Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 18

Hypothalamic Regulation of Hormonal Functions 281

the search for its ligand led to the isolation of a 31-amino-acid
polypeptide from the human hypothalamus. This polypeptide
stimulated prolactin secretion by an action on the anterior pitu-
itary receptor, but additional research is needed to determine if
it is the physiologic PRH. GnRH stimulates the secretion of FSH
as well as that of LH, and it seems unlikely that a separate follicle-
stimulating hormone-releasing hormone exists.
The structures of the six established hypophysiotropic hor-
mones are shown in Figure 18–11. The structures of the
genes and preprohormones for TRH, GnRH, somatostatin,
CRH, and GRH are known. PreproTRH contains six copies
of TRH. Several other preprohormones may contain other
hormonally active peptides in addition to the hypophysiotro-
pic hormones.
The area from which the hypothalamic releasing and inhib-
iting hormones are secreted is the median eminence of the
hypothalamus. This region contains few nerve cell bodies, but
many nerve endings are in close proximity to the capillary
loops from which the portal vessels originate.
The locations of the cell bodies of the neurons that project
to the external layer of the median eminence and secrete the
hypophysiotropic hormones are shown in Figure 18–12,
which also shows the location of the neurons secreting oxyto-
cin and vasopressin. The GnRH-secreting neurons are pri-
marily in the medial preoptic area, the somatostatin-secreting
neurons are in the periventricular nuclei, the TRH-secreting
and CRH-secreting neurons are in the medial parts of the
paraventricular nuclei, and the GRH-secreting and dopa-
mine-secreting neurons are in the arcuate nuclei.
Most, if not all, of the hypophysiotropic hormones affect
the secretion of more than one anterior pituitary hormone
(Figure 18–10). The FSH-stimulating activity of GnRH has
been mentioned previously. TRH stimulates the secretion of
prolactin as well as TSH. Somatostatin inhibits the secretion
of TSH as well as growth hormone. It does not normally
inhibit the secretion of the other anterior pituitary hormones,
but it inhibits the abnormally elevated secretion of ACTH in
patients with Nelson’s syndrome. CRH stimulates the secre-
tion of ACTH and
β
-LPH.

FIGURE 18–11
Structure of hypophysiotropic hormones in humans.
Preprosomatostatin is processed to a tetradecapeptide (soma-
tostatin 14, [SS14], shown above) and also to a polypeptide containing 28 amino acid residues (SS28).

TRH (pyro)Glu-His-Pro-NH 2

GnRH (pyro)Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH 2

Somatostatin Ala-Gly-Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys

Ser-Glu-Glu-Pro-Pro-Ile-Ser-Leu-Asp-Leu-Thr-Phe-His-Leu-Leu-Arg-Glu-Val-Leu-Glu-Met-Ala-Arg-Ala-Glu-Gln-Leu-

Ala-Gln-Gln-Ala-His-Ser-Asn-Arg-Lys-Leu-Met-Glu-Ile-Ile-NH 2

CRH

GRH Tyr-Ala-Asp-Ala-Ile-Phe-Thr-Asn-Ser-Tyr-Arg-Lys-Val-Leu-Gly-Gln-Leu-Ser-Ala-Arg-Lys-Leu-Leu-Gln-Asp-Ile-Met-

Ser-Arg-Gln-Gln-Gly-Glu-Ser-Asn-Gln-Glu-Arg-Gly-Ala-Arg-Ala-Arg-Leu-NH 2

PIH Dopamine

SS

FIGURE 18–12

Location of cell bodies of hypophysiotropic

hormone-secreting neurons projected on a ventral view of the

hypothalamus and pituitary of the rat.

AL, anterior lobe; ARC, arcu-

ate nucleus; BA, basilar artery; DA, dopamine; IC, internal carotid artery;

IL, intermediate lobe; MC, middle cerebral artery; ME, median emi-

nence; PC, posterior cerebral artery; Peri, periventricular nucleus; PL,

posterior lobe; PV, paraventricular nucleus; SO, supraoptic nucleus.

The names of the hormones are enclosed in boxes.

(Courtesy of LW

Swanson and ET Cunningham Jr.)

0.5 mm

Oxytocin

Vasopressin

SS

DA

CRH

GnRH

TRH

GRH

Peri

SO

PV

ME

ARC

IC

PL

IL

AL

BA

PC

MC

TRH

GRH

DA