Whole-body Corticosterone Research Articles

The role of phenotypic plasticity and corticosterone in coping with pond drying conditions in yellow-bellied toad (Bombina variegata, Linnaeus 1758) tadpoles.

Amphibian larvae inhabiting temporary ponds often exhibit the capacity to accelerate development and undergo metamorphosis in challenging conditions like desiccation. However, not all species exhibit this ability, the yellow-bellied toad (Bombina variegata) is one such example. The underlying mechanisms behind the inability to accelerate development under desiccation remain largely unexplored. The hypothalamic-pituitary-interrenal (HPI) axis and corticosterone (CORT), which act synergistically with thyroid hormone, are thought to facilitate metamorphosis in response to desiccation stress. In this study, we aimed to investigate whether modification in the HPI axis, particularly CORT levels, contributes to the absence of adaptive plasticity in B. variegata under desiccation stress. The study design included four treatments: high water level, high water level with exogenous CORT, low water level, and low water level with metyrapone (a CORT synthesis inhibitor). The main objective was to evaluate the effects of these treatments on whole-body corticosterone levels, life history, morphological traits, and oxidative stress parameters during the prometamorphic and metamorphic climax developmental stages. While low water level had no effect on total corticosterone levels, larval period, body condition index, and metamorphic body shape, it negatively affected metamorph size, mass, and growth rate. Our findings suggest that constant exposure to desiccation stress over generations may have led to modifications in the HPI axis activity in B. variegata, resulting in adaptation to changes in water level, evident through the absence of stress response. Consequently, CORT may not be a relevant stress indicator in desiccation conditions for this species.

Do antipredator responses of Euphlyctis cyanophlyctis tadpoles depend on the intensity of predation risk?

Successful coexistence with predators depends on the ability of prey to strike a balance between the costs of predator avoidance and the benefits of foraging and reproducing as there is a trade-off between these activities. Prey animals can balance this trade-off by responding to predation risk in a manner that matches the intensity of risk posed by the predator, which is known as the threat-sensitive predator avoidance hypothesis. To test this hypothesis, we exposed larval Indian skipper frogs (Euphlyctis cyanophlyctis) to different intensities of predation risk by increasing the number of predators and the amount of biomass consumed and recorded their behaviour, morphology, and metamorphic traits. We also quantified the whole-body corticosterone to assess the physiological basis of alterations in morphology, behaviour, and life-history traits. The results indicate that behavioural responses of larval skipper frogs increased with the intensity of risk conforming to the predictions of the threat-sensitivity hypothesis. However, the morphological responses did not vary with the intensity of risk and hence did not support the predictions of the hypothesis. Interestingly, tadpoles facing a higher level of predation risk metamorphosed at the largest size, but with increased larval duration. At the physiological level, corticosterone levels increased with increases in the intensity of risk. Thus, our results suggest that certain antipredator responses may vary with the intensity of risk while others may not. Further, the results of our study also show an association between physiological and behavioural responses of larval E. cyanophlyctis to predation risk. More importantly, we show that larval skipper frogs can assess the intensity of predation risk through threat-sensitive learning by associating conspecific alarm cues with odours of dragonfly nymphs.

Exposure to the Herbicide Atrazine Nonlinearly Affects Tadpole Corticosterone Levels

Abstract To determine whether the herbicide, atrazine, affects the stress hormone corticosterone, we exposed Osteopilus septentrionalis (Cuban Treefrog) tadpoles to four concentrations of atrazine and two controls (water and acetone) for three time durations (4, 28, and 100 h). Atrazine concentration, but not exposure duration, had significant nonlinear effects on whole-body corticosterone. Relative to controls, intermediate concentrations of atrazine (10.2 and 50.6 μg/L) tended to lower corticosterone, whereas the lowest (0.1 μg/L) and highest atrazine concentrations (102 μg/L) elevated corticosterone. These results indicate that atrazine exposure might dysregulate corticosterone, a hormone integral to vertebrate immunity, neurogenesis, and health.

