Glutathione‐dependent induction of local and systemic defense against oxidative stress by exogenous melatonin in cucumber (Cucumis sativus L.)

Melatonin is involved in defending against oxidative stress caused by various environmental stresses in plants. In this study, the roles of exogenous melatonin in regulating local and systemic defense against photooxidative stress in cucumber (Cucumis sativus) and the involvement of redox signaling were examined. Foliar or rhizospheric treatment with melatonin enhanced tolerance to photooxidative stress in both melatonin‐treated leaves and untreated systemic leaves. Increased melatonin levels are capable of increasing glutathione (reduced glutathione [GSH]) redox status. Application of H₂O₂ and GSH also induced tolerance to photooxidative stress, while inhibition of H₂O₂ accumulation and GSH synthesis compromised melatonin‐induced local and systemic tolerance to photooxidative stress. H₂O₂ treatment increased the GSH/oxidized glutathione (GSSG) ratio, while inhibition of H₂O₂ accumulation prevented a melatonin‐induced increase in the GSH/GSSG ratio. Additionally, inhibition of GSH synthesis blocked H₂O₂‐induced photooxidative stress tolerance, whereas scavenging or inhibition of H₂O₂ production attenuated but did not abolish GSH‐induced tolerance to photooxidative stress. These results strongly suggest that exogenous melatonin is capable of inducing both local and systemic defense against photooxidative stress and melatonin‐enhanced GSH/GSSG ratio in a H₂O₂‐dependent manner is critical in the induction of tolerance.     

Altered neutrophil apoptosis activity is reversed by melatonin in liver ischemia-reperfusion

Delayed neutrophil apoptosis has been implicated as the mechanism of the systemic inflammatory response. Herein, we examined the effect of melatonin on the neutrophil apoptosis in ischemia and reperfusion of the human liver. We studied seven patients receiving elective hepatectomy for liver tumor and ten patients receiving laparoscopic cholecystectomy for gallstones. Ten milli liters of blood was drawn isolation and incubation of the human neutrophils. Neutrophil apoptosis activity and CD18 expression and respiratory burst activity were assessed flow cytometrically. Another group of neutrophils included those from the patients receiving hepatectomy and isolated and incubated with melatonin. Neutrophil apoptosis is delayed from patients after hepatectomy or laparoscopic cholecystectomy when compared with that of the preoperative state. The decrease in the apoptosis activity is more severe in patients receiving hepatectomy as compared with those receiving laparoscopic cholecystectomy. Neutrophils from patients receiving hepatectomy or laparoscopic cholecystectomy are functionally activated. Melatonin can reverse the delayed process and enhance the apoptosis activity in neutrophils from patients receiving hepatectomy. This study demonstrates that melatonin enhances neutrophil apoptosis in patients receiving hepatectomy involving ischemia and reperfusion of the human liver.     

Melatonin protects against oxidative stress caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the mouse nigrostriatum

We tested the hypothesis that melatonin acts as a powerful hydroxyl radical (*OH) scavenger in vivo in the brain, and interferes with oxidative stress caused by the parkinsonian neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We investigated the effect of melatonin on in vitro *OH production employing a Fenton-like reaction in test tubes, and ex vivo *OH generation in isolated mitochondria induced by 1-methyl-4-phenyl pyridinium (MPP+), as well as on in vivo *OH formation in the mouse striatum following systemic administration of MPTP. We also measured reduced glutathione (GSH) levels, and superoxide dismutase (SOD) activity in the nucleus caudatus putamen (NCP) and substantia nigra (SN), 7 days following MPTP and/or melatonin administration. Melatonin caused a significant and dose-dependent inhibition of the production of *OH in the in vitro, ex vivo and in vivo experimental conditions. Melatonin caused no changes in monoamine oxidase-B activity, in vitro in mitochondrial P2 fractions or in vivo following systemic administration. MPTP treatment in mice caused a significant depletion of GSH, and increased the specific activity of SOD both in SN and NCP on the seventh day. MPTP-induced GSH depletion was dose-dependently blocked in SN and NCP by melatonin. Higher doses of melatonin exhibited a synergistic effect on MPTP-induced increase in the SOD activity in the SN. These results suggest that while GSH inhibition is a direct consequence of *OH generation following neurotoxin administration, the increase in SOD activity is a compensatory mechanism for removing superoxide radicals generated as the result of MPTP. Our results not only point to the potency of melatonin in blocking the primary insults caused by MPTP, but also provide evidence for triggering secondary neuroprotective mechanisms, suggesting its use as a therapeutic agent in neurodegenerative disorders, such as Parkinson's disease.     

