CH2 Hypothalamus & Posterior Pituitary

Question 1

What hormones are released by the posterior pituitary?

Answer 1

Oxytocin & Arginine Vasopressin

Question 2

Hormones released by the posterior pituitary control what?

Answer 2

Oxytocin (parturition & lactation) & Arginine Vasopressin (water balance)

Question 3

How does the hypothalamus send information to the posterior pituitary?

Answer 3

via Magnocellular Neurons of the Supraoptic N & Paraventricular Nucleus which have long axons that terminate in the posterior pituitary (forms the hypothalamo-hypophysial tract)

Question 4

What nuclei send axons from the hypothalamus to the posterior pituitary? What kind of neurons are they?

Answer 4

Magnocellular Neurons of the Supraoptic & Paraventricular Nuclei have long axons that terminate in the posterior pituitary

Question 5

How does the hypothalamus send information to the anterior pituitary?

Answer 5

via Parvicellular neurons which terminate in the median eminence. Neuropeptides released here are transported to the anterior pituitary by blood supply (long portal veins draining the median eminence, transporting peptides from the primary capillary plexus to the secondary plexus – that provides blood supply to the anterior pituitary)

Question 6

What forms the Hypothalamo-hypophysial Tract?

Answer 6

Unmyelinated axons of magnocellular neurons (induce the release of oxytocin, arginine vasopressin, neurophysins) from Paraventricular and Supraoptic Nuclei that end in the posterior pituitary

Question 7

What hormones induced by Parvicellular Neurons?

Answer 7

Hypophysiotropic Hormones (CRH, TRH, LHRH, GHRH, SS, DA) released in the median eminence induces the anterior pituitary to release hormones (ACTH, TSH, LH/FSH, GH, Prl)

Question 8

What are Hypophysiotropic Hormones and where are they found?

Answer 8

Hypophysiotropic Hormones are releasing/inhibiting neuropeptides released by Parvicellular Neurons that terminate in the median eminence to ultimately influence anterior pituitary hormone release

Question 9

What vessels connect posterior pituitary to anterior pituitary?

Answer 9

Short Portal Vessels provide venous connection from posterior to anterior pituitary. (allows neuropeptides released from posterior to affect anterior)

Question 10

How do hormones from the pituitary gland enter systemic circulation?

Answer 10

Blood from the anterior and posterior pituitary drains into the Intercavernous Sinus & then into the Internal Jugular Vein.

Question 11

Track Hypophysiotropic Hormones from production to production of Anterior Pituitary Hormones & from Anterior Pituitary Hormones to systemic circulation.

Answer 11

Hypophysiotropic Hormones are released into the primary capillary plexus of the superior hypophysial aa. (median eminence) via Parvicellular Neurons. Blood & hormones then go down the Long Hypophysial Portal Veins (infundibular stalk) to supply the anterior pituitary (secondary plexus). The secondary plexus is composed of fenestrated sinusoid capillaries and hormones diffuse and bind to anterior pituitary cells, which express G-protein-coupled receptors. They then (if releasing neurohormone is bound & not inhibiting neurohormone), release hormone which is drained into the Intercavernous Sinus & into the Internal Jugular Vein. hor

Question 12

Track Hormones from the posterior pituitary from production to system circulation.

Answer 12

Production in the posterior pituitary –> short portal vessels (ant. pituitary) –> intercavernous sinus –> internal jugular vein

Question 13

What hypophysiotropic hormones are released by the Parvicellular Neurons that control anterior pituitary function?

Answer 13

Corticotropin-releasing Hormone, Growth Hormone-releasing Hormone, Thyrotroping-releasing Hormone, Dopamine, Lutenizing Hormone-releasing Hormone, Somatostatin

Question 14

What is the result of inherited Familial Neurogenic Diabetes Insipidus?

Answer 14

Diabetes Insipidus is characterized by AVP deficiency caused by mutations in Neurophysins & improper targeting of the hormone to neurosecretory granules. This causes a build-up of AVP in the ER, leading to apoptosis

Question 15

What stimulates oxytocin release from the posterior pituitary?

Answer 15

sucking (tactile receptors) during breast feeding (lactation) & stretching of the cervix during childbirth (parturition - primary stimulus; positive feedback)

Question 16

What effects does oxytocin have on its targets?

Answer 16

myoepithelial cell contraction/milk ejection (breast); uterine contraction (uterus); positive autocrine feedback (oxytocin is also released within the hypothalamic supraoptic and paraventricular nuclei)

Question 17

What substances from other neurons inhibits oxytocin-releasing neurons (magnocellular neurons)?

