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.

Hypothalamus

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.

Illustration of a sagittal section of the brain showing the location of the hypothalamus and the pituitary.
Figure 31.1. The pituitary, shown in green in a mid-sagittal section, lies inferior to the hypothalamus, shown in blue. ‘Hypothalamus and Pituitary’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

View the hypothalamus using the BrainFacts.org 3D Brain

View the pituitary using the BrainFacts.org 3D Brain

Hormone Release

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.

Illustration showing GnRH, LH, and FSH hormone release from the hypothalamus and pituitary. Details in caption and text.
Figure 31.2. In the HPG axis, the hypothalamic parvocellular neurosecretory neurons release gonadotropin-releasing hormone (GnRH) into the hypophyseal portal circulation, causing the hormone-releasing endocrine cells in the anterior pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). ‘LH and FSH Release’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

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.

 

Illustration showing LH and FSH release from the pituitary causing hormone release from the gonads. Details in caption and text.
Figure 31.3. The gonads release either testosterone (testes) or estradiol (ovaries) into the bloodstream in response to release of LH and FSH by the anterior pituitary. ‘HPg Axis’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Hormone Action

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.

Animation 31.1. Steroid hormones can cross the phospholipid bilayer and bind to hormone receptors. The receptors dimerize, move to the nucleus, and interact with DNA, altering transcription of certain genes. ‘Estradiol Action’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Key Takeaways

  • 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

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