A new gonadotropin-releasing hormone (GnRH) superagonist in goldfish: influence of dialkyl- d-homoarginine at position 6 on gonadotropin-II and growth hormone release

Abstract

The two native forms of gonadotropin-releasing hormones (GnRH) present in goldfish, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), stimulate gonadotropin-II (GTH-II) and growth hormone (GH) release both in vivo and in vitro. In our previous study using perifused goldfish pituitary fragments, many mammalian GnRH antagonists, especially those with d-Arg 6, showed weak to strong stimulation of GTH-II and GH release. In the present study, the dose-related stimulation of GTH-II and GH release by [Ac-D(2)-Nal 1, 4Cl- d-Phe 2, d-Trp 3, d-Arg 6, Trp 7, d-Ala 10] mGnRH (analog J) and [Ac-D(2)-Nal 1, 4Cl- d-Phe 2, d-Trp 3, d-hArg(Et 2) 6, d-Ala 10] mGnRH (analog K) was demonstrated; the stimulatory potency of both analogs was significantly lower than that of native sGnRH. In the presence of analogs J and K, cGnRH-II stimulated GTH-II release was significantly suppressed. Further, GTH-II and GH stimulation by 2 μM of analog K was significantly suppressed by a ‘true’ GnRH antagonist, [Ac- δ 3-Pro 1, 4FD-Phe 2, d-Trp 3,6] mGnRH (analog E). These results indicate that analogs J and K increase GTH-II and GH release in goldfish by acting on GnRH receptors on gonadotrophs and somatotrophs. Since analog K, having [ d-hArg(Et 2) 6], strongly stimulated GTH-II release, the potency of [ d-hArg(Et 2) 6] or [ d-hArg(CH 2CF 3) 2 6] substituted analogs to stimulate GTH-II and GH release from the perifused goldfish pituitary fragments was tested. Among the peptides tested, [ d-hArg(Et 2) 6, Pro 9-NHEt] sGnRH had a higher potency in stimulating GTH-II release than any other analog tested in the present or in previous studies. For stimulation of GH release, [ d-hArg(Et 2) 6, Pro 9-NHEt] sGnRH and [ d-Arg 6, Pro 9-NHEt] sGnRH were the most potent analogs tested; analogs of mGnRH were less potent than sGnRH, indicating the importance of Trp 7, Leu 8 residues in the native peptide. These results suggest the importance of [ d-Arg 6] or alkylated [ d-Arg 6] in determining the intrinsic activity and potency of GnRH peptides in goldfish.