GENDER DIFFERENCES/ STARVATION and REPRODUCTION Flashcards
In terms of sexual differentiation, how are men and women different?
1) Chromosomal sex (genetic)
2) Gonadal sex (primary sexual differentiation
3) Hormonal sex
4) Phenotypic sex
5) Behavioural sex
The Jost paradigm
What anatomical (organisation effects are different) gender wise?
The AVPV is much bigger in FEMALES THAN MALES. Although in humans, a distinct AVPV is not as clear as in animal models, they are presumed to have one, just more diffuse. Present at birth but becomes more obvious during puberty.
THE AVPV IS NEEDED FOR THE GNRH/LH SURGE (ovulation). In females, the AVPV has higher DOPAMINE, GABA, GLUTAMATE AND KISSPEPTIN
What is kisspeptin? Where is it found? What does it bind to? What evidence is there to show its importance? Clinically, what can kisspeptin be used for?
• Is expressed in the hypothalamus, pituitary and placenta, and binds to g protein coupled receptor, GPR54.
It is particularly important in the AVPV
• It is a family of proteins that all have the same amino acid sequence meaning can bind to GPR54.
• MICE KO MODELS/ Human family in Saudi with a LOSS OF RECEPTOR→ FAIL TO GO THROUGH PUBERTY.
• Kisspeptin sits ABOVE GNRH and is the primary activator, thus mutations switch off all downstream signaling.
• As kisspeptin induces ovulation, can be used in IVF to prevent ovarian hyperstimulation syndrome
What do injections of kisspeptin do? Males versus females?
- KISSPEPTIN ICV INJECTED INCREASES THE LH SURGE IN RATS IN A DOSE DEPENDENT MANNER. Also found in HUMAN MEN and for FSH (not as dose dependent). LH surge does not occur with ablation
- For Females IV bolus of kisspeptin only stimulates the FSH/LH surge in PRE-OVULATORY (high oestrogen) - HYPERSTIMULATORY, not in follicular phase (even at very high doses)
- Kisspeptin-10 has similar plasma half-lives in men and women- so the reason why women only respond to kisspeptin in the preovulatory phase is not due to a difference in the clearance of kisspeptin in men and women
- Therefore, gonadotrophin secretion following kisspeptin-10 is sexually dimorphic
What organisational affects are different between men and women?
Both genders have kisspeptin neurons in the ARC however, only females have kisspeptin neurons in a substantial amount in the AVPV. ARC also needed to be fertile and go through puberty. KISSPEPTIN DISTRIBUTION IS SEXUALLY DIAMORPHIC
What happens in the arcuate nucleus?
Arcuate nucleus
Number of kiss1 cells:
1.Decrease when E2 levels rise (so levels are minimum around ovulation)
2.Increase during ovariectomy (OVX)
3.Decrease after OVX with hormone replacement (E/P)
Therefore, ARC kisspeptin neurons are negatively regulated by E2
What is co-secreted with kisspeptin in the arcuate nucleus? How has its importance been established?
Neurokinin B
• NEUROKININ B IS CO – EXPRESSED WITH KISSPEPTIN IN THE ARC
• Inactivating mutations also cause a failure to go through puberty. (Familial hypogonadotrophic hypogonadism)
• Show that neurokinin B plays a key role in reproduction
• In prepubertal monkeys, neurokinin B injections stimulate LH secretions
What is the name of the arcuate neuron where kisspeptin is secreted from? What else is secreted from there?
- Co-expressed with kisspeptin is neurokinin B and DYNORPHIN
- Kisspeptin+neurokinin B stimulate GnRH secretions, DYNORPHIN INHIBITS
- Unclear if dynorophin sexually diamorphic
Is there sexual dimorphism in neurokinin B signalling?
• Both size and number of kisspeptin and neurokinin B neurons bigger in post menopausal women than age matched men
• But men still produce testosterone, females no oestrogen so difficult to interpret these results
A study done in humans- took small numbers of post-mortem samples of children, adults, elderly
Increased levels of NKB peptide in women compared to men
Non-significant increase in number of NKB cells in infundibular nucleus
Therefore, contradictory evidence for NKB
How are the AVPV neurons regulated by oestrogen?
Number of Kiss1 cells:
1) Increase when E2 rise
2) Decrease during ovx
3) Increase during ovx with hormone replacement
Therefore AVPV neurons are positively regulated by E2.
Describe the timing of the organisational effects in terms of kisspeptin
Critical period of postnatal development in rodents( PN d7-10)
Castration of male at birth:»_space;» female AVPV Kiss1 neurones
Give T or E2 to females at birth»_space;» male AVPV Kiss1 neurones (ie no AVPV)
After the postnatal period, these manipulations do not alter the AVPV
Neonatal exposure of rodents to T or E2, causes defeminisation of hypothalamus
What is the activational effect? Evidence?
