Effects of cadmium on the reproductive axis of Japanese medaka (Oryzias latipes)

Cadmium (Cd) is a ubquitous element and a significant inorganic pollutant that has previously been found to bioaccumulate in reproductive organs of fish and disrupt important endocrine processes, especially those involved in synthesis, release and metabolism of hormones. Clearly, there is potential for reproductive effects in fish populations exposed to Cd, however, few studies have investigated the non-lethal consequences of Cd in fish. To this extent, adult male and female Japanese medaka were exposed to 0-10 ppb Cd for 7 weeks. Reproductive endpoints were monitored during weeks 6 and 7 of exposure and compared to physiological responses along the hypothalamus-pituitary-gonadal (HPG) axis, including plasma vitellogenin (VTG), hepatic estrogen receptor (ER), plasma steroids, gonadal-somatic indices (GSI), and gonadal steroid release. There were no observed effects on VTG and ER by long-term Cd exposure. However, gonadal steroid release was significantly decreased in males and females at all exposure concentrations and female plasma estradiol levels were significantly altered at concentrations higher than 5 ppb Cd. Overall, responses along the HPG axis were more sensitive to Cd exposure than the reproductive and developmental endpoints, which were not affected in this study, indicating that higher level impairment in fish might be relatively protected.