The decrease in plasma melatonin at metamorphic climax in Rana catesbeiana (bullfrog) tadpoles is induced by thyroxine.

Melatonin decreases in the plasma of Rana catesbeiana (bullfrog) tadpoles at the climax of metamorphosis when the thyroxine (T(4)) level peaks. Since melatonin inhibited thyroid function in vitro, it would be of interest to determine if the decline in plasma melatonin permits greater thyroid hormone secretion, or if the increasing levels of T(4) cause the climactic decrease in plasma melatonin. The reciprocal effects of administering T(4) or melatonin just prior to metamorphic climax were examined in tadpoles kept at 22 degrees C on an 18L:6D cycle. If melatonin functions as a thyroid antagonist at later metamorphic stages, administration of melatonin should decrease plasma T(4), whereas if T(4) causes the decline in plasma melatonin, T(4) treatment of tadpoles prior to climax should induce the climactic melatonin decrease prematurely. Once daily injection of 40 microg melatonin for 5 days at 19.30 h had no effect on metamorphic progress, or on plasma T(4) or melatonin levels, except for a transient rise in melatonin just after the injection. Immersion in 2.2x10(-4) M melatonin for 6 days accelerated metamorphosis and decreased plasma melatonin, but had no effect on plasma T(4). Administration of T(4) by injection of 0.2 microg, or immersion in a 6.3x10(-8) M solution accelerated metamorphosis more than melatonin immersion, raised plasma T(4) to climax levels, and induced a decrease in plasma melatonin. We conclude that rapid clearance of exogenous melatonin from the circulation in these experiments did not allow it to affect plasma T(4), and that there is clear evidence that the rise in T(4) induces the climax decrease in plasma melatonin. The finding that immersion in a high level of melatonin can lower plasma melatonin and accelerate metamorphosis, whereas a single daily injection does not, provides an explanation for some of the contradictory reports in the literature concerning melatonin's effect on tadpole metamorphic progress.     

UV radiation elevates arylalkylamine N-acetyltransferase activity and melatonin content in the two-spotted spider mite, Tetranychus urticae

Ultraviolet (UV) radiation produces reactive oxygen species (ROS) in mammals, where melatonin plays the role of a ROS scavenger. The melatonin synthetic enzyme arylalkylamine N-acetyltransferase (NAT) is a significant element in a possible ROS removal system. Changes in NAT activity and melatonin content were determined in the two-spotted spider mite Tetranychus urticae by irradiating it with monochromatic light using the Okazaki Large Spectrograph at the National Institute for Basic Biology, Okazaki, Japan. The NAT activity and melatonin content were suppressed by blue light (450nm). No effects of red light (650nm) on the NAT activity and melatonin content were observed. UV radiation had intensity-dependent dual effects on the NAT activity and melatonin content. In the UV-B (300nm) treatment, the NAT activity and melatonin content were suppressed at the intensity below 1mumolm(-2)s(-1) but elevated when the intensity was as high as 10mumolm(-2)s(-1). In the UV-A (350nm) treatment, the melatonin content was elevated when the intensity was as high as 10mumolm(-2)s(-1), though the NAT activity and melatonin content were suppressed at the intensity below 10 and 1mumolm(-2)s(-1), respectively. Elevation of the NAT activity and melatonin content by high intensity UV irradiation may indicate that the UV signals initiate melatonin synthesis for ROS removal in mites.     

Melatonin-improved reproductive performance in sheep bred out of season

The effects of melatonin implants on out-of-season breeding in New Zealand Romney composite ewes, was determined by comparison of reproductive performance in ewes treated with progesterone+equine chorionic gonadotrophin (eCG) (control; n=107), melatonin+progesterone+eCG (n=97) or melatonin+progesterone (n=96). Conception rates in melatonin+progesterone+eCG-treated ewes (67%) were higher than in the control ewes (P<0.01; 47%). Pregnancy rates were higher in melatonin+progesterone+eCG-treated ewes (55%; P<0.001) compared with the control ewes (40%). Fewer melatonin+progesterone-treated ewes displayed oestrus (14%; P<0.001) and subsequently became pregnant (6%). Oestrus rates in melatonin+progesterone-treated ewes (14%) were lower than both the melatonin+progesterone+eCG-treated (82%) and control ewes (86%; P<0.001), which were similar to each other. The number of foetuses per pregnant ewe was similar in all three treatment groups. Serum melatonin concentrations at Day -9 were higher in the ewes treated with melatonin and there was a large variation between individual ewes, but concentrations were similar for pregnant and nonpregnant ewes. The combination of higher conception rate and the trend for more lambs per pregnant ewes resulted in more lambs being born per ewe treated in melatonin+progesterone+eCG-treated ewes compared to the other two treatment groups. These results suggest that melatonin implants, in conjunction with administration of progesterone and eCG, may be suitable as a means of increasing the number of lambs born per ewe treated in an out-of-season breeding program in New Zealand sheep flocks while melatonin and progesterone is not.     

