2[125I]iodomelatonin binding sites in spleens of guinea pigs

Immunoregulatory properties of melatonin in the humoral immune system: A narrative review

Melatonin is the major product both synthesized and secreted by the pineal gland during the night period and it is the principal chronobiotic hormone that regulates the circadian rhythms and seasonal changes in vertebrate biology. Moreover, melatonin shows both a broad distribution along the phylogenetically distant organisms and a high functional versatility. At the present time, a significant amount of experimental evidence has been reported in scientific literature and has clearly shown a functional relationship between the endocrine, nervous, and immune systems. The biochemistry basis of the functional communication between these systems is the utilization of a common chemicals signals. In this framework, at present melatonin is considered to be a relevant member of the so-called neuro-endocrine-immunological network. Thus, both in vivo and in vitro investigations conducted in both experimental animals and humans, have clearly documented that melatonin has an important immunomodulatory role. However, most of the published results refer to information on T lymphocytes, i.e., cell-mediated immunity. On the contrary, fewer studies have been carried out on B lymphocytes, the cells responsible for the so-called humoral immunity. In this review, we have focused on the biological role of melatonin in the humoral immunity. More precisely, we report the actions of melatonin on B lymphocytes biology and on the production of different types of antibodies.

Melatonin as a Coadjuvant in the Treatment of Patients with Fibromyalgia

Fibromyalgia syndrome (FMS) is a chronic widespread pain syndrome that is accompanied by fatigue, sleep disturbances, anxiety, depression, lack of concentration, and neurocognitive impairment. As the currently available drugs are not completely successful against these symptoms and frequently have several side effects, many scientists have taken on the task of looking for nonpharmacological remedies. Many of the FMS-related symptoms have been suggested to be associated with an altered pattern of endogenous melatonin. Melatonin is involved in the regulation of several physiological processes, including circadian rhythms, pain, mood, and oxidative as well as immunomodulatory balance. Preliminary clinical studies have propounded that the administration of different doses of melatonin to patients with FMS can reduce pain levels and ameliorate mood and sleep disturbances. Moreover, the total antioxidant capacity, 6-sulfatoxymelatonin and urinary cortisol levels, and other biological parameters improve after the ingestion of melatonin. Recent investigations have proposed a pathophysiological relationship between mitochondrial dysfunction, oxidative stress, and FMS by looking at certain proteins involved in mitochondrial homeostasis according to the etiopathogenesis of this syndrome. These improvements exert positive effects on the quality of life of FMS patients, suggesting that the use of melatonin as a coadjuvant may be a successful strategy for the management of this syndrome.

Melatonin ameliorates aging-related impaired angiogenesis in gastric endothelial cells via local actions on mitochondria and VEGF-survivin signaling

Tissue injury healing is impaired in aging, and this impairment is caused in part by reduced angiogenesis. Melatonin, a neuroendocrine hormone that regulates sleep and circadian rhythm, is also produced in the gastrointestinal tract. The expression of melatonin receptors MT1 and MT2 in gastric endothelial cells and their roles in aging-related impairment of gastric angiogenesis have not been examined. We hypothesized that MT1 and MT2 expression is reduced in gastric endothelial cells of aging rats and that melatonin treatment can upregulate their expression and improve angiogenesis. We examined the expression of MT1 and MT2 in gastric endothelial cells (GECs) isolated from young and aging rats. We also examined the effects of melatonin treatment on angiogenesis, GEC mitochondrial function, expression of vascular endothelial growth factor (VEGF), its signaling receptor (VEGFR-2), and the inhibitor of apoptosis protein, survivin. Young and aging GECs expressed MT1 (in the cytoplasm and mitochondria) and MT2 (in nucleus and mitochondria). In aging GECs, MT1 and MT2 levels, in vitro angiogenesis, and mitochondrial membrane potential were significantly reduced (by 1.5-fold, 1.9-fold, 3.1-fold, and 1.63-fold, respectively) compared with young GECs. Melatonin treatment of aging GECs significantly increased MT1 and MT2 expression compared with the controls, induced nuclear translocation of MT1, and significantly ameliorated the aging-related impairment of angiogenesis and mitochondrial function. Aging GECs have significantly reduced MT1 and MT2 expression, angiogenesis, and mitochondrial membrane potential compared with young GECs. Treatment of aging GECs with melatonin increases expression of VEGF receptor and survivin and ameliorates aging-related impaired angiogenesis and mitochondrial function.NEW & NOTEWORTHY This study showed reduced expression of melatonin receptors MT1 and MT2, angiogenesis, and mitochondrial function in gastric endothelial cells (GECs) isolated from aging rats. Treatment of aging GECs with melatonin increases expression of VEGF receptor and survivin and ameliorates aging-related impaired angiogenesis and mitochondrial function. These studies provide new insight into the mechanisms of the aging-related impairment of angiogenesis and delayed tissue injury healing and provide a rationale for melatonin treatment to reverse these abnormalities.

Melatonin and its atheroprotective effects: a review

Atherosclerosis is a chronic vascular disease in which oxidative stress and inflammation are commonly implicated as major causative factors. Identification of novel strategies that contribute to plaque stabilization or inhibition represents a continuing challenge for the medical community. The evidence from the last decade highlights that melatonin influences the cardiovascular system, but its mechanisms of action have not been definitively clarified. Melatonin has atheroprotective effects by acting on different pathogenic signaling processes; these result from its direct free radical scavenger activity, its indirect antioxidant properties and its anti-inflammatory actions. In this review, we summarize the many pieces of the puzzle which identified molecular targets for prevention and therapy against the atherosclerotic pathogenic processes and we evaluate the data documenting that melatonin treatment has important actions that protect against atherosclerosis and atherosclerosis-related cardiovascular diseases.

The role of melatonin in the cells of the innate immunity: a review

Melatonin is the major secretory product synthesized and secreted by the pineal gland and shows both a wide distribution within phylogenetically distant organisms from bacteria to humans and a great functional versatility. In recent years, a considerable amount of experimental evidence has accumulated showing a relationship between the nervous, endocrine, and immune systems. The molecular basis of the communication between these systems is the use of a common chemical language. In this framework, currently melatonin is considered one of the members of the neuroendocrine-immunological network. A number of in vivo and in vitro studies have documented that melatonin plays a fundamental role in neuroimmunomodulation. Based on the information published, it is clear that the majority of the present data in the literature relate to lymphocytes; thus, they have been rather thoroughly investigated, and several reviews have been published related to the mechanisms of action and the effects of melatonin on lymphocytes. However, few studies concerning the effects of melatonin on cells belonging to the innate immunity have been reported. Innate immunity provides the early line of defense against microbes and consists of both cellular and biochemical mechanisms. In this review, we have focused on the role of melatonin in the innate immunity. More specifically, we summarize the effects and action mechanisms of melatonin in the different cells that belong to or participate in the innate immunity, such as monocytes-macrophages, dendritic cells, neutrophils, eosinophils, basophils, mast cells, and natural killer cells.

