Consequences of long-term treatment with agomelatine on depressive-like behavior and neurobiological abnormalities in pinealectomized rats

In Vitro Effects of Fentanyl on Aortic Viscoelasticity in a Rat Model of Melatonin Deficiency

Melatonin influences arterial biomechanics, and its absence could cause remodeling of the arterial wall, leading to increased stiffness. Direct effects of fentanyl on the aortic wall have also been observed previously. This study aimed to evaluate in vitro the effects of fentanyl on aortic viscoelasticity in a rat model of melatonin deficiency and to test the hypothesis that melatonin deficiency leads to increased arterial wall stiffness. The viscoelasticity was estimated in strip preparations from pinealectomized (pin, melatonin deficiency) and sham-operated (sham, normal melatonin) adult rats using the forced oscillations method. In the untreated aortic wall pin, the viscoelasticity was not significantly altered. However, combined with 10-9 M fentanyl, the pin increased the natural frequency (f0) and modulus of elasticity (E') compared to the sham-operated. Independently, fentanyl treatment decreased f0 and E' compared separately to untreated sham and pin preparations. The effects of fentanyl were neither dose-dependent nor affected by naloxone, suggesting a non-opioid mechanism. Furthermore, an independent effect of naloxone was also detected in the normal rat aortic wall, resulting in reduced E'. Additional studies are needed that may improve the clinical decisions for pain management and anesthesia for certain patients with co-occurring chronic low levels of blood plasma melatonin and some diseases.

The role of melatonin deficiency induced by pinealectomy on motor activity and anxiety responses in young adult, middle-aged and old rats

BACKGROUND

Aging affects anxiety levels in rats while the pineal gland, via its hormone melatonin, could modulate their inherited life "clock." The present study aimed to explore the impact of plasma melatonin deficiency on anxiety responses and the possible involvement of the hypothalamic-pituitary-adrenocortical (HPA) axis and heat shock proteins (Hsp) 70 and 90 in the frontal cortex (FC) and the hippocampus in young adult, middle-aged and elderly rats with pinealectomy.

RESULTS

Melatonin deficiency induced at different life stages did not affect the lifespan of rats. Pinealectomy abolished the circadian rhythm of motor activity, measured for 48 h in the actimeter, in young adult but not in middle-aged rats. Pinealectomy reduced the motor activity of the young adult rats during the dark phase and impaired the diurnal activity variations of old rats. The same generations (3- and 18 month-old rats with pinealectomy) had lower anxiety levels than the matched sham groups, measured in three tests: elevated-plus maze, light-dark test, and novelty-suppressed feeding test. While the activity of the HPA axis remained intact in young adult and middle-aged rats with melatonin deficiency, a high baseline corticosterone level and blunted stress-induced mechanism of its release were detected in the oldest rats. Age-associated reduced Hsp 70 and 90 levels in the FC but not in the hippocampus were detected. Pinealectomy diminished the expression of Hsp 70 in the FC of middle-aged rats compared to the matched sham rats.

CONCLUSIONS

Our results suggest that while melatonin hormonal dysfunction impaired the motor activity in the actimeter and emotional behavior in young adult and elderly rats, the underlying pathogenic mechanism in these generations might be different and needs further verification.

Physiological and pharmacological perspectives of melatonin

The pineal gland is a interface between light-dark cycle and shows neuro-endocrine functions. Melatonin is the primary hormone of pineal gland, secreted at night. The night-time melatonin peak regulates the physiological functions at dark. Melatonin has several unique features as it synchronises internal rhythm with daily and seasonal variations, regulates circadian rhythm and sleep-wake cycle. Physiologically melatonin involves in detoxification of free radicals, immune functions, neuro-protection, oncostatic effects, cardiovascular functions, reproduction, and foetal development. The precise functions of melatonin are exhibited by specific receptors. In relation to pathophysiology, impaired melatonin secretion promotes sleep disorder, cancer progression, type-2 diabetes, and neurodegenerative diseases. Several reports have highlighted the therapeutic benefits of melatonin specially related to cancer protection, sleep disorder, psychiatric disorders, and jet lag problems. This review will touch the most of the area of melatonin-oriented health impacts and its therapeutic aspects.

