Aging of the hypothalamic-pituitary-adrenal axis in nonhuman primates with depression-like and aggressive behavior

Hormonal changes in northern white-cheeked gibbons (Nomascus leucogenys) after the death of a group member

The small apes, gibbons and siamangs, are monogamous species with their social groups comprising of both parents and their offspring. Therefore, the loss of a member may elicit a stress response in the remaining members due to their strong bonds. Glucocorticoids (GCs) have been useful indicators of stress, but distinguishing between acute versus chronic stress may be limited when measuring these hormones alone. The adrenal hormone dehydroepiandrosterone-sulfate (DHEAS), a GC antagonist, has been implicated in the regulation of the stress response. Thus, the concomitant measurement of these hormones can help examine whether an event, such as the loss of a group member, elicited a stress response. In this brief report, we discuss the hormonal response of two zoo-housed northern white-cheeked gibbons (Nomascus leucogenys) (1 adolescent male and his mother) after the death of the adult male of the group. Baseline fecal samples were collected opportunistically from these two individuals 5 months prior, and 3 months following the death of their group member. A total of 25 samples were quantified for fecal GC metabolites (FGCMs) and DHEAS by enzyme immunoassay (EIA) to calculate the FGCMs:DHEAS ratio. Our results indicate an increase in FGCMs and FGCMs:DHEAS for the adolescent male, but not the adult female, following the death. Our findings suggest that the integration of FGCMs and DHEAS measurements can provide valuable information to interpret individual stress levels to the sudden change in the group's social structure.

Age, adrenal steroids, and cognitive functioning in captive chimpanzees (Pan troglodytes)

Background

Dehydroepiandrosterone-sulfate is the most abundant circulating androgen in humans and other catarrhines. It is involved in several biological functions, such as testosterone production, glucocorticoid antagonist actions, neurogenesis and neuroplasticty. Although the role of dehydroepiandrosterone-sulfate (DHEAS) in cognition remains elusive, the DHEAS/cortisol ratio has been positively associated with a slower cognitive age-decline and improved mood in humans. Whether this relationship is found in nonhuman primates remains unknown.

Methods

We measured DHEAS and cortisol levels in serum of 107 adult chimpanzees to investigate the relationship between DHEAS levels and age. A subset of 21 chimpanzees was used to test the potential associations between DHEAS, cortisol, and DHEAS/cortisol ratio in cognitive function, taking into account age, sex, and their interactions. We tested for cognitive function using the primate cognitive test battery (PCTB) and principal component analyses to categorize cognition into three components: spatial relationship tasks, tool use and social communication tasks, and auditory-visual sensory perception tasks.

Results

DHEAS levels, but not the DHEAS/cortisol ratio, declined with age in chimpanzees. Our analyses for spatial relationships tasks revealed a significant, positive correlation with the DHEAS/cortisol ratio. Tool use and social communication had a negative relationship with age. Our data show that the DHEAS/cortisol ratio, but not DHEAS individually, is a promising predictor of spatial cognition in chimpanzees.

Validation of an enzyme immunoassay for measurement of fecal dehydroepiandrosterone sulfate in gibbons and siamangs

Monitoring wildlife stress levels is essential to ensure their quality of life in captivity or in the wild. One promising method to assess the stress response is the comeasurement of glucocorticoids (GC) and dehydroepiandrosterone sulfate (DHEAS), adrenal hormones involved in the modulation of the stress response. Although noninvasive methods to measure GCs have been validated in several species, only a few studies have validated DHEAS assays. The aims of this study were (1) to describe an enzyme immunoassay (EIA) to measure DHEAS levels, (2) to validate this assay for fecal samples in gibbons and siamangs, and (3) to test hormonal stability after one freeze-thaw cycle and over time at two freezer temperatures (-20°C and -80°C). Subjects included 32 gibbons and siamangs from U.S. zoological parks. The EIA was validated analytically by parallelism and accuracy tests, and biologically by confirming a DHEAS response 1-2 days after a stressful event (accident, vaccination, or transportation) in three individuals. In addition, fecal DHEAS levels in a pregnant female were above nonpregnant/nonlactating levels and declined progressively the following parturition. The hormonal stability experiments revealed no significant changes in fecal DHEAS levels after one freeze-thaw cycle. Hormonal levels in fecal extracts were stable for 2 months, regardless of the storage temperature, with no significant differences between -20°C and -80°C conditions. The EIA described has high sensitivity and it is suitable for fecal DHEAS measurement in gibbons and siamangs, with a potential to be applied to other species.

