Circadian Rhythms & Melatonin

What are some effects melatonin has on target organs?

- Cytosol = detoxification, enzyme regulation - Nucleus = immune modulation - Membrane = seasonal reproduction, bone growth, blood pressure modulation - Free radical scavenging = protection against ionizing radiation, heavy metal toxicity and alcohol toxicity

What is melatonin?

- Involved in the entrainment of circadian rhythms of physiological functions - Synthesized in the pineal gland (on top of the thalamus) - Light information is transduced by the retina - Melanopsin-containing retinal ganglion cells typically respond to blue light and are responsible for suppressing melatonin release (in the presence of blue light; melatonin is not synthesized)

What are the hormones that are tightly regulated by the Circadian rhythms?

- Melatonin levels are highest at night and start to decrease when you wake up (triggered by the dark) - Cortisol increases over sleep time and is the highest in the mornings when you wake up - GH peaks immediately after you fall asleep and gradually decreases

What are Circadian rhythms?

- Physical, mental and behavioural changes that follow a roughly 24 hour cycle - Controlled by the suprachiasmatic nucleus of the hypothalamus; large axons come in from the retina to input information - Influence secretion of numerous hormones

How is melatonin released?

- Suprachiasmatic nucleus projects to the paraventricular nucleus which then projects to the thoracic level of the spinal cord and activates the sympathetic nervous system (releases norepinephrine) - Norepinephrine binds to target receptors in the pineal gland and increases cAMP and PKA - In the dark, NAT activates serotonin to convert to melatonin - In blue light, the ipRGC are activated and activate cells in the SCN which release GABA and inhibits the pathway - Melatonin release can travel back to the SCN and bind to receptors which allow it to control the circadian rhythms