Reproduction

Question 1

Which hormones are responsible for the female reproductive cycle? What happens to them?

Answer 1

1. Estradiol: goes high during proestrus (before progesterone)
2. Progesterone: goes high during estrus

Question 2

Where do Gonadotropin-releasing hormones (GnRH) project

Answer 2

1. Median eminence (terminating on portal vein)
2. Hypothalamic regions + cortex (necessary for feedback mechanisms)

The critical component of the HPG axis is the firing of GnRH neurons

GnRH I: projections to median eminence that are transported to pituitary to regulate luteinizing hormone (LH) and follicle stimulating hormone (FSH)

Question 3

What are the functions of LH and FSH in female system?

Answer 3

general function:
- Gonadotropins have two subunits
Alpha subunits are identical for LH, FSH, TSH, & hCG/ ß subunits confer biological activity
- Luteinizing Hormone (LH)
Essential for rupture of follicle and release of egg (ovulation) and maintenance of corpus luteum/ Responsible for androgen synthesis in theca cells (female) and Leydig cells (male)
- Follicle Stimulating Hormone (FSH)
Binds to receptors on granulosa cells to increase aromatase activity for E2 biosynthesis and follicular maturation/ Binds to receptors in Sertoli cells to drive sperm maturation

1. LH ruptures follicle and releases egg (ovulation)
2. FSH responsible for follicular maturation (sperm maturation in males)

Question 4

What kind of pattern of release does GnRH exhibit?

Answer 4

Pulsatile (once every 25 minutes)

• GnRH is released as a pulse approximately every 25 minutes from tissues cultures of MBH preparation or neuronal cells lines
• Intra-hypothalamic implants of these immortalized cells to GnRH mutant mice results in fertility

Question 5

What is a way to measure GnRH indirectly

Answer 5

Measure LH secretion (pulses are coupled)

Pulse of LH, as a surrogate measure of GnRH, is is associated with increase in multi-unit electrical activity in the mediobasal hypothalamus

Question 6

What experiment shows the importance of pulsing GnRH release?

Answer 6

Ovx monkeys, give GnRH at varying frequencies (pulsatile and continuous).

Continuous infusion of GnRH SUPRESSES LH and FSH.
One pulse per hr reinstated LH & FSH secretion.
Slowing pulse frequency suppressed LH and elevated FSH.

Question 7

What is responsible for the effect of GnRH pulse frequency on LH and FSH release?

Answer 7

GnRH-receptors are down-regulated either by continuous GnRH release or by diminished GnRH pulse frequency

Question 8

Describe negative feedback of GnRH. Role of GABAergic neurons?

Answer 8

1. Estradiol acts on pituitary (suppressing GnRH release) and hypothalamus (suppressing GnRH neuronal activity)
2. Within 1st 42 hours, E2 decreases autoinhibition of GABAergic neurons -> increase in GABAergic inhibition -> E2 negative feedback (leads to decrease in GnRH production)

Question 9

What is the importance of two separate estrogen receptors? What are they?

Answer 9

1. ER alpha and ER beta
2. Both are important for negative feedback (regulation of LH and GnRH)
3. In experiment, removing alpha and/or beta receptor INCREASED LH levels (b/c no feedback)

Question 10

Describe estradiol positive feedback

Answer 10

1. E2 decreases expression of GABA-producing genes
2. This reduces GABAergic inhibition on GnRH neurons

Ability of E2 to stimulate rather than suppress LH & FSH;
Basis for surge in LH & FSH at ovulation;
Dose & duration of E2 exposure are critical:
- Ovx monkeys received two sequential doses of E2, varying the duration of 2nd dose
- Negative feedback was initially evident followed in time by a surge in LH
Site of action:
- E2 upregulates GnRH-R
- Simultaneous measurements of LH and GnRH indicate that high levels of E2 overtime stimulate GnRH

Question 11

Describe action of E2 on kisspeptin expression (2)

Answer 11

1. E2 upregulates kiss expression in anteroventral periventricular nucleus (AVPV) > stimulates GnRH production > Positive feedback
2. E2 downregulates kiss expression in ARC > decreases GnRH production > negative feedback

from slide: effects of estradiol mediated by kisspeptin

• Kisspeptin induces cFos expression in GnRH neurons
• Kisspeptin receptors (GPR54) expressed in >60% of GnRH neurons
• Kisspeptin expression in arcuate nucleus and anteroventral periventricular nucleus (AVPV) that project to GnRH neurons

Question 12

Describe seasonal reproduction as a potential disruptor of the regular cycle

Answer 12

Most animals are seasonal breeders b/c melatonin slows GnRH production in the females April-August

Question 13

Describe evidence that stress affects reproduction (2)

Answer 13

1. In mice infused with corticotropin releasing factor (CRF/CRH), # of estrus cycles decreases and the length of the cycle increases
2. Ewes experience smaller LH oscillations when infused with cortisol (stress hormone) (cortisol + E2 are so strong they abolish the pusling of LH and therefore GnRH altogether)

Question 14

Describe evidence that social stress plays a role in female reproduction (2)

Answer 14

1. subordinates more sensitive to estradiol negative feedback
2. no status differences in positive feedback

Question 15

Describe evidence that caloric restriction affects female reproduction

Answer 15

LH pulse frequency much smaller in women with LOW caloric intake

Question 16

Describe evidence how cell’s ability to take up glucose affects female reproduction

Answer 16

2DG (glucose oxidation limiter) decreases plasma LH levels in rats and sheep

Question 17

kisspeptin as a mediator of metabolic signals

Answer 17

1. KiSS-1 neurons are responsive to decreased energy availability
2. Central kisspeptin administration restores HPG function during energy restriction
3. KiSS-1 neurons are targets of the adipocyte hormone, leptin
   - Leptin deficient mice – who are infertile – have reduced KiSS-1 expression in ARC and AVPV
   - Leptin increases KiSS-1 expression in these neurons

Question 18

Which estrogen receptor is important for sexual behavior (lordosis)? Experiment?

Answer 18

1. ER alpha

2. Mice with Era knockout or Era and Erb knockout exhibited less lordosis than WT and ERb (only) KO mice.

Question 19

What experiments show that CRH/CRF are important in sexual behavior?

Answer 19

1. Receptive behavior, lordosis lowered in mice injected with CRF
2. Time spent with males after sex decreases when mice are restrained (stressed)

Question 20

ovarian steroidogenesis: luteal phase and follicular phase

Answer 20

how much details?

Question 21

testicular steroidogenesis

Answer 21

• Leydig cells: respond to luteinizing hormone (LH) with steroid production (primarily testosterone)
• The production of androgens from cholesterol is identical to that in the adrenal and ovary, except that it continues from androstenedione to testosterone via 17ß-hydroxysteroid oxidoreductase
• Testosterone can then be converted (mostly in peripheral tissues) to:
  
  • DHT (dihydrotestosterone) by 5α-reductase, or to
  • estradiol (E2) by aromatase cytochrome P450 aromatase

Question 22

what signals mediate both negative and positive feedback systems?

Answer 22

1. pituitary effects on gonadotropin secretion mediated by ER and AR on gonadotropes
2. central effects on GnRH pulsatility less direct, as GnRH neurons express few steroid receptors