Impairment of spinal cord conduction velocity in diabetic rats

Peripheral neuropathy is a common and well-studied complication of diabetes mellitus, but the possibility that central neuropathy is also present has received scant attention. Based on recent evidence showing that insulin has a direct effect on axon formation and neuronal survival in vitro, it was predicted that functional neuropathy would be present in the spinal cord of diabetic animals. Although structural lesions are encountered in the spinal cord of diabetic patients at autopsy, the functional corollaries have essentially remained unstudied. We used a new procedure to study evoked spinal cord potentials in the rat, which revealed a significant retardation in conduction velocity in streptozocin (STZ)-induced diabetic animals. This retardation was not due to a toxic effect of STZ on the involved spinal cord sensory pathways, because insulin infusion prevented the development of spinal cord neuropathy. The kinetics and magnitude of decline in conduction velocity were similar in the spinal cord, saphenous nerve, and common peroneal nerve during the first 2 wk, suggesting that a common mechanism was involved. After 10 wk, a spontaneous improvement in function was observed in the spinal cord and common peroneal nerve but not in the saphenous nerve. Our results support the hypothesis that central nervous system dysfunction can occur along with peripheral sensory neuropathy in diabetes.