RESUMO
BACKGROUND: Studies have shown that Gingko biloba leaf extract (GbE) is effective in promoting the functions recovery of the brain that follows traumatic injury, in improving the dysfunctions of learning and memory of the elderly, and it is also effective in improving the plasticity in central nervous system (CNS). However, what is the effect on learning and memory functions of rats with type 2 diabetes mellitus?OBJECTIVE: To study the effects of GbE on the learning and memory dysfunction and glial fibrillary acidic protein (GFAP) expressions in hippocampus of diabetic rats.DESIGN: Complete-random design, controlled experimental study.SETTING: Department of Pharmacology, Guangxi Medical University.MATERIALS: A total of 84 male Wistar rats (180-220 g), 8 weeks old,SPF, were used in this study. GbE (containing 24.8% flavone glycosides and 6.2% diterpene lactone) was purchased from Guilin Xintejia Natural plants Pharmaceutical Factory, Guangxi Province, Lot No. 200405.METHODS: The experiment was completed at the Pharmacology Lab (Provincial Lab) of the Experimental Center of Guangxi Medical University from June 2004 to March 2005. ① A total of 70 rats were rendered diabetic by a single intraperitoneal injection of streptozotocin at a dose of 55 rmg/kg dissolved in citrate buffer (pH4.4) after 24 hours fasting. Tail vein blood glucose concentration was determined 4 days later using ONE TOUCH glucose meter. A total of 56 streptozotocin-treated rats with a blood glucose concentration of ＞ 15 mmol/L were recognized as type 2 diabetic rats. ② These diabetic rats were randomly divided into model group, insulin group, high-dose GbE group, and low-dose GbE group. There were 14 rats in each group. There was no difference in the blood glucose concentration among the groups. Another 14 male rats with an intraperitoneal injection of citrate buffer solution were served as control group. After division, drugs were given. Insulin 10 μ/kg was injected subcutaneously every day for 6 months. GbE 100 mg/kg and GbE 50 mg/kg were administered through intra-gastric method every day for 6 months.The diabetic group and control group were administered normal solution through intra-gastric method every day for 6 months. ③ Six months later,Morris water maze was operated on each group of rats. The Morris water maze consisted of a large circular pool [100 cmdimension, 60 cm height,filled to a depth of 42 cm with water at (25±1) ℃]. Within the pool a submerged platform (round, black, 8 cm diameter, 2 cm below the water surface) was hidden on a fixed location, 20 cm from the edge of the pool,in which milk powder was dissolved to obscure the platform. The rat could climb on the platform to escape from the necessity of swimming. The rats were trained to locate the hidden platform. The animals were received 4 trials (2 in the morning, and 2 in the afternoon) per day on 4 consecutive days. The rat was given a maximum of 90 s to find the hidden platform.On the 5th day, the rat's learning ability was examined by observing the time to find the hidden platform (escape latency) in 90 s and the platformfinding strategy (prompt and straight way, marked 4 scores; hesitating first and then straight way, marked 3 scores; random way, marked 2 scores;aimless way and around the pool border, marked 1 score). On the 8th day,the escape latency and the platform-finding strategy were observed to examine the rat's memory level. ④ After the Morris water maze test, 8 rats of each group were sacrificed by decapitation for RT-PCR of GFAP mRNA expression, and 6 rats of each group were sacrificed for immunohistochemistry of GFAP protein expression. GFAP mRNA expression level was analyzed by the expression ratio of the interest GFAP to the control β-actin according to the computer image analysis. The GFAP protein expression was analyzed by the volume density of GFAP in hippocampus. ⑤ Data were analyzed with one-way ANOVA and q-test.MAIN OUTCOME MESURES: The effects of GbE-on the performances of the water maze Morris of type 2 diabetic rats and both GFAP mRNA and protein expressions in hippocampus.RESULTS: A total of 14 streptozotocin-treated rats with a blood glucose concentration of ＜ 15 mmol/L were rejected from the study. ① The performance of diabetic rats in the Morris water maze was significantly impaired compared to control group, the results on the 5th day and the 8th day showed that both escape latency and platform-finding strategy scores were decreased (P ＜ 0.01). The escape latency of both insulin treatment and GbE treatments on the 5th day and the 8th day was shorter than that of diabetic group, the platform-finding strategy scores was higher than that of diabetic group (P ＜ 0.05-0.01). There was no marked difference among the insulin treatment and GbE treatments groups in performance of the water maze Morris (P ＞ 0.05). ② The levels of both GFAP mRNA and protein expressions in hippocampus: Statistical analyses indicated that the level of GFAP mRNA expression of diabetic rats was significantly higher than that of the 3 other groups (P ＜ 0.01). Compared to control group, the diabetic rats showed a high immunoreactivity, the GFAP body was enlarged markedly, apophysis was obviously longer, the expressed numbers were increased, and the volume density of GFAP was also increased significantly (P ＜ 0.01). Compared to the diabetic group, the insulin treatment and GbE treatments groups showed a low immunoreactivity, the GFAP body was markedly smaller, apophysis was obviously shorter, the expressed numbers were decreased, and the volume density of GFAP was also decreased significantly (P ＜ 0.01). There were no marked differences in both GFAP mRNA and protein expressions among the insulin treatment and GbE treatments groups (P ＞ 0.05).CONCLUSION: There is cognition impairment in type 2 diabetic rats, the responsive GFAP may take a part in the progress of the learning and memory dysfunction. GbE can decrease markedly the reactive hypertrophy of astrocytes of diabetic hippocampus and improve the learning and memory dysfunction in diabetic rats.