Behavioral diversity as a potential positive indicator of animal welfare in bottlenose dolphins

Accredited zoological facilities are committed to fully understanding the behavioral, mental, and physical needs of each species to continuously improve the welfare of the animals under their professional care and detect when welfare has diminished. In order to accomplish this goal, internally consistent and externally valid indicators of animal welfare are necessary to advance our understanding of the current welfare status of individual animals. Historically, efforts have focused on monitoring visible or observable signs of poor health or problem behavior, but lack of signs or problems does not necessarily demonstrate that an individual animal is thriving. The current study examined fecal hormone metabolite levels and behavior for two species of bottlenose dolphins (Tursiops truncatus and Tursiops aduncus) from 25 different accredited zoological facilities. At the time of the study, all facilities were accredited by the Alliance of Marine Mammal Parks and Aquariums and/or the Association of Zoos and Aquariums. This was part of the multi-institutional study 'Towards understanding of the welfare of cetaceans in zoos and aquariums" commonly referred to as the Cetacean Welfare Study. Behavioral diversity was calculated using the Shannon Diversity Index on species-appropriate behavioral events. Behavioral diversity was compared to the fecal metabolites of cortisol, aldosterone, and the ratio of cortisol to dehydroepiandrosterone (DHEA) as well as the stereotypic behavior of route tracing. Similar to previous studies on other species, there was a significant inverse relationship between behavioral diversity and both fecal cortisol metabolites and route tracing. Additionally, a significant inverse relationship also exists between behavioral diversity and the ratio of fecal cortisol to DHEA metabolites. Behavioral diversity and fecal aldosterone metabolites were not associated. Additional research is still needed to validate behavioral diversity as an indicator of positive animal welfare for bottlenose dolphins and across species. However, based on current results, facilities could utilize behavioral diversity combined with other measures of welfare to more comprehensively evaluate the welfare of bottlenose dolphins.

Sleep Disturbances and Depression Are Co-morbid Conditions: Insights From Animal Models, Especially Non-human Primate Model

The incidence rates of depression are increasing year by year. As one of the main clinical manifestations of depression, sleep disorder is often the first complication. This complication may increase the severity of depression and lead to poor prognosis in patients. In the past decades, there have been many methods used to evaluate sleep disorders, such as polysomnography and electroencephalogram, actigraphy, and videography. A large number of rodents and non-human primate models have reproduced the symptoms of depression, which also show sleep disorders. The purpose of this review is to examine and discuss the relationship between sleep disorders and depression. To this end, we evaluated the prevalence, clinical features, phenotypic analysis, and pathophysiological brain mechanisms of depression-related sleep disturbances. We also emphasized the current situation, significance, and insights from animal models of depression, which would provide a better understanding for the pathophysiological mechanisms between sleep disturbance and depression.

Comparison of the anatomical morphology of cervical vertebrae between humans and macaques: related to a spinal cord injury model

Non-human primates are most suitable for generating cervical experimental models, and it is necessary to study the anatomy of the cervical spine in non-human primates when generating the models. The purpose of this study was to provide the anatomical parameters of the cervical spine and spinal cord in long-tailed macaques (Macaca fascicularis) as a basis for cervical spine-related experimental studies. Cervical spine specimens from 8 male adult subjects were scanned by micro-computed tomography, and an additional 10 live male subjects were scanned by magnetic resonance imaging. The measurements and parameters from them were compared to those of 12 male adult human subjects. Additionally, 10 live male subjects were scanned by magnetic resonance imaging, and the width and depth of the spinal cord and spinal canal and the thickness of the anterior and posterior cerebrospinal fluid were measured and compared to the relevant parameters of 10 male adult human subjects. The tendency of cervical parameters to change with segmental changes was similar between species. The vertebral body, spinal canal, and spinal cord were significantly flatter in the human subjects than in the long-tailed macaques. The cerebrospinal fluid space in the long-tailed macaques was smaller than that in the human subjects. The anatomical features of the cervical vertebrae of long-tailed macaques provide a reference for establishing a preclinical model of cervical spinal cord injury.

