Three different estrogen receptors (ERs) have been characterized in teleost fish, i.e. Esr1, Esr2a and Esr2b. In this study we carried out in situ hybridizations in the brain and pituitary of the sea bass (Dicentrarchus labrax) to study the putative involvement of Esr2 subtypes in the control of gonadotropin secretion in fish. Our studies demonstrated that both receptors are expressed within the main hypophysiotrophic areas of the sea bass brain thus providing neuroanatomical basis for the involvement of Esr2 subtypes in the long (or indirect) regulatory feedbacks on pituitary function in the sea bass. The results revealed that Esr2b-mRNA distribution was restricted to the preoptic area and tuberal hypothalamus. On the contrary, Esr2a presented a widespread distribution and transcripts were detected within the ventral telencephalon, preoptic area, hypothalamus, thalamus, posterior tubercle, mesencephalon and rhombencephalon. New Esr2-expressing areas were described in all of the above areas. This paper is the first demonstration of Esr2a and Esr2b expression in the follicle-stimulating hormone beta-subunit (betaFSH)- and luteinizing hormone beta-subunit (betaLH)-expressing cells in the fish pituitary, thus suggesting the participation of both receptors in the direct effect of estrogen on the control of gonadotropin hormone synthesis.

Reproduction is controlled by two pituitary gonadotropin hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). This study reports the cloning, sequence analysis, and gene expression of gonadotropin (GTH) subunits from the European sea bass (Dicentrarchus labrax). The GTH subunits were cloned from a sea bass brain-pituitary cDNA library. The nucleotide sequences of the common alpha, the FSHbeta, and the LHbeta subunit cDNAs were 625, 521, and 591 base pair (bp) long, respectively, encoding for mature peptides of 94, 105, and 115 amino acids (aa), respectively. Sequence analysis showed that sea bass FSHbeta is more similar to higher vertebrate FSHbeta's (35-37%) than to LHbeta's (26-30%), whereas sea bass LHbeta is more similar to LHbeta's (40-53%) than to FSHbeta's (26-41%). Phylogenetic analysis of fish GTH sequences grouped the beta subunits into two groups, FSH and LH, distributed into four classes, corresponding to the accepted divisions of Elopomorphs, Ostariophysis, Salmonids, and Percomorphs. A dot-blot technique was developed to analyze GTH pituitary mRNA levels during the reproductive cycle of male sea bass. From October (initiation of gametogenesis) to February (spermiation), the expression of all three subunits in the pituitary increased in parallel, concomitantly with the gonadosomatic index (GSI) and the accumulation of LH protein in the pituitary, all values declining sharply at post-spermiation. This study demonstrates that the pituitary of sea bass contains two gonadotropin hormones and that both gonadotropins are probably involved in the control of gametogenesis, gamete maturation, and spermiation.