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

Photoperiod Conditions Modulate Serum Oxylipins Levels in Healthy and Obese Rats: Impact of Proanthocyanidins and Gut Microbiota

Seasonal rhythms are emerging as a key factor influencing gut microbiota and bioactive compounds functionality as well as several physiological processes such as inflammation. In this regard, their impact on the modulation of oxylipins (OXLs), which are important lipid mediators of inflammatory processes, has not been investigated yet. Hence, we aimed to investigate the effects of photoperiods on OXLs metabolites in healthy and obesogenic conditions. Moreover, we evaluated if the impact of proanthocyanidins and gut microbiota on OXLs metabolism is influenced by photoperiod in obesity. To this purpose, Fischer 344 rats were housed under different photoperiod conditions (L6: 6 h light, L12: 12 h light or L18:18 h light) and fed either a standard chow diet (STD) or a cafeteria diet (CAF) for 9 weeks. During the last 4 weeks, obese rats were daily administered with an antibiotic cocktail (ABX), an oral dose of a grape seed proanthocyanidin extract (GSPE), or with their combination. CAF feeding and ABX treatment affected OXLs in a photoperiod dependent-manner. GSPE significantly altered prostaglandin E2 (PGE2) levels, only under L6 and mitigated ABX-mediated effects only under L18. In conclusion, photoperiods affect OXLs levels influenced by gut microbiota. This is the first time that the effects of photoperiod on OXLs metabolites have been demonstrated.

Differential blood leukocyte populations based on individual variances and age

Blood was collected from the New York State Department of Health (NYSDOH) employees to assess variances in leukocyte numbers in January, May, and September throughout a year and over many years. Women and men of ages 20 to 80 volunteered to donate for this program. Most of the blood came from healthy individuals, and many remained healthy throughout the years of their blood donations. The major objective was to determine the extent that blood leukocyte numbers change so that transient vs more lingering changes may be helpful in assessing health status. Since some donors remained in the program for 14 years, age influences over time could be determined. Within a short period of 2-3 years, the flow cytometric immunophenotypic profile of blood lymphocyte is relatively stable with a CV% of < 20%. However, as humans age, the blood CD3⁺ T cell, CD8⁺ T cell, B cell, NKT cell, and CD4^-/CD8^- double-negative T cell (DN-T cell) subsets declined in cell numbers/μL, but the double-positive CD4⁺/CD8⁺ T cells (DP-T cells) increased in numbers. The extent and chronology of a variance, e.g., a subset exceeding its 75th or 90th percentile, might be indicative of a transient or chronic physiological or psychosocial stress affecting health or a developing pathology; however, because of the wide ranges of cell numbers/μL for each subset among individuals reported as healthy, everyone's immunity and health must be carefully evaluated. A CD4 to CD8 ratio (4/8R) of < 1 has been used to define an immunodeficiency such as HIV-induced AIDS, but a high 4/8R is less well associated with health status. A high 4/8R or granulocyte to lymphocyte ratio (GLR) might be an indicator of a stress, infection, or immune-related pathology. Sporadic and longitudinal increases of GLRs are reported. The results suggest that there are some age and sex differences in leukocyte numbers; stress influences on the blood profile of leukocytes likely exist. However, some values exceeding 2 standard deviations from means do not necessarily predict a health concern, whereas a longitudinal increase or decline might be indicative of a need for further evaluations.

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.

On the winter enhancement of adaptive humoral immunity: hypothesis testing in desert hamsters (Phodopus roborovskii: Cricetidae, Rodentia) kept under long-day and short-day photoperiod

