Cognitive flexibility is a function of the prefrontal cortex (PFC) that allows shifting strategies to adjust performance in changing environments. The dorsal raphe nucleus (DRN) provides most of the serotonergic inputs to the PFC. Clinical and pharmacological evidence suggests that prefrontal impairments or serotonergic alterations affect cognitive flexibility. However, the neuronal correlates of cognitive flexibility are largely unknown, and whether modulation of the neuronal activity in the DRN affects flexible behaviors remains unclear. We investigated how projections from the DRN modulate the activity of the PFC by recording the neuronal population activity of the PFC and the DRN in behavioral tasks involving rule-switching. Our aims comprised 1) to assess cognitive flexibility by measuring behavioral flexibility in a reversal learning and task-switching task. 2) To identify the cellular and network mechanisms underlying rule-switching in the PFC and DRN. 3) To study the effects of the PFC circuits under DRN modulation. We developed two tasks for head-restrained mice and recorded the extracellular activity of individual neurons in the PFC and DRN with silicon probes. In addition, we used optogenetics to investigate the DRN projections to the PFC. We found a broad firing selectivity to heterogeneous aspects of the task at the single unit level, including specific firing patterns associated with recognizing single rules. At the population level, we observed changes in neural activity states when mice adapted to the different contingency rules in the task, which was absent when animals showed inflexible behaviors.Grant: CS18-039 Vienna Science and Technology Fund (WWTF)