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

Sex differences in photoperiodic and stress-induced enhancement of immune function in Siberian hamsters

Siberian hamsters breed during the long days of spring and summer when environmental conditions (e.g., ambient temperatures, food availability) are favorable for reproduction. Environmental conditions may also influence the onset and severity of infection and disease, and photoperiodic alterations in immune function may comprise part of a repertoire of seasonal adaptations to help survive winter. In order to test the hypothesis that animals use day length to anticipate seasonal stressors and adjust immune function, we measured antigen-specific delayed-type-hypersensitivity (DTH) responses in the skin of male and female hamsters during long, "summer-like," or short, "winter-like" days, at baseline and following acute restraint stress. Sex steroid hormones were lower, and cortisol was higher, in males and females during short days. Baseline DTH was enhanced in short- compared to long-day males, and acute stress augmented this effect. In contrast, photoperiod alone did not influence the DTH response in females. As predicted, female hamsters exhibited significantly higher DTH responses than males during long days, but not during short days. However, this enhancement was observed in acutely stressed females only. Cortisol concentrations were significantly higher at baseline in females, and increased more in response to stress, compared to males in both photoperiods. These results suggest that photoperiod provides a useful cue by which stressors in the environment may be anticipated in order to adjust immune function. Furthermore, interactions among reproductive status and stress responses appear to mediate the expression of sex differences in immune responses in hamsters.

Chronopharmacology of melatonin in mice to maximize the antitumor effect and minimize the rhythm disturbance effect

The influence of dosing time on the antitumor effect and the rhythm disturbance effect of melatonin (MLT) was investigated in ICR male mice under a light/dark (12:12) cycle. In tumor-bearing mice, the antitumor effect of MLT (1 mg/kg intraperitoneal) was most effective in the dark phase; and the rhythm disturbance effect of MLT on the locomotor activity was more serious in the light phase than in the dark phase. The antitumor effect and the rhythm disturbance effect of MLT increased when the specific binding of MLT receptor in target tissues, tumor or suprachiasmatic nucleus, increased and they decreased when the level decreased. Furthermore, because luzindole, an MT1 and MT2 blocker, caused the antitumor effect or rhythm disturbance effect of MLT to decrease, it is suggested that the time-dependent change of the pharmacological effects of MLT were influenced by that of MLT receptor(s) function. On the other hand, there was no significant dosing time-dependent change of MLT concentration in tumor or brain after injection. Thus, the pharmacokinetic factor does not seem to contribute to the dosing time-dependent effect of MLT. These results suggest that by choosing the most suitable dosing time for MLT, the efficacy of the drug can be increased, and the toxicity of the drug can be decreased in certain experimental and clinical situations.

Photoperiod modulates melanoma growth in C57BL/6 mice

Seasonal variations can be found in almost any parameter of an organism's biochemistry, physiology, endocrinology, and behaviour. This phenomenon, generally called photoperiodism, results from one of the major functions of the circadian system, i.e. the translation of environmental information into rhythmic intraorganismic signals, which then regulate or influence physiology and pathology. We induced melanoma in three groups of syngeneic C57BL/6 mice synchronised to different photoperiods (8, 12, or 18 h of light within 24-h days) by subcutaneous injections of HFH18 melanoma cell suspensions. All animals from all three photoperiodic groups developed exponentially growing tumors. The average tumor volume on day 31 post injection was significantly smaller in animals exposed to light/dark conditions (LD) 8 : 16 h as compared with animals held in LD 18 : 6 h and intermediate in animals from the equinox group. These results indicate that C57BL/6 mice react to photoperiod, which can exert a significant effect on tumor growth.

Melatonin enhances cellular and humoral immune responses in the Japanese quail (Coturnix coturnix japonica) via an opiatergic mechanism

It is known that melatonin has important immunomodulatory properties in the Japanese quail. However, the mechanism of melatonin action on the immune system is not clearly understood in avian species. In mammals, the immunostimulatory properties of melatonin are mediated by the release of opioid peptides from activated T-lymphocytes. The present study was performed to determine if these same melatonin-induced opioids (MIO) are involved with the immunoenhancing effects of melatonin in quail. Three treatment groups were given melatonin (50 microg/ml) in the drinking water ad libitum along with naltrexone, a known opioid receptor-blocking agent. Melatonin was administered throughout the 3 week study and each bird received a daily intramuscular injection of naltrexone at a dose of 0.1, 1.0, or 10.0 mg/kg. In addition, three control groups were established that received only melatonin, naltrexone, or diluent. Evaluation of the cellular and humoral immune responses was initiated after 2 weeks of treatments. A cutaneous basophil hypersensitivity reaction to phytohemagglutinin (PHA-P) was measured to evaluate the cellular immune response. To evaluate the humoral immune response, primary antibody titers were determined 7 days post-intravenous injection with a Chukar red blood cell (CRBC) suspension. Both the cellular and humoral immune responses were significantly increased by 22 and 34%, respectively, upon melatonin exposure as compared to quail receiving diluent only. Concomitant administration of naltrexone and melatonin significantly reduced the immunoenhancing effect of melatonin across all naltrexone doses. We conclude that melatonin enhances a cellular and humoral immune response in Japanese quail via an opiatergic mechanism.

