Melatonin effects on thymus steroid receptors in the course of primary antibody responses: significance of circulating glucocorticoid levels

The neuroprotective and antiapoptotic effects of melatonin in cerebellar neurons involve glucocorticoid receptor and p130 signal pathways

Melatonin is an endocrine factor known to affect a number of physiological functions. The present studies have demonstrated an additional activity for pineal melatonin, specifically associated with the survival and differentiation of neuroblasts. Based on experimental data several conclusions might be drawn. First, melatonin negatively regulates the expression of glucocorticoid receptor (GR) in cerebellar granule neurons. Second, downregulation of GR is associated with a marked decrease in programmed cell death of the granule neurons. Third, melatonin upregulates the expression of p130, which is an essential factor for the initiation and maintenance of neuronal development and differentiation. Thus, melatonin function in postmitotic neurons involves several regulatory pathways with partially overlapping roles. The biological implications are discussed in light of these results.

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.

Bright light exposure during the daytime affects circadian rhythms of urinary melatonin and salivary immunoglobulin A

The effects of bright light exposure during the daytime on circadian urinary melatonin and salivary immunoglobulin A (IgA) rhythms were investigated in an environmental chamber controlled at a global temperature of 27 degrees C+/-0.2 degrees C and a relative humidity of 60%+/-5%. Seven diurnally active healthy females were studied twice, in bright and dim light conditions. Bright light of 5000 lux was provided by placing fluorescent lamps about 1 meter in front of the subjects during the daytime exposure (06:30-19:30) from 06:30 on day 1 to 10:30 on day 3. Dim light was controlled at 200 lux, and the subjects were allowed to sleep from 22:30 to 06:30 under both light exposure conditions. Urine and saliva were collected at 4h intervals for assessing melatonin and IgA. Melatonin excretion in the urine was significantly greater during the nighttime (i.e., at 06:30 on day 1 and at 02:30 on day 2) after the bright light condition than during the dim light condition. Furthermore, the concentration and the amount of salivary IgA tended to be higher in the bright light than in the dim light condition, especially during the night-time. Also, salivary IgA concentration and the total amount secreted in the saliva were significantly positively correlated with urinary melatonin. These results are consistent with the hypothesis that bright light exposure during the daytime enhances the nocturnal melatonin increase and activates the mucosal immune response.

Multiple domains of melatonin receptor are involved in the regulation of glucocorticoid receptor-induced gene expression

Melatonin, the principal hormone of the pineal gland, elicits potent anti-stress, anti-aging and oncostatic properties and influences various immunological and endocrinological functions. We have previously described the effects of melatonin on glucocorticoid receptors and suggested its potential influence on gene transcription. In the present study, the mechanistic basis for melatonin effects on glucocorticoid receptor (GR)-dependent gene expression was examined. Activation of the melatonin transduction pathway affects type I glucocorticoid receptor expression and reduces its transcriptional activity. Coexpression of the intact melatonin and glucocorticoid receptors with MMTV promoter construct reduced the GR transcriptional activity. N- and C-terminus deletions of melatonin receptor revealed the existence of regulatory sites mediating this process. These data identify for first time one of the molecular targets of melatonin action and suggest that melatonin signaling may involve relatively direct signal transmission from the cell surface to the nucleus.

Melatonin, immune modulation and aging

Melatonin is a hormone secreted by the pineal gland in response to photoperiods and influences many important biological processes. For one, Melatonin has been shown to produce resistance to cancer and infectious diseases in aged animals. Studies in animals have demonstrated melatonin-related mechanisms of action on immunoregulation. Additionally, melatonin has been successfully used in humans, along with interleukin-2, as a treatment of solid tumors. In vivo and in vitro studies show melatonin enhances both natural and acquired immunity in animals. Despite all of this intriguing evidence, melatonin's mechanism of action on the immune system is only partially defined. It does, however, appear to act through lymphocyte receptors, and perhaps, receptors on other immune tissues, to modulate immune cells. In order to understand immunomodulation and anti-cancer effects, information on melatonin and it's interactions with other endocrine hormones are summarized.

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

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

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

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

Specific binding of 2-[125I]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.

Tissue-specific modulation of rat glucocorticoid receptor binding activity by melatonin

The effect of melatonin and 2-Iodo-melatonin on nuclear and cytosolic glucocorticoid receptors in the brain, pituitary, thymus and liver has been examined. The results indicate that both melatonin and 2-Iodo-melatonin administration is associated with marked changes in the density and the affinity of cytosolic and nuclear forms of glucocorticoid receptors. These observations are discussed in the context of a possible involvement of pineal melatonin in the mechanisms regulating the behaviour and metabolism of steroid receptors.

Selective effect of melatonin on the proliferation of lymphoid cells

1. The present study was designed to investigate the effect of melatonin on the proliferation of normal lymphocytes and certain T-lymphomas and myelomas under in vitro conditions. 2. The results revealed that administration of 200 microM melatonin inhibited significantly the incorporation of [3H]thymidine into both normal mouse and human lymphocytes and T-lymphoblastoid cell lines. 3. On the contrary, melatonin provoked an increase of myeloma cell proliferation. 4. The influence of melatonin on hybridoma cell lines was negligible. 5. Collectively, these data demonstrated that the chief pineal indole affect selectively the processes of lymphoblastoid cell growth.