The presence of melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) and its precursors, serotonin (<em>5</em>-hydroxytryptamine) and <em>N</em>-<em>acetyl</em>serotonin, was demonstrated in extracts of human ovary using reverse-phase high-performance liquid chromatography coupled with fluorometric detection. In addition, activities of two melatonin-synthesizing enzymes, arylalkylamine <em>N</em>-<em>acetyl</em>transferase (<em>N</em>AT) and hydroxyindole-O-methyltransferase (HIOMT), were found in human ovary homogenates. The apparent Michaelis constants for the substrates of <em>N</em>AT and HIOMT in the human ovary were similar to those reported for the pineal glands of humans and other mammals. These findings strongly suggest that the human ovary, like the pineal gland, may synthesize melatonin from serotonin by the sequential action of <em>N</em>AT and HIOMT.

Mast cells take part of armamentarium immunologic for host defense against parasitic and bacterial infections. They are derived from bone marrow progenitors and can be activated by immunological and chemical stimuli in order to get its degranulation. The activation of mast cells generates a signalling cascade leaded to the rapid release of vasoactives and pro-inflammatory mediators. Melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) is a molecule with antioxidant, cytoprotective and immunomodulatory actions. It was initially known to be produced exclusively in the pineal gland but melatonin synthesis has been found in different sites of the organism, and a major source of extrapineal melatonin is the immune system. The aim of the present study was to prove if the rat mast cell line (RBL-2H3) synthesizes and releases melatonin, also to explain its possible mechanism of action. We report that both resting and stimulated mast cells synthesize and release melatonin. We also report that the necessary machinery to synthesize melatonin is present in mast cells and that these cells showed the presence of MT1 and MT2 melatonin membrane receptors. Those results indicated that the melatonin would be able to exert a regulatory effect on inflammatory reactions mediated by mast cells.

This review summarizes new findings related to beneficial effects of melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) on reproductive physiology. Recently many researchers have begun to study the local role of melatonin as an antioxidant. We focused on intra-follicular role of melatonin in the ovary. Melatonin, secreted by the pineal gland, is taken up into the follicular fluid from the blood. Reactive oxygen species (ROS) are produced within the follicles, during the ovulatory process. Melatonin reduces oxidative stress as an antioxidant, and contribute to oocyte maturation, embryo development and luteinization of granulosa cells. Our clinical study demonstrated that melatonin treatment for infertile women increases intra-follicular melatonin concentrations, reduces intra-follicular oxidative damage, and elevates fertilization and pregnancy rates. Melatonin treatment also improves progesterone production by corpus luteum in infertile women with luteal phase defect. Melatonin treatment could become a new cure for improving oocyte quality and luteal function in infertile women.

2-[12<em>5</em>I]Iodomelatonin binds with high affinity to a site possessing the pharmacological characteristics of a melatonin receptor in chicken retinal membranes. The specific binding of 2-[12<em>5</em>I]iodomelatonin is stable, saturable, and reversible. Saturation experiments indicated that 2-[12<em>5</em>I]iodomelatonin labeled a single class of sites with an affinity constant (Kd) of 434 +/- <em>5</em>6 pM and a total number of binding sites (Bmax) of 74.0 +/- 13.6 fmol/mg of protein. The affinity constant obtained from kinetic analysis was in close agreement with that obtained in saturation experiments. Competition experiments showed a monophasic reduction of 2-[12<em>5</em>I]iodomelatonin binding with a pharmacological order of indole amine affinities characteristic of a melatonin receptor: 2-iodomelatonin greater than 6-chloromelatonin greater than or equal to melatonin greater than or equal to 6,7-dichloro-2-methylmelatonin greater than 6-hydroxymelatonin greater than or equal to 6-methoxymelatonin much greater than <em>N</em>-<em>acetyl</em>tryptamine greater than <em>N</em>-<em>acetyl</em>-<em>5</em>-hydroxytryptamine greater than <em>5</em>-<em>methoxytryptamine</em> greater than <em>5</em>-hydroxytryptamine (inactive). The affinities of these melatonin analogs in competing for 2-[12<em>5</em>I]iodomelatonin binding sites were correlated closely with their potencies for inhibition of the calcium-dependent release of [3H]dopamine from chicken and rabbit retinas, indicating association of the binding site with a functional response regulated by melatonin. The results indicate that 2-[12<em>5</em>I]iodomelatonin is a selective, high-affinity radioligand for the identification and characterization of melatonin receptor sites.

