Figure 1. Activation of the Ca²⁺-dependent Cl⁻ current in individual cones decreases the size of light-induced feedback responses.

A, the cone membrane potential was clamped at −60 mV and stepped to a range of voltages from −80 mV to +30 mV in 10 mV steps. Left panel, whole-cell current traces in control conditions show that I_Cl(Ca) is a slowly activating current. Right panel, whole-cell current traces of a cone when I_Cl(Ca) is blocked by 100 μm niflumic acid. The slowly activating current is absent. B, average whole-cell I–V relations of cones after a pre-pulse of 1 min to either −20 mV (black trace, n= 5) or −80 mV (red trace, n= 5). From these curves both the peak amplitude and the half-activation potential of the I_Ca were determined. The peak amplitude did not differ significantly between the two V_Clamp values, but the half-activation potential of I_Ca was significantly shifted to positive potentials for a depolarized V_Clamp (−20 mV: −21.5 ± 1.6 mV; −80 mV: −26.6 ± 1.1 mV; n= 5; P= 0.01). C, average I–V relations of the pre-pulse-induced current (the difference between the current after the pre-pulse of −20 mV and −80 mV) in the presence or absence of niflumic acid. The pre-pulse-induced current (black trace) reverses at about the calculated value (−35 mV) for E_Cl (−33.9 ± 5.8 mV; n= 6), and can be partially blocked by 100 μm niflumic acid (red trace). D, activation if I_Cl(Ca) leads to changes in the communication between HCs and cones. Left panel, activation of I_Cl(Ca) leads to a sustained shift of I_Ca to positive potentials. Middle panel, the peak amplitude of I_Ca does not depend on the activation of I_Cl(Ca). Right panel, feedback responses in cones are significantly reduced when I_Cl(Ca) is activated. E, example of feedback responses to 100 ms full-field flash in a cone clamped at −40 mV, following either a pre-pulse of −20 mV (black trace) or −80 mV (red trace). A depolarizing pre-pulse significantly reduces the feedback responses compared with a hyperpolarizing pre-pulse (3.3 ± 0.8 pA vs. 16.6 ± 4.8 pA, respectively; n= 6; P= 0.021). F, the amplitude of light-induced feedback responses in cones at different clamping potentials and pre-pulse potentials. The amplitude of feedback is dependent on both the potential at which it is measured and the pre-pulse potential. Feedback is the largest for the most hyperpolarized pre-pulse.