Effects of aging on hypothalamic-pituitary-adrenal (HPA) axis activity and reactivity in virgin male and female California mice (Peromyscus californicus)

Light: An Extrinsic Factor Influencing Animal-based Research

Light is an environmental factor that is extrinsic to animals themselves and that exerts a profound influence on the regulation of circadian, neurohormonal, metabolic, and neurobehavioral systems of all animals, including research animals. These widespread biologic effects of light are mediated by distinct photoreceptors-rods and cones that comprise the conventional visual system and melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) of the nonvisual system that interact with the rods and cones. The rods and cones of the visual system, along with the ipRGCs of the nonvisual system, are species distinct in terms of opsins and opsin concentrations and interact with one another to provide vision and regulate circadian rhythms of neurohormonal and neurobehavioral responses to light. Here, we review a brief history of lighting technologies, the nature of light and circadian rhythms, our present understanding of mammalian photoreception, and current industry practices and standards. We also consider the implications of light for vivarium measurement, production, and technological application and provide simple recommendations on artificial lighting for use by regulatory authorities, lighting manufacturers, designers, engineers, researchers, and research animal care staff that ensure best practices for optimizing animal health and well-being and, ultimately, improving scientific outcomes.

Impact of Long-Term Dietary High Fat and Eicosapentaenoic Acid on Behavior and Hypothalamic-Pituitary-Adrenal Axis Activity in Amyloidogenic APPswe/PSEN1dE9 Mice

INTRODUCTION

Alzheimer's disease (AD) alters neurocognitive and emotional function and causes dysregulation of multiple homeostatic processes. The leading AD framework pins amyloid beta plaques and tau tangles as primary drivers of dysfunction. However, many additional variables, including diet, stress, sex, age, and pain tolerance, interact in ways that are not fully understood to impact the onset and progression of AD pathophysiology. We asked: (1) does high-fat diet, compared to low-fat diet, exacerbate AD pathophysiology and behavioral decline? And, (2) can supplementation with eicosapentaenoic (EPA)-enriched fish oil prevent high-fat-diet-induced changes?

METHODS

Male and female APPswePSdE9 mice, and their non-transgenic littermates, were randomly assigned to a diet condition (low-fat, high-fat, high-fat with EPA) and followed from 2 to 10 months of age. We assessed baseline corticosterone concentration during aging, pain tolerance, cognitive function, stress coping, and corticosterone response to a stressor.

RESULTS

Transgenic mice were consistently more active than non-transgenic mice but did not perform worse on either cognitive task, even though we recently reported that these same transgenic mice exhibited metabolic changes and had increased amyloid beta. Mice fed high-fat diet had higher baseline and post-stressor corticosterone, but diet did not impact cognition or pain tolerance. Sex had the biggest influence, as female mice were consistently more active and had higher corticosterone than males.

CONCLUSION

Overall, diet, genotype, and sex did not have consistent impacts on outcomes. We found little support for predicted interactions and correlations, suggesting diet impacts metabolic function and amyloid beta levels, but these outcomes do not translate to changes in behaviors measured here.

Vivarium Lighting as an Important Extrinsic Factor Influencing Animal-based Research

Light is an extrinsic factor that exerts widespread influence on the regulation of circadian, physiologic, hormonal, metabolic, and behavioral systems of all animals, including those used in research. These wide-ranging biologic effects of light are mediated by distinct photoreceptors, the melanopsin-containing intrinsically photosensitive retinal ganglion cells of the nonvisual system, which interact with the rods and cones of the conventional visual system. Here, we review the nature of light and circadian rhythms, current industry practices and standards, and our present understanding of the neurophysiology of the visual and nonvisual systems. We also consider the implications of this extrinsic factor for vivarium measurement, production, and technological application of light, and provide simple recommendations on artificial lighting for use by regulatory authorities, lighting manufacturers, designers, engineers, researchers, and research animal care staff that ensure best practices for optimizing animal health and wellbeing and, ultimately, improving scientific outcomes.