We tested the winter immunity enhancement hypothesis (WIEH) on male desert hamsters (Phodopus roborovskii) kept under long-day (LD) and short-day (SD) photoperiods. We assumed that under SD in a laboratory, the adaptive humoral immune responsiveness to the antigenic challenge would be enhanced due to the lack of winter physical stressors and food shortages and/or because of the action of an endogenous winter bolstering mechanism, while under LD the immune responsiveness would be suppressed by the activity of the reproductive system. The results support the WIEH in part. We did not find a difference in antibody production in response to sheep erythrocytes between SD and LD hamsters, but SD males had the lower number of granulocytes and the higher number of lymphocytes in white blood cell counts. Reproductive activity was lower in SD males. These males demonstrated an increase in their mass-specific resting metabolic rate, their mass-specific maximal metabolic rate and their level of cortisol. The result of a generalized linear model analysis indicates the negative effect on secondary immunoresponsiveness to sheep erythrocytes of mid-ventral gland size, the organ characterizing individual reproductive quality, and designates a tradeoff between antibody production and reproductive effort. The mass-independent maximal metabolic rate also negatively affected antibody production, indicating a tradeoff between maximal aerobic performance and the adaptive immune function. The higher stress in SD males seems to be the most likely reason for the lack of the effect of daylight duration on antibody production.

Prior exposure to long-day photoperiods alters immune responses and increases susceptibility to parasitic infection in stickleback

Seasonal disease and parasitic infection are common across organisms, including humans, and there is increasing evidence for intrinsic seasonal variation in immune systems. Changes are orchestrated through organisms' physiological clocks using cues such as day length. Ample research in diverse taxa has demonstrated multiple immune responses are modulated by photoperiod, but to date, there have been few experimental demonstrations that photoperiod cues alter susceptibility to infection. We investigated the interactions among photoperiod history, immunity and susceptibility in laboratory-bred three-spined stickleback (a long-day breeding fish) and its external, directly reproducing monogenean parasite Gyrodactylus gasterostei. We demonstrate that previous exposure to long-day photoperiods (PLD) increases susceptibility to infection relative to previous exposure to short days (PSD), and modifies the response to infection for the mucin gene muc2 and Treg cytokine foxp3a in skin tissues in an intermediate 12 L : 12 D photoperiod experimental trial. Expression of skin muc2 is reduced in PLD fish, and negatively associated with parasite abundance. We also observe inflammatory gene expression variation associated with natural inter-population variation in resistance, but find that photoperiod modulation of susceptibility is consistent across host populations. Thus, photoperiod modulation of the response to infection is important for host susceptibility, highlighting new mechanisms affecting seasonality of host-parasite interactions.

Season and sex have different effects on hematology and cytokines in striped hamsters (Cricetulus barabensis)

Animals in the temperate zones face seasonal variations in environments and hence their immune responses change seasonally. In the current study, seasonal changes in hematological parameters and cytokines in striped hamsters (Cricetulus barabensis) were examined to test the winter immunoenhancement hypothesis, which states that immune function tends to increase in fall and winter compared with other seasons. Male and female hamsters were captured from the wild in the fall and winter of 2014 and in the spring and summer of 2015. Maximum body mass in both sexes and relative fatness in female hamsters occurred in the summer, indicating that body condition was the best during this season. All hematological parameters were not different between male and female hamsters, and were also not affected by the interaction of season and sex except neutrophil granulocytes (GRAN). Red blood cells (RBC) and haematocrit (PCV) were higher in the fall and winter, and hemoglobin concentration (HGB) was the highest in winter in hamsters compared with the spring and summer, implying that their oxygen-carrying capacity and oxygen affinity of the blood increased during these seasons. Compared with other seasons, the number of white blood cells (WBC) was higher in winter than in summer, intermediate granulocytes (MID), the percent of MID (MID%), GRAN and the percent of GRAN (GRAN%) were the highest in winter, which all supported the winter immunoenhancement hypothesis. However, the count of lymphocytes (LYMF) was the highest in spring, being inconsistent with this hypothesis. IL-2 levels, but not TNF-α, were influenced by seasons, sex and their interaction in hamsters. Regardless of sex, IL-4 titres were higher in spring and summer than in fall and winter in hamsters. INF-γ titres in male hamsters did not differ between the spring and summer, while its titres in female hamsters was lower in spring in contrast with winter and summer. Higher IL-2 and IL-4 levels during the breeding seasons might be crucial in controlling the increased possibilities of infections in these seasons. In summary, season and sex had disparate effects on different hematological profiles and the levels of cytokines in hamsters.

