Optogenetic ion pumps differ with respect to the secondary pattern of K+ redistribution

We recently reported that strong activation of the optogenetic chloride pump, Halorhodopsin leads to a secondary redistribution of K+ ions into the cell, through tonically open, leak K+ channels. Here we show that this effect is not unique to halorhodopsin, but is also seen with activation of another electrogenic ion pump, archaerhodopsin. The two opsins differ however in the size of the rebound rise in extracellular potassium, [K+]o, after the end of activation, which is far larger with halorhodopsin than for archaerhodopsin activation. Multiple linear regression modelling indicates that most of the variance in the post-illumination surge in [K+]o was explained by the type of opsin, and almost nothing by the size of the preceding, illumination-induced drop in [K+]o. These data provide additional support for the hypothesis that intense chloride-loading of cells, as occurs naturally following intense bursts of GABAergic synaptic bombardment, or artificially following halorhodopsin activation, is followed by extrusion of both Cl- and K+ coupled together. We discuss this with respect to the pattern of [K+]o rise that occurs at the onset of seizure-like events. ### Competing Interest Statement The authors have declared no competing interest.