Evidence for the presence of two types of potassium channels in the rat optic nerve.

Single axon and whole nerve recording techniques were used to study the effects of 4-aminopyridine (4-AP) and tetraethylammonium (TEA) on action potential waveform and firing characteristics of rat optic nerve. 4-AP led to action potential broadening and a pronounced afterhyperpolarization (AHP) several hundreds of milliseconds in duration. In addition, some fibers showed repetitive firing and a prolonged depolarization immediately following the spike. Whole nerve sucrose gap recordings obtained in the presence of 4-AP showed action potential broadening followed by a postspike positivity indicative of intracellular hyperpolarization. The time course of the postspike positivity was similar to that of the AHP seen in single fiber recordings. TEA alone had little effect on action potential waveform, but reversibly blocked the 4-AP-induced postspike positivity. These findings indicate the presence of two pharmacologically distinct potassium channels on myelinated axons of the rat optic nerve, one sensitive to 4-AP and the other to TEA. These channels appear to have different functional roles, the 4-AP-sensitive channel participating in action potential repolarization, and the TEA-sensitive channel modulating repetitive firing patterns.