Beyond Glucocorticoids: Integrating Dehydroepiandrosterone (DHEA) into Animal Welfare Research

Animal welfare researchers are committed to identifying novel measures for enhancing the quality of life of individual animals. Recently, welfare scientists have emphasized the need for tracking multiple indicators of an animal's behavioral, emotional and mental health. Researchers are currently focused on integrating non-invasive physiological biomarkers to gain insight into an individual's welfare status. Most commonly, the animal welfare community has analyzed glucocorticoid hormones and their metabolites as a measure of stress. While glucocorticoids provide valuable information about hypothalamic-pituitary-adrenal (HPA) axis activity, there are limitations to utilizing these hormones as the sole measure of long-term stress and welfare. Other biomarkers, such as dehydroepiandrosterone and its sulfate ester-collectively referred to as DHEA(S)-help provide a more complete picture of HPA activity. DHEA(S) counteracts the effects glucocorticoids by having anti-aging, immune-enhancing and neuroprotective properties. Recent studies have examined the ratio of glucocorticoids to DHEA(S) as a way to better understand how the HPA axis is functioning. There is evidence that this ratio serves as an indicator of immune function, mental health, cognitive performance and overall welfare. We review studies that employed the glucocorticoid:DHEA(S) ratio, outline methodological considerations and discuss how researchers can integrate glucocorticoids, DHEA(S) and the glucocorticoid:DHEA(S) ratio into welfare assessments.

Mini-review: Aging of the neuroendocrine system: Insights from nonhuman primate models

The neuroendocrine system (NES) plays a crucial role in synchronizing the physiology and behavior of the whole organism in response to environmental constraints. The NES consists of a hypothalamic-pituitary-target organ axis that acts in coordination to regulate growth, reproduction, stress and basal metabolism. The growth (or somatotropic), hypothalamic-pituitary-gonadal (HPG), hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-thyroid (HPT) axes are therefore finely tuned by the hypothalamus through the successive release of hypothalamic and pituitary hormones to control the downstream physiological functions. These functions rely on a complex set of mechanisms requiring tight synchronization between peripheral organs and the hypothalamic-pituitary complex, whose functionality can be altered during aging. Here, we review the results of research on the effects of aging on the NES of nonhuman primate (NHP) species in wild and captive conditions. A focus on the age-related dysregulation of the master circadian pacemaker, which, in turn, alters the synchronization of the NES with the organism environment, is proposed. Finally, practical and ethical considerations of using NHP models to test the effects of nutrition-based or hormonal treatments to combat the deterioration of the NES are discussed.

Glucocorticoid Negative Feedback in Regulation of the Hypothalamic-Pituitary-Adrenal Axis in Rhesus Monkeys With Various Types of Adaptive Behavior: Individual and Age-Related Differences

The study of the mechanisms underlying the increased vulnerability of the individual to stressful environmental factors in different age periods is of great relevance for prevention and effective treatment of stress-dependent diseases that are widespread in the population of aging individuals. The purpose of our study was to investigate the individual and age-related features of the glucocorticoid negative feedback in regulation of the hypothalamic-pituitary-adrenal (HPA) axis, the key adaptive neuroendocrine system, in experiments with physically healthy young and old female rhesus monkeys with administration of mineracorticoid receptor (fludrocortisone) and glucocorticoid receptor (dexamethasone) agonists. We studied the monkeys with increased trait anxiety and depression-like behavior (DAB) characterized, as previously was shown, by the increased vulnerability to acute stress and the animals with normal standard behavior (SB) as the control. The pronounced individual differences in the reaction of HPA axis to fludrocortisone and dexamethasone in young animals were found. Young animals with DAB showed a lower sensitivity of HPA axis to the inhibitory effect of both fludrocortisone and dexamethasone compared with young animals with SB. At the same time, there were no significant intergroup differences in the concentration of ACTH and cortisol in response to placebo injection, i.e., in basal conditions. The old individuals with DAB demonstrated the essential relative resistance of HPA axis to fludrocortisone test and higher basal plasma levels of cortisol and ACTH in the evening (the period of HPA axis low circadian activity) compared to old SB animals. In the same time, the intergroup differences in the response of HPA axis to dexamethasone administration were leveled due to age-related increase in sensitivity of HPA axis to dexamethasone in animals with DAB. These data testify the pronounced intergroup and age differences in the feedback regulation of HPA axis, presumably resulting from unequal individual, and age-related changes in the activity of mineralcorticoid and glucocorticoid receptors in the brain structures supporting the functions of HPA axis. The maximum age disorders in functioning of the negative feedback mechanism in the regulation of HPA axis are characteristic of animals with DAB, which, apparently, underlie the increased vulnerability of these animals to stress exposure.

