Sleep and Circadian Rhythms

Updates and Additional Resources

  • PowerPoint for sleep and circadian rhythms has been updated.
  • PowerPoints for the remaining three lectures are uploaded but may be tweaked.
  • Additional reading folder under lectures contains videos for vision lecture revision:
    • Weasel and Hubel experiments.
    • Receptive fields of on and off-center retinal ganglion cells in edge detection.

Regulatory Forces of Sleep

  • Sleep is regulated by two main forces:
    • Homeostatic force: Sleep drive increases with prolonged wakefulness and decreases after sleep.
    • Circadian rhythm: Daily rhythm in sleep drive, influencing sleepiness at certain times of day, regardless of prior sleep.
  • Even with an irregular napping schedule (e.g., 7 minutes awake, 7 minutes napping), the circadian component influences sleepiness.

Circadian Rhythms: Definition and Health Contribution

  • The lecture will cover the definition of circadian rhythms and their impact on health and well-being.
  • The underlying mechanisms of the sleep-wake cycle will be illustrated using Drosophila melanogaster as a model organism.

Rhythms of Life and Time

  • Rhythms of life are determined by planetary and solar movements.
  • Daily rhythms: Earth's rotation causes daily cycles of light, dark, temperature changes.
  • Solar rhythms: Earth's orbit around the sun and axial tilt cause seasonal rhythms.
  • Monthly rhythms: Lunar cycles (approximately 29.5 days) affect tides and lunar rhythms.
    • Tidal rhythms: approximately 12.4-hour periodicity.
    • Interaction between tidal and lunar rhythms leads to circa semi-lunar and circadian rhythms.
  • The circadian rhythm is the most important rhythm for humans, adapted to daily cycles.

The Human Body Clock

  • The suprachiasmatic nucleus (SCN) in the hypothalamus serves as the brain's clock.
  • The SCN receives light information indirectly from intrinsically photosensitive retinal ganglion cells via the optic nerve.
  • The SCN synchronizes its internal rhythms using light information.
  • While the SCN is unique in having direct light input, most tissues in the body have clock circuitry and maintain daily rhythms.
  • The brain's clock circuits provide more reliable timekeeping due to their organization and communication; stochastic events are corrected by other clock-bearing neurons.
  • Clocks in the body receive time-of-day information from the brain via:
    • Rhythms in body temperature (approximately 1.5-degree amplitude).
    • Hormones secreted rhythmically.
    • Direct innervation from the central nervous system.

Synchronization and Optimization

  • The body's systems are coordinated so that hunger aligns with readiness for food intake.
  • Mental capacities peak when one is likely to be awake and engaging with the environment.
  • Muscle strength is maximal at appropriate times of day.
  • Clocks throughout the body optimizes tissue function for the time of day it is most in demand.

Chronomedicine

  • Chronomedicine recognizes that the body at midnight differs from the body at noon, with some predictability.
  • Blood pressure varies throughout the day.
  • Medication distribution and metabolism depend on the time of day of ingestion.
  • Fine-tuning medication delivery times can minimize side effects and maximize specific effects.

Disrupting the System

  • Disruptions occur by traveling across time zones, staying up late, getting up early, or having irregular sleep patterns.
  • Artificial light alters the light-dark schedule, disrupting timing.
  • Phase shifts lead to the need to reset the clock using a new light-dark schedule.
  • The brain clock adjusts relatively quickly, but clocks in other tissues (e.g., liver, lung, kidneys) rely on the brain clock and other mechanisms:
    • Body temperature rhythm changes.
    • Hormone changes.
    • Nerve signals
    • Rhythm of food intake.
  • Misalignment between the light-dark cycle and internal timekeeping leads to internal desynchrony.

Negative Outcomes of Disruption

  • Disrupted sleep-wake patterns impact cognitive function and mental health.
  • Negative outcomes are associated with increased risks for:
    • Cancer
    • Diabetes
    • Obesity
    • Stroke
    • Heart disease
    • Depression
    • Accidents
  • Decreased cognitive function increases the risk of accidents during tasks like driving.
  • The Exxon Valdez disaster was attributed to sleep deprivation.
  • Studying the brain clock is crucial due to its role in linking environmental light-dark information to internal organization.

