Elevated oxidative stress and sensorimotor deficits but normal cognition in mice that cannot synthesize ascorbic acid

Abstract

Oxidative stress is implicated in the cognitive deterioration associated with normal aging as well as neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. We investigated the effect of ascorbic acid (vitamin C) on oxidative stress, cognition, and motor abilities in mice null for gulono-gamma-lactone oxidase (Gulo). Gulo-/- mice are unable to synthesize ascorbic acid and depend on dietary ascorbic acid for survival. Gulo-/- mice were given supplements that provided them either with ascorbic acid levels equal to- or slightly higher than wild-type mice (Gulo-sufficient), or lower than physiological levels (Gulo-low) that were just enough to prevent scurvy. Ascorbic acid is a major anti-oxidant in mice and any reduction in ascorbic acid level is therefore likely to result in increased oxidative stress. Ascorbic acid levels in the brain and liver were higher in Gulo-sufficient mice than in Gulo-low mice. F(4)-neuroprostanes were elevated in cortex and cerebellum in Gulo-low mice and in the cortex of Gulo-sufficient mice. All Gulo-/- mice were cognitively normal but had a strength and agility deficit that was worse in Gulo-low mice. This suggests that low levels of ascorbic acid and elevated oxidative stress as measured by F(4)-neuroprostanes alone are insufficient to impair memory in the knockouts but may be responsible for the exacerbated motor deficits in Gulo-low mice, and ascorbic acid may have a vital role in maintaining motor abilities.

Figure 1. Neurochemical differences in Gulo−/− mice

(a) The low (0.033g/l) dose of ascorbic acid was enough for Gulo-low mice to maintain wild-type levels in liver but not in brain. Greater ascorbic acid supplementation (0.33g/l) in the Gulo-sufficient group resulted in higher levels of ascorbic acid than wild-type mice in liver and cerebellum. Gulo-sufficient mice had significantly higher ascorbic acid levels than Gulo-low mice in all areas assayed. (b) F₄-Neuroprostanes were elevated in the cortex in both groups of Gulo knockouts despite high ascorbic acid levels in the Gulo-sufficient group. In the cerebellum only Gulo-low mice had elevated neuroprostane levels. Bars represent group mean ± S.E.M. Group differences are depicted as follows: Gulo-low or wild-type control from Gulo-sufficient * P <.05, ** P <.01, *** P <.001.

Figure 2. Neurological and motor dysfunction in Gulo−/− mice

Gulo-sufficient mice had (a) a shorter stride length than wild-type control mice and (b) exhibited poorer grip strength on the horizontal beam, but did not differ from Gulo-low mice on these tasks. Impaired agility in the Gulo-sufficient group compared to wild-type mice was seen on the (c) rotarod and (d) inverted screen tasks. These deficits were exacerbated by subsistence levels of ascorbic acid supplementation in the Gulo-low group. Data shown are group means ± S.E.M. Group differences are depicted as follows: Gulo-low or wild-type control from Gulo-sufficient * P <.05, Gulo-low from wild-type control mice ⁺⁺ P <.01.

Figure 3. Normal cognitive function and anxiety in Gulo−/− mice

Gulo−/− and wild-type mice performed similarly on measures of cognitive function including (a) exploration (arm-entries) and alternation in the Y-maze, (b) hidden-platform learning during water maze acquisition, and (c–d) spatial memory for the location of the platform during a 60-s probe trial measured by (c) time spent in each quadrant and (d) search error. (e) Gulo mice showed no impairment in long-term contextual memory. (f) Gulo−/− and wild-type mice also performed similarly on the measures of anxiety in the elevated plus maze and locomotor activity chambers. Bars represent group mean ± S.E.M.