Stress and chytridiomycosis: Exogenous exposure to corticosterone does not alter amphibian susceptibility to a fungal pathogen

Recent emergence and spread of the amphibian fungal pathogen, Batrachochytrium dendrobatidis (Bd) has been attributed to a number of factors, including environmental stressors that increase host susceptibility to Bd. Physiological stress can increase circulating levels of the hormone, corticosterone, which can alter a host's physiology and affect its susceptibility to pathogens. We experimentally elevated whole-body levels of corticosterone in both larval and post-metamorphic amphibians, and subsequently tested their susceptibility to Bd. Larvae of three species were tested (Anaxyrus boreas, Rana cascadae, and Lithobates catesbeianus) and one species was tested after metamorphosis (R. cascadae). After exposure to Bd, we measured whole-body corticosterone, infection, mortality, growth, and development. We found that exposure to exogenous corticosterone had no effect on Bd infection in any species or at either life stage. Species varied in whole-body corticosterone levels and exposure to corticosterone reduced mass in A. boreas and R. cascadae larvae. Exposure to Bd did not affect mortality, but had a number of sublethal effects. Across species, larvae exposed to Bd had higher corticosterone levels than unexposed larvae, but the opposite pattern was found in post-metamorphic R. cascadae. Bd exposure also increased larval length in all species and increased mass in R. cascadae larvae. Our results indicate that caution is warranted in assuming a strong link between elevated levels of corticosterone and disease susceptibility in amphibians. The role of physiological stress in altering Bd prevalence in amphibian populations is likely much more complicated than can be explained by examining a single "stress" endpoint.

Stress hormones mediate predator-induced phenotypic plasticity in amphibian tadpoles

Amphibian tadpoles display extensive anti-predator phenotypic plasticity, reducing locomotory activity and, with chronic predator exposure, developing relatively smaller trunks and larger tails. In many vertebrates, predator exposure alters activity of the neuroendocrine stress axis. We investigated predator-induced effects on stress hormone production and the mechanistic link to anti-predator defences in Rana sylvatica tadpoles. Whole-body corticosterone (CORT) content was positively correlated with predator biomass in natural ponds. Exposure to caged predators in mesocosms caused a reduction in CORT by 4 hours, but increased CORT after 4 days. Tadpoles chronically exposed to exogenous CORT developed larger tails relative to their trunks, matching morphological changes induced by predator chemical cue; this predator effect was blocked by the corticosteroid biosynthesis inhibitor metyrapone. Tadpole tail explants treated in vitro with CORT increased tissue weight, suggesting that CORT acts directly on the tail. Short-term treatment of tadpoles with CORT increased predation mortality, likely due to increased locomotory activity. However, long-term CORT treatment enhanced survivorship, likely due to induced morphology. Our findings support the hypothesis that tadpole physiological and behavioural/morphological responses to predation are causally interrelated. Tadpoles initially suppress CORT and behaviour to avoid capture, but increase CORT with longer exposure, inducing adaptive phenotypic changes.

Roles of stress hormones in food intake regulation in anuran amphibians throughout the life cycle

Towards understanding the ontogeny of energy balance regulation in vertebrates we analyzed the responses of corticotropin-releasing factor (CRF) and corticosterone to food deprivation in the Western spadefoot toad ( Spea hammondii) at three developmental stages: premetamorphic tadpole, prometamorphic tadpole, and juvenile. Corticosterone responses to 5 days of food deprivation varied among developmental stages. Both pre- and prometamorphic tadpoles increased whole-body corticosterone content with food deprivation, but the magnitude of the response of premetamorphic tadpoles was significantly greater. By contrast, juvenile toads decreased plasma corticosterone concentration. Similarly, brain CRF peptide content tended to increase in food-deprived tadpoles but did not change in food-deprived juveniles. Therefore, there is an ontogenetic difference in the way the hypothalamic–pituitary–interrenal (HPI) axis responds to food deprivation in amphibians. In tadpoles, the HPI axis is activated in response to fasting as is seen in birds and mammals, and may be associated with mobilization of stored fuels and increased foraging. Juvenile toads do not respond to food deprivation by activating the HPI axis, but instead pursue a strategy of energy conservation that involves a reduction in plasma corticosterone concentration.

