Ongoing pain in streptozotocin model of diabetes in the rat: correlation with cutaneous cheminociception.

Streptozotocin (STZ) is commonly used to induce diabetes mellitus in experimental animal studies on peripheral diabetic neuropathy (PDN). Animals with STZ model of diabetes commonly develop changes in test stimulus-evoked pain behavior. However, it is still unclear whether rats with STZ model of diabetes have ongoing pain. Here we assessed whether STZ-induced diabetes induces ongoing pain-like behavior in male rats using conditioned place-preference (CPP) paradigm. CPP was tested in the fourth week of diabetes by pairing one chamber of the CPP device with vehicle and another chamber with either pregabalin (an established analgesic; 30 mg/kg i.p.; n = 9) or Chembridge-5861528 (a TRPA1 channel antagonist; 30 mg/kg i.p.; n = 9). After drug-pairings, the animals were allowed to choose which chamber they preferred. Mechanical sensitivity was assessed with monofilaments and chemonociception in the skin by determining mustard oil-induced pain behavior. Diabetic animals developed in two weeks mechanical hypersensitivity that changed into hyposensitivity by the fourth week. Mustard oil-induced sustained pain was reduced by the 4(th) week. After 4 weeks of diabetes, neither pregabalin nor the TRPA1 antagonist induced a significant overall change in the median CPP, although both drugs significantly reduced median withdrawal responses evoked by noxious mechanical stimulation. Pregabalin-induced CPP, however, had a significant positive correlation with the sustained pain behaviour induced by topical mustard oil. In conclusion, the present results suggest that the response to topical mustard oil may predict ongoing pain-like behavior in the STZ model of diabetes.