Expression levels of sex hormone receptors in brains of Japanese medaka, <I>Oryzias latipes</I> (Actinopterygii: Beloniformes: Adrianichthyidae)

Background. In teleost fishes, the brain is the target organ for sex steroid hormones. The actions of sex steroid hormones are mediated by their receptors and play an important role in the regulation of endocrine function in the brain. Japanese medaka, Oryzias latipes, is a species widely used in many fields of experimental biology, including neurobiology. In this study, we examined the mRNA expression levels of androgen and estrogen receptors in medaka brains. Materials and methods. The brains of adult fish were separated into three parts (forebrain, midbrain and hindbrain). The expression levels of androgen receptor (AR) and estrogen receptor (ER) beta from each part of the brain were determined using a semi-quantitative RT-PCR analysis. Results. AR and ER beta levels in males were higher in the forebrain and midbrain than in the hindbrain. In females, AR and ER beta levels were higher in the forebrain than in the midbrain and hindbrain. AR levels in the forebrain and midbrain of males were higher than those of females. Conversely, there was no difference in ER beta level between males and females. Conclusion. These data on hormone receptors provide the foundation for understanding the molecular basis of AR and ER beta mRNA expression levels in medaka brains. In addition, our results suggest that, in Japanese medaka, AR, but not ER beta, expression may exhibit sexual dimorphisms between males and females in the forebrains and midbrains.