Photoperiodic Effects on Diurnal Rhythms in Cell Numbers of Peripheral Leukocytes in Domestic Pigs

The photoperiod is known to modulate immune cell number and function and is regarded essential for seasonal disease susceptibility. In addition, diurnal variations in the immune system are regarded important for immune competence. Whereas few studies investigated the influence of season, none investigated the specific effect of the photoperiod on these diurnal immune rhythms until now. Therefore, the present study compared diurnal rhythms in cell numbers of peripheral leukocyte types in domestic pigs held either under long day conditions (LD) or short day conditions (SD). Cosinor analyses of cell numbers of various peripheral leukocyte subtypes investigated over periods of 50 h revealed distinct photoperiodic differences in diurnal immune rhythms. Relative amplitudes of cell numbers of total leukocytes, NK cells, T cells, and monocytes in blood were higher under SD than LD. In addition, cell counts of total leukocytes, NK cells, T cells including various T cell subtypes, and eosinophils peaked earlier relative to the time of lights-on under SD than LD. In contrast, diurnal rhythms of neutrophil counts did not show photoperiodic differences. Mesor values did not differ in any leukocyte type. Generalized linear mixed model analyses revealed associations of leukocyte counts with plasma cortisol concentration and activity behavior in most investigated cell types. Moreover, the present study demonstrated photoperiodic effects on diurnal rhythms in plasma cortisol concentrations and activity behavior, which is in agreement with human and primate studies. The results of the present study imply stronger rhythmicity in leukocyte counts in general under SD. Common intrinsic mechanisms seem to regulate photoperiodic effects on diurnal rhythms in leukocyte counts, except for neutrophils, in domestic pigs. Our results reveal considerable insights into the regulation of immune rhythms in diurnally active species.

Population-level seasonality in cardiovascular mortality, blood pressure, BMI and inflammatory cells in UK biobank

INTRODUCTION

The risk of mortality from cardiovascular disease (CVD) is higher in wintertime throughout the world, but it is not known if this reflects annual changes in diet or lifestyle, or an endogenous photoperiodic mechanism that is sensitive to changes in day length.

METHODS

Phenotypic data on cardiometabolic and lifestyle factors were collected throughout a 4 year time period from 502,642 middle-aged participants in UK Biobank. To assess the impact of seasonal environmental changes on cardiovascular risk factors, we linked these data to the outdoor temperature and day length at the time of assessment. Self-reported information on physical activity, diet and disease status were used to adjust for confounding factors related to health and lifestyle.

RESULTS

Mortality related to CVD was higher in winter, as were risk factors for this condition including blood pressure, markers of inflammation and body mass index (BMI). These seasonal rhythms were significantly related to day length after adjustment for other factors that might affect seasonality including physical activity, diet and outdoor temperature.

CONCLUSIONS

The risk of CVD may be modulated by day length at temperate latitudes, and the implications of seasonality should be considered in all studies of human cardiometabolic health. Key messages In this cross-sectional study in UK Biobank, we report annual variations in cardiovascular risk factors and mortality that were associated with day length independent of environmental and lifestyle factors. These seasonal changes in day length might contribute to annual patterns in cardiovascular disease and mortality.

Blue Light Enhances Bacterial Clearance and Reduces Organ Injury During Sepsis

OBJECTIVES

The physiology of nearly all mammalian organisms are entrained by light and exhibit circadian rhythm. The data derived from animal studies show that light influences immunity, and these neurophysiologic pathways are maximally entrained by the blue spectrum. Here, we hypothesize that bright blue light reduces acute kidney injury by comparison with either bright red or standard, white fluorescent light in mice subjected to sepsis. To further translational relevance, we performed a pilot clinical trial of blue light therapy in human subjects with appendicitis.

DESIGN

Laboratory animal research, pilot human feasibility trial.

SETTING

University basic science laboratory and tertiary care hospital.

SUBJECTS

Male C57BL/6J mice, adult (> 17 yr) patients with acute appendicitis.

