Melatonin regulates energy balance and attenuates fever in Siberian hamsters

Thermoregulatory costs of the innate immune response are modulated by winter food availability in a small passerine

In winter, a challenge to the immune system could pose a major energetic trade-off for small endotherms, whereby increasing body temperature (T_b ; i.e. eliciting fever) may be beneficial to fight off invading pathogens yet incur a cost for vital energy-saving mechanisms. Having previously shown that the availability and acquisition of energy, through manipulation of food predictability, influences the depth of rest-phase hypothermia in a wild bird in winter, we expected that the nocturnal thermoregulatory component of the acute-phase immune response would also be modulated by food availability. By manipulating winter food availability in the wild for great tits Parus major, we created an area offering a "predictable" and constant supply of food at feeding stations, while an unmanipulated area was subject to naturally "unpredictable" food. Birds were subject to an immune challenge shortly after dusk, and the thermoregulatory response was quantified via continuous recording of nocturnal T_b , using subcutaneous thermo-sensitive transponders. In response to immune challenge, all birds increased T_b above the level maintained prior to immune challenge (i.e. baseline). However, birds experiencing a naturally unpredictable food supply elevated T_b more than birds subject to predictable food resources, during the period of expected peak response and for the duration of the night. Furthermore, "unpredictable-food" females took longer to return to their baseline T_b . Assuming baseline nocturnal T_b reflects an individual's optimum, based on their available energy budget, the metabolic cost of eliciting an acute-phase response for "unpredictable-food" birds was more than double that of "predictable-food" birds. The absence of differences in absolute T_b during the peak response could support the idea of an optimal T_b for immune system activation. Alternatively, "predictable-food" birds could have acquired tolerance to endotoxin as a result of using feeding stations, thus affording them reduced costs associated with a smaller T_b increase. These findings shed new light on the trade-offs associated with food acquisition, thermoregulation and immune function in small-bodied endotherms. This knowledge is of increasing importance, given the predicted elevated pathogen risks associated with changes in climate and anthropogenic activities.

Influence of innate immune activation on endocrine and metabolic pathways in infancy

Prematurity is the leading cause of neonatal morbidity and mortality worldwide. Premature infants often require extended hospital stays, with increased risk of developing infection compared with term infants. A picture is emerging of wide-ranging deleterious consequences resulting from innate immune system activation in the newborn infant. Those who survive infection have been exposed to a stimulus that can impose long-lasting alterations into later life. In this review, we discuss sepsis-driven alterations in integrated neuroendocrine and metabolic pathways and highlight current knowledge gaps in respect of neonatal sepsis. We review established biomarkers for sepsis and extend the discussion to examine emerging findings from human and animal models of neonatal sepsis that propose novel biomarkers for early identification of sepsis. Future research in this area is required to establish a greater understanding of the distinct neonatal signature of early and late-stage infection, to improve diagnosis, curtail inappropriate antibiotic use, and promote precision medicine through a biomarker-guided empirical and adjunctive treatment approach for neonatal sepsis. There is an unmet clinical need to decrease sepsis-induced morbidity in neonates, to limit and prevent adverse consequences in later life and decrease mortality.

The paraventricular thalamus serves as a nexus in the regulation of stress and immunity

Many temperate zone animals exhibit seasonal rhythms in physiology and behavior, including seasonal cycles of reproduction, energetics, stress responsiveness, and immune function, among many others. These rhythms are driven by seasonal changes in the duration of pineal melatonin secretion. The neural melatonin target tissues that mediate several of these rhythms have been identified, though the target(s) mediating melatonin's regulation of glucocorticoid secretion, immune cell numbers, and bacterial killing capacity remain unspecified. The present results indicate that one melatonin target tissue, the paraventricular nucleus of the thalamus (PVT), is necessary for the expression of these seasonal rhythms. Thus, while radiofrequency ablations of the PVT failed to alter testicular and body mass response to short photoperiod exposure, they did block the effect of short day lengths on cortisol secretion and bacterial killing efficacy. These results are consistent with the independent regulation by separate neural circuits of several physiological traits that vary seasonally in mammals.

The effect of melatonin from slow-release implants on basic and TLR-4-mediated gene expression of inflammatory cytokines and their receptors in the choroid plexus in ewes

The present study concerns the effect of melatonin from slow-release implants on the expression of genes coding interleukin-1β (Il1B), inerleukin-6 (Il6), tumour necrosis factor α (Tnf) and their receptors: IL-1 receptor type I (Il1r1) and type II (Il1r2), IL-6 receptor (Il6r) and signal transducer (Il6st), TNFα receptor type I (Tnfrsf1a) and II (Tnfrsf1b) and retinoid-related orphan receptor α (RorA) and Rev.-erbα in the ovine choroid plexus (CP) under basal and lipopolysaccharide (LPS)-challenged conditions. Studies were performed on four groups: 1) sham-implanted and placebo-treated, 2) melatonin-implanted (Melovine, 18mg) and placebo-treated, 3) sham-implanted and LPS-treated (400ng/kg of body weight) and 4) melatonin-implanted and LPS-treated. Under basal conditions, we observed weak expression of Tnf, low expression of Il1B, Il6 and Il1r2 and intermediate expression of other cytokines receptors. LPS treatment induced (P≤0.05) expression in all cytokines and their receptors, except Il6r 3h after the administration. Melatonin attenuated (P≤0.05) LPS-induced up-regulation of Il6 but had no effect on other cytokines and their receptors and up-regulated (P≤0.05) Rev.-erbα expression under basal conditions. This indicates that melatonin from slow-release implants suppresses TLR4-mediated Il6 expression in the ovine CP via a mechanism likely involving clock genes.

