Insulin-induced hypoglycemic peripheral motor neuropathy in spontaneously diabetic WBN/Kob rats

Abstract

Intensive insulin therapy can lead to hypoglycemia, with patients sometimes developing hypoglycemic neuropathy. Spontaneously diabetic Wistar Bonn Kobori (WBN/Kob) rats develop diabetic peripheral motor neuropathy characterized by segmental demyelination and axonal degeneration. We examined the short-term effects of hypoglycemia on neuropathic changes in these rats. Spontaneous diabetic WBN/Kob rats received insulin implants for 40 d and were divided into 3 groups based on blood glucose levels: group N, normoglycemic to slightly hyperglycemic (150 to 250 mg/dL); group H, hypoglycemic to slightly hyperglycemic (50 to 200 mg/dL); and group D, nontreated spontaneously diabetic (350 to 420 mg/dL). Conduction velocity was measured in sciatic-tibial motor nerves; these nerves also underwent qualitative and quantitative histomorphologic analysis. Conduction velocity was not significantly different in N, D, and H groups. Morphologic analysis of the sciatic nerves of H rats showed severe changes, including axonal degeneration, myelin distention, and endoneurial fibrosis, that tended to occur in large, myelinated fibers. N and D rats showed relatively mild changes. The degree and distribution of degenerated nerve fibers in H rats were significantly higher than in N and D rats. These results suggest that hypoglycemia of less than 50 mg/dL induced severe peripheral neuropathy. Hypoglycemic lesions differed from the hyperglycemic lesions in diabetic WBN/Kob rats. This rat strain is an appropriate model for investigating the hypoglycemic peripheral neuropathy that can be associated with a diabetic condition.

Figure 1.

Blood glucose levels (mean ± 1 SD) in diabetic WBN/Kob rats with or without insulin treatment for 6 wk. Rats in group D maintained hyperglycemia (366 to 455 mg/dL) throughout the experimental period. The average blood glucose level of N rats dropped to 150 mg/dL and gradually increased to 250 mg/dL by the end of the experimental period. Average blood glucose in H rats sharply dropped to 50 mg/dL and remained at 50 to 200 mg/dL. *, P < 0.05.

Figure 2.

Representative sections of rat sciatic nerve. Nontreated spontaneously diabetic rats (group D; A, D) and normoglycemic to slightly hyperglycemic rats (group N; B, E) show only mild changes including slight myelin distention and endoneurial fibrosis around vessels. Small to large axons are uniformly positive for neurofilament in group D (G) and group N (H) rats. In contrast, hypoglycemic to slightly hyperglycemic rats (group H) had many degenerate fibers with endoneurial fibrosis (C); degenerate fibers were characterized by axonal atrophy with myelin distention (F). Longitudinal sections of nerves from group H rats contained many myelin ovoid formations and loss of immunopositivity for neurofilaments (I). Toluidine blue stain, A through F; Luxol fast blue and neurofilament immunostaining, G through I. Bar, 100 µm (D through F), 50 µm (G through I).

Figure 3.

Representative sections of sciatic nerves of hypoglycemic to slightly hyperglycemic rats (group H). (A) These affected fibers are bound together with hypertrophic Schwann cells containing myelin debri and vacuoles (arrows). Several thinly myelinated fibers are surrounded by a single hypertrophic Schwann cell (arrowheads). (C) A single hypertrophic Schwann cell surrounds both small, thinly myelinated fibers and medium fibers with myelin distention. Toluidine blue stain; bar, 100 µm.