Agomelatine decreases cocaine-induced locomotor sensitisation and dopamine release in rats

BACKGROUND

Agomelatine is a melatoninergic antidepressant approved to treat the major depressive disorder. Agomelatine exerts its behavioural, pharmacological, and physiological effects through the activation of MT₁ and MT₂ melatonin receptors and the blockade of 5-HT_2B and 5-HT_2C serotonin receptors. Some studies have reported that the activation of the MT₁ and MT₂ melatonin receptors decreased cocaine-induced locomotor activity and cocaine self-administration. These findings from another study showed that agomelatine decreased alcohol consumption. This study aimed to evaluate the effects of agomelatine administration on cocaine-induced behavioural (cocaine-induced locomotor activity and cocaine-induced locomotor sensitisation) and neurochemical (dopamine levels) effects.

METHODS

Male Wistar rats (250-280 g) received cocaine (10 mg/kg) during the induction and expression of locomotor sensitisation. Agomelatine (10 mg/kg) was administered 30 minutes before cocaine. After each treatment, locomotor activity was recorded for 30 minutes. Dopamine levels were determined in the ventral striatum, the prefrontal cortex (PFC), and the ventral tegmental area (VTA) by high-performance liquid chromatographic (HPLC) in animals treated with agomelatine and cocaine. Luzindole (30 mg/kg) was administered to block the agomelatine effect.

RESULTS

In this study, we found that agomelatine decreased cocaine-induced locomotor activity and the induction and expression of locomotor sensitisation. In addition, agomelatine decreased cocaine-induced dopamine levels. Luzindole blocked the agomelatine-induced decrease in the expression of locomotor sensitisation in rats.

CONCLUSION

Our results suggest (1) that agomelatine showed efficacy in decreasing cocaine psychostimulant effects and (2) that agomelatine can be a useful therapeutic agent to reduce cocaine abuse.

Night shift hormone: How does melatonin affect depression?

Melatonin is the main hormone secreted by the pineal gland that modulates the circadian rhythm and mood. Previous studies have shown the therapeutic effects of melatonin, or its important analogue, agomelatine, on depression. In this review study, we aimed to discuss the potential mechanisms of melatonin involved in the treatment of depression. It was noted that disrupted circadian rhythm can lead to depressive state, and melatonin via regulating circadian rhythm shows a therapeutic effect. It was also noted that melatonin induces antidepressant effects via promoting antioxidant system and neurogenesis, and suppressing oxidative stress, neuroinflammation, and apoptosis. The interaction effect between melatonin or agomelatine and serotonergic signaling has a significant effect on depression. It was noted that the psychotropic effects of agomelatine are induced by the synergistic interaction between melatonin and 5-HT_2C receptors. Agomelatine also interacts with glutamatergic signaling in brain regions involved in regulating mood and circadian rhythm. Interestingly, it was concluded that melatonin exerts both pro- and anti-inflammatory effects, depending on the grade of inflammation. It was suggested that synergistic interaction between melatonin and 5-HT_2C receptors may be able to induce therapeutic effects on other psychiatric disorders. Furthermore, dualistic role of melatonin in regulating inflammation is an important point that can be examined at different levels of inflammation in animal models of depression.

Metabolic footprint in young, middle-aged and elderly rats with melatonin deficit

The pineal gland is suggested to be an essential area involved in the programming of fertility, growth, aging, and death of mammals via the released hormone melatonin.The present study aimed to ascertain the effect of melatonin deficit on several physiological and metabolic parameters, closely associated with the aging process, at certain stages of ontogenesis. Sham and rats with pinealectomy, operated at ages 3, 14, and 18-months, respectively, were tested two months later. Sham rats demonstrated an age-related decline of muscle strength, exercise endurance, motor activity, food intake, calorimetric parameters, and impaired lipid profile. Pinealectomy reduced the maximal time to exhaustion and body weight gain while diminished motor activity, food intake, O₂ consumption, CO₂ production, and energy expenditure during the Dark phase in the youngest rat group. In addition, melatonin deficit elevated arterial blood pressure (systolic, diastolic, and mean arterial pressure) and increased serum glucose and triglyceride level in 3-month-old rats while decreased the liver enzyme activity in 14-month-old rats. In conclusion, the present study brought new insights confirming the complex impact of melatonin deficit on important physiological, metabolic and biochemical markers related to aging and demonstrated for the first time that the lack of melatonin hormone is harmful in young adult rats.

