Endocrinology 3 - Neurohypophysial system

Question 1

What is the neurohypophysis made of? And where are their cell bodies?

Answer 1

Nerve axons.

Cell bodies in the hypothalamus.

Question 2

What are 2 groups of cell bodies of the hypothalamic nuclei?

Answer 2

1. Paraventricular Nucleus (located near the 3rd Ventricle).

2. Supraoptic Nucleus (Above the optic chiasma).

Question 3

What are the 2 molecules associated with neurohypophysis?

Answer 3

1. Vasopressin.

2. Oxytocin.

Question 4

What are Magnocellular neurones?

Answer 4

1. They are large neurones that originate in the paraventricular or supraoptic nuclei.
2. They terminate at the neurohypophysis.
3. Have Herring bodies.

Question 5

What are Parvocellular neurones?

Answer 5

1. Average sized neurones that originate at the paraventricular.
2. They terminate at median eminence or other parts of the brain.

Question 6

What are Supraoptic neurones?

Answer 6

1. They are magnocellular and release neuroscretions at the neurohypophysis.
2. They have Herring bodies and are either vasopressinergic or oxytocinergic.

Question 7

What are Herring bodies?

Answer 7

Areas on the magnocellular neurone where neurosecretions can be stored on the way down to the neurohypophysis.

Question 8

What are Paraventricular neurones?

Answer 8

1. They have a magnocellular and parvocellular component.

2. Either vasopressinergic or oxytocinergic.

Question 9

How is Vasopressin synthesized?

Answer 9

Pre-prohormone => Prohormone => Hormone. This breakdown takes place in nerve axon.

Question 10

What does the vasopressin prohormone break down into?

Answer 10

Vasopressin, Neurophysin and Glycopeptide.

Question 11

How is Oxytocin synthesized?

Answer 11

Pre-prooxytocin splits into two (different neurophysin to vasopressin + no glycopeptide).

Question 12

What are the structural differences between vasopressin and oxytocin?

Answer 12

Differ by 2 AA. (AVP has Phenylalanine instead of isoleucine and AVP has Arginine instead of Leucine).

Question 13

What are the similarities of vasopressin and oxytocin? (3)

Answer 13

1. Both nonapeptides,
2. Initially synthesized as prohormones.
3. Prohormones cleaved into hormone and their neurophysin protein.

Question 14

What are the actions of Vasopressin? (6)

Answer 14

1. Stimulates water reabsorption by the principal cells in the renal collecting ducts (antidiuretic effect).
2. Powerful Vasoconstrictor.
3. Released by parvocellular neurones at median eminence = carried to adenohypophysis and acts with CRH to control release of corticotrophin.
4. Acts on other parts of brain and can effect behaviour.
5. Involved in blood clotting factor synthesis (von Willebrand and factor 7).
6. Hepatic Glycogenolysis - stressors such as dehydration release vasopressin (hence it is a stress hormone) + increases blood glucose levels.

Question 15

What is the function of a V1 receptor?

Answer 15

It is a G protein linked receptor and has 2 subgroups (a + b). It’s function is vasoconstriction and glycogenolysis.
G protein - PLC => PIP2 => IP3 + DAG => increased Ca2+ => activates PKC.

Question 16

What is the function of a V2 receptor?

Answer 16

G protein linked receptor to Adenylate Cyclase => converts ATP to cAMP => activates Protein Kinase A (PKA) => stimulates Aquaporin 2 production => accumulate + assemble into Aggraphores => migrate to cell membrane and water moves into cell => water goes into blood through AQP3 + 4.

Question 17

How does dehydration affect vasopressin levels?

Answer 17

Dehydration increases plasma osmolality => rise detected by Osmoreceptors => stimulates vasopressinergic neurones => vasopressin levels increase.

Question 18

How does blood pressure affect vasopressin levels?

Answer 18

Blood pressure falls => Baroreceptors (stretch receptors) become less active => vasopressin not inhibited => increased vasopressin release => increased sympathetic activity.

Question 19

What are the actions of oxytocin? (2)

Answer 19

1. Binds to oxytocin receptors on the smooth muscle cells in the uterus causes contraction which facilitates delivery.
2. Sucking of the nipple causes oxytocin release. Oxytocin binds to receptors in contractile myoepithelial cells that surround the ducts + alveoli involved in production of milk = milk ejection.

Question 20

What is the Neuroendocrine reflex arc involved with breast milk production + ejection?

Answer 20

Reflex arc from the nipple to the adenohypophysis which stimulates prolactin release = milk production. Reflex arc also goes to neurohypophysis which stimulates oxytocin release = ejection of milk.

Question 21

What is Diabetes insipidus?

Answer 21

Two types:
1. Central - no vasopressin produced.
2. Nephrogenic - tissue insensitivity.
Polydipsia (frequent thirst) and Polyuria (frequent urination) caused.

Question 22

What is SIADH (Syndrome of Inappropriate ADH)?

Answer 22

Too much ADH = decreases plasma osmolality = urine in concentrated.