Desensitization of pigment granule aggregation in Xenopus leavis melanophores: melatonin degradation rather than receptor down-regulation is responsible

Xenopus laevis melanophores express a high density (B(max) 1224 fmol/mg protein) of high-affinity (K(d) 37 pm) cell membrane melatonin receptors. Treatment of melanophores with melatonin resulted in a loss of membrane melatonin receptors reaching a maximum (approximately 60%) by 6 h. In addition to receptor loss, a decline in the potency of melatonin to produce pigment aggregation was observed on prolonged treatment. However, the loss of potency (3.8-fold in 24 h and 162-fold in 96 h) was much slower than loss of receptors, and was completely prevented by inclusion of eserine (100 microm), an inhibitor of melatonin deacetylation in the culture medium. Incubation of melanophores with [(3)H]-melatonin showed that eserine prevented metabolism of melatonin to 5-methoxytryptamine. These results indicate that although receptor density does decline on prolonged treatment, this is not responsible for the diminishing melatonin potency, which is entirely due to degradation of melatonin by deacetylation and subsequent deamination in melanophores.     

Antioxidative effect of melatonin on DNA and erythrocytes against free-radical-induced oxidation

The aim of this work is to investigate the antioxidative effect of melatonin (N-acetyl-5-methoxytryptamine) on the oxidation of DNA and human erythrocytes induced by 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH). First, the 50% inhibition concentration (IC50) of melatonin is measured by reacting with two radical species, i.e., 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) radical cation (ABTS*+) and 2,2'-diphenyl-1-picrylhydrazyl (DPPH). The IC50 of melatonin are 75microM and 300microM when melatonin reacts with ABTS*+ and DPPH, respectively. Especially, the reactions of melatonin with ABTS*+ and DPPH are the direct evidence for melatonin to trap radicals. Then, melatonin is applied to protect DNA and human erythrocytes against oxidative damage and hemolysis induced by 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH). The presence of melatonin prolongs the occurrence of the oxidative damage of DNA and hemolysis of erythrocytes, generating an inhibition period (t(inh)). The proportional relationship between t(inh) and the concentration of melatonin ([MLT]) is treated by the chemical kinetic equation, t(inh)=(n/R(i))[MLT], in which n means the number of peroxyl radical trapped by an antioxidant, and R(i) stands for the initiation rate of the radical reaction. It is found that every molecule of melatonin can trap almost two radicals in protecting DNA and erythrocytes. Furthermore, quantum calculation proves that the indole-type radical derived from melatonin is much stable than amide-type radical. Finally, melatonin is able to accelerate hemolysis of erythrocytes induced by hemin, indicating that melatonin leads to the collapse of the erythrocyte membrane in the presence of hemin. This may provide detailed information for the usage of melatonin and helpful reference for the design of indole-related drugs.     

Effects of melatonin on dopamine metabolism in the hypothalamus and the pituitary of the rainbow trout, Oncorhynchus mykiss

The present paper discusses the effect of a single melatonin treatment (0.5 mg/kg, i.p.) on the dopaminergic metabolism in the hypothalamus and pituitary of the rainbow trout. The effects of exogenous melatonin on dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) contents were compared with the variations in the content of these catecholamines associated to the natural increase in the endogenous melatonin from daytime (3 hr before lights off) to nighttime (3 hr after lights off). Animals treated with melatonin showed a rapid (maximal values at 30 min post-injection) and relatively sustained rise in plasma melatonin levels, which reached supraphysiological ranges. The increase in circulating melatonin was accompanied by a reduction in the amount of DOPAC in both the hypothalamus (30, 60, and 120 min after i.p. melatonin) and the pituitary (120 min after i.p. melatonin) as well as in the pituitary DOPAC/DA ratio (60 and 120 min after i.p. melatonin). Similarly, the increase in circulating melatonin levels from the daytime to nighttime was associated with decreases in the contents of DOPAC in both the hypothalamus and pituitary and in the DOPAC/DA ratio in the pituitary. These data suggest that the inhibition of the hypothalamic-pituitary dopaminergic metabolism may be a specific mechanism of melatonin action in the trout brain that might operate following changes in the secretion of the hormone from the pineal gland.     

