We have recently developed sensitive and specific radioimmunoassays (RIAs) for salmon gonadotropin-releasing hormone (sGnRH) and chicken GnRH-II (cGnRH-II) and have measured the contents of both GnRHs in the rainbow trout brain. Our results showed that contents of the two GnRHs are variable among different brain regions. Therefore, in order to confirm the differential distribution of the two GnRHs by a different technique, we examined the distribution of immunoreactive sGnRH and cGnRH-II in the brain of masu salmon by using immunocytochemical techniques. sGnRH immunoreactive (ir) cell bodies were scattered in the transitional areas between the olfactory nerve and the olfactory bulb, the ventral olfactory bulb, between the olfactory bulb and the telencephalon, the ventral telencephalon, and the preoptic area. These sGnRH-ir cell bodies were dispersed in a strip-like region running rostrocaudally in the most ventral part of the ventral telencephalon. sGnRH-ir fibers were distributed in the various brain regions from the olfactory bulb to the spinal cord. They were especially abundant in the olfactory bulb, ventral telencephalon, preoptic area, hypothalamus, deep layers of the optic tectum, and thalamus. sGnRH-ir fibers also innervated the pituitary directly. cGnRH-II-ir cell bodies were found in the nucleus of the medial longitudinal fasciculus (nMLF). The distribution of cGnRH-II-ir fibers was similar to that of sGnRH-ir fibers, except that cGnRH-II-ir fibers were absent in the pituitary. The number of cGnRH-II-ir fibers was much fewer than that of sGnRH-ir fibers. The results of the present immunocytochemical study are in basic agreement with those of our previous RIA study. Thus, we suggest that in masu salmon, sGnRH not only regulates gonadotropin (GTH) release from the pituitary but also functions as a neuromodulator in the brain, whereas cGnRH-II functions only as a neuromodulator.

Although reduced fetal growth in response to hypoxia has been appreciated for decades, we have a poor understanding of the effects of hypoxia on embryonic development and the underlying cellular and molecular mechanisms. Here we show that hypoxia treatment not only resulted in embryonic growth retardation but also caused significant delay in developmental zebrafish ͉ heart development ͉ craniofacial skeleton ͉ morphogenesis ͉ oxygen H ypoxic stress causes major metabolic changes in all organisms requiring oxygen. Hypoxic stress also influences fetal growth and development and the pathogenesis of several human diseases, including intrauterine growth restriction (IUGR) (1-3). Reduced birth weight is also observed at high altitudes (4). IUGR not only increases fetal and neonatal morbidity and mortality, but also increases the risk of having adult diseases, such as cardiovascular disease, type-2 diabetes, obesity, and hypertension (5).Recent evidence suggests that hypoxia may influence fetal growth through its connection to the insulin-like growth factor (IGF) signaling system. IGFs are well known fetal growth factors (6-9). Several groups have reported that the circulating level of IGF-binding protein (IGFBP)-1, a secreted protein that binds to IGF in extracellular environments, is elevated in IUGR fetuses (10-12) and that there is a striking inverse correlation between IGFBP-1 levels and fetal size (13). In addition, higher maternal serum IGFBP-1 levels are found at higher altitude (14). In vitro studies with cultured human cells and in vivo studies with mammalian animal models suggest that IGFBP-1 gene expression is elevated in hypoxic conditions (15)(16)(17) and that this up-regulation is mediated through the hypoxia-inducible factor (HIF)-1 pathway (16). Because IGFBP-1 binds IGFs with high affinity and inhibits IGF activities on cell growth in vitro (18,19) and because IGFBP-1-overexpressing transgenic mice had reduced birth weight (20-22), it was postulated that the elevated IGFBP-1 plays a major role in hypoxia-caused IUGR by binding fetal IGFs and inhibiting their growth-promoting activities (3,16,17). This appealing model, however, has not been directly tested in vivo, and a causative relationship between the elevated IGFBP-1 expression and IUGR has not been established. Moreover, the impact of hypoxia on early developmental processes, such as morphogenesis, is poorly understood, and the role of IGFBP-1, if any, in mediating the hypoxic effects on embryonic development is unknown.The zebrafish has now become an informative vertebrate model organism for the study of IGF signaling in early development (23). Zebrafish embryos develop externally, eliminating the complication of maternal compensation. Fast developing and transparent zebrafish embryos make it possible to manipulate environmental factors and observe the phenotypic changes in organ formation in real time. Furthermore, major components of the zebrafish IGFsignaling pathway, including IGF ligands, receptors, IGFBPs, and intracellular signal transdu...

Follicle-stimulating hormone (FSH) plays important roles in spermatogenesis. However, the biologic activity of FSH can vary in different vertebrate classes, and the definitive function of FSH has not been established. In this study, we investigated the functions of FSH on spermatogenesis using an in vitro culture system for Japanese eel testis. The eel Fsh receptor was expressed in testis tissue during the whole process of spermatogenesis, mainly by Leydig cells that produce steroid hormones and by Sertoli cells surrounding type A spermatogonia and early type B spermatogonia. In an in vitro organ culture, recombinant eel Fsh (r-eFsh) induced complete spermatogenesis from the proliferation of spermatogonia to spermiogenesis during 36 days of culture; also, spermatozoa were observed in the testicular fragments. Spermatogenesis induced by r-eFsh was inhibited by trilostane, a specific inhibitor of 3beta-hydroxysteroid dehydrogenase. However, trilostane did not inhibit spermatogenesis induced by 11-ketotestosterone. These results clearly show that the main function of FSH in eel is to induce spermatogenesis via stimulating androgen production.