Download The Hypothalamus and Pituitary Gland

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The Hypothalamus and Pituitary
Gland
∗
Steven Telleen
Based on The Pituitary Gland and Hypothalamus† by
OpenStax College
This work is produced by OpenStax-CNX and licensed under the
Creative Commons Attribution License 4.0‡
Abstract
By the end of this section, you will be able to:
• Explain the interrelationships of the anatomy and functions of the hypothalamus and the posterior
and anterior lobes of the pituitary gland
• Identify the two hormones released from the posterior pituitary, their target cells, and their principal
actions
• Identify the six hormones produced by the anterior lobe of the pituitary gland, their target cells,
their principal actions, and their regulation by the hypothalamus
The hypothalamuspituitary complex can be thought of as the command center of the endocrine system.
This complex secretes several hormones that directly produce responses in target tissues, as well as hormones
that regulate the synthesis and secretion of hormones of other glands.
In addition, the hypothalamus
pituitary complex coordinates the messages of the endocrine and nervous systems. In many cases, a stimulus
received by the nervous system must pass through the hypothalamuspituitary complex to be translated into
hormones that can initiate a response.
The
hypothalamus
is a structure of the diencephalon of the brain located anterior and inferior to
the thalamus (Figure 1 (HypothalamusPituitary Complex )). It has both neural and endocrine functions,
producing and secreting many hormones. In addition, the hypothalamus is anatomically and functionally
related to the pituitary gland (or hypophysis), a bean-sized organ suspended from it by a stem called the
infundibulum (or pituitary stalk). The pituitary gland is cradled within the sellaturcica of the sphenoid
bone of the skull. It consists of two lobes that arise from distinct parts of embryonic tissue: the posterior
pituitary (neurohypophysis) is neural tissue, whereas the anterior pituitary (also known as the adenohypophysis) is glandular tissue that develops from the primitive digestive tract. The hormones secreted by the
posterior and anterior pituitary, and the intermediate zone between the lobes are summarized in Table 1.
∗ Version
1.2: Jul 6, 2015 1:33 pm -0500
† http://cnx.org/content/m46699/1.3/
‡ http://creativecommons.org/licenses/by/4.0/
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HypothalamusPituitary Complex
Figure 1: The hypothalamus region lies inferior and anterior to the thalamus. It connects to the
pituitary gland by the stalk-like infundibulum. The pituitary gland consists of an anterior and posterior
lobe, with each lobe secreting dierent hormones in response to signals from the hypothalamus.
Pituitary Hormones
Pituitary lobe
Associated hormones
Chemical class
Eect
Anterior
Growth hormone (GH)
Protein
Promotes
growth
of
body tissues
Anterior
Prolactin (PRL)
Peptide
Promotes milk production
from
mammary
glands
continued on next page
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Anterior
3
Thyroid-stimulating
Glycoprotein
hormone (TSH)
Stimulates thyroid hormone release from thyroid
Anterior
Adrenocorticotropic
Peptide
hormone (ACTH)
Anterior
lease by adrenal cortex
Follicle-stimulating hor-
Glycoprotein
mone (FSH)
Anterior
Luteinizing
Antidiuretic
hormone
Glycoprotein
Stimulates
androgen
production by gonads
hormone
Peptide
(ADH)
Posterior
Stimulates gamete production in gonads
(LH)
Posterior
Stimulates hormone re-
Stimulates water reabsorption by kidneys
Oxytocin
Peptide
Stimulates uterine contractions
during
child-
birth
Intermediate zone
Melanocyte-stimulating
Peptide
hormone
Stimulates melanin formation in melanocytes
Table 1
1 Posterior Pituitary
The posterior pituitary is actually an extension of the neurons of the paraventricular and supraoptic nuclei
of the hypothalamus. The cell bodies of these regions rest in the hypothalamus, but their axons descend as
the hypothalamichypophyseal tract within the infundibulum, and end in axon terminals that comprise the
posterior pituitary (Figure 2 (Posterior Pituitary )).