Antioxidative effects of melatonin administration in elderly primary essential hypertension patients

The imbalance of the redox state of the aging organism may be involved in the development of primary essential hypertension. Melatonin, a potent antioxidant agent, was found to exert a hypotensive effect and improve the function of the cardiovascular system. The aim of this study was to determine the influence of melatonin supplementation on oxidative stress parameters in elderly primary essential hypertensive (EH) patients, controlled by a diuretic (indapamide) monotherapy. The levels of malondialdehyde (MDA) and reduced glutathione (GSH), activities of Cu-Zn superoxide dismutase (SOD-1), catalase (CAT) and glutathione peroxidase (GSH-Px) in erythrocytes, the plasma level of nitrate/nitrite, the content of carbonyl groups of plasma proteins and morning melatonin levels in the serum of 17 elderly EH patients were determined at the baseline and after the 15th and 30th days of melatonin supplementation (5 mg daily). Melatonin administration resulted in a significant increase in the morning melatonin concentration, SOD-1 and CAT activities, and a reduction in the MDA level. Statistically significant alterations in the levels of GSH, nitrate/nitrite and carbonyl groups and the activity of GSH-Px were not observed. These results indicate an improvement in the antioxidative defense of the organism by melatonin supplementation in the examined group and may suggest melatonin supplementation as an additional treatment supporting hypotensive therapy in elderly EH patients.     

Melatonin reduces torsion-detorsion injury in rat ovary: biochemical and histopathologic evaluation

This experimental study was designed to determine the effects of melatonin on the levels of malondialdehyde (MDA), reduced glutathione (GSH), xanthine oxidase (XO) after adnexial torsion/detorsion (ischemia/reperfusion, I/R) of the ovaries of in rats. Forty adult albino rats were divided into five groups: sham operation, torsion, I/R plus saline, I/R plus melatonin and torsion plus melatonin. Rats in the sham-operated group underwent a surgical procedure similar to the other groups but the adnexa was not occluded. Rats in the torsion group were killed after adnexal torsion for 3 hr. Melatonin and saline were injected intraperitoneally (10 mg/kg) 30 min before detorsion to the I/R plus melatonin group and I/R plus saline group respectively. After 3 hr of ovarian detorsion, the rats were killed and ovaries were removed. Melatonin was injected intraperitoneally (10 mg/kg) 30 min before torsion to the torsion plus melatonin group. After 3 hr of ovarian torsion, the rats were killed and ovaries were harvested. The tissue levels of MDA, GSH and XO were measured. MDA and XO levels in the I/R plus saline group increased significantly when compared with torsion and sham-operated groups (P < 0.001). MDA and XO levels in the I/R plus melatonin group were lower than I/R plus saline and differences between the two groups were statistically significant (P < 0.001). GSH levels in the I/R plus saline group decreased significantly when compared with ischemia and sham-operated groups (P < 0.001). GSH levels in the I/R plus melatonin treated rats were significantly higher than I/R plus saline and ischemia groups (P < 0.001). The tissue levels of XO, MDA and GSH were similar between ischemia and ischemia plus melatonin groups. Morphologically, polymorphonuclear neutrophil infiltration and vascular dilatation were obvious in the I/R-damaged ovaries, and the changes also partially reversed by melatonin. This study demonstrates that melatonin protects the ovaries against oxidative damage associated with reperfusion following an ischemic insult.     

Melatonin regulates the rhythmic migration of neutrophils in live zebrafish

The circadian clock plays a vital role in physiology and behavior such as the sleep–wake cycle and blood pressure and hormone levels. Immune responses also display circadian rhythmicity and particularly pineal melatonin contributes to immunological processes. Little attention, however, is given to mechanisms underlying rhythmic neutrophil responses to the injury. Here, we used a transgenic Tg(lyz:EGFP) zebrafish tail fin transection model to investigate whether the recruitment of neutrophils toward the injured site is regulated by the circadian clock. We found that migrating neutrophils display robust rhythmicity, peaking at darkness. Melatonin positively regulates rhythmic neutrophil migration, as evidenced that treatment with melatonin at low dosage can significantly enhance neutrophil recruitment toward the injured site, which is attenuated by luzindole treatment and in pinealectomized fish. Furthermore, using a transgenic zebrafish eyeball model, we observed that melatonin enhances secretion of two cytokines, TNF‐α and IL‐8, both of which markedly enhance neutrophil migration. Taken together, these results highlight a positive role of melatonin in rhythmic neutrophil migration and help obtain a better understanding of circadian regulation in immunology.     