Mitochondria and chloroplasts as the original sites of melatonin synthesis: a hypothesis related to melatonin's primary function and evolution in eukaryotes

Mitochondria and chloroplasts are major sources of free radical generation in living organisms. Because of this, these organelles require strong protection from free radicals and associated oxidative stress. Melatonin is a potent free radical scavenger and antioxidant. It meets the criteria as a mitochondrial and chloroplast antioxidant. Evidence has emerged to show that both mitochondria and chloroplasts may have the capacity to synthesize and metabolize melatonin. The activity of arylalkylamine N-acetyltransferase (AANAT), the reported rate-limiting enzyme in melatonin synthesis, has been identified in mitochondria, and high levels of melatonin have also been found in this organelle. From an evolutionary point of view, the precursor of mitochondria probably is the purple nonsulfur bacterium, particularly, Rhodospirillum rubrum, and chloroplasts are probably the descendents of cyanobacteria. These bacterial species were endosymbionts of host proto-eukaryotes and gradually transformed into cellular organelles, that is, mitochondria and chloroplasts, respectively, thereby giving rise to eukaryotic cells. Of special importance, both purple nonsulfur bacteria (R. rubrum) and cyanobacteria synthesize melatonin. The enzyme activities required for melatonin synthesis have also been detected in these primitive species. It is our hypothesis that mitochondria and chloroplasts are the original sites of melatonin synthesis in the early stage of endosymbiotic organisms; this synthetic capacity was carried into host eukaryotes by the above-mentioned bacteria. Moreover, their melatonin biosynthetic capacities have been preserved during evolution. In most, if not in all cells, mitochondria and chloroplasts may continue to be the primary sites of melatonin generation. Melatonin production in other cellular compartments may have derived from mitochondria and chloroplasts. On the basis of this hypothesis, it is also possible to explain why plants typically have higher melatonin levels than do animals. In plants, both chloroplasts and mitochondria likely synthesize melatonin, while animal cells contain only mitochondria. The high levels of melatonin produced by mitochondria and chloroplasts are used to protect these important cellular organelles against oxidative stress and preserve their physiological functions. The superior beneficial effects of melatonin in both mitochondria and chloroplasts have been frequently reported.

Melatonin membrane receptor type MT1 modulates cell-mediated immunity in the seasonally breeding tropical rodent Funambulus pennanti

OBJECTIVE

Despite the evidence for melatonin membrane receptors (MT1R and MT2R) on lymphoid tissues in a wide range of seasonal breeders, their specific potency has never been compared and correlated with cell-mediated immunity.

METHODS

We used luzindole, a nonselective MT2R antagonist, and 4-phenyl-2-propionamidotetralin (4P-PDOT), a selective MT2R antagonist, to assess the potency of the melatonin receptors MT1 and MT2 in melatonin-induced immunity under both in vivo as well as in vitro conditions.

RESULTS

Physiological doses (25 μg/100 g body weight in vivo and 100 and 500 pg/ml in vitro) of melatonin upregulated both MT1R and MT2R expression as well as splenocyte proliferation, while higher doses (100 and 500 μg/100 g body weight in vivo and 1 ng/ml in vitro) downregulated splenocyte proliferation and the expression of both receptors. Luzindole antagonized the expression of both MT1R and MT2R in a dose-dependent manner under in vivo as well as in vitro conditions, while 4P-PDOT blocked the expression of MT2R only during both experimental conditions. Splenocyte proliferation and IL-2 secretion (in vitro) followed the MT1R expression pattern, while the MT2R expression pattern showed no definite relation with either splenocyte proliferation or IL-2 secretion under in vivo and in vitro conditions.

CONCLUSION

Immune function in tropical rodents is directly regulated by melatonin via its high-affinity membrane receptor MT1. MT1R plays a directive role in mediating splenocyte proliferation and IL-2 release, while the MT2R subtype appears not to be required for the immunoenhancing role of melatonin.

The inhibition of apoptosis by melatonin in VSC4.1 motoneurons exposed to oxidative stress, glutamate excitotoxicity, or TNF-alpha toxicity involves membrane melatonin receptors

Loss of motoneurons may underlie some of the deficits in motor function associated with the central nervous system (CNS) injuries and diseases. We tested whether melatonin, a potent antioxidant and free radical scavenger, would prevent motoneuron apoptosis following exposure to toxins and whether this neuroprotection is mediated by melatonin receptors. Exposure of VSC4.1 motoneurons to either 50 microm H(2)O(2), 25 microm glutamate (LGA), or 50 ng/mL tumor necrosis factor-alpha (TNF-alpha) for 24 h caused significant increases in apoptosis, as determined by Wright staining and ApopTag assay. Analyses of mRNA and proteins showed increased expression and activities of stress kinases and cysteine proteases and loss of mitochondrial membrane potential during apoptosis. These insults also caused increases in intracellular free [Ca(2+)] and activities of calpain and caspases. Cells exposed to stress stimuli for 15 min were then treated with 200 nm melatonin. Post-treatment of cells with melatonin attenuated production of reactive oxygen species (ROS) and phosphorylation of p38, MAPK, and JNK1, prevented cell death, and maintained whole-cell membrane potential, indicating functional neuroprotection. Melatonin receptors (MT1 and MT2) were upregulated following treatment with melatonin. To confirm the involvement of MT1 and MT2 in providing neuroprotection, cells were post-treated (20 min) with 10 microm luzindole (melatonin receptor antagonist). Luzindole significantly attenuated melatonin-induced neuroprotection, suggesting that melatonin worked, at least in part, via its receptors to prevent VSC4.1 motoneuron apoptosis. Results suggest that neuroprotection rendered by melatonin to motoneurons is receptor mediated and melatonin may be an effective neuroprotective agent to attenuate motoneuron death in CNS injuries and diseases.

Visualization of melatonin's multiple mitochondrial levels of protection against mitochondrial Ca(2+)-mediated permeability transition and beyond in rat brain astrocytes

Melatonin protects cells against various types of oxidative stress-induced apoptosis due primarily to its ability to effectively scavenge pathological and disease condition-augmented generation of mitochondrial reactive oxygen species (mROS). Once produced, mROS indiscriminately damage mitochondrial components and more importantly they crucially activate directly the mitochondrial permeability transition (MPT), one of the critical mechanisms for initiating post mitochondrial apoptotic signaling. Whether or not melatonin targets directly the MPT, however, remains inconclusive, particularly during oxidative stress. This study, thus, investigated this possibility of an 'oxidation free Ca(2+) stress' in the presence of vitamin E after ionomycin exposure as a sole Ca(2+)-mediated MPT in order to exclude melatonin's primary antioxidative effects as well as Ca(2+)-mediated oxidative stress. The studies were carried out using cultured rat brain astrocytes RBA-1. With the application of laser scanning multiple fluorescence imaging microscopy, we visualized for the first time multiple mitochondrial protective effects provided by melatonin during Ca(2+) stress. First, melatonin, due to its primary antioxidative actions, completely prevented mCa(2+)-induced mROS formation during ionomycin exposure. Secondly, when melatonin(')s antioxidative effects were prevented due to the addition of vitamin E, melatonin significantly prevented mCa(2+)-mediated MPT and apoptosis suggesting its direct targeting of the MPT. Surprisingly, in the presence of cyclosporin A, a MPT inhibitor, melatonin reduced further mCa(2+)-mediated apoptosis during ionomycin exposure also suggesting its targeting beyond the MPT. As astrocytes are actively involve in regulating synaptic transmission and neurovascular coupling in the CNS, these multiple mitochondrial layers of protection provided by melatonin against mCa(2+)-and/or mROS-mediated apoptosis in astrocytes may be crucial for future therapeutic prevention and treatment of astrocyte-mediated neurodegenerative diseases in the CNS.

