Melengestrol acetate dysregulates HPP, HPT, and HPI axes, affecting the metamorphosis of <i>Xenopus (Silurana) tropicalis</i>

Abstract Tadpole development involves several morphological, biochemical, and behavioural transformations regulated by endocrine networks, primarily the hypothalamic-pituitary-thyroid (HPT) and hypothalamic-pituitary-interrenal (HPI) axes. Exposure to melengestrol acetate (MGA), a progesterone-like compound, disrupts these processes, resulting in asynchronous metamorphosis characterized by narrower heads, skin abnormalities, and absence of forelimb emergence (FLE). To examine endocrinal alterations associated with these effects, Xenopus (Silurana) tropicalis tadpoles were exposed to 1.7 μg/L MGA, and combinations of MGA with 24.9 μg/L metyrapone (MTP, a 21-hydroxylase inhibitor that inhibits endogenous corticosterone synthesis) or 43 μg/L mifepristone (RU486, a dual anti-progestogen and anti-glucocorticoid). MGA and MGA+MTP treatments induced asynchronous metamorphosis and blocked FLE, while MGA+RU486 attenuated MGA-associated phenotypic effects. Histological analysis revealed that FLE obstruction in MGA and MGA+MTP groups was associated with changes in the epithelium layer surrounding the forelimb. Gene expression analysis showed upregulated prl and downregulated crh, dio2, dio3, and trβ in MGA and MGA+RU486 groups. Circulating corticosterone was significantly reduced by MGA exposure and partially modulated in presence of RU486 co-treatment, without fully restoring control-like endocrine profiles. Together, these findings reveal that MGA disrupts tadpole metamorphosis through coordinated alterations across multiple endocrine axes, including the HPT, HPI, and hypothalamic-pituitary-prolactin (HPP) axes, highlighting the integrative nature of endocrine regulation during amphibian development.