HPG Axis Flashcards
Where is the hypothalamus located and how is it connected to the pituitary gland?
- Located in the forebrain (Diencephalon).
- Connected to the pituitary gland via the infundibulum.
What does the hypothalamus produce relating to reproductive hormones and where does it act?
- Produces Gonadotropin-releasing hormone (GnRH), a peptide hormone
- Travels to the anterior pituitary to regulate hormone secretion.
What different types of hormone producing cells does the anterior pituitary contain?
- Corticotropes → Regulate adrenal cortex function.
- Somatotropes → Produce growth hormone.
- Gonadotrophs → Stimulated by GnRH to release FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone).
What is the signalling connection with gonadotroph cells and GnRH signalling and what plays a regulatory role?
- GnRH receptors (GnRHR) on gonadotroph cells bind GnRH, triggering a signaling cascade.
- This leads to the production and release of FSH and LH into the bloodstream.
- Calcium (Ca²⁺) plays a regulatory role—increased Ca²⁺ enhances GnRH stimulation.
What are the effects of FSH and LH on the ovaries and testes?
FSH and LH travel to the gonads (ovaries and testes) and stimulate:
- Sex steroid production, including:
Androgens (e.g., testosterone, dihydrotestosterone, androstenedione).
- Progestogens (e.g., progesterone).
- Oestrogens (e.g., oestradiol, oestrone).
- Inhibins and Activins, which regulate FSH production.
What is the difference between sex steroid in males and females?
Both males and females produce the same sex steroids, but in different amounts.
Where do sex steroids originate from?
All sex steroids originate from cholesterol and undergo interconversion via a biosynthetic network.
Apart from hormone levels, what else impacts hormone effects?
Cell receptor expression and density, which vary throughout the menstrual cycle.
How do produced sex steroid regulate the GnRH- Gonadotroph interaction?
- Progesterone (P), Testosterone (T), and Oestrogen (E) regulate the GnRH-Gonadotroph interaction via negative feedback.
- These hormones are internalized into gonadotroph cells and suppress further hormone production, ensuring hormonal balance.
What family do Inhibin and Activin belong to and what roles do they have outside of reproductive processes?
- Both belong to the TGF-β family
- Play roles in tumor suppression, germ cell development, and red blood cell production.
What are the general roles of Inhibin and Activin?
- Inhibin: Inhibits FSH production and dampens FSH receptor response.
- Activin: Stimulates FSH production.
** Balance between Inhibin and Activin controls FSH levels.
What happens when GnRH neurons are destroyed in mice?
Destruction of GnRH neurons (e.g., through genetic mutations or viral infections) prevents gonad development.
What is the GnRH secretion pattern timeline?
- Minimal in infancy.
- Becomes pulsatile at puberty (hourly bursts from the hypothalamus).
- Pulsatile release of GnRH leads to pulsatile release of FSH and LH.
What happens when you modulate the pulse amplitude/frequency of GnRH neurons?
- Modulating pulse amplitude or frequency affects FSH/LH secretion.
- Drug treatments can alter hormonal pulses, affecting ovarian function.
What is the role of kisspeptin in GnRH secretion?
- Kisspeptin is a key regulator of reproduction, encoded by the Kiss1 gene.
- Produced in the hypothalamus, it binds to Kiss1R receptors on GnRH neurons, stimulating GnRH release.
- Steroid hormones regulate kisspeptin via feedback control, influencing GnRH output.
*Kisspeptin receptors are also found in other body tissues.
What is the role of RFRP-3 in GnRH secretion?
- RFRP-3 inhibits GnRH secretion, acting directly on gonadotrophs and gonadal tissues.
- Leads to reduced GnRH output, ultimately lowering FSH and LH levels.
How is research exploring how Kisspeptin is related to energy balance?
- Linked to energy balance and fertility.
- Puberty is metabolically gated—sufficient energy reserves are needed for reproductive function.
- Leptin levels (reflecting body fat stores) influence puberty onset:
Low body fat → delayed puberty.
Very low or very high body weight → reduced fertility.
**Research explores how energy status, neurodevelopment, and reproduction are interconnected.
