Pharmacological Doses Of Melatonin Research Articles

Melatonin in energy control: Circadian time-giver and homeostatic monitor.

Melatonin is a neurohormone synthesized from dietary tryptophan in various organs, including the pineal gland and the retina. In the pineal gland, melatonin is produced at night under the control of the master clock located in the suprachiasmatic nuclei of the hypothalamus. Under physiological conditions, the pineal gland seems to constitute the unique source of circulating melatonin. Melatonin is involved in cellular metabolism in different ways. First, the circadian rhythm of melatonin helps the maintenance of proper internal timing, the disruption of which has deleterious effects on metabolic health. Second, melatonin modulates lipid metabolism, notably through diminished lipogenesis, and it has an antidiabetic effect, at least in several animal models. Third, pharmacological doses of melatonin have antioxidative, free radical-scavenging, and anti-inflammatory properties in various in vitro cellular models. As a result, melatonin can be considered both a circadian time-giver and a homeostatic monitor of cellular metabolism, via multiple mechanisms of action that are not all fully characterized. Aging, circadian disruption, and artificial light at night are conditions combining increased metabolic risks with diminished circulating levels of melatonin. Accordingly, melatonin supplementation could be of potential therapeutic value in the treatment or prevention of metabolic disorders. More clinical trials in controlled conditions are needed, notably taking greater account of circadian rhythmicity.

Melatonin alters the excitability of mouse cerebellar granule neurons by inhibiting voltage-gated sodium, potassium, and calcium channels.

Besides its role in the circadian rhythm, the pineal gland hormone melatonin (MLT) also possesses antiepileptogenic, antineoplastic, and cardioprotective properties, among others. The dosages necessary to elicit beneficial effects in these diseases often far surpass physiological concentrations. Although even high doses of MLT are considered to be largely harmless to humans, the possible side effects of pharmacological concentrations are so far not well investigated. In the present study, we report that pharmacological doses of MLT (3 mM) strongly altered the electrophysiological characteristics of cultured primary mouse cerebellar granule cells (CGCs). Using whole-cell patch clamp and ratiometric Ca2+ imaging, we observed that pharmacological concentrations of MLT inhibited several types of voltage-gated Na+ , K+ , and Ca2+ channels in CGCs independently of known MLT-receptors, altering the character and pattern of elicited action potentials (APs) significantly, quickly and reversibly. Specifically, MLT reduced AP frequency, afterhyperpolarization, and rheobase, whereas AP amplitude and threshold potential remained unchanged. The altered biophysical profile of the cells could constitute a possible mechanism underlying the proposed beneficial effects of MLT in brain-related disorders, such as epilepsy. On the other hand, it suggests potential adverse effects of pharmacological MLT concentrations on neurons, which should be considered when using MLT as a pharmacological compound.

Melatonin enhances neural stem cell differentiation and engraftment by increasing mitochondrial function.

Neural stem cells (NSCs) are regarded as a promising therapeutic approach to protecting and restoring damaged neurons in neurodegenerative diseases (NDs) such as Parkinson's disease and Alzheimer's disease (PD and AD, respectively). However, new research suggests that NSC differentiation is required to make this strategy effective. Several studies have demonstrated that melatonin increases mature neuronal markers, which reflects NSC differentiation into neurons. Nevertheless, the possible involvement of mitochondria in the effects of melatonin during NSC differentiation has not yet been fully established. We therefore tested the impact of melatonin on NSC proliferation and differentiation in an attempt to determine whether these actions depend on modulating mitochondrial activity. We measured proliferation and differentiation markers, mitochondrial structural and functional parameters as well as oxidative stress indicators and also evaluated cell transplant engraftment. This enabled us to show that melatonin (25μM) induces NSC differentiation into oligodendrocytes and neurons. These effects depend on increased mitochondrial mass/DNA/complexes, mitochondrial respiration, and membrane potential as well as ATP synthesis in NSCs. It is also interesting to note that melatonin prevented oxidative stress caused by high levels of mitochondrial activity. Finally, we found that melatonin enriches NSC engraftment in the ND mouse model following transplantation. We concluded that a combined therapy involving transplantation of NSCs pretreated with pharmacological doses of melatonin could efficiently restore neuronal cell populations in PD and AD mouse models depending on mitochondrial activity promotion.

