What is the difference between an agonist and an antagonist?
Agonist and antagonist are polypeptide hormones, which selectively act at the level of the hypothalamus and pituitary to affect the secretion of gonadotropins from specific cells in the pituitary. Agonists have an initial effect in stimulating gonadotropin secretion, and increasing the levels of LH and FSH. This is short lived, however. Through a process referred to as “down regulation”, the continued administration of an agonist results in suppression of gonadotropin secretion, decreases in the levels of LH and FSH, and a drop in circulating estradiol to the menopausal level. Several agonist preparations are available, which may be administered intramuscularly, subcutaneously, or via the intra-nasal route, respectively. Agonists available in the U.S. include Lupron, Synarel, and Zolodex. Antagonists have direct blocking effects on the receptor for the hormone GnRH. As a result, it causes an immediate and profound suppression of LH and FSH secretions. Unlike agonist, there is no initia
