Sodium currents in Schwann cells from myelinated and non‐myelinated nerves of neonatal and adult rabbits.

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Abstract
1. Patch‐clamp methods were used to study sodium channels in Schwann cells obtained from four different tissue sources. Primary cultures of Schwann cells were prepared from the sciatic nerve and from the vagus nerve of neonatal and of adult rabbits. In the adult, the sciatic is predominantly myelinated whereas the vagus is predominantly non‐myelinated. Whole‐cell currents, and single‐channel currents in outside‐out membrane patches, were analysed. 2. No substantial differences were noted in the passive electrical properties (input resistance, cell capacitance, resting membrane potential) of the four groups of cells. Similarly, no substantial differences were found in the average properties of sodium currents (maximum current, maximum conductance, time‐to‐peak current, current‐voltage relation, h infinity relation) recorded from each type of cell in cultures less than 8 days old. At 10‐17 days a fall in the size of the sodium currents recorded from cells in the vagal cultures was found. 3. Exposure of the cells to proteolytic enzymes or collagenase, under conditions similar to those used when the cells were put in culture initially, substantially reduced the size of the peak sodium currents recorded from the cells 24 h later. 4. The results of experiments on Schwann cells with retracted processes indicated that sodium channels are present in the processes extending from each pole of the cell soma and that the plasmalemmal density of these channels in the processes is about the same as it is at the soma. 5. Recordings from outside‐out patches revealed no apparent differences in the properties of single‐channel sodium currents in patches from cells obtained from the four different sources. The single‐channel conductance was about 20 pS for each of the four groups. Ensemble currents from single‐channel records were similar in time course to those of whole‐cell currents. 6. Saxitoxin reduced the maximum sodium conductance in Schwann cells and bound to the cells with equally high affinity. The equilibrium dissociation constant was about 2 nM at 20‐22 degrees C. 7. It is argued that the expression of sodium channels in myelinating Schwann cells does not differ substantially from that of non‐myelinating Schwann cells.