EndocrinologyTopics

Endocrinology Topics

Pituitary and Pineal Glands

Anatomy and Developmental Origin

The pituitary gland, also known as the hypophysis, is located at the base of the brain just behind the nasal cavity. It is inside a pocket (sellaturcica) formed by the sphenoid bone. The human pituitary gland has two lobes: anterior and posterior. Each lobe has a different embryonic origin and secretes a different set of hormones. A third area, the pars intermedia, contains different cells than the anterior and posterior lobes. While in other animals the pars intermedia is a discrete third lobe, in humans consist of dispersed cycts in the anterior lobe.

Each lobe has a different origin. The posterior lobe originates from a neuroectoderm bud at the floor of the diencepahlon (i.e. future brain). The anterior lobe originates from an invagination of the oral ectoderm in the roof of the mouth, known as Rathke's Pouch.

Several key transcription factors are responsible for the development of the different anterior pituitary cell types. Some transcription factors are specific to the pituitary (ex. Pit-1), some become pituitary-specific (ex. Lhx3), some are pituitary-enriched, and some are not pituitary-enriched but act in combination with other factors.

Pit-1 (also knwn as Pou1F1) is an anterior pituitary-specific transcription factor. It is expressed in somatotrps, lactotropes and thyrotropes. The Snell mouse has a mutated Pit-1 genes (in chromosome 3p11). They are dwarfed and lack the three cell types controlled by Pit-1 and their hormones. Pit-1 activates the genes for GH, PRL and bTSH. May be the target of hypothalamic control mechanisms. Humans with Pit-1 mutations are of short stature, may have mental retardation, and have a combined pituitary hormone disease or CPHD (i.e. very low or missing GH, TSH and PRL). Pit-1 target genes include the hormone genes GH, bTSH, PRL and SL (fish) and the receptor genes GFR-R and TRb2. Pit-1 also regulates its own gene (cell type stability?) .

Porp-1 (Prophet of Pit-1. gene in chromosome 5q35) was identified from a mouse with genetic pituitary disease. It was an Ames dwarf mice with phenotype similar to the Snell and Jackson dwarfs. These mice do not have GH, TSH or PRL nor their source cells, and do not have Pit-1. Pro-1 is an anterior pituitary-specific transcription factor, and it is be mutated in some patiens with CPHD, thus having symptoms similar to patients with a Pit-1 mutation. Human patients also lack the gonadotropins FSH and LH.

Lhx3 is a homeodomain-containing transcription factor expressed in th Rathke's Pouch of the developing brain. It is pituitary-specific in adult. Lhx3 activates pituitary genes such as aGSU, PRL, Pit-1 and TSH. Mice with deleted Lhx3 genes have no anterior and intermediate pituitary (a few corticotropes remain) and they die at birth. (Sheng et al. 1996). Humans with mutated LHX3 genes have no GH, PRL, TSH, FSH, nor LH, but they do have ACTH. (Netchine et al. 2000). A characteristic symptom is a rigid cervical spine.

The anterior lobe is also known as the adenohypophysis and in humans has three parts: the main body of the lobe (pars distalis), an upper stalk (pars tuberalis) and the pars intermedia. There is no direct neural connection bethween the anterior lobe and the hypothalamus, but hypotalamic factors still control its function. The parvocellar neurosecretory system -- i.e the paraventricular nucleus (PVN), arcuate nucleus (AN), and medial preoptic nucleus (MPN) of hypothalamus -- makes hypothalamic hormones that regulate the anterior pituitary. Axons of the parvocellular system conver at the median eminece of the hypothalamus, where the hormones can reach the anterior lobe through the hypopheseal portal vein.

Comparative Pituitary Structure
Organisms	Anterior	Intermediate	Posterior
Teleost Fish	cells types arranged in zones		not a distinct lobe
Cyclostome Fish	cells embedded in plate beneath brain
Tetrapods birds mice and most other mammals humans, elephants, whales	intermingled cells	none distinct not distinct in adult	very distinct lobe

The posterior lobe is also known as the neurohypophysis and has two parts: the main body (pars nervosa) and the infundibular stalk which connects to the median eminence of the hypothalamus. It is made mostly of nerve axons that originate at the magnocellar secretory system in the hypothalamus, i.e. paraventricular nucleus (PVN) and supraoptic nucleus (SON).

The pituitary produces hormones that regulate growth, metaboilism, reproduction, stress response and lactation.

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Posterior Pituitary

The posterior pituitary produces two hormones: oxytocin (OT) and arginine vasopressin (AVP), also known as antidiuretic hormone (ADH). Neurons of the hypotalamic magnocellular system (PVN and SON) make these hormones. These neurons project thru axons through the infundibular stalk into the posterior pituitary, were the hormones are released.

