THE ENDOCRINE SYSTEM: HYPOTHALAMUS AND PITUITARY GLAND
CHP 6 (149-154)
Primary endocrine organs: secretes hormones. Secondary endocrine organs: secretion of hormones is
secondary to some other function. Examples: hypothalamus, pituitary gland, pineal gland.
Primary endocrine organs are located outside the nervous system (thyroid gland, parathyroid gland,
thymus, adrenal glands, pancreas, gonads).
Secondary endocrine glands include organs (heart, liver, stomach, small intestine, kidney, skin).
Primary Endocrine Organs
HYPOTHALAMUS AND PITUITARY GLAND
They function together to regulate every body system.
Hypothalamus is a primary endocrine gland because it secretes several hormones, which affects the
pituitary gland (connected to the hypothalamus by thin stalk of tissue – infundibulum).
The pituitary gland is divided into the anterior lobe and posterior lobe.
NEURAL CONNECTION BETWEEN HYPOTHALAMUS AND POSTERIOR PITUITARY
o Neural endings in the posterior pituitary gland secretes two peptide hormones: antidiuretic
hormone (ADH/vasopressin) and oxytocin.
o ADH is synthesized in the paraventricular nucleus, but less in the supraoptic nuecleus.
o Oxytocin is synthesized primarily in the supraoptic nucleus (less in the paraventricular nucleus).
o The peptides are packaged into secretory vesicles that are transported to the axon terminals.
Released by exocytosis when signaled by neurons (neurohormones).
o ADH regulates water reabsorption by the kidneys. Oxytocin stimulates uterine contractions and
milk letdown in the breasts.
BLOOD CONNECTION BETWEEN HYPOTHALAMUS AND ANTERIOR PITUITARY
o The anterior lobe and the cells of the hypothalamus that control it, secrete tropic hormones,
which are hormones that regulate the secretion of other hormones.
o Stimulating hormone: increases the secretion of another hormone.
o Inhibiting hormone: decreases the secretion of another hormone.
o The hypothalamus releases a tropic hormone that affects the release of another tropic hormone
from the anterior pituitary. It affects the release of a 3 hormone from another endocrine gland,
which effects target cells.
ANATOMY OF HYPOTHALAMIC-PITUITARY PORTAL SYSTEM:
o The hypothalamus and anterior lobe are connected by the hypothalamic-pituitary portal system.
o It is a specialized arrangement of blood vessels in which two capillary beds are located in series.
o After the tropic hormones are secreted into the capillary beds, they travel down the
infundibulum to the pituitary gland via portal vein, which they entecapillary bed.
o The portal system enables the hypothalamic tropic hormones to be delivered to the target cells
in the anterior pituitary, ensuring that they are not diluted or degraded.
o Therefore the tropic hormones are more concentrated in the portal blood delivered to the
anterior pituitary and have a greater effect on hormone release.
TROPIC HORMONES OF THE HYPOTHALAMUS AND ANTERIOR PITUITARY
o The anterior lobe is more like a “typical” endocrine gland in that hormones are synthesized and
secreted by cells located entirely within the pituitary gland.
o In the anterior lobe, intermingled cell types are responsible for synthesizing and secreting a
o Tropic hormones secreted by neurons in the hypothalamus regulate those hormones.
o Prolactin releasing hormone (PRH): stimulates the anterior pituitary to release prolactin, which
stimulates mammary gland development and milk secretion.
o Prolactin inhibiting hormone (PIH) / dopamine (catecholamine): inhibits the release of prolactin.
o Thyrotropin releasing hormone (TRH): stimulates release of thyroid stimulating hormone (TSH)
from the anterior pituitary. TSH then stimulates secretion of thyroid hormones.
o Corticotropin releasing hormone (CRH): stimulates release of adrencorticotropic hormone
(ACTH). ACTH stimulates secretion of insulin-like growth factors (IGFs) by the liver. THE ENDOCRINE SYSTEM: HYPOTHALAMUS AND PITUITARY GLAND
o Growth hormone releasing hormone (GHRH): stimulates secretion of growth hormone (GH) by
the anterior pituitary. It regulates growth and energy metabolism but functions as a tropic
hormone by stimulating secretion of IGFs.
