Mathematical modeling reveals how the speed of endocrine regulation should affect baseline and stress-induced glucocorticoid levels

Lack of glucocorticoid flexibility is indicative of wear-and-tear in Hyla versicolor tadpoles from agricultural environments

In habitats where stressors are frequent or persistent, it can become increasingly difficult for wildlife to appropriately match their endocrine responses to these more challenging environments. The dynamic regulation of glucocorticoid (GC) hormones plays a crucial role in determining how well individuals cope with environmental changes. Amphibians exposed to agricultural stressors can dampen aspects of their GC profile (baseline, agitation, recovery, stress responsiveness, and negative feedback) to cope in these stressful environments, but this dampening can lead to reductions in an individual's reactive scope and a loss of endocrine flexibility. Organic agriculture could potentially limit some of these effects, however, little is known about how amphibians respond physiologically to organic agricultural environments. We compared GC profiles of Hyla versicolor tadpoles from three treatments: natural ponds (<5% agriculture within 500m), ponds near organic farms, and ponds near conventional farms. We hypothesized that tadpoles would cope with agricultural habitats by dampening stress responsiveness and exhibiting more efficient negative feedback and that the magnitude of these changes in response would differ based on agricultural method. We found that tadpoles from conventional and organic ponds were less likely to downregulate GCs via negative feedback after stressor exposure than tadpoles from natural ponds. For agricultural tadpoles that did downregulate GCs after the stressor, we found lower stress responsiveness and faster downregulation to baseline corticosterone than tadpoles from natural ponds. These results point to an accumulation of wear-and-tear, leading to an overall reduction in reactive scope and limited GC flexibility in our agricultural tadpoles. Regardless of agricultural method used, agricultural tadpoles exhibited the same patterns of GC response, indicating that current efforts to incentivize farmers to switch to organic farming methods may not be sufficient to address negative agricultural impacts on amphibians.

Endocrine flexibility can facilitate or constrain the ability to cope with global change

Global climate change has increased average environmental temperatures world-wide, simultaneously intensifying temperature variability and extremes. Growing numbers of studies have documented phenological, behavioural and morphological responses to climate change in wild populations. As systemic signals, hormones can contribute to orchestrating many of these phenotypic changes. Yet little is known about whether mechanisms like hormonal flexibility (reversible changes in hormone concentrations) facilitate or limit the ability of individuals, populations and species to cope with a changing climate. In this perspective, we discuss different mechanisms by which hormonal flexibility, primarily in glucocorticoids, could promote versus hinder evolutionary adaptation to changing temperature regimes. We focus on temperature because it is a key gradient influenced by climate change, it is easy to quantify, and its links to hormones are well established. We argue that reaction norm studies that connect individual responses to population-level and species-wide patterns will be critical for making progress in this field. We also develop a case study on urban heat islands, where several key questions regarding hormonal flexibility and adaptation to climate change can be addressed. Understanding the mechanisms that allow animals to cope when conditions become more challenging will help in predicting which populations are vulnerable to ongoing climate change. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

A mathematical representation of the reactive scope model

Researchers have long sought to understand and predict an animal's response to stressful stimuli. Since the introduction of the concept of homeostasis, a variety of model frameworks have been proposed to describe what is necessary for an animal to remain within this stable physiological state and the ramifications of leaving it. Romero et al. (Horm Behav 55(3):375-389, 2009) introduced the reactive scope model to provide a novel conceptual framework for the stress response that assumes an animal's ability to tolerate a stressful stimulus may degrade over time in response to the stimulus. We provide a mathematical formulation for the reactive scope model using a system of ordinary differential equations and show that this model is capable of recreating existing experimental data. We also provide an experimental method that may be used to verify the model as well as several potential additions to the model. If future experimentation provides the necessary data to estimate the model's parameters, the model presented here may be used to make quantitative predictions about physiological mediator levels during a stress response and predict the onset of homeostatic overload.

The response to stressors in adulthood depends on the interaction between prenatal exposure to glucocorticoids and environmental context

Maternal stress during reproduction can influence how offspring respond to stress later in life. Greater lifetime exposure to glucocorticoid hormones released during stress is linked to greater risks of behavioral disorders, disease susceptibility, and mortality. The immense variation in individual's stress responses is explained, in part, by prenatal glucocorticoid exposure. To explore the long-term effects of embryonic glucocorticoid exposure, we injected Japanese quail (Coturnix japonica) eggs with corticosterone. We characterized the endocrine stress response in offspring and measured experienced aggression at three different ages. We found that prenatal glucocorticoid exposure affected (1) the speed at which the stress response was terminated suggesting dysregulated negative feedback, (2) baseline corticosterone levels in a manner dependent on current environmental conditions with higher levels of experienced aggression associated with higher levels of baseline corticosterone, (3) the magnitude of an acute stress response based on baseline concentrations. We finish by proposing a framework that can be used to test these findings in future work. Overall, our findings suggest that the potential adaptive nature of prenatal glucocorticoid exposure is likely dependent on environmental context and may also be tempered by the negative effects of longer exposure to glucocorticoids each time an animal faces a stressor.

