Optogenetics are used to activate or silence specific sets of neurons, intracellular signaling pathways, or to examine brain structure at an unprecedented detail. The only optogenetic approach lagging behind all others is the genetically encoded optical monitoring of multi-neuronal activity at a time resolution of single action potentials. Such ultrafast (<1 ms) resolution is important to understand network function in healthy and diseased nervous systems because timing of neuronal firing and synchrony are the foremost determinants of brain function. Techniques exist to measure membrane voltage and/or cellular activity using optical probes, but all have drawbacks either in their genetic targeting, optical sensitivity and/or temporal resolution. Recent developments using genetically encoded hybrid voltage sensor (GEVOS) methodology showed that this approach has an excellent potential to become an ultrafast voltage sensing system. The GEVOS technique can easily be adapted to work with multiple colors simultaneously, thus recording the activities of genetically distinct cell types in the same preparation. The overall objective of this proposal is to advance the GEVOS method so that it can be used with multiple colors simultaneously in at least two different genetically targetable cell types. Two major advances are sought after in this proposal: a technical/ methodological innovation (improve upon the GEVOS technique and extend it to two fluorescent proteins) and a scientific vision. The latter relates to gaining insights into the parallel functioning of local microcircuits and the optical recording of the concurrent behavior of pre- and postsynaptic elements during GABAergic inhibition. These studies will advance high temporal resolution optical voltage sensing (the other side of optogenetics) and will provide an unprecedented look at the functioning of cortical microcircuits with their specific components monitored at the same time..
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