The biased intracellular accumulation of the ?-subunit of salmon gonadotropin (GTH II) in the pituitary of rainbow trout Oncorhynchus mykiss, during gametogenesis

Steroid hormones stimulate gonadotrophs in juvenile male African catfish (Clarias gariepinus)

In juvenile African catfish (Clarias gariepinus), the pituitary LH content strongly increased after the beginning of spermatogonial proliferation. We hypothesized that a signal of testicular origin is involved in stimulating the gonadotrophs. We investigated the effects of castration and sex steroid treatment on gonadotrophs in juvenile males by quantifying LH production and release and LH subunit transcript levels and by examining gonadotroph morphology and proliferation. Castration reduced but did not abolish the maturation-associated elevation in pituitary LH content. Treatment with testosterone but not with 11-ketotestosterone, an otherwise potent androgen in fish, reversed the castration-induced decrease of pituitary LH levels. An increased pituitary LH content was accompanied by an increased number of cytologically mature gonadotrophs. However, no evidence was found for gonadotroph proliferation, so that quiescent gonadotrophs may have become activated. Although 11-ketotestosterone treatments had no effect in castrated males, this androgen attenuated gonadotroph activation in intact males. Because androgen production in juvenile catfish is downregulated by treatment with 11-ketotestosterone, its inhibitory effects on gonadotrophs in gonad-intact males may be due to suppression of Leydig cell testosterone production, which appears to be a limiting factor for the activation of catfish gonadotrophs. Aromatizable androgens may have opposite effects on fish (stimulatory) and mammalian (inhibitory) gonadotrophs.

Immunocytochemical and histological studies on the hypophyseal-gonadal system in the freshwater nile tilapia, oreochromis niloticus (L.), during sexual maturation and spawning in different habitats

The activities of the hypophyseal-gonadal system of O. niloticus in different habitats were investigated using immunocytochemical and histological techniques. The observed physico-chemical results indicated that Lake Nasser water characteristics are within the allowed and desired safety baseline levels. In contrast, Lake Manzalah exhibited high levels of Ca(2+), Mg(2+), SO(4)(2-) and heavy metals (Zn, Pb, and Cd). These water conditions affected the activity of the hypophyseal-gonadal axis of tilapia. The secretory and the synthetic activities of GTH and SL cells in the pituitary gland, in general, underwent obvious normal changes during gonad maturation and spawning of O. niloticus in Lake Nasser. In Lake Manzalah, the secretory activity of GTH and SL cells declined. This may be due to the effect of the high levels of heavy metals which interfere with the release of hormones and disturb the feedback mechanisms. In addition, the activity of both PRL and GH cells in Lake Nasser was higher than that of Lake Manzalah, but the synthetic activity of the ACTH and MSH cells was higher in Lake Manzalah. This may be related to the increased stress on fishes and the dark polluted water in Lake Manzalah. Both the environmental and the endocrine impacts in Lake Manzalah caused a decline in the gonadal activity as reflected by the decrease of sperm amount in the ripe testis, the appearance of oocytes in the testis and the degeneration of ripe oocytes (atresia) during the spawning season. J. Exp. Zool. 284:343-354, 1999. Copyright 1999 Wiley-Liss, Inc.

Growth hormone (GH) and gonadotropin subunit gene expression and pituitary and plasma changes during spermatogenesis and oogenesis in rainbow trout (Oncorhynchus mykiss)

In order to evaluate potential interactions between somatotropic and gonadotropic axes in rainbow trout (Oncorhynchus mykiss), changes in pituitary content of the specific messenger RNA of growth hormone (GH) and gonadotropin (GTH) alpha- and beta-subunits were studied during gametogenesis with respect to pituitary and plasma hormone concentrations. Quantitative analyses of mRNA and hormones were performed by dot blot hybridization and homologous RIA on individual fish according to stage of spermatogenesis and oogenesis. All transcripts were detectable in 9-month-old immature fish. GH, GTH IIbeta, and GTH alpha increased moderately throughout most of gametogenesis and then more dramatically at spermiation and during the periovulatory period. GTH Ibeta mRNA increased first from stage I to V in males and more abruptly at spermiation, while in females GTH Ibeta transcripts increased first during early vitellogenesis and again around ovulation. Pituitary GH absolute content (microgram/pituitary, not normalized with body weight) increased slowly during gametogenesis and more abruptly in males during spermiation. In the pituitary of previtellogenic females and immature males, GTH I beta peptide contents were 80- to 500-fold higher than GTH II beta peptide contents. GTH I contents rose regularly during the initial phases of vitellogenesis and spermatogenesis and then more abruptly in the final stages of gonadal maturation, while GTH II contents show a dramatic elevation during final oocyte growth and maturation, in postovulated females, and during spermiogenesis and spermiation in males. Blood plasma GTH II concentrations were undetectable in most gonadal stages, but were elevated during spermiogenesis and spermiation and during oocyte maturation and postovulation. In contrast, plasma GTH I was already high ( approximately 2 ng/ml) in fish with immature gonads, significantly increased at the beginning of spermatogonial proliferation, and then increased again between stages III and VI to reach maximal levels ( approximately 9 ng/ml) toward the end of sperm cell differentiation, but decreased at spermiation. In females, plasma GTH I rose strongly for the first time up to early exogenous vitellogenesis, decreased during most exogenous vitellogenesis, and increased again around ovulation. Our data revealed that patterns of relative abundance of GTH Ibeta mRNA and pituitary and plasma GTH I were similar, but not the GTH II patterns, suggesting differential regulation between these two hormones at the transcriptional and posttranscriptional levels. Pituitary and plasma GH changes could not be related to sexual maturation, and only a weak relationship was observed between GH and gonadotropin patterns, demonstrating that no simple connection exists between somatotropic and gonadotropic axes at the pituitary level during gametogenesis.