Cell type-specific expression of FoxP2 in the ferret and mouse retina

Although the anatomical and physiological properties of subtypes of retinal ganglion cells (RGCs) have been extensively investigated, their molecular properties are still unclear. Here, we examined the expression patterns of FoxP2 in the retina of ferrets and mice. We found that FoxP2 was expressed in small subsets of neurons in the adult ferret retina. FoxP2-positive neurons in the ganglion cell layer were divided into two groups. Large FoxP2-positive neurons expressed Brn3a and were retrogradely labeled with cholera toxin subunit B injected into the optic nerve, indicating that they are RGCs. The soma size and the projection pattern of FoxP2-positive RGCs were consistent with those of X cells. Because we previously reported that FoxP2 was selectively expressed in X cells in the ferret lateral geniculate nucleus (LGN), our findings indicate that FoxP2 is specifically expressed in the parvocellular pathway from the retina to the LGN. Small FoxP2-positive neurons were positive for GAD65/67, suggesting that they are GABAergic amacrine cells. Most Foxp2-positive cells were RGCs in the adult mouse retina. Dendritic morphological analyses suggested that Foxp2-positive RGCs included direction-selective RGCs in mice. Thus, our findings suggest that FoxP2 is expressed in specific subtypes of RGCs in the retina of ferrets and mice.