Seasonal patterns of invasive pneumococcal disease

Pneumococcal infections increase each winter, a phenomenon that has not been well explained. We conducted population-based active surveillance for all cases of invasive pneumococcal disease in seven states; plotted annualized weekly rates by geographic location, age, and latitude; and assessed correlations by time-series analysis. In all geographic areas, invasive pneumococcal disease exhibited a distinct winter seasonality, including an increase among children in the fall preceding that for adults and a sharp spike in incidence among adults each year between December 24 and January 7. Pneumococcal disease correlated inversely with temperature (r -0.82 with a 1-week lag; p<0.0001), but paradoxically the coldest states had the lowest rates, and no threshold temperature could be identified. The pattern of disease correlated directly with the sinusoidal variations in photoperiod (r +0.85 with a 5-week lag; p<0.0001). Seemingly unrelated seasonal phenomena were also somewhat correlated. The reproducible seasonal patterns in varied geographic locations are consistent with the hypothesis that nationwide seasonal changes such as photoperiod-dependent variation in host susceptibility may underlie pneumococcal seasonality, but caution is indicated in assigning causality as a result of such correlations.

Pineal melatonin secretion, but not ocular melatonin secretion, is sufficient to maintain normal immune responses in Japanese quail (Coturnix coturnix japonica)

Reports that plasma melatonin is an important immune regulator in avian species have been rather sparse and contradictory. Also, the primary source of immune-modulating melatonin has yet to be determined in birds. In Japanese quail (Coturnix coturnix japonica), the pineal gland and eyes contribute roughly two thirds and one third of the melatonin found in the blood, respectively. Two experiments were conducted to evaluate melatonin as an immune modulator in Japanese quail and to determine the primary source of immune-modulating melatonin in this species. Experiment 1 was designed to evaluate the involvement of the pineal gland and the eyes in immunocompetence. Each of three groups of quail was assigned a surgical treatment and the cellular and humoral immune responses were determined 8 weeks following surgery. The surgical treatments were pinealectomy (Px), sham pinealectomy (SH-Px), and ocular enucleation (eye removal (Ex)). Experiment 2 utilized exogenous melatonin as a replacement to reconstitute immune responses in surgically immunocompromised birds. In this experiment, 50.0 microg/ml of melatonin, or diluent only, was provided to Px and SH-Px birds in the drinking water ad libitum. The cellular and humoral immune responses were determined after 8 weeks of melatonin treatment. In both experiments, a cutaneous basophil hypersensitivity reaction to phytohemagglutinin was measured to evaluate the cellular immune response. To evaluate the humoral immune response, primary antibody titers were determined 7 days postintravenous injection with a Chukar red blood cell suspension. Flow cytometric analysis of peripheral blood lymphocytes was performed to determine the relative percentage of CD4(+) and CD8(+) T- and B-lymphocytes in all treatments of Experiment 2. In Experiment 1, both the SH-Px and Ex surgical treatments produced similar cellular and humoral immune responses, and these responses were significantly greater than those in Px-treated birds. Pinealectomy significantly reduced the cellular and humoral immune responses from SH-Px by 25.8% and 41.3%, respectively. In Experiment 2, Px again resulted in depressed cellular and humoral immune responses. In addition, Px significantly reduced CD8(+) T-lymphocyte numbers compared to SH-Px, while B-lymphocytes remained unchanged. Melatonin administration to Px birds increased the cellular (32.9%) and humoral (30.6%) immune responses to the level of control (SH-Px) birds, although this reconstitution was not due to increased CD8(+) T- or B-lymphocytes. From these data, it was clear that removal of the pineal gland, but not the eyes, reduced cellular and humoral immune responses, which were reconstituted to normal levels by exogenous melatonin. These data suggest that immunodepression is only observed in birds with two thirds of the plasma melatonin removed by pinealectomy. Removal of one third of the plasma melatonin (by ocular enucleation) is not sufficient to reduce cellular and humoral responses in the Japanese quail.

Photorefractoriness of immune function in male Siberian hamsters (Phodopus sungorus)

Short days induce multiple changes in reproductive and immune function in Siberian hamsters. Short-day reproductive inhibition in this species is regulated by an endogenous timing mechanism; after approximately 20 weeks in short days, neuroendocrine refractoriness to short-day patterns of melatonin develops, triggering spontaneous recrudescence of the reproductive system. It is unknown whether analogous mechanisms control immune function, or if photoperiodic changes in immune function are masked by prevailing photoperiod. In Experiment 1, 3 weeks of exposure to long days was not sufficient to induce long-day-like enhancement of in vitro lymphocyte proliferation in short-day adapted male Siberian hamsters. Experiment 2 tested the hypothesis that immunological photorefractoriness is induced by prolonged exposure to short days. Adult male hamsters were gonadectomized or sham-gonadectomized and housed in long (14 h light/day) or short (10 h light/day) photoperiods for 12, 32 or 40 weeks. Somatic and reproductive regression occurred after 12 weeks in short days, and spontaneous recrudescence was complete after 32-40 weeks in short days, indicative of somatic and reproductive photorefractoriness. In gonad-intact hamsters, 12 weeks of exposure to short days decreased the number of circulating granulocytes and increased the number of B-like lymphocytes. After 32 weeks in short days, these measures were restored to long-day values, indicative of photorefractoriness; castration eliminated these effects of photoperiod. In both intact and castrated hamsters, in vitro proliferation of splenic lymphocytes was inhibited by 12 weeks of exposure to short days. After 40 weeks in short days lymphocyte proliferation was restored to long-day values in intact hamsters, but remained suppressed in castrated hamsters. These results suggest that short-day-induced inhibition of lymphocyte function does not depend on gonadal regression, but that spontaneous recrudescence of this measure is dependent on gonadal recrudescence. In Experiment 3, in vitro treatment with melatonin enhanced basal proliferation of lymphocytes from male hamsters exposed to short days for 12 weeks, but had no effect on lymphocytes of photorefractory hamsters or long-day control hamsters. Lymphocytes of castrated hamsters were unresponsive to in vitro melatonin, suggesting that photoperiodic changes in gonadal hormone secretion may be required to activate mechanisms which permit differential responsiveness to melatonin depending on phase in the annual reproductive cycle. Together, these data indicate that, similar to the reproductive system, the immune system of male Siberian hamsters exhibits refractoriness to short days.

