Knockout of the Serotonin Transporter in the Rat Mildly Modulates Decisional Anhedonia

Serotonin transporter gene variance has long been considered an essential factor contributing to depression. However, meta-analyses yielded inconsistent findings recently, asking for further understanding of the link between the gene and depression-related symptoms. One key feature of depression is anhedonia. While data exist on the effect of serotonin transporter gene knockout (5-HTT^-/-) in rodents on consummatory and anticipatory anhedonia, with mixed outcomes, the effect on decisional anhedonia has not been investigated thus far. Here, we tested whether 5-HTT^-/- contributes to decisional anhedonia. To this end, we established a novel touchscreen-based go/go task of visual decision-making. During the learning of stimulus discrimination, 5-HTT^+/+ rats performed more optimal decision-making compared to 5-HTT^-/- rats at the beginning, but this difference did not persist throughout the learning period. During stimulus generalization, the generalization curves were similar between both genotypes and did not alter as the learning progress. Interestingly, the response time in 5-HTT^+/+ rats increased as the session increased in general, while 5-HTT^-/- rats tended to decrease. The response time difference might indicate that 5-HTT^-/- rats altered willingness to exert cognitive effort to the categorization of generalization stimuli. These results suggest that the effect of 5-HTT ablation on decisional anhedonia is mild and interacts with learning, explaining the discrepant findings on the link between 5-HTT gene and depression.