Effects of diazepam and its metabolites on nocturnal melatonin secretion in the rat pineal and Harderian glands. A comparative in vivo and in vitro study

We investigated the effects of diazepam (DZP) and its three metabolites: nordiazepam (NZP), oxazepam (OZP), and temazepam (TZP) on pineal gland nocturnal melatonin secretion. We looked at the effects of benzodiazepines on pineal gland melatonin secretion both in vitro (using organ perifusion) and in vivo in male Wistar rats sacrificed in the middle of the dark phase. We also examined the effects of these benzodiazepines on in vivo melatonin secretion in the Harderian glands. Neither DZP (10^-5-10^-6 M) nor its metabolites (10^-4-10^-5 M) affected melatonin secretion by perifused rat pineal glands in vitro. In contrast, a 10^-4 M suprapharmacological concentration of DZP increased melatonin secretion of perifused pineal glands by 70%. In vivo, a single acute subcutaneous administration of DZP (3 mg/kg body weight) significantly affected pineal melatonin synthesis and plasma melatonin levels, while administration of the metabolites under the same conditions did not. DZP reduced pineal melatonin content (-40%), N-acetyltransferase activity (-70%), and plasma melatonin levels (-40%), but had no affects on pineal hydroxyindole-O-methyltransferase activity. Neither DZP nor its metabolites affected Harderian gland melatonin content. Our results indicate that the in vivo inhibitory effect of DZP on melatonin synthesis is not due to the metabolism of DZP. The results also show that the control of melatonin production in the Harderian glands differs from that observed in the pineal gland.     

Melatonin Protects against Apoptosis-Inducing Factor (AIF)-Dependent Cell Death during Acetaminophen-Induced Acute Liver Failure

Acetaminophen (APAP) overdose is the most frequent cause of acute liver failure and is primarily caused by cytochrome P450 (CYP) 2E1-driven conversion of APAP into hepatotoxic metabolites. Several reports showed that melatonin attenuated APAP-induced acute liver failure. Nevertheless, the exact mechanism remains obscure. In the present study, we investigated the effects of melatonin on apoptosis-inducing factor (AIF)-dependent cell death in APAP-induced acute liver failure. Mice were intraperitoneally (i.p.) injected with different doses of melatonin (1.25, 5, 20 mg/kg) 30 min before APAP (300 mg/kg, i.p.). As expected, melatonin significantly alleviated APAP-induced cell death, as determined by TdT-mediated dUTP-biotin nick end labeling (TUNEL) assay. Further analysis showed that melatonin significantly attenuated APAP-induced activation of the serine/threonine kinase receptor interacting protein 1 (RIP1). In addition, melatonin inhibited APAP-induced hepatic c-Jun N-terminal kinase (JNK) phosphorylation and mitochondrial Bax translocation. Correspondingly, melatonin inhibited APAP-induced translocation of AIF from mitochondria to nuclei. Interestingly, no changes were induced by melatonin on hepatic CYP2E1 expression. In addition, melatonin had little effect on APAP-induced hepatic glutathione (GSH) depletion. In conclusion, melatonin protects against AIF-dependent cell death during APAP-induced acute liver failure through its direct inhibition of hepatic RIP1 and subsequent JNK phosphorylation and mitochondrial Bax translocation.     

