stress

Abstract

  • The brain plays a central role in stress and adaptation to stress, perceiving threats and determining behavioral and physiological responses.

  • Promotes adaptation (allostasis) but can contribute to pathophysiology (allostatic load/overload) when dysregulated, causing detrimental health outcomes.

  • Both adult and developing brains exhibit structural and functional plasticity in response to stress, which includes:

    • Neuronal replacement

    • Dendritic remodeling

    • Synapse turnover

  • Stress potentially disrupts neural circuits involved in cognition, decision making, anxiety, and mood regulation.

  • Changes can also affect systemic physiology through neuroendocrine, autonomic, immune, and metabolic mediators.

  • While short-term changes can be adaptive, long-term maladaptation may necessitate pharmacological and behavioral therapies.

  • Significant sex differences exist in brain responses to stressors, indicating varied susceptibility and resilience.

  • Adverse early-life experiences, combined with specific gene alleles, may yield lasting effects via epigenetic mechanisms.

  • Preventive measures are critical; the inherent plasticity of the brain offers hope for developing therapies that leverage brain-body interactions.

  • Comprehensive governmental and private sector policies are essential for promoting health and increasing healthspan.

Keywords

  • hippocampus, amygdala, prefrontal cortex, glucocorticoids, glutamate, epigenetics, lifecourse, adverse childhood experiences, sex differences, policy

Received

  • Date: 7 December 2016

  • Revised: 14 January 2017

  • Accepted: 16 January 2017

Introduction

  • The concept of "stress" is frequently referenced in various contexts, leading to ambiguity in understanding its implications.

  • Traditional definitions often focus on acute challenges and reactivity, emphasizing mediators such as adrenaline and cortisol without providing a holistic view of the interconnected systems and health behaviors involved.

  • The goals of this review are as follows:

    • Discuss multiple body systems affected by stress, focusing primarily on the brain's role.

    • Explore the concepts of allostasis and allostatic load/overload within a broader life course framework.

    • Examine key mechanisms such as structural plasticity, remodeling of brain architecture, the role of glucocorticoids, and the impact of excitatory amino acids.

    • Consider the life course perspective and the impact of epigenetic processes on health and disease trajectories, along with potential interventions.

What Do We Mean by "Stress?"

Classification of Stress

  • Stress can be categorized into three main types:

    • Good Stress:

    • Refers to positive challenges that promote growth (known as eustress).

    • It is associated with good self-esteem, impulse control, and effective decision-making capabilities.

    • Even adverse experiences can provide growth opportunities for resilient individuals.

    • Tolerable Stress:

    • Involves adverse events that can be managed with adequate coping strategies and social support.

    • Toxic Stress:

    • Occurs when there is insufficient support, leading to severe distress and potential long-lasting harm due to impaired brain architecture, especially affecting impulse control and judgment.

    • Increases the risk of allostatic overload, leading to a cascade of health issues.

Definitions of Allostasis and Allostatic Load

  • Allostasis:

    • The process through which the body achieves stability by making psychological and physiological changes in response to stressors.

  • Allostatic Load:

    • Refers to the cumulative wear-and-tear on the body and brain resulting from chronic exposure to stressors, highlighting the balance between physiological adaptation and damage.

Biphasic Stress Response

  • The stress response can exhibit biphasic characteristics where:

    • Acute Stress:

    • Acts protectively (e.g., enhancing immune response and promoting wound healing).

    • Chronic Stress:

    • Results in immune response suppression and may contribute to autoimmune conditions such as psoriasis.

Health-Promoting and Damaging Behaviors

  • A triadic classification assists in understanding outcomes shaped by one’s mindset and control over stressful experiences:

    • Generally ignored behaviors affecting health outcomes arise from lifestyle stressors such as circadian disruptions, ambient noise, environmental pollution, and social isolation.

  • Chronic stress can lead to negative behavioral adaptations, resulting in:

    • Anxiety

    • Depression

    • Sleep disturbances

    • Unhealthy dietary habits

  • Over time, these maladaptive behaviors can result in physical deterioration of health, necessitating an increase in pharmaceutical interventions (e.g., anxiolytics) to manage symptoms.

Central Role of the Brain

  • The brain functions as the primary organ that interprets risks and orchestrates responses to stress, which directly influences various health outcomes.

  • Alterations in brain architecture and neurochemical profiles induced by stress can lead to health-damaging consequences by regulating the metabolic, cardiovascular, and immune systems.

