Puberty and Kisspeptin Flashcards
What are the characteristics of adolescence.
• Heightened stress
• Drive for independence
• Increased salience of social and peer interaction
• Frequent negative emotional states
• Heightened emotional response
− Shown card with facial expressions → heightened amygdala response
− Task with monetary reward → heightened nucleus accumbens response to reward
• Physical changes
• Risky behavior
− Eg substance abuse, unprotected sex, harming others, injuries and death
− Engage in this because they have enhanced motivation to seek out incentives and new experiences
− 4 main causes of death in adolescence → motor vehicle accidents, homicide, suicide and unintentional injuries
• Brain development changes
− Amygdala → regulates environmental factors important for the person, and regulates emotions and sensitivity to others emotions.
− Nucleus accumbens → key part in reward pathway and incentives
− Prefrontal cortex → rational thinking side
➢ In adolescence, amygdala and nucleus accumbens regions develop faster than the prefrontal region
What defines puberty?
When an immature individual gains the behavioural and physical characteristics that will allow him or her to reproduce
• Secondary sex characteristics
• Physical growth
• Reproductive competence achieved
- Boys → onset seen as the point of the first ejactulation
- Girls → onset time of first menstruation
- Age around 12-13
Describe the HPG axis during puberty
Males
• Increase in pulsatile GnRH release at puberty activates tonic gonadotrophin secretion
• Simulates leydig cells to secrete testosterone, which in combination with FSH, initiates spermatogenesis
Females
• Incease in pulsatile GnRH release activates tonic gonadotrophin secretion
• LH and FSH responsible for folliculogenesis and estradiol secretion
• LH surge stimulates ovulation
Describe the HPG axis during menstruation
- FSH high at the start of the cycle → allows follicle maturation
- Follicle maturation gives estrogen
- Estrogen inhibits FSH, as we don’t want more than one follicle maturing
- At day 14, there is an LH surge → this allows ovulation
- Corpus luteum produces progesterone which maintains the lining of the uterus
Describe the HPG axis during adulthood
Female
• Slow pulses of GnRH stimulate an increase in FSH
• FSH high at the start of the cycle → allows follicle maturation
• Follicle maturation gives estrogen
− Theca expresses LH receptor - LH stimulate androgen production
− Granulosa expresses FSH receptor - FSH stimulates aromatase activity
− Testosterone is aromatized to estrogen
• Estrogen has negative feedback to arcuate nucleus, causing decrease in GnRH and decrease in FSH
• Eventually estrogen crosses a threshold and has positive feedback on the AVPV, giving more LH
• At day 14, there is an LH surge → this allows ovulation
• Corpus luteum produces progesterone which maintains the lining of the uterus
• Progesterone has a negative feedback – slows down the HPG axis so that if there is a pregnancy, we wont get ovulation.
• If a woman doesn’t get pregnant, progesterone levels decrease and the HPG axis starts again
Male
• LH and FSH stimulate production of testosterone → promotes spermatogenesis
• Testosterone then has negative feedback on axis, inhibiting GnRH and LH/FSH production
What are the results of changes to the HPG axis with age?
Female • Axis slows and eventually stops. No longer fertlile = menopause − Headaches and hot flushes − Backache − Risk of CVD − Loose teeth − Hair thinning − Smaller nipples − Abdomen loses muscle tone − Vaginal dryness − Body and pubic hair becomes thicker and darker − Bones become fragile
Male • Remain fertile their whole life, but axis may slow a little − Muscle mass decreases − Increased fat mass − Loss of libido − Impotence − Decreased attention − Increased risk of fractures − Abnormal spem production
What is the structure of the kisspeptin gene
- 145 amino acids, cleaved to produce a 54 amino acid peptide
- The first two exons are non-coding
- Exons 3 and 4 code for a precursor – prepro-kisspeptin
- This is then cleaved to form the kisspeptins of different lengths
- All the different kisspeptins have the same affinity for the receptor
Describe kisspeptin receptor signalling
• Signals through G-aq
- Receptor is activated. G-alpha subunit dissociates from the g-betagamma unit
- Phospholipase C activated
- Hydrolysis PIP2 to produce DAG
- DAG activates PKC
- Activates GnRH gene transcription
What is the distribution of kisspeptin?
