Increased neuromuscular activity reduces sprouting in partially denervatedmuscles

The effects of increasing neural activity on sprouting remain unclear and c ontroversial. In a rat model of partial denervation of skeletal muscles, we investigated the effect of neuromuscular activity on sprouting. Rat hindli mb muscles were partially denervated by avulsion of either L4 or L5 spinal root. Immediately after partial denervation, the rats were divided into thr ee groups: (1) normal caged activity, (2) running exercise on wheels, 8 hr daily, and (3) functional electrical stimulation (FES) of sciatic nerves, 2 0 Hz for 8 hr daily. At 1 month, muscle unit (MU) enlargement was quantitat ed electrophysiologically and histochemically. MU twitch force was increase d by four- to fivefold by partial denervation in extensively denervated tib ialis anterior (TA) and medial gastrocnemius (MG) and by approximately twof old in moderately denervated plantaris (PL) and soleus (SOL). For the exten sively denervated TA and MG muscles, MU enlargement, measured electrophysio logically, declined significantly after an average of 1757 +/- 310 m/d runn ing exercise and daily FES for 1 month. The detrimental effects on MU enlar gement were much less but significant in the moderately denervated PL and d id not reach statistical significance in the moderately denervated SOL musc le. Histochemical evaluation of sprouting showed a reduction in the number of sprouts in the extensively denervated TA muscle, but not the moderately denervated PL and SOL muscles, by increased neuromuscular activity. Thus, i ncreased neuromuscular activity is detrimental primarily in muscles that ar e extensively denervated, and the MUs are smaller than under conditions in which the muscles experience normal physiological levels of activation.