  • Examples of resilience and maladaptive changes include:

    • Healthy brains can adapt to experiences through structural changes (neuroplasticity).

    • Unhealthy brains may face issues of plasticity that necessitate intervention for recovery.

    • Chronic hyperactivation may be linked to irreversible damage associated with Alzheimer's disease.

  • Normal aging can lead to reversible changes which can be countered through lifestyle interventions, such as regular physical activity and social engagement.

Role of Adrenal Steroids

Glucocorticoid Receptors in the Hippocampus

  • Glucocorticoid receptors located in the hippocampus are closely correlated with memory, spatial awareness, and mood regulation.

  • Stress exposure leads to dendritic shrinkage and loss of spines in CA3 pyramidal neurons.

  • Neurogenesis is pivotal in facilitating functional adaptations in adult brains, where:

    • Acute stress may stimulate an increase in spine density within the amygdala while leading to divergent effects in various brain regions under prolonged stress exposure.

    • Chronic stress notably alters dendritic architecture within the medial prefrontal cortex (PFC), affecting higher-order cognitive functions.

Key Role of Excitatory Amino Acids

  • Glutamate:

    • Plays a critical role in modifying brain structure and function, but excessive levels can lead to neurotoxicity.

  • The relationship between stress and glutamate release is noteworthy:

    • Chronic restraint stress leads to a reduction in dendritic length of hippocampal neurons.

    • Stress-induced excess glutamate release is driven by adrenal cortex activation, correlating with depressive-like behaviors.

  • Potential interventions may include agents that enhance metabotropic receptors and serotonin reuptake inhibitors, targeting the pathways affected by glutamate dysregulation.

Insulin Resistance in the Brain and Excitatory Amino Acids

  • Insulin and various metabolic hormones substantially influence cognitive functions, especially in states of insulin resistance, which is characterized by:

    • Insulin resistance correlating with diminished memory and executive functioning, adversely impacting connectivity between the hippocampus and prefrontal regions.

  • Manipulating insulin receptor signaling can lead to significant cognitive impairments, suggesting the need for corrective nutritional and lifestyle changes to reverse cognitive decline and metabolic disruptions.

Life Course and Epigenetics of Individual Differences

Mechanisms of Epigenetics

  • Epigenetics refers to changes in gene activity that occur without altering the DNA sequence, driven by environmental factors, thus critical for understanding health outcomes.

  • The concept encompasses several mechanisms influencing individual health trajectories throughout their lifespan, primarily shaped by experiences, particularly during early life events like maternal care.

  • The quality of early maternal care significantly influences both behavioral and neurological developments, leading to traits that may affect mental and physical health later in life.

Sex Differences in the Brain

  • Notable disparities exist between sexes in their responses to stress and cognitive effects, highlighted by:

    • Differences in responses to chronic stress between males and females during numerous stress and cognitive tasks.

    • Hormonal influences that invariably alter the impact of stress on behavior and cognition.

    • Establishing control over stressors may help mitigate these observed differences, as stressors applied in uncontrollable contexts generally yield contrasting results between sexes.

When Things Go Wrong Over the Life Course

Adverse Childhood Experiences (ACEs)

  • ACEs significantly influence long-term health trajectories, particularly in relation to socioeconomic status, linking environmental context to physical health outcomes.

  • Research indicates that ACEs are associated with inflammatory responses, leading to various health issues including metabolic syndrome.

  • Children raised in low socioeconomic conditions often exhibit compromised cognitive and emotional development, which may correlate with neural structural changes, exemplified by smaller hippocampal volumes and other adverse alterations.

Relevance to the RDoC Framework

  • Key processes within the RDoC (Research Domain Criteria) framework can be significantly influenced by life course experiences and alterations in neural circuitry.

  • It is crucial for RDoC to place more emphasis on the interplay between hormonal, immune, and metabolic interactions affecting stress correlates.

Conclusion: Actions Against Stress

  • To develop effective interventions against stress, comprehensive improvements in environmental factors and personal lifestyle choices are essential.

  • Modifications at societal levels, via educational and health incentives, can bolster healthier lifestyle choices.

  • Individual recommendations involve enhancing sleep quality, fostering social supports, maintaining balanced diets, and encouraging moderate physical activities to combat health issues related to stress.

Acknowledgments and Conflicts of Interest

  • The author(s) declared no potential conflicts of interest.

  • Funding was received through NIH grants to support this research article and its publication.

References

(Full list of references from pages 7-11, as documented in the original transcript)

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