ANAT 322 Lecture Notes - Lecture 7: Cyclic Adenosine Monophosphate, Antral Follicle, Transmembrane Protein
7 Jun 2018
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ANAT 322 Winter 2016
Lectures
Lecture 7:
→ Neuroendocrine Control of Reproduction: Gonadotropin Regulation
4. In terms of negative feedback from rodent models, we do not have a clear answer because two
groups did the same experiments and got different results.
The first results suggest no negative feedback at the pituitary level. They compared mice with three
different genotypes; the wild-type, mice that lack the estrogen receptor everywhere (Global ERKO) and
they made a new model where they selectively knock out the ERα only in the gonadotrope cells of the
pituitary (Gonadotrope ERKO). They are looking at serum LH levels and we see that there is an increase
in LH in the global knock out which reflects a loss of negative feedback. If we remove the ovary, we
remove estrogen feedback, we have increase GnRH pulse generator activity, more GnRH activity and
more LH secretion. This is akin to removing the ligand but in this case instead of removing estrogen we
are removing the receptor. These animals have high estrogen levels but are not sensitive to it. This
experiment does not tell us whether the loss of feedback is at the pituitary level or the brain which is
why they make the other group of mice. The third group only had knock out of the receptor at the level
of the pituitary and the levels of LH do not go up. Based on these results we would conclude that
negative feedback is mainly at the brain level.
A second group of researchers did basically the same experiment and got different results. If we look
at the bottom panel, they compare wild-type to animals that lack the estrogen receptor in the
gonadotrope cells. They discovered that they have to populations of mice, some mice were completely
infertile and a second group with same genotype produced a half the number of offspring as the wild
type (shown in the upper right panel). When they looked at LH animals, the infertile group of mice had
levels similar to that of the global knockout so loss of negative feedback. The other group that were
subfertile were somewhere in between.
There is still no clear or definitive conclusion on whether there is negative feedback at the level fo the
pituitary.
5. The data is clear with positive feedback and there are positive feedbacks at the level of the pituitary.
The second group of researchers also looked at positive feedback and they look at animals lacking the
estrogen receptor at the gonadotropes in the pituitary. A wild-type female was tracked for several days
in a row and they looked at cells through a vaginal swab and looked at what stage of the estrous cycle
the animal is. Remember that in the afternoon of proestrus there is an LH surge driven by estrogen
positive feedback. The black line with the circles shows the estrous cycle whereas the line with the stars
shows the LH levels and in this particular animal there seems to be a surge every 5 days. When they
looked at animals that lacked the estrogen receptors, all LH surges were gone (period of 66 days). On
day three there is a blip but it is not real a LH surge but a pulse. So we need estrogen feedback at the
level of the pituitary for positive feedback.
6. Individuals with this disorder will have hypogonadotropic hypogonadism and this shows a control
fetus and a fetus that had a mutation in the KAL 1 gene. In the control, at 24 weeks of gestation, GnRH
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ANAT 322 Winter 2016
Lectures
euos ae alead at the edial asal hpothalaus. I Kalla’s patiets, the ells fail to igate
into the brain and are thus absent in the hypothalamus making the individuals GnRH deficient.
7. If you give pulsatile GnRH to these individuals, you can actually fully restore their fertility.
Women with the syndrome that has HH and was given pulsatile GnRH over the period of a month,
she was pushed into a normal menstrual cycle. She gets 12 pulses per day and we can look at her
gonadotropin levels in which the full circles are LH and the empty circles are FSH (upper panel). The
open symbols are estradiol and the closed ones are progesterone in the lower panel. As soon as she is
being given exogenous LHRH, you can see big picks in LH and FSH secretion. This reflects the fact that
the pituitary has not seen GnRH before and so it is really sensitive to it so we get an exaggerated
response that normalizes after. The increase in the gonadotropin secretion form the pituitary is
sufficient to trigger follicle development, once a dominant follicle is secreting large amounts of estrogen
we exceed a threshold which triggers an LH surge. We know that ovulation then occurs because we see
elevated progesterone in the second half of the treatment. Progesterone is a product of the corpus
luteum formed after a follicle has ovulated.
