One molecule, many derivatives: a never-ending interaction of melatonin with reactive oxygen and nitrogen species?

Therapeutic potential of melatonin agonists

Melatonin, a hormone secreted by the pineal gland, has been successfully employed to improve sleep in both normal patients and insomniacs, and for the treatment of circadian rhythm sleep disorders. Melatonergic MT₁ and MT₂ receptors exist in high concentrations in the suprachiasmatic nucleus of the hypothalamus and have been shown to be instrumental for the sleep-promoting and circadian rhythm-regulating effects of melatonin. A lack of consistency among reports on the therapeutic efficacy of melatonin has been attributed to differences in melatonin's bioavailability and the short half-life of the hormone. In view of the need for longer acting melatonergic agonists that improve sleep efficiency without causing drug abuse or dependency, ramelteon (Rozerem™, Takeda) was developed. Ramelteon, which acts via MT₁/MT₂ melatonergic agonism, has been found clinically effective for improving total sleep time and sleep efficiency in insomniacs. Agomelatine (Valdoxan™, Servier) is another MT₁/MT₂ melatonergic agonist that also displays antagonist activity at 5-HT_2C serotonin receptors. Agomelatine has been found effective in treating depression and sleep disorders in patients with major depressive disorder. A slow-release preparation of melatonin (Circadin™, Neurim) has been shown to be effective in treating sleep disorders in the elderly population.

The co-catalytic effect of chlorpromazine on peroxidase-mediated oxidation of melatonin: enhanced production of N1-acetyl-N2-formyl-5-methoxykynuramine

Accumulating evidence points to relationships between increased production of reactive oxygen or decreased antioxidant protection in schizophrenic patients. Chlorpromazine (CPZ), which remains a benchmark treatment for people with schizophrenia, has been described as a pro-oxidant compound. Because the antioxidant compound melatonin exerts protective effects against CPZ-induced liver disease in rats, in this investigation, our main objective was to study the effect of CPZ as a co-catalyst of peroxidase-mediated oxidation of melatonin. We found that melatonin was an excellent reductor agent of preformed CPZ cation radical (CPZ(*+)). The addition of CPZ during the horseradish peroxidase (HRP)-catalyzed oxidation of melatonin provoked a significant increase in the rate of oxidation and production of N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK). Similar results were obtained using myeloperoxidase. The effect of CPZ on melatonin oxidation was rather higher at alkaline pH. At pH 9.0, the efficiency of oxidation of melatonin was 15 times higher and the production of AFMK was 30 times higher as compared with the assays in the absence of CPZ. We suggest that CPZ is able to exacerbate the rate of oxidation of melatonin by an electron transfer mechanism where CPZ(*+), generated during the peroxidase-catalyzed oxidation, is able to efficiently oxidize melatonin.

Diurnal differences in melatonin effect on intracellular Ca2+ concentration in chicken spleen leukocytes in vitro

Melatonin plays a pleiotropic role in the immune system of mammals and birds. Endogenous and exogenous melatonin modulates lymphocyte proliferation via specific MT(1), MT(2) and Mel(1c) membrane receptors, although the mechanisms behind this process are poorly understood. The diurnal changes in the expression and function of melatonin membrane receptors within the immune system have so far received little attention. We investigated the day/night differences in melatonin membrane receptor mRNA expression in chicken lymphoid organs and cultured splenocytes and examined the in vitro effect of melatonin and 2-iodomelatonin on the intracellular Ca(2+) concentration ([Ca(2+)](i)) in chicken splenocytes. In whole organs, expression of all subtypes of Mel membrane receptors was observed, and the level did not change significantly with the time of day. Interestingly, we observed a significant increase in the expression of the transcripts of all receptor subtypes in cultured splenocytes isolated at night compared with cells obtained during the day. In chicken spleen leukocytes isolated during the day, melatonin and 2-iodomelatonin increased [Ca(2+)](i), with only 2-iodomelatonin being effective in the 'night' cells. Luzindole modulated the [Ca(2+)](i) increase caused by melatonin receptor agonists: it potentiated the stimulatory effect of melatonin during the day, but counteracted that evoked by 2-iodomelatonin at night. The results of this study demonstrate that melatonin can induce changes in [Ca(2+)](i) in chicken spleen leukocytes that should modulate proliferation. The effect of melatonin on [Ca(2+)](i) is less pronounced at night, possibly caused by receptor desensitization.

Melatonin prevents gestational hyperhomocysteinemia-associated alterations in neurobehavioral developments in rats

Chronic hyperhomocysteinemia is a risk factor in cardiovascular diseases and neurodegeneration. Among the putative mechanisms of homocysteine-induced neurotoxicity, an increased production of reactive oxygen species has been suggested. However, elevated homocysteine levels might disturb neurogenesis during brain development and lead to persistent congenital malformations in the fetus. In this study, we examined whether administration of melatonin inhibits maternal hyperhomocysteinemia-induced cognitive deficits in offspring. Hyperhomocysteinemia was induced in female rats by administration of methionine during pregnancy at a dose of 1 g/kg body weight dissolved in drinking water. Some animals received methionine plus 10 mg/kg/day melatonin subcutaneously throughout pregnancy. The levels of glial fibrillary acidic protein, S100B protein, and neural cell adhesion molecules were determined in the brain tissue from the pups. Learning and memory performances of the young-adult offspring were tested using the Morris water maze test. There were significant reductions in the expression of glial fibrillary acidic protein and S100 B protein in the brains of pups from hyperhomocysteinemic rat dams. Furthermore, maternal hyperhomocysteinemia altered the expression pattern of neural cell adhesion molecules in the fetal brain. In addition, maternal hyperhomocysteinemia significantly reduced learning abilities in offspring. Treatment with melatonin during pregnancy improved learning deficits and prevented the reduction of glial and neuronal markers induced by hyperhomocysteinemia. In conclusion, administration of melatonin throughout pregnancy reduces the effects of hyperhomocysteinemia on the development of fetal brain; therefore, it might be beneficial in preventing persistent congenital malformations.

Protective effect of melatonin against hippocampal injury of rats with intermittent hypoxia

Obstructive sleep apnea (OSA) patients suffer from intermittent hypoxia (IH) and neuropsychologic impairments. Oxidative stress is involved in the pathogenesis of OSA, so the application of an antioxidant may be useful. We evaluated the hypothesis that melatonin would reduce IH-induced hippocampal injury via an increased expression of antioxidant enzymes. Adult Sprague-Dawley rats that had received a daily injection of melatonin or vehicle were exposed to IH for 8 hr/day for 7 or 14 days. The serum and hippocampus were harvested for the measurement of malondialdehyde (MDA). Apoptotic cell death was studied histologically in hippocampal sections. The mRNA expression of inflammatory mediators including tumor necrosis factor-alpha, inducible nitric oxide synthase, cyclooxygenase-2 and antioxidant enzymes including glutathione peroxidase, catalase and copper/zinc superoxide dismutase were examined in the hippocampus by RT-PCR. The results show significant increases in levels of serum and hippocampal MDA, apoptotic cell death and mRNA levels of inflammatory mediators in hypoxic rats when compared with the normoxic controls. Also, mRNA levels of the antioxidant enzymes were decreased in hypoxic animals. In the melatonin-treated hypoxic rats, serum MDA levels were comparable with those in normoxic control rats. Also, melatonin treatment significantly reduced hippocampal MDA levels and totally prevented apoptosis. Moreover, there were a decreased expression of the inflammatory mediators and an elevated expression of antioxidant enzymes in the melatonin injected rats when compared with vehicle-treated animals. These results indicate that melatonin mitigates oxidative stress and the pathogenesis of IH-induced hippocampal injury via its antioxidant and anti-inflammatory properties which includes stimulation of transcriptional regulation of antioxidant enzymes.

