Reactivity to novelty during youth as a predictive factor of cognitive impairment in the elderly: a longitudinal study in rats

Allostasis, allostatic load, and the aging nervous system: role of excitatory amino acids and excitotoxicity

The adaptive responses of the body to challenges, often known as "stressors", consists of active responses that maintain homeostasis. This process of adaptation is known as "allostasis", meaning "achieving stability through change". Many systems of the body show allostasis, including the autonomic nervous system and hypothalamo-pituitary-adrenal (HPA) axis and they help to re-establish or maintain homeostasis through adaptation. The brain also shows allostasis, involving the activation of nerve cell activity and the release of neurotransmitters. When the individual is challenged repeatedly or when the allostatic systems remain turned on when no longer needed, the mediators of allostasis can produce a wear and tear on the body that has been termed "allostatic load". Examples of allostatic load include the accumulation of abdominal fat, the loss of bone minerals and the atrophy of nerve cells in the hippocampus. Circulating stress hormones play a key role, and, in the hippocampus, excitatory amino acids and NMDA receptors are important mediators of neuronal atrophy. The aging brain seems to be more vulnerable to such effects, although there are considerable individual differences in vulnerability that can be developmentally determined. Yet, at the same time, excitatory amino acids and NMDA receptors mediate important types of plasticity in the hippocampus. Moreover, the brain retains considerable resilience in the face of stress, and estrogens appear to play a role in this resilience. This review discusses the current status of work on underlying mechanisms for these effects.

Age-dependent effects of a chronic ultramild stress procedure on open-field behaviour in B6D2F1 female mice

Few studies have been devoted to the interaction between age and stress. However, in view of the age-related changes in various components of the stress responses, the effects of stress may not be constant with age. In this study, we used a dimensional approach to compare open-field behaviour of B6D2F1 female mice, aged 5-6, 11-12, 17-18 and 23-24 months, exposed to a chronic ultramild stress (CUMS) procedure, solely based on nonnociceptive socioenvironmental stressors. Three behavioural dimensions emerged from the principal-component analysis; these were labelled as motor reactivity, exploratory activity, and emotional reactivity. Despite a major effect of age on the three dimensions, we could not conclude that CUMS had any influence as a function the age of the subjects. At all ages, CUMS increased motor activity and had no clear-cut effect on emotional reactivity and exploratory activity. The results are discussed in terms of the influence of the nature of the stressors on behavioural responses to novelty.

Protective and damaging effects of mediators of stress. Elaborating and testing the concepts of allostasis and allostatic load

Stress is a condition of human existence and a factor in the expression of disease. A broader view of stress is that it is not just the dramatic stressful events that exact their toll but rather the many events of daily life that elevate activities of physiological systems to cause some measure of wear and tear. We call this wear and tear "allostatic load," and it reflects not only the impact of life experiences but also of genetic load; individual habits reflecting items such as diet, exercise, and substance abuse; and developmental experiences that set life-long patterns of behavior and physiological reactivity (see McEwen). Hormones associated with stress and allostatic load protect the body in the short run and promote adaptation, but in the long run allostatic load causes changes in the body that lead to disease. This will be illustrated for the immune system and brain. Among the most potent of stressors are those arising from competitive interactions between animals of the same species, leading to the formation of dominance hierarchies. Psychosocial stress of this type not only impairs cognitive function of lower ranking animals, but it can also promote disease (e.g. atherosclerosis) among those vying for the dominant position. Social ordering in human society is also associated with gradients of disease, with an increasing frequency of mortality and morbidity as one descends the scale of socioeconomic status that reflects both income and education. Although the causes of these gradients of health are very complex, they are likely to reflect, with increasing frequency at the lower end of the scale, the cumulative burden of coping with limited resources and negative life events and the allostatic load that this burden places on the physiological systems involved in coping and adaptation.

Genetic differences in response to novelty and spatial memory using a two-trial recognition task in mice

A two-trial memory task, based on a free-choice exploration paradigm in a Y-maze, was previously developed to study recognition processes in Sprague-Dawley rats. Because this paradigm avoids the use of electric shock or deprivation that may have nonspecific effects and does not require learning of a rule, it may be particularly useful for studying memory in mice. Four inbred strains (Balb/cByJ, DBA/2J, C57BL/6J, and SJL/J), an F1 hybrid (C57BL/6 x SJL/J), and one outbred strain (CD1) were used to validate this task in mice and to characterize a strain distribution in response to novelty and working memory. Exploration was measured with a short (2 min) intertrial interval (ITI) between acquisition and retrieval, while memory was examined with longer intervals (30 min, 1 h, and 2 h). A study of the time course of the response to novelty revealed varying degrees of preference and/or habituation to novelty among the different strains, with CD1 exhibiting a very high response to novelty and others showing lower (C57 x SJL hybrids) to complete absence (SJL) of exploration of novelty. Memory span, assessed with increasing ITIs, varied widely among strains from 30 min (C57 x SJL hybrids) to at least 2 h (C57 and BALB). Such demonstrated sensitivity to a wide range of behavioral phenotypes supports the use of this spatial memory task as an effective tool for the study of genetic influences on the response to novelty and recognition processes in mice.

