Ilya Buldyrev


August 2010

Document Type


Degree Name



Oregon Health & Science University


Action potentials in distinct classes of retinal ganglion cell (RGC) encode salient features in the visual environment. While the firing properties of many RGCs are well described, much less is known about the underlying synaptic mechanisms. NMDA receptor mediated excitation of RGCs has been reported, but the physiological significance remains unclear. Here we examine light-evoked synaptic inputs to the center receptive field of OFF brisk-sustained (OFF-BS) RGCs in the rabbit retina and determine the roles of NMDA, non-NMDA and inhibitory inputs in generating the spike output. We confirm that the excitatory inputs to these neurons are mediated by both NMDA and AMPA inputs. The analysis indicates that the NMDA and AMPA receptors are segregated to different synapses, which was evident from a slower rate of contrast adaptation for the NMDA inputs relative to AMPA inputs, and differential effects when GABAergic transmission was blocked - the NMDA inputs were suppressed, while AMPA inputs were enhanced. Moreover, NMDA antagonists blocked relatively more of the action potentials, and excitatory conductance at low than at high contrasts. OFF-BS cells also receive a dis-inhibitory glycinergic input driven by the ON pathway, which enhances contrast-sensitivity at high temporal frequencies, but produces little effect at low temporal frequencies. Thus, we demonstrate two novel retinal circuits that regulate contrast and temporal frequency sensitivity of a specific retinal ganglion cell type. The first relies on separate, NMDA receptor-containing synapses to enhance sensitivity to low contrasts, whereas the second uses a phase-reversed inhibitory input to maintain contrastsensitivity at high temporal frequencies.




Neuroscience Graduate Program


School of Medicine



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