Fig. 1

Surround illumination augments cone I _Ca. A I _Ca of a cone in newt retinal slice was recorded under the whole-cell voltage clamp condition at a holding potential of −40 mV. The Cs⁺-based pipette solution contained 20 mM BAPTA to minimize any Ca²⁺-activated currents. Depolarizing voltage steps from −50 mV to +8 mV (2 mV step) induced inward currents. I _Ca of a cone in newt retinal slice was recorded under the whole-cell voltage clamp condition at a holding potential of −40 mV. The I _Ca was abolished by extracellular cadmium, a blocker of Ca channels (plots with open squares shown in the right panel). Five representative traces, voltage-clamped at −40, −26, −24, −16 and −4 mV, are shown. Diffuse illumination (4,000 μm in diameter used for ‘surround illumination’: shorter bar) was applied every 4 s, while the spot illumination (30 μm in diameter: top bar) was maintained. An additional 2-mV depolarization was applied to mimic an ephaptic (field) effect (external voltage drop) after withdrawing the diffuse illumination. Note that at −4 mV (pink trace), diffuse illumination evoked an inward current, while a +2-mV pulse evoked an outward current. The current amplitude was sampled at the time indicated by the symbols to construct the I–V curves shown in B a and B b. B a Leak-subtracted I–V curve of cone I _Ca in the presence of the spot (filled squares) and during diffuse illumination (open squares). Inset shows activation curves fitted to the Boltzmann function derived from the I–V curves. B b Leak-subtracted I–V curve of cone I _Ca in the presence of the spot light (filled circles) and during a +2-mV depolarizing pulse (open circles). Inset shows activation curves fitted to the Boltzmann function derived from the I–V curves. Boxed inset isolation method of I–V curves of cone I _Ca obtained from a different cone in A. Top The I–V curvers were obtained in the control solution [filled squares (1)] and in a 3 mM Cd containing solution [open squares (2)]. Bottom (open circles) I–V curve of the cone I _Ca obtained by subtracting the I–V curve in a 3 mM Cd containing solution from that in the control solution [(1)–(2)]. Filled circles I–V curve obtained by subtracting the I–V curve from the extrapolated leakage current from that in the control solution [(1)–(3)] (from Hirasawa and Kaneko [28])