Antagonistic surround responses in different cones are mediated by feedback synapses from different horizontal cells

Cone photoreceptors are the first neurons along the visual pathway that exhibit center-surround antagonistic receptive fields, the basic building blocks for spatial information processing in the visual system. The surround responses in cones are mediated by the horizontal cells (HCs) via multiple feedback synaptic mechanisms. It has been controversial on which mechanisms are responsible for the surround-elicited depolarizing responses in cones (ΔV_Cone(s)), and whether the surround responses of various types of cones are mediated by the same HC feedback mechanisms. In this report, we studied ΔV_Cone(s)) of four types of cones in the salamander retina, and found that they are mediated by feedback synapses from A-type, B-type or A- and B-type HCs. ΔV_Cone(s) are observable in the presence of concomitant center light spots, and surround + center light stimuli of various intensity, size and wavelength differentially activate the feedback synapses from A- and B-type HCs to cones. We found that ΔV_Cone(s) of the L-cones are mediated by both A- and B-type HCs, those of the P- and S-cones by B-type HCs, and those of the A-cones by the A-type HCs. Moreover, our results suggest that B-type HCs mediate ΔV_Cone(s) through both GABAergic and GluT-ClC feedback synaptic mechanisms, and A-type HCs mediate ΔV_Cone(s) via the GluT-ClC feedback mechanism. Feedback synaptic mechanisms that increase calcium influx in cone synaptic terminals play important roles in mediating the antagonistic surround responses in the postsynaptic bipolar cells, but they may not generate enough current to depolarize the cones and significantly contribute to ΔV_Cone(s).