INTERVENTIONS

Mice underwent cecal ligation and puncture and were randomly assigned to a 24-hour photoperiod of bright blue, bright red, or ambient white fluorescent light. Subjects with appendicitis were randomized to receive postoperatively standard care or standard care plus high-illuminance blue light.

MEASUREMENTS AND MAIN RESULTS

Exposure to bright blue light enhanced bacterial clearance from the peritoneum, reduced bacteremia and systemic inflammation, and attenuated the degree of acute kidney injury. The mechanism involved an elevation in cholinergic tone that augmented tissue expression of the nuclear orphan receptor REV-ERBα and occurred independent of alterations in melatonin or corticosterone concentrations. Clinically, exposure to blue light after appendectomy was feasible and reduced serum interleukin-6 and interleukin-10 concentrations.

CONCLUSIONS

Modifying the spectrum of light may offer therapeutic utility in sepsis.

Short day length enhances physiological resilience of the immune system against 2-deoxy-d-glucose-induced metabolic stress in a tropical seasonal breeder Funambulus pennanti

Studies demonstrate the importance of metabolic resources in the regulation of reproduction and immune functions in seasonal breeders. In this regard, the restricted energy availability can be considered as an environmental variable that may act as a seasonal stressor and can lead to compromised immune functions. The present study explored the effect of photoperiodic variation in the regulation of immune function under metabolic stress condition. The T-cell-dependent immune response in a tropical seasonal breeder Funambulus pennanti was studied following the inhibition of cellular glucose utilization with 2-deoxy-d-glucose (2-DG). 2-DG treatment resulted in the suppression of general (e.g., proliferative response of lymphocytes) and antigen-specific [anti-keyhole limpet hemocyanin IgG titer and delayed-type hypersensitivity response] T-cell responses with an activation of the hypothalamic-pituitary-adrenal axis, which was evident from the increased levels of plasma corticosterone. 2-DG administration increased the production of inflammatory cytokines [interleukin (IL)-1β and tumor necrosis factor (TNF)-α] and decreased the autocrine T-cell growth factor IL-2. The immunocompromising effect of 2-DG administration was retarded in animals exposed to short photoperiods compared with the control and long photoperiod-exposed groups. This finding suggested that short photoperiodic conditions enhanced the resilience of the immune system, possibly by diverting metabolic resources from the reproductive organs toward the immune system. In addition, melatonin may have facilitated the energy "trade-off" between reproductive and immune mechanisms, thereby providing an advantage to the seasonal breeders for their survival during stressful environmental conditions.

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.

The influence of biological rhythms on host-parasite interactions

Biological rhythms, from circadian control of cellular processes to annual cycles in life history, are a main structural element of biology. Biological rhythms are considered adaptive because they enable organisms to partition activities to cope with, and take advantage of, predictable fluctuations in environmental conditions. A flourishing area of immunology is uncovering rhythms in the immune system of animals, including humans. Given the temporal structure of immunity, and rhythms in parasite activity and disease incidence, we propose that the intersection of chronobiology, disease ecology, and evolutionary biology holds the key to understanding host-parasite interactions. Here, we review host-parasite interactions while explicitly considering biological rhythms, and propose that rhythms: influence within-host infection dynamics and transmission between hosts, might account for diel and annual periodicity in host-parasite systems, and can lead to a host-parasite arms race in the temporal domain.

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.

Cell-autonomous iodothyronine deiodinase expression mediates seasonal plasticity in immune function

