Membrane properties and inhibitory connections of normal and upper cervically axotomized rubrospinal neurons in the rat.

Membrane properties and inhibitory synaptic connections of normal and axotomized rat rubrospinal neurons were examined using a coronal slice preparation. Rubrospinal neurons were axotomized at the C2 vertebral level in vivo. Retrograde labelling in vivo and intracellular biocytin injection following recording were combined to identify recorded axotomized rubrospinal neurons. Their input resistances decreased three and four days and became higher than normal four and 10 weeks following lesioning which coincided with a sequential increase and decrease of their soma area. On the other hand, although their membrane time-constant was reduced three and four days following lesioning, it returned to normal value four and 10 weeks following axotomy. Other than these, their membrane current-voltage relationship including an inward rectification in the hyperpolarizing direction was not altered. Normal rubrospinal neurons generated very fast spikes which were not affected by axotomy. Both normal and axotomized cells generated trains of repetitive spikes with a fast spike frequency adaptation at the beginning upon suprathreshold current injection. However, the slope of the steady-state spike frequency and applied current relationship was increased four and 10 weeks following axotomy which also showed an increased steady-state spike frequency in response to high-amplitude current injection. Synaptically, the amplitude and duration of the monosynaptic inhibitory potential evoked from nearby reticular formation were reduced following axotomy. In addition, fewer rubrospinal neurons were found to receive this inhibition 10 weeks following axotomy. Thus, our results show that spinal axotomy induces a time-dependent modification of the membrane properties and spike generating behaviour of rubrospinal neurons which probably represents an initial decrease and a later increase of their excitability. This is accompanied by a persistent decrease of synaptic inhibition which is expected to affect structures that remained innervated by the undamaged axon collaterals of these spinally axotomized neurons.