Effect of tryptophan depletion on conditioned threat memory expression: role of intolerance of uncertainty
Background Responding emotionally to danger is critical for survival. Normal functioning also requires flexible alteration of emotional responses when a threat becomes safe. Aberrant threat and safety learning occurs in many psychiatric disorders including post-traumatic stress disorder (PTSD), obsessive-compulsive disorder (OCD), and schizophrenia, where emotional responses can persist pathologically. Whilst there is evidence that threat and safety learning can be modulated by the serotonin systems, there have been few studies in humans. We addressed a critical clinically relevant question: How does pharmacological lowering of serotonin affect the retention of conditioned threat memory? Methods Forty-seven healthy participants underwent threat conditioning on Day 1 followed by an extinction session. Emotional responding was assessed by the skin conductance response (SCR). On Day 2, we employed acute dietary tryptophan depletion to lower serotonin temporarily, in a double-blind placebo-controlled randomized between-groups design. We then tested for the return of conditioned threat memory spontaneous recovery). We also measured self-reported intolerance of uncertainty, known to modulate threat memory expression. Results The expression of emotional memory was attenuated in participants who had undergone tryptophan depletion. Individuals who were more intolerant of uncertainty showed even greater attenuation of emotion following depletion. Conclusions These results support the view that serotonin is involved in predicting aversive outcomes and refine our understanding of the role of serotonin in the persistence of emotional responsivity, with implications for individual differences in vulnerability to psychopathology.