Activation of MT(2) melatonin receptors in rat suprachiasmatic nucleus phase advances the circadian clock

The aim of this study was to identify the melatonin receptor type(s) (MT(1) or MT(2)) mediating circadian clock resetting by melatonin in the mammalian suprachiasmatic nucleus (SCN). Quantitative receptor autoradiography with 2-[(125)I]iodomelatonin and in situ hybridization histochemistry, with either (33)P- or digoxigenin-labeled antisense MT(1) and MT(2) melatonin receptor mRNA oligonucleotide probes, revealed specific expression of both melatonin receptor types in the SCN of inbred Long-Evans rats. The melatonin receptor type mediating phase advances of the circadian rhythm of neuronal firing rate in the SCN slice was assessed using competitive melatonin receptor antagonists, the MT(1)/MT(2) nonselective luzindole and the MT(2)-selective 4-phenyl-2-propionamidotetraline (4P-PDOT). Luzindole and 4P-PDOT (1 nM-1 microM) did not affect circadian phase on their own; however, they blocked both the phase advances (approximately 4 h) in the neuronal firing rate induced by melatonin (3 pM) at temporally distinct times of day [i.e., subjective dusk, circadian time (CT) 10; and dawn, CT 23], as well as the associated increases in protein kinase C activity. We conclude that melatonin mediates phase advances of the SCN circadian clock at both dusk and dawn via activation of MT(2) melatonin receptor signaling.     

Reproductive phase dependent daily variation in melatonin receptors (Mel(1a) and Mel(1b)), androgen receptor (AR) and lung associated immunity of Perdicula asiatica

Our knowledge about the involvement of melatonin in the regulation of lung associated immune system (LAIS) is still poor though the melatonin receptor types (Mel(1a) and Mel(1b)) have been localized in lungs of some wild birds. We thought to explore the correlation between daily variation (within a 24h time scale) in peripheral melatonin and testosterone along with expression of melatonin receptors (Mel(1a) and Mel(1b)) and androgen receptor (AR) in lungs during reproductively active and inactive phases. Receptor expression of Mel(1b) was more prominent than Mel(1a) at all the time points during both the reproductive phases. The expression of AR was inversely related to both the melatonin and its receptor expression at the 24h time scale during both the reproductive phases. Results also reflected a parallel relationship of melatonin, melatonin receptors and all the immune parameters (total leukocyte count, lymphocyte count, % stimulation ratio) suggesting that peripheral melatonin might be responsible for daily periodicity of LAIS. The presence of androgen receptors in lung led us to propose that gonadal steroid does influence the LAIS. Therefore melatonin along with testosterone might be acting as a temporal synchronizer for daily rhythms in lung associated immunity in Perdicula asiatica during different reproductive phases.     

Plasma Melatonin Levels in Relation to the Light-Dark Cycle and Parental Background in Domestic Pigs

To study porcine melatonin secretion in a stable environment 3 daytime (10.00 – 15.00) and 3 nighttime (22.00 – 03.00) plasma samples were collected by jugular venipuncture from 15 gilts, 16 sows, 3 boars and 48 piglets (24 females and 24 males from 8 litters) and analysed for melatonin content. Nighttime melatonin concentrations were higher than daytime melatonin concentrations (p < 0.001), whereas no effect of sampling order could be discerned. The 3 adult Hampshire boars had higher melatonin concentrations during the day and the night, than the 31 adult Yorkshire females (p < 0.05). There was no clear difference between gilts and sows in plasma melatonin. The gilts from one of the litters had higher plasma melatonin concentrations than the gilts in 3 other litters (p < 0.05). Among the 48 piglets, the increase of nocturnal melatonin secretion differed between litters (p < 0.01), whereas the influence of father was not quite significant (p = 0.12). No difference in daytime melatonin concentrations between litters could be found, and there was no difference in melatonin levels between the male and female piglets. In conclusion, this study demonstrates that domestic pigs express a nocturnal increase of melatonin secretion in a standard stable environment. For some animals the amplitude of nighttime melatonin secretion was very low, although always higher than the daytime base levels. Furthermore, the levels of nighttime melatonin secretion differed between litters, which suggests a genetic background.     

