| Daidzein, one of the main soybean isoflavone in legume plants, is also animportant phytoestrogen. Its structure is similar to17β-estradiol (E2) of mammalianand thus possess potential estrogenic activity. To fulfill the need of developingaquaculture, more and more fishmeal was replaced by soy protein in fish feedproduction. Whether the high daidzein level in the soy protein affects growth of fish isstill unclear. In order to investigate the effects and mechanisms of daidzein on thegrowth of tilapia, juvenile tilapia were feed with diets containing0,10,100,1000g/g daidzein for eight weeks, and then the growth was examined. Furthermore, thesynthesis and secretion of GH (growth hormone), the expression of hypothalamicneuroendocrine factors (GnRHs, GnRHRs, NPY, NPYRs, PACAP, SS), the expressionof GHRs (growth hormone receprors) in hepatic and the synthesis and secretion ofIGF-I(insulin-like growth factor-I) were detected. The results showed that:1. After fed with diets containg0,10,100and1000g/g daidzein for8weeks,the body weight and length of female tilapia in10and100μg/g group had nosignificant difference from that in the control group, while a significant increase wasobserved in1000μg/g group. In male tilapia, there was also no significant differencein10g/g group, but an extremely significant increase was observed in100μg/ggroup, and a significant increase in1000μg/g group was observed too. Moreover,ingestion rate and feed efficiency ratio among all experimental groups exhibited nosignificant difference. The results showed that100μg/g and1000μg/g daidzein indiets could enhance the growth of male and female tilapia, and male tilapia were moresusceptible to lower level of daidzein.2. Real-time PCR was used to quantify mRNA levels of GH in pituitary, GnRH1,GnRH2, GnRHR1, GnRHR2, NPY, NPYR1, NPYR2, PACAP, SS in hypothalamic, and RIA was adopted to test levels of plasma GH. The results showed that theexpression level of pituitary GH was consistent with the growth of males and females,with a significant increase for female tilapia in1000g/g group and male tilapia in100g/g group; However, the level of plasma GH did not change dramatically formale and female tilapia in each group, respectively. Further, the expression levels ofGnRH1, GnRH2, NPY and NPYR1of male tilapia were increased by daidzein in100g/g group, thus may promote the synthesis of GH. But the expression of SS wasalso increased and the secretion of pituitary might be inhibited by SS, thus the plasmaGH levels had no change; Only GnRH1mRNA expression levels were increased in1000g/g female treatment group, this may promote the synthesis of pituitary GH.3. GHR1, GHR2, IGF-I mRNA expression levels in liver were examined byreal-time PCR following a8-week exposure to10,100,1000g/g dietary daidzein,and plasma IGF-I levels was examined by IRMA. The results showed that theexpression of hepatic IGF-I mRNA and plasma IGF-I levels were significantlyincreased in100g/g male group and1000g/g female group. Meanwhile, thecorresponding expressions of GHR2were also increased. It suggests that daidzeinmay increase the synthesis and secretion of IGF-I by stimulating the expression ofGHR2, and mediating the growth-promoting effect of GH.In summary, Diets containing100and1000g/g daidzein could significantlypromote the growth of Nile tilapia. And the effect on male was greater than female.The results suggested the potential mechanisms:(1) Daidzein may stimulate theexpression of growth hormone releasing factors in the upstream of growth axis,including GnRH1, GnRH2, NPY, NPYR1, to increase the expression of GH. But thepromoting for the GH release may be inhibited by SS.(2) Daidzein may stimulate theexpression of GHR2to increase the synthesis and secretion of IGF-I, thus mediate thegrowth-promoting effect of GH. |
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