conducted on rats, the cardiovascular effects of melatonin are modulatory, delayed, and indirect.

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.

Antidepressants and Circadian Rhythm: Exploring Their Bidirectional Interaction for the Treatment of Depression

Scientific evidence that circadian rhythms affect pharmacokinetics and pharmacodynamics has highlighted the importance of drug dosing-time. Circadian oscillations alter drug absorption, distribution, metabolism, and excretion (ADME) as well as intracellular signaling systems, target molecules (e.g., receptors, transporters, and enzymes), and gene transcription. Although several antidepressant drugs are clinically available, less than 50% of depressed patients respond to first-line pharmacological treatments. Chronotherapeutic approaches to enhance the effectiveness of antidepressants are not completely known. Even so, experimental results found until this day suggest a positive influence of drug dosing-time on the efficacy of depression therapy. On the other hand, antidepressants have also demonstrated to modulate circadian rhythmicity and sleep-wake cycles. This review aims to evidence the potential of chronotherapy to improve the efficacy and/or safety of antidepressants. It includes pre-clinical and clinical studies that demonstrate the relevance of determining the most appropriate time of administration for antidepressant drugs. In parallel, their positive influence on the resynchronization of disrupted circadian rhythms is also herein discussed. It is expected that this review will promote the investigation of chronotherapy for the treatment of depression, contribute to a better understanding of the relationship between antidepressants and circadian rhythms, and consequently promote the development of new therapeutics.

Molecular Link between Circadian Rhythmicity and Mood Disorders

BACKGROUND

The internal clock is driven by circadian genes [e.g., Clock, Bmal1, Per1-3, Cry1-2], hormones [e.g., melatonin, cortisol], as well as zeitgeber ['synchronisers']. Chronic disturbances in the circadian rhythm in patients diagnosed with mood disorders have been recognised for more than 50 years.

OBJECTIVES

The aim of this review is to summarise the current knowledge and literature regarding circadian rhythms in the context of mood disorders, focussing on the role of circadian genes, hormones, and neurotransmitters.

METHOD

The review presents the current knowledge and literature regarding circadian rhythms in mood disorders using the Pubmed database. Articles with a focus on circadian rhythms and mood disorders [n=123], particularly from 1973 to 2020, were included.

RESULTS

The article suggests a molecular link between disruptions in the circadian rhythm and mood disorders. Circadian disturbances, caused by the dysregulation of circadian genes, hormones, and neurotransmitters, often result in a clinical picture resembling depression.

CONCLUSION

Circadian rhythms are intrinsically linked to affective disorders, such as unipolar depression and bipolar disorder.

The effects of agomelatine and imipramine on liver cytochrome P450 during chronic mild stress (CMS) in the rat

BACKGROUND

The aim of our research was to determine the effects of chronic treatment with the atypical antidepressant agomelatine on the expression and activity of liver cytochrome P450 (CYP) in the chronic mild stress (CMS) model of depression, and to compare the results with those obtained for the first-generation antidepressant imipramine.

METHODS

Male Wistar rats were subjected to CMS for 7 weeks. Imipramine (10 mg/kg ip/day) or agomelatine (40 mg/kg ip/day) was administered to nonstressed or stressed animals for 5 weeks (weeks 3-7 of CMS). The levels of cytochrome P450 mRNA, protein and activity were measured in the liver.

RESULTS

Agomelatine and imipramine produced different broad-spectrum effects on cytochrome P450. Like imipramine, agomelatine increased the expression/activity of CYP2B and CYP2C6, and decreased the CYP2D activity. Unlike imipramine, agomelatine raised the expression/activity of CYP1A, CYP2A and reduced that of CYP2C11 and CYP3A. CMS modified the effects of antidepressants at transcriptional/posttranscriptional level; however, the enzyme activity in stressed rats remained similar to that in nonstressed animals. CMS alone decreased the CYP2B1 mRNA level and increased that of CYP2C11.

CONCLUSION

We conclude the following: (1) the effects of agomelatine and imipramine on cytochrome P450 are different and involve both central and peripheral regulatory mechanisms, which implicates the possibility of drug-drug interactions; (2) CMS influences the effects of antidepressants on cytochrome P450 expression, but does not change appreciably their effects on the enzyme activity. This suggests that the rate of antidepressant drug metabolism under CMS is similar to that under normal conditions.

Role of biological rhythm dysfunction in the development and management of bipolar disorders: a review

Disturbance of biological rhythms contributes to the onset of bipolar disorders and is an important clinical feature of the condition. To further explore the role of biological rhythms in bipolar disorders, 95 English articles published between 1968 and 2019 were retrieved from the PubMed database and analysed. We herein review the outcomes of studies on biological rhythm disturbance in bipolar disorders, including the epidemiology, aetiology, clinical features (eg, sleep, feeding and eating disorders) and treatment of the condition evaluated by patients’ self-report and biological indicators such as melatonin. Our report supports the characterisation of biological rhythm disturbance as a significant clinical feature affecting the onset and development of bipolar disorders and reviews classical and novel treatments, such as chronotherapy, that can be applied in the clinical practice. Our analysis indicates that a more comprehensive study of the pathophysiology, clinical phenomenology and treatment of biological rhythm disturbance is required.

Agomelatine treatment corrects impaired sleep-wake cycle and sleep architecture and increases MT1 receptor as well as BDNF expression in the hippocampus during the subjective light phase of rats exposed to chronic constant light

RATIONALE

Exposure to chronic constant light (CCL) has a detrimental impact on circadian rhythms of motor activity and sleep/wake cycles. Agomelatine is an atypical antidepressant showing a chronotropic activity.

OBJECTIVES

In this study, we explored the role of melatonin (MT) receptors and brain-derived neurotrophic factor (BDNF) in the brain in the mechanism underlying the effects of agomelatine on diurnal variations of motor activity, sleep/wake cycle, and sleep architecture in a rat model of CCL.

METHODS

In Experiment #1, home cage activity was monitored automatically with cameras for a period of 24 h. The diurnal rhythm of MT_1, MT₂ receptors, and BDNF expression in the hippocampus and frontal cortex (FC), was tested using the ELISA test. In Experiment #2, rats were equipped with electroencephalographic (EEG) and electromyographic (EMG) electrodes and recordings were made under basal conditions (12:12 LD cycle + vehicle), LL + vehicle and LL + agomelatine (40 mg/kg/day for 21 days).

RESULTS

The rats exposed to CCL showed an impaired diurnal rhythm of motor activity and sleep/wake cycle with reduced NREM sleep and delta power and increased REM sleep and theta power. The duration and number of episodes of the wake were diminished during the subjective dark phase in this group. The circadian rhythm of MT₁ and MT₂ receptors and their expression did not change in the hippocampus and FC under CCL exposure, while the BDNF levels in the hippocampus decreased during the subjective light phase. Agomelatine restored the diurnal rhythm of motor activity, disturbed sleep/wake cycle, and sleep architecture, which effect was accompanied by an increase in MT₁ receptor and BDNF expression in the hippocampus at 10:00 in CCL rats.

