Possible Role of Pineal and Extra-Pineal Melatonin in Surveillance, Immunity, and First-Line Defense.

Melatonin is a highly conserved molecule found in prokaryotes and eukaryotes that acts as the darkness hormone, translating environmental lighting to the whole body, and as a moderator of innate and acquired defense, migration, and cell proliferation processes. This review evaluates the importance of pineal activity in monitoring PAMPs and DAMPs and in mounting an inflammatory response or innate immune response. Activation of the immune-pineal axis, which coordinates the pro-and anti-inflammatory phases of an innate immune response, is described. PAMPs and DAMPs promote the immediate suppression of melatonin production by the pineal gland, which allows leukocyte migration. Monocyte-derived macrophages, important phagocytes of microbes, and cellular debris produce melatonin locally and thereby initiate the anti-inflammatory phase of the acute inflammatory response. The role of locally produced melatonin in organs that directly contact the external environment, such as the skin and the gastrointestinal and respiratory tracts, is also discussed. In this context, as resident macrophages are self-renewing cells, we explore evidence indicating that, besides avoiding overreaction of the immune system, extra-pineal melatonin has a fundamental role in the homeostasis of organs and tissues.

Rosette-Forming Glioneuronal Tumor in the Pineal Region: A Series of 6 Cases and Literature Review.

Resected lesions from the pineal region are rare specimens encountered by surgical pathologists, and their heterogeneity can pose significant diagnostic challenges. Here, we reviewed 221 pineal region lesions resected at New York-Presbyterian Hospital/Columbia University Irving Medical Center from 1994 to 2019 and found the most common entities to be pineal parenchymal tumors (25.3%), glial neoplasms (18.6%), and germ cell tumors (17.6%) in this predominantly adult cohort of patients. Six cases of a rare midline entity usually found exclusively in the fourth ventricle, the rosette-forming glioneuronal tumor, were identified. These tumors exhibit biphasic morphology, with a component resembling pilocytic astrocytoma admixed with variable numbers of small cells forming compact rosettes and perivascular pseudorosettes. Targeted sequencing revealed a 100% co-occurrence of novel and previously described genetic alterations in the mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) signaling pathways, suggesting a synergistic role in tumor formation. The most common recurrent mutation, PIK3CA H1047R, was identified in tumor cells forming rosettes and perivascular pseudorosettes. A review of the literature revealed 16 additional cases of rosette-forming glioneuronal tumors in the pineal region. Although rare, this distinctive low-grade tumor warrants consideration in the differential diagnosis of pineal region lesions.

The interplay between mast cells, pineal gland, and circadian rhythm: Links between histamine, melatonin, and inflammatory mediators.

Our daily rhythmicity is controlled by a circadian clock with a specific set of genes located in the suprachiasmatic nucleus in the hypothalamus. Mast cells (MCs) are major effector cells that play a protective role against pathogens and inflammation. MC distribution and activation are associated with the circadian rhythm via two major pathways, IgE/FcεRI- and IL-33/ST2-mediated signaling. Furthermore, there is a robust oscillation between clock genes and MC-specific genes. Melatonin is a hormone derived from the amino acid tryptophan and is produced primarily in the pineal gland near the center of the brain, and histamine is a biologically active amine synthesized from the decarboxylation of the amino acid histidine by the L-histidine decarboxylase enzyme. Melatonin and histamine are previously reported to modulate circadian rhythms by pathways incorporating various modulators in which the nuclear factor-binding near the κ light-chain gene in B cells, NF-κB, is the common key factor. NF-κB interacts with the core clock genes and disrupts the production of pro-inflammatory cytokine mediators such as IL-6, IL-13, and TNF-α. Currently, there has been no study evaluating the interdependence between melatonin and histamine with respect to circadian oscillations in MCs. Accumulating evidence suggests that restoring circadian rhythms in MCs by targeting melatonin and histamine via NF-κB may be promising therapeutic strategy for MC-mediated inflammatory diseases. This review summarizes recent findings for circadian-mediated MC functional roles and activation paradigms, as well as the therapeutic potentials of targeting circadian-mediated melatonin and histamine signaling in MC-dependent inflammatory diseases.

Are cortisol and melatonin involved in the immune modulation by the light environment in pike perch Sander lucioperca?

