Restoration of focal multiple innervation in rat muscles by transmission block during a critical stage of development.

1. The soleus, extensor digitorum longus and peroneus tertius muscles of the hind leg were paralysed by botulinum toxin injection in neonatal rats varying in age from 8 to 31 days. The soleus muscle was similarly paralysed in adult rats.2. Muscles were excised after different periods of block, neuromuscular transmission was assessed in vitro, and the nerve terminal size and amount of terminal sprouting and multiple innervation determined histologically using silver (Ag) and zinc iodide-osmium (ZIO) tetroxide stains.3. Recovery from the block induced by botulinum toxin was more rapid in immature multiply-innervated muscles than in muscles of older rats in which all multiple innervation had been eliminated.4. Terminals in muscles blocked during the first month after birth grew rapidly in size and developed a characteristic granulose morphology in the first few days following the block. This change did not occur in fully adult rat solei.5. Sprouts growing from the terminals were infrequent in muscles paralysed before day 16. Terminal sprouts were more frequent in muscles paralysed between 16 and 31 days of age, but were very infrequent in adult solei.6. In confirmation of Thompson, Kuffler & Jansen (1979) paralysis begun at 10 days was followed initially by a continued fall in multiple innervation detected electrophysiologically. After 2 days the percentage of muscle fibres with more than one input rose. The extra inputs did not come from terminal sprouts. They innervated the single synaptic site on each muscle fibre as they do during normal synaptogenesis.7. The amount of multiple innervation regained 5-10 days after the start of paralysis became progressively less the later paralysis started and was approximately equal to the level existing at the time the block was begun. Thus paralysis starting on or after day 16 (when all excess inputs have normally withdrawn) caused no return of multiple innervation.8. The return of multiple inputs and the swelling of the nerve terminals are presumably responses of the motoneurone to growth stimuli from inactive muscle. It is not clear whether the return of multiple innervation in the 10-15 day old rats is due to reactivation of inputs still in close contact with the end-plate before withdrawal or to regrowth of partly retracted nerve branches.9. A parallel is drawn between the limited period when inactivity can reinstate multiple innervation, and the critical period in the developing visual cortex.