Photoimmunomodulation and melatonin

The seasons, and daily physical rhythms can have a profound effect on the physiology of the living organism, which includes immune status. The immune system can be influenced by a variety of signals and one of them is photic stimulus. Light may regulate the immunity through the neuroendocrine system leading to the most recent branch of research the "Photoimmunomodulation". Mammals perceive visible light (400-700 nm) through some specialized photoreceptors located in retina like retinal ganglion cells (RGC). This photic signal is then delivered to the visual cortex from there to the suprachiasmatic nucleus (SCN) of the hypothalamic region. Melatonin--one of the universally accepted chronobiotic molecule secreted by the pineal gland is now emerging as one of the most effective immunostimulatory compound in rodents and as oncostatic molecule at least in human. Its synthesis decreases with light activation along with norepinephrine and acetylcholine. The changes in level of melatonin may lead to alterations (stimulatory/inhibitory) in immune system. The evidences for the presence of melatonin receptor subtypes on lymphoid tissues heralded the research area about mechanism of action for melatonin. Further, melatonin receptor subtypes-MT1 and MT2 was noted on pars tuberalis, SCN and on lymphatic tissues suggesting a direct action of melatonin in modulation of immunity by photoperiod as well. The nuclear receptors (ROR, RZR etc.) of melatonin are known for its free radical scavenging actions and might be indirectly controlling the immune function.

Melatonin prevents glutamate-induced oxytosis in the HT22 mouse hippocampal cell line through an antioxidant effect specifically targeting mitochondria

The pineal hormone melatonin has neuroprotective effects in a large number of models of neurodegeneration. Melatonin crosses the blood-brain barrier, shows a decrease in its nocturnal peaks in blood with age that has been associated with the development of neurodegenerative disorders, and has been shown to be harmless at high concentrations. These properties make melatonin a potential therapeutic agent against neurodegenerative disorders but the pathways involved in such neuroprotective effects remain unknown. In the present report we study the intracellular pathways implicated in the complete neuroprotection provided by melatonin against glutamate-induced oxytosis in the HT22 mouse hippocampal cell line. Our results strongly suggest that melatonin prevents oxytosis through a direct antioxidant effect specifically targeted at the mitochondria. Firstly, none of the described transducers of melatonin signalling seems to be implicated in the neuroprotection provided by this indole. Secondly, melatonin does not prevent cytosolic GSH depletion-dependent increase in reactive oxygen species (ROS), but it totally prevents mitochondrial ROS production despite the fact that the latter is much higher than the former. And finally, there is a high correlation between the concentration at which melatonin and closely related indoles exert a direct antioxidant effect in vitro and a neuroprotective effect against glutamate-induced oxytosis.

Ultrastructural clues for the potent therapeutic effect of melatonin on aging skin in pinealectomized rats

Recently we have reported a significant reduction in the thickness of epidermis and epidermis + dermis in the back, abdominal and thoracic skin of the long-term pinealectomized rats and the potent therapeutic effect of melatonin on the pinealectomy-induced morphometric changes. The present study was aimed to determine the fine structure of the abdominal and thoracic skin in pinealectomized rats and the effect of melatonin on skin ultrastructure. Rats were pinealectomized or sham operated (control) for 6 months. Half of the pinealectomized rats were treated with 4 mg/kg melatonin during the last month of the experiment. Pinealectomy resulted in prominent ultrastructural changes in the skin. Epidermal atrophy, disorganization and cytological atypia were obvious. Tonofilament distribution was not uniform, and intercellular space was narrow. Nuclear irregularity and heterochromatin condensation were detected. Many mitochondria were irregular and edematous with increased translucence of the matrix, either partial or total destruction of crests and frequently the presence of vacuoles, myelin figures and dense bodies. Microprojections of basal cells into the dermis were observed. The dermis was thin, and collagenous fibers were loosely arranged. The epidermis in melatonin administered pinealectomized rats was obviously thicker than that of pinealectomized rats. The cells of each layers had characteristic morphological and ultrastructural features. Nuclear irregularity and heterochromatin condensation were not seen. Mitochondria were generally normal in ultrastructural appearance but rarely vacuoles and myelin figures were observed. The dermis was thick, and collagenous fibers were closely packaged. This paper provides an additional ultrastructural evidence that the damage to mitochondria is the major contributory factor to skin aging and that melatonin has potent therapeutic effects in reducing age-related changes via protecting fine structure of the skin.

Changes in melatonin receptors in relation to the development of scoliosis in pinealectomized chickens

STUDY DESIGN

The 2[125I]iodomelatonin binding assay was used to investigate the involvement of melatonin receptors in the development of scoliosis in pinealectomized chickens.

OBJECTIVE

To compare the binding properties of melatonin receptors in the thoracic spinal cord between pinealectomized chickens that had scoliosis develop and those that did not.

SUMMARY OF BACKGROUND DATA

Surgical pinealectomy in young chickens induced scoliosis with a reported incidence of 50% to 100%. The factors regulating the development of scoliosis in this animal model are unclear. Melatonin receptors have been shown in the spinal cord of chickens, but their functions are still unknown. This study addresses the question as to whether spinal cord melatonin receptors are related to the pathogenesis of scoliosis in pinealectomized chickens.

METHOD

Control and pinealectomized chickens were kept under controlled lighting conditions and monitored for scoliosis development. At 9-11 weeks after pinealectomy, thoracic spinal cords were removed for 2[125I]iodomelatonin binding assay, and blood was collected for serum melatonin assay at either the middle of the light period or middle of the dark period.

RESULTS

Pinealectomy in young chickens produced: (1) loss of diurnal variations in serum melatonin levels, (2) 50% incidence of scoliosis, and (3) attenuation in the diurnal variations in the receptor affinity to melatonin. No differences were detected in the serum melatonin levels or binding of spinal cord melatonin receptors between the pinealectomized chickens that had scoliosis develop and those that did not.

CONCLUSION

Changes are detected in melatonin receptor binding after pinealectomy. However, these changes cannot account for the reason why scoliosis develops in some chickens after pinealectomy, while it does not in others. Neither low melatonin serum level nor changes in spinal cord melatonin binding can be a sole etiologic factor in the pathogenesis of scoliosis in pinealectomized chickens.

Melatonin and differential effect of L-thyroxine on immune system of Indian tropical bird Perdicula asiatica

Interaction of thyroxine and melatonin on immune status was noted in vivo and in vitro when peripheral melatonin was high and thyroxine low in plasma of male Perdicula asiatica during reproductively inactive phase. During this phase exogenous thyroxine (4 microg/100g. Bwt./day) and melatonin (25 microg/100g. Bwt./day) increased immune parameters (spleen weight, total leukocyte count, lymphocyte count, percent stimulation ratio) and increased splenocyte density in spleen. In vitro l-thyroxine (10(-6)M/ml) supplementation decreased the splenocyte proliferation which was reversed by melatonin (500 pg/ml) supplementation. In vivo l-thyroxine showed immunoenhancing effect while in vitro it decreased the splenocyte proliferation presenting a differential effect. In the absence of internal physiological conditions of the birds, T(4) showed a negative effect on splenocytes proliferation in vitro when treated alone. However, melatonin maintained its lymphoproliferative effect under both conditions. Thus, avian splenocyte exposed to different hormonal conditions in vitro might have produced different signal peptides other than in vivo, thereby making the result different.