Glucose and insulin modulate sickness responses in male Siberian hamsters

Mounting a sickness response is an energetically expensive task and requires precise balancing of energy allocation to ensure pathogen clearance while avoiding compromising energy reserves. Sickness intensity has previously been shown to be modulated by food restriction, body mass, and hormonal signals of energy. In the current study, we tested the hypothesis that sickness intensity is modulated by glucose availability and an endocrine signal of glucose availability, insulin. We utilized male Siberian hamsters (Phodopus sungorus) and predicted that pharmacological induction of glucoprivation with 2-deoxy-d-glucose (2-DG), a non-metabolizable glucose analog that disrupts glycolysis, would attenuate energetically expensive sickness symptoms. Alternatively, we predicted that treatment of animals with insulin would enhance energetically expensive sickness symptoms, as insulin would act as a signal of increased glucose availability. Upon experimental treatment with lipopolysaccharide (LPS), we found that glucose deprivation resulted in increased sickness-induced hypothermia as compared to control- and insulin-treated animals; however, it did not have any effects on sickness-induced anorexia or body mass loss. Insulin treatment resulted in an unexpectedly exaggerated sickness response in animals of lesser body masses; however, in animals of greater body masses, insulin actually attenuated sickness-induced body mass loss and had no effects on hypothermia or anorexia. The effects of insulin on sickness severity may be modulated by sensitivity to sickness-induced hypoglycemia. Collectively, these results demonstrate that both glucose availability and signals of glucose availability can modulate the intensity of energetically expensive sickness symptoms, but their effects differ among different sickness symptoms and are sensitive to energetic context.

Neuroendocrine-immune circuits, phenotypes, and interactions

Multidirectional interactions among the immune, endocrine, and nervous systems have been demonstrated in humans and non-human animal models for many decades by the biomedical community, but ecological and evolutionary perspectives are lacking. Neuroendocrine-immune interactions can be conceptualized using a series of feedback loops, which culminate into distinct neuroendocrine-immune phenotypes. Behavior can exert profound influences on these phenotypes, which can in turn reciprocally modulate behavior. For example, the behavioral aspects of reproduction, including courtship, aggression, mate selection and parental behaviors can impinge upon neuroendocrine-immune interactions. One classic example is the immunocompetence handicap hypothesis (ICHH), which proposes that steroid hormones act as mediators of traits important for female choice while suppressing the immune system. Reciprocally, neuroendocrine-immune pathways can promote the development of altered behavioral states, such as sickness behavior. Understanding the energetic signals that mediate neuroendocrine-immune crosstalk is an active area of research. Although the field of psychoneuroimmunology (PNI) has begun to explore this crosstalk from a biomedical standpoint, the neuroendocrine-immune-behavior nexus has been relatively underappreciated in comparative species. The field of ecoimmunology, while traditionally emphasizing the study of non-model systems from an ecological evolutionary perspective, often under natural conditions, has focused less on the physiological mechanisms underlying behavioral responses. This review summarizes neuroendocrine-immune interactions using a comparative framework to understand the ecological and evolutionary forces that shape these complex physiological interactions.

The role of the hypothalamic-pituitary-adrenal axis in modulating seasonal changes in immunity

Seasonal changes in environmental conditions are accompanied by significant adjustment of multiple biological processes. In temperate regions, the day fraction, or photoperiod, is a robust environmental cue that synchronizes seasonal variations in neuroendocrine and metabolic function. In this work, we propose a semimechanistic mathematical model that considers the influence of seasonal photoperiod changes as well as cellular and molecular adaptations to investigate the seasonality of immune function. Our model predicts that the circadian rhythms of cortisol, our proinflammatory mediator, and its receptor exhibit seasonal differences in amplitude and phase, oscillating at higher amplitudes in the winter season with peak times occurring later in the day. Furthermore, the reduced photoperiod of winter coupled with seasonal alterations in physiological activity induces a more exacerbated immune response to acute stress, simulated in our studies as the administration of an acute dose of endotoxin. Our findings are therefore in accordance with experimental data that reflect the predominance of a proinflammatory state during the winter months. These changes in circadian rhythm dynamics may play a significant role in the seasonality of disease incidence and regulate the diurnal and seasonal variation of disease symptom severity.

Mechanisms of fever production and lysis: lessons from experimental LPS fever

Fever is a cardinal symptom of infectious or inflammatory insults, but it can also arise from noninfectious causes. The fever-inducing agent that has been used most frequently in experimental studies designed to characterize the physiological, immunological and neuroendocrine processes and to identify the neuronal circuits that underlie the manifestation of the febrile response is lipopolysaccharide (LPS). Our knowledge of the mechanisms of fever production and lysis is largely based on this model. Fever is usually initiated in the periphery of the challenged host by the immediate activation of the innate immune system by LPS, specifically of the complement (C) cascade and Toll-like receptors. The first results in the immediate generation of the C component C5a and the subsequent rapid production of prostaglandin E2 (PGE2). The second, occurring after some delay, induces the further production of PGE2 by induction of its synthesizing enzymes and transcription and translation of proinflammatory cytokines. The Kupffer cells (Kc) of the liver seem to be essential for these initial processes. The subsequent transfer of the pyrogenic message from the periphery to the brain is achieved by neuronal and humoral mechanisms. These pathways subserve the genesis of early (neuronal signals) and late (humoral signals) phases of the characteristically biphasic febrile response to LPS. During the course of fever, counterinflammatory factors, "endogenous antipyretics," are elaborated peripherally and centrally to limit fever in strength and duration. The multiple interacting pro- and antipyretic signals and their mechanistic effects that underlie endotoxic fever are the subjects of this review.

Body mass affects seasonal variation in sickness intensity in a seasonally breeding rodent

Species that display seasonal variation in sickness intensity show the most intense response in the season during which they have the highest body mass, suggesting that sickness intensity may be limited by an animal's energy stores. Siberian hamsters (Phodopus sungorus) display lower body masses and less intense sickness when housed in short, winter-like days as opposed to long, summer-like days. To determine whether reduced sickness intensity displayed by short-day hamsters is a product of seasonal changes in body mass, we food restricted long-day hamsters so that they exhibited body mass loss that mimicked the natural photoperiod-induced loss of body mass in short-day hamsters. We then experimentally induced sickness with lipopolysaccharide (LPS) and compared sickness responses among long-day food-restricted and long- and short-day ad libitum fed groups, predicting that long-day food-restricted hamsters would show sickness responses comparable to those of short-day ad libitum fed hamsters and attenuated in comparison to long-day ad libitum fed hamsters. We found that long-day food-restricted hamsters showed attenuated LPS-induced anorexia, loss of body mass and hypothermia compared with long-day ad libitum fed animals; however, anorexia remained elevated in long-day food-restricted animals compared with short-day ad libitum fed animals. Additionally, LPS-induced anhedonia and decreases in nest building were not influenced by body mass. Results of hormone assays suggest that cortisol levels could play a role in the attenuation of sickness in long-day food-restricted hamsters, indicating that future research should target the roles of glucocorticoids and natural variation in energy stores in seasonal sickness variation.