Adrenal Aging and Its Implications on Stress Responsiveness in Humans

Normal aging results in subtle changes both in ACTH and cortisol secretion. Most notable is the general increase in mean daily serum cortisol levels in the elderly, without a noteworthy alteration in the normal circadian rhythm pattern. Glucocorticoid excess seen in the elderly population can have serious consequences in both the structural and functional integrity of various key areas in the brain, including the hippocampus, amygdala, prefrontal cortex, with consequent impairment in normal memory, cognitive function, and sleep cycles. The chronically elevated glucocorticoid levels also impinge on the normal stress response in the elderly, leading to an impaired ability to recover from stressful stimuli. In addition to the effects on the brain, glucocorticoid excess is associated with other age-related changes, including loss of muscle mass, hypertension, osteopenia, visceral obesity, and diabetes, among others. In contrast to the increase in glucocorticoid levels, other adrenocortical hormones, particularly serum aldosterone and DHEA (the precursor to androgens and estrogens) show significant decreases in the elderly. The underlying mechanisms for their decrease remain unclear. While the adrenomedullary hormone, norephinephrine, shows an increase in plasma levels, associated with a decrease in clearance, no notable changes observed in plasma epinephrine levels in the elderly. The multiplicity and complexity of the adrenal hormone changes observed throughout the normal aging process, suggests that age-related alterations in cellular growth, differentiation, and senescence specific to the adrenal gland must also be considered.

Age-related differences in stress responsiveness of the hypothalamic-pituitary-adrenal axis of nonhuman primates with various types of adaptive behavior

Aging is characterized by disturbances in the functioning of the hypothalamic-pituitary-adrenal (HPA) axis, associated with disturbances in the adaptation processes and increase of the probability of the onset of post-stress syndrome. However, the individual features of age-related disorders stress reactivity of HPA axis have not been studied. The purpose was to study individual characteristics of the HPA axis responsiveness to acute psycho-emotional stress exposure (restraint, ASE) at different age periods on the model of the young adult and old physically healthy female rhesus monkeys that differ in their behavioral responses to stress, i.e., with depression-like and anxiety-like behavior (DAB) on the one hand and healthy standard (control) adaptive behavior (SB) on the other hand. No significant intergroup differences were observed in HPA axis responses to ASE in young animals. During aging the monkeys with SB showed reduced ACTH response to the ASE, whereas the monkeys with DAB demonstrated its increase. The old animals with DAB in response to ASE demonstrated the most pronounced HPA axis disorders, such as the highest levels of corticotrophin (ACTH), the lowest levels of dehydroepiandrosterone sulfate (DHEAS), reduced cortisol (F) levels and the highest values of the F/DHEAS molar ratio. The ratio F/DHEAS positively correlates with the malondialdehyde concentration in erythrocytes that is considered as the biomarker of oxidative stress. Thus, these data allow us to consider the old monkeys with DAB as individuals with higher vulnerability to the adverse effects of ASE. In addition, depression-like and anxiety-like behavior of aged primates under mild/moderate stress along with reduced DHEAS plasma concentration and increased values of F/DHEAS ratio can be used to identify individuals with increased vulnerability to ASE and accelerated aging.

DHEA effects on brain and behavior: insights from comparative studies of aggression

Historically, research on the neuroendocrinology of aggression has been dominated by the paradigm that the brain receives sex steroid hormones, such as testosterone (T), from the gonads, and then these gonadal hormones modulate behaviorally relevant neural circuits. While this paradigm has been extremely useful for advancing the field, recent studies reveal important alternatives. For example, most vertebrate species are seasonal breeders, and many species show aggression outside of the breeding season, when the gonads are regressed and circulating levels of gonadal steroids are relatively low. Studies in diverse avian and mammalian species suggest that adrenal dehydroepiandrosterone (DHEA), an androgen precursor and prohormone, is important for the expression of aggression when gonadal T synthesis is low. Circulating DHEA can be converted into active sex steroids within the brain. In addition, the brain can synthesize sex steroids de novo from cholesterol, thereby uncoupling brain steroid levels from circulating steroid levels. These alternative mechanisms to provide sex steroids to specific neural circuits may have evolved to avoid the costs of high circulating T levels during the non-breeding season. Physiological indicators of season (e.g., melatonin) may allow animals to switch from one neuroendocrine mechanism to another across the year. DHEA and neurosteroids are likely to be important for the control of multiple behaviors in many species, including humans. These studies yield fundamental insights into the regulation of DHEA secretion, the mechanisms by which DHEA affects behavior, and the brain regions and neural processes that are modulated by DHEA. It is clear that the brain is an important site of DHEA synthesis and action. This article is part of a Special Issue entitled 'Essential role of DHEA'.

