Comparative Analysis Of Diabetic Neuropathic Pain In Rats And Mice

Diabetic neuropathic pain(DNP)is the main symptom of diabetic peripheral neuropathy(DPN),which is one of the most common complications of diabetes.Symptoms of DNP exhibited as spontaneous pain,allodynia,and hyperalgesia.The incidence of DNP in diabetic patients is about 10%-20% and 40%-60% in diabetic patients with documented neuropathy.The mechanisms underlying DNP remain elusive and the treatment drugs and other approaches are very limited.It is important in theory and clinical management to understand the development of DNP,however,based on the data and experiences reported by our previous study and many other labs in the filed in the whole world,the mostly used animal models of DNP in rats and mice are very unstable in incidence of development of neuroathy and neuropathic diabetic pain.Therefore,this study was to study and compare,in the most commonly used models in Sprague Dawley(SD)rats and C57BL/6 mice,the changes of the key indicators including blood glucose,pancreas function,incidence of diabetes and diabetic pain and accompanied neurochemical alterations.The purpose of this study was to provide a reliable models of diabetic neuropathic pain in rats and mice for studying mechanisms underlying DNP and developing new drug for treatment of DNP.We established STZ-induced diabetes models in SD rats and C57BL/6 mice.STZ was intraperitoneally injected in SD rats(single dose,70 mg/kg)and in C57BL/6 mice(40 mg/kg,daily for 5 consecutive days).The success model of diabetes was evaluated by the blood glucose(rat: >16.6mmol/L;mouse:>13.8mmol/L)combined with the pancreas damage examined by HE staining of pancreas and peripheral nerve neuropathy.DNP was evaluated by testing the mechanical sensitivity of the hind paw to the Von Frey Filament stimulation in the STZ rats and STZ mice.The significantly increased mechanical sensitivity(mechanical allodynia)was used as the behavioral sign of DNP.The results showed that,six weeks after STZ injection,incidences of diabetes,mortality and DNP in rats and mice,respectively,were as follows: diabetes,86.4% and 75.9%;mortality,13.6% and 0%;and DNP,56.3% and 85.4%.The most prominent phenomenon is that there were approximately half of the STZ rats did not show painful mechanical allodynia and such an incidence was greatly lower than that in mice(85.4%).These findings indicate that the previously reported studies on DNP using the STZ model in SD rats may have severe defects in their results and conclusions.On the basis of these observations,we continued to investigate the possible reasons underlying such difference in the mechanical sensitivity in the animals.Western blotting and immunohistochemical analyses showed that expression of GFAP and IBA1 in the spinal cord in painful SD rats and C57BL/6 mice were significantly greater than those in groups of na?ve control and non-pain STZ animals.There were no significant alterations in the signs of neurochemical alterations between control na?ve and non-pain STZ-animals.These results suggest that activation of the astrocytes and microglial cells is strongly associated with the development of DNP and may play important roles in production and persistence of DNP.In addition,Luxol Fast Blue staining showed a significance demyelination occurring in the sciatic nerves in both STZ-SD rats and STZ-C57BL/6 mice with mechanical allodynia,one of the behaviorally expressed symptoms of DNP.In conclusion,this study demonstrates the differences in incidences of diabetes,mortality,and DNP in SD rats and C57BL/6 mice following STZ treatment.The incidence of diabetes and mortality in those rats are significantly higher than that in C57BL/6 mice,while the incidence of DNP in STZ-rats is significantly lower than that in STZ-mice.Meanwhile,the high blood glucose can cause activation of astrocytes and peripheral nerve demyelination,which are positively related to the development of DNP.Considering that there are approximately half of the SD rats that previously received STZ treatment do not exhibit mechanical allodynia,thus the behaviorally expressed painful conditions has to be identified in each of animals to make sure the animal is with DNP.Actually,there should be no surprise for this low incidence of DNP because the low incidence of DNP is true in diabetic patients.It is thus clear that the previous publications focusing on STZ-induced diabetic pain in SD rats without identifying their behaviorally expressed painful symptoms in each of the included animals may result in inaccurate or incorrect results and conclusions.This study has provided reliable models of DNP in SD rats and C57BL/6 mice and is valuable for further studying mechanisms underlying DNP and developing new drug for treatment of DNP.