In the soleus muscle of the rat following section of the L5 ventral ramus (partial
denervation) the remaining motor axons increase their territory by sprouting. Nerve sprouts are first seen two to three days after the operation, their number peaks
at 10-14 days and subsequently remains at this level. The time course of the initial sprouting in partially denervated muscles is not altered by paralysing the muscles with
alpha-bungarotoxin, and the initial extent of the sprouting is, if anything, greater in the paralysed muscles. However, unlike in controls, this level of sprouting is not maintained and neuromuscular contacts are lost when muscles recover from the
paralysis. The loss of these contacts can be prevented by treatment of these partially denervated paralysed muscles with
leupeptin, an inhibitor of
calcium-activated neutral protease. Interestingly, more contacts are rescued when
leupeptin is applied 10 days after
alpha-bungarotoxin treatment, when sprouting has reached high levels, than at three days, when sprouting has just begun. The neuromuscular connections rescued by
leupeptin are functional. Maximum tetanic tension produced by untreated soleus muscles two to five months after partial
denervation is 66 +/- 9% of contralateral control muscles, but only 39 +/- 8% when the muscles were paralysed with
alpha-bungarotoxin for 12-14 days after partial
denervation. However, when partially denervated paralysed muscles were treated with
leupeptin three and 10 days after
alpha-bungarotoxin treatment their tension output is 74 +/- 3% and 81 +/- 8%, respectively. After partial
denervation alone, motor units are twice their normal size. Short-term
paralysis with
alpha-bungarotoxin prevents this increase in motor unit territory. However, the application of
leupeptin to the paralysed muscles rescues neuromuscular contacts, allowing motor unit size to remain expanded, at around 2-2.5-fold. Thus, following recovery from temporary
paralysis with
alpha-bungarotoxin, there is a sudden withdrawal of neuromuscular contacts and these can be rescued by treatment with
leupeptin.