Gregory E. Demas

Seasonal Changes in Immune Function

Winter is energetically demanding. Physiological and behavior adaptations have evolved among nontropical animals to cope with winter because thermoregulatory demands increase when food availability decreases. Seasonal breeding is central within the suite of winter adaptations among small animals. Presumably, reproductive inhibition during winter conserves energy at a time when the odds of producing viable young are low. In addition to the well-studied seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among many populations of mammals and birds in the field. Challenging winter conditions, such as low ambient temperatures and decreased food availability, can directly induce death via hypothermia, starvation, or shock. In some cases, survival in demanding winter conditions puts individuals under great physiological stress, defined here as an adaptive process that results in elevated blood levels of glucocorticoids. The stress of coping with energetically demanding conditions can also indirectly cause illness and death by compromising immune function. Presumably, the increased blood concentrations of adrenocortical steroids in response to winter stressors compromise immune function and accelerate catabolic mechanisms in the field, although the physiological effects of elevated glucocorticoids induced by artificial stressors have been investigated primarily in the laboratory. However, recurrent environmental stressors could reduce survival if they evoke persistent glucocorticoid secretion. The working hypothesis of this article is that mechanisms have evolved in some animals to cambat seasonal stress-induced immunocompromise as a temporal adaptation to promote survival. Furthermore, we hypothesize that mechanisms have evolved that allow individuals to anticipate periods of immunologically challenging conditions, and to cope with these seasonal health-threatening conditions. The primary environmental cue that permits physiological anticipation of season is the daily photoperiod; however, other environmental factors may interact with photoperiod to affect immune function and disease processes. The evidence for seasonal fluctuations in lymphatic organ size, structure, immune function, and disease processes, and their possible interactions with recurrent environmental stressor, is reviewed. Seasonal peaks of lymphatic organ size and structure generally occur in late autumn or early winter and seasonal minima are oberved prior to the onset of breeding. Although many of the field data suggest that immune function and disease processes are also enhanced during the winter, the opposite seasonal pattern is also observed in some studies. We propose that compromised immune function may be observed in some populations during particularly harsh winters when stressors override the enhancement of immune function evoked by short day lenghts. Because so many factors covary in field studies, assessment of our proposal that photoperiod mediates seasonal changes in immune function requires laboratory studies in which only photoperiod is varied. A review of the effects of photoperiod on immune function in laboratory studies reveals that exposure to short day lengths enhances immune function in every species examined. Short day exposure in small mammals causes reproductive inhibition and concomitant reduction in plasma levels of prolactin and steroid hormones, as well as alterations in the temporal pattern of pineal melatonin secretion. These hormones effect immune function, and influence the development of opportunistic disease, including cancer; however, it appears that either prolactin or melatonin secretion is responsible for mediating the effects of photoperiod on immune function. Taken together, day length appears to affect immune function in many species, including animals that typically do not exhibit reproductive responsiveness to day length. These data could have a major impact on understanding the etiology and progression of diseases in humans and nonhuman animals. The clinical significance of these data is also considered.

Metabolic costs of mounting an antigen-stimulated immune response in adult and aged C57BL/6J mice

Animals must balance their energy budget despite seasonal changes in both energy availability and physiological expenditures. Immunity, in addition to growth, thermoregulation, and cellular maintenance, requires substantial energy to maintain function, although few studies have directly tested the energetic cost of immunity. The present study assessed the metabolic costs of an antibody response. Adult and aged male C5BL/6J mice were implanted with either empty Silastic capsules or capsules filled with melatonin and injected with either saline or keyhole limpet hemocyanin (KLH). O2 consumption was monitored periodically throughout antibody production using indirect calorimetry. KLH-injected mice mounted significant immunoglobulin G (IgG) responses and consumed more O2 compared with animals injected with saline. Melatonin treatment increased O2 consumption in mice injected with saline but suppressed the increased metabolic rate associated with an immune response in KLH-injected animals. Melatonin had no effect on immune response to KLH. Adult and aged mice did not differ in antibody response or metabolic activity. Aged mice appear unable to maintain sufficient heat production despite comparable O2 production to adult mice. These results suggest that mounting an immune response requires significant energy and therefore requires using resources that could otherwise be allocated to other physiological processes. Energetic trade-offs are likely when energy demands are high (e.g., during winter, pregnancy, or lactation). Melatonin appears to play an adaptive role in coordinating reproductive, immunologic, and energetic processes.