Hypothalamic-pituitary-adrenal (HPA) axis activity and anxiety-like behavior during aging: A test of the glucocorticoid cascade hypothesis in amyloidogenic APPswe/PS1dE9 mice

Alzheimer's disease (AD) is a progressive, dementing, whole-body disorder that presents with decline in cognitive, behavioral, and emotional functions, as well as endocrine dysregulation. The etiology of AD is not fully understood but stress- and anxiety-related hormones may play a role in its development and trajectory. The glucocorticoid cascade hypothesis posits that levels of glucocorticoids increase with age, leading to dysregulated negative feedback, further elevated glucocorticoids, and resulting neuropathology. We examined the impact of age (from 2 to 10 months) and stressor exposure (predator odor) on hormone levels (corticosterone and ghrelin), anxiety-like behavior (open field and light dark tests), and memory-related behavior (novel object recognition; NOR), and whether these various measures correlated with neuropathology (hippocampus and cortex amyloid beta, Aβ) in male and female APPswe/PS1dE9 transgenic and non-transgenic mice. Additionally, we performed exploratory analyses to probe if the open field and light dark test as commonly used tasks to assess anxiety levels were correlated. Consistent with the glucocorticoid cascade hypothesis, baseline corticosterone increased with age. Predator odor exposure elevated corticosterone at each age, but in contrast to the glucocorticoid cascade hypothesis, the magnitude of stressor-induced elevations in corticosterone levels did not increase with age. Overall, transgenic mice had higher post-stressor, but not baseline, corticosterone than non-transgenic mice, and across both genotypes, females consistently had higher (baseline and post-stressor) corticosterone than males. Behavior in the open field test primarily showed decreased locomotion with age, and this was pronounced in transgenic females. Anxiety-like behaviors in the light dark test were exacerbated following predator odor, and female transgenic mice were the most impacted. Compared to transgenic males, transgenic females had higher Aβ concentrations and showed more anxiety-like behavior. Performance on the NOR did not differ significantly between genotypes. Lastly, we did not find robust, statistically significant correlations among corticosterone, ghrelin, recognition memory, anxiety-like behaviors, or Aβ, suggesting outcomes are not strongly related on the individual level. Our data suggest that despite Aβ accumulation in the hippocampus and cortex, male and female APPswePS1dE9 transgenic mice do not differ robustly from their non-transgenic littermates in physiological, endocrine, and behavioral measures at the range of ages studied here.

An acute dose of intranasal oxytocin rapidly increases maternal communication and maintains maternal care in primiparous postpartum California mice

Maternal-offspring communication and care are essential for offspring survival. Oxytocin (OXT) is known for its role in initiation of maternal care, but whether OXT can rapidly influence maternal behavior or ultrasonic vocalizations (USVs; above 50 kHz) has not been examined. To test for rapid effects of OXT, California mouse mothers were administered an acute intranasal (IN) dose of OXT (0.8 IU/kg) or saline followed by a separation test with three phases: habituation with pups in a new testing chamber, separation via a wire mesh, and finally reunion with pups. We measured maternal care, maternal USVs, and pup USVs. In mothers, we primarily observed simple sweep USVs, a short downward sweeping call around 50 kHz, and in pups we only observed pup whines, a long call with multiple harmonics ranging from 20 kHz to 50 kHz. We found that IN OXT rapidly and selectively enhanced the normal increase in maternal simple sweep USVs when mothers had physical access to pups (habituation and reunion), but not when mothers were physically separated from pups. Frequency of mothers’ and pups’ USVs were correlated upon reunion, but IN OXT did not influence this correlation. Finally, mothers given IN OXT showed more efficient pup retrieval/carrying and greater total maternal care upon reunion. Behavioral changes were specific to maternal behaviors (e.g. retrievals) as mothers given IN OXT did not differ from controls in stress-related behaviors (e.g. freezing). Overall, these findings highlight the rapid effects and context-dependent effect a single treatment with IN OXT has on both maternal USV production and offspring care.

Stress hypothesis overload: 131 hypotheses exploring the role of stress in tradeoffs, transitions, and health

