A variety of applications, systems, methods and constructs are implemented for use in connection with screening of ion-channel modulators. Consistent with one such system, drug candidates are screened to identify their effects on cell membrane ion channels and pumps. The system includes screening cells having light responsive membrane ion switches, voltage-gated ion switches and fluorescence producing voltage sensors. A chemical delivery device introduces the drug candidates to be screened. An optical delivery device activates the light responsive ion switches. An optical sensor monitors fluorescence produced by the voltage sensors. A processor processes data received from the optical sensor. A memory stores the data received from the optical sensor.