Melatonin increases striatal dopaminergic function in 6-OHDA-lesioned rats

Melatonin as a Chronobiotic and Cytoprotective Agent in Parkinson's Disease

This article discusses the role that melatonin may have in the prevention and treatment of Parkinson’s disease (PD). In parkinsonian patients circulating melatonin levels are consistently disrupted and the potential therapeutic value of melatonin on sleep disorders in PD was examined in a limited number of clinical studies using 2–5 mg/day melatonin at bedtime. The low levels of melatonin MT1 and MT2 receptor density in substantia nigra and amygdala found in PD patients supported the hypothesis that the altered sleep/wake cycle seen in PD could be due to a disrupted melatonergic system. Motor symptomatology is seen in PD patients when about 75% of the dopaminergic cells in the substantia nigra pars compacta region degenerate. Nevertheless, symptoms like rapid eye movement (REM) sleep behavior disorder (RBD), hyposmia or depression may precede the onset of motor symptoms in PD for years and are index of worse prognosis. Indeed, RBD patients may evolve to an α-synucleinopathy within 10 years of RBD onset. Daily bedtime administration of 3–12 mg of melatonin has been demonstrated effective in RDB treatment and may halt neurodegeneration to PD. In studies on animal models of PD melatonin was effective to curtail symptomatology in doses that allometrically projected to humans were in the 40–100 mg/day range, rarely employed clinically. Therefore, double-blind, placebo-controlled clinical studies are urgently needed in this respect.

Neuroprotective effects of melatonin against neurotoxicity induced by intranasal sodium dimethyldithiocarbamate administration in mice

Exposure to fungicide ziram (zinc dimethyldithiocarbamate) has been associated with increased incidence of Parkinson's disease (PD). We recently demonstrated that the intranasal (i.n.) administration of sodium dimethyldithiocarbamate (NaDMDC, a more soluble salt than ziram) induces PD-like behavioral and neurochemical alterations in mice. We now investigated the putative neuroprotective effects of melatonin on behavioral dificits and neurochemical alterations induced by i.n. NaDMDC. Melatonin treatment (3, 10 or 30 mg/kg, i.p.) was given 1 h before NaDMDC administration (1 mg/nostril) during 4 consecutive days and we evaluated early (up to 7 days) and late (up to 35 days) NaDMDC-induced behavioral and neurochemical alterations. Melatonin treatment protected against early motor and general neurological impairments observed in the open field and neurological score of severity, respectively, and late deficits in rotarod test. Melatonin prevented the NaDMDC-induced alterations in the striatal tyrosine hydroxylase immunocontent. Melatonin also protected against increased levels of oxidative stress markers (4-hydroxynonenal and 3-nitrotyrosine) in the striatum, as well as the NaDMDC-induced increase of 4-hydroxynonenal and TNF, markers of oxidative stress and inflammation, respectively, in the olfactory bulb. These results further detail the mechanisms underlying NaDMDC toxicity and demonstrate the neuroprotective effects of melatonin against the neuronal damage induced by NaDMDC.

Pharmacological Effects of Melatonin as Neuroprotectant in Rodent Model: A Review on the Current Biological Evidence

The progressive loss of structure and functions of neurons, including neuronal death, is one of the main factors leading to poor quality of life. Promotion of functional recovery of neuron after injury is a great challenge in neuroregenerative studies. Melatonin, a hormone is secreted by pineal gland and has antioxidative, anti-inflammatory, and anti-apoptotic properties. Besides that, melatonin has high cell permeability and is able to cross the blood-brain barrier. Apart from that, there are no reported side effects associated with long-term usage of melatonin at both physiological and pharmacological doses. Thus, in this review article, we summarize the pharmacological effects of melatonin as neuroprotectant in central nervous system injury, ischemic-reperfusion injury, optic nerve injury, peripheral nerve injury, neurotmesis, axonotmesis, scar formation, cell degeneration, and apoptosis in rodent models.

Melatonin: Clinical Perspectives in Neurodegeneration

Prevention of neurodegenerative diseases is presently a major goal for our Society and melatonin, an unusual phylogenetically conserved molecule present in all aerobic organisms, merits consideration in this respect. Melatonin combines both chronobiotic and cytoprotective properties. As a chronobiotic, melatonin can modify phase and amplitude of biological rhythms. As a cytoprotective molecule, melatonin reverses the low degree inflammatory damage seen in neurodegenerative disorders and aging. Low levels of melatonin in blood characterizes advancing age. In experimental models of Alzheimer's disease (AD) and Parkinson's disease (PD) the neurodegeneration observed is prevented by melatonin. Melatonin also increased removal of toxic proteins by the brain glymphatic system. A limited number of clinical trials endorse melatonin's potentiality in AD and PD, particularly at an early stage of disease. Calculations derived from animal studies indicate cytoprotective melatonin doses in the 40-100 mg/day range. Hence, controlled studies employing melatonin doses in this range are urgently needed. The off-label use of melatonin is discussed.

Melatonin protects against behavioral deficits, dopamine loss and oxidative stress in homocysteine model of Parkinson's disease

AIM

Hyperhomocysteinemia and homocysteine (Hcy) mediated dopaminergic neurotoxicity is a matter of concern in the pathophysiology of Parkinson's disease (PD). Our previous study established the involvement of oxidative stress in the substantia nigra (SN) of Hcy rat model of PD; however, the role of antioxidants, such as melatonin, was not tested in this model.

MAIN METHODS

Melatonin (10, 20 and 30mg/kg, i.p.) was administered to rats injected with Hcy in right SN (1.0μmol in 2μl saline) to investigate its potency in attenuating the behavioral abnormalities, dopamine depletion and oxidative stress prompted by Hcy.

