Melatonin-induced enhancement of antibody-dependent cellular cytotoxicity

Melatonin-A Potent Therapeutic for Stroke and Stroke-Related Dementia

Secreted by the pineal gland to regulate the circadian rhythm, melatonin is a powerful antioxidant that has been used to combat oxidative stress in the central nervous system. Melatonin-based therapies have been shown to provide neuroprotective effects in the setting of ischemic stroke by mitigating neuroinflammation and accelerating brain tissue restoration. Melatonin treatment includes injection of exogenous melatonin, pineal gland grafting and melatonin-mediated stem cell therapy. This review will discuss the current preclinical and clinical studies investigating melatonin-based therapeutics to treat stroke.

T. cruzi infection among aged rats: Melatonin as a promising therapeutic molecule

Although T. cruzi was identified as the cause of Chagas disease more than 100 years ago, satisfactory treatments still do not exist, especially for chronic disease. Here we review work suggesting that melatonin could have promise as a Chagas therapeutic. Melatonin has remarkably diverse actions. It is an immunomodulator, an anti-inflammatory, an antioxidant, a free radical scavenger, and has antiapoptotic and anti-aging effects. The elderly (aged 60 years or more) as a group are growing faster than any other age group. Here we discuss the major effects and the mechanisms of action of melatonin on aged T. cruzi-infected rats. Melatonin's protective effects may be consequences of its cooperative antioxidant and immunomodulatory actions. Melatonin modulates oxidative damage, inducing an antioxidant response and reversing age-related thymus regression. Its protective actions could be the result of its anti-apoptotic activity, and by its counteracting the excessive production of corticosterone. This review describes our work showing that host age plays an important and variable influence on the progression of systemic T. cruzi infection and supporting the hypothesis that melatonin should be considered as a powerful therapeutic compound with multiple activities that can improve host homeostasis during experimental T. cruzi infection.

Melatonin: Antioxidant and modulatory properties in age-related changes during Trypanosoma cruzi infection

The purpose of this study was to investigate the effects of melatonin on selected biomarkers of innate and humoral immune response as well as the antioxidant/oxidant status (superoxide dismutase-SOD and reduced glutathione levels (GSH) to understand whether age-related changes would influence the development of acute Trypanosoma cruzi (T. cruzi) infection. Young- (5 weeks) and middle-aged (18 months) Wistar rats were orally treated with melatonin (gavage) (05 mg/kg/day), 9 days after infection. A significant increase in both SOD activity and GSH levels was found in plasma from all middle-aged melatonin-treated animals. Melatonin triggered enhanced expression of major histocompatibility class II (MHC-II) antigens on antigen-presenting cell (APC) and peritoneal macrophages in all treated animals. High levels of CD4⁺ CD28-negative T cells (*P<.05) were detected in middle-aged control animals. Melatonin induced a significant reduction (***P<.001) in CD28-negative in CD4⁺ and CD8⁺ T cells in middle-aged control animals. Contrarily, the same group displayed upregulated CD4⁺ CD28⁺ T and CD8⁺ CD28⁺ T cells. Melatonin also triggered an upregulation of CD80 and CD86 expression in all young-treated groups. Significant percentages of B and spleen dendritic cells in middle-aged infected and treated animals were observed. Our data reveal new features of melatonin action in inhibiting membrane lipid peroxidation, through the reduction in 8-isoprostane, upregulating the antioxidant defenses and triggering an effective balance in the antioxidant/oxidant status during acute infection. The ability of melatonin to counteract the immune alterations induced by aging added further support to its use as a potential therapeutic target not only for T. cruzi infection but also for other immunocompromised states.

Evaluation of melatonin levels in saliva in gingivitis and periodontitis cases: A pilot study

Objective:

To evaluate the melatonin levels in saliva in gingivitis and periodontitis cases.

Background:

Melatonin has strong antioxidant, free radical scavenging, and immunomodulating properties, acts on osteoblasts directly to stimulate cell proliferation and synthesis of Type I collagen, and promotes bone formation.

Materials and Methods:

A total of thirty participants were selected and divided into three groups (control group, gingivitis group, and periodontitis group). In each group, ten participants were taken. Salivary melatonin was estimated in each of the three groups.

Results:

Results from this study showed that the mean levels of salivary melatonin increased as severity increased from control to periodontitis, i.e., the mean levels were highest in periodontitis followed by gingivitis and least in control group. The melatonin level of all participants was positively and significantly (P < 0.01) correlated with their gingival index (r = 0.85, P < 0.01) and probing depth (r = 0.72, P < 0.01).

Conclusion:

Salivary melatonin level varied with the severity of gingivitis and periodontitis. With increased severity of periodontal disease, the level of salivary melatonin also increased suggesting that salivary melatonin may act as a diagnostic biomarker for periodontal diseases.

Melatonin as an Antioxidant for Stroke Neuroprotection

Melatonin (N-acetyl-5-methoxytryptamine) is a hormone derived from the pineal gland that has a wide range of clinical applications. While melatonin was originally assessed as a hormone specializing in regulation of the normal circadian rhythm in mammals, it now has been shown to be an effective free radical scavenger and antioxidant. Current research has focused on central nervous system (CNS) disorders, stroke in particular, for potential melatonin-based therapeutics. As of now, the realm of potential therapy regimens is focused on three main treatments: exogenously delivered melatonin, pineal gland grafting, and melatonin-mediated stem cell therapy. All therapies contain both costs and benefits, and current research is still focused on finding the best treatment plan. While comprehensive research has been conducted, more research regarding the safety of such therapies is needed in order to transition into the clinical level of testing. Antioxidants such as traditional Chinese medicine, (-)-epigallocatechin-3-gallate (EGCG), and lavender oil, which have been used for thousands of years as treatment, are now gaining recognition as effective melatonin treatment alternatives. This review will further discuss relevant studies assessing melatonin-based therapeutics and provide evidence of other natural melatonin treatment alternatives for the treatment of stroke.

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.

