Mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) reduces circulating melatonin levels in the rat

Environmental disruption of reproductive rhythms

Reproduction is a key biological function requiring a precise synchronization with annual and daily cues to cope with environmental fluctuations. Therefore, humans and animals have developed well-conserved photoneuroendocrine pathways to integrate and process daily and seasonal light signals within the hypothalamic-pituitary-gonadal axis. However, in the past century, industrialization and the modern 24/7 human lifestyle have imposed detrimental changes in natural habitats and rhythms of life. Indeed, exposure to an excessive amount of artificial light at inappropriate timing because of shift work and nocturnal urban lighting, as well as the ubiquitous environmental contamination by endocrine-disrupting chemicals, threaten the integrity of the daily and seasonal timing of biological functions. Here, we review recent epidemiological, field and experimental studies to discuss how light and chemical pollution of the environment can disrupt reproductive rhythms by interfering with the photoneuroendocrine timing system.

Melatonin preserves ovarian tissues of rats exposed to chronic TCDD: An electron microscopic approach to effects of TCDD on ovarian cells

2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD) is a toxic agent and has disruptive effects on reproductive tissues in females. TCDD disrupts the hormonal regulation of the body and decreases the production of melatonin. In this study, we investigated the protective effects of melatonin supplements against the toxic effects of TCDD on ovaries of female rats. TCDD caused a significant decrease in the average number of corpora lutea and follicles per tissue section (2.1 ± 0.7; 2.3 ± 0.8, respectively), whereas these numbers were maintained in the melatonin supplemented group (5.0 ± 0.8; 5.1 ± 0.8, respectively) and were similar to the control group (5.3 ± 1.0; 5.9 ± 0.9, respectively). Electron microscopic analysis showed that the disruption of ultrastructure components such as cell membrane and organelles due to TCDD exposure was inhibited by melatonin supplements. This study suggested that melatonin has a protective and a possible ameliorative effect over histopathological damage of rat ovaries exposed to TCDD.

Melatonin as a Potent and Inducible Endogenous Antioxidant: Synthesis and Metabolism

Melatonin is a tryptophan-derived molecule with pleiotropic activities. It is present in almost all or all organisms. Its synthetic pathway depends on the species in which it is measured. For example, the tryptophan to melatonin pathway differs in plants and animals. It is speculated that the melatonin synthetic machinery in eukaryotes was inherited from bacteria as a result of endosymbiosis. However, melatonin's synthetic mechanisms in microorganisms are currently unknown. Melatonin metabolism is highly complex with these enzymatic processes having evolved from cytochrome C. In addition to its enzymatic degradation, melatonin is metabolized via pseudoenzymatic and free radical interactive processes. The metabolic products of these processes overlap and it is often difficult to determine which process is dominant. However, under oxidative stress, the free radical interactive pathway may be featured over the others. Because of the complexity of the melatonin degradative processes, it is expected that additional novel melatonin metabolites will be identified in future investigations. The original and primary function of melatonin in early life forms such as in unicellular organisms was as a free radical scavenger and antioxidant. During evolution, melatonin was selected as a signaling molecule to transduce the environmental photoperiodic information into an endocrine message in multicellular organisms and for other purposes as well. As an antioxidant, melatonin exhibits several unique features which differ from the classic antioxidants. These include its cascade reaction with free radicals and its capacity to be induced under moderate oxidative stress. These features make melatonin a potent endogenously-occurring antioxidant that protects organisms from catastrophic oxidative stress.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on hormones of energy balance in a TCDD-sensitive and a TCDD-resistant rat strain

One of the hallmarks of the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a drastically reduced feed intake by an unknown mechanism. To further elucidate this wasting syndrome, we followed the effects of a single large dose (100 μg/kg) of TCDD on the serum levels of several energy balance-influencing hormones, clinical chemistry variables, and hepatic aryl hydrocarbon receptor (AHR) expression in two rat strains that differ widely in their TCDD sensitivities, for up to 10 days. TCDD affected most of the analytes in sensitive Long-Evans rats, while there were few alterations in the resistant Han/Wistar strain. However, analyses of feed-restricted unexposed Long-Evans rats indicated several of the perturbations to be secondary to energy deficiency. Notable increases in ghrelin and glucagon occurred in TCDD-treated Long-Evans rats alone, which links these hormones to the wasting syndrome. The newly found energy balance regulators, insulin-like growth factor 1 and fibroblast growth factor 21 (FGF-21), appeared to function in concert in body weight loss-induced metabolic state, and FGF-21 was putatively linked to increased lipolysis induced by TCDD. Finally, we demonstrate a reverse set of changes in the AHR protein and mRNA response to TCDD and feed restriction, suggesting that AHR might function also as a physiological regulator, possibly involved in the maintenance of energy balance.

