外源性褪黑激素对黄鳝性腺发育及性腺激素分泌的影响

EFFECTS OF EXOGENOUS MELATONIN ON GONADAL DEVELOPMENT AND SECRETION OF GONADAL HORMONES IN THE RICEFIELD EEL (MONOPTERUS ALBUS) *

通过注射实验研究了外源性褪黑激素对黄鳝性腺发育及性腺激素分泌的影响。结果表明，处于不同性腺发育时期的黄鳝对外源褪黑激素的反应存在季节性差异。注射褪黑激素对Ⅳ期黄鳝的性腺发育具有剂量依存的促进与抑制的双重调节作用，但对Ⅴ期（排卵前期）个体没有显著影响。单次注射外源褪黑激素对黄鳝血清性类固醇激素（睾酮和雌二醇）水平具有一定的刺激作用，然而不同生殖时期的黄鳝对褪黑激素的反应差异很大，敏感时期出现在性腺发育的静止期。上述结果暗示，褪黑激素影响黄鳝性腺发育的关键时间可能是在性腺静止期，亦即Ⅳ期或／和Ⅳ期之前。

Effects of exogenous melatonin on gonadal development in the ricefield eel, a protogynous hermaphrodite, were studied in a long term injection experiment (twice every week for 5 weeks). The results demonstrated that melatonin had dose dependent dual effects, pro and anti gonadal development, on the eels originally at the fourth inactive stage (the last phase just before prespawning). The lowest dose of melatonin (0 1μg/g fish) improved the process of sex change in the fish, with half in the group becoming male. But higher doses (1 0 and 10 0μg/g fish) showed inhibitory effect on gonadal development, and the former had stronger effect (no male appeared). When the experimental fish were at prespawning stage, however, a significant effect of melatonin treatment was not discernible in terms of male percentage. It is concluded that exogenous melatonin showed different effects when original stage of the examined fish were different. In the course of gonadal development in the ricefield eel, concentrations of melatonin and sex steroids (testosterone and 17 beta estradiol) in blood seasonally changed. When the fish were at different stages, effects of exogenous melatonin on the levels of serum sex steroids were also different, and the most susceptible fish were in their inactive phases. The above results suggest certain correlation between serum melatonin and sex steroids, and the critical time for significant effect of melatonin on gonadal development in the ricefield eel may be at the inactive stage.