Raimondo, J.V., et al. Nat. Neurosci. advance online publication (24 June 2012).

Silencing neurons using light-driven pumps is known to be specific, fast and effective, but the consequences of silencing on neuron physiology remain under investigation. Raimondo et al. tested the effect of two of the most common optogenetic silencers—the light-driven outward proton pump Arch and inward Cl− pump NpHR3.0—on cells after the light-activation period. They monitored the electrophysiological properties of hippocampal neurons in rat brain slices before and after light-induced silencing, and they found that activation of NpHR3.0 increased the probability of spiking after photoactivation in the silenced cells. This effect was not observed using Arch and is thought to be due to the fact that high Cl− currents alter GABAergic transmission in these cells. These findings might help with the design of experiments that use these optogenetic tools.