Simulating physiological flexibility in the acute glucocorticoid response to stressors reveals limitations of current empirical approaches

Wild animals often experience unpredictable challenges that demand rapid and flexible responses. The glucocorticoid mediated stress response is one of the major systems that allows vertebrates to rapidly adjust their physiology and behavior. Given its role in responding to challenges, evolutionary physiologists have focused on the consequences of between-individual and, more recently, within-individual variation in the acute glucocorticoid response. However, empirical studies of physiological flexibility are severely limited by the logistical challenges of measuring the same animal multiple times. Data simulation is a powerful approach when empirical data are limited, but has not been adopted to date in studies of physiological flexibility. In this article, I develop a simulation that can generate realistic acute glucocorticoid response data with user specified characteristics. Simulated animals can be sampled continuously through an acute response and across as many separate responses as desired, while varying key parameters. Using the simulation, I develop several scenarios that address key questions in physiological flexibility. These scenarios demonstrate the conditions under which a single glucocorticoid trait can be accurately assessed with typical experimental designs, the consequences of covariation between different components of the acute stress response, and the way that context specific differences in variability of acute responses can influence the power to detect relationships between the strength of the acute stress response and fitness. I also describe how to use the simulation tools to aid in the design and evaluation of empirical studies of physiological flexibility.

The relative speed of the glucocorticoid stress response varies independently of scope and is predicted by environmental variability and longevity across birds

The acute glucocorticoid response is a key mediator of the coordinated vertebrate response to unpredictable challenges. Rapid glucocorticoid increases initiate changes that allow animals to cope with stressors. The scope of the glucocorticoid response - defined here as the absolute increase in glucocorticoids - is associated with individual differences in performance and varies across species with environment and life history. In addition to varying in scope, responses can differ enormously in speed; however, relatively little is known about whether speed and absolute glucocorticoid levels covary, how selection shapes speed, or what aspects of speed are important. We used corticosterone samples collected at 5 time points from 1750 individuals of 60 species of birds to ask i) how the speed and scope of the glucocorticoid response covary and ii) whether variation in absolute or relative speed is predicted by environmental context or life history. Among species, faster absolute glucocorticoid responses were strongly associated with a larger scope. Despite this covariation, the relative speed of the glucocorticoid response (standardized within species) varied independently of absolute scope, suggesting that selection could operate on both features independently. Species with faster relative glucocorticoid responses lived in locations with more variable temperature and had shorter lifespans. Our results suggest that rapid changes associated with the speed of the glucocorticoid response, such as those occurring through non-genomic receptors, might be an important determinant of coping ability and we emphasize the need for studies designed to measure speed independently of absolute glucocorticoid levels.

Optimal hormonal regulation when stressor cues are imperfect

The endocrine system uses information about the environment and the individual's state to regulate circulating concentrations of hormones, and then those hormones, through receptor binding, cause changes in the phenotype. How quickly individuals can up- and down-regulate their hormones can affect baseline and elevated hormone levels and presumably affects how successfully individuals can cope with a varying environment. To respond to environmental change, individuals first need to perceive and process cues about the state of the environment. Individuals may receive imperfect cues about the environment due to perceptual errors, variation in cues, or inexperience with novel stressors. In this paper we use a mathematical model to ask how these imperfect cues should affect how individuals regulate their glucocorticoid concentrations. We find imperfect cues can lead to changes in hormone regulation with individuals generally having higher baseline and lower elevated hormone levels as environmental cues become less reliable. Informational constraints and physiological constraints appear to have generally additive effects, with informational constraints having less of an impact as physiological constraints increase. Our results highlight the different means by which imperfect information can affect hormone regulation. We find that mistakes caused by imperfect cues are commonly responsible for changes in average hormone levels, but imperfect cues also cause individuals to be slower and less certain in their updated estimates of the environmental state, which affects hormone regulation. We also demonstrate the separate effects of false positive and false negative cues and how these are shaped by the relative fitness consequences of baseline and stress-induced hormone levels. Our model shows how given our assumptions imperfect stressor cues should affect endocrine flexibility and regulation, and we hope provides a piece for future conversations and models of endocrine regulation.

