Effects of Chronic Nerve Conduction Block on Formation of Neuromuscular Junctions and Junctional AChE in the Rat

The development of ectopic n.m.j.s. between the transplanted superficial fibular nerve and the soleus muscle has been studied in adult rats. Impulse conduction in the sciatic nerve was blocked chronically and synapse formation between the blocked fibular nerve and the paralysed soleus was compared with synapse formation between non-blocked fibular nerves and denervated soleus muscles. Nerves with conduction block readily made new n.m.j.s. Thus 6 and 10--14 days after the onset of the block the number of newly innervated muscle fibres, the percentage of innervated fibres responding with action potentials and the frequency of m.e.p.p.s. at new junctions were comparable to that observed during innervation by non-blocked nerves. Muscle fibres innervated by both the original soleus nerve and the foreign fibular nerve were regularly encountered in the impulse blocked preparations. Junctions formed by impulse blocked fibular nerves had either no or very little AChE activity 10--15 days after the onset of the block. The evidence for this was 1) weak staining for CHE; 2) prolonged rise time and 1/2 decay time of m.e.p.p.s; 3) positive correlation between m.e.p.p. amplitude and 1/2 decay time and 4) insensitivity to anticholinesterases. In contrast, junctions formed by non-blocked fibular nerves had strong AChE activity by these criteria at corresponding times. AChE activity at the original soleus endplates was much reduced 10--15 days after the onset of conduction block.