Targeted disruption of the mouse Mel(1b) melatonin receptor

Two high-affinity, G protein-coupled melatonin receptor subtypes have been identified in mammals. Targeted disruption of the Mel(1a) melatonin receptor prevents some, but not all, responses to the hormone, suggesting functional redundancy among receptor subtypes (Liu et al., Neuron 19:91-102, 1997). In the present work, the mouse Mel(1b) melatonin receptor cDNA was isolated and characterized, and the gene has been disrupted. The cDNA encodes a receptor with high affinity for melatonin and a pharmacological profile consistent with its assignment as encoding a melatonin receptor. Mice with targeted disruption of the Mel(1b) receptor have no obvious circadian phenotype. Melatonin suppressed multiunit electrical activity in the suprachiasmatic nucleus (SCN) in Mel(1b) receptor-deficient mice as effectively as in wild-type controls. The neuropeptide, pituitary adenylyl cyclase activating peptide, increases the level of phosphorylated cyclic AMP response element binding protein (CREB) in SCN slices, and melatonin reduces this effect. The Mel(1a) receptor subtype mediates this inhibitory response at moderate ligand concentrations (1 nM). A residual response apparent in Mel(1a) receptor-deficient C3H mice at higher melatonin concentrations (100 nM) is absent in Mel(1a)-Mel(1b) double-mutant mice, indicating that the Mel(1b) receptor mediates this effect of melatonin. These data indicate that there is a limited functional redundancy between the receptor subtypes in the SCN. Mice with targeted disruption of melatonin receptor subtypes will allow molecular dissection of other melatonin receptor-mediated responses.     

Diurnal actions of melatonin on epileptic activity in hippocampal slices of rats

Since melatonin receptors have been found in the hippocampus of mammals it has been suggested that melatonin can modulate neuronal functions of hippocampal cells. The effect of melatonin (10 nM/l and 1 microM/l) on frequency and amplitude of epileptiform field potentials (EFP) elicited by low Mg(2+) or by bicuculline was tested in the CA1 region of hippocampal slices of rats. In the low Mg(2+) model, melatonin, applied in a near physiological concentration of 10 nM/l, exerts no effect on EFP in slices prepared at night or during the day. In a concentration of 1 microM/l, however, melatonin enhances the frequency of EFP to approximately 140% in slices prepared during the day. This effect was suppressed through simultaneous administration of the melatonin receptor antagonist luzindole (10 microM/l). In contrast, melatonin did not affect epileptic activity in slices prepared at night. Epileptiform discharges elicited by blocking the GABAergic inhibition (bicuculline model) were not affected by melatonin, either during the day or at night. The results indicate that melatonin affects epileptic activity in a diurnal manner and that the action of melatonin is different in relation to the epilepsy model.     

Melatonin metabolism,signaling and possible roles in plants

Melatonin is a multifunctional biomolecule found in both animals and plants. In this review, the biosynthesis, levels, signaling, and possible roles of melatonin and its metabolites in plants is summarized. Tryptamine 5-hydroxylase (T5H), which catalyzes the conversion of tryptamine into serotonin, has been proposed as a target to create a melatonin knockout mutant presenting a lesion-mimic phenotype in rice. With a reduced anabolic capacity for melatonin biosynthesis and an increased catabolic capacity for melatonin metabolism, all plants generally maintain low melatonin levels. Some plants, including Arabidopsis and Nicotiana tabacum (tobacco), do not possess tryptophan decarboxylase (TDC), the first committed step enzyme required for melatonin biosynthesis. Major melatonin metabolites include cyclic 3-hydroxymelatonin (3-OHM) and 2-hydroxymelatonin (2-OHM). Other melatonin metabolites such as N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK), N-acetyl-5-methoxykynuramine (AMK) and 5-methoxytryptamine (5-MT) are also produced when melatonin is applied to Oryza sativa (rice). The signaling pathways of melatonin and its metabolites act via the mitogen-activated protein kinase (MAPK) cascade, possibly with Cand2 acting as a melatonin receptor, although the integrity of Cand2 remains controversial. Melatonin mediates many important functions in growth stimulation and stress tolerance through its potent antioxidant activity and function in activating the MAPK cascade. The concentration distribution of melatonin metabolites appears to be species specific because corresponding enzymes such as M2H, M3H, catalases, indoleamine 2,3-dioxygenase (IDO) and N-acetylserotonin deacetylase (ASDAC) are differentially expressed among plant species and even among different tissues within species. Differential levels of melatonin and its metabolites can lead to differential physiological effects among plants when melatonin is either applied exogenously or overproduced through ectopic overexpression.     

