Changes in axon fluorescence during activity: Molecular probes of membrane potential

TLDR: The fluorescence of dyes added to squid giant axons was studied during action potentials and voltage-clamp steps to find large changes in fluorescence that would allow optical monitoring of membrane potential in neurons and other cells.

Abstract: The fluorescence of dyes added to squid giant axons was studied during action potentials and voltage-clamp steps. One goal was to find fluorescence changes related to the increases in membrane conductance that underlie propagation. A second goal was to find large changes in fluorescence that would allow optical monitoring of membrane potential in neurons and other cells. Attempts were made to measure fluorescence changes using over 300 different fluorescent molecules and positive results were obtained with more than half of these. No evidence was found that would relate, any of the fluorescence changes to the increases in membrane conductance that accompany depolarization; most, instead, were correlated with the changes in membrane potential. The fluorescence changes of several dyes were relatively large; the largest changes during an action potential were 10−3 of the resting intensity. They could be measured with a signal-to-noise ratio of better than 10∶1 in a single sweep.