circadian Rhythms

Circadian Rhythms Learning Objectives

• Understand circadian rhythms and their mechanisms.

• Identify the role of the suprachiasmatic nucleus (SCN) in regulating circadian rhythms.

• Learn how light influences circadian rhythms through entrainment.

• Explore the molecular clock mechanisms involved in the maintenance of circadian rhythms.

• Understand neural pathways associated with sleep and wakefulness.

• Analyze the feedback loop that regulates sleep and wake cycles.

Circadian Systems Biological Rhythms

1. Definition of Circadian:

   • The term "circadian" is derived from Latin:

     • "circa" meaning "about"

     • "diem" meaning "day"

   • Refers to biological processes that follow a roughly 24-hour cycle.

2. History:

   • The term was coined in 1959 by Franz Halberg.

3. Types of Biological Rhythms:

   • Circatidal: Rhythms that cycle with the tides.

   • Circannual: Yearly rhythms.

   • Circalunal: Rhythms that follow the lunar cycle (waxing and waning of the moon).

     • Infradian rhythms: Longer than 24 hours.

     • Ultradian rhythms: Shorter than 24 hours.

   • Quote: "All biological clocks are adaptations to life on a rotating world."

Circadian Systems Rhythmic Outputs

• Mairan's Experiment (1729):

  • Jean-Jacques d'Ortous de Mairan tested a heliotrope plant in darkness to check if daily leaf movements were solar-dependent.

  • Results showed that the leaves still moved in a rhythmic manner, suggesting an endogenous origin.

• Current Knowledge:

  • The understanding of circadian rhythms is largely based on observed cellular and physiological processes, including:
    a) Mammalian Rhythms: Digestion, body temperature regulation, hormone secretion, and sleep onset timings.
    b) Cyanobacteria Rhythms: Processes like photosynthesis, nitrogen fixation, and cell division.

Circadian Rhythmicity

• Sleep/Wake Cycle:

• Illustrated by colors where red signifies sleep and blue denotes wakefulness.

• Most organisms display circadian rhythms lasting around 24 hours; for humans, it's slightly longer than 24 hours.

• Free-running rhythms can occur without external cues. Entrainment: The process of syncing the circadian rhythm with external signals, particularly light.

Core Biological Indicators

• Core body temperature and blood levels of growth hormone and cortisol exhibit circadian patterns.

Cells Involved in Circadian Entrainment

1. Role of Light:

   • Entrainment is primarily driven by light, not to be confused with the involvement of rods and cones in vision.

   • Light Responsive Retinal Ganglion Cells (RGCs):

     • These cells transduce necessary light signals.

     • A minor subset expresses opsins 4 and 5, which are important for light sensitivity.

2. Spectral Sensitivity:

   • Photosensitive RGCs are less sensitive to longer light wavelengths compared to traditional photoreceptors (rods and cones).

Retinal Ganglion Cells Projection

• RGCs extend from the eye at the optic nerve head and project to the SCN via the retinohypothalamic tract.

• The SCN acts as the pacemaker for circadian rhythms, generating the endogenous free-running rhythm.

• Function of SCN:

  • Regulates the secretion of melatonin from the pineal gland.

  • Influences other neural structures including the paraventricular nucleus (PVH), intermediolateral cell column (IML), and superior cervical ganglion (SCG).

SCN and Lesions

1. SCN/pacemaker Functionality:

   • SCN lesions lead to arrhythmic conditions.

   • When SCN neurons are cultured, they still exhibit rhythmic firing.

   • Reintroduction of SCN to lesioned animals restores circadian activity, regardless of species.

2. Melatonin Production:

   • Controlled by the SCN, variations in melatonin output coincide with light variations:

     • Melatonin assists in promoting sleep, reaching peak levels around 3 AM.

     • It regulates the sleep/wake cycle dynamics.

Regulation of Rhythms by Melatonin

1. Melatonin Secretion:

   • Even blind individuals exhibit a rhythm of melatonin secretion (approx. 24 hours).

2. Melatonin Average Levels:

   • Example data showing melatonin levels in pg/ml across certain days indicates daily fluctuations inherent in the circadian rhythm.

3. Consequences of Lack of Entrainment:

   • Leads to non-24-hour sleep-wake disorder, which presents unique challenges in circadian regulation.

Effects of Light on Circadian Clock

1. Phase-Dependent Effects:

   • Light exposure impacts the circadian clock's phase, influencing sleep regulation differently depending on timing.

     • Light pulse during light cycle: No impact.

     • Light pulse close to dark cycle: Phase delay.

     • Light pulse during dark cycle: Phase delay.

     • Light pulse close to the end of dark cycle: Phase advance.

   • Phase Response Curve (PRC): A graphical representation of how light influences circadian phases.

