Ganong's Review of Medical Physiology, 23rd Edition

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
CRH
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 pituitary
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 folliclestimulating
hormone-releasing hormone exists.
The structures of the six established hypophysiotropic hormones
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 hypophysiotropic
hormones.
The area from which the hypothalamic releasing and inhibiting
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 oxytocin
and vasopressin. The GnRH-secreting neurons are primarily
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 dopamine-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 secretion
of ACTH and β-LPH.
CHAPTER 18 Hypothalamic Regulation of Hormonal Functions 281
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
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
S S
FIGURE 18–11 Structure of hypophysiotropic hormones in humans. Preprosomatostatin is processed to a tetradecapeptide (somatostatin
14, [SS14], shown above) and also to a polypeptide containing 28 amino acid residues (SS28).
Oxytocin
Vasopressin
SO
0.5 mm
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, arcuate
nucleus; BA, basilar artery; DA, dopamine; IC, internal carotid artery;
IL, intermediate lobe; MC, middle cerebral artery; ME, median eminence;
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.)
SS
Peri
CRH
PV TRH
ME
DA
ARC
GRH
PL
BA
IL
AL
GnRH
IC
TRH
GRH
DA
PC
MC