60 YEARS OF NEUROENDOCRINOLOGY: Regulation of mammalian neuroendocrine physiology and rhythms by melatonin.

The isolation of melatonin was first reported in 1958. Since the demonstration that pineal melatonin synthesis reflects both daily and seasonal time, melatonin has become a key element of chronobiology research. In mammals, pineal melatonin is essential for transducing day-length information into seasonal physiological responses. Due to its lipophilic nature, melatonin is able to cross the placenta and is believed to regulate multiple aspects of perinatal physiology. The endogenous daily melatonin rhythm is also likely to play a role in the maintenance of synchrony between circadian clocks throughout the adult body. Pharmacological doses of melatonin are effective in resetting circadian rhythms if taken at an appropriate time of day, and can acutely regulate factors such as body temperature and alertness, especially when taken during the day. Despite the extensive literature on melatonin physiology, some key questions remain unanswered. In particular, the amplitude of melatonin rhythms has been recently associated with diseases such as type 2 diabetes mellitus but understanding of the physiological significance of melatonin rhythm amplitude remains poorly understood.

Pharmacological dose of melatonin reduces cytosolic calcium load in response to cholecystokinin in mouse pancreatic acinar cells.

Intracellular Ca(2+) overload has been considered a common pathological precursor of pancreatic injury. In this study, the effects of melatonin on Ca(2+) mobilization induced by cholecystokinin octapeptide (CCK-8) in freshly isolated mouse pancreatic acinar cells have been examined. Changes in intracellular free Ca(2+) concentration were followed by single cell fluorimetry. For this purpose, cells were loaded with the Ca(2+)-sensitive fluorescent dye fura-2-acetoxymethyl ester. In order to evaluate the contribution of Ca(2+) transport at the plasma membrane, at the endoplasmic reticulum (ER) or at the mitochondria, cells were incubated with CCK-8 alone or in combination with LaCl3, thapsigargin (Tps), or FCCP to, respectively, uncouple Ca(2+) transport at these localizations. The experiments were performed in the absence or in the presence of melatonin in combination with the stimuli mentioned. Our results show that the total Ca(2+) mobilization evoked by CCK-8 was attenuated by a 30% in the presence of 100 µM melatonin compared with the responses induced by CCK-8 alone. Upon inhibition of Ca(2+) transport into the ER by Tps, Ca(2+) mobilization was also reduced in the presence of melatonin. In the presence of LaCl3 plus melatonin, the total Ca(2+) mobilization induced by CCK-8 was significantly decreased, compared with the response obtained without melatonin but in the presence of LaCl3. No major differences were found when the cells were incubated with CCK-8 or Tps alone or in combination with LaCl3 plus melatonin and FCCP, compared with the responses obtained in the absence of FCCP. The initial Ca(2+) release from intracellular stores evoked by CCK-8 or Tps was not significantly reduced in the presence of melatonin. The effect of melatonin could be explained on the basis of a stimulated Ca(2+) transport out of the cell through the plasma membrane and by a stimulation of Ca(2+) reuptake into the ER. Accumulation of Ca(2+) into mitochondria might not be a major mechanism stimulated by melatonin. We conclude that melatonin alleviates intracellular Ca(2+) accumulation, a situation potentially leading to cell damage in the exocrine pancreas.

Corrections by melatonin of liver mitochondrial disorders under diabetes and acute intoxication in rats

