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,
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
Other hypothalamic neurons make
synaptic contact with other neurons, and their neuropep-
tide products function as neurotransmitters or as neuro-
modulators. They are called
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
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