11ß hydroxysteroid dehydrogenases regulate circulating glucocorticoids but not central gene expression

Regulation of glucocorticoids (GCs), important mediators of physiology and behavior at rest and during stress, is multi-faceted and dynamic. The 11ß hydroxysteroid dehydrogenases 11ß-HSD1 and 11ß-HSD2 catalyze the regeneration and inactivation of GCs, respectively, and provide peripheral and central control over GC actions in mammals. While these enzymes have only recently been investigated in just two songbird species, central expression patterns suggest that they may function differently in birds and mammals, and little is known about how peripheral expression regulates circulating GCs. In this study, we utilized the 11ß-HSD inhibitor carbenoxolone (CBX) to probe the functional effects of 11ß-HSD activity on circulating GCs and central GC-dependent gene expression in the adult zebra finch (Taeniopygia guttata). Peripheral CBX injection produced a marked increase in baseline GCs 60 min after injection, suggestive of a dominant role for 11ß-HSD2 in regulating circulating GCs. In the adult zebra finch brain, where 11ß-HSD2 but not 11ß-HSD1 is expressed, co-incubation of micro-dissected brain regions with CBX and stress-level GCs had no impact on expression of several GC-dependent genes. These results suggest that peripheral 11ß-HSD2 attenuates circulating GCs, whereas central 11ß-HSD2 has little impact on gene expression. Instead, rapid 11ß-HSD2-based regulation of local GC levels might fine-tune membrane GC actions in brain. These results provide new insights into the dynamics of GC secretion and action in this important model organism.

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.

11β-HSD Types 1 and 2 in the Songbird Brain

Glucocorticoid (GC) hormones act on the brain to regulate diverse functions, from behavior and homeostasis to the activity of the hypothalamic-pituitary-adrenal axis. Local regeneration and metabolism of GCs can occur in target tissues through the actions of the 11β-hydroxysteroid dehydrogenases [11 beta-hydroxysteroid dehydrogenase type 1 (11β-HSD1) and 11 beta-hydroxysteroid dehydrogenase type 2 (11β-HSD2), respectively] to regulate access to GC receptors. Songbirds have become especially important model organisms for studies of stress hormone action; however, there has been little focus on neural GC metabolism. Therefore, we tested the hypothesis that 11β-HSD1 and 11β-HSD2 are expressed in GC-sensitive regions of the songbird brain. Localization of 11β-HSD expression in these regions could provide precise temporal and spatial control over GC actions. We quantified GC sensitivity in zebra finch (Taeniopygia guttata) brain by measuring glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) expression across six regions, followed by quantification of 11β-HSD1 and 11β-HSD2 expression. We detected GR, MR, and 11β-HSD2 mRNA expression throughout the adult brain. Whereas 11β-HSD1 expression was undetectable in the adult brain, we detected low levels of expression in the brain of developing finches. Across several adult brain regions, expression of 11β-HSD2 covaried with GR and MR, with the exception of the cerebellum and hippocampus. It is possible that receptors in these latter two regions require direct access to systemic GC levels. Overall, these results suggest that 11β-HSD2 expression protects the adult songbird brain by rapid metabolism of GCs in a context and region-specific manner.

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.

Sex, estradiol, and spatial memory in a food-caching corvid

Estrogens significantly impact spatial memory function in mammalian species. Songbirds express the estrogen synthetic enzyme aromatase at relatively high levels in the hippocampus and there is evidence from zebra finches that estrogens facilitate performance on spatial learning and/or memory tasks. It is unknown, however, whether estrogens influence hippocampal function in songbirds that naturally exhibit memory-intensive behaviors, such as cache recovery observed in many corvid species. To address this question, we examined the impact of estradiol on spatial memory in non-breeding Western scrub-jays, a species that routinely participates in food caching and retrieval in nature and in captivity. We also asked if there were sex differences in performance or responses to estradiol. Utilizing a combination of an aromatase inhibitor, fadrozole, with estradiol implants, we found that while overall cache recovery rates were unaffected by estradiol, several other indices of spatial memory, including searching efficiency and efficiency to retrieve the first item, were impaired in the presence of estradiol. In addition, males and females differed in some performance measures, although these differences appeared to be a consequence of the nature of the task as neither sex consistently out-performed the other. Overall, our data suggest that a sustained estradiol elevation in a food-caching bird impairs some, but not all, aspects of spatial memory on an innate behavioral task, at times in a sex-specific manner.

