Controlling the number of melanopsin-containing retinal ganglion cells by early light exposure

•Early light exposure increases the number of melanopsin immunopositive ipRGCs.•Light exposure affects ipRGCs number only in early development.•The increase of ipRGC number induced by constant light is spiking activity dependent.

A small percentage of retinal ganglion cells (RGCs) express melanopsin and are intrinsically photosensitive (ipRGCs). Whether light can affect the development of ipRGCs is not clear. In the rat retina, we found constant light exposure during the first postnatal week significantly increased the number of melanopsin immunopositive ipRGCs. This increase was durable and specific for melanopsin immunopositive ipRGCs. BrdU labeling showed no proliferation of the melanopsin immunopositive ipRGCs during constant light exposure. Retrograde labeling from the superior colliculus showed that no other types of RGCs were induced to express melanopsin. Light exposure was effective in increasing melanopsin immunopositive ipRGCs only when it coincided with the apoptotic phase of RGC development. However, daily intravitreous injection of tetrodotoxin, blocking action potentials, abolished the light induced increase of melanopsin immunopositive ipRGCs. These findings indicate that early light exposure can increase the number of melanopsin immunopositive ipRGCs through a process dependent on intrinsic photosensitive spiking activity. Furthermore, the increase of melanopsin immunopositive ipRGCs is potentially induced by apoptosis suppression in ipRGCs or enhanced expression of melanopsin.