Cell-cell communication in the ovarian tissue and its regulation by pituitary prolactin : in case of GnRH and annexin 5

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In the ovarian tissue, the expression of annexin 5, a calcium-phospholipid binding protein and a novel target of GnRH, in the corpus luteum is suppressed by prolactin. During pseudopregnancy of rats in which two daily prolactin surges occur, inhibition of prolactin secretion by dopamine agonist, 2α-bromoergocriptine mesylate (CB-154), resulted in the augmentation of annexin 5 expression and the occurrence of TUNEL positive reaction in the same luteal cells. Local administration of GnRH antagonist, Cetrorelix, to unilateral ovarian bursa of a CB-154 administered pseudopregnant rat with an osmotic pump completely suppressed both apoptosis and annexin 5 expression, while they were observed in the luteal cells of the contralateral non-treated ovary. The direct effect of GnRH on luteal cells was examined by administering GnRH agonist (GnRHa) with an osmotic pump to hemilateral ovarian bursa of a pseudopregnant rat. GnRHa induced apoptosis and annexin 5 expression in the same luteal cells. Since the administration of progesterone did not suppressed apoptosis and annexin 5 expression in the CB-154 treated pseudopregnant rats, prolactin is thought to inhibit GnRH action directly in the ovary. Getting together, it is suggested that prolactin maintains the functional luteal phase at least partly by suppressing local GnRH action. (this work was published in Endocrinology 2003 144:3625-3631).