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 generat ion of color-coded responses in chromaticity horizontal cells. A simpl e cascade model of excitatory feedforward and inhibitory feedback syna pses had been suggested to underlie these observations. In this study, the photoresponses of cones and horizontal cells were recorded intrac ellularly from the turtle eyecup. Three different approaches were adop ted in order to test quantitatively the cascade model. Comparing linea rity functions between these neurons indicated multiple excitatory inp uts 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. T his observation disagrees with the basic assumption of the cascade mod el that assign the depolarizing photoresponses of R/G C-type horizonta l cells to a negative feedback pathway from L-type horizontal cells on to 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 s pectral type of cones. Only by assuming excitatory and inhibitory inpu ts from all spectral types of cones, the action spectra of all types o f horizontal cells could be reconstructed. These findings suggest that the negative feedback pathways from horizontal cells onto cones in th e 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.