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M. van Wyk, W. R. Taylor, H. Wässle; Asymmetries Between the Receptive-Field Circuitries of On and Off A-Type Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3621.
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While the receptive-field properties of the On and Off A-type ganglion cells have been well characterised, the synaptic mechanisms underlying the light responses in both cell types remain to be elucidated. In this study we examined the neuronal circuits of the On and Off A-type cells in the isolated dark-adapted mouse retina.
Neurons with large somata were targeted for recording under IR-DIC optics. A-type cells were identified by their distinct extracellular responses to visual stimuli. The light-evoked changes in synaptic inputs were measured by whole-cell patch-clamp recording. The dendritic morphology of physiologically identified neurons was visualised by adding Lucifer Yellow to the intracellular solution in the patch pipette.
At mesopic light intensities (0.001-0.9 cd/m2), On-cells received a large tonic excitatory synaptic input (~10 nS); this excitation was suppressed by a decrease in light intensity and enhanced by an increase in light intensity over the receptive-field centre, with no light-evoked changes in inhibitory synaptic input. Off-cells, on the other hand, received considerable tonic glycinergic inhibition (~10 nS), which was either up- or down-regulated to generate light responses, with little contribution from excitatory inputs. Both cell types had relatively large dendritic trees (~300 um diameter) with dendrites stratifying in the On- or Off-sublaminae, immediately vitreal to the levels of the starburst amacrine cells.
Although light-evoked extracellular responses recorded from On and Off A-type cells were mirror symmetric, the synaptic mechanisms driving the spike responses were markedly different between cell types. Light responses in On-cells were mediated through postsynaptic excitation while responses in Off-cells were generated through postsynaptic inhibition.
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