Subthreshold inward membrane currents in guinea-pig frontal cortex neurons

Area of Science:

• Neuroscience
• Electrophysiology
• Computational Neuroscience

Background:

• Cortical pyramidal neurons generate action potentials crucial for brain function.
• Subthreshold membrane potential dynamics influence neuronal excitability and information processing.
• The specific ionic currents responsible for subthreshold depolarization in these neurons remain incompletely understood.

Purpose of the Study:

• To investigate the inward currents active in the subthreshold voltage range of cortical pyramidal neurons.
• To determine the contribution of specific ionic currents to subthreshold depolarization leading to action potential firing.
• To explore the physiological relevance of these currents using dopamine as a pharmacological tool.

Main Methods:

• Performed current-clamp and single-electrode voltage-clamp recordings on guinea-pig frontal cortex slices.
• Utilized tetrodotoxin (TTX) and calcium-free extracellular solutions to isolate ionic currents.
• Investigated the effects of 10 microM dopamine on neuronal excitability and subthreshold currents.

Main Results:

• Subthreshold depolarization was partially blocked by TTX and calcium-free solutions, indicating involvement of both sodium and calcium currents.
• Identified a TTX-sensitive persistent sodium current (I(Na-p)) and a Ca2+-sensitive low-threshold calcium current (I(T)) active around -60 mV.
• Dopamine (10 microM) inhibited I(Na-p) and prevented action potential firing from -60 mV, while sparing I(T) and firing from -80 mV.

Conclusions:

• Subthreshold depolarization in cortical pyramidal neurons relies on both I(Na-p) and I(T), with I(Na-p) being dominant at potentials near -70 to -75 mV.
• I(Na-p) plays a critical role in initiating action potentials at more depolarized subthreshold potentials.
• Dopamine's selective inhibition of I(Na-p) provides a mechanism for modulating neuronal excitability and action potential generation.