ANAT 322 Lecture Notes - Lecture 8: Epididymis, Fetus, Estradiol Benzoate
7 Jun 2018
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ANAT 322 Winter 2017
Lectures
Lecture 8:
→ Neuroendocrine Control of Reproduction: Male Reproductive Axis
. Positie feedak is uiue to feales so they a hae LH suges hile ales a’t.
→ Regulation of Hypothalamic (GnRH)/Pituitary (LH/FSH) Function in Males
3. At the gonadal level we are talking about testis and the main action of the gonadotropins is to
regulate steroidogenesis. Now instead of follicle development, they will regulate spermatogenesis. The
main steroid that will come out of the testis is the androgen testosterone.
Unlike the case in females where we have Inhibin A and B, males only produce one Inhibin which is B
in the case of humans and rodents. In other species such as sheep, they make only Inhibin A.
Testosterone main negative feedback effects are at the level of the hypothalamus regulating GnRH
secretion whereas Inhibin B will have its effects in the pituitary on FSH but not LH (selective regulation).
4. GnRH is absolute critical for male reproductive physiology just as it is in females.
There are ways we can treat someone like this but nothing endogenous or natural can compensate
for the absence of GnRH and this is true in male rodents as well.
5. Hypogonadal mice have a large deletion within the GnRH gene and they cannot make the GnRH
decapeptide, or the preproprotein. They have small testes and a smaller seminal vesicles whose size is
directly proportional to how much testosterone is in the system (androgen-sensitive).
6. As in women, both GnRH and LH are released in a pulsatile fashion but it turns out that so is
testosterone. Here they are showing two examples of two male sheep and the GnRH pulses are
measured at the level of the medial eminence or at the pituitary portal vessels through cannulation.
Circulating levels of LH and testosterone are measured and we can see discrete pulses of all of these
three. The pulses of LH are of larger duration because its half-life is about 20 minutes whereas GnRH
half-life is of 10 minutes.
In response to an LH surge, the testis will produce a pulse of testosterone so there is a nice 1:1
correspondence.
7. As with females, there is potent steroid negative feedback on the GnRH pulse generator and here we
are looking at two castrated male sheep. Testosterone cannot be measured because the testis are
removed but GnRH secretion can be measured and they correlate. There are rapid GnRH pulsitivity
because feedback is removed when the source of testosterone is released. If testosterone is given back
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ANAT 322 Winter 2017
Lectures
to these castrated animals, we get negative feedback and we slow down the pulse generator of both
GnRH and LH.
8. As in females, kisspeptinergic system in the arcuate nucleus plays an important role in pulse
generation and mediation of negative feedback.
Here we are looking at LH levels in five different groups of mice so in the first case we have intact
males that were given a placebo injection and we can see the LH levels. If males are castrated and we
give them that vehicle there is anticipated increase in LH because we have removed negative feedback
and we increase the pulse generator.
The other three groups of animals were injected with increasing concentrations of the peptide 234
which is the kisspeptin receptor antagonist which blocks the increase in LH secretion. The increase in
pulse frequency and LH depends on kisspeptin.
9. Here we can see in-situ hybridization that shows mRNA for kisspeptin expression in the arcuate
nucleus. If we look at the left panel showing the arcuate nucleus divided by the third ventricle of intact
males, we can see dots that are clusters of silver grains (radioactive signal and decay that light up). In
the castrated male there is a large increase in the intensity of the signal that reflects loss of negative
feedback and increase kisspeptin expression within the arcuate but giving testosterone we have the
classical negative feedback again.
Androgens, just like estrogens, have negative feedback effects of kisspeptin within the arcuate
nucleus.
10. However, testosterone is called a prohormone because it has effects on its own but most of its
effects are mediated by its metabolites so it is a precursor form converted to an active form.
Testosterone in the ovary or in the female can be converted into estradiol via aromatase enzyme and
this is true in males as well. Androgens can also be converted into more biological active androgens
called dihydrotestosterone (DHT) by the activity of the 5α-reductases enzymes (1 and 2). DHT is the one
that mediates the androgenic effects of testosterone whereas estradiol mediates the estrogenic effects
of testosterone.
