Flash Memory: Photochemical Imprinting of Neuronal Action Potentials onto a Microbial Rhodopsin

We developed a technique, flash memory, to record a photochemical imprint of the activity state-firing or not firing-of a neuron at a user-selected moment in time. The key element is an engineered microbial rhodopsin protein with three states. Two non-fluorescent states, D-1, and D-2, exist in a voltage-dependent equilibrium. A stable fluorescent state, F, is reached by a photochemical conversion from D-2. When exposed to light of a wavelength lambda(write), population transfers from D-2 to F, at a rate determined by the D-1 reversible arrow D-2 equilibrium. The population of F maintains a record of membrane voltage which persists in the dark. Illumination at a later time at a wavelength lambda(read) excites fluorescence of F, probing this record. An optional third flash; at a wavelength lambda(reset) converts F back to D-2, for a subsequent write-read cycle. The flash memory method offers the promise to decouple the recording of neural activity from its readout. In principle, the technique may enable one to generate snapshots of neural activity in a large volume of neural tissue, e.g., a complete mouse brain, by circumventing the challenge of imaging a large volume with simultaneous high spatial and high temporal resolution. The proof-of-principle flash memory sensors presented here will need improvements in sensitivity, speed, brightness, and membrane trafficking before this goal can be realized.