Hormonal Regulation of Male Reproduction I Testosterone I Reproductive Physiology I The slides will discuss about the following: 1. Hypothalamic-Pituitary-testicular axis 2. Why pulsatile secretion of GnRH? 3. Functions of testosterone 4. Mechanism of action of testosterone 5. Metabolism of testosterone Also watch the YouTube video of same on HM Learning YouTube channel

2. Hypothalamic- Pituitary-Testicular axis- Negative
feedback loop

3. Why Pulsatile secretion of GnRH?
• Continuous secretion of GnRH
1. Downregulation of GnRHR on gonadotrophs
2. Receptor internalization and degradation in lysosomes
• Receptor replenishment- Return of the GnRH receptor to the cell
membrane
• Some GnRH receptors are shuttled back to the cell surface, and de
novo receptor synthesis continues from GnRH receptor gene
expression

4. Testosterone
• Male sex hormone which is produced by Leydig cells.
• Steroid hormone, precursor is cholesterol

5. Synthesis of testosterone
SCC

6. Functions of testosterone
• Negative feedback inhibition of hypothalamic-pituitary-testicular axis at
both hypothalamic (release of GnRH) and pituitary (release of LH)
level.
• Appearance of secondary sex characteristics.
• Anabolic effects
• Meiosis during spermatocytogenesis
• Maintenance of BTB
• Sertoli-spermatid adhesion- release of immature spermatids

7. Secondary sex characteristics

8. Mechanism of action of testosterone
1. Free form of testosterone diffuses across the membrane and bind to
its high affinity androgen receptor (AR) or get converted to
dihydrotestosterone and then bind to AR.
2. Homodimerization of AR
3. This AR-androgen complex will act as a transcription factor and then
bind to its hormone response element on DNA located in the
promoter regions of the target genes
4. Promotion of transcription of proteins exerting function of
testosterone (growth and development).

9. Metabolism of testosterone
• Occurs mainly in liver and prostate.
• About 2% testosterone is secreted daily in urine without any
metabolism.
• The remaining balance of testosterone and other androgens is
converted in the liver to 17-ketosteroids and in the prostate to DHT.
These metabolites of testosterone are excreted in urine and feces as
water soluble conjugates of sulfuric acid or glucuronic acid.

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