CONCLUSIONS

These findings support the value of agomelatine as an antidepressant that can adjust circadian homeostasis of motor activity and sleep/wake cycle in a CCL model.

Pineal gland and schizophrenia: A systematic review and meta-analysis

Melatonin (MLT), the main hormone of the pineal gland (PG), is assumed to support initiation and maintenance of sleep, and a stable sleep-wake cycle, exerting antioxidative and neuroprotective actions. Evidence demonstrates that sleep and circadian rhythm abnormalities are very common in schizophrenia patients. Some imaging studies suggest structural abnormalities of the PG in these patients as well. We aimed to critically appraise the literature on PG imaging and melatonin secretion in schizophrenia patients, in comparison to matched healthy controls, and to review placebo-controlled trials of add-on exogenous MLT treatment in schizophrenia patients. In this systematic review, twenty-nine studies were included. Meta-analytical evaluation of data was possible only for MLT secretion finding that midnight plasma levels were significantly reduced in individuals with schizophrenia as compared to healthy controls (Hedge`s g = 1.32, p < 0.01). Imaging studies demonstrated greater prevalence of enlarged calcifications (>1 cm) of the PG (2 out of 2 computed tomography studies) and smaller PG volume (2 out of 3 magnetic resonance studies) compared with healthy controls. Anatomic and functional abnormalities of the PG were not associated with duration of illness or with treatment factors, maybe suggesting them to be primary characteristics of the disease and genetically based. Add-on MLT treatment leads to a modest improvement of objective and subjective sleep quality, of metabolic adverse effects of antipsychotics, and of tardive dyskinesia symptoms in schizophrenia patients. It remains to be established whether MLT treatment in prodromal phases of the disease could prevent neurostructural abnormalities.

Prospects for circadian treatment of mood disorders

Disruption of circadian clocks is strongly associated with mood disorders. Chronotherapies targeting circadian rhythms have been shown to be very effective treatments of mood disorders, but still are not widely used in clinical practice. The mechanisms by which circadian disruption leads to mood disorders are poorly characterized and, therefore, may not convince clinicians to apply chronotherapies. Hence, in this review, we describe specific potential mechanisms, in order to make this connection more credible to clinicians. We believe that four major features of disrupted clocks may contribute to the development of mood disorders: (1) loss of synchronization to environmental 24-h rhythms, (2) internal desynchronization among body clocks, (3) low rhythm amplitude, and (4) changes in sleep architecture. Discussing these attributes and giving plausible examples, we will discuss prospects for relatively simple chronotherapies addressing these features that are easy to implement in clinical practice. Key messages In this review, we describe specific potential mechanisms by which disrupted clocks may contribute to the development of mood disorders: (1) loss of synchronization to environmental 24-h rhythms, (2) internal desynchronization among body clocks, (3) low rhythm amplitude, and (4) changes in sleep architecture. We provide prospects for relatively simple chronotherapies addressing these features that are easy to implement in clinical practice.

Agomelatine-induced modulation of brain-derived neurotrophic factor (BDNF) in the rat hippocampus

Brain-derived neurotrophic factor (BDNF) is a neurotrophin that serves as a survival factor for neurons. Agomelatine is a novel antidepressant as well as a potent agonist of melatonin (MT), MT1 and MT2 receptor types and an antagonist of the serotonin (5HT), 5-HT_2C receptor. The study herein established whether treatment with agomelatine alters hippocampal BDNF protein expression under chronic unpredictable mild stress (CUMS) condition. Twenty-one day treatment with agomelatine, fluoxetine or vehicle was assessed in 52 Sprague-Dawley rats undergoing CUMS. Ten naïve control rats were also evaluated after 21 days. The behavioral effects of treatments were studied using the open field test (OFT) on day 0, 7 and 21 and sucrose preference test on day 21. Hippocampal BDNF protein expression was measured using immunohistochemistry. The effect of the interventions on hippocampal neurons was histologically examined after H&E staining. Agomelatine mitigated the reduction in rearing behavior by CUMS in the OFT on day 7 as well as sucrose preference on day 21. The mean optical density value of BDNF was significantly higher in the CUMS + agomelatine group than the CUMS and CUMS + fluoxetine groups. The CUMS + agomelatine group had a significantly higher number of BDNF positive cells compared to naïve controls and CUMS group. Histology showed that hippocampal neurons in the CUMS + agomelatine and CUMS + fluoxetine groups were intact and few of them demonstrated karyopyknosis. Agomelatine-a novel antidepressant, but not fluoxetine, increased hippocampal BDNF level and of BDNF positive neurons in rats subject to CUMS.

Agomelatine treatment corrects symptoms of depression and anxiety by restoring the disrupted melatonin circadian rhythms of rats exposed to chronic constant light

Desynchronization of circadian rhythms is a hallmark of depression. The antidepressant agomelatine, which is an MT₁/MT₂ melatonin receptor agonist/5-HT_2C serotonin receptor antagonist has advantages compared to the selective serotonin reuptake inhibitors as a circadian phase-shifting agent. The present study was designed to explore whether agomelatine is able to have an antidepressant effect on rats exposed to chronic constant light (CCL) for 6 weeks. Focus is also placed on whether this activity affects diurnal rhythms of depressive-like symptoms and is associated with restoration of impaired circadian rhythms in plasma melatonin and corticosterone. We report that CCL induced a depressive-like symptoms associated with decreased grooming in the splash test during the subjective light/inactive phase. Anhedonia-like deficit in the saccharine preference test and increased immobility in the forced swimming test were both detected during the subjective dark/active phase. The disturbed emotional fluctuations due to CCL were corrected by agomelatine treatment (40 mg/kg, i.p. for 3 weeks). Agomelatine also restored novelty-induced hypophagia, which reflects an anxiety state, during the subjective Light and Dark phase, respectively, in rats exposed to CCL. Parallel to the observed positive influence on behavior, this melatonin analogue restored impaired circadian patterns of plasma melatonin but not that of corticosterone. These findings demonstrated the antidepressant-like effect of agomelatine in rats exposed to CCL possibly exerted via correction of melatonin rhythms and are suggestive of the therapeutic potential of this drug in a subpopulation of people characterized by a melatonin deficit.

Delayed sleep phase syndrome and bipolar disorder: Pathogenesis and available common biomarkers

Circadian rhythm disturbances are common in bipolar affective disorder (BD). Delayed sleep-wake phase syndrome (DSWPD) is the most prevalent circadian rhythm sleep-wake disorder (CRSWDs) and is frequently observed in BD. It is unclear whether DSWPD in BD is an independent process or is a consequence of BD. In this hypothetical review, we discuss the overlap between BD and DSWPD and potential common biomarkers for DSWPD and BD. The review will include a discussion of the genetics of DSWPD and BD. Biomarkers elucidating the pathophysiological processes occurring in these two disorders may offer insight into the etiology and prognosis of both conditions.

