Imaging Membrane Potential via Second Harmonic Generation - Membrane potential for brainiacs

Abstract:

Understanding how the brain works is one of the great unsolved scientific challenges. We need a way of mapping the voltage changes in neurons, with high sensitivity, high spatial resolution and high temporal resolution. Microelectrodes are currently the primary method for measuring membrane potentials; they give excellent sensitivity and temporal resolution, but poor spatial resolution. Optical microscopy has the potential to revolutionize this field by allowing the non-invasive, real-time, high resolution imaging of voltages along individual neurons, within their native networks. At present, the most effective optical probes for membrane potential are fluorescent calcium indicators. However changes in calcium concentration do not accurately reflect voltage transients, and provide no information on the voltage waveform. Second harmonic generation (SHG) imaging has emerged as a powerful alternative. Push-pull chromophores orientated in the neuronal plasma membrane generate a high contrast signal that is sensitive to the local electric field. The high polarizability and intense optical transitions of porphyrins make them excellent candidates for engineering efficient SHG voltage-sensitive probes. Our studies on porphyrin-based voltage probes led to dyes which exhibit strong SHG and have high affinities for biological membranes. We plan build on these results to create a new series of voltage-sensitive porphyrin-based dyes for studying neuronal networks.