The effects of methylene blue on learning and memory, plasticity, and oxidative stress in aged rats
Normal aging in humans and animal models has been characterized, in large part, by declines and impairments in memory. Oxidative damage has been implicated in the process of aging. As an organism ages, the levels of reactive oxygen species (ROS) increase in the brain and can lead to an accumulation of oxidative damage. Methylene blue (MB) is a dye that has powerful antioxidant properties that prevent free radical damage and may help reduce the oxidative damage associated with aging. Low-dose MB improves memory, increases energy metabolism, enhances metabolic functions, and has antioxidant defenses for neuroprotection. It is not known whether MB has any beneficial effects in aged rats. In this dissertation, I investigated the effects of MB on trace-fear conditioning (TFC), a task known to be impaired in aged rats, and synaptic plasticity in the hippocampus. Levels of oxidative damage and protein conformational changes were also assessed.
MB was fed to F344/BN F1 rats separated into two age groups---young (6 months) and aged (24 months). They were fed either Methylene blue (MB) or the blue dye Indigo carmine (IC, control) for 4 weeks. Subjects underwent trace-fear conditioning (a hippocampus-related task) and 24 hours later they were tested for memory, or induction of long-term potentiation (LTP). At the end of the experiments, the hippocampi were extracted to measure levels of oxidative stress. We found that the young MB-treated animals learned faster during trace-fear training and the memory of TFC was rescued when compared to aged matched controls. In addition, the MB-treated rats showed less oxidative damage than their age-matched controls as measured by lipid peroxidation levels and the aged MB treated rats showed less protein conformation changes as measured by protein surface hydrophobicity. These results suggest that MB enhances learning and memory in young rats and reduces oxidative stress across the ages in the rat hippocampus. Furthermore, MB did not enhance LTP in either the young or aged MB-treated rats, but MB did decrease protein conformational changes in the hippocampus of aged rats.
These results suggest that MB, when given orally for 4 weeks, has a regional effect in the hippocampus and can work to reduce levels of oxidative damage. MB did not enhance LTP and this indicates that the treatment was not sufficient to have effects in LTP plasticity, or possibly, there was a ceiling effect in which all rats attained the maximum amount of LTP and there was no room for enhancement.