KEY FINDINGS

Treatment of melatonin protected against nigral dopamine loss and replenished the striatal dopamine loss that resulted in amelioration of rotational behavioral bias in Hcy denervated animals. Melatonin administration significantly improved mitochondrial complex-I activity and protected the SN neurons from the toxic insults of oxidative stress induced by Hcy. Amelioration of oxidative stress by melatonin in Hcy-infused SN was bought by dose-dependently scavenging of hydroxyl radicals, restoration of glutathione level and elevation in the activity of antioxidant enzymes.

SIGNIFICANCE

The observations bring into light the significant neuroprotective potentials of melatonin in Hcy model of PD which is attributed to the attenuation of oxidative stress in SN.

Melatoninergic System in Parkinson's Disease: From Neuroprotection to the Management of Motor and Nonmotor Symptoms

Melatonin is synthesized by several tissues besides the pineal gland, and beyond its regulatory effects in light-dark cycle, melatonin is a hormone with neuroprotective, anti-inflammatory, and antioxidant properties. Melatonin acts as a free-radical scavenger, reducing reactive species and improving mitochondrial homeostasis. Melatonin also regulates the expression of neurotrophins that are involved in the survival of dopaminergic neurons and reduces α-synuclein aggregation, thus protecting the dopaminergic system against damage. The unbalance of pineal melatonin synthesis can predispose the organism to inflammatory and neurodegenerative diseases such as Parkinson's disease (PD). The aim of this review is to summarize the knowledge about the potential role of the melatoninergic system in the pathogenesis and treatment of PD. The literature reviewed here indicates that PD is associated with impaired brain expression of melatonin and its receptors MT₁ and MT₂. Exogenous melatonin treatment presented an outstanding neuroprotective effect in animal models of PD induced by different toxins, such as 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, paraquat, and maneb. Despite the neuroprotective effects and the improvement of motor impairments, melatonin also presents the potential to improve nonmotor symptoms commonly experienced by PD patients such as sleep and anxiety disorders, depression, and memory dysfunction.

Melatonin enhances L-DOPA therapeutic effects, helps to reduce its dose, and protects dopaminergic neurons in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism in mice

L-3,4-dihydroxyphenylalanine (L-DOPA) reduces symptoms of Parkinson's disease (PD), but suffers from serious side effects on long-term use. Melatonin (10-30 mg/kg, 6 doses at 10 hr intervals) was investigated to potentiate L-DOPA therapeutic effects in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism in mice. Striatal tyrosine hydroxylase (TH) immunoreactivity, TH, and phosphorylated ser 40 TH (p-TH) protein levels were assayed on 7th day. Nigral TH-positive neurons stereology was conducted on serial sections 2.8 mm from bregma rostrally to 3.74 mm caudally. MPTP caused 39% and 58% decrease, respectively, in striatal fibers and TH protein levels, but 2.5-fold increase in p-TH levels. About 35% TH neurons were lost between 360 and 600 μm from 940 μm of the entire nigra analyzed, but no neurons were lost between 250 μm rostrally and 220 μm caudally. When L-DOPA in small doses (5-8 mg/kg) failed to affect MPTP-induced akinesia or catalepsy, co-administration of melatonin with L-DOPA attenuated these behaviors. Melatonin administration significantly attenuated MPTP-induced loss in striatal TH fibers (82%), TH (62%) and p-TH protein (100%) levels, and nigral neurons (87-100%). Melatonin failed to attenuate MPTP-induced striatal dopamine depletion. L-DOPA administration (5 mg/kg, once 40 min prior to sacrifice, p.o.) in MPTP- and melatonin-treated mice caused significant increase in striatal dopamine (31%), as compared to L-DOPA and MPTP-treated mice. This was equivalent to 8 mg/kg L-DOPA administration in parkinsonian mouse. Therefore, prolonged, effective use of L-DOPA in PD with lesser side effects could be achieved by treating with 60% lower doses of L-DOPA along with melatonin.

Melatonin synergizes with low doses of L-DOPA to improve dendritic spine density in the mouse striatum in experimental Parkinsonism

The dopamine precursor, L-3,4-dihydroxyphenylalanine (L-DOPA), is the preferred drug for Parkinson's disease, but long-term treatment results in the drug-induced dyskinesias and other side effects. This study was undertaken to examine whether melatonin could potentiate low dose L-DOPA effects in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced experimental parkinsonism. Mice were treated with the parkinsonian neurotoxin, MPTP, and different doses of melatonin and low doses of L-DOPA. Behavior, striatal histology, and dopamine metabolism were evaluated on the 7th day. MPTP-induced striatal dopamine loss was not modified by melatonin administration (10-30 mg/kg; i.p. at 10-hr intervals, 6 times; or at 2-hr intervals, by day). However, low doses of L-DOPA (5 mg/kg, by oral gavage) administered alone or along with melatonin (10 mg/kg, i.p.) twice everyday for 2 days, 10 hr apart, after two doses of MPTP significantly attenuated striatal dopamine loss and provided improvements in both catalepsy and akinesia. Additionally, Golgi-impregnated striatal sections showed preservation of the medium spiny neurons, which have been damaged in MPTP-treated mouse. The results demonstrated that melatonin, but not L-DOPA, restored spine density and spine morphology of medium spiny neurons in the striatum and suggest that melatonin could be an ideal adjuvant to L-DOPA therapy in Parkinson's disease, and by the use of this neurohormone, it is possible to bring down the therapeutic doses of L-DOPA.

