Acetylcholine sensitivity of developing ectopic nerve‐muscle junctions in adult rat soleus muscles.

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Abstract
The development of junctional ACh [acetylcholine] sensitivity was studied during the formation of ectopic nerve-muscle junctions (n.m.j.) between the superficial fibular nerve and the denervated soleus muscle of adult rats. When the soleus nerve was cut 2 wk or more after implanting the fibular nerve, spontaneous m.e.p.p. [miniature end plate potential] and evoked e.p.p. [end plate potential] were first detected in the vicinity of the fibular nerve sprouts 2.5-3 days later. At this time, peaks of local ACh sensitivity greater than the high level of extrajunctional sensitivity induced by denervation were found near the sprouts of the fibular nerve. During the 1st week of foreign innervation, the extrajunctional sensitivity of the newly innervated muscle fibers fell, but the peaks of sensitivity in the region of the fibular nerve sprouts persisted. Many of these peaks occurred at sites of transmitter release from the fibular nerve terminals. Each innervated fiber had 1-8 such peaks. When the fibular nerve was cut 2 days or more after cutting the soleus nerve peaks of ACh sensitivity persisted in the region of the degenerated foreign nerve terminals even if the extrajunctional sensitivity was abolished by direct electrical stimulation of the muscle starting soon after cutting the fibular nerve. When the fibular nerve was left intact, more than half of the peaks of sensitivity formed initially in the region of the foreign nerve sprouts had disappeared 2-3 wk after cutting the soleus nerve. During the formation of ectopic n.m.j. in adult rat muscle the foreign nerve terminals may bring about 2 types of long-lasting change in the distribution and stability of the underlying ACh sensitivity in the muscle fiber membrane: an increase and stabilization of sensitivity at sites of transmitter release which occurs by the time functional transmission at the newly formed n.m.j. can be detected and a loss of sensitivity at some of the sites which takes place about 1-2 wk later.