Precocious induction of oocyte maturation and ovulation in rainbow trout (Salmo gairdneri) : problems when using 17 α-hydroxy-20 β-dihydroprogesterone

The efficiency of 17 a-hydroxy-20 s-dihydroprogesterone (17 a-20 s P) administered alone of after pituitary priming, was investigated in vivo before peripheral migration of the germinal vesicle 4 to 8 weeks prior to natural ovulation. Treatment with 17 a-20 s P alone (2 injections of 3 mg/kg at a 2-day interval) induced oocyte maturation in 94% of the fish, but only 25% ovulated. Treatment with 17 a-20 s P (3 mg/kg once) 2 days after pituitary priming (1 ml/kg of trout pituitary extract, TPE, containing 3 25 x 10-3 mg/ml of trout gonadotropin, t-GTH) induced maturation in all fish, 59 % of which ovulated. In both cases, fish in which ovulation did not follow oocyte maturation were killed 15 days after the first injection; ovarian follicles were either dissected by hand to remove mature oocytes, or incubated in vitro with prostaglandin F2a which induced successful ovulation. In all cases, oocytes obtained from in vivo ovulation, in vitro ovulation by PGF2a , or manual dissection were fertilized to some extent. These observations demonstrate that: (1) Fertilizable mature oocytes can be produced 4 to 6 weeks in advance of natural spawning by injection of 17 a-20 P in vivo; (2) Although ovulation can occur in vivo, or in vitro with PGF2a , the specific stimulus for ovulation is lacking in most of the fish injected with 17 a-20 s P only, and appears to some extent when a pituitary priming is given prior to 17 a-20 P. The possible involvement of gonadotropin in the synthesis and storage of some mediator (or its precursor) specific for the induction of ovulation is discussed in relation to the plasma gonadotropin level in the different groups of females.