Fiber-coupled light-emitting diode for localized photostimulation of neurons expressing channelrhodopsin-2

Channelrhodopsin-2 (ChR2) is a blue-light-gated ion channel that can be used to stimulate genetically defined neurons reproducibly, rapidly and non-invasively. Existing approaches for delivering light to cells expressing ChR2 rely upon microscopes, lasers, arc lamps and shutters, all of which are relatively expensive and are not readily scalable for use on more than one brain region or animal at a time. In this paper, we describe an inexpensive method for delivering blue light locally and with millisecond precision to cells expressing ChR2. We accomplished this by coupling the light from a high-intensity blue light-emitting diode (LED; XLamp XR-E from CREE) into an optical fiber. When positioned in proximity to ChR2-expressing HEK293 cells, this fiber-coupled LED provided localized illumination of up to 32mW/mm2 and generated ChR2 photocurrents as efficiently as wide-field mercury arc lamp illumination. This fiber-coupled LED was also used to photostimulate action potentials in ChR2-expressing dorsal root ganglia (DRG) sensory neurons. LED light power and pulse frequency were controlled with an inexpensive, custom-built amplifier circuit. This scalable fiber-coupled LED system can be used to deliver light independent of the microscope objective and could, in principle, deliver light in parallel to multiple brain regions or to multiple genetically engineered animals.