Effect of agomelatine and its interaction with the daily corticosterone rhythm on progenitor cell proliferation in the dentate gyrus of the adult rat

The role of the circadian system in the etiology of depression

Circadian rhythms have evolved in almost all organisms enabling them to anticipate alternating changes in the environment. As a consequence, the circadian clock controls a broad range of bodily functions including appetite, sleep, activity and cortisol levels. The circadian clock synchronizes itself to the external world mainly by environmental light cues and can be disturbed by a variety of factors, including shift-work, jet-lag, stress, ageing and artificial light at night. Interestingly, mood has also been shown to follow a diurnal rhythm. Moreover, circadian disruption has been associated with various mood disorders and patients suffering from depression have irregular biological rhythms in sleep, appetite, activity and cortisol levels suggesting that circadian rhythmicity is crucially involved in the etiology and pathophysiology of depression. The aim of the present review is to give an overview and discuss recent findings in both humans and rodents linking a disturbed circadian rhythm to depression. Understanding the relation between a disturbed circadian rhythm and the etiology of depression may lead to novel therapeutic and preventative strategies.

Adverse maternal environment affects hippocampal HTR2c variant expression and epigenetic characteristics in mouse offspring

BACKGROUND

An adverse maternal environment (AME) predisposes progeny towards cognitive impairment in humans and mice. Cognitive impairment associates with hippocampal dysfunction. An important regulator of hippocampal function is the hippocampal serotonergic system. Dysregulation of hippocampal serotonin receptor 2c (HTR2c) expression is linked with cognitive impairment. HTR2c contains multiple mRNA variants and isoforms that are epigenetically regulated including DNA methylation, histone modifications, and small nucleolar RNA MBII-52. We tested the hypotheses that AME increases HTR2c variant expression and alters epigenetic modifications along the HTR2c gene locus.

METHODS

We create an AME through maternal Western diet and prenatal environmental stress in the mouse. We analyzed hippocampal HTR2c and variants' expression, DNA methylation and histone modifications along the gene locus, and MBII-52 levels in postnatal day 21 offspring.

RESULTS

AME significantly increased the expressions of total HTR2c and full-length variants (V201 and V202) concurrently with an altered epigenetic profile along the HTR2c gene locus in male offspring hippocampi. Moreover, increased full-length variants' expression in AME males was in line with increased MBII-52 levels.

CONCLUSIONS

AME affects male offspring hippocampal expression of HTR2c and full-length variants via epigenetic mechanisms. Altered hippocampal HTR2c expression may contribute to cognitive impairment seen in adult males in this model.

IMPACT

The key message of our article is that an adverse maternal environment increases expression of total HTR2c mRNA and protein, alters proportions of HTR2c mRNA variants, and impacts HTR2c epigenetic modifications in male offspring hippocampi relative to controls. Our findings add to the literature by providing the first report of altered HTR2c mRNA variant expression in association with altered epigenetic modifications in the hippocampus of offspring mice exposed to an adverse maternal environment. Our findings suggest that an adverse maternal environment affects the expression of genes previously determined to regulate cognitive function through an epigenetic mechanism in a sex-specific manner.

The effects of agomelatine on endoplasmic reticulum stress related to mitochondrial dysfunction in hippocampus of aging rat model

BACKGROUND

Agomelatine, a novel antidepressant, is a melatonin MT receptor agonist and serotonin 5HT2C receptor antagonist. In this study, agomelatine was used to investigate the molecular mechanisms of hippocampal aging associated with endoplasmic reticulum (ER) stress, mitochondrial dysfunction, and apoptosis, all of which led to short-term memory impairment.

METHOD

Hippocampal aging was induced in male Wistar rats by d-galactose (D-gal) intraperitoneal injection (100 mg/kg) for 14 weeks. During the last 4 weeks of D-gal treatment, rats were treated with agomelatine (40 mg/kg) or melatonin (10 mg/kg). At the end of the experiment, all rats were assessed for short-term memory by using the Morris water maze test. Subsequently, rats were sacrified and the hippocampus was removed from each rat for determination of reactive oxygen species (ROS), malondialdehyde (MDA), and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays; and immunohistochemistry related to ER stress, mitochondrial dysfunction, and apoptosis.