The aim of the present work was to investigate the mechanisms of oxidative damage of the liver mitochondria under diabetes and intoxication in rats as well as to evaluate the possibility of corrections of mitochondrial disorders by pharmacological doses of melatonin. The experimental (30 days) streptozotocin-induced diabetes mellitus caused a significant damage of the respiratory activity in rat liver mitochondria. In the case of succinate as a respiratory substrate, the ADP-stimulated respiration rate V₃ considerably decreased (by 25%, p < 0·05) as well as the acceptor control ratio (ACR) V₃/V₂ markedly diminished (by 25%, p < 0·01). We observed a decrease of the ADP-stimulated respiration rate V₃ by 35% (p < 0·05), with glutamate as substrate. In this case, ACR also decreased (by 20%, p < 0·05). Surprisingly, the phosphorylation coefficient ADP/O did not change under diabetic liver damage. Acute rat carbon tetrachloride-induced intoxication resulted in considerable decrease of the phosphorylation coefficient because of uncoupling of the oxidation and phosphorylation processes in the liver mitochondria. The melatonin administration during diabetes (10 mg·kg⁻¹ body weight, 30 days, daily) showed a considerable protective effect on the liver mitochondrial function, reversing the decreased respiration rate V₃ and the diminished ACR to the control values both for succinate-dependent respiration and for glutamate-dependent respiration. The melatonin administration to intoxicated animals (10 mg·kg⁻¹ body weight, three times) partially increased the rate of succinate-dependent respiration coupled with phosphorylation. The impairment of mitochondrial respiratory plays a key role in the development of liver injury under diabetes and intoxication. Melatonin might be considered as an effector that regulates the mitochondrial function under diabetes.

Role of biological clocks in cancer processes and chronotherapy

Biological rhythms are considered in the rational use of therapeutic agents to optimize the treatment of several diseases. In cancer, several stages of cell cycle are regulated by the circadian clock. Clock genes play a vital role in cell proliferation and apoptosis. Therefore, mutation/abnormal function of circadian clock genes could result in tumor development. Circadian rhythmicity can be used as a marker rhythm for tumor progression, as rhythm impairment increases along with tumor malignancy. Administration of drugs at a particular circadian phase has achieved the best anti-tumor activity. Pharmacological doses of melatonin significantly increase antioxidant levels, thereby decreasing free radical formation. In some cancer cell types, melatonin is known to inhibit cancer cell growth under in vitro conditions. The protective effect of S-allylcysteine, diallyl disulphide and α-ketoglutarate on NDEA treated rats showed that alternations in the rhythmicities of lipid peroxidation (TBARS) and antioxidant levels (SOD, CAT, GSH and GPX) could be normalized.

Influences of chronic administration of melatonin on hormonal rhythms in rats

Pharmacological doses of melatonin—low (0.5 mg/kg body wt.) and high (1.0 mg/kg body wt.) doses were administered chronically for 45 days to Wistar rats, and 24 h rhythms of adrenocorticotrophic hormone, cortisol, growth hormone, prolactin and melatonin were studied under semi-natural conditions. Exogenous melatonin administration caused delays in the acrophases of growth hormone and melatonin rhythm itself, whereas advances in the acrophases of adrenocorticotrophic hormone, cortisol and prolactin were observed, thus indicating that chronic administration of melatonin could alter the characteristics of endocrine rhythms. Alterations in the amplitude and mesor values of these endocrine rhythms were also observed during melatonin administration. Modulation of melatonin rhythmicity (due to exogenous administration) could influence the hormonal rhythms as a modulated internal zeitgeber and could simulate/mimic the conditions of altered photoperiod in the animal; this could be the reason for altered acrophase values in the melatonin treated groups. Significant dose-dependent effects of melatonin were absent in the present study. It remains to be proven how exogenous administration of melatonin could influence the hormonal rhythms investigated in the present study.

Biochemotherapy with immunomodulating pineal hormones other than melatonin: 5-methoxytryptamine as a new oncostatic pineal agent

Several experiments have demonstrated that pineal gland plays a physiological anticancer role. Melatonin (MLT), its most investigated hormone, is a natural anticancer agent. However, MLT would not be the only endocrine molecule responsible for the anticancer property of the pineal gland. In fact, another pineal indole hormone, the 5-methoxytryptamine (5-MTT), has appeared to exert in vitro an antitumour activity superior to that of MLT itself. Previous studies have already shown the therapeutic anticancer action of MLT in association with chemotherapy also in human neoplasms. This study was performed to evaluate the influence of 5-MTT at physiological doses (1 mg/day orally during light phase) on the efficacy of chemotherapy with cisplatin plus etoposide in advanced non-small cell lung cancer patients with respect to that obtained in patients treated by chemotherapy alone or chemotherapy plus pharmacological doses of MLT (20 mg/day orally during the dark phase of the day). The study included 100 patients, who were randomised to receive chemotherapy alone or in association with MLT or 5-MTT. The overall response rate achieved in both patients concomitantly treated with MLT or 5-MTT was significantly higher with respect to that obtained in patients treated with chemotherapy alone. Moreover, both MLT and 5-MTT significantly reduced some chemotherapy-related toxicities, namely thrombocytopenia and neurotoxicity. This preliminary study shows that less known pineal hormone 5-MTT may exert at low doses the same anticancer therapeutic effect in association with chemotherapy, which may be obtained by pharmacological doses of the most investigated pineal hormone MLT.

