Light-dependant intraretinal ion regulation by melanopsin in young awake and free moving mice evaluated with manganese-enhanced MRI

PURPOSE: To test the hypothesis that in young, functionally blind mice, light-dependent intraretinal ion regulation occurs via melanopsin. METHODS: Postnatal day (P) 7 wild type (WT, C57Bl/6) and melanopsin knockout (KO, opn4−/−, B6129) mice were light or dark adapted. Awake and freely moving animals were injected intraperitoneally (ip) with MnCl(2). Four hours later, the mice in both groups were anesthetized and studied with manganese-enhanced MRI (MEMRI) to measure the extent of intraretinal uptake of manganese and whole retinal thicknesses. RESULTS: In control P7 mice, light exposure increased (p<0.05) retinal manganese uptake over that in dark. This difference was observed throughout most of the retina. In P7 KO mice, intraretinal manganese uptake did not differ from that in age-matched dark-adapted WT mice, and was not light-dependent. No differences in whole retinal thickness were noted between groups. CONCLUSIONS: First time evidence is presented which demonstrates intraretinal ion regulation by melanopsin in vivo.