Astrocyte activation: a key step in rotenone induced cytotoxicity and DNA damage

Astrocytes are the most abundant glial cells, which provide metabolic support for neurons. Rotenone is a botanical pesticide of natural origin, known to exhibit neurotoxic potential via inhibition of mitochondrial complex-I. This study was carried out to explore the effect of rotenone on C6 cells. The cell line C6 derived from rat glioma cells represents astrocyte-like cell. C6 cells were treated with rotenone (0.1, 1 and 10 μM) for 4 h. The effect of rotenone was studied on cell survival (MTT reduction and PI uptake); free radicals (ROS and RNS) and DNA damage (comet assay and Hoechst staining). The glial cell activation and apoptotic cell death was evaluated by expression of Glial fibrillary acidic protein (GFAP) and caspase-3 respectively. The treatment with rotenone resulted in decreased cell survival and increased free radical generation. Altered nuclear morphology and DNA damage were evident following rotenone treatment in Hoechst staining and Comet assay. Rotenone elevated expression of GFAP and caspase-3 that indicates glial cell activation and apoptosis, respectively. We further studied the effect of melatonin, an antioxidant, on the observed toxic effects. Co-incubation of antioxidant, melatonin (300 μM), significantly suppressed rotenone induced above-mentioned effects in C6 cells. Inhibitory effects of melatonin suggest that free radicals play a major role in rotenone induced astrocyte activation and cellular toxicity leading to apoptosis of astroglial cells.

Rotenone-induced apoptosis and role of calcium: a study on Neuro-2a cells

Rotenone causes cytotoxicity in astrocytic cell culture by glial activation, which is linked to free radical generation. The present study is an investigation to explore whether rotenone could also cause cellular toxicity in mouse neuroblastoma cells (Neuro-2a) under treatment similar to astroglial cells. The effect of rotenone (0.1, 1, and 10 μM) on mitochondrial dehydrogenase enzyme activity by MTT reduction assay, PI uptake, total reactive oxygen species (ROS)/superoxide levels, nitrite levels, extent of DNA damage (by comet assay), and nuclear morphological alteration by Hoechst staining was studied. Caspase-3 and Ca⁺²/calmodulin-dependent protein kinase II (CaMKIIα) gene expression was determined to evaluate the apoptotic cell death and calcium kinase, respectively. Calcium level was estimated fluorometrically using fura-2A stain. Rotenone decreased mitochondrial dehydrogenase enzyme activity and generated ROS, superoxide, and nitrite. Rotenone treatment impaired cell intactness and nuclear morphology as depicted by PI uptake and chromosomal condensation of Neuro-2a cells, respectively. In addition, rotenone resulted in increased intracellular Ca⁺² level, caspase-3, and CaMKIIα expression. Furthermore, co-exposure of melatonin (300 μM), an antioxidant to cell culture, significantly suppressed the rotenone-induced decreased mitochondrial dehydrogenase enzyme activity, elevated ROS and RNS. However, melatonin was found ineffective to counteract rotenone-induced increased PI uptake, altered morphological changes, DNA damage, elevated Ca⁺², and increased expression of caspase-3 and CaMKIIα. The study indicates that intracellular calcium rather than oxidative stress is a major factor for rotenone-induced apoptosis in neuronal cells.

Effect of chronic L-dopa or melatonin treatments after dopamine deafferentation in rats: dyskinesia, motor performance, and cytological analysis

The present study examines the ability of melatonin to protect striatal dopaminergic loss induced by 6-OHDA in a rat model of Parkinson's disease, comparing the results with L-DOPA-treated rats. The drugs were administered orally daily for a month, their therapeutic or dyskinetic effects were assessed by means of abnormal involuntary movements (AIMs) and stepping ability. At the cellular level, the response was evaluated using tyrosine hydroxylase immunoreactivity and striatal ultrastructural changes to compare between L-DOPA-induced AIMs and Melatonin-treated rats. Our findings demonstrated that chronic oral administration of Melatonin improved the alterations caused by the neurotoxin 6-OHDA. Melatonin-treated animals perform better in the motor tasks and had no dyskinetic alterations compared to L-DOPA-treated group. At the cellular level, we found that Melatonin-treated rats showed more TH-positive neurons and their striatal ultrastructure was well preserved. Thus, Melatonin is a useful treatment to delay the cellular and behavioral alterations observed in Parkinson's disease.

Melatonin as a neuroprotective agent in the rodent models of Parkinson's disease: is it all set to irrefutable clinical translation?

Parkinson's disease (PD), a neurodegenerative disorder, is characterized by the selective degeneration of the nigrostriatal dopaminergic neurons, continuing or permanent deficiency of dopamine, accretion of an abnormal form of alpha synuclein in the adjacent neurons, and dysregulation of ubiquitin proteasomal system, mitochondrial metabolism, permeability and integrity, and cellular apoptosis resulting in rigidity, bradykinesia, resting tremor, and postural instability. Melatonin, an indoleamine produced almost in all the organisms, has anti-inflammatory, anti-apoptotic, and anti-oxidant nature. Experimental studies employing 1-methyl 4-phenyl 1, 2, 3, 6-tetrahydropyridine (MPTP), 6-hydroxydopamine (6-OHDA), methamphetamine, rotenone, and maneb and paraquat models have shown an enormous potential of melatonin in amelioration of the symptomatic features of PD. Although a few reviews published previously have described the multifaceted efficacy of melatonin against MPTP and 6-OHDA rodent models, due to development and validation of the newer models as well as the extensive studies on the usage of melatonin in entrenched PD models, it is worthwhile to bring up to date note on the usage of melatonin as a neuroprotective agent in PD. This article presents an update on the usage and applications of melatonin in PD models along with incongruous observations. The impending implications in the clinics, success, limitations, and future prospective have also been discussed in this article.