Stress is ubiquitous and thus, not surprisingly, many hypotheses and models have been created to better study the role stress plays in life. Stress spans fields and is found in the literature of biology, psychology, psychophysiology, sociology, economics, and medicine, just to name a few. Stress, and the hypothalamic-pituitaryadrenal/interrenal (HPA/I) axis and sympathetic nervous system (SNS), are involved in a multitude of behaviors and physiological processes, including life-history and ecological tradeoffs, developmental transitions, health, and survival. The goal of this review is to highlight and summarize the large number of available hypotheses and models, to aid in comparative and interdisciplinary thinking, and to increase reproducibility by a) discouraging hypothesizing after results are known (HARKing) and b) encouraging a priori hypothesis testing. For this review I collected 214 published hypotheses or models dealing broadly with stress. In the main paper, I summarized and categorized 131 of those hypotheses and models which made direct connections among stress and/or HPA/I and SNS, tradeoffs, transitions, and health. Of those 131, the majority made predictions about reproduction (n = 43), the transition from health to disease (n = 38), development (n = 23), and stress coping (n = 18). Additional hypotheses were classified as stage-spanning or models (n = 37). The additional 83 hypotheses found during searches were tangentially related, or pertained to immune function or oxidative stress, and these are listed separately. Many of the hypotheses share underlying rationale and suggest similar, if not identical, predictions, and are thus not mutually exclusive; some hypotheses spanned classification categories. Some of the hypotheses have been tested multiple times, whereas others have only been examined a few times. It is the hope that multi-disciplinary stress researchers will begin to harmonize their naming of hypotheses in the literature so as to build a clearer picture of how stress impacts various outcomes across fields. The paper concludes with some considerations and recommendations for robust testing of stress hypotheses.

Glucocorticoid negative feedback as a potential mediator of trade-offs between reproduction and survival

A large increase in glucocorticoid hormones can inhibit or completely shut down breeding in wild animals. Because of its critical role in reducing glucocorticoids after exposure to stressors, hypothalamic-pituitary-adrenal (HPA) negative feedback could be an important mediator of the ecological trade-off between investing limited resources into survival/self vs. reproduction/offspring. Although assessing negative feedback in a standardized way using injections of the synthetic glucocorticoid dexamethasone is a straightforward procedure, we show that several different approaches are used to report negative feedback in the literature, and then demonstrate that this can in turn affect the statistical results and conclusions of a study. We then review six specific predictions about adaptive within- and across-species patterns in glucocorticoids based on the relative costs and benefits of maintaining or abandoning breeding attempts when animals are faced with prolonged strong stressors, and examine evidence for these predictions in the context of HPA negative feedback. Thus far, evidence supporting these predictions for negative feedback is mixed, with the strongest evidence supporting a link between poor body condition and weak negative feedback in breeding animals. However, more research is necessary to assess the importance of changes in HPA negative feedback, especially in reptile, fish, and amphibian species. Furthermore, future research would benefit from reporting negative feedback ability in a standardized way, or at least making raw data available for the computation of alternate measures, to more easily compare studies in this growing area of research.

Influence of Daytime LED Light Exposure on Circadian Regulatory Dynamics of Metabolism and Physiology in Mice

Light is a potent biologic force that profoundly influences circadian, neuroendocrine, and neurobehavioral regulation in animals. Previously we examined the effects of light-phase exposure of rats to white light-emitting diodes (LED), which emit more light in the blue-appearing portion of the visible spectrum (465 to 485 nm) than do broad-spectrum cool white fluorescent (CWF) light, on the nighttime melatonin amplitude and circadian regulation of metabolism and physiology. In the current studies, we tested the hypothesis that exposure to blue-enriched LED light at day (bLAD), compared with CWF, promotes the circadian regulation of neuroendocrine, metabolic, and physiologic parameters that are associated with optimizing homeostatic regulation of health and wellbeing in 3 mouse strains commonly used in biomedical research (C3H [melatonin-producing], C57BL/6, and BALB/c [melatonin-non-producing]). Compared with male and female mice housed for 12 wk under 12:12-h light:dark (LD) cycles in CWF light, C3H mice in bLAD evinced 6-fold higher peak plasma melatonin levels at the middark phase; in addition, high melatonin levels were prolonged 2 to 3 h into the light phase. C57BL/6 and BALB/c strains did not produce nighttime pineal melatonin. Body growth rates; dietary and water intakes; circadian rhythms of arterial blood corticosterone, insulin, leptin, glucose, and lactic acid; pO₂ and pCO₂; fatty acids; and metabolic indicators (cAMP, DNA, tissue DNA 3H-thymidine incorporation, fat content) in major organ systems were significantly lower and activation of major metabolic signaling pathways (mTOR, GSK3β, and SIRT1) in skeletal muscle and liver were higher only in C3H mice in bLAD compared with CWF. These data show that exposure of C3H mice to bLAD compared with CWF has a marked positive effect on the circadian regulation of neuroendocrine, metabolic, and physiologic parameters associated with the promotion of animal health and wellbeing that may influence scientific outcomes. The absence of enhancement in amelatonic strains suggests hyperproduction of nighttime melatonin may be a key component of the physiology.