Hormones in the nucleus. Immunologically demonstrable biogenic amines (serotonin, histamine) in the nucleus of rat peritoneal mast cells

Using 1-ethyl-3(3-dimethyl-aminopropyl)-carbodiimide (EDAC) fixation and immunocytochemical confocal microscopic study, bright serotonin and histamine fluorescence appeared in the nucleus of rat peritoneal mast cells. In case of paraformaldehyde fixation, this was not observed. The phenomenon can be explained by the cross-linking effect of EDAC, which did not allow the efflux of biogenic amines from the nucleus. This means that biogenic amines are present in the nucleus of mast cells, and this is supported by the flow cytometric measurement data of the whole cell. Other hormones studied (triiodothyronine, insulin, and endorphin) were not present in the nucleus. Four pharmaca with biogenic amine-influencing character in the central nervous system were used for studying the relation between the external (surrounding and cytoplasmic) and nuclear biogenic amine content of mast cells. Fluoxetine, a serotonin reuptake inhibitor depleted nuclear as well as cytoplasmic serotonin content. Clorgyline, a MAO-A inhibitor, decreased cytoplasmic serotonin content and weakened nuclear serotonin fluorescence. The tryptophan hydroxylase inhibitor, para-chlorophenylalanine (PCPA), and the mast cell degranulator, Compound 48/80, reduced cytoplasmic serotonin content without influencing nuclear content. Histamine fluorescence was influenced solely by fluoxetine. The results show that nuclear 5-HT content is dependent firstly of serotonin uptake and reuptake. To our knowledge, this is the first exact report on the presence of non-steroid-type-receptor-transported hormones inside the nucleus of a cell.

Melatonin in higher plants: occurrence and possible functions

Melatonin may be ubiquitous in the plant kingdom. This review considers the evaluation of methods of melatonin determination in plant material and possible melatonin functions in plants. Concerning the determination methods, the only reliable techniques are liquid chromatography--mass spectrometry or gas chromatography--mass spectrometry after some purification steps of the extract. Melatonin was shown to delay flower induction in some photoperiodic plants and in the dinoflagellate Lingulodinium it replaces, in part, the requirement of darkness for cyst formation. Melatonin may also have a function as an antioxidant and it may possess some auxin-like effects. Finally, it may act as a signal for interaction of plants with herbivores and pests. Further research is needed to clarify these potential functions.

A review of the multiple actions of melatonin on the immune system

This review summarizes the numerous observations published in recent years which have shown that one of the most significant of melatonin's pleiotropic effects is the regulation of the immune system. The overview summarizes the immune effects of pinealectomy and the association between rhythmic melatonin production and adjustments in the immune system as markers of melatonin's immunomodulatory actions. The effects of both in vivo and in vitromelatonin administration on non-specific, humoral, and cellular immune responses as well as on cellular proliferation and immune mediator production are presented. One of the main features that distinguishes melatonin from the classical hormones is its synthesis by a number of non-endocrine extrapineal organs, including the immune system. Herein, we summarize the presence of immune system-synthesized melatonin, its direct immunomodulatory effects on cytokine production, and its masking effects on exogenous melatonin action. The mechanisms of action of melatonin in the immune system are also discussed, focusing attention on the presence of membrane and nuclear receptors and the characterization of several physiological roles mediated by some receptor analogs in immune cells. The review focuses on melatonin's actions in several immune pathologies including infection, inflammation, and autoimmunity together with the relation between melatonin, immunity, and cancer.

Melatonin and mitochondrial function

Melatonin is a natural occurring compound with well-known antioxidant properties. In the last decade a new effect of melatonin on mitochondrial homeostasis has been discovered and, although the exact molecular mechanism for this effect remains unknown, it may explain, at least in part, the protective properties found for the indoleamine in degenerative conditions such as aging as well as Parkinson's disease, Alzheimer's disease, epilepsy, sepsis and other injuries such as ischemia-reperfusion. A common feature in these diseases is the existence of mitochondrial damage due to oxidative stress, which may lead to a decrease in the activities of mitochondrial complexes and ATP production, and, as a consequence, a further increase in free radical generation. A vicious cycle thus results under these conditions of oxidative stress with the final consequence being cell death by necrosis or apoptosis. Melatonin is able of directly scavenging a variety of toxic oxygen and nitrogen-based reactants, stimulates antioxidative enzymes, increases the efficiency of the electron transport chain thereby limiting electron leakage and free radical generation, and promotes ATP synthesis. Via these actions, melatonin preserves the integrity of the mitochondria and helps to maintain cell functions and survival.

Does neuroimmune dysfunction mediate seasonal mood changes in winter depression?

BACKGROUND

Animal studies have demonstrated seasonal changes in immune function mediated by nocturnal melatonin duration as a biological signal for photoperiod. Recent research has highlighted the potential role of neuroimmune dysfunction in depressive disorders. The etiology of winter depression (seasonal affective disorder, or SAD) is not known, but a number of studies have provided support for both photoperiod and neurotransmitter hypotheses.

HYPOTHESIS

A new hypothesis is presented that links the SAD data on melatonin, photoperiod, and neurotransmitters by proposing that seasonal increases in proinflammatory cytokines are critical in the pathophysiology of winter SAD.

TESTING THE HYPOTHESIS

In SAD patients, but not healthy subjects: proinflammatory cytokines will be increased and the Th1/Th2 balance will be shifted to the left in winter compared to summer; neuroimmune function will be correlated with nocturnal melatonin duration in SAD patients; and light treatment will correct neuroimmune dysfunction.

IMPLICATIONS OF THE HYPOTHESIS

Diagnostic tests for SAD may be developed using cytokine assays; neuroimmune dysfunction may be predictors of response to treatments; new treatments for SAD (immune or anti-inflammatory treatment) may be developed.

Pineal control of immune status and hematological changes in blood and bone marrow of male squirrels (Funambulus pennanti) during their reproductively active phase

In addition to pineal control of reproduction in seasonal breeders, melatonin is also known to influence various immune parameters. In the present experiment, we assessed the effect of exogenous melatonin treatment on different hematological parameters of peripheral blood and bone marrow cells, together with histological observations of spleen and thymus blastogenic response and stimulation ratio, and hormonal assays (melatonin and testosterone) of Indian palm squirrel (Funambulus pennanti) during their reproductively active phase when endogenous melatonin levels are low. Daily subcutaneous injection of melatonin (25 microg/100 g body mass.) at 17.30-18.00 h to adult male squirrels for 60 consecutive days during May-June significantly increased the lymphocyte count of blood and bone marrow and the blastogenic response/percent stimulation ratio of spleen and thymus. Histological observation showed densely packed thymocytes and splenocytes. During this period, peripheral testosterone level was high and melatonin was low establishing an inverse relationship as noted earlier for this squirrel. In pinealectomized squirrels, decreased total leukocyte count and percent lymphocyte count in peripheral blood and bone marrow, along with a decreased cell density in spleen and thymus was observed histologically. Further, melatonin treatment of pinealectomized squirrels resulted in restoration of the immune parameters in line with a normal control level. We suggest that during the reproductively active period of male Indian palm squirrels the lymphoid organs were sensitive to melatonin; hence, the exogenous melatonin treatment had an immuno-enhancing effect.

