Down-regulation of goldfish (Carassius auratus) prolactin gene expression by dopamine and thyrotropin releasing hormone.

Prolactin (PRL) is the most versatile hormone identified with multiple functions from osmoregulation in euryhaline fish to lactation in mammals. However, little is known about the basic physiological control of PRL in stenohaline freshwater fish. In this study, goldfish is used as a model for the study of PRL gene expression in stenohaline fish. We report herein the identification of the goldfish PRL (gfPRL) genomic sequence which possesses five exons with its 5'-flanking gene promoter region characterized in vitro using GH3 and GH4 cell-lines. Dopamine and thyrotropin releasing hormone were found to down-regulate the transcription of this gfPRL gene promoter in vitro. This was further confirmed by the decrease of PRL mRNA levels in vitro (in goldfish pituitary primary cells) and in vivo (intra-peritoneal injection) following the administrations of dopamine and thyrotropin releasing hormone. The Pit-1 binding sites of gfPRL are highly conserved with a consensus DNA sequence of 5'TATNCAT-3', as confirmed with the electrophoretic mobility shift assay using nuclear extract from the GH3 cell-line, may be responsible for the control of gfPRL gene promoter.