Structural and Functional Alterations in Rat Corticospinal Neurons Following Axotomy. Tseng, Guo-Fang and David A. Prince. National Taiwan University, College of Medicine, Department of Anatomy, 1 Section 1, Jen Ai Road, Taipei, Taiwan 10018, Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, CA 94305 .
APStracts 2:0250N, 1995.
SUMMARY AND CONCLUSIONS
1) The electrophysiological properties of rat corticospinal neurons (CSNs) were studied 3, 9 and 12 months following axotomy in the cervical spinal cord, using a combination of the in vitro neocortical slice technique, intracellular recordings, and a double-labelling method that allowed identification of CSNs studied in vitro. 2) CSNs retained the rhodamine-labelled microspheres employed as a retrograde marker and were functionally active in the longest survival group (1 year). 3) The somatic area of axotomized CSNs became progressively smaller, a reduction that amounted to 37% for all cells at 1 year. There were no obvious differences between normal and axotomized cells in terms of apical dendritic widths, numbers of apical dendritic branches, or basal dendritic arbors. Intracortical axonal arborizations of axotomized neurons were in general similar to those of normal CSNs in that most axons ended in layers V and VI with only occasional collaterals reaching supragranular layers. 4) Axotomized CSNs were grouped according to their spike firing patterns during depolarizing current pulses so that their electrophysiological behavior could be compared to that of regular spiking and adapting groups of normal CSNs. No significant differences were found in resting membrane potential, or spike parameters between axotomized neurons in any survival group and normal controls. Neurons surviving 1 year after axotomy had a higher input resistance (R N ) than normal CSNs. There was a reduction in the percentage of CSNs that generated prominent spike depolarizing afterpotentials in the axotomized group. 5) The steady state relationship between spike frequency and applied current ( f-I slope) became steeper over time and was significantly greater 9 months after axotomy in regular spiking (RS) and adapting neurons than in normal CSNs in the same groups. The increase in steady state f-I slope was in part related to increases in the R N of axotomized neurons. 6) There was a significant decrease in the generation of slow afterhyperpolarizations following trains of spikes in axotomized versus normal RS neurons, first detected at 3 months and also present in 9 month and 1 year survival groups. 7) Biphasic IPSPs were evoked in only 1 of 11 axotomized neurons in the 3-month group, 2 of 12 cells examined at 9 months and 3 of 15 neurons 1 year after axotomy. The proportions of neurons generating IPSPs were significantly smaller than in comparable groups of control CSNs. As a consequence, longer duration evoked excitatory postsynaptic potentials were generated by axotomized CSNs. 9) Results show that axotomized CSNs undergo alterations in intrinsic membrane properties and inhibitory synaptic electrogenesis that would tend to make them more responsive to excitatory inputs. 

Received 5 April 1995; accepted in final form 11 August 1995.
APS Manuscript Number J218-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.