[A profile of antidepressive effects of agomelatine and a current view on the mechanism of its action]

Agomelatine is one of the latest antidepressants (melatoninergic agonists) with a new mechanism of action. From the positions of classical monoaminoergic theory, tts mechanism of action is difficult to understand, because the drug increases the levels of monoamines and neurotrophic factors, while not affecting their reuptake and negative feedback, which control neurotransmission level. Besides the effect on suprachiasmatic nucleus, a relevant role in the mechanism of action of agomelatine plays its special functionally selective (with regard to intracellular signaling pathways) interaction with heteromeric complexes of serotonin 5-НТ2С and melatonin MT2 receptors in the hippocampus and cerebral cortex. Agomelatine is competitive to other modern antidepressants in the efficacy assessed by the percentage of complete responders and superior in the total frequency of remissions. Compared to other SSRI antidepressants, agomelatine is more effective for anhedonia. In these cases, agomelatine increases the level of brain-derived neurotrophic factor (BDNF) in the blood of responders.

Circadian Rhythm Disturbances in Mood Disorders: Insights into the Role of the Suprachiasmatic Nucleus

Circadian rhythm disturbances are a common symptom among individuals with mood disorders. The suprachiasmatic nucleus (SCN), in the ventral part of the anterior hypothalamus, orchestrates physiological and behavioral circadian rhythms. The SCN consists of self-sustaining oscillators and receives photic and nonphotic cues, which entrain the SCN to the external environment. In turn, through synaptic and hormonal mechanisms, the SCN can drive and synchronize circadian rhythms in extra-SCN brain regions and peripheral tissues. Thus, genetic or environmental perturbations of SCN rhythms could disrupt brain regions more closely related to mood regulation and cause mood disturbances. Here, we review clinical and preclinical studies that provide evidence both for and against a causal role for the SCN in mood disorders.

New targets for rapid antidepressant action

Current therapeutic options for major depressive disorder (MDD) and bipolar disorder (BD) are associated with a lag of onset that can prolong distress and impairment for patients, and their antidepressant efficacy is often limited. All currently approved antidepressant medications for MDD act primarily through monoaminergic mechanisms. Glutamate is the major excitatory neurotransmitter in the central nervous system, and glutamate and its cognate receptors are implicated in the pathophysiology of MDD, and in the development of novel therapeutics for this disorder. The rapid and robust antidepressant effects of the N-methyl-d-aspartate (NMDA) antagonist ketamine were first observed in 2000. Since then, other NMDA receptor antagonists have been studied in MDD. Most have demonstrated relatively modest antidepressant effects compared to ketamine, but some have shown more favorable characteristics. This article reviews the clinical evidence supporting the use of novel glutamate receptor modulators with direct affinity for cognate receptors: (1) non-competitive NMDA receptor antagonists (ketamine, memantine, dextromethorphan, AZD6765); (2) subunit (GluN2B)-specific NMDA receptor antagonists (CP-101,606/traxoprodil, MK-0657); (3) NMDA receptor glycine-site partial agonists (GLYX-13); and (4) metabotropic glutamate receptor (mGluR) modulators (AZD2066, RO4917523/basimglurant). We also briefly discuss several other theoretical glutamate receptor targets with preclinical antidepressant-like efficacy that have yet to be studied clinically; these include α-amino-3-hydroxyl-5-methyl-4-isoxazoleproprionic acid (AMPA) agonists and mGluR2/3 negative allosteric modulators. The review also discusses other promising, non-glutamatergic targets for potential rapid antidepressant effects, including the cholinergic system (scopolamine), the opioid system (ALKS-5461), corticotropin releasing factor (CRF) receptor antagonists (CP-316,311), and others.

Agomelatine in the treatment of major depressive disorder: an assessment of benefits and risks

Agomelatine (AGM) was approved for the treatment of major depressive disorder (MDD) in adults by the European Medicines Agency (EMA) in February 2009. It is an analogue of melatonin and features a unique pharmacodynamic profile with agonism on both types of melatonergic receptors (MT1/MT2) and antagonism at serotonergic 5-HT2C receptors. There is, however, an ongoing debate regarding the efficacy and safety of this novel antidepressant agent, originally evoked by claims of a significant publication bias underlying the assessment of AGM being an effective antidepressant. Indeed, two recent comprehensive metaanalyses of published and unpublished clinical trials found evidence for a relevant publication bias. However, due to its statistically significant advantage over placebo based on the results of these metaanalyses AGM must be referred to as an effective antidepressant agent in the acute phase of MDD. However, the effect sizes of AGM in the treatment of MDD were evaluated as being small in comparison to other antidepressant agents. In addition, there is insufficient evidence for the efficacy of AGM in relapse prevention of MDD. Apart from efficacy issues, AGM appears to have the potential to exhibit severe hepatotoxicity (the EMA has identified AGM-associated “hepatotoxic reactions” as a new safety concern in September 2013) that is currently poorly understood. Considering these aspects, it seems inappropriate to evaluate AGM as an antidepressant agent of first choice. Nevertheless, its unique mechanism of action with particular sleep modulating effects may represent a specific treatment strategy for patients with particular characteristics; further studies with thorough characterization of patients are needed to test this hypothesis.

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.

Could agomelatine be the ideal antidepressant?

Depressive disorders are a common cause of chronic and recurrent psychiatric dysfunction, constituting the fourth leading cause of global diseases. Depression is associated with a high rate of morbidity and mortality, and is a leading cause of global disability. Despite the effectiveness of most currently available antidepressants, many of them have a number of undesirable side effects. Agomelatine is the first melatonin (MT)(1)/MT(2) agonist having 5-hydroxytryptamine (5-HT)(2C) and 5-HT(2B) antagonist properties and antidepressant activity. Agomelatine is effective in several animal models of depression and anxiety. In addition, three large, multicenter, multinational, placebo-controlled studies and several double-blind, placebo-controlled trials of agomelatine have demonstrated that it is a clinically effective and well-tolerated antidepressant in acute trials. Since currently available antidepressants are not always adequate to cause complete remission of symptoms in severely depressed patients, the superior rate of response achieved with agomelatine in this group of patients underlines its future for clinical use in depressive disorders. In summary, the clinical advantage of agomelatine is attributed to its novel mechanism of action, which helps not only to exert antidepressant action, but also to regulate the sleep-wake rhythm.

