Neurotrophic factors enhance the survival of muscle fibers in EDL, but not SOL, after neonatal nerve injury.

Neonatal sciatic nerve crush results in a sustained reduction of the mass of both extensor digitorum longus (EDL) and soleus (SOL) muscles in the rat. Type IIB fibers are selectively lost from EDL. We have investigated the effects of ciliary neurotrophic factor (CNTF) combined with neurotrophin (NT)-3 or NT-4 on muscle mass, as well as the number, cross-sectional area, and distribution of muscle fiber types and the number of motor neurons innervating EDL and SOL 3 mo after transient axotomy 5 days after birth. Both NT treatments prevented the axotomy-induced loss of muscle mass in both EDL and SOL and of total number of muscle fibers in EDL but not in SOL. Although IIB fiber loss was not prevented, both NT treatments resulted in altered fiber type distribution. Both NT combinations also reduced the loss of EDL motor neurons. These data suggest that a differential distribution of NT receptors on either motor neurons or muscle fibers may lead to different levels of susceptibility to neonatal axotomy.