Effection Of Catalpol On The Expression Of IGF-Ⅰ And Akt In Dorsal Root Ganglia And Sciatic Nerve Of Diabetic Rats

Background and Objective:Catalpol is one of the activity monomers in the traditional Chinese medicine, it has been proved owing the effection of antioxidative and neuroprotective, but it hasn't been reported on the research about diabetic peripheral neuropathy (DPN). For the past few years, the relationship between the deficiency of insulin-like growth factors (IGFs) especially IGF-Ⅰwhich is one of the neurotrophic factors and the initiation and development of DPN caused attention under the state of diabetes. But the mechanism produced by catalpol is not wellknown whether it is carried out by activating downstream conduction pathway of IGF-Ⅰwhich named phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB/Akt). It may be contributed to study the pathogenesis of DPN deeply and to promote traditional Chinese medicine monomer clinical application by investigating the above problems. This research aims at investigating the intervention and mechanism of catalpol on DPN by viewing the expression of IGF-Ⅰand Akt in dorsal root ganglia (DRG) and sciatic nerve of diabetic rats.Methods:Male Sprague-Dawley (SD) rats were randomly divided into three groups: normal control group (NC, n=8), diabetic control group (DC, n=12), and diabetic group treated with catalpol (CI, n=12). The diabetic rat models (DC and CI) were induced by intraperitoneal injection of streptozotocin (STZ) (50mg/kg). Since 2 week (wk) point, the rats of CI group were administered with catalpol (5mg/kg) by intraperitoneal injection once daily for 2 weeks. All rats accepted the electrophysiologic examination of right sciatic nerve at the 4 wk point. Before sacrificed, rats blood was harvested from heart for measuring glycosylated hemoglobin (HbA1c) level. Then the left sciatic nerve, both L3, L4 and left L5 DRG were harvested tobe used for extraction of total RNA and reverse transcription-polymerase chain reaction (RT-PCR) of IGF-Ⅰand Akt1 gene. The right sciatic nerve and right L5 DRG were harvested for detection the expression of IGF-Ⅰand p-Akt immunohistochemically.Results:1. Body weight (g), blood glucose (mmol/L) and HbA1c (%): After diabetic model, body weight among three groups were similar (P>0.05), which was NC group (227.2±14.0), DC group (231.3±15.5), CI group (230.3±16.6). Blood glucose of DC group and CI group were 27.22±3.73 and 30.32±4.12 respectively, and were significantly (P < 0.001) higher than that of NC group (5.45±1.03). Before sacrificed, body weight of DC group and CI group were 271.5±48.4 and 269.2±33.4 respectively, and were significantly (P< 0.001) lower than that of NC group (345.3±20.9). Blood glucose of DC group and CI group were 27.77±6.43 and 27.46±6.04 respectively, and were significantly (P<0.001) higher than that of NC group (7.70±1.45). HbA1c of DC group and CI group were 3.10±0.43 and 2.98±0.31, and were significantly (P<0.001) higher than that of NC group (2.01±0.14). But there was no significant difference between DC group and CI group on body weight, blood glucose and HbA1c (P>0.05).2. Electrophysiologic examination data: At the 4 wk of diabetes, motor nerve conduction velocity (MNCV, m/s) of DC group and CI group were 35.18±3.20 and 41.23±2.86 respectively, and were significantly (P<0.05) lower than that of NC group (47.18±5.04). And the MNCV of CI group increased more than that of DC group (P<0.05). Sensory nerve conduction velocity (SNCV, m/s) of DC group and CI group were 32.80±4.38 and 39.25±2.63 respectively, and were significantly (P<0.05) lower than that of NC group (44.33±3.18). And the SNCV of CI group increased more than that of DC group too (P<0.05). The minimum latency of F wave (minFLat, ms) of DC group and CI group were 8.65±0.89 and 8.28±0.59 respectively, and were significantly (P<0.05) longer than that of NC group (6.82±0.26). But there was no statistical difference between CI group and DC group on the minFLat. 3. PT-PCR: In DRG tissue, the expression of IGF-ⅠmRNA in DC group and CI group decreasd significantly than that of NC group (P<0.05),and there was no statistical difference between CI group and DC group (P>0.05). The expression of Akt1 mRNA in DRG of DC group decreasd significantly than that of NC group and CI group (P<0.05), and there was no statistical difference between CI group and NC group (P>0.05). In the sciatic nerve of these groups, the expression of IGF-ⅠmRNA of DC group decreasd significantly than that of NC group and CI group (P<0.05), and there was no statistical difference between CI group and NC group (P>0.05). But the expression of Akt1 mRNA in sciatic nerve of every group was negative.4. The results of immunohistochemical techniques: In DRG tissue, the average optical density (AOD) of IGF-Ⅰin DC group and CI group were 14.16±1.32(×10^-2)and 17.69±1.22(×10^-2)respectively, and were significant- ly (P <0.05) lower than that of NC group [22.32±1.47(×10^-2)], and the AOD of CI group was higher than that of DC group significantly (P<0.05). The AOD of p-Akt in DC group and CI group were 7.31±0.69(×10^-2)and 9.76±1.42(×10^-2)respectively, and were significantly (P<0.05) lower than that of NC group[11.96±1.12(×10^-2)], and the AOD of CI group was higher than that of DC group significantly (P<0.05). In sciatic nerve of DC group and CI group, the AOD of IGF-Ⅰwere 14.36±2.21(×10^-2)and 18.62±1.73 (×10^-2)respectively, and were significantly (P<0.05) lower than that of NC group[20.82±1.39(×10^-2)], and the AOD of CI group was higher than that of DC group significantly (P<0.05). There were no statistical difference (P<0.05) of p-Akt in sciatic nerve among the DC, CI and NC group with the AOD 14.90±2.03(×10^-2),15.03±2.36(×10^-2)and15.45±1.29(×10^-2) respectively.Conclusions:1. Electrophysiologic abnormalities of sciatic nerve are present at the 4 wk point after diabetic model induced by STZ. And the expression of IGF-Ⅰdown-regulated in DRG and sciatic nerve. As well, the expression of Akt1 mRNA and its phosphorylated protein down-regulated in DRG.. So, DPN is probably related to the interruption of PI3K/Akt pathway.2. Catalpol has some neuroprotective effects on peripheral nerve of diabetic rats without influence on blood glucose, the mechanism may be that catalpol partially improves the activity of PI3K/Akt signal pathway via the IGF-Ⅰgene up-regulation.