[Melatonin. I. Physiology of its secretion]

Area of Science:

• Neuroendocrinology
• Chronobiology
• Hormone Research

Background:

• Melatonin is primarily synthesized by the pineal gland, acting as a key neuroendocrine transducer of environmental light-dark cycles.
• Its secretion follows an endogenous circadian rhythm but is significantly modulated by external light cues.
• Melatonin plays a crucial role in signaling day-night patterns to mammals.

Purpose of the Study:

• To elucidate the regulatory mechanisms of melatonin secretion.
• To identify factors influencing melatonin levels beyond the light-dark cycle.
• To understand the diverse physiological actions of melatonin.

Main Methods:

• Review of neuroanatomical pathways involved in melatonin synthesis.
• Analysis of physiological and pathological factors affecting melatonin secretion.
• Examination of melatonin's receptor-mediated actions in various organs.

Main Results:

• Light inhibits melatonin secretion during the day; norepinephrine stimulates synthesis at night.
• Factors such as electromagnetic fields, age, sex, certain medical conditions (e.g., Cushing syndrome), and substance use (alcoholism) can lower melatonin levels.
• Female sex, specific hormonal conditions (hypergonadotropic hypogonadism), exercise, and fasting are associated with increased melatonin secretion.

Conclusions:

• Melatonin secretion is intricately regulated by light, neurochemistry, and various endogenous and exogenous factors.
• Numerous physiological and pathological conditions can alter melatonin levels, impacting its signaling functions.
• Melatonin exerts widespread effects through specific receptors in multiple organ systems.