Maternal separation affects expression of stress response genes and increases vulnerability to ethanol consumption

INTRODUCTION

Maternal separation is an early life stress event associated with behavioral alterations and ethanol consumption. We aimed to expand the current understanding on the molecular mechanisms mediating the impact of postnatal stress on ethanol consumption.

METHODS

In the first experiment (T1), some of the pups were separated from their mothers for 6 hr daily (Maternal Separation group - MS), whereas the other pups remained in the cage with their respective mothers (Control group - C). In the second experiment (T2), mice from both groups were subjected to the model of free-choice between water and sucrose solution or between water and ethanol solution. Maternal behavior was assessed at the end of T1. At the end of both T1 and T2, pups were subjected to the light/dark box behavioral test and blood corticosterone concentrations were analyzed.

RESULTS

Our maternal separation protocol led to intense maternal care and affected weight gain of the animals. The expression of stress response genes was altered with higher levels of Crh and Pomc being observed in the hypothalamus, and higher levels of Crhr1, Crhr2, Htr2a and lower levels of Nr3c1 and Htr1a being observed in the hippocampus after T1. At the end of T2, we observed higher levels of Avp and Pomc in the hypothalamus, and higher levels of Crhr1, Crhr2, Nr3c1, Slc6a4, Bdnf and lower levels of Htr1a in the hippocampus. Additionally, maternal separation increased vulnerability to ethanol consumption during adolescence and induced changes in anxiety/stress-related behavior after T2. Furthermore, voluntary ethanol consumption attenuated stress response and modified expression of reward system genes: enhancing Drd1 and Drd2, and reducing Gabbr2 in the striatum.

CONCLUSION

Maternal separation induced behavioral changes and alterations in the expression of key genes involved in HPA axis and in the serotonergic and reward systems that are likely to increase vulnerability to ethanol consumption in adolescence. We demonstrated, for the first time, that ethanol consumption masked stress response by reducing the activity of the HPA axis and the serotonergic system, therefore, suggesting that adolescent mice from the MS group probably consumed ethanol for stress relieving purposes.

The behavioural constellation of deprivation: Causes and consequences

Socioeconomic differences in behaviour are pervasive and well documented, but their causes are not yet well understood. Here, we make the case that a cluster of behaviours is associated with lower socioeconomic status (SES), which we call “the behavioural constellation of deprivation.” We propose that the relatively limited control associated with lower SES curtails the extent to which people can expect to realise deferred rewards, leading to more present-oriented behaviour in a range of domains. We illustrate this idea using the specific factor of extrinsic mortality risk, an important factor in evolutionary theoretical models. We emphasise the idea that the present-oriented behaviours of the constellation are a contextually appropriate response to structural and ecological factors rather than a pathology or a failure of willpower. We highlight some principles from evolutionary theoretical models that can deepen our understanding of how socioeconomic inequalities can become amplified and embedded. These principles are that (1) small initial disparities can lead to larger eventual inequalities, (2) feedback loops can embed early-life circumstances, (3) constraints can breed further constraints, and (4) feedback loops can operate over generations. We discuss some of the mechanisms by which SES may influence behaviour. We then review how the contextually appropriate response perspective that we have outlined fits with other findings about control and temporal discounting. Finally, we discuss the implications of this interpretation for research and policy.

The role of the hypothalamus-pituitary-adrenal/interrenal axis in mediating predator-avoidance trade-offs

Maintaining energy balance and reproducing are important for fitness, yet animals have evolved mechanisms by which the hypothalamus-pituitary-adrenal/interrenal (HPA/HPI) axis can shut these activities off. While HPA/HPI axis inhibition of feeding and reproduction may have evolved as a predator defense, to date there has been no review across taxa of the causal evidence for such a relationship. Here we review the literature on this topic by addressing evidence for three predictions: that exposure to predators decreases reproduction and feeding, that exposure to predators activates the HPA/HPI axis, and that predator-induced activation of the HPA/HPI axis inhibits foraging and reproduction. Weight of evidence indicates that exposure to predator cues inhibits several aspects of foraging and reproduction. While the evidence from fish and mammals supports the hypothesis that predator cues activate the HPA/HPI axis, the existing data in other vertebrate taxa are equivocal. A causal role for the HPA axis in predator-induced suppression of feeding and reproduction has not been demonstrated to date, although many studies report correlative relationships between HPA activity and reproduction and/or feeding. Manipulation of HPA/HPI axis signaling will be required in future studies to demonstrate direct mediation of predator-induced inhibition of feeding and reproduction. Understanding the circuitry linking sensory pathways to their control of the HPA/HPI axis also is needed. Finally, the role that fear and anxiety pathways play in the response of the HPA axis to predator cues is needed to better understand the role that predators have played in shaping anxiety related behaviors in all species, including humans.

