Research reportAnxiety-like behavior and cognitive flexibility in adult rats perinatally exposed to increased serotonin concentrations

Serotonin (5-hydroxytryptamine, 5HT) is a biologically active amine that regulates the development of 5HT neurons and target tissues during neurogenesis, while later it assumes the function of a neurotransmitter. Serotonin mediates many essential behaviors common to all mammals, and is held responsible for anxiety-like behavior and cognitive rigidity. Proper serotonin levels, controlled through 5HT synthesis and metabolism, are crucial for normal brain development. In this study we investigated anxiety-like behavior and cognitive flexibility in adult animals after exposing their developing brains to increased 5HT concentrations. Wistar rats were treated subcutaneously from gestational day 12 to post-natal day 21 with the immediate 5HT precursor 5-hydroxytryptophan (5HTP, 25 mg/kg), a non-selective MAO inhibitor tranylcypromine (TCP, 2 mg/kg), or saline. After reaching adulthood, animals were tested for anxiety-like behavior (exploratory behavior, thigmotactic behavior, social contact, and reaction to stressful stimulus) and cognitive flexibility (ability for reversal learning). Results of the behavioral studies corresponded with our previous neurochemical findings. Treatment with 5HTP, which has induced mild reduction in cortical 5HT concentrations, caused reduction in only one aspect of anxiety-like behavior (increased exploratory activity). Treatment with TCP, which lead to drastic reduction in 5HT concentration/function, resulted in a highly anxiolytic phenotype (reduced thigmotaxis, reaction to stress, and social anxiety) with improved cognitive flexibility. Although further neurochemical, anatomical and gene-expression studies are needed to elucidate the mechanisms underlying the observed behavior, we hope that our results will contribute to the understanding of the role of serotonin in anxiety-like behavior and cognitive rigidity.

► Peinatal treatments with 5HTP or TCP exposed developing rat brains to increased 5HT concentrations. ► Adult animals were tested for anxiety-like behavior and cognitive flexibility. ► 5HTP-treatment reduced only one aspect of anxiety-like behavior: exploratory activity. ► TCP-treatment resulted in highly anxiolytic phenotype with improved cognitive flexibility. ► Reduced anxiety-like behavior observed in the treated rats corresponds to the reduced 5HT function found in their cortices.