Impact of Melatonin Deficit on Emotional Status and Oxidative Stress-Induced Changes in Sphingomyelin and Cholesterol Level in Young Adult, Mature, and Aged Rats

The pineal gland regulates the aging process via the hormone melatonin. The present report aims to evaluate the effect of pinealectomy (pin) on behavioral and oxidative stress-induced alterations in cholesterol and sphingomyelin (SM) levels in young adult, mature and aging rats. Sham and pin rats aged 3, 14 and 18 months were tested in behavioral tests for motor activity, anxiety, and depression. The ELISA test explored oxidative stress parameters and SM in the hippocampus, while total cholesterol was measured in serum via a commercial autoanalyzer. Mature and aged sham rats showed low motor activity and increased anxiety compared to the youngest rats. Pinealectomy affected emotional responses, induced depressive-like behavior, and elevated cholesterol levels in the youngest rats. However, removal of the pineal gland enhanced oxidative stress by diminishing antioxidant capacity and increasing the MDA level, and decreased SM level in the hippocampus of 14-month-old rats. Our findings suggest that young adult rats are vulnerable to emotional disturbance and changes in cholesterol levels resulting from melatonin deficiency. In contrast, mature rats with pinealectomy are exposed to an oxidative stress-induced decrease in SM levels in the hippocampus.

Agomelatine Prevents Amyloid Plaque Deposition, Tau Phosphorylation, and Neuroinflammation in APP/PS1 Mice

Agomelatine, an agonist of melatonergic MT1 and MT2 receptors and a selective 5-hydroxytryptamine 2C receptor antagonist, is widely applied in treating depression and insomnia symptoms in several neurogenerative diseases. However, the neuroprotective effect of agomelatine in Alzheimer’s disease (AD) is less known. In this study, a total of 30 mice were randomly divided into three groups, namely, wild type (WT), APP/PS1, and agomelatine (50 mg/kg). After 30 days, the Morris water maze was performed to test the cognitive ability of mice. Then, all mice were sacrificed, and the hippocampus tissues were collected for ELISA, Western blot, and immunofluorescence analysis. In this study, we found that agomelatine attenuated spatial memory deficit, amyloid-β (Aβ) deposition, tau phosphorylation, and neuroinflammation in the hippocampus of APP/PS1 mice. Further study demonstrated that agomelatine treatment upregulated the protein expression of DHCR24 and downregulated P-Akt, P-mTOR, p-p70s6k, Hes1, and Notch1 expression. In summary, our results identified that agomelatine could improve cognitive impairment and ameliorate AD-like pathology in APP/PS1 mice via activating DHCR24 signaling and inhibiting Akt/mTOR and Hes1/Notch1 signaling pathway. Agomelatine may become a promising drug candidate in the therapy of AD.

Antidepressant actions of melatonin and melatonin receptor agonist: Focus on pathophysiology and treatment

Depression has become one of the most commonly prevalent neuropsychiatric disorders, and the main characteristics of depression are sleep disorders and melatonin secretion disorders caused by circadian rhythm disorders. Abnormal endogenous melatonin alterations can contribute to the occurrence and development of depression. However, molecular mechanisms underlying this abnormality remain ambiguous. The present review summarizes the mechanisms underlying the antidepressant effects of melatonin, which is related to its functions in the regulation of the hypothalamic-pituitary-adrenal axis, inhibition of neuroinflammation, inhibition of oxidative stress, alleviation of autophagy, and upregulation of neurotrophic, promotion of neuroplasticity and upregulation of the levels of neurotransmitters, etc. Also, melatonin receptor agonists, such as agomelatine, ramelteon, piromelatine, tasimelteon, and GW117, have received considerable critical attention and are highly implicated in treating depression and comorbid disorders. This review focuses on melatonin and various melatonin receptor agonists in the pathophysiology and treatment of depression, aiming to provide further insight into the pathogenesis of depression and explore potential targets for novel agent development.