Rhodospirillum rubrum is a spiral anoxygenic photosynthetic bacterium that can exist under either aerobic or anaerobic conditions. The organism thrives in the presence of light or complete darkness and represents one of the oldest species of living organisms, possibly 2-3.<em>5</em> billion years old. The success of this prokaryotic species may be attributed to the evolution of certain indole compounds that offer protection against life-threatening oxygen radicals produced by an evolutionary harsh environment. Melatonin, <em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>, is an indolic highly conserved molecule that exists in protists, plants, and animals. This study was undertaken to determine the presence of an immunoreactive melatonin in the kingdom Monera and particularly in the photosynthetic bacterium, R. rubrum, under conditions of prolonged darkness or prolonged light. Immunoreactive melatonin was measured during both the extended day and extended night. Significantly more melatonin was observed during the scotophase than the photophase. This study marks the first demonstration of melatonin in a bacterium. The high level of melatonin observed in bacteria may provide on-site protection of bacterial D<em>N</em>A against free radical attack.

The metabolism of melatonin in the central nervous system is of interest for several reasons. Melatonin enters the brain either via the pineal recess or by uptake from the blood. It has been assumed to be also formed in some brain areas. <em>N</em>europrotection by melatonin has been demonstrated in numerous model systems, and various attempts have been undertaken to counteract neurodegeneration by melatonin treatment. Several concurrent pathways lead to different products. Cytochrome P(4<em>5</em>0) subforms have been demonstrated in the brain. They either demethylate melatonin to <em>N</em>-<em>acetyl</em>serotonin, or produce 6-hydroxymelatonin, which is mostly sulfated already in the C<em>N</em>S. Melatonin is de<em>acetyl</em>ated, at least in pineal gland and retina, to <em>5</em>-<em>methoxytryptamine</em>. <em>N</em>(1)-<em>acetyl</em>-<em>N</em>(2)-formyl-<em>5</em>-methoxykynuramine is formed by pyrrole-ring cleavage, by myeloperoxidase, indoleamine 2,3-dioxygenase and various non-enzymatic oxidants. Its product, <em>N</em>(1)-<em>acetyl</em>-<em>5</em>-methoxykynuramine, is of interest as a scavenger of reactive oxygen and nitrogen species, mitochondrial modulator, downregulator of cyclooxygenase-2, inhibitor of cyclooxygenase, neuronal and inducible <em>N</em>O synthases. Contrary to other nitrosated aromates, the nitrosated kynuramine metabolite, 3-acetamidomethyl-6-methoxycinnolinone, does not re-donate <em>N</em>O. Various other products are formed from melatonin and its metabolites by interaction with reactive oxygen and nitrogen species. The relative contribution of the various pathways to melatonin catabolism seems to be influenced by microglia activation, oxidative stress and brain levels of melatonin, which may be strongly changed in experiments on neuroprotection. Many of the melatonin metabolites, which may appear in elevated concentrations after melatonin administration, possess biological or pharmacological properties, including <em>N</em>-<em>acetyl</em>serotonin, <em>5</em>-<em>methoxytryptamine</em> and some of its derivatives, and especially the <em>5</em>-methoxylated kynuramines.

