Neural interactions between cone photoreceptors and horizontal cells in the turtle (Mauremys caspica) retina

Horizontal cells and cone photoreceptors in the vertebrate retina are interconnected by a complex network of synapses leading to the generation of color-coded responses in chromaticity horizontal cells. A simple cascade model of excitatory feedforward and inhibitory feedback synapses had been suggested to underlie these observations. In this study, the photoresponses of cones and horizontal cells were recorded intracellularly from the turtle eyecup. Three different approaches were adopted in order to test quantitatively the cascade model. Comparing linearity functions between these neurons indicated multiple excitatory inputs to each type of horizontal cells. The depolarizing photoresponses of R/G C-type horizontal cells were considerably faster than those of L-type horizontal cells but slower than those recorded from L-cones. This observation disagrees with the basic assumption of the cascade model that assign the depolarizing photoresponses of R/G C-type horizontal cells to a negative feedback pathway from L-type horizontal cells onto M-cones. Finally, the action spectra of each of the three types of horizontal cells could not be solely accounted for by input from one spectral type of cones. Only by assuming excitatory and inhibitory inputs from all spectral types of cones, the action spectra of all types of horizontal cells could be reconstructed. These findings suggest that the negative feedback pathways from horizontal cells onto cones in the turtle retina cannot solely account for the chromatic properties of the horizontal cells and support a direct inhibitory inputs from cones to turtle horizontal cells.