The pineal gland is the main organ involved in the transduction process converting environmental light information into a melatonin response. Since light environment was described as an important factor that could affect physiology of teleosts, and because melatonin is a crucial hormone regulating numerous physiological processes, we hypothesized that environmental light may act on both stress and circadian axes which in turn could influence the immune status of pike perch. Therefore, we investigated the effects of two light spectra (red and white) and two light intensities (10 and 100 lx) with a constant photoperiod 12L(8:00-20:00) /12D on pike perch physiological and immune responses. Samples were collected at 04:00 and 16:00 at days 1 and 30 of the experiment. Stress markers, plasma melatonin levels, humoral innate immune markers, and expression of key immune genes in the head kidney were assessed. Light intensity clearly affected pike perch physiology. This included negative growth performances, increase in stress status, decrease in plasma melatonin levels, and immune depression. Light spectrum had only little influences. These results demonstrate that high stress status may have impacted melatonin production and secretion by the pineal organ. The drop in circulating melatonin and the increase in stress status may both be involved in the immune suppression.

The Effect of Light Exposure at Night (LAN) on Carcinogenesis via Decreased Nocturnal Melatonin Synthesis.

In mammals, a master clock is located within the suprachiasmatic nucleus (SCN) of the hypothalamus, a region that receives input from the retina that is transmitted by the retinohypothalamic tract. The SCN controls the nocturnal synthesis of melatonin by the pineal gland that can influence the activity of the clock's genes and be involved in the inhibition of cancer development. On the other hand, in the literature, some papers highlight that artificial light exposure at night (LAN)-induced circadian disruptions promote cancer. In the present review, we summarize the potential mechanisms by which LAN-evoked disruption of the nocturnal increase in melatonin synthesis counteracts its preventive action on human cancer development and progression. In detail, we discuss: (i) the Warburg effect related to tumor metabolism modification; (ii) genomic instability associated with L1 activity; and (iii) regulation of immunity, including regulatory T cell (Treg) regulation and activity. A better understanding of these processes could significantly contribute to new treatment and prevention strategies against hormone-related cancer types.

Immune-pineal axis - acute inflammatory responses coordinate melatonin synthesis by pinealocytes and phagocytes.

Melatonin is well known for its circadian production by the pineal gland, and there is a growing body of data showing that it is also produced by many other cells and organs, including immune cells. The chronobiotic role of pineal melatonin, as well as its protective effects in vitro and in vivo, have been extensively explored. However, the interaction between the chronobiotic and defence functions of endogenous melatonin has been little investigated. This review details the current knowledge regarding the coordinated shift in melatonin synthesis from the pineal gland (circadian and monitoring roles) to the regulation of acute immune responses via immune cell production and autocrine effects, producing systemic interactions termed the immune-pineal axis. An acute inflammatory response drives the transcription factor, NFκB, to switch melatonin synthesis from pinealocytes to macrophages/microglia and, upon acute inflammatory resolution, back to pinealocytes. The potential pathophysiological relevance of immune-pineal axis dysregulation is highlighted, with both research and clinical implications, across several medical conditions, including host/parasite interaction, neurodegenerative diseases and cancer. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Immunoregulatory action of melatonin. The mechanism of action and the effect on inflammatory cells.

Literature data indicate a significant immunoregulatory role of melatonin. Melatonin exerts an effect directly affecting leucocytes bearing specific melatonin receptors or indirectly by means of melatonin regulating other hormones, opioids or cytokines. Despite numerous experiments, the influence of the hormone on the immune system is still controversial. Melatonin affects the immune response acting as both an activator and an inhibitor of the inflammatory process. The hormone acts as an "immunological buffer" activating impaired immunity in immunosuppression, chronic stress or old age as well as suppressing overreaction of the immune system. Melatonin mediates between neurohormonal and immune systems by means of the immune-pineal axis acting as a negative feedback mechanism. The axis connects development of the immune reaction with pineal activity and melatonin secretion induced by inflammatory mediators. The seasonal and circadian fluctuation of the melatonin level and the fluctuation related changes of the immune parameters can be responsible for some autoimmune and infectious diseases. In spite of that, there is a growing number of papers suggesting considerable therapeutic potential of melatonin in inflammatory disease treatment. This paper presents well-systematized information on the mechanism of melatonin action and its influence on cells involved in the inflammatory process - neutrophils and monocytes.