Evolutionary patterns of adaptive acrobatics and physical performance predict expression profiles of androgen receptor - but not oestrogen receptor - in the forelimb musculature

1. Superior physical competence is vital to the adaptive behavioral routines of many animals, particularly those that engage in elaborate socio-sexual displays. How such traits evolve across species remains unclear. 2. Recent work suggests that activation of sex steroid receptors in neuromuscular systems is necessary for the fine motor skills needed to execute physically elaborate displays. Thus, using passerine birds as models, we test whether interspecific variation in display complexity predicts species differences in the abundance of androgen and estrogen receptors (AR and ERα) expressed in the forelimb musculature and spinal cord. 3. We find that small-scale evolutionary patterns in physical display complexity positively predict expression of the AR in the main muscles that lift and retract the wings. No such relationship is detected in the spinal cord, and we do not find a correlation between display behavior and neuromuscular expression of ERα. Also, we find that AR expression levels in different androgen targets throughout the body - namely the wing muscles, spinal cord, and testes - are not necessarily correlated, providing evidence that evolutionary forces may drive AR expression in a tissue-specific manner. 4. These results suggest co-evolution between the physical prowess necessary for display performance and levels of AR expression in avian forelimb muscles. Moreover, this relationship appears to be specific to muscle and AR-mediated, but not ERα-mediated, signaling. 5. Given that prior work suggests that activation of muscular AR is a necessary component of physical display performance, our current data support the hypothesis that sexual selection shapes levels of AR expressed in the forelimb skeletal muscles to help drive the evolution of adaptive motor abilities.

In vivo detection of fluctuating brain steroid levels in zebra finches

This protocol describes a method for the in vivo measurement of steroid hormones in brain circuits of the zebra finch. A guide cannula is surgically implanted into the skull, microdilysate is collected through a microdialysis probe that is inserted into the cannula, and steroid concentrations in the microdialysate are determined using the enzyme-linked immunosorbent assay (ELISA). In some cases, the steroids measured are derived locally (e.g., neural estrogens in males), whereas in other cases, the steroids measured reflect systemic circulating levels and/or central conversion (e.g., the primary androgen testosterone and the primary glucocorticoid corticosterone). A reverse-microdialysis ("retrodialysis") method that can be used to deliver pharmacological agents into the brain to influence local steroid neurochemistry as well as behavior is also discussed.

Region-specific neural corticosterone patterns differ from plasma in a male songbird

The adrenal hormone corticosterone (CORT) acts on brain to mediate physiology and behavior. In songbirds, behavioral effects of CORT vary across species, environmental conditions, and life history stage, with several mechanisms proposed to account for these divergent results. Although blood CORT levels are well characterized, few studies measure CORT within the brain itself. Here we used in vivo microdialysis to measure CORT in two regions of the zebra finch brain, the hippocampus (HP) and caudal nidopallium (cNp). Our results show that we can successfully measure physiological levels of CORT in brain within 15- to 30-minute intervals of dialysate collection. Moreover, we found that levels in the cNp were generally lower than levels in the HP. Surprisingly, whereas plasma CORT levels increased in response to a standard stressor, no stress-induced surge was detected in the HP or cNp. In addition, although a diel CORT rhythm was observed in plasma, the rhythm in brain was attenuated and only observed when levels were integrated over a 4-hour time period. Regional differences in brain CORT levels were reflected in local mRNA expression levels of the CORT-inactivating enzyme 11β-hydroxysteroid dehydrogenase type 2 with levels elevated in the cNp relative to the HP. Region-specific CORT metabolism may therefore play a role in buffering the brain from CORT fluctuations.

Are roads and traffic sources of physiological stress for the Florida scrub-jay?

