Abstract
We have previously reported that the spinal angiotensin (Ang) system is involved in the modulation of streptozotocin (STZ)-induced diabetic neuropathic pain in mice. An important drawback of this model however is the fact that the neuropathic pain is independent of hyperglycemia and produced by the direct stimulation of peripheral nerves. Here, using the leptin deficient ob/ob mouse as a type 2 diabetic model, we examined whether the spinal Ang system was involved in naturally occuring diabetic neuropathic pain. Blood glucose levels were increased in ob/ob mice at 5-15 weeks of age. Following the hyperglycemia, persistent tactile and thermal hyperalgesia were observed at 11-14 and 9-15 weeks of age, respectively, which was ameliorated by insulin treatment. At 12 weeks of age, the expression of Ang-converting enzyme (ACE) 2 in the spinal plasma membrane fraction was decreased in ob/ob mice. Spinal ACE2 was expressed in neurons and microglia but the number of NeuN-positive neurons was decreased in ob/ob mice. In addition, the intrathecal administration of Ang (1-7) and SB203580, a p38 MAPK inhibitor, attenuated hyperalgesia in ob/ob mice. The phosphorylation of spinal p38 MAPK was also attenuated by Ang (1-7) in ob/ob mice. These inhibitory effects of Ang (1-7) were prevented by A779, a Mas receptor antagonist. In conclusion, we revealed that the Ang (1-7)-generating system is downregulated in ob/ob mice and is accompanied by a loss of ACE2-positive neurons. Furthermore, Ang (1-7) decreased the diabetic neuropathic pain through inhibition of p38 MAPK phosphorylation via spinal Mas receptors.