Lecture 17: Seasonal clocks I

Question 1

why must animals adapt to a changing environment?

Answer 1

increased fitness –> increased survival –> more offspring

Question 2

what are some seasonal events that must be timed?

Answer 2

migration, hibernation, change in colour of coats, sexual behaviours.

done via photoperiodism

Question 3

what is photoperiod?

Answer 3

duration of light and dark” or “day length”

most accurate environmental indicator of seasonal time.

Question 4

what is photoperiodism?

Answer 4

Use of day length cues to time seasonal changes in physiology and behaviour

Question 5

what is a photoperiodic species?

Answer 5

a species that uses the annual cycle of day length to time seasonal adaptations

Question 6

how do many species adapt to environmental changes?

Answer 6

in anticipation of the changes, ie temperature or food availability.

Question 7

Describe the Siberian Hamster.

Answer 7

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

Question 8

Describe the syrian hamster.

Answer 8

LD large testes, SD small testes.

Question 9

Describe lab tests on siberian hamsters

Answer 9

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.

Question 10

Seasonal breeding?

Answer 10

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.

Question 11

What is the result of a pinealectomy in syrian hamsters?

Answer 11

(Hoffman & Reiter 1965)

blocks response to photoperiod.

look at testis size, they remain large, no shrinkage in winter once 12 hour days occur.

Question 12

Pineal gland?

Answer 12

SLIDES????
pinealocytes??

produces melatonin.

SCN orchestrates rhythms in melatonin.

melatonin production is inhibited by light.

melatonin is derived from seratonin.

Question 13

what is the rate limiting enzyme in melatonin production?

Answer 13

NAT

??

Question 14

what regulates the duration of melatonin signal?

Answer 14

Photoperiod regulates duration of melatonin signal as light inhibits production.

Long photoperiod = short melatonin signal

Short photoperiod = long melatonin signal

Question 15

The melatonin endocrine message

How is it interpreted by the brain?

Answer 15

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.

Question 16

Melatonin infusion experiments on Siberian Hamster

Answer 16

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.

Question 17

how is reproduction regulated?

Answer 17

hypothalamus.

GnRH synthesised in hypothalamic neurons, GnRH –> LH/FSH release which acts on gonads and causes seasonal changes.

Question 18

Describe pinealectomised sheep, 4 year experiment

Answer 18

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

Question 19

What can melatonin be seen as in mammals?

Answer 19

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.

Question 20

Why would it be important that melatonin has greater effects at a particular time of the year?
Reindeer

Answer 20

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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