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Posterior Pituitary
Figure 2: Neurosecretory cells in the hypothalamus release oxytocin (OT) or ADH into the posterior
lobe of the pituitary gland. These hormones are stored or released into the blood via the capillary plexus.
The posterior pituitary gland does not produce hormones, but rather stores and secretes hormones
produced by the hypothalamus.
The paraventricular nuclei produce the hormone oxytocin, whereas the
supraoptic nuclei produce ADH. These hormones travel along the axons into storage sites in the axon
terminals of the posterior pituitary. In response to signals from the same hypothalamic neurons, the hormones
are released from the axon terminals into the bloodstream.
1.1 Oxytocin
When fetal development is complete, the peptide-derived hormone
ulates uterine contractions and dilation of the cervix.
oxytocin
(tocia- = childbirth) stim-
Throughout most of pregnancy oxytocin hormone
receptors are not expressed at high levels in the uterus. Toward the end of pregnancy the synthesis of oxytocin receptors in the uterus increases, and the smooth muscle cells of the uterus become more sensitive to its
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eects. Oxytocin is continually released throughout childbirth through a positive feedback mechanism. As
noted earlier, oxytocin prompts uterine contractions that push the fetal head toward the cervix. In response,
cervical stretching stimulates additional oxytocin to be synthesized by the hypothalamus and released from
the pituitary. This increases the intensity and eectiveness of uterine contractions and prompts additional
dilation of the cervix.
The positive feedback loop continues until birth when the stretching of the cervix
ceases.
Although the mother's high blood levels of oxytocin begin to decrease immediately following birth,
oxytocin continues to play a role in maternal and newborn health. First, oxytocin is necessary for the milk
ejection reex (commonly referred to as let-down) in breastfeeding women. As the newborn begins suckling
sensory receptors in the nipples transmit signals to the hypothalamus. In response oxytocin is secreted and
released into the bloodstream.
Within seconds smooth muscle cells in the mother's milk ducts contract
ejecting milk into the infant's mouth.
In addition to its eect on smooth muscle, oxytocin also has been shown to aect behavior. In both males
and females there is evidence that oxytocin contributes to parentnewborn bonding, known as attachment.
Oxytocin also is thought to be involved in feelings of love, closeness, and the establishment of pair bonding
when released as part of the sexual response.
Experiments with a single dose of nasally administered
oxytocin indicate that it also is involved in, at least temporarily, inducing trust, cooperation, and positive
social behavior among adults. However, the way in which oxytocin works in clinical therapies is not well
understood, and evidence shows that while some patients benet from extended, repeated use others do not,
some even becoming less social and more aggressive.
Oxytocin is an evolutionarily old hormone that is found even in the simple worm C. elegans, a popular
organism for studying neurological circuits because its entire nervous system contains only 302 neurons. In
this species oxytocin helps the individual worms nd and recognize mates. A function of oxytocin in the
mammalian brain seems to be the enhancement of specic neuronal circuits over others. In the hippocampus
of rats, oxytocin has been shown to selectively act on inhibitory interneurons reducing background noise and
improving signal transmission within specic brain circuits. These studies indicate that oxytocin helps the
brain focus in on socially relevant sensory input. It also is becoming clear that oxytocin often does not work
alone but requires other permissive or synergistic neurotransmitters, like serotonin or ADH, which may be
why some oxytocin-only clinical trials have had limited results.
1.2 Antidiuretic Hormone (ADH)
Antidiuretic hormone (ADH) was originally called vasopressin because it was rst identied for its role
in regulating blood pressure. The release of ADH is controlled by two negative feedback loops, one where the
regulated variable is blood pressure and the other where the regulated variable is blood
osmolarity (solute
concentration). Note that these two regulated variables are related. For example increasing water retention
to respond to increased osmolarity will also increase blood volume and consequently blood pressure. This is
a good example of the important role the hypothalamus plays in integrating competing needs in the attempt
to optimize whole organism homeostasis.