Behavioural trait of reactivity to novelty is related to hippocampal neurogenesis

The hippocampal formation is one of the brain areas where neurogenesis persists during adulthood, with new neurons being continuously added to the population of dentate granule cells. However, the functional implications of this neurogenesis are unknown. On the other hand, the hippocampal formation is particularly concerned with the detection of novelty, and there are indications that dentate granule cells play a significant role in this function. Recently, the existence of inter-individual differences in behavioural reactivity to novelty has been evidenced, related to differences in the reactivity of the hypothalamic-pituitary-adrenal axis (HPA). Rats that are highly reactive to novelty (HR) exhibit a prolonged corticosterone secretion in response to novelty and to stress when compared with low reactive rats (LR). Taking advantage of the existence of these inter-individual differences, we investigated whether neurogenesis in the dentate gyrus is correlated with the behavioural trait of reactivity to novelty. Rats were first selected according to their locomotor reactivity to a novel environment. Two weeks later, cell proliferation, evaluated by the incorporation of 5-bromo-2'-deoxyuridine (BrdU) in progenitors, was studied by immunohistochemistry. We found that cell proliferation in the dentate gyrus was negatively correlated with locomotor reactivity to novelty. Indeed, cell proliferation in LR rats was twice that observed in HR rats. In contrast, survival of nascent neurons was not influenced by the behavioural trait of reactivity to novelty. Using an unbiased stereology, we show that LR rats had more cells within the granule cell layer of the dentate gyrus than did HR rats. These results demonstrate the existence of inter-individual differences in neurogenesis and total granule cell number within the dentate gyrus. These differences in hippocampal plasticity can be predicted by the behavioural trait of reactivity to novelty.

Corticosteroids, the Aging Brain and Cognition

The hippocampal region of the brain is a useful model system for understanding the plasticity and resilience of brain cells to stress hormone action and aging. Hippocampal neurons show both structural and functional plasticity, and individual differences in hippocampal function are shaped by early life experiences. For human brain aging, there are new non-invasive imaging tools to relate to the animal models, and these can help to assess the vulnerability of the aging hippocampus in relation to stress and Alzheimer's disease.

Stress and the aging hippocampus

The "glucocorticoid cascade hypothesis" of hippocampal aging has stimulated a great deal of research into the neuroendocrine aspects of aging and the role of glucocorticoids, in particular. Besides strengthening the methods for investigating the aging brain, this research has revealed that the interactions between glucocorticoids and hippocampal neurons are far more complicated than originally envisioned and involve the participation of neurotransmitter systems, particularly the excitatory amino acids, as well as calcium ions and neurotrophins. New information has provided insights into the role of early experience in determining individual differences in brain and body aging by setting the reactivity of the hypothalamopituitary-adrenal axis and the autonomic nervous system. As a result of this research and advances in neuroscience and the study of aging, we now have a far more sophisticated view of the interactions among genes, early development, and environmental influences, as well as a greater appreciation of events at the cellular and molecular levels which protect neurons, and a greater appreciation of pathways of neuronal damage and destruction. While documenting the ultimate vulnerability of the brain to stressful challenges and to the aging process, the net result of this research has highlighted the resilience of the brain and offered new hope for treatment strategies for promoting the health of the aging brain.

Reaction of sleep-wakefulness cycle to stress is related to differences in hypothalamo-pituitary-adrenal axis reactivity in rat

Acute stress is known to modify sleep-wakefulness cycle, although with considerable interindividual differences. The origin of these individual differences remains unknown. One possibility is an involvement of the hypothalamo-pituitary-adrenal axis (HPA), as its reactivity is correlated with an individual's behavioral reactivity to stress, and it is known to influence the sleep-wakefulness cycle. The present study was designed to analyze relationships between natural differences in behavioral reactivity to stress associated with differential HPA reactivity and stress-induced changes in sleep-wakefulness. Adult rats were classified into two sub-groups according to their locomotor reactivity to a mild stress (novel environment): the 'low responders (LR)' and the 'high responders (HR)' animals exhibited different glucocorticoid secretion in response to stress. We show that immobilization stress induced an increase in wakefulness in LR animals and a decrease in wakefulness in HR animals. On the other hand, paradoxical sleep was increased in both LR and HR animals. Moreover, we observed that LR animals slept more than the HR animals, whereas the two groups had similar levels of paradoxical sleep. These results indicate that the response of the sleep-wakefulness cycle to stress is related to the behavioral reactivity to stress, in turn governed by the individual's reactivity of the HPA axis. The involvement of dopaminergic mechanisms is discussed.