Effects of aging on stress-related responses of serotonergic neurons in the dorsal raphe nucleus of male rats

Responses to various stressors in the brain change with age. However, little is known about the neural mechanisms underlying age-dependent changes in stress responses. It is known that serotonin, a stress-related transmitter, is closely related with the regulation of stress responses in the brain and that serotonergic function is modulated by various factors, including estrogen, in both sexes. In the present study, to elucidate the effects of aging on stress responses in serotonergic neurons, we examined the expression levels of tryptophan hydroxylase (TPH; a marker of serotonergic neurons) in the dorsal, ventral and lateral parts of the dorsal raphe nucleus (DRN) in young and old intact male rats. In young males, repeated restraint stress significantly increased the number of TPH-positive cells in all subdivisions of the DRN. In contrast, the stress-induced increase in TPH expression was only observed in the ventral part of the DRN in old males. Pretreatment with an estrogen receptor β antagonist had no effect on the number of TPH-positive cells in the dorsal and lateral DRN in young stressed males, whereas the antagonist decreased the number of TPH-positive cells in all DRN subdivisions in old stressed males. Our results suggest that the effects of repeated stress exposure on the expression of TPH in serotonergic neurons in the DRN change with age and that estrogenic effects via estrogen receptor β on TPH expression in stressed old males differ from those in young males.

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We examined the effect of aging on stress-induced TPH expression in male rats.

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The effect of stress exposure on TPH expression in the DRN changed with age.

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The effect of ER-β blockade on stress-induced TPH expression changed with age.

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Stress experience in young adulthood changed serotonergic response in old age.

Hypothalamic-pituitary-adrenal (HPA) axis and aging

Human aging is associated with increasing frailty and morbidity which can result in significant disability. Dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis may contribute to aging-related diseases like depression, cognitive deficits, and Alzheimer's disease in some older individuals. In addition to neuro-cognitive dysfunction, it has also been associated with declining physical performance possibly due to sarcopenia. This article reviews the pathophysiology of HPA dysfunction with respect to increased basal adrenocorticotropic hormone (ACTH) and cortisol secretion, decreased glucocorticoid (GC) negative feedback at the level of the paraventricular nucleus (PVN) of the hypothalamus, hippocampus (HC), and prefrontal cortex (PFC), and flattening of diurnal pattern of cortisol release. It is possible that the increased cortisol secretion is secondary to peripheral conversion from cortisone. There is a decline in pregnolone secretion and C-19 steroids (DHEA) with aging. There is a small decrease in aldosterone with aging, but a subset of the older population have a genetic predisposition to develop hyperaldosteronism due to the increased ACTH stimulation. The understanding of the HPA axis and aging remains a complex area with conflicting studies leading to controversial interpretations.

The prudent parent meets old age: a high stress response in very old seabirds supports the terminal restraint hypothesis

The reproductive success of wild animals usually increases with age before declining at the end of life, but the proximate mechanisms underlying those patterns remain elusive. Young animals are expected to invest less in current reproduction due to high prospects for future reproduction (the "restraint" hypothesis). The oldest animals may also show restraint when conditions are sub-optimal where even a small increase in reproductive investment may lead to death ("terminal restraint"). Alternatively, reproduction may be constrained by lack of experience and senescence (the "constraint" hypothesis). In two species of breeding seabirds, behavioural (time to return the offspring, calmness during restraint) and physiological (metabolism, glucose and corticosterone) parameters responded similarly to stress with advancing age, implying a generalized stress response. Across those parameters, birds were "shy" (high stress response) when young or old, and "bold" (low stress response) when middle-aged. Specifically, free corticosterone, the principal avian glucocorticoid responsible for directing energy away from reproduction and towards immediate survival following stress, was highest in both young and very old stressed birds. All age groups had a similar adrenal capacity to produce corticosterone, implying that middle-aged birds were showing restraint. Because the stress response, was highest at ages when the probability of current reproduction was lowest rather than at ages when the probability of future reproduction was highest we concluded that birds restrained reproductive investment based on current conditions rather than potential future opportunities. In particular, old birds showed terminal restraint when stressed. Hormonal cues promoted investment in adult survival over reproductive output at both the start and end of life consistent with the restraint hypothesis.