Gating current harmonics. I. Sodium channel activation gating in dynamic steady states

Area of Science:

• Neuroscience
• Biophysics
• Molecular Biology

Background:

• Voltage-gated ion channels are crucial for neuronal excitability.
• Gating currents reflect the conformational changes of these channels.
• Existing models, like Hodgkin-Huxley, describe channel kinetics but may not capture all complexities.

Purpose of the Study:

• To investigate the nonlinear properties of gating currents in squid giant axons.
• To compare experimental gating current data with predictions from established kinetic models.
• To identify potential molecular mechanisms underlying observed gating current behavior.

Main Methods:

• Preparation of internally perfused and pronase-treated squid giant axons.
• Recording of gating currents using sinusoidal voltage clamp techniques.
• Nonlinear analysis of current records to determine harmonic content (amplitude and phase).
• Comparison of experimental harmonic behavior with simulations from standard channel models.

Main Results:

• A dominant second harmonic in gating currents was observed around a mean membrane potential of +10 mV.
• Harmonic content diminished at extreme membrane potentials (<-60 mV and >+72 mV).
• Experimental harmonic behavior significantly differed from simulations based on standard sodium-channel kinetics.

Conclusions:

• The observed nonlinear harmonic behavior in giant axon gating currents is not fully explained by current kinetic models.
• The data strongly suggest the presence of at least two distinct molecular components with independent movement.
• Fundamental revisions to channel kinetic models may be necessary to accurately represent axonal gating current dynamics.