There are many other similar posterior pituitary hormones in animals other than humans, wiht different functions. For example, arginine vasotocin (AVT) has many putative roles in non-mammalian vertebrates and is found in the pineal gland an in fetal mammals. Vasotocin induces sexual activity (clasping) in salamanders, and oviposition (oviductal muscle contraction) in birds and reptles. Vasotocin and hydrins (longer, extended vasotocin molecules) control water balance in anphibians by controlling water reabsorption into skin and bladder. The hydrins have only been found in amphibians so far, not in birds nor reptiles.

Oxytocin is released from the posterior pituitary in response to active touch receptors in the nipple (milk ejection) or stretch receptors in the uterine cervix (uterine contraction). It also plays a role in maternal behaviours, sexual function (orgasm) and stress response. The same receptors trigger nervous signiling through the spinal cord, acting as a positive feedback loop into the hypothalamus to increase oxytocin release.

The precursor to oxytocin, prepro-oxyphysin, is made mostly in the PVN and processed to yield oxytocin as it travels along axons to the posterior pituitary. Another product of prepro-oxyphysin, neurophysin I, may help stabilize the granules that carry oxytocin to the pituitary.

The oxytocin receptor is a G-protein coupled receptor (GPCR) that activates the IP3/Ca pathway. As calcium levels increase in the cytosol, kinases get activated that stimulate the myosin/acting contraction of muscle cells. Pytocin is a synthetic agonis of OT receptors.

Vasopressin is also made from a precursor, prepro-pressophysin, that is processes as it travels to the posterior pituitary.

Vasopressing receptors are two GPCR: VR1 and VR2. VR2 is the kidney receptor, and activates cAMP pathways leading to aquaporin insertion in cell membranes. In the renal tubules, vasopressin receptors affect solute uptake in the ascending loop of Henle and water reabsorption in the collecting duct. It makes the distal convoluted tubule and collecting duct permeable to water, thus urine volume decreases.

There are many VR1 subtipes, located in all tissues except the kidney, and activate IP3/Ca pathways. Roles of ADH outside the kidneys include modulation of adrenocorticotropic hormone (ACTH) secretion (and maybe thyroid-stimulating hormone), promotion of glycogen conversion to phycogen-phosphate in the liver, and may have a role in memory.

Diabetes insipidus (DI) is a disease were the action of vasopressing is lacking, leading to increased urination (polyuria) and thirst (polydipsia). Central DI is due to a deffect in the hypothalamic/pituitary mechanisms that produce vasopressin, for example a mutation in the vasopressin gene. Peripheral DI is due to an unresponsive kidney, for example a mutated VR2 receptor gene or mutated aquaporin gene.

Central DI is treated with a nasal spray that supplies vasopressin. Peripheral (nephorgenic) DI is treated with the antidiuretic drug chlorpropamide (a sulplylourea) which increases kidney tubule sensitivity to vasopressin.

A syndrome of inapropriate ADH secretion (SIADH) can be caused by anesthetics, nicotine, narcotics or ADH-secreting tumors. SIADH may lead to mild or severe water retention, in the later case causing convulsins, coma and death. SIADH is treated by controlling water intake or if severe by using a renal poison like demeclocycline, which will induce nephrogenic DI.

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Anterior Pituitary - Lactogens and ACTH

The anterior pituitary produces at least six different hormones: growth hormone (GH), prolactin (PRL), adenocoticotrophic hormone (ACTH), thyroid-stimulating hormone (TSH, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). The release of these hormones is induced or inhibited by specific hypothalamic hormones and inhibited by themselves and by the products of their target organs.

Five different cell types are responsible for generating the anterior pituitary hormones. Somatotropes secrete growth hormone. Lactotropes secrete prolactin. Corticotropes secrete ACTH. Thyrotropes secrete TSH. Gonadotropes secrete LH and FSH. Somatotropes and lactotropes react with acidic dyes (acidophyls, pink color), while corticotropes, thyrotropes and gonadotrops react with basic dye (basophyls, blue color).

Diseases of the pituitary gland that can involve all or any of the cell types include tumors and McCune-Albright syndrome. Other diseases are specific to a cell type and usually feature a mutation in a hormone or receptor gene, for example growth disease are due to excess or lack of GH or its receptor.

The types and effects of pituitary tumors vary widely. A large tumor will cause pressure on the optic chiasm and loss of vision. A tumor made of one type of cells may hinder the growth and function of other pituitary cells, ex. a somatotrope tumor may hinder gonadotrope function and impeed puberty. Prolatinomas, the most common type of pituitary tumor, occur in 25% of the population, but clinically significant tumor occur only in 14 out of 100,000 people. The cause of pitutary tumors remains unknown. Most tumors are sporadic, i.e. they are not genetically passed from parents to children. Removal is not a complicated procedure, but somtime the whole pituitary is lost, requiring a wider range of hormone replacement therapy.