RESULTS

Agomelatine suppressed the expression of the aging-related proteins P16 and receptor for advanced glycation endproducts (RAGE), the expression of NADPH oxidase (NOX) 2 and 4, and ROS production. This treatment also shifted the morphology of astrocytes and microglia toward homeostasis. Furthermore, agomelatine decreased inositol-requiring enzyme 1 (pIRE1), protein kinase R-like endoplasmic reticulum kinase (pPERK), and chaperone binding immunoglobulin protein (BiP), leading to suppression of ER stress markers C/EBP homologous protein (CHOP) and caspase-12. Agomelatine reduced Ca2+ from the ER and stabilized the mitochondrial membrane stability, which was denoted by the BCL2 Associated X (Bax)/B-cell lymphoma 2 (Bcl2) balance. Agomelatine decreased cleaved caspase-3 production and the Terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL)-positive area, and glutamate excitotoxicity was prevented via suppression of N-methyl-d-aspartate (NMDA) receptor subunit expression. Agomelatine exhibited effects that were similar to melatonin.

CONCLUSION

Agomelatine improved neurodegeneration in a rat model of hippocampal aging by attenuating ROS production, ER stress, mitochondrial dysfunction, excitotoxicity, and apoptosis.

17β-Estradiol augments the neuroprotective effect of agomelatine in depressive- and anxiety-like behaviors in ovariectomized rats

RATIONALE AND OBJECTIVE

Estradiol decline has been associated with depression and anxiety in post-menopausal women. Agomelatine (Ago) is an agonist of the melatonergic MT1/MT2 receptors and an antagonist of the serotonergic 5-HT2c receptors. The present study aimed to evaluate the effects of combining Ago with 17β-estradiol (E2) on ovariectomy (OVX)-induced depressive- and anxiety-like behaviors in young adult female rats.

METHODS

OVX rats were treated with Ago (40 mg/kg/day, p.o.) for 10 days starting 1 week after surgery alone or combined with two doses of E2 (40 μg/kg/day, s.c.) given before behavioral testing.

RESULTS

Co-administration of E2 enhanced the anti-depressant and anxiolytics effects of Ago as evidenced by decreased immobility time in the forced swimming test, as well as increased time spent in the open arms and number of entries to open arms in the elevated plus-maze. In parallel, Ago increased hippocampal norepinephrine, dopamine, melatonin, and brain-derived neurotrophic factor (BDNF). Meanwhile, Ago-treated rats exhibited reduced hippocampal nuclear factor kappa beta (NF-kB) P65 expression and pro-inflammatory cytokine level. Ago upregulated estrogen receptor (ER α and β) mRNA expression in the hippocampus of OVX rats and elevated serum estradiol levels. Co-administration of E2 with Ago synergistically decreased NF-kB P65 expression and pro-inflammatory cytokines, and increased BDNF levels.

CONCLUSION

E2 augmented the neuroprotective effect of Ago in OVX rats via its anti-inflammatory and neurotrophic effects. The combined treatment of E2 and Ago should be further investigated as a treatment of choice for depression, anxiety, and sleep disturbances associated with menopause.

Protective Effects of Melatonin on Neurogenesis Impairment in Neurological Disorders and Its Relevant Molecular Mechanisms

Neurogenesis is the process by which functional new neurons are generated from the neural stem cells (NSCs) or neural progenitor cells (NPCs). Increasing lines of evidence show that neurogenesis impairment is involved in different neurological illnesses, including mood disorders, neurogenerative diseases, and central nervous system (CNS) injuries. Since reversing neurogenesis impairment was found to improve neurological outcomes in the pathological conditions, it is speculated that modulating neurogenesis is a potential therapeutic strategy for neurological diseases. Among different modulators of neurogenesis, melatonin is a particularly interesting one. In traditional understanding, melatonin controls the circadian rhythm and sleep-wake cycle, although it is not directly involved in the proliferation and survival of neurons. In the last decade, it was reported that melatonin plays an important role in the regulation of neurogenesis, and thus it may be a potential treatment for neurogenesis-related disorders. The present review aims to summarize and discuss the recent findings regarding the protective effects of melatonin on the neurogenesis impairment in different neurological conditions. We also address the molecular mechanisms involved in the actions of melatonin in neurogenesis modulation.