Blunted epidermal L-tryptophan metabolism in vitiligo affects immune response and ROS scavenging by Fenton chemistry, part 1: Epidermal H2O2/ONOO(-)-mediated stress abrogates tryptophan hydroxylase and dopa decarboxylase activities, leading to low serotonin and melatonin levels

Vitiligo is characterized by a progressive loss of inherited skin color. The cause of the disease is still unknown. To date, there is accumulating in vivo and in vitro evidence for massive oxidative stress via hydrogen peroxide (H(2)O(2)) and peroxynitrite (ONOO(-)) in the skin of affected individuals. Autoimmune etiology is the favored theory. Since depletion of the essential amino acid L-tryptophan (Trp) affects immune response mechanisms, we here looked at epidermal Trp metabolism via tryptophan hydroxylase (TPH) with its downstream cascade, including serotonin and melatonin. Our in situ immunofluorescence and Western blot data reveal significantly lower TPH1 expression in patients with vitiligo. Expression is also low in melanocytes and keratinocytes under in vitro conditions. Although in vivo Fourier transform-Raman spectroscopy proves the presence of 5-hydroxytryptophan, epidermal TPH activity is completely absent. Regulation of TPH via microphthalmia-associated transcription factor and L-type calcium channels is severely affected. Moreover, dopa decarboxylase (DDC) expression is significantly lower, in association with decreased serotonin and melatonin levels. Computer simulation supports H(2)O(2)/ONOO(-)-mediated oxidation/nitration of TPH1 and DDC, affecting, in turn, enzyme functionality. Taken together, our data point to depletion of epidermal Trp by Fenton chemistry and exclude melatonin as a relevant contributor to epidermal redox balance and immune response in vitiligo.

Sustained melatonin treatment blocks body mass, pelage, reproductive, and fever responses to short day lengths in female Siberian hamsters

Winter imposes physiological challenges on individuals including increased thermoregulatory demands, risk of infection, and decreased food availability. To survive these challenges, animals living outside the tropics must appropriately distribute their energetic costs across the year, including reproduction and immune function. Individuals of many species use the annual cycle of changing day lengths (photoperiod), which is encoded by the nightly duration of melatonin secretion, to adjust physiology. Siberian hamsters exposed to short days (SD) (long nights/prolonged endogenous melatonin secretion) enhance some aspects of immune function, but curtail other energetically expensive immune functions including the febrile response. The purpose of this study was twofold. First, we determined whether sustained melatonin treatment would inhibit the development of the SD phenotype in female hamsters as it does in males. Second, we examined whether the SD attenuation of fever would be blocked by continuous exposure to exogenous melatonin. Hamsters were implanted with melatonin or empty capsules, housed in either long days (LD) or SD for 8-9 weeks, and then challenged with lipopolysaccharide; body temperature and locomotor activity were recorded. Unlike hamsters with empty capsules, hamsters with melatonin implants did not respond to SD and maintained a LD phenotype including summer-like spleen, uterine and body masses, and pelage characteristics. Further, sustained melatonin treatment blocked the SD attenuation of febrile responses and prolonged the behavioral components of the sickness response. These results suggest that the daily fluctuations in endogenous melatonin may be masked by continuous exposure to exogenous melatonin, thus inhibiting functional photoperiodic responses to SD.

Melatonin reduces the severity of experimental amoebiasis

BACKGROUND

Melatonin has immunomodulatory effects but very little is known about its influence in protozoan infections, such as Entamoeba histolytica, which causes amoebiasis, a disease with significant morbidity and mortality. In this study, we evaluated the effects of exogenous melatonin interference in experimental amoebiasis and on interactions between human blood cells and E. histolytica trophozoites.

METHODS

The effect of melatonin was investigated in models of experimental amoebiasis in hamsters and rats by evaluating the area of necrosis induced by E. histolytica. The activity of melatonin on the interactions between leukocytes and amoebae was determined by examining leukophagocytosis. For in vitro tests, polymorphonuclear and mononuclear human blood leucocytes were incubated with E. histolytica trophozoites.

RESULTS

The areas of amoebic necrosis were significantly reduced in animals treated with melatonin. Melatonin treatment increased leukophagocytosis but was associated with a greater number of dead amoebae.

CONCLUSIONS

These results suggest that melatonin may play a beneficial role in the control of amoebic lesions, raising the possibility that this drug may be used as an adjuvant in anti-amoebic therapy.

The role of melatonin in immuno-enhancement: potential application in cancer

Melatonin, a neurohormone produced mainly by the pineal gland, is a modulator of haemopoiesis and of immune cell production and function, both in vivo and in vitro. Physiologically, melatonin is associated with T-helper 1 (Th1) cytokines, and its administration favours Th1 priming. In both normal and leukaemic mice, melatonin administration results in quantitative and functional enhancement of natural killer (NK) cells, whose role is to mediate defenses against virus-infected and cancer cells. Melatonin appears to regulate cell dynamics, including the proliferative and maturational stages of virtually all haemopoietic and immune cells lineages involved in host defense - not only NK cells but also T and B lymphocytes, granulocytes and monocytes - in both bone marrow and tissues. In particular, melatonin is a powerful antiapoptotic signal promoting the survival of normal granulocytes and B lymphocytes. In mice bearing mid-stage leukaemia, daily administration of melatonin results in a survival index of 30-40% vs. 0% in untreated mice. Thus, melatonin seems to have a fundamental role as a system regulator in haemopoiesis and immuno-enhancement, appears to be closely involved in several fundamental aspects of host defense and has the potential to be useful as an adjuvant tumour immunotherapeutic agent.

Melatonin synthesized by Jurkat human leukemic T cell line is implicated in IL-2 production

Human lymphocytes have recently been described as an important physiological source of melatonin (N-acetyl-5-methoxytryptamine), which could be involved in the regulation of the human immune system. On the other hand, stimulation of IL-2 production by exogenous melatonin has been shown in the Jurkat human lymphocytic cell line. Furthermore, both melatonin membrane and nuclear receptors are present in these cells. In this study, we show that the necessary machinery to synthesize melatonin is present and active in resting and stimulated Jurkat cells. Accordingly, we have found that cells synthesize and release melatonin in both conditions. Therefore, we investigated whether endogenous melatonin produced by Jurkat cells was involved in the regulation of IL-2 production. When melatonin membrane and nuclear receptors were blocked using specific antagonists, luzindole and CGP 55644, respectively, we found that IL-2 production decreased, and this drop was reverted by exogenous melatonin. Additionally, PHA activation of Jurkat cells changed the profile of melatonin nuclear receptor mRNA expression. A previous study showed that exogenous melatonin is able to counteract the decrease in IL-2 production caused by prostaglandin E2 (PGE2) in human lymphocytes via its membrane receptor. In our model, when we blocked the melatonin membrane receptor with luzindole, the inhibitory effect of PGE2 on IL-2 production was higher. Therefore, we have demonstrated the physiological role of endogenous melatonin in this cell line. These findings indicate that endogenous melatonin synthesized by human T cells would contribute to regulation of its own IL-2 production, acting as an intracrine, autocrine, and/or paracrine substance.