Melatonin reduces the neuronal loss, downregulation of dopamine transporter, and upregulation of D2 receptor in rotenone-induced parkinsonian rats

Parkinson's disease (PD) is a movement disorder resulting from nigrostriatal dopaminergic neurodegeneration. The impairment of mitochondrial function and dopamine synaptic transmission are involved in the pathogenesis of PD. Two mitochondrial inhibitors, 1-methyl-4-phenylpyridine (MPP(+)) and rotenone, have been used to induce dopaminergic neuronal death both in in vitro and in vivo models of PD. Because the uptake of MPP(+) is mediated by the dopamine transporter (DAT), we used a cell-permeable rotenone-induced PD model to investigate the role of DAT and dopamine D2 receptor (D2R) on dopaminergic neuronal loss. Rotenone subcutaneously infused for 14 days induced PD symptoms in rats, as indicated by reduced spontaneous locomotor activity (hypokinesis), loss of tyrosine hydroxylase (TH, a marker enzyme for dopamine neurons) immunoreactivity in the substantia nigra and striatum, obvious alpha-synuclein accumulation, downregulated DAT protein expression, and upregulated D2R expression. Interestingly, rotenone also caused significant noradrenergic neuronal loss in the locus coeruleus. Melatonin, an antioxidant, prevented nigrostriatal neurodegeneration and alpha-synuclein aggregation without affecting the rotenone-induced weight loss and hypokinesis. However, rotenone-induced hypokinesis was markedly reversed by the DAT antagonist nomifensine and body weight loss was attenuated by the D2R antagonist sulpiride. In addition, both antagonists significantly prevented the reduction of striatal TH or DAT immunoreactivity but not the loss of nigral TH- and DAT-immunopositive neurons. These results suggested that oxidative stress and DAT downregulation are involved in the rotenone-induced pathogenesis of nigrostriatal dopaminergic neurodegeneration, whereas D2R upregulation may simply represent a compensatory response.

Melatonin protects from the long-term consequences of a neonatal hypoxic-ischemic brain injury in rats

Among the main factors responsible for perinatal brain injury, inflammation, hypoxia-ischemia and formation of free radicals (FR) appear to play key roles. Melatonin, an endogenously produced indoleamine formed in higher amounts in adults than in neonates, is a potent FR scavenger as well as an indirect antioxidant. Herein, we examined whether melatonin provides significant protection against brain damage and its long-term consequences in a neonatal model of hypoxia-ischemia (HI). Seven day-old rats were subjected to permanent legation of the right common carotid artery followed to 2.5 hrs hypoxia 3 hrs later (HI). The neuroprotective effect of melatonin was evaluated 7 days after HI, or when rats reached adulthood, using behavioral and histological analyses. A beneficial effect was observed with 5 mg/kg melatonin administered before HI. The same dose repeated three times reduced further injury. A significant protective effect was found when 15 mg/kg melatonin was given 30 min before HI or when the same dose was given after HI and administration repeated after 24 and 48 hrs. The latter schedule of administration was used to assess the long-term protective effects. Melatonin did not affect growth rate and behavior at adulthood, but significantly improved the behavioral asymmetry and learning deficits induced by HI. Consistently, brain injury was significantly attenuated in the melatonin-treated ischemic group. The present study demonstrates that melatonin administration before or after HI in immature rats has an excellent and long-lasting benefit on ischemic outcomes suggesting that the drug could represent a potentially safe approach to perinatal brain damage in humans.

Melatonin preserves superoxide dismutase activity in hypoglossal motoneurons of adult rats following peripheral nerve injury

Peripheral nerve injury (PNI) produces functional changes in lesioned neurons in which oxidative stress is considered to be the main cause of neuronal damage. As superoxide dismutase (SOD) is an important antioxidative enzyme involved in redox regulation of oxidative stress, the present study determined whether melatonin would exert its beneficial effects by preserving the SOD reactivity following PNI. Adult rats subjected to hypoglossal nerve transection were intraperitoneally injected with melatonin at ones for 3, 7, 14, 30 and 60 days successively. The potential neuroprotective effects of melatonin were quantitatively demonstrated by neuronal nitric oxide synthase (nNOS), mitochondrial manganese SOD (Mn-SOD), and cytosolic copper-zinc SOD (Cu/Zn-SOD) immunohistochemistry. The functional recovery of the lesioned neurons was evaluated by choline acetyltransferase (ChAT) immunohistochemistry along with the electromyographic (EMG) recordings of denervation-induced fibrillation activity. The results indicate that following PNI, the nNOS immunoreactivity was significantly increased in lesioned neurons peaking at 14 days. The up-regulation of nNOS temporally coincided with the reduction of ChAT and SOD in which the Cu/Zn-SOD showed a greater diminution than Mn-SOD. However, following melatonin administration, the nNOS augmentation was successfully suppressed and the activities of Mn-SOD, Cu/Zn-SOD, and ChAT were effectively preserved at all postaxotomy periods. EMG data also showed a decreased fibrillation in melatonin-treated groups, suggesting a potential effect of melatonin in promoting functional recovery. In association with its significant capacity in preserving SOD reactivity, melatonin is suggested to serve as a powerful therapeutic agent for treating PNI-relevant oxidative damage.

Melatonin decreases mortality following severe subarachnoid hemorrhage

Aneurysmal subarachnoid hemorrhage (SAH) is a devastating disease that is associated with significant morbidity and mortality. There is substantial evidence to suggest that oxidative stress is significant in the development of acute brain injury following SAH. Melatonin is a strong antioxidant that has low toxicity and easily passes through the blood-brain barrier. Previous studies have shown that melatonin provides neuroprotection in animal models of ischemic stroke. This study hypothesizes that melatonin will provide neuroprotection when administered 2 hr after SAH. The filament perforation model of SAH was performed in male Sprague-Dawley rats weighing between 300 and 380 g. Melatonin (15 or 150 mg/kg), or vehicle was given via intraperitoneal injection 2 hr after SAH. Mortality and neurologic deficits were assessed 24 hr after SAH. A significant reduction in 24-hr mortality was seen following treatment with high dose melatonin. There was no improvement in neurologic scores with treatment. Brain water content and lipid peroxidation were measured following the administration of high dose melatonin to identify a mechanism for the increased survival. High dose melatonin tended to reduce brain water content following SAH, but had no effect on the lipid peroxidation of brain samples. Large doses of melatonin significantly reduces mortality and brain water content in rats following SAH through a mechanism unrelated to oxidative stress.

Melatonin mitigates oxidative damage and apoptosis in mouse cerebellum induced by high-LET 56Fe particle irradiation

Cerebellum is a vital organ responsible for the motor coordination and recently it has been reported to be involved in cognitive function. Reactive oxygen species are implicated in neurodegeneration and cognitive disorders because of higher vulnerability of neuronal tissues. Therefore, the present study aimed at investigating the role of melatonin against high-LET (linear energy transfer) (56)Fe particle irradiation-induced oxidative damage and apoptosis in the mouse cerebellum. Radiation-induced oxidative damage was examined using a neuronal-specific terminal deoxynucleotidyl transferase-mediated nick end-labeling (TUNEL), quantitative histopathology, DNA damage (comet assay), carbonyl content and 4-HAE + MDA (4-hydroxyalkenal + malondialdehyde) status of the cerebellum. Radiation exposure augmented the number of TUNEL positive cell, DNA migration in the comet tail and carbonyl and 4-HAE + MDA level in the cerebellum. Melatonin pretreatment significantly inhibited the oxidative damage to biomolecules as well as cerebellar apoptosis. Melatonin-treated irradiated mice showed higher counts of intact Purkinje cells as compared to vehicle-treated irradiated mice. In addition, radiation induced augmentation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) and a decline in the total antioxidant capacity in serum; these changes were also ameliorated by melatonin pretreatment. The present results provide evidence supporting the antioxidant and neuroprotective function of melatonin.

