Anxiolytic properties of agomelatine, an antidepressant with melatoninergic and serotonergic properties: role of 5-HT2C receptor blockade

The phase shift hypothesis for the circadian component of winter depression

The finding that bright light can suppress melatonin production led to the study of two situations, indeed, models, of light deprivation: totally blind people and winterdepressives. The leading hypothesis for winter depression (seasonal affective disorder, or SAD) is the phase shift hypothesis (PSH). The PSH was recently established in a study in which SAD patients were given low-dose melatonin in the afternoon/evening to cause phase advances, or in the morning to cause phase delays, or placebo. The prototypical phase-delayed patient as well as the smaller subgroup of phase-advanced patients, optimally responded to melatonin given at the correct time. Symptom severity improved as circadian misalignment was corrected. Orcadian misalignment is best measured as the time interval between the dim light melatonin onset (DLMO) and mid-sleep. Using the operational definition of the plasma DLMO as the interpolated time when melatonin levels continuously rise above the threshold of 10 pglmL, the average interval between DLMO and mid-sleep in healthy controls is 6 hours, which is associated with optimal mood in SAD patients.

Efficacy of agomelatine, a MT1/MT2 receptor agonist with 5-HT2C antagonistic properties, in major depressive disorder

Current antidepressants used in major depressive disorder (MDD) are still not efficacious enough for many patients due to high levels of treatment resistance and bothersome side-effects. Using a novel blinding method (interactive voice response system), this flexible-dosing study examined the effects of therapeutic doses of agomelatine, a new approach to depressive therapy offering potent melatonergic MT1/MT2 receptor agonism with 5-HT2C receptor antagonist properties, in patients with moderate-to-severe MDD. This 6-wk, double-blind, parallel-group study randomized 238 patients to 25 mg/d agomelatine (with dose adjustment at 2 wk to 50 mg/d in patients with insufficient improvement) or placebo. Depression severity was assessed using the Hamilton Depression Rating Scale (HAMD) and the Clinical Global Impression (CGI) scale. Agomelatine was significantly more efficacious than placebo, with an agomelatine-placebo difference of 3.44 (p<0.001) using the HAMD final total score. Compared with placebo, agomelatine also had a significant positive impact on CGI - Improvement (treatment difference=0.45) and CGI - Severity (treatment difference=0.50) (both p=0.006), response rate (54.3% vs. 35.5% with placebo, p<0.05) and time to first response (p=0.008). Similar results were seen in patients with the most severe MDD. Depressed mood and sleep items of the HAMD were also significantly improved with agomelatine, which was well tolerated with a safety profile similar to placebo at both doses. This study confirms that agomelatine is effective in treating major depression, including the most severely depressed patients, with a good safety and tolerability profile, therefore providing physicians with an effective pharmacological approach to antidepressant therapy.

Evidence of agomelatine's antidepressant efficacy: the key points

Depressive disorders are of the highest socioeconomic and health-economic importance, as they are the psychiatric disorders that most frequently cause psychosocial disability. Despite the progress that has been made, currently available pharmacotherapies for depression still have a limited antidepressant efficacy with a delayed onset of several weeks, and still cause side effects; these unmet needs represent important reasons to continue to search for novel treatment options. A disorganization of circadian rhythms has been suggested to play an important role in the pathophysiology of major depression, and complaints regarding disturbed sleep are frequent in depressed patients. As endogenous melatonin secretion underlies the regulation of circadian rhythms, compounds with activity at melatonergic receptors have been proposed as potential novel therapeutics. Agomelatine (S-20098), a compound with agonistic properties at MT1 and MT2 receptors and antagonistic properties at the 5-HT2C receptor, has been shown preclinically to exhibit robust antidepressant effects in several experimental paradigms. Clinical trials, including phase III studies, have now demonstrated the superior efficacy of agomelatine in comparison with placebo, and a similar efficacy in comparison with active comparators, for the treatment of major depression. Agomelatine was even effective in severely depressed patients. In all studies published so far, agomelatine was found to be safe and its overall tolerability profile was superior to that of selective serotonin reuptake inhibitors and selective serotonin and noradrenaline reuptake inhibitors.

