Melatonin: detoxification of oxygen and nitrogen-based toxic reactants

Reducing oxidative/nitrosative stress: a newly-discovered genre for melatonin

The discovery of melatonin and its derivatives as antioxidants has stimulated a very large number of studies which have, virtually uniformly, documented the ability of these molecules to detoxify harmful reactants and reduce molecular damage. These observations have clear clinical implications given that numerous age-related diseases in humans have an important free radical component. Moreover, a major theory to explain the processes of aging invokes radicals and their derivatives as causative agents. These conditions, coupled with the loss of melatonin as organisms age, suggest that some diseases and some aspects of aging may be aggravated by the diminished melatonin levels in advanced age. Another corollary of this is that the administration of melatonin, which has an uncommonly low toxicity profile, could theoretically defer the progression of some diseases and possibly forestall signs of aging. Certainly, research in the next decade will help to define the role of melatonin in age-related diseases and in determining successful aging. While increasing life span will not necessarily be a goal of these investigative efforts, improving health and the quality of life in the aged should be an aim of this research.

Melatonin protects against oxidative damage in a neonatal rat model of bronchopulmonary dysplasia

BACKGROUND

Oxidative stress plays an important role in the pathogenesis of bronchopulmonary dysplasia (BPD). Melatonin (MT) has direct and indirect free radical detoxifying activity. The present study was to investigate whether treatment with MT would attenuate hyperoxia-induced lung injury and the effect of MT on imbalance of oxidants/antioxidants in the lung of neonatal rats.

METHODS

BPD was induced by exposure to hyperoxia in neonatal rats (n=90). The rats were divided randomly into three groups (n=30 each): air-exposed control group, hyperoxia-exposed group, and hyperoxia-exposed MT-treated group. Lung specimens were obtained respectively on day 3, day 7, and day 14 after exposure (n=10 each). Activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT), and levels of myeloperoxidase (MPO), nitrite/nitrate, and malondialdehyde (MDA) were assayed. Histopathologic changes were observed in the tissues stained with hematoxylin and eosin and Masson's trichrome stain.

RESULTS

Increased levels of MPO, nitrite/nitrate, and MDA in the hyperoxia-exposed rats were significantly reduced by MT (P<0.05). Activities of GSH-Px, SOD, and CAT which did not change after exposure to hyperoxia were increased by MT (P<0.05). Furthermore, BPD associated histopathological alterations such as reduced total number of alveoli and interstitial fibrosis were obviously abated in the MT-treated group.

CONCLUSIONS

MT can reverse oxidants/antioxidants imbalance in damaged lung tissue and thus exert a beneficial effect on hyperoxia-induced lung disease in neonatal rats. With regard to humans, there may be a protective effect of MT on BPD.

Melatonin: an established antioxidant worthy of use in clinical trials

Oxidative stress plays a key role in the pathogenesis of aging and many metabolic diseases; therefore, an effective antioxidant therapy would be of great importance in these circumstances. Nutritional, environmental, and chemical factors can induce the overproduction of the superoxide anion radical in both the cytosol and mitochondria. This is the first and key event that leads to the activation of pathways involved in the development of several metabolic diseases that are related to oxidative stress. As oxidation of essential molecules continues, it turns to nitrooxidative stress because of the involvement of nitric oxide in pathogenic processes. Once peroxynitrite forms, it damages via two distinctive mechanisms. First, it has direct toxic effects leading to lipid peroxidation, protein oxidation, and DNA damage. This mechanism involves the induction of several transcription factors leading to cytokine-induced chronic inflammation. Classic antioxidants, including vitamins A, C, and E, have often failed to exhibit beneficial effects in metabolic diseases and aging. Melatonin is a multifunctional indolamine that counteracts virtually all pathophysiologic steps and displays significant beneficial actions against peroxynitrite-induced cellular toxicity. This protection is related to melatonin's antioxidative and antiinflammatory properties. Melatonin has the capability of scavenging both oxygen- and nitrogen-based reactants, including those formed from peroxynitrite, and blocking transcriptional factors, which induce proinflammatory cytokines. Accumulating evidence suggests that this nontoxic indolamine may be useful either as a sole treatment or in conjunction with other treatments for inhibiting the biohazardous actions of nitrooxidative stress.