Impaired circadian rhythm of melatonin secretion in sedated critically ill patients with severe sepsis

OBJECTIVE

Melatonin is involved in the regulation of the sleep-wake cycle and exhibits multiple interactions with the neuroendocrine and the immune system. Melatonin secretion in healthy individuals follows a stable circadian rhythm. Critical illness, continuous administration of drugs, and loss of external zeitgeber might impair the circadian rhythm of melatonin secretion in the intensive care unit (ICU), thereby compromising the physiologic stress-induced immune response.

DESIGN

Prospective, controlled clinical study.

SETTING

Medical intensive care unit in a university hospital.

PATIENTS

Seventeen septic, sedated ICU patients (group A); 7 nonseptic ICU patients (group B); and 21 control patients (group C) were studied.

MEASUREMENTS AND MAIN RESULTS

6-Sulfatoxymelatonin (aMT6s) was determined from urine samples taken at 4-hr intervals over a total period of 24 hrs. aMT6s was measured by enzyme-linked immunosorbent assay. Circadian mesors, phase amplitudes, and timing of the acrophase were assessed by cosinor analysis. Differences between groups were calculated by contingency data analysis and by analysis of variance. Circadian mesors of urinary aMT6s were 3904 +/- 1597, 2622 +/- 927, and 3183 +/- 1514 ng/4 hrs in groups A, B, and C, respectively (p = NS). aMT6s exhibited significant circadian periodicity in only 1/17 (6%) patients of group A but in 6/7 (86%) patients of group B and in 18/23 (78%) patients of group C (group A vs. groups B and C: p = .0001) Phase amplitudes were markedly lower in group A (1071 +/- 1005 ng/4 hrs) compared with group B (2284 +/- 581 ng/4 hrs, p = .009) and C (2838 +/- 2255 ng/4 hrs, p = .006). The acrophase was significantly delayed in patients of group A (10:35 am +/- 255 mins) compared with group B (05:43 am +/- 114 mins, p = .01) and group C (4:20 am +/- 107 mins, p < .0001). In sepsis survivors, aMT6s excretion profiles tended to normalize, but still lacked a significant circadian rhythm at ICU discharge.

CONCLUSION

The present study revealed striking abnormalities in urinary aMT6s excretion in septic ICU patients. In contrast, circadian rhythm was preserved in nonseptic ICU patients, indicating that impaired circadian melatonin secretion in septic patients is mainly related to the presence of severe sepsis and/or concomitant medication. Further investigations are required to examine the underlying pathophysiologic mechanism and the clinical implications of this finding.

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.

Influence of near-ultraviolet radiation on reproductive and immunological development in juvenile male Siberian hamsters

The aim of this study was to characterize the lenticular ultraviolet transmission of the Siberian hamster (Phodopus sungorus) and to probe the range of near-ultraviolet (UV-A, 315–400nm) and visible wavelengths (400–760nm) for modulating the photoperiodic regulation of its reproductive and immune systems. Ocular lenses from adult hamsters were found to transmit UV-A wavelengths at similar levels to visible wavelengths, with a short-wavelength cut-off of 300nm. Five separate studies compared the responses of juvenile male hamsters to long photoperiods (16h:8h L:D), short photoperiods (10h:14h L:D) and short photoperiods interrupted by an equal photon pulse of monochromatic light of 320, 340, 360, 500 or 725nm during the night. The results show that UV-A wavelengths at 320, 340 and 360nm can regulate both reproductive and immune short-photoperiod responses as effectively as visible monochromatic light at 500nm. In contrast, long-wavelength visible light at 725nm did not block the short-photoperiod responses. These results suggest that both wavelengths in the visible spectrum, together with UV-A wavelengths, contribute to hamster photoperiodism in natural habitats.

Melatonin modulation of lymphocyte proliferation and Th1/Th2 cytokine expression

Melatonin is hypothesized to play a role in neuroimmunomodulation. This study investigated the in vitro effects of melatonin (10(-12) - 10(-6) M) on human peripheral blood mononuclear cell (PBMC) proliferation and T helper type 1 and T helper type 2 (Th1/Th2) cytokine expression. In vitro doses of melatonin significantly increased PBMC proliferation (p<0.05) and decreased IL-10 production in culture supernatants (p<0.05). However, there was no effect of melatonin on the stimulated production of IFN-gamma or on the intracellular accumulation of the activation antigen CD69, IFN-gamma, or IL-10 as measured by flow cytometry. These data support the notion that physiologic doses of melatonin increase lymphocyte proliferation possibly due to decreases in production of the inhibitory cytokine IL-10.

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

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

Melatonin mediates seasonal changes in immune function

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

Effects of age and photoperiod on reproduction and the spleen in the marsh rice rat (Oryzomys palustris)

To examine the interactions between age and photoperiod on reproduction and spleen weights, we exposed adult male and female rice rats of various ages to photoperiods of 16:8-h light-dark photoperiods (16L:8D) or 12L:12D. After 10 wk, animals were killed and the following data were recorded: weights of testes, seminal vesicles, uterus, ovaries, body, and spleen and, in addition, vaginal patency. Young adult males displayed a greater degree of testicular and seminal vesicle regression in short photoperiods than did older males; the testes of most older males did not regress in response to short photoperiods. Spleen weight was unresponsive to short photoperiods in all males, but was affected by age. Females, however, exhibited reproductive organ regression and decreased vaginal patency in response to short photoperiods at all ages examined. Body weights were affected by photoperiod in young females, and, as in males, photoperiod had no effect on spleen weights. These data suggest that the reproductive response to photoperiod in adult male rice rats declines with age, whereas in adult females it does not.