Blood pressure is monitored by
baroreceptors (pressure receptors) in the aorta, the carotid arteries, and
the aerent arterioles in the kidneys that respond to the mechanical stretching of the cell membrane of the
sensory cells. A decrease in blood pressure reduces the stretching and triggers a signal to the hypothalamus
(integration center). The ADH (messenger) that is released targets two eectors. It stimulates contraction
of the smooth muscles in the blood vessels, increasing peripheral resistance. It also increases reabsorption
of water in the kidney tubules (details on this eector response is discussed in more detail in the osmolarity
section below) increasing blood volume. Both of these eects increase blood pressure which increases the
stretching of the baroreceptors and reduces the signal to the hypothalamus, thus reducing the release of
ADH.
Interestingly, the hypothalamus also can trigger the release of ADH in anticipation of the need for
increased blood pressure, for example to support the whole body needs in an emergency or excitement
(ght/ight) response. This is why chronic stress, which also triggers this same anticipatory response, can
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cause chronically high blood pressure.
Blood osmolarity is monitored by
osmoreceptorsspecialized cells within the hypothalamus and distal
convoluted tubules in the kidneys that are particularly sensitive to the concentration of sodium ions and
other solutes. Blood osmolarity may change in response to dehydration, the consumption of certain foods
and uids, as well as in response to disease, injury, medications, or other factors.
In response to high blood osmolarity the osmoreceptors signal the posterior pituitary (integration center) to release ADH (messenger).
For this regulated variable the target cells of ADH (eectors) are the
same tubular cells of the kidneys involved in responding to blood pressure. The eect is translocation of
vesicles containing aquaporin channels into the epithelial cell membranes increasing permeability to water
and therefore increased water reabsorption.
The more channels added to the membranes the more water
that is reabsorbed from the ltrate and returned to the blood leaving less to be excreted in the urine. The
reabsorbed water dilutes the blood osmolarity. As blood osmolarity decreases, the osmoreceptor signal levels
to the posterior pituitary change and prompt a corresponding decrease in the secretion of ADH. As a result,
fewer aquaporin channels are maintained in the epithelial cell membranes so permeability decreases and less
water is reabsorbed from the urine ltrate. Remember that, in the process of reducing osmolarity, the blood
volume and with it blood pressure also increases.
Drugs also can aect the secretion of ADH. For example, alcohol consumption inhibits the release of
ADH, resulting in increased urine production that can eventually lead to dehydration, low blood pressure,
and a hangover. A creative student intuitively noted this connection when he wrote on the wall above the
urinal at a popular bar where I went to college: "You don't buy beer, you only rent it." Caeine is another
commonly consumed drug that inhibits the release of ADH.
A disease called
diabetes insipidus
is characterized by chronic underproduction of ADH that causes
chronic dehydration. Because little ADH is produced and secreted, not enough water is reabsorbed by the
kidneys. Although patients feel thirsty and increase their uid consumption this doesn't eectively decrease
the solute concentration in their blood. Because ADH levels are not high enough to trigger water reabsorption
in the kidneys, the additional water ingested just passes out with the urine. This can result in electrolyte
imbalances in severe cases of diabetes insipidus.
Note: diabetes does not refer to a glucose imbalance but refers to the symptom of high
urine output. The more common form of diabetes, diabetes mellitus also is named after its symptom
of higher than normal urine output. In this case the high urine output is not caused by low ADH levels but
by excess glucose in the urine. This creates higher than normal urine osmolarity which pulls more water
out with it. Mellitus means "honey-sweet" in Latin, referring to the sweet avor of the urine caused by the
excess glucose. Before modern chemical techniques physicians would diagnose the disease using their built-in
chemical analyzer, taste, to test the urine.
Like oxytocin, ADH also has been shown to have behavioral eects in mammals. Many are related to
behaviors mediating social interaction.
There appear to be dierences in eect based on the sex of the
individual. In voles increased ADH seems to increase pair bonding in females, while it increases territorial
behavior in males.
