Relevance of hyperglycemia to early mechanical hyperalgesia in streptozotocin-induced diabetes

A modified von Frey filament test and an algesiometer paw pressure test were used to measure mechanical nociceptive withdrawal thresholds of the hind limb of control rats and rats injected with streptozotocin (STZ, 50 mg/kg). STZ treatment induced hyperglycemia (HG rats) in about 40% of treated animals. The rest of the STZ-treated and control rats remained normoglycemic (NG rats) throughout the entire experiment. No indications of mechanical hyperalgesia were observed in control groups of animals injected with physiological buffer only. However, both the behavioral tests used detected a 15-30% decrease in the mechanical nociceptive threshold of rats treated with STZ. Furthermore, mechanical nociceptive threshold changes were statistically indistinguishable between NG and HG rats. Glucose tolerance test did not reveal abnormalities of glucose metabolism in NG rats (compared to control animals). However, 1 week after STZ injection, the serum insulin level of NG rats was significantly lower than that of age-matched control rats (0.81 +/- 0.16 vs. 3.5 +/- 0.4 ng/mL; p < 0.01). These data strongly argue that systemic hyperglycemia is not the only factor triggering the development of mechanical hyperalgesia in the STZ rat model of diabetes. Other than hyperglycemia, consequences of insulinemia or insulinemia itself may play an important role in early impairment of mechanical nociception in this animal model.