Melatonin attenuates diabetes-induced oxidative stress in rabbits

Oxidative stress is considered to be the main cause of diabetic complications. As the role of antioxidants in diabetes therapy is still underestimated, the aim of the present investigation was to study the antioxidative action of melatonin in comparison with N-acetylcysteine (NAC) under diabetic conditions. Alloxan-diabetic rabbits were treated daily with either melatonin (1 mg/kg, i.p.), NAC (10 mg/kg, i.p.) or saline. Blood glutathione redox state and serum hydroxyl free radicals (HFR), creatinine and urea levels were monitored. After 3 wk of treatment animals were killed and HFR content, reduced glutathione/oxidized glutathione (GSH/GSSG) ratio as well as the activities of glutathione reductase, glutathione peroxidase and gamma-glutamylcysteine synthetase were estimated in both liver and kidney cortex. Diabetes evoked a several-fold increase in HFR levels accompanied by a significant decline in GSH/GSSG ratio in serum and the examined organs. In contrast to NAC, melatonin (at 1/10 the dose of NAC) attenuated diabetes-induced alterations in glutathione redox state and HFR levels, normalized creatinine concentration and diminished urea content in serum. Moreover, the indole resulted in an increase in glutathione reductase activity in both studied organs and in a rise in glutathione peroxidase and gamma-glutamylcysteine synthetase activities in the liver. In contrast to NAC, melatonin seems to be beneficial for diabetes therapy because of its potent antioxidative and nephroprotective action. The indole-induced increase in the activities of the enzymes of glutathione metabolism might be of importance for antioxidative action of melatonin under diabetic conditions.     

Mechanisms of protection by melatonin against acetaminophen-induced liver injury in mice

The present study was performed to determine whether melatonin protects mouse liver against severe damage induced by acetaminophen (APAP) administration and where melatonin primarily functions in the metabolic pathway of APAP to protect mouse liver against APAP-induced injury. Treatment of mice with melatonin (50 or 100 mg/kg, p.o.) 8 or 4 hr before APAP administration (750 mg/kg, p.o.) suppressed the increase in plasma alanine aminotransferase and aspartate aminotransferase activities in a dose- and a time-dependent manner. Melatonin treatment (100 mg/kg, p.o.) 4 hr before APAP administration remarkably inhibited centrilobular hepatic necrosis with inflammatory cell infiltration and increases in hepatic lipid peroxidation and myeloperoxidase activity, an index of tissue neutrophil infiltration, as well as release of nitric oxide and interleukin-6 into blood circulation at 9 hr after APAP administration. However, melatonin neither affected hepatic reduced glutathione (GSH) content nor spared hepatic GSH consumption by APAP treatment. Moreover, pretreatment with melatonin 4 hr before APAP administration did not influence the induction of hepatic heat shock protein 70 (HSP70) by APAP and melatonin alone did not induce HSP70 in mouse liver. These results indicate that exogenously administered melatonin exhibits a potent hepatoprotective effect against APAP-induced hepatic damage probably downstream of the activity of cytochrome P450 2E1, which works upstream of GSH conjugation in the pathway of APAP metabolism, via its anti-nitrosative and anti-inflammatory activities in addition to its antioxidant activity.     

Alleviation of maternal hyperthermia-induced early embryonic death by administration of melatonin to mice

Maternal hyperthermia induces early embryonic death via increased oxidative stress to the embryo. In this study, we examined whether melatonin administered to heat-stressed pregnant mice would reduce hyperthermia-induced embryonic death. Mice were heat stressed (12 hr at 35 degrees C, 60% relative humidity) on the day of mating and melatonin (3 mg/kg body weight) was injected subcutaneously every 2 hr during heat exposure. Thereafter, zygotes were collected, and in vitro developmental ability and intracellular glutathione (GSH) content were assessed. In addition, reactive oxygen species (ROS) levels and free radical scavenging activity (FRSA) in the oviduct as well as lipid peroxidation in the liver were measured. Melatonin administration was associated with a tendency for higher intracellular GSH content in zygotes (1.67 pmol/zygote) and a significantly higher percentage of embryos that developed to the morula or blastocyst stage (47.91%; P < 0.01) compared with the parameters in heat-stressed mice that were administered a placebo (1.48 pmol GSH/zygote and 14.78% development). Lipid peroxidation levels in the liver and ROS levels in the oviduct were the same in melatonin-treated stressed mice and the controls, while these parameters were significantly higher in heat-stressed mice that were not treated with melatonin. Furthermore, FRSA in the oviduct was significantly (P < 0.05) higher in the melatonin-treated mice than in the controls. These results suggest that administration of melatonin to heat-stressed mice alleviates hyperthermia-induced early embryonic death and that this is accomplished in part by maintaining a neutral redox status within the mother.     

