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Fig. 9 Summary of key findings and working model for the role of SNa1?34 in female zebrafish reproduction. Prohormone convertases selectively process the Scg2a precursor protein to the bioactive peptide SNa1?34. Immunocytochemistry revealed SNa and OXT are colocalized in preoptic neurons. We found that SNa + OXT neurons project to the neurointermediate lobe in the posterior pituitary. While direct evidence is still scant, localization of SNa and OXT in nerve terminals implies release into the circulation, where the peptides may influence ovulation and spawning. Some neuronal fibers immunoreactive for SNa but not OXT entered the proximal pars distalis (anterior pituitary). Regression analysis revealed that SNa1?34 (SNa on the figure) and Gnrh3 peptide levels in the brain increase concomitantly at the time of the Lh surge. In the pituitary, SNa1?34 peptide levels are highest prior to ovulation. Experimental data demonstrated that 3?h following i.p. injection of synthetic SNa1?34 increases mRNA levels for gnrh3 in the hypothalamus, lhb and cga in the pituitary, and lhcgr and npr in the ovary. These are key indicators of activation of genes at each level of the HPG axis that led to ovulation 6?h after a single SNa1?34 injection. SNa1?34 can enhance Lh cell activity through a neuroendocrine pathway (i) and by stimulating Gnrh3 neurons (ii). This model is supported by previous observations in mutant lines (27), where it was demonstrated that hypothalamic gnrh3, and pituitary lhb and cga are lower in scg2a-/- females compared with wild type. In the same mutant line, SNa1?34 injection in the subfertile scg2a-/-; scg2b-/- double mutant zebrafish enhanced spawning by ?3-fold, like the effects of the Lh analog hCG in the same line (27).