Peripheral nerve changes in experimental diabetes and the effects of an aldose reductase inhibitor
The object of this study was to investigate the effects of an aldose reductase inhibitor, sorbinil (Pfizer inc), in relation to some of the biochemical, structural and functional changes associated with diabetic nerves. An animal model, the streptozotocin-induced diabetic rat was used and the following studies were performed: Motor nerve conduction velocity (MNCV) was measured in the tibial nerve over a 6 month period. A group of rats were examined at the onset of the experiment (OC) to provide a baseline for comparison to all other groups. Age matched control (AMC) animals showed a 13% increase in MNCV during the first 3 months of the experiment with little increase thereafter. The diabetic animals (DC) did not significantly differ over the experimental period from the OC animals and were thus slower conducting than the corresponding AMC group. Administration of sorbinil (25 mg/Kg) to rats from the onset of diabetes had no effect on MNCV by 3 months but had normalised values by 6 months. Nerve glucose, sorbitol, fructose and myo-inositol levels were examined by GLC. Sorbinil had no effect on nerve glucose values but prevented the 10-fold increase in nerve sorbitol values observed with the DC animals. Sorbinil partially normalised nerve fructose values after 3 months of treatment and fully normalised them after 6 months. Myo-inositol (MI) levels showed a 45% reduction by 3 months of diabetes but were normal after 6 months. Sorbinil showed a tendency to restore the reduced MI values by 3 months. Morphometric profiles were examined in the tibial nerve. Axon areas demonstrated a 14% reduction at both 3 and 6 months of diabetes while myelin areas were increased by 13 and 22% respectively. Sorbinil treatment allowed normal axon growth and normalised myelin areas. MNCV was examined in the tibial and gastrocnemius nerves. As above, diabetes prevented the normal MNCV maturation in the tibial nerve. Sorbinil administration (25 mg/Kg) to rats initially diabetic for 2 months, was ineffective in restoring MNCV in the tibial nerve though a partial recovery was observed after 4 months of treatment. MNCV in the gastrocnemius nerve of the DC animals continued to fall as the experiment progressed, reaching a 33% reduction below the OC animals by 3 months. A spontaneous recovery was observed thereafter. Sorbinil partially normalised MNCV in the gastrocnemius nerve after 1 month. These changes exactly paralleled the changes in nerve MI levels. Sorbinil reversed the already elevated nerve sorbitol levels after 1 month of treatment though nerve fructose levels were only partially normalised after 4 months. A morphometric evaluation of the triceps surae nerve (containing fibres to gastrocnemius and soleus muscles) after 4 months of the experiment demonstrated an 18% increase of myelin area in the DC animals. Axon areas were unaffected by diabetes. Sorbinil treatment for 2 months partially normalised myelin areas. Sorbinil administration at doses of 7.5, 12.5 and 25 mg/Kg to rats that had been diabetic for 2 months did not normalise MNCV in the tibial nerve. However, 12.5 and 25 mg/Kg produced a significant improvement in MNCV of the gastrocnemius nerve after 1 month of treatment. 7.5 mg/Kg had no effect in this nerve. All doses of sorbinil produced a trend towards reversing the already elevated nerve sorbitol levels, though 25 mg/Kg was effective after 1 month of treatment whereas 12.5 mg/Kg required 2 months. 7.5 mg/Kg did not fully normalise nerve sorbitol levels. Nerve fructose values remained elevated, though treatment with 25 mg/Kg of sorbinil produced a reduction towards normal values. All 3 doses partially normalised MI levels. For all DC groups, sciatic nerve water content was significantly elevated after a 1 month experimental period. Sorbinil treatment, either given from the induction of diabetes or given after rats were initially diabetic for 2 months, had only a small effect on water content and values remained elevated compared with the controls.