Doublecortin expression continues into adulthood in horizontal cells in the rat retina

The doublecortin (DCX) protein is associated with microtubules, and is essential for neuronal migration, differentiation, and plasticity. In mammals, it is expressed in developing neurons and new immature neuroblasts in the adult brain, but not generally in mature neurons. In the retina, doublecortin is detectable as early as embryonic day 15 (E15), is highly expressed between E18 and E20, and is poorly expressed postnatally. In this study, we investigated immunohistochemically the expression and cellular localization of doublecortin in the adult rat retina. Doublecortin was expressed in the outer plexiform layer (OPL), and in cells in the outer border of the inner nuclear layer (INL). No other layers were labeled by anti-doublecortin antibodies. In double-labeling experiments, doublecortin expression co-localized with the expression of the marker for horizontal cells, calbindin D. By contrast, the marker for immature neuroblasts, polysialylated neural cell-adhesion molecule, was not expressed in horizontal cells. These results suggest that either horizontal cells have the capacity to continuously remodel their neurites or doublecortin has a different function in horizontal cells from the control of neuronal plasticity that it is known to modulate other neurites. In addition, doublecortin might be an alternative molecular marker for horizontal cells in the adult rat retina.