Optimizing the spatial resolution of Channelrhodopsin-2 activation.

Over the past few years, the light-gated cation channel Channelrhodopsin-2 (ChR2) has seen a remarkable diversity of applications in neuroscience. However, commonly used wide-field illumination provides poor spatial selectivity for cell stimulation. We explored the potential of focal laser illumination to map photocurrents of individual neurons in sparsely transfected hippocampal slice cultures. Interestingly, the best spatial resolution of photocurrent induction was obtained at the lowest laser power. By adjusting the light intensity to a neuron's spike threshold, we were able to trigger action potentials with a spatial selectivity of less than 30 microm. Experiments with dissociated hippocampal cells suggested that the main factor limiting the spatial resolution was ChR2 current density rather than scattering of the excitation light. We conclude that subcellular resolution can be achieved only in cells with a high ChR2 expression level and that future improved variants of ChR2 are likely to extend the spatial resolution of photocurrent induction to the level of single dendrites.