As seen in rats, there is no evidence of a strong correlation between visual acuity and learning in the mouse. The second generation of mice (F1) fed a diet poor in LNA was compared to a group fed laboratory chow. In adult mice, there was a significant difference in the retinal concentration of DHA, but the difference in b-wave electroretinograms ceased to be significant at the seventh week (Carrie, et al., 1999). The ability to learn was examined using the passive-avoidance test and the LNA-deficient group continued to significantly underperform the chow group, even as adults. Normalization of visual performance was not accompanied by an improvement in cognitive performance.
The data on behavior for the mouse is less consistent than the data on visual acuity, even if most studies point to beneficial effects of a diet enriched in DHA and its precursor LNA. Wainright (Wainright, et al., 1994) fed mice one of three diets through pregnancy, lactation, and weaning. They were the same basic diet but with (1) saturated fat, (2) adequate LA but deficient in LNA, and (3) sufficient LNA and LA (n-6/n-3 ratio of 3.7). Six weeks after weaning, two females from each litter were tested in the Morris water maze and the open field. Initially, mice fed the saturated diet were a little slower in the water maze, but that effect was transient. No difference was found in the open field or during reverse learning in the water maze.
In contrast, Bourre's group (Frances, et al., 1996a) compared OF1 mice fed a peanut-oil-based diet containing trace amounts of LNA with a group receiving 200 mg of LNA/ 100 g of feed from rape seed oil and found significant performance differences. Both groups had the same total fat and LA content in the diet and had been reared by dams receiving the same diet as their pups. There were 10 female mice in each group. The LNA-deficient group improved its performance over time, but in the Morris water maze (place-test version), it took them significantly longer to find the platform after 16 practice trials. The difference was not significant in the cued version of the test. When mice raised by the same scheme were tested (Frances, et al., 1996b) it was found that the n-3-sufficient mice but not n-3-deficient mice showed reduced exploration in a photocell Actimeter. In the forced swimming test, the n-3-sufficient mice were significantly less active, and the same was found (p < 0.061, one-sided test) for the number of escape attempts from a confined space as well as for activity in an open field (pooled data for 2 d). The authors concluded that n-3-sufficient mice showed greater habituation. There were 14 mice per group.
Frances et al. (Frances, et al., 1996b) used the same protocol to rear LNA-deficient and LNA-sufficient pups. Animals on the n-3-deficient diet showed significantly slower learning. Mice raised on a sufficient diet were quicker to escape the Rotarod. Paw coordination, muscular function, defensive behavior, and anxiety was not different between the groups.
Suzuki et al. (Suzuki et al., 1998) found that mice fed sardine oil for 12 mo were better at finding their way though a maze. They negotiated it more quickly and strayed down wrong paths less frequently when compared to a n-3-deficient group
7.2.3. Effect of Dietary DHA During n-3 Sufficiency on Behavior in the Mouse
Recently, Carrie and colleagues (Carrie, et al., 2000) studied the effect of addition of sardine oil (a source of long-chain n-3 fatty acids) to a diet containing adequate n-3 and n-6 fatty acids. The control was a diet with equivalent amounts of palm oil. Both diets contained 14% fat of which 31% and 2.0%, respectively, were n-3. The sardine oil diet contained 0.94 g DHA and 1.6 g of EPA per 100 g of feed. Groups of 12 female OF1 mice from each dietary treatment were tested in the open-field, Morris water maze, and avoidance tests as young adults, mature, and old. To avoid memory effects, mice were not subjected to the same test twice. The effect of the n-3 diet on young mice was a significant increase in exploration and locomotor activity. In older mice, exploration, as measured in the open-field test was the same in both groups. However, the locomotor activity of old mice was significantly lower in the fish-oil group. Perhaps this is an effect of superior memory in this group. If the hidden platform is removed from the Morris maze, the time that a mouse spends searching in that quadrant is a measure of memory. By this criterion mature mice from the fish-oil group were significantly better than the control group at remembering the location of the hidden platform. In old mice, the fish-oil group spent 45% more time in the training quadrant than the palm-oil group, but this did not reach significance at the 5% level. In active avoidance, the fish-oil-fed mice were significantly different from the control on the first day of training. Young fish-oil-fed mice had higher avoidance, whereas in the two other ages, mice from the palm-oil group were better at escaping before the electric chock. This could be a consequence of the increased locomotor activity. Mice that move about less may take longer to realize that the room of the cage they are in makes a difference. Also, n-3 fatty acids may make the mouse less averse to the electric stimulus. Yehuda and Carasso (Yehuda and Carasso, 1993) reported that rats injected with 25 mg/kg/d of LA and LNA in a ratio of 4 : 1 had a latency to lick the paw, after being placed on a 58°C hotplate, which was twice as long as a saline-injected control.
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