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.

Human lymphocyte-synthesized melatonin is involved in the regulation of the interleukin-2/interleukin-2 receptor system

Since melatonin was first isolated in 1958 up to the last few years, this substance was considered a hormone exclusive to the pineal gland. Although melatonin has lately been identified in a large number of extrapineal sites, its potential biological actions have not yet been studied. This paper shows that human lymphocyte-synthesized melatonin plays a crucial role modulating IL-2/IL-2 receptor system because when blocking melatonin biosynthesis by the tryptophan hydroxylase inhibitor, parachlorophenylalanine, both IL-2 and IL-2 receptor levels fell, restoring them by adding exogenous melatonin. Moreover, we demonstrated that this endogenous melatonin interfered with the exogenous melatonin effect on IL-2 production. Melatonin exerted these effects by a receptor-mediated action mechanism because both IL-2 and IL-2 receptor expressions significantly decreased when lymphocytes were incubated in the presence of the specific membrane and/or nuclear melatonin receptor antagonists, luzindole, and/or CGP 55644, respectively. Finally, we made the real significance of the membrane melatonin receptors in this process clear, so prostaglandin E(2)-induced inhibition on IL-2 production increased when we blocked the membrane receptors using luzindole. In conclusion, these data show that endogenous melatonin is an essential part for an accurate response of human lymphocytes through the modulation of IL-2/IL-2 receptor system.

Evidence of melatonin synthesis by human lymphocytes and its physiological significance: possible role as intracrine, autocrine, and/or paracrine substance

It has been historically assumed that the pineal gland is the major source of melatonin (N-acetyl-5-methoxytryptamine) in vertebrates. Melatonin plays a central role in fine-tuning circadian rhythms in vertebrate physiology. In addition, melatonin shows a remarkable functional versatility exhibiting antioxidant, oncostatic, antiaging, and immunomodulatory properties. Melatonin has been identified in a wide range of organisms from bacteria to human beings. Its biosynthesis from tryptophan involves four well-defined intracellular steps catalyzed by tryptophan hydroxylase, aromatic amino acid decarboxylase, serotonin-N-acetyltransferase, and hydroxyindole-O-methyltransferase. Here, for the first time, we document that both resting and phytohemagglutinin-stimulated human lymphocytes synthesize and release large amounts of melatonin, with the melatonin concentration in the medium increasing up to five times the nocturnal physiological levels in human serum. Moreover, we show that the necessary machinery to synthesize melatonin is present in human lymphocytes. Furthermore, melatonin released to the culture medium is synthesized in the cells, because blocking the enzymes required for its biosynthesis or inhibiting protein synthesis in general produced a significant reduction in melatonin release. Moreover, this inhibition caused a decrease in IL-2 production, which was restored by adding exogenous melatonin. These findings indicate that in addition to pineal gland, human lymphoid cells are an important physiological source of melatonin and that this melatonin could be involved in the regulation of the human immune system, possibly by acting as an intracrine, autocrine, and/or paracrine substance.

Relationship between salivary melatonin levels and periodontal status in diabetic patients

Among other functions, melatonin exerts both antioxidative and immunoregulatory roles. The indoleamine is secreted in the saliva, although its role into the mouth is not known. Diabetic patients frequently display oral cavity pathologies such as periodontal disease (PD), an inflammatory disease coursing with an increase in free radical production. Thus, we compared the degree of PD and interleukin-2 (IL-2) levels with melatonin concentrations in plasma and saliva of diabetic patients. A total of 43 diabetic patients (20 with type I and 23 with type II diabetes) and 20 age- and sex-matched controls were studied. Dental and medical history of all patients was in accordance with the criteria of the WHO. The periodontal status was evaluated by the Community Periodontal Index (CPI). Plasma and salivary melatonin levels were determined by specific commercial radioimmunoassays, and plasma IL-2 was measured using a commercial enzyme-linked immunosorbent assay kit. Diabetic patients had plasma and saliva melatonin levels of 8.98 +/- 7.14 and 2.70 +/- 2.04 pg/mL, respectively. These values were significantly lower (P < 0.001) than those obtained in plasma and saliva of controls (14.91 +/- 4.75 and 4.35 +/- 0.98 pg/mL, respectively). Plasma and salivary melatonin concentrations show a biphasic response in diabetic patients. Melatonin decreased in patients with a CPI index of 2, and then increased reaching highest levels in patients with a CPI index of 4. By contrast, IL-2 levels decreased from CPI index 1 to 4. The results indicate that, in diabetic patients, the presence of a marked impairment of the oral status, as assessed by the CPI index, is accompanied by an increase in plasma and salivary melatonin. The increase in salivary melatonin excretion may have a periodontal protective role.

Melatonin counteracts the inhibitory effect of PGE2 on IL-2 production in human lymphocytes via its mt1 membrane receptor

It is well known that melatonin plays a fundamental role in human neuro-immunomodulation. Thus, melatonin regulates the production of a large number of cytokines, including interleukin-2 (IL-2) in the human system. Both membrane and nuclear receptors for melatonin are present in lymphoid cells. However, most of these effects have been shown to be mediated by the putative nuclear receptor for the neurohormone. In this paper, we show that prostaglandin E2 (PGE2), a potent inflammatory mediator, inhibits IL-2 production in human lymphocytes by a cyclic AMP (cAMP)-dependent mechanism. In this model, melatonin counteracts the effects of PGE2 on IL-2 and cAMP production. We propose that the effect of melatonin is mediated by a membrane receptor, since similar results were obtained when cells were cultured in the presence of S 20098, a specific melatonin membrane receptor agonist. No effect was observed by using CGP 52608, a nuclear receptor agonist. Moreover, when cells were stimulated with phorbol myristate acetate (PMA), which has been shown to inhibit mt1 melatonin membrane receptor expression, the neurohormone failed to counteract the effect of PGE2. Therefore, we postulate, for the first time, a physiological role of the mt1 melatonin membrane receptor in the human immune system.