[The role of the pineal-thymus system in the regulation of autoimmunity, aging and lifespan]. / A tobozmirigy-csecsemomirigy rendszer szerepe az autoimmunitás, öregedés és élettartam szabályozásában.

Thymus is an immunoendocrine organ, the hormones of which mainly influence its own lymphatic elements. It has a central role in the immune system, the neonatal removal causes the collapse of immune system and the whole organism. The thymic nurse cells select the bone marrow originated lymphocytes and destroy the autoreactive ones, while thymus originated Treg cells suppress the autoreactive cells in the periphery. The involution of the organ starts after birth, however, this truly happens in the end of puberty only, as before this it is overcompensated by developmental processes. From the end of adolescence the involution allows the life, proliferation and enhanced functioning of some autoreactive cells, which gradually wear down the cells and intercellular materials, causing the aging. The enhanced and mass function of autoreactive cells lead to the autoimmune diseases and natural death. This means that the involution of thymus is not a part of the organismic involution, but an originator of it, which is manifested in the lifespan-pacemaker function. Thus, aging can be conceptualized as a thymus-commanded slow autoimmune process. The neonatal removal of pineal gland leads to the complete destruction of the thymus and the crashing down of the immune system, as well as to wasting disease. The involution of the pineal and thymus runs parallel, because the two organs form a functional unit. It is probable that the pineal gland is responsible for the involution of thymus and also regulates its lifespan determining role. However, the data reviewed here do not prove the exclusive role of the pineal-thymus system in the regulation of aging and lifespan, but only call attention to such possibility.

The Immunoendocrine Thymus as a Pacemaker of Lifespan.

The thymus develops from an endocrine area of the foregut, and retains the ancient potencies of this region. However, later it is populated by bone marrow originated lymphatic elements and forms a combined organ, which is a central part of the immune system as well as an influential element of the endocrine orchestra. Thymus produces self-hormones (thymulin, thymosin, thymopentin, and thymus humoral factor), which are participating in the regulation of immune cell transformation and selection, and also synthesizes hormones similar to that of the other endocrine glands such as melatonin, neuropeptides, and insulin, which are transported by the immune cells to the sites of requests (packed transport). Thymic (epithelial and immune) cells also have receptors for hormones which regulate them. This combined organ, which is continuously changing from birth to senescence seems to be a pacemaker of life. This function is basically regulated by the selection of self-responsive thymocytes as their complete destruction helps the development (up to puberty) and their gradual release in case of weakened control (after puberty) causes the erosion of cells and intercellular material, named aging. This means that during aging, self-destructive and non-protective immune activities are manifested under the guidance of the involuting thymus, causing the continuous irritation of cells and organs. Possibly the pineal body is the main regulator of the pacemaker, the neonatal removal of which results in atrophy of thymus and wasting disease and its later corrosion causes the insufficiency of thymus. The co-involution of pineal and thymus could determine the aging and the time of death without external intervention; however, external factors can negatively influence both of them.

[THE CHANGES OF THE INTERRELATIONS OF THE PINEAL GLAND AND THE ORGANS OF THE IMMUNE SYSTEM IN RATS IN RESPONSE TO MELATONIN ADMINISTRATION IN LIGHT REGIME DISTURBANCES].

In this work the correlation analysis was applied to detect the integrated response of the pineal gland (PG) and immunocompetent organs of male Wistar rats in response to administration of melatonin (MT) in light regime disturbances. Animals were kept for 14 days under natural or continuous light (CL). Then for 7 days they received the injections of either 0.9% solution of sodium chloride or MT, after which the rats were decapitated and the mass of their body, PG, thymus and spleen was determined. The lymphocyte subpopulations of the thymus and spleen were studied by flow cytometry. The amount of lipofuscin in PG was assessed by the intensity of autofluorescence in organ frozen sections in 560-600 nm wavelength range. It was found that under the influence of MT, the number of intraorgan correlations in the immune system increased, regardless of the light regime. In animals on CL treated with MT, the number of interorgan connections was reduced, while negative correlations appeared between PG lipofuscin content and cellular composition of the spleen. The synchronizing and adaptogenic effects of MT were most pronounced under conditions of CL.

[Interrelations of pineal gland morpho-functional indices and immune system organs in rats exposed to natural illumination regime and continuous illumination].

