A Comparative Study of Terminal Disruption After Partial Denervation in Slow- and Fast-twitch Muscles of Neonatal CFP Mice and Wistar Rats.

Abstract

After partial denervation of mature muscle, the remaining motoneurons grow terminal sprouts, which reinnervate denervated muscle fibers. In neonatal rat soleus muscle however, not only do the remaining motoneurons fail to reinnervate nearby muscle fibers, but they also display clear morphological and physiological evidence of terminal disruption. My ultimate aim is to attain a better understanding of this disruption by performing repeated in vivo observations of individually identified terminals in transgenic mice expressing fluorescent proteins in their motoneurons. For technical reasons, these in vivo imaging studies would ideally be conducted on CFP mouse sternomastoid (SM) muscle. I therefore tested whether neonatal partial denervation of CFP mouse SM muscle also causes terminal disruption. Two to five days after neonatal partial denervation, I fluorescently labeled nerve terminals and acetylcholine receptors (AChRs) to assess the effect on terminal morphology. Terminal disruption after partial denervation did not occur as extensively in CFP mouse SM muscle (a fast-twitch muscle) as it did in rat soleus muscle (a slow-twitch muscle). This relative absence of terminal disruption in CFP mouse SM suggested that the extent/occurrence of terminal disruption after neonatal partial denervation could depend on either the species being studied or muscle fiber type. To distinguish between these possibilities, I investigated terminal disruption after partial denervation of rat SM muscle. In both soleus and sternomastoid muscles of rats, neonatal partial denervation resulted in disruption of about half of the terminals, whereas in SM muscles from CFP mice only 8% of terminals showed evidence of disruption. Therefore, it appears that the nature of terminal disruption that takes place after partial denervation of neonatal muscles is not related to muscle fiber type but may be species-specific.