Neuroendocrine control of photoperiodic changes in immune function

Seasonal variation in immune function putatively maximizes survival and reproductive success. Day length (photoperiod) is the most potent signal for time of year. Animals typically organize breeding, growth, and behavior to adapt to spatial and temporal niches. Outside the tropics individuals monitor photoperiod to support adaptations favoring survival and reproductive success. Changes in day length allow anticipation of seasonal changes in temperature and food availability that are critical for reproductive success. Immune function is typically bolstered during winter, whereas reproduction and growth are favored during summer. We provide an overview of how photoperiod influences neuronal function and melatonin secretion, how melatonin acts directly and indirectly to govern seasonal changes in immune function, and the manner by which other neuroendocrine effectors such as glucocorticoids, prolactin, thyroid, and sex steroid hormones modulate seasonal variations in immune function. Potential future research avenues include commensal gut microbiota and light pollution influences on photoperiodic responses.

Photoperiodic time measurement and seasonal immunological plasticity

Seasonal variations in immunity are common in nature, and changes in day length are sufficient to trigger enhancement and suppression of immune function in many vertebrates. Drawing primarily on data from Siberian hamsters, this review describes formal and physiological aspects of the neuroendocrine regulation of seasonal changes in mammalian immunity. Photoperiod regulates immunity in a trait-specific manner, and seasonal changes in gonadal hormone secretion and thyroid hormone signaling all participate in seasonal immunomodulation. Photoperiod-driven changes in the hamster reproductive and immune systems are associated with changes in iodothyronine deiodinase-mediated thyroid hormone signaling, but photoperiod exerts opposite effects on select aspects of the epigenetic regulation of reproductive neuroendocrine and lymphoid tissues. Photoperiodic changes in immunocompetence may explain a proportion of the annual variance in disease incidence and severity in nature, and provide a useful framework to help understand brain-immune interactions.

Leptin mediates seasonal variation in some but not all symptoms of sickness in Siberian hamsters

Many seasonally breeding species, including Siberian hamsters (Phodopus sungorus), exhibit seasonal variation in sickness responses. One hypothesis regarding the mechanism of this variation is that sickness intensity tracks an animal's energetic state, such that sickness is attenuated in the season that an animal has the lowest fat stores. Energetic state may be signaled via leptin, an adipose hormone that provides a signal of fat stores. Siberian hamsters respond to extended housing in short, winter-like days by reducing fat stores and leptin levels, relative to those housed in long, summer-like days. Sickness responses are also attenuated in short-day hamsters as compared to long-day hamsters. We hypothesized that leptin provides a physiological signal by which seasonally breeding animals modulate sickness responses, such that animals with higher leptin levels show increased sickness intensity. To test this, we provided short-day hamsters with a long-day-like leptin signal and assessed their responses to lipopolysaccharide (LPS), a sickness-inducing antigen. We compared these responses to short-day vehicle-, long-day vehicle-, and long-day leptin-treated hamsters. Unexpectedly, LPS induced a hypothermic response (rather than fever) in all groups. Short-day vehicle-treated hamsters exhibited the greatest LPS-induced hypothermia, and leptin treatment attenuated this response, making hypothermia more long-day-like. Contrary to our hypothesis, short-day leptin-treated hamsters showed the least pronounced LPS-induced anorexia among all groups. These results suggest that leptin may mediate some but not all aspects of seasonal sickness variation in this species. Future studies should be targeted at determining roles of other energetic hormones in regulating seasonal sickness response variation.

Dim light at night disrupts the short-day response in Siberian hamsters

Photoperiodic regulation of physiology, morphology, and behavior is crucial for many animals to survive seasonally variable conditions unfavorable for reproduction and survival. The photoperiodic response in mammals is mediated by nocturnal secretion of melatonin under the control of a circadian clock. However, artificial light at night caused by recent urbanization may disrupt the circadian clock, as well as the photoperiodic response by blunting melatonin secretion. Here we examined the effect of dim light at night (dLAN) (5lux of light during the dark phase) on locomotor activity rhythms and short-day regulation of reproduction, body mass, pelage properties, and immune responses of male Siberian hamsters. Short-day animals reduced gonadal and body mass, decreased spermatid nuclei and sperm numbers, molted to a whiter pelage, and increased pelage density compared to long-day animals. However, animals that experienced short days with dLAN did not show these short-day responses. Moreover, short-day specific immune responses were altered in dLAN conditions. The nocturnal activity pattern was blunted in dLAN hamsters, consistent with the observation that dLAN changed expression of the circadian clock gene, Period1. In addition, we demonstrated that expression levels of genes implicated in the photoperiodic response, Mel-1a melatonin receptor, Eyes absent 3, thyroid stimulating hormone receptor, gonadotropin-releasing hormone, and gonadotropin-inhibitory hormone, were higher in dLAN animals than those in short-day animals. These results suggest that dLAN disturbs the circadian clock function and affects the molecular mechanisms of the photoperiodic response.

Melatonin treatment during early life interacts with restraint to alter neuronal morphology and provoke depressive-like responses

Stressors during early life induce anxiety- and depressive-like responses in adult rodents. Siberian hamsters (Phodopus sungorus) exposed to short days post-weaning also increase adult anxiety- and depressive-like behaviors. To test the hypothesis that melatonin and exposure to stressors early in life interact to alter adult affective responses, we administered melatonin either during the perinatal (gestational day 7 to postnatal day 14) or postnatal (day 15-56) periods and also exposed a subset of dams to restraint during gestation (1 h-2×/day for 4 days). During the final week of injections, depressive-like behaviors were assessed using the sucrose anhedonia and forced swim tests. Hamsters exposed to prenatal restraint and treated with melatonin only during the postnatal period increased depressive-like responses in the forced swim test relative to all other groups. Offspring from restrained dams increased the number of fecal boli produced during the forced swim test, an anxiety-like response. In the present study, prenatal restraint reduced CA1 dendritic branching overall and perinatal melatonin protected hamsters from this restraint-induced reduction. These results suggest that the photoperiodic conditions coincident with birth and early life stressors are important in the development of adult affective responses.

