The distribution of expression of doublecortin (DCX) mRNA and protein in the zebra finch brain

Using in situ hybridization, we measured the distribution of expression of doublecortin (DCX), a microtubule-associated protein, in zebra finch adult and nestling (P9-11) brains. In adult brain, DCX mRNA was detected mainly in the mesopallium (M), medial striatum (MSt), septum, Area X, diencephalon, telencephalic subventricular zone (SVZ), and Purkinje cells in the cerebellum. The expression at posthatch day 9 (P9) was heavy in almost the entire telencephalon and showed heavier expression in SVZ and song regions such as the high vocal center (HVC) and the robust nucleus of arcopallium (RA). Outside of the telencephalon at P9, we found distinct label in nucleus ovoidalis (OV), nucleus spiriformis lateralis (SpL), and nucleus subpretectalis (SP) in the midbrain, almost the entire diencephalon including nucleus dorsomedialis posterior thalami (DMP), stratum griseum et fibrosum superficiale (SGF) in optic tectum, and Purkinje cells in cerebellum. Most of the heavily labeled areas by in situ hybridization overlapped with immunohistochemical staining for DCX, indicating that DCX mRNA is probably translated into protein in those regions. No sex difference was found in DCX expression at P9 or in the adult except that Area X was labeled only in the adult male. The intensity of expression in the adult was significantly lower than that at P9, which suggests a particular role for DCX in early song bird brain development. If DCX is predominantly expressed in migrating neurons, as suggested from studies in mammals, the present results offer no evidence for a sex difference in neuronal migration.