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  • 1
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The in vivo and in vitro potency of native and modified forms of gonadotropin releasing hormone (GnRH) to release luteotropic hormone (LH) was studied in sea bass Dicentrarchus labrax in particular the hypothalamic fish-specific sea bream GnRH form (sbGnRH) and the general mesoencephalic form chicken GnRH-II (cGnRH-II). The potencies of the natives and their analogs (GnRHas) were referred to that of [D-Ala6, Pro9Net]-mGnRHa (LHRHa) at equivalent doses. Analogs of the native peptides [D-Arg6, Pro9Net]-cGnRH-II, [D-Ala6, Pro9Net]-cGnRH-II, [D-Trp6, Pro9Net]-sbGnRH and [D-Ala6, Pro9Net]-sbGnRH were effective in inducing in vivo LH release (at 15 µg kg−1 body mass), exhibiting longer lasting activity than their corresponding native forms. Injection of sbGnRH and cGnRH-II provoked a small but significant peak of circulating LH at 1·5 h after treatment (a.t.) decreasing down to basal levels at 4 h a.t. [D-Arg6, Pro9Net]-cGnRH-II, [D-Ala6, Pro9Net]-cGnRH-II and [D-Ala6, Pro9Net]-mGnRHa evoked a higher and a more sustained elevation of LH, peaking at 12 h a.t. and returning to basal levels between 48 and 72 h a.t. [D-Trp6, Pro9Net]-sbGnRH and [D-Ala6, Pro9Net]-sbGnRH also induced a significant surge of LH in plasma at 4 h a.t. turning to the basal levels at 24 h a.t. These rises, however, were of less amplitude and duration than the observed after treatment with cGnRH-II analogs and [D-Ala6, Pro9Net]-mGnRHa. The in vitro stimulation of dispersed pituitary cells with the different native and modified forms of GnRH resulted in a dose-dependent increase in the quantity of LH released at 24 h a.t. [D-Arg6, Pro9Net]-cGnRH-II and [D-Ala6, Pro9Net]-cGnRH-II induced the highest response of LH in vitro release followed by salmon GnRH (sGnRH), [D-Ala6, Pro9Net]-mGnRHa and [D-Trp6, Pro9Net]-sbGnRH. The lowest activity was exhibited by sbGnRH. Collectively, the in vitro biological activity (compared by their EC50) can be ordered as follows: [D-Arg6, Pro9Net]-cGnRH-II 〉 [D-Ala6, Pro9Net]-cGnRH-II 〉 sGnRH 〉 [D-Ala6, Pro9Net]-mGnRHa 〉 [D-Trp6, Pro9Net]-sbGnRH 〉 [D-Ala6, Pro9Net]-sbGnRH 〉 cGnRH-II 〉 sbGnRH.
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  • 2
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Spermiating male European sea bass Dicentrarchus labrax were treated with gonadotropin-releasing hormone agonists (GnRHa), either a GnRHa injection (IN; 25 μg kg−1 body mass) or one of three types of controlled-release GnRHa-delivery systems: fast release implants (EVAc; 1OO μg kg−1), slow release implants (EVSL; lOO μg kg−1) and slow release microspheres (MC; 50 μg kg−1). Luteinizing hormone (LH) release was highly stimulated by all GnRHa treatments, with elevated plasma levels lasting for 2 days in injected fish (IN) and 2, 4 and 6 weeks in controlled-release-treated fish (EVAc, MC and EVSL, respectively), correlating with a 1, 3, 5 and 5 week period of stimulation of milt production, respectively. Plasma levels of the androgens testosterone (T) and 11-ketotestosterone (11-KT), were not significantly affected by the GnRHa treatments. Plasma T was high at early spermiation and declined sharply near the end of this period. Plasma 11-KT levels declined continuously throughout the experiment. Levels of 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P), a proposed maturation-inducing steroid (MIS) in European sea bass, fluctuated around 0.2–1 ng ml−1 and were not greatly affected by the treatments. These results indicated a close correlation between sustained stimulation of LH release, achieved by GnRHa-delivery systems, and long-term enhancement of milt production. They also show an absence of changes in the common sex steroids, associated with elevated LH and enhanced spermiation.
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  • 3
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Prostaglandin series E (PGE) production by mixed populations of isolated sea bass Dicentrarchus labrax testicular cells was measured after culture at 20° C in L-15 for up to 24 h in the absence or presence of arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and/or gonadotropin (hCG). AA stimulated a significant dose- and time-dependent increase in PGE production. EPA stimulated PGE production only during the first 6 h although levels were 26·4% those induced with AA; DHA reduced PGE production. Although hCG alone had no effect, it enhanced AA-induced PGE formation and suppressed EPA-induced PGE production. These results suggest that essential PUFAs modulate significantly testicular PGE production in vitro and therefore may have important effects on steroidogenesis and spermiation in the male European sea bass.
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  • 4
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Fifteen tagged female sea bass Dicentrarchus labrax were sampled weekly from September to April and plasma vitellogenin (VTG), testosterone (T), 17β-estradiol (E2), and two potential maturation inducing steroids (MISs): 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and 17,20β,21-trihydroxy-4-pregnen-3-one (20βS) assayed. An oocyte sample was obtained via intraovarian cannulation at each sampling time from every female and the stage of development of the most advanced clutch of oocytes determined and related to VTG and hormone plasma levels for each female. The mean number of ovulations per female was 1·75+0·25 when those females that did not present ovulations were excluded and up to 4 ovulations detected in some females. The highest plasma levels of T (c. 6 ng ml-1) were observed during postvitellogenesis and the beginning of maturation while maximum plasma levels of E2 (〉5 ng ml-1) were obtained during late vitellogenesis. VTG plasma levels increased throughout vitellogenesis peaking (c. 2·5 mg ml-1) at postvittelogenesis. For the first time significant changes of plasma progestogens were detected in European sea bass during the sexual cycle. The highest plasma level of 17,20βP (c. 1·1 ng ml-1) was observed during postvitellogenesis while the highest level of 20βS (c. 1·4 ng ml-1) coincided with final maturation. These results suggest that 17,20βP and 20βS play a role in the early and final maturation, respectively, in the European sea bass.
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  • 5
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Exposure of sea bass Dicentrarchus labrax oocytes to 25, 50, 100, and 300 IU ml−1 of human chorionic gonadotropin (hCG) resulted in a dose-dependent increase (ED50=60 IU ml−1) in the maximal maturation and volume response rate over a 120-h incubation period. Oocytes responded in a similar dose-dependent manner to graded doses of homologous sea bass pituitary extract (ED50=3·3#10−2 PE ml−1). The response latency of sea bass oocytes to endogenous and exogenous gonadotropins was similar (t1/2≃45 and 48 h, respectively). Both 17α,20β-dihydroxy-4-pregnen-3-one (17,20βP) and 17α,20β,21-trihydroxy-4-pregnen-3-one (20βS) stimulated maturation in a dose- and time-dependent manner with t1/2 values less than those of hCG (15 h and 7·5 h at 100 ng ml−1, respectively); 17α,20α-dehydroxy-4-pregnen-3-one (17,20αP) was ineffective. 17,20βP was more potent at all doses tested. The stimulatory actions of gonadotropin and maturation-inducing steroid (MIS) involved both transcription and translation since maturation was significantly inhibited in the presence of actinomycin D and cycloheximide. Moreover, pituitary extract and hCG stimulated release of higher levels of 17,20βP in a dose- and time-dependent manner in follicle incubations as compared to 20βS suggesting that this steroid may in fact be the MIS in this species.
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