Effects Of Hormone Oxytocin On Content Of Serum Steroid Hormones, Vitellogenin And Vtg MRNA Expression In Oreochromis Niloticus

Nile tilapia（Oreochromis niloticus）, native to Africa, charactering with fast-growing, short reproductive cycle, adaptability, has become one of major fresh water fish in China. Tilapia easily reproduce in a natural state,but its artificial propagation is relatively difficult, this is because tilapia gonadal mature in asynchrony among individuals, with a short process of egg maturation in comparison with other conventional fishes, therefore,lack of access to a large number of mature eggs for in vitro fertilization. In this paper, 5 sets of standard doses of the hormones oxytocin were designed,the content change of serum steroid hormones（gonadotropin-releasing hormone, GnRH and gonadotropin GTH,progesterone P, estradiol E2） were investigated after injection, the best hormone oxytocin combination was evaluated. Secondly, three dose levels of the best combination of hormone oxytocin was designed, the best dose of oxytocin was evaluated by testing serum GnRH, GTH, P, E2 levels, and vitellogenin（VTG） content and Vtg mRNA expression levels in the liver,ovaries. The spawning rate and oxytocin effect time were investigated within 144 h after hormones injection in breeding season, meantime,morphology of the eggs squeezed from the injected females were observed.the major results are as follows:（1） 5 sets of different hormone combinations were designed with standard dose（1）DOM 6mg/Kg + LRH-A₂ 10ug/Kg,（2）DOM 6mg/Kg +GnRH-A₄ 8ug/Kg,（3） DOM 6mg/Kg +LRH-A₂ 10ug/Kg + HCG2000IU/Kg,（4） DOM 6mg/Kg + LRH-A₂ 10ug/Kg + HCG 2000IU/Kg +PG 6mg/Kg,（5）DOM 6mg/Kg + LRH-A₂ 10ug/Kg + HCG 2000IU/Kg +PG 6mg/Kg + GnRH-A₄ 8ug/Kg. Meantime, 1 time, 1.5 times and 2 times standard does of hormone combination（3） were also designed, serum steroid hormone（gonadotropin-releasing hormone GnRH, gonadotropin GTH, estradiol E2, progesterone P） contents in female Oreochromis niloticus were investigated at 0h, 24 h, 36 h, 48 h, 72 h after injection. Results showed that the most elevation of serum GnRH and E2 level was observed at 24 h after injection, GTH and P contents reached their maximum value at36 h after injection. There was significant difference between combination（3）,（4）,（5） and combination（1） and（2）. The group（1.5 times dose）resulted in more obvious impact of serum steroid hormone contents, while the group（2.0 time dose） failed to induce significant effects. The optimum hormone combination and dose for Oreochromis niloticus was DOM9mg/Kg + LRH-A₂ 15ug/Kg + HCG 3000IU/Kg, which would be helpful tilapia artificial spawning.（2） According to filter out the best combination of hormone oxytocin,different doses（1 times times 1.5 times, twice times） hormones oxytocin（6mg/Kg DOM + 10ug/Kg LRH-A₂ + 2000IU/Kg HCG） injection experiments, vitellogenin（VTG） content and Vtg mRNA expression levels in liver and ovary were measured after 0h, 24 h, 36 h, 48 h, 72 h injection of hormones. The results showed that VTG content, Vtg mRNA expression in experimental groups were significantly greater than those in the control group（P<0.05）. VTG content reached a maximum in the liver at 36 h after injection, while VTG content in ovary reached maximum values at 72h;VTG content in the 1.5 times group was significantly higher than that in 1.0times, 2.0 times groups（P<0.05）. Vtg mRNA expression level in the liver reached its maximum at 36 h after injection, Vtg mRNA expression in ovary reached its maximum at 48 h after injection, Vtg mRNA expression in 1.5times group was significantly higher than that in 1.0 times, 2.0 times groups. The above results showed that the hormone oxytocin had a significant effect on ovary development of Nile tilapia with the best dose in1.5 times group, which provided some information for artificial breeding of Nile tilapia.（3） In order to verify the practical effect on artificial propagation ofOreochromis niloticus by hormone combination injection, the spawning rate and oxytocin effect time were investigated within 144 h after hormones injection in breeding season, meantime, morphology of the eggs squeezed from the injected females were observed. Results showed, under water temperature of 21-29 ℃, the spawning rate was 57.5% in the experimental group, and mostly happened within 72 h after injection, while the spawning rate was 27.5% in the control with a irregular spawning time.At 72 h after injection of hormones, 50% female in the experimental group reached maturity, in comparison with 30% in control group. These results showed that hormones combination injection could effectively improve the spawning rate, which provided some base for artificial propagation of Nile tilapia.