Figure 1.

Activation of halorhodopsin in principal cell class induces strong modulation of extracellular [K⁺] during and after illumination. A, Recordings of extracellular [K⁺] in brain slices in which either halorhodopsin (blue, top) was expressed in all pyramidal cells, or in a slice with no opsin expression (black, bottom). Yellow bar represents the time of illumination (90 s period). Light activation of halorhodopsin induced a very marked reduction in [K⁺]_extra, followed by an even larger, rebound increase. Brain slices without opsins showed no light-induced fluctuations in [K⁺]_extra. B, The peak extracellular [K⁺], after the end of the period of halorhodopsin activation (mean peak = 7.98 mm). Lines represent mean +/– std.