Delayed melatonin administration promotes neuronal survival, neurogenesis and motor recovery, and attenuates hyperactivity and anxiety after mild focal cerebral ischemia in mice

Melatonin is a potent antioxidant with neuroprotective activity in animal models of ischemic stroke, which based on its lack of serious toxicity has raised hopes that it might be used for human stroke treatment in the future. This study investigated how subacute delivery of melatonin, starting at 24 hr after stroke onset, and continuing for 29 days (4 mg/kg/day; via drinking water), influences neuronal survival, endogenous neurogenesis, motor recovery and locomotor activity in C57Bl6/j mice submitted to 30-min middle cerebral artery occlusion. Histologic studies showed that melatonin improved neuronal survival and enhanced neurogenesis, even when applied 1 day after stroke. Cell survival was associated with a long-lasting improvement of motor and coordination deficits, evaluated by the grip strength and RotaRod tests, as well as with attenuation of hyperactivity and anxiety of the animals as revealed in open field tests. The robust functional neurologic improvements encourage proof-of-concept studies with melatonin in human stroke patients.

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.

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 pretreatment improves liver function and hepatic perfusion after hemorrhagic shock

Exogenous administration of pineal hormone melatonin (MEL) has been demonstrated to attenuate organ damage in models of I/R and inflammation by antioxidative effects. However, specific organ-protective effects of MEL with respect to hemorrhagic shock have not been investigated yet. In the present study, we evaluated the role of MEL pretreatment for hepatic perfusion, redox state, and function after hemorrhage and resuscitation, with emphasis on MEL receptor activation. In a model of hemorrhagic shock (MAP 35 +/- 5 mmHg for 90 min) and reperfusion (2 h), we measured nicotinamide adenine dinucleotide phosphate (reduced form; NADPH) autofluorescence, hepatic microcirculation, and hepatocellular injury by intravital microscopy, as well as plasma disappearance rate of indocyanine green (PDRICG) as a sensitive maker of liver function in rat. Pretreatment with 10 mg kg(-1) MEL (i.v.) 15 min before induction of hemorrhage resulted in a significantly improved PDR(ICG) compared with controls (MEL/shock, 15.02% min(-1) +/- 2.9 SD vs. vehicle/shock, 6.18 +/- 4.6 SD; P = 0.001). Intravital microscopy after reperfusion revealed an improved hepatic perfusion index, redox state, and reduced hepatocellular injury in pretreated animals compared with the vehicle group. Melatonin receptor antagonist luzindole (LZN; 2.5 mg kg(-1)) almost completely abolished the protective effects of MEL pretreatment with respect to liver function (MEL + LZN/shock PDR(ICG), 7.31% min(-1) +/- 3.4 SD). Beneficial effects regarding hepatic perfusion, redox state, and cellular injury were not influenced by LZN, indicating that they may depend on antioxidative effects of MEL. However, liver function after hemorrhage is effectively maintained by MEL pretreatment via receptor-dependent pathways.

Melatonin receptors mediate improvements of liver function but not of hepatic perfusion and integrity after hemorrhagic shock in rats

OBJECTIVE

Melatonin has been demonstrated to attenuate organ damage in models of ischemia and reperfusion. Melatonin treatment before hemorrhagic shock has been shown to improve liver function and hepatic perfusion. Proposed mechanisms of the pineal hormone involve direct inactivation of reactive oxygen species and induction of antioxidative enzymes. However, recent evidence suggests a strong influence of melatonin receptor activation for these effects. Specific protection of organ function by melatonin after hemorrhage has not been investigated yet. In this study, we evaluated whether melatonin therapy after hemorrhagic shock improves liver function and hepatic perfusion, with emphasis on melatonin receptor activation.

DESIGN

Prospective, randomized, controlled study.

SETTING

University research laboratory.

SUBJECTS

Male Sprague-Dawley rats, 200-300 g (n = 10 per group).

INTERVENTIONS

Animals underwent hemorrhagic shock (mean arterial pressure, 35 +/- 5 mm Hg for 90 mins) and were resuscitated with shed blood and Ringer's solution. At the end of shock, animals were treated with either melatonin (10 mg/kg, intravenously), melatonin receptor antagonist luzindole (2.5 mg/kg, intravenously) plus melatonin (10 mg/kg, intravenously), luzindole alone (2.5 mg/kg, intravenously), or vehicle.

