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

Neonatalsciatic nerve crush results in a sustained reduction of the mass ofboth extensor digitorum longus (EDL) and soleus (SOL) musclesin the rat. Type IIB fibers are selectively lost from EDL. We haveinvestigated the effects of ciliary neurotrophic factor (CNTF) combinedwith neurotrophin (NT)-3 or NT-4 on muscle mass, as well as the number,cross-sectional area, and distribution of muscle fiber types and thenumber of motor neurons innervating EDL and SOL 3 mo after transientaxotomy 5 days after birth. Both NT treatments prevented theaxotomy-induced loss of muscle mass in both EDL and SOL and of totalnumber of muscle fibers in EDL but not in SOL. Although IIB fiber losswas not prevented, both NT treatments resulted in altered fiber typedistribution. Both NT combinations also reduced the loss of EDL motorneurons. These data suggest that a differential distribution of NTreceptors on either motor neurons or muscle fibers may lead todifferent levels of susceptibility to neonatal axotomy.