Likewise in Syrian hamsters, ADH appears to increase aggression in males while it
decreases aggression in females.
2 Anterior Pituitary
The anterior pituitary originates from the digestive tract in the embryo and migrates toward the brain during
fetal development.
There are three regions: the pars distalis is the most anterior, the pars intermedia is
adjacent to the posterior pituitary, and the pars tuberalis is a slender tube that wraps the infundibulum.
Recall that the posterior pituitary does not synthesize hormones, but merely stores them. In contrast,
the anterior pituitary does manufacture hormones. However, the secretion of hormones from the anterior
pituitary is regulated by two classes of hormones. These hormones, secreted by the hypothalamus, are the
releasing hormones that stimulate the secretion of hormones from the anterior pituitary and the inhibiting
hormones that inhibit secretion.
Hypothalamic hormones are secreted by neurons, but enter the anterior pituitary through blood vessels
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(Figure 3 (Anterior Pituitary )). Within the infundibulum is a bridge of capillaries and veins that connect
hypophyseal portal system, allows
primary capillary plexus and be transported directly
the hypothalamus to the anterior pituitary. This network, called the
hormones secreted by the hypothalamus to enter the
to the
secondary capillary plexus in the anterior pituitary without rst entering the systemic circulation.
Branches of the superior hypophyseal artery form the hypophyseal portal system (see Figure 3 (Anterior
Pituitary )). Hypothalamic releasing and inhibiting hormones diuse into the primary capillary plexus and
travel through the
hypophyseal portal veins,
which carry them to the secondary capillary plexus where
they diuse out into the intracellular uid of the anterior pituitary. Hormones produced by target cells in the
anterior pituitary (in response to the hypothalamic releasing hormones) diuse into the secondary capillary
plexus and are carried into the systemic circulation via the
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Anterior Pituitary
Figure 3: The anterior pituitary manufactures seven hormones. The hypothalamus produces separate
hormones that stimulate or inhibit hormone production in the anterior pituitary. Hormones from the
hypothalamus reach the anterior pituitary via the hypophyseal portal system.
The anterior pituitary produces seven hormones. These hormones are part of the three-level hypothalamicpituitary axis presented in an earlier section. They are the thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), growth hormone
(GH), prolactin, and beta endorphin. The rst four hormones, TSH, ACTH, FSH, and LH are collectively
referred to as tropic hormones (trope- = turning) because they turn on the function of other endocrine
glands.
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2.1 Thyroid Stimulating Hormone
The activity of the thyroid gland is regulated by
thyroid-stimulating hormone (TSH),
also called
thyrotropin. TSH is released from the anterior pituitary in response to thyrotropin-releasing hormone (TRH)
from the hypothalamus. As discussed shortly, it triggers the secretion of thyroid hormones by the thyroid
gland.
In a classic negative feedback loop, elevated levels of thyroid hormones in the bloodstream then
trigger a drop in production of TRH and subsequently TSH.
2.2 Adrenocorticotropic Hormone
The
adrenocorticotropic hormone (ACTH), also called corticotropin, stimulates the adrenal cortex (the
more supercial layer of the adrenal glands) to secrete corticosteroid hormones such as cortisol. ACTH comes
from a precursor molecule known as pro-opiomelanotropin (POMC) which produces several biologically active
molecules when cleaved, including ACTH, melanocyte-stimulating hormone, and the brain opioid peptides
known as endorphins.
The release of ACTH is regulated by the corticotropin-releasing hormone (CRH) from the hypothalamus
in response to normal physiologic rhythms. A variety of stressors can also inuence its release, and the role
of ACTH in the stress response is discussed later in this chapter.
2.3 Follicle-Stimulating Hormone and Luteinizing Hormone
The endocrine glands secrete a variety of hormones that control the development and regulation of the
reproductive system (these glands include the anterior pituitary, the adrenal cortex, and the gonadsthe
testes in males and the ovaries in females). Much of the development of the reproductive system occurs during
puberty and is marked by the development of sex-specic characteristics in both male and female adolescents.