Brain:
• AVPV → more in females
• ARC → important for GnRH pulse generation
• Existence of scattered neurons has also been suggested in the POA, amygdala
Peripheral tissues (not yet known what the function is) • Placenta • ovary • Testis • Pituitary • Pancreas • Adipose
Describe the kisspeptin neurons
Differences in estrogen feedback
• Estrogen has positive feedback in the AVPV
• Estrogen and testosterone have negative feedback in the ARC
• Kisspeptin neurons have estrogen and testosterone receptors, but GnRH neurons don’t. So the feedback mechanisms act through kisspeptin
Differences in GnRH neuron activation
• AVPV kisspeptin neurons project into the cell body of the GnRH neuron
• ARC neurons project to the axon
Sexual dimorphism in Kiss1 in the AVPV
• 25x more kisspeptin in the brains of female rats
• Reason for this is thought to be because positive feedback in the AVPV mediates the LH surge for ovulation
Differences in neuropeptide expression
• Kisspeptin neurons in the AVPV also express galanine and tyrosine hydroxylase
• Kisspeptin neurons in the ARC express neurokinin B and dynorphin
What is the evidence for kisspeptin being a puberty trigger
• Loss of function mutations in kisspeptin receptor in some patients with hypogonadotrophic hypogonadism suggested role of kisspeptin in reproduction (Seminara et al, 2003)
- GnRH neurons express kisspeptin receptor
- Experiments involving GPR54 and kisspeptin knockout mice show that a functional kisspeptin receptor is necessary for GnRH secretion and release of LH and FSH
- GnRH neuronal activity is increased by kisspeptin
- Humans and mice with disruption in the receptor or kisspeptin fail to go through puberty
- Kisspeptin injections induced precocious puberty (Navarro et al, 2004), whereas injection of a kisspeptin antagonist delayed puberty
- During puberty, increased communication between kisspeptin and GnRH neurons
What is a model for puberty in rodents?
- Elevation of the endogenous kisspeptine tone leading to full activation of the HPG axis
- Increase in the sensitivity to the stimulatory effects of kisspeptin on GnRH/LH responses
- Enhancement of GPR54 signalling efficiency
- Increase in the number of kisspeptin neurons in the hypothalamus and projections to GnRH neurons
- For all these things to happen, we need an increase in estrogen
- So kisspeptin doesn’t trigger puberty, it just amplifies the increase in sex steroids
- Therefore, an unknown factor needs to activate the ovaries
Describe the neurological break model for puberty in primates
• Infancy → ARC GnRH pulse activity is robust, leading to intermittent release of KP, resulting in corresponding pattern of GnRH release into the portal circulation. This in turn drives pulsatile gonadotrophin secretion.
• Juvenile → A neurological break (central inhibition) holds the ARC and GnRH pulse generating mechanism in check, and pulsatile KP release is markedly suppressed. This leads to reduced GnRH release, and to a hypogonadotrophic state.
− This is a conceptual break, and may be accounted for by the imposition of an inhibitory input, or the loss of a stimulatory input
− Because this break is observed in neonatally castrated monkeys and agonadal humans, this break is independent of steroid → in contrast to the mouse model!
• Puberty → triggered when the brake is released, and GnRH pulse generation with robust intermittent release of KP is reactivated
− As discussed, the genetic evidence for the view that kisspeptin is critical for puberty onset is overwhelming.
− This has lead to the perception that kisspeptin signaling represents the key neural substret that controls the timing and onset of puberty
− But there is an altnernative possibility:
− While kisspeptin neurons are critical for puberty, this may just be because they are an integral component mediating GnRH release, which is necessary for the onset of puberty → kisspeptin is necessarily the master regulator.
Therefore seems reasonable to propose that:
- The primary role if kisspeptin is its role in generating GnRH pulses
- The time of puberty onset is dictated by kisspeptin-independent mechanisms
What are the potential neurological substrates of tonic inhibition
- The main mystery of the model is:
- what is the substrate that underlies gonadal-steroid independent reduction in GnRH pulse generation in the juvenile primary
- the signals responsible for timing the application and removal of the brake
Two labs have proposed a different substrate:
• Terasawa:
• Propose tonic inhibition by GABA is responsible
• GABA levels are higher when GnRH release is low in prepubertal monkeys
• GABA levels are lower after the onset of puberty
• Plant:
• Propose inhibition by neuropeptide Y is responsible
• mRNA and peptide levels of NPY are lower during the neonatal period and increase during the juvenile period, then decrease during puberty.
What is the neuronal substrate for Steroid Inhibition of GnRH Release in rodents?
