Low-threshold, slow-inactivating Na+ potentials in the cockroach giant axon

Area of Science:

• Neuroscience
• Insect electrophysiology
• Ion channel function

Background:

• The ventral giant axon of the cockroach abdominal nerve cord is a model for studying neuronal excitability.
• Potassium (K+) conductance plays a crucial role in shaping action potentials and neuronal repolarization.
• Understanding ion channel dynamics is key to deciphering neuronal signaling.

Purpose of the Study:

• To investigate the electrophysiological properties of the cockroach giant axon by manipulating K+ conductance.
• To characterize the nature of depolarizing currents and potentials, particularly plateau potentials and slow depolarizing responses.
• To identify potential differences in sodium (Na+) channel behavior and inactivation kinetics.

Main Methods:

• Intracellular potential recordings in cockroach giant axons.
• Pharmacological blockade of K+ conductance using tetraethylammonium (TEA) and 3,4-diaminopyridine.
• Complete blockade of K+ conductance to induce plateau potentials.
• Partial blockade of K+ conductance with TEA alone to evoke slow depolarizing responses.
• Manipulation of external ion concentrations (Ca2+, Na+) and application of tetrodotoxin (TTX).

Main Results:

• Complete K+ blockade resulted in decreased action potential threshold and a prolonged plateau potential dependent on Na+ but not Ca2+.
• Plateau potentials showed reduced input resistance and TTX sensitivity, with evidence of distinct Na+ inactivation kinetics.
• Partial K+ blockade revealed low-threshold, slow depolarizing responses sensitive to TTX and Na+-free solutions.
• A small, TTX-sensitive depolarizing response was observed even in normal conditions, masked by K+ conductance.

Conclusions:

• The cockroach giant axon possesses at least two populations of Na+ channels with differing activation kinetics and voltage dependence.
• A slow, low-threshold Na+-dependent potential, normally masked by K+ conductance, contributes to neuronal excitability.
• The relationship between plateau and slow potentials, and whether they arise from the same or different Na+ channel populations, requires further investigation.