Neurotoxins: free radical mechanisms and melatonin protection

Toxins that pass through the blood-brain barrier put neurons and glia in peril. The damage inflicted is usually a consequence of the ability of these toxic agents to induce free radical generation within cells but especially at the level of the mitochondria. The elevated production of oxygen and nitrogen-based radicals and related non-radical products leads to the oxidation of essential macromolecules including lipids, proteins and DNA. The resultant damage is referred to as oxidative and nitrosative stress and, when the molecular destruction is sufficiently severe, it causes apoptosis or necrosis of neurons and glia. Loss of brain cells compromises the functions of the central nervous system expressed as motor, sensory and cognitive deficits and psychological alterations. In this survey we summarize the publications related to the following neurotoxins and the protective actions of melatonin: aminolevulinic acid, cyanide, domoic acid, kainic acid, metals, methamphetamine, polychlorinated biphenyls, rotenone, toluene and 6-hydroxydopamine. Given the potent direct free radical scavenging activities of melatonin and its metabolites, their ability to indirectly stimulate antioxidative enzymes and their efficacy in reducing electron leakage from mitochondria, it would be expected that these molecules would protect the brain from oxidative and nitrosative molecular mutilation. The studies summarized in this review indicate that this is indeed the case, an action that is obviously assisted by the fact that melatonin readily crosses the blood brain barrier.

Chronic melatonin treatment and its precursor L-tryptophan improve the monoaminergic neurotransmission and related behavior in the aged rat brain

Melatonin has an important role in the aging process as a potential drug to relieve oxidative damage, a likely cause of age-associated brain dysfunction. As age advances, the nocturnal production of melatonin decreases potentially causing physiological alterations. The present experiments were performed to study in vivo the effects of exogenously administered melatonin chronically on monoaminergic central neurotransmitters serotonin (5-HT), dopamine (DA) and norepinephrine (NE) and behavioral tests in old rats. The accumulation of 5-hydroxy-tryptophan (5-HTP) and L-3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in rat brain. Also neurotransmitters 5-HT, DA and NE and some metabolites were quantified by HPLC. In control rats, an age-related decline was observed in neurochemical parameters. However, chronic administration of melatonin (1 mg/kg/day, diluted in drinking water, 4 wk) significantly reversed the age-induced deficits in all the monoaminergic neurotransmitters studied. Also, neurochemical parameters were analyzed after administration of melatonin biosynthesis precursor L-tryptophan (240 mg/kg/day, i.p., at night for 4 wk) revealing similar improvement effects to those induced by melatonin. Behavioral data corresponded well with the neurochemical findings since spatial memory test in radial-maze and motor coordination in rota-rod were significantly improved after chronic melatonin treatment. In conclusion, these in vivo findings suggest that melatonin and L-tryptophan treatments exert a long-term effect on the 5-HT, DA and NE neurotransmission by enhancing monoamine synthesis in aged rats, which might improve the age-dependent deficits in cognition and motor coordination.

Driving GDNF expression: the green and the red traffic lights

Glial cell line-derived neurotrophic factor (GDNF) is widely recognized as a potent survival factor for dopaminergic neurons of the nigrostriatal pathway that degenerate in Parkinson's disease (PD). In animal models of PD, GDNF delivery to the striatum or the substantia nigra protects dopaminergic neurons against subsequent toxin-induced injury and rescues previously damaged neurons, promoting recovery of the motor function. Thus, GDNF was proposed as a potential therapy to PD aimed at slowing down, halting or reversing neurodegeneration, an issue addressed in previous reviews. However, the use of GDNF as a therapeutic agent for PD is hampered by the difficulty in delivering it to the brain. Another potential strategy is to stimulate the endogenous expression of GDNF, but in order to do that we need to understand how GDNF expression is regulated. The aim of this review is to do a comprehensive analysis of the state of the art on the control of endogenous GDNF expression in the nervous system, focusing mainly on the nigrostriatal pathway. We address the control of GDNF expression during development, in the adult brain and after injury, and how damaged neurons signal glial cells to up-regulate GDNF. Pharmacological agents or natural molecules that increase GDNF expression and show neuroprotective activity in animal models of PD are reviewed. We also provide an integrated overview of the signalling pathways linking receptors for these molecules to the induction of GDNF gene, which might also become targets for neuroprotective therapies in PD.

Melatonin activates brain dopaminergic systems in the eel with an inhibitory impact on reproductive function

In the eel, a deficit in gonadotrophin-releasing hormone (GnRH) and a strong dopaminergic (DA) inhibition are responsible for the blockade of gonad development if silver eels are prevented from their reproductive migration. Environmental factors that eels encounter during their oceanic reproductive migration are thought to play an important role in the stimulation of eel pubertal development. We investigated the potential role of melatonin, a known mediator of the effects of external factors on reproductive function in vertebrates. We demonstrated that a long-term melatonin treatment increased brain tyrosine hydroxylase (TH, the rate limiting enzyme of DA synthesis) mRNA expression in a region-dependent way. Melatonin stimulated the dopaminergic system of the preoptic area, which is involved in the inhibitory control of gonadotrophin [luteinising hormone (LH) and follicle-stimulating hormone (FSH)] synthesis and release. Moreover, we showed that the increased TH expression appeared to be consistent with melatonin binding site distribution as shown by 2[(125)I]-melatonin labelling studies. On the other hand, melatonin had no effects on the two eel native forms of GnRH (mGnRH and cGnRH-II) mRNA expression. Concerning the pituitary-gonad axis, we showed that melatonin treatment decreased both gonadotrophin beta-subunit (LHbeta, FSHbeta) mRNA expression and reduced sexual steroid (11-ketotestosterone, oestradiol) plasma levels. This indicates that melatonin treatment had a negative effect on eel reproductive function. To our knowledge, the results of the present study provide the first evidence that melatonin enhances TH expression in specific brain regions in a non-mammalian species. By this mechanism melatonin could represent one pathway by which environmental factors could modulate reproductive function in the eel.

Genetic findings in Parkinson's disease and translation into treatment: a leading role for mitochondria?