Puberty is initiated by gonadotropin-releasing hormone (GnRH), a hormone produced and secreted by the
hypothalamus. GnRH stimulates the anterior pituitary to secrete
gonadotropinshormones that regulate
the function of the gonads. The levels of GnRH are regulated through a negative feedback loop; high levels
of reproductive hormones inhibit the release of GnRH. Throughout life, gonadotropins regulate reproductive
function and, in the case of women, the onset and cessation of reproductive capacity.
The gonadotropins include two glycoprotein hormones:
follicle-stimulating hormone (FSH)
stimu-
lates the production and maturation of sex cells, or gametes, including ova in women and sperm in men.
FSH also promotes follicular growth; these follicles then release estrogens in the female ovaries.
hormone (LH)
Luteinizing
triggers ovulation in women, as well as the production of estrogens and progesterone by
the ovaries. LH stimulates production of testosterone by the male testes.
2.4 Growth Hormone
The endocrine system regulates the growth of the human body, protein synthesis, and cellular replication.
A major hormone involved in this process is
growth hormone (GH), also called somatotropina protein
hormone produced and secreted by the anterior pituitary gland. Its primary function is anabolic; it promotes
protein synthesis and tissue building through direct and indirect mechanisms (Figure 4 (Hormonal Regulation
of Growth )). GH levels are controlled by the hypothalamus through the release of GHRH and GHIH (also
know as somatotropin and somatostatin respectively).
While the balance of GHRH and GHIH released from the hypothalamus is the primary determinant for
synthesis and release of Growth Hormone (GH), regulation of this balance is complex as the hypothalamus
integrates input from many dierent sources. No single negative feedback loop has been identied as the
primary controller, although
Insulin-like Growth Factor (IGF-1) has been shown to reduce GH secretion
in placebo-controlled experiments. IGF-1, which is released by the liver in response to GH, accounts for or
enhances many of the eects attributed to growth hormone. Generally, the actions of the combined GH/IGF1 axis promote cell growth and inhibit cell apoptosis. Therefore, it should be no surprise that higher levels
of IGF-1 are found in many cancers.
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Hormonal Regulation of Growth
Figure 4: Growth hormone (GH) directly accelerates the rate of protein synthesis in skeletal muscle
and bones. Insulin-like growth factor 1 (IGF-1) is activated by growth hormone and indirectly supports
the formation of new proteins in muscle cells and bone.
The feedback control is not as simple as the gure makes it look. The circulating IGF-1 level is only
one of many inputs integrated by the hypothalamus to establish the set point that maintains the balance
between GHRH and GHIH, and consequently the circulating levels of both hormones (GH and IGF-1) at
any given time. Other inputs include:
•
•
•
•
•
Genetic makeup
Time of day (more is released during sleep)
Age
Sex
Exercise status
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•
•
•
•
•
•
11
Stress levels
Nutrition levels
Body Mass Index (BMI)
Disease status
Estrogen status
Exposure to xenobiotic substances (articial signal molecules)
Because the amount of GH released is dependent on the relative balance between releasing and inhibiting
antagonists, the actual circulating levels can be increased either by increasing the levels of the agonist
(releasing signals) or decreasing the levels of the antagonist (inhibiting signals). The reverse also is true.
The circulating levels of GH can be reduced either by decreasing the levels of the agonist or increasing the
levels of the antagonist.
Specic factors known to stimulate release of GH are:
•
•
•
•
•
•
•
•
•
•
•
GHRH
Fasting (which may be connected to calorie restriction aects seen in aging experiments)
Ghrelin (the hunger hormone produced in the GI tract)
Androgen, both from male testes and the adrenal cortex in both sexes
Estrogen
Nicotinic agonists
Deep sleep
Niacin (vitamin B3)
Vigorous exercise
Clonidine and L-DOPA, both directly stimulate GHRH release
Hypoglycemic arginine and propranolol, both directly inhibit GHIH release
Specic factors known to inhibit release of GH are:
•
•
•
•
•
GHIH
Circulating GH and IGF-1 (negative feedback on the pituitary and hypothalamus)
Hyperglycemia
Glucocorticoids (adrenal stress hormones)
Dihydrotestosterone (DHT)
Growth hormone has a number of eects on the body related to growth and metabolism; however, many
of these eects may be more related to its stimulation of IGF-1 release than the direct results of the GH
itself.
