Hypothalamic-Pituitary-Gonadal Axis

Question 1

How does the HPG Axis work?

Where does the negative feedback occur?

Answer 1

• Hypothalamus releases GnRH → Pituitary releases LH and FHS → Gonads release sex steroids (Oestrogen, Progesterone, Testosterone)
• Sex steroids negatively feedback to Hypothalamus and Pituitary

Question 2

Gonadotropin Releasing Hormone (GnRH):
How is it secreted from the hypothalamus? How does this affect the pituitary?

How does the rate of pulsatile secretion affect LH and FSH?

What occurs to LH and FSH when GnRH is secreted CONTINUOUSLY?

How can Synthetic GnRH be given to affect the HPG axis?

Answer 2

• Pulsatile secretion every 30-120 mins, which stimulates pulses of LH and FSH release
• FAST pulse favours LH, SLOW pulse favours FSH
• Huge drop in LH and FSH secretion
• • GnRH agonists given continuously to downregulate LH/FSH pituitary secretion
  • GnRH antagonists block the receptor without stimulating a response

Question 3

GONADATROPHINS:
What are the main ones?

How do these differ in structure?

In Ovary:
What does FSH cause?

Where are LH and FSH receptors found? On stimulation, what occurs?

How does Ovulation occur?

What stops the Corpus Luteum (CL) from dying off?

What does the Corpus Luteum (CL) produce in response to LH?

In Testes:
Where are LH and FSH receptors found? On stimulation, what occurs?

Answer 3

• LH, FSH, HCG
• Peptide hormones with the same α-subunit, but a different β-subunit
• Follicular maturation and growth
• • LH receptors on THECA cells = Androgen synthesis
  • FSH receptors on GRANULOSA cells = Oestrogen synthesis
• When Oestrogen is high for a few days, there’s a temporary switch to positive feedback, causing a huge rise in LH = Ovulation
• Rescued by hCG in pregnancy
• Progesterone
• • LH receptors on LEYDIG cells = Testosterone synthesis
  • FSH receptors on SERTOLI cells = Oestrogen synthesis