Stress and HPA Axis

Stress Concepts - Brain's First Alert

• Stress: Body's response to perceived threats (stressors), disrupting homeostasis.
• Stressors:
  • Physical: Injury, infection, pain.
  • Psychological: Exams, loss, major life changes.
  • Social: Isolation, social rejection.
• Brain's Initial Rapid Response (First Alert):
  • Amygdala (threat detection) rapidly activates Sympathetic Nervous System (SNS).
  • Purpose: Mobilize energy for "Fight or Flight".
  • Adrenal medulla releases catecholamines: Adrenaline & Noradrenaline.
  • Effects: ↑Heart rate, ↑BP, ↑Respiration, ↑Glucose, sharpened senses.
• Key Brain Areas:
  • Amygdala: Primary threat sensor.
  • Locus Coeruleus: Norepinephrine source, triggers alertness.
  • Prefrontal Cortex (PFC): Evaluates threat, modulates response.

⭐ The "fight or flight" response, mediated by the SNS and catecholamines, is a rapid, short-lived adaptation crucial for immediate survival.

HPA Axis Anatomy & Hormones - Cortisol Command Chain

• Anatomical Trio:
  • Hypothalamus: Specifically, the Paraventricular Nucleus (PVN).
  • Anterior Pituitary: Contains corticotroph cells.
  • Adrenal Cortex: Zona Fasciculata layer produces cortisol.
• Hormonal Relay (The "Stress Circuit"):
  • Hypothalamus secretes CRH (Corticotropin-Releasing Hormone).
  • CRH stimulates Anterior Pituitary to release ACTH (Adrenocorticotropic Hormone).
  • ACTH targets Adrenal Cortex, triggering Cortisol (a glucocorticoid) synthesis.
• Cortisol's Role: Mobilizes energy (↑glucose), modulates immunity, reduces inflammation.
• Control Mechanism: Cortisol exerts negative feedback, inhibiting CRH and ACTH release. 📌 "Cortisol Cuts" CRH/ACTH.

⭐ Dexamethasone Suppression Test (DST): Used to assess HPA axis function. Failure to suppress cortisol after dexamethasone administration suggests HPA axis hyperactivity (e.g., Cushing's syndrome, severe depression).

HPA Axis Regulation & Dysregulation - Feedback Loop Faults

• Normal Regulation: Cortisol provides negative feedback to Hypothalamus (↓CRH) & Pituitary (↓ACTH).
• Dysregulation Triggers: Chronic stress, genetic vulnerability.
• Core Fault: Impaired negative feedback mechanism.
  • Reduced glucocorticoid receptor (GR) sensitivity or density (hippocampus, hypothalamus, pituitary).
  • Result: Cortisol fails to adequately inhibit CRH/ACTH secretion.
• Consequences:
  • Sustained HPA axis hyperactivity → ↑Cortisol (hypercortisolemia).
  • Associated with: Depression, Anxiety Disorders, PTSD.
  • Hippocampal damage (due to cortisol toxicity) further impairs feedback.

⭐ Dexamethasone Suppression Test (DST) non-suppression (failure of cortisol to suppress ACTH/cortisol after dexamethasone) is a common biological marker in major depression, indicating HPA axis hyperactivity and impaired feedback.

HPA Axis in Psychiatry - Mood Under Stress

• Core Pathway: Stress activates Hypothalamus (CRH) → Pituitary (ACTH) → Adrenal glands (Cortisol).
• Cortisol's Role: Primary stress hormone; regulates metabolism, immune response. Provides negative feedback to HPA axis.
• Dysregulation & Mood Disorders:
  • Depression: Often ↑ cortisol (hypercortisolemia), impaired negative feedback (e.g., Dexamethasone Suppression Test non-suppression). Linked to hippocampal volume ↓.
  • Anxiety (PTSD): Variable findings; some show ↓ cortisol, others altered diurnal rhythm or enhanced negative feedback.
  • Bipolar Disorder: HPA axis hyperactivity common during mood episodes.
• Mechanisms of Damage: Chronic stress → sustained ↑ cortisol → neuroinflammation, ↓ neurogenesis, altered neurotransmitter function (serotonin, norepinephrine, dopamine).

⭐ Persistent HPA axis hyperactivity, leading to elevated cortisol levels, is strongly implicated in the pathophysiology of Major Depressive Disorder and is linked to structural changes like hippocampal atrophy.

High‑Yield Points - ⚡ Biggest Takeaways

• The HPA axis (Hypothalamic-Pituitary-Adrenal) is the body's central stress response system.
• CRH from hypothalamus triggers pituitary ACTH, which stimulates adrenal cortisol release.
• Cortisol exerts negative feedback on the hypothalamus and pituitary.
• Chronic stress causes HPA axis dysregulation, linked to depression and anxiety disorders.
• Dexamethasone Suppression Test (DST) evaluates HPA axis; non-suppression often seen in depression.
• Hippocampal glucocorticoid receptors (GR) are vital for feedback; chronic stress can ↓ GR sensitivity/number.