Melatonin reduces non-adrenergic, non-cholinergic relaxant neurotransmission by inhibition of nitric oxide synthase activity in the gastrointestinal tract of rodents in vitro

The aim of the present study was to investigate the effects of melatonin on non-adrenergic, non-cholinergic (NANC) relaxant neurotransmission in the gastrointestinal tract, which is mainly mediated by nitrergic and peptidergic mechanisms. Melatonin (10(-7)-10(-3) M) had no effect on the basal tonus of the rat gastric fundus smooth muscle. Relaxant responses following electrical stimulation(40 V; 0.5 ms pulse duration; 10 s stimulation duration) under NANC conditions on a 5-hydroxytryptamine (5-HT, 10(-7) M) contraction plateau were elicited at frequencies in the range of 0.5-16 Hz. Melatonin significantly reduced these inhibitory NANC responses (16 Hz without melatonin: -103 +/- 6.3%; melatonin 10(-5) M: -80.4 +/- 7.5%; melatonin 10(-4) M: -39.1 +/- 17.1%). Intracellular recording was carried out in a mouse colonic preparation. Electrical neural stimulation of the mouse colonic neurons caused biphasic intracellular hyperpolarization in smooth-muscle cells. The initial fast component is apamin-sensitive, and the following slow component is dependent on nitrergic mechanisms, as it is abolished in the presence of NG-nitro-L-arginine (L-NNA). Melatonin significantly reduced the nitric oxide-dependent slow component of neurally transmitted hyperpolarization, whereas the initial fast component was left unchanged. In a synaptosomal preparation of the enteric nervous system of rat intestine, enzymatic nitric oxide synthase (NOS) activity was significantly reduced by melatonin at concentrations ranging from 10(-7) to 10(-4) M (basal preparation including cofactors: 61.2 +/- 9.4 fmol/mg; melatonin 10(-4) M: 39.2 +/- 6.9 fmol/mg). Reverse transcriptase-polymerase chain reaction (RT-PCR) studies were conducted to investigate the melatonin receptors (mt(1), MT(2) and MT(3)) present in the esophagus, stomach and ileum of the rat. The presence of mt1 mRNA expression alone, but not of mRNA expression for MT(2) or MT(3), was demonstrated in the tissues. In conclusion, this study demonstrates that melatonin reduces the functional inhibitory NANC response. It shows that this effect may be the result of a reduction of the nitrergic component of the smooth-muscle inhibitory junction potential (IJP) and related to direct inhibition of NOS activity in enteric synaptosomes. The presence of mt1 receptor transcripts adds supportive evidence for a possible physiological role of melatonin within the enteric nervous system.

Melatonin receptor subtype MT2 (Mel 1b) and not mt1 (Mel 1a) is associated with melatonin-induced enhancement of cell-mediated and humoral immunity

Individuals of many vertebrate species undergo seasonal changes in immune function in addition to marked seasonal changes in reproductive, metabolic, and other physiological processes. Despite growing evidence that photoperiod mediates seasonal changes in immunity, little is known regarding the neuroendocrine mechanisms underlying these changes. Enhanced immune function in short days is correlated with increased duration of nightly melatonin secretion, and recent studies indicate that melatonin can act directly on immune cells to enhance immune function. It remains unknown, however, which melatonin receptor subtype mediates immune enhancement by melatonin. The present study examined the contribution of specific melatonin receptor subtypes, mt1 (Mel 1a) and MT2 (Mel 1b), in mediating melatonin-induced enhancement of cell-mediated and humoral immune function in mice. Melatonin enhanced both splenocyte proliferation and anti-keyhole limpet hemocyanin (KLH) IgG concentrations in both wild-type (WT) and mice lacking a functional gene for melatonin receptor mt1 (mt1 -/-), suggesting that the mt1 receptor does not mediate these responses. In addition, luzindole, an MT2 receptor antagonist, attenuated melatonin-induced enhancement of splenocyte proliferation in both WT and mt1 -/- mice. Taken together, these results suggest that receptor subtype mt1 is not necessary for mediating melatonin-induced enhancement of immune function and provide the first evidence for a specific melatonin receptor subtype, MT2, that may be involved in melatonin-induced immune enhancement.

Melatonin enhancement of splenocyte proliferation is attenuated by luzindole, a melatonin receptor antagonist

In addition to marked seasonal changes in reproductive, metabolic, and other physiological functions, many vertebrate species undergo seasonal changes in immune function. Despite growing evidence that photoperiod mediates seasonal changes in immune function, little is known regarding the neuroendocrine mechanisms underlying these changes. Increased immunity in short days is hypothesized to be due to the increase in the duration of nightly melatonin secretion, and recent studies indicate that melatonin acts directly on immune cells to enhance immune parameters. The present study examined the contribution of melatonin receptors in mediating the enhancement of splenocyte proliferation in response to the T cell mitogen Concanavalin A in mice. The administration of luzindole, a high-affinity melatonin receptor antagonist, either in vitro or in vivo significantly attenuated the ability of in vitro melatonin to enhance splenic lymphocyte proliferation during the day or night. In the absence of melatonin or luzindole, splenocyte proliferation was intrinsically higher during the night than during the day. In the absence of melatonin administration, luzindole reduced the ability of spleen cells to proliferate during the night, when endogenous melatonin concentrations are naturally high. This effect was not observed during the day, when melatonin concentrations are low. Taken together, these results suggest that melatonin enhancement of splenocyte proliferation is mediated directly by melatonin receptors on splenocytes and that there is diurnal variation in splenocyte proliferation in mice that is also mediated by splenic melatonin receptors.

Melatonin mediates seasonal changes in immune function

Field studies indicate that immune function is compromised and the prevalence of many diseases are elevated during winter when energetic stressors are extensive. Presumably, individuals would enjoy a survival advantage if seasonally recurring stressors could be anticipated and countered by shunting energy reserves to bolster immune function. The primary environmental cue that permits physiological anticipation of season is daily photoperiod, a cue that is mediated by melatonin. However, other environmental factors, including low food availability and ambient temperatures, may interact with photoperiod to affect immune function and disease processes. This paper will review laboratory studies that consistently report enhanced immune function in short day lengths. Prolonged melatonin treatment mimics short days, and both in vitro and in vivo melatonin treatment enhances various aspects of immune function, especially cell-mediated immune function, in nontropical rodents. Reproductive responsiveness to melatonin appears to affect immune function. In sum, melatonin may be part of an integrative system to coordinate reproductive, immunologic, and other physiological processes to cope successfully with energetic stressors during winter.

Pharmacological characterization, molecular subtyping, and autoradiographic localization of putative melatonin receptors in uterine endometrium of estrous rats

The objective of this study was to determine the biochemical characteristics, subtypes, and localization of melatonin receptors in the rat uterus in estrous stage. Autoradiography with the melatonin ligand, 2-[125I]iodomelatonin, showed that melatonin receptors were localized in the rat uterine endometrium. Binding of 2-[125I]iodomelatonin in crude membrane preparations of rat uterine endometrium in estrous stage was stable, saturable, reversible and of high affinity. Rosenthal analysis yielded an equilibrium dissociation constant (Kd) of 28.9 +/- 3.59 pmol/l (n = 8) and a maximum number of binding sites (Bmax) of 1.6 +/- 0.15 fmol/mg protein (n = 8). The Kd value determined from kinetic analysis was 16.5 +/- 3.02 pmol/l (n = 3). Competition studies using various indoles and neurotransmitters demonstrated that 2-iodomelatonin, melatonin, 6-chloromelatonin, 6-hydroxymelatonin and N-acetylserotonin showed significant inhibition of the 2-[125I]iodomelatonin binding, while the other indole compounds tested had no significant inhibition. The expression of rat uterine endometrial melatonin receptor subtypes was studied by reverse transcription-polymerase chain reaction (RT-PCR) using mt1 and MT2 receptor gene-specific primers. mt1 receptor cDNA was amplified and confirmed by nucleotide sequencing. These findings indicate that mt1 receptors were present in the rat uterine endometrium, and suggest that melatonin plays an integral part in uterine physiology.

