Circadian rhythms in animals Flashcards
What is an actogram?
Shows ability to keep time graph is days=Y-axis and time= X-axis
What is an endogenous oscillator?
Inside rhythm.
An endogenous oscillator is one that is internal to the organism and not a reflection environmental inputs.
What is entrainment?
The daily rhythms of most plants and animals are synchronised to a period of exactly 24 hours, a process called entrainment. The clock synchronises using environmental time cues called Zeitgebers
What is Zeitgeber? (giver of time)
a rhythmically occurring natural phenomenon which acts as a cue in the regulation of the body’s circadian rhythms.
Light as a Zeitgeber makes peripheral clocks ‘slave oscillators’ to the SCN but they can also be uncoupled e.g. Food as a Zeitgeber can entrain liver, lung, pancreas etc on 2nd clock
Circadian genes: CLOCK, BMAL1 (CYC), CRY (TIM), PER
(endogenous clock)
- CLOCK & BMAL1 heterodimerize → CLOCK-BMAL1 (these bind together to form different molecule which then binds to other receptors to promote transcription of other genes e.g PER and CRY
- CLOCK-BMAL1 → promotes PER, CRY + other gene transcription
- Excess PER & CRY → they heterodimerize PER-CRY
- PER-CRY blocks CLOCK-BMAL (hence their own transcription)
And the whole thing takes about 24hr = endogenous clock!
then they degrade and the whole thing starts again
What is the SCN?
Suprachiasmatic nucleus (SCN) in hypothalamus=the master clock in mammals.
In mammals, the SCN is the master pacemaker→
-rest-activity, daily variations in metabolism,
-body temperature, hormone secretion
Describe some peripheral clocks
Clocks: circadian genes regulating gene expression
→Not unique to the SCN!
* Cardiovascular system: e.g. vasoconstriction, heart rate
* Osmoregulation: e.g. glomerular filtration rate, urine vol.
* Digestive system: e.g. GI tract motility, ghrelin production
* Immune system: e.g. lymphocyte population levels
* Pancreas, ovaries, skeletal muscles…
How does light get to the SCN?
retinohypothalamic pathway
It also stimulates the pineal gland by an indirect route to superior retinal ganglion. pineal gland produces melatonin.
Intrinsically photosensitive and retinal ganglion cells react to
mostly blue light and give us awareness of day time.
How do we know it’s night
SCN sends signals to pineal gland to prevent secretion of melatonin during the day. Melatonin rises during the night.
How is the parietal eye (pineal/third eye) used?
It is a photosensory organ that occurs as a part of the pineal complex in some vertebrates, most notably in many lizards
How does cortisol affect sleep?
In the adrenal gland. The body’s melatonin (sleep hormone) and cortisol level (stress hormone) normally follows a regular, 24-hour circadian rhythm or pattern. Melatonin eases you into sleep—and helps you stay asleep—while cortisol helps get you up and keep you awake
What if we had constant light?
We have a less melatonin being released and slightly higher levels of cortisol but no peak in comparison to regular light levels.
Describe the differences in a nocturnal animal?
Melatonin is the ‘hormone of darkness’ (not sleep!)
→ Nocturnal animals have the same melatonin but opposite
glucocorticoid profile to us
What does Melatonin do?
Role in circadian rhythms
* Circulation, lipid/water soluble, receptors throughout body
* Short half-life (20-30min)
* Inducing sleep, more REM (by strong rhythm, not soporific)
* Helps synchronise peripheral clocks (cortisol stronger?)
* Buffers shifts (e.g. jet lag), “circadian glue” to lower drift
Melatonin… what does it actually do?
But stronger role in seasonality?
* Pinealectomy → lose seasonal physiology
* Receptor density seasonal in photoperiodic animals
* ‘Onset’ induces CRY expression; offset induces PER expression
= molecular basis for photoperiod ‘measurement’
* Receptors in Pars Tuberlaris → prolactin regulation; and
dorsomedial hypothalamus → luteinizing hormone
