Hypothalamic Pituitary Axis Flashcards
What are names for the anterior and posterior pituitary?
Anterior: pars distalis, adenohypophysis
Posterior: pars nervosa, neurohypophysis
What are trophic hormones?
Hormones that stimulate the production of hormones from the anterior pituitary
What is the relationship between the hypothalamus and posterior pituitary?
NEUROENDOCRINE: Cell bodies are based in the hypothalamus which is where hormones are synthesized, and axons extend down infundibulum and terminate in the posterior pituitary.
What is the difference between the paraventricular nucleus and the supraoptic nucleus?
Both produced in the hypothalamus, but…
paraventricular: secretes oxytocin (OT)
supraoptic: secretes vasopressin (AVP/ADH)
What are neurophysins? What is the Neurohypophysis?
Neurophysin: Precursor peptides of OT and ADH produced in the hypothalamus
Neurohypophysis: nerve terminals for the secretion vesicles containing neurophysins and proteolytic enzymes
How is AVP/ADH released?
- Reduced extracellular fluid volume results in increased plasma osmolality, which triggers vasopressin release
- Reduced extracellular fluid volume results in decrease in left atrial volume, which results in low arterial blood pressure, which triggers vasopressin release
What does AVP/ADH do?
Increases H2O reabsorption in renal tubules and vasoconstriction of vascular smooth muscle to maintain blood pressure.
When is oxytocin released?
Birth canal distension and infant suckling induces positive feedback, triggering oxytocin release
What does oxytocin do?
Increase uterine muscle contraction during birth and milk ejection from mammary gland
How does oxytocin influence behaviour?
- Increases maternal behaviour in presence of estrogen
- Released during sexual arousal in both sexes
- Acts as neuromodulators to influence social recognition, memory, and affiliative behaviours
How does AVP influence behaviour?
- Stimulates release of ACTH (regulates cortisol and androgen production)
- Plays a greater role in males in social recognition and memory
- Aggression, courtship, scent marking, and learning
How does anterior pituitary hormone make it to the blood?
Neurosecretory neurons in hypothalamus secrete into the hypothalamic-hypophysial portal system into the anterior pituitary and either captures them in vesicles for storage or releases them into veins.
What are examples of anterior pituitary cells and the hormones they make?
Corticotroph: ACTH
Thyrotroph: TSH
Gonadotroph: FSH, LH
Lactotroph: PRL (acidophile)
Somatotroph: GH (acidophile)
What are hypophysiotrophic hormones?
Hormones that can regulate hypophyseal hormones (secreted by anterior pituitary) via stimulation or inhibition.
Includes TRH (regulates TSH), CRH (regulates ACTH), GnRH (regulates FSH, LH), GRHR (regulates GH).
What is the pathway for TSH?
TRH stimulates release of TSH from anterior pituitary which travels to thyroid gland and promotes release T3 and T4 which alters metabolic rate
What is the pathway for ACTH?
CRH stimulates release of ACTH from anterior pituitary which travels to adrenal cortex and promotes release of cortisol which alters metabolic actions
What is the pathway for growth hormone?
GRHR stimulates release of growth hormone from anterior pituitary which travels to…
- Liver, influence release of somatomedins that affect bone and soft tissue’
- many other tissues and promotes or inhibits metabolic actions
What is the pathway for LH and FSH?
GnHr stimulates release of LH and FSH from anterior pituitary. LH and FSH go to gonads (ovaries/testes) and induces production of sex hormones (estrogen, progesterone, testosterone) and gamete production.
What is the pathway for prolactin?
Prolactin is released by anterior pituitary and travels to mammary glands for breast growth and secretion.
What is somatostatin and dopamine’s role in anterior pituitary endocrinology?
Somatostatin: inhibits growth hormone
Dopamine: inhibits prolactin
Which hypophyseal hormones belong in the growth hormone family?
Growth hormone and prolactin (prolactin comes in post translational variants including normal, cleaved or spliced)
What is the structure and examples of hypophyseal hormones belong in the glycoprotein family?
Structure: alpha and beta subunit (alpha is conserved, beta has variety)
Examples: Follicle stimulating hormone (FSH), Leutinizing hormone (LH), Thyroid stimulating hormone (TSH), Human chorionic gonadotropin (hCG - pregnancy test)
What is a pars intermedia?
