Israel Oceanographic and Limnological Research, National Center for Mariculture, PO Box 1212, Eilat 88112, Israel.
Gen Comp Endocrinol. 2012 Jan 1;175(1):55-64. doi: 10.1016/j.ygcen.2011.09.013. Epub 2011 Oct 10.
A controlled-release implant loaded with GnRH agonist (GnRHa) was used to induce spawning in Atlantic bluefin tuna (Thunnus thynnus) during two consecutive reproductive seasons. The fish were implanted underwater and sampled between days 2 and 8 after treatment. At the time of GnRHa treatment, females were in full vitellogenesis and males in spermiation. There was a rapid burst of pituitary luteinizing hormone (LH) release at day 2 after treatment in GnRHa-treated fish, and circulating LH remained elevated up to day 8 after treatment. In contrast, control fish had significantly lower levels in the plasma, but higher LH content in the pituitary, as observed in many other cultured fishes that fail to undergo oocyte maturation, ovulation and spawning unless induced by an exogenous GnRHa. Plasma testosterone (T) and 17β-estradiol (E(2)) were elevated in response to the GnRHa treatment in females, while 11-ketotestosterone (11-KT) but not T was elevated in males. Even though oocyte maturation and ovulation did occur in GnRHa-induced fish, no significant elevations in 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) or 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S), in either the free, conjugated or 5β-reduced,3α-hydroxylated forms was observed in fish sampled within 6 days after treatment. Interestingly, a significant peak in plasma free 17,20β-P levels occurred in both males and females at day 8 after treatment. Histological sections of the ovaries in these females contained oocytes at the migrating germinal vesicle stage, suggesting the role of this hormone as a maturation-inducing steroid in Atlantic bluefin tuna. In conclusion, the GnRHa implants activated effectively the reproductive endocrine axis in captive Atlantic bluefin tuna broodstocks, through stimulation of sustained elevations in plasma LH, which in turn evoked the synthesis and secretion of the relevant sex steroids leading to gamete maturation and release.
一种载 GnRH 激动剂（GnRHa）的控释植入物被用于在两个连续的繁殖季节中诱导大西洋蓝鳍金枪鱼（Thunnus thynnus）产卵。鱼在水下植入，并在处理后第 2 天至第 8 天之间进行采样。在 GnRHa 处理时，雌鱼处于完全卵黄发生期，雄鱼处于精子发生期。在 GnRHa 处理的鱼中，处理后第 2 天出现促黄体生成素（LH）的快速爆发释放，处理后第 8 天血浆 LH 仍保持升高。相比之下，对照鱼的血浆水平明显较低，但垂体中的 LH 含量较高，这在许多其他未能进行卵母细胞成熟、排卵和产卵的养殖鱼类中观察到，除非受到外源性 GnRHa 的诱导。雌鱼对 GnRHa 处理的反应导致血浆睾酮（T）和 17β-雌二醇（E2）升高，而雄鱼的 11-酮睾酮（11-KT）升高，但 T 不升高。尽管在 GnRHa 诱导的鱼中发生了卵母细胞成熟和排卵，但在处理后 6 天内采样的鱼中，17,20β-二羟-4-孕烯-3-酮（17,20β-P）或 17,20β,21-三羟-4-孕烯-3-酮（20β-S）的游离、共轭或 5β-还原、3α-羟化形式均无显著升高。有趣的是，处理后第 8 天，雄性和雌性鱼的血浆游离 17,20β-P 水平均出现显著峰值。这些雌鱼的卵巢组织学切片含有处于迁移性生发泡期的卵母细胞，表明这种激素作为大西洋蓝鳍金枪鱼的成熟诱导类固醇发挥作用。总之，GnRHa 植入物通过刺激血浆 LH 的持续升高，有效地激活了圈养大西洋蓝鳍金枪鱼亲鱼的生殖内分泌轴，从而引发相关性激素的合成和分泌，导致配子成熟和释放。
