In vivo evidence for Urotensin II as a regulator of reproduction in GIFT tilapia ( Oreochromis niloticus )

Urotensin II (UII) is a conserved vasoactive neuropeptide secreted by the caudal neurosecretory system (CNSS) of bony fish, yet its reproductive functions in teleost remain poorly defined. Here, we investigated the regulatory role of UII in the brain-pituitary-gonadal (BPG) axis of genetically improved farmed tilapia (GIFT, Oreochromis niloticus) using acute intraperitoneal injection, receptor antagonism with urantide, and sustained release via PLGA microspheres. Acute tUII administration significantly up-regulated mRNA expressions of gnrh1/2/3 in the brain, fshβ, lhβ, and gthα in the pituitary, and gonadal receptors (fshr, lhr), with dose-dependent biphasic responses. These stimulatory effects were abolished by urantide, confirming UT receptor-mediated signaling. Long-term delivery of tUII resulted in persistent activation of hypothalamic and pituitary genes, induction of steroidogenic and receptor genes in gonads, and significant elevation of serum estradiol in females and testosterone in males. Histological analyses further revealed accelerated spermatogenesis and oocyte maturation in tUII-treated groups. Moreover, UII up-regulated sex-differentiation genes, including dmrt1 in testis and foxl2 in ovaries, indicating its dual involvement in gonadal development and sexual differentiation. Together, these findings identify UII as a novel upstream regulator of tilapia reproductive axis and a potential target for manipulating puberty, gonadal maturation, and sex differentiation in aquaculture.