McCune-Albright sindrome is due to a mutation of the adenylyl cyclase-stimulating stimulating G-alpha protein (Gas) that causes decreased GTPase activity, thus Gas is always active. This leads to constitutivelly adenylyl cyclase activity in one or more endocrine gland: pituitary, adrenal cortex, gonads, thyroid. etc. The syndrome cause precocious puberty, hyperthyroidism, Cushing's disease, bone dysplacia, etc. Many of these diseases will be due in part to constitutively high cAMP levels in the pituitary.

Expression of Human GH and CS Genes
GH-N	anterior pituitary
GH-V	placenta
CS-A	placenta
CS-B	placenta
CS-L	placenta

Growth hormone and prolactin belong to the same superfamily of hormones, which also includes chorionic somatotropin (CS). While there is a single gene for PRL in human chromosome 6, there are multiple genes for CS and GH on human chromosome 17.

Mice have a large family (>24) of prolactin-related genes on chromosome 13. Four "classical" members of this family are prolactin, placental lactogen I, placental lactogen IV and placental lactogen II. "Non-classical" members include proliferin. Most are expressed in placenta and sometimes in other tisssues. Rats and cows have similar genes.

The human prolactin (PRL) gene is expressed in the pituitary, placenta and immune system. The main role of PRL in humans s in mammary gland development and maintenance of lactation. Other roles include regulation of lymphocytes, osmotic effects in the kidneys, liver function, the corpus luteum, and gonadal steroid production.

The PRL receptor is a 7-transmembrane helix receptor, member of the cytokine receptor superfamily, that activates the JAK/STAT pathway. IThere are two forms of PRLR: short and long. The long form appears to be the important one for STAT5 activation and milk production.

Knockout of the PRL-R gene produces multiple reproductive defects in mice (Ormandy et al. 1997). Heterozygous females showed almost complete failure of lactation due to poor mammary gland develkopment after their first, but not subsequent pregnancies. Homozygous females were sterile due to failure of embryonic implantation, and had multiple reproductive abnormalities: irregular cycles, reduced fertilization rates, defective preimplantation embryonic development, and lack of pseudopregnancy. Half of the homozygous males displayed delayed fertility. Therefore the PRL and its ligands are key regulators of mammalian reproduction.

STAT5a is mandatory for adult mammary development and lactogenesis (Liu et al. 1997). STAT5a amdf STAT5b are expressed in mammary tissue during pregnancy. Homozygous STAT5a female mice develop normally and physically indistinguishable from wild type animals. But they are sterile due to complete lack of embryionic implantation and have the same reproductive abnormalities as homozygous PRLR knockout mice. Homozygous males developed normally.

A prolactinoma is a bening tumor of the pituitary gland made of lactotrops, thus producing high levels of PRL. Symptoms are caused by too much PRL in the blood (hyperprolactinemia) or by pressure of thetumor on surrounding tissues. In woman, hyperprolactinemia causes infertility and changes in mestruation: menstrual flow changes, and/or periods become irregular or disapear altogether. Women that are not pregnant or nursing may produce milk. Some women experience loss of libido or intercourse may be painful due to vaginal dryness. The most common symptom in men is impotence, although the patient may not notice this until preblems of pressure in brain structures and loss of vision develops.

Chorionic somatotropin (CS), also known as chorionic somatommamotropin or placental lactrogen (PL), is secreted by the placenta during pregnancy to augment maternal functions. It is made in the syncytiotrophoblastic epithelial cells of the chorionic villi of the placenta beguining at the 6th week of pregnancy. CS levels are very high at mid to late pregnancy, reaching levels 3 times higher than growth hormone.

The primary role of CS may be to stimulate mammary gland development without causing milk secretion. PRL may then initiate milk secretion after parturition. CS may also alter the maternal metabolism by countracting maternal insulin to ensure adequate glucose, amino acids and mineral availability for the fetus and promote fetal growth.

Adrenocorticotropic hormone (ACTH), also known as corticotropin, adenocorticotropin or cortical-stimulating hormone, is a 39 amino acid peptide hormone secreted by corticotrope cells of the anterior pituitary. ACTH is one product of the pre-pro-opiomelanocortin (POMC) gene. It shares 13 amino acids with aMSH, but there is little evidence that ACTH plays a role in normal melanocite function.

The ACTH receptor is a GCPR of the MSH-R family (>4 members) that activates the cAMP pathway. It is located in cells of the adrenal cortex.

ACTH is positivelly regulated by corticotropin releasing hormone (CRH), also known as corticotropin releasing factor (CRF), a 41 amino acid peptide hormone from PVN of the hypothalamus. CRH was identified by Vale et al. (1981). CRH synergizes with vasopressin to promote ACTH release, and can also stimulate MSH release. CRH is related to the frog skin peptide sauvagine and the fish peptide urotensin I.

