Abstract
Purpose :
While rods and cones degenerate in retinitis pigmentosa and age-related macular degeneration, retinal ganglion cells (RGCs) and bipolar cells survive, providing multiple possible substrates for vision restoration. However, RGCs undergo physiological remodeling, exhibiting hyperactive firing. Here we ask if PR degeneration also affects the synapse between Type 6 ON-cone bipolar cells (CBC6) and ON αRGCs.
Methods :
We used rd1 (P60) and rd10 (P90) mice which exhibit photoreceptor degeneration. CBC6 cells were identified through selective genetic expression of ChR2/EYFP fusion protein. To measure voltage-gated voltage-gated Ca2+ current (ICa), we employed whole-cell patch clamp on CBC6 cells in slices. To measure synaptic transmission, we optogenetically stimulated CBC6 in flat-mount retinas and recorded excitatory post-synaptic currents (EPSCs) in ON αRGCs. To measure synaptic density, we visualized synaptic ribbons in CBC6 cells with immunohistological staining with anti-CTBP2 (RIBEYE).To block retinoic acid (RA) signaling, we intravitreally injected the RA receptor inhibitor BMS 493.
Results :
RIBEYE staining showed no change in the number or density of CBC6 synaptic ribbons adjacent to dye-filled ON αRGCs. However, the intrinsic ICa of CBC6 cells was 70% smaller in rd1 than in wild-type (WT) mice. Optogenetically evoked EPSCs recorded in ON αRGCs were ~75% and ~50% smaller in rd1 and rd10 respectively. Paired-pulse stimulation caused synaptic depression in WT but none in rd1 and rd10 retinas. RA has been implicated as a key trigger of retinal remodeling. Spatial representation of multiple CBC6 inputs onto an individual ON αRGC was unchanged in rd1 retinas, but temporal filtering was dramatically altered. Blocking RA signaling with BMS 493 restored both CBC6 ICa and synaptic transmission in rd1 mice.
Conclusions :
Photoreceptor degeneration dramatically alters signaling in the downstream ON pathway. Bipolar cells have less calcium current thereby reducing synaptic transmission. These changes will attenuate signals initiated by surviving photoreceptors or by vision restoration tools such as optogenetics, optopharmocology, stem cell therapy, and subretinal implants. Fortunately, blocking RA signaling may suppress remodeling, mitigating these vision-corrupting effects.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.