Effects of salmon gonadotropin-releasing hormone on follicle stimulating hormone secretion and subunit gene expression in coho salmon (Oncorhynchus kisutch)

Previous work has indicated that, during the process of gametogenesis in sa lmon, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are d ifferentially synthesized and released. Although substantial information is available on the regulation of LH in many fish species, relatively little is known about the regulation of FSH biosynthesis and secretion or the regu lation of two types of cr subunit in salmon. In this study, the effects of salmon gonadotropin-releasing hormone (sGnRH) on in vitro secretion of FSH, and alpha 1, alpha 2, LH beta, and FSH beta subunit gene expression were i nvestigated in coho salmon (Oncorhynchus kisutch) using primary pituitary c ell cultures. To quantify FSH beta, LH beta, alpha 1, and alpha 2 subunit t ranscript levels, a multiplex RNase protection assay (RPA) was developed. P robes for the beta subunits of coho salmon FSH and LH were available from p revious studies. To generate probes for the alpha subunit RPAs, alpha 1 and alpha 2 subunit cDNAs were cloned using reverse transcriptase PCR. Release of FSH and LH into cell culture medium was quantified by radioimmunoassays . The effects of sGnRH on gonadotropin release and gene expression were tes ted at two points during the spring (April and May) prior to spawning in th e autumn; a period when plasma and pituitary FSH levels are increasing and females are in early stages of secondary oocyte growth. In both experiments , sGnRH increased steady-state mRNA levels of FSH beta, alpha 1, and alpha 2, whereas LH beta mRNA levels were not detectable. Secretion of FSH was st imulated by sGnRH in a concentration-dependent manner. Medium LH was not de tectable in the first experiment (April) and was measurable only after sGnR H treatment in the second experiment (May). Control levels of medium FSH an d transcripts for FSH beta and alpha subunits increased approximately fourf old between April and May, whereas alpha 2 transcript levels remained relat ively constant, suggesting that the seasonal increase in FSH release may in volve increased production of alpha 1. Therefore, sGnRH has direct stimulat ory effects on both secretion of FSH and FSH subunit biosynthesis, most lik ely due to increased transcription. However, alterations in rates of transc ript degradation cannot be ruled out. (C) 2000 Academic Press.