Answer 17

endogenous opioids, nitric oxide, & gamma-aminobutyric acid

Question 18

What stimuli inhibit oxytocin release from the posterior pituitary?

Answer 18

severe pain, increased body temperature, and loud noise

Question 19

What kind of receptors does Oxytocin bind to?

Answer 19

Gq/11-protein coupled receptors

Question 20

How does oxytocin cause muscle contraction?

Answer 20

oxytocin binds to Gq/11-protein coupled receptors –> activates phospholipase C –> phospholipase C interacts with phosphatidylinositol bisphosphate to produce diacylglyceride (DAG) and inositol triphosphate(IP3) –> IP3 binds to IP3 calcium channels, releasing Ca++ –> Ca++ binds to calmodulin, which causes contraction via MLCK in smooth muscle

Question 21

What does Arginine Vasopressin do?

Answer 21

Increase water reabsorption/urine concentration, increase vascular resistance (vasoconstriction)

Question 22

What are the 3 receptors for AVP and their effects?

Answer 22

V1R (V1a): coupled to Gq/11 for vasoconstriction; V2R: coupled to Gs for water reabsorption; V3R (V1b) coupled to Gq/11 for vasoconstriction

Question 23

How does V2R differ from V1R & V3R?

Answer 23

V2R is expressed in the collecting duct’s principal cells of the kidney. Activates cAMP formation and phosphorylation and insertion of Aquaporin 2 (AQP2) in luminal membrane. (V1R & V3R ultimately increase cytosolic levels of calcium)

Question 24

Describe how AVP increases water reabsorption.

Answer 24

AVP binds to V2R G-protein-coupled receptor in Principal Cells of the distal tubule. Gs stimulates Adenylate Cyclase, which forms cAMP –> cAMP activates protein kinase A PKA –> PKA phosphorylates aquaporin 2 AQP2, leading to its insertion into luminal cell membrane and increase in water permeability –> water that is reabsorped leave through AQP3 and AQP4, which are in the basolateral membrane of principal cells

Question 25

List the aquaporins and their key features.

Answer 25

AQP1 - apical and basoalteral membranes of proximal tubules and descending loop of Henle (90% of water resorption); AQP2 - EXCLUSIVELY expressed in collecting ducts, and ONLY aquaporin directly regulated by ADH/AVP located in luminal membrane; AQP3+AQP4 - basoalteral membranes of epithelial cells in collecting ducts to enhance water reabsorption following AQP2 insertion into the luminal membrane

Question 26

What stimulates release of AVP?

Answer 26

increase of plasma osmolarity & decrease in BP

Question 27

What structures detect changes in plasma osmolarity?

Answer 27

hypothalamus (osmoreceptor neurons) & lamina terminalis (subfornical organ, median preoptic nucleus, organum vasculosum lamina terminalis)

Question 28

What 3 structures of the lamina terminalis help detect changes in plasma osmolarity?

Answer 28

subfornical organ, median preoptic nucleus, organum vasculosum lamina terminalis

Question 29

What structures in vasculature detect changes in blood pressure? How do they induce AVP release?

Answer 29

pressure-sensitive receptors in the cardiac atria, aorta, and carotid sinus (barorecptors); a decrease in blood pressure decreases the stretch of baroreceptors and decrease their firing rate. This signal is carried back by the vagus and glossopharyngeal nerves. Reduced stimulation of the baroreceptor produces a decrease in TONIC INHIBITION of AVP release.

Question 30

What structure in the Kidney detects changes in blood pressure? How does it induce AVP release?

Answer 30

Macula Densa in the kidney detects decrease in blood pressure. It stimulates release of Renin from Juxtaglomerular Apparatus. Renin converts angiotensinogen (from liver) to angiotensin I. Angiotensin I is converted to angiotensin II by antiotensin-converting enzyme (lungs). Angiotensin II sensitizes osmoreceptors, leading to enhanced AVP release.

Question 31

What causes Neurogenic Diabetes Insipidus?

Answer 31

diseases affect the hypothalamic-neurohypophysial tract, which decreases AVP release. Can be traumatic, inflammatory, infectious, &/ cancer related.

Question 32

What causes Renal (Nephrogenic) Diabetes Insipidus?

Answer 32

Renal insensitivity to the antidiuretic effect of AVP; AVP production and release are not effected (plasma levels are fine), but responsiveness at the distal tubule is impaired. Usually X-linked impairment of AVP2 receptor gene or can be mutation in AQP2 gene. Can also be acquired.

Question 33

What is the effect of SIADH?

Answer 33

Syndrome of Inappropriate Antidiuretic Hormone Secretion results in an increase or excess in the release of ADH. This presents as production of very small volumtes of concentrated urine and dilutional hyponatremia.