Epigenetic regulation: a new research area for melatonin?

Epigenetic, modifications of DNA and histones, i.e. heritable alterations in gene expression that do not involve changes in DNA sequences, are known to be involved in disease. Two important epigenetic changes that contribute to disease are abnormal methylation patterns of DNA and modifications of histones in chromatin. Epimutations, such as the hypermethylation and epigenetic silencing of tumor suppressor genes, have revealed a new area for cancer treatment. Studies using DNA methyltransferase inhibitors such as procaine, hydralazine, and RG108 have had promising outcomes against cancer therapy. Melatonin, one of the most versatile molecules in nature, may hypothetically be involved in epigenetic regulation. In this review, the potential role of melatonin in inhibiting DNA methyltransferase and epigenetic regulation is discussed.

Signal transduction pathways involved in protective effects of melatonin in C6 glioma cells

Melatonin (N-acetyl-5-methoxytryptamine), an indole hormone, is the chief secretory product of the pineal gland and is an efficient free radical scavenger and antioxidant, both in vitro and in vivo. The role of melatonin as an immunomodulator is, in some cases, contradictory. Although melatonin is reported to influence a variety of inflammatory and immune responses, evidence supporting its effects on important glioma cells-derived mediators is incomplete. We studied, in rat glioma cell line (C6), the role of melatonin (100 microm-1 mm) in the regulation of the expression of nitric oxide synthase (NOS) caused by incubation with lipopolysaccharide (LPS)/interferon (IFN)-gamma (1 microg/mL and 100 U/mL, respectively) and defined the mode of melatonin's action. Treatment with LPS/IFN-gamma for 24 hr elicited the induction of inducible (iNOS) activity as determined by nitrite and nitrate (NO(x)) accumulation in the culture medium. Preincubation with melatonin abrogated the mixed cytokines-mediated induction of iNOS. The effect of melatonin was concentration-dependent. Moreover, Western blot analysis showed that melatonin inhibited LPS/IFN-gamma-induced expression of COX-2 protein, but not that of constitutive cyclooxygenase. Inhibition of iNOS and COX-2 expression was associated with inhibition of activation of the transcription factor nuclear factor kappa B (NF-kappaB). The ability of melatonin to inhibit NF-kappaB activation was further confirmed by studies on the degradation of the inhibitor of NF-kappaB, IkappaB-alpha. Increased production of lipid peroxidation products using thiobarbituric acid assay were found in cellular contents from activated cultures. Lipid peroxidation was decreased by melatonin treatment in a concentration-dependent manner. Moreover, several genes having roles in heat-shock response were downregulated in melatonin-treated cells, such as 70 proteins, reflecting the reduced oxidative stress in these cells. The mechanisms underlying in vitro the neuroprotective properties of melatonin involve modulation of transcription factors and consequent altered gene expression, resulting in downregulation of inflammation.

Cardiovascular diseases: protective effects of melatonin

This brief review considers some of the cardiac diseases and conditions where free radicals and related reactants are believed to be causative. The report also describes the beneficial actions of melatonin against oxidative cardiovascular disorders. Based on the data available, melatonin seems to have cardioprotective properties via its direct free radical scavenger and its indirect antioxidant activity. Melatonin efficiently interacts with various reactive oxygen and reactive nitrogen species (receptor independent actions) and it also upregulates antioxidant enzymes and downregulates pro-oxidant enzymes (receptor-dependent actions). Moreover, melatonin enters all cells and subcellular compartments and crosses morphophysiologic barriers. These findings have implications for the protective effects of melatonin against cardiac diseases induced by oxidative stress. Melatonin attenuates molecular and cellular damages resulting from cardiac ischemia/reperfusion in which destructive free radicals are involved. Anti-inflammatory and antioxidative properties of melatonin are also involved in the protection against a chronic vascular disease, atherosclerosis. The administration of melatonin, as a result of its antioxidant features, has been reported to reduce hypertension and cardiotoxicity induced by clinically used drugs. The results described herein help to clarify the beneficial effects of melatonin against these conditions and define the potential clinical applicability of melatonin in cardiovascular diseases.

Melatonin and the hair follicle

Melatonin, the chief secretory product of the pineal gland, has long been known to modulate hair growth, pigmentation and/or molting in many species, presumably as a key neuroendocrine regulator that couples coat phenotype and function to photoperiod-dependent environmental and reproductive changes. However, the detailed effects and mechanisms of this surprisingly pleiotropic indole on the hair follicle (HF) regarding growth control and pigmentation have not yet been completely understood. While unspecific melatonin binding sites have long been identified (e.g., in goat and mouse HFs), specific melatonin membrane MT2 receptor transcripts and both protein and mRNA expression for a specific nuclear melatonin binding site [retinoid-related orphan receptor alpha (RORalpha)] have only recently been identified in murine HFs. MT1, known to be expressed in human skin cells, is not transcribed in mouse skin. After initial enzymologic data from hamster skin related to potential intracutaneous melatonin synthesis, it has recently been demonstrated that murine and human skin, namely human scalp HFs in anagen, are important sites of extrapineal melatonin synthesis. Moreover, HF melatonin production is enhanced by catecholamines (as it classically occurs in the pineal gland). Melatonin may also functionally play a role in hair-cycle control, as it down-regulates both apoptosis and estrogen receptor-alpha expression, and modulates MT2 and RORalpha expression in murine skin in a hair-cycle-dependent manner. Because of melatonin's additional potency as a free radical scavenger and DNA repair inducer, the metabolically and proliferatively highly active anagen hair bulb may also exploit melatonin synthesis in loco as a self-cytoprotective strategy.

Melatonin reverses lipopolysaccharide-induced gastro-intestinal motility disturbances through the inhibition of oxidative stress

A growing number of reports demonstrate that a pro-inflammatory and oxidative condition is related to the pathogenesis and the progression of endotoxin-induced septic shock and that antioxidants may have therapeutic potential in lipopolysaccharide (LPS)-induced sepsis. Melatonin has been shown to possess potent antioxidant properties in several models of inflammation in mice and rats. In the present study we focused on the possible protective mechanism of melatonin in preventing gastrointestinal (GI) disturbances induced by LPS in mice. In fact, mice treated with LPS showed a reduced gastric emptying of solid beads. Also the geometric center, representing the relative distribution of the solid beads throughout the entire GI tract, was significantly reduced in LPS-treated mice confirming that sepsis leads to a disturbed GI motility in mice. Melatonin completely reversed the LPS-induced motility disturbance. This beneficial effect of melatonin is associated with a reduction in lipid peroxidation, MAPK activation, NF-kappaB activation, iNOS transcription and expression and nitrite production in intestinal tissue from septic mice. These results demonstrate that melatonin prevents the LPS-induced GI disturbances in mice switching off the pro-oxidant pathways induced by the endotoxin. Therefore, it is reasonable to propose melatonin as a molecule with therapeutic potential for the treatment of systemic inflammation by interfering at the earliest steps of activation of the oxidative and pro-inflammatory cascade.

