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Current-clamp analysis of a time-dependent rectification in rat optic nerve.
D L Eng1, T R Gordon, J D Kocsis
1Department of Neurology, Yale University School of Medicine, New Haven, CT 06510.
The Journal of Physiology
|February 1, 1990
Summary
Rat optic nerves show a unique inward current when hyperpolarized, dependent on sodium and potassium ions. This current, distinct from typical channels, enhances nerve excitability and may stabilize neuronal activity during intense signaling.
Area of Science:
- Neuroscience
- Cellular Electrophysiology
- Axonal Physiology
Background:
- Understanding the ionic mechanisms underlying neuronal excitability is crucial for comprehending nervous system function.
- Optic nerve fibers, particularly myelinated axons, possess unique electrophysiological properties that are not fully elucidated.
- Previous studies have identified various ion channels, but the specific conductances activated during hyperpolarization remain an area of investigation.
Purpose of the Study:
- To investigate the nature of a time-dependent conductance increase observed in rat optic nerves upon hyperpolarization.
- To characterize the ionic basis and channel properties of this conductance.
- To determine the functional implications of this conductance for axonal excitability.
Main Methods:
- Intra-axonal and whole-nerve recordings were performed on isolated rat optic nerves using a sucrose-gap chamber.
- Constant-current pulses were applied to induce hyperpolarization and elicit inward currents.
- Ionic substitutions (Na+, K+), pharmacological agents (TTX, TEA, 4-AP, Cs+, Ba2+, Cd2+, Mg2+), and varying ion concentrations were used to probe the conductance properties.
Main Results:
- A prominent time-dependent inward current and conductance increase were observed during hyperpolarization.
- This conductance was dependent on both external sodium (Na+) and potassium (K+), with maximal current at 5 mM [K+]o.
- The inward current was insensitive to TTX, TEA, and 4-AP but was completely blocked by Cs+ and partially by Ba2+, suggesting a non-conventional channel mechanism.
- Intra-axonal recordings confirmed the axonal origin of the conductance, and stimulus-response curves showed increased excitability during the conductance increase.
Conclusions:
- Rat optic nerve myelinated fibers exhibit a significant time-dependent conductance increase upon hyperpolarization.
- This conductance relies on Na+ and K+ and is characteristic of an inwardly rectifying mechanism, distinct from classical voltage-gated channels.
- The observed increase in axonal excitability suggests a role in maintaining or stabilizing neuronal function during high-frequency activity.