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chapter31

Endocrine Metabolism II: Hypothalamus and Pituitary

F I G U R K 3 1 -1

Hypothalamic-pituitary regulatory system. Hypothalamic releasing hormones and release-inhibiting hormones are

synthesized in various neurons. These hypothalamic hormones are transported to the anterior pituitary via a portal

venous system and the anterior pituitary target cells either release or inhibit the release of specific hormones into the

general circulation. The posterior pituitary hormones are synthesized and packaged in cell bodies in the hypothalamus

and then are transported to nerve endings in the posterior pituitary. Afterward they are released following appropriate

stimuli. The explanation for the abbreviations appears in the text. [Slightly modified and reproduced with permission

from A. W. Norman and G. Litwack,

Hormones,

2nd ed. New York: Academic Press, 1997, p. 89.]

where their dilated nerve endings are closely positioned

next to capillaries that drain the tissue. Peptides synthe-

sized in these magnocellular neurons are transported to

the neurohypophysis by axoplasmic flow, then released

into the general circulation. These neuropeptides are called

“

neurohypophyseal

Other hypothalamic neurons make

synaptic contact with other neurons, and their neuropep-

tide products function as neurotransmitters or as neuro-

modulators. They are called

“neuroregulatory.”

Pyroglutamic

acid

o

o

amide

Hypophysiotropic Peptides (Table 31-1)

Thyrotropin-releasing hormone

(TRH) is a tripeptide

amide with the following structure:

All of its components are essential for activity. TRH is

resistant to intestinal peptidase action and is active when

taken orally. TRH is produced by the parvicellular neu-

rons of the PVN and the periventricular nucleus, and is