All-optical reporting of inhibitory receptor driving force in the nervous system

Ionic driving forces provide the net electromotive force for ion movement across receptors, channels, and transporters, and are a fundamental property of all cells. In the nervous system, fast synaptic inhibition is mediated by chloride permeable GABA_A and glycine receptors, and single-cell intracellular recordings have been the only method for estimating driving forces across these receptors (DF_GABAA). Here we present a tool for quantifying inhibitory receptor driving force named ORCHID: all-Optical Reporting of CHloride Ion Driving force. We demonstrate ORCHID's ability to provide accurate, high-throughput measurements of resting and dynamic DF_GABAA from genetically targeted cell types over multiple timescales. ORCHID confirms theoretical predictions about the biophysical mechanisms that establish DF_GABAA, reveals differences in DF_GABAA between neurons and astrocytes, and affords the first in vivo measurements of intact DF_GABAA. This work extends our understanding of inhibitory synaptic transmission and demonstrates the potential for all-optical methods to assess ionic driving forces.