Memory loss in old rats is associated with brain mitochondrial decay and RNA/DNA oxidation: partial reversal by feeding acetyl-L-carnitine and/or R-alpha -lipoic acid

Abstract

Accumulation of oxidative damage to mitochondria, protein, and nucleic acid in the brain may lead to neuronal and cognitive dysfunction. The effects on cognitive function, brain mitochondrial structure, and biomarkers of oxidative damage were studied after feeding old rats two mitochondrial metabolites, acetyl-l-carnitine (ALCAR) [0.5% or 0.2% (wt/vol) in drinking water], and/or R-alpha-lipoic acid (LA) [0.2% or 0.1% (wt/wt) in diet]. Spatial memory was assessed by using the Morris water maze; temporal memory was tested by using the peak procedure (a time-discrimination procedure). Dietary supplementation with ALCAR and/or LA improved memory, the combination being the most effective for two different tests of spatial memory (P < 0.05; P < 0.01) and for temporal memory (P < 0.05). Immunohistochemical analysis showed that oxidative damage to nucleic acids (8-hydroxyguanosine and 8-hydroxy-2'-deoxyguanosine) increased with age in the hippocampus, a region important for memory. Oxidative damage to nucleic acids occurred predominantly in RNA. Dietary administration of ALCAR and/or LA significantly reduced the extent of oxidized RNA, the combination being the most effective. Electron microscopic studies in the hippocampus showed that ALCAR and/or LA reversed age-associated mitochondrial structural decay. These results suggest that feeding ALCAR and LA to old rats improves performance on memory tasks by lowering oxidative damage and improving mitochondrial function.

Figure 1

Morris water maze test in relation to age and treatment. (A) Time on day 4 taken to find the hidden platform. (B) Time spent at the former platform position in the transfer test. (C) Time to find the visible platform. Data are mean ± SEM of 9 rats in young and old, 5 in LA (0.1%), and 6 in ALCAR (0.2%) and ALCAR + LA groups. Higher doses, 0.2% LA and/or 0.5% ALCAR, showed similar results (data not shown). Statistical differences were examined with two-tailed Student t test. ***, P < 0.001 vs. young rats; #, P < 0.05 and ##, P < 0.01 vs. old rats.

Figure 2

Peak procedure test related to age and treatment. Response rate functions plotted separately for sound-signaled trials (A) and light-signaled trials (B) obtained during the last 10 days of the test. (C) Peak rate over the 20 days of peak procedure testing. Each data point averages 2 days of testing. (D) Peak rate of the last 10 days averaged. Data are mean ± SEM of 6 in young, 7 in old, 4 in LA (0.2%), and 5 in ALCAR (0.5%) and ALCAR + LA groups. Treatment with lower doses, 0.1% LA and/or 0.2% ALCAR, showed similar results (data not shown). Statistical differences were examined with two-tailed Student t test. *, P < 0.05 and ***, P < 0.001 vs. young rats; #, P < 0.05 vs. old unsupplemented rats.

Figure 3

Immunostaining relative to age and treatment for oxidized nucleic acids in neurons. (A) Representative photographs of oxo8G immunoreactivity in area CA1 of the hippocampus and adjacent white matter, from individual rats selected from young, old, old + ALCAR (0.5%), old + LA (0.2%), and old + ALCAR (0.5%) + LA (0.2%) groups. (B) CA1 sections pretreated with either DNase or RNase before incubation with Ab. (C) Extent of immunoreactivity to oxo8G in the hippocampus [CA1, CA3, CA4, dentate gyrus (DG), cerebral cortex (CX), and white matter (WM) in rat brain]. [Bar = 50 μm.] Values are mean ± SEM of 5 animals for young and old groups, 3 for old + ALCAR and old + LA groups, and 2 for the old + ALCAR + LA group. The Mann–Whitney U test was used to compare values. *, P < 0.005 vs. young rats; #, P < 0.05 vs. old control rats.