Augmentation of SSRI effects on serotonin by 5-HT2C antagonists: mechanistic studies

The treatment of depression may be improved by using an augmentation approach involving selective serotonin reuptake inhibitors (SSRIs) in combination with compounds that focus on antagonism of inhibitory serotonin receptors. Using microdialysis coupled to HPLC, it has recently been shown that the systemic co-administration of 5-HT(2C) antagonists with SSRIs augmented the acute effect of SSRIs on extracellular 5-HT. In this paper, we have investigated the mechanism through which this augmentation occurs. The increase in extracellular 5-HT was not observed when both compounds were locally infused. However, varying the route of administration for both compounds differentially revealed that an augmentation took place when the 5-HT(2C) antagonist was locally infused into ventral hippocampus and the SSRI given systemically, but not when systemic 5-HT(2C) antagonist was co-administered with the local infusion of citalopram. This suggests that the release of extracellular serotonin in ventral hippocampus may be controlled by (an)other brain area(s). As 5-HT(2C) receptors are not considered to be autoreceptors, this would implicate that other neurotransmitter systems are involved in this process. To investigate which neurotransmitter systems were involved in the interaction, systemic citalopram was challenged with several glutamatergic, GABA-ergic, noradrenergic, and dopaminergic compounds to determine their effects on serotonin release in ventral hippocampus. It was determined that the involvement of glutamate, norepinephrine, and dopamine in the augmentation did not seem likely, whereas evidence implicated a role for the GABA-ergic system in the augmentation.

Serotonin 5-HT(2C) receptors regulate anxiety-like behavior

Central serotonin (5-hydroxytryptamine, 5-HT) systems have been implicated in the pathophysiology and treatment of anxiety disorders, which are among the world's most prevalent psychiatric conditions. Here, we report that the 5-HT(2C) receptor (5-HT(2C)R) subtype is critically involved in regulating behaviors characteristic of anxiety using male 5-HT(2C)R knockout (KO) mice. Specific neural substrates underlying the 5-HT(2C)R KO anxiolytic phenotype were investigated, and we report that 5-HT(2C)R KO mice display a selective blunting of extended amygdala corticotropin-releasing hormone neuronal activation in response to anxiety stimuli. These findings illustrate a mechanism through which 5-HT(2C)Rs affect anxiety-related behavior and provide insight into the neural circuitry mediating the complex psychological process of anxiety.

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.

Agomelatine: an antidepressant with a novel mechanism of action

Despite advances in understanding potential disease mechanisms in depression and in therapeutic approaches to the management of major depressive disorder, the disease continues to carry an enormous personal, social and economic burden. Agomelatine is a melatonergic antidepressant with melatonin (MT1 and MT2) agonistic and serotonin (5-HT2C) antagonistic properties. It represents a promising approach to treating depression through a unique synergistic mechanism. There is also preliminary evidence to suggest that agomelatine has anxiolytic properties in depressed patients. Clinical data confirm that agomelatine, in a dose range of 25–50 mg daily, is an effective antidepressant with a very favorable side-effect profile. In particular, sleep restorative effects in the absence of daytime sedation and a favorable effect on sexual function suggest that agomelatine will offer a high efficacy–tolerability index for patients with major depressive disorder. Such results have been anticipated based on preclinical studies using various animal models of both depression and anxiety.

Circadian genes, rhythms and the biology of mood disorders

For many years, researchers have suggested that abnormalities in circadian rhythms may underlie the development of mood disorders such as bipolar disorder (BPD), major depression and seasonal affective disorder (SAD). Furthermore, some of the treatments that are currently employed to treat mood disorders are thought to act by shifting or "resetting" the circadian clock, including total sleep deprivation (TSD) and bright light therapy. There is also reason to suspect that many of the mood stabilizers and antidepressants used to treat these disorders may derive at least some of their therapeutic efficacy by affecting the circadian clock. Recent genetic, molecular and behavioral studies implicate individual genes that make up the clock in mood regulation. As well, important functions of these genes in brain regions and neurotransmitter systems associated with mood regulation are becoming apparent. In this review, the evidence linking circadian rhythms and mood disorders, and what is known about the underlying biology of this association, is presented.