MEASUREMENTS AND MAIN RESULTS

After 2 hrs of reperfusion, either liver function was assessed by plasma disappearance rate of indocyanine green or intravital microscopy of the liver was performed for evaluation of hepatic perfusion, hepatocellular redox state, and hepatic integrity. Compared with vehicle controls, melatonin therapy after hemorrhagic shock significantly improved plasma disappearance rate of indocyanine green, hepatic redox state, hepatocellular injury, and hepatic perfusion index. Coadministration of luzindole completely abolished the protective effect with respect to liver function only, and improvements regarding hepatic redox state, perfusion, and integrity were comparable with melatonin treatment alone.

CONCLUSIONS

Melatonin therapy after hemorrhagic shock improves liver function, hepatic perfusion, redox state, and hepatic integrity. With respect to liver function, beneficial effects of the pineal hormone seem to be dependent on melatonin receptor activation.

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.

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.

Photocatalytic mechanisms of indoleamine destruction by the quinalphos metabolite 2-hydroxyquinoxaline: a study on melatonin and its precursors serotonin and N-acetylserotonin

The redox-active quinalphos main metabolite, 2-hydroxyquinoxaline, is particularly effective under excitation by light. We have studied the photocatalytic destruction of melatonin and its precursors, because the cytoprotective indoleamine has been detected in high quantities in mammalian skin. In photooxidation reactions, in which melatonin, N-acetylserotonin and serotonin are destroyed by 2-hydroxyquinoxaline, the photocatalyst is virtually not consumed. Rates of melatonin and serotonin destruction are not changed by the singlet oxygen quencher 1,4-diazabicyclo-(2,2,2)-octane, indicating that this oxygen species is not involved in the primary reactions, so that the persistence of 2-hydroxyquinoxaline has to be explained by redox cycling. This should imply formation of an organic radical, presumably the quinoxaline-2-oxyl radical, from which 2-hydroxyquinoxaline is regenerated by electron abstraction from indolic radical scavengers. Electron donation by 2-hydroxyquinoxaline is demonstrated by reduction of the 2,2'-azino-bis-(3-ethylbenzthiazolinyl-6-sulfonic acid) cation radical under ultrasound excitation. The compound 2-hydroxyquinoxaline interacts with the specific superoxide anion scavenger Tiron. Formation of oligomeric products from melatonin and serotonin is strongly inhibited by sodium dithionite. Products from photocatalytic indolamine conversion are predominantly dimers and oligomers. No kynuramines were detected in the case of serotonin oxidation, and melatonin's otherwise prevailing oxidation product N(1)-acetyl-N(2)-formyl-5-methoxykynuramine, another cytoprotective metabolite, is only formed in relatively small quantities. The proportion between products from melatonin is changed by 1,4-diazabicyclo-(2,2,2)-octane: singlet oxygen, also formed under the influence of excited 2-hydroxyquinoxaline, only affects secondary reactions.

The effect of pH on horseradish peroxidase-catalyzed oxidation of melatonin: production of N1-acetyl-N2-5-methoxykynuramine versus radical-mediated degradation

There is a growing body of evidence that melatonin and its oxidation product, N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK), have anti-inflammatory properties. From a nutritional point of view, the discovery of melatonin in plant tissues emphasizes the importance of its relationship with plant peroxidases. Here we found that the pH of the reaction mixture has a profound influence in the reaction rate and products distribution when melatonin is oxidized by the plant enzyme horseradish peroxidase. At pH 5.5, 1 mm of melatonin was almost completely oxidized within 2 min, whereas only about 3% was consumed at pH 7.4. However, the relative yield of AFMK was higher in physiological pH. Radical-mediated oxidation products, including 2-hydroxymelatonin, a dimer of 2-hydroxymelatonin and O-demethylated dimer of melatonin account for the fast consumption of melatonin at pH 5.5. The higher production of AFMK at pH 7.4 was explained by the involvement of compound III of peroxidases as evidenced by spectral studies. On the other hand, the fast oxidative degradation at pH 5.5 was explained by the classic peroxidase cycle.

