Endocrinology 3 - The Hypothalamo-neurohypophysial axis

Question 1

What are the two hypothalamic nuclei associated with the posterior pituitary gland?

Answer 1

• Paraventricular nuclei

- Supra-optic nuclei

Question 2

What is a hypothalamic nucleus?

Answer 2

A collection of cell bodies.

Question 3

What types of neurones are present the paraventricular neurones?

Answer 3

• The majority are magnocellular, which pass down to the neurohypophysis via the medial eminence.
• Some are parvocellular, These are small and pass to other parts of the brain or the median eminence.

Question 4

What types of parvocellular neurones terminate in the median eminence?

Answer 4

Some vasopressinergic neurones.

Question 5

What types of neurones are supraoptic neurones?

Answer 5

These neurones are magnocellular.

Question 6

Describe the position and structure of magnocellular neurones.

Answer 6

• They pass through the median eminence and terminate in the neurohypophysis.
• They have herring bodies along the axon.

Question 7

What are herring bodies?

Answer 7

Granules that accumulate the newly synthesised hormones within the
axon/dendrites, forming swellings which then release the hormones into the general circulation.

Question 8

What hormones are produced by the supraoptic neurones and the paraventricular neurones?

Answer 8

They produce vasopressin and oxytocin. Therefore, they are vasopressinergic or oxytocinergic.

Question 9

Compare the chemical structures of vasopressin and oxytocin.

Answer 9

• Both have 9 amino acids, with a connecting disulfide bridge.
• There are 2 amino acids different - Vasopressin contains Phe and Arg, while oxytocin contains Ile and Leu.

Question 10

Describe the process of synthesis of vasopressin.

Answer 10

• Pre-provasopressin has a signal peptide so it is converted to provasopressin.
• Provasopressin contains arginine vasopressin, neurophysin and glycopeptide.

Question 11

What is different between oxytocin synthesis and vasopressin synthesis?

Answer 11

Oxytocin synthesis has different neurophysin and the glycopeptide is absent.

Question 12

What happens in the cell upon vasopressin binding to V1 receptors on cells?

Answer 12

• Linked by Gg proteins to phospholipase C.

- IP3 is produced alongside DAG, which causes Ca2+ release and PKC production.

Question 13

What are the two types of V1 receptors, and what are their affects on tissues?

Answer 13

• V1a causes vasoconstriction in arteriel/arteriolar smooth muscle, as well as glycogenolysis.
• V1b causes production of adrenocorticotrophic hormone

Question 14

What is the effect when vasopressin binds to V2 receptors?

Answer 14

• Water reabsorption in the collecting duct (ADH)

Question 15

Describe the process that occurs when vasopressin binds to V2 receptors in the collecting duct.

Answer 15

• Vasopressin binds to Gs receptors on the basolateral membrane
• Adenyl cyclase converts ATP to cAMP, which activates protein kinase A
• This stimulates synthesis of aquaporin 2 in aggraphores, which insert aquaporin into the apical membrane so water can be reabsorbed.
• Aquaporin 3 and 4 in the basolateral membrane transport the water into the blood.

Question 16

List the physiological actions of oxytocin.

Answer 16

• Major effects are in the uterus and mammary gland.

- Minor effects in the cardiovascular system and kidney

Question 17

Describe the physiological action of oxytocin in the uterus.

Answer 17

• Here oxytocin is acting on myometrial cells, causing a rhythmic contraction form fundus to cervix.
• This increases prostanoid production, and dilation of the cervix

Question 18

Which hormones suppress and enhance oxytocin?

Answer 18

• Progesterone suppresses oxytocin

- Oestrogen enhances oxytocin

Question 19

Describe the physiological action of oxytocin on the mammary gland.

Answer 19

Here oxytocin causes contraction in the myoepithelial cells, resulting in milk ejection.

Question 20

How is oxytocin controlled by neuro-endocrine reflex?

Answer 20

• Sucking causes a neural response to the hypothalamus, causing the production of oxytocin to increase and the cells contract to release milk

Question 21

How can oxytosin be used in the kidney and cardiovascular systems?

Answer 21

• It is similar to vasopressin, so may be used to increase uptake of water from urine (anti-diuretic).
• It can treat tachycardia and increase vasodilation, so blood flow to the organs increases.

Question 22

Describe the action of oxytocin in the CNS

Answer 22

Causes tend and befriend, meaning it increases affiliative behaviour. This is like fight or flight but in women.

Question 23

What are the major clinical uses of oxytocin?

Answer 23

• Induction of labour at term by IV infusion
• Prevention treatment of post-partum haemorrhage (supresses bleeding)
• Encourage release of milk in breast feeding mothers

Question 24

Describe the homeostatic control of vasopressin in the kidney.

Answer 24

• Increased plasma osmolarity stimulates release of vasopressin, this results in a decrease in plasma osmolarity so vasopressin is no longer released.
• Decreased arterial blood pressure detected by baroreceptors stimulates release of vasopressin, which increases vasoconstriction and therefore increases the blood pressure (stimulus is removed - vasopressin is no longer produced)

Question 25

What does a lack of oxytocin cause?

Answer 25

Partuition (giving birth) and milk ejection effects must be replaced by other means.

Question 26

What happens if there is a lack of circulating vasopressin?

Answer 26

• Diabetes insipidus (large water consumption, constant thirst and large amounts of dilute urine passed)
• This is central (/cranial), meaning too little is produced.

Question 27

What is the second type of diabetes insipidus, other than cranial?

Answer 27

• Nephrogenic diabetes insipidus, where the kidney has a resistance to vasopressin
• In this case, the levels of vasopressin will be high.