No abstract

Specific and sensitive radioimmunoassays (RIAs) were newly developed for two types of gonadotropin-releasing hormone (GnRH), namely, seabream (sb) GnRH and chicken (c) GnRH-II. We employed these two RIAs together with a previously reported RIA for salmon (s) GnRH to study the presence and regional distribution of these three GnRHs in the brains and pituitaries of four perciform fishes (red seabream, Pagrus major; black seabream, Acanthopagrus schlegeli; striped knifejaw, Oplegnathus fasciatus; and Nile tilapia, Oreochromis niloticus), as well as clarify seasonal changes in levels of these GnRHs in the brain and pituitary of red seabream. All three GnRHs were found in brains of all fishes examined, with regional distributions in the brains of the three GnRHs being rather similar. sbGnRH was abundant in telencephalon and hypothalamus. cGnRH-II was concentrated from the middle to posterior part of the brain and distributed throughout the brain. sGnRH was concentrated in the olfactory bulb and distributed all over the brain, as was cGnRH-II. The dominant form of GnRH in the pituitary was sbGnRH, with levels 500- to 2400-fold higher than those of sGnRH, while cGnRH-II was undetectable in all four species. In the brain and pituitary of female red seabream, levels of both brain and pituitary sbGnRH increased from October (immature phase) and reached a peak in April (spawning phase), reflecting the increase in gonadosomatic index and vitellogenesis. However, levels of sbGnRH remained high only in the pituitary of completely regressed fish in June. Levels of both sGnRH and cGnRH-II in the brain were higher in the regressed phase and remained lower during the spawning phase. From these and previous results, it appears that sbGnRH is physiologically the most important form of GnRH in reproduction in red seabream and, probably, in other perciforms also.

No abstract

A cDNA encoding a putative gonadoliberin receptor was cloned from the pituitary of the African catfish. Conceptual translation predicts a protein of 379 amino acids which shows typical characteristics of GTP-binding-protein-coupled receptors. The isolated cDNA was stable expressed in human embryonic kidney (HEK) 293 cells which were used for studies on gonadoliberin-activated second messenger systems (inositol phosphate production; increase in CAMP andor intracellular Ca2' ). The isolated cDNA encoded a functional receptor, designated catfish gonadoliberin receptor (cfGnRH-R), which had an amino acid sequence similarity of 38 % with mammalian gonadoliberin receptors. In contrast to its mammalian counterparts which lack an intracellular carboxy-terminal domain, the cfGnRH-R contains an additional 49 amino acid residues. From the two endogenous gonadoliberins in African catfish, chicken gonadoliberin-11 had a several hundredfold higher potency than catfish gonadoliberin to activate cfGnRH-R-associated second messenger systems in transfected HEK 293 cells. This is in line with the previously determined higher gonadotropin-release capacity of chicken gonadoliberin-I1 in catfish. Stimulation of second messenger systems with chicken gonadoliberin-11, but not with catfish gonadoliberin, resulted in a biphasic effect and chicken gonadoliberin-I1 led to a higher maximum stimulation than catfish gonadoliberin. Challenging cfGnRH-R simultaneously with chicken gonadoliberin-I1 and catfish gonadoliberin did not lead to additive effects. In contrast, two types of mutual inhibitory effects were recorded. These data indicate that a single cognate cfGnRH-R couples with distinct efficacies to signal transduction systems upon stimulation by the two endogenous gonadoliberins which, in addition, may interact negatively.Keywords: GTP-binding-protein-coupled receptor; gonadoliberin receptor; cDNA ; pituitary ; transfection.Gonadoliberin is the central regulator of reproduction in vertebrates. It is produced by neuroendocrine neurons in the ventral forebrain which release the hormone into the hypophyseal portal blood system or, in the case of teleost fish, directly in the vicinity of pituitary gonadotrophs. Nine different molecular forms of this decapeptide have been characterized in vertebrate brains so far. In highly evolved eutherian mammals only mammalian gonadoliberin (Glp-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH,) Note. The novel nucleotide sequence published here has been submitted to the EMBL data bank and is available under accession number x97497. which all show a high degree of sequence similarity and display the seven transmembrane structure characteristic for GTP-binding-protein-(G-protein)-coupled receptors [3 -91. lnteraction of gonadoliberin with its receptor results in the activation of a G protein that catalyzes phosphatidylinositol turnover. This stimulates the pituitary gonadotrophs to release gonadotropins, which in turn regulate the activity of the gonads. No information is available on the structure of gonadoliberi...