Sleep deprivation attenuates endotoxin-induced cytokine gene expression independent of day length and circulating cortisol in male Siberian hamsters (Phodopus sungorus)

Sleep is restorative, whereas reduced sleep leads to negative health outcomes, such as increased susceptibility to disease. Sleep deprivation tends to attenuate inflammatory responses triggered by infection or exposure to endotoxin, such as bacterial lipopolysaccharide (LPS). Previous studies have demonstrated that Siberian hamsters (Phodopus sungorus), photoperiodic rodents, attenuate LPS-induced fever, sickness behavior and upstream pro-inflammatory gene expression when adapted to short day lengths. Here, we tested whether manipulation of photoperiod alters the suppressive effects of sleep deprivation upon cytokine gene expression after LPS challenge. Male Siberian hamsters were adapted to long (16 h:8 h light:dark) or short (8 h:16 h light:dark) photoperiods for >10 weeks, and were deprived of sleep for 24 h using the multiple platform method or remained in their home cage. Hamsters received an intraperitoneal injection of LPS or saline (control) 18 h after starting the protocol, and were killed 6 h later. LPS increased liver and hypothalamic interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF) gene expression compared with vehicle. Among LPS-challenged hamsters, sleep deprivation reduced IL-1 mRNA levels in liver and hypothalamus, but not TNF. IL-1 attenuation was independent of circulating baseline cortisol, which did not increase after sleep deprivation. Conversely, photoperiod altered baseline cortisol, but not pro-inflammatory gene expression in sleep-deprived hamsters. These results suggest that neither photoperiod nor glucocorticoids influence the suppressive effect of sleep deprivation upon LPS-induced inflammation.

The effect of season on inflammatory response in captive baboons

INTRODUCTION

Highly seasonal animals demonstrate predictable changes in immune function that coincide with changes in photoperiod. Little is known about the effect of season on immune response in baboons. The objective of this study was to determine the effect of season on inflammatory response in baboons.

MATERIALS AND METHODS

Peripheral blood mononuclear cell cytokine response following immune stimulation and serum markers of inflammation were assessed during each season in two groups of young male baboons: one housed under natural light and one in a controlled environment of 12 hours light:12 hours dark.

RESULTS

A seasonal immune rhythm was evident in both groups, with a greater TNF-α and IL-6 response to stimulation and serum CRP concentration in June and September compared with December.

CONCLUSIONS

Season is an important experimental confounder, and therefore, time of year should be controlled when designing studies and analyzing data from immune studies in baboons.

Short photoperiods attenuate central responses to an inflammogen

In most parts of the world, environmental conditions vary in a predictable seasonal manner. Thus, seasonal variation in reproductive timing and immune function has emerged in some species to cope with disparate seasonal demands. During the long days of spring and summer when food availability is high and thermoregulatory demands low, Siberian hamsters invest in reproduction, whereas during the harsh short days of winter hamsters divert energy away from reproductive activities and modify immune capabilities. Many seasonal adaptations can be recapitulated in a laboratory setting by adjusting day length (photoperiod). Early-life photoperiods are important sources of seasonal information and can establish an individual's developmental trajectory. Siberian hamsters housed under short days (SD; 8 h light/day) recover more rapidly than long-day (LD; 16 h light/day) hamsters from immune activation with lipopolysaccharide (LPS). SD hamsters attenuate fever response, reduce cytokine production, and abrogate behavioral responses following LPS injection. The mechanism by which SD Siberian hamsters attenuate febrile response remains unspecified. It is possible that periphery-to-brain communication of inflammatory signals is altered by exposure to photoperiod. Rather than testing photoperiod effects on each of the multiple routes by which immunological cues are communicated to the CNS, we administered LPS intracerebroventricularly (i.c.v.) following adolescent exposure to either 6 weeks of SD or LD. Injection of LPS i.c.v. led to a similar immune reaction in SD hamsters as previously reported with intraperitoneal injection. Short days attenuated the response to LPS with diminished fever spike and duration, as well as decreased locomotor inactivity. Furthermore, only LD hamsters demonstrated anhedonic-like behavior following LPS injection as evaluated by decreased preference for a milk solution. These results suggest that photoperiodic differences in response to infection are due in part to changes in central immune activation.

Influence of photoperiod on hormones, behavior, and immune function

Photoperiodism is the ability of plants and animals to measure environmental day length to ascertain time of year. Central to the evolution of photoperiodism in animals is the adaptive distribution of energetically challenging activities across the year to optimize reproductive fitness while balancing the energetic tradeoffs necessary for seasonally-appropriate survival strategies. The ability to accurately predict future events requires endogenous mechanisms to permit physiological anticipation of annual conditions. Day length provides a virtually noise free environmental signal to monitor and accurately predict time of the year. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system. Thus, melatonin is the fulcrum mediating redistribution of energetic investment among physiological processes to maximize fitness and survival.

Early life experiences affect adult delayed-type hypersensitivity in short and long photoperiods

Environmental experiences during development provide animals with important information about future conditions. Siberian hamsters are photoperiodic rodents that dramatically adjust their physiology and behavior to adapt to seasonal changes. For example, during short winter-like days, hamsters enhance some components of immune function putatively to cope with increasing environmental challenges. Furthermore, early life stress alters the developmental course of the immune system. Overall, immune function is typically suppressed in response to chronic stress, but responses vary depending on the type of stress and components of immune function assessed. This led us to hypothesize that delayed-type hypersensitivity (DTH), an antigen-specific, cell-mediated immune response, would be differentially modulated in hamsters that underwent early life maternal separation (MS) in either short or long photoperiods. At birth, hamsters were assigned to either short (SD; 8 h light/day) or long (LD; 16 h light/day) photoperiods and either daily 3 h MS, daily 15-min brief maternal separation (BMS), or no manipulation from postnatal day 2 through 14. In adulthood DTH was assessed. Hamsters reared in short days enhanced DTH responses. MS and BMS attenuated DTH responses in both short and long days. However, BMS long-day female hamsters did not suppress pinna swelling, suggesting a protective effect of female sex steroids on immune function. As is typical in short days, reproductive tissue was regressed. Reproductive tissue mass was also decreased in long-day MS female hamsters. Furthermore, MS altered photoperiod-induced changes in body mass. Taken together, these findings suggest that manipulations of early life mother-pup interactions in Siberian hamsters result in physiological changes and suppressed cell-mediated immunity.