Melatonin serum levels in rheumatoid arthritis

The pineal hormone melatonin (MLT) exerts a variety of effects on the immune system. MLT activates immune cells and enhances inflammatory cytokine and nitric oxide production. Cytokines are strongly involved in the synovial immune and inflammatory response in rheumatoid arthritis (RA) and reach the peak of concentration in the early morning, when MLT serum level is higher. Nocturnal MLT serum levels were evaluated in 10 RA patients and in 6 healthy controls. Blood samples were obtained at 8 and 12 p.m., as well as at 2, 4, 6, and 8 a.m. MLT serum levels at 8 p.m. and 8 a.m. were found to be higher in RA patients than in controls (p < 0.05). In both RA patients and healthy subjects, MLT progressively increased from 8 p.m. to the first hours of the morning, when the peak level was reached (p < 0.02). However, MLT serum level reached the peak at least two hours before in RA patients than in controls (p < 0.05). Subsequently, in RA patients, MLT concentration showed a plateau level lasting two to three hours, an effect not observed in healthy controls. After 2 a.m., MLT levels decreased similarly in both RA patients and healthy subjects. Several clinical symptoms of RA, such as morning gelling, stiffness, and swelling, which are more evident in the early morning, might be related to the neuroimmunomodulatory effects exerted by MLT on synovitis and might be explained by the imbalance between cortisol serum levels (lower in RA patients) and MLT serum levels (higher in RA patients).

Melatonin: from basic research to cancer treatment clinics

Melatonin, the chief secretory product of the pineal gland, is a direct free radical scavenger, an indirect antioxidant, as well as an important immunomodulatory agent. In both in vitro and in vivo investigations, melatonin protected healthy cells from radiation-induced and chemotherapeutic drug-induced toxicity. Furthermore, several clinical studies have demonstrated the potential of melatonin, either alone or in combination with traditional therapy, to yield a favorable efficacy to toxicity ratio in the treatment of human cancers. This study reviews the literature from laboratory investigations that document the antioxidant and oncostatic actions of melatonin and summarizes the evidence regarding the potential use of melatonin in cancer treatment. This study also provides rationale for the design of larger translational research-based clinical trials.

Melatonin and the enhancement of immune responses in immature male chickens

Understanding the role of melatonin in affecting different physiological functions, especially immune responses, is becoming increasingly important in the basic and applied sciences. Enhancing the immune response will result in increasing disease resistance and, therefore, improve production efficiency. The purpose of the present study was to investigate the effects of melatonin, administered during the light or dark period, on BW, feed consumption (FC), and immune responses of immature chickens. Eight-week-old Cornell White Leghorn males were used in this study. The doses of melatonin were 0, 5, 10, 20, and 40 mg/kg BW. Melatonin was administered s.c. every 24 h for 7 consecutive d. The chicks were randomly divided into two groups; one group received injection during the middle of the light period, and the other group received injection during the middle of the dark period. All birds received 16 h light and 8 h darkness during a 24-h period. Body weights were measured before and after melatonin treatment, and FC was also measured. After the seven injections, blood samples were collected from the brachial vein, and total white blood cell (WBC) counts, differential cell counts, and activities of T and B lymphocytes were measured. Body weight was not significantly affected by dose of melatonin or time of injection. Furthermore, melatonin did not significantly affect FC; however, FC was significantly lower in the group that was injected in the dark vs. light period. The WBC counts of birds injected with 40 mg melatonin/kg BW were significantly higher than the WBC counts of saline-injected birds. The heterophil/lymphocyte (H/L) ratios of birds injected during the light period were significantly higher than those of birds injected during the dark period. T- and B-lymphocyte proliferation were significantly higher in birds injected with 40 mg melatonin/kg BW compared to saline-injected birds. These results indicate that melatonin in vivo is important in enhancing not only circulating WBC but also activities of B and T lymphocytes of immature male chickens without adversely affecting BW.

Effect of melatonin treatment on 24-h variations in responses to mitogens and lymphocyte subset populations in rat submaxillary lymph nodes

Wistar male rats were injected s.c. with melatonin (30 microg) or vehicle, 1 h before lights off, for 11 days. Ten days after beginning melatonin treatment, rats received Freund's complete adjuvant or its vehicle s.c., and after 2 days, they were sacrificed at six different time intervals throughout a 24-h cycle. The mitogenic effect of lipopolysaccharide (LPS) and concanavalin A (Con A), the activity of ornithine decarboxylase (ODC) and the relative size of lymphocyte subset populations were measured in submaxillary lymph nodes. In control rats, the mitogenic effects of LPS and Con A and ODC activity peaked during the afternoon. Injection of Freund's adjuvant induced a 10-h shift in the diurnal rhythm of the mitogenic effect of LPS to attain maximal values at night. Melatonin pretreatment blunted the daily variations in the mitogenic activity of Con A or LPS and, when given to Freund's adjuvant-injected rats, augmented mesor and amplitude of diurnal rhythm in ODC activity. Maxima in B cell number occurred at night whereas those of T and B-T cell number occurred during the afternoon. During the early phase of immunization tested, the number of B cells augmented and the amplitude of its diurnal rhythmicity increased both after immunization and following melatonin pretreatment. Maxima of 24-h rhythms in CD4+ and CD4+/CD8+ cell populations occurred during the afternoon while those of CD8+ cells occurred at late night. Melatonin significantly augmented CD4+ cell number and decreased CD8+ cell number; it therefore augmented the CD4+:CD8+ ratio. The results suggest that pretreatment with a pharmacological dose of melatonin exerts immunomodulating effects at an early, preclinical, phase of Freund's adjuvant-induced arthritis in rats.

