Christine R. Lattin

Hypothalamus–pituitary–adrenal axis activity and the subsequent response to chronic stress differ depending upon life history stage

The hypothalamus-pituitary-adrenal (HPA) axis is modulated seasonally in many species, and chronic stress can alter HPA functioning. However, it is not known how these two factors interact - are there particular life history stages when animals are more or less vulnerable to chronic stress? We captured wild house sparrows (Passer domesticus) in Massachusetts during six different life history stages: early and late winter, pre-laying, breeding, late breeding, and molt. At each time point, we tested HPA function by measuring baseline and stress-induced corticosterone (CORT), negative feedback in response to an injection of dexamethasone, and maximum adrenal response through an injection of adrenocorticotropic hormone. We then brought birds into captivity as a model for chronic stress, and repeated the four tests 5 days later. At capture, all HPA variables varied seasonally. Birds showed increased negative feedback during breeding and late winter compared to pre-laying. Furthermore, birds during the late breeding period had down-regulated their HPA axis, perhaps in preparation for molt. After 5 days of captivity, house sparrows lost ∼11% of initial body mass, although birds lost more weight during molt and early winter. Overall, captive sparrows showed elevated baseline CORT and increased negative feedback, although negative feedback did not show a significant increase during any individual life history stage. During most of the year, adrenal sensitivity was unaffected by captivity. However, during late breeding and molt, adrenal sensitivity increased during captivity. Taken together, these data provide further support that HPA function naturally varies throughout the year, with the interesting consequence that molting birds may potentially be more vulnerable to a chronic stressor such as captivity.

Feather coloration in museum specimens is related to feather corticosterone

Colorful ornaments in birds are often sexually selected signals of quality, and variation in ornament expression may be mediated by physiological stress through the secretion of corticosterone. However, testing for links between ornamentation and corticosterone often requires sampling live animals, and such physiological measures may not be matched in the time span in which they were sampled (e.g., very dynamic plasma corticosterone vs. plumage coloration, which is relatively static). Here, we use museum specimens to test for a link between the color of a sexual ornament and feather corticosterone at the time of ornament formation. In red-winged blackbirds, Agelaius phoeniceus, carotenoid-based epaulets appear to be important in male–male social interactions, territory maintenance, and female choice. We measured reflectance spectra of adult male epaulets and plucked adjacent feathers for corticosterone analysis via radioimmunoassay. We controlled for differences in the number of mates, specimen age, and geography by selecting only males with one mate and only birds collected in Florida during a 3-year period. Epaulet hue and red chroma did not vary with feather corticosterone, but males whose epaulets scored high for mean brightness and red brightness had significantly lower corticosterone than males with low brightness scores. This correlation with brightness but not hue or chroma is consistent with an effect of corticosterone (CORT) on feather microstructure, with elevated CORT leading to lower reflectance of white light from the keratin matrix surrounding the carotenoid pigments.

Seasonal Variation in Corticosterone Receptor Binding in Brain, Hippocampus, and Gonads in House Sparrows (Passer domesticus)

ABSTRACT. Both baseline and stress-induced concentrations of corticosterone (CORT) vary seasonally in a predictable fashion in many wild birds. Hypotheses about why these patterns exist include the “behavior hypothesis,” which predicts that animals will down-regulate stress-induced CORT when CORT-induced behaviors are too likely to cause reproductive failure; and the “preparative hypothesis,” which posits that baseline and stress-induced CORT will both be high at times of year with a higher incidence of predictable stressors. We tested predictions made by the behavior and predictive hypotheses about the CORT sensitivity of tissues involved in breeding: whole brain, hippocampus, and gonads. We used radioligand binding assays to examine glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) binding in free-living House Sparrows (Passer domesticus) at several different life history stages. We found lowest GR binding in whole brain during breeding; this suggests relative insensitivity of brain tissue to CORT at this time of year, which is consistent with predictions made by the behavior hypothesis. We found highest GR binding in whole brain in the pre-egg-laying period, which is consistent with the preparative hypothesis, given that this life stage is associated with a predictable increase in the likelihood of stressful events such as threats to territory and nest sites. However, we found no seasonal changes in GR or MR binding in gonads or hippocampus. Our results suggest that down-regulation of brain GR could be one way birds limit the negative effects of CORT release on breeding behavior, but further studies are necessary to understand the anatomic specificity of these changes.