Developmental Changes in Interrenal Responsiveness in Anuran Amphibians

Basal activity of the hypothalamo-pituitary-interrenal (HPI) axis changes over development in larval amphibians, but development of the responsiveness of this axis to an external stressor has not been studied. We compared developmental changes in whole-body corticosterone content of two anuran amphibian species, Rana pipiens (family Ranidae) and Xenopus laevis (family Pipidae). We also examined developmental changes in the responsiveness of the HPI axis by subjecting tadpoles of different developmental stages to a laboratory shaking/confinement stress and to ACTH injection. We measured whole-body corticosterone content as an indicator of the activity of the HPI axis. Whole-body corticosterone content of R. pipiens remained low during premetamorphosis and prometamorphosis but increased dramatically at metamorphic climax and remained elevated in juvenile frogs. By contrast, whole-body corticosterone content of X. laevis was highest during premetamorphosis, declined at the onset of prometamorphosis, increased at metamorphic climax and remained at climax levels in juvenile frogs. Premetamorphic and prometamorphic tadpoles of both species showed strong corticosterone responses to both shaking stress and ACTH injection. The magnitude and pattern of response differed among developmental stages, with premetamorphic tadpoles of both species showing greater responsiveness to stress and ACTH. Our results show that interrenal responsiveness is developed in premetamorphic tadpoles, suggesting that at these stages tadpoles are capable of mounting an increase in stress hormone production in response to changes in the external environment. Our results also highlight the importance of comparative studies in understanding the development of the stress axis.

Small changes in whole-body corticosterone content affect larval Rana pipiens fitness components

In amphibians, large changes in tissue corticosterone content (caused by treatment with large doses of hormone) alter tadpole growth and development, but the effects of smaller changes on growth, development, behavior, and morphology are unknown. In the current study, we exposed pre-metamorphic Rana pipiens tadpoles to moderate doses (62 and 125 nM) of exogenous corticosterone by adding it to the rearing water. We then analyzed effects on growth, development, behavior, morphology, and the endogenous corticosterone response to exogenous adrenocorticotropic releasing hormone (ACTH). A 50% elevation in whole-body corticosterone content was associated with slowed growth and development, increased tail muscle depth, and a diminished corticosterone response to ACTH. Behavior was unaffected by corticosterone administration. Treatment with the corticoid synthesis inhibitor metyrapone (MTP) reduced whole-body corticosterone content by 50% and was associated with increased size at metamorphosis but no change in time to metamorphosis. Our findings support the hypothesis that corticoids can mediate growth, developmental, and morphological responses of tadpoles to changing environmental conditions. Our results also demonstrate that even small changes in corticosterone content can have important implications for amphibian fitness.

Role for corticoids in mediating the response of Rana pipiens tadpoles to intraspecific competition.

Competition is known to decrease growth and development rate in tadpoles, but the physiological basis for this phenomenon is poorly understood. We hypothesized that competition results in increased production of stress hormones and that these hormones are responsible for the suppression of growth and development. To test this hypothesis, we measured whole-body corticosterone content in premetamorphic Leopard frog (Rana pipiens) tadpoles raised at two different population densities and three different food levels. Whole body corticosterone content was elevated in tadpoles subjected to either limited food (at low density) or high density. Within the low and intermediate food treatments, high density reduced tadpole growth and slowed development. Limited food slowed growth and development at all densities. Blocking corticoid synthesis by treating tadpoles with metyrapone (MTP) reversed the growth suppression caused by high density (tested in the intermediate food level treatment) but did not alter the effect of density on development rate. MTP treatment did not alter the depressive effect of limited resources on growth or development. Our results suggest that elevated corticoid biosynthesis mediates the negative effect of increased population density (i.e., increased intraspecific competition) on tadpole growth.

Sublethal effects of chronic exposure to an organochlorine compound on northern leopard frog (Rana pipiens) tadpoles

Global contamination with organochlorine compounds (OCs) has posed developmental and reproductive problems in wildlife worldwide. However, little is known about the impact of OCs or other pollutants on amphibians, despite mounting concerns about amphibian population declines and developmental deformities in the wild. Wildlife populations may be affected critically by sublethal impacts of anthropogenic disturbances, yet little research has focused on such effects in amphibians. In the current study, northern leopard frog (Rana pipiens) tadpoles were chronically exposed to a polychlorinated biphenyl (PCB) congener, 77-TCB, and effects on behavior, morphology, competitive performance, and corticosterone content were determined. R. pipiens activity levels and feeding rates were decreased by 77-TCB exposure, but morphology of mouthparts and body proportions were unaffected. 77-TCB enhanced growth and altered competitive interactions between R. pipiens and wood frog (Rana sylvatica) tadpoles. R. pipiens tadpoles exposed to 77-TCB showed decreased whole-body corticosterone content compared to controls both before and after injection with adrenocorticotropic hormone (ACTH). All of the factors examined in the current study play critical roles in tadpole development, growth, survivorship, and eventual reproductive success, suggesting negative population-level consequences for amphibians in PCB-contaminated habitats.