Melatonin increases tissue accumulation and toxicity of cadmium in the bank vole (<Emphasis Type="Italic">Clethrionomys glareolus</Emphasis>)

Recent study has shown that a short photoperiod increases the accumulation and toxicity of cadmium (Cd) in the bank vole as compared to a long photoperiod. Since many of the effects of photoperiod on physiological processes in small mammals are transduced by the pineal gland and its hormone melatonin, in this study the effect of subchronic melatonin injection (7 mol/kg/day for 6 weeks) on the hepatic, renal and intestinal Cd accumulation in the bank voles raised under a long photoperiod and exposed to dietary Cd (0.9 mol/g) was examined. Simultaneously, histological examinations of the liver and kidneys, and analyses of metallothionein (MT) and lipid peroxidation were carried out. Melatonin co-treatment brought about a significant increase in the hepatic (61%), renal (79%) and intestinal (77%) Cd concentrations as compared to those in the Cd alone group. However, the concentrations of MT in the liver and kidneys of the Cd + melatonin co-treated bank voles did not differ from those in the Cd alone group. Also, histopathological changes in the liver (infiltration of leukocytes) and kidneys (glomerular swelling and a focal tubular cell degeneration) as well as an increase (2-fold) in the renal lipid peroxidation occurred only in animals from the Cd + melatonin group. These data indicate that (1) subchronic melatonin injection has similar effect on the tissue accumulation and toxicity of Cd to that produced by a short photoperiod and (2) the Cd-induced toxicity in the liver and kidneys of melatonin co-treated bank voles is probably due to increased Cd accumulation and decreased synthesis of MT.     

Constitutive dimerization of human serotonin 5-HT4 receptors in living cells

Serotonin 5-HT4 receptor isoforms are G protein-coupled receptors (GPCRs) with distinct pharmacological properties and may represent a valuable target for the treatment of many human disorders. Here, we have explored the process of dimerization of human 5-HT4 receptor (h5-HT4R) by means of co-immunoprecipitation and bioluminescence resonance energy transfer (BRET). Constitutive h5-HT4(d)R dimer was observed in living cells and membrane preparation of CHO and HEK293 cells. 5-HT4R ligands did not influence the constitutive energy transfer of the h5-HT4(d)R splice variant in intact cells and isolated plasma membranes. In addition, we found that h5-HT4(d)R and h5-HT4(g)R which structurally differ in the length of their C-terminal tails were able to form constitutive heterodimers independently of their activation state. Finally, we found that coexpression of h5-HT4R and beta2-adrenergic receptor (beta2AR) led to their heterodimerization. Given the large number of h5-HT4R isoforms which are coexpressed in a same tissue, our results points out the complexity by which this 5-HTR sub-type mediates its biological effects.     

Reduction of carbon tetrachloride-induced nephropathy by melatonin administration

The aim of this study was to investigate possible protective effects of melatonin on carbon tetrachloride (CCl4)-induced renal damage in rats. A total of 24 animals were divided into three equal groups: the control rats received pure olive oil subcutaneously, rats in the second group were injected with CCl4 (0.5 ml kg-1, s.c. in olive oil) and rats in the third group were injected with CCl4 (0.5 ml kg-1) plus melatonin (25 mg kg-1, s.c. in 10% ethanol) every other day for 1 month. At the end of the experimental period, the animals were sacrificed and blood samples were collected. The kidneys were removed and weighed. Urea and creatinine levels were determined in blood samples. Histopathological examination of the kidney was performed using light microscopic methods. Administration of CCl4 significantly increased relative kidney weight (g per 100 g body weight) and decreased serum urea levels compared to controls (p<0.01). Melatonin treatment significantly (p<0.01) reduced relative kidney weight, and it produced a statistically equal (p=0.268) relative weight with the kidneys of control rats. CCl4 administration alone also caused histopathologically prominent damage in the kidney compared to the control group. Glomerular and tubular degeneration, interstitial mononuclear cell infiltration and fibrosis, vascular congestion around the tubules, and interstitial haemorrhage in perivascular areas were observed in the renal cortex and cortico-medullary border. However, the affect of CCl4 on the medulla was limited. Melatonin provided protection against CCl4-induced renal toxicity as was evident by histopathological evaluation. In view of the present findings, it is suggested that melatonin protects kidneys against CCl4 toxicity.     

