Purpose: Alterations in retinal ganglion cell response patterns were profiled in dystrophic Royal College of Surgeons (hereafter RCS) rats over the first 100 postnatal days as a baseline for retinal rescue and vision restoration strategies. This method enabled the evaluation of the extent to which postreceptoral neuronal attributes in degenerating retinas mirror inferred declines in photoreceptor function.
Methods: Single-unit responses from large retinal ganglion cells were recorded from age-matched dystrophic RCS (RCS-rdy(-)) and congenic RCS-p(+) (hereafter wild-type or wt) rats were recorded in vitro under visual control. Cells were profiled with conventional spatial and flux stimulus modulations.
Results: Ganglion cell single unit and population attributes alter slowly over the course of photoreceptor degeneration in dystrophic RCS rats, with significant decreases in apparent receptive field size, contrast sensitivity, and threshold sensitivity detected by the first month of life. Spatial frequency tuning and contrast responses were extremely weak by postnatal day (P)76, paralleled by a progressive decline in signal-to-noise (S-N) ratio to roughly unity by postnatal day (P)107. This decline was only a simple loss of responsivity, as background firing rates increased substantially over time. Whereas wt retinas were dominated by ON-center cells (15/23 cells), dystrophic animals were dominated by OFF-center cells by P47 (24/27 cells).
Conclusions: The first definitive signs of degeneration in dystrophic RCS rats are parallel decreases in ganglion cell threshold sensitivity and receptive field size, followed by deterioration in spatial summation. Arguably, these changes can be qualitatively explained as photoreceptor signaling losses. However, the apparent shift in population profile from ON- to OFF-center ganglion cells long before loss of the b-wave at P90 implies that a reactive mechanism such as bipolar cell rewiring and/or transformation of neuronal phenotypes occur during the early phase of photoreceptor stress, before rod and cone death.