Propagation of action potentials in the dendrites of neurons from rat spinal cord slice cultures. Larkum, Matthew E., Marc G. Rioult and Hans-R. Lscher. Department of Physiology, University of Bern, Bhlplatz 5, CH-3012 Bern, Switzerland.
APStracts 2:0282N, 1995.
SUMMARY AND CONCLUSIONS
1 . We examined the propagation of action potentials in the dendrites of ventrally located presumed motoneurons of organotypic rat spinal cord cultures. Simultaneous patch electrode recordings were made from the dendrite and soma of individual cells. In other experiments, we visualized the membrane voltage over all the proximal dendrites simultaneously using a voltage- sensitive dye and an array of photodiodes. Calcium imaging was used to measure the dendritic rise in Ca 2+ accompanying the propagating action potentials. 2 . Spontaneous and evoked action potentials were recorded using high resistance patch electrodes with separations of 30-423 [mu]m between the somatic and dendritic electrodes. 3 . Action potentials recorded in the dendrites varied considerably in amplitude but were larger than would be expected if the dendrites were to behave as passive cables (sometimes little or no decrement was seen for distances of more than 100 [mu]m). Because the amplitude of the action potentials in different dendrites was not a simple function of distance from the soma, we suggest that the conductance responsible for the boosting of the action potential amplitude varied in density from dendrite to dendrite and possibly along each dendrite. 4 . The dendritic action potentials were usually smaller, broader and arrived later at the dendritic electrode than at the somatic electrode irrespective of whether stimulation occurred at the dendrite or soma or as a result of spontaneous synaptic activity. This is clear evidence that the action potential is initiated at or near the soma and spreads out into the dendrites. The conduction velocity of the propagating action potential was estimated to be 0.5 m/s. 5 . The voltage time courses of previously recorded action potentials were generated at the soma using voltage-clamp before and after applying 1 [mu]M TTX over the soma and dendrites. TTX reduced the amplitude of the action potential at the dendritic electrode to a value in the range expected for dendrites that behave as passive cables. This indicates that the conductance responsible for the actively propagating action potentials is a Na + conductance. 6 . The amplitude of the dendritic action potential could also be initially reduced more than the somatic action potential using 1-10 mM QX-314 (an intracellular sodium channel blocker) in the dendritic electrode as the drug diffused from the dendritic electrode towards the soma. Furthermore, in some cases the action potential elicited by current injection into the dendrite had two components. The first component was blocked by QX-314 in the first few seconds of the diffusion of the blocker. 7 . In some cells, an after-depolarizing potential (ADP) was more prominent in the dendrite than in the soma. This ADP could be reversibly blocked by 1 mM Ni 2+ or by perfusion of a nominally Ca 2+ -free solution over the soma and dendrites. This suggests that the back- propagating action potential caused an influx of Ca 2+ predominantly in the dendrites. 8 . Using a voltage-sensitive dye (di-8-ANEPPS) and an array of photodiodes, the action potential was tracked along all the proximal dendrites simultaneously. The results confirmed that the action potential propagated actively in contrast to similarly measured hyperpolarizing pulses that spread passively. There were also indications that the action potential was not uniformly propagated in all the dendrites suggesting the possibility that the distribution of Na + channels over the dendritic membrane is not uniform. 9 . Calcium imaging with the Ca 2+ fluorescent indicator Fluo-3 showed a larger percentage change in fluorescence in the dendrites than in the soma. Both bursts and single action potentials elicited sharp rises in fluorescence in the proximal dendrites suggesting that the back-propagating action potential causes a concomitant rise in intracellular calcium concentration ([Ca 2+ ] i ). This might have important consequences for the modulation of metabolic processes which in turn might affect the modulation of synaptic transmission by the postsynaptic neuron. 10 . In summary, the results indicate that the dendrites of motoneurons have non-uniformly distributed Na + and Ca 2+ conductances. These are activated by the spreading action potentials which are generated at or near the soma and propagate back into the dendrites. This has important consequences for the processing of synaptic input by motoneurons as well as for their intrinsic firing properties. 

Received 27 April 1995; accepted in final form 9 August 1995.
APS Manuscript Number J285-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.