P. Grafe et al., FUNCTION OF THE HYPERPOLARIZATION-ACTIVATED INWARD RECTIFICATION IN NONMYELINATED PERIPHERAL RAT AND HUMAN AXONS, Journal of neurophysiology, 77(1), 1997, pp. 421-426
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.