HPG Axis and GnRH Function
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Questions and Answers

What role does the hypothalamic–pituitary–gonadal (HPG) axis primarily play in the body?

  • Regulating metabolic rates
  • Stimulating appetite
  • Regulating reproduction and fertility (correct)
  • Controlling body temperature

What triggers the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)?

  • Estrogen release
  • Testosterone activation
  • Growth hormone stimulation
  • Gonadotropin-releasing hormone (GnRH) (correct)

How is GnRH released in the body?

  • In a pulsatile fashion (correct)
  • Continuously at a constant rate
  • Only during sleep
  • In response to stress

What happens to GnRH shortly after it is released?

<p>It is broken down by proteolysis (D)</p> Signup and view all the answers

What is the function of gonadotropins in males and females?

<p>They act via a two-cell system. (C)</p> Signup and view all the answers

What hormone primarily regulates testosterone secretion from the testes?

<p>Luteinizing hormone (LH) (D)</p> Signup and view all the answers

During which life stage does testosterone secretion experience a perinatal surge?

<p>Early neonatal life (B)</p> Signup and view all the answers

What is the primary consequence of the increases in testosterone levels during male puberty?

<p>Somatic changes in males (A)</p> Signup and view all the answers

What portion of circulating testosterone is bound to sex hormone-binding globulin (SHBG)?

<p>45% (D)</p> Signup and view all the answers

What happens to circulating testosterone levels after middle age?

<p>They decrease gradually (B)</p> Signup and view all the answers

What is the role of the androgen-AR complex once it binds to hormone response elements on DNA?

<p>It acts as a transcription factor (D)</p> Signup and view all the answers

Which effect does testosterone NOT have on the skeletal system?

<p>Decreased osteoblast growth (B)</p> Signup and view all the answers

Which of the following describes how androgens impact the nervous system?

<p>They promote remyelination of neurons (B)</p> Signup and view all the answers

What mechanism occurs when either testosterone or DHT binds to androgen receptors?

<p>Dimerization and exposure of zinc finger domains (C)</p> Signup and view all the answers

How do androgens generally affect the body's immunological response?

<p>They promote maturation of T cells (D)</p> Signup and view all the answers

Which of the following is NOT a symptom of hypogonadism in adult men?

<p>Increased energy levels (D)</p> Signup and view all the answers

What is Klinefelter's Syndrome primarily associated with?

<p>Presence of extra X chromosomes (A)</p> Signup and view all the answers

Which condition results from iron overload affecting testosterone production?

<p>Hemochromatosis (D)</p> Signup and view all the answers

Which option is a DIRECT cause of primary hypogonadism?

<p>Testicular injury (A)</p> Signup and view all the answers

What is a known effect of aging on testosterone levels in men?

<p>Gradual decline in serum testosterone levels (D)</p> Signup and view all the answers

Which of the following hormones are affected by Kallmann Syndrome?

<p>Luteinizing hormone and follicle-stimulating hormone (D)</p> Signup and view all the answers

Which of the following symptoms is associated with testosterone deficiency?

<p>Hot flashes (B)</p> Signup and view all the answers

What can result from the treatment of cancer with chemotherapy?

<p>Reduced testosterone and sperm production (C)</p> Signup and view all the answers

What are the sex steroids synthesized and secreted in females and males?

<p>Estradiol and progesterone in females, testosterone in males (B)</p> Signup and view all the answers

Which hormone is crucial for the proliferation of Sertoli cells during early puberty in boys?

<p>Follicle-stimulating hormone (FSH) (D)</p> Signup and view all the answers

What is the primary function of the luteinizing hormone (LH) in males?

<p>Stimulates testosterone production in Leydig cells (A)</p> Signup and view all the answers

Which of the following accurately describes the action of FSH in the male reproductive system?

<p>FSH binds to receptors on Sertoli cells, increasing Leydig cell numbers (A)</p> Signup and view all the answers

What effect does negative feedback from sex steroids have in the body?

<p>Inhibits hypothalamic and pituitary activity (A)</p> Signup and view all the answers

Which protein is crucial for the action of steroid hormones in Sertoli cells?

<p>Androgen-binding protein (ABP) (D)</p> Signup and view all the answers

How do Leydig cells respond to an increase in luteinizing hormone (LH)?