CFR receptors are GPCRs that activate the cAMP pahtway. Splice variants of the CFR-R exists but their role is unclear. They are found in the pituitary, hypothalamus and some other tissue, ex. placenta, but its role in these other tissue is also unclear.

ACTH regulates the cells of the adrenal cortex, which are steroidogenic and secrete steroid hormones that affect carbohydrate and mineral metabolism: aldosterone, cortisol and corticosterone.

Excess ACTH leads to Cushing's syndrome, typified by excess cortisol, pathological alterations in glucose metabolism, and hyperpigmentation. Symptoms include obesity, fatigue, high blood pressure, bruising, female menstrual problems, moon face and "buffalo hump". Cushings syndrome can be caused by pituitary adenomas that secrete ACTH (then known as Cushing's disease) or by ACTH-secreting tumors in other tissues (ex. lungs), or by adrenal tumors secreating excess cortisol. It is treated by removal of the tumor (adenomectomy or hypophysectomy).

Absense of ACTH leads to Addison's disease (AD), typified by low cortisol and pathological alterations of cellular metabolism. Symptoms include low blood pressure, weakness, headaches, fainting, fever and pain. AD is rare (6/100,000), usually caused by adrenal failure (primary AD) due to autoimmune disease or another serious illnesss, ex. tuberculosis. It can also be cuased by insuficient ACTH (secondary AD). Patients with primary AD can have high levels of ACTH due to the lack of negative feedback, and may even suffer hyperpigmentation.

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Growth Hormone

GH, also known as somatotrophin or somatotropin (STH or ST), is a 191 amino acid protein hormone that regulates linear growth and metabolism. GH is made by somatotropes of the anterior pituitary, and accounts for 4-10% of the anterior pituitary weight (5-10 mg). It is secreted in bursts, in a 24 hour cyclic pattern (high release rates during sleep, almost no plasma GH between peaks). GH circulates in plasma bound to GH binding protein (GHBP).

Smith (1926) noticed that hypophysectomized rats failed to grow and pituitary extracts reversed this effect. Now we know that GH stimulates proliferation of the epiphyseal cartilage in the growth plates at the ends of long bones. This leads to growth as ossification occurs in the diaphyseal and epiphyseal ossification centers. GH also stimulates amino acid uptake by muscles and liver.

The GH receptor (GH-R) is a 7-transmembrane helix receptor that dimerizes and activates the JAK/STAT pathway.

GH signaling is controlled by many different mechanisms, both permisive an inhibitory. The main positive control is GHRH, the main negative control somatostain, both from the hypothalamus.

Growth Hormone Releasing Factor (GRF), also knwon as Growth Hormone Relesing Hormone (GHRH) or somatocrinin, is the main positive control of GH release. GHRH is a 44 amino acid neuropeptide released by the hypothalamus that promotes somatotrope proliferation and GH secretion. It is related to the secretin/glucagon family of peptides.

The GHRH receptor (GRH-R) is a GPCR that activates the cAMP signaling pathway in somatotropes. It is found in the membrane of liver cells. The little dwarf mouse (lit mouse) has a mutation in the extracellular domain of the GHRH-R. Some human patients with short stature and GH deficiency also have mutated GHRH-R.

Some small synthetic molecules will act as GH secretagogues (GHS), i.e. act like GHRH to promore GH secretion (ex. GHRP-6, a hexapeptide). They may activate the IP/CA pathway instead of the cAMP pathway, thus they are acting thru a different receptor. A GHS-R was cloned from pigs by Howard et al. (1996), and is found in the pituitary gland and hypothalamus, maybe in more tissues.

Kojima et al. (1999) found a new peptide, gherelin, that binds the GHS-R. Gherelin is made by endocrine neurons in the stomach, and its mRNA was also found in the hypothalamus. It is a 28 amino acid peptide, produced from a 117 amino acid precursor, with a novel n-octanoyl modification. Human gherelin has 26 amino acids of the rat peptide. In rats, gherelin can specifically activate the GHS-R and cause subsequent release of calcium. Tschop et al. (200) suggested that gherelin may also signal the hypothalamus when an increase in metabolic efficiency is necessary.

Somatostatin (SST), also known as somatotropin release-inhibiting factor (SRIF), is a 14 amino acid neuropeptide released by the hypothalamus that inhibits GH secretion from somatotrps. SST receptors inhibit the cAMP pathway. It also has other roles in the gut, pancreas and other tissues and is conserved in vertebrates.

The cell proliferation promoted by GH in the epiphyseal cartilage can be mesasured by the incorporation of radioactive sulfur because GH stimulates the liver to produce insuline-like growth factors (IGFs). IGFs are small peptides that circulate in plasma bound to IGF binding protein (IGFBP).

