Compromised decision-making and increased gambling proneness following dietary serotonin depletion in rats

Psycho-genetic studies have revealed a role for the brain serotonin system in gambling proneness and poor decision-making. We assessed whether manipulation of brain serotonin levels in rats affected perfor- mance in operant-based tasks for decision-making and gambling proneness. Male Wistar rats were exposed to an L-tryptophan (TRP) deficient diet (0.0 g/kg; T group) or to a control, L-tryptophan con- taining diet (2.8 g/kg; Tþ group). The same rats were tested for decision-making performance in the rodent Iowa Gambling Task (rIGT) using home-cage operant panels, and subsequently for gambling proneness in a Probabilistic Delivery Task (rPDT) using classic Skinnerboxes. At sacrifice, monoamines and metabolites were evaluated with HPLC analysis, confirming a drastically reduced serotonin synthesis, as well as altered dopamine turnover in the prefrontal cortex of T rats. As expected, control rats (Tþ) progressively chose the option with the best long-term payoff in the rIGT, and also shifted from “Large & Luck-Linked” (LLL) to “Small & Sure” (SS) reinforcers in the rPDT. In contrast, depleted animals (T) exhibited a weaker improvement of performance in the rIGT and maintained a sub-optimal attraction for LLL reinforcer in the rPDT. Comparing individual performances in both tests, we found a significant correlation between the two tasks in control (Tþ) but not in depleted (T) rats. The present study revealed that (1) brain 5-HT depletion leads to poor decision-making and to gambling proneness; (2) the relationship between these two traits, shown in the control group, was disrupted in 5-HT depleted rats. The data are discussed in terms of changes within forebrain loops involved in cognitive and motivational/affective processes.