Protective effect of melatonin against adriamycin toxicity in the rat

Adriamycin, an anthracyclinic antibiotic frequently used in quimioterapeutic treatments is highly toxic; it inhibits protein synthesis and provokes prooxidant effects. Melatonin has recently been shown to have high antioxidative properties. We tested if melatonin is able to neutralize the oxidative damage induced by a single dose (20 mg/kg, i.p.) of adriamycin preceded (3 days) and followed (7 days) by a low pharmacological dose (50 microg/kg, i.p.) of melatonin. After the administration of a single dose of adriamycin (20 mg/kg i.p.) to male Wistar rats, the reduced to oxidized glutathione (GSH/GSSG) ratio and the glutathione peroxidase (GPx, E.C. 1.11.1.9.) activity in the brain, intestine, heart, kidney, and lung were significantly reduced. When the treatment of adriamycin was preceded and followed by low pharmacological doses of melatonin, the decrease in the GSH/GSSG ratio was significantly reduced but the reduction in GPx activity was not attenuated. A significant increase in lipid peroxidation products was observed in brain, heart, and kidney tissues after a single administration of adriamycin, which was attenuated by pre- and post-treatment with a low pharmacological dose of melatonin. Our results demonstrate that oxidative damage induced by the antitumor drug, adriamycin, can be reduced by low pharmacological doses of melatonin.     

Neutrophils of Centenarians Show Function Levels Similar to Those of Young Adults

OBJECTIVES: To analyze several functions and antioxidant parameters of peripheral blood neutrophils from healthy centenarians (men and women) and compare them with those of healthy young (aged 25–35) and middle‐aged (aged 65–75) men and women. DESIGN: Cross‐sectional study. SETTING: Community‐based. PARTICIPANTS: Twenty‐one healthy centenarians (8 men), 30 young adults (15 men), and 30 middle‐aged adults (15 men). MEASUREMENTS: Several neutrophil functions (adherence, chemotaxis, phagocytosis, and stimulated and nonstimulated intracellular superoxide anion levels) and antioxidant parameters (glutathione levels and catalase activity) were measured in peripheral blood neutrophil suspension in the three study groups. RESULTS: Neutrophil functions of the middle‐aged group were worse than those of young adults and centenarians (lower chemotaxis and phagocytosis and higher adherence and superoxide anion levels). The neutrophil functions of the centenarians were closer to those of the young adults. Age‐related differences in neutrophil functions were fundamentally similar in men and women, except for intracellular superoxide anion levels, which were lower in young adult women than in young adult men. With normal aging, total glutathione levels decrease, but the centenarians in this study showed levels similar to those of young adults. Centenarians showed the highest catalase activity of the three groups. CONCLUSION: Progressive impairment of the immune system accompanies aging. The better preservation of function and antioxidant systems in the neutrophils of centenarians could play a key role in the longevity of these subjects.     

Melatonin-induced reduction in age-related accumulation of oxidative damage in mice

The ever-increasing understanding of oxygen radical-linked diseases, including the biological process of aging, has stimulated general interest in modulating these biological events. Melatonin has been reported to have antioxidant properties in addition to its known hormonal activities. However, reports on low-level chronic administration with its anti-aging influence are scanty. Hence, the present study was aimed to investigate the influence of low-dose chronic administration (0.10 mg/kg bodyweight/day for 3 months) of melatonin again stage-induced oxidative stress in mice tissues, namely brain, liver, spleen and kidney. Sixteen-month-old mice were supplemented with melatonin (0.10 mg/kg body weight/day) for three months and then autopsied (at the age of19 months) for the biochemical estimation of lipid peroxidation, reduced glutathione (GSH), glutathione disulphide (GSSG), glutathione peroxidase (GSH-Px) and serum phosphatase activity. Results indicate that age-induced augmentation (compared to 6–8-week-old mice)in the level of lipid peroxidation, GSSG and acid phosphatase is significantly (P < 0.001)ameliorated in melatonin-treated mice. Age-induced decline in the level of GSH, GSH-Px and alkaline phosphatase activity is inhibited significantly by the long-term administration of melatonin. The findings indicate that low-dose chronic administration of melatonin acts as a free radical scavenger and anti-aging agent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Melatonin attenuates ifosfamide-induced Fanconi syndrome in rats

