MOTOR DENERVATION INDUCES ALTERED MUSCLE-FIBER TYPE DENSITIES AND ATROPHY IN A RAT MODEL OF NEUROPATHIC PAIN

Loose ligation of a sciatic nerve in rats (chronic constriction injury ; CCI) provokes sensory, autonomic, and motor disturbances like those observed in humans with partial peripheral nerve injury. So far, it is unknown whether these motor disturbances result from (mechanical) all odynia or from damage to the motor neuron. These considerations prompt ed us to assess, in CCI rats, the density of motor axons in both the l igated sciatic nerve and the ipsilateral femoral nerve. To this end, w e determined the number of cholinesterase positive fibres. II has been demonstrated previously that muscle fibre type density may be used as a measure of motor denervation and/or hypokinesia. Therefore, the myo fibrillar ATPase reaction was employed to assess fibre type density in biopsies obtained from the lateral gastrocnemius muscle (innervated b y sciatic nerve) and rectus femoris muscle (innervated by femoral nerv e). We observed axonal degeneration of motor fibres within the loosely ligated sciatic nerve, both at an intermediate (day 21) and at a late stage (day 90) after nerve injury. The reduction in the number of mot or nerve fibres was more pronounced distal to the site of the ligature s than proximal. A (less pronounced) reduction of motor fibres was obs erved in the ipsilateral (non-ligated) femoral nerve. In line with the se findings, we observed altered fibre type densities in muscle tissue innervated by the ligated sciatic nerve as well as the non-ligated fe moral nerve indicative of motor denervation rather than hypokinesia. T he findings of this study suggest that the motor disorder induced by p artial nerve injury involves degeneration of motor nerve fibres not on ly within the primarily affected nerve but also within adjacent large peripheral nerves. This spread outside the territory of the primarily affected nerve suggests degeneration of motor neurons at the level of the central nervous system. (C) 1998 Elsevier Science Ireland Ltd.