Melatonin attenuates the focal cerebral ischemic injury by inhibiting the dissociation of pBad from 14-3-3

It has recently been reported that melatonin protects neuronal cells from damage by enhancing Akt activation, thus mediating antiapoptosis signals. However, there is little information regarding the effects of melatonin on the activation of genes further downstream in the Akt signaling pathway in ischemic brain injury. This study investigated whether melatonin modulates the antiapoptotic signal through Akt and its downstream targets, Bad and 14-3-3. Adult male rats were treated with melatonin (5 mg/kg) prior to middle cerebral artery occlusion (MCAO). Brains were collected at 24 hr after MCAO and infarct volumes were analyzed. Our results confirm that melatonin significantly reduces infarct volume and decreases the positive reaction of TUNEL staining in the cerebral cortex. Signal pathway activation was measured by phosphorylation of Akt at Ser(473) and Bad at Ser(136) using Western blot analysis. Melatonin prevented the injury-induced decrease of pAkt and pBad levels. However, melatonin did not affect the expression of 14-3-3, which acts as an antiapoptotic factor through interaction with Bad. Immunoprecipitation analysis showed that the interaction between pBad and 14-3-3 increased in the presence of melatonin, compared to that of control animals. Our findings suggest that melatonin prevents cell death because of brain injury and that these protective effects are mediated through maintaining the interaction between pBad and 14-3-3, thus blocking activation of the apoptotic pathway.

Melatonin in relation to the "strong" and "weak" versions of the free radical theory of aging

That free radicals and the damage they inflict are related to deteriorative cellular and organismal changes associated with aging and also with the development of a variety of age-related diseases is widely debated. There seems to be little doubt that free radical mutilation of essential molecules contributes to these conditions. Numerous investigators, on the basis of their experimental results, have drawn this conclusion. If the free radical theory of aging and disease development has validity, antioxidants could presumably be successfully used to delay the molecular destruction, cellular loss, and organismal death. In the current review we summarize the experimental data related to the utility of melatonin in protecting against reactive oxygen and reactive nitrogen species-induced cellular damage. While the data supporting a role for melatonin in forestalling aging and prolonging life span per se is not compelling, the findings related to melatonin's ability to reduce the severity of a variety of age-related diseases that have as their basis free radical damage is convincing. To date, the bulk of these investigations have been performed in experimental models of diseases in animals. It is now imperative that similar studies be conducted using humans whose quality of life may benefit from treatment with melatonin.

Melatonin inhibits nicotinic currents in cultured rat cerebellar granule neurons

There is limited data regarding the effects of melatonin on the activity of neuronal acetylcholine receptors (nAChRs) themselves. This study analyzes the effects of low concentrations of melatonin on nicotine-evoked currents from cerebellar granule neurons (CGNs) in culture. Using electrophysiological and Ca(2+)-imaging techniques, it was found a subset of rat CGNs to which nicotine application elicited both intracellular Ca(2+) transients and inward whole-cell currents. These responses were mediated by heteromeric nAChRs, as assessed by their sensitivity to nicotine and time constant of current decay. Preincubating the cells with low melatonin concentrations (down to 1 pm) significantly reduced the current amplitude in a dose-dependent manner, without affecting the receptor's apparent affinity and voltage-dependency, nor the current's rise and decay time course. The inhibitory effect of melatonin was significantly reduced by luzindole, a competitive antagonist of both MT(1) and MT(2) melatonin receptors. In conclusion, melatonin inhibits nicotinic currents through non-alpha7 heteromeric nAChRs expressed by CGNs in culture, an effect that appears to be at least partially mediated by melatonin membrane receptors. Direct modulation of nicotinic receptors is accomplished at doses that are likely to be physiologically relevant, thus providing a mechanism through which melatonin circadian rhythmic levels could modulate cholinergic activity.

Intracellular signaling pathways involved in cell growth inhibition of human umbilical vein endothelial cells by melatonin

Melatonin, an indolamine mainly produced in the pineal gland, has received a great deal of attention in the last decade because of its oncostatic effects, which are due to its immunomodulatory, antiproliferative, antioxidant and its possible antiangiogenesis properties. Herein, we document its antiproliferative action on human umbilical vein endothelial cells (HUVECs). Moreover, the possible cell signaling pathways when melatonin inhibited HUVEC proliferation were explored in this study. Primary HUVECs were isolated, cultured, purified and identified before the studies were performed. HUVECs were found to possess G-protein-coupled membrane receptors for melatonin (MT1 and MT2) and also nuclear melatonin receptors (RORalpha and RORbeta, especially RORbeta). No obvious expression of RORgamma was found. We investigated the membrane receptors and several intracellular signaling pathways including mitogen-activated protein kinases (MAPK)/extracellular signal-related kinases (ERK), phosphoinositol-3-kinase (PI3K)/Akt and protein kinases C (PKC) involved in antiproliferative action of melatonin on HUVECs. The blockade of these pathways using special inhibitors decreased cell growth. Furthermore, the constitutive activation of nuclear factor kappa B (NF-kappaB) contributed to the proliferation of HUVECs. High concentrations of melatonin inhibited both NF-kappaB expression and its binding ability to DNA, possibly through inactivation of ERK/Akt /PKC pathways. Taken together, high concentrations of melatonin markedly reduced HUVEC proliferation; the antiproliferative action of melatonin was closely correlated with following pathway: melatonin receptors/ERK/PI3K/Akt/PKC/ NF-kappaB.

Melatonin in the skin: synthesis, metabolism and functions

Melatonin, a ubiquitous methoxyindole, is produced by and metabolized in the skin. Melatonin affects skin functions and structures through actions mediated by cell-surface and putative-nuclear receptors expressed in skin cells. Melatonin has both receptor-dependent and receptor-independent effects that protect against oxidative stress and can attenuate ultraviolet radiation-induced damage. The widespread expression and pleiotropic activity of the cutaneous melatoninergic system provides for a high level of cell-specific selectivity. Moreover, intra-, auto- and para-crine mechanisms equip this system with exquisite functional selectivity. The properties of endogenous melatonin suggest that this molecule is an important effector of stress responses in the skin. In this way, melatonin actions may counteract or buffer both environmental and endogenous stressors to maintain skin integrity.

Association of ischemia-modified albumin and melatonin in patients with ST-elevation myocardial infarction

INTRODUCTION

It has been proposed that reactive oxygen species (ROS) generated during myocardial ischemia-reperfusion modify the N-terminus of serum albumin resulting in ischemia-modified albumin (IMA) formation. Likewise, several recent publications provide evidence that melatonin, a circadian endogenously produced indolamine, is a direct scavenger of ROS. We sought to investigate the relationship between IMA and melatonin in ST- elevation myocardial infarction (STEMI).

METHODS

We compared IMA and melatonin levels in 27 patients with STEMI undergoing primary angioplasty and 20 age- and gender-matched healthy normal subjects. Blood samples were drawn at 02:00h (night period) and 09:00h (day period) while patients were resting, to assess IMA and melatonin.

RESULTS

In both groups, melatonin concentrations maintained a diurnal variation, but the difference between nocturnal and diurnal levels was less in the STEMI-patients than in the control group (p<0.001). In contrast to findings about melatonin, IMA levels showed no diurnal variations in control subjects. However, the STEMI group showed a diurnal fluctuation with significantly higher levels at 02:00h (p<0.01). The association between IMA and melatonin remained statistically significant after adjustment for cardiovascular risk factors. An inverse correlation between IMA and melatonin at 02:00h and at 09:00h was observed, with respective r-values of -0.42 (p<0.03) and -0.57 (p<0.002).

CONCLUSIONS

Circulating IMA is negatively correlated to melatonin in STEMI-patients. Our results suggest that melatonin might exert a beneficial effect as a radical scavenger in a human model of myocardial ischemia-reperfusion.