The presence of melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) in plants has been clearly demonstrated. However, while this indoleamine has been intensively studied in animals, especially in mammals, the same is not true in the case of plants, where one of the most interesting aspects is its possible role as antioxidative molecule in physiological processes. Some data reflect the possible protective role that melatonin may exert in some stress situations such as ultraviolet (UV)-radiation, induced senescence and copper stress. The present work was designed to establish how the melatonin content changes in plants as a result of chemically induced stress. For this, barley plants were exposed in different treatments to the chemical-stress agents: sodium chloride, zinc sulphate or hydrogen peroxide. After different times, the content of melatonin in treated roots and control roots were determined using liquid chromatography (LC) with time-of-flight/mass spectrometry and LC with fluorescence detection for identification and quantification, respectively. The data show that the melatonin content in roots increased due to stress, reaching up to six times the melatonin content of control roots. Induction was time dependent, while hydrogen peroxide (10 mm) and zinc sulphate (1 mm) were the most effective inducers. The capacity of roots to absorb melatonin from soil was also studied. The data establish, for first time, that the chemical-stress agents assayed can induce the biosynthesis of melatonin in barley roots and produce a significant increase in their melatonin content. Such an increase in melatonin probably plays an important antioxidative role in the defense against chemically induced stress and other abiotic/biotic stresses.

The distribution and characterization of specific melatonin binding sites were studied using 12<em>5</em>I-melatonin. Autoradiography revealed only three sites of specific melatonin binding in brain: the suprachiasmatic nuclei, the median eminence, and the small part of choroid plexus at the caudal end of the fourth ventricle. Two other sites were detected outside the C<em>N</em>S: the anterior pituitary and the retina. The specific binding of 12<em>5</em>I-melatonin was saturable and reversible. The dissociation constant (KD) of the binding sites was 60 pM. The concentration of the binding sites (Bmax) in the median eminence was 26 fmol/mg protein, and in the pituitary 3 fmol/mg protein. Specificity of the binding sites was tested by displacement of 12<em>5</em>I-melatonin. The order of potency--melatonin much less than <em>N</em>-<em>acetyl</em>-<em>5</em>-hydroxytryptamine less than <em>5</em>-<em>methoxytryptamine</em> much less than <em>5</em>-hydroxytryptamine = 3,4-dihydroxyphenylethylamine = noradrenaline--shows high specificity of the binding sites for melatonin.

Melatonin, <em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>, is a molecule with diverse physiological functions. This neuro-hormone affects reproductive performance in a wide variety of species. In most animals, but not exclusively all, melatonin has an antigonadotrophic effect. The seasonal changes in the number of hours per day that melatonin is secreted mediate the temporal coupling of reproductive activity to seasonal changes in day-length. These observations stimulated a search for a role for the pineal gland and melatonin in human reproduction. Clinical experience related to this issue has yielded inconclusive and sometimes conflicting results. This article reviews the current available evidence concerning the effects of melatonin on human reproductive processes (e.g., puberty, ovulation, pregnancy, and fertility). Possible reasons for the vagueness and elusiveness of the clinical effects are discussed.