Agomelatine: Its Role in the Management of Major Depressive Disorder

Circadian rhythm abnormalities, as shown by sleep/wake cycle disturbances, constitute one the most prevalent signs of depressive illness; advances or delays in the circadian phase are documented in patients with major depressive disorder (MDD), bipolar disorder, and seasonal affective disorder (SAD). The disturbances in the amplitude and phase of rhythm in melatonin secretion that occur in patients with depression resemble those seen in chronobiological disorders, thus suggesting a link between disturbed melatonin secretion and depressed mood. Based on this, agomelatine, the first MT1/MT2 melatonergic agonist displaying also 5-HT2C serotonergic antagonism, has been introduced as an antidepressant. Agomelatine has been shown to be effective in several animal models of depression and anxiety and it has beneficial effects in patients with MDD, bipolar disorder, or SAD. Among agomelatine's characteristics are a rapid onset of action and a pronounced effectiveness for correcting circadian rhythm abnormalities and improving the sleep/wake cycle. Agomelatine also improves the 3 functional dimensions of depression—emotional, cognitive, and social—thus aiding in the full recovery of patients to a normal life.

Mode of action of agomelatine: synergy between melatonergic and 5-HT2C receptors

OBJECTIVES

The association between depression and circadian rhythm disturbances is well established and successful treatment of depressed patients is accompanied by restoration of circadian rhythms. The new antidepressant agomelatine is an agonist of melatonergic MT₁/MT₂ receptors as well as an antagonist of serotonergic 5-HT2C receptors. Animal studies showed that agomelatine resynchronizes disturbed circadian rhythms and reduces depression-like behaviour.

METHODS

This review analyzes results from different experimental studies.

RESULTS

Recent data on the effects of agomelatine on cellular processes involved in antidepressant mechanisms have shown that the drug is able to increase the expression of brain-derived neurotrophic factor in prefrontal cortex and hippocampus, as well as the expression of activity-regulated cytoskeleton associated protein (Arc) in the prefrontal cortex. In line with this, prolonged treatment with agomelatine increases neurogenesis within the hippocampus, particularly via enhancement of neuronal cell survival. Agomelatine attenuates stress-induced glutamate release in the prefrontal/frontal cortex. Treatment with 5-HT2C antagonists or melatonin alone failed to reproduce these effects.

CONCLUSIONS

The unique mode of action of agomelatine may improve the management of major depression by counteracting the pathogenesis of depression at cellular level, thereby relieving the symptoms of depression. These effects are suggested to be due to a synergistic action on MT₁/MT₂ and 5-HT2C receptors.

Circadian rhythms and mood regulation: insights from pre-clinical models

Affective disorders such as major depression, bipolar disorder, and seasonal affective disorder are associated with major disruptions in circadian rhythms. Indeed, altered sleep/wake cycles are a critical feature for diagnosis in the DSM IV and several of the therapies used to treat these disorders have profound effects on rhythm length and stabilization in human populations. Furthermore, multiple human genetic studies have identified polymorphisms in specific circadian genes associated with these disorders. Thus, there appears to be a strong association between the circadian system and mood regulation, although the mechanisms that underlie this association are unclear. Recently, a number of studies in animal models have begun to shed light on the complex interactions between circadian genes and mood-related neurotransmitter systems, the effects of light manipulation on brain circuitry, the impact of chronic stress on rhythms, and the ways in which antidepressant and mood-stabilizing drugs alter the clock. This review will focus on the recent advances that have been gleaned from the use of pre-clinical models to further our understanding of how the circadian system regulates mood.

Chronic agomelatine and fluoxetine induce antidepressant-like effects in H/Rouen mice, a genetic mouse model of depression

The novel antidepressant agomelatine behaves as an agonist at melatonergic MT(1) and MT(2) receptors and as an antagonist at serotonin 5-HT(2C) receptors. This study investigated the effects of agomelatine and fluoxetine in a genetic model of depression called H/Rouen mice Male and female H/Rouen (helpless line) and NH/Rouen (nonhelpless line) mice, received once daily for 3 weeks agomelatine (10 and 50 mg/kgi.p.), fluoxetine (10 mg/kgi.p.) or vehicle. Immobility duration in the tail suspension test (TST) was assessed on day 1 (D1), day 8 (D8), day 15 (D15) and day 22 (D22). Locomotor activity in a novel environment was assessed on day 18 (D18) and anhedonia (2-bottle sucrose preference test) was considered after the end of chronic treatment, from days 22 to 25. Agomelatine (50 mg/kg) significantly reduced immobility at D15 (p<0.01), and D22 (p<0.001) in treated H/Rouen mice whereas agomelatine at 10 mg/kg did not induce a statistically significant change. Fluoxetine reduced immobility at D8 (p<0.01), D15 (p<0.001) and D22 (p<0.001). Locomotor activity was unchanged in all treated groups as compared to vehicle groups. In the sucrose test, there was a significant decrease in sucrose preference in H/Rouen mice compared with NH/Rouen mice receiving vehicle. Both agomelatine doses (10 mg/kg (p=0.05) and 50 mg/kg (p<0.001) as well as fluoxetine (p<0.001) significantly increased the sucrose preference in H/Rouen mice as compared with H/Rouen mice that had received vehicle. These data indicate that the novel antidepressant agomelatine has antidepressant-like properties in H/Rouen mice, a genetic model of depression.

A systematic, updated review on the antidepressant agomelatine focusing on its melatonergic modulation

Objective:

To present an updated, comprehensive review on clinical and pre-clinical studies on agomelatine.

Method:

A MEDLINE, Psycinfo and Web of Science search (1966-May 2009) was performed using the following keywords: agomelatine, melatonin, S20098, efficacy, safety, adverse effect, pharmacokinetic, pharmacodynamic, major depressive disorder, bipolar disorder, Seasonal Affective Disorder (SAD), Alzheimer, ADHD, Generalized Anxiety Disorder (GAD), Panic Disorder (PD), Obsessive-Compulsive Disorder (OCD), anxiety disorders and mood disorder.

Study collection and data extraction:

All articles in English identified by the data sources were evaluated. Randomized, controlled clinical trials involving humans were prioritized in the review. The physiological bases of melatonergic transmission were also examined to deepen the clinical comprehension of agomelatine’ melatonergic modulation.

Data synthesis:

Agomelatine, a melatonergic analogue drug acting as MT₁/MT₂ agonist and 5-HT_2C antagonist, has been reported to be an effective antidepressant therapy.

Conclusions:

Although a bias in properly assessing the “sleep core” of depression may still exist with current screening instruments, therefore making difficult to compare agomelatine’ efficacy to other antidepressant ones, comparative studies showed agomelatine to be an intriguing option for depression and, potentially, for other therapeutic targets as well.

The pharmacological properties of antidepressants

Antidepressant drugs represent one of the main forms of effective treatment for the amelioration of depressive symptoms. Most available antidepressants increase extracellular levels of monoamines. However, it is now recognized that monoamine levels and availability are only part of the story, and that antidepressants whose mechanism of action is mainly based on the modulation of monoaminergic systems may not be able to satisfy the unmet needs of depression. Therefore, a number of compounds, developed for their potential antidepressant activity, are endowed with putative mechanisms of action not affecting traditional monoamine targets. This article briefly reviews, within a mechanistic perspective, the pharmacological profiles of representative antidepressants from each class, including monoamine oxidase inhibitors, tricyclics, norepinephrine reuptake inhibitors, selective serotonin reuptake inhibitors, norepinephrine and serotonin reuptake inhibitors, antidepressants interacting with dopaminergic, melatonergic, glutamatergic, or neuropeptide systems. The undesirable side effects of currently used antidepressants, which can often be a reason for lack of compliance, are also considered.