The effects of bright light and nighttime melatonin administration on cardiac activity

Although melatonin has an important physiological role in the facilitation of sleep, its precise mechanism of action is not clear. To investigate the potential contribution of melatonin to influence cardiac autonomic activity in the evening, 16 young healthy subjects participated in a repeated measures design where cardiac autonomic activity, heart rate and blood pressure were examined during three experimental conditions. An initial baseline condition involved dim light exposure (< 10 lux), permitting the normal nocturnal rise in endogenous melatonin. In other sessions, subjects were exposed to bright light (> 3000 lux) to suppress melatonin secretion and administered a placebo or melatonin (5 mg) capsule at the estimated time of increase in endogenous melatonin (wake time + 14 hours). Heart rate, pre-ejection period (a measure of cardiac sympathetic activity) and respiratory sinus arrhythmia (a measure of parasympathetic activity) were not significantly altered in response to the three melatonin levels. While melatonin had no effect on diastolic blood pressure, systolic blood pressure was maximally decreased by 6 +/- 1.93 mmHg (mean +/- SEM, p < 0.005) 150 minutes after exogenous melatonin. The results indicate that melatonin does not directly modulate cardiac autonomic activity, but may rather act directly on the cardiovascular system.     

Anti-apoptotic effect of melatonin on preimplantation development of porcine parthenogenetic embryos

In the present study, we investigated the effect of melatonin on the preimplantation development of porcine parthenogenetic and somatic cell nuclear transfer (SCNT) embryos. Parthenogenetic embryos were cultured in mNCSU-23 supplemented with various concentrations of melatonin for 7 days. The results revealed that 100 pM was the optimal concentration, which resulted in significantly increased cleavage and blastocyst formation rates. Additionally, 100 pM melatonin provided the highest increase in total cell number of blastocysts. Therefore, the subsequent experiments were performed with 100 pM melatonin. ROS level in 2-8 cell stage embryos in the presence or absence of melatonin was evaluated. Embryos cultured with melatonin showed significantly decreased ROS. Blastocysts cultured with melatonin for 7 days were analyzed by the TUNEL assay. It was observed that melatonin not only increased (P < 0.05) the total cell number but also decreased (P < 0.05) the rate of apoptotic nuclei. Blastocysts cultured with melatonin were assessed for the expression of apoptosis-related genes Bcl-xl and Bax, and of pluripotency marker gene Oct-4 by real-time quantitative PCR. Analysis of data showed that the expression of Bcl-xl was higher (1.7-fold) compared to the control while the expression of Bax was significantly decreased relative to the control (0.7-fold) (P < 0.05). Moreover, the expression of Oct-4 was 1.7-fold higher than the control. These results indicated that melatonin had beneficial effects on the development of porcine parthenogenetic embryos. Based on the findings of parthenogenetic embryos, we investigated the effect of melatonin on the development of porcine SCNT embryos. The results also demonstrated increased cleavage and blastocyst formation rates, and the total cell numbers in blastocysts were significantly higher when the embryos were cultured with melatonin. Therefore, these data suggested that melatonin may have important implications for improving porcine preimplantation SCNT embryo development.     

Effects of melatonin implants in pony mares. 1. Acute effects

The effects of melatonin implant treatment over a four week period on LH, estradiol (E2) and progesterone (P4) secretion during the breeding season were studied in ovary-intact and ovariectomized pony mares. Mares with melatonin implants had significantly higher daytime melatonin concentrations than mares with sharm implants (P = 0.0065). In ovariectomized mares, LH secretion did not differ between mares with melatonin and sham implants. In ovary-intact mares, melatonin implants altered the pattern of LH secretion (P = 0.0023) in such a way that an increase in LH secretion was observed during the periovulatory period. Estradiol and P4 secretion were unaffected by melatonin implants. These results suggest that constant administration of melatonin may enhance the secretion of LH during the periovulatory surge but does not adversely affect E2, P4 or basal LH secretion in mares during the breeding season.     