Distributed forebrain sites mediate melatonin-induced short-day responses in Siberian hamsters

The pineal hormone melatonin (MEL) is the key initiator in regulating seasonal photoperiodic responses; however, the central sites that mediate short day (SD) winter-like responses, such as testicular regression and decreases in white adipose tissue (WAT) mass, by Siberian hamsters are not precisely known. WAT is innervated by the sympathetic nervous system, and several forebrain sites that are part of the sympathetic nervous system outflow to WAT coexpress MEL(1a) receptor mRNA [e.g. suprachiasmatic nucleus, subzona incerta (SubZi), dorsomedial nucleus of the hypothalamus, nucleus reunions and paraventricular nuclei of the thalamus]. We tested the involvement of these sites in MEL-triggered SD responses. A long duration, SD-like MEL signal was applied site specifically for 5 wk, with sc and third ventricle MEL application serving as positive controls. Whereas SD MEL signals delivered to each of these sites were able to induce testicular regression, all but the paraventricular nuclei of the thalamus also trigger SD-induced decreases in body mass. Third ventricle, sc, suprachiasmatic nucleus, or SubZi MEL application also decreased WAT mass, and only sc and SubZi MEL application decreased food intake. Collectively these data suggest a distributed system of MEL-sensitive brain sites sufficient to mediate these SD responses, the redundancy of which suggests its importance for appropriate seasonal responses critical for overwintering.

Food deprivation alters thermoregulatory responses to lipopolysaccharide by enhancing cryogenic inflammatory signaling via prostaglandin D2

We tested the hypothesis that food deprivation alters body temperature (T(b)) responses to bacterial LPS by enhancing inflammatory signaling that decreases T(b) (cryogenic signaling) rather than by suppressing inflammatory signaling that increases T(b) (febrigenic signaling). Free-feeding or food-deprived (24 h) rats received LPS at doses (500 and 2,500 microg/kg iv) that are high enough to activate both febrigenic and cryogenic signaling. At these doses, LPS caused fever in rats at an ambient temperature of 30 degrees C, but produced hypothermia at an ambient temperature of 22 degrees C. Whereas food deprivation had little effect on LPS fever, it enhanced LPS hypothermia, an effect that was particularly pronounced in rats injected with the higher LPS dose. Enhancement of hypothermia was not due to thermogenic incapacity, since food-deprived rats were fully capable of raising T(b) in response to the thermogenic drug CL316,243 (1 mg/kg iv). Neither was enhancement of hypothermia associated with altered plasma levels of cytokines (TNF-alpha, IL-1beta, and IL-6) or with reduced levels of an anti-inflammatory hormone (corticosterone). The levels of PGD(2) and PGE(2) during LPS hypothermia were augmented by food deprivation, although the ratio between them remained unchanged. Food deprivation, however, selectively enhanced the responsiveness of rats to the cryogenic action of PGD(2) (100 ng icv) without altering the responsiveness to febrigenic PGE(2) (100 ng icv). These findings support our hypothesis and indicate that cryogenic signaling via PGD(2) underlies enhancement of LPS hypothermia by food deprivation.

Seasonal changes in vertebrate immune activity: mediation by physiological trade-offs

Animals living in temporally dynamic environments experience variation in resource availability, climate and threat of infection over the course of the year. Thus, to survive and reproduce successfully, these organisms must allocate resources among competing physiological systems in such a way as to maximize fitness in changing environments. Here, we review evidence supporting the hypothesis that physiological trade-offs, particularly those between the reproductive and immune systems, mediate part of the seasonal changes detected in the immune defences of many vertebrates. Abundant recent work has detected significant energetic and nutritional costs of immune defence. Sometimes these physiological costs are sufficiently large to affect fitness (e.g. reproductive output, growth or survival), indicating that selection for appropriate allocation strategies probably occurred in the past. Because hormones often orchestrate allocations among physiological systems, the endocrine mediators of seasonal changes in immune activity are discussed. Many hormones, including melatonin, glucocorticoids and androgens have extensive and consistent effects on the immune system, and they change in systematic fashions over the year. Finally, a modified framework within which to conduct future studies in ecological immunology is proposed, viz. a heightened appreciation of the complex but intelligible nature of the vertebrate immune system. Although other factors besides trade-offs undoubtedly influence seasonal variation in immune defence in animals, a growing literature supports a role for physiological trade-offs and the fitness consequences they sometimes produce.

Gonadal hormone-dependent and -independent regulation of immune function by photoperiod in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit changes in reproductive and immune function in response to seasonal variations in day length. Exposure to short days induces gonadal regression and inhibits testosterone secretion. In parallel, short days enhance immune function: increasing leukocyte numbers and attenuating cytokine and behavioral responses to infection. We examined whether photoperiodic changes in leukocyte phenotypes and sickness behaviors are dependent on concurrent photoperiodic changes in gonadal function. Male hamsters were gonadectomized or sham-gonadectomized and either exposed to short days (9 h light/day; SD) or kept in their natal long-day (15 h light/day; LD) photoperiod for 10-13 wk. Blood samples were obtained for leukocyte enumeration, and hamsters were challenged with bacterial LPS, which induced behavioral (anorexia, reductions in nest building) and somatic (weight loss) sickness responses. Among gonad-intact hamsters, exposure to SD increased total and CD62L+ lymphocytes and CD3+ T lymphocytes in blood and significantly attenuated LPS-induced sickness responses. Independent of photoperiod, castration alone increased total and CD62L+ lymphocyte and CD3+ T lymphocyte numbers and attenuated somatic and anorexic sickness responses. Among castrated hamsters, SD exposure increased lymphocyte numbers and suppressed sickness behaviors. In castrated hamsters, the magnitude of most immunological effects of SD were diminished relative to those evident in gonad-intact hamsters. The SD phenotype in several measures of immunity can be instated via elimination of gonadal hormones alone; however, photoperiodic effects on immune function persist even in castrated hamsters. Thus, photoperiod affects the immune system and neural-immune interactions underlying sickness behaviors via gonadal hormone-dependent and -independent mechanisms.