Annual rhythms in morbidity and mortality are well-documented, and host defense mechanisms undergo marked seasonal phenotypic change. Siberian hamsters (Phodopus sungorus) exhibit striking immunological plasticity following adaptation to short winter day lengths (SD), including increases in blood leukocytes and in the magnitude of T cell-mediated immune responses. Thyroid hormone (TH) signaling is rate-limited by tissue-level expression of iodothyronine deiodinase types II and III (dio2, dio3), and dio2/dio3 expression in the central nervous system gate TH-dependent transduction of photoperiod information into the neuroendocrine system. THs are also potent immunomodulators, but their role in seasonal immunobiology remains unexamined. Here we report that photoperiod-driven changes in triiodothyronine (T3) signaling mediate seasonal changes in multiple aspects of immune function. Transfer from long days (LD) to SD inhibited leukocyte dio3 expression, which increased cellular T4→T3 catabolism. T3 was preferentially localized in the lymphocyte cytoplasm, consistent with a non-nuclear role of T3 in lymphoid cell differentiation and maturation. Exposure to SD upregulated leukocyte DNA methyltransferase expression and markedly increased DNA methylation in the dio3 proximal promoter region. Lastly, to bypass low endogenous T3 biosynthesis in LD lymphocytes, LD hamsters were treated with T3, which enhanced T cell-dependent delayed-type hypersensitivity inflammatory responses and blood leukocyte concentrations in a dose-dependent manner, mimicking effects of SD on these immunophenotypes. T3 signaling represents a novel mechanism by which environmental day length cues impact the immune system: changes in day length alter lymphoid cell T3-signaling via epigenetic transcriptional control of dio3 expression.

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.

Light and the outcome of the critically ill: an observational cohort study

Introduction

Light before and during acute illness has been associated with both benefit and harm in animal models and small human studies. Our objective was to determine the associations of light duration (photoperiod) and intensity (insolation) before and during critical illness with hospital mortality in ICU patients. Based on the 'winter immunoenhancement' theory, we tested the hypothesis that a shorter photoperiod before critical illness is associated with improved survival.

Methods

We analyzed data from 11,439 patients admitted to 8 ICUs at the University of Pittsburgh Medical Center between June 30, 1999 and July 31, 2004. Daily photoperiod and insolation prior to and after ICU admission were estimated for each patient by using data provided by the United States Naval Observatory and National Aeronautics and Space Administration and direct measurement of light gradient from outside to bedside for each ICU room. Our primary outcome was hospital mortality. The association between light and risk of death was analyzed using multivariate analyses, adjusting for potential confounders, including severity of illness, case mix, and ICU type.

Results

The cohort had an average APACHE III of 52.9 and a hospital mortality of 10.7%. In total, 128 ICU beds were analyzed; 108 (84%) had windows. Pre-illness photoperiod ranged from 259 to 421 hours in the prior month. A shorter photoperiod was associated with a reduced risk of death: for each 1-hour decrease, the adjusted OR was 0.997 (0.994 to 0.999, p = 0.03). In the ICU, there was near complete (99.6%) degradation of natural light from outside to the ICU bed. Thus, light exposure once in the ICU approached zero; the 24-hour insolation was 0.005 ± 0.003 kWh/m² with little diurnal variation. There was no association between ICU photoperiod or insolation and mortality.

Conclusions

Consistent with the winter immunoenhancement theory, a shorter photoperiod in the month before critical illness is associated with a reduced risk of death. Once in the ICU, patients are exposed to near negligible natural light despite the presence of windows. Further studies are warranted to determine the underlying mechanisms and whether manipulating light exposure, before or during ICU admission, can enhance survival.

Endotoxin elicits ambivalent social behaviors

The acute phase response to infection is reliably accompanied by decreases in social investigation; however, social behavior is commonly assayed in inescapable environments using unfamiliar social stimuli. In this experiment, male Wistar rats were raised from weaning with 2 familiar, same-sex conspecifics. In adulthood, rats were implanted with radiotelemetry devices that permitted localization in space, and were challenged with LPS treatments (150 μg/kg, i.p.) in a novel, semi-natural arena which afforded the treated (Focal) animal exclusive control of social exposure, and the ability to avoid social interactions. LPS reliably elicited thermoregulatory responses (transient hypothermia and fever) during the scotophase following injection, but did not yield changes in the proportion of time spent engaged in social interactions: both LPS- and saline-treated rats spent approximately 10% of the night with their familiar cagemates. Injection treatments markedly altered the spatial distribution of activity: LPS-treated rats exhibited significant increases in the amount of time spent as far as possible from their cagemates. The data suggest that sickness responses to LPS may give rise to a transient state of social ambivalence-characterized by a persistent motivation to engage in social contact, but also by increased avoidance of social environments. Selective maintenance of social motivation illustrates plasticity in the expression of sickness behaviors and may be adaptive in social species.