Melatonin protects against epirubicin-induced cardiotoxicity

We investigated the cytoprotective effect of melatonin in epirubicin-induced cardiotoxicity using four experimental groups of male Wistar rats: untreated control rats, epirubicin-treated rats, epirubicin+melatonin-treated rats, and melatonin-treated rats. We examined the histopathological and biochemical effects of melatonin on the epirubicin-induced changes and measured the levels of the lipid peroxidation end-product (malondialdehyde, MDA), an indicator of nitric oxide (NO) synthesis (nitrite/nitrate production), and reduced glutathione (GSH) in the heart. We also studied the extracellular matrix components (fibronectin, laminin) in the heart. Vacuole formation, mitochondrial deformation and degeneration, and disordered myofibrillary structures were detected ultrastructurally in the epirubicin-treated group. The degeneration was reduced in the heart tissues of the epirubicin+melatonin group. Epirubicin increased the nitrite/nitrate production, but did not change the MDA and GSH levels significantly. Melatonin treatment lowered the nitrite/nitrate concentrations, while increasing the GSH levels, which exceeded the levels in epirubicin+melatonin-treated rats. We conclude that the epirubicin increased the nitrozative stress, not the oxidative stress, in heart tissue, and the cardioprotective effect of melatonin was partially attributed to the suppression of epirubicin-induced nitrozative stress. These results suggest that melatonin partially protects against epirubicin-induced cardiotoxicity.

Expression and cellular localization of melatonin-synthesizing enzymes in the rat lens

Melatonin (N-acetyl-5-methoxytryptamine) prevents oxidative stress-induced cataract development, and previous studies have suggested that the ocular lens synthesizes melatonin. In the present study, we examined whether two key enzymes in melatonin biosynthesis, arylalkylamine N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase (HIOMT), are expressed in the lens of adult male rats. Reverse transcriptase-polymerase chain reaction analyses demonstrated that both AANAT and HIOMT mRNAs are expressed in the lens. Western blotting for AANAT protein showed that the lens, like the pineal gland, contains this enzyme protein with a molecular mass of 24 kDa. Immunohistochemistry revealed that AANAT protein is localized to the lens cortical fiber cells. Serotonin, which is a substrate for AANAT and a melatonin precursor, was also found in this region. These findings demonstrate that the lens expresses AANAT and HIOMT, and suggest that the cortical fiber cells are the main melatonin-synthesizing sites in the lens. Locally synthesized melatonin in the lens cortical fiber cells may protect the lens itself from cataract development.

In vivo protective effect of melatonin on cadmium-induced changes in redox balance and gene expression in rat hypothalamus and anterior pituitary

Cadmium (Cd) is widely used in industrial applications and is an important side contaminant of agricultural products. As an endocrine disruptor, Cd modifies pituitary hormone release. It has been shown that this metal causes oxidative stress in primary cultures of anterior pituitary cells. To examine whether Cd induces redox damage in the hypothalamic-pituitary axis in vivo and to evaluate the efficacy of the antioxidant molecule melatonin to prevent Cd activity, rats were exposed to Cd (5 p.p.m. in drinking water) with or without melatonin (3 microg/mL drinking water) for 1 month. In the anterior pituitary, Cd increased lipid peroxidation and mRNA levels for heme oxygenase-1 (HO-1) at both time intervals tested (09:00 and 01:00 hr, beginning of rest span and middle of activity span, respectively). Melatonin administration prevented the Cd-induced increase in both parameters. In the hypothalamus, Cd affected the levels of mRNA for HO-1 by decreasing it in the evening. Melatonin reduced hypothalamic HO-1 gene expression. Cd treatment augmented gene expression of nitric oxide synthase (NOS)1 and NOS2 in the pituitary whereas melatonin decreased it, impairing the activity of Cd. Exposure to Cd increased the levels of hypothalamic NOS1 mRNA at 09:00 hr and decreased the levels of NOS2 mRNA at 01:00 hr, with melatonin treatment preventing Cd effects. Cd treatment decreased plasma thyroid-stimulating hormone levels at both examined times, while melatonin reversed the effect of Cd at 09:00 hr and partially counteracted the effect at 01:00 hr. There were important variations between day and night in the expression of all the genes tested in both tissues. Melatonin treatment was effective reducing all examined effects of Cd, documenting its effectiveness to protect the rat hypothalamic-pituitary axis from the toxic metal effects.

