Table of Contents
- Strain and Stress
- Levels of Hormonally Regulated Cycles
- Role of the Hypothalamus
- Role of the Pituitary Gland
- Role of the Adrenal Cortex
- Cortisol and its Function
- Regulation of the Hypothalamus Pituitary Adrenal Axis
- Diseases of the Hypothalamus Pituitary Adrenal Axis
- Popular Exam Questions Concerning the HPA Axis
Strain and Stress
Through the liberation of glucocorticoids, the feedback control loop between pituitary gland, hypothalamus, and adrenal cortex plays a central role in the reaction of the human body to stress. In this context, stress is primarily an unspecific reaction to a great amount of possible stressors like e.g. injuries, cold, hunger, work, but also psychic triggers like social stress.
It has been proven in experiments that the body uses the same reaction pattern if people are exposed to greater competition in their work environment. Specifically, it increases glucocorticoids and catecholamines levels (e.g. adrenalin) in the blood.
Stress is a reaction of the central nervous system and is mostly mediated by the sympathetic nervous system and the endocrinological pathway via the hypothalamus, the pituitary gland, and finally the adrenal cortex. Thus, this pathway is very relevant for the physiological stress reaction, but it also interacts with other areas like the immune system or the electrolyte household.
Levels of Hormonally Regulated Cycles
The hypothalamus pituitary adrenal axis is – like most endocrinological regulated cycles – organized in 3 tiers.
The hypothalamus is the upmost instance and connects the nervous system with the endocrine system. The hormones which are released here and are transported to the pituitary gland via axons, are called releasing hormones (RH) or liberines.
The next instance is the pituitary gland. In the anterior lobe of the pituitary gland (adenohypophysis), hormones are produced, which stimulate the release of hormones at downstream endocrine glands. These hormones are referred to as glandotropic hormones or tropines. Hormones which do not have an effect on endocrine glands, but directly influence the target organ, are called effector hormones.
The posterior lobe of the pituitary gland secretes such effector hormones, to wit oxytocin and antidiuretic hormone (ADH). Most effector hormones originate from endocrine glands like the thyroid gland, the adrenal cortex, or the ovaries/the testicles.
Role of the Hypothalamus
The supreme regulating authority of the axis is the hypothalamus. The hypothalamus is a part of the interbrain (diencephalon) and receives information from different centers of the cortex. The information is then processed to a hormonal response.
This response consists of a releasing hormone that is transported via axons in the pituitary gland and stimulates synthesis and secretion of hormones. In the case of the HPA axis, the responsible hormone is corticotropin-releasing hormone (CRH).
At rest, the secretion of CRH occurs in a pulsatile manner at a frequency of roughly 7-10 per days and follows a circadian rhythm with the highest levels in early morning and, else, in cases of stress reaction.
Role of the Pituitary Gland
The middle authority in this cascade is the pituitary gland. If CRH reaches the anterior lobe of the pituitary gland, synthesis and secretion of the glandotropic hormone ACTH (adrenocorticotropic hormone) is increased via a G-protein coupled receptor. The synthesis occurs in the pituitary gland via splitting of a precursor peptide called proopiomelanocortin (POMC), which is also used for the peptides ß-endorphin, ß-lipotropin, and melanocyte-stimulating hormone (MSH).
The latter determines the hyperpigmentation frequently observed at insufficiency of the adrenal cortex (Addison’s disease) since more POMC-peptides and, thus, also more MSH is secreted at elevated ACTH-production. The liberation of ACTH follows the pulsatile pattern of CRH, an increase of cortisol in the blood occurs only a few minutes after the secretion of ACTH.
Role of the Adrenal Cortex
The adrenal gland as an endocrine gland is a paired organ which lies within the kidney capsule and directly abuts on the kidney. While the adrenal medulla is regulated by the sympathetic nervous system and is responsible for the release of catecholamines into the blood, so-called steroid hormones are produced in the adrenal cortex.
Concerning their biosynthesis, steroid hormones all originate from cholesterol and run through similar steps of synthesis. Histologically, the adrenal cortex has three different layers which mirror the production locations of different steroid hormones:
- Zona glomerulosa: location of biosynthesis of mineral corticoids (e.g. aldosterone)
- Zona fasciculata: location of biosynthesis of glucocorticoids (e.g. cortisol)
- Zona reticularis: location of biosynthesis of androgens
Thus, cortisol is the final effector hormone and forms ACTH-dependent in the zona fasciculata of the adrenal cortex. Besides the effect on biosynthesis of cortisol, ACTH also makes for a sufficient provision of NADPH, an important cofactor of glucocorticoid synthesis, and for an increased activity of an esterase, which provides cholesterol, the original substance.
