The Hypothalamo-Neurohypophysial System

Question 1

What are the two hypothalamic nuclei associated with posterior pituitary gland and where do their axons terminate?

Answer 1

Paraventricular nucleus - axons pass through median eminence and terminate in neurohypophysis
Supraoptic nucleus - axons pass through the median eminence and terminate just above the optic chiasma in neurohypophysis

Question 2

Two types of neurones in paraventricular nucleus?

Answer 2

Magnocellular - larger, terminate on neurohypophysis (supraoptic plexus neurones are also magnocellular)
Parvocellular - smaller, terminate on primary capillary plexus

Question 3

What do the paraventricular and supraoptic neurones produce?

Answer 3

Both can be either vasopressinergic or oxytocinergic and release either vasopressin or oxytocin

Question 4

Process of vasopressin production?

Answer 4

Pre-provasopressin is synthesised and processed in granules to form pro-vasopressin
Provasopressin then forms vasopressin, neurophysin proteins, and glycopeptide in EQUIMOLAR amounts
These are released as neurosecretions of hormone and neurophysin proteins

Question 5

What is similar and different about oxytocin synthesis?

Answer 5

The initial events (preprohormone to prohormone to hormone) are the same
The neurophysin produced is different
NO GLYCOPEPTIDE PRODUCED

Question 6

Similarities in vasopressin and oxytocin structures?

Answer 6

Both have a ring structure with 6 amino acids

Both have a small attached chain of pre-amino acids

Question 7

Differences in vasopressin and oxytocin structures?

Answer 7

Differ by 2 amino acids in different locations
There is a difference in the ring structure, Vasopressin has Phenylalanine replaced by Isoleucine in Oxytocin
There is a difference in the chain structure, Vasopressin has Arginine replaced by Leucine in Oxytocin

Question 8

Principal physiological action of vasopressin?

Answer 8

Increased water reabsorption in the principal cells of the collecting duct of the kidney

Question 9

Some other actions of vasopressin throughout the body?

Answer 9

Vasoconstriction
Corticotrophin (ACTH) release
Synthesis of blood clotting factors (VIII and Von Willebrand factor)
Hepatic glycogenolysis

Question 10

What are the different vasopressin receptors and their locations?

Answer 10

V1a receptors - vascular smooth muscle, hepatocytes, CNS parvocellular neurones
V1b receptors - Adenohypophysial corticotroph cells
V2 receptors - Collecting duct principal cells

Question 11

Process of V1 receptor activation?

Answer 11

Vasopressin binds
G-protein-linked receptor which activates phospholipidase C enzyme
IP3, DAG produced which increase cytoplasmic Ca2+ and other mediators
Produces a cellular response

Question 12

Process of V2 receptor activation?

Answer 12

Vasopressin binds
G-protein linked receptor which activates Adenylate cyclase
Increases ATP to cAMP conversion
cAMP activates protein kinase A which activates cellular response
Cellular response is increased synthesis of aquaporins, particularly AQP2

Question 13

Which membranes do V2 receptors lie on and AQP2 molecules insert into?

Answer 13

Basolateral membrane of principal cell for receptor (vasopressin in blood)
Apical membrane for vasopressin-dependent AQP2 (water in lumen moves down conc gradient through AQP2)

Question 14

Where do AQP3/4 channels lie and why?

Answer 14

In BASOLATERAL MEMBRANE for water movement out of cell into blood

Question 15

Process of synthesis of AQP2 to insertion into apical membrane?

Answer 15

AQP2 synthesised
Packaged into aggraphores which migrate to apical membrane
Fuse with membrane leading to insertion of AQP2 into membrane

Question 16

How is vasopressin stimulated to release to change water reabsorption and process of doing so?

Answer 16

Increased plasma osmolarity detected by osmoreceptors in the brain
Increased activation of neurones
Increased VP secretion into circulation
Increased water reabsorption in kidneys
--> DECREASE in plasma osmolarity

Question 17

How is vasopressin stimulated to release to alter vasoconstriction and process of doing so?

Answer 17

Decreased arterial blood pressure detected by baroreceptors (decreased stimulus of sympathetic NS)
Increased VP secretion
Increased vasoconstriction
–> INCREASE in arterial blood pressure

Question 18

Difference in prolactin and oxytocin effects on breast milf production/

Answer 18

Prolactin - SYNTHESIS

Oxytocin - EJECTION

Question 19

Oxytocin actions on uterus and breast?

Answer 19

Uterus - during parturition, myometrial cells contract, baby is delivered
Breast - during lactation, myoepithelial cells contract, milk ejected

Question 20

Neuroendocrine reflex arc of oxytocin? (same arc as prolactin with different end response)

Answer 20

Suckling stimulates tactile receptors
Activation of neural afferent limb
Increased oxytocin release from neurohypophysis
Activation of endocrine efferent limb
Milk ejection response

Question 21

Comparative clinical importance of oxytocin and vasopressin and any conditions arising from problems?

Answer 21

Oxytocin - not hugely important, can be replaced by artificial milk and delivery of baby can be induced
Vasopressin - very important, insufficient = diabetes insipidus, too much = SIADH (syndrome of inappropriate ADH)

Question 22

What is diabetes insipidus and characteristic symptoms?

Answer 22

Condition where you lose too much water in urine due to reduced water reabsorption
Polyuria - excessive urination
Polydipsia - excessive drinking

Question 23

What are the two types of DI?

Answer 23

Central - caused by having/producing no vasopressin

Nephrogenic - caused by tissue insensitivity to vasopressin