Transient
paralysis of the soleus muscle in neonatal rats leads to permanent
muscle weakness, loss of muscle fibres and motoneuron death. Application of
leupeptin, an inhibitor of a
calcium-activated neutral protease, to the neuromuscular junction is known to enhance the maintenance of neuromuscular contacts during development and axonal sprouting. Here, we show that treatment of soleus muscles with
leupeptin as they recover from a period of
paralysis rescues motoneurons that would otherwise die. The number of motoneurons to the soleus muscle was established by retrograde labelling with
horseradish peroxidase eight to 10 weeks after recovery from
paralysis. There were only 38.4 (+/-2.8 S.E.M., n=5) motoneurons innervating the soleus muscle that had been paralysed with
alpha-bungarotoxin, compared to 58.2 (+/-3.1 S.E.M., n=5) to the control untreated soleus. Thus, the number of motoneurons to the soleus muscle on the
alpha-bungarotoxin-treated side was 66.9% (+/-6.2% S.E.M., n=5) of the control side. In those animals where
paralysis of the soleus muscle was followed three days later by treatment with
leupeptin, the number of labelled motoneurons on the treated side of the spinal cord was 61.5 (+/-4.6 S.E.M., n=4) and that on the contralateral untreated control side was 59 (+/-3.8 S.E.M., n=4). This improvement in motoneuron survival in the
leupeptin-treated animals is also confirmed by counts of the number of motor units in the soleus muscle obtained by recording muscle tension. In animals that had their soleus muscles paralysed at birth, only 21 (+/-0.7 S.E.M., n=5) motor units were present, compared to 30 motor units in control muscles. When the paralysed soleus muscle was subsequently treated with
leupeptin, the number of remaining motor units in the muscle was 29.8 (+/- 1.0 S.E.M., n=5). In addition, the force output of the soleus muscles that had undergone a period of neonatal
paralysis was calculated for both the NaCl- and
leupeptin-treated animals. The results showed that
paralysis at birth results in a reduction in weight and force output of the soleus muscle, which is not improved following treatment with
leupeptin. This study shows that application of
leupeptin to the soleus muscle after
alpha-bungarotoxin-induced
paralysis rescues motoneurons to the soleus that would otherwise die. This effect is most likely due to stabilization of their neuromuscularjunctions.