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Age‐Related Responses of Skeletal Muscle After Ectopic Innervation, with Particular Reference to 16S Acetylcholinesterase, in Adult Rats

Abstract: The formation of ectopic junctions between the foreign fibular nerve and the soleus muscle of young (35‐day‐old) and mature (200‐day‐old) adult rats was induced by severing the normal nerve 4 weeks after implanting the foreign nerve. The various molecular forms of ace‐tylcholinesterase (AChE) were studied both at the implanted region and at the original denervated endplates. The velocity of contraction was also studied. In young rats the 16S form was first detected in the ectopic junctions around day 5 after reinnervation; this form rapidly increased during the following weeks, reaching a plateau by day 20. By contrast, in mature rats the appearance of the 16S AChE was dramatically delayed; in fact, it could not be observed before day 80 after reinnervation. (The 16S AChE form appeared at day 20 after reinnervation in the original denervated endplates of young rats; however, at the same time, no effect was observed in mature animals.) The original, slow muscle fibers of the soleus became faster upon reinnervation; this change occurred also much earlier in younger than in mature rats. Our results indicate a loss of plasticity in the skeletal muscle of mature rats. We suggest caution in the use of the ectopic innervation model to study development in mature adult rats.