GnRH

Question 1

Give a brief description of the history of GnRH.

Answer 1

• Master controller of reproduction.
• Characterised in 1971.
• R Guillemin and A Schally, Nobel prize in physiology and Medicine 1977.)

Question 2

Which chromosome is the gene for GnRH on?

Answer 2

Chromosome 8

Question 3

How many GnRH isoforms are there in vertebrates?

Answer 3

23 isoforms.

Question 4

Outline the neuroendocrine role of the HPG axis.

Answer 4

Paracrine - placenta/gonads
Autocrine - prostate/breast
Neurotransmitter - regions of the brain.

Question 5

Outline GnRH structure.

Answer 5

Preprohormone - cleaved and consecrated as mature GnRH (decapeptide) and GAP (GnRH associated protein.) May stimulate and increase increase prolactin secretion.

Question 6

Where do GnRH neurones cells originate from?

Answer 6

Embryonic period - Originate outside the CNS in the medial olfactory placode. (area of thickening of the embryonic epithelial later from which the organ/structure later develops)
Cells migrate - via cribiform plate to olfactory bulb to medio-basal hypothalamus. (Numerous genes involved, e.g KAL1, FGFR1, FGF8, PROK2, PROKR2, CHD7, NELF.)

Question 7

Name the genes responsible for homeostasis and GnRH secretion.

Answer 7

DAX1, PC1, LEPR, LEP, KISS1R, FGFR1, PROKR2, PROK2, TACR3, TAC3, GNRH1.

Question 8

Outline an in vitro experiment to demonstrate GnRH neurone migration.

Answer 8

In vitro culture explants from fetal monkey placed.
Immunostained.
Migration of GnRH neurones out of explant.

Question 9

Name the syndrome associated with unsuccessful GnRH neurone migration.

Answer 9

Kallman’s syndrome. KAL1 gene. Premature termination of migration. Associated with anosmia (indicative of malformation of the olfactory bulb and tract.) Leads to hypogonadotrophic hypogonadism.

Question 10

Which system contains GnRH neurones.

Answer 10

Parvocellular system containing medial pre optic and arcuate nucleus.

Question 11

What is the importance of the hypothalamus in GnRH neurone production.

Answer 11

Hypothalamus has both neural and vascular connections within the pituitary. Magnocellular neurones arise from paraventricular nucleus and supraoptic nucleus. These project to the posterior pituitary. Oxytocin and ADH made here and stored in Post Pit. Hypothal and ant pit - hypophyseal portal circulation.

Question 12

Outline GnRH release.

Answer 12

• Processed and packaged into storage granules.
• Transported down axons to the external zone of the median eminence.
• GnRH released in synchonised pulses from GnRH neurone endings into hypophyseal portal system.
• Circhoral pulses (every 30-120m)

Question 13

What is the GnRH Pulse Generator

Answer 13

Collection of hypothalamic neurones producing endogenous secretory rhythms. Occurs in the mediobasal hypothalamus.

Question 14

What is the half life of GnRH?

Answer 14

2-4 minutes.

Question 15

What is the function of GnRH?

Answer 15

Stimulates synthesis and secretion of gonadotrophins.

Question 16

Between which 2 hormones is there a close temporal relationship?

Answer 16

GnRH and LH. Been found in monkeys, ewes and humans.

Question 17

Describe the structure of the GnRHR.

Answer 17

• GPCR.
• 7 transmembrane domain structure.
• No carboxyl terminal chain.
• Type 1 - full length
• Type 2 - missense truncation.
• Resistance to desensitisation.
• Expressed on gonadotroph cells of AP.

Question 18

Why are there GNRHRs in different tissues, e.g, breast, ovarian, placenta.

Answer 18

It is thought that in evolutionary terms maybe there was a direct regulation of the gonads as an early function and then the neuroendocrinological role in regulating the pituitary came as a later evolutionary development.

Question 19

Name the different tumour cell lines of GnRHR?

Answer 19

Breast tumor cell lines (MCF-7 and MDA-MB 468) and prostate tumor cell lines (PC-3 and LNCaP).

Question 20

Describe how GnRH pulsatility regulates gonadotrophin production.

Answer 20

• Slow frequency/low amplitude GnRH (120m) = FSHB gene expression.
• Fast frequency (30m) = LHB transcription.
• Wildt L et al 1981.
• Determines dimerisation and glycosylation of subunits.
  JC Marshall on rats, proved glycosylation was needed for hormones to work.

Question 21

Describe how GnRH pulsatility varies over the menstrual cycle.

Answer 21

(1) Early Follicular phase – pulses slow (every 90-120mins) »FSH
(2) Mid-late F phase – pulse freq increases (every hr.) »LH
(3) After ovulation – pulses slow (every 3-5h) »FSH production
(4) End of luteal phase – increase in GnRH pulse secretion » FSH release
- Marshall JC 1991.

Question 22

Outline the feedback in males.

Answer 22

In MALES:
LH stimulates Leydig cells to produce Testosterone
FSH stimulates Sertoli cells to produce inhibin B, activin, ABP & transferrin & spermatogenesis
Testosterone from Leydig cells reduces LH secretion (Rhesus monkey experiments)
Mainly Inhibin that↓FSH secretion from pituitary
Activin ↑FSH secretion from pituitary

Question 23

Where are the speculated sites of steroid feedback?

Answer 23

• Either anterior pituitary or hypothalamus
• Oestrogen & progesterone receptors on anterior pituitary and hypothalamus
• Inhibin receptors found only in pituitary

Question 24

How does feedback of steroids occur?

Answer 24

• In anterior pituitary by direct regulation of GnRHR
• In hypothalamus by directly affecting GnRH neurones or indirectly by changing activity of other neural system that influence GnRH release

Question 25

Outline the mechanisms of feedback on the hypothalamus.

Answer 25

GnRH neurones only express ERβ
Need ERα for positive feedback
Erα is crucial for reproductive function
Erα knock-out mice
E2 must act on other afferents that project onto GnRH neurones

E2 indirectly stimulates GnRH neurones via other neuronal inputs:
Kisspeptin
Other hypothalamic neurotransmitters- e.g., GABAnergic neurons