Hypo-pituitary Ovarian Axis

Question 1

Describe the basic concept of the HPOA

Answer 1

The effects of the hypothalamus, pituitary gland, and ovaries as a whole

Question 2

What is the hypo-pituitary?

Answer 2

The combination of hypothalamus and the pituitary gland.

Question 3

Describe the relationship between the anterior pituitary and the hypothalamus

Answer 3

Neurons exist in hypothalamus that link up with blood vessels in the pituitary stalk. These supply blood to the pituitary.

When appropriately stimulated, hypothalamic neurons secrete releasing and inhibiting hormones into the blood vessels.

These hormones travel through the anterior pituitary where they stimulate or inhibit the release of hormones from the gland.

Question 4

Describe the relationship between the anterior pituitary and the hypothalamus

Answer 4

Neurons exist in hypothalamus that link up with blood vessels in the pituitary stalk. These supply blood to the pituitary.

When appropriately stimulated, hypothalamic neurons secrete releasing and inhibiting hormones into the blood vessels.

These hormones travel through the anterior pituitary where they stimulate or inhibit the release of hormones from the gland.

This happens by GnRH binding to GnRH receptors on the cells (gonadotrophs) in the anterior pituitary which then secrete LH/FSH

Question 5

What does GnRH control?

Answer 5

Release of LH and FSH –> Control follicle development and steroid production

Question 6

What does TRH control?

Answer 6

Release of TSH –> Control thyroid function

Question 7

What does CRH control?

Answer 7

Release of ACTH from adrenals

Question 8

What does GHRH control?

Answer 8

Release of GH which controls growth

Question 9

List the main pituitary hormones with reproductive effect

Answer 9

Gonadotrophins: LH and FSH - direct actions on ovaries

GH: Indirect effect

Prolactin: Direct effect on ovarian function

Question 10

Describe the structure and half life of gonadotrophins.

Answer 10

LH and FSH have a common alpha-subunit and a different beta-subunit.

LH = alpha + beta (LH)
FSH = alpha + beta (FSH)

LH half life: 30 mins
FSH half life: 2 hours

Difference in half life can be explained by the different amounts of glycosylation. FSH is more heavily glyosylated and therefore cannot be cleared so quickly.

This is very important in ART because if equal amounts of LH and FSH are injected into the body, the FSH will remain for much longer.

Question 11

What is a half life?

Answer 11

The amount of time it takes for the concentration of a substance to half in the blood

Question 12

What is glycosylation?

Answer 12

Sugar residues on a molecule

Question 13

What is a gonadotroph?

Answer 13

Cells in the anterior pituitary that secrete gonadotropins (FSH and LH)

Question 14

How do we know that some gonadotrophs secrete LH, some FSH, and some secrete both?

Answer 14

Fluorescent antibodies are used to bind to the beta-subunit of the LH and FSH. We can then visualise which gonadotrophs have both of these hormones, which only have LH, and which only have FSH.

Question 15

Describe the role of secretory granules in LH secretion

Answer 15

Gonadotrophs carry LH within secretory granules. When the cell is stimulated by GnRH, the granules move towards the membrane of the cell and release the appropriate amount of LH into the bloody (in a 1:1 manner with GnRH)

Question 16

Describe what pusatile 1:1 induction of LH refers to.

Answer 16

An experiment: portal vein cannulation model. This works by taking blood from between the hypothalamus and pituitary. This can be assayed for GnRH and LH. This shows a 1:1 relationship between the two hormones.

GnRH neurons in the hypothalamus secrete GnRH hormones in a pulsatile fashion to secrete LH from the pituitary,

Question 17

Describe the two forms of GnRH release from the hypothalamus.

Answer 17

There are two different ways that GnRH secretion is controlled.

One is the usual pulsatile release where small amounts of GnRH lead to small amounts of LH but there is also a surge release around the time of ovulation where a very large amount of LH is required.

Pulse generator;
Responsive to endocrine, metabolic, immune, and behavioural cues. This is the usual way how small pulses of GnRH modulate small pulses of LH into the circulatory system.

Surge generator;
Responsive to ovulatory oesetradiol increase. This causes a massive release of GnRH and a massive release of LH into the circulation. This LH surge causes ovulation and final maturation of the oocyte and is in turn caused by a surge of oestrodial and GnRH.

Question 18

Describe the phenomenon of “downregulation of the receptor” with regard to GnRH/LH/FSH.

Answer 18

In an experiment, GnRH was changed from a pulsatile to a continuous stimulation. This led to a significant decline in LH and FSH levels.

This phenomenon is called “downregulation of the receptor”.

It seems to be related to the way the GnRH receptors are processed.

