Published online before print 17 October 2002.

Prolactin (PRL) in fish is considered to be an osmoregulatory hormone, although some studies suggest that it may influence the production of steroid hormones in the gonads. The objective of the present study was to establish if PRL is involved in repro-duction of the gilthead seabream—a protandrous hermaphro-dite. Adult and juvenile gilthead seabream received implants of estradiol-17b (E2) for 1 wk during the breeding season, and the mRNA expressions of PRL and PRL receptor (sbPRLR) were de-termined. Northern blot analysis revealed a single pituitary PRL transcript, the expression of which was significantly reduced by E2 treatment in adults but significantly increased in juvenile fish. In adult gonads, four sbPRLR transcripts of 1.1, 1.3, 1.9, and 2.8 kilobases were observed. A competitive reverse transcrip-tion-polymerase chain reaction was developed and used to de-termine how E2 treatment alters expression of the gonadal sbPRLR gene. Seabream PRLR was detectable in all samples an-alyzed by this assay. Levels of sbPRLR mRNA increased signifi-cantly (50-fold) after E2 treatment in adults, but a 24-fold de-crease was measured in juveniles. Immunohistochemistry using specific polyclonal antibodies raised against an oligopeptide from the extracellular domain of sbPRLR detected the receptor in spermatogonia and oocytes. Taken together, the preceding re-sults suggest that in the seabream, PRL may act on both testis and ovary via its receptor and that the stage of maturity influ-ences this process. The full characterization and relative impor-tance of the different transcripts of sbPRLR in eliciting the action of PRL in the gonads remain to be elucidated. estradiol, pituitary, prolactin, prolactin recepto