Selenium-mercury interactions in man and animals

Selenium-mercury interactions were most extensively studied in relation to alleviation of Hg toxicity by added selenium. This presentation considers the influence of mercury on endogenous selenium, on its tissue and cellular "status" after lifelong or acute exposure to mercury vapor (Hg o). Discussed are data obtained from (1) humans living near or working in a mercury mine, and (2) rats experimentally exposed in the mine. Mercury vapor is unique--or similar to methylmercury--because of its ability to penetrate cell membranes and so invade all cells, where it is oxidized in the biologically active form (Hg++) by catalase. Such in situ-generated ions can react with endogenously generated highly reactive Se metabolites, like HSe-, and render a part of the selenium unavailable for selenoprotein synthesis. Data on human populations indicate that in moderate Hg exposure combined with an adequate selenium supply through diet, Se bioavailability can be preserved. On the other hand, the results of an acute exposure study emphasize the dual role of selenium in mercury detoxification. Besides the well-known Se coaccumulation through formation of nontoxic Hg-Se complexes, we observed noticeable Se (co)excretion, at least at the beginning of exposure. The higher Hg accumulation rate in the group of animals with lower basal selenium levels can also point to selenium involvement in mercury excretion. In such conditions there is a higher probability for decreased selenoprotein levels (synthesis) in some tissues or organs, depending on the synthesis hierarchy.

Melatonin: New Places in Therapy

The fact that the full extent of the function of the pineal gland has not yet been elucidated, has stimulated melatonin research worldwide. This review introduces melatonin's mechanism of action, direct and indirect antioxidant actions as well as the antioxidant properties of its metabolites, 6-hydroxymelatonin (6-OHM) and N-acetyl-N-formyl-5-methoxykynurenamine (AFMK). At present the mechanism of action is proposed to be receptor-, protein- and nonprotein-mediated. From its popular role in the treatment of jetlag, melatonin is now implicated in the reduction of oxidative stess, both as a free radical scavenger and antioxidant. Melatonin's direct scavenging action in respect of the following will be discussed: superoxide anions, hydrogen peroxide, hydroxyl radicals, singlet oxygen, peroxy radicals and nitric oxide/peroxy nitrite anions. In addition melatonin also possesses indirect antioxidant activity and the role of its metabolites, AFMK and 6-OHM will be presented. It is these free radical scavenging and antioxidant properties of melatonin that has shifted the focus from that of merely strengthening circadian rhythms to that of neuroprotectant: a new place in therapy.

N-nitroso-melatonin releases nitric oxide in the presence of serotonin and its derivatives

A novel reaction was observed between 5-hydroxytryptophan derivatives like serotonin and N-nitroso-melatonin (NOMela). This reaction decreased the concentration of serotonin by about 50% and generated initially as detectable products nitric oxide and melatonin with stoichiometrical yields. The other expected product, a serotonin-derived radical, could not be detected by electron spin resonance (ESR) spectrometry, probably because the self-decay of phenoxyl type radicals proceed at the diffusion-controlled limit. From the facts that the decay rate of NOMela corresponded very well with the nitric oxide releasing rate and that nitrite was the only thermodynamically stable nitrogen oxide-containing product, it is concluded that the NOMela-serotonin reaction proceeded quantitatively. The observed reaction might be a possibility to counteract a pharmacologically abnormal high serotonin concentration in various diseases.

Melatonin inhibits free radical-mediated mitochondrial-dependent hepatocyte apoptosis and liver damage induced during malarial infection

We showed earlier that malarial infection significantly induces liver apoptosis mediated by oxidative stress mechanisms. Thus, a nontoxic antioxidant-antiapoptotic molecule may be beneficial for hepatoprotection. Melatonin remarkably prevents hepatocyte apoptosis in mice induced during malaria as indicated by caspase 3 and TUNEL assays as well as transmission electron microscopy (TEM) of the liver tissue. The mitochondrial apoptotic pathway, which plays a critical role in liver cell death during malarial infection, was almost completely suppressed by melatonin as it corrects both the overexpression of Bax and down-regulation of bcl-2 as revealed by semiquantitative RT-PCR. Fluorometric studies using JC-1 documented that melatonin also restores mitochondrial transmembrane potential (DeltaPsim) in malaria-infected mice liver. The antiapoptotic effect of melatonin is associated with its antioxidant role because melatonin protects liver from oxidative stress induced during malaria by scavenging the hydroxyl radicals, preventing the depletion of reduced glutathione, inhibiting lipid peroxidation and protein carbonyl formation. The effective antioxidant dose of melatonin to protect liver from oxidative stress during malaria is 20 times lower than that of known antioxidants, vitamin C and vitamin E. Apoptosis of hepatocytes during malarial infection is well correlated with dysfunction of the liver while melatonin offers hepatoprotective effects as indicated by different liver function tests. Thus, melatonin may well be effective in combating oxidative stress-induced apoptosis and liver damage during malaria infection.

Phytoremediative capacity of plants enriched with melatonin

Melatonin is an environmentally friendly-molecule with broad spectrum antioxidant capacity. Melatonin is widely present in the plant kingdom. High levels of melatonin exist in an aquatic plant, the water hyacinth, which is highly tolerant of environmental pollutants. Elevated levels of melatonin probably help plants to protect against environmental stress caused by water and soil pollutants. To investigate the potential relationships between melatonin supplementation and environmental tolerance in plants, pea plants were treated with high levels of copper in the soil. The results show that copper contamination kills pea plants; however, melatonin added to the soil significantly enhanced their tolerance to the copper contamination and, therefore, increased their survival. Based on the theory and these preliminary data, we speculate that melatonin could be used to improve the phytoremediative efficiency of plants against different pollutants. Since melatonin is safe to animals and humans as well as being inexpensive, it may be a feasible and cost-effective approach to clean environmental contaminations.

Pineal melatonin and the innate immune response: the TNF-alpha increase after cesarean section suppresses nocturnal melatonin production

The nocturnal surge of melatonin is the endocrine expression of the circadian system and is essential for organizing the timing of various endogenous processes. Previous works suggest that, in the beginning of a defense response, the increase in circulating tumor necrosis factor-alpha (TNF-alpha) leads to a transient block of nocturnal melatonin production and promotes a disruption of internal time organization. In the present paper, the concentration of melatonin and cytokines [TNF-alpha, interferon-gamma (IFN-gamma), interleukin (IL)-2, IL-4, IL-5, IL-10, IL-12] in the colostrum (postdelivery day 3) and in the milk (postdelivery days 10, 15, 20 and 30) obtained at midday and midnight from mothers who gave birth by vaginal or cesarean section were compared. The nocturnal melatonin surge observed 3 days after vaginal delivery was absent after cesarean section. IL-12 presented no daily variation in either case, while daily variations in IFN-gamma, IL-10, IL-4 and IL-5 were observed after vaginal delivery and cesarean section. On the other hand, the increase in TNF-alpha after cesarean section resulted in suppression of the nocturnal melatonin surge. Daily variation of IL-2 was only observed after recovery of the nocturnal melatonin surge, 30 days after cesarean section. The present paper supports the hypothesis of a cross-talk between the pineal gland and the immune system, which could represent a putative immune-pineal axis.

Melatonin effect on bovine embryo development in vitro in relation to oxygen concentration

Melatonin promotes mouse embryo development in vitro. An effect of melatonin on bovine embryo development is described here. Slaughterhouse derived oocytes were subjected to standard in vitro maturation and fertilization procedures. Presumptive zygotes were cultured for 2 days in CR1aaLA medium supplemented with melatonin (10(-4) m) or without melatonin (control). Culture was performed under two different gas atmospheres containing physiological (7%) or atmospheric (20%) oxygen concentrations (2x2 factorial analysis). After day 2, embryos from each treatment group developed to at least four-cell stage, were cultured without melatonin until day 10 at optimum 7% O2 atmosphere. Blastocyst formation rates of presumptive zygotes and of four-cell embryos were calculated for each group. Significant interactions between oxygen tension and the melatonin treatment were found. Out of four-cell embryos put into in vitro culture after initial incubation in medium containing melatonin, decreased blastocyst rate was observed in melatonin group (47.7%) compared with control (67.7%; P=0.0327) when lower oxygen concentration was applied. A beneficial effect of melatonin was observed in 20% O2: out of 61 embryos, 42 (68.9%) developed to the blastocyst stage after treatment in melatonin versus 32 of 63 (50.8%; P=0.0458) blastocysts that developed in control group. In conclusion, beneficial or harmful effects of melatonin on bovine embryo development in vitro were observed, depending on the oxygen tension during the treatment.

