Effects of gonadotrophin-releasing hormone during the pubertal development of the male African catfish (Clarias gariepinus): gonadotrophin and androgen levels in plasma.

The sensitivity of the pituitary to gonadotrophin-releasing hormone (GnRH) and that of the testis to gonadotrophin (GTH) was monitored in African catfish in vivo at different stages of pubertal development (20, 21, 24, 31, 39, 42 and 49 weeks of age). The fish were injected i.p. with chicken GnRH-II (cGNRH-II) or catfish GnRH (cfGnRH), their two endogenous GnRHs. Blood samples were collected to quantify LH-like GTH-II and three androgens 11-ketotestosterone (11-KT), testosterone and 11 beta-hydroxyandrostenedione (OHA). The testes of 20- and 21-week-old fish contained spermatogonia alone, or spermatogonia and spermatocytes, or -in a limited number of specimens--some spermatids as well. Spermatozoa were first observed in the testes of 24-week-old fish and became predominant as the fish attained full maturity (49 weeks of age). In 20- to 24-week-old fish, significantly elevated plasma GTH-II levels were only recorded after treatment with cGnRH-II. In 31- to 49-week-old fish, injection of both GnRHs led to increased plasma GTH-II levels, but cGnRH-II was always more effective than cfGnRH. Whereas basal GTH-II plasma levels hardly changed throughout the study, GnRH-stimulated levels increased with the age of the fish. Plasma concentrations of 11-KT were not different from controls in 20- and 21-week-old males despite their elevated GTH-II levels following injection of cGnRH-II. The first significant increase in levels of 11-KT after cGnRH-II treatment was observed in 24-week-old fish and, after cfGnRH treatment, in 39-week-old fish.(ABSTRACT TRUNCATED AT 250 WORDS)

Two gonadotropin-releasing hormones in the African catfish, Clarias gariepinus: localization, pituitary receptor binding, and gonadotropin release activity.

Two GnRH peptides have recently been identified in brain extracts of the African catfish, chicken-II GnRH ([His5,Trp7,Tyr8]GnRH, cGnRH-II) and catfish GnRH ([His5,Asn8]GnRH, cfGnRH). Using three experimental approaches, we investigated whether both peptides are involved in the regulation of pituitary gonadotropin secretion. First, the presence of cfGnRH and cGnRH-II in the pituitary was studied by biochemical and immunocytochemical techniques, as GnRH reaches the pituitary via axonal transport in teleost fish. Pituitary extracts contained cfGnRH- and cGnRH-II-immunoreactive material, showing the same HPLC retention times as the respective synthetic GnRH peptides; cfGnRH was present in 37-fold higher amounts than cGnRH-II. Using single and double labeling immunocytochemical techniques, both peptides were localized in the same peptidergic nerve fibers and often within the same secretory granules in the vicinity of the gonadotropes. Second, the two peptides were tested for their capacity to induce an increased secretion of the LH-like gonadotropin-II (GTH-II). In vivo studies showed that both GnRHs released GTH-II, but 100-fold higher cfGnRH than cGnRH-II doses were necessary to induce similar increases in circulating GTH-II levels. In vitro experiments using pituitary tissue fragments in a perifusion system also revealed a clearly higher GTH-II-releasing capacity of cGnRH-II compared to that of cfGnRH. Third, the peptides were tested for their ability to displace [125I]salmon GnRH analog ([D-Arg6,Trp7,Leu8,Pro9-NEt] GnRH, sGnRHa), a high affinity GnRH receptor ligand, from catfish pituitary membrane preparations. Chicken GnRH-II competed with [125I]sGnRHa for pituitary GnRH-binding sites, whereas cfGnRH did so only slightly. The present data show that cGnRH-II is the more potent GTH-II secretagogue, although a role for cfGnRH in the regulation of GTH-II secretion cannot be excluded. The high biological activity of cGnRH-II may be related to the regulation of GTH-II secretion surges, such as those associated with spawning, whereas cfGnRH may be involved in regulating moderate changes in GTH-II plasma levels. The peptides' potency differences appear to be related to their different binding affinities for the pituitary GnRH receptor.