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 opposite polarities to stimulation with different wavelengths of light. The origin of these different polarities in the same bipolar cell has always been a mystery. Here we show that an intracellularly recorded and HRP-injected, red-ON, blue/green-OFF bipolar cell of the turtle retina made invaginating (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 metabotropic glutamate receptors, and sign-conserving transmission from M-cones at wide-cleft basal junctions via ionotropic receptors. To explain the pronounced blue sensitivity of the bipolar cell, computer simulations were performed using a sign-conserving input from a yellow/blue chromaticity-type (H3) horizontal cell. The response properties of the red-ON, blue/green-OFF bipolar cell could be quantitatively reproduced by this means. The simulation also explained the asymmetry in L- and M-cone inputs to the bipolar cell as found in the ultrastructural analysis and assigned a putative role to H3 horizontal cells in color processing in the turtle retina.
Haverkamp
S
S
Neuroanatomy, Max-Planck-Institute für Hirnforschung, Frankfurt, Germany.
Möckel
W
W
Ammermüller
J
J
D000818:Animals; D016015:Logistic Models; D008854:Microscopy, Electron; D010775:Photic Stimulation; D012160:Retina / Q000166:cytology / Q000502:physiology*; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D013569:Synapses / Q000502:physiology*; D014426:Turtles / Q000502:physiology*