Repositioning of an anti-depressant drug, agomelatine as therapy for brain injury induced by craniotomy

Traumatic brain injury (TBI) leads to the disruption of blood-brain barrier integrity and therefore results in increased brain water content (brain edema). Brain edema is a significant factor for increased intracranial pressure (ICP), which ultimately causes functional disability and death. The decompressive craniotomy (DC) is a surgical procedure widely used for treating increased ICP following TBI. The life-saving craniotomy itself results in brain injury. The objective of this study is to investigate the effect of agomelatine against craniotomy induced brain injury. The craniotomy was performed by a variable speed micro-motor dental driller of 0.8 mm drill bit. The present study, in addition to blood-brain permeability, brain water content (edema) and histological examination of the brain, also estimated locomotor activity, oxidant, and antioxidant parameters. Results show that the craniotomy induced increase in the blood-brain barrier permeability, brain water content (edema), oxidative stress (lipid peroxide and nitric oxide) and impaired antioxidant mechanisms (superoxide dismutase, catalase, and reduced glutathione) in rats. The craniotomy was also found to increase neuronal cell death indicated by augmented chromatolysis and impaired locomotor activity. Administration of agomelatine after the craniotomy ameliorated histopathological, neurochemical and behavioral consequences of craniotomy. Thus agomelatine is effective against brain injury caused by craniotomy.

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.

Sleep and hippocampal neurogenesis: Implications for Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia and currently there are no effective disease-modifying treatments available. Hallmark symptoms of AD include impaired hippocampus-dependent episodic memory and disrupted sleep and circadian rhythms. The pathways connecting these symptoms are of particular interest because it is well established that sleep and circadian disruption can impair hippocampus-dependent learning and memory. In rodents, these procedures also markedly suppress adult hippocampal neurogenesis, a form of brain plasticity that is believed to play an important role in pattern separation, and thus episodic memory. A causal role for sleep disruptions in AD pathophysiology is suggested by evidence for sleep-dependent glymphatic clearance of metabolic waste products from the brain. This review explores a complementary hypothesis that sleep and circadian disruptions in AD contribute to cognitive decline by activating neuroendocrine and neuroinflammatory signaling pathways that suppress hippocampal neurogenesis. Evidence for this hypothesis underscores the promise of sleep, circadian rhythms, and neurogenesis as therapeutic targets for remediation of memory impairment in AD.

Effects of melatonin and its analogues on neural stem cells

Neural stem cells (NSCs) are multipotent cells which are capable of self-replication and differentiation into neurons, astrocytes or oligodendrocytes in the central nervous system (CNS). NSCs are found in two main regions in the adult brain: the subgranular zone (SGZ) in the hippocampal dentate gyrus (DG) and the subventricular zone (SVZ). The recent discovery of NSCs in the adult mammalian brain has fostered a plethora of translational and preclinical studies to investigate novel approaches for the therapy of neurodegenerative diseases. Melatonin is the major secretory product synthesized and secreted by the pineal gland and shows both a wide distribution within phylogenetically distant organisms from bacteria to humans and a great functional versatility. Recently, accumulated experimental evidence showed that melatonin plays an important role in NSCs, including its proliferation, differentiation and survival, which are modulated by many factors including MAPK/ERK signaling pathway, histone acetylation, neurotrophic factors, transcription factors, and apoptotic genes. The purpose of this review is to summarize the beneficial effects of melatonin on NSCs and further to discuss the potential usage of melatonin and its derivatives or analogues in the treatment of CNS neurodegenerative diseases.

The Neurobiology of Bipolar Disorder: An Integrated Approach

Bipolar disorder is a heterogeneous condition with myriad clinical manifestations and many comorbidities leading to severe disabilities in the biopsychosocial realm. The objective of this review article was to underline recent advances in knowledge regarding the neurobiology of bipolar disorder. A further aim was to draw attention to new therapeutic targets in the treatment of bipolar disorder. To accomplish these goals, an electronic search was undertaken of the PubMed database in August 2015 of literature published during the last 10 years on the pathophysiology of bipolar disorder. A wide-ranging evaluation of the existing work was done with search terms such as "mood disorders and biology," "bipolar disorder and HPA axis," "bipolar disorder and cytokines," "mood disorders and circadian rhythm," "bipolar disorder and oxidative stress," etc. This endeavor showed that bipolar disorder is a diverse condition sharing neurobiological mechanisms with major depressive disorder and psychotic spectrum disorders. There is convincing evidence of crosstalk between different biological systems that act in a deleterious manner causing expression of the disease in genetically predisposed individuals. Inflammatory mediators act in concert with oxidative stress to dysregulate hormonal, metabolic, and circadian homeostasis in precipitating and perpetuating the illness. Stress, whether biologically or psychologically mediated, is responsible for the initiation and progression of the diathesis. Bipolar spectrum disorders have a strong genetic component; severe life stresses acting through various paths cause the illness phenotype.