Impact of a long photoperiod during lactation on immune status of piglets

The effects of a long photoperiod treatment around parturition and throughout lactation on immune status of piglets were studied. Sows were assigned to 2 light regimens: i) standard short photoperiod (SP, n = 17), 8 h of daily light from d 112 of gestation until d 23 of lactation; and ii) long photoperiod (LP, n = 17), 23 h of daily light from d 112 of gestation to d 4 of lactation and 16 h thereafter. In front of the crates, under the side heat lamps and behind the sow, light intensities were 59 ± 5, 109 ± 6, and 44 ± 6 lx, respectively. On d 15 of lactation and at weaning (d 23), 2 piglets of similar BW per litter were selected and immunized intramuscularly with ovalbumin (OVA). Blood samples (5 mL serum and 10 mL whole blood) were taken at d 15 and d 23 of lactation, and at d 30, 37, and 44 of age after weaning to evaluate the antibody response to OVA and measure phagocytosis, lymphocyte proliferative response, and different circulating blood lymphocyte populations of piglets. Results showed that phagocytosis was increased in piglets submitted to LP (P < 0.05). A treatment × time interaction (P < 0.001) indicated that SP piglets developed a better IgG response to OVA than LP piglets. The percentage of B lymphocytes was also increased (P = 0.02) in SP piglets compared with piglets exposed to LP during lactation; the lymphocyte response to OVA tended to be enhanced (P = 0.07) over time in SP piglets. Different subpopulations of CD8+ lymphocytes were markedly increased in SP piglets at 23 d of age compared with piglets exposed to LP (treatment × time: P < 0.05). These results suggest that exposure of piglets to LP during lactation seems to reduce the capacity of piglets to develop a strong immune response to novel antigens. This may have important consequences on the ability of piglets to resist an infection after weaning.

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.

Immune responses to lipopolysaccharide challenge in a tropical rodent (Funambulus pennanti): photoperiod entrainment and sex differences

Annual variation in day length (photoperiod) triggers changes in the immune system of seasonal breeders. The rationale behind this study was to delineate any sex differences in immune responses of photoperiodically entrained animals challenged against lipopolysaccharide (LPS)-induced inflammatory stress. We observed that photoperiodically entrained [short day, SD, 10 h light (L):14 h dark (D); long day, LD, 16 h L:8 h D; and natural day length, NDL, 12 h L:12 h D] male and female Indian palm squirrels, Funambulus pennanti, presented sexual dimorphism in immune status after LPS-induced stress. Females presented high humoral (anti-keyhole limpet hemocyanin immunoglobulin) and cellular immunity (lymphocyte proliferation) compared with the males of all photoperiodic conditions. Female squirrels showed reduced pro-inflammatory cytokine levels (interleukin-1β, interleukin-6, and tumor necrosis factor-α) than the males suggesting their high efficiency to recover from LPS-induced inflammatory stress. Increased duration of melatonin secretion and corticosterone concentration in squirrels experiencing SD evidently supported survival of squirrels as compared with control (NDL) and LD squirrels of both sexes. Decreased immune status in both sexes under LD condition might be due to a short melatonin signal mimicking the LDs of summer. Thus, we infer that photoperiodic entrainment via the levels of melatonin and corticosterone synergistically supported more the survival of female squirrels under LPS-induced stress.

Can photoperiod predict mortality in the 1918-1920 influenza pandemic?

