Anterior and Posterior Pituitary Hormone Regulation

The Thyroid and Adrenal Axes

Metabolism and stress responses are controlled through similar stimulatory pathways:

• Thyroid Axis: The hypothalamus releases TRH (Thyrotropin-Releasing Hormone), which targets thyrotrophs in the anterior pituitary. This triggers the release of TSH (Thyroid-Stimulating Hormone).
• Adrenal Axis: To manage stress, the hypothalamus releases CRH (Corticotropin-Releasing Hormone). This hormone targets corticotrophs, stimulating them to release ACTH (Adrenocorticotropic Hormone). ACTH then travels to the adrenal cortex, prompting the production and release of cortisol.

Exam Tip: Steroid Hormone Signaling Note that while the pituitary hormones (like ACTH or TSH) are often peptides, their downstream targets like cortisol, testosterone, and estrogen are steroids. Unlike peptide hormones that bind to surface receptors, steroid hormones typically bind to intracellular receptors to influence gene expression via response elements on DNA.

The Gonadal Axis: Reproduction and Development

The reproductive system is regulated by the release of gonadotropins, which are controlled by GnRH (Gonadotropin-Releasing Hormone). GnRH targets gonadotrophs in the anterior pituitary, which then release two distinct hormones with different targets:

1. FSH (Follicle-Stimulating Hormone): Targets granular follicular cells in the ovaries to produce estrogens and progesterone.
2. LH (Luteinizing Hormone): In males, LH targets Leydig cells to produce testosterone and targets Sertoli cells to initiate spermatogenesis.

The Prolactin Axis

Prolactin (PRL) is essential for lactation. Its regulation is unique because it is heavily influenced by inhibitory signals.

• Mechanism: While GnRH can play a role, the release of Prolactin (PRL) from lactotrophs is primarily regulated by the inhibitory action of dopamine.
• Target: Once released, PRL targets the mammary glands to increase both milk production and milk secretion.

The Posterior Pituitary: Storage and Release

Unlike the anterior pituitary, the posterior pituitary does not synthesize its own hormones. Instead, it functions as a storage and release site for hormones that are produced directly in the cell bodies of neurons located within the hypothalamus. These hormones are transported down axons into the posterior pituitary and released into the bloodstream when triggered by nerve impulses.

The primary hormones released by the posterior pituitary include:

• AVP (Arginine Vasopressin / Antidiuretic Hormone): This hormone targets the collecting ducts of the kidneys to increase water permeability, helping the body retain water.
• OT (Oxytocin): This hormone has two primary roles: stimulating contractions in the uterus and triggering milk ejection in the mammary glands.

Summary Table of Anterior Pituitary Regulation

For quick review, the following table summarizes the complex relationships within the adenohypophysis: