Pineal Gland

Question 1

Pineal Gland

Answer 1

• Small endocrine gland located near the brain’s thalamus
• Highly vascularized
• Weak BBB
• Attached to the posterior end of the root of the third ventricle of the brain
• Neuronal connections to the suprachiasmatic nucleus (SCN)
• Functions as key regulator of circadian and seasonal rhythms

Question 2

Pinealocytes

Answer 2

Secrete melatonin (influences various physiological processes)

Question 3

Suprachiasmatic Nucleus (SCN)

Answer 3

• Primary circadian pacemaker
• Receives light information from the retina via the retinohypothalamic tract (RH)
• Indirect connection: involving multiple relay stations:
  1. SCN -> paraventricular nucleus (PVN)
  2. PVN -> intermediolateral cell column (IML) of spinal cord
  3. IML -> superior cervical ganglion (SCG)
  4. SCG -> pineal gland (via sympathetic neurons)

Question 4

How does the dark impact melatonin synthesis?

Answer 4

• SCN stimulates norepinephrine release in the pineal gland
• Activates beta-adrenergic receptors, increasing cAMP levels
• Enhances serotonin N-acetyltransferase (SNAT) activity
• Melatonin synthesis
• Negative feedback

Question 5

How does the light impact melatonin synthesis?

Answer 5

• SCN inhibits signaling cascade
• Decrease in melatonin production

Question 6

Explain melatonin synthesis.

Answer 6

• Synthesized from tryptophan:
  1. Tryptophan -> 5-hydroxytryptophan -> serotonin
  2. Serotonin -> N-acetylserotonin -> melatonin
• Serotonin N-acetyltransferase (SNAT) converts serotonin to N-Acetylserotonin (NAS) and is the RLS
• Step that is regulated by dark-light cycle is the conversion of serotonin to N-acetyl serotonin

Question 7

What are the melatonin receptors and what is the pathway?

Answer 7

• MT1 and MT2 (GPCRs)
• MT1: regulates sleep and circadian rhythms (higher affinity)
• MT2: influences phase shifting and seasonal responses
• Pathways: cAMP inhibition -> decreased PKA activity -> MAPK signaling -> modulation of calcium channels

Question 8

What are the target organs for melatonin?

Answer 8

1. Brain: hypothalamus (circadian rhythm regulation) and pituitary (hormonal modulation)
2. Peripheral organs: reproductive organs (seasonal breeding: melatonin inhibits GnRH), immune system (immune response regulation), cardiovascular (blood pressure regulation), GI (motility and secretions)

Question 9

Sleep Disorders

Answer 9

• Disruption in the SCN-pineal pathway can lead to sleep disorders and circadian misalignment
• Nighttime light exposure suppresses melatonin production

Question 10

Jet Lag

Answer 10

Circadian rhythm resynchronization