Evidence for membrane potential changes in isolated synaptic membrane ghosts monitored with a merocyanine dye

Area of Science:

• Neuroscience
• Biophysics
• Membrane Biology

Background:

• Synaptic plasma membranes are crucial for neuronal communication.
• Ion gradients across these membranes drive electrical signaling.
• Merocyanine-540 is a fluorescent probe sensitive to membrane potential.

Purpose of the Study:

• To investigate the effect of potassium (K+) ions on synaptic plasma membrane ghosts.
• To use merocyanine-540 fluorescence to assess membrane potential changes.
• To estimate the relative permeability of sodium (Na+) and potassium (K+) ions.

Main Methods:

• Isolation of synaptic plasma membrane ghosts from rat brain cortex.
• Incubation of membrane ghosts in solutions with varying K+ and Na+ concentrations.
• Measurement of merocyanine-540 fluorescence intensity changes.
• Assessment of ion effects using gramicidin D and sonication.

Main Results:

• Increased external K+ concentration led to a proportional increase in merocyanine-540 fluorescence.
• The K+-induced fluorescence increase was greater in K+-enriched membranes compared to Na+-enriched ones.
• Other cations like Rb+, NH4+, and Cs+ also induced fluorescence changes, with varying effectiveness.
• Gramicidin D and sonication significantly reduced the K+-induced fluorescence changes.

Conclusions:

• K+-induced fluorescence increase in synaptic plasma membrane ghosts reflects membrane depolarization.
• The findings suggest a specific interaction between K+ ions and the membrane leading to potential changes.
• The estimated permeability ratio PNa/PK was found to be less than 0.03, indicating low Na+ permeability relative to K+.