All-optical reporting of chloride ion driving force in the nervous system.

Abstract Ionic driving forces provide the net electromotive force for ion movement across membranes and are therefore a fundamental property of all cells. In the nervous system, chloride driving force (DF Cl ) determines inhibitory signaling, as fast synaptic inhibition is mediated by chloride-permeable GABA A and glycine receptors. Here we present a new tool for all-Optical Reporting of CHloride Ion Driving force (ORCHID). We demonstrate ORCHID’s ability to provide accurate, high-throughput measurements of resting and dynamic DF Cl from genetically targeted cell types over a range of timescales. ORCHID confirms theoretical predictions about the biophysical mechanisms that establish DF Cl , reveals novel differences in DF Cl between neurons and astrocytes under different network conditions, and affords the first in vivo measurements of intact DF Cl in mouse cortical neurons. This work extends our understanding of chloride homeostasis and inhibitory synaptic transmission and establishes a precedent for utilizing all-optical methods to assess ionic driving force.