Delayed loss of spinal motoneurons after peripheral nerve injury in adult rats: a quantitative morphological study

The existence of retrograde cell death in sensory dorsal root ganglion (DRG ) cells after peripheral nerve injury is well established. However, with re spect to retrograde motoneuron death after peripheral nerve injury, availab le data are conflicting. This may partly be due to the cell counting techni ques used. In the present study, quantitative morphometric methods have bee n used to analyse retrograde motoneuron death induced by spinal nerve injur y in adult rats. For comparison, DRG cells were also included in the study. The C7 spinal nerve was transected about 10 mm distal to the DRG and expos ed to the fluorescent tracer fast blue in order to retrogradely label the s pinal motoneurons and DRG cells of the C7 segment. At 1-16 weeks postoperat ively, the nuclei of fast-blue-labelled C7 motoneurons and DRG cells were c ounted in consecutive 50-mum-thick serial sections. For comparison, the phy sical disector technique and measurements of neuronal density were also use d to calculate motoneuron number. The counts of fast-blue-labelled motoneur ons revealed a delayed motoneuron loss amounting to 21% and 31% after 8 and 16 weeks, respectively (P<0.001). The remaining motoneurons exhibited 20% (P<0.05) soma atrophy. Using the physical disector technique, the motoneuro n loss was 23% (P<0.001) after 16 weeks. Calculations of neuronal density i n Nissl-stained sections failed to reveal any motoneuron loss, although aft er correction for shrinkage of the ventral horn a 14% (P<0.001) motoneuron loss was found. The fast-blue-labelled DRG neurons displayed 51% (P<0.001) cell loss after 16 weeks, and the remaining cells showed 22% (P<0.001) soma atrophy. In summary, cervical spinal nerve injury induces retrograde degen eration of both motoneurons and DRG cells. However, to demonstrate the moto neuron loss adequate techniques for cell counts have to be employed.