Melatonin as a naturally occurring co-substrate of quinone reductase-2, the putative MT3 melatonin membrane receptor: hypothesis and significance

The nature of the MT3 melatonin receptor/binding site has been a long pondered mystery for scientists. Even though it is a presumptive membrane receptor, neither its transduction cascade nor its biological consequences, after its stimulation, have been uncovered. Moreover, solid data support the idea that the MT3 melatonin binding site is an enzyme, quinone reductase 2 (QR2), rather than a membrane melatonin receptor. Based on the data available and our preliminary studies, we hypothesize that melatonin is a co-substrate of QR2. We surmise that melatonin binds to a co-substrate binding site (MT3 binding site) donating an electron to the enzyme co-factor, flavin adenine dinucleotide (FAD). FAD can be reduced to either FADH or FADH2 while melatonin is converted to N1-acetyl-N2-formyl-5-methoxykynuramine and/or cyclic 3-hydroxymelatonin. QR2 is considered to be a detoxifying and antioxidant enzyme and its behavior changes depending on available co-substrates. As a naturally occurring substance, melatonin's levels fluctuate with the light/dark cycle, with aging and with health/disease state. As a result, these alterations in melatonin production under physiological or pathological conditions would probably influence the activity of QR2.

Melatonin protects against common deletion of mitochondrial DNA-augmented mitochondrial oxidative stress and apoptosis

Defected mitochondrial respiratory chain (RC), in addition to causing a severe ATP deficiency, often augments reactive oxygen species (ROS) generation in mitochondria (mROS) which enhances pathological conditions and diseases. Previously, we demonstrated a potent endogenously RC defect-augmented mROS associated dose-dependently with a commonly seen large-scale deletion of 4977 base pairs of mitochondrial DNA (mtDNA), i.e. the common deletion (CD). As current treatments for CD-associated diseases are rather supplementary and ineffective, we investigated whether melatonin, a potential mitochondrial protector, provides beneficial protection for CD-augmented mitochondrial oxidative stress and apoptosis particularly upon the induction of a secondary oxidative stress. Detailed mechanistic investigations were performed by using laser scanning dual fluorescence imaging microscopy to provide precise spatial and temporal resolution of mitochondrial events at single cell level. We demonstrate, for the first time, that melatonin significantly prevents CD-augmented mROS formation under basal conditions as well as at early time-points upon secondary oxidative stress induced by H2O2 exposure. Thus, melatonin prevents mROS-mediated depolarization of mitochondrial membrane potential (DeltaPsim) and subsequent opening of the mitochondrial permeability transition pore (MPTP) and cytochrome c release. Moreover, melatonin prevents depletion of cardiolipin which appears to be crucial for postponing later MPTP opening, disruption of the mitochondrial membrane and apoptosis. Finally, the protection provided by melatonin is superior to those caused by the suppression of mitochondrial Ca2+ regulators including the mitochondrial Na+-Ca2) exchanger, the MPTP, and the mitochondrial Ca2+ uniporter and by antioxidants including vitamin E and mitochondria-targeted coenzyme Q, MitoQ. As RC defect-augmented endogenous mitochondrial oxidative stress is centrally involved in a variety of pathological conditions and diseases, melatonin thus may serve as a therapeutic drug to benefit many clinical conditions that involve malfunction of the mitochondria.

Possible therapeutic value of melatonin in mild cognitive impairment: a retrospective study

Mild cognitive impairment (MCI) is an etiologically heterogeneous syndrome characterized by cognitive impairment preceding dementia. Approximately 12% of MCI patients convert to Alzheimer's disease (AD) or other dementia disorders every year. In the present report we retrospectively examined the initial and final neuropsychological assessment of 50 MCI outpatients, 25 of whom had received daily 3-9 mg of a fast-release melatonin preparation p.o. at bedtime for 9-18 months. Melatonin was given in addition to the standard medication prescribed by the attending psychiatrist. Patients treated with melatonin showed significantly better performance in Mini Mental State Examination and the cognitive subscale of the Alzheimer's Disease Assessment Scale. After application of a battery of neuropsychological tests including Mattis' test, Digit-symbol test, Trail A and B tasks and the Rey's verbal test, better performance was found in melatonin-treated patients, except for the Digit-symbol test score which remained unchanged. Abnormally high Beck Depression Inventory scores decreased in melatonin-treated patients, concomitantly with an improvement in wakefulness and sleep quality. The results suggest that melatonin can be a useful add-on drug for treating MCI in a clinical setting.

Melatonin therapy in fibromyalgia

Fibromyalgia (FM) is a painful syndrome that is more common in women than in men. Whether FM has an organic basis or whether it is psychosomatic is debated. Of the numerous treatments that have been tried, rarely have any been completely effective in relieving the pain of FM. Preliminary evidence indicates that melatonin, a molecule that is endogenously produced and is available as an over-the-counter supplement, may be effective in treating the pain associated with this syndrome. Although melatonin is commonly known as a sleep aid (sleep/wake problems are common in FM sufferers), it has a variety of other beneficial effects that may account for its potential benefits in the treatment of FM.

Prevention by melatonin of hepatocarcinogenesis in rats injected with N-nitrosodiethylamine

N-nitrosodiethylamine (NDEA) is a potent carcinogenic agent that induces liver cancer. To evaluate the chemopreventive function of melatonin in this experimental model, Wistar male rats received a single i.p. injection of NDEA or vehicle followed by weekly s.c. injections of carbon tetrachloride or vehicle for 6 weeks. Melatonin (5 mg/kg body weight) or its vehicle (0.5 mL saline) was given i.p. on a daily basis 2 hr before lights off for 20 wk. At the end of this period the rats were killed and liver and blood samples were taken for histological and biochemical studies. As markers for liver function, the activity of aspartate transaminase (AST) and alanine transaminase (ALT) and the levels of alpha-fetoprotein were measured in serum. To assess lipid peroxidation and the antioxidant status in liver and blood, the levels of thiobarbituric acid reactive substances (TBARS) and of reduced glutathione (GSH) were measured. The activity of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione S-transferase (GST) was assessed in liver and erythrocyte fraction of NDEA-treated rats. NDEA administration inhibited body weight, macro- and microscopically detectable liver tumors and increased levels of plasma AST, ALT and alpha-fetoprotein. NDEA treatment decreased liver TBARS levels and CAT and SOD activities and increased liver GSH levels and GST and GPx activities. Plasma TBARS were augmented, while plasma GSH levels and the activities of erythrocyte CAT, SOD, GST and GPx decreased, in NDEA-treated rats. Melatonin administration significantly curtailed tumor development and counteracted all the biochemical effects.

The dark side of light at night: physiological, epidemiological, and ecological consequences

Organisms must adapt to the temporal characteristics of their surroundings to successfully survive and reproduce. Variation in the daily light cycle, for example, acts through endocrine and neurobiological mechanisms to control several downstream physiological and behavioral processes. Interruptions in normal circadian light cycles and the resulting disruption of normal melatonin rhythms cause widespread disruptive effects involving multiple body systems, the results of which can have serious medical consequences for individuals, as well as large-scale ecological implications for populations. With the invention of electrical lights about a century ago, the temporal organization of the environment has been drastically altered for many species, including humans. In addition to the incidental exposure to light at night through light pollution, humans also engage in increasing amounts of shift-work, resulting in repeated and often long-term circadian disruption. The increasing prevalence of exposure to light at night has significant social, ecological, behavioral, and health consequences that are only now becoming apparent. This review addresses the complicated web of potential behavioral and physiological consequences resulting from exposure to light at night, as well as the large-scale medical and ecological implications that may result.

