Endocrine disruptors affect larval zebrafish behavior: Testing potential mechanisms and comparisons of behavioral sensitivity to alternative biomarkers

Larval zebrafish (Danio rerio) are a tool for assessing endocrine disruption during early development. Here, we investigated the extent to which a simple light/dark behavioral test at five days post fertilization could compliment current methods within the field. We exposed fertilized embryos to hormones (17β-estradiol, testosterone, dihydrotestosterone, 11-ketotestosterone, thyroxine, triiodothyronine, progesterone, and hydrocortisone) and other relevant compounds (17α ethinylestradiol, bisphenol A, bisphenol S, nonylphenol, flutamide, nilutamide, linuron, drospirenone, potassium perchlorate, mifepristone, and fadrozole) to screen for behavioral effects between 96 and 118 h post fertilization (hpf). With the exception of progesterone, all the hormones tested resulted in altered behaviors. However, some inconsistencies were observed regarding the age of the larvae at testing. For example, the xenoestrogens 17α- ethinylestradiol and nonylphenol had behavioral effects at 96 hpf, but not at 118 hpf. Furthermore, although thyroxine exposure had pronounced effects on behavior, the thyroid disruptor potassium perchlorate did not. Finally, we were unable to demonstrate a role of nuclear receptors following testosterone and 17α- ethinylestradiol exposure, as neither the androgen receptor antagonist flutamide nor the general estrogen receptor inhibitor fulvestrant (ICI) could rescue the observed behavioral effects, respectively. Similarly, molecular markers for androgen and estrogen disruption were upregulated at concentrations below which behavioral effects were observed. These results demonstrate hormones and endocrine disruptors can alter the behavior of larval zebrafish, but the mechanistic pathways remain unclear.