PROSTAGLANDINS IN HYPOTHALAMO-PITUITARY AND OVARIAN-FUNCTION

Some aspects of prostaglandin (PG) functions are reviewed including: 1) the role of PGs in the hypothalamic and pituitary control of gonadotropin secretions; and 2) their roles in ovulation, 3) in luteinization, and 4) in corpus luteum regression. PGE1 is known for its role in stimulation of increased cyclic adenosine 3',5' monophosphate (cAMP) and hormone secretion in the anterior pituitary. Direct effects of PGs on the secretion of luteinizing hormone, follicle stimulating hormone, and adrenal cortex hormones are not clearly known, but surmised. Such actions may not be the direct effects of PGs on pituitary action. Instead, more studies on receptor functions for PGs in pituitary cells are needed. Systemic administration of PGs has been shown to increase circulating levels of gonadotropins, adrenal cortex hormones, prolactin, follicle stimulating hormone, and luteinizing hormone; and in general, PGs of the E series are more potent than those of the F series. This response to systemic administration seems to be caused by an hypothalamic site of action, a conclusion based on several observations, including the observation that direct application of PGs to brain tissue causes a mimicking of endogenous PG effects of gonadotropin secretion. PGs also play a role in ovulation. Elevated PG levels in follicular tissues are induced by gonadotropins; cyclic nucleotides may be involved in mediating the action of gonadotropins on follicular PG production; a recognized time lag after exposure of the follicle to gonadotropin or cyclic nucleotides indicates that macromolecular synthesis may be involved in follicular PG production; and plasminogen activator may play a role in the process of follicular rupture that leads ot ovulation. The role of PGs in luteinization has been suggested by experiments which showed that granulosa cells cultured with PGE1 and PGE2 luteinized. PGs, particularly PGF2 alpha, cause luteal regression in many species, except perhaps in humans. And PGF2 alpha may be an antagonist of gonadotropin action in the corpus luteum. A proposed mechanism of PGF2 alpha-induced luteolysis in rats is also presented.