The objective of this investigation was to determine the complex response of the pineal gland (PG) and of the organs of the immune system in Wistar rats in response to a violation of the illumination regime in the experiment. Animals were kept under natural light regime and continuous illumination for 14 days. After that rats were sacrificed and the mass of the body, PG, gonads, thymus and spleen was measured. Thymus and spleen cell subpopulations were determined by flow cytometry. The lipofuscin content in PG was determined by measuring an autofluorescence intensity in frozen tissue sections in the wavelength range of 505-545 nm using a confocal laser scanning microscope LSM 510 META (Carl Zeiss). The correlation analysis showed an increase in the amount and the change of sign and direction of relations between the indices of the state of PG and the immune system. This indicates the up-regulation of the intensity of inter-system relationships and the change of migration and differentiation vector of immunocompetent cells.

The pineal regulation of the immune system: 40 years since the discovery.

The first observation on the relationship between the pineal gland and the immune system was done by the author of this paper in the late sixties and early seventies of the last century. After neonatal pinealectomy the thymus has been destroyed and wasting disease developed. Since that time a flood of experiments justified the observation and pointed to the prominent role of pineal in the regulation of the immune system. Melatonin, the hormone of the pineal gland stimulates immune processes acting to the immune cells' cytokine production, the haemopoiesis, and immune cell-target cell interactions. Melatonin receptors have been demonstrated and their localization and function were justified. Melatonin production by and melatonin receptors on (and in) the immune cells was proved. Melatonin agonists have been synthesized and the use of melatonin as adjuvant in the therapy of diseases connected to the immune system (cancers included) has been started. The paper summarizes the most important studies and discusses the interrelations of the data. The discussion points to the possibility of packed transport of the pineal hormone by the immune cells and to the adventages of local regulation by this transport.

The concept of the immune-pineal axis tested in patients undergoing an abdominal hysterectomy.

OBJECTIVE: Activation of the immune-pineal axis induces a transient reduction in nocturnal melatonin in the plasma during the proinflammatory phase of an innate immune response to allow the proper migration of leukocytes to the lesion site. This transient reduction should be regulated by inflammatory mediators, which are responsible for the fine-tuning of the process. In the present study, we measured the pre- and postoperative serum concentrations of melatonin, tumor necrosis factor (TNF) and cortisol in women who underwent an elective hysterectomy and correlated the variation in melatonin with postoperative pain. METHODS: We evaluated 12 women who had an abdominal hysterectomy. Blood was collected at 10.00 and 22.00 h 1 week and 1 day before the surgery, on the 1st and 2nd days after the surgery and at 22.00 h on the day of the surgery. RESULTS: On the night after the surgery, there was no melatonin detected at 22.00 h. High TNF levels were accompanied by a lower nocturnal melatonin output, higher postoperative pain according to a visual analog scale and the request of higher doses of analgesics. In addition, low cortisol levels were accompanied by a lower nocturnal melatonin output. CONCLUSION: Our results confirm that the same antagonistic pattern between TNF and glucocorticoids observed in cultured pineal glands also occurs in humans. This integrative pattern suggests that the cross talk between the immune and endocrine system orchestrates longitudinal changes in pineal activity, reinforcing the hypothesis of an immune-pineal axis.

Melatonin: buffering the immune system.

Melatonin modulates a wide range of physiological functions with pleiotropic effects on the immune system. Despite the large number of reports implicating melatonin as an immunomodulatory compound, it still remains unclear how melatonin regulates immunity. While some authors argue that melatonin is an immunostimulant, many studies have also described anti-inflammatory properties. The data reviewed in this paper support the idea of melatonin as an immune buffer, acting as a stimulant under basal or immunosuppressive conditions or as an anti-inflammatory compound in the presence of exacerbated immune responses, such as acute inflammation. The clinical relevance of the multiple functions of melatonin under different immune conditions, such as infection, autoimmunity, vaccination and immunosenescence, is also reviewed.

Glia-pinealocyte network: the paracrine modulation of melatonin synthesis by tumor necrosis factor (TNF).

