Feedback effects of horizontal cell membrane potential on cone calcium currents studied with simultaneous recordings

Horizontal cell (HC) to cone feedback helps establish the center-surround arrangement of visual receptive fields. It has been shown that HC activity influences cone synaptic output by altering the amplitude and voltage dependence of the calcium current (I_Ca) in cones. In this study, we obtained voltage-clamp recordings simultaneously from cones and HCs to directly control the membrane potential of HCs and thereby measure the influence of HC membrane potential changes on I_Ca in adjacent cones. Directly hyperpolarizing voltage clamped HCs produced a negative activation shift and increased the amplitude of I_Ca in cones. Both of these effects were abolished by enhancing extracellular pH buffering capacity with HEPES. In contrast, addition of the gap junction blocker, carbenoxolone, did not significantly alter the shifts or amplitude changes in cone I_Ca produced by changes in HC membrane potential. These results support the hypothesis that changes in the HC membrane potential alter the voltage dependence and amplitude of cone I_Ca by altering extracellular pH levels at the synapse.