In general the GH/IGF-1 axis stimulates growth of all internal organs, including the brain.
This
hypertrophy (increase in cell size) and hyperplasia
(increase in cell number) via transcription activation (activating protein synthesis) and by optimizing
the nutrient and energy resources needed to carry out both types of growth.
is accomplished both by directly stimulating cell
The GH/IGF-1 axis causes an increase in calcium retention by the body and stimulates bone mineralization. IGF-1 receptors have been identied on osteocytes, which likely play a role in these anabolic and
mineralization aects related to bone growth. Additionally, IGF-1 promotes the uptake of amino acids by
all cells, which are the required building blocks for protein synthesis. Increasing amino acid uptake is one of
the "insulin-like" eects of IGF-1, as insulin also promotes the uptake of amino acids. Skeletal muscle and
cartilage cells are particularly sensitive to stimulation from IGFs.
Cell growth and division require energy, and GH/IGF-1 promote mobilization and management of the
body's energy resources to support these functions. In addition to glucose as an energy source, most cells in
the body can also use fatty acids and a few other molecules as an energy source. The exception are the cells
of the nervous system, which also are the biggest consumer of body energy. Under normal circumstances
muscles (the other large consumer of energy) and other cells rely mostly on fatty acids as their energy source,
leaving most of the glucose for the nervous system. However, when whole body energy needs increase, so
does the utilization of other energy molecules, like glucose. To protect the nervous system from depletion of
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glucose by uptake from other parts of the body both a
glucose sparing eect and a diabetogenic eect
are activated.
GH/IGF-1 create the glucose sparing eect by targeting the adipose cells causing them to initiate
lipolysis (the breakdown of lipids back into fatty acids). This increases the level of fatty acids in the blood
and therefore reduces the uptake of glucose by non-neuronal cells.
The
diabetogenic eect is a result of GH activity and refers to the active inhibition of glucose uptake
by liver and muscle cells and the creation and release of "new" glucose molecules into the blood from other
sources, primarily glycogen and non-carbohydrate molecules such as amino acids, pyruvate, lactate, and
glycerol. Conversion of glycogen back to glucose is
molecules is
gluconeogenesis.
glycogenolysis,
while conversion of non-carbohydrate
This combination of eects, while not caused by insulin reactions, results in
elevated blood glucose levels similar to those seen in diabetes mellitus, hence the name diabetogenic eect.
Dysfunction of the endocrine system's control of growth can result in several disorders.
gigantism
For example,
is a disorder in children that is caused by the secretion of abnormally large amounts of GH,
resulting in excessive growth. A similar condition in adults is
acromegaly,
a disorder that results in the
growth of bones in the mandible, hands, and feet in response to excessive levels of GH in individuals whose
other bones have stopped growing. Abnormally low levels of GH in children can cause growth impairment,
a disorder called
:
pituitary dwarsm (also known as growth hormone deciency).
Growth hormone is the focus of much popular attention and controversy.
Human growth
hormone (hGH) is an anabolic agent, and as such is one of the performance enhancing hormones
banned for use by athletes. Anyone who has followed sports is aware of the many titles and awards
that have been stripped from athletes in a wide variety of sports, both professional and amateur,
because articial growth hormone (or an articial growth hormone stimulator or precursor) was
detected in the athlete's blood.
In livestock production growth hormone is creating controversy for its use to increase meat (muscle)
and milk production bound for human consumption. Testing of milk from dairy cows that have
been given bovine growth hormone (BGH) shows that the hormone does not show up in the milk.
