[Motoneuronal death following prevention of reinnervation during postnatal development]

The hypothesis that survival of motoneurons depends on targets postnatally was tested in Wistar rats of known age. Under ether anesthesia, the cut distal end of a medial gastrocnemius (MG) nerve was anchored to the lateral gastrocnemius (LG) muscle to prevent reinnervation. The compound action potential (CAP) in response to stimulation of the MG nerve was recorded from L4 and L5 ventral roots to evaluate the degree of motoneuronal death in the nerve. CAPs of MG nerves were greatly reduced 1 to 8 months after the operation in the rats operated on at 6 or 9 days of age, which indicates that the majority of MG motoneurons died after the operation. In contrast, the CAPs in the animals which were operated on at older than 12 days were similar to normal animals. However, the number of motor units that remained in the implanted MG nerve was 1.3 on the average in the rats operated on at 12-days-old. In normal rats, it was 87. Thus, the results indicate that in the rats operated on at 12 days of age, a majority of MG motoneurons survived without making functional synapses. We conclude that there is a sharp critical period, between 9 and 12 days after birth, during which motoneuronal target dependency changes.