Characterization of membrane melatonin receptor in mouse peritoneal macrophages: inhibition of adenylyl cyclase by a pertussis toxin-sensitive G protein

Melatonin binding sites were characterized in mouse peritoneal macrophages. Binding of 2-[125I]melatonin by macrophages fulfills all criteria for binding to a receptor site. Thus, binding was dependent on time, temperature and cell concentration, stable, reversible, saturable and specific. Stoichiometric studies showed a high-affinity binding site with a Kd of 0.58-0.71 nM. These data are in close agreement with data obtained from kinetic studies (Kd = 0.29 nM). The affinity of these binding sites suggests that they may recognize the physiological concentrations of melatonin in serum. Moreover, binding experiments using macrophage crude membranes showed that melatonin bound specifically to the membranes. Additionally, in competition studies we observed a low-affinity binding site (Kd = 2.02 microM). Melatonin inhibited significantly forskolin-stimulated cyclic AMP accumulation in a dose-dependent manner. This effect was blocked by luzindole, an antagonist of the melatonin membrane receptor. Pretreatment of macrophages with pertussis toxin blocked the inhibitory effect of melatonin. Pertussis toxin ADP-rybosilation and Western blot experiments demonstrated both alpha(i1/2) and alpha(i3/o) G protein subunits expression in mouse peritoneal macrophages membranes. Our results demonstrate the existence of melatonin receptors in mouse peritoneal macrophages, and a pertussis toxin-sensitive melatonin signal transduction pathway that involves the inhibition of adenylyl cyclase.

The effect of melatonin chronic treatment upon macrophage and lymphocyte metabolism and function in Walker-256 tumour-bearing rats

Melatonin is the main hormone involved in the neuroendocrine-immune axis. It also presents antitumour activity. To evaluate the role of melatonin on the progression of Walker-256 tumour in rats we determined the effect of the hormone on some biochemical and functional aspects of macrophage and lymphocytes from cachectic rats. An important finding observed in immune cells from tumour-bearing (TB) rats is the impairment on glutamine and glucose metabolism in such cells. These changes are very similar to those observed in pinealectomized rats (PNX). The increased production of lactate and the flux of glucose through the Krebs cycle and the reduction in glutamine consumption seems to be involved in the immunosuppression presented in the TB and PNX animals. Melatonin treatment restored the changes observed in the metabolism of glucose and glutamine and stimulated the proliferation of lymphocytes from tumour-bearing rats. The results indicate that the effect of melatonin upon tumour growth involves the stimulation of the immune system by the hormone.

Role of melatonin in mediating seasonal energetic and immunologic adaptations

Winter is energetically demanding and stressful; thermoregulatory demands increase when food availability usually decreases. Physiological and behavioral adaptations, including termination of breeding, have evolved among nontropical animals to cope with the energy shortages during winter. Presumably, selection for the mechanisms that permit physiological and behavioral anticipation of seasonal ambient changes have led to current seasonal breeding patterns for many populations. In addition to the well-studied seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among field populations of mammals and birds. Energetically challenging winter conditions can directly induce death via hypothermia, starvation, or shock; surviving these demanding conditions likely puts individuals under great physiological stress. The stress of coping with energetically demanding conditions may increase adrenocortical steroid levels that could indirectly cause illness and death by compromising immune function. Individuals would enjoy a survival advantage if seasonally recurring stressors could be anticipated and countered by bolstering immune function. The primary environmental cue that permits physiological anticipation of season is daily photoperiod, a cue that is mediated by melatonin. However, other environmental factors may interact with photoperiod to affect immune function and disease processes. Immune function is compromised during the winter in field studies of birds and mammals. However, laboratory studies of seasonal changes in mammalian immunity consistently report that immune function is enhanced in short day lengths. To resolve this apparent discrepancy, we hypothesize that winter stressors present in field studies counteract short-day enhancement of immune function. Prolonged melatonin treatment mimics short days, and also enhances rodent immune function. Reproductive responsiveness to melatonin appears to affect immune function. In sum, melatonin may be part of an integrative system to coordinate reproductive, immunologic, and other physiological processes to cope successfully with energetic stressors during winter.

Investigation into the contractile response of melatonin in the guinea-pig isolated proximal colon: the role of 5-HT4 and melatonin receptors

1. The interaction of melatonin (N-acetyl-5-methoxytryptamine) with 5-hydroxytryptamine4 (5-HT4) receptors and/or with melatonin receptors (ML1, ML2 sites) has been assessed in isolated strips of the guinea-pig proximal colon. In the same preparation, the pharmacological profile of a series of melatonin agonists (2-iodomelatonin, 6-chloromelatonin, N-acetyl-5-hydroxytryptamine (N-acetyl-5-HT), 5-methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT)) was investigated. 2. In the presence of 5-HT1/2/3 receptor blockade with methysergide (1 microM) and ondansetron (10 microM), melatonin (0.1 nM-10 microM), 5-HT (1 nM-1 microM) and the 5-HT4 receptor agonist, 5-methoxytryptamine (5-MeOT: 1 nM-1 microM) caused concentration-dependent contractile responses. 5-HT and 5-MeOT acted as full agonists with a potency (-log EC50) of 7.8 and 8.0, respectively. The potency value for melatonin was 8.7, but its maximum effect was only 58% of that elicited by 5-HT. 3. Melatonin responses were resistant to atropine (0.1 microM), tetrodotoxin (0.3 microM), and to blockade of 5-HT4 receptors by SDZ 205,557 (0.3 microM) and GR 125487 (3, 30 and 300 nM). The latter antagonist (3 nM) inhibited 5-HT-induced contractions with an apparent pA2 value of 9.6 GR 125487 antagonism was associated with 30% reduction of the 5-HT response maximum. Contractions elicited by 5-HT were not modified when melatonin (1 and 10 nM) was used as an antagonist. 4. Like melatonin, the four melatonin analogues concentration-dependently contracted colonic strips. The rank order of agonist potency was: 2-iodomelatonin (10.8) > 6-chloromelatonin (9.9) > or = N-acetyl-5-HT (9.8) > or = 5-MCA-NAT (9.6) > melatonin (8.7), an order typical for ML2 sites. In comparison with the other agonists, 5-MCA-NAT had the highest intrinsic activity. 5. The melatonin ML1B receptor antagonist luzindole (0.3, 1 and 3 microM) had no effect on the concentration-response curve to melatonin. Prazosin, an alpha-adrenoceptor antagonist possessing moderate/ high affinity for melatonin ML2 sites did not affect melatonin-induced contractions at 0.1 microM. Higher prazosin concentrations (0.3 and 1 microM) caused a non-concentration-dependent depression of the maximal response to melatonin without changing its potency. Prazosin (0.1 and 1 microM) showed a similar depressant behaviour towards the contractile responses to 5-MCA-NAT. 6. In the guinea-pig proximal colon, melatonin despite some structural similarity with the 5-HT4 receptor agonist 5-MeOT, does not interact with 5-HT4 receptors (or with 5-HT1/2/3 receptors). As indicated by the rank order of agonist potencies and by the inefficacy of luzindole, the most likely sites of action of melatonin are postjunctional ML2 receptors. However, this assumption could not be corroborated with the use of prazosin as this 'ML2 receptor antagonist' showed only a non-concentration-dependent depression of the maximal contractile response to both melatonin and 5-MCA-NAT. Further investigation with the use of truly selective antagonists at melatonin ML2 receptors is required to clarify this issue.

