Abstract
Purpose: :
Our goal was to determine the synaptic interactions underlying cone opponent receptive field structure in the trichromatic primate retina.
Methods: :
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.
Results: :
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.
Conclusions: :
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.
Keywords: ganglion cells • color vision • synapse