Differential effects of fluvoxamine and other antidepressants on the biotransformation of melatonin

Melatonin, the predominant product of the pineal gland, is involved in the maintenance of diurnal rhythms. Nocturnal blood concentrations of melatonin have been shown to be enhanced by fluvoxamine, but not by other serotonin reuptake inhibitors. Because fluvoxamine is an inhibitor of several cytochrome P450 (CYP) enzymes, the authors studied the biotransformation of melatonin and the effects of fluvoxamine on the metabolism of melatonin in vitro using human liver microsomes and recombinant human CYP isoenzymes. Melatonin was found to be almost exclusively metabolized by CYP1A2 to 6-hydroxymelatonin and N-acetylserotonin with a minimal contribution of CYP2C19. Both reactions were potently inhibited by fluvoxamine, with a Ki of 0.02 microM for the formation of 6-hydroxymelatonin and 0.05 microM for the formation of N-acetylserotonin. Other than fluvoxamine, fluoxetine, paroxetine, citalopram, imipramine, and desipramine were also tested at 2 and 20 microM. Among the other antidepressants, only paroxetine was able to affect the metabolism of melatonin at supratherapeutic concentrations of 20 microM, which did not reach by far the magnitude of the inhibitory potency of fluvoxamine. The authors concluded that fluvoxamine is a potent inhibitor of melatonin degradation. Because this inhibitory action is also found in vivo, fluvoxamine might be used as an enhancer of melatonin, which might offer new therapeutic possibilities of fluvoxamine.

Pineal modulation of thymus and immune function in a seasonally breeding tropical rodent, Funambulus pennanti

The immune system driven by cytokines is now known to be influenced by various other endocrine glands and its hormones. Results of the present study indicate a bidirectional relation between the pineal-thymus axis and the immune system status of an Indian tropical rodent, Funambulus pennanti, during winter months (reproductive inactive phase), when it faces maximum challenges from nature. Pinealectomy during the reproductive inactive phase inhibited thymus and spleen functions, which resulted in significant changes in leukocyte and lymphocyte counts and T-cell-mediated immune function (measured in terms of delayed-type hypersensitivity response to oxazolone). Blastogenic responses of lymphoid cells (thymocytes, splenocytes, and lymph node cells) also decreased following ablation of the pineal gland. To check the definite role of the pineal gland we injected melatonin into pinealectomized squirrels, and the suppressed immune function was significantly restored. Neuroendocrine control of the pineal gland on the histocompatible tissues in this seasonal breeder, F. pennanti, suggests an adaptive mechanism of the immune system for survival in the tropical zone. J. Exp. Zool. 289:90-98, 2001.

Seasonal variation in host susceptibility and cycles of certain infectious diseases

Seasonal cycles of infectious diseases have been variously attributed to changes in atmospheric conditions, the prevalence or virulence of the pathogen, or the behavior of the host. Some observations about seasonality are difficult to reconcile with these explanations. These include the simultaneous appearance of outbreaks across widespread geographic regions of the same latitude; the detection of pathogens in the off-season without epidemic spread; and the consistency of seasonal changes, despite wide variations in weather and human behavior. In contrast, an increase in susceptibility of the host population, perhaps linked to the annual light/dark cycle and mediated by the pattern of melatonin secretion, might account for many heretofore unexplained features of infectious disease seasonality. Ample evidence indicates that photoperiod-driven physiologic changes are typical in mammalian species, including some in humans. If such physiologic changes underlie human resistance to infectious diseases for large portions of the year and the changes can be identified and modified, the therapeutic and preventive implications may be considerable.

Analysis of the distribution of peripheral blood lymphocyte subsets associated with chronic ethanol treatment in rats with a disturbed circadian cycle

The present study examined the composition of lymphoid subsets in the peripheral blood of alcohol-preferring (PRF) and non-preferring (NPF) rats, in an experimental model of alcoholism involving the disruption of the circadian cycle. The absolute and relative number of lymphocytes in specific subsets (CD3, CD4, CD8, NK, CD45RA) were measured using the flow cytometry method. When control animals with a disrupted circadian cycle (KN) were compared with a normal diurnal cycle group (KD), it was noticed that this disruption led to an increase in the absolute number of lymphocytes T (CD3(+), CD4(+) and CD8(+) cells) and lymphocytes B (CD45RA(+)). After the period of time when the alcohol preference was seen, there was a change in response - as measured by the numbers and the percentage of lymphoid subsets in NPF rats - involving a lowering of NK and CD45RA(+) cells. It seems that these animals exhibit higher sensitivity towards prolonged ethanol intoxication. However, the PRF animals - for whom the analysed values were close to those of the control group (KN) - tolerated the toxic effects of ethanol better and this may be related to their genetic predisposition.

Influence of melatonin on immunotoxicity of lead

The results suggested that immunotoxicity induced by lead [Pb, as Pb(NO(3))(2)] was significantly restored or prevented by melatonin (MLT). MLT (10 or 50 mg/kg) was orally administered to ICR mice daily for 28 days, and Pb was also administered at 35 mg/kg in the same way 2 h after the administration of MLT, and the normal mice were given vehicle. Within the Pb plus MLT-treated group, the body weight gains and the relative thymus weights were significantly increased when compared with the treatment of Pb alone. The relative spleen and liver weights were increased by the treatment of Pb alone, and then restored to normal value by MLT treatment. Hemagglutination (HA) titer, plaque-forming cell response to sheep red blood cell (SRBC), and secondary IgG antibody response to BSA were significantly enhanced in the Pb plus MLT-treated mice, as opposed to when compared with the treatment of Pb alone. The mitogenic response of splenic T cell to concanavalin A and that of B cells to lipopolysaccharide was remarkably increased by MLT treatment when compared with treatment of Pb alone. Splenic CD4(+)cells were significantly increased by MLT treatment when compared with treatment of Pb alone. In case of CD8(+) cells, the slight enhancement was observed in MLT treatment. Splenic T and B cells were significantly increased by MLT treatment when compared with the treatment of Pb alone. The natural killer cell, phagocytic activity and the number of peripheral leukocytes were significantly enhanced in Pb plus MLT-treated mice when compared with the treatment of Pb alone.

