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Lec. 3+4 - Hypothalamic and pituitary hormones
Aim of the lecture
1. To know the types of hormones produced by hypothalmus and pituitary hormones
2.Synthesis and metabolism of hypothalamus and pituitary hormones
3.Action and regulation of hypothalamus and pituitary hormones.
The hypothalamus is a small region located within the brain that controls many bodily
functions, including body temperature maintenance, sexual functions and behaviors,
blood pressure and heart rate, the sleep-wake cycle, and emotional states (e.g., fear, pain,
anger, and pleasure).
Hypothalamic hormones play pivotal roles in the regulation of many of those functions.
Because the hypothalamus is part of the central nervous system, the hypothalamic
hormones actually are produced by nerve cells (i.e., neurons). In addition, because signals
from other neurons can modulate the release of hypothalamic hormones, the
hypothalamus serves as the major link between the nervous and endocrine systems.
It is connected to the anterior lobe of the pituitary gland by hypophysal portal blood
vessels and with the posterior lobe of pitutary gland by axons of its neurons. The portal
vessels and axons pass through a stalk to the pitutary lobes thus forming a hypothalamo-
hypophysial axis. This is most important for homeostatsis as it regulates the most major
physiological activities in the body. It also indicates a close connection and co-ordination
between the hormonal and nervous systems
Hypothalamus secretes releasing and release-inhibiting hormones that control the anterior
lobe of the pituitary and Two other hormones: are synthesised by the neurons in the
hypothalamus and stored in their axon ends, and released by the posterior pitutiary
lobe.The following hormones are released into the blood in the capillaries and travel
immediately – in portal veins – to a second capillary bed in the anterior lobe of the
pituitary, where they exert their effects.
Thyrotropin-releasing hormone (TRH)
Gonadotropin-releasing hormone (GnRH)
Growth hormone-releasing hormone (GHRH)
Corticotropin-releasing hormone (CRH)
Somatostatin
Dopamine
Vasopressin and oxytocin are two hormone that are synthesised by the neurons
in the hypothalamus and stored in their axon ends in the posterior lobe of the
pituitary gland.
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1.Thyrotropin-releasing hormone (TRH)
TRH is a tripeptide When it reaches the anterior lobe of the pituitary it stimulates the
release of thyroid-stimulating hormone (TSH) and prolactin (PRL) from the anterior
pituitary gland
2.Gonadotropin-releasing hormone (GnRH)
GnRH is a peptide hormone. Its secretion at the onset of puberty triggers sexual
development, and from then on it is essential for normal sexual physiology in both males
and females. In both sexes, its secretion occurs in periodic pulses usually occurring every
1–2 hours
3.Growth hormone-releasing hormone (GHRH)
GHRH stimulates cells in the anterior lobe of the pituitary to secrete growth hormone
(GH).
4.Corticotropin-releasing hormone (CRH)
CRH is a peptide hormone., its acts on cells in the anterior lobe of the pituitary to release
adrenocorticotropic hormone (ACTH)
5.Somatostatin
Somatostatin is a mixture of two peptides, Somatostatin acts on the anterior lobe of the
pituitary to inhibit the release of growth hormone (GH)
Somatostatin is also secreted by cells in the pancreas and in the intestine where it inhibits
the secretion of a variety of other hormones
6.Dopamine
Dopamine is a derivative of the amino acid tyrosine. Its principal function in the
hypothalamus is to inhibit the release of prolactin (PRL) from the anterior lobe of the
pituitary.
Hormones of the Pituitary Gland
The pituitary gland is pea-sized structure located at the base of the brain. In humans, it
consists of two lobes: Anterior & Posterior lobes.
The Anterior Lobe
The anterior lobe contains six types of secretory cells. All of them secrete their hormone
in response to hormones reaching them from the hypothalamus.
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1.Thyroid Stimulating Hormone (TSH)
TSH (also known as thyrotropin) is a glycoprotein consisting of:
1) a beta chain of 118 amino acids and
2) an alpha chain of 92 amino acids. The alpha chain is identical to that found in two other
pituitary hormones, FSH and LH; is its beta chain that gives TSH its unique properties.
The secretion of TSH is stimulated by the arrival of thyrotropin releasing hormone
(TRH) from the hypothalamus.
