Cortisol and corticosterone in immune organs and brain of European starlings: developmental changes, effects of restraint stress, comparison with zebra finches

Heat stress and poultry production: a comprehensive review

The impact of global warming on poultry production has gained significant attention over the years. However, our current knowledge and understanding of the mechanisms through which heat stress (HS) resulting from global warming affects the welfare, behavior, immune response, production performance, and even transgenerational effects in poultry are still incomplete. Further research is needed to delve deeper into these mechanisms to gain a comprehensive understanding. Numerous studies have investigated various biomarkers of stress in poultry, aiming to identify reliable markers that can accurately assess the physiological status and well-being of birds. However, there is a significant amount of variation and inconsistency in the results reported across different studies. This inconsistency highlights the need for more standardized methods and assays and a clearer understanding of the factors that influence these biomarkers in poultry. This review article specifically focuses on 3 main aspects: 1) the neuroendocrine and behavioral responses of poultry to HS, 2) the biomarkers of HS and 3) the impact of HS on poultry production that have been studied in poultry. By examining the neuroendocrine and behavioral changes exhibited by poultry under HS, we aim to gain insights into the physiological impact of elevated temperatures in poultry.

Different Types of Glucocorticoids to Evaluate Stress and Welfare in Animals and Humans: General Concepts and Examples of Combined Use

The main glucocorticoids involved in the stress response are cortisol and cortisone in most mammals and corticosterone in birds and rodents. Therefore, these analytes are currently the biomarkers more frequently used to evaluate the physiological response to a stressful situation. In addition, "total glucocorticoids", which refers to the quantification of various glucocorticoids by immunoassays showing cross-reactivity with different types of glucocorticoids or related metabolites, can be measured. In this review, we describe the characteristics of the main glucocorticoids used to assess stress, as well as the main techniques and samples used for their quantification. In addition, we analyse the studies where at least two of the main glucocorticoids were measured in combination. Overall, this review points out the different behaviours of the main glucocorticoids, depending on the animal species and stressful stimuli, and shows the potential advantages that the measurement of at least two different glucocorticoid types can have for evaluating welfare.

Transportation Stress Increases Fos Immunoreactivity in the Paraventricular Nucleus, but Not in the Nucleus of the Hippocampal Commissure in the Pekin Duck, Anas platyrhynchos domesticus

Commercial poultry undergo transportation during their life, and the effects of transportation can negatively impact poultry production and welfare. In order to maintain physiological homeostasis, the hypothalamic−pituitary−adrenal axis (HPA) works to respond to stressors. Previous studies by others have shown contradictory effects of transportation on corticosterone release. However, recent studies from our lab and by others have shown that cortisol may also be an important hormone in the avian HPA. The purpose of our current study was to determine the effects of transportation stress on the stimulation of brain nuclei that regulate the HPA in birds, and on glucocorticoid (GC) secretion. To test this hypothesis, we collected blood and brain samples from developer drakes and hens (N = 10 per sex/time point): 24 h prior to transportation, immediately after transportation, 24 h after transportation, and 1 week after transportation. Serum GC levels and fos immunocytochemistry (ICC) within the nucleus of the hippocampal commissure (NHpC) and paraventricular nucleus (PVN) were measured. Data were analyzed using a two-way ANOVA. Post hoc analysis was completed using a Fisher’s PLSD with a p < 0.05 considered significant. We observed a sex difference (p < 0.05) in both corticosterone and cortisol secretion in Pekin ducks, although neither GC showed a significant increase in secretion associated with transportation. However, we did observe a significant (p < 0.05) increase in fos-like immunoreactivity for 24 h in the PVN, but not in the NHpC. Further studies are required to determine the specific role that GCs play in the avian stress response and the short-term stressors that could have long-term physiological effects on birds.

Sex differences in serum glucocorticoid levels and heterophil:lymphocyte ratios in adult pekin ducks (Anas platyrhynchos domesticus)

