The brain metabolic enhancer methylene blue improves discrimination learning in rats

Abstract

Methylene blue (MB) is a metabolic enhancer that has been demonstrated to improve memory retention when given post-training in low doses in a variety of tasks in rats, including inhibitory avoidance, spatial memory (in both normal and metabolically-impaired subjects), object recognition, and habituation to a familiar environment. MB has been also shown to improve memory retention of extinction of fear conditioning in the rat. No experiments have been conducted to determine the effects of MB on more complex learning such as in discrimination tasks that require repeated days of training. This study examined the effects of daily MB on spatial discrimination memory in a baited holeboard maze. Following three days of discrimination training, subjects treated daily with post-training MB (1 mg/kg) reliably discriminated between rewarded (baited) and non-rewarded (unbaited) trials as indicated by a greater number of correct responses on rewarded trials than non-rewarded trials during the last three days of discrimination training. No such discrimination effects were observed in the saline-treated control group during the same training period. To determine whether the memory-enhancing effects of MB are associated with an increase in metabolic energy capacity in the brain, cytochrome c oxidation was measured in brains from rats treated with 1 mg/kg MB or saline for three days. The number of daily injections was chosen based on the behavioral data which revealed group differences three days after the beginning of MB treatment. Brain cytochrome oxidase activity in the MB-treated group was approximately 70% higher than in saline-treated rats. The findings suggest that repeated post-training MB may improve memory consolidation between days of learning by an induction in the enzyme cytochrome oxidase, leading to increased metabolic capacity in brain regions requiring more energy during discrimination learning.

Figures

Figure 1

The baiting pattern in holeboard utilized for all trials. Dark circles are baited holes, and open circles are unbaited holes.

Figure 2

Acquisition curve for all subjects showing means ± standard errors for reference memory scores (number of nose pokes to baited holes/total number of nose pokes) for the training phase of the experiment, prior to injections, in the holeboard. During this phase there were 5 daily trials with 4 of the 16 holes baited in a fixed baiting pattern (Figure 1).

Figure 3

Means ± standard errors for reference memory scores (number of nose pokes to baited holes/total number of nose pokes) in rewarded versus non-rewarded trials in saline administered subjects for the discrimination training phase of the experiment. Saline-administered rats did not demonstrate a strong discrimination between rewarded and non-rewarded trials.

Figure 4

Means ± standard errors for reference memory scores (number of nose pokes to baited holes/total number of nose pokes) in rewarded versus non-rewarded trials in methylene blue-treated subjects for the discrimination training phase of the experiment. Methylene blue-administered rats reliably discriminated between rewarded and non-rewarded trials on days 10-12 of the experiment. *p < .05