Hypothalamic Pituitary Axis Flashcards
What does corticotropin releasing hormone do?
It induces the release of ACTH
What does Gonadotropin-releasing hormone do?
Induces the release of LH and FSH
What does Growth Hormone Releasing Hormone do?
Induces release of GH…BIG STRETCH
What does Somatostatin do?
Inhibits the release of GH and TSH
What does Dopamine do?
Inhibits the release of prolactin
What does Thyrotropin do?
Induces the release of TSH and prolactin
Effect of cortisol on hypothalamic hormones
inhibits CRH
Effect of the sex hormones testosterone and progesterone on hypothalamic hormones
Inhibit GnRH (Gonadotropin releasing hormone)
What does GHRH do to other hypothalamic hormones?
Trick question, it just inhibits itself.
What does prolactin do to hypothalamic hormones?
Inhibits GnRH but stimulates dopamine
What do somatomedins do to hypothalamic hormones?
Stimulate Somatostatin release
Effect of growth hormone on hypothalamic hormones
Stimulates somatostatin
Discuss the two types of Empty Sella Syndrome
In primary ESS, increased pressure in the sella turcica flattens the pituitary along the walls of the cavity
In secondary ESS, we see a regression of the pituitary secondary to injury or radiation. Both give the impression of an empty sella on imaging.
This syndrome occasionally results in endocrine dysfunction
What is the embryonic origin of the anterior pituitary (adenohypophysis)
Forms from the embryonic invagination of pharyngeal epithelium (oral ectoderm) called the pouch of Rathke
What is the embryonic origin of the posterior pituitary (neurohypophysis)
Forms from the neuroectoderm derived from the hypothalamus. During development, it fuses with the pouch of Rathke
Discuss the response types we see in the anterior and posterior pituitary
With the anterior pituitary, we get activation by circulating hormones from the hypothalamus.
With the posterior pituitary, since it is composed of neural tissue, we get release of hormones in response to neurotransmission.
All anterior pituitary hormones are what type of hormone?
Peptide hormones
What does adenocorticotropic hormone do?
ACTH induces synthesis of adrenal cortical hormones (cortisol, androgens, aldosterone) in response to CRH. It regulates the size, integrity, and synthetic function of the adrenal cortex
What does Follicle Stimulating hormone do?
FSH stimulates follicle growth in ovaries and spermatogenesis in testes
What does luteinizing hormone do?
LH Promotes testosterone synthesis in testes, promotes estrogen/progesterone synthesis in the ovaries.
Surge causes ovulation and maintains the corpus luteum
What does growth hormone do when released from the anterior pituitary?
Promotes protein synthesis and tissue growth
What does Thyroid Stimulating Hormone do?
Stimulates growth of the thyroid gland and synthesis and secretion of thyroid hormones
What does Prolactin do when released from the anterior pituitary?
Promotes additional breast development during pregnancy in preparation for milk secretion, and directly stimulates lactation.
It also inhibits ovulation indirectly by inhibiting GnRH
ACTH is inhibited by ____
Cortisol
FSH is inhibited by ____, ____, and ____
Inhibin, estrogen, progesterone
LH is stimulated by ___ and inhibited by ___, ___, and ___.
Inhibited and stimulated by estrogen
Inhibited by testosterone and progesterone
Growth hormone is inhibited by ____
Somatomedins
Thyroid Stimulating Hormones are inhibited by _____
Thyroid hormones
Prolactin is stimulated by ____ and ____ and inhibited by ___
Stimulated by TRH and the action of suckling and inhibited by Dopamine
Cell populations of the anterior pituitary can be subdivided even further on imaging by using special stains. Periodic Acid Schiff (PAS) stains can identify three groups.
Discuss the acidophil group
Stains Orange
Somatotropes (GH) and lactotropes (Prolactin)
Cell populations of the anterior pituitary can be subdivided even further on imaging by using special stains. Periodic Acid Schiff (PAS) stains can identify three groups.
Discuss the basophil group
Stains purple
B-FLAT
Gonadotropes - FSH, LH
Corticotropes - ACTH
Thyrotropes - TSH
Cell populations of the anterior pituitary can be subdivided even further on imaging by using special stains. Periodic Acid Schiff (PAS) stains can identify three groups.
Discuss the chromophobe group
No stain reaction.
We see this with empty cells, meaning they lack cytoplasmic granules so there is no stain. Former acidophils and basophils do this when they have released all of their hormone-containing granules.
