Transient effects of methyltestosterone injection on different reproductive parameters of the hermaphrodite fish Kryptolebias marmoratus

To elucidate the action mechanism of environmental androgenic chemicals on fish reproductive activity by transient stimulation in heavily polluted areas, individuals of the hermaphrodite fish Kryptolebias marmoratus were injected once with six concentrations of methyltestosterone (MT) (0.1, 1, 5, 10, 50, and 100 mu g/g BW) intraperitoneally. The fish were sampled at intervals of 7, 15, and 30 days after a single injection. At 7 days after injection, mature oocytes were not observed in the MT-exposed groups except for the group exposed to 0.1 mu g MT, while testicular development was not remarkably different between any of the groups. Also, at 7 days after injection, hepatic estrogen receptor alpha (ER alpha) and vitellogenin (VTG) mRNA abundance decreased significantly in the MT-exposed groups despite no significant difference in plasma 17 beta-estradiol (E2) levels between any of the groups. This significant difference in VTG mRNA between the control and the MT-exposed groups persisted until 30 days after injection, although ER alpha mRNA abundance was not statistically different between any groups at 30 days after injection. Our results clearly show that a single injection of MT inhibits ovarian development rather than testicular development in the hermaphroditic gonad of K. marmoratus. Furthermore, our results demonstrate that a single injection of MT interfered with hepatic VTG mRNA synthesis mediated by the suppression of hepatic ER alpha mRNA transcription.