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Aspects of the reproductive endocrinology of the Thai carp, Puntius gonionotus (Bleeker) Sukumasavin, Naruepon


Several aspects of the reproductive endocrinology of the Thai carp (Puntius gonionotus Bleeker) were investigated. The annual cycle of gonadal development, changes in plasma hormone levels in male and female fish reared in ponds at Kalasin Freshwater Fisheries Station, Kalasin, Thailand and environmental parameters were observed for a period of 20 months. The interaction of a salmon gonadotropin-releasing hormone analog (D-Arg⁶, Pro⁹ NEt-sGnRH; sGnRHA) and domperidone (Dom) on the induction of gonadotropin (GtH) secretion and spawning in female fish was investigated. Further, long- and short-term changes in plasma hormone levels during sGnRHA and Dom induced spawning were examined. Moreover, the biological activities of 12 different mammalian, avian and piscine GnRHs and their analogs were also compared. In females, gonadal recrudescence was highly correlated with the changes in air temperature and daylength, but spawning coincided with the occurrence of rainfall. Histological analysis of the ovary revealed that oocyte development was of the asynchronous type, thus suggesting that the Thai carp can spawn several times in a spawning season. It was not possible to distinguish pronounced seasonal changes in plasma hormone levels during this study. Also, there were no correlations between seasonal changes in plasma hormone levels and any reproductive parameter. Gonadal development in males was highly correlated with that of females. The structure of the testis corresponds to the lobular type. Histological analysis revealed a continuous spermatogenetic activity. Plasma hormones levels in males exhibited a bimodal pattern of seasonal changes. However, there were no correlations between the changes in plasma reproductive hormones, testicular development and environmental parameters. Injection of sGnRHA and Dom alone increased plasma GtH levels significantly, but the magnitude of increasing GtH levels was insufficient to induce spawning, except in the case of the highest dose of domperidone. Administration of various combinations of sGnRHA and Dom increased both the plasma GtH level and the number of fish spawning, suggesting that dopamine plays an important role as the gonadotropin release inhibitory factor in this species. Plasma E₂ and T levels in females increased significantly following an elevation of GtH during the induction period. During the short-term observations, both E₂ and T peaked immediately at the time of ovipositon. Plasma 17α, 20β dihydroxy-4-pregnene-3-one was undetectable throughout the induction period. In males, a significant increase in plasma GtH, T and 11-ketotestosterone (11-KT) was observed at the onset of spawning. Both T and 11-KT continued to increase during the process of spawning, while GtH immediately decreased to a lower level. The changes in plasma 11-KT were synchronous with the occurrence of spawning, suggesting that 11-KT is probably the major androgen during the process of spawning in male Thai carp. High concentrations (25 μg/kg) of either native GnRHs or their analogs in combination with Dom (25 mg/kg) were equally effective in inducing spawning. At 10 μg/kg, however, sGnRH and chicken GnRH-11, in combination with 10 mg/kg Dom, were found to be the most potent of the native GnRHs. GnRHs with a substitution in position 6 with hydrophobic or aromatic D-amino acids possess greater potencies compared to native forms. Furthermore, [D-Ala⁶]-mGnRHA and [D-Trp⁶]-mGnRHA were found to be the most potent peptides.

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