Effect of damage to the pituitary stalk
Damage to the pituitary stalk can lead to decreased secretion of pituitary hormones secondary to disruption of the hypophyseal portal system (anterior pituitary) and hypothalamic neurons (posterior pituitary).
The exception is prolactin secretion, which is increased in a situation of damage.
Discuss the link between growth hormone and IGF-1
GH acts directly on tissues to decrease glucose uptake and mobilize fatty acids. Its growth promoting effect is primarily mediated via insulin-like growth factor-1 (IGF-1), formerly known as somatomedin, and it stimulates protein synthesis at the organ level and in chondrocytes for linear growth.
IGF-1 is made in the liver in response to GH. Both hormones independently stimulate protein synthesis in muscle and increase lean body mass.
Discuss what we mean by counterregulatory hormone
These are hormones released during non-homeostatic conditions to restore a balance of some kind, and usually refers to hypoglycemic response hormones
GH, for example, is released in response to hypoglycemia to increase serum glucose levels by promoting glycogenolysis, gluconeogenesis, lipolysis, and ketogenesis
Discuss the structure and formation of ACTH
Synthesized from a larger precursor, POMC (proopiomelanocortin), which also derives Beta lipotropin and Beta endorphin.
A bioactive moiety, alpha-MSH (melanocyte stimulating hormone) is present on the N-terminal end of ACTH.
Discuss the structural homology of the 6 major hormones of the anterior pituitary
The anterior pituitary gland produces six major hormones that can be classified into three groups based on structural homology
- Glycoproteins - FSH, LH, TSH - All have an identical alpha subunit but a unique beta unit
- Somatomammotropins - GH and Prolactin (PRL) - Structurally related peptides that belong to the same cytokine-hematopoietin family
- ACTH-related peptides - ACTH, MSH, lipotropin - all formed by cleavage of POMC
_____ shares the same alpha subunit with FSH, LH, and TSH
hCG
Discuss synthesis and release of hormones from the posterior pituitary
Hormones of the posterior pituitary are synthesized in hypothalamic neurons (called magnocellular neurons due to their large cell bodies) and transported via axoplasmic flow to axon terminals in the posterior lobe of the pituitary.
Oxytocin and ADH are synthesized from larger precursors (prohormones) which are cleaved within the vesicles to produce active forms.
What hypothalamic nuclei are responsible for hormone production?
Paraventricular nucleus and the supraoptic nucleus
Discuss the actions of Oxytocin and how we release it
Two main actions:
- Promotes contractions of the uterine myometrium during labor
- Stimulates contraction of myoepithelial cells in the breast, facilitating milk release to suckling infant
Exogenous stimuli drive oxytocin secretion. Signals include suckling infant and dilation of the cervix as seen in childbirth
Discuss ADH osmolarity regulation
The osmotic concentration of extracellular fluid is sensed by specialized neurons within or adjacent to the hypothalamus. The size of these neurons changes with extracellular osmolality, resulting in nerve signals increasing or decreasing ADH secretion as appropriate.
Increase in ADH occurs when we have a high osmolality. ADH increases water reabsorption and decreases plasma osmolality.
ADH also responds to huge decreases in blood volume, unlike the RAS, which reacts to small changes.
ADH also constricts arterioles.
How does ADH decrease plasma osmolality?
ADH increases permeability to water in the distal tubules and collecting ducts of the kidney, thereby increasing water reabsorption and decreasing plasma osmolality. It stimulates membrane bound V2-receptors, triggering an increase in intracellular cAMP. Special vesicles containing aquaporins (permeable water pores) are inserted into the luminal aspect of the cell membrane. Free diffusion of water occurs from tubule to peritubular fluid, decreasing ECF osmolality.
How does ADH constrict arterioles?
At high concentrations, ADH constricts arterioles by acting through the V1 receptor, and is consequently referred to as arginine vasopressin
Discuss how ADH responds to large changes in blood volume
ADH is secreted in response to >10% changes in blood volume. Secretion is stimulated by signals from atrial stretch receptors and baroreceptors.
Hypovolemia with resultant reduced atrial stretch results in decreased secretion of atrial natriuretic peptide (ANP). Low levels of ANP support increased secretion of ADH.
Baroreceptors of the aortic and carotid sinus sense decreased arterial pressure due to hypovolemia, resulting in decreased afferent signals to the medulla and consequent increase in ADH secretion and sympathetic stimulation.
