Optogenetic determination of dynamic and cell-type-specific chloride equilibrium potentials

Optogenetics has revolutionized neurobiological research by providing tools for modulating neuronal activity. As these tools utilise light-activated ion fluxes, they afford new opportunities to examine the nature of transmembrane ion gradients. Traditional investigation into the equilibrium potential for chloride (ECl) has been limited to studying endogenous chloride-permeable receptors. Here we demonstrate the utility of using a light-activated chloride channel, stGtACR2, to probe somatic ECl in rodent. This agonist-independent optogenetic strategy is validated in vitro and in vivo , captures differences in ECl dynamics following manipulations of endogenous chloride fluxes, and reveals distinct resting ECl across genetically-defined neuronal subpopulations. Using this approach to challenge chloride homeostasis, we uncover cell-specific ECl dynamics that are supported by the differential expression of endogenous handling mechanisms. Our findings establish an optical method for investigating transmembrane chloride gradients and thereby expand the repertoire of optogenetics. ### Competing Interest Statement The authors have declared no competing interest.