GnRH Agonists And Antagonists

Biology 101

In order to understand, at least a little, what happens when fertility drugs are used, we must get a picture of what happens when the fertilization process takes place within a woman's body. In very simple terms, we know that an egg is released from an ovary and is fertilized by the sperm in the fallopian tube. After that point, in a normal pregnancy, the egg travels back down into the uterus where it adheres to the wall of the womb and begins to grow. About 40 weeks later a baby is born. Simple.

If Things Don't Work Properly, Help Is Needed

The reason fertility drugs are needed is because there is something malfunctioning in the process of ovulation (the release of the egg from the ovary) in the woman, or there is a problem with the sperm in the man. We know that the entire process of ovulation takes place as the result of hormones triggering actions and in this article we will talk about certain hormones and their function, as well as the drugs developed to address the malfunction of some of these hormones.

The Role Of The Hypothalamus And Pituitary Gland

In a woman, ovulation may not occur because the hypothalamus, located in the brain directly above the pituitary gland, does not secrete gonadotropin-releasing hormone (GnRH). GnRH is responsible for stimulating the pituitary gland to produce the hormones that trigger ovulation. Those hormones are luteinizing hormone (LH) and follicle stimulating hormone (FSH). At the pituitary gland, GnRH, through frequency pulses (secreting in small amounts every hour or so), stimulates the secretion of the gonadotropins, FSH and LH. Low frequency GnRH pulses lead to FSH release and high frequency GnRH pulses encourage LH release. In women, the frequency of the pulses varies during the menstrual cycle and there is a large surge of GnRH just before ovulation. This surge is vital for proper reproductive function and controls the complex process of follicle growth and ovulation in a woman.

The Role Of GnRH And The Use Of GnRH Agonists And Antagonists

The stimulation of GnRH to release FSH and LH induces follicular development. If there is a problem with ovulation and a woman is seeking fertility treatment, then often a GnRH analogue is given with other drugs such as FSH to enhance ovulation stimulation for IVF or IUI. There are two types of analogues, GnRH-agonists and GnRH-antagonists. The GnRH-agonists cause an initial surge in both FSH and LH production but then cause the body to cease production of these hormones, thus preventing ovulation and limiting estrogen. Used during IVF treatments, they give the doctor control over ovulation in order to facilitate conception.

GnRH-antagonists work against the hormones FSH and LH, suppressing ovulation. Used also during IVF treatments, they prevent the eggs being ovulated and "lost" or absorbed back into the body before they can be harvested. This type of drug can also be used during IUI treatment and, if estrogen levels are too high, the drug is effective in reducing the risk of ovarian hyperstimulation syndrome.

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