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

In order to evaluate potential interactions between somatotropic and gonado tropic axes in rainbow trout (Oncorhynchus mykiss), changes in pituitary co ntent of the specific messenger RNA of growth hormone (GH) and gonadotropin (GTH) alpha- and beta-subunits were studied during gametogenesis with resp ect to pituitary and plasma hormone concentrations. Quantitative analyses o f 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 II beta, and GTH alpha increased moderately throughout most of gametogenesis a nd then more dramatically at spermiation and during the periovulatory perio d. GTH 1 beta mRNA increased first from stage I to V in males and more abru ptly at spermiation, while in females GTH 1 beta transcripts increased firs t during early vitellogenesis and again around ovulation. Pituitary GH abso lute content (mu g/pituitary, not normalized with body weight) increased sl owly during gametogenesis and more abruptly in males during spermiation. In the pituitary of previtellogenic females and immature males, GTH I beta pe ptide contents were 80- to 500-fold higher than GTH II beta peptide content s. GTH I contents rose regularly during the initial phases of vitellogenesi s and spermatogenesis and then more abruptly in the final stages of gonadal maturation, while GTH II contents show a dramatic elevation during final o ocyte growth and maturation, in postovulated females, and during spermiogen esis and spermiation in males. Blood plasma GTH II concentrations were unde tectable in most gonadal stages, but were elevated during spermiogenesis an d spermiation and during oocyte maturation and postovulation. In contrast, plasma GTH I was already high (approximate to 2 ng/ml) in fish with immatur e gonads, significantly increased at the beginning of spermatogonial prolif eration, and then increased again between stages III and VI to reach maxima l levels (approximate to 9 ng/ml) toward the end of sperm cell differentiat ion, but decreased at spermiation. In females, plasma GTH I rose strongly f or the first time up to early exogenous vitellogenesis, decreased during mo st exogenous vitellogenesis, and increased again around ovulation. Our data revealed that patterns of relative abundance of GTH I beta mRNA and pituit ary and plasma GTH I were similar, but not the GTH II patterns, suggesting differential regulation between these two hormones at the transcriptional a nd posttranscriptional levels. Pituitary and plasma GH changes could not be related to sexual maturation, and only a weak relationship was observed be tween GH and gonadotropin patterns, demonstrating that no simple connection exists between somatotropic and gonadotropic axes at the pituitary level d uring gametogenesis. (C) 1999 Academic Press.