Resumption and progression of meiosis and circulating levels of steroids and prostaglandin F₂ of Piaractus mesopotamicus induced by hypophysation with prostaglandin F₂

We have previously shown an increase on the ovulation rate of pacu, Piaractus mesopotamicus, using hypophysation with prostaglandin F-2 alpha (PGF(2 alpha)) (HPG protocol) rather than hypophysation (H); only, however, a persistent lack of ovulation in some females could not be further explained by PGF(2 alpha) and steroid plasma levels at time of ovulation. Considering this, here we compared the meiotic process and steroids and PGF(2 alpha) levels during spawning induction using both protocols. To that, H and HPG were treated with conventional hypophysation (0.6 mg/kg priming and 5.4 mg/kg resolving dose), and the latter received synthetic PGF(2 alpha) (2 mL/fish) at resolving dose. A negative control (saline solution), a SPG group (receiving only PGF(2 alpha)) and a PPG group (receiving priming and PGF(2 alpha)) were included. An initial PGF(2 alpha) peak between one to four hours post-resolving dose (1-4 hr) occurred in groups receiving PGF(2 alpha) (HPG, SPG and PPG). Only H and HPG showed a 17 alpha-20 beta-dihydroxy-4-pregnen-3-one (DHP) peak 4 hr, concomitant with meiosis resumption, and a final PGF(2 alpha) peak 12 hours. The HPG protocol resulted in higher PGF(2 alpha) plasma levels for longer periods (1-4 and 12 hr) than H (single peak 12 hr), occasioning in the former higher and concomitant levels of both PGF(2 alpha) and DHP at 4 h. Considering that DHP is known to induce expression of prostaglandin receptors in fish, the concomitant plasma peaks of DHP and PGF(2 alpha) may also explain the ovulation rate gain previously published, using HPG protocol, which can be further improved adjusting the priming dose and defining the exact interval of PGF(2 alpha) ovulatory action in this species.