Origin of the Subunit Nonlinearity in Retinal Ganglion Cell Receptive Fields

Abstract

 

Purpose

 

Retinal ganglion cells signal visual information according to their receptive field properties. A fundamental property of many receptive fields, including those of alpha/Y-type ganglion cells (alpha cells), is a nonlinear subunit structure that encodes high-spatial frequency texture and motion. The subunits are thought to be the presynaptic bipolar cells, but the mechanism for the nonlinearity remains incompletely understood.

 

Methods

 

Here, we investigated the synaptic basis of the nonlinear subunits by combining whole-cell recording of mouse alpha cells and two-photon fluorescence imaging of a glutamate sensor (iGluSnFR; Marvin et al., Nat. Methods, 2013) expressed on their dendrites. ON and OFF alpha cells were targeted for whole-cell patch clamp recordings and filled with a fluorescent dye (red). We stimulated the retina with patterned light and simultaneously recorded evoked excitatory currents and iGluSnFR fluorescence responses (green) locally on the recorded cell's dendrites. We also screened spatio-temporal patterns of glutamate release broadly across the inner plexiform layer (IPL), by recording stimulus-evoked fluorescence responses from the ganglion cell layer to the amacrine cell layer at 2 µm intervals.

 

Results

 

The subunit nonlinearity of ON and OFF alpha cells originated at the level of the bipolar cell output, after the point of spatial integration of photoreceptor inputs by the bipolar cell (Fig.1). The nonlinearity differed between OFF and ON pathways: OFF synapses showed strong rectification, whereas ON synapses showed weak rectification but substantial temporal asymmetries in the rise and fall of glutamate release. ON synapses showed a TPMPA-sensitive inhibitory surround that could be activated by high-spatial frequency gratings (Fig.1C, arrowhead). We found nonlinear synaptic release at glutamate synapses at all levels of the IPL.

 

Conclusions

 

Nonlinear synaptic release from bipolar cells generates the spatial-subunit nonlinearity of Y-type ganglion cells. While the nature of the nonlinearity differs both between and within the ON and OFF layers, nonlinear synaptic release is ubiquitous across the IPL and can support nonlinear responses in wide-field ganglion cells in layers with transient as well as sustained synaptic release.

 

 

Figure 1. A Contrast-reversing grating (A) evoked frequency-doubled responses in alpha cells (B) but not in bipolar cells (C).

 

Keywords: 435 bipolar cells • 531 ganglion cells • 673 receptive fields