The Role of Melatonin on Behavioral Changes and Concomitant Oxidative Stress in icvAβ1-42 Rat Model with Pinealectomy

One of the pathological hallmarks of Alzheimer’s disease (AD) associated with its progression that contributes to β-amyloid (Aβ) generation is oxidative stress (OS). Clinical data suggest that melatonin is a potent antioxidant that might be effective in the adjunctive therapy of this neurodegenerative disease. The present study aimed to explore the role of melatonin on behavioral changes and markers of OS in three rat models, namely, pinealectomy (pin) model of melatonin deficit, intracerebroventricular (icv)Aβ_1-42 model of AD, and combination of both pin and Aβ_1-42 model (pin+icvAβ_1-42). The chronic injection with vehicle/melatonin (50 mg/kg, i.p. for 40 days) started on the same day of sham/pin and icv vehicle/Aβ_1-42 infusion procedures. Anxiety in the open field and the elevated plus-maze test and cognitive responses in the object recognition test were tested between the 30th–35th day after the surgical procedures. Markers of OS in the frontal cortex (FC) and hippocampus were detected by the ELISA method. Melatonin treatment corrected the exacerbated anxiety response only in the pin+icvAβ_1-42 model while it alleviated the cognitive impairment in the three models. Pinealectomy disturbed the antioxidant system via enhanced SOD activity and decreased GSH levels both in the FC and hippocampus. The Aβ_1-42 model decreased the SOD activity in the FC and elevated the MDA level in the two brain structures. The pin+icvAβ_1-42 model impaired the antioxidant system and elevated lipid peroxidation. Melatonin supplementation restored only the elevated MDA level of icvAβ_1-42 and pin+icvAβ_1-42 model in the hippocampus. In conclusion, our study reveals that the pin+icvAβ_1-42 rat model triggers more pronounced anxiety and alterations in markers of OS that may be associated with melatonin deficit concomitant to icvAβ_1-42-induced AD pathology.

Reduced neuroinflammation and enhanced neurogenesis following chronic agomelatine treatment in rats undergoing chronic constant light

Experimental studies have revealed the involvement of neuroinflammation mediated by activated microglia in the pathophysiology of depression, suggesting a novel target for treatment. The atypical antidepressant Agomelatine (Ago) has an advantage compared to the classical antidepressants due to its chronobiotic activity and unique pharmacological profile as a selective agonist at the melatonin receptors and an antagonist at the 5HT_2C receptors. We have recently revealed that Ago can exert a potent antidepressant effect in rats exposed to a chronic constant light (CCL). In the present study, we hypothesized that the anti-inflammatory activity of this melatonin analog on activated neuroglia in specific brain structures might contribute to its antidepressant effect in this model. Chronic Ago treatment (40 mg/kg, i.p. for 21 days) was executed during the last 3 weeks of a 6-week period of CCL exposure in rats. The CCL-vehicle-treated rats showed a profound neuroinflammation characterized by microgliosis and astrogliosis in the hippocampus, basolateral amygdala (BL) and partly in the piriform cortex (Pir) confirmed by immunohistochemistry. With the exception of the Pir, the CCL regime was accompanied by neuronal damage, identified by Nissl staining, in the hippocampus and basolateral amygdala and impaired neurogenesis with reduced dendritic complexity of hippocampal neuroprogenitor cells detected by doublecortin-positive cells in the dentate gyrus (DG) subgranular zone compared to the control group. Ago reversed the gliosis in a region-specific manner and partially restored the suppressed DG neurogenesis. Ago failed to produce neuroprotection in CCL exposed rats. The present results suggest that the beneficial effects of Ago represent an important mechanism underlying its antidepressant effect in models characterized by impaired circadian rhythms.

Chronic agomelatine treatment alleviates icvAβ-induced anxiety and depressive-like behavior through affecting Aβ metabolism in the hippocampus in a rat model of Alzheimer's disease