<p>By proliferating and increasing testosterone levels (B)</p> Signup and view all the answers

What is the role of P-450 aromatase in the male reproductive system?

<p>Converts testosterone into estradiol (C)</p> Signup and view all the answers

Flashcards

Androgens

Testosterone and dihydrotestosterone (DHT) are the primary male sex hormones called androgens, responsible for male characteristics.

LH's role in testosterone production

LH (luteinizing hormone) controls the production of testosterone by stimulating the conversion of cholesterol to pregnenolone in Leydig cells within the testes.

Testosterone transport

Most testosterone in the bloodstream is bound to proteins, primarily Sex Hormone-Binding Globulin (SHBG) and albumin, with only a small amount circulating freely.

Testosterone and puberty

Testosterone levels rise during puberty, leading to male secondary sexual characteristics like muscle growth and deepening of the voice.

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Inhibins

Inhibins are hormones produced by the testes that regulate testosterone production by providing negative feedback on the hypothalamus and pituitary gland.

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What is the HPG axis?

The Hypothalamic-Pituitary-Gonadal (HPG) axis controls reproduction and fertility by regulating the production of sex hormones.

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Where is GnRH produced and what does it do?

GnRH is released from the hypothalamus, specifically the arcuate nucleus, and triggers the production of LH and FSH by the pituitary gland.

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How does GnRH signal to the pituitary gland?

GnRH's receptor is a G-protein coupled receptor that activates a signaling cascade, ultimately leading to the release of LH and FSH.

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What are FSH and LH, and what do they do?

FSH and LH are hormones produced by the anterior pituitary gland. They control the development and function of the gonads, which produce sex hormones.

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How do FSH and LH act on the gonads?

The effects of FSH and LH on the gonads vary in males and females, but both involve a two-cell system for the production of sex hormones.

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What is the mechanism of androgen action?

Testosterone and dihydrotestosterone (DHT) bind to androgen receptors (AR) in the nucleus, forming a complex that acts as a transcription factor.

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What is the role of the androgen receptor (AR)?

Androgen receptors (AR) are proteins that bind to testosterone and DHT, triggering changes in gene expression. These changes lead to development of male secondary sexual characteristics.

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How does testosterone affect bone?

Testosterone increases bone mineral density (BMD) by stimulating osteoblast activity and reducing osteocyte apoptosis.

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What is the effect of testosterone on the immune system?

Testosterone supports immune function by enhancing T cell maturation and reducing autoantibody proliferation.

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How does testosterone affect the nervous system?

Testosterone reduces inflammation by boosting anti-inflammatory markers (like IL-10) and reducing reactive oxygen species (ROS). This contributes to neuronal health and potentially slows down neurodegenerative processes.

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Hypogonadism

A condition in men characterized by low levels of testosterone.

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Primary Hypogonadism

Hypogonadism where the problem lies within the testicles themselves, leading to reduced testosterone production.

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Secondary Hypogonadism

Hypogonadism where the problem lies in the pituitary gland, which doesn't send the proper signals to the testicles.

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Klinefelter's Syndrome

A genetic condition in men where they have an extra X chromosome, leading to underdeveloped testicles and low testosterone.

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Mumps Orchitis

A viral infection that can inflame the testicles, potentially causing damage and reduced testosterone production.

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Hemochromatosis

A condition where the body accumulates too much iron, leading to testicular dysfunction and low testosterone.

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Kallmann Syndrome

A rare condition where the hypothalamus doesn't develop properly, affecting the release of hormones that control testosterone production.

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Pituitary Disorders

A disorder of the pituitary gland that affects the release of hormones that control testosterone production.

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Gonadotropins

Hormones produced by the pituitary gland that stimulate the production of sex hormones in the gonads.

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Sex steroids

Sex hormones produced by the ovaries in females (estradiol and progesterone) and the testes in males (testosterone). They are responsible for the development and maintenance of sex characteristics.

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Negative feedback loop of sex steroids

A feedback mechanism where high levels of sex steroids inhibit the production of gonadotropins (LH and FSH). It helps regulate the production of sex hormones.

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Luteinizing hormone (LH)

A type of gonadotropin responsible for stimulating the production of testosterone in the testes.

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Follicle-stimulating hormone (FSH)

A type of gonadotropin responsible for stimulating Sertoli cells in the testes, which are essential for sperm development.