Effects of lighting conditions and melatonin supplementation on the cellular and humoral immune responses in Japanese quail Coturnix coturnix japonica

Two experiments were conducted to determine the effects of lighting conditions and melatonin supplementation on the cellular and humoral immune responses in Japanese quail. The first experiment was designed to evaluate differing light regimes as immune modulators in both adult and juvenile quail. The cellular and humoral immune responses were determined for three lighting conditions; short days (8:16LD), long days (16:8LD), and constant light (LL). In the second experiment, melatonin was administered in varying doses to adult quail placed in LL. The doses used in this experiment were 0.0, 0.5, 5.0, and 50.0 microg/ml melatonin given in the drinking water for 16 h per day for 2 weeks. The cellular and humoral immune responses were evaluated after 1 week of melatonin treatment. In both experiments, a cutaneous basophil hypersensitivity reaction to phytohemagglutinin (PHA-P) was measured to evaluate the cellular immune response. To evaluate the humoral immune response, primary antibody titers were calculated 7 days postintravenous injection with a Chukar red blood cell suspension. In the adult birds of experiment 1, both the 8:16LD and 16:8LD treatments produced similar cellular and humoral immune responses but these responses were significantly greater than those observed in LL. The juvenile birds held under 8:16LD also had significantly greater cellular and humoral immune responses as compared to juvenile birds held in LL. In experiment 2, there was a clear melatonin dose response on immune function in LL. The humoral immune response increased to a peak at the 5.0 microg/ml dose while the cellular immune response increased across all dose levels. From the present study it was clear that quail placed in daily light-dark cycles (LD), possessing a diurnal rhythm of melatonin, had significantly elevated immune responses as compared to those birds in LL. Furthermore, melatonin supplemented to birds exposed to LL was immuno-enhancing. This suggests that melatonin may be a mediator of the differences seen between LD and LL lighting conditions and may have important immune modulating properties.

Influence of melatonin on immunotoxicity of cadmium

The results suggested that immunotoxicity induced by Cd was significantly restored or prevented by MLT. MLT (10 or 50 mg/kg) was orally administered to ICR mice daily for 28 consecutive days, and cadmium (Cd, as [Cd(AC)(2)]) was also administered at 25 mg/kg by the same route 2 h after the administration of MLT, and the normal mice were given vehicle. Within the Cd plus MLT-treated group, the body weight gains and relative thymus weights were significantly increased when compared with the treatment of Cd alone. The relative spleen and liver weights were increased by treatment of Cd alone, then restored to normal value by MLT treatment. Hemagglutination (HA) titer, primary IgM antibody response to SRBC, and secondary IgG antibody response to BSA was significantly increased with the Cd plus MLT-treated mice, as opposed to when compared with treatment of Cd alone. The NK cell and phagocytic activity used for evaluation of non-specific immunocompetence was significantly increased in Cd plus MLT-treated mice when compared with the treatment of Cd alone. The number of peripheral leukocytes was significantly increased in Cd plus MLT-treated mice when compared with treatment of Cd alone.

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.

In vitro melatonin treatment enhances splenocyte proliferation in prairie voles

The seasonal effects of photoperiod on reproduction are mediated by melatonin, and it is hypothesized that increased immune function in short days is due to the increase in the duration of nightly melatonin secretion. Melatonin can act both directly and indirectly on target tissue within the immune system. The present study sought to tease apart the direct and indirect effects of melatonin on one aspect of immune function by examining the influence of in vitro melatonin on splenocyte proliferation in female prairie voles held in long (LD 16:8) or short (LD 8:16) days. Splenocyte proliferation in response to the T-cell mitogen concanavalin A was enhanced by the addition of melatonin in vitro, as compared to cultures receiving no melatonin. Body mass increased in short-day housed prairie voles, indicating that the animals were responsive to photoperiod. However, photoperiod did not affect splenocyte proliferation in the present study. These results support the hypothesis that melatonin exerts a direct effect on splenocyte proliferation, potentially via high-affinity melatonin receptors localized on splenocytes. The findings also indicate that, irrespective of photoperiod, melatonin exerts direct effects on splenocytes to enhance immune function.

Identification of G-protein coupled receptor subunits in normal human dental pulp

To respond appropriately to their environment, dental pulp cells must integrate informational input from multiple ligands, such as neuropeptides, growth factors, and vasoactive amines. These ligands act through multiple intracellular signaling pathways. G-protein coupled receptor subunits play a major role in this process, providing a mechanism for coordinated regulation of both messengers and effectors. Increasing number of neuropeptides have been found in pulpal tissue. However, there is no data about molecular identification of G-protein subunits in human dental pulp. To identify the postreceptor mechanism involved in dental pulp cell signal transduction, we performed a Western blot analysis of different G-protein subunits. Biopsy specimens of human dental pulp were prepared and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by analysis with appropriate antibodies. We detected G alpha q/alpha 11, short and long forms of G alpha s, beta common, Gio-3, and Gil-2 antigens with a molecular weight approximately 42 kDa, 42 and 45 kDa, 36 kDa, 40 kDa, and 40 kDa, respectively. These results indicate that human pulp cells possess the cellular machinery to respond to sensory neuropeptides when they are released from the peptidergic nerve fibers. On this basis, the relationships of postdevelopmental, age-dependent, and pathophysiological disorders of G-proteins subunits in dental pulp could be studied.