New therapeutic targets for mood disorders

Existing pharmacological treatments for bipolar disorder (BPD) and major depressive disorder (MDD) are often insufficient for many patients. Here we describe a number of targets/compounds that clinical and preclinical studies suggest could result in putative novel treatments for mood disorders. These include: (1) glycogen synthase kinase-3 (GSK-3) and protein kinase C (PKC), (2) the purinergic system, (3) histone deacetylases (HDACs), (4) the melatonergic system, (5) the tachykinin neuropeptides system, (6) the glutamatergic system, and (7) oxidative stress and bioenergetics. The paper reviews data on new compounds that have shown antimanic or antidepressant effects in subjects with mood disorders, or similar effects in preclinical animal models. Overall, an improved understanding of the neurobiological underpinnings of mood disorders is critical in order to develop targeted treatments that are more effective, act more rapidly, and are better tolerated than currently available therapies.

Impact of the novel antidepressant agomelatine on disturbed sleep-wake cycles in depressed patients

BACKGROUND

Disturbance of sleep-wake cycles is common in major depressive disorder (MDD), usually as insomnia, but also as hypersomnia or reduced daytime alertness. Agomelatine, an MT(1) and MT(2) receptor agonist and 5-HT(2C) receptor antagonist, represents a novel approach in MDD, with proven antidepressant efficacy and a positive impact on the sleep-wake cycle. We review the effects of agomelatine 25/50 mg/day on objective and subjective measures of the sleep-wake cycle in MDD.

SUBJECTIVE MEASURES

Agomelatine improved all aspects of the sleep-wake cycle from as early as 1 week in randomized trials versus selective serotonin reuptake inhibitors and venlafaxine, particularly getting off to sleep and quality of sleep, with an improvement in daytime alertness.

OBJECTIVE MEASURES

Agomelatine's effect on sleep architecture in MDD has been measured by polysomnography (PSG). There were significant improvements in sleep efficiency, slow-wave sleep (SWS), and the distribution of delta activity throughout the night, but no change in amount or latency of rapid eye movement (REM) sleep. Furthermore, the slow-wave sleep was resynchronized to the first sleep cycle of the night.

CONCLUSION

Agomelatine, a novel antidepressant, improves disturbed sleep-wake cycles in MDD. The improvement of both nighttime sleep and daytime functioning with agomelatine are promising features of this antidepressant regarding the management of MDD.

The effect of melatonergic and non-melatonergic antidepressants on sleep: weighing the alternatives

In DSM-IV the occurrence of disturbed sleep is one of the principal diagnostic criteria for major depressive disorder (MDD). Further, there is evidence of reciprocity between the two conditions such that, even in the absence of current depressive symptoms, disturbed sleep often predicts their development. The present review discusses the effects of antidepressants on sleep and evaluates the use of the recently developed melatonin agonist-selective serotonin antagonists on sleep and depression. Although many antidepressants such as the tricyclics, monoamine oxidase inhibitors, serotonin-norepinephrine reuptake inhibitors, several serotonin receptor antagonists and selective serotonin reuptake inhibitors (SSRIs) have all been found successful in treating depression, their use is often associated with a disruptive effect on sleep. SSRIs, currently the most widely prescribed of the antidepressants, are well known for their instigation or exacerbation of insomnia. The recently introduced novel melatonin agonist and selective serotonin antagonist antidepressant, agomelatine, which has melatonin MT(1) and MT(2) receptor agonist and 5-HT(2c) antagonist properties, has been useful in treating patients with MDD. Its rapid onset of action and effectiveness in improving the mood of depressed patients has been attributed to its ability to improve sleep quality. These properties underline the use of melatonin analogues as a promising alternative for the treatment of depression.

Potential novel therapeutics for bipolar disorders

Existing pharmacological treatments for bipolar disorder (BPD), a severe recurrent mood disorder, are in general insufficient for many patients. Despite adequate doses and treatment duration, many individuals with this disease continue to experience mood episode relapses, residual symptoms, and functional impairment. This chapter reviews a number of targets/compounds that could result in putative novel treatments for BPD, including the dynorphin opioid neuropeptide system, the glutamatergic system, the purinergic system, the cholinergic system (muscarinic and nicotinic systems), the oxidative stress system, and the melatonergic system. The arachidonic acid cascade and intracellular signaling cascades (including glycogen synthase kinase 3 and protein kinase C) are also reviewed, as are agents that affect multiple targets (e.g., modafinil, Uridine RG2417). Further study of these and similar agents may improve our understanding of relevant drug targets and their clinical utility as potential therapeutics for this devastating disorder.

Agomelatine: innovative pharmacological approach in depression

Currently available antidepressant agents such as tricyclic antidepressants (TCAs) act primarily through monoaminergic systems in the brain, and have proved to be suboptimal for the management of major depressive disorder (MDD). Such agents are also active at non-target receptor sites, contributing to the development of often serious adverse events. Even the newer selective serotonin reuptake inhibitors (SSRIs), which also act through monoaminergic systems, have suboptimal antidepressant efficacy, and the adverse events that do occur often negatively influence adherence. Although the pathophysiology of depression is not completely understood, it is increasingly recognized that monoamine deficiency/disruption is not the only pathway involved. Recognition that circadian rhythm desynchronization also plays a key role in mood disorders has led to the development of agomelatine, which is endowed with a novel mechanism of action distinct from that of currently available antidepressants. Agomelatine is an agonist of the melatonergic MT(1) and MT(2) receptors, as well as a 5-HT(2C) receptor antagonist. The antidepressant activity of agomelatine is proposed to stem from the synergy between these sets of receptors, which are key components of the circadian timing system. Agomelatine has shown antidepressant-like activity in a number of animal models of depression, such as the learned helplessness model, the chronic mild stress model, the forced swim test and the chronic psychosocial stress test. Moreover, agomelatine has been found to restore normal circadian rhythms in animal models of a disrupted circadian system, and has proved beneficial in an animal model of delayed sleep phase syndrome. Likewise, it has been shown to improve disturbed sleep-wake rhythms in depressed patients. Moreover, current pharmacological and clinical data strongly support the use of agomelatine in the management of MDD.