Melatonin increases cleavage rate of porcine preimplantation embryos in vitro

Melatonin has been used to promote in vitro embryo development in different species. This study determined the effects of melatonin on in vitro porcine embryo development; in particular, cleavage rate, blastocyst rate, and blastocyst cell number. Starting 5 hr after insemination, porcine zygotes were cultured in porcine zygote medium 3 (PZM-3) culture medium supplemented with melatonin at increasing concentrations (10(-12) M, 10(-9) M, 10(-6) M, 10(-3) M). Melatonin at a concentration of 10(-9) M had a positive effect on cleavage rates, while the highest concentration of melatonin (10(-3) M) significantly decreased cleavage rates. Although blastocyst rates were not increased by 10(-9) M melatonin, blastocyst cell numbers were significantly higher for embryos subjected to 10(-9) M melatonin. The expression levels of the pro-apoptotic gene BAX and anti-apoptotic gene BCL2L1 in blastocysts were not affected by the presence of melatonin in the culture medium. To further study the protective properties of 10(-9) M melatonin against stressful conditions, hydrogen peroxide (0.01 mm) and heat (40 degrees C) were used during embryo culture. The addition of melatonin to embryos subjected to 40 degrees C for 3 hr increased cleavage rates, but had no protective effect for embryos subjected to 0.01 mm H(2)O(2), probably because the physiological levels of melatonin could not counteract the pharmacological levels of H(2)O(2). Our data indicate that 10(-9) M melatonin has a positive effect on porcine embryo cleavage rates and blastocyst total cell numbers and it might have a protective effect against heat stress.

Melatonin inhibits oxidative stress and apoptosis in fetal brains of hyperhomocysteinemic rat dams

Moderate hyperhomocysteinemia is a risk factor for neurodegenerative diseases and complications during pregnancy. Increased homocysteine levels during pregnancy may elevate developmental risk on fetal brain structure and function. However, little is known about the mechanism of action of homocysteine on the degeneration of the fetal brain. Hence in this study, we examined the effects of maternal hyperhomocysteinemia on oxidative stress and apoptosis in brain tissues and investigated whether administration of melatonin to the mother would prevent homocysteine-induced oxidative cerebral damage in pups. Hyperhomocysteinemia was induced in female rats by administration of methionine at a dose of 1 g/kg body weight dissolved in drinking water during pregnancy. Some animals received methionine plus 10 mg/kg/day melatonin subcutaneously throughout pregnancy. After delivery, the level of lipid peroxidation (malondialdehyde + 4-hydroxyalkenals) was determined in different subfractions of pup brains. Furthermore, DNA fragmentation, levels of Bcl-2 protein and p53 mRNA expression were determined to evaluate apoptosis. Significant elevation was found in the levels of lipid peroxidation in subcellular fractions of the brain of pups of hyperhomocysteinemic dams. Increased DNA fragmentation and p53 mRNA expression was observed in the brain of pups of homocysteine-treated rats, while a significant reduction was seen in the levels of anti-apoptotic Bcl-2 levels. Melatonin administration prevented markers of oxidative stress and biochemical signs of apoptosis. In conclusion, therapeutic administration of melatonin protects against the induction of oxidative stress and neural tissue injury and might prevent congenital malformations of fetal brain caused by maternal hyperhomocysteinemia.

Melatonin location in egg phosphatidylcholine liposomes: possible relation to its antioxidant mechanisms

Although it is known that the antioxidant properties of melatonin can be modulated by its effect on membrane fluidity, there are few studies on this subject reported in the literature and they are controversial. In this study, viscosimetry and nuclear magnetic resonance (NMR) techniques were used to determine melatonin's effect and location on egg phosphatidylcholine bilayers mobility. Melatonin decreases the dynamic viscosity of the lipid dispersion. (31)P-NMR line width analysis indicated that melatonin induces a slight but uniform restriction of the lipid motional freedom in the polar head. However, melatonin changes in choline (13)C dynamics was only observed through chemical shift analysis. On the other hand, melatonin can induce an increase in the lipid nonpolar chain mobility, as observed by (13)C and (1)H relaxation time analysis. These results suggest the interfacial location of melatonin in the membrane. Additionally, the results of the analysis of the lipid (1)H-fitted exponential relaxation times suggest that melatonin promotes a molecular rearrangement of the bilayers. The melatonin effect and location in the lipid membrane may explain its antioxidant properties against lipid peroxidation induced by reactive species.

Beneficial properties of melatonin in an experimental model of pancreatic cancer

Pancreatic cancer is a major health problem because of the aggressiveness of the disease and the lack of effective systemic therapies. Melatonin has antioxidant activity and prevents experimental genotoxicity. However, the effect of melatonin in pancreatic cancer has not been tested. Pancreatic carcinogenesis was induced by N-nitrosobis (2-oxopropyl)amine (BOP) in Syrian hamsters. Melatonin was administered during the BOP-induction phase (12 wk) and/or following the postinduction phase (12 wk). Different parameters of oxidative stress including lipid peroxides (LPO) and antioxidants (superoxide dismutase, catalase, reduced glutathione and glutathione peroxidase) were determined in pancreatic tissue. Also, the presence of atypical hyperplasia (AH), well and moderately differentiated adenomacarcinoma (ADC-WD and ADC-MD, respectively) were studied. The administration of BOP induced an intense oxidative stress and ADC induction in the pancreas. The administration of melatonin during the induction or postinduction phase reduced LPO and improved the antioxidant status, as well as drastically reducing the presence of ADC but some AH remained. In conclusion, treatment with melatonin reduced oxidative damage and cancer nodules induced by BOP in the pancreas.

Renal deterioration caused by carcinogens as a consequence of free radical mediated tissue damage: a review of the protective action of melatonin

This brief review summarizes some of the publications that document the preventive role of melatonin in kidney damage caused by carcinogens such as 2-nitropropane, arsenic, carbon tetrachloride, nitrilotriacetic acid and potassium bromate. Numerous chemicals generate excessive free radicals that eventually induce renal worsening. Melatonin partially or totally prevents free radical mediated tissue damages induced by many carcinogens. Protective actions of melatonin against the harmful effects of carcinogens are believed to stem from its direct free radical scavenging and indirect antioxidant activities. Dietary or pharmacologically given melatonin may attenuate the oxidative stress, thereby mitigating the subsequent renal damage.

Melatonin and thyroxine response to pollution in white stork nestlings (Ciconia ciconia): aspects of rhythmicity and age

There is growing evidence that ubiquitous environmental contaminants may interfere with vertebrate endocrine systems. The selected endocrine biomarkers are used to indicate the condition of free-ranging populations of wildlife, including avian species. The aim of this study was to determine the impact of environment quality on serum thyroxine (T4) and melatonin (Mel) in white stork nestlings (Ciconia ciconia) living in different locations: small villages in natural areas surrounded by forests and crop fields, near the city and near the copper smelter. We extended our analyses to examine the hormones' day-night changes in conjunction with chicks' age. Total serum T4 and Mel was measured by RIA. T4 level, as a decisive measure of thyroid hormone productivity, was significantly lower in the nestlings exposed to pollutants from the copper smelter. Mel, as a well-known scavenger of free radicals, was elevated in the nestlings in the area near the copper smelter. This study indicates that alteration in T4 and Mel levels could be a useful marker of exposure of nestling wild storks to different toxic substances in field studies. Mel is postulated to be a susceptible defensive molecule as a protective mechanism for organisms.

