The Rod Pathway Contributes to Aberrant Ganglion Cell Activity in Early Retinal Degeneration

Abstract

Purpose : Aberrant tonic and oscillatory spike activity emerges in retinal ganglion cells (RGCs) during the progression of inherited photoreceptor degenerations. This aberrant activity degrades residual vision and may impede the success of vision restoration. In this study, we tested whether the rod pathway drives aberrant activity in AII amacrine cells (AII-ACs) and RGCs during early degeneration in the rd10 mouse.

Methods : We used rd10 mice at postnatal (P) day 21-60, an age range when rod bipolar cells (RBCs) undergo morphological and functional changes. Age-matched C57BL6/J mice (WT) were used as controls. Whole-cell recordings were made from AII-ACs in retinal slices and spike recordings were made from alpha-like RGCs in wholemount. Two-photon imaging was used to record spontaneous calcium signals from RGCs after bulk-loading of Cal520 AM. The Ca²⁺-permeable AMPA receptor blocker, IEM1460 (IEM, 50 uM), was used to block RBC input to AII-ACs, while sparing cone-driven input. Paired and unpaired t-tests were used for statistical analyses.

Results : We observed periodic voltage fluctuations (frequency: ~3-12 Hz) in 14/20 AII-ACs in rd10 retina as early as P21-25, which were rarely seen in WT (1/10 AII-ACs). These fluctuations were suppressed by IEM in 6/9 rd10AII-ACs, suggesting they were driven in part by the rod pathway. Since AII-ACs relay signals to RGCs, we tested whether IEM could suppress aberrant RGC activity in the rd10 retina at a timepoint after complete rod degeneration (P44-55). On-type RGCs had higher spontaneous spike rates in rd10 compared to WT (rd10,31.3 Hz ±19.3, n=12 cells; vs WT, 3.6 Hz ±3.7, n=10 cells; mean±S.D., p=0.003). IEM significantly reduced spontaneous firing in rd10 On-RGCs (~62% suppression; n=12 cells, p=0.001) and unmasked residual cone-driven light-evoked activity. Ca²⁺ imaging in the rd10 (n=2 mice; P45-55) revealed spontaneous sustained Ca²⁺ events lasting >30s (~7.5% of 293 imaged cells), which were blocked by IEM. Similar spontaneous Ca²⁺events were absent from WT retina (n=2327 cells, 3 mice; P30-60).

Conclusions : Our results suggest that the rod pathway drives aberrant spontaneous excitatory activity in a sub-population of RGCs in the rd10 retina. Blocking the rod bipolar to AII-AC synapse may improve detection of residual cone signals in the degenerating retina by silencing much of the aberrant spontaneous activity.

This is a 2021 ARVO Annual Meeting abstract.