Melatonin as a principal component of red light therapy

Melatonin is well recognized for its role as a potent antioxidant and is directly implicated in the free radical theory of aging [1] [Reiter RJ, Pablos MI, Agapito TT, Guerrero JM. Melatonin in the context of the free radical theory of aging. Ann N Y Acad Sci 1996;786:362-78]. Moreover, melatonin has been shown to retard age-related increases in lipid peroxidation and oxidative damage [2] [Okatani Y, Wakatsuki A, Reiter RJ. Melatonin protects hepatic mitochondrial respiratory chain activity in senescence-accelerated mice. J Pineal Res 2002;32:143-8] and to act directly upon the immune system [3] [Poon AM, Liu ZM, Pang CS, Brown GM, Pang SF. Evidence for a direct action of melatonin on the immune system. Biol Signals 1994;3:107-17]. This report focuses on characterizing documented functions of melatonin in the context of red light therapy and proposes that melatonin is a potential mediator of red light's therapeutic effects, a hypothesis that is as yet untested. Red light therapy (670 nm, 4J/cm(2)) has been shown to restore glutathione redox balance upon toxicological insult and enhance both cytochrome c oxidase and energy production, all of which may be affected by melatonin. The red light treatment has also been successfully implemented in the clinical setting for its effectiveness in reducing both the number of incidences and severity of oral mucositis resulting in part from the chemotherapy and/or radiation administered prior to bone marrow transplants. Moreover, red light therapy improves wound healing and is being further tested for its ability to ameliorate toxicant-induced retinal and visual cortical neuron damage. Researchers in the growing field of light therapy may be in a position to draw from and collaborate with melatonin researchers to better characterize this alternative treatment.

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

Melatonin is a highly conserved molecule. Its presence can be traced back to ancient photosynthetic prokaryotes. A primitive and primary function of melatonin is that it acts as a receptor-independent free radical scavenger and a broad-spectrum antioxidant. The receptor-dependent functions of melatonin were subsequently acquired during evolution. In the current review, we focus on melatonin metabolism which includes the synthetic rate-limiting enzymes, synthetic sites, potential regulatory mechanisms, bioavailability in humans, mechanisms of breakdown and functions of its metabolites. Recent evidence indicates that the original melatonin metabolite may be N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) rather than its commonly measured urinary excretory product 6-hydroxymelatonin sulfate. Numerous pathways for AFMK formation have been identified both in vitro and in vivo. These include enzymatic and pseudo-enzymatic pathways, interactions with reactive oxygen species (ROS)/reactive nitrogen species (RNS) and with ultraviolet irradiation. AFMK is present in mammals including humans, and is the only detectable melatonin metabolite in unicellular organisms and metazoans. 6-hydroxymelatonin sulfate has not been observed in these low evolutionary-ranked organisms. This implies that AFMK evolved earlier in evolution than 6-hydroxymelatonin sulfate as a melatonin metabolite. Via the AFMK pathway, a single melatonin molecule is reported to scavenge up to 10 ROS/RNS. That the free radical scavenging capacity of melatonin extends to its secondary, tertiary and quaternary metabolites is now documented. It appears that melatonin's interaction with ROS/RNS is a prolonged process that involves many of its derivatives. The process by which melatonin and its metabolites successively scavenge ROS/RNS is referred as the free radical scavenging cascade. This cascade reaction is a novel property of melatonin and explains how it differs from other conventional antioxidants. This cascade reaction makes melatonin highly effective, even at low concentrations, in protecting organisms from oxidative stress. In accordance with its protective function, substantial amounts of melatonin are found in tissues and organs which are frequently exposed to the hostile environmental insults such as the gut and skin or organs which have high oxygen consumption such as the brain. In addition, melatonin production may be upregulated by low intensity stressors such as dietary restriction in rats and exercise in humans. Intensive oxidative stress results in a rapid drop of circulating melatonin levels. This melatonin decline is not related to its reduced synthesis but to its rapid consumption, i.e. circulating melatonin is rapidly metabolized by interaction with ROS/RNS induced by stress. Rapid melatonin consumption during elevated stress may serve as a protective mechanism of organisms in which melatonin is used as a first-line defensive molecule against oxidative damage. The oxidative status of organisms modifies melatonin metabolism. It has been reported that the higher the oxidative state, the more AFMK is produced. The ratio of AFMK and another melatonin metabolite, cyclic 3-hydroxymelatonin, may serve as an indicator of the level of oxidative stress in organisms.