GnRH receptors - as with gonadatrophin receptors - are G-protein coupled receptors.
They are characterised by receptors that pass through the plasma membrane 7 times and a long tail within the cellular matrix which interacts with the G-proteins which in turn interact with several intra-cellular signalling molecules.
When GnRH binds to its receptor, it is thought to be internalised into the cell cytoplasm by vesicles in the cell. This may then lead to the GnRH being stripped off the receptor and the receptor being recycled to the cell surface OR the receptor and GnRH complex are lysed and destroyed in the cell.
Therefore, if there was a very high level of GnRH binding to the cell, the number of receptors on the surface would be lower and the cell will become unresponsive to GnRH.
In ART, this process is exploited by giving patients potent GnRH agonists that cause an initial sustained stimulation which will eventually down-regulate and desensitise GnRH receptors with a consequent decrease in hormone levels.

Question 19

Describe the phenomenon of “downregulation of the receptor” with regard to GnRH/LH/FSH.

Answer 19

In an experiment, GnRH was changed from a pulsatile to a continuous stimulation. This led to a significant decline in LH and FSH levels.

This phenomenon is called “downregulation of the receptor”.

It seems to be related to the way the GnRH receptors are processed.

GnRH receptors - as with gonadatrophin receptors - are G-protein coupled receptors.
They are characterised by receptors that pass through the plasma membrane 7 times and a long tail within the cellular matrix which interacts with the G-proteins which in turn interact with several intra-cellular signalling molecules.
When GnRH binds to its receptor, it is thought to be internalised into the cell cytoplasm by vesicles in the cell. This may then lead to the GnRH being stripped off the receptor and the receptor being recycled to the cell surface OR the receptor and GnRH complex are lysed and destroyed in the cell.
Therefore, if there was a very high level of GnRH binding to the cell, the number of receptors on the surface would be lower and the cell will become unresponsive to GnRH.
In ART, this process is exploited by giving patients potent GnRH agonists that cause an initial sustained stimulation which will eventually down-regulate and desensitise GnRH receptors with a consequent decrease in hormone levels.

GnRH antagonists, on the other hand, block receptors which immediately stops LH secretion.

Question 20

Compare the secretory pathways of FSH and LH

Answer 20

FSH is released constitutively in response to a steady stimulation of GnRH.

LH is highly regulated and secreted in a pulsatile fashion in response to a pulsatile stimulation of GnRH.

Question 21

Role of FSH

Answer 21

FSH role: Stimulation of follicle development.

It does this in two ways

1. Proliferation
2. Differentiation (stimulation of transformation of cells to differentiated status)

It does this by inducing proliferation at low doses and differentiation and high doses.

FSH receptors are only found on granulosa cells of ovarian follicles.

Question 22

Role of LH

Answer 22

LH role:

• Stimulation of steroid secretion from follicle/CL
• Pre-ovulatory surge (ovulation)

LH receptors are on theca cells.
They are also on the granulosa cells of oestrogenic pre-ovulatory follicles. This is very important as it allows the selection of just one ovulatory follicle in our species.

Question 23

Name the hormones released from the ovary

Answer 23

Inhibin (protein)

• Inhibin B
• Inhibin A

Steroid hormones

• Progesterone
• Androgens
• Oestrogens

Question 24

Hormones released from the ovary

Where are inhibin A and B released from?

Answer 24

• Inhibin B (granulosa cells in small follicles)

- Inhibin A (granulosa cells in large follicles - luteal cells in primates)

Question 25

Hormones released from the ovary

Where is progesterone released from?

Answer 25

• Progesterone (granulosa, theca, luteal cells)

Question 26

Hormones released from the ovary

Where are androgens released from?

Answer 26

Androgens (theca cells)

Question 27

Hormones released from the ovary

Where are oestrogens released from?

Answer 27

Oestrogens (granulosa cells follicles - luteal cells primates)

Question 28

Describe steroidogenesis from cholesterol to oestrogen

Answer 28

Cholesterol

• Side-chain cleavage -

Progesterone

• 17alpha-hydroxylase -

Androgen

• Aromatase -

Oesterogen

Question 29

Describe the localisation of steroidogenesis

Answer 29

Localisation of steroidogenesis occurs mainly in mitochondria and smooth ER.
The substrate for most steroid production is cholesterol that is circulated in the peripheral blood. However cholesterol can also be directly synthesised by ovarian somatic cells from acetate.
The cholesterol is transferred into the mitochondria under the influence of an enzyme called StAR. Within the mitochondria, the first stage of steroidogenesis occurs with the conversion of cholesterol to pregnenolone. This is then converted to progesterone in the smooth ER which can be converted to androgens and then to oestrogens.