Regarding the mechanisms of ifosfamide (IFO)-induced nephrotoxicity and hemorrhagic cystitis, several hypotheses have been put forward, among which oxidative stress and depletion of glutathione (GSH) are suggested. This investigation elucidates the role of free radicals in IFO-induced toxicity and the protection by melatonin. Wistar albino rats were injected intraperitoneally with saline (0.9% NaCl; control-C group), melatonin (Mel group; 10 mg/kg daily for 5 days) or ifosfamide (50 mg/kg daily for 5 days; IFO group) or IFO + Mel. On the 5th day (120 hr) after the first IFO dose, animals were killed by decapitation and trunk blood was collected. Kidney and bladder tissues were obtained for biochemical and histological analysis. Urine was collected 24 hr before the rats were killed. The results demonstrated that IFO induced a Fanconi syndrome (FS) characterized by wasting of sodium, phosphate, and glucose, along with increased serum creatinine and urea. Melatonin markedly ameliorated the severity of renal dysfunction induced by IFO with a significant decrease in urinary sodium, phosphate, and glucose and increased creatinine excretion. Moreover, melatonin significantly improved the IFO-induced GSH depletion, malondialdehyde accumulation and neutrophil infiltration in both renal and bladder tissues. In the kidney, Na+,K+ -ATPase activity which was significantly reduced by IFO, was increased with melatonin treatment. Increased collagen contents of the kidney and bladder tissues by IFO treatment were reversed back to the control levels with melatonin. Our results suggest that IFO causes oxidative damage in renal and bladder tissues and melatonin, via its antioxidant effects, protects these tissues. These data suggest that melatonin may be of therapeutic use in preventing acquired FS due to IFO toxicity.     

In vivo neutrophil dysfunction in cirrhotic patients with advanced liver disease

Bacterial infections are frequent, life-threatening complications in cirrhotic patients. This study investigated in vivo neutrophil migration and phagocytic activity in cirrhotic patients with advanced liver disease, in liver transplant recipients, and in healthy volunteers, by use of the skin window technique. Complement receptor type III (CR3) expression was also measured in blood and elicited neutrophils. Neutrophil migration to skin windows and neutrophil in vivo phagocytosis of heat-killed Escherichia coli were significantly decreased in cirrhotic patients compared with healthy controls. Neutrophil migration and phagocytosis were decreased in cirrhotic patients with previous episodes of bacterial infection compared with noninfected patients. Expression of CR3 in circulating neutrophils was significantly higher in cirrhotic patients, whereas it was significantly reduced in elicited neutrophils of cirrhotic patients with previous bacterial infection. These data suggest that deficient neutrophil recruitment to the infection site and impaired phagocytic activity may contribute to bacterial infections in cirrhotic patients with advanced liver disease.     

The protective effect of melatonin on ischemia-reperfusion injury in the groin (inferior epigastric) flap model in rats

Inadequate blood perfusion and ischemia-reperfusion (I/R) injury in the surgical skin flap are believed to be the major factors that cause harmful changes within the tissue and vasculature, resulting in flap necrosis. Reactive oxygen radical species (ROS), in part, are believed to play an important role in this injury. Melatonin, in many physiological conditions, has been shown to have direct and indirect antioxidative effects and free-radical-scavenging properties. Therefore, it may have a beneficial effect on I/R-induced flap injury. In this study, the possible protective effects of melatonin were investigated in I/R injury of rat epigastric (axial pattern) flaps. Ischemia was achieved for 12 h by occlusion of inferior epigastric artery. Melatonin or vehicle was administered 1 h before flap elevation and was continued for 6 days after ischemia. I/R injury elevated malondialdehyde (MDA), an end product of lipid peroxidation, and nitric oxide (NO) levels while the glutathione (GSH) content was reduced. Myeloperoxidase (MPO) activity, which is known to be related to tissue neutrophil accumulation, was found to be statistically higher in the I/R group when compared with the sham group. Administration of melatonin significantly decreased MDA, NO and MPO levels and elevated the GSH content. Moreover, melatonin reduced the flap necrosis area, which was determined using a planimetric method. In conclusion, melatonin, a potent scavenger of free radicals, plays a major role in preventing the inferior epigastric arterial I/R-induced flap necrosis, based on planimetric flap survival and biochemical results. The beneficial effects of melatonin in I/R injury implies the involvement of free radicals in flap damage.     