It is becoming more common for poultry scientists to utilize direct and indirect measures of stress hormones to monitor bird welfare. However, it has been clear that our understanding of the avian hypothalamic-pituitaryadrenal axis (HPA) is insufficient as evidenced by the many conflicting reports regarding stress responses, such as transportation stress, in poultry. It has long been assumed that the poultry HPA functions similarly to that of mammals, but now we know that there are considerable differences in the avian HPA compared to mammals. Synthesis and release of glucocorticoids (GC) are stimulated by adrenocorticotropin hormone (ACTH); GC are synthesized from a common pathway that begins with cholesterol and pregnenolone. The synthesis of one of the glucocorticoids does not depend upon the synthesis of the other. The purpose of our study was to test the hypothesis that ACTH will stimulate both corticosterone and cortisol release in ducks. To test this hypothesis, we injected artificial ACTH (cosyntropin; 0.0625 mg/kg, 0.031 mg/kg, or 0.016 mg/kg or saline as control) intramuscularly into adult drakes and hens (N = 10/sex/dose). Both glucocorticoids (GC) were assayed in serum using previously verified ELISAs. Blood smears were also assessed for heterophil to lymphocyte ratios (HLR). Data were analyzed by repeated measures 3-way ANOVA with Fishers PLSD as an ad hoc test. We observed that both GC were secreted in significantly (p = 0.0002) different patterns in a dose-dependent manner compared to controls, and that there was a significant (p = 0.0001) sex difference in both GC compared to saline controls. Further, we observed that all doses of ACTH elicited a significant (p = 0.004) sex difference in the HLR response compared to controls, but no dose-dependent effects were noted. Our data suggest that ducks, at least, may utilize more than just corticosterone to maintain physiological homeostasis in response to stress. Further, the time course of the stressor to release GC and subsequent HLR response may be dependent upon sex. More detailed analyses of the HPA are necessary in all avian species to better understand stress responses as we utilize biological bases for welfare assessments and stress responses.

Glucocorticoid production in the thymus and brain: Immunosteroids and neurosteroids

Glucocorticoids (GCs) regulate a myriad of physiological systems, such as the immune and nervous systems. Systemic GC levels in blood are often measured as an indicator of local GC levels in target organs. However, several extra-adrenal organs can produce and metabolize GCs locally. More sensitive and specific methods for GC analysis (i.e., mass spectrometry) allow measurement of local GC levels in small tissue samples with low GC concentrations. Consequently, is it now apparent that systemic GC levels often do not reflect local GC levels. Here, we review the use of systemic GC measurements in clinical and research settings, discuss instances where systemic GC levels do not reflect local GC levels, and present evidence that local GC levels provide useful insights, with a focus on local GC production in the thymus (immunosteroids) and brain (neurosteroids). Lastly, we suggest key areas for further research, such as the roles of immunosteroids and neurosteroids in neonatal programming and the potential clinical relevance of local GC modulators.

Sex differences in glucocorticoid responses to shipping stress in Pekin ducks

Some concerns have been raised recently about the assay of corticosterone vs. cortisol in poultry species. Thus, we tested the hypothesis that ducks secrete both glucocorticoids. First, we validated two commercially ELISA kits for the two glucocorticoids by first charcoal stripping duck serum in order to remove all steroid hormones. We ran serial dilutions of spiked, charcoal-stripped serum on kits of opposite glucocorticoid as well as a serial dilution using the respective ELISA buffer of the opposite assay kit. We found that the glucocorticoid standard curve in duck serum matched the respective curve in that kit's own buffer. However, when the opposite hormone was run in each kit in both duck serum or ELISA buffer, a near zero slope was obtained. Second, we further validated the presence of both glucocorticoids using mass spectrometry. Third, we tested the hypothesis that exogenous ACTH would stimulate the release of both corticosterone and cortisol. And, fourth, we tested the hypothesis that each glucocorticoid would have different serum levels in response to shipping stress. To test this hypothesis, we collected serum from 10 drakes and 10 hens from 2 flocks (N = 20 per time point per sex): 24 h prior to shipping, at shipping as ducks were walked off the truck, 24 h after shipping, and 1 wk after shipping. Data were analyzed by 2-way repeated measures ANOVA. Surprisingly, we also observed a sex difference in both glucocorticoid levels in that hens showed higher (P < 0.01) serum levels than did drakes at all-time points in response to either ACTH or transportation. Finally, no differences were observed in either glucocorticoid levels associated with shipping in either sex. The fact that both glucocorticoids are released in measurable amounts lends to the possibility that they may be differentially regulated, or at least there is a sex difference in the neural pathways associated with glucocorticoid release in ducks. Although corticosterone is the likely predominate glucocorticoid in ducks, serious attention should be given to the role of cortisol in poultry. Further consideration of sex, age, and timing of blood collection to stressor needs to be considered when assessing glucocorticoid levels in any avian species.

