Sodium and gating current time shifts resulting from changes in initial conditions

Area of Science:

• Neuroscience
• Biophysics
• Ion Channel Physiology

Background:

• The squid giant axon is a model system for studying neuronal excitability.
• Sodium channels are crucial for action potential generation.
• Gating currents represent the movement of charged particles within ion channels.

Purpose of the Study:

• To investigate the effect of hyperpolarizing prepulses on sodium and gating currents.
• To elucidate the relationship between gating current charge movement and sodium current kinetics.
• To explore the early steps of sodium channel activation.

Main Methods:

• Electrophysiological recordings of sodium and gating currents in the squid giant axon.
• Application of depolarizing pulses following varying hyperpolarizing prepulses.
• Analysis of current delays, shapes, and charge movement.
• Modeling of sodium channel activation using a sequential six-state model.

Main Results:

• Hyperpolarizing prepulses significantly delayed both sodium and gating currents without altering their shape.
• Delays reached saturation at approximately 45 microseconds for prepulses to -140 mV.
• A strong correlation was observed between the magnitude of gating current charge movement and the increased time delay of the sodium current.

Conclusions:

• The gating current is intimately linked to the process of sodium channel opening.
• These findings provide insights into the kinetics of early, electrophysiologically hidden steps in sodium channel activation.
• A six-state sequential model accurately reproduces the observed gating and sodium current time shifts and their correlation.