Melatonin's O-methyl and <em>N</em>-<em>acetyl</em> residues are not only the basis of its amphilicity enabling the molecule to enter all organs and all subcellular compartments, but are also decisive for its antioxidant properties. We have compared melatonin's redox chemistry with that of several structural analogs: tryptamine, <em>N</em>-<em>acetyl</em>tryptamine, serotonin, <em>N</em>-<em>acetyl</em>serotonin, <em>5</em>-<em>methoxytryptamine</em>, 6-chloromelatonin and 2-iodomelatonin. Scavenging of hydroxyl radicals (*OH) was measured in a scavenger competition assay based on ABTS cation radical (ABTS(*)+) formation. The capability of undergoing single-electron transfer reactions was studied using an ABTS(*)+ reduction assay, reflecting the more general property of scavenging organic cation radicals. Direct scavenging of superoxide anions (O2(*)-), under non-catalyzed conditions, was investigated in a hematoxylin autoxidation assay. Measurements of chemiluminescence were used for studying scavenging of O2(*)- under catalyzed conditions, either by hemin-mediated interaction or by combination with the respective indolyl cation radicals. Light emission was determined in the absence or presence of the *OH scavenger dimethylsulfoxide and the O2(*)- scavenger Tiron. Products formed by oxidation of the respective indoles in a moderately alkaline, hemin-catalyzed H2O2 system were analyzed by thin-layer chromatography and fluorometry. Absence of either the O-methyl or the <em>N</em>-<em>acetyl</em> residue causes marked diminutions in the capacities of scavenging *OH and ABTS(*)+ as well as in chemiluminescence emitted during oxidation. The importance of the <em>N</em>-<em>acetyl</em> group is insofar remarkable as it seems, at first glance, to be isolated from the indolic moiety; interactions between side chain and indolic moiety are therefore decisive for melatonin's redox properties. The <em>5</em>-hydroxylated compounds are not generally more efficient scavengers, but particularly better reducers of ABTS(*)+; in the alkaline H2O2 system generating *OH and O2(*)-, melatonin was much more rapidly oxidized than the <em>5</em>-hydroxylated and non-substituted analogs. Oxidative products formed from any of the compounds studied contained much less of substituted kynuramines as in the case of melatonin, indicating that radical chain termination by O2(*)- is considerably more efficient with melatonin. These findings are supported by measurements of chemiluminescence, which largely reflects pyrrole ring cleavage as a result of combination with superoxide anions. In this regard, only 6-chloromelatonin equalled melatonin, whereas the efficiency of 2-iodomelatonin was much lower, another indication for the importance of 2,3-dioxygenation.

Sleep disorders are a group of conditions that affect the ability to sleep well on a regular basis and cause significant impairments in social and occupational functions. Although currently approved medications are efficacious, they are far from satisfactory. Benzodiazepines, antidepressants, antihistamines and anxiolytics have the potential for dependence and addiction. Moreover, some of these medications can gradually impair cognition. Melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) is an endogenous hormone produced by the pineal gland and released exclusively at night. Exogenous melatonin supplementation is well tolerated and has no obvious short- or long-term adverse effects. Melatonin has been shown to synchronize the circadian rhythms, and improve the onset, duration and quality of sleep. It is centrally involved in anti-oxidation, circadian rhythmicity maintenance, sleep regulation and neuronal survival. This narrative review aims to provide a comprehensive overview of various therapeutic functions of melatonin in insomnia, sleep-related breathing disorders, hypersomnolence, circadian rhythm sleep-wake disorders and parasomnias. Melatonin offers an alternative treatment to the currently available pharmaceutical therapies for sleep disorders with significantly less side effects.

High-affinity 2-12<em>5</em>I-iodomelatonin binding sites in homogenates of purified cell nuclei from rat liver were localized and characterized using biochemical binding techniques. Binding at these sites was found to be rapid, reversible, saturable, and to demonstrate pharmacological selectivity. At 0 degrees C, binding reached equilibrium in about 10 min. Scatchard analysis of the data at equilibrium revealed a single class of binding sites with a dissociation constant of KD = 190 +/- 47 pM, Bmax = 9.8 +/- 0.6 fmol/mg protein, and a Hill coefficient of nH = 1.02 +/- 0.034. Kinetic analysis of the association and dissociation curves indicated a kinetic KD = 148 +/- 41 pM, which is in good agreement with the value obtained at equilibrium. The specific binding of 2-12<em>5</em>I-iodomelatonin (4<em>5</em> pM) (0.<em>5</em>1 +/- 0.04 fmol/mg protein) was significantly improved (0.79 +/- 0.04 fmol/mg protein) when the homogenates of purified liver cell nuclei were preincubated with D<em>N</em>ase (2 micrograms/ml at 37 degrees C for 20 min) before being used in binding experiments. After the addition of either proteinase K or trichloroacetic acid to D<em>N</em>ase-treated purified cell nuclear homogenates, the specific binding disappeared. This suggests that the specific binding of 2-12<em>5</em>I-iodomelatonin in liver cell nuclei is associated with nuclear protein. Competition experiments show that <em>N</em>-<em>acetyl</em>-serotonin (Ki = 81.3 nM) was more potent than <em>5</em>-hydroxytryptamine (Ki>> 1 microM) and <em>5</em>-<em>methoxytryptamine</em> (Ki>>) 10 microM) in inhibiting 2-12<em>5</em>I-iodomelatonin binding (Ki melatonin = 146 pM). These data indicate that specific 2-12<em>5</em>I-iodomelatonin binding sites exist in the cell nuclei of rat liver, and that they may comprise a locus for the intracellular action of melatonin. The correlation between the KD and Bmax values with melatonin concentrations in nuclei suggest that these binding sites may be a physiological melatonin receptor, which could explain the described genomic effects of the pineal hormone.

