The role of the Ca²⁺-activated Cl^- conductance in the membrane potential and light response of mouse rods

Abstract

To characterize the function of the Ca²⁺-activated Cl^- current I_Cl(Ca) in mammalian rod photoreceptors, we made patch-clamp recordings from retinal slices of mice (Mus musculus) of both sexes that lack Ano2 (TMEM16B). Depolarizing voltage ramps in solutions blocking K⁺ currents elicited a large outward current inhibited by the Cl^-- channel blocker niflumic acid; this current was absent in Ano2^-/- rods. The membrane potential of Ano2^-/- rods was 10 – 15 mV more depolarized in darkness than WT or Cx36^-/- rods, indicating a substantial resting Cl^- permeability. Rod outer segment photocurrents were similar in waveform and amplitude in Ano2^-/- and Cx36^-/- rods, but photovoltages in Ano2^-/- rods were nearly doubled. Measurements of light-response reversal potentials in rods with and without Ano2 suggest that the outer-segment conductance is nearly linear with a reversal potential of -9 mV; and that $$mathtex$$\Delta g_{Cl}$$mathtex$$ increases during the light response. Using these results, we estimated E_Cl from permeabilized-patch recordings of reversal potentials of Cx36^-/- rods to have a mean value of -35 mV near the rod resting potential, but other evidence suggests that E_Cl may be more positive by as much as 10 - 15 mV. Thus activation of I_Cl(Ca) during the light response would be depolarizing. At dim intensities, the photocurrents of downstream rod bipolar cells were larger and about twice as sensitive in Ano2^-/- retinas with reduced nonlinearity. These experiments show that Ca²⁺-activated Cl^- currents in mammalian rods have more important roles in photoreceptor physiology than previously appreciated.

Significance Statement To characterize the function of the Ca²⁺-activated Cl^- channel in mammalian rods, we recorded from Ano2^-/- mice with a disrupted channel gene. We show that these Cl^- channels make a surprisingly large contribution to the rod resting permeability. Moreover, measurements of reversal potentials indicate that light produces an increase in Cl^- conductance, which can only occur if the Ca²⁺ concentration near the channels is increasing even as the rod membrane potential is hyperpolarizing. We describe a novel method to make the first measurement of E_Cl in a mammalian rod, which is near to or somewhat positive of E_rest. Thus, channel activation would depolarize the rod. Ca²⁺-activated Cl^- channels have more important roles in mammalian photoreceptor physiology than previously appreciated.