Question 30

Androgens are potentially transformed to oesterogens as it depends on the cell type. Explain.

Answer 30

Two-cell, Two-gonadotrophin hypothesis

The theca cell (under the influence of LH) produces androgens from cholesterol BUT lacks the aromatase enzyme that would convert the androgen to oestrodial. The androgen is then transferred across the basement membrane of follicle to the granulosa cell where it is converted to oestrodial (under the influence of FSH).

Question 31

Describe what inhibin is and its structure in relation to activin

Answer 31

Inhibin is an ovarian protein that inhibits FSH production.
Activin is a closely related protein which enhances FSH synthesis
They are both dimers
Activin has two beta subunits
Inhibin has a beta subunit and an alpha subunit

Inhibin A = Alpha subunit + Beta A subunit
Inhibin B = Alpha subunit + Beta B subunit

Question 32

Describe what inhibin is and its structure in relation to activin

Answer 32

Inhibin is an ovarian protein that inhibits FSH production.
Activin is a closely related protein which enhances FSH synthesis
They are both dimers
Activin has two beta subunits
Inhibin has a beta subunit and an alpha subunit

Inhibin A = Alpha subunit + Beta A subunit
Inhibin B = Alpha subunit + Beta B subunit

Activin A: Beta A + Beta A
Activin B: Beta B + Beta B
Acitivin AB: Beta A + Beta B

Question 33

Describe the FSH feedback relationship

Answer 33

Synergistic negative feedback acting at the level of the piuitary modulated by oestradiol and inhibin A

When we have the secretion of either oestrodial or inhibin from the ovary, we have a negative effect on FSH release from the pituitary gland.
If we have an ovary with lots of follicles secreting lots of oestrodial and inhibin, secretion of FSH will be inhibited, and follicle development will be inhibited in the ovary.
Conversely, if the ovary contains very few ovarian follicles, the pituitary will increase the level of FSH secretion and lead to a stimulation of ovarian follicle development.

Question 34

The FSH negative feedback mechanism is known as “synergistic” - explain why.

Answer 34

Synergistic: Oestrodial and inhibin act together to control FSH release

If the ovaries of an organism are removed, this means there is low amounts of E2 and inhibin being produced which causes the pituitary gland to overproduce FSH.

In a study (Mann et al, 1991), ovarectomised animals had their FSH levels tested without any artificial inh or E2, +inh, and +E2. The results showed that FSH levels were lower with the artificial inh and E2 but the levels were only at a steady low (comparable to the control) when both E2 and inh were used in combination.

Question 35

Describe how passive immunisation against E2 and Inh can affect FSH production.

Answer 35

Passive immunisation
If you passively immunise against inhibin and E2, you get the same results of FSH as if you had ovarectomised someone. However if you just immunise against E2 OR Inh, you still get FSH production - just on a lower scale. This further emphasises the importance of BOTH Inh and E2 in FSH production.
However you only get the same results as an ovarectomised patient for around 12 hours. After that, the FSH levels return to a similar level as the control.

The way this happens is as follows;
When a patient is immunised against E2 and Inh, the feedback dial is blocked so lots of FSH is produced which leads to lots of follicle development in the ovary so the ovary produces lots of E2 and Inh. This can eventually override the immunisation and bring FSH back under control.

This shows how tightly controlled the FSH system is in monogatry species because they must only release one oocyte per cycle. Too much FSH will lead to multiple ovulation which will increase the chances of multiple pregnancies (dangerous).

Question 36

Describe how oestrodial and progesterone affect the release of LH in both a pulsatile and surge manner.

Answer 36

There is a synergistic negative feedback system mediated by oestrodial and progesterone.
Oestrodial works in a negative fashion in terms of the control of pulsatile LH release.

High levels of oestrodial in periphary circulation, we have a negative inhibitory effect on LH pulsatile release initially. Then this will switch to a positive surge effect when oestrodial levels reach a certain threshold in the peripheral circulation.

In contract, progesterone can be thought of as an antagonist of oestrodial as it has a negative effect on both pulsatile and surge release of LH.

Question 37

Feedback in different stages of menstrual cycle

Answer 37

In the follicular phase, the ovary contains some small follicles but mainly a large preovulatory sized follicle secreting large amounts of oestrodial and large amounts of inhibin but no corpus luteum secreting progesterone. Therefore it is mainly the actions of the single large preovulatory follicle that is causing the release of LH and FSH.

In contrast, in the luteal phase, we have a large corpus luteum secreting large amounts of oestrodial, progesterone, and inhibin. This leads to a large negative feedback situation where FSH and LH secretion is low and there is a suppression of follicle development.