STRUCTURAL AND FUNCTIONAL ALTERATIONS IN RAT CORTICOSPINAL NEURONS AFTER AXOTOMY

1. The electrophysiological properties of rat corticospinal neurons (C SNs) were studied 3, 9, and 12 mo after axotomy in the cervical spinal cord, with the use of a combination of the in vitro neocortical slice technique, intracellular recordings, and a double-labeling method tha t allowed identification of CSNs studied in vitro. 2. CSNs retained th e rhodamine-labeled microspheres employed as a retrograde marker and w ere functionally active in the longest survival group (1 yr). 3. The s omatic area of axotomized CSNs became progressively smaller, a reducti on that amounted to 37% for all cells at 1 yr. There were no obvious d ifferences between normal and axotomized cells in terms of apical dend ritic 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 i n layers V and VI with only occasional collaterals reaching supragranu lar layers. 4. Axotomized CSNs were grouped according to their spike f iring patterns during depolarizing current pulses so that their electr ophysiological behavior could be compared with that of regular spiking and adapting groups of normal CSNs. No significant differences were f ound in resting membrane potential, or spike parameters between axotom ized neurons in any survival group and normal controls. Neurons surviv ing 1 yr after axotomy had a higher input resistance (R(N)) than norma l 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 gre ater 9 mo after axotomy in regular spiking (RS) and adapting neurons t han in normal CSNs in the same groups. The increase in steady-state f- I slope was in part related to increases in the RN Of axotomized neuro ns. 6. There was a significant decrease in the generation of slow afte rhyperpolarizations following trains of spikes in axotomized versus no rmal RS neurons, first detected at 3 mo and also present in 9 mo and 1 yr survival groups. 7. Biphasic inhibitory postsynaptic potentials (I PSPs) were evoked in only 1 of 11 axotomized neurons in the 3-mo group , 2 of 12 cells examined at 9 mo, and 3 of 15 neurons 1 yr after axoto my. The proportions of neurons generating IPSPs were significantly sma ller than in comparable groups of control CSNs. As a consequence, long er duration evoked excitatory postsynaptic potentials were generated b y axotomized CSNs. 8. Results show that axotomized CSNs undergo altera tions in intrinsic membrane properties and inhibitory synaptic electro genesis that would tend to make them more responsive to excitatory inp uts.