Hypothalamic & Pituitary Hormone Overview
The
hypothalamus collects information from throughout the body and uses it to regulate
pituitary hormone secretion.
Hypothalamic neuroendocrine cell axons terminate in the median eminence and posterior pituitary, where they secrete various neurohormones.
– 5 hypothalamic hormones act on the anterior pituitary lobe.
– 2 hypothalamic hormones are released by the posterior pituitary lobe.
The anterior lobe, sometimes referred to as the adenohypophysis because of its gland-like components, comprises "-troph" cells that receive inhibitory and/or releasing signals from the hypothalamus via the hypothalamic-hypophyseal portal system.
– The anterior lobe synthesizes and releases 6 peptide hormones that regulate growth, reproduction, and metabolism.
The posterior lobe, sometimes referred to as the neurohypophysis because it comprises nervous tissue, releases 2 peptide hormones that are synthesized in large-bodied neurons with cell bodies in the hypothalamus.
"Tropic" means that a hormone acts on other endocrine cells, as opposed to having direct actions on target organs; this will be an important function of several hypothalamic and anterior pituitary lobe hormones.
First, we draw the hypothalamus and pituitary gland; indicate anterior and posterior lobes.
Within the anterior lobe reside the following clusters of endocrine cells:
Somatotrophs, Corticotrophs, Thyrotrophs, Lactotrophs, and Gonadotrophs.
– Be aware that some authors end these cells with the suffix "tropes" instead of "trophs" (for example, corticotropes).
Hypothalamic hormones that act on the anterior lobe:
– Growth-hormone Releasing hormone (GHRH)
– Growth-hormone Inhibiting hormone (GHIH)
– Corticotropin-releasing hormone (CRH)
– Thyrotropin-releasing hormone (TRH)
– Prolactin-Inhibiting hormone (PIH)
– Gonadotropin-Releasing hormone (GnRH).
Notice that there are 2 inhibitory hormones and 4 releasing hormones in this group.
Anterior Lobe Hormones
Growth-hormone releasing hormone:
Simulates somatotroph release of
growth hormone (aka, somatotropic hormone, aka, somatotropin).
Growth hormone has multiple effects throughout the body: it facilitates bone and muscle growth and has diabetogenic effects on metabolism (it increases blood glucose).
Growth hormone inhibiting hormone, aka, somatostatin, inhibits somatotrophic cell release of growth hormone.
Corticotropin-releasing hormone triggers corticotroph release of adrenocorticotropic hormone (ACTH) (aka, corticotropin).
ACTH, in turn, stimulates the
adrenal glands to release cortisol, which has a variety of effects, including reduction of inflammatory and immune responses and increases in gluconeogenesis, lipolysis, and proteolysis.
- ACTH release is inhibited via short and long negative feedback loops (see the links in our notes for more details).
Thyrotropin-Releasing hormone stimulates thyroprophic release of thyroid-stimulating hormone (TSH).
Thyroid-stimulating hormone stimulates
thyroid gland production and release of
thyroid hormone, which promotes growth and maturation, increases metabolism, and stimulates beta-adrenergic receptors.
Prolactin-inhibiting hormone, which is dopamine, tonically inhibits lactotroph release of
prolactin.
- Dopamine's effects ensure that prolactin is secreted in low levels in males and non-pregnant/non-breastfeeding females; be aware that prolactin receptors are located in various tissues, including the prostate and immune cells, but its exact effects are uncertain.
- In pregnant and breastfeeding women, dopamine's effects are inhibited, and lactotrophs release prolactin, which stimulates breast development and lactogenesis (milk formation).
Gonadotropin-releasing hormone (GnRH) promotes gonadotroph release of
follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which act on the gonads.
Follicle stimulating hormone works at the testes to support
Sertoli cell growth sperm cell proliferation.
- In the ovaries, FSH promotes granulosa cell growth, follicle maturation, and aromatase synthesis; aromatase is the enzyme that converts androgens to estrogens.
- In the testes, Luteinizing hormone promotes Leydig cell secretion of testosterone.
- In the ovaries, LH has multiple effects:
It promotes thecal cell secretion of androgens, which are then converted to estrogens by the nearby granulosa cells;
– The LH surge induces ovulation.
– Post-ovulation, LH promotes the development and functioning of the Corpus Luteum (notice that "luteinizing" hormone promotes the "luteum").
Posterior Lobe Hormones
Neurons in the hypothalamus produce Oxytocin and Anti-diuretic hormone (aka, vasopressin), which are transported to the posterior pituitary, which releases them into the blood.
Oxytocin is responsible for
myometrial contractions during childbirth to expel the fetus, and, during lactation, it promotes milk ejection.
– Recall that prolactin was necessary for milk production and secretion; oxytocin is required for its ejection.
Anti-diuretic hormone (ADH) regulates body water and blood pressure via the following mechanisms:
- In the kidney, ADH acts on the nephron collecting ducts to promote water reabsorption, thus increasing body water (and, therefore, blood volume, blood pressure, and cardiac output).
- Vasoconstriction is a secondary function of ADH, which binds to vascular smooth muscle receptors (hence its alternative name, vasopressin); in hypovolemic shock, increased endogenous production of ADH is an important compensatory mechanism.
For full references, please see our tutorial on Hypothalamic and Pituitary Hormones.