Inhibition of neuronal nitric oxide synthase activity by N1-acetyl-5-methoxykynuramine, a brain metabolite of melatonin

We assessed the effects of melatonin, N(1)-acetyl-N (2)-formyl-5-methoxykynuramine (AFMK) and N(1)-acetyl-5-methoxykynuramine (AMK) on neuronal nitric oxide synthase (nNOS) activity in vitro and in rat striatum in vivo. Melatonin and AMK (10(-11)-10(-3) m), but not AFMK, inhibited nNOS activity in vitro in a dose-response manner. The IC(50) value for AMK (70 microm) was significantly lower than for melatonin (>1 mm). A 20% nNOS inhibition was reached with either 10(-9) m melatonin or 10(-11) m AMK. AMK inhibits nNOS by a non-competitive mechanism through its binding to Ca(2+)-calmodulin (CaCaM). The inhibition of nNOS elicited by melatonin, but not by AMK, was blocked with 0.05 mm norharmane, an indoleamine-2,3-dioxygenase inhibitor. In vivo, the potency of AMK to inhibit nNOS activity was higher than that of melatonin, as a 25% reduction in rat striatal nNOS activity was found after the administration of either 10 mg/kg of AMK or 20 mg/kg of melatonin. Also, in vivo, the administration of norharmane blocked the inhibition of nNOS produced by melatonin administration, but not the inhibition produced by AMK. These data reveal that AMK rather than melatonin is the active metabolite against nNOS, which may be inhibited by physiological levels of AMK in the rat striatum.

Urinary metabolites and antioxidant products of exogenous melatonin in the mouse

Exogenous melatonin is widely used for sleep disorders and has potential value in neuroprotection, cardioprotection and as an antioxidant. Here, a novel method is described for the determination of melatonin and six metabolites in mouse urine by use of LC-MS/MS and GC-MS. LC-MS/MS is used for the measurement of melatonin, N1-acetyl-5-methoxykynuramine (AMK), N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and 6-hydroxymelatonin (6-HMEL), while GC/MS is used for the determination of N-[2-(5-methoxy-2-oxo-2,3-dihydro-1H-indol-3-yl)-ethyl]-acetamide (2-OMEL) and cyclic 3-hydroxymelatonin (3-HMEL) with detection limits on column of 0.02-0.5 pmol, depending on the metabolite. Following oral administration of melatonin to mice, a 0-24 hr urine collection revealed the presence of melatonin (0.2% dose), 6-HMEL (37.1%) and NAS (3.1%) comprising >90% of the total metabolites; AMK and AFMK were also detected at 0.01% each; 2-OMEL was found at 2.2% of the dose, which is >100 times more than the AMK/AFMK pathway, and comprises >5% of the melatonin-related material detected in mouse urine. 3-HMEL was largely found as a sulfate conjugate. These studies establish sensitive assays for determination of six melatonin metabolites in mouse urine and confirm the potential for antioxidant activity of melatonin through the identification in vivo of AMK and AFMK, ring-opened metabolites with a high capacity for scavenging reactive oxygen species.

Melatonin protects against pressure ulcer-induced oxidative injury of the skin and remote organs in rats

Pressure ulcers (PU) cause morphological and functional alterations in the skin and visceral organs; the damage is believed to be due to ischemia/reperfusion (I/R) injury. In this study, we examined the role of oxidative damage in PU and the beneficial effect of treatment with the antioxidant melatonin. PU were induced by applying magnets over steel plates that were implanted under the skin of rats; this compressed the skin and caused ischemia. Within a 12-hr period, rats were subjected to five cycles of I/R (2 and 0.5 hr respectively), followed by an additional 12 hr of ischemia (to simulate the period at sleep at night). This protocol was repeated for 3 days. In treatment groups, twice a day during reperfusion periods, melatonin (5 mg per rat) was either applied locally as an ointment on skin, or administered i.p. (10 mg/kg). At the end of the experimental period, blood and tissue (skin, liver, kidney, lung, stomach, and ileum) samples were taken for determination of biochemical parameters and for histological evaluation. Local treatment with melatonin inhibited the increase in malondialdehyde levels; an index of lipid peroxidation, myeloperoxidase activity; an indicator of tissue neutrophil infiltration, and the decrease in glutathione; a key antioxidant, in the skin induced by PU, but was less efficient in preventing the damage in visceral organs. However, systemic treatment prevented the damage in the visceral organs. Significant increases in creatinine, blood urea nitrogen, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase and collagen levels in animals with PU were prevented by melatonin treatment. The light microscopic examination exhibited significant degenerative changes in dermis and epidermis in the PU rats. Tissue injury was decreased especially in the locally treated group. Findings of the present study suggest that local and/or systemic melatonin treatment may prove beneficial in the treatment of PU.

Melatonin counteracts inducible mitochondrial nitric oxide synthase-dependent mitochondrial dysfunction in skeletal muscle of septic mice

Mitochondrial nitric oxide synthase (mtNOS) produces nitric oxide (NO) to modulate mitochondrial respiration. Besides a constitutive mtNOS isoform it was recently suggested that mitochondria express an inducible isoform of the enzyme during sepsis. Thus, the mitochondrial respiratory inhibition and energy failure underlying skeletal muscle contractility failure observed in sepsis may reflect the high levels of NO produced by inducible mtNOS. The fact that mtNOS is induced during sepsis suggests its relation to inducible nitric oxide synthase (iNOS). Thus, we examined the changes in mtNOS activity and mitochondrial function in skeletal muscle of wild-type (iNOS(+/+)) and iNOS knockout (iNOS(-/-)) mice after sepsis. We also studied the effects of melatonin administration on mitochondrial damage in this experimental paradigm. After sepsis, iNOS(+/+) but no iNOS(-/-) mice showed an increase in mtNOS activity and NO production and a reduction in electron transport chain activity. These changes were accompanied by a pronounced oxidative stress reflected in changes in lipid peroxidation levels, oxidized glutathione/reduced glutathione ratio, and glutathione peroxidase and reductase activities. Melatonin treatment counteracted both the changes in mtNOS activity and rises in oxidative stress; the indole also restored mitochondrial respiratory chain in septic iNOS(+/+) mice. Mitochondria from iNOS(-/-) mice were unaffected by either sepsis or melatonin treatment. The data suggest that inducible mtNOS, which is coded by the same gene as that for iNOS, is responsible for mitochondrial dysfunction during sepsis. The results also suggest the use of melatonin for the protection against mtNOS-mediated mitochondrial failure.

Preventive effect of melatonin on bleomycin-induced lung fibrosis in rats

Oxidative stress has an important role in the pathogenesis of idiopathic pulmonary fibrosis. Melatonin has direct and indirect free radical-detoxifying activity. The present study investigated whether melatonin treatment attenuates bleomycin-induced lung fibrosis in rats. A group of rats was given one dose of bleomycin while the control animals were given saline. The first dose of melatonin (4 mg/kg/day) was given 2 days before the bleomycin injection. At day 14, fibrotic changes were evaluated using Aschoft's criteria and lung hydroxyproline content. Bleomycin produced a 2.7-fold rise in the fibrosis score that was decreased 65% by melatonin (P < 0.05) and a 1.4-fold increase in hydroxyproline content which was completely prevented by melatonin. Protein carbonyl and thiobarbituric acid reactive substances levels, which were significantly elevated in the bleomycin treated rats, were significantly attenuated by melatonin. Bleomycin administration significantly reduced the activities of catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) in lung tissue. The reduction in CAT activity was prevented by melatonin but SOD and GSH-Px were not influenced. These results revealed that melatonin may prevent the development of bleomycin-induced lung fibrosis via the repression of protein and lipid peroxidation.

Antioxidative protection by melatonin: multiplicity of mechanisms from radical detoxification to radical avoidance

Melatonin has been shown to protect against oxidative stress in various, highly divergent experimental systems. There are many reasons for its remarkable protective potential. Signaling effects comprise the upregulation of antioxidant enzymes, such as superoxide dismutases, peroxidases, and enzymes of glutathione supply, down-regulation of prooxidant enzymes, such as nitric oxide synthases and lipoxygenases, and presumably also the control of quinone reductase 2. Other mechanisms are based on direct interactions with several reactive oxygen and nitrogen species. Among these reactions, the capacity of easily undergoing single-electron transfer reactions is of particular importance. Electron donation by melatonin is not only an aspect of direct radical scavenging, but additionally represents the basis for formation of the protective metabolites AFMK (N1-ace-tyl-N2-formyl-5-methoxykynuramine) and AMK (N1-acetyl-5-methoxykynuramine). Recent investigations on mitochondrial metabolism indicate that melatonin as well as AMK are capable of supporting the electron flux through the respiratory chain, of preventing the breakdown of the mitochondrial membrane potential, and of decreasing electron leakage, thereby reducing the formation of superoxide anions. Radical avoidance is a new line of investigation, which exceeds mitochondrial actions and also comprises antiexcitatory effects and contributions to the maintenance of internal circadian phase relationships.

