ELIMINATION OF MOTOR-NERVE TERMINALS IN NEONATAL MICE EXPRESSING A GENE FOR SLOW WALLERIAN DEGENERATION (C57BL WLD(S))/

Degeneration of motor terminals after nerve section occurs much more s lowly than normal in young adult mice of the C57BI/Wld(S) strain. This observation prompted us to re-examine the possible role of degenerati on and intrinsic axon withdrawal during neonatal synapse elimination. Polyneuronal innervation was assayed by two methods: intracellular rec ording of end-plate potentials in cut-muscle fibre preparations of iso lated hemidiaphragm and soleus muscles; and in silver-stained preparat ions of triangularis sterni and transversus abdominis muscle fibres, N o differences in the rate of synapse elimination were detected in unop erated Wlds compared with CBA, C3H/HE and BALB/c mice. At 3 days of ag e, >80% of fibres were polyneuronally innervated. By 7 days this decli ned to similar to 20% of hemidiaphragm, 50% of triangularis sterni and 60% of soleus fibres. Nearly all fibres were mononeuronally innervate d by 15 days. The mean number of terminals per triangularis sterni mus cle fibre 7 days after birth was 1.55 +/- 0.07 in Wlds and 1.56 +/- 0. 09 in wild-type mice, Three to 4 days after sciatic nerve section, nea r-normal numbers of motor units were evident in isometric tension reco rdings of the soleus muscle, and intracellular recordings revealed man y polyneuronally innervated fibres. Mononeuronally and polyneuronally innervated fibres were also observed in silver-stained preparations of soleus and transversus abdominis muscles made 3-4 days after sciatic or intercostal nerve section. We conclude (i) that the W/ds gene has n o direct impact on the normal rate of postnatal synapse elimination, ( ii) that Wallerian degeneration and synapse elimination must occur by distinct and different mechanisms, and (iii) that muscle fibres are ab le to sustain polyneuronal synaptic inputs even after motor axons have become disconnected from their cell bodies.