Context Anthropogenic disturbances induce physiological and behavioural responses in numerous species. The negative effects of human disturbance are of special concern to threatened and endangered species. Aims The present study aims to compare physiological stress measures and reproductive success of Florida scrub-jays (Aphelocoma coerulescens) living near roads with jays that live away from roads. Specifically, it aims to test whether roads are stressful. Methods We assessed physiological measures that can serve as indicators of ‘stress’ to determine whether a highway that bisects our study site had physiological effects on adult male Florida scrub-jays. We captured male breeders from three territory types, including scrub habitat that (1) bordered a highway with a grassy shoulder that created an ‘edge’ habitat (roadside), (2) bordered human-maintained habitat (a plowed firebreak of sand with adjacent pasture) that served as a control for the edge-effect of the road (pasture) and (3) contained only natural scrub habitat (interior). We measured baseline concentrations of the stress hormone corticosterone (CORT), body mass, and a suite of longitudinal body measures to generate a body condition index in males from each territory type over three breeding seasons. Key results Roadside jays had greater body mass than did interior and pasture jays, although there were no differences in overall baseline CORT concentrations or body condition among territory classes. There was no difference in clutch initiation date or size and nestling and independent-young survival. Conclusions Our results suggest that the road through our study site has physiological effects on Florida scrub-jays; however, there is mixed evidence as to whether it acts as a chronic stressor. Implications Our findings provide evidence that roads and road-associated disturbance has neutral or potentially beneficial physiological effects of roads on Florida scrub-jays. Knowledge of these effects of roads and disturbance on jays will hopefully provide additional opportunities to improve conservation of this species.

Dietary protein restriction impairs growth, immunity, and disease resistance in southern leopard frog tadpoles

The immune system is a necessary, but potentially costly, defense against infectious diseases. When nutrition is limited, immune activity may consume a significant amount of an organism's energy budget. Levels of dietary protein affect immune system function; high levels can enhance disease resistance. We exposed southern leopard frog [Lithobates sphenocephalus (=Rana sphenocephala)] tadpoles to high and low protein diets crossed with the presence or absence of the pathogenic amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd) and quantified: (1) tadpole resistance to Bd; (2) tadpole skin-swelling in response to phytohaemagglutinin (PHA) injection (a measure of the T cell-mediated response of the immune system); (3) bacterial killing ability (BKA) of tadpole blood (a measure of the complement-mediated cytotoxicity of the innate immune system); and (4) tadpole growth and development. Tadpoles raised on a low-protein diet were smaller and less developed than tadpoles on a high-protein diet. When controlled for developmental stage, tadpoles raised on a low-protein diet had reduced PHA and BKA responses relative to tadpoles on a high-protein diet, but these immune responses were independent of Bd exposure. High dietary protein significantly increased resistance to Bd. Our results support the general hypothesis that host condition can strongly affect disease resistance; in particular, fluctuations in dietary protein availability may change how diseases affect populations in the field.

Age-related differences in baseline and stress-induced corticosterone in Florida scrub-jays

In physiological studies of free-living species, it is essential to consider the context of the life history stage at which an individual was observed in order to link measures of physiology with ecological parameters. One such measure that is important to consider is the age of an individual. We tested whether baseline or stress-induced corticosterone levels vary with age in free-living Florida scrub-jays (Aphelocoma coerulescens) during the pre-breeding period. Corticosterone (CORT), the primary avian stress hormone, is released in response to stressful stimuli, and stimulates gluconeogenesis; however, it also serves as a chemical messenger that can influence other physiological processes, reproduction, and behavior. We monitored both baseline CORT levels longitudinally throughout a five-year period and stress-induced CORT responses over a shorter two-year period. We predicted that older jays would have lower baseline CORT levels and a dampened stress response compared to younger birds, as has been shown in other avian species. We found no significant differences in baseline CORT levels with age. We found a decrease in total corticosterone responses to a stressor with age, however, the oldest birds in the population showed greater total corticosterone responses to a stressor. These results may be a product of age-related changes in physiological processes related to the stress response or a result of selection acting on the population, resulting in only the most responsive individuals surviving to old age.