The pineal gland, a circumventricular organ, plays an integrative role in defense responses. The injury-induced suppression of the pineal gland hormone, melatonin, which is triggered by darkness, allows the mounting of innate immune responses. We have previously shown that cultured pineal glands, which express toll-like receptor 4 (TLR4) and tumor necrosis factor receptor 1 (TNFR1), produce TNF when challenged with lipopolysaccharide (LPS). Here our aim was to evaluate which cells present in the pineal gland, astrocytes, microglia or pinealocytes produced TNF, in order to understand the interaction between pineal activity, melatonin production and immune function. Cultured pineal glands or pinealocytes were stimulated with LPS. TNF content was measured using an enzyme-linked immunosorbent assay. TLR4 and TNFR1 expression were analyzed by confocal microscopy. Microglial morphology was analyzed by immunohistochemistry. In the present study, we show that although the main cell types of the pineal gland (pinealocytes, astrocytes and microglia) express TLR4, the production of TNF induced by LPS is mediated by microglia. This effect is due to activation of the nuclear factor kappa B (NF-kB) pathway. In addition, we observed that LPS activates microglia and modulates the expression of TNFR1 in pinealocytes. As TNF has been shown to amplify and prolong inflammatory responses, its production by pineal microglia suggests a glia-pinealocyte network that regulates melatonin output. The current study demonstrates the molecular and cellular basis for understanding how melatonin synthesis is regulated during an innate immune response, thus our results reinforce the role of the pineal gland as sensor of immune status.

Psychoneuroendocrine modulation of regulatory T lymphocyte system: in vivo and in vitro effects of the pineal immunomodulating hormone melatonin.

BACKGROUND: At present, it is known that cancer-related immunosuppression would mainly depend on an immunosuppressive action mediated by a subtype of CD4+ lymphocytes, the so-called regulatory T lymphocytes (T-reg), which are identified as CD4+CD25+ cells. Moreover, it has been shown that anticancer immunity is under psychoneuroendocrine regulation, mainly mediated by the pineal hormone melatonin (MLT). This study was performed to investigate the in vivo and in vitro effects of MLT on T-reg generation. MATERIALS AND METHODS: We evaluated the in vivo effects of MLT (20 mg/daily orally in the evening) in 20 patients with untreatable metastatic solid tumor and the in vitro effects of MLT incubation (at 10 and 100 pg/ml) of pure lymphocyte cultures on T-reg cell count. RESULTS: MLT induced a statistically significant decline in mean T-reg cell numbers in patients who achieved disease control, whereas no effect was seen in those who had progressed. In contrast, no in vitro effect of MLT incubation was apparent. CONCLUSION: This preliminary study would suggest that MLT may exert in vivo an inhibitory action on T-reg cell generation in cancer patients which is associated with a control of the neoplastic progression, whereas no direct effect was seen in vitro on lymphocyte differentiation. This finding would suggest that MLT may counteract T-reg cell generation in vivo by inhibiting macrophage activity which is involved in stimulating T-reg cell production.

TLR4 and CD14 receptors expressed in rat pineal gland trigger NFKB pathway.

Nuclear factor-kappa B (NFKB), a pivotal player in inflammatory responses, is constitutively expressed in the pineal gland. Corticosterone inhibits pineal NFKB leading to an enhancement of melatonin production, while tumor necrosis factor (TNF) leads to inhibition of Aa-nat transcription and the production of N-acetylserotonin in cultured glands. The reduction in nocturnal melatonin surge favors the mounting of the inflammatory response. Despite these data, there is no clear evidence of the ability of the pineal gland to recognize molecules that signal infection. This study investigated whether the rat pineal gland expresses receptors for lipopolysaccharide (LPS), the endotoxin from the membranes of Gram-negative bacteria, and to establish the mechanism of action of LPS. Here, we show that pineal glands possess both CD14 and toll-like receptor 4 (TLR4), membrane proteins that bind LPS and trigger the NFKB pathway. LPS induced the nuclear translocation of p50/p50 and p50/RELA dimers and the synthesis of TNF. The maximal expression of TNF in cultured glands coincides with an increase in the expression of TNF receptor 1 (TNFR1) in isolated pinealocytes. In addition, LPS inhibited the synthesis of N-acetylserotonin and melatonin. Therefore, the pineal gland transduces Gram-negative endotoxin stimulation by producing TNF and inhibiting melatonin synthesis. Here, we provide evidence to reinforce the idea of an immune-pineal axis, showing that the pineal gland is a constitutive player in the innate immune response.

Association of mast cells with calcification in the human pineal gland.