Protective effect of melatonin in acetic acid induced colitis in rats

Possible protective effects of exogenous melatonin on colonic inflammation were studied in rats. Colitis was induced by intracolonic (i.c.) instillation of 4% acetic acid (AA) and the resulting injury was assessed after 1 and 48 hr. Diffuse hyperemia and bleeding with erosions and ulcerations were observed in the colons of vehicle-treated rats. Melatonin administered in doses of 5 and 10 mg/kg reduced significantly the extent of gross mucosal damage after intraperitoneal as well as i.c. dosing. The inflammation induced increase in colonic wet weight was also reduced by melatonin treatment. In the early phase of colonic inflammation (60 min), melatonin partly prevented the decrease of reduced glutathione (GSH) content and limited lysosomal enzyme, N-acetyl-glucosaminidase and cathepsin D, activities induced by AA, with no changes in proteins or acid phosphatase activity. Increase of myeloperoxidase activity (MPO) caused by colonic inflammation was prevented by melatonin given i.c. As observed 48 hr after AA exposure, there was no difference between the effect of vehicle and melatonin on the content of GSH. Colitis did not influence the melatonin content of the colon. After administration of exogenous melatonin, plasma, pineal and gut melatonin tended to increase. The results indicate that melatonin participates in various defense mechanisms against colonic inflammatory processes by preserving the important endogenous antioxidant reserve of GSH, by preventing lysosomal enzyme disruption, by inhibiting enhanced MPO activity, thus reducing the extent of colonic damage, mainly in the early phase of colitis.     

Melatonin stimulates the activity of the detoxifying enzyme glutathione peroxidase in several tissues of chicks

Abstract: The pineal hormone melatonin has been shown to directly scavenge free radicals and to stimulate, in the mammalian brain, at least one enzyme, glutathione peroxidase, which reduces free radical generation. In the present studies, we examined the effect of melatonin on glutathione peroxidase activity in several tissues of an avian species. Melatonin (500 μg/kg), when injected into chicks, increased glutathione peroxidase activity within 90 min in every tissue examined. Tissue melatonin levels, measured by radioimmunoassay, also increased following its peripheral administration. Depending on the tissue, the measured increases in melatonin varied from 75% to 1,300% over the control values. The melatonin-induced increases in glutathione peroxidase activity varied with the tissue and were between 22% and 134%. These percentage increases in glutathione peroxidase activity were directly correlated with tissue melatonin content. These results suggest that melatonin induces the activity of the detoxifying enzyme, glutathione peroxidase, in several tissues in the chick. The findings also suggest that melatonin would reduce the generation of highly toxic hydroxyl radicals by metabolizing its precursor, hydrogen peroxide. Because of this ability to stimulate glutathione peroxidase activity, melatonin should be considered as a component of the antioxidative defense system in this avian species.     

Melatonin reduces dimethylnitrosamine-induced liver fibrosis in rats

Increased deposition of the extracellular matrix components, particularly collagen, is a central phenomenon in liver fibrosis. Stellate cells, the central mediators in the pathogenesis of fibrosis are activated by free radicals, and synthesize collagen. Melatonin is a potent physiological scavenger of hydroxyl radicals. Melatonin has also been shown to be involved in the inhibitory regulation of collagen content in tissues. At present, no effective treatment of liver fibrosis is available for clinical use. We aimed to test the effects of melatonin on dimethylnitrosamine (DMN)-induced liver damage in rats. Wistar albino rats were injected with DMN intraperitoneally. Following a single dose of 40 mg/kg DMN, either saline (DMN) or 100 mg/kg daily melatonin was administered for 14 days. In other rats, physiologic saline or melatonin were injected for 14 days, following a single injection of saline as control. Hepatic fibrotic changes were evaluated biochemically by measuring tissue hydroxyproline levels and histopathogical examination. Malondialdehyde (MDA), an end product of lipid peroxidation, and glutathione (GSH) and superoxide dismutase (SOD) levels were evaluated in blood and tissue homogenates. DMN caused hepatic fibrotic changes, whereas melatonin suppressed these changes in five of 14 rats (P < 0.05). DMN administration resulted in increased hydroxyproline and MDA levels, and decreased GSH and SOD levels, whereas melatonin reversed these effects. When melatonin was administered alone, no significant changes in biochemical parameters were noted. In conclusion, the present study suggests that melatonin functions as a potent fibrosuppressant and antioxidant, and may be a therapeutic choice.