MELATONIN

Introduction:
• Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine.
• Hormone secreted by the pineal gland.
• Chemical structure identified in 1958.
• Expressed rhythmically throughout the day.
• Extrapineal melatonin : retinal , gastrointestinal and harderian
gland melatonin.

Precursor: Tryptophan.
Enzymes involved:
Melatonin biosynthesis
Precursor: Tryptophan.
Enzymes involved:
• Tryptophan-5-hydroxylase
• 5HTP-decarboxylase
• Serotnin-N-acetyltransferase
• Hydroxyindole-O-methyltransferase
Rate limiting step:
Serotonin-N-acetyltransferase

• Blood melatonin is mainly bound to albumin (70%). Circulating melatonin can
reach all body tissues including brain and is able to cross the blood–brain
barrier to modulate brain activity.
• Melatonin secretion starts at 3-4 months of age when nighttime sleeping
consolidates.
• Peak levels at 1-3 years of age.
• Slightly lower levels through early adulthood and marked decline in levels
afterwards.
Melatonin secretion :
The 24-Hour Cycle of Melatonin Production

Melatonin catabolism :
Melatonin

Regulation of melatonin synthesis

Regulation of melatonin synthesis

MELATONIN ACTION :
Melatonin has a variety of means by which it influences the physiology of organism:
1. RECEPTOR MEDIATED ACTIONS
2. RECEPTOR INDEPENDENT ACTIONS
1. Receptor mediated action :
Major membrane receptors
They are members of the G protein coupled
receptor family with seven transmembrane
domains.
• MT1 and MT2 receptor
• MT3 receptor
MT3, is seemingly identical to the cystosolic
enzyme, quinone reductase 2 (QR2)

Melatonin released in the pineal gland travels back to the SCN, where it interacts
with MT1 and MT2 receptors
MT1 suppresses SCN firing, reducing alertness
MT2 is responsible for circadian phase shifts
Mechanism of MT1 and MT2 receptor mediated action

Non receptor mediated action of melatonin

Melatonin receptors and signal transduction mechanism

Antioxidant activity
Mediates many hormone
functions
Immunity
Control circardian rhythymicity
and seasonality
Vital part of many biochemical systems
including sleep and learning.
An endogenous synchroniser
Functions of melatonin

Potential medical indications
• Cancer
• Immunology
• Menstrual cycle - fertility
• Puberty onset
• Learning and memory
• Treatment of circadian rhythm
disorders- jet lag
• Mood disorders-SAD.

CONCLUSION:
•Melatonin is aften referred to as “a time giver hormone” and also “ the
hormone of darkness”.
•Finally, melatonin is generally classified as a hormone, it is in fact a molecule
with paracrine, autocrine and antioxidant actions.
•Considering its diverse actions via both receptor and receptor-independent
actions, to classify melatonin exclusively as a hormone seems inappropriate.

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