The role of the melatoninergic system in epilepsy and comorbid psychiatric disorders

Low-dose 7,8-Dihydroxyflavone Administration After Status Epilepticus Prevents Epilepsy Development

Temporal lobe epilepsy often manifests months or even years after an initial epileptogenic insult (e.g., stroke, trauma, status epilepticus) and, therefore, may be preventable. However, no such preventive treatment is currently available. Aim of this study was to test an antioxidant agent, 7,8-dihydroxyflavone (7,8-DHF), that is well tolerated and effective in preclinical models of many neurological disorders, as an anti-epileptogenic drug. However, 7,8-DHF also acts as a TrkB receptor agonist and, based on the literature, this effect may imply an anti- or a pro-epileptogenic effect. We found that low- (5 mg/kg), but not high-dose 7,8-DHF (10 mg/kg) can exert strong anti-epileptogenic effects in the lithium-pilocarpine model (i.e., highly significant reduction in the frequency of spontaneous seizures and in the time to first seizure after status epilepticus). The mechanism of these different dose-related effects remains to be elucidated. Nonetheless, considering its excellent safety profile and antioxidant properties, as well as its putative effects on TrkB receptors, 7,8-DHF represents an interesting template for the development of effective and well-tolerated anti-epileptogenic drugs.

Envisioning the role of inwardly rectifying potassium (Kir) channel in epilepsy

Epilepsy is a devastating neurological disorder characterized by recurrent seizures attributed to the disruption of the dynamic excitatory and inhibitory balance in the brain. Epilepsy has emerged as a global health concern affecting about 70 million people worldwide. Despite recent advances in pre-clinical and clinical research, its etiopathogenesis remains obscure, and there are still no treatment strategies modifying disease progression. Although the precise molecular mechanisms underlying epileptogenesis have not been clarified yet, the role of ion channels as regulators of cellular excitability has increasingly gained attention. In this regard, emerging evidence highlights the potential implication of inwardly rectifying potassium (Kir) channels in epileptogenesis. Kir channels consist of seven different subfamilies (Kir1-Kir7), and they are highly expressed in both neuronal and glial cells in the central nervous system. These channels control the cell volume and excitability. In this review, we discuss preclinical and clinical evidence on the role of the several subfamilies of Kir channels in epileptogenesis, aiming to shed more light on the pathogenesis of this disorder and pave the way for future novel therapeutic approaches.

The Anticonvulsant Effect of a Novel Indole-Related Compound in the Kainate-Induced Status Epilepticus in Mice: The Role of the Antioxidant and Anti-inflammatory Mechanism

We synthesized a series of novel indole compounds containing aroylhydrazone moieties and evaluated them in mice to check their anticonvulsant activity. In the present study the most potent C3-modified derivative 3e, containing 2-furyl fragment was evaluated in kainate (KA)-induced status epilepticus (SE) and the consequences on oxidative stress and inflammation in the hippocampus in mice were explored. Melatonin was used as positive control while the melatonin receptor antagonist Luzindol was studied alone or in combination with melatonin or 3e, respectively. After intraperitoneal (i.p.) pre-treatment with melatonin 3e, Luzindol + melatonin and Luzindol + 3e for 7 days (melatonin and 3e-30 mg kg^-1 or 60 mg kg^-1, Luzindol 10 mg kg^-1) the animals were i.p. injected with KA (30 mg kg^-1, i.p.). The 3e decreased the SE-induced seizure intensity while melatonin suppressed seizures at the higher dose of 60 mg kg^-1. Luzindol blocked the anticonvulsant effect of both Mel and 3e. The dose-dependent antioxidant effect of 3e measured by reduced glutathione (GSH) and total GSH in the hippocampus, was comparable to the effect of melatonin. Luzindol fully blocked the effect of melatonin but affected partially the antioxidant activity of 3e. The KA-induced increased amplifier of neuroinflammation high-mobility group box protein 1 (HMGB1) was neither alleviated by melatonin, nor by 3e. The activation by this DNA-binding protein receptor for advanced glycation end products (RAGE) was not affected by SE, melatonin and 3e pre-treatment. Our results suggest that the novel indole derivate 3e, containing 2-furyl fragment, might be clinically useful as an adjunct therapy against SE and concomitant oxidative stress.