In mammals, body weight is correlated with IGF-I levels rather than GH secretory capacity. For example, Eigenmann et al. (1984) found that standard poodles exhibited six times the mean plasma IGF-I than toy poodles. All dog secreted similar, normal amounts of GH after clonidine (alpha-2 adrenergic agonist that stimulates the hypothalamus?) administration.

Either IGF-I and IGF-II knockout mice have slow growth in utero. IGF-I knockout mice also have very poor growth after birth. Therefore IGF-II is mostly a fetal growth factor, while IGF-I is an important growth factor at all stages.

In summary, GH may act directly on tissues or indirectly by promoting IGF generation in the liver. GH acts directly to induce lipolysis in fat cells and carbohydrate catabolism in liver and muscle, both ani-insuline diabertogenic action. IGFs induce protein synthesis in muscle, lipolysis in fat cells and growth and cartilage proliferation in bone.

Growth diseases include gigantism, acromegaly, pituitary dwarfism and Laron dwarfism. Some rare syndromes are due to lack or insensitivity to IGFs.

Gigantism and acromegalia are due to an excess of GH. Gigantism results from excess GH early in life, i.e. when body growth is ongoing. Often the pituitary tumors producing the excess GH also cause deficiency of other hormones by impeeding growth of neighboring cell types. These tumors often prevent gonadotropin production, causing sex hormone deficiency and delaying or preventing puberty. At the same time, bones do not mature aand growth plates continue to grow, leading to symetrical body enlargement.

Acromegaly results from excess GH in adults, when body growth has stopped. Since long bones are already fused, overgrowth is only seen in certain areas, most commonly as a protruding jaw, thickened fingers, thickened skin and overgrown viceral organs.

Dwarfism may be due to either deficiency or insensitivity to GH or IGFs. Pituitary dwarfism is usually due to either a defect in the GHRH-R or a mutation on the GH gene, leading to GH deficiency. In the past, such aptients were treated with pituitary extracts, but there was a adnger of aquiring a prion disease. Today recombinant GH protein is available, although it is expensive and must be injected.

Genomic Imprinting
A gene is imprinted if its pattern of expression depends on whether it passed the previous generation in a male or female germline. The ability of somatic cells to distinguish the parental origin of a gene copy requires a reversible DNA modification that is acquired in the germline of each sex and maintained through many rounds of cell replication. The mechanism appears to involve DNA methylation.

Laron dwarfism is due to a defect in the GH receptor extracellular domain, which by itself functions as the GH binding protein (GHBP) that transport GH in plasma. Patients are very short for their age and have an abdominal fat pad due to metabolic disease. When the GH-R/GHBP gene is disrupted in mice, homozygous are much smaller than heterozygous or wild-type.

Missregulation of IGF-II is associated with developmental abnormalities. The murine IGF-II gene (Igf 2) is expressed only from the paternal copy, and the Igf 2 receptor gene (Igf 2r) is expressed from the maternal copy. The human IGF 2 gene was shown to be imprinted like its murine homologue. Normally, genomic imprinting keeps the maternal gene inactive. Imprinting failure will lead to overproduction of IGF-II. In humans this condition is known as Beckwith-Wiedemann Syndrome. Symptoms include: overall overgrowth, organ overgrowth, skeletal abnormalities and increased risk of tumors. A mouse model that overexpresses IGF-II also shows organ ovegrowth, skeletal abnormalities and increased risk of tumor.

GH insensitivity associated with STAT5b has only been reported in one patient, who has an homozygous deletion of exons 4 and 5 due to missense mutations (Woods et al. 1996). At 15 years old, the boy was very small, and had sensoneural hearing loss and mental retardation due to severe prenatal and postnatal growth failure. His parents were first cousings, carring heterozygous mutations. IGF-I was undetectable. GH measured for 12 hours showed abnormally high peaks and absence of undetectable values between peaks, i.e. a very high average GH level. Other laboratory studies were normal. Bone age was only minimally delayed, supporting the hypothesis tat GH stimulates bone maturation directly (?). This patient was later treated witrh IGF-I with good response.

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Pituitary Glycoprotein Hormones

The three pituitary glycoprotein hormones are thyroid stimulating hormone (TSH), follicule stimulating hormone (FSH) and and luthenizing hormone (LH). A fourth glycoprotein hormone, chorionic gonadotropin (CG), is placental and the basis for pregnancy tests. They have in common an alpha subunit known as alpha glycoprotein subunit (aGSU), bound to the hormone-specific beta subunit: bTSH, bFSH or bLH. These hormones have many disulfide bridges, 5 in alpha and 6 in the beta subunits, and their sugar moieties are important for activity.

