Effects of alpha-lipoic acid on associative and spatial memory of sham-irradiated and 56Fe-irradiated C57BL/6J male mice

Abstract

Cranial irradiation with (56)Fe, a form of space radiation, causes hippocampus-dependent cognitive changes. (56)Fe irradiation also increases reactive oxygen species (ROS) levels, which may contribute to these changes. Therefore, we investigated the effects of the antioxidant alpha lipoic acid (ALA) on cognition following sham-irradiation and irradiation. Male mice were irradiated (brain only) with (56)Fe (3 Gy) or sham-irradiated at 6-9 months of age. Half of the mice remained fed a regular chow and the other half of the mice were fed a caloric-matched diet containing ALA starting two-weeks prior to irradiation and throughout cognitive testing. Following cognitive testing, levels of 3-nitrotyrosine (3NT), a marker of oxidative protein stress, and levels of microtubule-associated protein (MAP-2), a dendritic protein important for cognition, were assessed using immunohistochemistry and confocal microscopy. ALA prevented radiation-induced impairments in spatial memory retention in the hippocampal and cortical dependent water maze probe trials following reversal learning. However, in sham-irradiated mice, ALA treatment impaired cortical-dependent novel object recognition and amygdala-dependent cued fear conditioning. There was a trend towards lower 3NT levels in irradiated mice receiving a diet containing ALA than irradiated mice receiving a regular diet. In the hippocampal dentate gyrus of mice on regular diet, irradiated mice had higher levels of MAP-2 immunoreactivity than sham-irradiated mice. Thus, ALA might have differential effects on the brain under normal physiological conditions and those involving environmental challenges such as cranial irradiation.

Figure 1

Novel Object Recognition of sham-irradiated and irradiated mice receiving a regular diet or a diet containing ALA. *P < 0.05 versus the other two objects.

Figure 2

Cued fear conditioning of sham-irradiated and irradiated mice receiving a regular diet or a diet containing ALA. Freezing measures of sham-irradiated and irradiated female and male mice with or without ALA supplementation during the pretone (white bars) and tone (black bars) in the cued fear conditioning task are shown. *P < 0.05.

Figure 3

Spatial memory retention of sham-irradiated and irradiated mice receiving a regular diet or a diet containing ALA in for the first hidden platform location (Probes 1 and 2) and second hidden platform location (Probes 3 and 4) in the water maze probe trials is shown. Spatial memory retention was measured as the percent time spent in the target quadrant where the platform was previously located compared to the other three non-target quadrants. *P < 0.05 versus the three non-target quadrants.

Figure 4

3NT immunoreactivity in the amygdala, enthorhinal cortex, and dentate gyrus of sham-irradiated and irradiated mice receiving a regular diet or a diet containing ALA. Mean pixel intensities are shown.

Figure 5

Representative images of MAP-2 immunoreactivity in the dentate gyrus of sham-irradiated and irradiated mice receiving a regular diet or a diet containing ALA.

Figure 6

MAP-2 immunohistochemistry (IR) in the dentate gyrus of sham-irradiated and irradiated male with or without LA supplementation. In non-LA supplemented male mice, irradiation increased MAP-2 IR in the dentate gyrus of the hippocampus (*P < 0.05 versus sham-irradiated non-LA supplemented mice, Fisher’s PLSD following ANOVA).