Short- and long-term effects of developmental corticosterone exposure on avian physiology, behavioral phenotype, cognition, and fitness: A review

A growing body of evidence from across taxa suggests that exposure to elevated levels of glucocorticoids during early development can have long-term effects upon physiological and behavioral phenotypes. Additionally, there is some, though limited, evidence that similar early exposure can also negatively impact cognitive ability. Following pioneering mammalian studies, several avian studies have revealed that the responsiveness of the hypothalamo-pituitary-adrenal (HPA) axis as an adult can be explained by levels of corticosterone, the avian glucocorticoid, the individual experienced as a nestling or even as an embryo via yolk exposure. Studies also suggest that perinatal exposure to corticosterone can have effects upon avian ‘personalities’ or coping styles, and findings from mammalian studies suggest that these long-term effects are mediated epigenetically via altered expression of relevant DNA sequences. Although a consistent pattern across-species has yet to emerge, recent work in Florida scrub-jays Aphelocoma coerulescens found that baseline corticosterone levels in 11-day-old nestlings explained 84% of the variation in ‘personality’ (bold vs. timid) when those individuals were tested approximately seven months later. Nestlings with elevated corticosterone levels were more timid than those individuals that as nestlings experienced relatively low corticosterone levels. Some researchers have suggested that parents might use such mechanisms to ‘program’ their offsprings’ phenotype to best fit prevailing environmental conditions. This review will visit what is known about the links between stressful developmental conditions that result in exposure to elevated corticosterone and the short- and long-term effects of this steroid hormone upon central nervous system function and whether alterations thereof are beneficial, deleterious, or neutral. It will concentrate on examples from birds, although critical supporting studies from the mammalian literature will be included as appropriate.

Corticosterone, brood size, and hatch order in free-living Florida scrub-jay (Aphelocoma coerulescens) nestlings

It is well known that variation in developmental conditions can have profound effects upon lifetime fitness. In altricial avian species, nestlings undergo a substantial portion of development in the nest after hatching, often in the presence of nest-mates. This can result in the formation of brood hierarchies based on age, size, and competitive ability. Measurement of baseline corticosterone (CORT) levels in developing birds may provide a means to assess whether individuals within a brood experience stress due to sibling competition or nutritional state. However, few studies have attempted to correlate corticosterone concentrations with brood hierarchies in free-living, developing birds. We investigated the degree to which corticosterone levels in nestling Florida scrub-jays (Aphelocoma coerulescens) were predicted by brood size, hatching order, and body mass. Nestling corticosterone levels were inversely correlated with body mass but did not differ among brood sizes. Within broods of two, second-hatched nestlings had lower CORT than first-hatched nestlings, although there was no effect of hatch order in broods of three or four nestlings. The results of this study suggest that conditions within the nest, particularly those associated with within-brood hierarchies, are manifested through differential body condition and nestling corticosterone secretion. The consequences of this variation in nestling corticosterone are unknown, but development of the adult phenotype, as well as life-long survival, may be impacted.

Repeatability of baseline and stress-induced corticosterone levels across early life stages in the Florida scrub-jay (Aphelocoma coerulescens)

Recent studies have posited that the pattern of glucocorticoid secretion within an individual represents a stable, fixed physiological trait. To test this hypothesis, we assessed the repeatability of baseline and stress-induced corticosterone (CORT) secretion across developmental stages and years in Florida scrub-jays. We sampled individuals from multiple cohorts repeatedly from the age of 11 days post-hatch up to 4 years of age. We found a significant degree of repeatability within individuals in stress-induced corticosterone levels, i.e., the amount of hormone secreted during a standardized stress protocol (corrected integrated corticosterone). However, baseline corticosterone levels were not statistically repeatable, although there was some indication that nestling levels predicted levels at 1 year of age. The results of this study indicate that stress-induced CORT levels are consistent within individual scrub-jays, and the degree to which a young jay mounts an acute stress response appears to be somewhat "set" by the age of nutritional independence. Thus stress-induced corticosterone secretion appears to be a stable, repeatable trait within individuals and as such may be subject to natural selection.

