Armando Menendez-Pelaez

Melatonin stimulates brain glutathione peroxidase activity

Exogenously administered melatonin causes a 2-fold rise in glutathione peroxidase activity within 30 min in the brain of the rat. Furthermore, brain glutathione peroxidase activity is higher at night than during the day and is correlated with high night-time tissue melatonin levels. Glutathione peroxidase is thought to be the principal enzyme eliminating peroxides in the brain. This antioxidative enzyme reduces the formation of hydroxyl radicals formed via iron-catalyzed Fenton-type reactions from hydrogen peroxide by reducing this oxidant to water. Since the hydroxyl radical is the most noxious oxygen radical known, induction of brain glutathione peroxidase might be an important mechanism by which melatonin exerts its potent neuroprotective effects.

Neurohormone melatonin prevents cell damage: effect on gene expression for antioxidant enzymes.

It is well known that porphyrins cause a toxic light‐mediated effect due to their capability to generate free radicals. Several reports have proved that melatonin is a potent free radical scavenger. The aim of this work has been to study the ability of melatonin to prevent the cell damage caused by porphyrins in the Harderian gland of female Syrian hamsters. Cell injury was evaluated estimating the percentage of damaged cells found in the gland and analyzing the degree of this damage at ultrastructural level. To explain the mechanism by which this hormone could prevent the cell damage caused by porphyrins, its capability to both decrease porphyrin synthesis and increase the mRNA levels for antioxidant enzymes was evaluated. Our results demonstrate that melatonin administration decreases the percentage of damaged cells, porphyrin synthesis, and aminolevulinate synthase (ALA‐S) mRNA levels and increases the mRNA levels for manganese superoxide‐dismutase and copper‐zinc superoxide dismutase. When observed under an electron microscope, the lesions in the clear cells of the treated females were much less severe than in the corresponding cells of the control animals. Melatonin exerts a cytoprotective effect by inhibiting the ALA‐S gene expression (and so porphyrin synthesis) and by raising the mRNA levels for several antioxidant enzymes.—Antolín, I., Rodríguez, C., Sáinz, R. M., Mayo, J. C., Uría, H., Kotler, M. L., Rodríguez‐Col‐ unga, M. J., Tolivia, D., Menéndez‐Peláez, A. Neurohormone melatonin prevents cell damage: effect on gene expression for antioxidant enzymes. FASEB J. 10, 882‐890 (1996)

Distribution of melatonin in mammalian tissues : the relative importance of nuclear versus cytosolic localization

Abstract: Besides its presence in the pineal gland, melatonin has been found in a variety of other tissues as well. The indoleamine also has been identified in invertebrates including an unicellular organism where it exhibits a diurnal rhythm. Although melatonin is mainly known for its effects on seasonal reproduction and endocrine physiology, there is evidence showing that this ubiquitously acting hormone is also a potent free radical scavenger, thereby providing protection from oxidative attack to DNA and other biomolecules. Through the years, melatonin was thought to be exclusively cytosolic. However, careful examination of some of these pioneering reports revealed a nuclear localization of melatonin in different tissues including the retina and Harderian glands. Using a very sensitive immunocytochemical method, we have also found that melatonin is located in the nucleus of many cells where it may bind to nuclear components. The use of cell fractionation studies followed by radioimmunoassay confirmed these results. The administration of exogenous melatonin resulted in a marked increase in the nuclear melatonin content without a concomitant change in the cytosolic fraction. In addition to its ability to scavenge free radicals, its location in the nucleus suggests possible genomic actions.

Melatonin increases gene expression for antioxidant enzymes in rat brain cortex

Kotler M, Rodríguez C, Sáinz RM, Antolín I, Menéndez‐Peláez A. Melatonin increases gene expression for antioxidant enzymes in rat brain cortex. J. Pineal Res. 1998; 24:83–89.

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.

Characterization of high-affinity melatonin binding sites in purified cell nuclei of rat liver

Acuna‐Costroviejo D, Reiter RJ, Menéndez‐Peláez A, Pablos MI, Burgos A. Characterization of high‐affinity melatonin binding sites in purified cell nuclei of rat liver. J. Pineal Res. 1994: 16: 100–112.