MLT and the immune-hematopoietic system

It is now well recognized that a main actor in the continuous interaction between the nervous and immune systems is the pineal hormone MLT. T-helper cells bear G-protein coupled MLT cell membrane receptors and, perhaps, MLT nuclear receptors. Activation of MLT receptors enhances the release of T-helper cell type 1 (Th1) cytokines, such as gamma-interferon and interleukin-2 (IL-2), as well as of novel opioid cytokines which crossreact immunologically with both interleukin-4 (IL-4) and dynorphin B. MLT has been reported also to enhance the production of interleukin-1 (IL-1), interleukin-6 (IL-6) and interleukin-12 (IL-12) in human monocytes. These mediators may counteract stress-induced immunodepression and other secondary immunodeficiencies, protect mice against lethal viral and bacterial diseases, synergize with IL-2 in cancer patients and influence hematopoiesis. In cancer patients, MLT seems to be required for the effectiveness of low dose IL-2 in those neoplasias that are generally resistant to IL-2 alone. Hematopoiesis is apparently influenced by the action of the MLT-induced-opioids (MIO) on kappa-opioid receptors present on stromal bone marrow macrophages. Most interestingly, gamma-interferon and colony stimulating factors (CSFs) may modulate the production of MLT in the pineal gland. A hypothetical pineal-immune-hematopoietic network is, therefore, taking shape. From the immunopharmacological and ethical point of view, clinical studies on the effect of MLT in combination with IL-2 or other cytokines in viral disease including human immunodeficiency virus-infected patients and cancer patients are needed. In conclusion, MLT seems to play a crucial role in the homeostatic interactions between the brain and the immune-hematopoietic system and deserves to be further studied to identify its therapeutic indications and its adverse effects.

Therapeutic potential of melatonin in immunodeficiency states, viral diseases, and cancer

Maintenance of health depends on the ability to respond appropriately to environmental stressors via reciprocal interactions between the body and the brain. In this context, it is well recognized that the pineal hormone melatonin (MLT) plays an important role. T-helper cells bear G-protein-coupled MLT cell membrane receptors and, perhaps, MLT nuclear receptors. Activation of MLT receptors enhances the release of T-helper cell cytokines, such as gamma-interferon and interleukin-2 (IL-2), as well as activation of novel opioid cytokines which crossreact immunologically with both interleukin-4 and dynorphin B. MLT has been reported also to enhance the production of interleukin-1, interleukin-6 and interleukin-12 in human monocytes. These mediators may counteract secondary immunodeficiencies, protect mice against lethal viral and bacterial diseases, synergize with IL-2 against cancer and influence hematopoiesis. Hematopoiesis is influenced by MLT-induced-opioids (MIO) acting on kappa 1-opioid receptors present on bone marrow macrophages. Clinically, MLT could amplify the anti-tumoral activity of low dose IL-2, induce objective tumor regression, and prolong progression-free time and overall survival. MLT seems to be required for the effectiveness of low dose IL-2 in those neoplasias that are generally resistant to IL-2 alone. Similar findings were obtained in a study in which MLT was combined with gamma-interferon in metastatic renal cell carcinoma. In addition, MLT in combination with low-dose IL-2 was able to neutralize the surgery-induced lymphocytopenia in cancer patients. IL-2 treatment in patients results in activation of the immune system and creates the most suitable biological background for MLT. The finding that MLT stimulates IL-12 production from human monocytes only if incubated in presence of IL-2 further supports this concept. On the other hand, high concentrations of MLT have been found in human breast cancer tissue. The MLT concentration, which was 3 orders of magnitude higher than that present in the plasma, correlated positively with good prognostic markers such as estrogen receptor status and nuclear grade. Whether this relates to the immunoneuroendocrine action of MLT remains to be established. Clinical studies are needed on the effect of MLT in combination with IL-2 or other cytokines in cancer patients and viral diseases including HIV-infected patients.

Melatonin is responsible for the nocturnal increase observed in serum and thymus of thymosin alpha1 and thymulin concentrations: observations in rats and humans

This paper shows that melatonin regulates both thymosin alpha1 and thymulin production as well as the expression of the prothymosin alpha gene. The results revealed the following facts: (a) The concentrations of thymosin alpha1 in both serum and thymus of rat showed a nyctohemeral profile with peak values late at night and basal values during the day. The concentrations of thymulin in rat serum also showed a 24-h rhythm with an increase in their values at night. This rhythmical character for thymosin alpha1, and thymulin was also found in the human serum. (b) Rats injected with melatonin during the day exhibited a significant increase in the concentrations of both peptides. Moreover, continuous light exposure on the animals at daytime and pinealectomy cause a decrease in thymosin a1 and thymulin concentrations with regards to those found in control rats. (c) Melatonin regulates the expression of the prothymosin alpha gene, analyzed by Northern blot. These results suggest that melatonin may be involved in the regulation of immune functions by increasing the thymic peptides production.

Effects of photoperiod and melatonin on lymphocyte activities in male broiler chickens

Understanding the role of the pineal gland in regulating the immune response and the role of photoperiod in influencing pineal gland secretions are becoming increasingly important. The purposes of the present experiments were to investigate the effects of different photoperiod regimens on T- and B-lymphocyte activities in broiler chickens. Next, the influence of different photoperiod regimens on the responsiveness of lymphocytes to melatonin in vitro was examined. The effect of melatonin in vitro on lymphocyte activities was also studied, regardless of the photoperiod received. Finally, the effects of photoperiod on the profiles of different splenocyte cell types were investigated. To study the effect of photoperiod on lymphocyte activities, different photoperiod regimens were used. These were: constant lighting, 23 h light:1 h darkness; intermediate lighting, 12 h light:12 h darkness; and intermittent lighting, 1 h light:3 h darkness. Peripheral blood and splenic lymphocyte activities were tested at 3 and 6 wk of age by performing a mitogen cell-proliferation assay with a polyclonal T-cell mitogen, concanavalin A (Con A), and T-dependent B-cell mitogen, pokeweed mitogen (PWM). To study the effect of photoperiod on the responsiveness of lymphocytes to melatonin in vitro or the effect of melatonin in vitro on lymphocyte activities regardless of photoperiod received, lymphocytes from the chickens that were exposed to the different photoperiod regimens were incubated with mitogen and different concentrations of melatonin. To study the effect of photoperiod on profiles of different cell types, the percentages of splenocyte subpopulations from birds exposed to different photo-periods were determined using flow cytometry with CD4+, CD8+, CD3+, and B-cell markers. The results of these studies indicate that splenic T and B lymphocytes from 6-wk-old chickens grown in intermittent lighting had higher activities than those from chickens grown in constant lighting. Peripheral blood and splenic lymphocytes from chickens raised under constant lighting were more responsive to melatonin in vitro than those from chickens raised under intermittent lighting. This difference in response may be due to lower levels of melatonin in birds receiving constant lighting, making them more sensitive to melatonin in vitro. Melatonin in vitro enhanced the mitogenic response of peripheral blood T lymphocytes from 6-wk-old chickens, splenic T lymphocytes from 3-wk-old chickens, and splenic T and possibly B lymphocytes from 6-wk-old chickens. Finally, intermittent lighting increased the percentages of splenic CD4+, CD8+, and CD3+ cells but not B-cell subpopulations at 6 wk of age, presumably because of increased levels of melatonin in birds receiving intermittent lighting. Our results re-emphasize the importance of melatonin in regulating host immune response; this regulation could be accomplished through exposing broiler chicks to intermittent lighting.

