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J. D. Crook, M. Manookin, O. S. Packer, D. M. Dacey; Excitatory Synaptic Conductances Mediate ‘Blue-Yellow’ and ‘Red-Green’ Opponency in Macaque Monkey Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5178.
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© ARVO (1962-2015); The Authors (2016-present)
Our goal was to determine the synaptic interactions underlying cone opponent receptive field structure in the trichromatic primate retina.
We recorded from identified midget (long (L) vs middle (M) wavelength cone opponent) and small bistratified (short wavelength (S)-ON vs L+M-OFF cone opponent) ganglion cells in whole cell voltage clamp and measured the synaptic conductances evoked by selective modulation of the S, L or M cones.
For the bistratified S-ON ganglion cells our main goal was to directly determine whether the L+M-OFF response originates presynaptically via surround inhibition or postsynaptically by excitatory OFF-bipolar input. S cone increments elicited sustained ON-excitatory and ON-feedforward inhibitory conductances. No evidence was found for ‘crossover’ inhibition at S decrement consistent with the absence of S-OFF bipolar circuitry. L+M cone stimuli elicited sustained OFF-excitation and both feedforward and crossover inhibition. Bath application of L-AP4, the mGluR6 receptor agonist and GABAa/c (GABAzine/TPMPA) and glycine (strychnine) receptor antagonists abolished ON excitatory and all inhibitory currents but spared a clear L+M OFF-excitatory conductance presumably arising from known OFF bipolar synaptic inputs. Similarly for midget cells our goal was to determine whether L vs M cone antagonism arises pre- or postsynaptically or by some combination of both synaptic mechanisms. L and M cone selective stimuli modulated sustained excitatory L and M conductances in counterphase. Block of all inner retinal feedforward inhibition evoked by L and M cone stimuli spared and amplified clear L vs M opponent excitatory conductances.
For the blue-ON cell the S vs L+M receptive field arises postsynaptically by converging excitatory S-ON and L+M-OFF bipolar inputs to the bistratified tree. By contrast L vs M cone opponency arises presynaptically for the midget ganlgion cell presumably from the unequal distribution of randomly arranged L and M cones to the centers vs surrounds of midget bipolar receptive fields. Inner retinal inhibitory pathways appear to play little, if any, role in constructing either blue-yellow or red-green cone opponency.
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