Hormonal dynamics reveal a stimulatory role for secretoneurin in zebrafish ovulation

Surge release of luteinizing hormone (Lh) from the pituitary is essential for fertility as it triggers ovulation. While secretoneurin (SN) is a phylogenetically conserved secretogranin-2-derived peptide that stimulates Lh, its role in ovulation has not been established. To directly compare periovulatory changes in the classical hormones to the emerging reproductive neuropeptides SNa and SNb, simultaneous mass spectrometry measurement of 9 peptides and 5 steroids was conducted in female zebrafish. Regression analysis indicated that levels of SNa1–34 in the brain peaked when type 3 gonadotropin-releasing hormone (Gnrh3) increased (R² = 0.71) at the time of the Lh surge, 3.5 h before ovulation. Levels of the naturally occurring derivative SNa1–14 were highest at ovulation, while SNb1–31 was invariable. The bioactivities of SNa1–34 and SNa1–14 were investigated. After injection of SNa1–34 in females that had been isolated from males, 61% (11/18) ovulated within 6 h, which was like the effects of the Lh analog human chorionic gonadotropin (72%; 13/18 females). SNa1–34 injection induces ovulation by increasing time-dependent expression of gnrh3 in the brain, a likely direct stimulation of chorionic gonadotropin alpha (cga) and luteinizing hormone b (lhb) subunit in pituitary, and via the subsequent time-dependent increase in nuclear progesterone receptor (npr) in ovaries. In contrast, SNa1–14 exhibited far fewer effects on gene expression and did not induce ovulation. Our results support the proposal that SN is a reproductive hormone.