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

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          Abstract

          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.

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          Most cited references 51

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          Extrapineal melatonin: sources, regulation, and potential functions.

          Endogenous melatonin is synthesized from tryptophan via 5-hydroxytryptamine. It is considered an indoleamine from a biochemical point of view because the melatonin molecule contains a substituted indolic ring with an amino group. The circadian production of melatonin by the pineal gland explains its chronobiotic influence on organismal activity, including the endocrine and non-endocrine rhythms. Other functions of melatonin, including its antioxidant and anti-inflammatory properties, its genomic effects, and its capacity to modulate mitochondrial homeostasis, are linked to the redox status of cells and tissues. With the aid of specific melatonin antibodies, the presence of melatonin has been detected in multiple extrapineal tissues including the brain, retina, lens, cochlea, Harderian gland, airway epithelium, skin, gastrointestinal tract, liver, kidney, thyroid, pancreas, thymus, spleen, immune system cells, carotid body, reproductive tract, and endothelial cells. In most of these tissues, the melatonin-synthesizing enzymes have been identified. Melatonin is present in essentially all biological fluids including cerebrospinal fluid, saliva, bile, synovial fluid, amniotic fluid, and breast milk. In several of these fluids, melatonin concentrations exceed those in the blood. The importance of the continual availability of melatonin at the cellular level is important for its physiological regulation of cell homeostasis, and may be relevant to its therapeutic applications. Because of this, it is essential to compile information related to its peripheral production and regulation of this ubiquitously acting indoleamine. Thus, this review emphasizes the presence of melatonin in extrapineal organs, tissues, and fluids of mammals including humans.
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            Prolonged glucocorticoid exposure reduces hippocampal neuron number: implications for aging.

            The hippocampus of the rat loses neurons with age, a loss which may eventuate in some of the functional impairments typical of senescence. Cumulative exposure to corticosterone (CORT) over the lifespan may be a cause of this neuronal loss, as it is prevented by adrenalectomy at mid-age. In this study, we demonstrate that prolonged exposure to CORT accelerates the process of cell loss. Rats were injected daily with sufficient CORT to produce prolonged elevations of circulating titers within the high physiological range. Animals treated for 3 months (chronic subjects) resembled aged rats in a number of ways. First, both groups had extensive and persistent depletions of CORT receptors in the hippocampus; in the case of chronic rats, no recovery of receptor concentrations occurred 4 months after the end of steroid treatment. Second, autoradiographic analysis revealed that the receptor depletion was due, in part, to a loss of CORT-concentrating cells, especially in the CA3 cell field. Remaining cells bound significantly less [3H]corticosterone than did those of control rats. Finally, analysis of size distributions of hippocampal cell bodies indicated that chronic subjects lost neurons of the same size as those lost in the aged hippocampus. Furthermore, chronic subjects also had increased numbers of small, darkly staining cells of CA3; these corresponded in size to the dark glia whose numbers increase in the aged hippocampus, and which are thought to infiltrate in response to neuronal damage or destruction. Thus, this study supports the hypothesis that cumulative exposure to CORT over the lifespan may contribute to age-related loss of neurons in the hippocampus, and that prolonged stress or exposure to CORT accelerates this process.
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              Genetic animal models for absence epilepsy: a review of the WAG/Rij strain of rats.

              Based on the reviewed literature and the data presented in this paper, conclusions can be drawn with respect to the validity of the WAG/Rij strain of rats as a model for absence epilepsy in humans. The view that the WAG/Rij model has "face validity" is supported by the simultaneous presence of clinical and electroencephalographic signs characterizing absences in rat and humans, by the decrease in responsiveness during the presence of spike-wave discharges in both species, by the agreement between model and patient with respect to the preferential occurrences of spike-wave discharges at transitions in states of vigilance, by the corresponding modulation of spike-wave discharges by physical and mental activities in both and, finally, by the fact that in both humans and rats absence epilepsy is inherited. Against this view, however, argue two points. In rats, absences appear after puberty and are maintained during life, while in humans the seizures occur before puberty and then disappear or convert to more serious forms of epilepsy. The second point is the frequency difference of the spikes and waves in the discharge train: 8-10 Hz in the rat and 3 Hz in the human (though there are no a priori reasons why the frequency of spike waves in the burst must be the same in all species). The absence model also has predictive validity, based on pharmacological data that demonstrate the specificity of certain drugs as being effective in convulsive epilepsies and not in absence epilepsy. So far, all drugs affect spike-wave activity the same way in rats and humans, with lamotrigine being, perhaps, the only exception. Furthermore, sleep deprivation is a powerful provocation for the initiation of spike-wave discharges in both rats and humans. Potential explanations for the presence of absence seizures in rats have been found at the levels of activities in networks and nuclei; of neurons, membrane properties, and ion channels; of proteins and enzymes; and, finally, of genes and chromosomes. Further descriptions of the cellular processes can be found extensively in the literature (e.g., McCormick and Contreras, 2001) and those of the thalamo-cortico-thalamic network in this review as well as in others (Avanzini et al., 1999). Considering the extensive involvement of the phenomena under study with theoretical issues such as the relationship between sleep spindles and spike-wave discharges, and the origin of seizure activity, it can be concluded that the model also has construct validity as far as the present neurobiological theories holding for absence epilepsy in humans are concerned. The WAG/Rij model can therefore be recommended for continued use in evaluating antiepileptic drugs for monotherapy and polytherapy, as well as for the toxicological side effects of putative new antiabsence drugs.
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                Author and article information

                Journal
                Int J Mol Sci
                Int J Mol Sci
                ijms
                International Journal of Molecular Sciences
                MDPI
                1422-0067
                06 July 2018
                July 2018
                : 19
                : 7
                Affiliations
                [1 ]I.R.C.C.S. Neuromed, 86077 Pozzilli, Italy; moyanova.slavianka@ 123456neuromed.it (S.M.); antonio.defusco85@ 123456gmail.com (A.D.F.); ines.santolini@ 123456neuromed.it (I.S.); robertacelli85@ 123456hotmail.it (R.C.); domenico.bucci@ 123456neuromed.it (D.B.); federica.mast@ 123456neuromed.it (F.M.); giuseppe.battaglia@ 123456neuromed.it (G.B.); ferdinandonicoletti@ 123456hotmail.com (F.N.)
                [2 ]Department of Physiology and Pharmacology, Sapienza University of Rome, 00161 Rome, Italy
                [3 ]Institute of Neurobiology, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
                Author notes
                [* ]Correspondence: janetchekalarova@ 123456gmail.com ; Tel.: +359-2979-2172; Fax: +359-2-719-109
                [†]

                These authors contributed equally to this work.

                [‡]

                Current address: Center for Synaptic Neuroscience and Technology, Istituto Italiano di Tecnologia, Genova 16132, Italy and Department of Experimental Medicine, University of Genova, Genova 16132, Italy.

                Article
                ijms-19-01973
                10.3390/ijms19071973
                6073874
                29986414
                © 2018 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                Categories
                Article

                Molecular biology

                absence seizures, pinealectomy, depression, melatonin, mt receptors

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