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 relea se from nerve terminals prevents the death of motoneurones following neonat al nerve injury. Whether this increase in motoneurone survival is reflected in an improvement in the reinnervation of muscle was studied here. The mus cles in one hindlimb of newborn rats were treated with 4-aminopyridine. Thr ee days later, the sciatic nerve was crushed in the treated leg. When the a nimals were seven, 14 and 21 days of age, the soleus and extensor digitorum longus muscles were removed and processed for GAP-43 (a 43-kDa growth-asso ciated protein) and synaptophysin immunocytochemistry. Both GAP-43 and syna ptophysin were expressed in normal soleus and extensor digitorum longus mus cles at seven days. Synaptophysin was still expressed at 14 days, but GAP-4 3 expression had declined. Following nerve injury at three days of age, the re was no GAP-43 or synaptophysin immunoreactivity in nerve terminals at se ven 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, w ith mean terminal areas of 47.5 +/- 3.3 and 49.8 +/- 2.6 mum(2), respective ly. In animals in which nerve crush was preceded by 4-aminopyridine treatme nt, at 21 days there were 32.9 +/- 2.6 GAP-43-immunoreactive terminals in t he soleus and 44.9 +/- 2.3 in the extensor digitorum longus, with a mean ar ea of 122.7 +/- 6.6 mum(2) in the soleus and 136.2 +/- 9.7 mum(2) in the ex tensor digitorum longus. These results indicate that in muscles pretreated with 4-aminopyridine, pri or to nerve crush at three days, there are significantly more terminals, wh ich occupy a larger area than in untreated muscles. Thus, increasing transm itter release prior to nerve injury significantly improved the ability of a xons to reinnervate muscle. (C) 2001 IBRO. Published by Elsevier Science Lt d. All rights reserved.