Effects of progestogen neurosteroids on locomotor activity in zebrafish embryos and larvae

The steroid hormone progesterone (P4) and related compounds called progestogens are well known for their effects on the reproductive system. However, their physiological roles outside reproduction are less understood. Additionally, there is limited information on the toxicological repercussions of environmental exposure to exogenous progestogens and how such exposure might impact the development and survival of animals living in contaminated habitats. Two progesterone-based neurosteroids, allopregnanolone and tetrahydrodeoxycorticosterone (THDOC), are known to enhance γ-aminobutyric acid type A (GABA_A) receptor activity, inducing neuronal hyperpolarization. In this study, locomotor parameters in zebrafish embryos and larvae were used as endpoints to assess the inhibitory effects of pregnane neurosteroids. Specifically, spontaneous tail coiling in embryos at 24 h post-fertilization (hpf) and swimming activity in larvae aged 120-168 hpf were evaluated. Motility assays in embryos showed that P4 increased spontaneous tail coiling, whereas larvae exhibited an anesthetic-like loss of motility. This effect was both age- and dose-dependent for P4, deoxycorticosterone (DOC), 5α-dihydroprogesterone, and the membrane progesterone receptor agonist, ORG-OD-02-0, but not for other steroids tested. Removal of the steroids after the onset of anesthesia led to recovery of motility in larvae, suggesting that the observed effects are not due to a non-specific toxicity. Our results suggest that P4 targets the mPR, which acts in association with the GABA_A receptor to mediate the loss of locomotor behavior. This study provides further insight into how neuroactive compounds can affect locomotor behaviors during early developmental stages in nonmammalian species.