Melatonin role in experimental arthritis.

Our perception of the function of the pineal gland and its hormone melatonin has attained a new dimension during the last decade. Through melatonin, the pineal becomes a principal organ present in vertebrates involved in the control of rhythmic adaptations to daily and seasonal cycles. Melatonin is synthesized and secreted during the dark period of the light/dark cycle. The rhythmic nocturnal melatonin secretion is directly generated by the circadian clock and is entrained to a 24-hour period by the light-dark cycle. The periodic secretion of melatonin may be used as a circadian mediator to any system than can "read" the message. Melatonin acts as an arm of the circadian clock, giving a time-related signal to a number of body functions; one of them is the circadian organization of the immune response. This review discusses melatonin role in rheumatoid arthritis. Animal studies employing Freund's complete mycobacterial adjuvant (FCA) as a model of rheumatoid arthritis are described. Immune and neuroendocrine circadian rhythms were examined in FCA-injected rats, both in the preclinical phase of arthritis (2-3 days after FCA injection) as well as in the acute phase of the disease (18 days after FCA injection). In arthritic rats, the 24-h organization of immune and neuroendocrine responses becomes altered. Significant effects of immune response on diurnal rhythmicity of adenohypophysial and hypophysiotropic hormones occurred in arthritic rats. Melatonin treatment prevented alteration of 24-h rhythms of serum ACTH, prolactin and luteinizing hormone in rats injected with FCA. In addition, melatonin treatment prevented alteration of the 24-h variation in hypothalamic monoamine transmitter turnover during the preclinical phase of Freund's adjuvant arthritis in rats. A comparison between the inflammatory and immune responses elicited by physiological and pharmacological doses of melatonin in FCA arthritis is reported. Pinealectomized rats exhibited a significantly less pronounced inflammatory response, which was restored to normal by a low melatonin dose (0.3 microg/ml of drinking water), whereas a high melatonin dose (30 microg/ml) that resulted in a 50-60-fold increase in plasma melatonin, augmented the inflammatory and immune response. These results should be considered in the light of recent reports that rheumatoid arthritis patients have increased nocturnal plasma levels of melatonin and that their synovial macrophages respond to melatonin with an increased cytokine production.

Myocardial ischemia-reperfusion in rats: reduction of infarct size by either supplemental physiological or pharmacological doses of melatonin.

Myocardial ischemia-reperfusion (I/R) represents a clinically relevant problem associated with thrombolysis, angioplasty and coronary bypass surgery. I/R injury is believed to be a consequence of free radical generation in the heart especially during the period of reperfusion. The pineal secretory product, melatonin, is known to be a potent free radical scavenger and pharmacological concentrations have been shown to reduce the I/R-induced cardiac damage in isolated rat hearts. However, the physiological role of melatonin in the prevention of this damage is unknown. Rats were pinealectomized or sham-operated (control) 2 months before the I/R studies. To produce cardiac damage, the left main coronary artery was occluded for 30 min, followed by 120 min reperfusion, in anesthetized rats. Infarct size, expressed as the percentage of the risk zone, was found significantly higher in pinealectomized rats (49+/-3.4%) than in the control group (34+/-3.6%). Melatonin administration (4 mg/kg, either before ischemia or reperfusion) to pinealectomized rats significantly reduced the infarct size values and returned them to the control values. On the other hand, melatonin administration (4 mg/kg) to sham-operated rats failed to attenuate significantly the I/R-induced infarct size. These results suggest that physiological melatonin concentrations are important in reducing the I/R-induced myocyte damage, while pharmacological concentrations of melatonin did not add to the beneficial effect. As melatonin levels have been reported to decrease with age, melatonin replacement therapy may attenuate I/R-induced myocardial injury, especially in older patients.

Glutamate induces oxidative stress not mediated by glutamate receptors or cystine transporters: protective effect of melatonin and other antioxidants.

