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Gonadotropin involvement in the control of oocyte maturation and ovulation in coho salmon (Oncorynchus kisutch) : neuroendocrine control of gonadotropin secretion, effects on steroid production and properties of ovarian gonadotropin binding sites Van Der Kraak, Glen John


The involvement of gonadotropin in the regulation of reproductive development in coho salmon (Oncorhynchus kisutch) was studied by monitoring changes in plasma gonadotropin and steroid hormone levels and the induction of oocyte maturation and ovulation following the administration of gonadotropin releasing-hormones (Gn-RH). Further studies examined the properties of gonadotropin binding sites in ovaries from adult coho salmon and immature chum salmon (O. keta). Intraperitoneal injections of luteinizing hormone-releasing hormone (LH-RH), its active analog des-Gly¹⁰ [D-Ala⁶]LH-RH-ethylamide (LH-RHA DAIa⁶) and chum salmon Gn-RH caused an increase in plasma gonadotropin levels. LH-RHA DAla⁶, which had a more prolonged effect on the maintenance of elevated plasma gonadotropin levels than LH-RH and chum salmon Gn-RH, induced oocyte maturation and ovulation. Intraperitoneal injections of pimozide, a dopamine receptor antagonist, also increased plasma gonadotropin levels suggesting that endogenous dopamine may inhibit gonadotropin secretion. Pimozide potentiated the effects of LH-RHA DAla& on gonadotropin release. Studies comparing the effects of partially purified salmon gonadotropin (SG-G100) and LH-RHA DAla⁶ on the induction of ovulation demonstrated that the induction of ovulation was dependant on the duration rather than the magnitude of the initial increase in plasma gonadotropin levels. Plasma testosterone and 17α,20βdihydroxy-4-pregnene-3-one (17α2OβP) levels were increased and 17β-estradiol levels decreased following an elevation of plasma gonadotropin levels. High levels of 17α20βP were associated with both spontaneous and gonadotropin induced oocyte maturation. Declining 17β-estradiol production appears to determine the time of oocyte maturation and ovulation by having a permissive effect on 17α20βP production. The presence of a gonadotropin receptor was demonstrated for the first time in the salmonid ovary. Gonadotropin binding to immature chum salmon ovaries was a saturable process as the uptake of ¹²⁵I-labeled salmon gonadotropin was reduced .in a dose dependant manner by unlabeled gonadotropin. Gonadotropin binding was attributed to a single class of high affinity binding sites present in limited numbers. Similar sites were not present in liver, kidney or muscle. The ability of various teleost and mammalian gonadotropins to decrease the binding of ¹²⁵I-labeled salmon gonadotropin to ovarian tissue was in agreement with the ability of these hormone preparations to stimulate steroid production in vitro. Gonadotropin binding sites were demonstrated in thecal and granulosa cell layers from adult coho salmon, although the levels of binding were lower than those for immature chum salmon ovaries.

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