Distribution of the orphan nuclear receptor Nurr1 in medaka (Oryzias latipes): Cues to the definition of homologous cell groups in the vertebrate brain

The orphan nuclear receptor Nurr1 has been extensively studied in mammals a nd shown to contribute to the differentiation of several cell phenotypes in the nervous and endocrine systems. In this study, the gene homologous to t he mammalian Nurr1 (NR4A2) was isolated in the teleost fish medaka (Oryzias Latipes), and the distribution of its transcripts was analyzed within brai ns of embryos and adults. Nurr1 has a widespread distribution in the medaka brain. Large amounts of Nurr1 transcripts were found in the intermediate n ucleus of the ventral telencephalon, preoptic magnocellular nucleus, ventra l habenula, nucleus of the periventricular posterior tuberculum, and nuclei of glossopharyngeal and vagus nerves. To search for homologous cell groups between teleost fish and tetrapods brains, the colocalization of Nurr1 and tyrosine hydroxylase (TH) transcripts was analyzed. Neither Nurr1 nor TH e xpression was detected in the ventral midbrain, but both transcripts were p resent in the periventricular nucleus of the posterior tuberculum. This obs ervation supports the hypothesis that this nucleus is homologous to dopamin ergic mesencephalic nuclei of mammals. The presence of Nurr1 in the preopti c magnocellular nucleus of medaka and paraventricular hypothalamic nucleus of mammals reinforces the hypothesis of homology between these areas. TH an d Nurr1 transcripts are also co-localized, among others, in the nucleus of the paraventricular organ and nucleus of the vagus nerve. This work suggest s that the differentiating role of Nurr1 in the central nervous system is c onserved in gnathostomes.