Abstract
Purpose :
Glaucoma is characterized by progressive dysfunction of the retina; animal models shown that synapses between bipolar and ganglion cells are being dismantled early after intraocular pressure (IOP) elevation. We propose to investigate in detail, and on a large-scale, the relative arrangement of excitatory (E) and inhibitory (I) synapses in the inner retina, as well as in specific bipolar cell populations, to understand whether the degenerative process preferentially impacts specific retinal circuits.
Methods :
Transient IOP elevation was induced in albino CD1 mice by photocoagulation of the episcleral and limbal vessels. After 14 days, retinal whole mounts were collected and immunolabeled for the excitatory and inhibitory synaptic proteins PSD95, RibeyeA, Gephyrin, and VGAT. Individual bipolar cell types were immunolabeled using PKC-alpha and Synaptotagmin-2 antibodies. Pairwise statistical comparisons use Wilcoxon-Mann-Whitney test.
Results :
The average density of inhibitory (I) and excitatory (E) synapses in the inner plexiform player (IPL) of lasered eyes was reduced compared to control eyes (E: P=0.1, I: P=0.01), but their loss is non-uniform across the IPL, with a spatial pattern that is sublamina-specific. Axons of rod bipolar cells significantly lose both ribbon and gephyrin density (E: P=0.04, I: P=0.04), while Type-6 ON cone bipolar axons preserve both (E: P=0.6; I: P=0.6). In contrast, Type-2 OFF cone bipolar axons preserve their ribbon density (E: P=0.4) , while significantly increasing inhibitory inputs (I:,P=0.005).
Conclusions :
Excitatory and inhibitory synapses are unevenly disassembled in the inner retina, with more excitatory synapses lost in the OFF sublamina and more inhibitory synapses lost in the central region of the IPL<!-- I’m not sure which data above you are referring to with this phrase? -->. Within these distributions, specific bipolar cell types are either unaffected, equally affected, or asymmetrically affected in their E/I balance. These findings will help focus the investigation of specific cell types and microcircuits experiencing high synapse loss and ultimately guide the design of stimuli to probe their function in early glaucoma.
This is a 2021 ARVO Annual Meeting abstract.