Steroid profiling of glucocorticoids in microdissected mouse brain across development

Corticosterone is produced by the adrenal glands and also produced locally by other organs, such as the brain. Local levels of corticosterone in specific brain regions during development are not known. Here, we microdissected brain tissue and developed a novel liquid chromatography tandem mass spectrometry method (LC-MS/MS) to measure a panel of seven steroids (including 11-deoxycorticosterone (DOC), corticosterone, and 11-dehydrocorticosterone (DHC) in the blood, hippocampus (HPC), cerebral cortex (CC), and hypothalamus (HYP) of mice at postnatal day (PND) 5, 21, and 90. In a second cohort of mice, we measured the expression of three genes that code for steroidogenic enzymes that regulate corticosterone levels (Cyp11b1, Hsd11b1, and Hsd11b2) in the HPC, CC, and HYP. There were region-specific patterns of steroid levels across development, including higher corticosterone levels in the HPC and HYP than in the blood at PND5. In contrast, corticosterone levels were higher in the blood than in all brain regions at PND21 and PND90. Brain corticosterone levels were not positively correlated with blood corticosterone levels, and correlations across brain regions increased with age. Local corticosterone levels were best predicted by local DOC levels at PND5, but by local DHC levels at PND21 and PND90. Transcripts for the three enzymes were detectable in all samples (with highest expression of Hsd11b1) and showed region-specific changes with age. These data demonstrate that individual brain regions fine-tune local levels of corticosterone during early development and that coupling of glucocorticoid levels across regions increases with age.

The stressed brain: regional and stress-related corticosterone and stress-regulated gene expression in the adult zebra finch (Taeniopygia guttata)

Glucocorticoids (CORT) are well-known as important regulators of behaviour and cognition at basal levels and under stress. However, the precise mechanisms governing CORT action and functional outcomes of this action in the brain remain unclear, particularly in model systems other than rodents. In the present study, we investigated the dynamics of CORT regulation in the zebra finch, an important model system for vocal learning, neuroplasticity and cognition. We tested the hypothesis that CORT is locally regulated in the zebra finch brain by quantifying regional and stress-related variation in total CORT across brain regions. In addition, we used an ex vivo slice culture system to test whether CORT regulates target gene expression uniquely in discrete regions of the brain. We documented a robust increase in brain CORT across regions after 30 minutes of restraint stress but, interestingly, baseline and stress-induced CORT levels varied between regions. In addition, CORT treatment of brain slice cultures differentially affected expression of three CORT target genes: it up-regulated expression of FKBP5 in most regions and SGK1 in the hypothalamus only, whereas GILZ was unaffected by CORT treatment across all brain regions investigated. The specific mechanisms producing regional variation in CORT and CORT-dependent downstream gene expression remain unknown, although these data provide additional support for the hypothesis that the songbird brain employs regulatory mechanisms that result in precise control over the influence of CORT on glucocorticoid-sensitive neural circuits.

Measurement of 11-dehydrocorticosterone in mice, rats and songbirds: Effects of age, sex and stress

Glucocorticoids (GCs) are secreted into the blood by the adrenal glands and are also locally-produced by organs such as the lymphoid organs (bone marrow, thymus, and spleen). Corticosterone is the primary circulating GC in many species, including mice, rats and birds. Within lymphoid organs, corticosterone can be locally produced from the inactive metabolite, 11-dehydrocorticosterone (DHC). However, very little is known about endogenous DHC levels, and no immunoassays are currently available to measure DHC. Here, we developed an easy-to-use and inexpensive immunoassay to measure DHC that is accurate, precise, sensitive, and specific. The DHC immunoassay was validated in multiple ways, including comparison with a mass spectrometry assay. After assay validations, we demonstrated the usefulness of this immunoassay by measuring DHC (and corticosterone) in mice, rats and song sparrows. Overall, corticosterone levels were higher than DHC levels across species. In Study 1, using mice, we measured steroids in whole blood and lymphoid organs at postnatal day (PND) 5, PND23, and PND90. Corticosterone and DHC showed distinct tissue-specific patterns across development. In Studies 2 and 3, we measured circulating corticosterone and DHC in adult rats and song sparrows, before and after restraint stress. In rats and song sparrows, restraint stress rapidly increased circulating levels of both steroids. This novel DHC immunoassay revealed major changes in DHC concentrations during development and in response to stress, which have important implications for understanding GC physiology, effects of stress on immune function, and regulation of local GC levels.

