Cone loss in adult retina changes bipolar cell morphology and increases ganglion cell excitability

Abstract

Purpose : Complete photoreceptor death leads to disorganization of the retina, but early stages of this process are not understood. To understand the relationship between cone loss and retinal reorganization, we killed a subset of cones in adult mice and measured morphological and functional changes throughout the retinal circuit.

Methods : We used Grm6-tdTomato mice to visualize bipolar cells, crossed with mice expressing the diphtheria toxin receptor (DTR) under the cone opsin promoters. We injected these mice with diphtheria toxin at P30 to cause cell death (cones/mm²: control=11156±1657; DTR=5254±1885; p<0.0005). We used confocal imaging to assess morphology of type 6 cone bipolar (CB) cell dendrites, axon terminals, synaptic proteins, and pre- and post-synaptic partners. To assess retinal function, we made cell-attached and whole-cell patch-clamp recordings from ON-sustained ganglion cells (ON-S GCs) in response to 10ms flashes of rod or cone stimuli.

Results : After cone loss, type 6 CBs show a significant decrease in dendrite number (control=3.4±0.97; DTR=2.9±1.12; p<0.02) and a significant increase in dendrite length (µm: control=9.0±3.5; DTR=11.7±5.4; p<<0.0005) as compared to controls. Dendrites without cone contacts lose localization of the postsynaptic glutamate receptor (mGluR6) in dendritic tips (location of mGluR6 along the dendrite where soma=0, tip=1: control=0.85±0.16; DTR=0.68±0.31; p<0.0005). Imaging revealed no changes in the number of ribbons in type 6 CB axon terminals (control=92.9±17; DTR=92.7±14.7; p>0.05) and no differences in the density of postsynaptic PSD95 in ON-S GCs (puncta/µm²: control=0.30±0.02; DTR=0.35±0.05; p>0.05). In response to cone stimulation, intensity-response maximum excitatory currents onto ON-S GCs were on average 69% reduced in amplitude (pA: control=2023±918; DTR=632±492; p<0.02) while spike rates were on average 44% reduced (Hz: control=21.6±1.9; DTR=12.12±7.4; p<0.0005). In response to rod stimulation, maximum excitatory currents (control=1352±692; DTR=1652±916; p>0.05) and spike rates (control=94±44; DTR=72±22; p>0.05) were unchanged.

Conclusions : Within 60 days of cone death, type 6 CB dendrites show structural change. Cone loss affects ON-S GC current amplitude more than spike rate. Changes in the cone pathway do not impact the rod pathway. These results indicate that ON-S GCs may compensate for lost input with intrinsic scaling of excitability.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.