Therapeutic potential of agomelatine in epilepsy and epileptic complications

Epilepsy is a chronic neurologic disorder which often induces numerous adverse long-term neurologic effects, such as behavioral and cognitive deficits, increased predisposition to additional seizures, and cell injury or death. Cognitive dysfunction, depression, anxiety and sleep disorders are some of the highly prevalent and most disabling complications of epilepsy. The mechanisms that lead to the generation of epileptic comorbidities are poorly understood. Treatment for epileptic complications still remains a challenge because of the poor adherence and drug interactions associated with multi drug prescriptions and also for the fear of worsening seizures by the individual medications for complications. Melatonin, an endogenous hormone secreted by pineal gland has a prominent role in epilepsy. Agomelatine is a novel antidepressant which acts as melatonin MT1 and MT2 receptor agonist and serotonin 5Ht2C receptor antagonist. The combined action at MT1/2 and 5HT2C receptors, reduction in the depolarization-evoked release of glutamate, strong neuroprotective action and possible antioxidant properties of agomelatine could make it a potential agent in the treatment of epilepsy. The effect of agomelatine on hippocampal neuronal cell survival and neurogenesis, neuroprotective effect in hippocampus and frontal cortex and the antioxidant potential may contribute to the protective action of agomelatine against epilepsy induced memory decline. Agomelatine is proven to be an antidepressant and it has relieved anxiety symptoms and improved the quality of sleep in patients with depressive disorder. The action of agomelatine as a melatonin agonist and the consequent circadian resynchronizing property as well as its action as 5-HT2C receptor antagonist, could possibly suggest an antidepressant and anxiolytic action of agomelatine in epilepsy induced depressive behavior and anxiety. Since one of the many causes of sleep disruption in epilepsy is circadian rhythm disturbances and sleep promoting and circadian effects of melatonin is attributed to the MT1 and MT2 subtypes of human melatonin receptors, agomelatine may also have a promising effect on epilepsy induced sleep disruptions. Thus with all these potential pharmacological actions, agomelatine could be recommended as a potential drug to treat epilepsy and its complications.

Agomelatine, a novel intriguing antidepressant option enhancing neuroplasticity: a critical review

OBJECTIVES

The treatment of major affective disorders, commonly associated with high disability and elevated social costs may be still considered unsatisfactory. Among all antidepressant drugs, predominantly acting through monoaminergic mechanisms, agomelatine is of particular interest due to another alternative mechanism of action. Targeting melatonergic receptors, agomelatine play a crucial role in synchronizing circadian rhythms, known to be altered in depressed subjects.

METHODS

A critical review of the literature focusing on efficacy, safety and tolerability of agomelatine in major affective disorders was performed. Additionally, we focused on the potential of agomelatine in enhancing neuroplasticity mechanisms and promote neurogenesis. A total of 136 articles from peer-reviewed journals were identified, of which 50 were assessed for eligibility and 21 were included.

RESULTS

Agomelatine, a melatonergic analogue drug acting as MT1/MT2 agonist and 5-HT2C antagonist, has been reported to be effective as antidepressant drug. Studies confirmed not only clinical efficacy but also safety and tolerability of agomelatine. Also, it enhances neuroplasticity mechanisms and adult neurogenesis in brain areas such as hippocampus and prefrontal cortex.

CONCLUSIONS

Agomelatine actually represents an intriguing option in the treatment of affective disorders.

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.

Neuroplasticity and major depression, the role of modern antidepressant drugs

The pathophysiology of depression has been traditionally attributed to a chemical imbalance and critical interactions between genetic and environmental risk factors, and antidepressant drugs suggested to act predominantly amplifying monoaminergic neurotransmission. This conceptualization may be currently considered reductive. The current literature about the pathophysiological mechanisms underlying depression, stress-related disorders and antidepressant treatment was examined. In order to provide a critical overview about neuroplasticity, depression and antidepressant drugs, a detailed Pubmed/Medline, Scopus, PsycLit, and PsycInfo search to identify all papers and book chapters during the period between 1980 and 2011 was performed. Pathological stress and depression determine relevant brain changes such as loss of dendritic spines and synapses, dendritic atrophy as well as reduction of glial cells (both in number and size) in specific areas such as the hippocampus and prefrontal cortex. An increased dendritic arborisation and synaptogenesis may instead be observed in the amygdala as a consequence of depression and stress-related disorders. While hippocampal and prefrontal functioning was impaired, amygdala functioning was abnormally amplified. Most of molecular abnormalities and biological changes of aberrant neuroplasticity may be explained by the action of glutamate. Antidepressant treatment is associated with neurogenesis, gliogenesis, dendritic arborisation, new synapse formation and cell survival both in the hippocampus and prefrontal cortex. Antidepressants (ADs) induce neuroplasticity mechanisms reversing the pathological effects of depression and stress-related disorders. The neuroplasticity hypothesis may explain the therapeutic and prophylactic action of ADs representing a new innovative approach to the pathophysiology of depression and stress-related disorders.