The influence of age and season on fecal dehydroepiandrosterone-sulfate (DHEAS) concentrations in Japanese macaques (Macaca fuscata)

Dehydroepiandrosterone (DHEA) and its sulfate, DHEAS, are the most abundant steroid hormones in primates, providing a large reservoir of precursors for the production of androgens. DHEAS levels decline with age in adult humans and nonhuman primates, prompting its consideration as a biomarker of senescence. However, the mechanisms responsible for this age-related decrease and its relationship to reproduction remain elusive. This research investigated DHEAS concentrations in fecal samples in order to determine age-related changes in captive Japanese macaques, as well as to assess the possible influence of seasonality. The subjects were 25 female Japanese macaques (2weeks to 14years-old) housed outdoors in social groups at the Primate Research Institute. We collected three fecal samples from each animal during the breeding season (October to December) and three additional samples from adult females during the non-breeding season (May to June). The hormonal concentrations were determined using enzyme immunoassay. DHEAS concentration was negatively correlated with age, but we did not find a significant difference between breeding and non-breeding seasons. Neonatal macaques had the highest DHEAS concentrations of all age groups. We suggest that elevated neonatal DHEAS is possibly a residue from fetal adrenal secretion and that, as in humans, it might assist in neurobiological development.

Stress responsiveness of the hypothalamic-pituitary-adrenal axis: age-related features of the vasopressinergic regulation

The hypothalamic-pituitary-adrenal (HPA) axis plays a key role in adaptation to environmental stresses. Parvicellular neurons of the hypothalamic paraventricular nucleus secrete corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) into pituitary portal system; CRH and AVP stimulate adrenocorticotropic hormone (ACTH) release through specific G-protein-coupled membrane receptors on pituitary corticotrophs, CRHR1 for CRH and V1b for AVP; the adrenal gland cortex secretes glucocorticoids in response to ACTH. The glucocorticoids activate specific receptors in brain and peripheral tissues thereby triggering the necessary metabolic, immune, neuromodulatory, and behavioral changes to resist stress. While importance of CRH, as a key hypothalamic factor of HPA axis regulation in basal and stress conditions in most species, is generally recognized, role of AVP remains to be clarified. This review focuses on the role of AVP in the regulation of stress responsiveness of the HPA axis with emphasis on the effects of aging on vasopressinergic regulation of HPA axis stress responsiveness. Under most of the known stressors, AVP is necessary for acute ACTH secretion but in a context-specific manner. The current data on the AVP role in regulation of HPA responsiveness to chronic stress in adulthood are rather contradictory. The importance of the vasopressinergic regulation of the HPA stress responsiveness is greatest during fetal development, in neonatal period, and in the lactating adult. Aging associated with increased variability in several parameters of HPA function including basal state, responsiveness to stressors, and special testing. Reports on the possible role of the AVP/V1b receptor system in the increase of HPA axis hyperactivity with aging are contradictory and requires further research. Many contradictory results may be due to age and species differences in the HPA function of rodents and primates.

Effects of adult onset mild calorie restriction on weight of reproductive organs, plasma parameters and gene expression in male mice

Calorie restriction (CR) extends lifespan and delays onset of age-related diseases in various organisms, even when started later in life. Despite benefits for health and lifespan, CR's negative impact on reproduction is documented in some animals. Studies employing approximately 40% CR detected a delay in sexual maturation and impairment of fertility, which were combined with extension of the reproductive period. In contrast, mild CR (10-20%) is apparently not deleterious to reproduction. Hence, we hypothesized that mild CR started at 8 months of age would prolong reproductive capabilities and improve health parameters of male mice. To test this hypothesis, we assessed the effects of 10 and 20% CR on reproductive organ weights, selected plasma parameters and hepatic/testicular gene expression in normal male mice of heterogeneous genetic background. Starting at 8 months of age (adult), mice were assigned to 3 regimen groups: 10% CR (n = 8), 20% CR (n = 9) or ad libitum (AL; n = 8). Four months of CR were sufficient to reduce glycemia in a non-fasted protocol. Mild CR initiated in adulthood did not significantly impact final body weight, most of the analyzed plasma parameters or weight of androgen-dependent organs. Moreover, CR did not interfere with expression of the assessed testicular genes, or most of the hepatic genes, but it did cause an increase in the levels of peroxisome proliferator-activated receptor gamma (Pparg) and mouse sulfotransferase (mSTa); and a decrease in glucose-6-phosphatase-α (G6pc) mRNA, which might signify improvement of body condition. The important finding of our study was that a mild CR regimen, as low as 10 and 20%, was sufficient to impair glycemia in a non-fasted state, and also the levels of plasma IGF-1, corroborating the concept that mild CR has the potential for improving health and longevity, even when started later in life.