Hypothalamo-Neurohypophysial System

Question 1

What is the anatomy of the pituitary gland?

Answer 1

3rd ventricle.
Hypothalamus
optic chiasma/Mammillary body
Anterior lobe ( adenohypophysis) and posterior lobe ( neurohypophysis)

Question 2

what is the anterior and posterior lobe made of?

what is the median eminence

Answer 2

• anterior: made up of secretory cells
• posterior: made up of nerve axons

median eminence: location of the primary capillary plexus which is part of a portal system linking the median eminence to the anterior pituitary.

Question 3

what is the neurohypophysis made of and how is it structured?

Answer 3

made up of nerve axons and some other cells.
the nerves have their cell bodies in the hypothalamus and their nerve axons pass down through the pituitary stalk into the neurohypophysis.

Question 4

What is a hypothalamic nuclei?

Answer 4

A collection of cell bodies that send their axons to a particular place.

Question 5

State the two main hypothalamic nuclei.

Answer 5

Supraoptic: lying above the optic chiasma
Paraventricular: located close to the 3rd ventricle

Question 6

What is the other hypothalamic nucleus where the biological clock resides?

Answer 6

Suprachiasmatic nucleus

Question 7

What two molecules are produced by the neurohypophysis?

Answer 7

Vasopressin

Oxytocin

Question 8

What are the two types of neurone and how do they differ?

Answer 8

Parvocellular:
Average sized
Terminate in the median eminence and other parts of the brain
ONLY from paraventricular nucleus

Magnocellular:
LARGE
Terminate in neurohypophysis
Nuclei in both paraventricular AND supraoptic nuclei
-release neurosecretions into the general circulation

Question 9

Describe supraoptic neurones.

Answer 9

ALL MAGNOCELLULAR and terminate in the neurohypophysis
They are either oxytocinergic or vasopressinergic. The neurosecretions are hormones because they release directly into the circulation.

Question 10

What is a key feature of magnocellular neurones?

Answer 10

Herring Bodies - areas where neurosecretions can be stored on their way down to the neurohypophysis
- are specific to magnocellular neurones from the supraoptic and paraventricular nuclei.

Question 11

Describe the synthesis of Vasopressin. What other molecules are produced when the prohormone is cleaved and where does this takes place?

Answer 11

Vasopressin is synthesised from Pre-provasopressin to form vasopressin
Cleaved to produce:
Arginine vasopressin
Glycopeptide ( may be a prolactin releasing factor but little is known about it)
Neurophysin: ( large protein which is specific to the provasopressin molecule)

takes place in nerve axon

Question 12

Describe the synthesis of Oxytocin.

Answer 12

pre-prooxytocin splits into 2.
Synthesised from pre-prooxytocin
Cleaved to produce: 
Oxytocin 
Neurophysin (different to that produced from pre-provasopressin) 
Does NOT produce glycopeptide

Question 13

State the main differences between Arginine Vasopressin and Oxytocin.

Answer 13

AVP has PHENYLALANINE instead of ISOLEUCINE
AVP has ARGININE instead of LEUCINE.
-AVP prohormone is cleaved to produce three molecules ( AVP, neurophysin, glycoprotein)
Oxytocin prohormone is cleaved to produce 2 molecules ( oxytocin and neurophysin)

Question 14

State some similarities between Arginine Vasopressin and Oxytocin.

Answer 14

They are both nonapeptides
They are both synthesised from prohormones
Prohormones are cleaved to produce neurophysin and hormones

Question 15

What is the main effect of vasopressin?

Answer 15

• action is in the renal collecting ducts
  -stimulates water reabsorption by principal cells.
  -Increased water reabsorption in the kidney collecting ducts
  ANTIDIURETIC EFFECT

Question 16

What are some other effects of vasopressin?

Answer 16

• Vasoconstriction
• Synthesis of blood clotting factors, therefore vasopressin can be used to treat certain blood clotting disorders
• Corticotrophin release
• Hepatic glycogenolysis: released by certain stressors such as dehydration, so it increases blood glucose conc.

Question 17

What are the different types of vasopressin receptor and which cells express these receptors?

Answer 17

V1a
MOST IMPORTANT
-vasculature (blood vessels) and brain
arterial/arteriolar smooth (vasoconstriction), CNS neurones (behavioural) and hepatocytes (glycogenolysis)

V1b
Involved in control of corticotrophin release
Adenohypophysial cells (corticotrophs)

V2
Involved in antidiuretic effect, kidney collecting duct cells
-clotting factors

Question 18

Describe how V1 and V2 receptors work.

Answer 18

hormones affect the brain by getting into the cerebrospinal fluid.
V1= V1a and v1b

V1 = Gq protein linked receptor
- linked via G protein to phospholipase(PLC; PIP3 —> IP3 + DAG; increase in [Ca2+] and others such as protein kinase C

V2 = Gs protein linked receptor
-linked via G protein to(adenylate cyclase; convert ATP to cAMP; cAMP activates PKA, and this activates other intracellular mediators.

Question 19

Explain how vasopressin acts on cells in the kidney collecting duct.

Answer 19

-V2 receptors found in the basolateral membrane
-AVP binds to V2 on collecting duct cells
Activates adenylate cyclase —> increase in cAMP —-> activate PKA
—> increased synthesis of AQUAPORIN 2
AQP2 —> assembled into aggraphores —> aggraphores migrate to apical membrane —> water moves in —> water moves out of cell down concentration gradient via AQP3 + AQP4

Question 20

What are the two main functions of vasopressin and how is vasopressin release stimulated?

Answer 20

Water Reabsorption 
Vasoconstriction 
Stimuli: 
Dehydration=Increase in plasma osmolality 
Fall in blood pressure

Question 21

where are osmoreceptors located?

Answer 21

-lie outside the blood brain barrier, circumventricular organs

Question 22

What are the two main actions of oxytocin?

Answer 22

Oxytocin is a CONTRACTOR molecule. Main actions:
Contraction of the myometrial cells in the uterus during CHILDBIRTH (acts on smooth muscle in the uterus to cause contraction which facilitates delivery)
MILK EJECTION - contraction of myoepithelial cells in the breast during lactation

Question 23

Explain these two actions of oxytocin.

Answer 23

Oxytocin is release in massive amounts during delivery
It acts on the myometrial cells to cause contraction
Prolactin stimulates milk PRODUCTION but NOT milk ejection
Oxytocin stimulates the contractile myoepithelial cells around the ducts and alveoli to cause MILK EJECTION

Question 24

What is a stimulus for oxytocin release when breastfeeding?

Answer 24

Stimulation of tactile receptors around the nipple passes message via a neuroendocrine reflex arc to the neurohypophysis and stimulates oxytocinergic neurons, resulting in oxytocin release.

NOTE: this is a different neuroendocrine arc to the one that stimulates release of prolactin from the adenohypophysis

Question 25

State two ways of dealing with a lack of oxytocin.

Answer 25

SIADH (Syndrome of inappropriate antidiuretic hormone secretion ): - too much ADH produced, decreases plasma osmolality, increases urine concentration
Diabetes Insipidus: - characterised by polydypsia and polyuria, in central diabetes insipidus - NO VASOPRESSIN produced

Question 26

What hormones can suppress and enhance the release of oxytocin

Answer 26

Supressed by progesterone

enhanced by oestrogen