Anatomically separate intermediate pituitary found in many animals, including humans during development. Predominant endocrine product is alpha MSH.
What does alpha MSH induce?
Melanin synthesis, immune response, and decrease in food intake
What occurs during normal human growth?
- protein, fat and cartilage synthesis
- Cell proliferation - hyperplasia (increase in cell #) and hypertrophy (increased cell size)
- Bone lengthening - increased extracellular matrix
What id normal growth influenced by?
- genetic resolve
- diet and nutrient transfer
- disease and stress
- multiple layers of hormonal control
What hormones influence growth rate?
Growth hormone, testicular androgens (male) adrenal androgens (female), testosterone and estrogen
What is the structure and timing of release of growth hormone?
Structure: 191 AA long polypeptide produced in somatotrophs, most abundant anterior pituitary hormone transported via binding protein
Timing: spontaneous secretion over a 24h period but peaks in the first 90 mins of sleep.
What is the diurnal rhyhtm’s influence on GH pathways?
DAY: exercise stress and low blood glucose cause growth hormone INHIBITING hormone to inhibit anterior pituitary from releasing growth hormone
NIGHT: high blood AA and low blood fatty acids cause growth hormone RELEASING hormone to tell anterior pituitary to release growth hormone.
What are the pathways of growth hormone in the liver and metabolism?
Liver: GH promotes release of somatomedins which increase cell division, increase protein synthesis and increase bone growth
Metabolism: increase fat breakdown and decrease glucose uptake by muscle cells
What is the somatomedin hypothesis?
Growth hormone does not have a direct effect on growth of any tissue, but acts indirectly via somatomedins - IGF I and II
What is the difference between IGF I and IGF II?
IGF I: tissue specific regulation, increase during pubertal growth spurt and plays a large role in adult growth
IGF II: important during fetal development and plays a smaller role in adult growth
What are the three parts of a bone?
Epiphysis: end of long bone
Epiphyseal plate: site of bone growth between epiphysis and diaphysis
Diaphysis: mature shaft of a long bone.
What are osteoblasts?
Bone cells that produce enzymes (osteoid), collagen, and proteins to provide a framework for hydroxyapetite crystals to widen bones. Eventually they turn into mature bone cells - osteocytes.
What are chondrocytes?
cartilage cells in epiphyseal plate that regulates bone length growth
What is the dual effector theory? What is the papthway?
Definition: Bone growth requires GH and IGF I
Pathway:
- GH causes development of IGF I responsivenss and expression of IGF I gene
- Local production of IGF I
- Stimulation of clonal expansion via autocrine/paracrine mechanisms (hyperplasia)
- Cytoplasmic maturation (hypertrophy)
What is the growth pathway when the anterior pituitary is hypoactive?
Less GH is released and travels to liver, less somatomedin (IGF I and II) travels to bone cells, causes hypopituitary dwarfism
What is the growth pathway when the anterior pituitary is hyperactive?
Increase in GH released which travels to liver and increases somatomedins released which travels to bone cells and causes gigantism (at infant stage) or acromegaly (adult)
What is the pathway of growth hormone when liver has a lesion?
Normal amount of GH is released by anterior pituitary and travels to liver. Liver reduces release of IGF which moves to bones and causes laron dwarfism
What is the thyroid hormone’s affect on growth?
Influences somatic growth by supplying energy but is largely permissive.
Hypothyroidism = reduced growth
What is insulin’s affect on growth?
Involved in carboydrate metabolism.
Deficiency can block growth and excess can promote growth, however there is a potential cross reactivity with IGF receptors as they are so similar in structure.
What is Androgen and estrogen’s effect on growth?
Arrest long bone length increase by closure of epiphyseal plate
How does prolactin affect growth?
Influences mammary gland growth and other aspects of the immune system.
How does placental lactogen influence growth?
Influences neonatal development, maternal glucose and amino acid supply, and peaks mid pregnancy.
What are other growth hormones?
- Neurotropic factors (NGF’s)
- Erythropoietin
-Platelet derived growth factors (vascular injury repair + atherosclerosis)
- epidermal growth factors
- Tumour derived growth factors: angiogenesis
