Circadian Rhythm

What 1938 experiment showed presence of a 24 hour body clock?

• 2 men locked themselves inside a cave for 32 days in attempt to force a 28 hour day.

  • failed, the body still resorted to a 24 hour day.

What is a circadian rhythm?

• autonomous and persistent rhythm without external cues. (CT/circadian time)

  • however, external cues can influence it (light/temp), this is called ZT (zeitgbers time)

What are the 3 proteins in mammals responsible for circadian rhythm?

• basic functions?

mPER1 // mPER2 // PER3

• 1 + 2 are critical for daily activity

• 3 is less involved, functions mostly in secondary clocks

What are Cryptochromes and their role in mammals?

• PER interaction?

CRY1 + CRY2 = blue light sensitive proteins.

• they’re essential for rhythmicity.

• CRY and PER form a complex.

What are the transcriptional regulators of PER / CRY?

• feedback loop?

CLOCK and BMAL-1 = promotors of Per expression.

• after PER/CRY are transcribed → associate → re-enter nucleus.

  • the PER/CRY complex inhibits CLOCK and BMAL-1 ∴ negative feedback of Per expression.

• CK1ε = negative regulator of PER stability // promotes per breakdown.

What is FASPS?

• features + cause

Familiar Advanced Sleep Phase Syndrome

• shorter rhythms (4-6 hours ahead)

• mutation on hper2, the CK1ε binding region ∴ less PER breakdown.

Where is the master clock?

• how does light entrain the master clock in flies vs humans?

Superchaismatic Nucleus (SCN)

• flies: Cry reacts to blue light, promoting Per degradation, by targeting its binding partner ‘timeless’

• humans: CRY1 / CRY2 aren’t responsive to blue light. Blue light cant penetrate to SCN neurons.

  • light at the retina entrains SCN.

How do human RGCs respond to light in order to entrain the SCN?

Most RGCs are blind to light, however ipRGCs are photosensitive.

• they express melanopsin → strongly absorbs blue light.

• there are 5 types of ipRGCs, some do not contribute to photic entrainment.

• the M1 subtype that DONT express BRN3B, are involved in circadian entrainment.

What route is used by these BRN3B lacking ipRGCs to reach the SCN?

The retino-hypothalamic route.

• They reach the ventrolateral area of the SCN, which is shielded from the dorsomedial shell.

What do the BRN3B positive ipRGCs do and where do they project?

They are also activated in response to blue light, and project to the perihabenular nucleus (Phb).

• this is implicated in targeting mood. Mouse -/- models exhibit depression.

• this route is independent of the SCN, and not involved in circadian entrainment.

How does light reset the circadian clock?

Light promotes expression of Per genes via TF’s such as CRE.

• light during dark phases resets the clock

How do neurons from the core vs shell of the SCN communicate?

The SCN core is entrained by light, (retino-hypothalamic route), and signals to the SCN shell to give it time cues.

• SCN core secretes VIP → binds VIP-R in SCN shell. GABA also to signal.

How does the pineal gland promote circadian phase shifts?

Pineal Gland stimulation uses serotonin as a precursor to produce melatonin.

• melatonin stimulates MT1 and MT2 receptors in the SCN .

• this reduces the firing rate of the SCN → phase shift.

What are 3 human circadian readouts?

1. Melatonin - plasma melatonin increases during the late evening.

2. Body temperature - preotic area of hypothalamus recieves SCN input.

3. Cortisol - secreted by hypothal-pitu-adrenal gland → increses upon waking up

Why are most mammals nocturnal, how does this work?

The rodent SCN is connected with orexin expressing neurons. orexin promotes wakefulness.

• in noctural mammals, SCN and orexin neuron activity is inverse. orexin is upregulated during the night.

  • during day, SCN active and inhibits orexinergic neurons in lateral hypothalamus.

  • at night, orexin inhibits SCN and promotes wakefullness via VLPO inhibition.

What is sleep? How can we record it?

Reduced state of physcial activity and responsiveness.

• EEGs record brain activity.

• as we fall asleep, our brain waves reduce in frequency but increase in amplitude.

What is REM sleep?

Rapid Eye Movement: associated with wake like brain activity, vivid dreams and muscle tone.

• defects in the REM phase is associated with sleep paralysis.

Why do we need to sleep?

Sleep promotes metabolite clearance, synaptic pruning, restoring synaptic baseline. Sleep deficits can negatively impact cognition, attention and learning.

What are peripheral clocks, give 2 examples:

Clocks that retain their own rhythm upon extraction. They maintain their rhythm through environment.

• hair growth // colonic motility.

How can the circadian rhythm be impacted in AD and PD?

• AD - ‘sundowning’ = symptom whereby patients become agitated during the last hours of the day. delayed circadian rhythm accompany this.

• PD - sleep disturbances correlate with lewy body accumulation.

What is Fatal Insomnia?

• symptoms // characteristics // treatment ?

Rare prion disease caused by double mutation of the PrPc protein.

• rapid insomnia, dementia and coma within 3 years.

• spindles at NREM III lost. Likely due to thalamic damage.

• doxycycline in mice rescues rhythm but doesnt prevent disease progression.