Metformin Eased Cognitive Impairment Induced by Chronic L-methionine Administration: Potential Role of Oxidative Stress

Abstract

Chronic administration of L-methionine leads to memory impairment, which is attributed to increase in the level of oxidative stress in the brain. On the other hand, metformin is a commonly used antidiabetic drug with strong antioxidant properties. In the current study, we tested if chronic metformin administration prevents memory impairment induced by administration of L-methionine. In addition, a number of molecules related to the action of metformin on cognitive functions were examined. Both metformin and L-methionine were administered to animals by oral gavage. Testing of spatial learning and memory was carried out using radial arm water maze (RAWM). Additionally, hippocampal levels or activities of catalase, thiobarbituric acid reactive substances (TBARs), glutathione peroxidase (GPx), glutathione (GSH), oxidized glutathione (GSSG) and GSH/GSSG ratio were determined. Results showed that chronic L-methionine administration resulted in both short- and long- term memory impairment, whereas metformin treatment prevented such effect. Additionally, L-methionine treatment induced significant elevation in GSSG and TBARs, along with reduction in GSH/GSSG ratio and activities of catalase, and GPx. These effects were shown to be restored by metformin treatment. In conclusion, L-methionine induced memory impairment, and treatment with metformin prevented this impairment probably by normalizing oxidative stress in the hippocampus.

Keywords: L-methionine; Memory; hippocampus; learning; maze; metformin.; oxidative stress.

Fig. (1)

Plasma levels of homocysteine. Levels of homocystiene were significantly increased in the L-metioninine (L-Meth) and Lmethionine and metformin treated (L-Meth+Met) groups compared to the control and metformin treated (Met) groups. Data are expressed as mean ± SEM from 10-12 animals. * indicates significant difference (P < 0.05) from other groups.

Fig. (2)

Performance of animal groups during the RAWM. In all groups, the number of errors declined as the learning proceeded learning (trials 1-12), with no significant change among groups. In the memory tests performed after 30min, 5hr, and 24hr after the end of the acquisition, the number of errors committed by L-Meth group was significantly higher than that of L-Meth+Met, Met, and control groups. Data are expressed as mean ± SEM from 10-12 animals. * indicates significant difference (P < 0.05) from other groups.

Fig. (3)

Hippocampal Catalase Activity. Catalase activity was significantly reduced in L-Meth group compared to L-Meth+Met, Met, and control groups. Data are expressed as mean ± SEM from 10-12 animals. * indicates significant difference (P < 0.05) from other groups.

Fig. (4)

Hippocampal levels of different forms of glutathione. No change was observed in the GSH levels among different groups (A). In the L-Meth group, the levels of GSSG (B), and GSH/GSSG ratio (B) were significantly elevated compared to other groups. However, levels of GSSG and GSH/GSGG ratio were significantly higher in L-Meth+Met group compared to Met and control groups, indicating that metformin treatment only partially restored GSSG and GSH/GSSG ratio. Data are expressed as mean ± SEM from 10- 12 animals. * indicates significant difference (P < 0.05) from other groups.

Fig. (5)

Hippocampal GPx Activity. L-Meth group showed significant reduction in GPx activity compared to L-Meth+Met, Met, and control groups. Each point is the mean ± SEM of 10-12 rats. * indicates significant difference compared to all other groups, (P < 0.05).

Fig. (6)

Levels of TBARS in the hippocampus. The levels of TBARS were elevated in the L-Meth group compared Met, and control groups. Metformin treatment prevented this elevation in TBARS levels (L-Meth+Met group). Data are expressed as mean ± SEM from 10-12 animals. * indicates significant difference (P < 0.05) from other groups.