Improving the reliability and utility of streptozotocin-induced rat diabetic model

Abstract

The Streptozotocin- (STZ-) induced diabetic model is widely used; however, unexplained acute toxicity has given the model an unreliable reputation. To improve the reliability and utility of this model, we characterize the age dependence of STZ toxicity and introduce novel endpoints to assess diabetic complications and reveal possible mechanisms for diabetic development. Diabetes was induced by STZ injection into male, 6 to 23 weeks old, Sprague-Dawley rats. Their metabolic (glucose, lipids, and hormones), inflammatory (cytokines), histologic and behavioral endpoints were observed for 1.2 years. Analgesic compounds were assessed for efficacy treating neuropathy. Acute mortality, within a week of STZ injection (50–65 mg/kg i.v.), was inversely correlated to animal age. Only 3% of rats, age 6–11 weeks, died in the week following STZ injection, whereas 83% of rats 12 to 17 weeks old and 91% of rats 18 weeks or older died in the same week. Partial model recovery (normalized insulin, glucose and food/water intake) was observed starting at week 36; however, pain scores, kidney enlargement, and cataract formation continued to show progression consistent with the diabetic state. Unique noninvasive observational measurements, such as haircoat quality and diarrhea scores, served as useful endpoints for this model. The increased plasma cytokines (such as TNF-α, IL-4, and IL-6) and inflammatory cell infiltration into the pancreatic islets are strong evidence of inflammation in the STZ-induced diabetic model. Pancreatic tissue staining revealed total islet area reduction and confirmed STZ-specific pancreatic toxicity; however, the β-cell density per area in pancreatic islets and insulin levels statistically increased over time in the diabetic rats, suggesting a mechanism for partial recovery of diabetic symptoms. Voltage-gated sodium channel (NaV1.7 specific, peripherally restricted) blocker, CC4148, inhibited neuropathy without side effects as compared to a nonspecific sodium channel inhibitor, Mexiletine, or GABA analog, Pregabalin, which inhibited neuropathy with side effects.