Maturation of transmission in reinnervated mouse soleus muscle.

After the tibial nerve of the mouse was cut unilaterally and immediately resutured, reinnervation of soleus muscle proceeded rapidly and muscle isometric contraction characteristics reached normal levels within 2 months. In contrast, synaptic transmission remained immature since resistance to presynaptic (magnesium) or postsynaptic (curare) blocking solutions remained reduced. Results suggest that release probability and transmitter stores were smaller than normal. To study the effect of training, animals were allowed to run in wheels. Running caused a delay in reinnervation at 18-20 days, which was, however, abolished by 4 weeks. On the other hand, exercise counteracted development of denervation atrophy. The safety margin of transmission in runners was higher than in nonrunners at 4 weeks, indicating enhanced maturation, but was lower at 2 months of reinnervation. These results suggest that recovery of muscle precedes maturation of synaptic transmission.