Anxiogenic-like effects of mCPP microinfusions into the amygdala (but not dorsal or ventral hippocampus) in mice exposed to elevated plus-maze

Serotonin (5-HT) can either increase or decrease anxiety-like behaviour in animals, actions that depend upon neuroanatomical site of action and 5-HT receptor subtype. Although systemic studies with 5-HT(2) receptor agonists and antagonists suggest a facilitatory role for this receptor subtype in anxiety, somewhat inconsistent results have been obtained when such compounds have been directly applied to limbic targets such as the hippocampus and amygdala. The present study investigated the effects of the 5-HT(2B/2C) receptor agonist mCPP bilaterally microinjected into the dorsal hippocampus (DH: 0, 0.3, 1.0 or 3.0nmol/0.2microl), the ventral hippocampus (VH: 0, 0.3, 1.0 or 3.0nmol/0.2microl) or the amygdaloid complex (0, 0.15, 0.5, 1.0 or 3.0nmol/0.1microl) in mice exposed to the elevated plus-maze (EPM). Test sessions were videotaped and subsequently scored for conventional indices of anxiety (percentage of open arm entries and percentage of open arm time) and locomotor activity (closed arm entries). Results showed that mCPP microinfusions into the DH or VH failed to affect any behavioural measure in the EPM. However, when injected into the amygdaloid complex, the dose of 1.0nmol of this 5HT(2B/2C) receptor agonist increased behavioural indices of anxiety without significantly altering general activity levels. This anxiogenic-like effect of mCPP was selectively and completely blocked by local injection of a behaviourally-inactive dose of SDZ SER-082 (10nmol/0.1microl), a preferential 5-HT(2C) receptor antagonist. These data suggest that 5HT(2C) receptors located within the amygdaloid complex (but not the dorsal or ventral hippocampus) play a facilitatory role in plus-maze anxiety in mice.

Role for the Clock gene in bipolar disorder

Nearly all patients with bipolar disorder have severely disrupted circadian rhythms. Treatment with mood stabilizers can restore these daily rhythms, and this is correlated with patient recovery. However, it is still uncertain whether clock abnormalities are the cause of bipolar disorder or if these rhythm disruptions are secondary to alterations in other circuits. Furthermore, the mechanism by which the circadian clock might influence mood is still unclear. With cloning and characterization of the circadian genes and recent advances in molecular biology, we are starting to understand this strong association between circadian rhythms and bipolar disorder. Recent human genetic and mouse behavioral studies indicate that the Clock gene is particularly relevant in the mood disruptions associated with this disorder. Furthermore, it appears that Clock expression outside of the central pacemaker of the suprachiasmatic nucleus (SCN) is involved in mood regulation. In this chapter, the evidence linking circadian rhythms, the Clock gene, and bipolar disorder is discussed, along with the possible biology that underlies this connection.

Interactions of buprenorphine and dipotassium clorazepate on anxiety and memory functions in the mouse

Buprenorphine, a partial mu-receptor agonist widely substituted for heroin in the treatment of addiction, is often misused in combination with benzodiazepines. Improved hedonic properties may result, but only at the cost of increased buprenorphine toxicity. In order to elucidate the appeal of the benzodiazepine-buprenorphine combination, the present study looked at its neuropsycho-pharmacological effects on various emotional and cognitive parameters in the mouse. On the basis of previous dose-response studies, the regimen used was buprenorphine 0.3mg/kg, s.c. plus dipotassium clorazepate 1, 4 and 16 mg/kg, i.p. Anxiety-like behaviour was assessed using the black and white test box, and memory processes were examined via the spontaneous alternation paradigm in the Y-maze, and passive avoidance tests. Spontaneous locomotor activity was also evaluated. High doses of clorazepate impaired buprenorphine-induced hyperactivity and anxiogenic-like effects. They also increased buprenorphine-induced spontaneous alternation impairment, but did not modify its impact on long-term memory processes. These results suggest that the positive reinforcement experienced with the buprenorphine-benzodiazepine combination may be attributable, at least in part, to an increase in buprenorphine's sedative effect associated with a decrease in anxiogenicity.

