Physiology Chapter 2: Hypothalamic Hormones

Hypothalamic Hormones and Their Functions

• Hypothalamic hormones regulate anterior pituitary trophic hormones, which determine target gland secretion.
• The hypothalamus is located at the base of the brain in the diencephalon, housing neurons critical for hormone regulation.
• The hypothalamic median eminence forms the vascular link between the hypothalamus and the anterior pituitary gland.

The Pituitary Gland

• The pituitary gland, referred to as the "master gland," resides in the sella turcica of the sphenoid bone and weighs approximately 0.5 g.
• Its weight can double during puberty or pregnancy.
• The pituitary is divided into two parts:
  • Adenohypophysis (anterior pituitary) - accounts for 75% of gland weight.
  • Neurohypophysis (posterior pituitary) - originates from the brain as an outgrowth from the hypothalamus.

Vascular Connection

• The hypothalamic-hypophysial portal system links the hypothalamus to the anterior pituitary, facilitating the transport of hormones.
• The anterior pituitary is the most vascularized tissue in mammals, receiving 0.8 ml/g/min from arterial blood supplied via internal carotid arteries.

Hypothalamic Hormones Regulating the Anterior Pituitary

• Thyrotropin Releasing Hormone (TRH):

  • A tripeptide composed of glutamic acid, histidine, and proline, with a structure of 242 amino acids.
  • Stimulates the release of Thyroid Stimulating Hormone (TSH).
  • Located in the medial portions of the paraventricular nuclei, with a half-life of 6 minutes.

• Gonadotropin Releasing Hormone (GnRH):

  • A linear decapeptide that stimulates LH and FSH secretion, dependent on its pulse frequency.
  • Secreting neurons reside in the preoptic area of the anterior hypothalamus, with a half-life of 2-4 minutes.

• Dopamine:

  • Produced in the arcuate nuclei, acting as the primary inhibitor of prolactin release (PRL).
  • Has a half-life of 1-2 minutes.

• Corticotropin Releasing Hormone (CRH):

  • A 41-amino acid peptide that undergoes extensive synthesis, increasing during late pregnancy and delivery.
  • Regulates ACTH secretion, with neurons located in the paraventricular nuclei.

• Growth Hormone Releasing Hormone (GHRH):

  • A peptide hormone comprising 108 amino acids, stimulating GH release from somatotrophs in the anterior pituitary.
  • Contains a major isoform of 44 amino acids and has a half-life of 3-7 minutes.

Other Hormones and Their Actions

• Somatostatin:
  • A tetradecapeptide with 14 amino acids, inhibits multiple hormones including GH, TSH, insulin, and glucagon.
  • Found in delta cells of pancreatic islets, with a half-life of 2-3 minutes.

Mechanism of Action

• Hypothalamic hormones bind to specific plasma membrane receptors on target cells, triggering intracellular signaling pathways.
• They activate phosphatidylinositol metabolism and the adenylate cyclase system, elevating intracellular calcium levels.
• The actions of hypothalamic hormones are regulated by various negative-feedback systems to maintain homeostasis.

Explore the critical role of hypothalamic hormones in regulating the secretion of anterior pituitary trophic hormones in this quiz. Understand how these hormones influence target gland activity and the intricate connections between the hypothalamus and the pituitary gland. Test your knowledge on this essential topic in physiology!