Seasonal Changes in Adiposity: the Roles of the Photoperiod, Melatonin and Other Hormones, and Sympathetic Nervous System

It appears advantageous for many non-human animals to store energy body fat extensively and efficiently because their food supply is more labile and less abundant than in their human counterparts. The level of adiposity in many of these species often shows predictable increases and decreases with changes in the season. These cyclic changes in seasonal adiposity in some species are triggered by changes in the photoperiod that are faithfully transduced into a biochemical signal through the nightly secretion of melatonin (MEL) via the pineal gland. Here, we focus primarily on the findings from the most commonly studied species showing seasonal changes in adiposity—Siberian and Syrian hamsters. The data to date are not compelling for a direct effect of MEL on white adipose tissue (WAT) and brown adipose tissue (BAT) despite some recent data to the contrary. Thus far, none of the possible hormonal intermediaries for the effects of MEL on seasonal adiposity appear likely as a mechanism by which MEL affects the photoperiodic control of body fat levels indirectly. We also provide evidence pointing toward the sympathetic nervous system as a likely mediator of the effects of MEL on short day-induced body fat decreases in Siberian hamsters through increases in sympathetic drive on WAT and BAT. We speculate that decreases in the SNS drive to these tissues may underlie the photoperiod-induced seasonal increases in body fat of species such as Syrian hamsters. Clearly, we need to deepen our understanding of seasonal adiposity, although, to our knowledge, this is the only form of environmentally induced changes in body fat where the key elements of its external trigger have been identified and can be traced to and through their transduction into a physiological stimulus that ultimately affects identified responses of white adipocyte physiology and cellularity. Finally, the comparative physiological approach to the study of seasonal adiposity seems likely to continue to yield significant insights into the mechanisms underlying this phenomenon and for understanding obesity and its reversal in general.

Beyond phytohaemagglutinin: assessing vertebrate immune function across ecological contexts

1. Over the past decade, there has been a substantial increase in interest in the immune system and the role it plays in the regulation of disease susceptibility, giving rise to the field of eco-immunology. 2. Eco-immunology aims to understand changes in host immune responses in the broader framework of an organisms evolutionary, ecological and life-history contexts. 3. The immune system, however, is complex and multifaceted and can be intimidating for the nonimmunologist interested in incorporating immunological questions into their research. Which immune responses should one measure and what is the biological significance of these measures? 4. The focus of this review is to describe a wide range of eco-immunology techniques, from the simple to the sophisticated, with the goal of providing researchers with a range of options to consider incorporating in their own research programs. 5. These techniques were chosen because they provide relatively straightforward, biologically meaningful assessments of immune function, many of which can be performed across a range of ecological contexts (i.e. field vs. laboratory) and in a wide range of vertebrate animals without relying on species-specific reagents. 6. By incorporating assessments of immune function into their specific research questions, animal ecologists will gain a more comprehensive understanding of organism-environment interactions.

SCN Efferents to Peripheral Tissues: Implications for Biological Rhythms

The suprachiasmatic nucleus (SCN) is the principal generator of circadian rhythms and is part of an entrainment system that synchronizes the animal with its environment. Here, the authors review the possible communication of timing information from the SCN to peripheral tissues involved in regulating fundamental physiological functions as revealed using a viral, transneuronal tract tracer, the pseudorabies virus (PRV). The sympathetic nervous system innervation of the pineal gland and the sympathetic outflow from brain to white adipose tissue were the first demonstrations of SCN-peripheral tissue connections. The inclusion of the SCN as part of these and other circuits was the result of lengthened postviral injection times compared with those used previously. Subsequently, the SCN has been found to be part of the sympathetic outflow from the brain to brown adipose tissue, thyroid gland, kidney, bladder, spleen, adrenal medulla, and perhaps the adrenal cortex. The SCN also is involved in the parasympathetic nervous system innervation of the thyroid, liver, pancreas, and submandibular gland. Individual SCN neurons appear connected to more than one autonomic circuit involving both sympathetic and parasympathetic innervation of a single tissue, or sympathetic innervation of two different peripheral tissues. Collectively, the results of these PRV studies require an expansion of the traditional roles of the SCN to include the autonomic innervation of peripheral tissues and perhaps the modulation of neuroendocrine systems traditionally thought to be controlled solely by hypothalamic stimulating/inhibiting factors.

Stroke in Estrogen Receptor-α–Deficient Mice

Background and Purpose —Recent evidence suggests that endogenous estrogens or hormone replacement therapy can ameliorate brain damage from experimental stroke. Protective mechanisms involve enhanced cerebral vasodilation during ischemic stress as well as direct preservation of neuronal viability. We hypothesized that if the intracellular estrogen receptor subtype-α (ERα) is important to estrogen’s signaling in the ischemic brain, then ERα-deficient (knockout) (ERαKO) female mice would sustain exaggerated cerebral infarction damage after middle cerebral artery occlusion. Methods —The histopathology of cresyl violet–stained tissues was evaluated after reversible middle cerebral artery occlusion (2 hours, followed by 22 hours of reperfusion) in ERαKO transgenic and wild-type (WT) mice (C57BL/6J background strain). End-ischemic cerebral blood flow mapping was obtained from additional female murine cohorts by using [14C]iodoantipyrine autoradiography. Results —Total hemispheric tissue damage was not altered by ERα deficiency in female mice: 51.9±10.6 mm3 in ERαKO versus 60.5±5.0 mm3 in WT. Striatal infarction was equivalent, 12.2±1.7 mm3 in ERαKO and 13.4±1.0 mm3 in WT mice, but cortical infarction was paradoxically smaller relative to that of the WT (20.7±4.5 mm3 in ERαKO versus 30.6±4.1 mm3 in WT). Intraocclusion blood flow to the parietal cortex was higher in ERαKO than in WT mice, likely accounting for the reduced infarction in this anatomic area. There were no differences in stroke outcomes by region or genotype in male animals. Conclusions —Loss of ERα does not enhance tissue damage in the female animal, suggesting that estrogen inhibits brain injury by mechanisms that do not depend on activation of this receptor subtype.

