Activated autophagy in spinal cord contributes to maintaining diabetic neuropathic pain in rats model

Abstract

To observe the role of autophagy in maintaining diabetic neuropathic pain in rats model. A total of 44 male Sprague-Dawley rats were randomly divided into diabetic neuropathic (n = 36) and normal control (n = 8) groups. Diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ) (60 mg/kg body weight, i.p) freshly dissolved in citrate buffer (pH = 4.5). For assessing the presence of mechanical hyperalgesia in diabetic rats, mechanical paw-withdrawal threshold (MWT) in response to punctuate mechanical stimuli was measured. At Week 4 post-injection, the rats with mechanical pain threshold decreasing over 50% as compared to baseline were designated as diabetic neuropathic pain rats. They were randomly divided into three groups of neuropathic pain (DP), neuropathic pain plus rapamycin (DR) and neuropathic pain plus 3-methyladenine (3-MA) (DA). The DR group received an intraperitoneal injection of rapamycin (1 mg/kg body weight) for Day 1 to Day 14 after grouping. At the same timepoint, the DA group had an intraperitoneal injection of 3-MA (2 mg/kg body weight) and the other group phosphate buffered saline (PBS) (1 ml/kg body weight). MWT was measured at week 1, 2, 3, 4 after STZ injection and at day 1, 3, 5, 7, 9, 14 after rapamycin, 3-MA or PBS injections. Spinal cord tissues were used to examine the expression of LC3, Beclin-1 and P62 protein by Western blot at Day 14 after medication. The mechanical threshold of group DR decreased further from Day 3 to Day 14 after rapamycin injection compared to baseline [(4.8±0.8), (4.3±0.7), (4.1±0.6), (3.6±0.5), (3.3±0.6) vs (5.3±0.9) g, P<0.05]. The mechanical threshold of group DA began to increase from Day 5 to Day 14 after 3-MA injection [(7.0±0.8), (7.7±1.0), (9.1±0.9), (9.6±1.1) vs (5.3±0.6) g, P<0.05]. The expressions of LC3-II and Beclin-1 protein in spinal cord of rapamycin-treated rats was significantly higher than those of non-supplemented diabetics (1.32±0.12 vs 1.02±0.11; 1.03±0.08 vs 0.80±0.06, P<0.05). Otherwise the expressions of these protein in spinal cord of 3-MA-treated rats were significantly lower than those of non-supplemented diabetics (0.70±0.09 vs 1.02±0.11; 0.55±0.05 vs 0.80±0.06, P<0.05). Up-regulated autophagy in spinal cord partially contributes to the maintenance of diabetic neuropathic pain.