Nevertheless, many people still avoid milk from cows given BGH, and advertising milk and milk
products as BGH-free has become a popular marketing strategy.
Growth hormone currently is
approved by the FDA only for use in cattle (beef ), so all legally sold poultry and eggs are "hormone
free," even when they are not advertised as such.
An additional o-label use of hGH is as a purported anti-aging supplement. While the research
does show an increase in muscle mass and a decrease in fat deposits in elderly users, overall antiaging eects have not been supported.
Additionally, at least one study at Stanford University
indicated that muscle mass increases were not accompanied by muscle strength increases and may
be a result of water retention by muscle cells rather than additional contractile protein synthesis.
2.5 Prolactin
As its name implies,
prolactin (PRL) promotes lactation (milk production) in women.
During pregnancy,
it contributes to development of the mammary glands, and after birth, it stimulates the mammary glands
to produce breast milk.
However, the eects of prolactin depend heavily upon the permissive eects of
estrogens, progesterone, and other hormones.
And as noted earlier, the active ejection of milk occurs in
response to stimulation from oxytocin.
In a non-pregnant woman, prolactin secretion is inhibited by prolactin-inhibiting hormone (PIH), which
is actually the neurotransmitter dopamine, and is released from neurons in the hypothalamus. Only during
pregnancy do prolactin levels rise in response to prolactin-releasing hormone (PRH) from the hypothalamus.
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3 Intermediate Pituitary: Melanocyte-Stimulating Hormone
The cells in the zone between the pituitary lobes secrete a hormone known as melanocyte-stimulating hormone
(MSH) that is formed by cleavage of the pro-opiomelanocortin (POMC) precursor protein, the same precursor
protein that gives rise to ACTH. Local production of MSH in the skin is responsible for melanin production
in response to UV light exposure.
The role of MSH made by the pituitary is more complicated.
For
instance, people with lighter skin generally have the same amount of MSH as people with darker skin.
Nevertheless, this hormone is capable of darkening of the skin by inducing melanin production in the skin's
melanocytes. Women also show increased MSH production during pregnancy; in combination with estrogens,
which commonly leads to darker pigmentation of the skin of the areolas and labia minora.
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4 Summary of the pituitary hormones and their principal eects
Major Pituitary Hormones
Figure 5: Major pituitary hormones and their target organs.
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:
1
Visit this link
to watch an animation showing the role of the hypothalamus and the pituitary gland.
Which hormone is released by the pituitary to stimulate the thyroid gland?
5 Chapter Review
The hypothalamuspituitary complex is located in the diencephalon of the brain. The hypothalamus and
the pituitary gland are connected by a structure called the infundibulum, which contains vasculature and
nerve axons. The pituitary gland is divided into two distinct structures with dierent embryonic origins.
The posterior lobe houses the axon terminals of hypothalamic neurons.
It stores and releases into the
bloodstream two hypothalamic hormones: oxytocin and antidiuretic hormone (ADH). The anterior lobe is
connected to the hypothalamus by vasculature in the infundibulum and produces and secretes six hormones.
Their secretion is regulated by releasing and inhibiting hormones from the hypothalamus. The six anterior
1 http://openstaxcollege.org/l/roleofhypo
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pituitary hormones are: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic
hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL).
6 Interactive Link Questions
Exercise 1
(Solution on p. 18.)
2
Visit this link
to watch an animation showing the role of the hypothalamus and the pituitary
gland. Which hormone is released by the pituitary to stimulate the thyroid gland?
7 Review Questions
Exercise 2
(Solution on p. 18.)
The hypothalamus is functionally and anatomically connected to the posterior pituitary lobe by a
bridge of ________.
a. blood vessels
b. nerve axons
c. cartilage
d. bone
Exercise 3
(Solution on p. 18.)
Which of the following is an anterior pituitary hormone?
a. ADH
b. oxytocin
c. TSH
d. cortisol
Exercise 4
(Solution on p. 18.)
How many hormones are produced by the posterior pituitary?
a. 0
b. 1
c. 2
d. 6
Exercise 5
(Solution on p. 18.)
