Circannual rhythms and photoperiodism Flashcards
What are 4 examples of circannual rhythms?
Seaweed: growth rate
Starling: Testicular width
Sheep: LH
Golden-mantled ground squirrel: hibernation
What was observed when raising Hamster and Sheep in summer (LD), winter (SD) and prolonged SD conditions?
Hamster
Summer (LD): high prolactin, agouti pelage
Winter (SD): low prolactin, white pelage
Prolonged SD, refractoriness
Sheep
Summer (LD): high prolactin, horn growth
Winter (SD): low prolactin, no horn growth
Prolonged LD: refractoriness
What is the effect of photoperiod on the fertility of male hamsters? How was this measured?
wt (FRP 24 h) hamsters
and
mutant (FRP 20h) hamsters
measure testes mass over hour of darkness per cycle
testes atrophy if the night is longer than 12 hrs in wt, 10 hrs in mutant
A mutation in the circadian clock alters photoperiodic time measurement
What encodes day length information? How?
Pineal melatonin rhythm
Eyes receive more or less sunlight, signalls to the pineal gland via SCN and causes melatonin to be secreted during the night
Depending on species, length of day effects reproductive status
What is the Pars tuberalis? What is expressed here?
A (possible) decoder of photoperiodic information
Melatonin receptor and thyroid hormone receptors are expressed here
How does TSHb expression differ during long/short days in the pars tuberalis?
Short days: weaker, peak during ZT 4
Long days: overal more expression, peak at ZT 16
In both cases, peak is just before end of light
What is the effect of photoperiodic induction of Eya3 on TSHb expression?
Eya3 induction potentiates TSHb expression
TSHb promoter has a conserved D box and MEF3 elements
TSHb expression is regulated by Tef and Eya/Six1
What is TEF?
Transcription factor that binds D box
How is Eya 3 controlled transcriptionally?
Via E boxes and D elements
Describe the model for photoperiodic induction of Eya3 expression in the pars tuberalis
Photoperiod is detected by retinae
Transduced in Pars Tuberalis of Pineal gland
Darkness promotes melatonin secretion
Melatonin represses Eya3
End of night = end of melatonin = spike of Eya3
Eya3 + TEF + tshb = Big spike of TSHb
Long photoperiod = less melatonin = more eya3 = more TSHb
Short photoperiod = more melatonin = less eya3 = less TSHb
How does melatonin affect Eya3 expression?
It suppresses it (possibly via cAMP)
Describe the PT-dependent photoperiodic regulation of reproductive function in mammals
- The retinae receive photopeiod, which is transmitted to SCN
- SCN signals to IML/SCG/pineal gland to release melatonin during the night
- Pars Tuberalis receives melatonin, which secretes TSH in a seasonal/melatonin-dependent manner, which transmits TH to to GnRH neurons
- GnRH neurons send gonadotropins which activate the gonads
What was observed when transferring mice on a short photoperiod to a long photoperiod with an 8 hour advance in phase? 8 hour delay?
What are the long term effects of dark-phase delay on TSHb expression?
Advance: TSHb mRNA increases until end of day
Delay: Not much change immediately
Long term: the Pars Tuberalis clock gradually phase shifts (advances) when dark period is shortened by phase delay, TSHb expression increases
What are the circannual molecular changes in the Pars Tuberalis and Hypothalamus when kept in constant conditions?
How do TSHb and Dio2 expression levels vary during LP and SP conditions, as well as during constant conditions (Subjective sumer/subjective winter)
Body Temperature: remained constant
Body weight: constant in subjective winter, peaks during subjective summer
Testis size: increases during subjective summer, falls off(?) during winter)
Photoperiodic cycle:
- TSHb: expression decreased during SP compared to LP
- Dio2: Expression decreased during SP, peaks in morning in LP and peaks in eveining in SP
Constant conditions
- TSHb: higher in subjective summer
- Dio2: Higher in subjective summer
What is Dio2?
A tanycyte
Controls the output of the hypothalamic-pituitary-thyroid axis
