ACTION-POTENTIAL PROPAGATION THROUGH EMBRYONIC DORSAL-ROOT GANGLION-CELLS IN CULTURE .2. DECREASE OF CONDUCTION RELIABILITY DURING REPETITIVE STIMULATION

1. The reliability of the propagation of action potentials (AP) throug h dorsal root ganglion (DRG) cells in embryonic slice cultures was inv estigated during repetitive stimulation at 1-20 Hz. Membrane potential s of DRG cells were recorded intracellularly while the axons were stim ulated by an extracellular electrode. 2. In analogy to the double-puls e experiments reported previously, either one or two types of propagat ion failures were recorded during repetitive stimulation, depending on the cell morphology. In contrast to the double-pulse experiments, the failures appeared at longer interpulse intervals and usually only aft er several tens of stimuli with reliable propagation. 3. In the period with reliable propagation before the failures, a decrease in the cond uction velocity and in the amplitude of the afterhyperpolarization (AH P), an increase in the total membrane conductance, and the disappearan ce of the action potential ''shoulder'' were observed. 4. The reliabil ity of conduction during repetitive stimulation was improved by loweri ng the extracellular calcium concentration or by replacing the extrace llular calcium by strontium. The reliability of conduction decreased b y the application of cadmium, a calcium channel blocker, 4-amino pyrid ine, a fast potassium channel blocker, or apamin or muscarine, the blo ckers of calcium-dependent potassium channels. The reliability of cond uction was not effected by blocking the sodium potassium pump with oua bain or by replacing extracellular sodium with lithium. 5. In the peri od with reliable propagation cadmium, apamin, and muscarine reduced th e amplitude of the AHP. The shoulder of the action potential was more pronounced and not sensitive to repetitive stimulation when extracellu lar calcium was replaced by strontium. It disappeared when cadmium was applied. 6. In DRG somata changes of the intracellular Ca2+ concentra tion were monitored by measuring the fluorescence of the Ca2+ indicato r Fluo-3 with a laser-scanning confocal microscope. During repetitive stimulation, an accumulation of intracellular calcium occurred that re covered very slowly (tens of seconds) after the AP trains. 7. Computer model simulations performed in analogy to the experimental protocols produced conduction failures during repetitive stimulation only when t he calcium currents during the APs were reduced. 8. From these finding s it is concluded that conduction failures during repetitive stimulati on are dependent on an accumulation of intracellular calcium leading t o an inactivation of calcium currents, combined with small contributio ns of an accumulation of extracellular potassium and a summation of sl ow potassium conductances.