Discovery of Internal Intracellular Rhythms

  • In 1971, Seymour Benzer and his postdoc Ronald Konopka conducted a genetic screen for mutations affecting rhythmic behavior in fruit flies.
  • They studied population-based changes in hatching, observing that flies preferentially hatch at dawn.
  • Mutations were found that abolished, sped up, or slowed down this rhythm.
  • Three mutations mapped to the same locus on the X chromosome.

Mutations and Timekeeping

  • Normal flies exhibit a peak in hatching at a specific time of day.
  • Mutations led to:
    • Arrhythmic hatching.
    • Increased hatching frequency (reduced periodicity).
    • Lengthened period.
  • The mutations all mapped to the same locus and did not complement each other.
  • The arrhythmic property was recessive relative to the other mutations.

Impact on Rest-Activity Rhythms

  • Researchers investigated whether the mutations affected timekeeping itself, rather than just hatching behavior.
  • Flies were placed in glass cubes with an infrared beam, and movement was tracked by beam interruptions.
  • The arrhythmic mutant disrupted the activity rhythm for individual flies, indicating the disruption of general timekeeping.
  • Short and long period mutations altered activity rhythms accordingly.
  • A locus termed "period" was identified as involved in daily timekeeping, aligning with control over timekeeping.

Properties of Daily Timekeepers

  • Control over overt rhythms: daily rhythms in blood pressure, cognitive strength, muscle strength, heart rate, and sleep-wake activity.
  • Entrainability: synchronization to the environmental light-dark cycle.
  • Autonomous timekeeping: maintenance of daily rhythm even without environmental cues (e.g., constant darkness).

Experimentation in Constant Darkness

  • Flies are placed in a device where movement in a glass tube is tracked via an infrared beam.
  • The number of events (beam interruptions) is plotted over time.
  • Flies in light-dark cycles synchronize their activity with the onset and offset of light, with a siesta in the middle of the day.
  • In constant darkness, flies maintain the rhythm but drift, indicating an internal period slightly shorter than 24 hours.
  • Human intrinsic clocks tend to run a bit slower than 24 hours.
  • Light cues allow adjustment to stay in 24-hour entrainment.

Temperature Robustness

  • Internal timekeeping remains robust despite changing temperatures.
  • The rhythm of approximately 24 hours stays the same at 18 degrees Celsius versus 29 degrees Celsius.
  • This is important because biochemical reactions typically speed up with increased temperature; mechanisms counteract this effect to maintain accurate timekeeping.

Molecular Mechanisms: Delayed Negative Feedback Loop

  • The Nobel Prize in 2017 was awarded to Jeff Hall, Michael Rosbash, and Michael Young for their discoveries of the molecular mechanisms controlling the circadian rhythm.
  • The circadian clock at the molecular level is a delayed negative feedback loop of gene expression.

Molecular Components and Processes

  • Transcriptional activators: Clock and Cycle drive expression of target genes via enhancer sites.
    • These are heterodimeric basic helix-loop-helix transcription factors that bind to CG palindromic sites.
    • Target genes include period (per) and timeless (tim).
  • Period and timeless are transcribed and translated in the cytoplasm.
  • Period is bound by double time (DBT): a kinase that phosphorylates serine and threonine residues of period, making it unstable and leading to degradation.
    • DBT is a casein kinase 1 delta/epsilon kinase.
  • Timeless binds to period: This protects period from phosphorylation by double time, stabilizing the complex.
  • The heterotrimeric complex (period, timeless, and double time) accumulates and enters the nucleus.
  • Once in the nucleus, the complex inhibits clock and cycle activity, displacing them from DNA and inactivating them.