Molecular Clock Mechanism

1. Clock-Controlled Genes (ccg):

   • Govern biological processes regulated by circadian mechanisms.

   • Comprises dimers of two primary proteins: BMAL1 and CLOCK.

     • These are basic helix-loop-helix (bHLH) PAS transcription factors.

   • They bind to E boxes (CACGTG) in ccg promoters, essential for the transcription process.

   • BMAL1 Dynamics: Levels peak in the morning and decline throughout the day, while CLOCK is consistently present throughout.

2. Pacific Cycle of Regulation:
   a) PER and CRY Proteins:

   • Circadian rhythms are also controlled by the protein complex comprised of PERIOD (PER), CRYPTOCHROME (CRY), and CASEIN KINASE 1 epsilon/delta (CK1 e/d).

   • PER accumulates in the cytoplasm during the day and interacts with CK1.

   • The presence of CRY is vital for preventing PER from being unstable, thus it stabilizes the complex, facilitating its transport to the nucleus where transcription inhibition occurs.

3. Interlocking Cycles:

   • The regulation involves two interlocking cycles: a) First Cycle:

     • The BMAL1:CLOCK complex upregulates per and cry genes via E boxes.

     • The PER:CRY:CK1 complex then inhibits their own transcription by inhibiting actions by the BMAL1:CLOCK complex.
       b) Second Cycle:

     • The specific upregulation of rev-erb a and ROR (retinoid-related orphan receptor) genes by BMAL:CLOCK.

     • The interaction of these proteins at their respective binding sites influences bmal 1 gene expression (upregulation by ROR alone and suppression when both ROR and REV-ERBa are present).

Neural Pathways in Sleep/Wake Regulation

1. EEG Patterns During Sleep:

   • The first hour of sleep features several brain wave patterns:

     • Beta Waves: High frequency (15-60 Hz) and low amplitude (~30μV).

     • Theta Waves: Falling frequency (4-8 Hz) and rising amplitude (50-100μV).

     • Sleep Spindles: Increasing frequency (10-12 Hz) and rising amplitude (50-150μV).

     • Delta Waves: Low frequency (0.5-4 Hz) and low amplitude (~100-150μV).

   • Characterized by rapid eye movements (REM).

2. Physiological Changes During Sleep Stages:

   • Notable changes through the stages of sleep:

     • Duration of REM sleep increases over the night (10 to 50 minutes).

     • Stage IV sleep occurs primarily during the first two cycles.

   • Heart rate and respiration slow during non-REM sleep, while both increase during REM sleep, with penile erections observed.

3. Central Role of Neural Circuits:

   • The activation of specific cholinergic neurons can awaken sleep states.

   • Electrical activity in the thalamus can trigger sleep in awake animals.

4. Key Nuclei in Sleep Regulation:

   • Relevant brain structures include:

     • Reticular Activating System (cholinergic), Locus Coeruleus (noradrenergic), and Raphe Nuclei (serotonin).

     • Activation of these networks promotes wakefulness, while decreased activity in these structures facilitates non-REM sleep.

     • The tuberomammillary nucleus (TMN), modulated by orexin-secreting neurons, plays a crucial role.

     • Antihistamines may induce drowsiness by inhibiting the TMN network.

5. Hypothalamic Neurons Impacting Sleep:

   • The Ventrolateral Preoptic Nucleus (VLPO) is critical for sleep.

   • Lesions in the VLPO result in chronic insomnia.

   • These neurons work by inhibiting pathways necessary for wakefulness.

Connection of Circadian System and Sleep/Wake Cycle

• The Dorsomedial Hypothalamus influences circadian behavior in diurnal and nocturnal animals.

  • Diurnal species activate the SCN through RGCs, while the VLPO may respond to the same signals in nocturnal species.

Thalamocortical Neurons and Sleep Cycle

1. Thalamic Activity Modes:

   • Thalamocortical neurons operate in two activity modes:

     • Tonic activity—indicates wakefulness, activated by brainstem inputs.

     • Oscillatory pulses—indicate sleep, occurring when input diminishes.

2. Thalamocortical Feedback Loop:

   • This loop involves excitatory interactions between thalamocortical neurons, pyramidal cortical neurons, and thalamic reticular neurons.

   • Thalamic reticular neurons provide inhibitory control over other thalamocortical neurons, which is essential for sleep spindle activity and the generation of specific oscillations.

K-complexes and Cortical Synchronization

• These structures facilitate synchronization within the thalamo-cortical network and generate sleep oscillations integrated with delta waves and sleep spindles.

• K-complexes serve to suppress neuronal activity related to wakefulness and help reset cortical synaptic thresholds, which is vital for memory consolidation.

Theories on Sleep Function