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      Differential Actions of a Mammalian Gonadotropin-Releasing Hormone Antagonist on Gonadotropin-ll and Growth Hormone Release in Goldfish, Carassius auratus

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          In goldifish the two native forms of gonadotropin-releasing hormone (GnRH), salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), stimulate both gonadotropin-II (GTH-II) and growth hormone (GH) release. Modifications of GnRH structure at positions 1, 2, 3, and 6 often result in an antagonist in goldfish, an observation well documented in mammalian studies. In a preliminary study in goldfish, a mammalian GnRH antagonist, [Ac-D(2)Nal<sup>1</sup>, 4Cl-D-Phe<sup>2</sup>, D(3)-Pal<sup>3,6</sup>, Arg<sup>5</sup>, D-Ala<sup>10</sup>]-mGnRH (analog L) weakly stimulated GTH-II release, and strongly inhibited GH release. The objectives of the present study were to study the dose-related actions of analog L on GTH-II and GH release in the goldfish, the specificity of inhibition of native GnRH actions, and to test whether analog L can act directly on goldfish pituitary cells. In a goldfish pituitary fragments perifusion system, analog L at different concentrations, given as 2-min pulses or as 30-min prolonged treatments, stimulated GTH-II and inhibited GH release in a dose-dependent manner. Analog L at 2 µ M concentration (45 min) significantly suppressed sGnRH- and cGnRH-II-stimulated GTH-II as well as GH release. Analog L specifically inhibited GnRH-stimulated GH release, without having any significant effects on the GH release induced by either SKF38393, a dopamine Dl receptor agonist, or thyrotropin-releasing hormone. The GTH-II stimulatory and GH-inhibitory actions of analog L were significantly suppressed by a ‘true’ GnRH antagonist (Ac-Δ<sup>3</sup>-Pro<sup>1</sup>, 4FD-Phe<sup>2</sup>, D-Trp<sup>3,6</sup>)-mGnRH. Further, analog L stimulated GTH-II release and suppressed GH release from the enzymatically dispersed goldfish pituitary cells, indicating the direct actions of analog L at the pituitary cell level. Analog L also displaced <sup>125</sup>I-(D-Arg<sup>6</sup>, Pro<sup>9</sup> NHEt)-sGnRH bound to crude goldfish pituitary membrane preparations in a dose-related manner. In conclusion, contrary to its action as a potent GnRH antagonist in mammals, analog L has GTH-II stimulatory action in goldfish. Analog L by acting via GnRH receptors at the pituitary cell level differentially acts on GTH-II and GH release, suggesting functional differences in the properties of the GnRH receptors on GTH and GH cells. Analog L also specifically inhibits sGnRH and cGnRH-II actions on GTH-II and GH release.

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          Author and article information

          S. Karger AG
          08 April 2008
          : 59
          : 6
          : 561-571
          aDepartment of Zoology, University of Alberta, Edmonton, Canada; bThe Clayton Foundation Laboratories for Peptide Biology, The Salk Institute, La Jolla, Calif., USA
          126706 Neuroendocrinology 1994;59:561–571
          © 1994 S. Karger AG, Basel

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          Page count
          Pages: 11
          Sex Steroids and Regulation of Gonadotropin


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