Effects of oxytocin on semen release response in African catfish (Clarias gariepinus)

In silurid fishes, semen collection is practically impossible, even after hormonal stimulation. Instead, males are killed and testes macerated to obtain sperm. To understand the endocrine control of semen release in catfishes, we investigated the role of smooth muscle contractors in semen release and semen quality of African catfish (Clarias gariepinus). In in vitro experiments, testis slices were incubated with oxytocin (1 and 10 IU), isotocin (2 and 20 ug), vasopressin (0.2 and 2 ug), epinephrine (1 and 10 ug), PGF2alpha (1 and 10 ug), purified Clarias LH (300 ng) and partly purified Clarias pituitary extract (containing 300 ng LH). Only oxytocin increased sperm concentration of the medium (assessed by optical density measurements) compared to control incubations. Oxytocin was then tested in vivo in two groups of fish: normal males, and males that had been treated with 17alpha-methyltestosterone during larval stages to inhibit seminal vesicle development (MT males). Both groups of fish received two doses of carp pituitary suspension (8 and 10 mg/kg, respectively i.m.) with or without subsequent oxytocin treatment (5 IU/kg i.v.; cPS-OT treatment and cPS treatment, respectively). There was no effect of oxytocin on the number of strippable males. Of cPS and cPS-OT treated fish, 87% MT males and 60% normal males were strippable. The stripped semen volume was low in both groups but MT males produced higher (P < 0.001) hatching rates (63.1%) than did normal males (2.1%).