Treatment of the neuromuscular junction with 4-aminopyridine results in improved reinnervation following nerve injury in neonatal rats

During early postnatal development, nerve injury results in the death of a large proportion of motoneurones and poor recovery of muscle function. Our previous results have shown that premature enhancement of transmitter release from nerve terminals prevents the death of motoneurones following neonatal nerve injury. Whether this increase in motoneurone survival is reflected in an improvement in the reinnervation of muscle was studied here. The muscles in one hindlimb of newborn rats were treated with 4-aminopyridine. Three days later, the sciatic nerve was crushed in the treated leg. When the animals were seven, 14 and 21days of age, the soleus and extensor digitorum longus muscles were removed and processed for GAP-43 (a 43-kDa growth-associated protein) and synaptophysin immunocytochemistry. Both GAP-43 and synaptophysin were expressed in normal soleus and extensor digitorum longus muscles at seven days. Synaptophysin was still expressed at 14 days, but GAP-43 expression had declined. Following nerve injury at three days of age, there was no GAP-43 or synaptophysin immunoreactivity in nerve terminals at seven days. By 21 days, there were 17.3+/-2.1 GAP-43-positive terminals per section in the soleus and 17.7+/-1.4 in the extensor digitorum longus, with mean terminal areas of 47.5+/-3.3 and 49.8+/-2.6 microm(2), respectively. In animals in which nerve crush was preceded by 4-aminopyridine treatment, at 21 days there were 32.9+/-2.6 GAP-43-immunoreactive terminals in the soleus and 44.9+/-2.3 in the extensor digitorum longus, with a mean area of 122.7+/-6.6 microm(2) in the soleus and 136.2+/-9.7 microm(2) in the extensor digitorum longus. These results indicate that in muscles pretreated with 4-aminopyridine, prior to nerve crush at three days, there are significantly more terminals, which occupy a larger area than in untreated muscles. Thus, increasing transmitter release prior to nerve injury significantly improved the ability of axons to reinnervate muscle.