Stress Response System and acupuncture. The role of Hypothalamic – Pituitary – Adrenal (HPA) axis.
Pathologic conditions related to stress and homeostasis has been described since 1911 from W.B. Cannon. Later on, Hans Seyle (1936) proposed the general adaptation syndrome that examined actions and consequences of environmental, physical, and psychological stressors on the healthy organism. The general idea is that each organism has the ability to manage stress successfully. The failure to do so (in cases of excessive or continued stress) can lead to pathologies related to stress (stress syndromes) like anxiety disorders, depression syndrome, fatigue syndrome, sleep disorders, gastrointestinal diseases, allergies, idiopathic pain disorders likefibromyalgia, TMJ syndrome, chronic pelvic pain, chronic headaches, chronic tinnitus, interstitial cystitis, autoimmune disorders etc (Chrousos 1999, 2002). Hypertension, diabetes, peptic ulcer, irritable bowel syndrome, bronchial asthma and many skin disorders are some of the most common disorders made worse by stress together with altered mood, motor, autonomic and neuroendocrine dysfunction (Diatchenko et al, 2006).
Today we use two scientific terms to describe the endogenous forces responsible for maintaining the internal stability and proper function of an organism during an acute or chronic stress condition: The homeostasis (Cannon 1911, Seyle 1936, McEwen and Wingfield 2003, McEwen 2002) and the allostasis (Sterling and Eyer 1988). Homeostasis regulates set points in the body in response to slowly evolving environmental changes in the context of daily fluctuations such as temperature, glucose or oxygen in the blood and blood pH (Robyn Klein 2006).The Allostatic mechanism responds to unexpected challenges in order to adapt to stress beyond everyday life homeostasis such as severe temperature drop or sudden reaction to an accident or exposure to a pathogen. The theory of allostasis provides to scientists an explanation for how exposure to unexpected or severe stressors, can result in wear and tear on tissues and organs, leading to a wide range of pathological conditions. Regulation of stress response involves two body systems:
The hypothalamus – pituitary – adrenal axis (HPA axis) and the central and peripheral components of autonomic nervous system. According to Chrousos (2004), the central components of the stress system are located
1) in the hypothalamus and the brain stem and include the corticotrophin-releasing hormone (CRH) and arginine – vasopressin (AVP) neurons, and
2) in the locus coeruleus (LC) and other catecholaminergic cell groups of the medulla and pons, parts of central sympathetic system. The peripheral limps of the HPA axis together with the efferent sympathetic/adrenomedullary system represent the peripheral components (Pavlovsky & Friedman, 2007). Stress Response System interact with endocrine system (reproductive, growth and thyroid axis), immune system (can influence inflammatory reactions), the mesocorticolimbic dopaminergic system (reward system), the amygdale/hypocampus complex (fear, learning and memory), the arcuate nucleus proopiomelanocortin (POMC), the thermoregulatory center and the appetite-satiety regulation center.
All these systems can modify the activity of the stress response system and fine-tunes the adaptive (homeostatic or allostatic) response of an organism. Chronic hyper activation of the stress system leads to chronic stress syndrome, with prolonged production of CRH and circulatory, endocrine, psychiatric, metabolic and immune dysfunction (Chrousos 1992, 2004). Hypothalamus is the principal regulator of autonomic function in the CNS and it is through the hypothalamus that the limbic system and the neocortex influence autonomic functions. Hypothalamus regulates autonomic function through an neuroanatomical way (control descending pathways to the autonomic centers of brain stem and spinal cord) and a neurohumoral way (production and release of neurohormones like oxytocin and vasopressin by the magnocellular neurosecretory system and the regulation of the adenohypophysis via the production and release of hypothalamic hypophysiotropic hormones (HHHs) by the parvocellular secretory system).
Clinically speaking the hypothalamus:
- Participate in four classes of reflexes (milk ejection and uterine contraction, urine flow, feedback loops with peripheral endocrine gland and central effects with hormones on behavior).
- Regulate the autonomic nervous system and emotional behavior.
- Regulate homeostatic processes of an organism such as temperature, feeding and thirst.
- Regulate tissue osmolarity and vascular volume.
- Regulate endocrine functions and immune response.
In TCM, the yin-yang theory summarizes the concept of homeostasis. It is the low of unity of opposites, i.e. the low of dynamic balance that can be applied to every phenomenon and every function of an organism. Excitation and inhibition, sympathetic and parasympathetic, vasoconstriction and vasodilatation, muscle contraction and muscle relaxation are pairs of functions. Between them a dynamic balance should be maintained.
In modern medicine instead of yin-yang we use the term homeostasis, metabolism and adaptability where hypothalamus controls every one of them. It is generally believed today, that the therapeutic (and partly analgesic) effect of acupuncture (to relieve and treat various inflammatory diseases and functional disturbances) is related with its ability to regulate HPA axis (Jeoung-Woo Kangk, 2004). There are reports suggesting that electroacupuncture (E/A) activate HPA axis and sympathetic adrenomedullary system and release glucocorticoids, the final effectors of the HPA axis with potent anti-inflammatory properties (Liao et al, 1979, 1981). Was also reported that E/A modulate the secretion rate of catecholamines (substances with anti-inflammatory properties through β – adrenoreceptor activation) from adrenal medulla, influencing sympathetic activity due to trauma of the needle (Mori et al 2000, Sato at al 1996). On the other hand, other reports that examined the parasympathetic, long term effect of acupuncture in humans and animals do not show such important effect on HPA axis (ACTH and cortisole levels), failing to verify the existence or not of this specific neural pathway of action of acupuncture.