Comparative biological effects and potency of 17α- and 17β-estradiol in fathead minnows

17β-Estradiol is the most potent natural estrogen commonly found in anthropogenically altered environments and has been the focus of many toxicological laboratory studies. However, fewer aquatic toxicological data on the effects of 17α-estradiol, a diastereoisomer of 17β-estradiol, exists in the literature even though it has been found in the aquatic environment, sometimes at higher concentrations than 17β-estradiol. The central objective of this study was to determine how the anatomical, physiological, and behavioral effects of exposure to 17α-estradiol compare to the well-documented effects of 17β-estradiol exposures in aquatic vertebrates. A 21-day flow-through exposure of mature male and female fathead minnows to three concentrations each of 17α- and 17β-estradiol (averaged measured concentrations 27, 72, and 150 ng/L for 17α-estradiol, and 9, 20, and 44 ng/L for β-estradiol, respectively) yielded significant, concentration-dependent differences in plasma vitellogenin concentrations among estradiol-exposed males when compared to fish from an ethanol carrier control. Interstitial cell prominence in the testis of fish was elevated in all estradiol treatments. Aggressiveness of male fish to defend nest sites appeared depressed in many of the higher concentration estradiol treatments (albeit not significantly). No clear effects were observed in female fish. Based on plasma vitellogenin data, it appears that 17β-estradiol is 8–9 times more potent than 17α-estradiol and that the lowest observable effect concentration (LOEC) for 17α-estradiol in fathead minnows is greater than 25 ng/L and may be less than 75 ng/L.