As its name suggests, TSH stimulates the thyroid gland to secrete thyroid hormone. It
does this by binding to transmembrane G-protein-coupled receptors (GPCRs) on the
surface of the cells of the thyroid.
Hormone deficiencies
A deficiency of TSH causes hypothyroidism: inadequate levels of T
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and T
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2.Follicle-Stimulating Hormone (FSH)
FSH is a glycoprotein hormone consisting of:
1) the same alpha chain found in TSH (and LH)
2) A beta chain of 118 amino acids, which gives it its unique properties.Synthesis and
release of FSH is triggered by the arrival from the hypothalamus of gonadotropin-
releasing hormone (GnRH). The effect of FSH depends on one's sex.
FSH in females in sexually-mature females, FSH (assisted by LH) acts on the ovarian
follicle to stimulate it to release the hormone estrogens.
FSH in males: In sexually-mature males, FSH stimulates the production of sperm. (with
the aid of testosterone)
3.Luteinizing Hormone (LH):
LH is synthesized within the same pituitary cells as FSH and under the same stimulus
(GnRH). It is also a glycoprotein consisting of :
1) the same alpha subunit found in FSH and TSH
2) a beta chain of 121 amino acids that is responsible for its properties.
The effects of LH also depend on sex.
o LH in females in sexually-mature females,
a surge of LH triggers the release of the ovum from the ovary(ovulation) in the middle
of the menstrual cycle;
stimulates the now-empty follicle to develop into the corpus luteum, which secretes
progesterone during the second half of the menstrual cycle.
o LH in males LH acts on the interstitial cells of the testes stimulating them to synthesize
and secrete the male sex hormone, testosterone.
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4.Prolactin (PRL)
Prolactin is a peptide hormone. During pregnancy it helps in the preparation of the
breasts for future milk production. After birth, prolactin promotes the synthesis of milk.
Prolactin secretion is stimulated by TRH
o hyperprolactinemia may be caused by physiological cause as stress and pregnancy or
by pathological causes as prolactin secreting tumor of the pituitary gland and drugs.
o Hyperprolactinemia leads to amenorrhea,sexual dysfunction and infertility.
5.Growth Hormone (GH)
growth hormone (also called somatotropin) is a protein hormone. The GH-secreting cells
are stimulated to synthesize and release GH by the intermittent arrival of growth
hormone releasing hormone (GHRH) from the hypothalamus. GH promotes body
growth by:
o binding to receptors on the surface of liver cells.
o This stimulates them to release insulin-like growth factor-1 (IGF-1)
o IGF-1 acts directly on the ends of the long bones promoting their growth.
In childhood:
o hyposecretion of GH produces a short stature.
o Growth retardation can also result from an inability to respond to GH.
o hypersecretion leads to gigantism (in childhood)
In adults, a hypersecretion of GH or GHRH leads to acromegaly.
6.ACTH — the adrenocorticotropic hormone
ACTH is a peptide of 39 amino acids. It is cut from a larger precursor
proopiomelanocortin (POMC).ACTH acts on the cells of the adrenal cortex,
stimulating them to produce
o glucocorticoids, like cortisol;
o mineralocorticoids, like aldosterone;
o androgens (male sex hormones).
Production of ACTH depends on the arrival of corticotropin-releasing hormone (CRH)
from the hypothalamus.
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The Posterior Lobe of the pituitary gland
The posterior lobe of the pituitary releases two hormones, both synthesized in the
hypothalamus:
Vasopressin
Vasopressin is a peptide of 9 amino acids , It is also known as arginine vasopressin
(AVP) and the antidiuretic hormone (ADH).
Vasopressin acts on the collecting ducts of the kidney to facilitate the reabsorption of
water into the blood. This it acts to reduce the volume of urine formed (giving it its name
of antidiuretic hormone).
A deficiency of vasopressin or inheritance mutation for its receptor leads to excessive
loss of urine, a condition known as diabetes insipidus. The most severely-afflicted
patients may urinate as much as 30 liters of urine each day. The disease is accompanied
by thirst, and patients must continually drink water to avoid dangerous dehydration.
Oxytocin
Oxytocin is a peptide hormone
It acts on certain smooth muscles:
o stimulating contractions of the uterus at the time of birth;
o stimulating release of milk when the baby begins to suckle.