The synthesis of cortisol begins in the mitochondrion with hydroxylation of cholesterol to pregnolon. This is the pacemaker reaction of cortisol synthesis, mediated by the enzyme desmolase. The following reaction steps take place in the cytosol. Eventually, a steroid hormone with 21 C-atoms forms with a characteristic OH-group at the C11-atom.
Cortisol and its Function
Cortisol (also: hydrocortisone) is a steroid hormone and one of the most important representatives of glucocorticoids. Like all remaining steroid hormones, cortisol also forms from the scaffold of cholesterol. It is produced in the adrenal cortex, which is responsible for synthesis of steroid hormones. Further glucocorticoids are cortisone and corticosterone as well as synthetic compounds, which are used medically.
The effects of cortisol are numerous. Besides the effect on the sugar-, amino acid-, and lipid-metabolism, it especially has an influence on the immune system and the anti-inflammatory components. As a steroid hormone, cortisol is lipophilic and binds to an intracellular receptor. In the blood, it is transported by binding to a protein called transcortin.
If cortisone reaches the target cell as an effector hormone, it unfolds its effect via regulation of gene expression on the level of transcription and is, thus, long lasting. Cortisone does not bind to a membranous G-protein coupled receptor, but to a glucocorticoid receptor in the cytosol, which acts as a transcription factor for enzymes if it is activated.
Concerning metabolism, cortisone mostly has catabolic effects in the periphery, while centrally, that is in the liver, its effects are mostly anabolic. While proteolysis is induced in muscle cells and lipolysis is promoted in fat tissue, increased gene expression of enzymes of gluconeogenesis and glycogen synthesis can be observed in the liver.
Glucose intake in the muscles is decreased. As a consequence, the blood sugar level rises, which ensures good supply of the CNS. Due to the decreased glucose intake and utilization in the periphery and increased glycogen synthesis, cortisone can be classified as an antagonist to insulin since it has similar effects to glucagon.
Additionally, there is an anti-inflammatory, that is an antiphlogistic effect of cortisone. This is due to an inhibition of lymphocyte proliferation, an inhibition of the synthesis of interleukins, and an inhibition of cyclooxygenase-2 (COX-2). This mechanism should physiologically inhibit an overshooting immune reaction.
Concerning its influence on other organ systems, cortisol has growth slowing effects on the bones and even causes osteoporosis, promotes alertness in the CNS, and can increase contractile force of the heart via an increase in the effect of catecholamines.
Every day, the body produces roughly 12-30 mg of cortisol, which has a half-life period of ca. 90 minutes and is then inactivated in the liver. Glucocorticoids, which are given for treatment, lose a part of their effect at first contact with the liver, which is called the ‘first pass effect’. The metabolites are then excreted via the liver or the kidneys.
A single measurement of the cortisol level is not very meaningful since it follows a physiological circadian rhythm. The maximum plasma concentration of cortisol with peak values of 25 µg/dl is reached early in the morning roughly half an hour after waking up (cortisol awakening response). During the course of the day, the level drops.
Regulation of the Hypothalamus Pituitary Adrenal Axis
To ensure an appropriate response of the HPA axis, but also prevent an overshooting release of cortisol with negative consequences for the body simultaneously, there are several regulating mechanisms, which intertwine here.
The most important of these mechanisms surely is the negative feedback. This concept can often be observed in endocrinology. It describes the inhibiting effect of a hormone on a superior organ in order to eventually inhibit its own synthesis.
In case of the glucocorticoid cascade, the increased release of cortisol has an inhibiting effect on the hypothalamus and the pituitary gland. As a consequence, less CRH and ACTH are secreted and biosynthesis of cortisol is reduced. As long as all involved organs function properly, the plasma levels of cortisol always remain within acceptable limits for the body.
On the other side, there are activators of glucocorticoid synthesis, which cause an increased synthesis whih is independent of stress reaction or physiological circadian activity. Catecholamines are an exaample for this, stimulating the secretion of ACTH. Also, mediators of the immune system like IL-1 and TNF-α have a stimulating effect in all three instances.
