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Delayed depolarization and slow sodium currents in cutaneous afferents
O Honmou1, D A Utzschneider, M A Rizzo
1Department of Neurology, Yale University School of Medicine, New Haven, Connecticut 06510.
Journal of Neurophysiology
|May 1, 1994
Summary
Potassium channel blocker 4-aminopyridine (4-AP) induces a delayed depolarization and bursting in cutaneous afferent axons, but not muscle afferents. This suggests distinct voltage-gated sodium channel properties underlie different nerve fiber types.
Area of Science:
- Neuroscience
- Electrophysiology
- Cellular Biology
Background:
- Action potential generation and propagation are fundamental to neuronal communication.
- Voltage-gated ion channels, particularly sodium (Na+) and potassium (K+) channels, play critical roles in shaping action potentials.
- Cutaneous and muscle afferent fibers exhibit distinct physiological properties.
Purpose of the Study:
- To investigate the differential effects of potassium channel blockade on action potential properties in cutaneous and muscle afferent axons.
- To characterize the ionic mechanisms underlying delayed depolarization and bursting activity in response to 4-aminopyridine (4-AP).
- To compare sodium current kinetics in cutaneous afferent neurons versus randomly selected dorsal root ganglion neurons.
Main Methods:
- Intraaxonal and whole-nerve sucrose gap recordings were performed on rat sural nerve (SN), anterior tibial nerve (ATN), and deafferented ATN (dATN) in vitro.
- Application of potassium channel blockers 4-aminopyridine (4-AP) and tetraethylammonium (TEA).
- Whole-cell patch-clamp recordings of sodium currents in dorsal root ganglion neurons and retrogradely labeled cutaneous afferent neurons.
Main Results:
- 4-AP induced a prominent delayed depolarization and burst firing in cutaneous afferents (SN) but only slight prolongation in muscle afferents (ATN/dATN).
- The recovery time for 4-AP-induced delayed depolarization in SN was significantly longer than the action potential recovery time.
- TEA reduced the afterhyperpolarization (AHP) in SN axons treated with 4-AP, leading to repetitive firing.
- Cutaneous afferent neurons exhibited kinetically distinct fast and slow sodium currents or a singular slow sodium current, unlike most randomly selected neurons with a singular fast sodium current.
Conclusions:
- Distinct voltage-gated sodium channel populations contribute to the unique electrophysiological properties of cutaneous afferent axons.
- Potassium channel function significantly influences action potential repolarization and the emergence of bursting activity in specific afferent types.
- These findings highlight functional heterogeneity in ion channel expression and function among different sensory neuron populations.