Physiological effects of tourism and associated food provisioning in an endangered iguana

Feeding wildlife is an increasingly popular tourism-related activity despite a limited understanding of potential impacts. Here we demonstrate variable differences in physiological values and endoparasitic infection rates between iguanas from tourist-visited and fed populations versus non-visited populations. The responses from iguanas inhabiting visited islands could compromise health over time.

Intracellular glucocorticoid receptors in spleen, but not skin, vary seasonally in wild house sparrows (Passer domesticus)

Over the short-term and at physiological doses, acute increases in corticosterone (CORT) titres can enhance immune function. There are predictable seasonal patterns in both circulating CORT and immune function across many animal species, but whether CORT receptor density in immune tissues varies seasonally is currently unknown. Using radioligand binding assays, we examined changes in concentrations of glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) in spleen and skin in wild-caught house sparrows in Massachusetts during six different life-history stages: moult, early winter, late winter, pre-egg-laying, breeding and late breeding. Splenic GR and MR binding were highest during the pre-laying period. This may help animals respond to immune threats through increased lymphocyte proliferation and/or an increase in delayed-type hypersensitivity reactions, both of which CORT can stimulate and in which spleen is involved. A decrease in splenic GR and MR during the late breeding period coincides with low baseline and stress-induced CORT, suggesting immune function in spleen may be relatively CORT-independent during this period. We saw no seasonal patterns in GR or MR in skin, suggesting skins response to CORT is modulated primarily via changes in circulating CORT titres and/or via local production of CORT in response to wounding and other noxious stimuli.

There is no correlation between glucocorticoid receptor mRNA expression and protein binding in the brains of house sparrows (Passer domesticus)

The stress response represents an animals attempt to cope with a noxious stimulus through a rapid release of corticosterone or cortisol (CORT) into the bloodstream, resulting in a suite of physiological and behavioral changes. These changes are mediated in large part through CORTs binding to two different intracellular receptors, the high-affinity mineralocorticoid receptor (MR) and the lower-affinity glucocorticoid receptor (GR). We tested the hypothesis that GR and MR mRNA expression would correlate with functional protein expression in neuronal tissue of wild-caught house sparrows (Passer domesticus). To test this hypothesis, we performed a parallel procedure in which protein concentrations were quantified in one half of house sparrow brains (n=16) using radioligand binding assays, and mRNA levels were quantified in the other brain half using reverse-transcriptase quantitative PCR (RT-qPCR). Two reference genes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and TATA-box binding protein (TBP), were used for relative quantification of GR and MR mRNA. Quantifications showed that these two reference genes were not correlated with each other. Furthermore, there was no correlation between mRNA and protein levels for GR or MR using either reference gene, suggesting that regulation of mRNA and protein levels for MR and GR is not tightly linked. This study provides insight into the importance of regulatory steps between mRNA expression and the creation and stability of a functional protein. The overall conclusion is that mRNA expression cannot be used as a proxy for GR or MR binding in house sparrows.

Testing the role of patch openness as a causal mechanism for apparent area sensitivity in a grassland specialist

Area sensitivity, species being disproportionately present on larger habitat patches, has been identified in many taxa. We propose that some apparently area-sensitive species are actually responding to how open a habitat patch is, rather than to patch size. We tested this hypothesis for Bobolinks (Dolichonyx oryzivorus) by comparing density and occupancy to a novel openness index, patch area, and edge effects. Bobolink density and occupancy showed significant relationships with openness, but logistic models based on an openness occupancy threshold had greater explanatory power. Thresholds remained approximately consistent from June through August, and shifted to be more open in September. Variance partitioning supported the openness index as unique and relevant. We found no relationships between measures of body condition (body mass, body size, circulating corticosterone levels) and either openness or area. Our findings have implications for studies of area sensitivity, especially with regards to inconsistencies reported within species: specifically, (1) whether or not a study finds a species to be area sensitive may depend on whether small, open sites were sampled, and (2) area regressions were sensitive to observed densities at the largest sites, suggesting that variation in these fields could lead to inconsistent area sensitivity responses. Responses to openness may be a consequence of habitat selection mediated by predator effects. Finally, openness measures may have applications for predicting effects of habitat management or development, such as adding wind turbines, in open habitat.