Glutamate is responsible for most of the excitatory synaptic activity and oxidative stress induction in the mammalian brain. This amino acid is increased in the substantia nigra in parkinsonism due to the lack of dopamine restraint to the subthalamic nucleus. Parkinson's disease also shows an increase of iron levels in the substantia nigra and a decrease of glutathione, the antioxidant responsible for the ascorbate radical recycling. Considered together, these facts could make the antioxidant ascorbate behave as a pro-oxidant in parkinsonism. Since both glutamate and ascorbate are present in the synaptosomes and neurons of substantia nigra, we tested 1) if glutamate is able to induce oxidative stress independently of its excitatory activity, and 2) if ascorbate may have synergistic effects with glutamate when these two molecules co-exist. Brains were homogenized in order to disrupt membranes and render membrane receptors and intracellular signaling pathways non-functional. In these homogenates glutamate induced lipid peroxidation, indicating that this amino acid also may cause oxidative stress not mediated by its binding to glutamate receptors or cystine transporters. Ascorbate also induced lipid peroxidation thus behaving as a pro-oxidant. Both substances together produced an additive effect but they did not synergize. Given that melatonin is a potent physiological antioxidant with protective effects in models of neurotoxicity, we tested the role of this secretory product on the pro-oxidant effect of both compounds given separately or in combination. We also checked the protective ability of several other antioxidants. Pharmacological doses of melatonin (millimolar), estrogens, pinoline and trolox (micromolar) prevented the oxidant effect of glutamate, ascorbate, and the combination of both substances. Potential therapeutic application of these results is discussed.

EFFECT OF MELATONIN ON 24H CHANGES IN PLASMA PROTEIN LEVELS DURING THE PRECLINICAL PHASE OF FREUND'S ADJUVANT ARTHRITIS IN RATS

The 24h rhythms in plasma protein concentration were examined in rats on the third day after injection of Freund's complete adjuvant or adjuvant's vehicle, performed 3h after light on. In rats treated with adjuvant's vehicle, peak values of albumin and gamma globulin occurred during the nocturnal activity span (P <.02 and P <. 0001, respectively), while those of alpha-1, alpha-2, and beta globulins were found late during the rest span (P <. 002, P <. 0001, and P <. 0004, respectively). Freund's adjuvant administration abolished temporal changes in plasma albumin and beta globulin levels. It also decreased the amplitude of daily changes in alpha-1 and alpha-2 globulin (P <. 05) and diminished mean values of alpha-2 globulin (P <. 01). Pretreatment of rats with melatonin (30 μg daily) for 11 days, 11h after light on, counteracted mycobacterial adjuvant-induced suppression of the 24h rhythms in albumin and alpha-1, alpha-2, and beta globulins. The results further support the existence of preventive properties of a pharmacological dose of melatonin in situations in which a lost of circadian rhythmicity is expected. (Chronobiology International, 18(3), 435–446, 2001)

Anti-depressant action of melatonin in chronic forced swimming-induced behavioral despair in mice, role of peripheral benzodiazepine receptor modulation

The possible antidepressant effect of physiological and pharmacological doses of melatonin was investigated in the Porsolt forced swimming-induced behavioral despair test. The duration of immobility period of BALB/c and C57BL/6J mice during a 6-min swim test was measured at noon (11:00–12:00 h), early dark (20:00–21:00 h) and at midnight (1:00–2:00 h), respectively. The circadian time cycle did not alter the duration of immobility in either strains of mice. Similarly, exogenously administered melatonin (10–1000 μg/kg≅50 nM to 5 μM/mouse), a dose that could act on high affinity melatonin receptors, did not modify the duration of immobility period at any of the time intervals studied in either strains of the mice. This suggested that neither circadian variation influenced the duration of immobility period of BALB/c and C57BL/6J mice nor at physiological doses melatonin showed any anti-depressant action. Acute administration of higher doses of melatonin (2.5–10 mg/kg) failed to induce any anti-depressant activity in mice which were subjected to forced swimming test for the first time. However, daily administration of melatonin (2.5–10 mg/kg) prior to swimming test significantly reversed the increase in immobility period that was observed on chronic exposure to swimming test. This effect was comparable with the effect of GABA-benzodiazepine (BZ) receptor agonists. Similarly, like GABAergic drugs, acute administration of melatonin also showed anti-depressant activity in a mice which were exposed to chronic forced swimming test. The anti-depressant action of melatonin was sensitive to reversal by peripheral BZ receptor antagonist, PK11195. Whereas, flumazenil failed to reverse the anti-depressant action of melatonin, thereby suggesting that central BZ receptor were not involved in its action. In conclusion the study showed that at pharmacological doses melatonin has anti-depressant action in chronic forced swimming-induced despair behavior by an action involving peripheral BZ receptors.

