Lec 6: Circadian Rhythm

Circadian Rhythms

Biological processes that follow a roughly 24-hour cycle

• Found in bacteria, protoza, plants, fungi, animals

• In humans have circadian rhythms of behavior, alertness, mood, and body temp, hormone levels

They are endogenous

• Continue even when environment is constant, not just when it changes

Fruit fly experiment

Gene called period/per on X chromosome shows 24 hr cycle

• transcribed mostly early in night, mRNA most abundant ~10 pm

• protein product PER most abundant 6 hrs later ~4 am

PER (Period) Protein

• PER represses transcription of per

• When PER is high, less per mRNA that will eventually become PER is transcribed

• So it inhibits own gene's transcription

• PER and its mRNA drive each others cycling through transcription-translation feedback loop/TTFL

Timeless gene IN FLYS (tim)

A protein that binds to PER, participating in the feedback loop that regulates circadian rhythms

• tim mRNA and TIM oscillate like to per and PER

relationship of TIM and PER IN FLYS

• TIM binds PER → dimer PER/TIM repress transcription of tim and per

  • ~4 am (6 hours after mRNA abundancy) high levels of PER/TIM shut off per and tim

  • = PER/TIM levels falling

  • = No more repression of per and tim

  • = per and tim levels rise in late evening

  • = another peak in PER/TIM next morning at 4 am

Missing either PER/TIM will result in no oscillation

So they oscillate in a 24hr rhythm

PER/TIM block (+) transc. factors (CLK, CYC)

Clock/clk gene codes protein CLK, cycle/cyc gene codes CYC

• In day, dimer CLK-CYC binds DNA & stimulates per and tim transcription

• In night, PER/TIME blocks CLK-CYC bound to DNA = per, tim repression

dbt lengthens FLY cycle

dbt lengthens the per/tim cycle

• DBT from gene doubletime/dbt binds PER causing it to break down

• PER levels rise slower = do not peak until 6 hrs after per = overall cycle length near 24 hrs

TTFL (Transcription-Translation Feedback Loop)

A biochemical mechanism in which gene expression is regulated by the proteins it produces, modulating circadian rhythms.

Mammalian homolog of TTFL

• We have similar gene to per but instead of PER forming dimmer with TIM, we have CRY from cry gene

• Mammalian homologs of clk, cyc,dbt are clk, cmal1, ck1e

• In mice and other mammals CLK/BMAL1 dimer (same thing as CLK/CYC in flies) stimulates transcription of per and cry when not blocked by PER/CRY

• CK1e (same thing as DBT) slow rise of PER protien levels

Synchronization of cellular clocks

• Kept in sync by many external factors (ex. light, temp, feeding, exercie, social interactions = zeitgeber cues)

• Main zeitgeber “time giver”, is light sensed by melanopsin retinal ganglion cells that project to master clock

Master clock

Suprachiasmatic nucleus (SCN) of hypothalamus

The Suprachiasmatic nucleus (SCN) of hypothalamus

SIts abv the optic chiasm

• “Master clock”

SCN and light

Retinal signals cause chemical changes in SCN cells → PER/CRY breakdown

• If drop in PER/CRY after 4 am… (PER/CRY levels fall) → clock set fwd b/c of accelerated breakdown of PER/CRY b/c protein levels are lower than supposed to be

• if drop in PER/CRY happen in evening…. (PER/CRY levels rise) → set clock back

SCN, light, dark

SCN neurons that receive retinal projections send these signals to other neurons in SCN so all clocks are adjusted, neural signals pass to other brain areas that send neural/hormonal signals all over body

• Entrainment = process of making clocks around body all in synchrony

Darkness; Pineal body

Pineal body is in back of diencephalon

• Secretes darkness hormone “melatonin”

  • starts at dusk, blood levels rise 8-fold peaking at 2 am, fall back to daylight levels by 8 am

• Melatonin acts via melatonin receptors in SCN to reset master clock toward night time

Melatonin pills

During jet lag, SCN master clock adjusts itself gradually to new schedule of light/dark by only 1 hr per day

• take the pill at the time u want to go to bed but using the clock of the place you’re used to

Diurnal vs Nocturnal animals

Diurnal: active during day

Nocturnal: active during night

Chronotypes

sleep @ diff times throughout day-night cycle

likely evolved for safety of herd

Sleepiness and master clock

Daylight

• SCN indirectly excite neuron in laternal hypothalamus so release orexin

Darkness

• other cells in LH active, project throughout brain releasing neuropeptide melanin-concentrating hormone (MCH) → sleep inducing

Orexin and MCH inhibit each other

Sleep pressure

The physiological drive to sleep that builds up during wakefulness and decreases during sleep.

Awake

• breakdown of ATP in brain cause adenosine build up → sleepy (as long as awake, adenosine level build = sleep pressure)

Asleep

• ATP level restored, adenosine level fall

Adenosine

A neuromodulator that accumulates during wakefulness and promotes sleepiness.

Orexin

A neuropeptide involved in regulating wakefulness and arousal; its loss can lead to narcolepsy.

Melatonin

A hormone secreted by the pineal gland that helps regulate sleep-wake cycles; its levels rise in darkness.

Caffeine

A stimulant that blocks adenosine receptors, temporarily reducing feelings of sleepiness.

• Blocks adenosine receptors but doesn’t lower adenosine levels

• So… when caffeine wears off we crash

Has half-life of 6 hrs:

• Blood level is 1/2 in 6 h

• 1/4 in 12 h

• 1/8 in 18 h

4 stages of sleep

1. rapid-eye movement (REM)

2. non-REM (split into 3 stages); 3 is deep NREM

REM sleep

A sleep stage marked by rapid eye movements, dreaming, and increased brain activity.

• eyes move

• dream

• erratic 30-40 hz brain waves

• muscle tone vanishes so no acting out in dreams

• only birds and mammals have

First REM stage occurs after about 90 min, as night progresses sleep gets shallower and REM stages longer

Non-REM (NREM) sleep

A stage of sleep characterized by slow brain waves and the absence of dreams.

• dreamless

• slower brain waves

• 3 stages

  • stage 3 is deep NREM with regular 2-4 hz brain waves