Misaligned hormonal rhythmicity: Mechanisms of origin and their clinical significance

Rhythmic hormonal secretion is key for sustaining health. While a central pacemaker in the hypothalamus is the main driver of circadian periodicity, many hormones oscillate with different frequencies and amplitudes. These rhythms carry information about healthy physiological functions, while at the same time they must be able to respond to external cues and maintain their robustness against severe perturbations. Since endocrine disruptions can lead to hormonal misalignment and disease, understanding the clinical significance of these rhythms can help support diagnosis and disease management. While the misalignment of dynamic hormone profiles can be quantitatively analysed though statistical and computational techniques, mathematical modelling can provide fundamental understanding about the mechanisms underpinning endocrine rhythms, particularly around the question of what makes them robust to some perturbations but fragile to others. In this study, I will review the key challenges of understanding hormonal rhythm misalignment from a mathematical perspective, including their causes and clinical significance. By reviewing modelling examples of coupled endocrine axes, I will address the question of how perturbations in one endocrine axis propagate to another, leading to the more complex issue of disentangling the contribution of each endocrine system to a robust dynamic environment.

An evolutionary perspective on stress responses, damage and repair

Variation in stress responses has been investigated in relation to environmental factors, species ecology, life history and fitness. Moreover, mechanistic studies have unravelled molecular mechanisms of how acute and chronic stress responses cause physiological impacts ('damage'), and how this damage can be repaired. However, it is not yet understood how the fitness effects of damage and repair influence stress response evolution. Here we study the evolution of hormone levels as a function of stressor occurrence, damage and the efficiency of repair. We hypothesise that the evolution of stress responses depends on the fitness consequences of damage and the ability to repair that damage. To obtain some general insights, we model a simplified scenario in which an organism repeatedly encounters a stressor with a certain frequency and predictability (temporal autocorrelation). The organism can defend itself by mounting a stress response (elevated hormone level), but this causes damage that takes time to repair. We identify optimal strategies in this scenario and then investigate how those strategies respond to acute and chronic exposures to the stressor. We find that for higher repair rates, baseline and peak hormone levels are higher. This typically means that the organism experiences higher levels of damage, which it can afford because that damage is repaired more quickly, but for very high repair rates the damage does not build up. With increasing predictability of the stressor, stress responses are sustained for longer, because the animal expects the stressor to persist, and thus damage builds up. This can result in very high (and potentially fatal) levels of damage when organisms are exposed to chronic stressors to which they are not evolutionarily adapted. Overall, our results highlight that at least three factors need to be considered jointly to advance our understanding of how stress physiology has evolved: (i) temporal dynamics of stressor occurrence; (ii) relative mortality risk imposed by the stressor itself versus damage caused by the stress response; and (iii) the efficiency of repair mechanisms.

Integrating theoretical and empirical approaches for a robust understanding of endocrine flexibility

There is growing interest in studying hormones beyond single 'snapshot' measurements, as recognition that individual variation in the endocrine response to environmental change may underlie many rapid, coordinated phenotypic changes. Repeated measures of hormone levels in individuals provide additional insight into individual variation in endocrine flexibility - that is, how individuals modulate hormone levels in response to the environment. The ability to quickly and appropriately modify phenotype is predicted to be favored by selection, especially in unpredictable environments. The need for repeated samples from individuals can make empirical studies of endocrine flexibility logistically challenging, but methods based in mathematical modeling can provide insights that circumvent these challenges. Our Review introduces and defines endocrine flexibility, reviews existing studies, makes suggestions for future empirical work, and recommends mathematical modeling approaches to complement empirical work and significantly advance our understanding. Mathematical modeling is not yet widely employed in endocrinology, but can be used to identify innovative areas for future research and generate novel predictions for empirical testing.

Perceived Stress Associated Factors in Workers at a Public University

ABSTRACT Objective: To describe the predictors of perceived stress in a broad sample of workers at a Brazilian public university. Method: Cross-sectional study carried out with a convenience sample of workers at a public university in Brazil. To be included in the present study, the worker had to be an administrative technician. From March to August, 2017, workers were surveyed, and 929 participants answered the questionnaires of sociodemographic characterization, work and health conditions, perceived stress (Perceived Stress Scale), depression (Beck Depression Inventory), and anxiety (Beck Anxiety Inventory). Results: The multiple linear regression showed that higher perceived stress was associated with being younger and male, occupying a higher or technical position, and presenting higher levels of depression and anxiety. Conclusion: These findings have implications for occupational health nurses and other health professionals to identify workers at risk for chronic and mental illness through predictors of perceived stress and to guide institutions in planning practical actions for stress management interventions.