Exposure to a 50-hz magnetic field induces a circadian rhythm in 6-hydroxymelatonin sulfate excretion in mice

The effect of magnetic field (MF) exposure on melatonin production was studied in female CD(2)F(1)(BALB/c x DBA/2) mice. The mice were exposed to a 50 Hz MF at 100 microT for 52 days and nocturnal urine was collected 1, 3, 7, 14, 16 and 23 days after the beginning of MF exposure. The animal room was illuminated for 12 h daily at 200 lux. To study the circadian rhythm of melatonin production, night and day samples of urine were collected once, at about 40 days after the beginning of MF exposure. Urinary 6-hydroxy melatonin sulfate (6-OHMS) was determined to assess melatonin production. The pineal glands were analyzed for melatonin content at the middle of the dark period. No statistically significant peak of melatonin was observed in either group. The light-regulated natural melatonin rhythm was absent in sham-exposed mice. The MF exposure caused a significant day-night difference in the 6-OHMS levels, but did not affect the total excretion of 6-OHMS during the 24-hour period. A possible interpretation of the findings is that MF exposure increases the sensitivity of the pineal gland to light in this strain normally insensitive to the circadian light variations. Further studies on interaction of light and MF exposure might help in understanding the inconsistencies of earlier research on MFs and melatonin.

Expression, cellular distribution and induction of cytochrome p4501A (CYP1A) in gilthead seabream, Sparus aurata, brain

The presence and induction of cytochrome p4501A (CYP1A) in the brain of a teleost fish, the seabream, Sparus aurata, was studied. Cerebral CYP1A expression of control fish or fish exposed to various concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was assessed at the enzyme activity level-measured as 7-ethoxyresorufin-O-deethylase; at the protein level-measured by means of Western blot and ELISA; and at the mRNA level-estimated by means of RT-PCR. Cellular localization of CYP1A in the brain tissue was studied using immunohistochemistry. In non-exposed control fish, expression of CYP1A could be demonstrated only in the olfactory bulbs. After TCDD exposure, the olfactory bulbs still showed the highest expression levels of CYP1A, however, other brain regions were now CYP1A-positive as well. Immunohistochemical examination of brain tissue sections from control fish demonstrated CYP1A immunoreactive fibers in the ventral telencephalon, in the glomerular layer of the olfactory bulbs, and in the endothelia of the cerebral vascular system. The same structures reacted positive in TCDD-exposed fish, but cell bodies and fibers from additional brain areas including telencephalon, diencephalon, mesencephalon and cerebellum showed CYP1A immunostaining. In the pituitary of TCDD-treated fish, putative GTH cells were positive for CYP1A, whereas in control fish no staining of the adenohypophysis was observed. The present findings provide evidence for basal expression of CYP1A in the telencephalon of Sparus aurata, and for the presence of inducible CYP1A in all other major brain regions, including the pituitary.

Occupational and environmental agents as endocrine disruptors: experimental and human evidence

In the last few years great concern has arisen from the description of adverse endocrine effects of several occupational and environmental chemical agents on human and/or wildlife health. Such agents may exert their effects directly, specifically binding to hormone receptors, and/or indirectly, by altering the structure of endocrine glands and/or synthesis, release, transport, metabolism or action of endogenous hormones. Many studies have been focused on the outcomes of the exposure to those chemicals mimicking estrogenic or androgenic actions. Nonetheless, the disruption of other hormonal pathways is not negligible. This paper reviews the experimental and human evidence of the effects of occupational and environmental chemical agents on hypothalamus-pituitary unit, pineal gland, parathyroid and calcium metabolism and adrenal glands.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on the expression of cytochrome P450 1A1, the aryl hydrocarbon receptor, and the aryl hydrocarbon receptor nuclear translocator in rat brain and pituitary