Effect of pinealectomy, superior cervical ganglionectomy, or melatonin treatment on 24-hour rhythms in ornithine decarboxylase and tyrosine hydroxylase activities of rat spleen

Diurnal variations in splenic ornithine decarboxylase and tyrosine hydroxylase activities were examined in rats subjected to pinealectomy, bilateral superior cervical ganglionectomy, or their respective sham operations. Rats were treated with Freund's complete adjuvant or its vehicle 2 days before sacrifice. After immunization, splenic ornithine decarboxylase activity was augmented 5-6-fold. In both immunized and nonimmunized sham-operated rats, significant diurnal variations in ornithine decarboxylase activity were detectable, with a maximum at early morning, acrophases after Cosinor analysis varying from 0845 to 1048h. In pinealectomized or superior cervical ganglionectomized, immunized rats, ornithine decarboxylase activity attained values 22-27% lower than those of immunized sham-operated controls, while amplitude decreased significantly by 27-30%. Administration of melatonin (30 microg/animal s.c. at late evening for 11 days in immunized rats) significantly augmented mesor levels of splenic ornithine decarboxylase activity and increased the amplitude of the diurnal rhythm both in pinealectomized and in superior cervical ganglionectomized rats. Melatonin treatment also augmented rhythm mesor in immunized, sham-ganglionectomized rats, as well as rhythm amplitude in immunized and nonimmunized, sham-ganglionectomized rats. Splenic tyrosine hydroxylase activity attained its maximum at late afternoon and early night, with acrophases varying from 1800 to 2023h. Immunization significantly increased mesor values of splenic tyrosine hydroxylase activity, whereas neither pinealectomy nor superior cervical ganglionectomy affected circadian rhythm parameters. Melatonin treatment augmented mesor values of tyrosine hydroxylase rhythm and increased its amplitude in pinealectomized, ganglionectomized, or sham-operated rats. The results are compatible with the view that the pineal gland plays a role in circadian changes of immune responsiveness in rat spleen via an immunopotentiating effect of melatonin on splenic cell proliferation.

Expression of the melatonin receptor in Xenopus laevis: a comparative study between protein and mRNA distribution

Reverse transcription-polymerase chain reaction (RT-PCR) analysis performed on total RNA from different tissues of Xenopus laevis showed that the melatonin receptor gene cloned from dermal melanophores is expressed in the whole brain, skin, and retina, and that apart from the ovary, there is no expression in tissues having origin outside the central nervous system. Comparative studies using in vitro autoradiography and in situ hybridization demonstrated that the melatonin receptor is expressed with discrete allocation in Xenopus brain. Though the distribution pattern of the specific messenger RNA conforms well with that of the corresponding receptor protein, it is not always coincident.

Melatonin and 2[125I]iodomelatonin binding sites in the human colon

2[125I]Iodomelatonin binding sites were identified in the mucosa of the human colon from Chinese patients with carcinoma of the rectum or colon using biochemical receptor assay and autoradiography. Melatonin in the colonic mucosa/submucosa and muscle layers were quantitated by radioimmunoassay. The binding of 2[125I]iodomelatonin to the membrane preparations of the human colonic mucosa/submucosa was stable, saturable, reversible and of high affinity. Rosenthal analysis from saturation studies performed at 21 degrees C yielded an equilibrium dissociation constant (Kd) of 61.7 +/- 4.48 pmol/L (n = 3) and maximum number of binding sites (B(max)) of 1.65 +/- 0.51 fmol/mg protein (n = 3). The linearity of the Rosenthal plots and unity of the Hill coefficient suggested that 2[125I]iodomelatonin was bound to a single class of binding sites. The radioligand binding was displaced by 2-iodomelatonin (Ki = 0.02 nmol/L), melatonin (0.65 nmol/L), 6-chloromelatonin (Ki = 5.33 nmol/L), 6-hydroxymelatonin (Ki = 33.8 nmol/L) and N-acetylserotonin (Ki = 122 nmol/L). The characteristic of the binding sites were similar to those reported in the jejunum of duck, chicken, and human but of higher affinity than those in the mouse colon. Autoradiography localizes the binding to the mucosa of the human colon. Radioimmunoassay revealed a melatonin concentration of 467 +/- 99 pg/g wet tissue of human colon (n = 6). Our findings suggest that melatonin may influence the human colonic functions through interaction with its receptors in the mucosa.

Specific binding of 2-[125I]iodomelatonin by rat spleen crude membranes: day-night variations and effect of pinealectomy and continuous light exposure

Melatonin binding sites were characterized in rat spleen crude membranes. The specific binding of 2-[125I]iodomelatonin by spleen crude membranes fulfills all the criteria for binding to a receptor site. Thus, binding was dependent on time and temperature, stable, specific, and increased under constant light exposure and after pinealectomy. In competition studies, the specific binding of 2-[125I]iodomelatonin to spleen crude membranes was inhibited by increasing concentrations of native melatonin. Scatchard analysis showed that the data were compatible with the existence of two classes of binding sites: a high affinity site with a Kd of 0.53 nM and a binding capacity of 2.52 pM, and a low-affinity site with a Kd of 374 nM and binding capacity of 820 pM. Moreover, binding of 2-[125I]iodomelatonin exhibited day-night variations with the highest binding observed late during the light period, and the lowest binding was observed late at night. However, binding of 2-[125I]iodomelatonin to membranes remained high when animals were kept under light exposure at night. Results support the hypothesis of a regulatory role of melatonin on the immune system in which melatonin downregulates its own binding site.

The influence of season, photoperiod, and pineal melatonin on immune function

In addition to the well-documented seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among many animal populations. Challenging winter conditions (i.e., low ambient temperature and decreased food availability) can directly induce death via hypothermia, starvation, or shock. Coping with these challenges can also indirectly increase morbidity and mortality by increasing glucocorticoid secretion, which can compromise immune function. Many environmental challenges are recurrent and thus predictable; animals could enhance survival, and presumably increase fitness, if they could anticipate immunologically challenging conditions in order to cope with these seasonal threats to health. The annual cycle of changing photoperiod provides an accurate indicator of time of year and thus allows immunological adjustments prior to the deterioration of conditions. Pineal melatonin codes day length information. Short day lengths enhance several aspects of immune function in laboratory studies, and melatonin appears to mediate many of the enhanced immunological effects of photoperiod. Generally, field studies report compromised immune function during the short days of autumn and winter. The conflict between laboratory and field data is addressed with a multifactor approach. The evidence for seasonal fluctuations in lymphatic tissue size and structure, as well as immune function and disease processes, is reviewed. The role of pineal melatonin and the hormones regulated by melatonin is discussed from an evolutionary and adaptive functional perspective. Finally, the clinically significance of seasonal fluctuations in immune function is presented. Taken together, it appears that seasonal fluctuations in immune parameters, mediated by melatonin, could have profound effects on the etiology and progression of diseases in humans and nonhuman animals. An adaptive functional perspective is critical to gain insights into the interaction among melatonin, immune function, and disease processes.

Sensitivity of human peripheral blood mononuclear leukocytes to visible light

Overnight light exposure of cultured human peripheral blood mononuclear leukocytes [PBML], significantly increased basal [3H]thymidine incorporation and upon stimulation with phytohemagglutinin [PHA]. Melatonin (10(-9) to 10(-5) M) enhanced the light-induced increase of [3H]thymidine incorporation, while serotonin (10(-9) to 10(-7) M) stimulated [3H]thymidine incorporation in the dark. The wavelengths responsible of this effect were restricted to the blue-green zone of the spectrum. The stimulatory effect of visible light on PHA-induced DNA replication had a circannual rhythm, being maximal during winter. In winter, white light also reduced melatonin and serotonin binding to PBML membranes and switched the PBML indole metabolism towards serotonin and 5-hydroxy-indole-acetic acid [HIAA] synthesis, with a concomitant decrease of melatonin production.