After either thyroidectomy (thyroid removal) or hypophysectomy, tadpols do not become frogs. Putting back either te thyroid extract or the pituitary extract in the water were the tadpoles live restore their ability to become frogs (pituitary extract works only in frogs with intact thyroids). This experiment demonstrated that an agent from the pituitary gland stimulates the thyroid gland.

TSH is made in the thyrotropes of the pituitary. By stimulating the cAMP pathway, TSH promotes general growth of the thyroid glan and activates many of its functions: iodine uptake, hormone synthesis on thyroglobulin (TG) in the thyroid colloid, uptake of TG-thyroid hormone into thyroid follicles, lysosomal release of thyroid hormone from TG, and release of thyroid hormone into the bloodstream.

TSH is positivelly regulated by thyrotropin releasing hormone (TRH) from the hypothalamus. TRH is made from a precursor pro-TRH that gives several copies of the bioactive TRH. The TRH receptor actvates IP3/Ca pathways. TRH was identified by Guillemin and Schally (1969). TSH is negativelly regulated by somatostatin (also regulates GH).

TRH probably has other roles in lower organisms, ex. cyclostome fish have TRH but not TSH, snails have TRH but no pituitary. The TSH-regulating function of TRH may be a relatively new one in evolutionary terms. TRH may also play a role in the brain as it is related to epileptic seizures.

FSH and LH are made in the pituitary gonadotrops, and are known collectivelly as gonadotropins. Both the FSH receptor and LH receptors are GPCR that activate the cAMP signaling pathway.

LH is also known as lutropin. In females LH induces ovulation, initiates steroidogenesis in the ovarian follicle and mantains the secretory functions of the corpus luteum. In males LH stimulates Leydig cells to produce testosterone. The half-life of LH is 60 minutes.

FSH is also known as follitropin. IN females FSH stimulates development of ovarian follicles and secretion of estrogen. In males FSH stimulates spermatogenesis and sex hormone binding globulin. The half-life of FSH is 170 minutes.

The gonadotropins are positively regulated by gonadotropin releasing hormone (GnRH), also known as LH releasing hormone (LHRH), from the hypothalamus. The gonadal steroids as well as inhibin (also from the gonads) are negative regulators. The gonads also secrete activins which may positivelly feedback to the pituitary. At certain times, estrogens may positivelly feedback.

GnRH is a decapeptide derived from a 92 amino acids precursor. Its structure was first reported by Matsuo et al. (1997). Pro-GnRH is cleaved to GnRH and GnHR associated peptide (GAP), packaged into storage vesicles (in the Golgi) and released in response to neurotransmitters by exocitosis. The vesicles are guided to the membrane along microtubules, then fuse with the membrane, the membrane lyses, and GnHR is released into the pituitary portal vein. GnRH is released episodically, at about 90 minutes intervals.

GnHR regulates gonadotropic activity in birds, mammals and amphibians. Injecting anti-GnRH antibodies stops ovulation in females, atrophies testicles in males and lowers both FSH and LH levels in both sexes. In frogs, the concentration of GnHR varies with the seasonal reproductive cycle. GnHR may play other physiological roles in fish. An GnHR variant, cII-GnHR, is present in most vertebrates (?).

The GnRH receptor is a GCPR that activates the cAMP pathway. The carboxy terminus (inside cell) is shorter in mamals and longer in birds and fish. GnRH-R gene mutations cause hypogonadotropic hypogonadism (HH) in humans, i.e. delayed or abssent sexual development. The mutant receptors are missfolded but not completely "dead", rather they are not efficiently targeted to the cell membrane. The short carboxyl end is involved in this: if you put the C-end of a fish GnHR-R onto a mutant human GnHR-R, it sometimes increases activity. This disease is very hard to treat. P. Michael Conn (Oregon)used small molecule drugs originally designed as GnRH weak antagonists to help fold the receptors, allowing them to reach the membrane and have some activity (after the drug is removed. This approach may work for other diseases like cystic fibrosis..

The transcription factor NGFI-A is a widespread a zinc finger trancription factror (also known as Egr-1) that seems to regulate bLH gene espression. At the same time the cell-specific nuclear receptor SF-1 seems to synergize with NGFI-A to increase bLH expression. Mice with a mutated NGFI-A transcription factor appear normal at first, but females are infertile: they have no estrous cycle, their uteri are 30% smoler than wild type, and their progesterone levels are reduced, although estrogen is normal. They have normal levels of bFSH and GnRH-R, but bLH cannot be detected. Treatment with pregnant mare serum increases uterine size in NGFI-A knockout mice, suggesting a pituitary defect. Knockout males have some LH but their interstitial cells are atrophied. Still, the seminal vesicles are fine and the animals are fertile. When the animals were ovarectomizzed to remove the gonadal negative feedback on gonadotropins, FSH levels increased 5 times in both wild type and knockout animals, but LH increased 5 times only in wild type animals.

