Increased activity improves recovery of partially denervated fast rat muscles

Partial denervation of the neonatal rat extensor digitorum longus muscles by removing the L4 spinal nerve and thus 80% of its innervation [A.L. Connold, T.J. Fisher, S. Maudarbocus, G. Vrbová, Response of developing fast muscles to partial denervation, Neuroscience 46 (1992) 981-988; F. Tyc, G. Vrbová, The effect of partial denervation of developing rat fast muscles on their motor unit properties, J. Physiol. 482 (1995) 651-660] results in its permanent weakness. The possibility that the weakness that follows partial denervation is due to the effects of reducing activity of the muscle during a critical stage of development was studied here. Partial denervation was carried out in 3-day-old pups by removing the L4 spinal nerve. To enhance motor activity two days later some animals had injections of l-Dopa twice a day for 8 days. This treatment induced locomotor activity for at least 2 h/day. The muscles from treated and untreated animals were examined 2 months later. There was a significantly smaller reduction of weight and force production in the muscles from l-Dopa treated animals. Both twitch and tetanic force developed by the EDL muscle from the treated group was twice that of the control untreated group. This effect was due mainly to the larger size of the motor units (MUs) in the l-Dopa treated muscles compared to the controls. The mean motor unit force in the untreated group was 69% of that in the control muscle, whereas this value was 127% in the l-Dopa treated animals. Thus it appears that the activity induced by treatment with l-Dopa could to some extent prevent the loss of weight and force output seen after partial denervation of young fast muscles.