Reduced pupil light response to red light correlates with cone defect in Rho^-/- mice

This study characterizes retinal degeneration progression in Rhodopsin knockout (Rho^-/-) mouse model from 1 to 12 months, comparing molecular markers with functional measures from electroretinogram (ERG) and pupil light response recordings. ERG analyses confirmed the absence of rod activity and a significant decline in photopic responses, indicating cone dysfunction that became undetectable after 3 months. Similarly, immunohistology showed cone alteration at 3 months. Gene expression analysis by RT-QPCR revealed a rapid decrease of rod transducin transcript already from 1 month, while the decline in the cone marker Gnat2 was delayed until 6 months. Western blot analysis indicated an earlier reduction in GNAT2 cone protein expression, detectable from 1 month with a significant drop at 2 months. Gene and protein expression of melanopsin, the photopigment of intrinsically photosensitive retinal ganglion cells (ipRGCs), remained stable during rod degeneration, protein level decreasing only at the later ages of 8 and 12 months. Consistent with these findings, pupil response to high-intensity blue stimuli remained stable across age groups, reflecting ipRGCs stability. However, the early dynamics of the pupil response to red and lower blue light stimuli exhibited progressive alterations after 3 months, correlating with cone marker decline. A significant decrease in the derivative area under the curve (dAUC) for low-intensity blue light and low to medium-intensity red stimuli was observed after 3 months, indicating reduced cone contribution with age. This study highlights the correlation between structural and functional cone decline and the maintenance of ipRGCs post-photoreceptor degeneration, providing insights for future therapeutic approaches.