Beyond the monoaminergic hypothesis: agomelatine, a new antidepressant with an innovative mechanism of action

There are many potentials for the development of more effective, better tolerated, and more rapidly acting antidepressants. As there is large prevalence of circadian dysfunction in various affective disorders, including depression, one of the approaches is the development of antidepressant drugs with melatonergic agonist properties. Agomelatine, with its melatonergic agonistic (at both MT(1) an MT(2) receptors) and 5-HT(2C) antagonistic properties, represents a new concept for the treatment of depression. The antidepressant action of agomelatine has been initially demonstrated in animal models of depression, such as the forced swim - the learned helplessness - and the chronic mild stress paradigms. Subsequent studies demonstrated that the antidepressant activity of agomelatine does not solely depend on its agonistic action at melatonergic receptors, but also on its antagonistic activity at 5-HT(2C) receptors. Agomelatine also exhibits anxiolytic properties that bear a striking resemblance to those of selective 5-HT(2C) receptor antagonists. In patients with major depressive disorder, agomelatine had efficacy at least comparable to that seen with available antidepressants. Interestingly, agomelatine demonstrated antidepressant efficacy not only in patients with a moderate depressive episode but also in a more severe depressed subpopulation of patients. The treatment effect increased with the severity of the disease. Agomelatine also rapidly regulates the sleep-wake cycle without causing sedation and improves daytime condition. Agomelatine has an excellent safety profile, is weight neutral, does not affect sexual functioning and does not cause discontinuation syndrome. Collectively, its efficacy, together with its excellent tolerability, makes agomelatine an especially promising antidepressant for the near future.

Influence of the novel antidepressant and melatonin agonist/serotonin2C receptor antagonist, agomelatine, on the rat sleep-wake cycle architecture

RATIONALE

The novel antidepressant, agomelatine, behaves as an agonist at melatonin MT(1) and MT(2) receptors and as an antagonist at serotonin (5-HT)(2C) receptors. In animal models and clinical trials, agomelatine displays antidepressant properties and re-synchronizes disrupted circadian rhythms.

OBJECTIVES

The objectives of this study were to compare the influence of agomelatine upon sleep-wake states to the selective melatonin agonists, melatonin and ramelteon, and to the selective 5-HT(2C) receptor antagonist, S32006.

METHODS

Rats were administered with vehicle, agomelatine, ramelteon, melatonin, or S32006, at the onset of either dark or light periods. Polygraphic recordings were performed and changes determined over 24 h, i.e., number and duration of sleep-wake episodes, latencies to rapid eye movement (REM) and slow-wave (SWS) sleep, power band spectra of the electroencephalogram (EEG), and circadian changes.

RESULTS

Administered at light phase onset, no changes were induced by agomelatine. In contrast, administered shortly before dark phase, agomelatine (10 and 40 mg/kg, per os) enhanced duration of REM and SWS sleep and decreased wake state for 3 h. Melatonin (10 mg/kg, per os) induced a transient enhancement in REM sleep followed by a reduction in REM and SWS sleep and an increase in waking. Ramelteon (10 mg/kg, per os) provoked a transient increase in REM sleep. Finally, S32006 (10 mg/kg, intraperitoneally), administered at dark phase onset, mimicked the increased SWS provoked by agomelatine, yet diminished REM sleep.

CONCLUSIONS

Agomelatine possesses a distinctive EEG profile compared with melatonin, ramelteon, and S32006, possibly reflecting co-joint agonist and antagonist properties at MT(1)/MT(2) and 5-HT(2C) receptors, respectively.

Bipolar disorder: candidate drug targets

Current pharmacotherapy for bipolar disorder is generally unsatisfactory for a large number of patients. Even with adequate modern bipolar pharmacological therapies, many afflicted individuals continue to have persistent mood episode relapses, residual symptoms, functional impairment, and psychosocial disability. Creating novel therapeutics for bipolar disorder is urgently needed. Promising drug targets and compounds for bipolar disorder worthy of further study include both systems and intracellular pathways and targets. Specifically, the purinergic system, the dynorphin opioid neuropeptide system, the cholinergic system (muscarinic and nicotinic systems), the melatonin and serotonin [5-hydroxytryptamine receptor 2C] system, the glutamatergic system, and the hypothalamic-pituitary adrenal axis have all been implicated. Intracellular pathways and targets worthy of further study include glycogen synthase kinase-3 protein, protein kinase C, and the arachidonic acid cascade.

Circadian rhythm disturbances in depression

OBJECTIVE

The aim of this article is to review progress in understanding the mechanisms that underlie circadian and sleep rhythms, and their role in the pathogenesis and treatment of depression.

METHODS

Literature was selected principally by Medline searches, and additional reports were identified based on ongoing research activities in the authors' laboratory.

RESULTS

Many physiological processes show circadian rhythms of activity. Sleep and waking are the most obvious circadian rhythms in mammals. There is considerable evidence that circadian and sleep disturbances are important in the pathophysiology of mood disorders. Depressed patients often show altered circadian rhythms, sleep disturbances, and diurnal mood variation. Chronotherapies, including bright light exposure, sleep deprivation, and social rhythm therapies, may be useful adjuncts in non-seasonal and seasonal depression. Antidepressant drugs have marked effects on circadian processes and sleep.

CONCLUSIONS

Recent progress in understanding chronobiological and sleep regulation mechanisms may provide novel insights and avenues into the development of new pharmacological and behavioral treatment strategies for mood disorders.

Agomelatine, an innovative pharmacological response to unmet needs

Most of the available antidepressants, with different pharmacological profiles, such as inhibitors of serotonin reuptake (SSRIs) or norepinephrine reuptake (NRIs) or both (SNRIs), have limitations leading some patients to drop out of treatment. Another direction of research has therefore been undertaken, based initially on the fact that affective disorders are most often characterized by abnormal patterns of circadian rhythms. This consideration has led to the synthesis of agomelatine, a novel antidepressant combining melatonergic MT(1) and MT(2) agonism and serotonergic 5-HT(2C) antagonism. The antidepressant effects of agomelatine have been investigated in different animal models, including chronic mild stress, forced swimming, learned helplessness and psychosocial stress. All studies reported an antidepressant-like effect of agomelatine. A resynchronizing activity of agomelatine was seen in animal models for delayed sleep phase syndrome and in several original models of circadian disturbance, such as rodents infected by trypanosome or old hamsters. This activity of agomelatine on circadian rhythms was further confirmed in humans. Furthermore, several randomized, double-blind, placebo-controlled and comparator-controlled studies of agomelatine in the treatment of major depressive disorder indicate that agomelatine is effective and well tolerated.

Agomelatine: a preliminary review of a new antidepressant

Agomelatine is a new antidepressant that is a potent agonist of melatonin receptors and an antagonist of the serotonin 5-HT(2C) receptor subtype. It is in late-phase trials for the treatment of major depressive disorder (MDD). Symptoms of depression significantly improved with agomelatine compared with placebo in large placebo-controlled trials, and agomelatine appears to be as efficacious in treating MDD as other antidepressants but with fewer adverse effects. Agomelatine appears to improve sleep quality and ease of falling asleep, as measured subjectively in depressed patients. Polysomnographic studies have shown that agomelatine decreases sleep latency, decreases wake after sleep onset (WASO), and improves sleep stability as measured by changes in the cyclic alternating pattern. Agomelatine is generally well tolerated in patients with MDD; in clinical trials, adverse events were generally mild to moderate in nature, with an overall frequency close to that of placebo. Discontinuation of agomelatine because of adverse effects occurred at a similar rate to placebo.

