IN-VITRO CHARACTERIZATION OF GONADOTROPIN-RELEASING-HORMONE ANTAGONISTS IN GOLDFISH, CARASSIUS-AURATUS

The two native forms of GnRH, salmon GnRH and chicken GnRH-II, in the brain and pituitary of goldfish are both active in stimulating gonadot ropin-II (GTH-II) and GH release. The objective of the present study w as to characterize GnRH antagonists for their ability to inhibit sGnRH - and cGnRH-II-induced GTH-II and GH release in goldfish using a pitui tary fragments perifusion system. Contrary to expectations, putative G nRH antagonists with D-Arg6 stimulated GTH-II and GH release in nearly all cases. [Ac-DELTA3-Pro1,4FD-Phe2,D-Trp3,6]mammalian (m) GnRH inhib ited sGnRH- and cGnRH-II-stimulated GTH-II release in a dose-dependent manner, with ED50 values of 242 +/- 48 and 169 +/- 17 nm, respectivel y. [Ac-DELTA3-Pro1,4FD-Phe2,D-Trp3,6]mGnRH also inhibited GH release s timulated by sGnRH (ED50, 128 +/- 74 nm) and cGnRH-II (ED50, 157 +/- 6 7 nm). The degree of inhibition was higher in sexually regressed fish compared to postspawning fish. [D-p-Glu1,D-Phe2,D-Trp3,6]mGnRH suppres sed both sGnRH- and cGnRH-II-induced GTH-II release with ED50 values o f 326 +/- 96 and 249 +/- 74 nm, respectively. [Ac-DELTA3-Pro1,4FD-Phe2 ,D-Trp3,6]sGnRH inhibited sGnRH and cGnRH-II stimulated GTH-II release , but stimulated GH release. On the other hand, (2)-Nal1,4Cl-D-Phe2,D- (3)Pal3,6,Arg5,D-Ala10]mGnRH weakly stimulated GTH-II release, but str ongly inhibited basal GH release. These results indicate that [Ac-DELT A3-Pro1,4FD-Phe2,D-Trp3,6]mGnRH has clear antagonistic activity on sGn RH and cGnRH-II stimulation of GTH-II and GH release in vitro. The dif ferential actions of a few GnRH analogs on GTH-II and GH release indic ate that the properties of the GnRH receptors on GTH and GH cells may be different. The amino acid in position 6 plays an important role in determining the nature of intrinsic activity of GnRH peptides, and sub stitution of D-Arg6 normally produces agonistic analogs