Synaptic Inputs to OFF Parasol Ganglion Cells in Macaque Retina: An Analysis Using Serial Blockface Scanning Electron...

Ganglion cells are the projection neurons of the retina, and there are multiple types that differ in their morphology, light responses and central projections. Parasol cells are one of the major retinal ganglion cell types in primates. The presynaptic bipolar cells have been well-characterized, but less is known about the amacrine cells that provide the majority of their inputs. The goal of this study was to identify the amacrine cells presynaptic to the OFF subtype of parasol cells.

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.

All the OFF parasol cells in the volume were reconstructed. All the synaptic inputs of two OFF parasol cells were analyzed. They received 80% or more of their input from amacrine cells and the remainder from bipolar cells, almost entirely from the Off diffuse type. Many of the presynaptic amacrine cells were reconstructed sufficiently to be classified as wide-field or narrow-field, and the latter type predominated. Five specific types of presynaptic amacrine cells were identified as AII, A4, knotty bistratified type 1, A13 and wiry type 1. Notably, the same types of amacrine cells are also presynaptic to OFF midget ganglion cells, another major type.

These findings suggest that differences between the light responses of midget and parasol ganglion cells likely arise from differences in the presynaptic bipolar cell types.