Melatonin activates Th1 lymphocytes by increasing IL-12 production

Melatonin could act on immune system by regulating cytokine production of immunocompetent cells. The hormone enhances IL-2, IFN-gamma and IL-6 production by cultured human mononuclear cells. As enhancement of IL-6 production is related to monocyte activation by melatonin, the hormone acts on human lymphoid cells causing a Th1-type response. This paper shows that melatonin seems to promote a Th1-response by increasing IL-12 production. The hormone enhances IL-12 production by cultured monocytes under suboptimal stimulation in a dose-dependent way. The effect of the hormone increases when PBMCs are incubated with melatonin before monocyte isolation. Enhanced IL-12 production by melatonin can also be shown in cultured human mononuclear cells.

Involvement of nuclear binding sites for melatonin in the regulation of IL-2 and IL-6 production by human blood mononuclear cells

Many functional studies show that melatonin plays a fundamental role in neuroimmunomodulation. In this paper, we have extended our studies on the influence of melatonin on IL-2 and IL-6 production by human peripheral blood mononuclear cells (PBMCs) by comparing the effects of the specific membrane receptor agonist S 20098, the RZR/ROR(alpha) receptor agonist CGP 52608, and structurally related thiazolidinediones. Melatonin bound to membranes as well as to nuclei of human PBMCs with about the same affinity (IC50 values around 5 nM). S 20098 bound to PBMC membranes but not to PBMC nuclei, although the affinity was at least 100 times lower than that of melatonin; this compound did not stimulate cytokine production. In contrast, all four CGP compounds did not bind to PBMC membranes, while binding to nuclei exhibited IC50 values comparable to those of melatonin. The thiazolidinediones activating the RZR/ROR(alpha) receptor (CGP 52608, CGP 53079) also increased IL-2 and IL-6 production. CGP 55644 had no effect on cytokine production and antagonized the effects of CGP 52608 on IL-2 and IL-6 production; moreover, CGP 55644 decreased the enhanced IL-2 production caused by melatonin. Results obtained in monocyte cultures resembled closely those shown in PBMCs. The results reported in this paper confirm the involvement of a nuclear mechanism in the melatonin effects on cytokine production in human PBMCs. We have also shown a synergistic effect of S 20098 and CGP 52608, suggesting a possible link between nuclear and membrane melatonin receptors in PBMCs.

Specific binding of melatonin by purified cell nuclei from spleen and thymus of the rat

In the present paper, we show that pineal hormone melatonin interacts with purified cell nuclei from rat spleen and thymus. Binding of 2-[125I]iodomelatonin ([125I]melatonin) by cell nuclei 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, and specificity. Results suggested binding to single classes of binding sites. The dissociation constants (Kd) for the binding sites in the spleen and thymus nuclei were 68 and 102 pM, respectively. These data are in close agreement with data obtained from kinetic studies, in which the kinetically derived values of the dissociation constant in the spleen and thymus nuclei were 166 and 537 pM, respectively. The affinities for melatonin of these nuclear binding sites suggest that they may recognize the physiological concentrations of melatonin in the tissues. Finally, we have demonstrated that binding of [125I]melatonin by the nuclei is displaced by CGP 52608, a specific ligand of the putative nuclear melatonin receptor RZR/ROR. Results strongly suggest that in addition to membrane receptor-related mechanisms, nuclear receptors may be involved in the regulation of immune system by melatonin.

The photoperiod transducer melatonin and the immune-hematopoietic system

The pineal neurohormone melatonin functionally synchronizes the organism with the photoperiod. It is now well recognized that melatonin also plays an important immunoregulatory role. T-helper cells bear G-protein coupled melatonin cell-membrane receptors and, perhaps, melatonin nuclear receptors. Activation of melatonin receptors enhances the release of T-helper cell type 1 (Th1) cytokines, such as gamma-interferon and interleukin-2, as well as of novel opioid cytokines which crossreact immunologically with both interleukin-4 and dynorphin B. Melatonin has also been reported to enhance the production of interleukin-6 from human monocytes. These mediators may counteract secondary immunodeficiencies, protect mice against lethal viral and bacterial diseases, synergize with interleukin-2 in cancer patients and influence hematopoiesis. Hematopoiesis is apparently influenced by the action of the melatonin-induced opioids on kappa-opioid receptors present on stromal bone marrow cells. Most interestingly, gamma-interferon and colony stimulating factors may modulate the production of melatonin in the pineal gland. A hypothetical pineal-immune-hematopoietic network is, therefore, taking shape. From the immunopharmacological point of view, a call is made for clinical studies on the effect of melatonin in viral disease including human immunodeficiency virus-infected patients and cancer patients. In conclusion, melatonin seems to be an important immunomodulatory hormone which deserves to be further studied to identify its relevance in immune-based diseases, its therapeutic indications, and its adverse effects.

Exogenous melatonin enhances cell-mediated, but not humoral, immune function in adult male deer mice (Peromyscus maniculatus)

Many nontropical rodent species display seasonal changes in reproductive physiology and metabolism, as well as in immune function. Field studies of seasonal changes in immune function typically report decreased immune function in the short days of winter compared to summer; presumably, reduced immunity in winter reflects increased glucocorticoid secretion in response to environmental stressors. In contrast, laboratory studies of photoperiodic changes in immunity invariably demonstrate increased immune function in short compared to long days. Although the precise mechanisms regulating short-day enhancement of immune function are not known, it is hypothesized that increased immunity is due to the increased duration of melatonin secretion in short compared to long days. However, melatonin can act both directly (i.e, via melatonin receptors located on lymphatic tissue) and indirectly (i.e., via alterations in gonadal steroids) to affect immune function. The present study examined the effects of exogenous melatonin administration on both cell-mediated and humoral immune function in adult male deer mice (Peromyscus maniculatus), as well as the role of gonadal steroid hormones in mediating these effects. Mice either were castrated to remove circulating androgens or received sham operations and were implanted with empty capsules or capsules containing melatonin. Individual mice implanted with melatonin underwent reproductive regression and displayed enhanced splenocyte proliferation to the T-cell mitogen concanavalin A; antigen-specific serum immunoglobulin M production was unaffected by melatonin treatment. Castration had no effect on either cell-mediated or humoral immune function. Taken together, these results suggest that exogenous melatonin enhances cell-mediated, but not humoral, immune function in adult male deer mice and that this effect is independent of gonadal steroid hormones. These results are consistent with a direct effect of melatonin on immunity.