The influence of nest attendance and provisioning on nestling stress physiology in the Florida scrub-jay

Stressful conditions during development may have lasting consequences for an animal's lifetime fitness. We investigated the degree to which parental provisioning and nest attendance influenced baseline levels of the stress hormone corticosterone in nestling Florida scrub-jays. Provisioning rates of male and female breeders and nest attendance of female breeders were recorded during focal watches conducted between days 3 and 5 post-hatch. A small blood sample was taken from each nestling on day 11 post-hatch and used to quantify levels of baseline corticosterone. The proportion of time spent by female breeders at a considerable distance from the nest was positively related to nestling corticosterone levels. Nestling corticosterone was also negatively related to parental provisioning rate, although this effect seemed to be secondary to the effect of the female's time away from the nest. These results indicate that parental behavior contributes to nestling stress physiology, which may in turn direct the formation of the adult phenotype and influence an individual's chances of survival.

Development of the adrenal stress response in the Florida scrub-jay (Aphelocoma coerulescens)

Nestlings of altricial species undergo a period of substantial growth and development in the nest after hatching. The hypothalamic-pituitary-adrenal axis regulates the release of stress hormones such as corticosterone, which in adults is critical in allowing an animal to respond to a stressor. However, activation of this axis in young birds may be detrimental to growth and possibly survival. The developmental hypothesis predicts that altricial nestlings should display a dampened corticosterone response to stress as a means of protection against the potentially harmful effects of elevated corticosterone. We examined this hypothesis in Florida scrub-jays, a cooperatively breeding species with altricial young. Blood samples were collected from nestlings, nutritionally independent young, and yearlings for measurement of corticosterone levels. Baseline corticosterone levels did not differ between age-classes; however, stress-induced corticosterone levels were highest in yearlings, intermediate in independent young, and lowest in nestlings. The nestling stress response was also of a shorter duration than the response in independent young and yearlings. This variation in stress responsiveness across ages may be an adaptive mechanism to protect the developing bird from the negative effects of corticosterone on growth and cognitive development.

Environment, glucocorticoids, and the timing of reproduction

Glucocorticoids mediate glucose availability under stressful and non-stressful conditions and, therefore, are essential for life. However, data across taxa demonstrate that chronic or elevated secretion of corticosterone or cortisol (CORT) can have negative effects at many levels and can trigger physiological or behavioral responses that may delay or, even halt reproduction. We present a brief overview of the effects that glucocorticoids, primarily the avian form, corticosterone, can have on the reproductive axis. Considerable data have demonstrated that environmental perturbations can result in elevated CORT levels that alter a bird's investment in current reproduction. Studies in our laboratory have shown a link between CORT and timing of reproduction in Florida scrub-jays: in "bad" years, clutch initiation dates are positively correlated with baseline CORT levels of female breeders. Also, population-level differences in CORT levels may explain timing of reproduction as lower CORT levels in suburban-dwelling jays are coupled with early breeding. Most research on stress and CORT concentrates on transient effects of CORT secretion. However, developmental CORT exposure, either from the yolk or embryo, may have long-term effects upon adult phenotype. For example, CORT levels in nestling scrub-jays predicts later 'personality,' as levels were highly correlated (r(2)=0.84) with fearfulness at 7 months of age. One can imagine that such 'personality' traits might also translate into differential success in gaining a territory or a mate. While speculative, it may be that early CORT exposure effectively programs adult behaviors that have wide ranging effects, including upon reproduction.

Corticosterone administration does not affect timing of breeding in Florida scrub-jays (Aphelocoma coerulescens)

Providing supplemental food to Florida scrub-jays (Aphelocoma coerulescens) causes a reliable advance in clutch initiation of 1 to 2 weeks. In some years, supplemental food appeared to not only advance laying date but also decrease baseline concentrations of corticosterone (CORT) relative to controls. The coincidence of low CORT levels and early breeding led us to hypothesize that CORT serves to communicate information about environmental conditions to the hypothalamic-pituitary-gonadal (HPG) axis, which ultimately influences the timing of breeding. To test this hypothesis, we administered small oral doses of CORT three times each day to female breeders that were provisioned with supplemental food. We compared clutch initiation dates of the CORT-dosed females to females with supplementation but no exogenous CORT and to females with neither CORT nor supplemental food. CORT administration had a strong temporary effect on circulating CORT concentrations but clutch initiation did not differ between the two groups of supplemented birds, both of which laid eggs approximately 10 days earlier than nonsupplemented birds. Furthermore, during the year of our study we found no reduction in baseline CORT concentrations in our undosed supplemental groups, as had been observed in past studies.