Winter day lengths enhance T lymphocyte phenotypes, inhibit cytokine responses, and attenuate behavioral symptoms of infection in laboratory rats

Annual variations in day length (photoperiod) trigger changes in the immune and reproductive system of seasonally-breeding animals. The purpose of this study was to determine whether photoperiodic changes in immunity depend on concurrent photoperiodic responses in the reproductive system, or whether immunological responses to photoperiod occur independent of reproductive responses. Here we report photoperiodic changes in enumerative, functional, and behavioral aspects of the immune system, and in immunomodulatory glucocorticoid secretion, in reproductively non-photoperiodic Wistar rats. T-cell numbers (CD3+, CD8+, CD8+CD25+, CD4+CD25+) were higher in the blood of rats housed in short as opposed to long-day lengths for 10 weeks. Following a simulated bacterial infection (Escherichia coli LPS; 125 microg/kg) the severity of several acute-phase sickness behaviors (anorexia, cachexia, neophobia, and social withdrawal) were attenuated in short days. LPS-stimulated IL-1beta and IL-6 production were comparable between photoperiods, but plasma TNFalpha was higher in long-day relative to short-day rats. In addition, corticosterone concentrations were higher in short-day relative to long-day rats. The data are consistent with the hypothesis that photoperiodic regulation of the immune system can occur entirely independently of photoperiodic regulation of the reproductive system. In the absence of concurrent reproductive responses, short days increase the numbers of leukocytes capable of immunosurveillance and inhibition of inflammatory responses, increase proinflammatory cytokine production, increase immunomodulatory glucocorticoid secretion, and ultimately attenuate behavioral responses to infection. Seasonal changes in the host immune system, endocrine system, and behavior may contribute to the seasonal variability in disease outcomes, even in reproductively non-photoperiodic mammals.

Photoperiodic regulation of behavioral responsiveness to proinflammatory cytokines

Symptoms of bacterial infection include decreases in body mass (cachexia), induction of depressive-like hedonic tone (anhedonia), decreases in food intake (anorexia), and increases in body temperature (fever). Recognition of bacteria by the innate immune system triggers the release of proinflammatory cytokines which induce these sickness behaviors via actions at central and peripheral targets. In Siberian hamsters, exposure to short day lengths decreases both the production of proinflammatory cytokines and the magnitude of the symptoms of infection. Short-day attenuation of sickness behaviors may arise solely from decreased production of cytokines; alternatively, substrates responsible for the generation of sickness behaviors may be less responsive to cytokines in short days. To discriminate among these hypotheses, Siberian hamsters were treated with either bacterial lipopolysaccharide (LPS; 25 microg) or recombinant mouse IL-1beta (rIL-1beta; 100 ng) following 11 weeks of exposure to long (15 h light/day) and short (9 h light/day) photoperiods. Replicating earlier work, the magnitude and/or duration of LPS-induced anorexia, anhedonia, cachexia, and fever were greater in long-day relative to short-day hamsters. A comparable short-day attenuation of sickness behaviors and fever was obtained in response to rIL-1beta treatment, despite treatment with identical concentrations of cytokine. These data suggest that hamsters subjected to immunoenhancing short days exhibit diminished symptoms of infection not solely because infections elicit lower levels of cytokine production, but also because the substrates upon which cytokines act become relatively refractory.

Pineal-dependent and -independent effects of photoperiod on immune function in Siberian hamsters (Phodopus sungorus)

Siberian hamsters (Phodopus sungorus) exhibit reproductive and immunological responses to photoperiod. Short (<10-h light/day) days induce gonadal atrophy, increase leukocyte concentrations, and attenuate thermoregulatory and behavioral responses to infection. Whereas hamster reproductive responses to photoperiod are dependent on pineal melatonin secretion, the role of the pineal in short-day induced changes in immune function is not fully understood. To examine this, adult hamsters were pinealectomized (PINx) or sham-PINx, and transferred to short days (9-h light/day; SD) or kept in their natal long-day (15-h light/day; LD) photoperiod. Intact and PINx hamsters housed in LD maintained large testes over the next 12 weeks; sham-PINx hamsters exhibited gonadal regression in SD, and PINx abolished this effect. Among pineal-intact hamsters, blood samples revealed increases in leukocyte, lymphocyte, CD62L+ lymphocyte, and T cell counts in SD relative to LD; PINx did not affect leukocyte numbers in LD hamsters, but abolished the SD increase in these measures. Hamsters were then treated with bacterial lipopolysaccharide (LPS), which induced thermoregulatory (fever), behavioral (anorexia, reductions in nest building), and somatic (weight loss) sickness responses in all groups. Among pineal-intact hamsters, febrile and behavioral responses to LPS were attenuated in SD relative to LD. PINx did not affect sickness responses to LPS in LD hamsters, but abolished the ameliorating effects of SD on behavioral responses to LPS. Surprisingly, PINx failed to abolish the effect of SD on fever. In common with the reproductive system, PINx induces the LD phenotype in most aspects of the immune system. The pineal gland is required for photoperiodic regulation of circulating leukocytes and neural-immune interactions that mediate select aspects of sickness behaviors.

Sequence variants in the melatonin-related receptor gene (GPR50) associate with circulating triglyceride and HDL levels

The gene encoding the melatonin-related receptor (GPR50) is highly expressed within hypothalamic nuclei concerned with the control of body weight and metabolism. We screened GPR50 for mutations in an obese cohort and identified an insertion of four amino acid residues (TTGH) at position 501, two common coding polymorphisms (T528A and V602I), and one noncoding polymorphism (C-16X2GPR50T). Single-nucleotide polymorphisms were then typed in 500 English Caucasian subjects, and associations were sought to intermediate obesity phenotypes. Although no association was seen with body mass index, carriers of two copies of the mutant allele at C-16X2GPR50T, Ins501Del, and A1582G had significantly higher fasting circulating triglyceride levels (P < 0.05). In a separate set of 585 subjects, the associations were replicated, with statistically significant effects of similar magnitude and direction. The association of C-16X2GPR50T with fasting triglycerides was highly significant (P < 0.001). In addition, a significant association between C-16X2GPR50T and circulating HDL levels was observed in the combined population, with C-16X2GPR50T carriers having significantly lower circulating HDL-cholesterol levels (1.39 mM) than wild-type subjects (1.47 mM) (P < 0.01). These findings suggest a previously unexpected role for this orphan receptor in the regulation of lipid metabolism that warrants further investigation.