Beneficial behavioural and neurogenic effects of agomelatine in a model of depression/anxiety

Agomelatine (S20098) is a novel antidepressant drug with melatonergic agonist and 5-HT2C receptor antagonist properties, displaying antidepressant/anxiolytic-like properties in animal models and in humans. In a depression/anxiety-like mouse model in which the response of the HPA axis is blunted, we investigated whether agomelatine could reverse behavioural deficits related to depression/anxiety compared to the classical selective serotonin reuptake inhibitor, fluoxetine. Adult mice were treated for 8 wk with either vehicle or corticosterone (35 μg/ml.d) via drinking water. During the final 4 wk, animals were treated with vehicle, agomelatine (10 or 40 mg/kg i.p.) or fluoxetine (18 mg/kg i.p.) and tested in several behavioural paradigms and also evaluated for home-cage activity. Our results showed that the depressive/anxiety-like phenotype induced by corticosterone treatment is reversed by either chronic agomelatine or fluoxetine treatment. Moreover, agomelatine increased the dark/light ratio of home-cage activity in vehicle-treated mice and reversed the alterations in this ratio induced by chronic corticosterone, suggesting a normalization of disturbed circadian rhythms. Finally, we investigated the effects of this new antidepressant on neurogenesis. Agomelatine reversed the decreased cell proliferation in the whole hippocampus in corticosterone-treated mice and increased maturation of newborn neurons in both vehicle- and corticosterone-treated mice. Overall, the present study suggests that agomelatine, with its distinct mechanism of action based on the synergy between the melatonergic agonist and 5-HT2C antagonist properties, provides a distinct antidepressant/anxiolytic spectrum including circadian rhythm normalization.

Synergistic mechanisms involved in the antidepressant effects of agomelatine

Agomelatine is a novel and clinically effective antidepressant drug with melatonergic (MT1/MT2) agonist and 5-HT(2C) receptor antagonist properties. Both receptorial components are widely expressed in the central nervous system and it seems that this compound could act synergistically on both the melatonergic and the 5-HT(2C) receptors. In this review we will briefly summarize the preclinical evidence suggesting that the molecular-cellular effects of agomelatine and in turn its antidepressant activity are the result of a synergistic action between its agonism at MT1/MT2 and antagonism at 5-HT(2C) receptors. The antidepressant properties of agomelatine related to its effect on neurogenesis, cell survival, brain-derived neurotrophic factor (BDNF), activity-regulated cytoskeleton associated protein (Arc) and stress-induced glutamate release, appear to be due to this synergistic action. Compared with traditional antidepressants which also affect these parameters, agomelatine is the only one able to resynchronize these effectors at distinct levels, circuital and intracellular. This suggests that agomelatine effects in restoring circadian rhythms and relieving depressive symptoms result from a synergistic interaction between melatonergic and serotonergic receptors.

Chronic agomelatine treatment corrects behavioral, cellular, and biochemical abnormalities induced by prenatal stress in rats

RATIONALE AND OBJECTIVES

The rat model of prenatal restraint stress (PRS) replicates factors that are implicated in the etiology of anxious/depressive disorders. We used this model to test the therapeutic efficacy of agomelatine, a novel antidepressant that behaves as a mixed MT1/MT2 melatonin receptor agonist/5-HT(2c) serotonin receptor antagonist.