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related substances are ubiquitous environmental pollutants causing a wide variety of pathological alterations, with the most severe being progressive anorexia and body weight loss. These features suggest a possible involvement of the nervous system and neuroendocrine-related organs including the pituitary gland. However, so far there is little evidence for direct effects of TCDD on these areas. In the present study, male Sprague-Dawley rats were treated with a single oral dose of TCDD (10 microg/kg) and euthanized 1, 3, or 28 days after treatment. The expression of cytochrome P450 1A1 (CYP1A1), the aryl hydrocarbon receptor (AHR), and the aryl hydrocarbon receptor nuclear translocator (ARNT) were analyzed in different brain regions and pituitaries using semiquantitative RT-PCR and Western blotting. Relative levels of CYP1A1 mRNA and protein were dramatically increased in the pituitary. A significant increase in CYP1A1 mRNA was also detected in all the brain regions examined including olfactory bulb, striatum-caudate, hypothalamus, hippocampus, cortex, cerebellum, and substantia nigra. The increase in the expression was time-dependent with the highest level observed 1 day after TCDD treatment. The AHR and ARNT mRNAs were detected in the same areas but in contrast to CYP1A1 the changes in AHR and ARNT mRNA expression were limited to the 28-day time point. The present results provide evidence for the presence of CYP1A1, AHR, and ARNT in the central nervous system and in the pituitary, suggesting that TCDD may exert a direct effect on these regions.

Reproductive, neuroendocrine, and immune consequences of acute exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in the Siberian hamster

The present study tested the hypothesis that acute treatment with 2, 3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) impairs fertility, disrupts the nocturnal melatonin rhythm, and suppresses lymphocyte function. Adult Siberian hamsters administered 2 or 100 microg TCDD/kg body weight/0.2 ml sesame oil had a delayed latency to first litter and an increased adult mortality compared to hamsters given 0.1 microg/kg or vehicle. Within 75 days of TCDD treatment, full reproductive capabilities were achieved. Moreover, the nocturnal melatonin rhythm was not disrupted in adults administered TCDD or in their progeny. Lymphocyte activity varied with respect to time of day and treatment. Lymphocyte proliferation was enhanced at night irrespective of TCDD treatment; during the day, 2 wk after the 2-microg/kg treatment, blastogenesis was reduced compared to that in the 0.1-microg/kg group or in vehicle-treated controls. In contrast, TCDD did not affect the mixed lymphocyte reaction in response to allogeneic antigen when assessed at 2 and 20 wk post-treatment. Thus, findings indicate that TCDD produced acute effects on fertility, mortality, and systemic lymphocyte proliferation, but long-lasting effects on specific aspects of reproductive, neuroendocrine, and immune cell functions were not observed.

Inconsistent suppression of nocturnal pineal melatonin synthesis and serum melatonin levels in rats exposed to pulsed DC magnetic fields

The purpose of these experiments was to determine whether the exposure of rats at night to pulsed DC magnetic fields (MF) would influence the nocturnal production and secretion of melatonin, as indicated by pineal N-acetyltransferase (NAT) activity (the rate limiting enzyme in melatonin production) and pineal and serum melatonin levels. By using a computer-driven exposure system, 15 experiments were conducted. MF exposure onset was always during the night, with the duration of exposure varying from 15 to 120 min. A variety of field strengths, ranging from 50 to 500 microT (0.5 to 5.0 G) were used with the bulk of the studies being conducted using a 100 microT (1.0 G) field. During the interval of DC MF exposure, the field was turned on and off at 1-s intervals with a rise/fall time constant of 5 ms. Because the studies were performed during the night, all procedures were carried out under weak red light (intensity of <5 microW/cm2). At the conclusion of each study, a blood sample and the pineal gland were collected for analysis of serum melatonin titers and pineal NAT and melatonin levels. The outcome of individual studies varied. Of the 23 cases in which pineal NAT activity, pineal melatonin, and serum melatonin levels were measured, the following results were obtained; in 5 cases (21.7%) pineal NAT activity was depressed, in 2 cases (8.7%) studies pineal melatonin levels were lowered, and in 10 cases (43.5%) serum melatonin concentrations were reduced. Never was there a measured rise in any of the end points that were considered in this study. The magnitudes of the reductions were not correlated with field strength (i.e., no dose-response relationships were apparent), and likewise the reductions could not be correlated with the season of the year (experiments conducted at 12-month intervals under identical exposure conditions yielded different results). Duration of exposure also seemed not to be a factor in the degree of melatonin suppression. The inconsistency of the results does not permit the conclusion that pineal melatonin production or release are routinely influenced by pulsed DC MF exposure. In the current series of studies, a suppression of serum melatonin sometimes occurred in the absence of any apparent change in the synthesis of this indoleamine within the pineal gland (no alteration in either pineal NAT activity or pineal melatonin levels). Because melatonin is a direct free radical scavenger, the drop in serum melatonin could theoretically be explained by an increased uptake of melatonin by tissues that were experiencing augmented levels of free radicals as a consequence of MF exposure. This hypothetical possibly requires additional experimental documentation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters melatonin metabolism in fish hepatocytes