Precocious puberty is the early but normal sequence of pubertal events occurring before age 8 in girls and age 10 in boys. The causes are often unknown (idiopathic), or may be due to brain disease (tumors secretein GnHR). Pseudoprecocious puberty occurs when there is a peripheral hypersecretion of estrogens or androgens. Delayed puberty can be treated with GnRH.

New hormones <add info later>

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MSH and the Pineal Gland

Melanotropes in the intermediate pituitary produce alpha-melanotropin, also known as alpha-melanocyte stimulating hormone (aMSH), a 13 amino acid melanotropic peptide:

Ac-SYSMEHFRWGKPV-NH2

While the intermediate lobe not distinct in adult humans, melanotropes are part of the anterior pituitary. MSH is also found in arcuate nucleus of the hypothalamus. aMSH is probably not a major player in human pigmentation since people do not get lighter after hypophysectomy, but it is important in skin coloration in other animals.

Adrenocorticotropic hormone (ACTH), aMSH, bMSH, gMSH and bLPH are all derived from pre-pro-opiomelanocortin (POMC), and they all have some melanotropic activity. aMSH is derived from proteolytic processing of ACTH, which also generates the CLIP peptide, its role if any still unclear.

aMSH controls melanin pigmentation in skin of many vertebrates. Melanocytes, also known as melanophores, contain the colored polymer melanin which accumulates in melanosomes (vacuoles containin MDH). Melanosomes can be concentrated or dispersed within the cell or secreted into surrounding cells. After a ligth stimuli activates MSH release, melanosomes are released into adjacent keratinocyte cells. In humans they function as a UV sunscreen. In amphibians this occurs during background adaptation (camouflage). Injection of aMSH into a yellow-colored mouse will induce dark melanin pigmentation and turn the fur brown.

MSH may have other roles in humans regarding behavior, anti-inflammatory activity and pregnancy (made in placenta). MSH receptors (MSH-R) found in melanocytes are GPCR structurally related to ACTH-R, and they also activate the cAMP pathway.

Melanin concentrating hormone (MCH) is a 19 amino acid peptide that has opposite effect to MSH in fish.
But in frogs and mammals, MCH may have MSH-like activity.

aMSH promotes brown pigmentation in weasels: they have a brown coat in the summer; white in the winter. Daylight perceived produces a signal from the pineal gland sent to pituitary to regulate aMSH secretion. The size of the intermediate pituitary seems to correlated with ability to change color: chameleon lizards (ex. Anolis carolinensis) have very large intermediate pituitary.

MSH secretion is predominantly controlled negative by dopamine (like PRL). Dopamine receptor antagonists (such as chlorpromazine) promote MSH secretion. Dopamine agonists (such as ergot alkaloids) inhibit MSH secretion.

The pineal gland is a pea-sized, pine-cone shaped gland located in the center of the brain, on the edge of the tird ventricle. Pinealocytes are the true pineal cells, but the pineal gland also contains astrocytes (brain support cells) and nerve input fibers. Pineal tumors may be true pineal tumors, astrocytomas or pineal cysts.

The French philosopher Rene Descartes (1596-1650) thought that the pineal was the "seat of the rational soul" because it was the only "unpaired" structure of the brain.

When McCord & Allen (1917) added bovine pineal extract to pond water, the tadpoles' skin lightened (melanosome aggregation). In frogs and fish, the pineal is a photoreceptor (3rd eye) that sends direct signals to the brain. In birds the pineal is a photoendocrine transducer that receives direct light signaling. In mammals the pineal is also a neuroendocrine transducer but receive its light signal indirectly through the eye.

Frogs: ligth --------------------------> pineal --> brain

Birds: light --------------------------> pineal --> hormones

Mammals: ligth --> eye --> brain --> pineal --> hormones (melatonin)

In humans, light is received by the retina, which sends along a sympathetic nerve pathway to the superior cervical ganglion. From there the signals are transmitted to the pineal gland, which then secretes the hormone melatonin. Melatonin is its own negative regulator (negative feedback loop).

Melatonin was discovered by Lerner (Yale, 1958). It is synthesized in the pineal gland from the amino acid tryptophan by pineal-specific enzymes. The rate-limiting step of meltaning synthesis is catalized by N-acetyltransferase (NAT). The pineal gland is very enriched in NAT, which catalizes the conversion of serotonin to N-acetylserotonin.

Melatonin activates three types of inhibitory GPCR, inativating the cAMP pathway. The physiological effects of melatonin includes control of circadian rhythms (entrains the SCN clock), seasonal reproduction, thyroid physiology, retinal physiology, and hypnotic effects.