Caloric restriction and melatonin substitution: Effects on murine circadian parameters

Aging effects have been reported in endocrine, metabolic and behavioral circadian rhythms. The effects of age on the circadian system have been investigated primarily in rats and hamsters and only seldom in mice. Our aim was to assess the effects of two common "anti-aging" treatments, namely caloric restriction (CR) and melatonin substitution, on the circadian system of mice. Animals were subjected to phase delays of the light-dark cycle and constant darkness (DD). The most pronounced change in the murine circadian system was the length of the endogenous period, tau, which increased with age regardless of treatment. CR had diverse effects e.g., enabling a more rapid phase shift response while concomitantly leading to a fragmented circadian phenotype with considerable activity during the rest (light) phase. Melatonin enforced the adaptation to the light/dark cycle, thus facilitating a rapid re-entrainment to phase delayed lighting conditions. Interestingly, the melatonin-substituted animals displayed an increase in locomotor activity under constant darkness and in 50% of all cases a biphasic (split) activity pattern. These results contribute to the phenotypic evaluation of two very different approaches to intervene in the age-related degeneration of the mammalian circadian system. As both CR and melatonin have negative and positive effects on the behavioral expression of clock function (i.e., fragmentation of rhythms vs. faster re-entrainment), their usefulness in managing age-related circadian disorders may be limited.

Photoperiodic regulation of circulating leukocytes in juvenile Siberian hamsters: mediation by melatonin and testosterone

The reproductive system of Siberian hamsters (Phodopus sungorus) undergoes rapid phenotypic responses to changes in day length that occur around the time of weaning. The present experiments tested whether the immune system of Siberian hamsters is similarly photoperiodic early in life and whether photoperiodic changes in melatonin or gonadal hormone secretions mediate any such responses to day length. Circulating blood leukocyte concentrations (WBC) were measured in juvenile male Siberian hamsters that were gestated in long-days (LD), transferred to short-days (SD) on the day of birth, and subsequently either remained in SD or were transferred from SD to LD at 18 days of age (day 18). WBC values were comparable between LD and SD hamsters on day 18. Between day 18 and day 32, SD hamsters exhibited a 3-fold increase in WBC, whereas LD hamsters failed to undergo a significant increase in WBC during this interval. WBC of LD hamsters was significantly lower than that of SD hamsters on day 25 and on day 32. In LD housed males, peripheral injections of melatonin delivered so as to extend the nocturnal duration of elevated endogenous melatonin secretion (i.e., provided in late afternoon) on days 18-31 increased WBC as measured on day 32. Peripubertal (day 17) gonadectomy abolished the immunosuppressive effect of LD exposure on WBC, and treatment with silastic implants containing testosterone suppressed WBC independent of photoperiod treatment. These data indicate that juvenile Siberian hamsters are immunologically responsive to photoperiod and that the leukocyte responses to day length are the result of melatonin-mediated effects of photoperiod on testicular hormone secretion.

Physiological and metabolic functions of melatonin

Melatonin is a lipophilic hormone, mainly produced and secreted at night by the pineal gland. Melatonin synthesis is under the control of postganglionic sympathetic fibers that innervates the pineal gland. Melatonin acts via high affinity G protein-coupled membrane receptors. To date, three different receptor subtypes have been identified in mammals: MT1 (Mel 1a) and MT2 (Mel 1b) and a putative binding site called MT3. The chronobiotic properties of the hormone for resynchronization of sleep and circadian rhythms disturbances has been demonstrated both in animal models or in clinical trials. Several other physiological effects of melatonin in different peripheral tissues have been described in the past years. In this way, it has been demonstrated that the hormone is involved in the regulation of seasonal reproduction, body weight and energy balance. This contribution has been focused to review some of the physiological functions of melatonin as well as the role of the hormone in the regulation of energy balance and its possible involvement in the development of obesity.

Peripubertal Immune Challenges Attenuate Reproductive Development in Male Siberian Hamsters (Phodopus sungorus)1

Differential allocation of energy to reproduction versus host defense is assumed to drive the seasonal antiphase relation between peak reproductive function and immunocompetence; however, evidence supporting this assumption is only correlational. These experiments tested whether photoperiod affects immune responses to antigens in peripubertal Siberian hamsters, whether such activation of the immune system exacts energetic and reproductive costs, and whether such costs vary seasonally. Male Siberian hamsters were raised from birth in long (LD) or short days (SD), which respectively initiate or inhibit the onset of puberty. To elicit a specific immune response, hamsters were injected with a novel antigen (keyhole limpet hemocyanin [KLH]) as juveniles. Reproductive development was attenuated and body temperature was elevated in LD hamsters relative to saline-injected control animals. In contrast, KLH treatments affected neither thermoregulation nor reproductive development in photoinhibited SD hamsters. In experiment 2, juvenile male hamsters were challenged with bacterial lipopolysaccharide (LPS) in order to elicit an innate immune response. Febrile and anorexic responses to LPS were greater in reproductively stimulated LD hamsters relative to reproductively inhibited SD hamsters. LPS treatments attenuated somatic and testicular development in LD hamsters, but did not significantly affect circulating testosterone concentrations. In contrast, LPS treatments were without effect on somatic and reproductive development in SD hamsters. These experiments indicate that photoperiod affects antigen-specific antibody production, febrile responses to LPS, and sickness behaviors in juvenile Siberian hamsters, and that peripubertal activation of the immune system exacts energetic and metabolic costs that can diminish the magnitude of somatic and reproductive maturation in LD. The data also underscore the importance of seasonally dependent life history factors in assessing physiological tradeoffs.

