Different types of synapses with different spectral types of cones underlie color opponency in a bipolar cell of the turtle retina

Electrophysiologically, color-opponent retinal bipolar cells respond with o pposite polarities tb stimulation with different wavelengths of light. The origin of these different polarities in the same bipolar cell has always be en a mystery. Here we show that an intracellularly recorded and HRP-injecte d, red-ON, blue/green-OFF bipolar cell of the turtle retina made invaginati ng (ribbon associated) synapses exclusively with L-cones. Non-invaginating synapses resembling wide-cleft basal junctions were made exclusively with M -cones. Input from S-cones was not seen. From these results we suggest sign -inverting transmission from L-cones at invaginating synapses via metabotro pic glutamate receptors, and sign-conserving transmission from M-cones at w ide-cleft basal junctions via ionotropic receptors. To explain the pronounc ed blue sensitivity of the bipolar cell. computer simulations were performe d using a sign-conserving input from a yellow/blue chromaticity-type (H3) h orizontal cell. The response properties of the red-ON, blue/green-OFF bipol ar cell could be quantitatively reproduced by this means. The simulation al so explained the asymmetry in L- and M-cone inputs to the bipolar cell as f ound in the ultrastructural analysis and assigned a putative role to H3 hor izontal cells in color processing in the turtle retina.