EFFECT OF TRANSIENT NEONATAL MUSCLE PARALYSIS ON THE GROWTH OF SOLEUS MOTONEURONS IN THE RAT

The postnatal growth of soleus motoneurones was studied during normal development and following transient paralysis of the soleus muscle in neonatal rats. Paralysis was achieved by implanting a silicon strip containing cr-bungarotoxin alongside the soleus muscle in rat pups within 3-6 h of birth. The soleus muscle was completely paralysed for at least 24 h, and by 9 days neuromuscular transmission was fully restored. The soma size of normal and target-deprived soleus motoneurones was compared at intervals during the first 3 postnatal weeks and in adults, using the retrograde horseradish peroxidase technique. There was a four-fold increase in the soma area of normal motoneurones during the first 3 postnatal weeks, with the greater part of the increase occurring between 7 and 14 days. At 3 days, the distribution of soma areas was unimodal and became bimodal by 21 days. Paralysis during the first postnatal week did not significantly affect the developmental changes in motoneurone soma area or their distribution up to 3 weeks of age. Thus, motoneurones deprived of functional neuromuscular contact appear to grow normally during the early postnatal period, although previous results show that at later stages (2-3 months of age), many of these motoneurones die and the remaining cells are smaller than normal.