(A) A new method called PARIS can be used to study gap junctions between cells (black squares). It is a genetically encoded system in which one cell (the ‘actuator’) contains a light-gated proton pump (ArchT). If the actuator cell is coupled to another (‘receiver’) cell, shining light onto the pump (blue trace) causes protons (H+, black circles) to flow, generating a measurable electrical current (black trace, pA). (B) In an alternate detection method, a fluorescent sensor (pHluorin, green oval) can be placed in the receiver cell. Once activated, the pump transports protons out of the actuator cell; if the cells are coupled, this induces a change in the pH of the receiver cell that increases the fluorescence of the sensor (green trace). (C) PARIS can be used to determine whether gap junction coupling (GJC) exists between different cell types by using light (blue beam) to stimulate a particular type of neuron and checking for fluorescence (green) from another type of neuron. For example, Wu et al. showed that excitatory projection neurons (ePN) in the Drosophila brain (black shape, left hemisphere) form gap junctions with excitatory local neurons (eLN) but not with other nearby cell types (KC, glia, iLN).