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder and in most patients its aetiology remains unknown. Molecular genetic studies in familial forms of the disease identified key proteins involved in PD pathogenesis, and support a major role for mitochondrial dysfunction, which is also of significant importance to the common sporadic forms of PD. While current treatments temporarily alleviate symptoms, they do not halt disease progression. Drugs that target the underlying pathways to PD pathogenesis, including mitochondrial dysfunction, therefore hold great promise for neuroprotection in PD. Here we summarize how the proteins identified through genetic research (alpha-synuclein, parkin, PINK1, DJ-1, LRRK2 and HTRA2) fit into and add to our current understanding of the role of mitochondrial dysfunction in PD. We highlight how these genetic findings provided us with suitable animal models and critically review how the gained insights will contribute to better therapies for PD.

Melatonin protects SK-N-SH neuroblastoma cells from amphetamine-induced neurotoxicity

Several hypotheses regarding the mechanism underlying amphetamine-induced neurotoxicity have been proposed. One of them is based on the observation of free radical formation and oxidative stress produced by auto-oxidation of dopamine (DA). The formation of DA-related reactive oxygen species (ROS) such as superoxide and hydroxyl radicals appears to play an important role in amphetamine-induced neurotoxicity. Melatonin, the main secretory product of pineal gland, is well known for its protective effects that are currently attributed mainly to its radical scavenging and antioxidant properties. The present study was conducted to investigate the protective effects of melatonin on d-amphetamine (AMPH)-induced neurotoxicity in cultured human dopaminergic neuroblastoma SK-N-SH cells. Our data indicate that AMPH significantly reduces cell viability, induces oxidative stress (enhances ROS production and malondialdehyde levels), up-regulates alpha-synuclein expression and decreases intracellular ATP levels. However, pretreatment of SK-N-SH cells with melatonin prevents AMPH-induced loss of cell viability and induction of oxidative stress, while reducing alpha-synuclein expression and increasing ATP production. These results suggest that the antioxidant properties of melatonin may provide a protective mechanism against AMPH-induced neuronal degeneration.

Modulation of tyrosine hydroxylase expression by melatonin in human SH-SY5Y neuroblastoma cells

We have previously reported in vivo preservation of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis, following treatment with physiological doses of melatonin, in a 6-hydroxydopamine model of Parkinson's disease. Based on these findings, we postulated that melatonin would similarly modulate the expression of TH in vitro. Therefore, using human SH-SY5Y neuroblastoma cells which can differentiate into dopaminergic neurons following treatment with retinoic acid, we first examined whether these cells express melatonin receptors. Subsequently, the physiological dose-dependent effects of melatonin on TH expression were examined in both undifferentiated and differentiated cells. The novel detection of the G protein-coupled melatonin MT(1) receptor in SH-SY5Y cells by RT-PCR was confirmed by sequencing and Western blotting. In addition, following treatment of SH-SY5Y cells with melatonin (0.1-100 nM) for 24h, Western analysis revealed a significant increase in TH protein levels. A biphasic response, with significant increases in TH protein at 0.5 and 1 nM melatonin and a reversal at higher doses was seen in undifferentiated cells; whereas in differentiated cells, melatonin was effective at doses of 1 and 100 nM. These findings suggest a physiological role for melatonin in modulating TH expression, possibly via the MT(1) receptor.

Oxidative stress and dopamine depletion in an intrastriatal 6-hydroxydopamine model of Parkinson's disease

Although the etiology of Parkinson's disease (PD) is unknown, a common element of most theories is the involvement of oxidative stress, either as a cause or effect of the disease. There have been relatively few studies that have characterized oxidative stress in animal models of PD. In the present study a 6-hydroxydopamine (6-OHDA) rodent model of PD was used to investigate the in vivo production of oxidative stress after administration of the neurotoxin. 6-OHDA was injected into the striatum of young adult rats and the production of protein carbonyls and 4-hydroxynonenal (HNE) was measured at 1, 3, 7, and 14 days after administration. A significant increase in both markers was found in the striatum 1 day after neurotoxin administration, and this increase declined to basal levels by day 7. There was no significant increase found in the substantia nigra at any of the time points investigated. This same lesion paradigm produced dopamine depletions of 90-95% in the striatum and 63-80% in the substantia nigra by 14-28 days post-6-OHDA. Protein carbonyl and HNE levels were also measured in middle-aged and aged animals 1 day after striatal 6-OHDA. Both protein carbonyl and HNE levels were increased in the striatum of middle-aged and aged animals treated with 6-OHDA, but the increases were not as great as those observed in the young adult animals. Similar to the young animals, there were no increases in either marker in the substantia nigra of the middle-aged and aged animals. There was a trend for an age-dependent increase in basal amounts of oxidative stress markers when comparing the non-lesioned side of the brains of the three age groups. These results support that an early event in the course of dopamine depletion following intrastriatal 6-OHDA administration is the generation of oxidative stress.

Bax and Bcl-2 expression and TUNEL labeling in lumbar enlargement of neonatal rats after sciatic axotomy and melatonin treatment