• Circulating E2 is much higher during the pre-ovulatory phase of menstrual cycle, which heightens sensitivity to effects of GnRH (same GnRH signal, but during ovulation more E2. Pituitary very sensitive- so LH surge)
• i.e. there is a reversible reason which may (at least partially) account for the differential effects of kisspeptin during the female menstrual cycle (men don’t have these levels of oestrogen, ACTIVATIONAL EFFECT THAT FEMALES HAVE A LOT OF OESTROGEN PRE-OVULATION)
Mid cycle, oestrogen levels go very high whilst GnRH stay constant. Knobil lesioned monkey so that GnRH could not pass through. Discovered that need pulsatile oestrogen and GnRH to ovulate→ PITUITARY GLAND NEEDS PRIMING IS AN ACTIVATIONAL EFFECT (amount of E2 priming the pituitary determines how much GnrH is released)
What 3 euro-peptides are known to influence fertility?
Ghrelin, leptin and PYY
What are the actions of ghrelin?
Chronic administration of PYY decreases food intake in rats and humans. There is high PYY after the fed state→ PYY is a potent stimulator of reproductive function.
There has not been any strong human studies, however, in rodents they found PYY stimulates GnRH secretion
What are the actions of leptin on reproduction? How long does it take for its actions?
Long-term signal of energy stores, produced from adipocytes. Ob/ob and db/db mouse has hypogonadotrophic hypogonadism (low gnrh, low FSH/LH and low sex steroids). Leptin replacement in the mouse/ human restores function (return of LH pulses).
Acute starvation for 72 hours is enough to reduce leptin levels and inhibit reproduction
What commonly occurs in females that suppresses reproductive function? How can this be overcome? What does this lead to
Hypothalamic amenorrhea. 30% of women of reproductive age. Mainly reversible due to exercise and weight loss causing inadequate gnrh secretion.
In these women, can give leptin to signal to body that have sufficient body reserves and can restore LH pulsatile secretion.
BUT GNRH NEURONS DO NOT HAVE LEPTIN RECEPTORS→ KISSPEPTIN. THERE IS KISSPEPTIN DEFICIENCY ASSOCIATED WITH HYPOTHALAMIC AMENORHEA.
WOMEN WITH HA ARE 4 TIMES MORE RESPONSIVE TO KISSPEPTIN THAN HEALTHY WOMEN.
May also involve a lack GLUTAMATE In ventral premamillary nucleus
Where is GnRH found in the hypothalamus?
GnRH is distributed in the hypothalamus quite diffusely and not concentrated in specific neurons, relatively small number in relation to other neurons.
What is the main GnRH? Where is it?
Main GnRH is GnRH1 (GnRH1 originates outside the brain, migrates during embryogenesis). Most of the neurons are in the pre-optic area and project into the median eminence.
What is the other GnRH? Where is it found, actions?
Most abundant in hypothalamus is GnRH 2. It originates in the brain (midbrain) and the neurons project to other parts of the brain→ more neurotransmitter/ neuro modulator function. Thought to have more of a behavioural effect rather than stimulating gonadotrophin. Gabaminergic neurons impinge on these neurons.
What is GnRH made as? How does it signal
Formation: Made as a very long precursor, single peptide. Has a processing site, and a 56 amino acid associated with it (GAP – GnRH associated peptide, function not known but thought to maybe modulate prolactin)
Mainly G protein receptors, many signaling pathways. With some of the mediators there is a lag phase.
What are the major actions of GnRH?
1) Stimulation of gonadotrophin release (LH and FSH)
2) Priming the gonadotrophs
3) Stimulation of gonadotrophin synthesis
4) Induction and maintenance of the secretory morphology of the gonadotrophs
What does priming involve? What simple evidence is there for it?
Priming – increasing the responsiveness of the gonadotroph to itself, is oestrogen dependent and involves increasing pituitary receptors as well as alteration of the second messenger system, increasing the readily available pool of LH and increasing protein synthesis
Using the same dose of Gnrh, you can see that you get an increase in the responsiveness of cells through increased amount of LH released.
In follicular phase, oestrogen is around
and GnRH increases. Luteal phase- progesterone causes marked inhibition of
GnRH
What does induction of the secretory morphology involve?
1) Increase in the number of GnRH receptors
2) Increased number and size of gonadotrophs
3) Increased mRNA and Protein
4) Increased glycosylation
Demonstrated in the hypogonadal (hpg) mouse- discovered by accident- mutation so no GnRH produced.
If give GnRH, can demostrate all these things happen in mouse- increase GnRH receptors and gonads start to grow.
What happens if you give continuous GnRH versus pulsatile?
Continuous then LH reduces, probably due to desensitization to the receptors
What experiment was there to show GnRH being pulsatile?
