Distribution of estrogen receptor 2 mRNAs (Esr2a and Esr2b) in the brain and pituitary of the sea bass (Dicentrarchus labrax).

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.