Spatial short-term memory in rats: effects of learning trials on metabolic activity of limbic structures.

The oxidative metabolism was assessed in the hippocampus and related regions in rats that were trained in a spatial short-term memory task in the water maze following distinct training schedules. The cytochrome oxidase (COx) histochemistry was evaluated in groups of rats that received a daily session made up of either two or three learning trials. An untreated group was added to determine baseline levels of COx. We found that the 3-Trials group exhibited a better performance concerning the differences in latency between trials 1 and 2. Trained groups showed higher COx activity than the untreated group in the medial prefrontal cortex, dentate gyrus, CA1, and the mammillary region. However, a decrease in COx activity was found in the dentate gyrus, CA1, and supramammillary region of the 3-Trials group. In addition, COx activity levels found in this group were similar to those of the untreated group in some thalamic nuclei. Most of the regions that presented significant correlations between COx activity and behavioral scores were found in the 3-Trials group. These findings suggest an influence of task difficulty in the oxidative metabolism of brain regions involved in spatial learning.