Towards the Development of Mixed MT1-Agonist/MT2-Antagonist Melatonin Receptor Ligands

Herein we report attempts to optimize the pharmacological properties of 5-(2-hydroxyethoxy)-N-acetyltryptamine (5-HEAT), a melatonin receptor ligand previously described by us. Several 5-substituted and 2,5-disubstituted N-acyltryptamines were synthesized and evaluated in vitro for the human cloned MT(1) and MT(2) receptors. From this series of N-acyltryptamines the 2-bromo derivative (5 c) retains the interesting efficacy profile of 5-HEAT and shows increased melatonin receptor affinities; it represents one of the first examples of a high-affinity MT(1) agonist/MT(2) antagonist. Some other full agonists for both melatonin receptors which exhibit similar or increased affinity relative to that of melatonin were obtained.

Melatonin affects the immobility time of rats in the forced swim test: the role of serotonin neurotransmission

The efficacy of melatonin or its derivatives in depressive patients has been recently considered for clinical application. However, the evidence for its effect on experimental models of depression is not consolidated. Here, the effects of melatonin on the model of forced swim test (FST) paradigm were studied in male rats of the Wistar strain after acute intraperitoneal (i.p.) administration of 0.1, 0.5 or 1 mg/kg of the hormone. Melatonin at doses of 0.5 and 1 mg/kg, but not of 0.1 mg/kg, decreased the immobility of rats in the FST paradigm suggesting a possible antidepressant-like activity. The dose of 0.5 mg/kg appeared to be as potent as clomipramine 50 mg/kg in reducing the immobility time of rats in the FST paradigm. The effect of melatonin on immobility time of rats in the FST paradigm was abolished by the simultaneous injection of the non-selective melatonin antagonist, luzindole (0.25 mg/kg, subcutaneously). Similarly, administration of small quantities of serotonin (5-HT, 5 ng/1 microl) or of the 5-HT(2A)/5-HT(2C) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (2 ng/1 microl) injected into the amygdale totally suppressed the reduction of immobility time in the FST paradigm induced by melatonin 0.5 mg/kg. These results may suggest that effects of melatonin on the behavioral reaction of rats in the FST paradigm are due to an interaction of the hormone with central 5-HT neurotransmission.

Melatonin and human rhythms

Melatonin signals time of day and time of year in mammals by virtue of its pattern of secretion, which defines 'biological night.' It is supremely important for research on the physiology and pathology of the human biological clock. Light suppresses melatonin secretion at night using pathways involved in circadian photoreception. The melatonin rhythm (as evidenced by its profile in plasma, saliva, or its major metabolite, 6-sulphatoxymelatonin [aMT6s] in urine) is the best peripheral index of the timing of the human circadian pacemaker. Light suppression and phase-shifting of the melatonin 24 h profile enables the characterization of human circadian photoreception, and circulating concentrations of the hormone are used to investigate the general properties of the human circadian system in health and disease. Suppression of melatonin by light at night has been invoked as a possible influence on major disease risk as there is increasing evidence for its oncostatic effects. Exogenous melatonin acts as a 'chronobiotic.' Acutely, it increases sleep propensity during 'biological day.' These properties have led to successful treatments for serveal circadian rhythm disorders. Endogenous melatonin acts to reinforce the functioning of the human circadian system, probably in many ways. The future holds much promise for melatonin as a research tool and as a therapy for various conditions.