o Growth hormone inhibiting hormone (GHIH)/somatostatin: inhibits the secretion of growth
o Gonadotropin releasing hormone (GnRH): stimulates release of follicle stimulating hormone
(FSH) and lutenizing hormone (LH) by the anterior pituitary. LH stimulates ovulation in females,
and secretion of sex hormones (estrogen, progesterone, androgens) by the gonads. FSH
promotes the development of egg cells and sperm cells. It stimulates the secretion of estrogen
and inhibin (both sexes).
Feedback loops regulate the multistep pathways of which tropic hormones are produced.
Short loop negative feedback: inhibits of hypothalamus tropic hormone by anterior pituitary tropic
hormone. Prevents the buildup of excess anterior pituitary tropic hormone.
Long loop negative feedback: hormone whose secretion is stimulated by the tropic hormone generally
feeds back to the hypothalamus and the anterior pituitary to inhibit secretion of the tropic hormone,
limiting its own secretion.
Corticotropin releasing hormone (CRH) stimulates the release of ACTH. In turn, stimulates the release of
cortisol from the adrenal gland.
If cortisol levels are high, negative feedback loops cause a decrease in the release of CRH and ACTH. Net
result, cortisol inhibits its own secretion.
Cortisol provides negative feedback only to its own tropic hormones.
CHP 19 (544-545)
EFFECTS OF ADH ON WATER REABSORPTION
ADH (vasopressin) regulates the permeability of the late distal tubules and collecting ducts to water.
Without ADH, the apical membrane of the principal cells is impermeable to water.
ADH stimulates synthesis of aquaporin-2 and its insertion to the apical membrane of principal cells in the
late distal tubules and collecting ducts, allowing water to be permeable.
Aquaporin-2 is stored in the cytoplasmic vesicles of principle cells.
ADH acts on receptors on the basolateral membrane. The receptors are coupled to G proteins to activate
adenylate cyclase, catalyzing the producing of cAMP. cAMP activates protein kinase A, which stimulates
aquaporin-2 to apical side by exocytosis.
ADH also stimulates production of aquaporin-2.
High [ADH] – water reabsorption is high, urine output is low. Low [ADH] – water reabsorption is low,
urine output is high.
REGULATION OF ADH SECRETION
Changes in the osmolarity of extracellular fluid are strongest stimuli for ADH release.
Osmoreceptors in the hypothalamus monitor the osmolarity of extracellular fluid.
If osmolarity increases, ADH secretion is stimulated and increases in water reabsorption.
If osmolarity decreases, ADH secretion is inhibited, which decreases water reabsorption and increases
Plasma levels of ADH depend on signals arising in baroreceptors that detect blood volume/pressure.
Decreased baroreceptor activity stimulates increases in ADH, which increases water reabsorption and
minimizes the stimuli for the release of ADH.
Increased baroreceptor activity decreases ADH secretion, resulting in water excretion to decrease blood
volume and blood pressure.
CHP 22 (662-663)
Suckling stimulates tactile receptors in the nipples, which project to the hypothalamus and excite
neurosecretory cells that extend to he posterior pituitary and secrete oxytocin.
Oxytocin travels in the blood stream and stimulates myoepithelial cells to contract the alveoli. THE ENDOCRINE SYSTEM: HYPOTHALAMUS AND PITUITARY GLAND
Suckling stimulates the production of milk by stimulating the release of prolactin releasing hormone
(PRH) by the hypothalamus and inhibiting the release of prolactin inhibiting hormone (PIH).
Both of these stimulate the anterior pituitary to secrete prolactin, which induces milk production by cells
in the alveoli.
Rising estrogen levels stimulate deposition of fatty tissue in the breasts. Progesterone promotes
development of the alveoli.
CHP 21 (620-625)