Electrophysiology of degenerating neurones in the vagal motor nucleus of the guinea-pig following axotomy.

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1. The electrophysiological properties of motoneurones in the dorsal motor nucleus of the vagus in the guinea-pig were studied at different times following cervical vagotomy. The results were compared both to normal neurones and to results obtained at the same time from intact neurones located in the contralateral nucleus. 2. The input resistances of axotomized neurones are significantly higher than those of normal neurones (66 +/- 29 compared to 45 +/- 17 M omega). This difference was seen during the first month following axotomy without any sign of a time-dependent process. On the other hand, no change in resting potential was observed. 3. Significant reduction in action potential amplitude was observed 1 month after axotomy (from 97.8 +/- 8 to 87 +/- 7 mV) and was followed by slow recovery lasting more than 1 year. Neither the Na+ conductance nor the voltage-dependent K+ conductance responsible for the fast rise and fall of the action potential, respectively, were affected by axotomy. 4. One month after axotomy the action potential duration in axotomized neurones was found to be shorter than that of normal neurones (0.9 +/- 0.1 ms compared to 1.1 +/- 0.04 ms). We show that this decrease in duration reflects a reduction in the depolarizing hump on the falling phase of the action potential, which is known to express the Ca2+ conductance activated during the action potential. A slow recovery of the spike duration was observed, although an age-dependent reduction in duration was also observed in neurones in the contralateral nucleus. 5. Two K+ conductances, the Ca2+-dependent and the A type, decrease 1 month after axotomy and follow a similar time course of recovery to that of the reduction in action potential duration and amplitude. 6. The firing pattern of axotomized neurones undergoes profound alteration, manifested as an increase in firing duration as a response to a rectangular current pulse. Examination of these alterations reveals that the reduction in both K+ conductances is responsible for the observed changes. 7. The results are discussed within the framework of the degenerative response known to take place in the nucleus following axotomy. We hypothesize that the observed phenomena reflect an increase in intracellular Ca2+ concentration which, in turn, inactivates the Ca2+ and K+ conductances. Furthermore this rise in intracellular Ca2+ may eventually be responsible for cell death.

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