Successively postadministered melatonin prevents disruption of hepatic antioxidant status in rats with bile duct ligation

We have reported that orally administered melatonin exerts a therapeutic effect on cholestatic liver injury in rats treated with bile duct ligation (BDL) possibly through its antioxidant and anti-inflammatory actions. Herein, we examined whether successively postadministered melatonin prevents the disruption of hepatic antioxidant status in BDL-treated rats. Wistar rats with BDL were killed 5 and 13 days after BDL. Melatonin (10 or 100 mg/kg body weight) was orally administered to rats with and without BDL everyday for 8 days, starting 5 days after BDL. The hepatic concentrations of thiobarbituric acid reactive substances, an index of lipid peroxidation, and reduced glutathione increased 5 days after BDL and further increased at 13 days. Hepatic vitamin E concentration and catalase and Se-glutathione peroxidase (Se-GSH-Px) activities were similarly reduced at 5 and 13 days after BDL. Hepatic ascorbic acid concentration and the hepatic activities of Cu,Zn- and Mn-superoxide dismutases, glutathione reductase, and glucose-6-phosphate dehydrogenase decreased 13 days after BDL. Melatonin postadministered to BDL-treated rats attenuated all these changes observed at 13 days after the treatment more effectively at the higher dose than at the lower dose. Melatonin administered to BDL-untreated rats increased the hepatic Se-GSH-Px activity at both doses and the hepatic activities of Cu,Zn- and Mn-superoxide dismutases at the higher dose. These results indicate that successively postadministered melatonin at pharmacological doses prevents the disruption of hepatic antioxidant status in rats with BDL through its direct and indirect antioxidant action, which may contribute to its therapeutic effect of BDL-induced cholestatic liver injury.

Melatonin, a potent agent in antioxidative defense: actions as a natural food constituent, gastrointestinal factor, drug and prodrug

Melatonin, originally discovered as a hormone of the pineal gland, is also produced in other organs and represents, additionally, a normal food constituent found in yeast and plant material, which can influence the level in the circulation. Compared to the pineal, the gastrointestinal tract contains several hundred times more melatonin, which can be released into the blood in response to food intake and stimuli by nutrients, especially tryptophan. Apart from its use as a commercial food additive, supraphysiological doses have been applied in medical trials and pure preparations are well tolerated by patients. Owing to its amphiphilicity, melatonin can enter any body fluid, cell or cell compartment. Its properties as an antioxidant agent are based on several, highly diverse effects. Apart from direct radical scavenging, it plays a role in upregulation of antioxidant and downregulation of prooxidant enzymes, and damage by free radicals can be reduced by its antiexcitatory actions, and presumably by contributions to appropriate internal circadian phasing, and by its improvement of mitochondrial metabolism, in terms of avoiding electron leakage and enhancing complex I and complex IV activities. Melatonin was shown to potentiate effects of other antioxidants, such as ascorbate and Trolox. Under physiological conditions, direct radical scavenging may only contribute to a minor extent to overall radical detoxification, although melatonin can eliminate several of them in scavenger cascades and potentiates the efficacy of antioxidant vitamins. Melatonin oxidation seems rather important for the production of other biologically active metabolites such as N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK) and N¹-acetyl-5-methoxykynuramine (AMK), which have been shown to also dispose of protective properties. Thus, melatonin may be regarded as a prodrug, too. AMK interacts with reactive oxygen and nitrogen species, conveys protection to mitochondria, inhibits and downregulates cyclooxygenase 2.

