Restorative effects of reinnervation on the size and dendritic arborization patterns of axotomized cat spinal alpha-motoneurons.

In a preceding paper [Brännström, et al. (1992) J. Comp. Neurol. 318:439-451] a marked reduction in dendritic size was observed in cat spinal motoneurons following permanent axotomy. The aim of the present study was to analyse the possible restorative effects of peripheral reinnervation on the size and dendritic branching patterns of cat spinal motoneurons which had been deprived of neuromuscular contact for an extended period of time. In adult cats the medial gastrocnemius (MG) nerve was transected and ligated. After 6 weeks the nerve was allowed to reinnervate its muscle through a nerve graft. With approximately 6 weeks needed for muscle reinnervation [Foehring, et al. (1986) J. Neurophysiol. 55:947-965], the MG motoneurons were devoid of neuromuscular contact for altogether about 12 weeks. Two years later reinnervated MG alpha-motoneurons were intracellularly labelled with horseradish peroxidase to allow quantitative analyses of the cell bodies and dendritic trees. Comparisons were made with previous data from normal and permanently axotomized MG motoneurons. The reinnervated motoneurons exhibited positive correlations between dendritic stem diameter, on one hand, and combined length, volume, membrane area, and number of end branches of the whole dendrite, on the other. By using the regression equations for these correlations, the total dendritic size of whole reinnervated motoneurons could be estimated. Such calculations showed that in comparison with the reduction in dendritic size found at 12 weeks after permanent axotomy (Brännström et al., see above), peripheral reinnervation caused the dendritic volume and membrane area to return to normal values. However, the values for combined dendritic length and number of dendritic end branches were still reduced by more than 25% as compared to the normal situation. The results indicate that following reinnervation of the target muscle, the axotomized motoneurons did not recover their original number of dendritic branches. The normalization of dendritic membrane area and volume was instead accomplished by two other mechanisms, namely an increase in dendritic diameters and an increased number of dendrites per neuron.