The Effects of Melatonin on the Antioxidant Systems in Experimental Spinal Injury

Melatonin has been recently shown by various in-vivo and in-vitro studies to exert potent neutralising effects on hydroxyl radicals, stimulate glutathione peroxidase (GSH-Px) activity, and protect catalase (CAT) from the destructive activity of hydroxyl radicals in neural tissue. We aimed to investigate the possible effects of pharmacological dose of melatonin on some of the antioxidant defence systems in an in-vivo study of experimental spinal injury. Seven groups of adult male Sprague Dawley rats were used in the following scheme: Group I: Naïve (n = 6), Group II: Lesion (n = 8), Group III: Melatonin (n = 5), Group IV: Melatonin + Lesion (n = 8), Group V: Placebo + Lesion (n = 5), Group VI: Sham operation (n = 5), and Group VII: Placebo (n = 5). Experimental spinal injury was induced at level T7-T8 by 5 sec compression of the total cord with an aneurism clip on anaesthetised and laminectomized animals. The total 10mg/kg dose of melatonin (Sigma) dissolved in alcohol-water was administered i.p. four times in 2.5 mg/kg doses, at 20min pre-, at the time of and at 1h and 2h post-compression. At 24±2h post-injury, the rats were euthanized and the lesioned segments of cord were dissected and homogenised with special care taken to distribute equal amount of injured tissue in each sample for analysis of reduced glutathione (GSH), oxidised glutathione (GSSG), superoxide dismutase (SOD), and CAT activity. Compression injury decreased GSH/GSSG ratio significantly (p <. 0001). Melatonin, by itself, significantly decreased GSSG content (p <. 05) and increased CAT activity (p <. 05) in the naive rats. Melatonin treatment decreased GSSG activity, thus elevating GSH/GSSG ratio, and also increased SOD and CAT activity without reaching statistical significance in the lesioned animals. In conclusion, pharmacological dose of systemically applied melatonin seemed to support some features of the antioxidant defence systems in our hands.

Therapeutic potential of melatonin receptor agonists and antagonists

1. Summary In humans, the hormone melatonin is produced and secreted from the pineal gland following a circadian rhythm with high levels during the normal hours of sleep at night and low levels during the day. The strong correlation between the sleep-wake cycle and the sleep-promoting effects of physiological and pharmacological doses of melatonin has lead to the hypotheses that melatonin can be used to treat insomnia and circadian sleep disorders. Insomnia is associated with daytime sleepiness and decrements in mental and physical performance. In the last decade, efforts from a number of pharmaceutical companies have aimed to discover and develop high affinity melatonin receptor agonists. Melatonin receptor agonists currently in development for the treatment of sleep disorders are non-selective in that they activate multiple melatonin receptor subtypes. This review highlights the main pharmacological properties and clinical uses of these melatonin analogues, and provides an assessment of the therapeutic potential of subtype-selective melatonin receptor agonists and antagonists. The efficacy and effectiveness of melatonin receptor agonists should be greatly increased and the side-effects reduced by targeting receptor subtypes mediating specific functions. The current need for the synthesis and characterisation of high affinity and subtype-selective melatonin receptor antagonists to further elucidate the physiological role of melatonin is also discussed. The discovery of drugs for the treatment of sleep disorders which are devoid of side-effects and which have novel mechanisms of action, is of current

Effect of Atenolol on Nocturnal Sleep and Temperature in Young Men: Reversal by Pharmacological Doses of Melatonin

