Lecture 2: Hypothalamus Pituitary Axis

How does the hypothalamus control secretion of anterior pituitary hormones

Hypothalamus controls secretion of Anterior Pituitary Hormones via secretion of hypothalamic hormones (synthesized in neurons in the hypothalamus) that travel in the hypothalamo-pituitary portal vessels to the Anterior Pituitary and target specific cells in the anterior pituitary

Hypothalmic hormones that target anterior pituitary

Corticotropin-releasing hormone (CRH)

Gonadotropin-releasing hormone (GnRH)

Growth hormone releasing hormone (GHRH)

Somatostatin

Thyrotropin-releasing hormone

Dopamine

Corticotropin-releasing hormone (CRH)

stimulates secretion of Adrenocorticotropic Hormone (ACTH) from anterior pituitary

Gonadotropin-releasing hormone (GnRH)

stimulates secretion of Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) from anterior pituitary

Growth hormone releasing hormone (GHRH)

stimulates secretion of Growth Hormone from anterior pituitary

Somatostatin

inhibits secretion of Growth Hormone from anterior pituitary

Thyrotropin-releasing hormone

stimulates secretion of thyroid-stimulating hormone (TSH) from anterior pituitary

Dopamine

inhibits secretion of prolactin from anterior pituitary

What do hypothalmic neurons synthesize that signal the posterior pituitary

Hypothalamic neurons synthesize Vasopressin and Oxytocin

How are hormones transported from hypothalamus to posterior pituitary

These hormones are transported via axonal transport to neuron terminals in the posterior pituitary

Antidiuretic Hormone (ADH)/Arginine Vasopressin (AVP)

major target cells are the renal tubule cells. ADH stimulates the insertion of water channels (aquaporin) into renal tubule cells, thus stimulating more water reabsorption. ADH also stimulates contraction of vascular smooth muscle and thus the pressor or vasoconstriction function of ADH

Oxytocin

major target cells are the female breast for milk ejection. Oxytocin also stimulates uterine contractions

Follicle-stimulating hormone/ Luteinizing hormone (FSH/LH)

target cells: gonads

• Female gonad produces estradiol and progesterone

• Male gonad produces testosterone

Growth Hormone

target cells: liver and body’s cells

stimulates secretion of insulin-like growth factor 1 (IGF-1)

Thyroid-stimulating hormone (TSH)

target cells: thyroid gland

stimulates secretion of thyroid hormones T3 and T4

Prolactin

target cells: breast

Adrenocorticotropic hormone (ACTH)

target cells: adrenal cortex

stimulates secretion of Glucocorticoids

Types of feedback in hypothalmic-pituitary target gland feedback

Hierarchical Control (Long loop) feedback regulation

Simple (Short loop) feedback regulation

Hierarchical Control (Long loop) feedback regulation

final hormone produced by a peripheral gland inhibits both the pituitary and the hypothalamus

Simple (Short loop) feedback regulation

pituitary trophic hormone inhibits the secretion of its own stimulating hormone from the hypothalamus

Categories of endocrine disorders

alterations in secretion

altered responsiveness of the target cell

Hyposectretion

The endocrine gland secretes too little hormone

Example: low iodine intake can cause hypothyroidism

Hypersecretion

The endocrine gland secretes too much hormone

Example: increased GH secretions in gigantism and acromegaly

Hyporesponsiveness

target cell has low response (decreased sensitivity) to a hormone

Example: insulin resistance in type 2 diabetes mellitus

Hyperresponsiveness

A target cell has too much response (increased sensitivity) to a hormone

Example: increased responsiveness to epinephrine in patients with hyperthyroidism, resulting in elevated resting heart rate