Increased pineal calcifications and decreased pineal melatonin biosynthesis, both age related, support the notion of a pineal bio-organic timing mechanism. The role of calcification in the pathogenesis of pineal gland dysfunction remains unknown but the available data document that calcification is an organized, regulated process, rather than a passive aging phenomenon. The cellular biology and micro-environmental conditions required for calcification remain poorly understood but most studies have demonstrated evidence that mast cells are strongly implicated in this process. The aim of the present study was to examine the phenotype of mast cells associated with early stages and with the progressive development of calcification in the human pineal gland. The study was performed on pineal samples of 170 fetuses and children whose brains were autopsied and diagnosed during 1998-2002. The representative cerebral and pineal specimens were stained with haematoxylin and eosin or the von Kossa staining technique and for the distribution of mast cell tryptase, mast cell chymase, histamine H4 receptor and vascular network using biotinylated Ulex europaeus agglutinin. Tryptase mast cells were found in all stages of pineal gland development independently of the presence of local tissue lesions. All of them were always localized in the close vicinity of the blood vessels and expressed immunoreactivity to histamine H4 receptor antibody. Immunolocalization of mast cells by chymase antibody (and following dual immunostaining with both chymase and tryptase antibodies) demonstrated that these cells were few in number and were located in the subcapsular region of the gland. In our study, all functional mast cells that underwent activation and were co-localized with deposits of calcium did not contain chymase. All of them were stained with tryptase and represent the MC-T phenotype. Tryptase mast cells and extracellular tryptase were often associated with areas of early and more advanced stages of calcification. Our results lead to the conclusion that the tryptase mast cells play a major role in the pineal calcification process as sites where this process starts and as a source of production of numerous biologically active substances including tryptase that participate in calcification.

The immune-pineal axis: stress as a modulator of pineal gland function.

The temporal organization of mammals presents a daily adjustment to the environmental light/dark cycle. The environmental light detected by the retina adjusts the central clock in the suprachiasmatic nuclei, which innervate the pineal gland through a polysynaptic pathway. During the night, this gland produces and releases the nocturnal hormone melatonin, which circulates throughout the whole body and adjusts several bodily functions according to the existence and duration of darkness. We have previously shown that during the time frame of an inflammatory response, pro-inflammatory cytokines, such as tumor necrosis factor-alpha, inhibit while anti-inflammatory mediators, such as glucocorticoids, enhance the synthesis of melatonin, interfering in the daily adjustment of the light/dark cycle. Therefore, injury disconnects the organism from environmental cycling, while recovery restores the light/dark information to the whole organism. Here, we extend these observations by evaluating the effect of a mild restraint stress, which did not induce macroscopic gastric lesions. After 2 h of restraint, there was an increase in circulating corticosterone, indicating activation of the hypothalamus-pituitary-adrenal (HPA) axis. In parallel, an increase in melatonin production was observed. Taking into account the data obtained with models of inflammation and stress, we reinforce the hypothesis that the activity of the pineal gland is modulated by the state of the immune system and the HPA axis, implicating the darkness hormone melatonin as a modulator of defense responses.

Photoperiod-related changes in hormonal and immune status of male Siberian hamsters, Phodopus sungorus.

Previously we have demonstrated that in Siberian hamsters some immune measures, especially the development of experimentally evoked peritonitis, varied in a photoperiod- and gender-dependent manner. The aim of the present study was to investigate whether the photoperiod-related differences in the activity of inflammation-involved immune cells are in this species attributed to the changes in the pineal gland function and/or hormonal status. Male hamsters housed in short day (SD), compared with those from long day (LD) conditions, exhibited significantly reduced plasma testosterone concentration and elevated cortisol and melatonin levels, the latter resulting from increased activity of hydroxyindole-O-methyltransferase (HIOMT). In LD hamsters but not in those from SD, an intraperitoneal (i.p.) injection of zymosan evoked a well-pronounced peritonitis expressed by increased free radical (ROS) production by peritoneal leukocytes (PTLs) stimulated in vitro with PMA. ROS production by these cells was additionally stimulated by both in vivo and in vitro treatment with melatonin and the latter was partially reversed by melatonin receptor antagonist luzindole. To conclude, in Siberian hamsters melatonin seems to exert rather immunostimulatory than anti-inflammatory effect, therefore other mechanisms, e.g. immunosuppressive effect of glucocorticoids, may underlay the compromised immune status observed in SD in this species.