FUNCTION OF THE HYPERPOLARIZATION-ACTIVATED INWARD RECTIFICATION IN NONMYELINATED PERIPHERAL RAT AND HUMAN AXONS

The function of time-dependent, hyperpolarization-activated inward rec tification was analyzed on compound potentials of nonmyelinated axons in the mammalian peripheral nervous system. Isolated rat vagus nerves and fascicles of biopsied human sural nerve were tested in a three-cha mbered, Vaseline-gap organ bath at 37 degrees C. Inward rectification was assessed by recording the effects of long-lasting hyperpolarizing currents on electrical excitability with the use of the method of thre shold electrotonus (program QTRAC, copyright Institute of Neurology, L ondon, UK) and by measuring activity-dependent changes in conduction v elocity and membrane potential. Prominent time-dependent, cesium-sensi tive inward rectification was revealed in rat vagus and human sural ne rve by recording threshold electrotonus to 200-ms hyperpolarizing curr ent pulses. A slowing of compound action potential conduction was obse rved during a gradual increase in the stimulation frequency from 0.1 t o 3 Hz. Above a stimulation frequency of 0.3 Hz, this slowing of condu ction was enhanced during bath application of 1 mM cesium. Cesium did not alter action potential waveforms during stimulation at frequencies <1 Hz. Cesium-induced slowing in action potential conduction was corr elated with membrane hyperpolarization. The hyperpolarization by cesiu m was stronger during higher stimulation frequencies and small in unst imulated nerves. These data show that a cesium-sensitive, time-depende nt inward rectification in peripheral rat and human nonmyelinated nerv e fibers limits the slowing in conduction seen in such axons at action potential frequencies higher than similar to 0.3 Hz.