Antidepressant-like effects of agomelatine (S 20098) in the learned helplessness model

To confirm the antidepressant-like activity of agomelatine (S 20098), a melatonin agonist and 5-hydroxytryptamine2C antagonist, already reported in the chronic mild stress and forced swimming tests, the effects of agomelatine were investigated in the learned helplessness test and compared with those of imipramine, melatonin and a selective 5-hydroxytryptamine2C antagonist, SB-242 084. Agomelatine was administered for 5 days either once a day or twice a day, and the effects of pretreatment by a melatonin receptor antagonist, S 22153 (20 mg/kg/day), were studied. A deficit in avoidance learning was observed in helpless control animals. Agomelatine (10 mg/kg/day) administered once a day significantly reduced this deficit with an effect similar to that of imipramine. Effects of agomelatine were abolished by S 22153 pretreatment. Melatonin or SB-242 084 did not reduce the deficit of helpless control animals. These results confirm the antidepressant-like activity of agomelatine and suggest a role of melatonin receptors in its mechanism of action.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Serotonin 5-HT2C receptors as a target for the treatment of depressive and anxious states: focus on novel therapeutic strategies

Serotonin (5-HT)2C receptors play an important role in the modulation of monoaminergic transmission, mood, motor behaviour, appetite and endocrine secretion, and alterations in their functional status have been detected in anxiodepressive states. Further, 5-HT2C sites are involved in the actions of several classes of antidepressant. At the onset of treatment, indirect activation of 5-HT2C receptors participates in the anxiogenic effects of selective 5-HT reuptake inhibitors (SSRIs) as well as their inhibition of sleep, sexual behaviour and appetite. Conversely, progressive down-regulation of 5-HT2C receptors parallels the gradual onset of clinical efficacy of SSRIs. Other antidepressants, such as nefazodone or mirtazapine, act as direct antagonists of 5-HT2C receptors. These observations underpin interest in 5-HT2C receptor blockade as a strategy for treating depressive and anxious states. This notion is supported by findings that 5-HT2C receptor antagonists stimulate dopaminergic and adrenergic pathways, exert antidepressant and anxiolytic actions in behavioural paradigms, and favour sleep and sexual function. In addition to selective antagonists, novel strategies for exploitation of 5-HT2C receptors embrace inverse agonists, allosteric modulators, ligands of homo/heterodimers, modulators of interactions with 'postsynaptic proteins', dual melatonin agonists/5-HT2C receptor antagonists and mixed 5-HT2C/alpha2-adrenergic antagonists. Intriguingly, there is evidence that stimulation of regionally discrete populations of 5-HT2C receptors is effective in certain behavioural models of antidepressant activity, and promotes neurogenesis in the hippocampus. This article explains how these ostensibly paradoxical actions of 5-HT2C antagonists and agonists can be reconciled and discusses both established and innovative strategies for the exploitation of 5-HT2C receptors in the improved management of depressed and anxious states.

Anxiolytic-like action of the antidepressant agomelatine (S 20098) after a social defeat requires the integrity of the SCN

In rats, social defeat by an aggressive opponent induces a state of anxiety, shown by a decrease in time spent on active explorative behaviour, an increase in immobility, a clear decrease in frequency of all active behavioural parameters (enhanced passivity). We tested the hypothesis whether acute or sub-chronic agomelatine would antagonize the negative consequences of a social defeat. As many chronobiological actions of melatonin and its receptor agonist agomelatine require the integrity of the suprachiasmatic nuclei (SCN), we examined whether the anxiolytic-like action of agomelatine 1 day after a social defeat is still present in SCN-lesioned rats. Sub-chronic administration of agomelatine caused a clear reduction of the social defeat induced behavioural consequences. A single agomelatine injection prior to the post-defeat test was less effective and a single melatonin injection was hardly effective. SCN lesion did not affect the anxiety reaction after a social defeat. Thus, sub-chronic agomelatine treatment or a single agomelatine injection reduced a state of anxiety and passivity caused by asocial defeat. The defeat-induced behavioural changes do not depend on the SCN but agomelatine showed its anxiolytic action only in sham-lesioned animals, which indicates that the anxiolytic-like action of agomelatine requires the integrity of the SCN. Mechanisms sustaining this activity are discussed.

Phase-shifts of 24-h rhythms of hormonal release and body temperature following early evening administration of the melatonin agonist agomelatine in healthy older men

OBJECTIVE

Older adults are less responsive to the phase-shifting effects of light than younger subjects and may have difficulties adapting to abrupt time shifts. This study aims to determine whether the potent melatonin agonist agomelatine (S-20098) is capable of phase-shifting overt circadian rhythms in older adults.

SUBJECTS AND DESIGN

Eight healthy elderly men participated in a double-blind, two-period, cross-over study of 15 days of daily administration of either agomelatine (50 mg) or placebo at 1830 h.

MEASUREMENTS

At the end of each treatment period, the 24-h profiles of body temperature and of the plasma levels of GH, PRL, cortisol and TSH were collected and sleep was monitored polygraphically.

RESULTS

Phase-advances, averaging nearly 2 h, were observed for the temperature profile and for the variables characterizing the temporal organization of cortisol secretion following agomelatine administration. A similar trend was observed for the circadian rise of plasma TSH. There was no effect of agomelatine on any of the sleep variables. Agomelatine stimulated GH secretion during the wake period and was associated with a transient elevation of PRL levels.

CONCLUSIONS

Melatonin agonists such as agomelatine may be useful to phase-shift at least some overt circadian rhythms in older adults.

