Changes in mRNA levels of ovarian steroidogenic enzymes during artificial maturation of Japanese eel Anguilla japonica

Changes in the levels of serum steroids and ovarian steroidogenic enzyme mRNA were analyzed in Japanese eel Anguilla japonica induced to undergo oogenesis by chum salmon pituitary homogenate (SPH) treatment. Serum 17alpha-hydroxyprogesterone, 17alpha,20beta-dihydroxy-4-pregnen-3-one, androstenedione and estrone levels were barely or not detectable during artificial maturation. In contrast, serum estradiol-17beta (E-2) and testosterone (T) levels increased after SPH injections and peaked at the migratory nucleus (MN) stage. 3beta-Hydroxysteroid dehydrogenase mRNA levels did not change between eels in different stages of oogenesis. mRNA levels of P450 cholesterol side-chain cleavage (P450sec), P450 17alpha-hydroxylase/C17-20 lyase (P450c17), 17beta-hydroxysteroid dehydrogenase type I (17beta-HSD-I) and P450 aromatase (P450arom) were low before SPH injections. P450scc, P450c17 and 17beta-HSD-I transcript abundance increased after SPH treatment and peaked at the MN stage. P450arom mRNA levels, however, peaked at the mid-vitellogenic stage and slightly decreased towards the MN stage. We suggest that these increases in P450scc and P450c17 mRNA levels may account for high T levels at the MN stage. In turn, the high T levels may permit the production of E-2 in spite of low P450arom mRNA levels towards the end of oogenesis. Steroidogenesis in artificially maturing eels appears to proceed unlike that in other teleosts, but whether or not this is an artifact remains as yet unknown.