8. The hypothalamic pituitary axis

Question 1

Describe the location of the pituitary.

Answer 1

Beneath the hypothalamus in a socket of bone - the sella turcica.

Question 2

What are the embryological origins of the anterior and posterior lobes of the pituitary.

Answer 2

anterior lobe: projection of ectoderm (Rathke’s pouch) growing upward from roof of mouth

posterior lobe: ectodermal tissue growing downwards from diencephalon of developing brain

Question 3

Which structure connects the hypothalamus and pituitary?

Answer 3

infandibulum (pituitary stalk)

Question 4

What does the posterior pituitary consist of?

Answer 4

1. axons and terminals that originate in hypothalamus

2. specialised glial cells - pituicytes

Question 5

Which 2 hormones are released from the posterior pituitary (and what hormone type are they)?

Answer 5

1. oxytocin (peptide)

2. anti-diuretic hormone (peptide)

Question 6

Where are oxytocin and ADH synthesised in the hypothalamus?

Answer 6

• ADH: supraoptic nucleus

- oxytocin: paraventricular nucleus

Question 7

What stimulates oxytocin release from the posterior pituitary?

Answer 7

1. suckling (milk let-down reflex) - stimulus is transmitted via neurons from the breast to the hypothalamus
2. during childbirth: pressure on cervix and uterine wall

Question 8

What is the effect of oxytocin release?

Answer 8

1. activates oxytocin Rs on myoepithelial cells surrounding mammary alveoli… contraction to squeeze milk into duct system
2. during childbirth: activates oxytocin Rs on uterine SM cells… powerful uterine contractions

Question 9

Why is synthetic oxytocin (pitocin) often administered after birth?

Answer 9

increase uterine tone and control bleeding

Question 10

What is the effect of ADH release?

Answer 10

reduction in urine production:
activates Rs on distal tubular epithelium of collecting ducts in kidneys… translocation of aquaporin water channels in plasma membrane of collecting duct cells (increased permeability)… reabsorption of water into blood

Question 11

Why does drinking alcohol increase urination and lead to dehydration?

Answer 11

inhibits ADH release from posterior pituitary

Question 12

How is blood osmolality regulated?

Answer 12

OsmoRs in hypothalamus detect changes in plasma osmolality…

1. If high: stimulates ADH release from posterior pituitary (and stimulates thirst)… increased H2O reabsorption from collecting ducts
2. If low: inhibits ADH release from posterior pituitary (and inhibits thirst)… decreased H2O reabsorption from collecting ducts

Question 13

How is hypothalamus control of anterior pit. different to control of posterior pit.?

Answer 13

• posterior pit.: neuronal control

- anterior pit.: tropic hormone control

Question 14

Where are hypothalamus hormones destined for AP stored?

Answer 14

median eminence just above AP

Question 15

What are hormones stored in median eminence released into?

Answer 15

hypophyseal portal system

Question 16

What type of R do hypothalamic tropic hormones bind to in the AP?

Answer 16

hormone-specific GPCRs

Question 17

Name the 7 hypothalamic tropic hormones.

Answer 17

1. thyrotropin-releasing hormone (TRH)
2. corticotropin-releasing hormone (CRH)
3. gonadotropin-releasing hormone (GnRH)
4. prolactin-releasing hormone (PRH)
5. prolactin release-inhibiting hormone (PIH)
6. growth hormone-releasing hormone (GHRH)
7. growth hormone inhibitory hormone (GHIH) / somatostatin

Question 18

Describe the downstream effects of TRH release.

Answer 18

i. TRH binds R on AP thyrotropes…
ii. release of TSH (thyroid stimulating hormone)…
iii. binds R on thyroid follicular cells…
iv. T3 and T4 release

Question 19

Describe the downstream effects of CRH release.

Answer 19

i. CRH binds R on AP corticotropes…
ii. release of ACTH (adrenocorticotropin hormone)…
iii. binds R on adrenal cortex cells…
iv. glucorticoid (mainly cortisol) release

Question 20

Describe the downstream effects of GnRH release.

Answer 20

i. GnRH binds R on AP gonadotropes…
ii. release of LH (luteinising hormone) and FSH (follicle stimulating hormone)…
iii. bind Rs on gonad cells…
iv. LH: stimulates progesterone and oestrogen production and ovulation (females) or testosterone production (males)
FSH: stimulates gamete production (eggs or sperm)

Question 21

What system regulates hormone release in the HPA?

Answer 21

negative feedback loops:

1. ultra-short loop: hypothalamic tropic hormone limits own production in autocrine/paracrine fashion within hypothalamus
2. short loop: AP hormone inhibits production of hypothalamic releasing hormone or stimulates production of hypothalamic inhibiting hormone
3. direct long loop: peripheral effector hormone inhibits production of AP hormone
   indirect long loop: peripheral effector hormone inhibits production of hypothalamic tropic hormone