Peripheral axotomy in neonatal rats induces neuronal death. We studied the anti-apoptotic protein Bcl-2 and cell death promoter Bax in spinal cord of neonatal rats after sciatic transection and treatment with melatonin, a neuroprotective substance. Pups were unilaterally axotomized at P2 and received melatonin (1 mg/kg; sc) or vehicle 1 h prior to lesion, immediately after, at 1 h, 2 h and then once daily. Rats were sacrificed at 3 h, 6 h, 24 h, 72 h and 5 days postaxotomy. Intact animals were used as controls. Lumbar enlargement was processed for Nissl staining, immunohistochemistry and RT-PCR for Bax or Bcl-2 and TUNEL reaction. Motoneurons (MN) of lesioned (L) and normal (N) sides were counted, and MN survival ratio (MSR=L/N) was calculated. Bax and Bcl-2 showed cytoplasmic immunoreactivity (IR). Bax IR was noticeable in small cells but less evident in MN. In unlesioned pups, some Bax-positive small cells (B+) and TUNEL-positive nuclei (T+) were mainly seen in the dorsal horn. In lesioned animals given vehicle, Bax mRNA levels and numbers of B+ and T+ were increased in comparison with intact controls at 24 h postaxotomy. The basal IR for Bax in MN was not changed by axotomy. Bcl-2 IR was noted in all cells and, like Bcl-2 mRNA, was unaltered after lesion. Melatonin reduced MN loss at 24 h, 72 h and 5 days and T+ at 24 h after lesion but did not interfere with Bax or Bcl-2 expression. These results suggest that (1) sciatic transection at P2 increases Bax mRNA and the amount of B+ and T+ in the lumbar enlargement, (2) Bax IR in immature MN is not altered by axotomy and (3) melatonin protects MN and dorsal horn cells through a mechanism independent of Bax and Bcl-2.

Inhibition of mitochondria responsible for the anti-apoptotic effects of melatonin during ischemia-reperfusion

OBJECTIVE

To investigate a possible mechanism responsible for anti-apoptotic effects of melatonin and provide theoretical evidences for clinical therapy.

METHODS

Ischemia-reperfusion mediated neuronal cell injury model was constructed in cerebellar granule neurons (CGNs) by deprivation of glucose, serum and oxygen in media. After ischemia, melatonin was added to the test groups to reach differential concentration during reperfusion. DNA fragmentation, mitochondrial transmembrane potential, mitochondrial cytochrome c release and caspase-3 activity were observed after subjecting cerebellar granule neurons to oxygen-glucose deprivation (OGD).

RESULTS

The results showed that OGD induced typical cell apoptosis change, DNA ladder and apoptosis-related alterations in mitochondrial functions including depression of mitochondrial transmembrane potential (its maximal protection ratio was 73.26%) and release of cytochrome c (its maximal inhibition ratio was 42.52%) and the subsequent activation of caspase-3 (its maximal protection ratio was 59.32%) in cytoplasm. Melatonin reduced DNA damage and inhibited release of mitochondrial cytochrome c and activation of caspase-3. Melatonin can strongly prevent the OGD-induced loss of the mitochondria membrane potential.

CONCLUSION

Our findings suggested that the direct inhibition of mitochondrial pathway might essentially contribute to its anti-apoptotic effects in neuronal ischemia-reperfusion.

Physiological neuroprotection by melatonin in a 6-hydroxydopamine model of Parkinson's disease

There is considerable evidence that pharmacological doses of the pineal hormone, melatonin, are neuroprotective in diverse models of neurodegeneration including Parkinson's disease. However, there is limited information about the effects of physiological doses of this hormone in similar models. In this study, rats were chronically treated with melatonin via drinking water following partial 6-hydroxydopamine lesioning in the striatum. The two doses of melatonin (0.4 microg/ml and 4.0 microg/ml) were within the reported physiological concentrations present in the serum and cerebrospinal fluid respectively. At 2 weeks after surgery, the higher dose of melatonin significantly attenuated rotational behavior in hemi-parkinsonian rats compared to similarly lesioned animals receiving either vehicle (P < 0.001) or the lower dose of melatonin (P < 0.01). Animals were perfused or sacrificed 10 weeks after commencing melatonin treatment for immunohistochemical or mRNA studies. Animals treated with 4.0 microg/ml melatonin exhibited normal tyrosine hydroxylase (TH) immunoreactivity in the lesioned striatum, whereas little or no TH immunofluorescence was visible in similarly lesioned animals receiving vehicle. In contrast, semiquantitative RT-PCR analysis revealed no group differences in TH mRNA, suggesting spontaneous recovery of this transcript as observed previously in partially lesioned animals. There were no significant differences in striatal GDNF mRNA levels between sham and lesioned animals. However, there was a significant (P < 0.01) increase in GDNF mRNA expression in the intact contralateral striata of lesioned animals treated with vehicle. Interestingly, melatonin treatment attenuated this novel compensatory contralateral increase in striatal GDNF expression, presumably due to its neuroprotective effect. These findings support a physiological role for melatonin in protecting against parkinsonian neurodegeneration in the nigrostriatal system.

Potential neuroprotective effect oft-butylhydroquinone against neurotoxicity?Induced by 1-methyl-4-(2?-methylphenyl)-1,2,3,6-tetrahydropyridine (2?-methyl-MPTP) in mice

Dopaminergic damage inducing Parkinson's disease (PD) is ubiquitous neurodegenerative disorder, characterized by the progressive loss of dopaminergic neurons in the nigrostriatal pathways. The etiology and pathogenic factors implicated in dopaminergic damage are still unexplored to develop causal therapeutic strategies aimed to halt its progressive loss. The neurotoxicity induced by 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP), which is more potent neurotoxic than MPTP in mice, is one of the most valuable models for analyzing pathological feature of dopaminergic damage. Herein, we investigated the neuroprotective effect of the potent antioxidant tertiary butylhydroquinone (TBHQ) against 2'CH3-MPTP-induced neurotoxicity in mice as well as the possible mechanism underlying such neurotoxicity. Male albino mice were injected with two doses of 2'CH3-MPTP (20 mg/kg, i.p.) for two consecutive days. Animals were killed after 2 weeks from the last dose of 2'CH3-MPTP. Control animals received 10 mL/kg body weight i.p. of distilled water. In both groups, brain stems containing the nigrostriatal pathways were dissected and reduced glutathione (GSH), malonyldialdehyde (MDA) contents, and superoxide dismutase (SOD) activity were estimated. Also, brain stem histopathological and histochemical changes were examined. The results of this study revealed that i.p. injection of 2'CH3-MPTP caused decrease in the brain stem content of GSH. On the other hand, the content of MDA and SOD activity was increased as compared with control groups. Also, 2'CH3-MPTP showed severe histopathological changes including swelling of cytoplasm, interstitial edema, and complete loss of the neurons with reactive microglial proliferation and gliosis. Furthermore, histochemical examination of brain stem qualitatively showed depletion of dopaminergic neurons of nigrostriatum. Oral administration of TBHQ (100 mg/kg) prior to 2'CH3-MPTP for 7 days caused normalization of GSH content and SOD activity and ameliorated the MDA content but still above the control value. Pretreatment with TBHQ slightly mitigated the histopathological and histochemical changes observed in 2'CH3-MPTP-treated mice. Based on these observations, it can be concluded that the antioxidant TBHQ has the ability to reverse the oxidative stress caused by 2'CH3-MPTP in mice while failed to challenge the histopathological and histochemical changes induced by that toxicant.