Chronic stress alters concentrations of corticosterone receptors in a tissue-specific manner in wild house sparrows (Passer domesticus)

The physiological stress response results in release of glucocorticoid hormones such as corticosterone (CORT). Whereas short-term activation of this response helps animals cope with environmental stressors, chronic activation can result in negative effects including metabolic dysregulation and reproductive failure. However, there is no consensus hormonal profile of a chronically stressed animal, suggesting that researchers may need to look beyond hormone titers to interpret the impacts of chronic stress. In this study, we brought wild house sparrows (Passer domesticus) into captivity. We then compared glucocorticoid and mineralocorticoid receptor concentrations in sparrows exposed either to a standardized chronic stress protocol (n=26) or to standard husbandry conditions (controls; n=20). We used radioligand binding assays to quantify receptors in whole brain, liver, kidneys, spleen, gonads, gastrocnemius and pectoralis muscle, omental and subcutaneous fat, and bib and back skin. In most tissues, CORT receptors did not differ between controls and stressed animals, although we found marginal increases in receptor density in kidney and testes in stressed birds at some time points. Only in pectoralis muscle was there a robust effect of chronic stress, with both receptor types higher in stressed animals. Increased pectoralis sensitivity to CORT with chronic stress may be part of the underlying mechanism for muscle wasting in animals administered exogenous CORT. Furthermore, the change in pectoralis was not paralleled by gastrocnemius receptors. This difference may help explain previous reports of a greater effect of CORT on pectoralis than on other muscle types, and indicate that birds use this muscle as a protein reserve.

Are receptor concentrations correlated across tissues within individuals? A case study examining glucocorticoid and mineralocorticoid receptor binding.

Hormone receptors are a necessary (although not sufficient) part of the process through which hormones like corticosterone create physiological responses. However, it is currently unknown to what extent receptor concentrations across different target tissues may be correlated within individual animals. In this study, we examined this question using a large dataset of radioligand binding data for glucocorticoid receptors (GRs) and mineralocorticoid receptors (MRs) in 13 different tissues in the house sparrow (Passer domesticus) (n=72). Our data revealed that individual house sparrows tended to exhibit higher or lower receptor binding across all tissues, which could be part of what creates the physiological and behavioral syndromes associated with different hormonal profiles. However, although statistically significant, the correlations between tissues were very weak. Thus, when each tissue was independently regressed on receptor concentrations in the other tissues, multivariate analysis revealed significant relationships only for sc fat (for GR) and whole brain, hippocampus, kidney, omental fat, and sc fat (for MR). We also found significant pairwise correlations only between receptor concentrations in brain and hippocampus, and brain and kidney (both for MR). This research reveals that although there are generalized individual consistencies in GR and MR concentrations, possibly due to such factors as hormonal regulation and genetic effects, the ability of 2 different tissues to respond to the same hormonal signal appears to be affected by additional factors that remain to be identified.

Experimentally reducing corticosterone mitigates rapid captivity effects on behavior, but not body composition, in a wild bird

&NA; Wild animals and captives display physiological and behavioral differences, and it has been hypothesized, but rarely tested, that these differences are caused by sustained elevation of the hormone corticosterone. We used repeated computed tomography (CT) imaging to examine body composition changes in breeding male and female wild house sparrows (Passer domesticus; n = 20) in response to two weeks of captivity, and assessed behavioral changes using video recordings. Half of the birds received the drug mitotane, which significantly decreased stress‐induced corticosterone titers compared to controls. Based on the CT images, fat volumes increased, and pectoralis muscle density and heart and testes volumes decreased, over the two weeks of captivity in both groups of birds. However, beak‐wiping, a behavior that can indicate anxiety and aggression, showed increased occurrence in controls compared to mitotane‐treated birds. While our results do not support the hypothesis that these body composition changes were primarily driven by stress‐induced corticosterone, our data suggest that experimentally reducing stress‐induced corticosterone may mitigate some captivity‐induced behavioral changes. Broadly, our results emphasize that researchers should take behavioral and physiological differences between free‐living animals and captives into consideration when designing studies and interpreting results. Further, time in captivity should be minimized when birds will be reintroduced back to the wild. HighlightsWe used mitotane to reduce corticosterone in some wild birds transferred to captivity.Repeated computed tomography (CT) imaging was used to assess body composition.Anxiety‐related behaviors increased more in controls than in mitotane‐treated birds.Body composition changed over the two weeks of captivity in both groups of birds.Reducing corticosterone may mitigate some captivity‐induced behavioral changes.