Remodeling of inhibitory synapses at retinal bipolar cell terminals after loss of afferent input.

Abstract

Purpose : Bipolar cells in the retina receive photoreceptor information and are responsible for transferring this information to amacrine cells and ganglion cells in the inner retina. The axon terminals of bipolar cells make specialized ribbon synapses with amacrine and ganglion cell processes. Output at these ribbon sites is regulated by distinct subsets of inhibitory synapses. In this study our purpose was to determine the impact of photoreceptor loss on the organization of inhibitory synapses at retinal bipolar cell terminals.

Methods : To understand how retinal bipolar cells alter their inhibitory synaptic connections in response to loss of their primary input partners, we utilized the rd1 photoreceptor degeneration mouse line (mice homozygous for the rd1 mutation) and performed immunolabeling for distinct GABA receptor types. To visualize individual ON bipolar cells, we crossed the rd1 line to the Grm6-tdtomato line, where the mGluR6 promoter drives tdtomato expression in ON bipolar cells. We quantified receptor levels at the one-month and two-month time-point. Images were acquired on a Leica SP8 confocal microscope and stacks were analyzed with Image J (Fiji/NIH) and Amira (Thermo Fisher Scientific). Receptor amounts were determined within individual bipolar cell terminals and protein expression was correlated to mRNA levels by qPCR. We performed single-cell electrophysiology to determine the bipolar cell receptor type-specific responses to GABA puff applications. Markers for OFF bipolar cell types were used to compare synaptic protein levels in the ON vs OFF laminae of the degenerating retina.

Results : We observed bipolar cell type-specific changes in inhibitory synaptic proteins across terminals of the rd1 retina. We also observed compartment-specific alterations in bipolar cell axon terminals as compared to their dendrites in the outer retina. Protein levels of GABA receptor types showed differences across the receptor subsets and across the time points studied.

Conclusions : During retinal degeneration, bipolar cells undergo significant changes in their inhibitory synaptic connectivity and receptor organization. These changes are receptor-type specific, bipolar cell type-specific and compartment-specific.

This is a 2021 ARVO Annual Meeting abstract.