Direct inhibition of the mitochondrial permeability transition pore: a possible mechanism responsible for anti‐apoptotic effects of melatonin

Melatonin, the secretory product of the pineal gland, is known to be neuroprotective in cerebral ischemia, which is so far mostly attributed to its antioxidant properties. Here we show that melatonin directly inhibits the mitochondrial permeability transition pore (mtPTP). mtPTP contributes to the pathology of ischemia by releasing calcium and cytochrome c (cyt c) from mitochondria. Consistently, NMDA-induced calcium rises were diminished by melatonin in cultured mouse striatal neurons, similar to the pattern seen with cyclosporine A (CsA). When the mouse striatal neurons were subjected to oxygen-glucose deprivation (OGD), melatonin strongly prevented the OGD-induced loss of the mitochondrial membrane potential. To assess the direct effect of melatonin on the mtPTP activity at the single channel level, recordings from the inner mitochondrial membrane were obtained by a patch-clamp approach using rat liver mitoplasts. Melatonin strongly inhibited mtPTP currents in a dose-dependent manner with an IC50 of 0.8 microM. If melatonin is an inhibitor of the mtPTP, it should prevent mitochondrial cyt c release as seen in stroke models. Rats underwent middle cerebral artery occlusion (MCAO) for 2 h followed by reperfusion. Melatonin (10 mg/kg ip) or vehicle was given at the time of occlusion and at the time of reperfusion. Indeed, infarct area in the brain sections of melatonin-treated animals displayed a considerably decreased cyt c release along with less activation of caspase-3 and apoptotic DNA fragmentation. Melatonin treatment diminished the loss of neurons and decreased the infarct volume as compared with untreated MCAO rats. Our findings suggest that the direct inhibition of the mtPTP by melatonin may essentially contribute to its anti-apoptotic effects in transient brain ischemia.

Protective effect of melatonin in a chronic experimental model of Parkinson's disease

Parkinson's disease is a chronic condition characterized by cell death of dopaminergic neurons mainly in the substantia nigra. Among the several experimental models used in mice for the study of Parkinson's disease 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine- (MPTP-) induced parkinsonism is perhaps the most commonly used. This neurotoxin has classically been applied acutely or sub-acutely to animals. In this paper we use a chronic experimental model for the study of Parkinson's disease where a low dose (15 mg/kg bw) of MPTP was administered during 35 days to mice to induce nigral cell death in a non-acute way thus emulating the chronic condition of the disease in humans. Free radical damage has been implicated in the origin of this degeneration. We found that the antioxidant melatonin (500 microg/kg bw) prevents cell death as well as the damage induced by chronic administration of MPTP measured as number of nigral cells, tyrosine hydroxylase levels, and several ultra-structural features. Melatonin, which easily passes the blood-brain barrier and lacks of any relevant side-effect, is proposed as a potential therapy agent to prevent the disease and/or its progression.

Melatonin attenuates MPP+-induced neurodegeneration and glutathione impairment in the nigrostriatal dopaminergic pathway

In this study we selected a rat model of Parkinson's disease (PD) by using intrastriatal infusion of the 1-methyl-4-phenyl-pyridinium ion (MPP+) to investigate the neuroprotective action of melatonin and its inhibitory activity on MPP+-impaired glutathione (GSH) system in the nigrostriatal system. Results show that MPP+ caused not only a severe neuronal injury in the striatum and in the ipsilateral substantia nigra (SN), but it also induced a significant decrease in GSH levels and an increase in the GSSG/GSH ratio 3 days after intrastriatal MPP+ infusion. Intraperitoneal co-administration of melatonin (10 mg/kg, five times) significantly attenuated MPP+-induced nigrostriatal neurotoxicity and GSH impairment. Depletion of cytosolic GSH by L-buthionine sulfoximine (BSO) did not cause neuronal damage by itself. It, however, when co-administrated with MPP+, potentiated the GSH reduction in the striatum, without aggravating nigrostriatal neurodegeneration induced by MPP+. Moreover, the MPP+-caused neuronal damage was positively correlated with a rising ratio of GSSG/GSH, but not with a drop of GSH. These results suggest that the MPP+-triggered oxidative stress may play a more important role than the loss of the antioxidant GSH in determining neuronal injury. Interestingly, the neuronal damage and oxidative stress elicited by co-treatment of BSO with MPP+ were effectively reduced by melatonin. Our results hence provide direct evidence showing that melatonin attenuates MPP+-induced nigrostriatal dopaminergic injury by its ability to impede the increase of GSSG/GSH ratio; therefore melatonin may have therapeutic implications in PD.

