Introduction

• The pineal gland, also called the epiphysis, is a small endocrine gland in the brain.
• It has attracted scientific interest because of its possible role in regulating several brain functions.
• Its functions are not completely understood.
• Its best-known function is the production of melatonin, which helps regulate the sleep-wake cycle and other daily rhythms.
• In fish, amphibians, and reptiles, the pineal gland can act as a light-sensitive organ.
• In birds, it contributes to the biological clock.
• In mammals, including humans, it functions mainly as an endocrine gland.
• Further research is needed to understand its role in human physiology and evolution.

Physiological Anatomy

• The pineal gland develops from an outgrowth of the roof of the third ventricle, between the habenular and posterior commissures. It forms part of the epithalamus, along with these two commissures.
• The gland is attached to the brain by a stalk, which connects it with the habenular and posterior commissures. Its tissue contains pinealocytes, neuroglial cells, and supporting parenchymal tissue.
• Pinealocytes are the main secretory cells and produce melatonin.
• The gland is relatively large during infancy and childhood. It becomes smaller around puberty and remains small in adults. Melatonin levels are higher in children, about 250 pg/mL, compared with approximately 70 pg/mL in adults.

Hormones of the Pineal Gland

• The pineal gland mainly secretes melatonin. It may also produce several other peptides that can influence the hypothalamus and other endocrine glands.
• Pineal peptides include:
  • Arginine vasotocin and arginine vasopressin
  • Oxytocin
  • ACTH, α-MSH and β-endorphin
  • Angiotensin II and renin-like substances
  • TRH-like and GnRH-like peptides

• These findings suggest that the pineal gland may have a wider role in endocrine regulation.

Melatonin

1. Melatonin is chemically known as N-acetyl-5-methoxytryptamine and is formed from tryptophan. It is produced by pinealocytes and released into the blood and cerebrospinal fluid.
2. Melatonin secretion follows a daily rhythm. Levels are highest during the night, particularly from about 11 pm to 7 am.
3. Light and darkness regulate melatonin production through the retinohypothalamic pathway. Light signals reach the suprachiasmatic nucleus (SCN), the body’s main biological clock. The SCN sends signals through the spinal cord to the superior cervical ganglion.
4. Sympathetic nerve fibers from this ganglion reach the pineal gland and regulate melatonin secretion, mainly through norepinephrine and β-adrenergic receptors.

Melatonin Receptors

• Two main melatonin receptor types are described: MT1 (ML1) and MT2 (ML2).

1. MT1 receptors mainly act through changes in cAMP. They include MT1A and MT1B subtypes.
2. MT2 receptors act mainly through the IP3-DAG signaling pathway.

Functions of Pineal Gland

1. Regulation of biological rhythms: Melatonin secretion changes throughout the day and helps synchronize body functions with the light-dark cycle.
2. Effect on reproductive function: Melatonin may suppress gonadal activity and influence the timing of puberty. Reduced pineal activity is associated with the onset of pubertal changes.
3. Abnormal pineal function has been reported in some cases of precocious puberty, although the exact relationship is not fully established.
4. Skin pigmentation: In some lower animals, melatonin affects melanophores and changes skin color. This role is not clearly established in humans.