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Leydig cells

Cells in the testes that produce testosterone. They are the main target of LH.

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Sertoli cells

Cells in the testes that support the development of sperm cells. They are the main target of FSH.

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Spermatogenesis

The process of sperm production, taking place within the seminiferous tubules in the testes.

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Study Notes

Hypothalamic-Pituitary-Gonadal (HPG) Axis

  • The HPG axis is crucial for regulating reproduction, fertility, and the formation of gonadal sex steroids.
  • Gonadotropin-releasing hormone (GnRH) is secreted from the hypothalamus' arcuate nucleus.
  • The anterior pituitary gland produces luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
  • The gonads (testes and ovaries) produce estrogen and testosterone.

GnRH

  • GnRH is a peptide hormone that activates gonadotrope cells.
  • Its receptor is a G-protein-coupled receptor.
  • GnRH stimulates phospholipase C, leading to calcium and protein kinase C mobilization.
  • This activation of proteins is involved in the synthesis and secretion of LH and FSH.
  • GnRH is degraded by proteolysis within a few minutes.
  • GnRH activity is low during childhood, activated at puberty/adolescence.
  • GnRH is released in a pulsatile fashion.

Pituitary Gonadotropins

  • Anterior pituitary gonadotrope cells respond to GnRH by secreting FSH and LH.
  • FSH and LH affect males and females differently, but both act via a two-cell system.
  • Gonadotropins stimulate sex steroid synthesis and secretion: estradiol and progesterone in females, and testosterone in males.
  • Negative feedback of sex steroids inhibits the hypothalamus and pituitary.

Male Gonadotropin Effects

  • FSH stimulates Sertoli cells, which support spermatogenesis.
  • LH stimulates Leydig cells, which produce testosterone.
  • Testosterone is converted to estradiol by aromatase.
  • 5α-reductase converts testosterone to dihydrotestosterone (DHT).
  • DHT and testosterone act on androgen receptors (ARs).

Testosterone

  • Testosterone plays a role in adult levels during three periods of male life: first trimester of intrauterine life, early neonatal life, and continually after puberty.
  • Adult testosterone levels decline with age.
  • Testosterone is primarily produced by Leydig cells.
  • Testosterone is bound to specific proteins (45% to SHBG, 55% to albumin and corticosteroid-binding globulin).
  • A small fraction of testosterone circulates freely.
  • Once inside a cell, testosterone either binds to the AR or converts to DHT.
  • The androgen-AR complex acts as a transcription factor, binding to hormone response elements in DNA.

Follicle-Stimulating Hormone (FSH)

  • FSH is a pituitary glycoprotein hormone.
  • The primary target of FSH in the testis is the Sertoli cell.
  • FSH indirectly increases the number of Leydig cells, which is key to pubertal development.
  • FSH stimulates Sertoli cells, promoting important genes and protein production for steroid hormone synthesis and action.
  • FSH supports androgen-binding protein (ABP), P-450 aromatase (converting testosterone to estradiol), growth factors, inhibin production.

Androgens

  • Androgens, primarily testosterone and DHT, are responsible for male secondary characteristics.
  • Androgens influence protein synthesis, amino acid utilization and skeletal muscle strength.
  • They affect erythropoietin (EPO) and hematocrit.
  • They affect bone mineral density (BMD), osteoblast growth, and osteocyte apoptosis.
  • They affect T cell maturation and autoantibody production.
  • Testosterone influences lipogenesis and nervous system activity, and plays a role in tumor growth.

Hypogonadism

  • Hypogonadism is a disorder characterized by a defect in testosterone synthesis and can be primary (testis failure) or secondary (pituitary failure).
  • Symptoms include loss of skeletal muscle, reduced bone mineral density, reduced sperm levels, sexual dysfunction, fatigue, depression, and testicular atrophy, among others.
  • Causes include Klinefelter syndrome, undescended testes, mumps orchitis, hemochromatosis, physical injury, and cancer.

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Description

This quiz covers the hypothalamic-pituitary-gonadal (HPG) axis and the role of gonadotropin-releasing hormone (GnRH) in regulating reproduction and fertility. Test your knowledge on how GnRH influences pathways involving luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well as the functions of the gonads in hormone production.

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