Implication of sestrin3 in epilepsy and its comorbidities

Epilepsy is a serious neurological disorder affecting about 1% of the population worldwide. Epilepsy may arise as a result of acquired brain injury, or as a consequence of genetic predisposition. To date, genome-wide association studies and exome sequencing approaches have provided limited insights into the mechanisms of acquired brain injury. We have previously reported a pro-epileptic gene network, which is conserved across species, encoding inflammatory processes and positively regulated by sestrin3 (SESN3). In this study, we investigated the phenotype of SESN3 knock-out rats in terms of susceptibility to seizures and observed a significant delay in status epilepticus onset in SESN3 knock-out compared to control rats. This finding confirms previous in vitro and in vivo evidence indicating that SESN3 may favour occurrence and/or severity of seizures. We also analysed the phenotype of SESN3 knock-out rats for common comorbidities of epilepsy, i.e., anxiety, depression and cognitive impairment. SESN3 knock-out rats proved less anxious compared to control rats in a selection of behavioural tests. Taken together, the present results suggest that SESN3 may regulate mechanisms involved in the pathogenesis of epilepsy and its comorbidities.

Revisiting the role of neurotransmitters in epilepsy: An updated review

Epilepsy is a neurologicaldisorder characterized by persistent predisposition to recurrent seizurescaused by abnormal neuronal activity in the brain. Epileptic seizures maydevelop due to a relative imbalance of excitatory and inhibitory neurotransmitters. Expressional alterations of receptors and ion channelsactivated by neurotransmitters can lead to epilepsy pathogenesis.

AIMS

In this updated comprehensive review, we discuss the emerging implication of mutations in neurotransmitter-mediated receptors and ion channels. We aim to provide critical findings of the current literature about the role of neurotransmitters in epilepsy.

MATERIALS AND METHODS

A comprehensive literature review was conducted to identify and critically evaluate studies analyzing the possible relationship between epilepsy and neurotransmitters. The PubMed database was searched for related research articles.

KEY FINDINGS

Glutamate and gamma-aminobutyric acid (GABA) are the main neurotransmitters playing a critical role in the pathophysiology of this balance, and irreversible neuronal damage may occur as a result of abnormal changes in these molecules. Acetylcholine (ACh), the main stimulant of the autonomic nervous system, mediates signal transmission through cholinergic and nicotinic receptors. Accumulating evidence indicates that dysfunction of nicotinic ACh receptors, which are widely expressed in hippocampal and cortical neurons, may be significantly implicated in the pathogenesis of epilepsy. The dopamine-norepinephrine-epinephrine cycle activates hormonal and neuronal pathways; serotonin, norepinephrine, histamine, and melatonin can act as both hormones and neurotransmitters. Recent reports have demonstrated that nitric oxide mediates cognitive and memory-related functions via stimulating neuronal transmission.

SIGNIFICANCE

The elucidation of the role of the main mediators and receptors in epilepsy is crucial for developing new diagnostic and therapeutic approaches.

Evaluation of neurobiological and antioxidant effects of novel melatonin analogs in mice

Based on the pharmacophore model of melatonin (MT₁) receptor, we recently synthesized a series of indole derivatives that showed anticonvulsant activity with low neurotoxicity and hepatotoxicity in rodents. In the present study, the three most potent C3-modified derivatives with hydrazine structure 3c, 3e, and 3f, with 2-chlorophenyl, 2-furyl, and 2-thienyl fragments, respectively, were selected, and their neurobiological activity was explored in mice. In Experiment #1, the dose-dependent anxiolytic effect of a single i.p. administration of the novel compounds at doses of 10, 30, and 60 mg/kg were studied in the open field (OF) test. In Experiment#2, the analgesic effect of 3c, 3e, and 3f (30–100 mg/kg) was tested in the hot plate test and formalin test. Experiment#3 was designed to assess the antidepressant-like activity of 3c, 3e, and 3f (10–60 mg/kg). The forced swimming test (FST) and tail suspension test (TST)-induced effect on markers of oxidative stress in the frontal cortex (FC), and the hippocampus was evaluated. Melatonin was used in the same doses as melatonin analogs in all three experiments as a positive control. Desipramine (10 mg/kg) was also applied as a control in the FST. The three melatonin analogs bearing hydrazide/hydrazone substitution at 3C of the indol scaffold demonstrated improved antidepressant-like activity compared to the melatonin. The tested substances are devoided of anxiolytic effects. The antioxidant activity of the melatonin analogs and analgesic potential is comparable to that of melatonin. The 3C substitution with hydrazide/hydrazone moiety substantially contributes to the antidepressant and antioxidant activity of the melatonin analogs.