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.

Role of photoperiod and the pineal gland in T cell-dependent humoral immune reactivity in the Siberian hamster

The present study tested the hypothesis that antibody production in response to xenoantigen is modulated by daylength and dependent upon the pineal gland. Alter injection of sheep erythrocytes (SRBC), serum immunoglobulin (Ig) concentrations were 5-fold lower in hamsters in short versus long days. Pinealectomy (Pinx) abolished the nocturnal melatonin rhythm, blocked short-day-mediated testis regression, and eliminated the short-day reduction in Ig production after SRBC treatment. Antibody titers in response to SRBC were equivalently augmented in short-day Pinx and long-day sham hamsters. The results indicate that photoperiodic effects on T cell-dependent humoral immunity are dependent upon the pineal gland. These findings raise the possibility that day length-associated changes in some immune system functions are mediated by the pineal melatonin rhythm.

Melatonin influences interleukin-12 and nitric oxide production by primary cultures of rheumatoid synovial macrophages and THP-1 cells

Because some of the clinical symptoms related to rheumatoid arthritis (RA) synovitis, such as joint morning stiffness and gelling, might be related to the effects exerted by the diurnal rhythmicity of the neurohormone melatonin (MLT) on synovial immune cell activation, we decided to evaluate the influence of MLT on the production of IL-12 and nitric oxide (NO) on primary cultures of RA synovial macrophages. Synovial macrophages were also prestimulated with lipopolysaccaride (LPS). Results were compared with those obtained on cultured human myeloid monocytic cells (THP-1). A significant increase in IL-12 (p = 0.01) was found in media of MLT-stimulated synovial macrophages versus RMPI-treated synovial macrophage controls. Interestingly, a significant decrease of IL-12 (p < 0.0001) was observed in media of synovial macrophages previously activated with LPS and then treated with MLT, when compared to synovial macrophages treated with LPS alone. A significant increase in NO levels (p = 0.01) was found in MLT-stimulated synovial macrophages versus RMPI-treated synovial macrophage controls. Interestingly, a nonsignificant increase of NO levels was observed in media of synovial macrophages previously activated with LPS and then treated with MLT, when compared to cynovial macrophages treated with LPS alone. Finally, a significant increase in IL-12 (p = 0.03) and NO (p = 0.002) concentrations was observed in media of MLT-stimulated THP-1 cells versus RMPI-treated controls. Our results therefore show that MLT induces IL-12 secretion and NO production by unstimulated cultured RA synovial macrophages and human monocytic myeloid THP-1 cells. The unexpected and opposite effects on IL-12 and NO production in RA synovial macrophages treated with LPS may be related to dose-dependent mechanisms exerted by MLT or to altered cell priming in RA macrophages; these are matters of our further research. This study strongly supports the role of MLT in immune response modulation and in particular suggests a close relationship between diurnal rhythmicity of neuroendocrine pathways, cytokine and reactive oxygen intermediate production by monocyte/macrophages, and synovial arthritis symptomatology, at least in RA.

The pineal secretory product melatonin reduces hydrogen peroxide-induced DNA damage in U-937 cells

Melatonin, the chief secretory product of the pineal gland, is a potent and efficient endogenous radical scavenger. Thus, melatonin was shown to protect different biomolecules, such as DNA, membrane lipids, and cytosolic proteins, from oxidative damage induced by oxygen-derived free radicals. In order to study the protective role of melatonin in hydrogen peroxide (H2O2)-induced DNA damage, U-937 cells were treated with different concentrations of H2O2, either in the presence or absence of melatonin, and DNA damage was assessed using the cytokinesis-block micronucleus technique. Melatonin diminished H2O2-induced micronuclei production both in short and long treatments. Additionally, melatonin concentrations higher than 1 microM were capable of protecting cells from spontaneous micronuclei production. These data suggest that melatonin, an endogenous antioxidant and nontoxic compound, may have an important role in protecting cells from genetic damage due to free radicals, supporting the idea of this hormone as a possible therapeutic agent in preventing aging and age-related diseases.

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

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

Normative melatonin excretion: a multinational study

The present study on overnight urinary melatonin was conducted on the most geographically dispersed population to date, over a 1 year period, also covering a broad age range (18-62 years). An inverse relationship between melatonin and age, as well as between melatonin and weight was observed for both genders. Females as a whole, had higher melatonin values than males. Furthermore, the excretion of melatonin exhibited a bimodal distribution, distinguishing two groups of individuals: low and high melatonin excretors. The cut-off point was set at 0.25 nmol/l for ages up to 40 years and at 0.20 nmol/l for subjects above this age. Since melatonin may be involved in several physiological and pathological processes, it could be of importance to detect the type of melatonin excretion that prevails in various conditions, using a simple noninvasive procedure such as the overnight urinary measurement. For that purpose, this large sample could serve as a worldwide reference databank across different ages and locations.