If you give testosterone to a person you do not know if the effects are due to DHT or E2 or both
metabolites which is relevant to the double-labelling experiment shown at the lower panel. The red cells
are kisspeptin expressing cells and the silver grains show kisspeptin cells that express the ERα or the
androgen receptor (AR) and these cells express both receptors so we cannot really tell if it is the
estrogenic or the androgenic metabolite that mediate the effect of testosterone.
You can compare intact animals to castrate (increased kisspeptin expression) and now we give back
to the animal DHT which cannot be metabolized into estrogen or estradiol. We can see that estradiol is
more potent at inhibiting kisspeptin expression in the arcuate nucleus of the male than DHT. This
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ANAT 322 Winter 2017
Lectures
suggests that in the male, testosterone effects on suppressing kisspeptin expression in the arcuate is
principally mediated by estrogens.
11. As in females, we have a population of kisspeptin neurons in the arcuate nucleus that stimulates
GnRH secretion which will stimulate LH pulses that will stimulate testosterone synthesis and secretion in
a pulsatile manner in the testis. Testosterone feedbacks onto the kisspeptin population in the arcuate
nucleus via estrogens.
→ Gonadotropin Regulation of Male Reproductive Physiology
13. Both LH and FSH are dimeric proteins so they share an α subunit they share and a β that differs so
the best way to selectively knock out one or the other is to target the β subunit.
These animals have similar but not identical phenotypes. If we look at the LH knock out, we can see
that the testis are smaller in this animal and the more abundant cell in the testis is sperm at different
stages of development so smaller testis means impaired spermatogenesis. In these animals there are no
mature sperm so LH is required for sperm production. Circulating or plasma testosterone and the
amount in the testis, these are very low so these males are infertile. The FSH knock out also have small
testis relative to wild-type but they have low sperm count but they do not have the absence of sperm.
Testosterone levels in the FSH deficient mice is normal therefore LH alone regulates testosterone
production.
Azoospermia means that there is no sperm during ejaculation (LH knock out) and oligospermia means
slow sperm count (FSH knock out). LH is required for spermatogenesis but FSH is important for
quantitative normal spermatogenesis.
14. The situation in humans is similar but not identical so in the case of LH deficiency it is the same but
not in FSH. It is uncommon to find mutations in genes that inhibit reproduction because if you do not
reproduce you do not pass genes to other.
Sexual infantilism and azoospermia (no sperm when ejaculating) and has a lot of characteristics
associated with pre-pubescent individuals but cannot be tell as easily as with GnRH deficiency because
the person is tall, has a mass, but he has a high voice and a small penis which is indicative of
testosterone or androgen deficiency.
Biochemical characteristics is that he has low testosterone, high FSH and undetectable LH. Normally
we would do some tests in the clinic such as giving the individual an injection of GnRH and they saw
there was stimulation of FSH secretion so this tells us that the GnRH receptor is functional but there is
no LH secretion so something is specifically wrong with LH. In this particular case, there is a mutation
that prevented the dimerization of the subunits so the person cannot produce the LH dimer.
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Document Summary
Neuroendocrine control of reproduction: male reproductive axis (cid:1006). Positi(cid:448)e feed(cid:271)a(cid:272)k is u(cid:374)i(cid:395)ue to fe(cid:373)ales so they (cid:272)a(cid:374) ha(cid:448)e lh su(cid:396)ges (cid:449)hile (cid:373)ales (cid:272)a(cid:374)"t. Regulation of hypothalamic (gnrh)/pituitary (lh/fsh) function in males: at the gonadal level we are talking about testis and the main action of the gonadotropins is to regulate steroidogenesis. Now instead of follicle development, they will regulate spermatogenesis. The main steroid that will come out of the testis is the androgen testosterone. Unlike the case in females where we have inhibin a and b, males only produce one inhibin which is b in the case of humans and rodents. In other species such as sheep, they make only inhibin a. Here they are showing two examples of two male sheep and the gnrh pulses are measured at the level of the medial eminence or at the pituitary portal vessels through cannulation.