Which of the following hormones contributes to the regulation of the body's uid and electrolyte
balance?
a. adrenocorticotropic hormone
b. antidiuretic hormone
c. luteinizing hormone
d. all of the above
2 http://openstaxcollege.org/l/roleofhypo
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8 Critical Thinking Questions
Exercise 6
(Solution on p. 18.)
Compare and contrast the anatomical relationship of the anterior and posterior lobes of the pituitary gland to the hypothalamus.
Exercise 7
Name the target tissues for prolactin.
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(Solution on p. 18.)
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Solutions to Exercises in this Module
to Exercise (p. 16)
Thyroid-stimulating hormone.
to Exercise (p. 16)
B
to Exercise (p. 16)
C
to Exercise (p. 16)
A
to Exercise (p. 16)
B
to Exercise (p. 17)
The anterior lobe of the pituitary gland is connected to the hypothalamus by vasculature, which allows
regulating hormones from the hypothalamus to travel to the anterior pituitary. In contrast, the posterior
lobe is connected to the hypothalamus by a bridge of nerve axons called the hypothalamichypophyseal
tract, along which the hypothalamus sends hormones produced by hypothalamic nerve cell bodies to the
posterior pituitary for storage and release into the circulation.
to Exercise (p. 17)
The mammary glands are the target tissues for prolactin.
Glossary
Denition 1: acromegaly
disorder in adults caused when abnormally high levels of GH trigger growth of bones in the face,
hands, and feet
Denition 2: adrenocorticotropic hormone (ACTH)
anterior pituitary hormone that stimulates the adrenal cortex to secrete corticosteroid hormones
(also called corticotropin)
Denition 3: antidiuretic hormone (ADH)
hypothalamic hormone that is stored by the posterior pituitary and that signals the kidneys to
reabsorb water
Denition 4: follicle-stimulating hormone (FSH)
anterior pituitary hormone that stimulates the production and maturation of sex cells
Denition 5: gigantism
disorder in children caused when abnormally high levels of GH prompt excessive growth
Denition 6: gonadotropins
hormones that regulate the function of the gonads
Denition 7: growth hormone (GH)
anterior pituitary hormone that promotes tissue building and inuences nutrient metabolism (also
called somatotropin)
Denition 8: hypophyseal portal system
network of blood vessels that enables hypothalamic hormones to travel into the anterior lobe of the
pituitary without entering the systemic circulation
Denition 9: hypothalamus
region of the diencephalon inferior to the thalamus that functions in neural and endocrine signaling
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Denition 10: infundibulum
stalk containing vasculature and neural tissue that connects the pituitary gland to the hypothalamus
(also called the pituitary stalk)
Denition 11: insulin-like growth factors (IGF)
protein that enhances cellular proliferation, inhibits apoptosis, and stimulates the cellular uptake
of amino acids for protein synthesis
Denition 12: luteinizing hormone (LH)
anterior pituitary hormone that triggers ovulation and the production of ovarian hormones in
females, and the production of testosterone in males
Denition 13: osmoreceptor
hypothalamic sensory receptor that is stimulated by changes in solute concentration (osmotic pressure) in the blood
Denition 14: oxytocin
hypothalamic hormone stored in the posterior pituitary gland and important in stimulating uterine
contractions in labor, milk ejection during breastfeeding, and feelings of attachment (also produced
in males)
Denition 15: pituitary dwarsm
disorder in children caused when abnormally low levels of GH result in growth retardation
Denition 16: pituitary gland
bean-sized organ suspended from the hypothalamus that produces, stores, and secretes hormones
in response to hypothalamic stimulation (also called hypophysis)
Denition 17: prolactin (PRL)
anterior pituitary hormone that promotes development of the mammary glands and the production
of breast milk
Denition 18: thyroid-stimulating hormone (TSH)
anterior pituitary hormone that triggers secretion of thyroid hormones by the thyroid gland (also
called thyrotropin)
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