Harnessing temperament to elucidate the complexities of serotonin function

This review highlights the utility of applying concepts of temperament and personality traits in healthy individuals to functional studies of serotonin (5-hydroxytryptamine, 5-HT), in an effort to better elucidate the complex roles of 5-HT, and ultimately advance our understanding of psychopathology. We highlight empirical demonstrations of multifaceted and trait-dependent emotional and behavioural effects of manipulating 5-HT in humans, with emphasis on studies employing the technique of acute dietary tryptophan depletion, and additionally selective serotonin reuptake inhibitors. Relevant evidence from studies of 5-HT in non-human animals is also discussed. We show how the effects of central 5-HT manipulations affect behaviour depending not only upon situational context but also on pre-existing temperament and personality traits such as empathy, psychopathy, neuroticism, impulsivity, and intolerance of uncertainty. These effects can be related to the concept of the baseline (or rate-) dependency of neurochemical effects on behavioural control. We speculate about the neurochemical substrates for some of these trait-dependent effects, as well as their clinical significance.