Effects of Rod and Melanopsin Loss on Nonvisual Light Responses in Rpe65 Knockout Mice

Abstract

Abstract: : Purpose: Rpe65 knockout mice are unable to regenerate 11–cis–retinaldehyde and as a result retain only a small degree of rod photoreceptor function. Because intrinsically photosensitive melanopsin (Opn4)–expressing retinal ganglion cells (ipRGCs), together with rods and cones, provide light information to the circadian system, our aim was to determine whether residual rod function or ipRGCs mediate circadian responses to light in Rpe65 knockouts. Methods: We generated mice carrying all possible combinations of Rpe65 and Opn4 wild type and knockout alleles combined with the presence or absence of the rdta transgene, which results in rapid postnatal loss of rod photoreceptors. Animals were tested for entrainment to light:dark cycles and circadian photosensitivity using a phase shift assay. Morphology and number of melanopsin–positive retinal ganglion cells were examined with immunocytochemistry in a subset of mice. Results: Rpe65–/– mice showed attenuated circadian photosensitivity, and numbers of melanopsin–positive ganglion cells were reduced in knockouts compared to controls. Surprisingly, the elimination of rods from the Rpe65–/– retina which occurs in Rpe65–/–;rdta mice, resulted in restoration of both circadian responses and number of melanopsin–containing cells to wild type levels. Rpe65–/–;Opn4–/– mice failed to show phase shifting responses to light, but in a light:dark cycle became robustly diurnal. Conclusions: These results demonstrate an important interaction between rods and melanopsin–ipRGCs, suggesting that rods in an inactive or "dark" state may inhibit the development and/or functioning of the melanopsin system. They further reveal a possible role for rods in light–dependent behavioral masking.

Keywords: circadian rhythms • transgenics/knock-outs • photoreceptors