Antioxidant compounds protect dopamine neurons from death due to oxidative stress in vitro

Using tissue culture models of oxidative stress caused by serum deprivation or MPTP/MPP+ toxicity, the present study establishes that the antioxidants epigallocatechin gallate, lazaroids U74389G and U83836E, reservatrol, MnTBAP, MCI 186, trolox, and melatonin protect 68-100% of dopamine (DA) neurons from cell death. In contrast, the nitric oxide inhibitor LY83583, the caspase inhibitors Z-VAD-FMK, Ac-DQMD-CHO and Z-DEVD-FMK, and the CDK-5 inhibitor, roscovotine were not neuroprotective, although death was often delayed by 1 day in vitro. We conclude that antioxidants are more effective at preventing cell death in vitro than are inhibitors at later stages in the death cascade.

Induction of GDNF mRNA expression by melatonin in rat C6 glioma cells

In order to determine the physiological effect of melatonin on glial cell line-derived neurotrophic factor (GDNF), which is reportedly up-regulated by high doses of this hormone, concentration-dependent studies were carried out in cultured cells. RT-PCR studies indicated that, in addition to GDNF, rat C6 glioma cells express both of the G protein-coupled melatonin receptor subtypes, MT1 and MT2. When C6 cells were treated with physiological (0.05-1 nM) or higher (10 and 100 nM) concentrations of melatonin for 24 h, a significant induction of relative GDNF mRNA levels (n = 4) was detected by semi-quantitative RT-PCR. These findings suggest that induction of GDNF is involved in physiological neuroprotection by melatonin. Given the potency of GDNF in maintaining nigrostriatal dopaminergic integrity, understanding the mechanisms of its induction by melatonin could provide novel therapies for Parkinson's disease.

Melatonin induces tyrosine hydroxylase mRNA expression in the ventral mesencephalon but not in the hypothalamus

We have evaluated the effect of chronic administration of melatonin in terms of mRNA expression for tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, and in the terms of dopamine (DA) transporter (DAT) by means of in situ hybridization. Experimental rats received daily late afternoon injections of 1.5 mg/kg melatonin for 30 days and analysis were performed in the ventral mesencephalon including the substantia nigra (SN) and ventral tegmental area (VTA), and hypothalamus. In the ventral mesencephalon, melatonin treatment significantly induced TH mRNA levels in individual dopaminergic neurons in SN and VTA. In contrast, DAT mRNA levels remained at control levels. Striatal synaptosomal DA uptake was not modified by melatonin treatment as compared with controls. Analysis of glutamic acid decarboxylase (GAD) mRNA in SN, the biosynthetic enzyme for GABAergic neurons, revealed no effect of melatonin treatment on mRNA levels for this marker. In the hypothalamus, we performed mRNA quantitation for TH in arcuate nucleus (Arc) and supraoptic nucleus (SO). Melatonin treatment failed to alter mRNA levels in either area. We detected weak but significant mRNA levels for DAT in Arc, SO, zona incerta (ZI) and periventricular hypothalamic nucleus (Pe). However, because of the low levels of mRNA in hypothalamic areas we were unable to perform a reliable measurement of DAT mRNA levels in response to melatonin treatment. We conclude that melatonin administration, that combines antioxidant capacity and a tissue-specific TH inducing effect, may be useful as a pharmacological agent to protect dopaminergic neurons from degeneration.

Melatonin-dopamine interactions: from basic neurochemistry to a clinical setting

To review the interaction between melatonin and the dopaminergic system in the hypothalamus and striatum and its potential clinical use in dopamine-related disorders in the central nervous system. Medline-based search on melatonin-dopamine interactions in mammals. Melatonin. the hormone produced by the pineal gland at night. influences circadian and seasonal rhythms, most notably the sleep-wake cycle and seasonal reproduction. The neurochemical basis of these activities is not understood yet. Inhibition of dopamine release by melatonin has been demonstrated in specific areas of the mammalian central nervous system (hypothalamus, hippocampus, medulla-pons, and retina). Antidopaminergic activities of melatonin have been demonstrated in the striatum. Dopaminergic transmission has a pivotal role in circadian entrainment of the fetus, in coordination of body movement and reproduction. Recent findings indicate that melatonin may modulate dopaminergic pathways involved in movement disorders in humans. In Parkinson patients melatonin may, on the one hand, exacerbate symptoms (because of its putative interference with dopamine release) and, on the other, protect against neurodegeneration (by virtue of its antioxidant properties and its effects on mitochondrial activity). Melatonin appears to be effective in the treatment of tardive dyskinesia. a severe movement disorder associated with long-term blockade of the postsynaptic dopamine D2 receptor by antipsychotic drugs in schizophrenic patients. The interaction of melatonin with the dopaminergic system may play a significant role in the nonphotic and photic entrainment of the biological clock as well as in the fine-tuning of motor coordination in the striatum. These interactions and the antioxidant nature of melatonin may be beneficial in the treatment of dopamine-related disorders.

Dehydroepiandrosterone and Melatonin: Two Neurohormones

Complementary or alternative medicines in the United States have received prominent attention during recent years. Though there seems to be little rigorous support for the long-term efficacy and safety of these treatments in humans,1-2 the popularity of these natural products persists. It has been estimated that, for 1997, 12 percent of the U.S. population (approximately 24 million people) had used herbal medicines within the previous 12 months, and that 5 billion dollars had been spent on these products during that same year.3 Two of the alternative medicines included are dehydroepiandrosterone (DHEA) and melatonin. DHEA is a hormone produced primarily by the adrenal glands, and is a precursor for endogenous steroid production (e.g., testosterone, estrogen). DHEA is also believed to exert central pharmacologic effects. Melatonin is a hormone produced by the pineal gland and is involved in circadium rhythm maintenance and sleep regulation. Because of extensive physiological effects, these two neuro-hormones are being advocated for the treatment of several medical disorders, as well as for routine ailments. The purpose of this article is to discuss salient, pharmacotherapeutic-related characteristics of each hormone, and to briefly review supporting evidence from studies using human subjects.