Effects of melatonin and agomelatine in anxiety-related procedures in rats: interaction with diazepam

The anxiolytic potential of melatonin and agomelatine, a potent MT(1/2) receptor agonist, and their combined effects with diazepam, were investigated in rats using the punished drinking test, the safety signal withdrawal operant paradigm, the elevated-plus-maze and hypophagia-induced novelty. In the punished drinking test, evening injections of melatonin (80 mg/kg, IP, but not 20 and 40 mg/kg) and agomelatine (40 mg/kg, IP) increased the number of foot shocks received. However, neither melatonin (40-80 mg/kg) nor agomelatine (20-40 mg/kg) released response suppression during the period associated with the safety signal withdrawal and affected rats' behaviour in the elevated-plus-maze. Furthermore, agomelatine (40 mg/kg) did not enhance food consumption in unfamiliar environment. However, the co-administration of melatonin (80 mg/kg) or agomelatine (20-40 mg/kg) with diazepam, at a dose (0.25 mg/kg) inactive on its own, induced an anxiolytic-like effect in the punished drinking test and the elevated plus-maze. These results indicate that, although mostly devoid of anxiolytic-like action per se, melatonin and agomelatine can potentiate the anxiolytic effects of diazepam.

Agomelatine, a new antidepressant, induces regional changes in hippocampal neurogenesis

BACKGROUND

Antidepressant treatments increase neural plasticity and adult neurogenesis, especially in the hippocampus. Here, we determined the effects of agomelatine (S-20098), a new antidepressant, on various phases of neurogenesis in the dentate gyrus of adult rat.

METHODS

Animals were injected with agomelatine for different time periods. Immunostaining for bromodeoxyuridine, neuron specific nuclear protein, and glial fibrillary acid protein, as well as for the highly polysialylated form of neuronal cell adhesion molecule and doublecortin, was used to detect changes in cell proliferation, neurogenesis, and survival. Cell death was estimated by terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick end labeling and cresyl violet staining.

RESULTS

Chronic (3 weeks) but not acute (4 hours) or subchronic (1 week) administration of agomelatine increased cell proliferation and neurogenesis in the ventral dentate gyrus, a region notably implicated in response to emotion, which is consistent with the antidepressant-anxiolytic properties of the drug. Extending agomelatine treatment over several weeks, however, increases survival of newly formed neurons in the entire dentate gyrus. Finally, agomelatine treatment does not affect mature granule cells.

CONCLUSIONS

This study shows that an antidepressant can affect differentially various stages of neurogenesis in the dorsal and ventral hippocampus. Altogether, these changes lead to a pronounced augmentation in the total number of new granule cells.

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.

Reassessing the melatonin pharmacophore—Enantiomeric resolution, pharmacological activity, structure analysis, and molecular modeling of a constrained chiral melatonin analogue

3-(Acetylaminomethyl)-2-(ethoxycarbonyl)-6-methoxy-1,3,4,5-tetrahydrobenzo[cd]indole (2) is a rigid melatonin analogue that as a racemate displays about the same affinity and intrinsic activity of melatonin (1) in in vitro experiments. We report here the resolution of the racemate by preparative medium pressure liquid chromatography (MPLC) and the X-ray determination of the R absolute configuration of the (-)-enantiomer. The two enantiomers were separately tested as MT1 and MT2 ligands, and the (+)-(S)-2 showed a potency comparable to that of melatonin and about three orders of magnitude greater than that of its enantiomer. The information obtained by crystallographic analysis and NMR studies about the conformational preference for 2 and by the pharmacological characterization of (R)-2 and (S)-2 was employed in a molecular modeling study, aimed at reassessing the melatonin receptor pharmacophore model for agonist compounds. Chiral enantioselective agonists reported in the literature were also included in the study.