Prevention of immune dysfunction and vitamin E loss by dehydroepiandrosterone and melatonin supplementation during murine retrovirus infection

Female C57BL/6 mice infected with the LP-BM5 leukaemia retrovirus developed murine acquired immune-deficiency syndrome (AIDS). Dehydroepiandrosterone (DHEA) and melatonin (MLT) modify immune dysfunction and prevent lipid peroxidation. We investigated whether DHEA and MLT could prevent immune dysfunction, excessive lipid peroxidation, and tissue vitamin E loss induced by retrovirus infection. Retrovirus infection inhibited the release of T helper 1 (Th1) cytokines, stimulated secretion of Th2 cytokines, increased hepatic lipid peroxidation, and induced vitamin E deficiency. Treatment with DHEA or MLT alone, as well as together, largely prevented the reduction of B- and T-cell proliferation as well as of Th1 cytokine secretion caused by retrovirus infection. Supplementation also suppressed the elevated production of Th2 cytokines stimulated by retrovirus infection. DHEA and MLT simultaneously reduced hepatic lipid peroxidation and prevented vitamin E loss. The use of DHEA plus MLT was more effective in preventing retrovirus-induced immune dysfunction than either DHEA or MLT alone. These results suggest that supplementation with DHEA and MLT may prevent cytokine dysregulation, lipid oxidation and tissue vitamin E loss induced by retrovirus infection. Similarly, hormone supplementation also modified immune function and increased tissue vitamin E levels in uninfected mice.

The role of granulocyte-macrophage-colony stimulating factor, cortisol, and melatonin in the regulation of the circadian rhythms of peripheral blood cells in healthy volunteers and patients with breast cancer

The circulating blood cells show highly reproducible circadian rhythms. However, the factors that regulate these rhythms are not well understood. In the current study, we examined the diurnal variations of peripheral blood cells (white blood cells, neutrophils, lymphocytes), granulocyte-macrophage-colony stimulating factor (GM-CSF), and melatonin levels, and considered the role of melatonin on these rhythms in healthy volunteers and in patients with early breast cancer. Fourteen premenopausal patients with early stage breast cancer (T2, N1 tumors) and 10 premenopausal healthy volunteers were included in the study. Blood samples were taken every 4 hr for a period of 24 hr. Peripheral blood cells were counted by automated analyser and also from peripheral blood films. GM-CSF levels were measured by ELISA and melatonin levels by radioimmunoassay (RIA). Serum melatonin, cortisol, and GM-CSF levels, and peripheral blood cell counts showed significant circadian rhythms in healthy volunteers. Except for GM-CSF, these circadian rhythms were found not to be suppressed in early breast cancer patients. While there were significant correlations of serum GM-CSF and cortisol levels with peripheral blood cell counts in healthy volunteers, only lymphocyte counts were found to be significantly correlated with serum GM-CSF and cortisol levels in patients with breast cancer. Serum melatonin levels were found to be significantly correlated with lymphocyte counts in both groups. Our results suggest that peripheral blood cells show significant circadian rhythms in both healthy volunteers and in patients with stage II (T2, N1) breast cancer, and GM-CSF, cortisol, and melatonin may have a role in the regulation of peripheral blood cell counts.

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.

Time-dependent effects of melatonin on immune measurements in male Syrian hamsters

Adult, male Syrian hamsters received daily subcutaneous melatonin (25 microg) injections or vehicle injections at 08:00 or 17:00 hr for 11 weeks. Body weights were measured weekly throughout the experiment and testes weights, spleen weights, and serum was collected at the end of the experiment. The spleens were sectioned and immunocytochemically analyzed for immunoglobulin G and serum levels of interferon-gamma, interleukin-2, and interleukin-4 were determined in heterologous mouse assays. Melatonin injections at 17:00 hr, but not at 08:00 hr, increased body weights, decreased testes weights and serum testosterone levels, and had no effect on immunoglobulin G content in the spleen. Likewise, melatonin injections at 17:00 hr, but not at 08:00 hr, increased serum interferon-gamma levels, had no effect on interleukin-2 levels, and appeared to increase interleukin-4 levels. Since melatonin injections at 08:00 hr were ineffective in altering immune measurements and correlations between reproductive measures and immune measures were high, the most parsimonious explanation for these results is that melatonin injections at 17:00 hr depressed reproductive hormone levels and these depressed levels altered immune measures.

Short photoperiod affects reproductive function but not dehydroepiandrosterone concentrations in male deer mice (Peromyscus maniculatus)

Cessation of breeding is central among the suite of winter-coping strategies used by small rodents to survive energy-demanding winter conditions. Animals use photoperiod to predict the onset of winter and initiate, well in advance of deteriorating conditions, seasonal adaptations. Exposure to short photoperiod leads to regression of the reproductive system in long-day breeding animals. Likewise, exposure to short days leads to enhanced immune function among several rodent species studied. Because dehydroepiandrosterone (DHEA) increases immune function in virtually all studies reported to date, we sought to determine if DHEA concentrations might be influenced by photoperiod, thereby suggesting a mechanism whereby short photoperiod may enhance immune function. Male deer mice (Peromyscus maniculatus) were exposed to either short or long days for 10 weeks. Short photoperiods caused significant reduction in all reproductive organs measured relative to animals housed in long days. However, DHEA concentrations did not differ between short- and long-day mice. Taken together, these data suggest that short-day enhancement of immune function in deer mice is independent of DHEA concentrations.

Circadian variations in the rat serum total antioxidant status: correlation with melatonin levels

In this paper, we show for the first time, a nyctohemeral rhythm in serum total antioxidant status (TAS) in rats which parallels the 24-H melatonin cycle. Both TAS and melatonin in rat serum exhibited 24 hr variations with nocturnal peak values at 05.00 hr and low basal values during the day. When rats were maintained under light exposure (>500 lux) from 20.00 h to 05.00 hr, serum TAS was significantly reduced when compared with control rat killed in darkness. Moreover, when animals were maintained under continuous light exposure for 5 days and killed at 05.00 hr, serum TAS exhibited an additional decrease when compared with control rats. Since administering exogenous melatonin also increased TAS in the rat serum, results suggest that melatonin may be relevant in terms of participating in the antioxidative capacity of the rat serum.