Recently, we reported that the atypical antidepressant agomelatine (Ago) exerted a beneficial impact on behavioral changes and concomitant neuropathological events in icvSTZ rat model of sporadic Alzheimer diseases (AD). In the present study, we aimed to explore the effect of Ago (40 mg/kg, i.p. for 30 days) on beta-amyloid (Aβ) metabolism in icvAβ_1-42 rat model of AD. The melatonin analogue was administered either simultaneously with Aβ_1-42 (AβAgo1) or 30 days later during the late stage of the progression of AD (AβAgo2). Treatment with Ago in the early stage of AD attenuated anxiety and depressive-like responses but was inefficient against Aβ-induced impairment of hippocampus-dependent spatial memory. The melatonin analogue, administered both during the early and the late stage of AD, corrected to control level the elevated Aβ_1-42 in the frontal cortex (FC) and the hippocampus. The concentration of α-secretase was enhanced by AβAgo1 compared to the sham- and Aβ-veh groups in the hippocampus. No changes in the concentration of β-secretase in the FC and the hippocampus as well as of γ-secretase in the FC were observed among groups. Both the AβAgo1 and AβAgo2 attenuated to control level the Aβ-induced increased concentration of γ-secretase in the hippocampus. AβAgo1 exerted also structure-specific neuroprotection observed mainly in the CA1, septal CA3b subfield of the dorsal hippocampus and septo-temporal piriform cortex (Pir) and partially in the temporal CA3c, septal and temporal Pir. These findings suggest that Ago treatment in the early stage of AD can exert beneficial effects on concomitant behavioral impairments and neuroprotection in associated brain structures. The antidepressant administration both in the early stage and after the progression of AD affected Aβ metabolism via decreasing of γ-secretase concentration in the hippocampus.

Chronic Piromelatine Treatment Alleviates Anxiety, Depressive Responses and Abnormal Hypothalamic-Pituitary-Adrenal Axis Activity in Prenatally Stressed Male and Female Rats

The prenatal stress (PNS) model in rodents can induce different abnormal responses that replicate the pathophysiology of depression. We applied this model to evaluate the efficacy of piromelatine (Pir), a novel melatonin analog developed for the treatment of insomnia, in male and female offspring. Adult PNS rats from both sexes showed comparable disturbance associated with high levels of anxiety and depressive responses. Both males and females with PNS demonstrated impaired feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) axis compared to the intact offspring and increased glucocorticoid receptors in the hippocampus. However, opposite to female offspring, the male PNS rats showed an increased expression of mineralocorticoid receptors in the hippocampus. Piromelatine (20 mg/kg, i.p., for 21 days injected from postnatal day 60) attenuated the high anxiety level tested in the open field, elevated plus-maze and light-dark test, and depressive-like behavior in the sucrose preference and the forced swimming tests in a sex-specific manner. The drug reversed to control level stress-induced increase of plasma corticosterone 120 min later in both sexes. Piromelatine also corrected to control level the PNS-induced alterations of corticosteroid receptors only in male offspring. Our findings suggest that the piromelatine treatment exerts beneficial effects on impaired behavioral responses and dysregulated HPA axis in both sexes, while it corrects the PNS-induced changes in the hippocampal corticosteroid receptors only in male offspring.

Pineal gland protects against chemically induced oral carcinogenesis and inhibits tumor progression in rats

Clinical investigations suggest that melatonin suppression and circadian dysfunction may be related to cancer development in shift workers. Studies also show that melatonin suppression after pinealectomy increases cancer incidence in preclinical models. However, no study evaluated the influence of pinealectomy on oral cancer development. In the current study, we investigated the effects of pinealectomy on oral squamous cell carcinoma (OSCC) occurrence and progression in rats. Rats submitted to sham surgery were used as control. Pinealectomy promoted an increase of 140% in OSCC occurrence when compared to sham animals. Tumors from pinealectomized rats displayed a higher volume and thickness than the tumors from sham-operated animals. Pinealectomy induced atrophy of the epithelium adjacent to the oral lesions. Pinealectomized rats showed higher mean number of tumor-associated macrophages and eosinophils in the invasive front of OSCC. In addition, nuclear overexpression of ERK1/2 and p53 was also observed in the front of carcinomas from pinealectomized rats. These results reveal that pineal gland plays a protective role against oral carcinogenesis. The melatonin suppression caused by the pinealectomy might contribute to oral cancer development by acting on ERK1/2 and p53 pathways and regulating tumor inflammation.

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.

Agomelatine alleviates neuronal loss through BDNF signaling in the post-status epilepticus model induced by kainic acid in rat