Van Den Heuvel, C., K. J. Reid and D. Dawson. Effect of atenolol on nocturnal sleep and temperature in young men: Reversal by pharmacological doses of melatonin. Physiol Behav 61(6)795–802, 1997.—To examine the physiological role of melatonin in sleep, nocturnal melatonin secretion was suppressed using 100 mg oral atenolol in two studies. In Study 1, nocturnal sleep was recorded in 8 young men over 4 nights. Subjects received atenolol on one of the last 2 nights and showed significantly increased total wake time (TWT) and wakefulness after sleep onset (WASO), as well as decreased REM sleep and slow-wave sleep (SWS). When melatonin (total 5 mg) was given after atenolol on the other night, the changes in TWT, WASO, REM, and SWS were reversed. In Study 2, sleep onset latencies (SOL) and core temperature (Tc) of 10 young men were measured for 3 nonconsecutive nights. In a cross-over design, atenolol given on one night significantly suppressed urinary 6-sulphatoxymelatonin (6s-aMT) production and increased hourly measures of Tc and SOL relative to baseline night values. Oral melatonin (3 mg), administered after atenolol, reversed the changes in Tc and SOL. These results suggest that endogenous melatonin may assist in the maintenance of normal sleep architecture (Study 1) and also increase nocturnal sleep propensity by hypothermic effects (Study 2).

Specific binding of 2-[ 125I]iodomelatonin by rat splenocytes: characterization and its role on regulation of cyclic AMP production

In the present paper we show that pineal hormone melatonin interacts with rat splenocytes through high-affinity binding sites. Binding of 2-[ 125I]iodomelatonin ([ 125I]MEL) by splenocytes fulfills all criteria for binding to a receptor site. Binding exhibited properties such as dependence on time and temperature as well as reversibility, saturability, high affinity, specificity, and increased under constant light exposure. Results suggest binding to a single class of binding sites without cooperative interactions. The dissociation constant ( K d) for the single site was 0.34 nM with a binding capacity of 2.25 fmol/107 cells. These data are in close agreement with data obtained from kinetic studies, in which the kinetically derived value of the dissociation constant was 0.20 nM. The affinity of these binding sites suggests that they may recognize the physiological concentrations of melatonin in serum. Moreover, pharmacological doses of melatonin also inhibited cyclic AMP production stimulated by forskolin, a potent activator of adenylate cyclase system. The demonstration of [ 125I]MEL binding sites in the spleen, in addition to those described in blood mononuclear cells and thymus, provides evidence to support a direct mechanism of action of melatonin on immune system.

Elevated rectal temperature produced by all-night bright light is reversed by melatonin infusion in men

Early morning rectal body temperature is lowest when melatonin levels are highest in humans. Although pharmacological doses of melatonin are hypothermic in humans, the relationship between endogenous melatonin and temperature level has not been investigated. We measured rectal body temperature in nine normal men whose melatonin levels were suppressed by all-night sleep deprivation in bright light and compared values with those seen in sleep in the dark, sleep deprivation in dim light (to control for the stimulatory effect of wakefulness on temperature), and sleep deprivation in bright light with an infusion of exogenous melatonin that replicated endogenous levels. Minimum rectal temperature, calculated from smoothed temperature data from 2300 to 0515 h, was greater in bright-light sleep deprivation, resulting in suppression of melatonin, than in conditions of sleep deprivation in dim light or sleep in the dark. An exogenous melatonin infusion in bright light returned the minimum temperature to that seen in dim-light sleep deprivation. A nonsignificant elevation in mean and minimum temperature was noted in all conditions of sleep deprivation relative to sleep. We conclude that melatonin secretion contributes to the lowering of core body temperature seen in the early morning in humans.

Melatonin and circadian control in mammals.

Although pinealectomy has little influence on the circadian locomotor rhythms of laboratory rats, administration of the pineal hormone melatonin has profound effects. Evidence for this comes from studies in which pharmacological doses of melatonin are administered under conditions of external desynchronization, internal desynchronization, steady state light-dark conditions, and phase shifts of the zeitgeber. Taken together with recent findings on melatonin receptor concentration in the rat hypothalamus, particularly at the level of the suprachiasmatic nuclei, these results suggest that melatonin is a potent synchronizer of rat circadian rhythms and has a direct action on the circadian pacemaker. It is possible, therefore, that the natural role of endogenous melatonin is to act as an internal zeitgeber for the total circadian structure of mammals at the level of cell, tissue, organ, whole organism and interaction of that organism with environmental photoperiod changes.