Abstract
Purpose: :
To define the spatial and temporal interactions between the excitatory and inhibitory pathways that lead to the unique functional properties of the Local Edge Detector (LED). The LED is a small receptive-field ganglion cell that generates sluggish ON-OFF responses to light flashes and displays edge-mediated center-surround antagonism.
Methods: :
We used whole-cell patch clamp to measure excitatory and inhibitory currents in response to flashed disks of varying diameter, and to luminance-neutral inverting gratings. Grating stimuli were designed to activate inner plexiform inhibitory activity by measuring the suppression of excitation elicited by a flashed spot place over the dendritic field. Care was taken to design gratings to bypass outer plexiform activation by being net luminance neutral and having stripe widths of 50 um, below the spatial resolution of horizontal cells. Specific feed forward and fed back excitatory or inhibitory currents were dissected using the pharmacological blockers SR95531, TPMPA, Strychnine, to block GABAa, GABAc, glycine, and APB to suppress ON bipolar activity.
Results: :
Feedback inhibition elicited by surround grating stimuli suppressed both ON and OFF center excitatory currents. This feedback was mediated at both GABAa and GABAc receptors along pathways with a spatial extent of 700um. Feedforward glycinergic inhibition at ON and OFF was elicited by stimuli that were spatially coincident with excitation over a region spanning the dendritic field of about 150 um. This inhibition was transient and effectively shunted the effect of excitatory currents for 150-200 ms. All synaptic components, including excitation, feedforward and feedback inhibition, responded to sub-receptive field features of approximately 50 um.
Conclusions: :
Feature sizes of approximately 50 um, about the dimension of bipolar dendrites, can activate excitatory and inhibitory components of the LED circuit. This suggests that the bipolar-to-LED and many of the amacrine-to-LED pathways are nonlinear. This may allow textured scene features to drive inhibition to shape the spatial and temporal responses of the LED. Specifically, the suppression of excitation by wide field ON and OFF feedback inhibition allows the LED to detect edges of textured areas in addition to general luminance edges. However, feedforward inhibition is driven by rapid changes in the center, essentially limiting responses to scene features which persist for more than 200 ms.
Keywords: retinal connections, networks, circuitry • ganglion cells • electrophysiology: non-clinical