31 HPG Axis
Control of gonadal hormone release relies on activation of the hypothalamic-pituitary-gonadal (HPG) axis. Gonadal hormones are important for development of the body and brain, changes during puberty, and the activation of some behavior in adulthood like reproductive behavior and aggression.
As a refresher, the hypothalamus, which is located inferior to the thalamus, integrates information from many regions of the central nervous system and maintains homeostasis in the body. They hypothalamic regulation of gonadal hormones and sex behavior is managed via hormone release by the pituitary gland.
Gonadal hormone release relies on anterior pituitary function. In the hypothalamus, the parvocellular neurosecretory cells release a hormone called gonadotropin-releasing hormone (GnRH) into the hypophyseal portal circulation. When GnRH reaches the anterior pituitary, it causes the endocrine cells of the pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH) into the general circulation.
The LH and FSH travel through the circulatory system and can act on the gonads, either the testes in males or ovaries in females. In response to the pituitary hormones, the testes release testosterone, an androgen, and the ovaries release estradiol, an estrogen, into the blood stream. After puberty, the LH and FSH are also critical for the maturation of sperm and egg cells.
Once the gonadal hormones enter the circulation, they are able to act on cells that express either androgen receptors or estrogen receptors. Like cortisol, testosterone and estradiol are steroid hormones and can cross the phospholipid bilayer. Inside the cell, the hormones bind to receptors which then dimerize and move to the nucleus The receptors can bind to DNA at special promotor regions and act as transcription factors, turning on specific genes.
- The hypothalamus directly controls release of gonadal hormones by controlling hormone release from the anterior pituitary
- The hypothalamus releases gonadotropin-releasing hormone (GnRH)
- The anterior pituitary releases luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
- The ovaries release estradiol
- The testes release testosterone
- Gonadal hormones bind to receptors and alter DNA transcription