Recently, we have reported that while agomelatine (Ago) is unable to prevent development of epilepsy it exerts a strong neuroprotective and anti-inflammatory response in the KA post-status epilepticus (SE) rat model. In the present study, we aimed to explore whether the brain-derived neurotrophic factor (BDNF) in the hippocampus is involved in the neuroprotective effect of Ago against the KA-induced SE and epileptiform activity four months later in rats. Lacosamide (LCM) was used as a positive control. The EEG-recorded seizure activity was also evaluated in two treatment protocols. In Experiment#1, Ago given repeatedly at a dose of 40 mg/kg during the course of SE was unable neither to modify EEG-recorded epileptiform activity nor the video- and EEG-recorded spontaneous seizures four months later compared to LCM (50 mg/kg). However, both Ago and LCM inhibited the expression of BDNF in the mossy fibers and also prevented neuronal loss in the dorsal hippocampal and the piriform cortex after SE. In Experiment#2, acute injection of Ago and LCM on epileptic rats, characterized by high seizure rates, did not prevent EEG-recorded paroxysmal events while only LCM decreased either absolute or relative powers of gamma (28-60 Hz) and high (HI) (60-120 Hz) frequency bands to baseline in the frontal and parietal cortex, respectively. Our results suggest that the protection against neuronal loss in specific limbic regions and overexpressed BDNF in the mossy fibers resulting from the repeated treatment with Ago and LCM, respectively, during SE is not a prerequisite for alleviation of epileptogenesis and development of epilepsy. In addition, a reduction of gamma and HI bands in the frontal and parietal cortex is not associated with EEG-recorded paroxysmal events after acute injection of LCM.

Antidepressant agomelatine attenuates behavioral deficits and concomitant pathology observed in streptozotocin-induced model of Alzheimer's disease in male rats

Experimental findings suggest that the melatonin system has a beneficial role in models of Alzheimer's disease (ADs). The aim of the present study was to explore whether the atypical antidepressant agomelatine (Ago), which is a melatonin MT₁ and MT₂ agonist and 5-HT_2C antagonist, is effective against behavioral, biochemical and histological impairments in streptozotocin (STZ)-induced model of ADs in male rats. Male Sprague Dawley rats were treated intraperitoneally (i.p.) with Ago (40 mg/kg) for 30 days starting three months following the intracerebroventricular (icv) injection of STZ. Chronic Ago treatment reduced anxiety-like behavior of STZ-treated rats in the elevated plus maze, increased the preference to saccharine and corrected the spatial memory impairment in the eight-arm radial arm maze test. This melatonin analogue restored STZ-induced biochemical changes, including an increase of beta amyloid (Aβ) protein, and signal markers of inflammation (TNF-alpha and IL-1 beta). Ago exerted partial neuroprotection, specifically in the temporal CA3b subfield of the dorsal hippocampus and temporal piriform cortex. The ability of Ago to alleviate behavioral symptoms and concomitant neuropathological events observed in a model of sporadic ADs suggests that this melatonin alternative can be considered a promising adjuvant in this disease.

Abnormal Hippocampal Melatoninergic System: A Potential Link between Absence Epilepsy and Depression-Like Behavior in WAG/Rij Rats?

Absence epilepsy and depression are comorbid disorders, but the molecular link between the two disorders is unknown. Here, we examined the role of the melatoninergic system in the pathophysiology of spike and wave discharges (SWDs) and depression-like behaviour in the Wistar Albino Glaxo from Rijswijk (WAG/Rij) rat model of absence epilepsy. In WAG/Rij rats, SWD incidence was higher during the dark period of the light-dark cycle, in agreement with previous findings. However, neither pinealectomy nor melatonin administration had any effect on SWD incidence, suggesting that the melatoninergic system was not involved in the pathophysiology of absence-like seizures. Endogenous melatonin levels were lower in the hippocampus of WAG/Rij rats as compared to non-epileptic control rats, and this was associated with higher levels of melatonin receptors in the hippocampus, but not in the thalamus. In line with the reduced melatonin levels, cell density was lower in the hippocampus of WAG/Rij rats and was further reduced by pinealectomy. As expected, WAG/Rij rats showed an increased depression-like behaviour in the sucrose preference and forced swim tests, as compared to non-epileptic controls. Pinealectomy abolished the difference between the two strains of rats by enhancing depression-like behaviour in non-epileptic controls. Melatonin replacement displayed a significant antidepressant-like effect in both WAG/Rij and control rats. These findings suggest that a defect of hippocampal melatoninergic system may be one of the mechanisms underlying the depression-like phenotype in WAG/Rij rats and that activation of melatonin receptors might represent a valuable strategy in the treatment of depression associated with absence epilepsy.

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.

