Early Appearance of Nonvisual and Circadian Markers in the Developing Inner Retinal Cells of Chicken

Abstract

The retina is a key component of the vertebrate circadian system; it is responsible for detecting and transmitting the environmental illumination conditions (day/night cycles) to the brain that synchronize the circadian clock located in the suprachiasmatic nucleus (SCN). For this, retinal ganglion cells (RGCs) project to the SCN and other nonvisual areas. In the chicken, intrinsically photosensitive RGCs (ipRGCs) expressing the photopigment melanopsin (Opn4) transmit photic information and regulate diverse nonvisual tasks. In nonmammalian vertebrates, two genes encodeOpn4: theXenopus(Opn4x) and the mammalian (Opn4m) orthologs. RGCs express bothOpn4genes but are not the only inner retinal cells expressingOpn4x: horizontal cells (HCs) also do so. Here, we further characterize primary cultures of both populations of inner retinal cells (RGCs and HCs) expressingOpn4x. The expression of this nonvisual photopigment, as well as that for different circadian markers such as the clock genesBmal1,Clock,Per2, andCry1, and the key melatonin synthesizing enzyme, arylalkylamineN-acetyltransferase (AA-NAT), appears very early in development in both cell populations. The results clearly suggest that nonvisual Opn4 photoreceptors and endogenous clocks converge all together in these inner retinal cells at early developmental stages.