Biphasic effects of 17β-estradiol and xenoestrogens on dopaminergic neurons in developing zebrafish

Due to the increasing pollution of aquatic environments by estrogen-like chemicals (xenoestrogens (XEs)), it is crucial to investigate their bioaccumulation, ecological impact, and potential endocrine-disrupting effects on aquatic organisms. Here, we investigated the effects of 17β-estradiol (E 2 ) and the XEs bisphenol A (BPA) and diethylstilbestrol (DES) on the nervous system, with a particular focus on dopaminergic (DAergic) neurons during early development of zebrafish. Our results revealed that a low dose of E 2 (10 −4 μM) significantly increased tyrosine hydroxylase (TH), the rate-limiting enzyme for dopamine (DA) synthesis, at 48 hpf, whereas a high dose (1 μM) significantly reduced TH expression. A similar pattern was observed for both BPA and DES, with DES exhibiting a more potent effect compared to BPA and E 2 . Specifically, DES increased TH expression at 10 −4 μM and reduced TH expression at concentrations starting from 0.1 μM, whereas BPA increased TH expression at 10 −3 μM and reduced TH expression at 1 μM. These effects were further validated through qRT-PCR analysis. The changes in TH expression correlated with alterations in motor activity, including the response to tactile stimulation at 72 hpf and swimming distance at 6 dpf, except that low doses had no effect on swimming distance. Notably, all effects caused by E 2 , BPA, or DES, at both low and high doses, were mediated through estrogen receptors (ER). While replacing E 2 with embryonic medium (EM) did not rescue the effect on locomotor activity, replacing or adding L-dopa (a DA precursor) completely rescued the effect. Moreover, co-incubation with buspirone (a partial agonist for serotonin (5-HT) 1A receptors) resulted in partial rescue, indicating that both DA and 5-HT signaling are involved in modulating locomotor activity, with DAergic neurons playing a central role. In conclusion, we demonstrated that E 2 and XEs regulate DA neurons through ER in a biphasic manner, modulating locomotor activity during early zebrafish development. Our findings thus highlight the potential toxicological impact of XEs, as their disruption on DAergic neurons and estrogen signaling pathways can lead to altered motor behavior, developmental abnormalities, and long-term neurological effects.