Used Rhesus monkey as their model- by taking regular blood samples, they found GnRH was released in a pulsatile fashion
Put lesions in hypothalamus to destroy GnRH producing cells
If give GnRH in pulsatile fashion/ 1 pulse per hour- LH and FSH secretion maintained in pulsatile manner
If give GnRH continuously/5 pulses per hour, LH and FSH levels fall
When you reinstate pulsatile delivery to 1 pulse per hour, LH and FSH levels rise again
Can correlate LH pulses with electrical activity in hypothalamus
Then looked at less frequent pulses:
1 pulse per hour- LH secretion normal
1 pulse every 3 hours- LH reduced
Reinstate to 1 pulse per hour- LH went back up
Different profile with FSH- 1 pulse every 3 hrs- FSH increased
What is now known about the half lives of FSH and LH?
LH- half hour.
FSH- 3 hrs
What happens if you over-expose the monkeys to GnRH?
Over exposure of the gonadotrophs to GnRH renders the cells refractory to stimulation
Which compounds are involved in the regulation of GnRH?
- Catecholamines
- GABA
- Glutamate
- Endogenous Opioid peptides (EOP)
- Galanin
- Neuropeptide Y
- Neurotensin
- Substance P
What is kisspeptins role in GnRH?
• Kisspeptin is essential for GnRH secretion, and involved in pulse generation of GnRH
Where is the GNRH pulse generator?
- KNDy neurons - 3 peptides shown to be co-localised in a single subpopulation in the ARC (sheep)
- K-kisspeptin STIMULATORY
- N - Neurokinin B STIMULATORY
- Dy - Dynorphin (DYN) INHIBITORY
- Kisspeptin and neurokinin B are essential
What has an inhibitory effect on GnRH?
Gonadotrophin inhibitory hormone (GnIH)
Hypothalamic peptide that Binds to GPR147 (Identified in mammals and other vertebrates)
Contact interactions of GnIH & GnRH neurons→ Inhibits GnRH secretion
Acts at the hypothalamus and pituitary gland
From ovaries, gonads exert negative feedback
Important for species that rely on the environment to switch on menstruation, for example day length (photo – light)
How does oestrogen exert negative feedback?
Knobil continued to investigate- ovariectomised rhesus monkeys (Lose negative feedback- so get robust pulsatile secretion of LH)
• Infusion of oestradiol- inhibitory effect on LH secretion after a lag, which is maintained
• When stop infusion, takes while for effects of oestradiol to be cleared from system
Rhesus monkeys implanted with capsules containing oestrogen, all same dose but different time
When exposed to low levels of oestrogen- exerts its negative feedback effect
When exposed to oestrogens for long period of time, positive feedback effect of oestrogen- increases LH and FSH
. Thought through kisspeptin
What affect do steroids have on GnRH feedback?
Hypothalamus
Steroid implants into medial basal hypothalamus suppress LH & FSH, reverse the increase in pro-GnRH mRNA induced by ovariectomy, alter GnRH neuronal activity.
Kisspeptin neurons in ARC mediate the –ve feedback actions of E2 (express ER).
How do you show negative feedback also happening at the level of the pituitary?
Pituitary
Negative feedback effects in lesioned animals (lesion animals and destroy GnRH neurones. Then give exogenous GnRH. Still get negative feedback- therefore neg feedback also exerted at pituitary)
What evidence is there that oestrogen’s negative feedback may be occurring through other mediators?
•GnRH neurons don’t have ERa receptors to mediate effects- so effects via other neuronal systems with ER)
What effects do oestrogen and progesterone generally have in the non pre-oestrous phase?
Progesterone slows the pulse frequency
Oestrogen reduces the pulse amplitude.
In general terms, what does stress do to the reproductive axis?
disrupts HPG axis - depression of gonadotrophin secretion (slowing of GnRH/LH pulse frequency), oligospermia, amenorrhoea and consequently infertility.
What evidence is there for the effect of stress on the axis?
• If give endotoxin, Increasing CRH, causes a dose related decrease in GnRH in MBH (medio basal hypothalamus) and ME (median eminence)
What evidence is there for the effect of stress on LH secretion?
• LH secretion: Castrated rats (If castrated/gonadectomised- removing the negative feedback so LH goes up- easier to measure and increases sensitivity of assay)→When stress added- inescapable foot shock- LH levels decreased and remain low: Therefore stress reduces LH secretion
What further evidence is there for the effect of stress on the axis?
• Reversal of stress effect- (CRH antagonist): CRH antagonist injected into brain before onset of stress. Stress: reduction in LH secretion normally in controls. Pretreated with CRH antagonist- effect reversed : Suggesting that CRH acts centrally to inhibit LH secretion
What evidence is there for the mechanisms behind the CRH effects?
Neuroanatomical evidence, projections between CRH and GnRH neurons.
Effects via other neuronal systems
What is the anatomical evidence between the CRH and GnRH neurons?
- however very few neurons project from the PVN to the pre-optic area.
- PVN may not be important, because lesions in PVN fail to prevent stress-induced inhibition of LH. •Some areas outside hypothalamus also contain CRH neurones- BNST (Bed nucleus of the Stria Terminalis) and Locus coeruleus
- Both contain high density of CRH neurons, implicated in GnRH secretion. Projections to POA do happen from these areas, but not CRH neurones, CRH probably acting via interneurons.
