Puberty and seasonality

Question 1

Discuss the hypothalamic-pituitary-gonadal axis in males and females

Answer 1

Female
• surge centre & tonic centre: contain groups of GnRH secreting neurons
• Post-puberty  LH surge seen prior to ovulation, high amplitude peaks. In the periods between the LH surges, there are basal secretions of Lh
- Fetal ovary produces eostradiol, binds to AFP-a, complex is too big to cross BBB, surge center develops as a result
- Fetal testis: produce testosterone, doesn’t bind to AFP-a, crosses BBB and is converted to oestradiol, surge center is defeminized as a result
in low amplitude peaks
Male
• NO SURGE CENTRE
• Post puberty  LH secretion is seen in a relatively consistent episodic pattern, followed rapidly by a peak in testosterone (LH binds to leydig cells in male to trigger production)

Question 2

Describe the endocrinological and anatomical changes that occur during puberty

Answer 2

Changes at puberty:
- There are various types of neurons which synapse onto the GnRH neurons. At puberty there is a change in the firing of these neurons, changes stimulation of the GnRH neurons
- GnRH neurons produce GnRH, act on pituitary to stimulate release of gonadotrophins, act on gonads: causing above changes
Before puberty (females):
- Both surge center and tonic centre affected
- Low amplitude low frequency pulses seen
After puberty (female):
- GnRH from the tonic centre is at a higher level: frequency increased compared to pre puberty,
- Surge centre: a pre ovulatory surge of GnRH is seen and low frequency episodes are seen the rest of the time. The surge centre controls the prevolatory surge of GnRH, and so it controls the pre ovulatory surge of LH.

Question 3

Describe the onset of puberty in females

Answer 3

• Prior to puberty the limiting factor is the amount of GnRH being produced by the hypothalamus.
• The pituitary and gonads are functional but the pituitary isn’t receiving enough GnRH from the hypothalamus to be stimulated into action.
  GnRH pulses in the pre-pubescent female:
  • Pre-puberty, the Tonic centre stimulates LH pulses
  Amplitude of LH pulses can = post-puberty levels
  BUT
  • Frequency of GnRH pulses low
  Stimulus not enough for LH and FSH release at high levels
  Oestradiol secretion by ovarian follicles very low- Below the minimum threshold needed to stimulate GnRH neurones in surge centre.
  Transition to puberty:
• The negative feedback effect of low oestrogen is reduced
• Allows the tonic center to produce GnRH, GnRH levels increase, resulting in increases in LH and FSH
• Levels get higher and higher
• Ultimately, oestradiol levels will begin to increase and continue to do so until the threshold level is reached
• The surge centre is then stimulated, which results in increased in GnRH and so increased LH and FSH
• Ovulation then occurs and so Puberty begins

Question 4

Describe the onset of puberty in males

Answer 4

• Testosterone cross BBB where it is converted to oestrogen
• Pre puberty: testosterone/oestrogen have a negative effect on GnRH secretion
• Approaching puberty:
  o Neurons become less sensitive to the negative feedback and so GnRH levels increase. This causes an increase in LH and FSH. Puberty is then triggered.

Question 5

Describe the relationship between female body size and the onset of puberty

Answer 5

• The amount of body fat present and the level of body maturation is important in the initation of puberty. Individual must grow to a larger size in order for there to be enough energy present for these processes to begin and be supported
• The GnRH neurons are sensitive to the metabolic status. There are three types of pre-synaptic neurons:
  o Glucose sensitive neurones
  o Fatty acid sensitive neurones
  o Leptin sensitive neurones: produced by adipose tissue
• Leptin, fatty acids and glucose are thought to promote activity in the kisspeptin neurons. The kisspeptins synapse on the GnRH neurons.
• Example:
  o Fat tissue: fat produces leptin: leptin can act on neuropeptide y neurons, synapse onto the kisspeptin neurons, synapse onto GnRH neurons
  o Neuropeptide y: important in appetite regulation
  o Uncertain how leptin acts on the neurons
  o BG sensitive neurones: synapse onto kisspeptin neurones: synapse onto GNRH
  o Kisspeptin neurones are important as they impact on GNRH neuron activity:
  o In this way, nutritional status influences GnRH activity and so influence the age of puberty

Question 6

Give social and environmental factors which impact the onset of puberty

Answer 6

• Season when animal born
sheep
• Photoperiod that animal experiences at puberty
sheep (short day length trigger)
• The timing of puberty can be influenced by photoperiod
• Spring born ewe lambs reach puberty when day length is decreasing (6 months later)
• Autumn born ewe lambs wont reach puberty until 12 months when day length is decreasing- takes longer for puberty to begin as require decreased day length
• Presence and absence of opposite sex just prior to puberty can accelerate its onset
swine and cattle (pheromonal and visual stimulus alongside other signals already discussed)
• Density of same sex groups
Swine

Question 7

Describe the onset of seasonality

Answer 7

• Prior to the onset of seasonality, there is a period of hypothalamic dormancy:
• Before breeding season, hypothalamus must be able to produce GnRH
• Consequently GnRH can initiate the HPG axis

Question 8

Give examples of types of seasonal breeders

Answer 8

• Polyoestrus: cycles all year round
• Seasonal polyoestrus
long day: mares, breed in spring
• Seasonal polyoestrus
short day: ewes, breed in autumn
• Mono-oestrus: cycle once a year, small amount of seasonality effect here
• Queen: not fully polyoestrus as diagram says, affected to some extent by photoperiod
• Males also affected by seasonality. Males tend to be ready to mate for longer than females
Seasonal effects on the male:
• Low testosterone production
• Reduced or cessation of sperm production
• Reduced sperm motility
• Testes involute: testes reduce in size
• Behavioural factors – decreased willingness to breed

Question 9

Describe anoestrus in females

Answer 9

• No oestrous cycles
• Ovaries inactive
• Due to decreasing GnRH, LH & FSH

Question 10

Describe the role of melatonin in seasonality

Answer 10

Melatonin:
• changing photoperiod changes production of melatonin
ultradian: < 1 day
circadian & annual
• Changing photoperiod is sensed by the retina & hypothalamus
• melatonin synthesised by pineal gland
• declining daylight (increasing dark) leads to increasing melatonin BUT species response is variable

Question 11

Discuss the effects of melatonin in long day breeders

Answer 11

require increasing photoperiod for cyclicity
declining melatonin (due to decreased levels of darkness) stimulates GnRH
short gestation (not mares, they have long gestation)
mate in spring
give birth early summer

Question 12

Discuss melatonin’s effects on short day breeders

Answer 12

require declining photoperiod for cyclicity
	increasing melatonin (darkness) stimulates GnRH
	mate in winter
	ceases after winter solstice
	as daylength increases, melatonin decreases, GnRH & gonadotrophins (FSH & LH) decline

Question 13

Describe the role of kisspeptin neurons in long and short day breeders

Answer 13

In long day breeders:
• Stimulates kisspeptin neurons- increased kisspeptin 10, increased GnRH, increased FSH and LH, increased cyclicity
In short day breeders:
• Inhibits kisspeptin neurons- decreased kisspeptin 10, decreased GNRH, decreased FSH and LH, anoestrus results.