Physiological concentrations of melatonin inhibit the norepinephrine-induced activation of prostaglandin E2 and cyclic AMP production in rat hypothalamus: a mechanism involving inhibiton of nitric oxide synthase

In this paper, we summarize the results of in vitro studies showing that physiological concentrations of melatonin inhibit the norepinephrine-induced activation of prostaglandin E2 (PGE2) and cyclic AMP production in rat medial basal hypothalamus (MBH). Interestingly, a concentration of melatonin as low as 1 nM, which is roughly equivalent to the nocturnal serum physiological concentration of the hormone in the rat, significantly inhibit PGE2 and cyclic AMP production in the MBH. The suppressive effect of melatonin may be mediated by an inhibition of nitric oxide synthase (NOS) activity, since the stimulatory effect of sodium nitroprusside (SNP), a spontaneous generator of NO, was not prevented by melatonin. Melatonin also inhibited NOS activity in rat MBH in a dose-dependent manner. The results suggest the existence of a new or an ancillary means by which melatonin may regulate the physiology of the hypothalamus-pituitary unit.

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.

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.

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

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

Presence of the pineal hormone melatonin in rat cochlea: its variations with lighting conditions

Within the normal inner ear, there are elements that belong to the immune system. Different inner ear disorders can be explained by autoimmune mechanisms, affecting both humoral and cellular immunity. Melatonin, the principal hormone of the pineal gland, modulates the immune system and extensively participates in the autoimmune processes related to type II collagen. Therefore, we have studied the presence of melatonin in rat cochlea, proving that its concentrations change depending on lighting conditions. Rats show high levels when confined to darkness and low levels when subject to continuous light exposure. The results correlate with the concentration of melatonin in peripheral circulation. Further experimental and clinical studies are necessary to clarify the role and the possible therapeutic applications of melatonin.

Role of melatonin in mediating seasonal energetic and immunologic adaptations

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

Immune responsiveness of splenocytes after chronic daily melatonin administration in male Syrian hamsters

The interrelationships between the immune system and the pineal hormone, melatonin, have been explored recently. The present studies investigated the effects of daily melatonin injections on reproductive and spleen function in male Syrian hamsters. Testes weights and serum testosterone levels were depressed after 8-10 weeks of daily melatonin injections. Melatonin-treated hamsters exhibited increased splenic lymphoproliferative responses to a polyclonal T-cell mitogen (concanavalin A (Con-A)), but decreased proliferation following stimulation with a polyclonal B-cell mitogen (lipopolysaccharide). It appears that daily melatonin injections in male hamsters increase the T-cell-mediated immune capacity while reducing the antibody-mediated immune potential. These data suggest that chronic, daily melatonin alters immune system responsiveness in hamsters by shifting the balance of cellular and humoral reactivity.

Inhibition of cerebellar nitric oxide synthase and cyclic GMP production by melatonin via complex formation with calmodulin

Constitutive rat cerebellar nitric oxide synthase (NOS) activity is shown to be inhibited by physiological concentrations of the pineal hormone melatonin. The inhibition was dose-dependent and was coupled to an inhibition of the cyclic GMP production activated by L-arginine. Results also show that calmodulin appears to be involved in this process because its presence in the incubation medium was able to prevent the effect of melatonin on both NOS activity and cyclic GMP production. Moreover, polyacrylamide gel electrophoresis studies suggest that melatonin can interact with calmodulin modifying the binding of the peptide to the synthetic NOS peptide encompassing the calmodulin-binding domain of constitutive NOS from rat cerebellum, the natural mechanism by which calmodulin activates cerebellar NOS.

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.

Binding of [125I]iodocyanopindolol by rat harderian gland crude membranes: kinetic characteristics and day-night variations

The Harderian glands are innervated by sympathetic fibers originating in the superior cervical ganglia. The aim of this study is to characterize the beta-adrenergic receptors in the rat Harderian gland. The characteristics of beta-adrenergic receptors were determined in crude membrane preparations from rat Harderian gland, using [125I]iodocyanopindolol ([125I]CYP) as radioligand. The binding of the ligand to the receptor is rapid, reversible, saturable, specific and dependent on time, temperature and membrane concentration. At 30 degrees C, stoichiometric data suggest the presence of one binding site with a Kd value of 0.29 nM and Bmax of 32 pmol/L. The interaction shows a high degree of specificity for beta-adrenergic agonists and blockers, as suggested by competitive displacement experiment with isoproterenol (IC50 = 19.1 nM), propranolol (IC50 = 28.1 nM), and norepinephrine (IC50 = 96.3 nM). Clonidine, yohimbine, methoxamine, and prazosin are ineffective at concentrations up to 1 microM. In the other hand, binding of [125I]CYP by Harderian gland membranes exhibits day-night variations. Binding values are low during the daytime and increase progressively late in the evening to reach a maximum at 2200 h (2 h after the onset of dark period), but decreased to the end of the dark period (0600 h). In conclusion, the results presented in this paper show the functional and pharmacological characterization of beta-adrenergic receptors in the rat Harderian gland. This neurotransmitter may play a physiological role at this level regulating, at least, processes such as a thyroid hormone metabolism.

Melatonin reduces nitric oxide synthase activity in rat hypothalamus

In this report, rat hypothalamic nitric oxide synthase (NOS) activity is shown to be partially inhibited by physiological concentrations of the pineal hormone melatonin. In vitro studies demonstrate that 1 nM melatonin, which approximates the physiological concentration of the hormone at night, significantly inhibited NOS activity. In vivo studies show that administering melatonin or collecting the hypothalamus from animals at night, when endogenous melatonin levels are elevated, results in a significant decrease of NOS activity. Results also show that calmodulin may be involved in this process since its presence in the incubation medium prevents the inhibitory effect of melatonin on NOS activity.