Lecture 15: Seasonal clocks I Flashcards
why must animals adapt to a changing environment?
increased fitness –> increased survival –> more offspring
what are some seasonal events that must be timed?
migration, hibernation, change in colour of coats, sexual behaviours.
done via photoperiodism
what is photoperiod?
duration of light and dark” or “day length”
most accurate environmental indicator of seasonal time.
what is photoperiodism?
Use of day length cues to time seasonal changes in physiology and behaviour
what is a photoperiodic species?
a species that uses the annual cycle of day length to time seasonal adaptations
how do many species adapt to environmental changes?
in anticipation of the changes, ie temperature or food availability.
Describe the Siberian Hamster.
During Long days: High body weight High fat reserves high sex hormones active reproduction agouti pelage/coat
During short days: Low body weight Low fat reserves Low sex hormones inactve reproduction white pelage/coat
Describe the syrian hamster.
LD large testes, SD small testes.
Describe lab tests on siberian hamsters
We can model photoperiodic regulation of physiology in the lab.
Short photoperiod
(8h light 16h dark)
Long photoperiod
(16h light 8h dark)
SD show white pelage, small testes.
LD show agouti pelage and large testes.
Seasonal breeding?
The same photoperiod can produce a different physiological response in different species.
Long day breeders – hamster and quail, short gestation, breed in summer so birth in summer.
Short day breeders – sheep, reindeer, deer, large animals. Longer gestation, breed in winter to time birth of offspring to summer.
What is the result of a pinealectomy in syrian hamsters?
(Hoffman & Reiter 1965)
blocks response to photoperiod.
look at testis size, they remain large, no shrinkage in winter once 12 hour days occur.
Pineal gland?
SLIDES????
pinealocytes??
produces melatonin.
SCN orchestrates rhythms in melatonin.
melatonin production is inhibited by light.
melatonin is derived from seratonin.
what is the rate limiting enzyme in melatonin production?
NAT
??
what regulates the duration of melatonin signal?
Photoperiod regulates duration of melatonin signal as light inhibits production.
Long photoperiod = short melatonin signal
Short photoperiod = long melatonin signal
The melatonin endocrine message
How is it interpreted by the brain?
Simmoneaux and Ribelayga 2003
Photoperiodic changes may depend on:
Duration hypothesis:
The absolute duration of the melatonin signal
Coincidence hypothesis:
The presence of a time-window of sensitivity to melatonin
Amplitude hypothesis:
The amplitude of the nocturnal melatonin peak
think amplitude is wrong.
duration is correct? listen to podcast.
Melatonin infusion experiments on Siberian Hamster
Pinealectomy (Px) removes all circulating melatonin signals.
Animals can no longer respond to photoperiod.
Replace with infusion of melatonin each day of long or short durations.
Mimics winter or summer respectively.
Measure reproductive or endocrine response.
saline + 6h melatonin -> large testis.
9/12h surfficient for regression of testis.
Melatonin provides an internal representation for external photoperiod change
Long duration signal administered to Px animals are read as “winter-like”
Melatonin needs to be present as a continuous un-interrupted signal each night.
remained large with a 3/2hr break, 1hr had some shrinkage but far less.
Duration of melatonin signal appears to be key.
how is reproduction regulated?
hypothalamus.
GnRH synthesised in hypothalamic neurons, GnRH –> LH/FSH release which acts on gonads and causes seasonal changes.
Describe pinealectomised sheep, 4 year experiment
Replaced melatonin signal for 90 days per year.
Timed at either Spring Summer Autumn Winter
Duration of infusion represented time of year.
Measured output - luteinising hormone (LH) - reproductive activity.
Controls all fertile over winter period.
no melatonin replacement and pinealectomy - Evidence of a free running cycle in Px (circannual) but not synchronised between individuals.
Spring - sync but wrong time of year, to early.
Summer - gooooood stuff. all synced at right time of year.
Autumn - varibale timing, early or late, not sync.
Winter - asymmetric, randoms periods and length.
THEREFORE:
Melatonin synchronises the rhythm but its effectiveness varies on time of year given!
Melatonin entrains circannual rhythms
What can melatonin be seen as in mammals?
A short period of melatonin information can entrain a circannual system
The timing of that signal appears to be is important
Melatonin is therefore a seasonal “Zeitgeber” in mammals.
Why would it be important that melatonin has greater effects at a particular time of the year?
Reindeer
Reindeer experience polar day (24h light) and polar night (24h dark) – especially those that live up in Svalbard
3-4 months with no photoperiod information.
90% of births occur in a 10 day period, robust seasonal rhythm.
can’t maintain clock in LL/DD.
weak circadian clock since no need for it.
same for ptarmigan birds.
but must have strong circannual clocks as life depends on seasonal awareness and preperation.
Do this via melatonin rhythms.
Melatonin non existent in constant light.
Constant dark, levels are pinned to a high point.
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