Pineal hormone melatonin is an important regulator of endocrine and circadian rhythms in vertebrates. Since liver is assumed to be the major organ in the metabolism of this indole hormone, we investigated the effect of the known Ah-receptor agonist, 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) on melatonin metabolism in fish hepatocytes as well as the in vitro effect of melatonin on trout hepatic microsomal cytochrome P4501A (CYP1A) catalyst. Primary cell cultures of rainbow trout hepatocytes were exposed to [3H]melatonin (1 nM to 1 microM) alone and in combination with TCDD (50 pM) at 15 degrees C for 24 or 48 h. Analysis of melatonin and its metabolites in the culture medium and hepatocytes by HPLC revealed that about 96% of the added [3H]melatonin was metabolised after 24 h in both control and TCDD treated cultures. 3H-radioactivity was found mainly in the culture medium and less than 5% of the total 3H-radioactivity retained inside hepatocytes. Of the HPLC separated metabolites, one coeluted with 6-hydroxymelatonin and one unknown metabolite eluted after 6-hydroxymelatonin. In addition, two other metabolites were more water-soluble than 6-hydroxymelatonin and were considered to be conjugated products. Treatment of the hepatocytes with TCDD increased the amount of the major oxidated product, 6-hydroxymelatonin, about 2.5-fold after 24 h and 1.2-fold after 48 h exposure, respectively when compared with the control cultures. Whereas the amount of the unknown metabolite eluting after 6-hydroxymelatonin decreased about 1.3-fold after 24 h and 1.2-fold after 48 h exposure, respectively. Melatonin alone did not affect P4501A associated EROD-activity or CYP1AmRNA levels in the primary hepatocyte cultures. TCDD-treatment increased EROD-activity 3 to 5-fold and respective CYP1AmRNA content 6 to 14-fold, when compared with the control or melatonin-treated cultures. Furthermore, melatonin competitively inhibited EROD-activity in liver microsomes with a Ki value of 62.06+/-3.78 microM. The results show that TCDD alters metabolic degradation of melatonin in hepatocytes and suggest that P4501A may be an important P450 isoenzyme involved in oxidative metabolism of melatonin in fish liver.

Non-carcinogenic effects of TCDD in animals

Exposure to TCDD and related chemicals leads to a plethora of effects in multiple species, tissues, and stages of development. Responses range from relatively simple biochemical alterations through overtly toxic responses, including lethality. The spectrum of effects shows some species variability, but many effects are seen in multiple wildlife, domestic, and laboratory species, ranging from fish through birds and mammals. The same responses can be generated regardless of the route of exposure, although the administered dose may vary. The body burden appears to be the most appropriate dosimetric. Many of the effects often attributed to TCDD are associated with relatively high doses: lethality, wasting, lymphoid and gonadal atrophy, chloracne, hepatotoxicity, adult neurotoxicity, and cardiotoxicity. Changes in multiple endocrine and growth factor systems have been reported in a manner which is tissue, sex, and age-dependent. The most sensitive adverse effects observed in multiple species appear to be developmental, including effects on the developing immune, nervous, and reproductive systems. Such effects have been observed at maternal body burdens in the range of 30-80 ng/kg in both non-human primates and rodents. Biochemical effects on cytokine expression and metabolizing enzymes occur at body burdens which are within a factor of ten of the clearly adverse developmental responses. Thus, effects on the immune system, learning, and the developing reproductive system of multiple animals occur at body burdens which are close to those present in the background human population.

A mathematical model of production, distribution, and metabolism of melatonin in mammalian systems

Melatonin is a neuroendocrine hormone which is currently receiving considerable attention as a treatment for jet lag, a treatment for insomnia and, by some, a possible "magic bullet" for delaying the effects of aging and preventing cancer. Production of melatonin is focused primarily in the pineal gland with very wide daily shifts in production controlled by the day/night cycle. The potential for increased disease as a consequence of lower or higher than average production of this hormone has not been well studied, although potential environmental agents may modulate circulating levels (e.g., electric and magnetic fields). In this manuscript, a physiologically realistic mathematical model for the production, distribution, and metabolism of melatonin is developed as a precursor to a future study of the role of chemicals and environmental agents in altering this system. Values for key aspects of the system (e.g., diurnal rates of production of the hormone in the pineal gland) were obtained from the literature and the model was validated against data on circulating levels. The mathematical equations and model parameters are presented.