How demanding is the brain on a reversal task under day and night conditions?

• We studied day and night effects on spatial reversal learning.
• The metabolic brain consume by cytochrome c-oxidase (CO) was shown.
• No behavioral differences in the time of the day between groups were found.
• CO differences were shown in cortical regions, hippocampus, striatum and thalamus.
• Orbitofrontal cortex was revealed as a key structure in the process.

Reversal learning has been studied as the process of learning to inhibit previously rewarded actions. These behavioral studies are usually performed during the day, when animals are in their daily period rest. However, how day or night affects spatial reversal learning and the brain regions involved in the learning process are still unknown. We conducted two experiments using the Morris Water Maze under different light-conditions: naïve group (CN, n = 8), day group (DY, n = 8), control DY group (CDY, n = 8) night group (NG, n = 8), and control NG group (CNG, n = 7). Distance covered, velocity and latencies to reach the platform were examined. After completing these tasks, cytochrome c-oxidase activity (CO) in several brain limbic system structures was compared between groups. There were no behavioral differences in the time of day when the animals were trained. However, the metabolic brain consumption was higher in rats trained in the day condition. This CO increase was supported by the prefrontal cortex, thalamus, dorsal and ventral striatum, hippocampus and entorhinal cortex, revealing their role in the performance of the spatial reversal learning task. Finally, the orbitofrontal cortex has been revealed as a key structure in reversal learning execution.