Prospects for the treatment of depression

Antidepressant drugs represent the principal form of treatment for major depressive disorder. While there are a plethora of medications available for this task, current drugs have many shortcomings. In the face of these deficiencies there is an ongoing search for new agents. The search has been guided, in part, by drug design based on existing agents and their putative mechanism of action. This has been less than fruitful in addressing inadequacies of existing medications as it has not produced compounds which are novel in terms of pharmacological mechanisms. Recent insights from molecular biological approaches hold promise for the discovery of novel compounds, in particular the so-called neurogenesis hypothesis suggests novel therapeutic approaches. Although significantly modified over the years, the monoamine hypothesis of depression and antidepressant drug action still remains an important driving force behind the development of new compounds. Several recently marketed agents and some in early-phase development tend to conform to these existing mechanistic hypotheses. Clearly the place of these agents in the treatment of depression is dependent on issues such as short- and long-term safety and efficacy. Duloxetine has been developed as a dual monoamine re-uptake inhibitor. Agomelatine is a compound with major effects on the circadian system as well as effects on subtypes of the serotonin receptor system. While the mechanism of action of this compound is not certain, recent evidence would suggest that the drug exerts its effects through antagonist actions at serotonin receptors. Compounds based on the hypothalamic pituitary adrenal axis, substance P antagonism and other neuropeptides have potential application for the treatment of depression but require further development before that potential is realized.

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.

Clinical efficacy of agomelatine in depression: the evidence

Despite the advances of recent decades, there is still an urgent need for antidepressants with improved efficacy, safety and tolerability. Agomelatine is a new antidepressant with an innovative pharmacological profile. It is the first melatonergic antidepressant, and is a potent agonist of melatonin receptors (MT1 and MT2) with 5-HT2C antagonist properties. The efficacy of 25 mg/day agomelatine in treating major depressive disorder (MDD) has been demonstrated in a number of placebo-controlled studies. Evidence of improvement in depressive symptoms was observed in a dose-ranging study in which 25 mg/day agomelatine was significantly better than placebo, whatever the rating scale used (Hamilton Rating Scale for Depression, Clinical Global Impression, and Montgomery-Asberg Depression Rating Scale). These results have been confirmed in two similarly designed placebo-controlled studies. Agomelatine also produces a significant improvement in anxiety compared to placebo, according to Hamilton Rating Scale for Anxiety scores. The efficacy of agomelatine has been studied in subpopulations with more severe depression, demonstrating its efficacy in these difficult-to-treat patients. In view of the available data on agomelatine, this antidepressant can be regarded as an innovative treatment for MDD patients, offering a new approach in the management of depressed patients.

Comment déveloper un antidépresseur au mécanisme d’action innovant : l’exemple de l’agomélatine

There are now many potentials for the development of more effective, better tolerated, and more rapidly acting antidepressants acting in association and/or beyond the monoamine hypothesis. One of these possibilities is the development of antidepressant drugs with melatonin agonist property. This holds much promise since various affective disorders, including depression, are characterized by abnormal patterns of circadian rhythms. In line with this, the melatoninergic agonist properties of agomelatine, an antidepressant with proven clinical efficacy, may represent a new concept for the treatment of depression. By way of behavioral studies in rodents, it has been shown that administration of agomelatine can mimic the action of melatonin in the synchronization of circadian rhythm patterns. Interest in agomelatine has increased in recent times due to its prospective use as a novel antidepressant agent, as demonstrated in a number of animal studies using well-validated animal models of depression (including the forced swimming test, the learned helplessness, the chronic mild stress). Interestingly, the melatoninergic agonist property of agomelatine may not, alone, be sufficient to sustain its clear antidepressant-like activity. Recent results from receptor binding and in vivo studies gave support to the notion that agomelatine's effects are also mediated via its function as a competitive antagonist at the 5-HT2C receptor. Finally, thanks to its absence of binding with a broad range of receptors and enzymes, agomelatine is particularly safe and devoid of all the deleterious effects reported with tricyclics and SSRIs.

