Postnatal dendritic development in lumbar motoneurons in mutant superoxide dismutase 1 mouse model of amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by motoneuron (MN) degeneration and muscle paralysis. Cu/Zn superoxide dismutase (SOD1) mutant mice develop an ALS-like phenotype similar to that seen in human. Recently it has been shown in SOD1 mice that the excitability and morphology of spinal MNs are altered at postnatal age, long before overt clinical symptoms. In the present study, we examined the morphology of lumbar MNs in wild-type (WT) and SOD1(G85R) mice at two postnatal ages (P3-P4 and P8-P9) when differences in size and excitability were reported. Detailed morphological analysis was performed in MNs intracellularly labeled with Neurobiotin and 3D-reconstructed using the Neurolucida™ system. We showed that SOD1 MNs exhibited longer terminal segments than in the WT MNs at age P3-P4. This excessive elongation was followed by pathological ramification of all individual dendrites between P4 and P8. Abnormal bifurcations occurred at the terminal tips, and distance between consecutive bifurcations remained stable. Most dendrites of SOD1 MNs performed in average two successive iterations of bifurcation during this short time. Our results suggest that the SOD1 MNs switch from excessive elongation to overbranching in a few days at early postnatal life, which might be a compensatory reaction to the possible variation of synaptic input.