Local glucocorticoid production in lymphoid organs of mice and birds: Functions in lymphocyte development

Circulating glucocorticoids (GCs) are powerful regulators of immunity. Stress-induced GC secretion by the adrenal glands initially enhances and later suppresses the immune response. GC targets include lymphocytes of the adaptive immune system, which are well known for their sensitivity to GCs. Less appreciated, however, is that GCs are locally produced in lymphoid organs, such as the thymus, where GCs play a critical role in selection of the T cell antigen receptor (TCR) repertoire. Here, we review the roles of systemic and locally-produced GCs in T lymphocyte development, which has been studied primarily in laboratory mice. By antagonizing TCR signaling in developing T cells, thymus-derived GCs promote selection of T cells with stronger TCR signaling. This results in increased T cell-mediated immune responses to a range of antigens. We then compare local and systemic GC patterns in mice to those in several bird species. Taken together, these studies suggest that a combination of adrenal and lymphoid GC production might function to adaptively regulate lymphocyte development and selection, and thus antigen-specific immune reactivity, to optimize survival under different environmental conditions. Future studies should examine how lymphoid GC patterns vary across other vertebrates, how GCs function in B lymphocyte development in the bone marrow, spleen, and the avian bursa of Fabricius, and whether GCs adaptively program immunity in free-living animals.

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.

Determinants and significance of corticosterone regulation in the songbird brain

Songbirds exhibit significant adult neuroplasticity that, together with other neural specializations, makes them an important model system for neurobiological studies. A large body of work also points to the songbird brain as a significant target of steroid hormones, including corticosterone (CORT), the primary avian glucocorticoid. Whereas CORT positively signals the brain for many functions, excess CORT may interfere with natural neuroplasticity. Consequently, mechanisms may exist to locally regulate CORT levels in brain to ensure optimal concentrations. However, most studies in songbirds measure plasma CORT as a proxy for levels at target tissues. In this paper, we review literature concerning circulating CORT and its effects on behavior in songbirds, and discuss recent work suggesting that brain CORT levels are regulated independently of changes in adrenal secretion. We review possible mechanisms for CORT regulation in the avian brain, including corticosteroid-binding globulins, p-glycoprotein activity in the blood-brain barrier and CORT metabolism by the 11ß hydroxysteroid dehydrogenases. Data supporting a role for CORT regulation within the songbird brain have only recently begun to emerge, suggesting that this is an avenue for important future research.

Locally elevated cortisol in lymphoid organs of the developing zebra finch but not Japanese quail or chicken

Glucocorticoids are important for production of functional lymphocytes and immunity. In altricial neonates, adrenal glands are unresponsive and local glucocorticoid synthesis in lymphoid organs may be necessary to support lymphocyte development. Precocial neonates, in contrast, have fully responsive adrenal glucocorticoid production, and lymphoid glucocorticoid synthesis may not be necessary. Here, we found that in altricial zebra finch hatchlings, lymphoid organs had dramatically elevated endogenous glucocorticoid (and precursor) levels compared to levels in circulating blood. Furthermore, while avian adrenals produce corticosterone, finch lymphoid organs had much higher levels of cortisol, an unexpected glucocorticoid in birds. In contrast, precocial Japanese quail and chicken offspring did not have locally elevated lymphoid glucocorticoid levels, nor did their lymphoid organs contain high proportions of cortisol. These results show that lymphoid glucocorticoids differ in identity, concentration, and possibly source, in hatchlings of three different bird species. Locally-regulated glucocorticoids might have species-specific roles in immune development.

Contextual modulation of social and endocrine correlates of fitness: insights from the life history of a sex changing fish

Steroid hormones are critical regulators of reproductive life history, and the steroid sensitive traits (morphology, behavior, physiology) associated with particular life history stages can have substantial fitness consequences for an organism. Hormones, behavior and fitness are reciprocally associated and can be used in an integrative fashion to understand how the environment impacts organismal function. To address the fitness component, we highlight the importance of using reliable proxies of reproductive success when studying proximate regulation of reproductive phenotypes. To understand the mechanisms by which the endocrine system regulates phenotype, we discuss the use of particular endocrine proxies and the need for appropriate functional interpretation of each. Lastly, in any experimental paradigm, the responses of animals vary based on the subtle differences in environmental and social context and this must also be considered. We explore these different levels of analyses by focusing on the fascinating life history transitions exhibited by the bi-directionally hermaphroditic fish, Lythrypnus dalli. Sex changing fish are excellent models for providing a deeper understanding of the fitness consequences associated with behavioral and endocrine variation. We close by proposing that local regulation of steroids is one potential mechanism that allows for the expression of novel phenotypes that can be characteristic of specific life history stages. A comparative species approach will facilitate progress in understanding the diversity of mechanisms underlying the contextual regulation of phenotypes and their associated fitness correlates.