Evaluation of the anticonvulsant effect of novel melatonin derivatives in the intravenous pentylenetetrazol seizure test in mice

The design of new pharmacologically active compounds with affinity to melatonin receptors has become an area of great interest during the last decade. Recently, we reported that newly synthesized melatonin derivatives, containing aroylhydrazone moiety in the indole scaffold, with the highest affinity to the elaborated pharmacophore model, possess an anticonvulsant activity in the maximal electroshock (MES) and 6Hz test in mice. We aimed further to explore the effect of these melatonin derivatives and the role of melatonin receptors on seizure threshold measured by the timed intravenous pentylenetetrazole (iv PTZ) infusion test in mice. Carbamazepine (CBZ) and melatonin were used as positive controls. Three out of six compounds, 3c, 3f, and 3e, respectively, dose-dependently increased the PTZ-induced seizure thresholds for myoclonic twitch, clonic, and tonic seizures comparable to the effect of CBZ and melatonin. The anticonvulsant effect of 3c, 3f, and 3e was blocked by the non-selective melatonin receptor antagonist luzindol suggesting the involvement of melatonin receptors in the activity of these compounds. Also docking study of 3c, 3f and 3e in the melatonin-binding site of melatonin receptor confirm the possible mechanism of action of these compounds involving melatonin receptors. Our previous and present results suggest that 3c, 3f, and 3e can be considered promising agents with anticonvulsant activity on melatonin receptors in the brain.

Chronobiology of limbic seizures: Potential mechanisms and prospects of chronotherapy for mesial temporal lobe epilepsy

Mesial Temporal Lobe Epilepsy (mTLE) characterized by progressive development of complex partial seizures originating from the hippocampus is the most prevalent and refractory type of epilepsy. One of the remarkable features of mTLE is the rhythmic pattern of occurrence of spontaneous seizures, implying a dependence on the endogenous clock system for seizure threshold. Conversely, circadian rhythms are affected by epilepsy too. Comprehending how the circadian system and seizures interact with each other is essential for understanding the pathophysiology of epilepsy as well as for developing innovative therapies that are efficacious for better seizure control. In this review, we confer how the temporal dysregulation of the circadian clock in the hippocampus combined with multiple uncoupled oscillators could lead to periodic seizure occurrences and comorbidities. Unraveling these associations with additional research would help in developing chronotherapy for mTLE, based on the chronobiology of spontaneous seizures. Notably, differential dosing of antiepileptic drugs over the circadian period and/or strategies that resynchronize biological rhythms may substantially improve the management of seizures in mTLE patients.

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.

Melatonin produces a rapid onset and prolonged efficacy in reducing depression-like behaviors in adult rats exposed to chronic unpredictable mild stress

The present study was aimed at evaluating the rapidity and duration of melatonin as an antidepressant in a rat model of depression. The rats were subjected to a six-week period of unpredictable mild stress followed by melatonin treatment. Three groups of rats were included in this study: Controls (CON - no stress exposure), Chronic Unpredictable Mild Stress (CUS) and CUS followed by melatonin (MT). Stressors consisted of exposure to rotation on a shaker, placement in a chamber maintained at 4°C, lights off for 3h, lights on overnight, exposure to an aversive odor, 45° tilted cages, food and water deprivation and crowding and isolated housing. Subsequently, the saline vehicle (CUS) or melatonin was administered at a dose of 10mg/kg for 14days period. Body weight and behavioral tests were used to evaluate depression-like behavior and its recovery following melatonin treatment. While body weight increases were significantly lower in rats exposed to CUS versus CON, body weights of the MT group increased significantly following melatonin treatment as compared with the CUS group. With regard to results obtained with behavioral assays indicative of depression, rapid and long-term functional recoveries in depression were observed in the MT as compared to the CUS group. The results indicate that not only does melatonin induce an antidepressant-like action within this rat model of depression, but does so with a rapid onset and prolonged efficacy. As most current treatments for depression require an extended period of administration, our current results suggest that melatonin may prove to be a particularly effect agent to promote a rapid onset and prolonged behavioral benefits in the treatment of depression.

Monoaminergic Mechanisms in Epilepsy May Offer Innovative Therapeutic Opportunity for Monoaminergic Multi-Target Drugs

A large body of experimental and clinical evidence has strongly suggested that monoamines play an important role in regulating epileptogenesis, seizure susceptibility, convulsions, and comorbid psychiatric disorders commonly seen in people with epilepsy (PWE). However, neither the relative significance of individual monoamines nor their interaction has yet been fully clarified due to the complexity of these neurotransmitter systems. In addition, epilepsy is diverse, with many different seizure types and epilepsy syndromes, and the role played by monoamines may vary from one condition to another. In this review, we will focus on the role of serotonin, dopamine, noradrenaline, histamine, and melatonin in epilepsy. Recent experimental, clinical, and genetic evidence will be reviewed in consideration of the mutual relationship of monoamines with the other putative neurotransmitters. The complexity of epileptic pathogenesis may explain why the currently available drugs, developed according to the classic drug discovery paradigm of "one-molecule-one-target," have turned out to be effective only in a percentage of PWE. Although, no antiepileptic drugs currently target specifically monoaminergic systems, multi-target directed ligands acting on different monoaminergic proteins, present on both neurons and glia cells, may represent a new approach in the management of seizures, and their generation as well as comorbid neuropsychiatric disorders.