Melatonin and its metabolites including 6-hydroxymelatonin (6(OH)M), <em>N</em>(1)-<em>acetyl</em>-<em>N</em>(2)-formyl-<em>5</em>-methoxykynuramine (AFMK) and <em>5</em>-<em>methoxytryptamine</em> (<em>5</em>MT) are endogenously produced in human epidermis. This production depends on race, gender and age. The highest melatonin levels are in African-Americans. In each racial group they are highest in young African-Americans [30-<em>5</em>0 years old (yo)], old Caucasians (60-90 yo) and Caucasian females. AFMK levels are the highest in African-Americans, while 6(OH)M and <em>5</em>MT levels are similar in all groups. Testing of their phenotypic effects in normal human melanocytes show that melatonin and its metabolites (10(-<em>5</em>) M) inhibit tyrosinase activity and cell growth, and inhibit D<em>N</em>A synthesis in a dose dependent manner with 10(-9) M being the lowest effective concentration. In melanoma cells, they inhibited cell growth but had no effect on melanogenesis, except for <em>5</em>MT which enhanced L-tyrosine induced melanogenesis. In conclusion, melatonin and its metabolites [6(OH)M, AFMK and <em>5</em>MT] are produced endogenously in human epidermis and can affect melanocyte and melanoma behavior.

Reactive oxygen species (ROS) play an important role in physiological processes, but - when being in excess - ROS cause oxidative damage to molecules. Under physiological conditions, the production and detoxification of ROS are more-or-less balanced. Also in the thyroid, ROS and free radicals participate in physiological and pathological processes in the gland. For example, hydrogen peroxide (H2O2) is crucial for thyroid hormone biosynthesis, acting at different steps of the process. Additionally, H2O2 is believed to participate in the Wolff-Chaikoff's effect, undergoing in conditions of iodide excess in the thyroid. Much evidence has been accumulated indicating that oxidative stress is involved in pathomechanism of thyroid disease, e.g., Graves' disease, goiter formation or thyroid cancer. Melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) - the main secretory product of the pineal gland - is a well-known antioxidant and free radical scavenger, widely distributed in the organism. Mutual relationships between the pineal gland and the thyroid have - for a long time - been a subject of intensive research. The abundant to-date's evidence relates mostly to the inhibitory action of melatonin on the thyroid growth and function and - to a lesser extent - to the stimulatory effects of thyroid hormones on the pineal gland. It is highly probable that under physiological conditions melatonin and, possibly, other antioxidants regulate ROS generation for thyroid hormone synthesis. We believe that melatonin may protect against extensive oxidative damage in the course of certain thyroid disorders or in case of a harmful action of some external factors on the thyroid. Thus, oxidative damage and the protective action of antioxidants, melatonin included, may occur during both physiological and pathological processes in the thyroid, however, this assumption, requires further studies.