Amplitude of the seasonal change in day length increases with distance from the equator, and changes in day length markedly alter immune function in diverse nonhuman animal models of infection. Historical records of mortality data, ambient temperature, population density, geography, and economic indicators from 42 countries during 1918-1920 were analyzed to determine relative contributions toward human mortality during the "Spanish" influenza pandemic of 1918-1920. The data identify a strong negative relation between distance from the equator and mortality during the 1918-1920 influenza pandemic, which, in a multiple regression model, manifested independent of major economic, demographic, and temperature variables. Enhanced survival was evident in populations that experienced a winter nadir day length ≤10 h light/day, relative to those that experienced lower amplitude changes in photoperiod. Numerous reports indicate that exposure to short day lengths, typical of those occurring outside the tropics during winter, yields robust and enduring reductions in the magnitude of cytokine, febrile, and behavioral responses to infection. The present results are preliminary but prompt the conjecture that, if similar mechanisms are operant in humans, then they would be predicted to mitigate symptoms of infection in proportion to an individual's distance from the equator. Although limitations and uncertainties accompany regression-based analyses of historical epidemiological data, latitude, per se, may be an underrecognized factor in mortality during the 1918-1920 influenza pandemic. The author proposes that some proportion of the global variance in morbidity and mortality from infectious diseases may be explained by effects of day length on the innate immune response to infection.

Seasonal changes in haematology, lymphocyte transferrin receptors and intracellular iron in Ironman triathletes and untrained men

We investigated whether 12 months of chronic endurance training would affect haematology, CD4(+) lymphocyte transferrin receptor (CD71) expression, CD4(+) intracellular iron and the incidence of upper respiratory tract illnesses (URTI) in Ironman triathletes compared with untrained men. Resting venous blood samples were taken from 15 Ironman triathletes (TR 30 ± 5 year) and 12 untrained men (UT 30 ± 6 year) every 4 weeks for 12 months. Erythrocyte, leukocyte and platelet concentration, haematocrit, haemoglobin (Hb) and mean corpuscular haemoglobin (MCHC) were measured with a full blood count. CD4(+) lymphocytes were analysed for changes in transferrin receptor (CD71) expression (CD4(+)CD71(+)), and intracellular iron (Fe(3+)), by flow cytometry. The TR group had significantly lower Hb, MCHC, and platelets for 10, 9 and 11 months, respectively; lower CD4(+)CD71(+) (3 months) and Fe(3+) (1 month), respectively; higher CD4(+)CD71(+) (1 month); a higher lymphocyte count for 4 months. There were no between-group differences in other variables. In both groups haematology and lymphocytes increased during spring, early summer and winter and decreased during late summer/late winter, with an inverse relationship between CD4(+)CD71(+) and Fe(3+). The TR group reported significantly fewer URTI than the UT. Low Hb and MCHC suggest an iron deficiency which may affect triathlete performance. Monthly changes in lymphocytes, CD4(+)CD71(+) and Fe(3+) suggested that spring, summer and late autumn are associated with CD4(+) proliferation. There may be seasonal relationships between haematology and lymphocyte function, independent of endurance training, possibly affecting performance but not the incidence of URTI.

Sex differences in hormonal responses to social conflict in the monogamous California mouse

Monogamous species are usually considered to be less likely to exhibit sex differences in behavior or brain structure. Most previous studies examining sex differences in stress hormone responses have used relatively sexually dimorphic species such as rats. We examined the stress hormone responses of monogamous California mice (Peromyscus californicus) to resident-intruder tests. We also tested males and females under different photoperiods, because photoperiod has been shown to affect both aggression and stress hormone responses. Females, but not males showed a significant increase in corticosterone levels immediately following a resident-intruder test. Males but not females showed elevated corticosterone levels under short days. Females tested in aggression tests also showed a significant increase in plasma oxytocin levels, but only when housed in long days. This was consistent with our observation that females but not males had more oxytocin positive cells in the paraventricular nucleus (PVN) when housed under long days. Our data show that sex differences in glucocorticoid responses identified in other rodents are present in a monogamous species.

