central control of reproduction: the HPG axis

Question 1

how do sex steroid such as progesterone change the function of cells

Answer 1

changing of gene expression via nuclear receptors

Question 2

how do sex steroid such as testosterone travel in the blood

Answer 2

lipid soluble so travel bound to proteins in blood

Question 3

in order for a hormone such as GnRH to have a effect on a cell what do they require

Answer 3

to have receptors present in the correct cell type

Question 4

GnRH comes from:

Answer 4

hypothalamus

Question 5

FSH and LH come from

Answer 5

anterior pituitary

Question 6

oestrogen, progesterone, inhibin, ocytosin, relaxin and testosterone come from

Answer 6

ovary

Question 7

testosterone (and other androgens), inhibin and oestrogen come from

Answer 7

testis

Question 8

prostaglandin, progesterone, oestrogen, eCG and hCG come from

Answer 8

uterus/placenta

Question 9

gonadotrophins (FSH, LH, CG) differ by what structure

Answer 9

all share same alpha chain but biological activity is conferred by different beta chains

Question 10

gonadotrophins get into the cell via

Answer 10

G-protein linked receptors (water soluble)

Question 11

describe how steroids are synthesised

Answer 11

cascade of changes from precursor (cholesterol) via enzymatic conversions

Question 12

how are steroids metabolised

Answer 12

• need to stop functioning and be made solible
• once steroid has acted on target, goes to liver to become soluble and excreted

Question 13

what are prostaglandins synthesised from

Answer 13

arachidonic acid

Question 14

how do prostaglandins get into cells

Answer 14

bind to cell membrane G protein coupled receptors

Question 15

prostaglandins have what functions in reproduction

Answer 15

• ovulation
• partuition
• luteolysis

Question 16

outline the HPG axis

Answer 16

• hypothalamus releases GnRH in response to stimuli (positive feedback, nutrition/season/stress)
• GnRH acts on pituitary (via portal system)
• pituitary releases gonadotrophins (FSH/LH)
• FSH/LH act on gonads
• gonads release sex steroids to act on target organs
• negative or positive feedback loop ensues

Question 17

discuss hormone production in the testis

Answer 17

• leydig cells respond to LH to produce testosterone
• sertoli cells respond to FSH to produce inhibin

Question 18

is the HPT axis positive or negative feedback loop

Answer 18

negative

Question 19

where does GnRH production occur in the female

Answer 19

• surge (preovulatory) centre of hypothalamus
• tonic centre of hypothalamus (produces basal low levels of GnRH

Question 20

discuss GnRH levels in the female

Answer 20

basal secretion = numerous small pulses that are regular and brief initiated by tonic centre until stimulated by increased oestrogen
- large preovulatory surge leads to ovulation

Question 21

under what circumstances is the hypothalamic-pituitary-ovarian axis positive feedbakc loop

Answer 21

during the follicular phase
- approaching ovulation there is a critical switch
- rising E2 becomes positive leading to LH surge and ovulation
- inhibin specifically inhibits FSH

Question 22

in the luteal phase, which hormone dominates to exert negative feedback

Answer 22

progesterone

Question 23

how is the negative feedback loop removed

Answer 23

• in absence of fertilisation, the corpus luteum undergoes structural and functional regression
• luteolysis
• progesterone decreases
• removes negaitve feedback in preparation for rising oestradiol levels