Short-day increases in aggression are inversely related to circulating testosterone concentrations in male Siberian hamsters (Phodopus sungorus).

Many nontropical rodent species display seasonal changes in both physiology and behavior that occur primarily in response to changes in photoperiod. Short-day reductions in reproduction are due, in part, to reductions in gonadal steroid hormones. In addition, gonadal steroids, primarily testosterone (T), have been implicated in aggression in many mammalian species. Some species, however, display increased aggression in short days despite basal circulating concentrations of T. The goal of the present studies was to test the effects of photoperiod on aggression in male Siberian hamsters (Phodopus sungorus) and to determine the role of T in mediating photoperiodic changes in aggression. In Experiment 1, hamsters were housed in long and short days for either 10 or 20 weeks and aggression was determined using a resident-intruder model. Hamsters housed in short days for 10 weeks underwent gonadal regression and displayed increased aggression compared to long-day-housed animals. Prolonged maintenance in short days (i.e., 20 weeks), however, led to gonadal recrudescence and reduced aggression. In Experiment 2, hamsters were housed in long and short days for 10 weeks. Half of the short-day-housed animals were implanted with capsules containing T whereas the remaining animals received empty capsules. In addition, half of the long-day-housed animals were castrated whereas the remaining animals received sham surgeries. Short-day control hamsters displayed increased aggression compared to either castrated or intact long-day-housed animals. Short-day-housed T treated hamsters, however, did not differ in aggression from long-day-housed animals. Collectively, these results confirm previous findings of increased aggression in short-day-housed hamsters and suggest that short-day-induced increases in aggression are inversely related to gonadal steroid hormones.

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

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

Spatial memory deficits in segmental trisomic Ts65Dn mice

Spatial memory was assessed in the segmental trisomic 16 mouse (Ts65Dn), a potential model for Down syndrome (DS), using the 12-arm radial maze (RAM). Ts65Dn mice have a portion of mouse chromosome 16 syntenic to the distal end of human chromosome 21 triplicated. On each of 8 daily trials of the RAM, Ts65Dn mice made fewer correct choices than control mice and performed at or near chance levels, indicating a deficit in spatial working memory. On trials 9 and 10, Ts65Dn mice performed as well as control mice on the initial 12 choices, but required a greater number of choices to complete the RAM. The improved performance of Ts65Dn mice on trials 9 and 10 was lost when the animals were retested after a 50-day retention period, suggesting that long-term memory is also defective. These results are not likely explained by differences in either response bias or perceptual discrimination. Ts65Dn and control mice displayed comparable levels of performance in spontaneous alternation in a T-maze, demonstrating that simple spatial memory was not impaired. In the elevated plus maze, Ts65Dn mice did not display higher anxiety levels which could affect their performance in the RAM. In fact, Ts65Dn mice visited open arms on the elevated plus maze more frequently and spent more time on open arms than did control mice. Taken together, these results provide evidence for short- and long-term spatial memory deficits in Ts65Dn mice.

Behavioral and physiological responses to experimentally elevated testosterone in female dark-eyed juncos (Junco hyemalis carolinensis)

Testosterone mediates the expression of many fitness-related traits in male vertebrates and is thought to account for numerous sex differences in trait expression. Testosterone is also secreted by females; however, far less is known regarding its effects on female physiology and behavior. Using a bird species in which the effects of testosterone on males are well characterized, the dark-eyed junco (Junco hyemalis), we tested whether an increase in exogenous testosterone in females would alter the phenotypic expression of a suite of behavioral and physiological traits. We found that increased testosterone levels in female dark-eyed juncos led to decreased cell-mediated immune function and increased intrasexual aggression, hypothalamo-pituitary-adrenal (HPA) axis responsiveness, baseline corticosterone and corticosterone-binding globulin (CBG) levels. Furthermore, immunosuppression following testosterone implantation was negatively correlated with total and free testosterone but did not appear to be related to either total or free corticosterone. These results demonstrate that the phenotypic impact of elevated testosterone is not confined to males in dark-eyed juncos, and that the impact in adults can be similar in males and females. We discuss these results in the context of potential endocrine-immune interactions and the evolution of sexual dimorphism.