Entrainment of Circadian Programs

Circadian rhythms were recently proposed as a measure of physiological state and prognosis in disorders of consciousness (DOC). So far, melatonin regulation was never assessed in vegetative state (VS). Aim of our research was to investigate the nocturnal melatonin levels and light-induced melatonin suppression in a cohort of VS patients. We assessed six consecutive patients (four men, age 33.3?±?9.3 years) with post-traumatic VS and nine age-matched healthy volunteers (five men, age 34.3?±?8.9 years) on two consecutive nights: one baseline and one light exposure night. During baseline, night subjects were in bed in a dim (<5?lux) room from 10?pm to 8?am. Blood samples were collected hourly 00:30–3:30?am (00:30?=?MLT1; 1:30?=?MLT2; 2:30?=?MLT3; and 3:30?=?MLT4). Identical setting was used for melatonin suppression test night, except for the exposure to monochromatic (470?nm) light from 1:30 to 3:30?am. Plasma melatonin levels were evaluated by radioimmunoassay. Magnitude of melatonin suppression was assessed by melatonin suppression score (caMSS) and suppression rate. We searched for group differences in melatonin levels, differences between repeated samples melatonin concentrations during baseline night and light exposure night, and light-induced suppression of melatonin secretion. During baseline night, controls showed an increase of melatonin (MLT4 vs MLT1, p?=?0.037), while no significant changes were observed in VS melatonin levels (p?=?0.172). Baseline night MLT4 was significantly lower in VS vs controls (p?=?0.036). During light-exposure night, controls displayed a significant suppression of melatonin (MLT3 and MLT4 vs MLT2, p?=?0.016 and 0.002, respectively), while VS patients displayed no significant changes. The magnitude of light-induced suppression of melatonin levels was statistically different between groups considering control adjusted caMSS (p?=?0.000), suppression rate (p?=?0.002) and absolute percentage difference (p?=?0.012). These results demonstrate for the first time that VS patients present an alteration in night melatonin secretion and reduced light-induced melatonin suppression. These findings confirm previous studies demonstrating a disruption of the circadian system in DOC and suggest a possible benefit from melatonin supplementation in VS.     

Radioprotective effect of melatonin assessed by measuring chromosomal damage in mitotic and meiotic cells.

This study was taken to evaluate the radioprotective effects of melatonin. Male adult albino mice were treated (intraperitoneal, i.p.) with 10 mg/kg melatonin either 1 h before or 1/2 h after exposure to 1.5 Gy of gamma-irradiation. Control, melatonin, irradiated and melatonin plus irradiation groups were sacrificed 24 h following treatment. The incidence of micronuclei (MN) in bone marrow cells was determined in all groups. The results show that melatonin caused a significant reduction in micronuclei polychromatic erythrocytes (MNPCE) when animals were treated with melatonin before and not after exposure to radiation. Mitotic and meiotic metaphases were prepared from spermatogonial and primary spermatocytes, respectively. Examination and analysis of metaphases showed no mutagenic effect of melatonin on chromosomal aberration (CA) frequency in spermatogonial chromosomes. Administration of one single dose of melatonin to animals before irradiation lowered total CA from 46 to 32%. However, no significant effect was observed when melatonin was given after irradiation. Similarly, the frequency of CA in meiotic metaphases decreased from 43.5% in the irradiated group to 31.5% in the irradiated group treated with melatonin 1 h before irradiation, but no change was observed when melatonin was administered after irradiation. The data obtained in this study suggest that melatonin administration confers protection against damage inflicted by radiation when given prior to exposure to irradiation and not after, and support the contention that melatonin radioprotection is achieved by its ability as a scavenger for free radicals generated by ionizing radiation.     

Some Perturbations That Disturb the Circadian Melatonin Rhythm

The circadian melatonin rhythm is highly reproducible and generally not easily altered. The few perturbations that are capable of significantly changing either the amplitude or the pattern of the 24-h melatonin rhythm are summarized herein. Aging alters cyclic melatonin production by decreasing the amplitude of the nocturnal melatonin peak in all species in which it has been studied. The best known acute suppressor of nocturnal melatonin is light exposure. The brightness of light required to acutely depress pineal melatonin production is species dependent; of the visible wavelengths, those in the blue range (∼500-520 nm) seem most effective in suppressing melatonin production. Nonvisible, nonionizing radiation in the extremely low frequency range (e.g., 60 Hz) seems also capable of altering pineal melatonin synthesis. Hormones have relatively little influence on the circadian production of melatonin, although either adrenalectomy or hypo-physectomy does attenuate the amplitude of the melatonin cycle. Exercise at the time of high melatonin production rapidly depresses pineal concentrations of the indole without influencing its synthesis; the mechanism of this suppression remains unknown.