Give a brief account of synthesis, storage, secretion and functions of the posterior pituitary hormones.

Synthesis and Storage

The posterior pituitary hormones, Oxytocin and Vasopressin (ADH), are synthesized not in the posterior pituitary itself, but in specialized neurosecretory cells located in the hypothalamus.

• Synthesis in Hypothalamus:
  • Oxytocin: Primarily synthesized by the magnocellular neurons in the paraventricular nucleus (PVN) of the hypothalamus.
  • Vasopressin (ADH): Primarily synthesized by the magnocellular neurons in the supraoptic nucleus (SON) of the hypothalamus.
• Precursor Molecules: Both hormones are initially synthesized as larger inactive precursor proteins (e.g., pro-oxytocin and pro-vasopressin) along with their respective neurophysins (neurophysin I for oxytocin and neurophysin II for vasopressin).
• Axonal Transport: These precursor molecules, packaged into secretory vesicles, are transported down the axons of the hypothalamic neurons through the infundibulum (pituitary stalk) to the posterior pituitary. Cleavage of the precursor molecules into active hormones and their neurophysins occurs during this transport.
• Storage in Herring Bodies: Upon reaching the posterior pituitary, the active hormones are stored in specialized swellings at the axon terminals, known as Herring bodies. These bodies represent the terminal ends of the axons.

Secretion

The secretion of oxytocin and vasopressin from the posterior pituitary is a neuroendocrine process, triggered by specific physiological stimuli.

• Neural Stimulation: When appropriate physiological stimuli occur, the hypothalamic neurons generate action potentials. These electrical signals travel down the axons to the Herring bodies in the posterior pituitary.
• Exocytosis: The action potentials cause the depolarization of the axon terminals, leading to an influx of calcium ions. This calcium influx triggers the fusion of the secretory vesicles with the cell membrane, releasing the hormones (Oxytocin and Vasopressin) into the surrounding capillary plexus via exocytosis.
• Entry into Bloodstream: From the capillaries, the hormones enter the systemic circulation to act on their target organs.
• Regulation:
  • Oxytocin: Secretion is primarily regulated by positive feedback mechanisms, such as uterine stretching during labor (Ferguson reflex) and suckling during lactation.
  • Vasopressin (ADH): Secretion is mainly regulated by changes in plasma osmolarity (detected by osmoreceptors in the hypothalamus) and blood volume/pressure (detected by baroreceptors). Increased osmolarity or decreased blood volume/pressure stimulates ADH release.

Functions of Posterior Pituitary Hormones

1. Oxytocin

Oxytocin is a versatile peptide hormone often referred to as the "love hormone" due to its roles in social bonding, but its primary physiological functions are reproductive.

• Uterine Contractions: Stimulates strong contractions of the smooth muscle of the uterus during childbirth, facilitating labor and delivery. Its receptors in the uterus significantly increase towards the end of pregnancy.
• Milk Ejection Reflex (Lactation): Causes the contraction of myoepithelial cells surrounding the alveoli in the mammary glands, leading to the "milk let-down" reflex during breastfeeding.
• Maternal Bonding: Plays a crucial role in promoting maternal-infant bonding and attachment after childbirth.
• Social Behavior: Involved in social recognition, trust, and pair-bonding in both males and females.
• Male Reproductive Function: Influences sperm transport and plays a role in ejaculation by contracting the vas deferens.

2. Vasopressin (Antidiuretic Hormone - ADH)

Vasopressin is a key regulator of the body's water balance and blood pressure.

• Water Reabsorption in Kidneys: Acts on the collecting ducts and distal convoluted tubules of the kidneys, increasing their permeability to water. This leads to increased water reabsorption back into the bloodstream, thereby reducing urine output and concentrating the urine. This is its primary antidiuretic effect.
• Regulation of Plasma Osmolarity: By controlling water reabsorption, ADH helps maintain the osmotic balance of body fluids.
• Vasoconstriction: At higher concentrations, ADH also acts on vascular smooth muscle, causing vasoconstriction, which helps to increase blood pressure. This effect is particularly important in situations of severe blood loss or hypotension.
• Thirst Regulation: High levels of ADH can also stimulate the sensation of thirst, encouraging water intake.

Feature	Oxytocin	Vasopressin (ADH)
Primary Synthesis Site	Paraventricular Nucleus (PVN)	Supraoptic Nucleus (SON)
Main Functions	Uterine contractions, milk ejection, social bonding	Water reabsorption, vasoconstriction, blood pressure regulation
Key Stimuli for Secretion	Uterine stretching, nipple suckling	Increased plasma osmolarity, decreased blood volume/pressure
Target Organs	Uterus, mammary glands, brain	Kidneys (collecting ducts, distal tubules), blood vessels