Abstract
Purpose :
In primates, the OFF midget retinal ganglion cells (OFF mRGCs) have the highest spatial density and the smallest dendritic arbors. Their axons provide the input to the parvocellular pathway mediating the highest acuity vision. This study aimed to understand the basis for their light responses by identifying the presynaptic amacrine and bipolar cells.
Methods :
Central retinal tissue from an adult macaque was processed for serial block-face scanning EM, and a volume of images of the inner retina located 2 mm temporal to the center of the fovea was analyzed using Viking software. Neurons were visualized using an open-source Matlab program (SBFSEM-tools) and identified morphologically.
Results :
Ten OFF mRGCs and many of their presynaptic cells were reconstructed. Both midget and diffuse types of bipolar cells provided input, as described by Tsukamoto and Omi (2015). Presynaptic narrow-field amacrine cells included AII, knotty type 2, and knotty bistratified type 1 (vGluT3) types of the macaque retina. A major input came from narrow-field amacrine cells that ramified in strata 2, 3 and 4; these resembled the A4 cells of the human retina. Other presynaptic narrow-field amacrine cells resembled human A8 cells. Axons and long dendrites of wide-field amacrine cells provided significant input. Some could be traced to somas of A13, wiry type 1, wispy, and semilunar type 1 amacrine cells. The presynaptic amacrine and bipolar cells also made synapses with other types of retinal ganglion cells, a finding suggesting that the amacrine and bipolar cells presynaptic to OFF midget ganglion cells are not specialized for this role.
Conclusions :
Most narrow-field amacrine are expected to provide glycinergic inhibition to the OFF mRGCs. The excitatory synapses from vGluT3 cells and diffuse bipolar cells are expected to generate OFF MGC receptive field centers larger than their dendritic arbors. The synapses from wide-field amacrine cells are expected to generate the extra-classical surrounds of their receptive fields and provide a tonic, inhibitory input mediated by GABA. Degenerative diseases of the peripheral retina would reduce these inputs to central OFF mRGCs, and this may compromise visual function in these patients.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.