Differential Expression of the Melatonin Receptor in Human Monocytes

Earlier studies have shown that the pineal hormone melatonin activates human monocytes. It is reported here that melatonin induces the secretion of IL-1, IL-6, and TNF in fresh and 1-day in vitro cultured monocytes that also express the melatonin receptor (Kd = 270 ± 60 pM; 42,000–48,000 receptors/cell). However, when monocytes were cultured in vitro for 2 days, the number of receptors decreased to 11,000 receptors/cell, with the same Kd. LPS activation of fresh or 1-day cultured monocytes did not result in any increase in melatonin receptor number. LPS activation of 2-day cultured monocytes led to an increase in the number of melatonin receptors, from 11,000 receptors/cell to the plateau of 42,000 to 48,000 receptors/cell. The loss of receptors by 2-day cultured monocytes was not irreversible. Melatonin did not induce the release of IL-1, TNF, or IL-6 in monocytes cultured in vitro for 3 days and for up to 15 days, and these long time cultured monocytes did not express the melatonin receptors even after activation by LPS. The loss of melatonin receptors by monocytes cultured in vitro for 3 days and for up to 15 days was irreversible. Therefore, it is shown for the first time that human monocytes express melatonin receptors. Furthermore, human monocytes express melatonin receptors differentially depending on their state of maturation, and it appears that in vitro monocyte differentiation and maturation negatively affect human monocyte melatonin receptor expression.

Melatonin in osteosarcoma: an effective drug?

The pineal indole amine melatonin has been shown to have oncostatic properties in a wide variety of neoplasms. Melatonin levels start to diminish before the onset of puberty and continue to decline during puberty. There appears to be a relationship between the rate of bone growth and the incidence of osteosarcoma (which was found to be highest in the long bones of the leg in the 10-14-year age group). It is hypothesized that the simultaneous decrease in melatonin levels (with diminishing oncostatic protection), concurrent with the exponential increase in bone growth during puberty (i.e. increased rate of cell proliferation), could be a factor in the typical age distribution of osteosarcoma. Melatonin is of value in combined chemotherapy, because it is non-toxic and can augment the anti-cancer action and decrease the side-effects of many other chemotherapeutic drugs. It is hoped that melatonin, as an adjunct to the routine chemotherapy of osteosarcoma, will help to improve the prognosis of this too often fatal disease.

The validity of melatonin as an oncostatic agent

The validity of melatonin as a prominent, naturally occurring oncostatic agent is examined in terms of its putative oncostatic mechanism of action, the correlation between melatonin levels and neoplastic activity, and the outcome of therapeutically administered melatonin in clinical trials. Melatonin's mechanism of action is summarized in a brief analysis of its actions at the cellular level, its antioxidative functions, and its indirect immunostimulatory effects. The difficulties of interpreting melatonin levels as a diagnostic or prognostic aid in cancer is illustrated by referral to breast cancer, the most frequently studied neoplasm in trials regarding melatonin. Trials in which melatonin was used therapeutically are reviewed, i.e., early studies using melatonin alone, trials of melatonin in combination with interleukin-2, and controlled studies comparing routine therapy to therapy in combination with melatonin. A table compiling the studies in which melatonin was used in the treatment of cancer in humans is presented according to the type of neoplasm. Melatonin's suitability in combination chemotherapy, where it augments the anticancer effect of other chemotherapeutic drugs while decreasing some of the toxic side effects, is described. Based on the evidence derived from melatonin's antiproliferative, antioxidative, and immunostimulatory mechanisms of action, from its abnormal levels in cancer patients and from clinical trials in which melatonin was administered, it is concluded that melatonin could indeed be considered a physiological anticancer substance. Further well-controlled trials should, however, be performed in order to find the link between its observed effects and the underlying mechanisms of action and to define its significance as a therapeutic oncostatic agent.

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.

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.

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.

Specific binding of 2-[125I]melatonin by partially purified membranes of rat thymus

Melatonin binding sites were characterized in partially purified rat thymus membranes. The specific binding of 2-[125I]iodomelatonin ([125I]MEL) to thymus membranes was dependent on time and temperature, stable, saturable, and reversible. Concentration-dependent binding of [125I]MEL to thymus membranes was saturable and resulted in a linear Scatchard plot, suggesting binding to a single class of binding sites. The Kd for this single site was 0.47 nM with a binding capacity of 1.01 pM. In competition studies, the specific binding of [125I]MEL to thymus membranes was inhibited by increasing concentrations of native melatonin. Scatchard analysis showed that, unlike in saturation studies with [125I]MEL, data were compatible with the existence of two classes of binding sites: a high-affinity site with a Kd of 1.72 +/- 0.25 nM and a binding capacity of 1.40 +/- 0.18 pM, and a low-affinity site with a Kd of 1226 +/- 325 nM and a binding capacity of 460 +/- 87 pM. Interestingly, Kd and BC values of the high-affinity binding site described by competition studies are similar to those obtained by saturation studies with [125I]MEL. Binding of [125I]MEL to thymus membranes was specific as indicated by the fact no other melatonin precursor or derivative was as potent as melatonin in inhibiting the binding of [125I]MEL to membranes. Results strongly suggest that melatonin is involved in regulation of thymus activity.