The pineal hormone, melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>), shows potent receptor-dependent and -independent actions, which participate in blood pressure regulation. The antihypertensive effect of melatonin was demonstrated in experimental and clinical hypertension. Receptor-dependent effects are mediated predominantly through MT1 and MT2 G-protein coupled receptors. The pleiotropic receptor-independent effects of melatonin with a possible impact on blood pressure involve the reactive oxygen species (ROS) scavenging nature, activation and over-expression of several antioxidant enzymes or their protection from oxidative damage and the ability to increase the efficiency of the mitochondrial electron transport chain. Besides the interaction with the vascular system, this indolamine may exert part of its antihypertensive action through its interaction with the central nervous system (C<em>N</em>S). The imbalance between the sympathetic and parasympathetic vegetative system is an important pathophysiological disorder and therapeutic target in hypertension. Melatonin is protective in C<em>N</em>S on several different levels: It reduces free radical burden, improves endothelial dysfunction, reduces inflammation and shifts the balance between the sympathetic and parasympathetic system in favor of the parasympathetic system. The increased level of serum melatonin observed in some types of hypertension may be a counter-regulatory adaptive mechanism against the sympathetic overstimulation. Since melatonin acts favorably on different levels of hypertension, including organ protection and with minimal side effects, it could become regularly involved in the struggle against this widespread cardiovascular pathology.

The mammalian pineal hormone melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>), an aminoindole produced by the metabolism of serotonin (<em>5</em>-hydroxytryptamine), has been shown to be a potent scavenger for the highly toxic hydroxyl radical. Three substances that are very important in animal physiology (e.g., in brain metabolism) are noradrenaline, histamine, and serotonin; all three occur in plants. Here we report that serotonin, tryptamine, and melatonin were found in some edible and medicinal plants in Egypt. The results of this screening showed that the pulp of underripe and ripe yellow banana contains <em>5</em>-hydroxytryptamine at concentrations of 31.4 and 18.<em>5</em> ng/g, respectively. Corn, rice, barley grains, and ginger showed the highest concentrations of melatonin, at 187.8, 149.8, 87.3, 142.3 ng/100 g, respectively. On the other hand, potato samples were free from all indolamines. Pomegranate and strawberry showed a low level of indolamines (8-12 microg/g serotonin, 4-9 microg/g tryptamine, and 13-29 ng/100 g melatonin).

The inflammatory-associated factors interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor-α (T<em>N</em>F-α) are widely reported to be associated with intervertebral disc (IVD) degeneration (IVDD). <em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em> (melatonin) is a natural hormone secreted by the pineal gland which has been shown to participate in several physiological and pathological progresses, such as aging, anti-inflammation, anti-apoptosis and autophagy regulation. However, the effects of melatonin on IVD remain unclear. In the present study, we treated human nucleus pulposus cells (<em>N</em>PCs) with melatonin and discovered that melatonin could modulate extracellular matrix (ECM) remodeling induced by IL-1β by enhancing collagen II and aggrecan expression levels and by downregulating matrix metalloproteinase-3 (MMP-3) levels. These findings were verified by western blot and immunofluorescence assays. Intraperitoneal injection of melatonin mitigated IVDD in the rat tail puncture model. X-ray and magnetic resonance imaging (MRI), as well as hematoxylin-eosin (H&E), Safranine O-Green, Alcian blue and Celium red staining methods were adopted to evaluate IVDD grades, the structural integrity of nucleus pulposus (<em>N</em>P) and annulus fibrosus (AF) and the damage and calcification of the cartilage endplate. Melatonin reduced inflammatory cell aggregation and the release of the inflammatory factors IL-1β, IL-6, T<em>N</em>F-α as determined by immunohistochemistry. In conclusion, the present study demonstrated that melatonin could modulate ECM remodeling by IL-1β in vitro and attenuate the IVDD and induction of inflammation in a rat tail puncture model in vivo. The data demonstrated that melatonin may contribute to the restoration processs of IVD following damage and may be used as a potential novel therapy for IVDD.

Melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>), 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 (<em>N</em>OS) caused by incubation with lipopolysaccharide (LPS)/interferon (IF<em>N</em>)-gamma (1 microg/mL and 100 U/mL, respectively) and defined the mode of melatonin's action. Treatment with LPS/IF<em>N</em>-gamma for 24 hr elicited the induction of inducible (i<em>N</em>OS) activity as determined by nitrite and nitrate (<em>N</em>O(x)) accumulation in the culture medium. Preincubation with melatonin abrogated the mixed cytokines-mediated induction of i<em>N</em>OS. The effect of melatonin was concentration-dependent. Moreover, Western blot analysis showed that melatonin inhibited LPS/IF<em>N</em>-gamma-induced expression of COX-2 protein, but not that of constitutive cyclooxygenase. Inhibition of i<em>N</em>OS and COX-2 expression was associated with inhibition of activation of the transcription factor nuclear factor kappa B (<em>N</em>F-kappaB). The ability of melatonin to inhibit <em>N</em>F-kappaB activation was further confirmed by studies on the degradation of the inhibitor of <em>N</em>F-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.

Melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) is an important biological hormone in many abiotic stress responses and developmental processes. In this study, the protective roles of melatonin were investigated by measuring the antioxidant defense system and photosynthetic characteristics in maize under salt stress. The results indicated that <em>N</em>aCl treatment led to the decrease in plant growth, chlorophyll contents and photochemical activity of photosystem II (PSII). However, the levels of reactive oxygen species increased significantly under salt stress. Meanwhile, we found that application of exogenous melatonin alleviated reactive oxygen species burst and protected the photosynthetic activity in maize seedlings under salt stress through the activation of antioxidant enzymes. In addition, 100 μM melatonin-treated plants showed high photosynthetic efficiency and salinity. Immunoblotting analysis of PSII proteins showed that melatonin application alleviated the decline of 34 kDa PSII reaction center protein (D1) and the increase of PSII subunit S protein. Taken together, our study promotes more comprehensive understanding in the protective effects of exogenous melatonin in maize under salt stress, and it may be involved in activation of antioxidant enzymes and regulation of PSII proteins.

Melatonin, <em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>, an evolutionally old molecule, is produced by the pineal gland in vertebrates, and it binds with high affinity to melatonin receptors, which are members of the GPCR family. Among the multiple effects attributed to melatonin, we will focus here on those that are dependent on the activation of the two mammalian MT1 and MT2 melatonin receptors. We briefly summarize the latest developments on synthetic melatonin receptor ligands, including multi-target-directed ligands, and the characterization of signalling-biased ligands. We discuss signalling pathways activated by melatonin receptors that appear to be highly cell- and tissue-dependent, emphasizing the impact of system bias on the functional outcome. Different proteins have been demonstrated to interact with melatonin receptors, and thus, we postulate that part of this system bias has its molecular basis in differences of the expression of receptor-associated proteins including heterodimerization partners. Finally, bias at the level of the receptor, by the expression of genetic receptor variants, will be discussed to show how a modified receptor function can have an effect on the risk for common diseases like type 2 diabetes in humans.

Melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) plays important role in multiple plant developmental processes and stress responses. We investigated the possible mediatory role of melatonin in growth, photosynthesis, and the response to cold stress in rice by using three different experiments: soaking seed; immersing roots, and spraying to leaves with 0, 20, or 100 μM melatonin. After 6 days of cold stress, the growth of rice seedlings was significantly inhibited, but this inhibition was alleviated by exogenous melatonin. Furthermore, exogenous melatonin pretreatment alleviated the accumulation of reactive oxygen species, malondialdehyde and cell death induced by cold stress. Melatonin pretreatment also relieved the stress-induced inhibitions to photosynthesis and photosystem II activities. Further investigations showed that, antioxidant enzyme activities and non-enzymatic antioxidant levels were increased by melatonin pretreatments. The treatment methods of seed soaking and root immersion were more effective in improving cold stress resistance than the spraying method. The results also indicated the dose-dependent response of melatonin on rice physiological, biochemical, and photosynthetic parameters.