Fasting increases aggression and differentially modulates local and systemic steroid levels in male zebra finches

Aggression enables individuals to obtain and retain limited resources. Studies of the neuroendocrine regulation of aggression have focused on territorial and reproductive contexts. By contrast, little is understood concerning the neuroendocrine regulation of aggression over other resources, such as food. Here, we developed a paradigm to examine the role of steroids in food-related aggression. In groups of male zebra finches, a 6-hour fast decreased body mass and increased aggressive interactions among subjects that competed for a point source feeder. Fasting also dramatically altered circulating steroid levels by decreasing plasma testosterone but not estradiol (E2). By contrast, both plasma corticosterone and dehydroepiandrosterone (DHEA) concentrations were elevated with fasting. Interestingly, short-term access to food (15 minutes) after fasting normalized circulating steroid levels. Fasting increased corticosterone levels in a wide range of peripheral tissues but increased DHEA levels specifically in adrenal glands and liver; these effects were quickly normalized with refeeding. DHEA can be metabolized within specific brain regions to testosterone and E2, which promote the expression of aggression. We measured E2 in microdissected brain regions and found that fasting specifically increased local E2 levels in 3 regions: the periaqueductal gray, ventral tegmental area, and ventromedial nucleus of the hypothalamus. These regions are part of the vertebrate social behavior network and regulate the expression of aggression. Together, these data suggest that fasting stimulates secretion of DHEA from the adrenals and liver and subsequent conversion of DHEA to E2 within specific brain regions, to enable individuals to compete for limited food resources.

Extra-adrenal glucocorticoids and mineralocorticoids: evidence for local synthesis, regulation, and function

Glucocorticoids and mineralocorticoids are steroid hormones classically thought to be secreted exclusively by the adrenal glands. However, recent evidence has shown that corticosteroids can also be locally synthesized in various other tissues, including primary lymphoid organs, intestine, skin, brain, and possibly heart. Evidence for local synthesis includes detection of steroidogenic enzymes and high local corticosteroid levels, even after adrenalectomy. Local synthesis creates high corticosteroid concentrations in extra-adrenal organs, sometimes much higher than circulating concentrations. Interestingly, local corticosteroid synthesis can be regulated via locally expressed mediators of the hypothalamic-pituitary-adrenal (HPA) axis or renin-angiotensin system (RAS). In some tissues (e.g., skin), these local control pathways might form miniature analogs of the pathways that regulate adrenal corticosteroid production. Locally synthesized glucocorticoids regulate activation of immune cells, while locally synthesized mineralocorticoids regulate blood volume and pressure. The physiological importance of extra-adrenal glucocorticoids and mineralocorticoids has been shown, because inhibition of local synthesis has major effects even in adrenal-intact subjects. In sum, while adrenal secretion of glucocorticoids and mineralocorticoids into the blood coordinates multiple organ systems, local synthesis of corticosteroids results in high spatial specificity of steroid action. Taken together, studies of these five major organ systems challenge the conventional understanding of corticosteroid biosynthesis and function.

DHEA and estradiol levels in brain, gonads, adrenal glands, and plasma of developing male and female European starlings

Traditionally, sexual differentiation of the brain was thought to be driven by gonadal hormones, particularly testosterone (T). However, recent studies in songbirds suggest that other steroids may also be important. For example, dehydroepiandrosterone (DHEA) can be synthesized by the gonads, adrenal glands, and/or brain and locally metabolized into T and 17β-estradiol (E(2)). Here, we examined DHEA and E(2) levels in the brain, peripheral tissues, and plasma of wild European starlings (Sturnus vulgaris). In Study 1, samples were collected from males and females at P0 (day of hatch), P6, and P8. In Study 2, samples were collected at P4. At P0, DHEA levels in the diencephalon were higher in males than females. DHEA levels were generally high in the gonads and adrenals, and they were higher in testes than ovaries at P8. Further, E(2) levels were non-detectable in most brain samples, suggesting that DHEA was not metabolized to E(2) or that locally produced E(2) was rapidly inactivated. At P4, DHEA levels in telencephalic regions were lower in males than females. Taken together, these data suggest that sex differences in peripheral DHEA secretion and neural DHEA metabolism at specific ages during development might play a role in sexual differentiation of the songbird brain.