Negative Feedback and Delay

  • Clock and cycle (transcriptional activators) make their own inhibitors (period and timeless).
  • There is a significant delay (at least six hours) between the activation phase (transcription of period and timeless) and the inhibition phase (proteins enter the nucleus).
  • When clock and cycle activity is inhibited, transcription stops, and no new protein is made.
  • The existing (old) protein is unstable and degrades:
    • The inhibition goes away when period and timeless run out.
    • Clock and Cycle become active again and starts the cycle over.

Turnover of Proteins

  • Period turnover:
    • Mediated by an SCF complex, an E3 ubiquitin ligase.
    • Ubiquitin is added to lysine residues of the substrate protein.
    • Forms a chain of polyubiquitin.
    • The chain is recognized by a proteasome, that degrades the proteins.
    • The E1, E2, and E3 ubiquitin ligases have an adaptor called an F-box protein which allows the SCF complex to target different substrates.
    • Double time-mediated phosphorylation of period makes it recognizable by the F-box protein and the SCF complex, leading to its turnover.
  • Timeless turnover:
    • Binds in a light-dependent way to the blue light photoreceptor cryptochrome.
    • In the presence of blue light, the binding with cryptochrome activates target tunnels for degradation that will destabilize period.
    • The binding leads to a phosphorylation event by an unknown kinase.
    • Allows timeless to be recognized by an F-box protein known as JetBlack or Jet, forming a link to an SCF complex and degradation.

Synchronizing to the Environmental Cycle

  • Light-dependent turnover of timeless synchronizes the clock to the environmental light-dark cycle.
  • Mechanism:
    • Light in the early night (late dusk):
      • Cryptochrome seeing light activates timeless for degradation, destabilizing in turn period.
      • Disrupts the accumulating period/timeless complex (ready to enter the nucleus).
      • Destabilizes period.
      • Delays the buildup of the complex, delaying negative feedback (phase delay).
    • Light in the late night:
      • Cannot prevent negative feedback (which has already occurred).
      • Enhances the resolution of negative feedback.
      • Speeds up the return to the active state.
      • Light in the late night advances the system by moving it faster to a stage where clock and cycle can be active again.

Mammalian Clock

  • The fruit fly clock is a paradigm for timekeeping across organisms, including humans.
  • The mammalian clock has similar components:
    • Clock and Cycle.
    • Binding sites.
    • Period
    • Casein kinase one, epsilon and delta.
  • Differences:
    • Instead of timeless, there is Cry (not homologous to Drosophila Cry, but acts like timeless).
    • Cryptochrome in mammals is not a photoreceptor.
    • Light-mediated signals from the eyes can reset brain clocks in flies and humans.
    • In the suprachiasmatic nucleus, light enhances the expression of period one and period two, synchronizing it with the external light-dark cycle.

Linking to Overt Rhythms (e.g., Sleep)

  • Clock control genes (genes controlled by the clock) are expressed rhythmically, making their functions rhythmic.

Connecting Clock Neurons to the Sleep-Wake Cycle in Fruit Flies

  • Three sets of neurons with clocks communicate with each other using neuropeptides:
    • Pigment dispersing factor (PDF): Equivalent to VIP in the human SCN.
    • Small neuropeptide F (sNPF): communicates with dorsal neurons.
  • Small ventral lateral neurons (sLN) and dorsolateral neurons (lLN) use sNPF.
  • Dopaminergic cells receive input from both light-active and dark-active pacemakers.
  • Dopaminergic neurons link to a central complex (ellipsoid body structure).
  • The central complex outputs to the ventral dopaminergic circuit, controlling rest and activity rhythms.
  • Dopamine relays timekeeping information from clocks to the activity circuit, controlling the sleep-wake rhythms.

Additional Circuit

  • Dorsal neurons project to the pars intercerebralis (equivalent to the hormonal axis in the human hypothalamus).
  • Corticotropin-releasing hormone equivalent (CRH) is released.
  • Goes to cells that express another neuropeptide (Hugin).
  • Feeds back on the clock itself.
    • Hugin is equivalent to human neuropeptide Y.
    • This is an external feedback loop.

Conclusion

  • Similar, potentially more complex circuits likely act in the human brain.