Effects of combination of melatonin and dexamethasone on secondary injury in an experimental mice model of spinal cord trauma

This study investigates the effects of combination therapy with melatonin and dexamethasone on the degree of spinal cord injury caused by the application of vascular clip in mice. Spinal cord injury in mice resulted in severe trauma, characterized by edema, neutrophil infiltration, and apoptosis (measured by terminal deoxynucleotidyltransferase-mediated UTP end labeling staining, and immunoreaction of Bax, Bcl-2, and Fas Ligand). Infiltration of the spinal cord tissue with neutrophils (measured as increase in myeloperoxidase activity) was associated with enhanced immuno- histochemical and functional alterations revealed, respectively, by an increased of tumor necrosis factor (TNF)-alpha immunoreactivity, NOS as well as nitrotyrosine and loss of hind leg movement in spinal cord injury (SCI)-operated mice. In contrast, the degree of neutrophil infiltration at different time points, cytokine expression, histologic damage iNOS expression, apoptosis, was markedly reduced in the tissues obtained from SCI-treated mice with the combination therapy, and the motor recovery was also ameliorated. No anti-inflammatory effect was observed in animals treated with melatonin (10 mg/kg) or with dexamethasone (0.025 mg/kg) alone. This study shows that the combination therapy with melatonin and dexamethasone reduces the degree of secondary damage associated with spinal cord injury in mice, and supports the possible use of melatonin in combination with steroids to reduce the dose and the side effects related with the use of steroids for the management of inflammatory disease.

Melatonin inhibits MPP+-induced caspase-mediated death pathway and DNA fragmentation factor-45 cleavage in SK-N-SH cultured cells

Neurodegenerative diseases such as Parkinson's disease are illnesses associated with high morbidity and mortality with few, or no effective, options available for their treatment. In addition, the direct cause of selective dopaminergic cell loss in Parkinson's disease has not been clearly understood. Taken together, several studies have demonstrated that melatonin has a neuroprotective effect both in vivo and in vitro. Accordingly, the effects of melatonin on 1-methyl, 4-phenyl, pyridinium ion (MPP(+))-treated cultured human neuroblastoma SK-N-SH cell lines were investigated in the present study. The results showed that MPP(+) significantly decreased cell viability. By contrast, an induction of phosphorylation of c-Jun, activation of caspase-3 enzyme activity, cleavage of DNA fragmentation factors 45 and DNA fragmentation were observed in MPP(+)-treated cells. These changes were diminished by melatonin. These results demonstrate the cellular mechanisms of neuronal cell degeneration induced via c-Jun-N-terminal kinases and caspase-dependent signaling, and the potential role of melatonin on protection of neuronal cell death induced by this neurotoxin.

Cellular mechanisms involved in the melatonin inhibition of HT-29 human colon cancer cell proliferation in culture

The antiproliferative and proapoptotic properties of melatonin in human colon cancer cells in culture were recently reported. To address the mechanisms involved in these actions, HT-29 human colon cancer cells were cultured in RPMI 1640 medium supplemented with fetal bovine serum at 37 degrees C. Cell proliferation was assessed by the incorporation of [(3)H]-thymidine into DNA. Cyclic nucleotide levels, nitrite concentration, glutathione peroxidase and reductase activities, and glutathione levels were assessed after the incubation of these cells with the following drugs: melatonin membrane receptor agonists 2-iodo-melatonin, 2-iodo-N-butanoyl-5-methoxytryptamine, 5-methoxycarbonylamino-N-acetyltryptamine (GR-135,531), and the antagonists luzindole, 4-phenyl-2-propionamidotetralin, and prazosin; the melatonin nuclear receptor agonist CGP 52608, and four synthetic kynurenines analogs to melatonin 2-acetamide-4-(3-methoxyphenyl)-4-oxobutyric acid, 2-acetamide-4-(2-amino-5-methoxyphenyl)-4-oxobutyric acid, 2-butyramide-4-(3-methoxyphenyl)-4-oxobutyric acid and 2-butyramide-4-(2-amino-5-methoxyphenyl)-4-oxobutyric acid. The results show that the membrane receptors are not necessary for the antiproliferative effect of melatonin and the participation of the nuclear receptor in this effect is suggested. Moreover, the antioxidative and anti-inflammatory actions of melatonin, counteracting the oxidative status and reducing the production of nitric oxide by cultured HT-29 cells seem to be directly involved in the oncostatic properties of melatonin. Some of the synthetic kynurenines exert higher antiproliferative effects than melatonin. The results reinforce the clinical interest of melatonin due to the different mechanisms involved in its oncostatic role, and suggest a new synthetic pathway to obtain melatonin agonists with clinical applications to oncology.

Melatonin inhibits lipopolysaccharide-induced CC chemokine subfamily gene expression in human peripheral blood mononuclear cells in a microarray analysis

Melatonin possesses a number of important biologic activities including oncostatic, anti-oxidant, and immunostimulatory actions. This study was designed to assess the effects of melatonin on inflammation-related gene expression in lipopolysaccharide (LPS)-stimulated human peripheral blood mononuclear cells (PBMCs), using CombiMatrix 2K Human Inflammation chip. After pretreatment with melatonin (100 microm) for 4 hr, cells were incubated with LPS (1 microg/mL) for 24 hr. We compared gene expression profiles between LPS-treated, melatonin-treated, LSP/melatonin-treated, and control groups. LPS induced the upregulation of 95 genes, compared with controls. Melatonin pretreatment in LPS-stimulated PBMCs suppressed the expression of 23 genes more than twofold. Interestingly, melatonin showed a suppressive effect on the expression of CC chemokine subfamily genes, including CCL2/MCP1, CCL3/MIP1 alpha, CCL4/MIP1 beta, CCL5/RANTES, CCL8/MCP2, CCL20/MDC, and CCL22/MIP3 alpha, in LPS-stimulated PBMCs. This result was confirmed by reverse transcriptase polymerase chain reaction. Among the CC chemokine subfamily genes, particularly, the expression of CCL2 and CCL5 was markedly downregulated by melatonin in LPS-stimulated PBMCs. The secretion levels of CCL2 and CCL5 were measured using enzyme-linked immunosorbent assay. Stimulation of PBMCs by LPS induced the secretion of CCL2 (2334.3 +/- 161.4 pg/mL, mean +/- S.E.M.), whereas melatonin pretreatment (153.0 +/- 3.8 pg/mL) inhibited the LPS-induced secretion of CCL2. Melatonin pretreatment (2696.2 +/- 385.3 pg/mL) also inhibited the LPS-induced secretion of CCL5 (4679.6 +/- 107.5 pg/mL). Taken together, these results suggest that melatonin may have a suppressive effect on LPS-induced expression of CC chemokine genes, especially CCL2 and CCL5, which may explain its beneficial effects in the treatment of various inflammatory conditions.

Melatonin antagonizes apoptosis via receptor interaction in U937 monocytic cells

Among the non-neurological functions of melatonin, much attention is being directed to the ability of melatonin to modulate the immune system, whose cells possess melatonin-specific receptors and biosynthetic enzymes. Melatonin controls cell behaviour by eliciting specific signal transduction actions after its interaction with plasma membrane receptors (MT(1), MT(2)); additionally, melatonin potently neutralizes free radicals. Melatonin regulates immune cell loss by antagonizing apoptosis. A major unsolved question is whether this is due to receptor involvement, or to radical scavenging considering that apoptosis is often dependent on oxidative alterations. Here, we provide evidence that on U937 monocytic cells, apoptosis is antagonized by melatonin by receptor interaction rather than by radical scavenging. First, melatonin and a set of synthetic analogues prevented apoptosis in a manner that is proportional to their affinity for plasma membrane receptors but not to their antioxidant ability. Secondly, melatonin's antiapoptotic effect required key signal transduction events including G protein, phospholipase C and Ca(2+) influx and, more important, it is sensitive to the specific melatonin receptor antagonist luzindole.