In humans, melatonin concetrations vary in both daily cycles and sesonal cycles. Melatonin concentrations are higher at night. The seasonal variations are less well understood.

Melatonin synthesis is regulated by . During dark periods, norepinephrie signaling from the cervical ganglia activates the cAMP pathway in pinealocytes. Increased cAMP levels activate protein kinase A (PKA), which in tur activates the transcription factor CREB. The NAT gene has a CREB responsive element (CRE) in its promoter, leading to increased levels of NAT and melatonin. The ICER protein inhibits NAT gene transcription, and the CREM gene that is the source of ICER also has a CRE in its promoter (negative feedback). ICER also blocks its own transcription. Less is known about another inhibitor of NAT transcription, FRA-2.

The sensitivity of the CREM gene to CREB stimulation is "entrained" by memory of night lengths. The nuclear response (CREM gene and CREB protein) to cAMP changes sensitivity to adrenergic signals depending on the duration of the previous night. This mechanism still not fully understood.

Whitman & Axelrod (1963) proposed that melatonin is controlled by light signals and regulates gonadal function. This was based on their observations in rodents. Increased light levels would decrease pineal gland size and increase ovary size. Removal of the pineal gland increased ovary size, while melatonin treatment decreased ovary size. The reproductive cycle of the male Syrian hjamster is regulated by seasonal variation in the amount of daylight. High levels of melatonin in winter correspond to decreased testicular size and a sexually quiecent phase. Low leves of melatonin from March to November correlate with increased testicular size and an increased level of sex activity.

The link between the pineal gland and human reproduction remains very controversial. In 1898 Otto Huebner described a boy with precocious puberty with a pineal tumor. Other similar cases have been documentes since, almost always males. Nevertheless, no link has been found between pineal/melatonin and GnRH, and there is no data showing that pineal tumors overproduce melatonin. Pineal tumors may make an LH-like molecule. It has been observed than in Finland, a contry with a long light cycle, the conception rate is high in summer.

There may be a link between melatonin and aging but this is also very controversial. Mice given melatonin live ~20% longer than contros, but this only works for mice
that lack the melatonin gene (e.g. BALB/C). In C3H mice (have melatonin), the extra melatonin leads to a decreased lifespan and reproductive tract tumors.

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Need more practice? Answer the following review questions:

1- Where is the pituitary gland located?
At the base of the brain just behind the nasal cavity. It is inside a pocket (sellaturcica) formed by the sphenoid bone.

2- Describe the anatomy of the human pituitary gland.
Has two lobes: anterior and posterior. A third area, the pars intermedia, contains different cells than the anterior and posterior lobes. While in other animals the pars intermedia is a discrete third lobe, in humans consist of dispersed cycts in the anterior lobe.

3- Describe the embryonic origin of the pituitary gland.
Each lobe has a different origin. The posterior lobe originates from a neuroectoderm bud at the floor of the diencepahlon (i.e. future brain). The anterior lobe originates from an invagination of the oral ectoderm in the roof of the mouth, known as Rathke's Pouch.

4- What is Pit-1?
Pit-1 (also knwn as Pou1F1) is an anterior pituitary-specific transcription factor expressed in somatotrps, lactotropes and thyrotropes, that activates the genes for GH, PRL and bTSH.

5- Describe the effects of Pit-1 mutation in mice.
The Snell mouse has a mutated Pit-1 genes: they are dwarfed and lack the three cell types controlled by Pit-1 and their hormones.

6- Describe the effects of Pit-1 mutation in humans.
Humans with Pit-1 mutations are of short stature, may have mental retardation, and have CPHD.

7- What is CPHD?
Combined pituitary hormone disease or CPHD is when there is very low or missing GH, TSH and PRL.

8- What is the cause(s) of inherited CPHD?
Mutations in the pituitary-specific transcription factors Pit-1, Prop-1 or Lhx3.

8- List 7 Pit-1 target genes.
GH
bTSH
PRL
SL (fish)
GFR-R
TRb2
Pit-1

9- What is Prop-1?
Prop-1 (Prophet of Pit-1) is a anterior pituitary-specific trascription factor needed for development of lactotropes, somatotropes, thryotropes and (in humans) gonadotropes.

10- List 5 hormones and 1 trascription factor missing in patients wit muated Prop-1.
PRL
GH
TSH
FSH
LH
Pit-1 (trans factor)

was identified from a mouse with genetic pituitary disease. It was an Ames dwarf mice with phenotype similar to the Snell and Jackson dwarfs. These mice do not have GH, TSH or PRL nor their source cells, and do not have Pit-1. Pro-1 is an anterior pituitary-specific transcription factor, and it is be mutated in some patiens with CPHD, thus having symptoms similar to patients with a Pit-1 mutation. Human patients also lack the gonadotropins FSH and LH.