Posterior Pituitary

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Posterior Pituitary

• Post Pituitary differs from Anterior Pituitary
  in
• Embryological origin
• Function
• Vasculature
• Hormones

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Posterior Pituitary (neurohypophysis)

• Cell bodies in SON (ADH) PVN (Oxytocin) of
  hypothalamus
• Consists of neural endings with
• associated blood vessels
• Acts as storage area, secretory
• granules travel down axon
• Connects to hypothalamus via
• hypothalamic-hypophyseal tract
• Processes extend through infundibulum and end in
  Post. Pit

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Primary positive signal
Fig. 4-10A Page 117
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Oxytocin and Vasopressin (Antidiuretic Hormone)
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Vasopressin (ADH)
Posterior Pituitary Hormone, nonapeptide
Arginine 8 is essential for antidiuretic activity
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Oxytocin
Posterior Pituitary Hormone, nonapeptide
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Posterior Pituitary Hormones

• Vasopressin (VP or AVP) aka Antidiuretic Hormone
  (ADH)
• VP neurons are majority of supraoptic nucleus
• Controls readsorption of water by kidney
• Regulates osmotic pressure (via sodium levels)
• Regulates blood pressure (secreted when pressure
  decreases)
• Oxytocin (OT)
• OT neurons are majority of paraventricular
  nucleus
• Milk ejection (suckling)
• Uterine myometrial contraction (parturition)
• Behavorial effects

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Oxytocin

• Ejection of milk stimulated by suckling/ nipple
  stimulation
• Pulsatile oxytocin release
• Role in parturition
• Induces contraction of myometrium
• Contractions during labor

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Oxytocin

• Human Sexual Response
• Oxytocin may have a role in sexual arousal,
  orgasm sexual satiety/satisfaction
• CNS maternal behavior, penile erection,
  copulatory behavior, yawning, trust
• Maternal Behavior
• Nest building, retrieval, crouching, cleaning
• Oxy induces maternal behavior in virgin female
  rats (effect is amplified by E)

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Neuroendocrine Reflex Arc of Milk Let-Down
Takes only milliseconds from suckling to
hypothalamus OT reaches its target a few seconds
later OT binds receptors on myoepithelial cells
of mammary gland
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Neuroendocrine Reflex
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Oxytocin Uterine Contractions

• Oxytocin contracts muscle layer of uterus
• Assists in labor, probably does not initiate it
• Oxytocin receptors in uterine tissue increased by
  E, decreased by P
• Uterus can make its own oxytocin
• levels increase 150x during pregnancy
• exceed levels of OXY in hypothalamus

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Neuroendocrine Reflex
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Antidiuretic Hormone (Arginine Vasopressin)

• Diuresis urine production
• Antidiuretic prevents urine formation
• ADH prevents large changes in water balance
  (dehydration or overload)
• Target Kidney distal and collecting ducts
• Increases water permeability reabsorption
• Decreases urine production
• Increases blood volume

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Vasopressin (AVP) stimulates synthesis of
aquaporin-2 (AQP) water channel proteins and
their transport to the apical surface of
collecting duct principal cells
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Baroregulation of ADH/Vasopressin

• Blood Volume/Pressure Regulation
• Baroreceptors located in left atrium, aortic arch
  carotid sinus detect changes in pressure can
  regulate ADH secretion
• Hypotension (decreased blood pressure) causes
  increase in ADH secretion
• ADH acts to correct hypovolemia

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Osmoregulation of Vasopressin

• Osmoreceptors in the anterior hypothalamus
  respond to changes in blood osmolality of 1 or
  less.
• A 2 decrease in total body water will increase
  plasma osmolality
• Plasma ADH/Vasopressin will rise to 2-4 pg/ml and
  urine will be maximally concentrated.

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ADH/Vasopressin

• Other Regulators of Secretion
• Hemorrhage (increases ADH secretion)
• Alcohol (suppresses ADH secretion)

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ADH/Vasopressin

• Other Factors Altering Secretion
• Diabetes insipidus- (tasteless, not sweet)
• Decreased ADH output or response
• High urine volume (dilute)

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Diabetes Insipidus

• Excessive loss of water (up to 25 liters/day)
• Types
• 1. Pit/Hypothal Lack of ADH/VP secretion
• 2. Nephrogenic ADH resistance in the kidney
• Causes
• Pit/Hypothal Acquired
• Head trauma, Pit. Stalk lesion, Tumor in hypothal
  or pit
• ADH levels low
• 2. Nephrogenic Hereditary
• Receptor (V2) malfunction in tubules
• ADH normal or elevated

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Diabetes Insipidus Treatment

• Pit/Hypothal
• Mild salt depletion
• Adequate hydration
• Vasopressin therapy (IM nasal)
• Nephrogenic
• Vasopressin therapy ineffective in AVP receptor
  mutations
• Mild salt depletion
• Adequate hydration
• Infusion of hypotonic solutions (severe cases)

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Other Factors Altering ADH Secretion

• SIADH (syndrome of inappropriate ADH secretion)
• Hypersecretion of ADH
• Person retains water
• Blood volume pressure increase
• Causes Tumors, tuberculosis, head trauma,
  anesthesia, surgery, pain, brain edema,
  meningitis, etc.