Chronic agomelatine treatment prevents comorbid depression in the post-status epilepticus model of acquired epilepsy through suppression of inflammatory signaling

Inflammatory signal molecules are suggested to be involved in the mechanism underlying comorbid depression in epilepsy. In the present study, we tested the hypothesis that the novel antidepressant agomelatine, a potent melatonin MT₁ and MT₂ receptor agonist and serotonin 5HT_2C receptor antagonist, can prevent depressive symptoms developed during the chronic epileptic phase by suppressing an inflammatory response. Chronic treatment with agomelatine (40 mg/kg, i.p.) was initiated an hour after the kainate acid (KA)-induced status epilepticus (SE) and maintained for a period of 10 weeks in Wistar rats. Registration of spontaneous motor seizures was performed through a video (24 h/day) and EEG monitoring. Antidepressant activity of agomelatine was explored in the splash test, sucrose preference test (SPT) and forced swimming test (FST) while anxiolytic effect was observed through the novelty suppression-feeding test (NSFT) during chronic phase in epileptic rats. The frequency of motor seizures detected by video and EEG recording did not differ between vehicle and Ago group. Rats with registered spontaneous motor seizures showed symptoms typical for depressive behavior that included decreased grooming, anhedonia during the dark period and hopeless-like behavior. Epileptic rats exhibited also anxiety with novelty-induced hypophagia. This behavioral deficit correlated with increased signal markers of inflammation (plasma levels of interleukin (IL)-1β and activated glia in brain), while plasma corticosterone levels were not changed. Agomelatine treatment during epileptogenesis exerted a clear antidepressant effect by suppressing all behavioral hallmarks, reducing plasma IL-1β levels and preventing microgliosis and astrogliosis in specific limbic regions. The present results suggest that agomelatine treatment starting after SE can provide an effective therapy of comorbid depression in chronic epileptic condition through suppression of inflammatory signaling.

Anxiolytic effect of anacardic acids from cashew (Anacardium occidentale) nut shell in mice

Antianxiety drugs currently in use are associated with a number of serious side effects. Present study was designed to evaluate the efficacy of anacardic acids (AAs) isolated from cashew nut (Anacardium occidentale L.) shell liquid (CNSL) to treat anxiety as well as its role in oxidative stress in mice model. Anxiolytic effect of AA was evaluated using rota-rod and a set of behavioral tests in male Swiss albino mice at the doses of 10, 25, and 50 mg/kg. Flumazenil was used to evaluate the possible involvement of GABAergic system in the mechanism of action of AA. The effect of AA on oxidative stress in mice was evaluated by determining the concentration of malondialdehyde (MDA), reduced glutathione, and catalase (CAT) activity. The detection of DNA damage of the treated animals was performed using alkaline comet test in the hippocampus and frontal cortex of the animals. The results demonstrated that AA did not produce myorelaxant and sedative effects, nor did it cause a decrease in locomotor activity. The anxiolytic effect of AA was well-evident in all tests, especially at higher dose levels (25 and 50 mg/mg). Flumazenil reversed the anxiolytic effect of AA at all doses. In addition, AA reduced oxidative stress by decreasing the concentration of MDA and increasing the levels of reduced glutathione (GSH) and CAT activity. Statistical analysis by Pearson's correlation indicated a positive correlation between anxiolytic effect of AA to its antioxidant and lipid peroxidation inhibitory activity. Furthermore, increased CAT activity and GSH concentrations in the hippocampus and frontal cortex of mice was also complementary to the reduced genotoxic damage observed in the study. In comet assay, AA did not increase in DNA damage. In conclusion, the results supported that AA possesses GABA_A receptor mediated anxiolytic activity with the lack of myorelaxation and genotoxicity. © 2018 IUBMB Life, 70(5):420-431, 2018.

Effects of agomelatine on pentylenetetrazole-induced kindling, kindling-associated oxidative stress, and behavioral despair in mice and modulation of its actions by luzindole and 1-(m-chlorophenyl) piperazine