Characteristics of 2-[125I]iodomelatonin binding sites in the pigeon spleen and modulation of binding by guanine nucleotides

2-[125]Iodomelatonin binding sites in membrane preparations of pigeon spleen have been characterized. The binding was stable, saturable, reversible, and of high affinity. Rosenthal and Hill analyses showed that the radioligand-receptor interaction involved a single class of binding sites. Analysis of the binding results of spleens collected during mid-light revealed an equilibrium dissociation constant (Kd) of 36.6 +/- 4.8 pmol/l (mean +/- sem, n = 10) and a maximum density (Bmax) of 2.3 +/- 0.2 fmol/mg protein. There was no significant difference in the Kd (46.9 +/- 5.0 pmol/l) or the Bmax values (2.4 +/- 0.3 fmol/mg protein) for spleens collected during mid-dark (n = 9), although the mid-dark serum and pineal melatonin levels were significantly higher (P < 0.05) than the corresponding mid-light values. Kinetic analysis showed a Kd of 8.6 +/- 2.0 pmol/l (n +/- 4), in agreement with that derived from the saturation studies. Except for inhibition by 2-iodomelatonin, melatonin, 6-chloromelatonin, 6-hydroxymelatonin and N-acetylserotonin, the other indoles or neurotransmitters tested have little inhibition on the binding. In addition, guanosine 5'-O-(3-thiophosphate) (GTP gamma S), a nonhydrolysable analog of GTP, was found to inhibit the binding in a dose-dependent manner. Saturation studies revealed that this is due to a decrease in both the affinity and density of the binding sites. These data suggest that a single type of melatonin receptor is found in the pigeon spleen and that the site is coupled to a guinine nucleotide binding protein (G-protein). Our findings support a direct pineal melatonin action on the immune system.

Binding of 2-[125I]melatonin by rat thymus membranes during postnatal development

Previously, we have demonstrated the presence of melatonin binding sites in thymus membranes of adult rats. In this paper, we show that the binding of melatonin by thymus membranes changes during postnatal development. Maximum binding was observed in newborn rats; thereafter, binding decreased progressively during the first weeks of life and exhibited the lowest values in adult animals. Stoichiometric studies showed that the decrease in melatonin binding was due to changes in the binding capacity (2.5-fold) rather than to changes in the affinity of the receptor for the ligand. The results suggest a physiological role of melatonin in regulating thymus activity early during postnatal development.

Neonatal pinealectomy impairs murine antibody-dependent cellular cytotoxicity

The pineal gland, through its principal hormone melatonin, is able to modulate different immune functions. We have previously demonstrated that exogenous melatonin induces a significant enhancement of murine antibody-dependent cellular cytotoxicity (ADCC). In order to determine whether the pineal gland plays a physiological role in ADCC regulation, we studied the influence of neonatal pinealectomy on this activity. The results presented here indicate that ablation of the pineal gland during the first week of life significantly reduces ADCC levels in adult mice. This impairment appears around 60 days of age, suggesting that sexual hormones may be involved in the pineal effect. Moreover, the administration of melatonin to pinealectomized mice restores ADCC levels regardless of the hour and seasonal time of injection. On the basis of the data reported here, a physiological regulation of ADCC by the pineal gland can be assumed.

The immunoneuroendocrine role of melatonin

A tight, physiological link between the pineal gland and the immune system is emerging from a series of experimental studies. This link might reflect the evolutionary connection between self-recognition and reproduction. Pinealectomy or other experimental methods which inhibit melatonin synthesis and secretion induce a state of immunodepression which is counteracted by melatonin. In general, melatonin seems to have an immunoenhancing effect that is particularly apparent in immunodepressive states. The negative effect of acute stress or immunosuppressive pharmacological treatments on various immune parameters are counteracted by melatonin. It seems important to note that one of the main targets of melatonin is the thymus, i.e., the central organ of the immune system. The clinical use of melatonin as an immunotherapeutic agent seems promising in primary and secondary immunodeficiencies as well as in cancer immunotherapy. The immunoenhancing action of melatonin seems to be mediated by T-helper cell-derived opioid peptides as well as by lymphokines and, perhaps, by pituitary hormones. Melatonin-induced-immuno-opioids (MIIO) and lymphokines imply the presence of specific binding sites or melatonin receptors on cells of the immune system. On the other hand, lymphokines such as gamma-interferon and interleukin-2 as well as thymic hormones can modulate the synthesis of melatonin in the pineal gland. The pineal gland might thus be viewed as the crux of a sophisticated immunoneuroendocrine network which functions as an unconscious, diffuse sensory organ.

[125I]-labelled iodomelatonin binding sites in the duck bursa of Fabricius: binding characteristics and diurnal variation

Melatonin receptors in membrane preparations of the duck bursa of Fabricius were studied by using [125I]-labelled iodomelatonin as the radioligand. Specific binding of [125I]-labelled iodomelatonin in the membrane preparations of bursa was stable, saturable, reversible and of high affinity. Scatchard analysis of the specific binding revealed an equilibrium dissociation constant (Kd) of 48.5 +/- 7.4 pmol/l and a total number of binding sites (Bmax) of 1.38 +/- 0.12 fmol/mg protein at mid-light. A diurnal study showed that the Bmax of [125I]-labelled iodomelatonin binding sites at mid-light was 94.4% higher (P < 0.05) than that at mid-dark. There was no significant difference in the mid-light and mid-dark Kd values. The Kd value determined by kinetic analysis was 42.0 +/- 9.3 pmol/l at mid-light. The pharmacological characteristics indicated that [125I]-labelled iodomelatonin binding sites are highly specific for melatonin. Our results suggest that the bursa is a target organ of melatonin action.

Constant light exposure increases 2-[125I]iodomelatonin binding sites in the guinea pig spleen

The effect of constant light exposure on 2-[125I]iodomelatonin binding sites in the guinea pig spleen was investigated. Guinea pigs exposed to constant light for 2-3 weeks showed an increase in the number of splenic 2-[125I]iodomelatonin binding sites, without any difference in the binding affinity, in comparison to animals kept under 12 h light/12 h darkness condition, whereas the splenic indices were similar in both groups. This light dependence of the splenic 2-[125I]iodomelatonin binding sites suggests that the binding sites are melatonin receptors and that the light dependent immunomodulatory effect of melatonin is mediated via a direct action on the lymphoid tissue.

A brief survey of pineal gland-immune system interrelationships

The present paper summarizes evidence that support the hypothesis of the existence of bilateral interactions between pineal gland and the immune system. Both in vivo and in vitro experiments show that the pineal gland, via its hormone melatonin, enhances immune function. Mechanisms involved in this immunostimulatory effect are not well understood, but some evidence suggests the existence of specific binding sites for melatonin on immune cells. Moreover, the release of opioid peptides and interleukin-2 by T-helper cells may also participate in this mechanism by activating, at least natural killer activity and antibody-dependent cellular cytotoxicity. Some immune signals, i.e., gamma-interferon, may be involved in regulating pineal function, thereby representing a regulatory mechanism in the opposite direction. The physiological and clinical significance of these data remains to be studied.