Sodium currents in axon-associated Schwann cells from adult rabbits

1. Patch-clamp and electron-microscopic studies were carried out on individual axon-Schwann-cell complexes 2-6 h after they were isolated from the sciatic nerves of rabbits 5, 10 and 20 weeks old. 2. Under Hoffman modulation contrast optics Schwann cells associated with both myelinated and non-myelinated axons could be seen. Frequently, fine cable-like structures about 1 micron in diameter, which are presumably axons, could be seen in isolation from a Schwann cell. 3. Cross-sectional electron-microscopic studies directly demonstrated the presence of axons engulfed by Schwann cells. For Schwann cells associated with non-myelinated axons, multiple fine axons (approximately 1 micron) could be seen enclosed by one or few turns of spiralling tongues of Schwann cells. Schwann cells associated with a single large myelinated axon showed characteristic compact myelin wrappings. No membrane fusion between Schwann cells and the axons could be detected. 4. Giga-seals could readily be formed when a patch pipette was pressed against the body region of a Schwann cell associated with either non-myelinated or myelinated axons. In contrast, giga-seals were only infrequently obtained on fine cable-like structures (1 micron) visually identified to be separated from the Schwann cell body. 5. Whole-cell recordings made from the body region of a Schwann cell revealed a TTX-sensitive fast inward current. Intriguingly, the expression of this current appeared to be dependent on the type of associated axon; this current was detectable in virtually all recordings made at the body region of Schwann cells associated with small non-myelinated axons, but not from those associated with large myelinated axons. 6. The inward current was like a neuronal sodium current; it had voltage-gated kinetics similar to the Hodgkin-Huxley sodium current, and exhibited a reversal potential close to the expected Nernstian potential for sodium ions. 7. From the observed size of the whole-cell membrane capacity and the electron-microscopic observations that the surface area of the Schwann cell at the body region was much larger than that of a 1 micron non-myelinated axon, it was argued that the whole-cell recordings were from Schwann cells rather than from single axons. Furthermore, the peak sodium current density was similar to that of Schwann cells cultured from new-born rabbits in which axons were presumed to be absent. 8. The results suggested that Schwann cells normally associated with non-myelinated axons in the rabbit sciatic nerves maintain an active synthesis of neuronal-like sodium channels throughout normal development.