The hypothalamus produces releasing and inhibiting hormones that reach the anterior pituitary through the hypothalamic portal system. The superior hypophyseal arteries, which are branches of the internal carotid and posterior communicating arteries, make up the primary plexus. Two groups of the parvocellular neurons secrete their hormones and inhibiting factors into the primary plexus, near the median eminence and above the optic chiasm. The blood from the primary plexus drains into the hypophyseal portal veins. The portal veins run outside of the pituitary stalk and enter the anterior pituitary, forming the secondary plexus. Once these hormones reach the anterior pituitary, they either stimulate or inhibit the anterior pituitary gland cells.
The anterior pituitary produces seven different hormones by five principle types of glandular cells:
1. Thyrotrophs - synthesize Thyroid Stimulating Hormone (TSH) 2. Corticotrophs - synthesize Adrenocorticotropin (ACTH) and Melanocyte Stimulating Hormone (MSH) 3. Somatotrophs - synthesize Growth Hormone (GH) 4. Gonadotrophs - synthesize Follicle Stimulating hormone (FSH) and Luteinizing Hormone (LH) 5. Lactotrophs - synthesize Prolactin (PRL)
There are five releasing hormones and two inhibiting hormones produced by the hypothalamus that regulate the glandular cells in the anterior pituitary:
1. Growth Hormone Releasing Hormone (GHRH) - stimulates release of human growth hormone (hGH) 2. Growth Hormone Inhibiting Hormone (GHIH) or Somatostatin - inhibits release of hGH and TSH 3. Thyrotropin Releasing Hormone (TRH) - stimulates release of TSH 4. Gonadotropin Releasing Hormone (GnRH) - stimulates release of LH & FSH 5. Prolactin Releasing Hormone (PRH) - stimulates release of PRL 6. Prolactin Inhibiting Hormone (PIH) or Dopamine - inhibits release of PRL 7. Corticotropin Releasing Hormone (CRH) - stimulates release of ACTH
All the hormones mentioned above are peptide hormones except Dopamine, which is a catecholamine. All the hypothalamic releasing and inhibiting hormones act on the pituitary gland cells by G Protein mediated hormone action. TRH and GnRH act on thyrotrophs, lactotrophs and gonadotrophs through Gp mediated action. CRH and GHRH act on corticotrophs and somatotrophs through Gs mediated action. Somatostatin and dopamine act through Gi mediated action. Click here to see a diagram of hypothalamic/pituitary hormone release.
Control over the hypothalamic and pituitary hormones is achieved by positive and negative feedback loops. Positive feedback control systems generally disrupt the steady state of hormone release. It is terminated by an endpoint and has zero or even number of inversions. Negative feedback, on the other hand, holds the system at a steady state and has odd number of inversions. It has a negative effect on the controller (in this case the controller is the hypothalamus).
|This diagram represents the negative feedback loops involved in the regulation of hypothalamic-pituitary hormones. The hypothalamus releases it's releasing hormones into the hypophyseal portal system, which carries these substances to the anterior pituitary. The anterior pituitary responds to the releasing hormones by synthesizing and releasing the corresponding pituitary hormones. The pituitary hormones are dumped into the blood which carries them to the target tissues. The target tissues respond to the hormone by initiating some sort of an action. This action is the controlled variable in the system. It can feedback the controller, in this case more than one (hypothalamus and pituitary), and decrease its activities. An example of such a system is the regulation of the thyroid gland. The hypothalamus releases TRH which stimulates the anterior pituitary. The anterior pituitary releases TSH into the blood which carries it to the thyroid gland. Thyroid hormones are produced and released into the blood, which spread throughout the body and induce all kinds of actions, including increasing body temperature. High body temperature is sensed by the temperature center in the hypothalamus which signals the TRH producing cells to decrease TRH production. Thyroid hormones have similar effects on the hypothalamus and the anterior pituitary. Also, TSH which is produced by the anterior pituitary can inhibit TRH synthesis and release. (You can read more about the thyroid gland and its hormones by clicking here).|
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