| Co-Investigator(Kenkyū-buntansha) |
AOKI Takasi Miyazaki University, Faculty of Agriculture, 農学部, 助教授 (00051805)
NAITO Nobuko Showa University School of Medicine, 医学部, 講師 (30053903)
NAKAI Yasumitsu Showa University School of Medicine, 医学部, 教授 (60053807)
HIRANO Tetsuya University of Tokyo, Ocean Research Institute, 海洋研究所, 教授 (70013571)
NAGAHAMA Yoshitaka National Institute for Basic Biology, Laboratory of reproductive Biology, 教授 (50113428)
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| Research Abstract |
Duality of teleost gonadotropins (GTHs): Two chemically distinct GTHs homologous to mammalian GTHs were isolated form salmon, carp and bonito pituitary glands, designated GTH I and GTH II and their cDNAs were cloned. They are composed of and subunits. Sequence studies show that two -subunits have approximately 30% homology and suggest that the divergence of the GTH gene took place earlier than the time of divergence of teleosts from the mainline of evolution leading to tetrapods. Salmon GTH I and GTH II were equipotent in stimulating estradiol production, whereas GTH II appears to be more potent in stimulating 17 , 20 -diohprog synthesis. In salmonids, the ratio of plasma levels and pituitary contents of GTHs and the secretory control by a GnRH suggest that GTH I is the predominant GTH during vitellogenesis and early stages of spermatogenesis, whereas GTH II is predominant at the time of spermiation and ovulation. GTH I and GTH II are found in distinctly separate cells. In trout, GTH I
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is expressed first in ontogeny, whereas GTH II cells appear coincident with the onset of spermatogenesis and vitellogenesis, and increase dramatically at the time of final reproductive maturation.
Mechanism of GTH actions: Two steroid hormones, estradiol-17 and 17 , 20 -diOHprog are involved in the regulation of vitellogenesis and ovulation, respectively. During vitellogenesis, GTH stimulates the thecal cells to produce testosterone, which are converted to estradiol-17 in granulosa cells by enhancing aromatase enzyme activity. A distinct steroidogenic shift occurs in follicles just prior to oocyte maturation. GTH stimulate thecal cells to produce 17 -OHprog, which is converted to 17 ,20 -diOHprog in granulosa cells by enhancing 20 -OHsteroid dehydrogenase activity. Two steroid hormones are involved in the regulation of two important processes of male germ cell development: 11-ketotestosterone for spermatogenesis and 17 ,20 -diOHprog for spermiation. Analysis of steroid biosynthetic pathways of the testis showed a shift in testicular steroid metabolic enzyme activity in response to GTH. During spermatogenesis, the testis synthesized 11-ketotestosterone, 17 ,20 -diOHprog production results from a change in the pathway, immediately prior to spermiation. GTH stimulates the production of these steroid hormones in ovary and testis via an adenylate cyclase-cAMP-dependent step.
Thyroid hormones: A bioassay of salmon TSH was established based on the increase in plasma thyroxine levels in juvenile rainbow throut. Thyroid hormones are found in the yolk of unfertilized eggs of all teleost studied. Treatment of juvenile salmonids with thyroid hormones caused an increase in oxygen consumption and body silvering, but no consistent effect was seen in seawater adaptability. The tissue and plasma levels of the hormones of chum salmon fry increased just before downstream migration. A marked increase in the tissue levels of the hormones was also observed during metamorphic climax of the flounder larvae, and injection of thyroxine or ovine TSH into pelagic larvae of the flounder accelerated the metamorphosis to benthic larvae. Less
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