HPG axis

Question 1

List the releasing hormones produced by the hypothalamus…(not just reproductive)

Answer 1

GnRH - Gonadotropin releasing hormone
PRH - Prolactin releasing hormone
PIH - Prolactin Inhibiting hormone
Somatostatin 
GHRH - Growth Hormone releasing hormone
TRH -Thyrotropin releasing hormone
CRH - Corticotropin

These hormones act on the pituitary to stimulate release of pituitary hormones on glands and tissues throughout the body.

Question 2

List the roles of the hypothalamus-pituitary unit…

Answer 2

Regulates function of: thyroid, adrenal, reproductive glands

Controls: somatic growth, lactation, milk secretion and water metabolism

Question 3

List the hormones produced by the anterior pituitary gland

Answer 3

Prolactin
Growth hormone
TSH
ACTH
FSH
LH

Question 4

Describe the nature of GnRH release by the hypothalamus

Answer 4

Release is pulsatile - 1/hr average

Rapid pulse frequency favours LH secretion
Slower pulse frequency favours FSH release

Question 5

What would be the effect of a continuous GnRH release?

Answer 5

Inhibits gonadotrope function

…this is the basis of long acting exogenous GnRH agonists and antagonists

Question 6

Describe the effect of LH on the testes

Answer 6

Stimulates Leydig cells - as they only have LH receptors - to produce Testosterone

Question 7

Describe the function of Testosterone

Answer 7

Stimulates Sertoli cells to promote spermatogenesis
Negative feedback on GnRH and LH release
Maintains internal genitalia
Has metabolic and behavioural effects as well

Question 8

Describe the effects of FSH on the testes

Answer 8

Maintains Sertoli cells

Induces LH/hCG receptor formation in Sertoli cells

Question 9

Decribe the role of Sertoli cells

Answer 9

Control the environment milleu in seminiferous tubules and promote spermatogenesis
Produces Inhibin, which inhibits FSH (negative feedback as FSH stimulates Sertoli cells)

Question 10

Describe the role of Inhibin in the male

Answer 10

Stimulates LH release
Inhibits FSH release
Secretion correlates to rate of spermatogenesis

Question 11

Describe the effect of LH on the ovaries

Answer 11

Stimulates Theca interna cells to produce androgens (androstenedione)

Question 12

Describe the effect of FSH on the ovaries

Answer 12

Stimulates Granulosa cells to convert androgens into Oestrogen

Question 13

The G-protein mechanism that LH and FSH stimulate is…

Answer 13

Gs … Adenylyl cyclase … cAMP production …

FSH: cAMP stimulates Aromatase in Granulosa cells (which converts androgens into Oestrogen)

LH: cAMP stimulates Cholesterol uptake into Theca cells, which can then be converted into androgens in Theca cells, and eventually Oestrogen in Granulosa cells

Question 14

Outline the hormonal control during the Preovulatory phase

Answer 14

Follicle producing a lot of Oestrogen…positive feedback on GnRH, FSH and LH release.
LH surge stimulates Theca and Granulosa cells…high prod. of Oestrogen and Inhibin.

Question 15

Outline the hormonal control during the Luteal phase

Answer 15

The Corpus Luteum produces produces Oestrogen and Progesterone under LH stimulation

Presence of Progesterone stops the +ve feedback action of Oestrogen and enhances the -ve feedback
Therefore FSH levels drop - preventing further follicles from developing