Photoperiod history differentially impacts reproduction and immune function in adult Siberian hamsters

Seasonal changes in numerous aspects of mammalian immune function arise as a result of the annual variation in environmental day length (photoperiod), but it is not known if absolute photoperiod or relative change in photoperiod drives these changes. This experiment tested the hypothesis that an individual's history of exposure to day length determines immune responses to ambiguous, intermediate-duration day lengths. Immunological (blood leukocytes, delayed-type hypersensitivity reactions [DTH]), reproductive, and adrenocortical responses were assessed in adult Siberian hamsters (Phodopus sungorus) that had been raised initially in categorically long (15-h light/day; 15L) or short (9L) photoperiods and were subsequently transferred to 1 of 7 cardinal experimental photoperiods between 9L and 15L, inclusive. Initial photoperiod history interacted with contemporary experimental photoperiods to determine reproductive responses: 11L, 12L, and 13L caused gonadal regression in hamsters previously exposed to 15L, but elicited growth in hamsters previously in 9L. In hamsters with a 15L photoperiod history, photoperiods < or = 11L elicited sustained enhancement of DTH responses, whereas in hamsters with a 9L photoperiod history, DTH responses were largely unaffected by increases in day length. Enhancement and suppression of blood leukocyte concentrations occurred at 13L in hamsters with photoperiod histories of 15L and 9L, respectively; however, prior exposure to 9L imparted marked hysteresis effects, which suppressed baseline leukocyte concentrations. Cortisol concentrations were only enhanced in 15L hamsters transferred to 9L and, in common with DTH, were unaffected by photoperiod treatments in hamsters with a 9L photoperiod history. Photoperiod history acquired in adulthood impacts immune responses to photoperiod, but manifests in a markedly dissimilar fashion as compared to the reproductive system. Prior photoperiod exposure has an enduring impact on the ability of the immune system to respond to subsequent changes in day length.

Influence of the olfactory bulbs on blood leukocytes and behavioral responses to infection in Siberian hamsters

Surgical removal of the olfactory bulb alters several aspects of immunological activity. This study investigated the role of the olfactory bulbs in the control of behavioral responses to simulated infection, and the environmental modulation of sickness behaviors by changes in day length. Adult male Siberian hamsters (Phodopus sungorus) were subjected to bilateral olfactory bulbectomy (OBx) or a sham surgical procedure, and were then exposed to long(15 h light/day; LD) or short (9 h light/day; SD) photoperiods for 8–12 weeks, after which circulating leukocytes and behavioral responses (anorexia, anhedonia, cachexia) to simulated gram-negative bacterial infections (i.p. lipopolysaccharide [LPS] treatment;0.625 mg/kg) were quantified. OBx treatment altered the effects of photoperiod on immune function in a trait-specific manner. LPS-induced anorexia was exacerbated in SD-OBx hamsters; LPS-induced anhedonia was exacerbated in LD-OBx hamsters; and photoperiodic differences in circulating leukocytes and LPS-induced cachexia were eliminated by OBx. Plasma cortisol concentrations did not differ between LD and SD hamsters, irrespective of olfactory bulb integrity. The data indicate that photoperiod affects immune function via OB-dependent and -independent mechanisms, and that changes in cortisol production are not required for photoperiodic changes in sickness behaviors to manifest.

Affective and adrenocorticotrophic responses to photoperiod in Wistar rats

The present study tested the hypothesis that seasonal intervals of exposure to modest changes in photoperiod, typical of those experienced by humans living in temperate latitudes (10-14 h light/day), engage changes in emotional behaviour of Wistar rats, a commonly-used animal model for investigations of affective physiology. Short day lengths (<or= 12 h light/day) induced behavioural despair in a forced-swim test, exploratory anxiety in an open field arena, and anhedonia in a two-bottle sucrose preference task, relative to longer day lengths. Plasma adrenocorticotrophic hormone was lower in short-day relative to long-day rats, but testosterone and corticosterone concentrations were comparable across treatments. In common with animals that engage reproductive responses to day length, reproductively nonresponsive mammals such as Wistar rats exhibit changes in affective state following small changes in day length. Wistar rats may provide an animal model for the study of seasonal mood regulation because the neuroendocrine, depressive, anxious and anhedonic responses of Wistar rats to short days bear similarities to those observed in some human populations. Standard laboratory husbandry practices (exposure to a 12 : 12 h light/dark cycle) may inadvertently deliver a chronic background depressive and anxiogenic stimulus.