Parasol and smooth monostratified ganglion cells in macaque retina

Abstract

Purpose : Parasol and smooth monostratified cells are two types of primate retinal ganglion cells that are similar in many respects and make important contributions to vision, particularly to the perception of motion. The goal of this study is to describe the neural circuits that provide their inputs.

Methods : In pieces of living, peripheral macaque retina, ON-type parasol and smooth monostratified ganglion cells were identified visually, injected with Neurobiotin and analyzed by confocal microscopy. A connectome from temporal, parafoveal macaque retina was generated using serial blockface scanning electron microscopy and annotated using the Viking software package. Parasol ganglion cells and 2 types of wide-field amacrine cells with somas in the ganglion cell layer (GCL) were reconstructed and identified by their distinctive morphology.

Results : The intracellular injection experiments showed that ON-type smooth monostratified ganglion cells were tracer-coupled to amacrine cells. All of them appeared to be the same type, wiry type 2 cells. Their somas were located in the GCL, and their long, straight terminal dendrites branched infrequently. They costratified with the dendrites of the injected smooth monostratified ganglion cells and made contacts with the dendrites of ON parasol cells. In the connectome, 5 adjacent ON parasol cells were reconstructed and identified by their characteristic morphology. One of the wide field amacrine cells was the large polyaxonal cell shown previously to be coupled to ON parasol cells, also known as semilunar type 2. This polyaxonal cell made a synapse onto one of the ON parasol cells, and many other amacrine cell dendrites with the same ultrastructure and large diameter also made synapses onto the ON parasol cells. Three other reconstructed cells were wiry type 2 cells, which made 4 synapses onto the ON parasol cells. Other amacrine cells with the same morphology and ultrastructure also made synapses onto the ON parasol cells.

Conclusions : ON-type parasol and smooth monostratified cells are coupled to different types of wide-field amacrine cells, and semilunar type 2 cells make both chemical and electrical synapses onto ON parasol cells. The wiry type 2 amacrine cells coupled to smooth monostratified cells are presynaptic to ON parasol cells. Taken together, the latter 2 findings suggest that the smooth monostratified cells inhibit ON parasol cells through their network of coupled wiry amacrine cells.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.