Hypothalamic Regulation of the Pituitary Gland

Control of the Pituitary Gland
1. Directly by neuronal input
2. Indirectly by local blood flow
3. Pituitary releases chemical mediators into the circulation
  1. Act on target organs, causing the release of hormones
Pituitary Gland
1. Hypophysis
2. Small endocrine gland at the base of the brain
3. Ventral to the hypothalamus, within a pit of bone called the sella turcica
4. Infundibulum is the pituitary 'stalk'
  1. Consists of blood vessels and axons, connecting the pituitary to the hypothalamus
5. Two lobes
  1. Anterior
    1. Adenohypophysis
    2. Epithelial tissue
    3. Parvicellular neurons in the hypothalamus secrete mediators (hypophysiotropic hormones) into the portal system
      1. The portal system is a network of blood vessels between the hypothalamus and the pituitary. The hypothalamus releases peptides into this system at the median eminence. The vessels are at the capillary bed at the bottom of the third ventricle, and travel up the infundibulum, allowing the passage of hormones to cause stimulation or inhibition of pituitary hormone release. These hormones cause peripheral hormone release in target organs
        Negative feedback loop - peripheral hormones bind to the hypothalamus or pituitary, causing a decrease in the release of chemical mediators
      2. These hormones then act upon the pituitary to secrete hormones
  2. Posterior
    1. Neurohypophysis
    2. Neural tissue
    3. Magnocellular neurons in the hypothalamus project neurons to the pituitary and releases mediators
      1. These are stored locally, to be released later
Hypothalamic-Pituitary Axis
1. Releases hormones in rhythms
  1. Ultradian
    1. Release occurs in pulses, every 1-3 hours
    2. e.g. TRH, GHRH
  2. Circadian
    1. Release occurs once or twice per day
    2. e.g. CRH
  3. Seasonal
    1. e.g. GnRH, LH, FSH
    2. Usually reproductive hormones
Thyroid Axis
1. TRH
  1. Tripeptide released from the paraventricular nucleus of the hypothalamus
  2. Acts upon a G-protein coupled receptor (GPCR) on thyrotropes in the anterior pituitary
    1. Causes the release of TSH
2. TSH
  1. Glycoprotein (with α and β subunits) that act on the TSH receptor of the thyroid gland
3. Thyroid gland
  1. Releases triiodothyronine (T3) and thyroxine (T4) into the blood
    1. Receptors all over the body
    2. Important during development and for maintaining metabolic rate
Somatotrophic Axis
1. GRH
  1. Peptide released from the arcuate nucleus of the hypothalamus
  2. Acts upon somatotropes in the anterior pituitary to release growth hormone (GH)
    1. Somatostatin can also be released, reducing GH release
2. Growth hormone (GH)
  1. Peptide that acts upon a cytokine receptor in the liver, releasing IGF-1
    1. IGF-1 acts on a tyrosine kinase
    2. GH and IGF-1 act together during development and growth
Posterior Pituitary
1. Anatomical and functional extension of the hypothalamus
2. Paraventricular (PVN) and Supraoptic (SON) nuclei in theh hypothalamus synthesise oxytocin and vasopressin, which are packaged into vesicles
  1. These are transported down axons into the infundibulum via the median eminence before being secreted into the capillaries of the posterior pituitary for later release
  2. Oxytocin
    1. Causes rhythmic contractions
      1. Milk ejection reflex
        Mammary myoepithelial cells
      2. Labour
        Uterine muscle
    2. 'Pulses'
      1. Lots of oxytocin neurons in the PVN/SON activated at the same time, followed by 'quiet' periods
      2. Released into the posterior pituitary and blood flow to act on receptors in target tissues
    3. Acts on lactotropes in the anterior pituitary to stimulate the release of prolactin during milk production
    4. Release is affected by mood
      1. Anxiety inhibits release
  3. Vasopressin
    1. Regulates fluid balance
    2. Baroreceptors in the aorta or brainstem, or osmotic imbalance via circumventricular organs will trigger release
    3. Acts upon V2 receptors in the kidney to reduce urine output
      1. Kidney also releases renin, which converts angiotensinogen (liver protein) into angiotensin I and then angiotensin II
        Signals thirst and vasopressin release
    4. Loss of vasopressin secretion e.g. by PVN/SON damage or defective V2 receptor signalling can lead to Diabetes insipidus
3. Does not produce its own hormones

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Hypothalamic Regulation of the Pituitary Gland

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	Created by Lydia Buckmaster almost 11 years ago