The pineal neurohormone melatonin prevents in vivo and in vitro apoptosis in thymocytes

Abstract: Recently, melatonin was found to be the most potent physiological free radical scavenger known to date. In this work, we attempted to define the role this neurohormone plays in the regulation of apoptosis, since the effect of bcl‐2, the main gene implicated in its inhibition, acts via an antioxidant mechanism. We investigated the role of melatonin in cell death of thymus, a well known model for the study of apoptosis. Two sets of experiments were carried out: in vivo experiments, performed with Wistar rats, and in vitro experiments, performed with primary cultures of young Wistar rat thymocytes treated with glucocorticoids in order to induce apoptosis. Morphometrical studies in semithin sections of thymus and analysis of DNA fragmentation by gel electrophoresis show that physiological apoptosis occurring in thymus of 65 days old rats, is prevented by the daily administration of melatonin beginning when the rats were 25 days old. Also, we found that at a concentration of 10−7 M, melatonin decreases by 35% the percentage of apoptotic cells induced by glucocorticoids in cultured thymocytes of 25 day old rats. 10−9 M melatonin decreases cell death by 20%. Finally, melatonin at 10−11 Mdid not have any effect. Several hypothesis are discussed to explain this effect: direct interaction of melatonin with glucocorticoid receptors in the thymus; induction of interleukin‐4 release; direct genomic action modulating the expression of apoptosis‐inhibiting genes; an effect on nitric oxide synthase; and finally, the antioxidant action of melatonin. Since apoptosis is a possible mechanism involved in neuronal death shown in several neurodegenerative diseases such as Parkinson or Alzheimers diseases, investigative efforts should be directed to the possible role of melatonin in inhibiting cell death in tissues other that the thymus. Melatonin might be a potent therapeutic agent in some of these conditions.

N-Acetyltransferase activity, hydroxyindole-O-methyltransferase activity, and melatonin levels in the Harderian glands of the female Syrian hamster: Changes during the light: Dark cycle and the effect of 6-parachlorophenylalanine administration

The activities of NAT and HIOMT and the melatonin content of the Harderian glands of female Syrian hamsters were studied. When hamsters were kept under a light:dark cycle of 14:10 (lights on at 06.00 h), NAT activity exhibited a sharp, short term rise at one hour after lights on. Simultaneously, the activity of HIOMT, which forms melatonin, exhibited a rapid decline. Melatonin levels, like HIOMT activity, also showed a precipitous drop at one hour after light onset. After the respective changes, both NAT and HIOMT activity reverted back to night time levels. Melatonin levels remained depressed for several hours but by 1400 h (8 hours after lights on), nighttime melatonin values were re-established. Treatment of female hamsters with PCPA, a trytophan hydroxylase inhibitor, led to depressed levels of Harderian melatonin without affecting the activities of either NAT or HIOMT.

5-Aminolevulinate synthase mRNA levels in the Harderian gland of Syrian hamsters: Correlation with porphyrin concentrations and regulation by androgens and melatonin

The levels of 5-aminolevulinate synthase mRNA were investigated in the Harderian glands of male and female Syrian hamsters by using a cDNA clone from rat liver. Female hamsters showed higher levels of mRNA than those in males, while the administration of testosterone to female hamsters led to a reduction in mRNA levels. Castration of male hamsters caused a marked elevation of mRNA levels, whereas both the exposure to constant darkness or melatonin injections to castrated males partially prevented the effects of castration. Porphyrin concentration of Harderian glands showed a strong correlation with the mRNA levels of 5-aminolevulinate synthase in all the animals studied. These results lead to the conclusion that in this system, porphyrin metabolism is controlled through hormonal regulation of 5-aminolevulinate synthase gene expression.

Sexual Dimorphism in N-Acetyltransferase Activity, Hydroxyindole-O-methyltransferase Activity, and Melatonin Content in the Harderian Gland of Syrian Hamsters: Changes following Gonadectomy

Abstract The activities of N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) and the melatonin content of the Harderian glands of intact and gonadectomized male and female Syrian hamsters were studied. NAT activity in intact male Harderian glands was twice that of the female. Prepubertal or adult castrated males exhibited a decrease in NAT activity to a level comparable to that seen in the female. Testosterone implants in the castrated males led to a recovery of the original male NAT levels. Intact male hamsters had very low levels of Harderian HIOMT activity and melatonin content in comparison with the glands of the females. Prepubertal gonadectomy but not castration of adult males raised the levels of HIOMT activity and the melatonin content to those of the females. Bilateral ovariectomy had no effect on melatonin content, NAT activity, or HIOMT activity in the female hamster Harderian gland.