Evidence for Membrane Potential Changes in Isolated Synaptic Membrane Ghosts Monitored with a Merocyanine Dye

Changes in the fluorescence intensity of merocyanine-540 were measured in suspensions of synaptic plasma membrane ghosts isolated from rat brain cortex. With preincubation of the membrane ghosts in isotonic KCl or NaCl solution, K- and Na-enriched ghosts samples were prepared. In suspensions of both sort of synaptic membrane ghosts, merocyanine-540 showed a fluorescence emission peak at a wavelength of 590 nm. Under a fixed total concentration of NaCl and KCl, high external K+ induced an increase in the fluorescence intensity, such an increase being proportional to logarithm of K+ concentration. Replacing K+ by Rb+, NH4+ or Cs+, a similar effect was observed. Rb+ was about as effective as K+; NH4 is about 2/3 and Cs+ 1/4 as effective. But the changes in fluorescence with increasing K+ concentration were larger in K-ghosts than in Na-ghosts. K+-induced fluorescence changes were very small when gramicidin D was added to the suspension. K+-induced fluorescence changes were not observed in ultrasonicated ghost suspensions. Such findings seem to indicate that the K+ -ions induced fluorescence increases reflect the depolarization in the isolated synaptic plasma membrane ghosts. Furthermore, the permeability ratio PNa/PK(PNa, PK: the permeability constant for Na+ and K+) was estimated to be smaller than 0.03.