2,3-Butanedione monoxime does not protect cardiomyocytes under oxidative stress

Heart muscle ischemia-reperfusion provokes a pronounced cardiomyocyte oxidative stress. In the present study, we examined a possible protective effect of the cardioprotective drug, 2,3-butanedione monoxime (BDM), on the cultured neonatal cardiac myocytes exposed to oxidative stress induced by hypochlorous acid (HOCl), that may be formed by activated polymorphonuclear neutrophils in myocardium ischemic-reperfusion areas, and a useful model oxidant, tert-butyl hydroperoxide (tBHP). Using isolated rat cardiomyocytes substantial cytotoxicity of HOCl and tBHP was demonstrated: The concentrations of HOCl and tBHP causing a 50% decrease of cardiomyocyte cell viability were estimated to be 55 +/- 5 microM and 36 +/- 6 microM, respectively. The cell viability measured immediately after the tBHP oxidative treatment was significantly higher than that measured after 22 h of cell post-incubation in a fresh culture medium. This showed delayed cell death after removing tBHP. Hypochlorous acid treatment of cardiomyocytes did not change cellular viability during the cellular post-incubation in a fresh medium. Even a long-term (22 h) incubation of oxidatively damaged cardiomyocytes with BDM (5 mM) added after the HOCl removal did not recover the viability of the HOCl-exposed cells. In the presence of BDM, the cytotoxicity of HOCl significantly increased probably due to a direct reaction of both compounds and toxic chlorinated derivative formation. 2,3-Butanedione monoxime (5 mM) did not reduce cytotoxicity of tBHP, either. Such well-known antioxidative agents as melatonin or glutathione considerably prevented oxidant-induced cell death in a concentration-dependent manner.     

Protective effects of melatonin and caffeic acid phenethyl ester against retinal oxidative stress in long-term use of mobile phone: A comparative study

There are numerous reports on the effects of electromagnetic radiation (EMR) in various cellular systems. Melatonin and caffeic acid phenethyl ester (CAPE), a component of honeybee propolis, were recently found to be potent free radical scavengers and antioxidants. Mechanisms of adverse effects of EMR indicate that reactive oxygen species may play a role in the biological effects of this radiation. The present study was carried out to compare the efficacy of the protective effects of melatonin and CAPE against retinal oxidative stress due to long-term exposure to 900 MHz EMR emitting mobile phones. Melatonin and CAPE were administered daily for 60 days to the rats prior to their EMR exposure during our study. Nitric oxide (NO, an oxidant product) levels and malondialdehyde (MDA, an index of lipid peroxidation), were used as markers of retinal oxidative stress in rats following to use of EMR. Superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) activities were studied to evaluate the changes of antioxidant status in retinal tissue. Retinal levels of NO and MDA increased in EMR exposed rats while both melatonin and CAPE caused a significant reduction in the levels of NO and MDA. Likewise, retinal SOD, GSH-Px and CAT activities decreased in EMR exposed animals while melatonin and CAPE caused a significant increase in the activities of these antioxidant enzymes. Treatment of EMR exposed rats with melatonin or CAPE increased the activities of SOD, GSH-Px and CAT to higher levels than those of control rats. In conclusion, melatonin and CAPE reduce retinal oxidative stress after long-term exposure to 900 MHz emitting mobile phone. Nevertheless, there was no statistically significant difference between the efficacies of these two antioxidants against to EMR induced oxidative stress in rat retina. The difference was in only GSH-Px activity in rat retina. Melatonin stimulated the retinal GSH-Px activity more efficiently than CAPE did.     

The brain's calendar: neural mechanisms of seasonal timing

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioural circadian rhythms. The pacemaker of the SCN oscillates with a near 24 h period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the SCN is the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes. The present review focuses on the neuronal organization and dynamic properties of the biological clock and the means by which it is synchronized with the environmental lighting conditions. It is shown that the functional activity of the biological clock is entrained to the seasonal photic cycle and that photoperiod (day length) may act as an effective zeitgeber. Furthermore, new insights are presented, based on electrophysiological and molecular studies, that the mammalian circadian timing system consists of coupled oscillators and that the clock genes of these oscillators may also function as calendar genes. In summary, there are now strong indications that the neuronal changes and adaptations in mammals that occur in response to a seasonally changing environment are driven by an endogenous circadian clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.     

