Light-induced changes in membrane current in cone outer segments of tiger salamander and turtle

The peak change in membrane conductance of vertebrate photoreceptors in response to the isomerization of a single photopigment has been estimated from recordings of membrane potential to be about 8 pS in rods and 6 pS in cones. For rods the estimate has been largely confirmed by Yau et al. by directly recording membrane current responses of rod outer segments to single photoisomerizations. However, no similar measurements have been reported for cone outer segments. Here, we report on direct recordings of membrane currents of single cone outer segments using an extracellular patch electrode. The technique was similar to that described by Yau et al. for toad rods. We have measured dark currents of up to 40 pA, and calculate that the conductance change resulting from a single photoisomerization is less than 1 pS.