Photoperiod influences the effects of exercise and food restriction on an antigen-specific immune response in Siberian hamsters

Environmental conditions influence the onset and severity of illness and infection and may compromise survival. Energetically challenging conditions during winter may directly induce death through hypothermia, starvation, or shock. The ability to forecast and prepare for the arrival of challenging conditions associated with winter (e.g. low temperatures, decreased food) likely confers survival advantages. Siberian hamsters (Phodopus sungorus) stop reproduction and reduce body mass (approximately 25%) during short, winter-like day lengths, resulting in energetic savings. Hamsters also increase circulating glucocorticoids and lymphocytes (e.g. T cells, NK cells), and exhibit enhanced antigen-specific delayed-type-hypersensitivity (DTH) responses in the skin during short days (SDs). We tested the hypothesis that Siberian hamsters use SD lengths to signal the onset of winter to mediate the energetic trade-offs among body mass, reproductive function, and immune function. Long-day (LD; 16 h light, 8 h dark) and SD (8 h light, 16 h dark) hamsters were either food restricted (25%) or provided ad libitum (ad lib) food for 4 wk; half of all hamsters in each food condition had voluntary access to a running wheel, and half remained sedentary. SD hamsters enhanced DTH responses compared with LD hamsters under sedentary ad lib conditions. Exercise enhanced DTH in LD hamsters regardless of food intake. Furthermore, food-restriction did not significantly influence DTH in LD hamsters. In contrast, food-restriction suppressed DTH in SD hamsters regardless of activity condition, and exercise modestly enhanced DTH only in SD hamsters with ad lib access to food. In sum, moderate energetic deficiency suppressed DTH in SD (but not LD) hamsters, and this suggests that hamsters may have evolved to enhance immune responses during winter in preparation for increased metabolic stressors.

Photoperiod controls the induction, retention, and retrieval of antigen-specific immunological memory

Changes in day length affect several measures of immunity in seasonally breeding mammals. In Siberian hamsters (Phodopus sungorus), short day lengths suppress specific secondary antibody responses to the keyhole limpet hemocyanin (KLH) antigen and enhance cutaneous delayed-type hypersensitivity (DTH) responses to dinitrofluorobenzene (DNFB). These experiments tested whether day length affects secondary antibody and DTH responses by altering immune function solely during the interval after the initial exposure to each antigen, solely during the interval after the second exposure, or during both stages of the respective immune responses. Adult male Siberian hamsters were exposed to either a long (16 h light/day; LD) or a short (8 h light/day; SD) photoperiod for 7.5 wk before receiving an initial exposure to each antigen (KLH injection, cutaneous DNFB treatment; separate groups of animals for each antigen). A subset of LD hamsters was transferred to the SD photo-period, and a subset of SD hamsters was transferred to the LD photoperiod. Other hamsters remained in LD or SD. Eight weeks later, all hamsters were challenged with a second subcutaneous injection of KLH or a second application of DNFB to the ear, and immune responses were measured. Exposure to SD during the primary antibody response did not affect secondary IgG responses, but SD exposure during the secondary response significantly suppressed IgG production independent of day length during the initial KLH treatment. In contrast, exposure to SD during the DNFB challenge enhanced the ensuing DTH response, but this enhancement depended on the photoperiod prevailing during the initial exposure. Exposure to SD during the sensitization stage did not enhance DTH in hamsters subsequently exposed to LD. The data suggest that short photoperiods have enduring effects on immune responsiveness and on the establishment and retention of immunological memory.

Photoperiod alters the time course of brain cyclooxygenase-2 expression in Siberian hamsters

Fever is initiated by activation of the arachidonic acid cascade and the biosynthesis of prostaglandins within the brain. Inducible cyclooxygenase (COX-2) is a rate-limiting enzyme in prostaglandin synthesis, and the number of blood vessels expressing COX-2 correlates with elevated body temperature following peripheral lipopolysaccharide (LPS). Despite its importance in host defense, fever is energetically expensive and we hypothesized that fever may be limited by available metabolic resources. During winter, when competing metabolic demands are constrained by low temperatures and food availability, it was predicted that fever duration would be reduced in seasonally breeding Siberian hamsters (Phodopus sungorus). We measured LPS-induced COX-2 expression in blood vessels of hamsters to test whether photoperiodic alterations in fever duration are centrally mediated, or whether they reflect changes in peripheral modulation of body temperature. Hamsters housed in long, 'summer-like' or short, 'winter-like' day lengths for 10 weeks were injected with LPS, and brains were collected 2, 4, or 8 h later. COX-2 expression was comparably increased in long- and short-day hamsters by 2 h and 4 h post-LPS; however, short-day hamsters exhibited significantly fewer COX-2-positive cells and blood vessels by 8 h post-LPS compared to long-day hamsters, corresponding with reduced fever duration in short-day hamsters. Cortisol concentrations increased more than two-fold in short-day compared to long-day hamsters by 4 h; this increase may have contributed to the decrease in COX-2 expression observed by 8 h in short days. We conclude that short photoperiods significantly altered the time course of central COX-2 protein expression in hamsters in a manner consistent with reduced fever duration.

Photoperiodic adjustments in immune function protect Siberian hamsters from lethal endotoxemia

Seasonal changes in day length enhance or suppress components of immune function in individuals of several mammalian species. Siberian hamsters (Phodopus sungorus) exhibit multiple changes in neuroendocrine, reproductive, and immune function after exposure to short days. The manner in which these changes are integrated into the host response to pathogens is not well understood. The present experiments tested the hypothesis that short-day changes in immune function alter the pathogenesis of septic shock and survival after challenge with endotoxin. Male and female Siberian hamsters raised in long-day photoperiods were transferred as adults to short days or remained in their natal photoperiod. Six to 8 weeks later, hamsters were injected i.p. with 0, 1, 2.5, 10, 25, or 50 mg/kg bacterial lipopolysaccharide (LPS) (the biologically active constituent of endotoxin), and survival was monitored for 96 h. Short days significantly improved survival of male hamsters treated with 10 or 25 mg/kg LPS and improved survival in females treated with 50 mg/kg LPS. Transfer from long to short days shifted the LD50 in males by approximately 90%, from 5.3 to 9.9 mg/kg, and in females from 11.1 to 15.0 mg/kg (+35%). Long-day females were more resistant than were males to lethal endotoxemia. In vitro production of the proinflammatory cytokine TNFalpha in response to LPS stimulation was significantly lower in macrophages extracted from short-day relative to long-day hamsters, as were circulating concentrations of TNFalpha in vivo after i.p. administration of LPS, suggesting that diminished cytokine responses to LPS in short days may mitigate the lethality of endotoxemia. Adaptation to short days induces changes in immune parameters that affect survival in the face of immune challenges.