Mitochondria in homeostasis of reactive oxygen species in cell, tissues, and organism

The recent knowledge on mitochondria as the substantial source of reactive oxygen species, namely superoxide and hydrogen peroxide efflux from mitochondria, is reviewed, as well as nitric oxide and subsequent peroxynitrite generation in mitochondria and their effects. The reactive oxygen species formation in extramitochondrial locations, in peroxisomes, by cytochrome P450, and NADPH oxidase reaction, is also briefly discussed. Conditions are pointed out under which mitochondria represent the major ROS source for the cell: higher percentage of non-phosphorylating and coupled mitochondria, in vivo oxygen levels leading to increased intensity of the reverse electron transport in the respiratory chain, and nitric oxide effects on the redox state of cytochromes. We formulate hypotheses on the crucial role of ROS generated in mitochondria for the whole cell and organism, in concert with extramitochondrial ROS and antioxidant defense. We hypothesize that a sudden decline of mitochondrial ROS production converts cells or their microenvironment into a "ROS sink" represented by the instantly released excessive capacity of ROS-detoxification mechanisms. A partial but immediate decline of mitochondrial ROS production may be triggered by activation of mitochondrial uncoupling, specifically by activation of recruited or constitutively present uncoupling proteins such as UCP2, which may counterbalance the mild oxidative stress.

Melatonin and nitric oxide: two required antagonists for mitochondrial homeostasis

The presence of nitric oxide (NO* ) in the mitochondria led to analysis of its source and functions in mitochondrial homeostasis. Studies have revealed the existence of a mtNOS isoform with similar features to nNOS, with some post-traslational modifications, although without the typical signal peptide responsible for addressing proteins to mitochondrion. This isoform may account for the physiological production of NO* related to the respiratory control. During inflammatory conditions there is an excess of NO* in the mitochondria responsible for an increase in reactive oxygen and nitrogen species in sufficient amounts to compromise mitochondrial function. These conditions led to the discovery of the presence of an inducible mtNOS isoform with kinetic properties similar to iNOS. Experiments with knockout mice lacking either nNOS or iNOS further confirmed the existence of these two mtNOS isoforms in mitochondria. Although the increase in NO* in sepsis by inducible mtNOS may have important regulatory functions including the redistribution of oxygen into other pathways under hypoxia, it causes the production of excess NO* that is deleterious for the cell. Melatonin, an endogenous antioxidant, regulates mitochondrial respiration and bioenergetics and protects mitochondria from excess NO* by controlling the activity of mtNOS.

5-HT in systemic hypertension: foe, friend or fantasy?

Since its discovery by Erspamer in the 1930s and identification by Page in the 1950s, 5-HT (5-hydroxytryptamine; serotonin) has been an elusive candidate as a substance that plays a role in the disease of high blood pressure, also known as hypertension. In both animal and human hypertension, arterial contraction to 5-HT is profoundly enhanced. Additionally, 5-HT is a vascular smooth muscle cell mitogen. Because both increased arterial contractility and smooth muscle growth contribute to the disease of hypertension, it is logical to believe that 5-HT is a potential cause of disease, and thus a foe. However, decades of research have produced conflicting results as to the potential role of 5-HT in hypertension. This review will discuss historical findings which both support and refute the involvement of 5-HT in hypertension, and pose some new questions that may reveal novel ways for 5-HT to modify vascular control of blood pressure.

Nitric oxide level in the nasal and sinus mucosa after exposure to electromagnetic field

OBJECTIVE

The purpose of this study was to examine the changes in nitric oxide (NO) level in the nasal and paranasal sinus mucosa after exposure radiofrequency electromagnetic fields (EMF).

STUDY DESIGN AND SETTING

Thirty male Sprague-Dawley rats were randomly grouped as follows: EMF group (group I; n, 10), EMF group in which melatonin received (group II; n, 10) and the control (sham operated) group (group III; n, 10). Groups I and II were exposed to a 900 MHz. Oral melatonin was given in group II. Control rats (group III) were also placed in the tube as the exposure groups, but without exposure to EMF. At the end of 2 weeks, the rats were sacrificed, and the nasal and paranasal sinus mucosa dissected. NO was measured in nasal and paranasal mucosa.

RESULTS

The nasal and paranasal sinus mucosa NO levels of group I were significantly higher than those of the control group (group III) ( P < 0.05). However, there was no statistically significant difference between group II and the control group (group III) regarding NO output ( P > 0.05).

CONCLUSION

Exposure to EMF released by mobile phones (900 MHz) increase NO levels in the sinus and nasal mucosa.

SIGNIFICANCE

Increased NO levels may act as a defense mechanism and presumably related to tissue damage. In addition, melatonin may have beneficial effect to prevent these changes in the mucosa.