Immunomodulatory role of melatonin: specific binding sites in human and rodent lymphoid cells

This paper reviews the evidence that supports the hypothesis of the existence of specific binding sites for melatonin on immune cells. These binding sites have been described in human blood lymphocytes and granulocytes, and thymus, spleen, and bursa of Fabricius from different rodents and birds. The dissociation constant values of these binding sites are in the 0.1-1 nM range, suggesting that melatonin may play a physiological role in lymphocyte regulation. Moreover, melatonin binding sites appear to be modulated by guanine nucleotides. Therefore, in addition to other mechanisms described for the regulation of immune function by melatonin, a direct mechanism of regulation can be involved via binding of melatonin by immunocompetent cells.

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 (Kd) for the single site was 0.34 nM with a binding capacity of 2.25 fmol/10(7) 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.

Differential effects of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) on the 2-[125I]iodomelatonin binding sites in the chicken bursa of Fabricius and spleen

Effects of 10 and 50 mumol/l guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) on 2-[125I]iodomelatonin binding in the chicken bursa of Fabricius and spleen were tested. In the chicken bursa of Fabricius, GTP gamma S increased the Kd of 2-[125I]iodomelatonin binding sites without affecting the Bmax value. In contrast, GTP gamma S caused both an increase in Kd and a reduction in Bmax of 2-[125I]iodomelatonin binding sites in the chicken spleen. Our results suggest the existence of subtypes of melatonin receptors with different receptor-G protein-effector complexes in the avian primary and secondary lymphoid systems.

Modulation of sympathetic neurotransmission by melatonin

Melatonin is of considerable interest for its regulatory influence on a variety of physiological processes including biological rhythms and neuroendocrine functions. We showed that melatonin potentiates sympathetic neurotransmission in the prostatic portion of the rat vas deferens, by increasing contractions in response to noradrenaline and ATP released by acetylcholine stimulation of presynaptic nicotinic receptors. Melatonin in vitro (100 pg/ml; for 4 h) increased the maximal acetylcholine-induced contraction only when the hypogastric ganglion was present, and this effect was blocked by cycloheximide (100 micrograms/ml). Melatonin also modulated the sympathetic trophic influence on smooth muscle, since it reduced [35S]methionine incorporation into the vas deferens in the hypogastric ganglion-vas deferens preparation. Thus, it is suggested that the regulation of protein synthesis might be one of the mechanisms whereby melatonin modulates endogenous rhythms and synchronizes them to the environmental light cycle.

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

High-affinity 2-125I-iodomelatonin binding sites in homogenates of purified cell nuclei from rat liver were localized and characterized using biochemical binding techniques. Binding at these sites was found to be rapid, reversible, saturable, and to demonstrate pharmacological selectivity. At 0 degrees C, binding reached equilibrium in about 10 min. Scatchard analysis of the data at equilibrium revealed a single class of binding sites with a dissociation constant of KD = 190 +/- 47 pM, Bmax = 9.8 +/- 0.6 fmol/mg protein, and a Hill coefficient of nH = 1.02 +/- 0.034. Kinetic analysis of the association and dissociation curves indicated a kinetic KD = 148 +/- 41 pM, which is in good agreement with the value obtained at equilibrium. The specific binding of 2-125I-iodomelatonin (45 pM) (0.51 +/- 0.04 fmol/mg protein) was significantly improved (0.79 +/- 0.04 fmol/mg protein) when the homogenates of purified liver cell nuclei were preincubated with DNase (2 micrograms/ml at 37 degrees C for 20 min) before being used in binding experiments. After the addition of either proteinase K or trichloroacetic acid to DNase-treated purified cell nuclear homogenates, the specific binding disappeared. This suggests that the specific binding of 2-125I-iodomelatonin in liver cell nuclei is associated with nuclear protein. Competition experiments show that N-acetyl-serotonin (Ki = 81.3 nM) was more potent than 5-hydroxytryptamine (Ki > 1 microM) and 5-methoxytryptamine (Ki >> 10 microM) in inhibiting 2-125I-iodomelatonin binding (Ki melatonin = 146 pM). These data indicate that specific 2-125I-iodomelatonin binding sites exist in the cell nuclei of rat liver, and that they may comprise a locus for the intracellular action of melatonin. The correlation between the KD and Bmax values with melatonin concentrations in nuclei suggest that these binding sites may be a physiological melatonin receptor, which could explain the described genomic effects of the pineal hormone.

Specific 2-[125I]iodomelatonin binding sites in the duck adrenal gland

Using 2-[125I]iodomelatonin as the radioligand, putative melatonin receptors in the duck adrenal gland were investigated. 2-[125I]Iodomelatonin binding to the membrane preparations of duck adrenals collected at mid-light was specific, rapid, saturable, stable, reversible and of high affinity. Scatchard analyses showed one class of binding sites with an equilibrium dissociation constant of 27.4 +/- 2.9 pmol/l and a maximum number of binding sites of 3.38 +/- 0.26 fmol/mg protein. Binding of 2-[125I]iodomelatonin in different subcellular fractions demonstrated the following descending order of density: mitochondrial > nuclear > microsomal >>> cytosol. Pharmacological studies indicated that these sites were highly specific to melatonin. As 2-[125I]iodomelatonin is a specific agonist of melatonin, it is proposed that the sites studied are adrenal melatonin receptors.

Modulation of 2[125I]iodomelatonin binding sites in the guinea pig spleen by melatonin injection is dependent on the dose and period but not the time

Effects of dose, time and period of melatonin injection on 2[125I]iodomelatonin binding sites in the guinea pig spleen were studied. Guinea pigs (Dunkin-hartley), kept under 12h light/12 h darkness, were given daily intraperitoneal injections of either vehicle or 0.01, 0.1 or 1 mg melatonin/kg body weight in either early (1 hour after onset of light period) or late light period (1 hour before offset of light period) for 2 or 7 days. To study the effect of opioid antagonist on the binding, intraperitoneal injections of 2 or 20 mg naltrexone/kg body weight alone or together with 0.1 mg melatonin/kg body weight was given daily in late light period for 2 days. 2[125I]Iodomelatonin binding assays were performed on spleen membrane preparations and radioimmunoassays of melatonin levels were carried out in serum and pineal glands collected during mid-light. High dose (1 mg/kg body weight) of melatonin injection elevated the mid-light serum melatonin levels without affecting pineal melatonin levels. Early light injection group had a higher mid-light serum melatonin level. Melatonin injection for 2 days at either time points caused a dose-dependent decrease in Bmax and increase in Kd of 2[125I]iodomelatonin binding sites in the spleen. The response was independent of the time of injection. A greater suppression of binding was achieved by injecting melatonin for 7 days. Naltrexone did not affect the binding by itself and was not able to reverse the melatonin-induced suppression of binding in the spleen. The modulation of the splenic 2[125I]iodomelatonin binding sites by exogenous melatonin suggests that melatonin may act directly on the immune system to affect its function.

G-protein linked melatonin binding sites in the chicken lung

[125I]Iodomelatonin binding was first demonstrated in the chicken lung membrane preparations. The binding was saturable, reversible, rapid, and of high affinity. The sites of binding distributed widely in different subcellular fractions except the cytosol fraction. Scatchard plots are linear indicating a dissociation constant (Kd) of 8.11 +/- 0.73 pmol/l and a maximum number of binding sites of 1.29 +/- 0.16 fmol/mg protein (n = 9). The Kd estimated from the kinetic study was 11.2 pmol/l. Guanosine 5'-O-(3-thiotriphosphate) significantly increased the Kd values while the density of binding sites was not affected indicating that the binding sites may be linked to a guanine nucleotide binding protein.