Hormones as regulators of brain development: life-long effects related to health and disease

The life-long interplay between genes and the environment is instrumental in shaping the structure and function of the body, and these interactions apply to the brain as a plastic and ever-changing organ of the body. Hormones are key regulators of gene expression throughout the body, and the actions of hormones on the brain are instrumental in shaping sex differences and in determining the effects of stress on brain function, including the rate of brain aging. This article also introduces a new term, allostatic load, to describe the cost of adaptation to stressors. Allostasis (stability through change) refers to the output of hormones and autonomic regulators that help to maintain homeostasis, and allostatic load is the consequence of the overactivity of these systems when they are not shut off properly or are forced to be hyperactive by stressors. Key brain areas like the hippocampus are vital to the processing of information that affects how each individual adapts to and responds to potentially stressful life events, and the response of the brain through its control of endocrine and autonomic function in turn determines the degree of allostatic load that an individual will experience. This allostatic load in turn works with the intrinsic genetic susceptibility to determine the progression toward declining health.

Inter-individual differences in the effects of acute stress on the sleep-wakefulness cycle in the rat

It has been described that an acute immobilization stress (IS) modifies subsequent paradoxical sleep (PS). However, its effects are complex because some subjects remain unaffected. This discrepancy might result from constitutive inter-individual psychobiological differences. In order to test this hypothesis, an inter-individual analysis of sleep patterns and their modifications after 60 min IS has been performed. Even though global analysis showed a PS increase after IS, inter-individual analysis evidenced different PS reactivity; subjects which had the least PS during control recordings were those with the largest PS increase. Unlike global analysis, an inter-individual study evidenced different modifications of wakefulness and slow wave sleep according to individuals. Subjects presenting the highest amount of wakefulness in control conditions (the lowest amount of slow wave sleep) decreased their wakefulness amount, while subjects with the lowest amount of wakefulness increased it. Thus, individual characteristics of the sleep-wakefulness cycle should be considered while studying its modifications induced by different treatments.

Early and later adoptions have different long-term effects on male rat offspring

Both prenatal and postnatal environmental factors exert complex influences on the development of an organism. Previous studies have demonstrated that intervening events during the prenatal period can have different and even opposite effects than similar intervening events occurring in the postnatal period. We have reported previously that early postnatal adoption prevents prenatal stress-induced long-term impairments in glucocorticoid feedback. To characterize further the effects of adoptions during the postnatal period, adoptions have been performed at different times, and the effect on the postnatal ontogeny of the hypothalamo-pituitary-adrenal axis has been investigated. Adoptions were performed during the first hour after birth (A1) and on the fifth (A5) and twelfth (A12) days after birth. At each of these times, other litters (S1, S5, S12) underwent a "separation" controlling for the 1 min maternal separation necessary for the adoptions. Locomotor behavior, cognition, and stress-induced corticosterone secretion in the adult male offspring have been examined, along with maternal behavior. Early adoption (A1) was found to prevent the prolonged stress-induced secretion of corticosterone evident in early separated (S1) offspring. Similarly, A1 rats demonstrated lower novelty-induced locomotion and improved recognition performance in a Y-maze compared to S1 offspring. However, later adoption (A5, A12) prolonged stress-induced corticosterone secretion, increased the locomotor response to novelty, and disrupted cognitive performance in the offspring. Only the early adoption increased maternal licking behavior, a factor that may have a protective effect on the pups. Taken together, these results suggest that the same postnatal manipulation realized at different times can induce different, or even opposite, effects on the behavioral and neuroendocrine characteristics of the adult offspring.

Impact of social environment characteristics on neuroendocrine regulation

OBJECTIVE

This article reviews evidence relating social environment characteristics to patterns of neuroendocrine regulation. To date, although there has been considerable interest in the effects of social ties and support on health and longevity, less attention has been given to the effects of such social environment characteristics on actual physiologic parameters.

METHOD

Animal and human studies from 1960s to the present are reviewed for evidence linking social environment characteristics to patterns of hypothalamic-pituitary-adrenal (HPA) axis, sympathetic nervous system (SNS), and cardiovascular activity.

RESULTS

Community and laboratory-based studies document that characteristics of the social environment influence patterns of neuroendocrine reactivity. These effects seem to be highly sensitive to aspects of the social environment such as relative social status, the relative stability of the social ordering and, importantly, the quality of social relationships. Although supportive social relationships are often associated with attenuated patterns of HPA and SNS activation, the converse also seems to be true as nonsupportive social interactions are frequently associated with enhanced reactivity.

CONCLUSION

Available evidence regarding links between social environment characteristics and neuroendocrine regulation documents a link between the social and biological realms that may have important consequences for health and longevity. The data provide support for the hypothesis that observed associations between social ties and health and longevity result, at least partially, from the positive influence of such social environment characteristics in reducing neuroendocrine reactivity. The evidence regarding nonsupportive or hostile social relationships highlights the importance of taking a broader view of the potential health effects of the social environment, one that encompasses the potential for both positive and negative effects.