Antidepressant-like effects of agomelatine, melatonin and the NK1 receptor antagonist GR205171 in impulsive-related behaviour in rats

RATIONALE

Substance P receptor [neurokinin1 (NK1-R)] antagonists and melatonin(1/2) receptor (MT(1/2)-R) agonists have been claimed to be potential antidepressants (ADs). In animals, these compounds are active in validated models responsive to ADs, such as forced swimming test and chronic mild stress paradigms. Classical AD drugs are also known to be effective in pathologies characterized by an impulse control deficiency. In line with this clinical observation, previous studies demonstrated that classical ADs increased the capacity to wait for food reward in rats subjected to a paradigm aimed at assessing impulsive-related behaviour.

OBJECTIVES

This study was conducted to investigate the effects of two MT(1/2)-R agonists, melatonin and agomelatine, and a NK1-R antagonist, GR205171, on tolerance to delay of food reward in rats.

METHODS

Fasting rats were trained in a T-maze and allowed to choose between two magnitudes of reward: immediate but small reward (two pellets) vs 25-s delayed but large reward (ten pellets). Under this alternative, vehicle-injected rats selected the large-but-delayed reinforcer in less than 40% of the trials.

RESULTS

Like the established ADs clomipramine (8 mg kg(-1), i.p.) and fluvoxamine (4 mg kg(-1), i.p.), melatonin (3 and 10 mg kg(-1), i.p.), agomelatine (10 and 30 mg kg(-1), i.p.) and GR205171 (30 mg kg(-1) but not 10 mg kg(-1), s.c.) significantly increased the number of choices of the large-but-delayed reward. The effect of melatonin (3 mg kg(-1), i.p.) was not counteracted by the MT(1/2)-R antagonist S22153 (40 mg kg(-1), i.p.) that exerted no effect on its own.

CONCLUSION

These results suggest that MT(1/2)-R agonists and NK1-R antagonists enhance rats' tolerance to delay of gratification, an effect which may reflect their ability to improve impulse control. Further investigations are necessary to clarify the neurobiological mechanisms responsible for this effect.

The regional and cellular expression profile of the melatonin receptor MT1 in the central dopaminergic system

The physiological effects of pineal melatonin are primarily mediated by melatonin receptors located in the brain and periphery. Even though there are a number of studies demonstrating the regulatory role of melatonin in the development of dopaminergic behaviors, such as psychostimulant-induced diurnal locomotor sensitization or drug seeking, little is known about the contribution of melatonin receptors (i.e., MT1) to this role. Therefore, as a first step in understanding the functional role of melatonin receptors in dopaminergic behaviors, we focused on determining the expression pattern of MT1 receptors in the dopaminergic system of the human and rodent brain. Regional (e.g., nucleus accumbens shell) and cellular (e.g., tyrosine hydroxylase immunopositive cells) expression of MT1 mRNA was characterized by applying the immuno-laser capture microdissection (immuno-LCM) technique coupled with nested RT-PCR. Moreover, employing quantitative Western immunoblotting and RT-PCR, we found that the mouse MT1 receptor expression presents diurnal variations (i.e., low mRNA and high protein levels at night, ZT21). The dopaminergic system-based presence of MT1 receptor proteins was not limited to rodents; we found these receptors in postmortem human brain as well. Further research is needed to understand the regional/cellular functional role of melatonin receptors in the regulation of dopaminergic behaviors, using models such as melatonin receptor knockout mice.

Pharmacotherapy of seasonal affective disorder

Seasonal affective disorder is a common variant of recurrent major depressive disorder or bipolar disorder. Treatment with bright artificial light has been found to be effective in this condition. However, for patients who do not respond to light therapy or those who lack compliance, conventional drug treatment with antidepressants also has been proposed. Substances with selective serotonergic or noradrenergic mechanisms should be preferred over older antidepressants. Although there are a number of open and controlled studies evaluating different compounds, these studies were often limited by relatively small sample sizes. Furthermore, there are no studies specifically addressing bipolar seasonal depression. This article will review the published literature on pharmacotherapy of seasonal affective disorder.