Antidepressant action of agomelatine (S 20098) in a transgenic mouse model

The aim of this study was to evaluate the efficacy of agomelatine (S 20098) to accelerate reversal of the neuroendocrinological, behavioural and cyclical changes seen in a transgenic mouse model of the neuroendocrine characteristics of depression. The effects of agomelatine were assessed in transgenic mice with low glucocorticoid receptor (GR) function, after acute stress or induced phase shift, and compared to desipramine and melatonin. Mice were injected 2 h before the onset of the dark period with agomelatine (10 mg/kg, i.p.), desipramine (10 mg/kg, i.p.), melatonin (10 mg/kg, i.p.) or vehicle (hydroxy-ethyl-cellulose (HEC) 1%) each day for 21 to 42 days. Agomelatine was effective in reversing the transgenic mouse behavioural changes noted in the Porsolt forced swim test as well as in the elevated plus maze. Both the number of open arm entries and the total time spent in open arms of the elevated plus maze is greatly increased in transgenic mice. The mean time spent in open arms is exquisitely sensitive to reversal by agomelatine and desipramine. Agomelatine also markedly accelerated readjustment of circadian cycles of temperature and activity following an induced phase shift. This action of agomelatine was superior to that of melatonin while desipramine was without effect. The accelerating effect of agomelatine was particularly notable if treatment was started 3 weeks prior to the induced phase shift. Agomelatine treatment did not cause any major change in corticosterone or adrenocorticotropic hormone (ACTH) concentrations nor in vasopressin (AVP), corticotropin-releasing hormone (CRH), GR and mineralocorticoid receptor (MR) mRNAs levels, which make it unlikely that the mechanism of agomelatine action is related to hypothalamic-pituitary-adrenocortical (HPA) axis changes. The present study shows that agomelatine displays some characteristics of antidepressant drug action in the transgenic mouse model, effects that could be partially related to its chronobiotic properties.

Differential effects of the novel antidepressant agomelatine (S 20098) versus fluoxetine on 5-HT1A receptors in the rat brain

Agomelatine (S 20098) is a novel antidepressant drug with melatonin receptor agonist and 5-HT(2C) receptor antagonist properties, but actual mechanisms underlying its antidepressant action are unknown. Because functional desensitization of 5-HT(1A) autoreceptors in the dorsal raphe nucleus (DRN) occurs after chronic administration of several classes of antidepressants, we investigated whether this adaptive change could also be induced by agomelatine. Neither acute nor chronic treatment with agomelatine (10 mg/kg i.p. for 14 days or 50 mg/kg i.p. for 21 days) changed the density of 5-HT(1A) receptors and their coupling with G proteins in the DRN and the hippocampus in rats. Moreover, these treatments did not affect the basal electrophysiological characteristics and the responses to 5-HT(1A) receptor stimulation of DRN and hippocampal neurons in brain slices. Parallel experiments with melatonin (10 mg/kg i.p. for 14 days) and fluoxetine (5 mg/kg i.p. for 14 days) as reference compounds showed that the former was unable to affect 5-HT(1A) receptors whereas the latter decreased both the 5-HT(1A) receptor-mediated [(35)S]GTP-gamma-S binding and the potency of ipsapirone, a 5-HT(1A) receptor agonist, to inhibit neuronal firing in the DRN. These data indicate that the antidepressant action of agomelatine is not mediated through the same mechanisms as SSRIs or tricyclics.

The novel melatonin agonist agomelatine (S20098) is an antagonist at 5-hydroxytryptamine2C receptors, blockade of which enhances the activity of frontocortical dopaminergic and adrenergic pathways

Agomelatine (S20098) displayed pKi values of 6.4 and 6.2 at native (porcine) and cloned, human (h)5-hydroxytryptamine (5-HT)2C receptors, respectively. It also interacted with h5-HT2B receptors (6.6), whereas it showed low affinity at native (rat)/cloned, human 5-HT2A (<5.0/5.3) and 5-HT1A (<5.0/5.2) receptors, and negligible (<5.0) affinity for other 5-HT receptors. In antibody capture/scintillation proximity assays, agomelatine concentration dependently and competitively abolished h5-HT2C receptor-mediated activation of Gq/11 and Gi3 (pA2 values of 6.0 and 6.1). As measured by [3H]phosphatidylinositol depletion, agomelatine abolished activation of phospholipase C by h5-HT2C (pKB value of 6.1) and h5-HT2B (pKB value of 6.6) receptors. In vivo, it dose dependently blocked induction of penile erections by the 5-HT2C agonists (S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine (Ro60,0175) and 1-methyl-2-(5,8,8-trimethyl-8H-3-aza-cyclopenta[a]inden-3-yl) ethylamine (Ro60,0332). Furthermore, agomelatine dose dependently enhanced dialysis levels of dopamine in frontal cortex of freely moving rats, whereas they were unaffected in nucleus accumbens and striatum. Although the electrical activity of ventrotegmental dopaminergic neurons was unaffected agomelatine, it abolished their inhibition by Ro60,0175. Extracellular levels of noradrenaline in frontal cortex were also dose dependently enhanced by agomelatine in parallel with an acceleration in the firing rate of adrenergic cell bodies in the locus coeruleus. These increases in noradrenaline and dopamine levels were unaffected by the selective melatonin antagonist N-[2-(5-ethyl-benzo[b]thien-3-yl)ethyl] acetamide (S22153) and likely flect blockade of 5-HT2C receptors inhibitory to frontocortical dopaminergic and adrenergic pathways. Correspondingly, distinction to agomelatine, melatonin showed negligible activity 5-HT2C receptors and failed to modify the activity of adrenergic and dopaminergic pathways. In conclusion, in contrast to melatonin, agomelatine behaves as an antagonist at 5-HT2B and 5-HT2C receptors: blockade of the latter reinforces frontocortical adrenergic and dopaminergic transmission.

Effect of agomelatine in the chronic mild stress model of depression in the rat

Chronic mild stress (CMS), a well-validated model of depression, was used to study the effects of the melatonin agonist and selective 5-HT(2C) antagonist agomelatine (S 20098) in comparison with melatonin, imipramine, and fluoxetine. All drugs were administered either 2 h before (evening treatment) or 2 h after (morning treatment) the dark phase of the 12-h light/dark cycle. Chronic (5 weeks) evening treatment with agomelatine or melatonin (both at 10 and 50 mg/kg i.p.) dose-dependently reversed the CMS-induced reduction in sucrose consumption. The magnitude and time course of the action of both drugs was comparable to that of imipramine and fluoxetine (both at 10 mg/kg i.p.); however, melatonin was less active than agomelatine at this dose. The effect of evening administration of agomelatine and melatonin was completely inhibited by an acute injection of the MT(1)/MT(2) antagonist, S 22153 (20 mg/kg i.p.), while the antagonist had no effect in animals receiving fluoxetine or imipramine. When the drugs were administered in the morning, agomelatine caused effects similar to those observed after evening treatment (with onset of action faster than imipramine) but melatonin was ineffective. Moreover, melatonin antagonist, S 22153, did not modify the intakes in stressed animals receiving morning administration of agomelatine and in any other control and stressed groups tested in this study. These data demonstrate antidepressant-like activity of agomelatine in the rat CMS model of depression, which was independent of the time of drug administration. The efficacy of agomelatine is comparable to that of imipramine and fluoxetine, but greater than that of melatonin, which had no antidepressant-like activity after morning administration. While the evening efficacy of agomelatine can be related to its melatonin receptors agonistic properties, its morning activity, which was not inhibited by a melatonin antagonist, indicates that these receptors are certainly required, but not sufficient to sustain the agomelatine efficacy. It is therefore suggested that the antidepressant-like activity of agomelatine depends on some combination of its melatonin agonist and 5-HT(2C) antagonist properties.