In view of well-evidenced antiepileptic effects of melatonin and few reports of anticonvulsant action of agomelatine, the present study investigated whether agomelatine protects against pentylenetetrazole (PTZ)-induced kindling in mice and kindling-associated oxidative stress, depression, and impairment of spatial memory. In order to explore whether effects are mediated by melatonergic or serotonergic mechanisms, 1-(m-chlorophenyl) piperazine (mCPP), selective 5HT2c receptor agonist and luzindole, melatonergic receptor antagonist, were taken as pharmacological tools. In view of few hepatotoxic reports on agomelatine, the study evaluated effects on hepatic enzyme levels. Swiss strain albino mice were injected with PTZ (25mg/kg, i.p.) once every two days for 5weeks to induce kindling. The effects of agomelatine (10mg/kg, p.o.) alone and in combination with luzindole (2.5mg/kg, i.p.) or mCPP (7mg/kg, i.p.) on seizure severity during induction and % incidence of animals kindled at the end of 5weeks were recorded. Modified forced swim test was used for studying depression-like behavior while spontaneous alternation behavior was used for studying effects on spatial memory. Serum AST and ALT concentrations, cortical and hippocampal malondialdehyde, and reduced glutathione were measured. Agomelatine 10mg/kg, p.o. effectively delayed development of kindling, reduced seizure severity, and decreased % incidence. Luzindole reversed the protective effects of agomelatine while mCPP failed to show such a reversal, indicating melatonergic (and not serotonergic) mechanisms in the observed effects. Agomelatine also showed antioxidant effects that can partially contribute to its anticonvulsant action. In addition, it alleviated PTZ-kindling-associated behavioral despair and favorably modulated liver enzymes. Its effects on improvement of kindling-associated spatial memory could possibly be related to its effects on locomotor activity. Agomelatine, thus, could be explored as an adjunct to antiepileptic drugs for seizure control and for alleviating epilepsy-associated depression.

Agomelatine protects against neuronal damage without preventing epileptogenesis in the kainate model of temporal lobe epilepsy

Recent studies about the novel antidepressant agomelatine, which is a mixed MT₁ and MT₂ melatonin receptor agonist and 5HT_2C serotonin receptor antagonist possessing an anticonvulsant and neuroprotective action, suggest that it may have potential to contribute against epileptogenesis and epilepsy-induced memory impairment. In order to ascertain whether protection of some brain structures could suppress epileptogenesis, in the present study, we evaluated the effect of chronic post-status treatment with agomelatine on epileptogenesis, behavioral and neuronal damage induced by kainate acid (KA) status epilepticus (SE). Agomelatine/vehicle treatment (40mg/kg, i.p.) started one hour after SE and continued up to 10weeks in Wistar rats. Latency for onset of spontaneous motor seizures (SMS) and their frequency was detected by a 24-h video-recording. Locomotor activity, anxiety and hippocampus-dependent spatial memory in open field (OF), elevated plus maze (EPM), light-dark test (LDT) and radial arm maze (RAM) test, respectively, were evaluated during the last two weeks after SE. Agomelatine significantly decreased the latency for onset of SMS and increased the seizure frequency during the 2nd and the 3rd week of treatment. The MT₁ and MT₂ receptor agonist and serotonin 5HT_2C receptor antagonist exacerbated the KA-induced hyperlocomotion and impulsive behavior and it was unable to prevent spatial memory impairment of epileptic rats. However, agomelatine induced a neuroprotection in the dorsal hippocampus, specifically in the CA1, septal CA2 and partially in the CA3c region, the hilus of the dentate gyrus, piriform cortex and septo-temporal and temporal basolateral amygdala. Our findings suggest that the beneficial impact against SE-induced neuronal loss exerted by agomelatine is not crucial for the suppression of epileptogenesis and its deleterious consequences in KA model of temporal lobe epilepsy.

Update on melatonin receptors: IUPHAR Review 20

Melatonin receptors are seven transmembrane-spanning proteins belonging to the GPCR superfamily. In mammals, two melatonin receptor subtypes exist - MT1 and MT2 - encoded by the MTNR1A and MTNR1B genes respectively. The current review provides an update on melatonin receptors by the corresponding subcommittee of the International Union of Basic and Clinical Pharmacology. We will highlight recent developments of melatonin receptor ligands, including radioligands, and give an update on the latest phenotyping results of melatonin receptor knockout mice. The current status and perspectives of the structure of melatonin receptor will be summarized. The physiological importance of melatonin receptor dimers and biologically important and type 2 diabetes-associated genetic variants of melatonin receptors will be discussed. The role of melatonin receptors in physiology and disease will be further exemplified by their functions in the immune system and the CNS. Finally, antioxidant and free radical scavenger properties of melatonin and its relation to melatonin receptors will be critically addressed.