- We know for puberty in rats, you need an increase in estrogen
- Majority of kisspeptin neurons express ER-alpha
- Mice with an ER-alpha knockout have higher Kisspeptin mRNA during the juveline phase → associated with high circulating LH and a dramatic advancement of the age of vaginal opening
- So in the mouse, prepubertal estrogen suppresses pulsatile GnRH release by acting on kisspeptin neurons
- Some time prior to puberty, the GnRH pulse generating mechanism escapes from this suppression as a result of estrogen induced increased kisspeptin activity, which amplifies GnRH activity
How is leptin the missing link?
- We know that under metabolic distress, you get low kisspeptin and slower puberty
- But kisspeptin is linked to reproduction, not food intake, so what is the link?
Leptin • Secreted by adipose • Signal of energy abundance • Secreted from body fat stores • Activates GnRH neurons → first indication that leptin could be a missing link
Ob/Ob mice:
• Congenital lack of leptin
• Altered puberty and are infertile
• Suppressed kisspeptin in the ARC
- No leptin receptors are found in GnRH neurons, however they are found on kisspeptin neruons
- Leptin activation of GnRH may therefore occur via kisspeptin neurons
Evidence for missing link:
• Low leptin levels associated with reduced hypothalamic expression of kisspeptin
• Administration of leptin enhances kisspeptin levels
However, this has been contended by Donato et al,
• Mice with genetic leptin elimination did not show alterations in puberty onset
• However, this mouse congenitally lacks leptin, so developmental compensatory mechanisms may have taken place.
Current model of puberty initiation:
• Leptin secreted by adipiose
• Acts on kisspeptin neurons in the ARC to stimulate kisspeptin release
• This stimulates GnRH production
• This explains why obese children start puberty early
Describe kisspeptin in pregnancy and lactation
Pregnancy
• 1000x more kisspeptin in the first trimester, rising to 10,000x more in the second
• However, there is a decrease in gonadotorphin levels
• Thought that because of the massive increase, the HPG axis because insensitive to kisspeptin
• Kisspeptin inhibits the migration of trophoblast cells in vitro
Lactation
• Decrease kisspeptin in lactating women
• Decrease in responsiveness to kisspeptin by GnRH neurons
• Leads to inhibition of the HPG axis → explain why breasfeeding women don’t have periods
• Breastfeeding women use up to 1000 calories a day to produce milk
• Isnt enough energy left to grow a baby
• HPG axis therefore needs to shut down
How does kisspeptin change during stress?
- When people are stressed, there is decreased kisspeptin levels
- Leads to decreased gonadotrophin secretion
- Thought this occurs though changes in CRH/glucocorticoids
Outline the cardiovascular effects of kisspeptin
- In the human CVS, expression of both receptor and peptide has been identified in the coronary artery, aorta, umbilical vein.
- Kisspeptin elicits vasoconstrictor effects in the coronary artery and umbilical vein
- Kisspeptin acts as a positive ionotropic agent in the human and mouse heart
- So may function as a cardiovascular transmitter
Outline the pancreas effects of kisspeptin
• High expression of peptide and receptor detected in human mouse islet endocrine cells where it can potentiate secretion of insulin
Describe HPG axis senescence
Rodents
• Onset of menopause caused by defective LH surge
• Low LH means low estrogen, decreasing activation of kisspeptin neurons
• This slows down the HPG axis
Humans
• Ovaries senesce, reduces estrogen
• This reduces negative feedback on ARC, giving increased kisspeptin
• This increases gonadotrophin levels
• This has been linked with neurodegenerative diseases such as Alzheimers
What is the role of kisspeptin in pre-eclampsia and PCOS?
- Reports of changes in kisspeptin in pre-eclampsia
* Raised kisspeptin in polycystic ovary syndrome → correlates with its association with leptin and obesity
What is the role of gonadal hormones in the adolescent brain?
− They drive sexual maturation, sexual dimorphisms in the brain, change brain structure and effect social, reproductive and emotional behavior.
• Studies by Neufang et al, 2009 → demonstrated that in boys with higher than normal levels of T, the amygdala is larger. These boys may be more sensitive to emotions
• Daily T levels fluctuate in a circadian rhythm (high in night, low in the day) → changes to this may lead to changes in behavior
− Boys with low T slope (levels decrease more slowly) → incrased anxiety, depression and attention problems
− Adolescents with higher peaks of hormones through the day → increased risk taking activities, higher social dominance.
− Girls with high T slope (levels increase rapidly) → increased disruptive behavior tendencies.
