The Endocrine System
333
FSH
and
LH
GnRH
Growth
hormone
TSH
ACTH
Prolactin
+
GHRH
+
SS
TRH
+
DA
CRH
+
Corticotropin-releasing hormone (CRH)
Thyrotropin-releasing hormone (TRH)
Growth hormone-releasing hormone (GHRH)
Somatostatin (SS)
Gonadotropin-releasing hormone (GnRH)
Dopamine (DA)*
Major known hypophysiotropic hormones
Stimulates secretion of ACTH
Stimulates secretion of TSH
Stimulates secretion of GH
Inhibits secretion of GH
Stimulates secretion of LH and FSH
Inhibits secretion of prolactin
Major effect on anterior pituitary
*Dopamine is a catecholamine; all the other hypophysiotropic hormones are peptides. Evidence exists
for PRL-releasing hormones, but they have not been unequivocally identified. One possibility is that TRH
serves this role in addition to its actions on TSH.
Hypothalamus
Anterior pituitary
Figure 11–17
The effects of defi nitely established hypophysiotropic hormones on the anterior pituitary. The hypophysiotropic hormones reach the anterior
pituitary via the hypothalamo-pituitary portal vessels. The
B
and
E
symbols indicate stimulatory and inhibitory actions, respectively.
hypothalamic neurons, pass down axons to the neuron
terminals, and are released in response to action potentials
in the neurons. Two crucial differences, however, distin-
guish the two systems: (1) The axons of the hypothalamic
neurons that secrete the posterior pituitary hormones leave
the hypothalamus and end in the posterior pituitary, whereas
those that secrete the hypophysiotropic hormones remain in
the hypothalamus, ending in the median eminence. (2) Most
of the posterior pituitary capillaries into which the posterior
pituitary hormones are secreted immediately drain into the
main bloodstream, which carries the hormones to the heart
for distribution to the entire body. In contrast, the hypo-
physiotropic hormones enter capillaries in the median emi-
nence of the hypothalamus that do not directly join the main
bloodstream, but empty into the hypothalamo-pituitary por-
tal vessels, which carry them to the anterior pituitary. Once
the hypophysiotropic hormones leave the capillaries in the
anterior pituitary, they bathe the pituitary cells. If a cell has
an appropriate receptor for a given hypophysiotropic hormone,
that cell will respond by increasing or decreasing the secretion
of its pituitary hormone. If a pituitary hormone is secreted, it
will diffuse into the same capillaries that delivered the hypo-
physiotropic hormone. These capillaries then drain into veins,
which enter the general blood circulation, where the pituitary
hormones can come into contact with their target cells.
There are multiple, discrete hypophysiotropic hormones,
each secreted by a particular group of hypothalamic neurons
and infl uencing the release of one or, in at least one case, two of
the anterior pituitary hormones. For simplicity,
Figure 11–17
Figure 11–16
Hormone secretion by the anterior pituitary is controlled by
hypophysiotropic hormones released by hypothalamic neurons and
reaching the anterior pituitary by way of the hypothalamo-pituitary
portal vessels.
Hypothalamo-pituitary
portal vessels
Hypothalamic
neuron
Arterial
inflow
from heart
Capillaries
in anterior
pituitary
Capillaries
in median
eminence
Anterior
pituitary
gland
cells
Venous
outflow to
heart
Anterior
pituitary
Hypothalamus
K
EY
Hypophysiotropic hormone
Anterior pituitary hormone
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