This tells us that estrogens also have positive feedback effects in the human pituitary because she
does not have GnRH coming from the brain. The GnRH stimulus is invariant and the reason we get a
surge is because the high levels of estrogen change the sensitivity of the pituitary. The only explanation
for this is that pituitary is more sensitive to GnRH and a change in the activity of the brain is not needed
and the pituitary is sufficient.
8. There is a positive feedback at the pituitary level and at the brain but the jury is still out on whether
there is negative feedback effects at the pituitary level.
10. Members of this family are glycoprotein hormones, they are both dimeric proteins that have two
subunits and they share the α subunit. The hormone specific β subunits confer the unique biological
activity to either LH or FSH.
CGA and LHβ are products of two different genes and whether LH reaches the ovaries or the testis, it
stimulates androgen production (androstenedione and testosterone, respectively).
FSH because it has a different β subunit, it has different behaviors in the circulation and it has a
longer half-life which is why we do not compare it or make it analogous to GnRH secretion. Granulosa
cells directly surround the ovary and Sertoli cells are the support cells for the matured sperm.
12. They are made by the same cell type, gonadotropins, and each cell that makes LH also makes FSH so
when GnRH reaches the cell, there is the potential for both hormones to come out.
We need to separately control the synthesis and secretion of these two hormones and it is the β
subunit that forms the foundation of the differential regulation.
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13. Experiment to emphasize that the pituitary want to see a pulsatile release in which the monkeys had
pituitary lesions and are hooked to a pump that sends pulsatile GnRH and the pituitary secretes both but
if give continuous GnRH then there is downregulation of the system.
The rate of pulsatile secretion makes a difference so the pituitary responds differently depending on
how frequently the pulses come in.
14. First we do one pulse per hour which is fast and in response the pituitary secretes more LH than FSH.
If we slow down the rate of the pulses or the pulse generator for one pulse every three hours then it is
the opposite. Therefore, at high GnRH pulses (high frequency) the pituitary prefers to secrete LH and
vice versa.
It is still not clearly known how the gonadotropes decode frequency of the pulse generator.
15. GnRH pulse frequency changes over the menstrual cycle and we can use LH pulse frequency as a
surrogate of GnRH frequency.
If we look early in the follicular phase we have about one pulse every 75 min, then the pulse
generator starts to speed up and at the late follicular phase it becomes 1 pulse every 50 min right before
the LH surge which reflects positive feedback. At the mid luteal phase the pulse generator slows down at
one pulse every 6.7 hours so pulses are very infrequent and this is mainly because of the strong negative
feedback of both progesterone and estradiol.
16. The pituitary sensitivity to pulse frequency is not just at the level of the secretion but also at the
production of the subunits of the hormone.
Experiment where rat pituitary cells are placed in culture and they are treated with GnRH at different
pulse frequencies. Then they looked at the gene expression or mRNA levels for the subunits and at
higher pulse frequency there is high levels of LHβ production but if the pulse frequency is slow down
then you have higher levels of FSHβ production. This is reflected of what is seen at the level of secretion.
17. One pulse per hour favours LH over FSH but then why does FSH goes up if it is a fast pulse
frequency?
If we have 1 pulse every 75 minutes then that is kind of one pulse per hour which is nearly optimal for
LH. If we measure the total end values at the end of the luteal phase versus the beginning of the
follicular phase, in fact, there is an increase in the amount of LH but we do not notice it because there is
a large increase in FSH.
Pulse feue is ipotat ut it a’t eplai the phsiolog. Thee is a seod hooal
regulatory system which is important for the selective regulation of FSH.
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Document Summary
Neuroendocrine control of reproduction: gonadotropin regulation: in terms of negative feedback from rodent models, we do not have a clear answer because two groups did the same experiments and got different results. The first results suggest no negative feedback at the pituitary level. They are looking at serum lh levels and we see that there is an increase in lh in the global knock out which reflects a loss of negative feedback. If we remove the ovary, we remove estrogen feedback, we have increase gnrh pulse generator activity, more gnrh activity and more lh secretion. This is akin to removing the ligand but in this case instead of removing estrogen we are removing the receptor. These animals have high estrogen levels but are not sensitive to it. This experiment does not tell us whether the loss of feedback is at the pituitary level or the brain which is why they make the other group of mice.