Hormones and immune function: implications of aging

Aging is associated with a decline in immunity described as immunosenescence. This is paralleled by a decline in the production of several hormones, as typically illustrated by the menopausal loss of ovarian oestrogen production. However, other hormonal changes that occur with aging and that potentially impact on immune function include the release of the pineal gland hormone melatonin and pituitary growth hormone, adrenal production of dehydroepiandrosterone and tissue-specific availability of active vitamin D. It remains to be established whether hormonal changes with aging actually contribute to immunosenescence and this area is at the interface of fact and fiction, clearly inviting systematic research efforts. As a step in this direction, the present review summarizes established facts on the physiology of secretion and function of hormones that, in most cases, decline with aging and that are likely to affect the immune system.     

Melatonin reduces the increase in 8-hydroxy-deoxyguanosine levels in the brain and liver of kainic acid-treated rats

In the present study, the effect of melatonin on oxidative DNA damage induced by kainic acid (KA) treatment was investigated. 8-hydroxy-deoxyguanosine (8-OH-dG) is a main product of oxidatively damaged DNA and was used as the endpoint in these studies. The levels of 8-OH-dG were found to be elevated in the hippocampus and frontal cortex of rats treated with KA. These elevated levels were significantly reduced in animals that were co-treated with melatonin. Thus, there was no difference in 8-OH-dG levels in the brain of control rats compared to those treated with KA (10 mg/kg) plus melatonin (10 mg/kg). The levels of 8-OH-dG also increased in the liver of rats treated with KA. This rise in oxidatively damaged DNA was also prevented by melatonin administration. Melatonin's ability to reduce KA-induced increases in neural and hepatic 8-OH-dG levels presumably relates to its direct free radical scavenging ability and possibly to other antioxidative actions of melatonin.     

Melatonin receptor pharmacology: toward subtype specificity

The recent cloning of three distinct melatonin receptor subtypes (Mel_1a, Mel_1b and Mel_1c) which are part of a new family of G-protein coupled receptors, and probably mediate the physiological actions of the hormone, has spurred interest in the design of analogues with subtype selectivity. The 5-methoxyl and N-acetyl groups of melatonin are important for binding to and activation of the receptor. The indole nucleus serves to hold these two groups at the correct distance from one another and allows them to adopt the required orientation for interaction with the receptor binding pocket. We have investigated the subtype selectivity of a number of analogues of melatonin in which the structure has systematically been modified in order to probe the similarities and differences in the interaction of ligand and receptor subtype. At all three subtypes 5-methoxyl and N-acetyl groups of melatonin are important for high affinity binding. However, replacing the 5-methoxyl group (eg with 5-H, 5-OH, 5-Me or 5-BzO) reduces affinity much less at the Mel_1b receptor subtype than at either Mel_1a or Mel_1c cloned subtypes. This suggests differences between the Mel_1b and Mel _1a/1c subtypes in the size and shape of the binding pocket or in the manner in which melatonin interacts with the receptor at this position. Further studies have revealed that analogues with longer N-acyl carbon chains behave similarly at each subtype. These observations suggest that the ‘pocket’ into which the N-acetyl group fits is very similar for each subtype. Substitutions at the 2-position on the indole ring improved affinity at each receptor subtype but did not give selective analogues. The systematic ‘mapping’ of the requirements for binding at each receptor subtype should allow the design of more selective agonists and antagonists, which will be valuable tools for the characterization and classification of functional melatonin receptors.     

How do the suprachiasmatic nuclei of the hypothalamus integrate photoperiodic information?

The suprachiasmatic nucleus of the hypothalamus (SCN) plays an essential role in the generation and maintenance of circadian rhythms in mammals. The SCN activity is also dependent upon the photoperiod. The duration of the SCN sensitive phase to light, in term of Fos induction, is variable and tied to the length of the night. The question is how and by which pathways can photoperiod influence SCN? It is possible following the theoretical model of evening and morning component of the clock that the SCN build itself the photoperiodic signal. That the SCN integrate the photoperiodic information through indirect neural or neuroendocrine pathways is also to consider. Data in favor of these different interpretations are presented.