ACTIVITY OF VERTEBRATE GONADOTROPIN-RELEASING HORMONES AND ANALOGS WITH VARIANT AMINO-ACID-RESIDUES IN POSITION-5, POSITION-7 AND POSITION-8 IN THE GOLDFISH PITUITARY

All non-mammalian vertebrates as well as marsupial mammals have two or more forms of gonadotropin-releasing hormone (GnRH) in the brain. Goldfish brain and pituitary contains two molecular forms of GnRH, salmon GnRH ([Trp7, Leu8]m-GnRH; s-GnRH) and chicken GnRH-II ([His5, Trp7, Tyr8]m-GnRH; cII-GnRH). Both sGnRH and cII-GnRH stimulate gonadotropin (GtH) as well as growth hormone (GH) release from the goldfish pituitary. The purpose of the present study was to study the activity of the five known forms of GnRHs as well as analogs of mammalian GnRH (m-GnRH) with variant amino acid residues in positions 5, 7 and 8 in terms of binding to GnRH receptors, and release of GTH and GH from the perifused fragments of goldfish pituitary in vitro. All five vertebrate GnRH peptides stimulated both GtH and GH release in a dose-dependent manner, although their potencies were very different. cII-GnRH was somewhat more active than s-GnRH in releasing GtH, whereas s-GnRH tended to have a greater potency than cII-GnRH in terms of GH release. Both chicken GnRH-I (cI-GnRH) and lamprey GnRH (l-GnRH) were significantly less potent than mGnRH, s-GnRH and cII-GnRH in releasing GtH and GH. cII-GnRH binds with higher affinity for the high affinity binding sites compared to all other native peptides. The activity of [Trp7]-GnRH was similar to both s-GnRH and cII-GnRH in releasing GtH and GH. Substitution of His5 resulted in a significant decrease in GtH releasing potencies compared to mGnRH, sGnRH and cII-GnRH. [His5]-GnRH also had lower GH releasing potency than mGnRH and sGnRH. Tyr8, His8 and Leu8 substitutions caused significant decreases in GtH releasing potencies compared to mGnRH, s-GnRH and cII-GnRH, but did not cause a significant change in GH releasing potency. The combination of [His5, Trp7]-GnRH had GtH and GH releasing activities similar to m-GnRH, s-GnRH and cII-GnRH. However, [His5, Tyr8]-GnRH had significantly lower GtH but not GH releasing potency compared to m-GnRH, s-GnRH and cII-GnRH. In terms of binding affinity, there were no correlations between binding affinities of the high or low affinity binding sites and GH or GtH releasing activities of the peptides used in the present study, suggesting that full molecular structure of native peptides are required for receptor recognition; in this context, previous studies demonstrated a good correlation between high affinity GnRH receptor binding and GTH release activity for GnRH analogs containing Trp7, Leu8 residues and various D-amino acids at position 6. Viewing all of the data together it is apparent that the presence of tryptophan in position 7 of the native GnRH peptides in goldfish, s-GnRH and cII-GnRH, is essential for the high potency of these peptides in releasing GtH from the goldfish pituitary. It is further evident that in goldfish the GH releasing activities of GnRH peptides are much less affected by position 8 substitutions than are the GtH releasing activities; position 8 substitutions in the m-GnRH molecule with histidine, leucine, methionine or tyrosine produces analogs that are significantly less potent in terms of GtH and are in this way more selective for the release of GH. These findings support the hypothesis that the GnRH receptors on somatotrophs and gonadotrophs in goldfish are different and have different requirements for optimal ligand binding and activity.