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?

It is broken down by proteolysis Signup and view all the answers

What is the function of gonadotropins in males and females?

They act via a two-cell system. Signup and view all the answers

What hormone primarily regulates testosterone secretion from the testes?

Luteinizing hormone (LH) Signup and view all the answers

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

Early neonatal life Signup and view all the answers

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

Somatic changes in males Signup and view all the answers

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

45% Signup and view all the answers

What happens to circulating testosterone levels after middle age?

They decrease gradually Signup and view all the answers

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

It acts as a transcription factor Signup and view all the answers

Which effect does testosterone NOT have on the skeletal system?

Decreased osteoblast growth Signup and view all the answers

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

They promote remyelination of neurons Signup and view all the answers

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

Dimerization and exposure of zinc finger domains Signup and view all the answers

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

They promote maturation of T cells Signup and view all the answers

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

Increased energy levels Signup and view all the answers

What is Klinefelter's Syndrome primarily associated with?

Presence of extra X chromosomes Signup and view all the answers

Which condition results from iron overload affecting testosterone production?

Hemochromatosis Signup and view all the answers

Which option is a DIRECT cause of primary hypogonadism?

Testicular injury Signup and view all the answers

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

Gradual decline in serum testosterone levels Signup and view all the answers

Which of the following hormones are affected by Kallmann Syndrome?

Luteinizing hormone and follicle-stimulating hormone Signup and view all the answers

Which of the following symptoms is associated with testosterone deficiency?

Hot flashes Signup and view all the answers

What can result from the treatment of cancer with chemotherapy?

Reduced testosterone and sperm production Signup and view all the answers

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

Estradiol and progesterone in females, testosterone in males Signup and view all the answers

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

Follicle-stimulating hormone (FSH) Signup and view all the answers

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

Stimulates testosterone production in Leydig cells Signup and view all the answers

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

FSH binds to receptors on Sertoli cells, increasing Leydig cell numbers Signup and view all the answers

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

Inhibits hypothalamic and pituitary activity Signup and view all the answers

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

Androgen-binding protein (ABP) Signup and view all the answers

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

By proliferating and increasing testosterone levels Signup and view all the answers

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

Converts testosterone into estradiol Signup and view all the answers

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.