RESULTS

Adult PRS rats showed behavioral, cellular, and biochemical abnormalities that were consistent with an anxious/depressive phenotype. These included an increased immobility in the forced swim test, an anxiety-like behavior in the elevated plus maze, reduced hippocampal levels of phosphorylated cAMP-responsive element binding protein (p-CREB), reduced hippocampal levels of mGlu2/3 and mGlu5 metabotropic glutamate receptors, and reduced neurogenesis in the ventral hippocampus, the specific portion of the hippocampus that encodes memories related to stress and emotions. All of these changes were reversed by a 3- or 6-week treatment with agomelatine (40-50 mg/kg, i.p., once a day). Remarkably, agomelatine had no effect in age-matched control rats, thereby behaving as a "disease-dependent" drug.

CONCLUSIONS

These data indicate that agomelatine did not act on individual symptoms but corrected all aspects of the pathological epigenetic programming triggered by PRS. Our findings strongly support the antidepressant activity of agomelatine and suggest that the drug impacts mechanisms that lie at the core of anxious/depressive disorders.

Corticotropin-releasing factor neurogenesis during midlife development in salmon: genetic, environmental and thyroid hormone regulation

Salmon parr-smolt transformation (smoltification) is a mid-life transitional stage between life in freshwater and seawater that entails a wide range of neural, endocrine and physiological modifications. In salmon, the neuroendocrine corticotropin-releasing factor (CRF) system regulates pituitary adrenocorticotrophic hormone and thyrotrophin release. Four experimental groups of Atlantic salmon, Salmo salar, were used to investigated CRF neurogenesis and its regulation during smoltification. We compared: (i) developmental stages (parr and early-smolt) in anadromous controls; (ii) a developmentally arrested model: anadromous reared under continuous light (LL) with anadromous controls; (iii) a natural hypoendocrine/incomplete smolt development salmon model (landlocked) with anadromous controls; and (iv) landlocked treated with thyroxine to anadromous control smolt levels. CRF neurogenesis between groups was studied with bromodeoxyuradine (BrdU) incorporation followed by double-labelling CRF and BrdU immunhistochemistry. The rate of CRF neurogenesis in the preoptic area (POA) increased from parr to early-smolts in anadromous salmon. By contrast, neurogenesis was inhibited in the LL group and reduced in the landlocked salmon. The administration of thyroxine in landlocked salmon to match anadromous levels increased the rate of CRF neurogenesis to anadromous levels. In conclusion, newly-formed CRF cells in the POA during smoltification are associated with increased retinal innervation to the POA and endocrine responsiveness to increased photoperiod. Both genetic and environmental factors influence the degree of salmon brain development. Thyroid hormones increase CRF neurogenesis during this critical period of development in salmon. We hypothesise that a positive-feedback of thyroid hormones on CRF neurogenesis may be an important event in reaching the developmental climax during critical periods.

Inhibition of hippocampal neurogenesis by sleep deprivation is independent of circadian disruption and melatonin suppression

Procedures that restrict or fragment sleep can inhibit neurogenesis in the hippocampus of adult rodents, although the underlying mechanism is unknown. We showed that rapid-eye-movement (REM) sleep deprivation (RSD) by the platform-over-water method inhibits hippocampal cell proliferation in adrenalectomized rats with low-dose corticosterone clamp. This procedure also greatly disrupts daily behavioral rhythms. Given recent evidence for circadian clock regulation of cell proliferation, we asked whether disruption of circadian rhythms might play a role in the anti-neurogenic effects of sleep loss. Male Sprague-Dawley rats were subjected to a 4-day RSD procedure or were exposed to constant bright light (LL) for 4 days or 10 weeks, a non-invasive procedure for eliminating circadian rhythms of behavior and physiology in this species. Proliferating cells in the granule cell layer of the dentate gyrus were identified by immunolabeling for the thymidine analogue 5-bromo-2-deoxyuridine. Consistent with our previous results, the RSD procedure suppressed cell proliferation by ∼50%. By contrast, although LL attenuated or eliminated daily rhythms of activity and sleep-wake without affecting daily amounts of REM sleep, cell proliferation was not affected. Melatonin, a nocturnally secreted neurohormone that is inhibited by light, has been shown to promote survival of new neurons. We found that 3-weeks of LL eliminated daily rhythms and decreased plasma melatonin by 88% but did not significantly affect either total cell survival or survival of new neurons (doublecortin+). Finally, we measured cell proliferation rates at the beginning and near the end of the daily light period in rats entrained to a 12:12 light/lark (LD) cycle, but did not detect a daily rhythm. These results indicate that the antineurogenic effect of RSD is not secondary to disruption of circadian rhythms, and provide no evidence that hippocampal cell proliferation and survival are regulated by the circadian system or by nocturnal secretion of pineal melatonin.