Manipulating transmitter release at the neuromuscular junction of neonatal rats alters the expression of ChAT and GAP-43 in motoneurons.

During early postnatal development, motoneurons innervating rat hindlimb muscles die following injury to the sciatic nerve. However, prematurely enhancing transmitter release from nerve terminals of neonatal rats renders motoneurons less vulnerable to nerve injury, whereas reducing transmitter release increases their susceptibility to injury. Thus, alterations in transmitter release may have an influence on motoneuron phenotype. Here we investigated the relationship between the vulnerability of motoneurons to injury, and the expression of proteins associated with axonal growth and neuromuscular transmission. We examined the effect of agents that affect transmitter release from nerve terminals and that have been shown to influence the expression of transmitter and growth related proteins in developing motoneurons in response to nerve injury. In newborn rats, implants containing either 4-aminopyridine (4-AP), to increase transmitter release, or magnesium sulphate (MgSO4), to decrease release, were applied to the soleus muscle in one hindlimb. The effect of these treatments on the activity of choline acetyltransferase (ChAT) in nerve terminals in the soleus muscle was measured using a radiochemical assay. Levels of GAP-43 in sciatic nerve were also assessed, by Western blot analysis. The results showed that during normal development, there was a gradual increase in ChAT activity during the second week of postnatal development, whereas GAP-43 levels declined sharply between postnatal days 12-14. However, following 4-AP treatment, there was a dramatic increase in ChAT activity in nerve terminals contacting the treated soleus muscles and the levels of GAP-43 in the sciatic nerve declined at an earlier age than normal. Conversely, following treatment with MgSO4 the normal increase in ChAT activity that occurs during the second postnatal week was delayed, and GAP-43 levels in the sciatic nerve were maintained for significantly longer than normal. Thus, manipulating transmitter release from nerve terminals in neonatal rats alters the normal pattern of expression of transmitter and growth related proteins in developing motoneurons. This alteration in protein expression may influence both the maturation of motoneurons and their ability to withstand nerve injury.