The pineal hormone melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) is normally secreted at night: in animals it serves to transmit information about light-dark cycles to body physiology Suitable timed administration will alleviate 'jet-lag' severity ratings in humans, the major effect being to improve sleep. It has been suggested that this may be mediated by melatonin re-entraining the endogenous circadian oscillator. We have examined this possibility by feeding melatonin to a blind individual (HK) with a free-running temperature rhythm and a pronounced 3<em>5</em>-day cycle in his ability to fall asleep at 'normal' times. Our results show a clear stabilizing effect of melatonin on sleep onset time with elimination of day time sleep, but no entrainment of rectal temperature or urinary cortisol rhythms. Thus melatonin may act on the timing mechanism of sleep onset, rather than as a entrainer of all circadian rhythms. It may well help shiftworkers to sleep at inappropriate phases of their circadian oscillators.

In animals, melatonin (<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em>) has several physiological roles, mostly related with circadian and seasonal rhythms. In 199<em>5</em>, it was detected in a variety of edible plants, and it is known that melatonin from plant foods is absorbed from the gastrointestinal tract and incorporated in the blood stream. This indoleamine also crosses the blood-brain barrier and the placenta, being incorporated at the subcellular level. The possibility of modulating blood melatonin levels in mammals and avians through the ingestion of plant foodstuffs seems to be an interesting prospect. However, data concerning the melatonin content of edible plants are scarce and have not been contrasted. Obtained with very different analytical techniques, in some cases inappropriate, the quantitative data show a high degree of variation. Possibly for the first time in plants, we have used liquid chromatography with time-of-flight/mass spectrometry to identify melatonin. This sophisticated technique, combined with the more commonly used liquid chromatography with fluorescence detection for melatonin quantification, has permitted us to describe the distribution of this compound in different organs and zones in plants. Also, changes in melatonin levels with age and the possible influence of a light/dark photoperiod or constant darkness on its levels are studied. The proposal, applied here to lupin (Lupinus albus L.) and barley (Hordeum vulgare L.), may also serve as a model for application to other plant foodstuffs.

<em>N</em>-<em>acetyl</em>-<em>5</em>-<em>methoxytryptamine</em> (melatonin) is an endogenous indoleamine produced by all vertebrate organisms. Its production in the pineal gland has been extensively investigated but other organs also synthesize this important amine. Melatonin's functions in organisms are diverse. The actions considered in the current review relate to its ability to function in the reduction of oxidative stress, i.e., molecular damage produced by reactive oxygen and reactive nitrogen species. <em>N</em>umerous publications have now shown that not only is melatonin itself an efficient scavenger of free radicals and related reactants, but so are its by-products cyclic 3-hydroxymelatonin, <em>N</em>1-<em>acetyl</em>-<em>N</em>2-formyl-<em>5</em>-methoxykynuramine, and others. These derivatives are produced sequentially when each functions in the capacity of a free radical scavenger. These successive reactions are referred to as the antioxidant cascade of melatonin. That melatonin has this function within cells has been observed in studies employing time lapse conventional, confocal and multiphoton fluorescent microscopy coupled with the use of appropriate mitochondrial-targeted fluorescent probes. The benefits of melatonin and its metabolites have been described in the brain where they are found to be protective in models of Parkinson's disease, Alzheimer's disease and spinal cord injury. The reader is reminded, however, that data not covered in this review has documented beneficial actions of these amines in every organ where they have been tested. The outlook for the use of melatonin in clinical trials looks encouraging given its low toxicity and high efficacy.