Diseases of the Hypothalamus Pituitary Adrenal Axis
Hypercortisolism (Cushing Syndrome)
An excessively increased cortisol level is referred to as Cushing syndrome. The causes are complex. However, a major part of the cases is the consequence of immunosuppressive therapy with glucocorticoids, which is called the iatrogenic Cushing syndrome. Thus, there is a maximum dose in therapy guidelines, the so-called Cushing threshold, which should not be exceeded if possible.
Another frequent cause is, for example, a tumor in the anterior lobe of the pituitary gland, which secretes ACTH and causes anomalously increased plasma levels of cortisol. This central Cushing syndrome is also referred to as Morbus Cushing.
The symptoms of such an increased glucocorticoid level are very impressive and characteristic for this disease. The elevated blood sugar level of the patients can be explained by the effect of the hormone. This can – among others – lead to diabetes mellitus type II. Also, further consequences are osteoporotic changes and muscle weakness due to proteolysis.
Concerning their outward appearance, patients often display a distinct central obesity, a so-called buffalo hump, and a moon face. Also, edemas and high blood pressure are part of the clinical picture since glucocorticoids bind to the mineralocorticoid receptor if the concentration is high enough.
Therapy often aims for the cause of the Cushing syndrome. In the case of iatrogenic Cushing syndrome, one tries to reduce the glucocorticoid amount. Adenomas of the pituitary gland are mostly removed surgically.
Hypocortisolism (Insufficiency of the adrenal cortex)
In contrast to the clinical picture mentioned above, there is a lack of glucocorticoids at insufficiency of the adrenal cortex. This can acutely manifest and can even have lethal consequences if not treated appropriately. This disease is divided into a primary (level of the adrenal gland), secondary (level of the pituitary gland), and a tertiary form (level of the hypothalamus).
The primary insufficiency of the adrenal cortex is by far the most frequent form, which is referred to as Addison’s disease. Several causes can lead to this disease, but in roughly 70 % of the cases, an autoimmune disease with antibodies against the cells of the adrenal gland is the underlying cause. Also, tumors and infections are possible causes.
An acute and fulminant manifestation with possible circulatory shock, frequently associated with triggering factors like disease or stress, is called Addisonian crisis and is an endocrinological emergency.
Symptoms of hypocortisolism are often of an unspecific nature, like general weakness and weight loss, hypotension, abdominal pain, nausea, vomiting, and distinct ‘salt hunger’. Hyperpigmentation is typical for Addison’s disease since more MSH is produced due to increased levels of ACTH as a consequence of absent cortisol feedback, as mentioned before. Thus, Addison’s disease is sometimes referred to as ‘bronze disease’.
Diagnostically, the primary form can be delimited from the other forms with ACTH-diagnostics and the detection of autoantibodies. Therapy is performed with continuous substitution of gluco- and mineralocorticoids.
Popular Exam Questions Concerning the HPA Axis
The answers can be found below the references.
1. Which statement concerning the HPA axis is not true?
- ACTH secreted by the pituitary gland increases biosynthesis of adrenalin.
- The hypothalamus liberates the hormone CRH in a pulsatile manner.
- Cortisol inhibits ACTH- and CRH-release.
- Cortisol is synthesized out of cholesterol.
- ACTH is a glandotropic hormone.
2. Where is the hormone cortisol synthesized?
- In the pituitary gland.
- In the adrenal medulla.
- In the zona glomerulosa of the adrenal cortex.
- In the zona fasciculata of the adrenal cortex.
- In the hypothalamus.
3. Which is no typical effect of cortisol?
- Increase in blood sugar level
- Proteolysis of muscles
- Overshooting inflammatory reaction
- Induction of transcription of certain enzymes
- Synergism with adrenalin
Kleine, Rossmanith: Hormone und Hormonsystem, Springer Verlag
Rassow et al: Duale Reihe Biochemie, Thieme Verlag, 2. Auflage
Löffler, Petrides: Biochemie und Pathobiochemie, Springer Verlag, 9. Auflage
Kirchner, Mühlhäußer: BASICS Biochemie, Elsevier Verlag
MEDI-LEARN: Biochemie, 4. Auflage
Haupt-VO Funktionelle Pathologie Endokrinologie der Med-Uni-Wien.ac.at
Correct anwers: 1A, 2D, 3B