Effects of p-nonylphenol, methoxychlor, and endosulfan on vitellogenin induction and expression in sheepshead minnow (Cyprinodon variegatus)

Temporal and dose-response relationships of vitellogenin (VTG) mRNA inducti on and subsequent plasma VTG accumulation were established for sheepshead m innows (Cyprinodon variegatus) treated with p-nonylphenol (an alkylphenol) and the organochlorine pesticides methoxychlor and endosulfan. Thirty-two a dult male fish per treatment were continuously exposed to measured concentr ations of 0.64, 5.4, 11.8, 23.3, and 42.7 mug/L p-nonylphenol; 1.1, 2.5, 5. 6, 12.1, and 18.4 mug/L methoxychlor; and in two separate tests, 15.9, 36.3 , 68.8, 162, 277, 403, 590, and 788 mug/L endosulfan using an intermittent flow-through dosing apparatus. Separate triethylene glycol (50 mul/L) and 1 7 beta -estradiol (65.1 ng/L) treatments served as the negative and positiv e controls, respectively. Four fish were randomly sampled from each test co ncentration on days 2, 5, 13, 21, 35, and 42 of exposure, and levels of hep atic VTG mRNA induction and serum VTG accumulation were determined for each individual. Overall, fish exposed to p-nonylphenol or methoxychlor demonst rated a rapid, dose-dependent synthesis of VTG mRNA up to day 5 of exposure , followed by a relatively constant dose-dependent expression through day 4 2. Both chemicals showed a dose-dependent increase in plasma VTG over the e ntire time course of exposure, with significantly elevated VTG levels by th e fifth day of exposure to p-nonylphenol at concentrations of 5.4 mug/L or greater and to methoxychlor at concentrations of 2.5 mug/L or greater. Expo sure to 0.64 mug/L p-nonylphenol resulted in highly variable plasma VTG lev els of less than 6 mg/ml. Exposures with endosulfan failed to induce measur able levels of either hepatic VTG mRNA or serum VTG at the chemical concent rations tested. Our results demonstrate that the sheepshead minnow bioassay is a suitable estuarine/marine teleost model for in vivo screening of pote ntially estrogenic substances.