Steroid concentrations in plasma, whole blood and brain: effects of saline perfusion to remove blood contamination from brain

The brain and other organs locally synthesize steroids. Local synthesis is suggested when steroid levels are higher in tissue than in the circulation. However, measurement of both circulating and tissue steroid levels are subject to methodological considerations. For example, plasma samples are commonly used to estimate circulating steroid levels in whole blood, but steroid levels in plasma and whole blood could differ. In addition, tissue steroid measurements might be affected by blood contamination, which can be addressed experimentally by using saline perfusion to remove blood. In Study 1, we measured corticosterone and testosterone (T) levels in zebra finch (Taeniopygia guttata) plasma, whole blood, and red blood cells (RBC). We also compared corticosterone in plasma, whole blood, and RBC at baseline and after 60 min restraint stress. In Study 2, we quantified corticosterone, dehydroepiandrosterone (DHEA), T, and 17β-estradiol (E₂) levels in the brains of sham-perfused or saline-perfused subjects. In Study 1, corticosterone and T concentrations were highest in plasma, significantly lower in whole blood, and lowest in RBC. In Study 2, saline perfusion unexpectedly increased corticosterone levels in the rostral telencephalon but not other regions. In contrast, saline perfusion decreased DHEA levels in caudal telencephalon and diencephalon. Saline perfusion also increased E₂ levels in caudal telencephalon. In summary, when comparing local and systemic steroid levels, the inclusion of whole blood samples should prove useful. Moreover, blood contamination has little or no effect on measurement of brain steroid levels, suggesting that saline perfusion is not necessary prior to brain collection. Indeed, saline perfusion itself may elevate and lower steroid concentrations in a rapid, region-specific manner.

Corticosterone and cortisol binding sites in plasma, immune organs and brain of developing zebra finches: intracellular and membrane-associated receptors

Glucocorticoids (GCs) affect the development of both the immune and nervous systems. To do so, GCs bind to intracellular receptors, mineralocorticoid receptors (MR) and glucocorticoid receptors (GR). In addition, GCs bind to membrane-associated corticosteroid receptors (mCR). Two well-known GCs are corticosterone and cortisol. Whereas corticosterone is the primary GC in zebra finch plasma, cortisol is the primary GC in zebra finch lymphoid organs and is also present in the brain and plasma during development. Here, we characterized binding sites for corticosterone and cortisol in plasma, liver, lymphoid organs, and brain of developing zebra finches. In tissues, we examined both intracellular and membrane-associated binding sites. For intracellular receptors, there were MR-like sites and GR-like sites, which differentially bound corticosterone and cortisol in a tissue-specific manner. For mCR, we found little evidence for membrane-associated receptors in immune organs, but this could be due to the small size of immune organs. Interestingly, cortisol, but not corticosterone, showed a low amount of specific binding to bursa of Fabricius membranes. For neural membranes, corticosterone bound to one site with low affinity but a relatively high B(max), and in contrast, cortisol bound to one site with high affinity but a lower B(max). Our results indicate that intracellular and membrane-associated receptors differentially bind corticosterone and cortisol suggesting that corticosterone and cortisol might have different roles in immune and nervous system development.

Effects of environmental change on wildlife health

Environmental change has negatively affected most biological systems on our planet and is becoming of increasing concern for the well-being and survival of many species. At an organism level, effects encompass not only endocrine disruptions, sex-ratio changes and decreased reproductive parameters, but also include teratogenic and genotoxic effects, immunosuppression and other immune-system impairments that can lead directly to disease or increase the risk of acquiring disease. Living organisms will strive to maintain health by recognizing and resolving abnormal situations, such as the presence of invading microorganisms or harmful peptides, abnormal cell replication and deleterious mutations. However, fast-paced environmental changes may pose additional pressure on immunocompetence and health maintenance, which may seriously impact population viability and persistence. Here, we outline the importance of a functional immune system for survival and examine the effects that exposure to a rapidly changing environment might exert on immunocompetence. We then address the various levels at which anthropogenic environmental change might affect wildlife health and identify potential deficits in reproductive parameters that might arise owing to new immune challenges in the context of a rapidly changing environment. Throughout the paper, a series of examples and case studies are used to illustrate the impact of environmental change on wildlife health.