Absence of Nerve - muscle Interaction Influences the Survival of Developing Motoneurons

Following sciatic nerve crush at birth, approximately 70% of motoneurons to the soleus and 60% of motoneurons to the tibialis anterior (TA) and to the extensor digitorum longus (EDL) die. However, following nerve injury at 5 days, there is negligible motoneuron death. We investigated whether the interaction between the nerve and its target during these 5 days is an important factor for the ability of the motoneuron to survive injury. Nerve - muscle interaction was blocked shortly after birth by alpha-bungarotoxin (BTX) and the effect on motoneuron survival after subsequent injury was examined. It was confirmed that sciatic nerve crush at 5 days produced no significant reduction in motoneuron numbers. However, if nerve crush was preceded by paralysis with alpha-bungarotoxin, the number of surviving motoneurons after nerve injury at 5 days was substantially reduced. On the operated side only 43 +/- 6.68% of the motoneurons of the soleus pool survived and even fewer, 14 +/- 5.0%, in the TA and EDL pool. In a control group of animals paralysed with alpha-bungarotoxin at birth but receiving no nerve crush, there was no appreciable reduction in the motoneuron numbers at 28 days in either motor pool. It is concluded that blocking of nerve - muscle interaction by paralysis in early postnatal life reduces the motoneurons' ability to survive nerve injury later in life, and that this effect is more severe for the motoneurons to the TA and EDL than to the soleus.