title: Pituitary gonadotropin and testicular gonadotropin receptor expression in Atlantic cod (Gadusmorhua L.) during the first reproductive season: Effects of photoperiod modulation.
abstract: Pituitary mRNA levels of gonadotropin β-subunits and of their cognate receptors in the testis were studied during puberty in Atlantic cod under normal and experimental photoperiod conditions that suppressed, delayed or accelerated testis maturation. Results are discussed in context with changes in testicular histology and plasma androgen levels, considered as end points of gonadotropic regulation. Up-regulation of fshb was closely associated with the onset of puberty, decreased when spermatogenesis was completed and reached minimum levels after spawning. These results demonstrate, for the first time using an experimental approach, that activation of Fsh-dependent signaling is associated with spermatogonial proliferation and formation of spermatogenic cysts. Changes in fshr expression were less prominent and could be explained by changes in the cellular composition and RNA content of cod testis tissue. At more advanced stages of development (spermiogenesis, spermiation and spawning), lhb and, one month later, lhcgr transcript levels increased and reached peak values in spawning fish, in a positive feedback loop involving plasma androgens and Lh/Lhcgr-dependent signaling. This loop was broken by a loss of lhb expression at the end of the spawning season. Continuous light (LL) from summer solstice, ~8 months prior to spawning, suppressed the start of testis maturation and the changes in gonadotropin and receptor mRNA levels, while LL from winter solstice initially up-regulated lhb and lhcgr expression, before resulting in a precocious termination of the spawning season and low expression of all four genes. Our studies provide experimental evidence for a clear functional discrimination of cod gonadotropins.
authors: Utrecht University, Faculty of Sciences, Department of Biology, Division Developmental Biology, P.O. Box 80058, 3508 TB Utrecht, The Netherlands.
authors: Fernanda Ferreira Loureiro
authors: FF
authors: de Almeida
authors: Eva
authors: E
authors: Andersson
authors: Christian
authors: C
authors: Mittelholzer
authors: Orjan
authors: O
authors: Karlsen
authors: Geir Lasse
authors: GL
authors: Taranger
authors: Rüdiger W
authors: RW
authors: Schulz