Expression of cyclooxygenase-2 and prostaglandin receptor EP4b mRNA in the ovary of the medaka fish, Oryzias latipes: Possible involvement in ovulation

In vitro ovulation of mature medaka ovarian follicles was inhibited by inhibitors of cyclooxygenase (COX) or by an antagonist of the prostaglandin E2 receptor (EP). Of the three medaka COX genes, ptgs2 was most dominantly expressed in the fish ovary. The ptgs2 transcript was detected in all sizes of growing follicles. In a 24-h spawning cycle, large-sized follicles contained ptgs2 mRNA at a fairly constant level. The levels of COX enzyme activity and prostaglandin E2 were also constant in the large-sized follicles during the spawning cycle. The expression of prostaglandin E2 receptor EP4b (ptger4b) mRNA was drastically upregulated in the large-sized follicles as the ovulation time approached. The current results indicate that prostaglandin E2, which might be produced by COX-2, is involved in the ovulation of medaka, and that EP4b is likely the receptor responsible for exerting the action of prostaglandin E2 in the process.