title: Tryptophan depletion disinhibits punishment but not reward prediction: implications for resilience.
abstract: BACKGROUND
  We have previously shown that tryptophan depletion enhances punishment but not reward prediction (Cools et al. in Neuropsychopharmacology 33:2291-2299, 2008b). This provided evidence for a valence-specific role of serotonin (which declines under depleted tryptophan) in aversive processing. Recent theoretical (Dayan and Huys in PLoS Comput Biol 4:e4, 2008) and experimental (Crockett et al. in J Neurosci 29:11993-11999, 2009) approaches have, however, further specified this role by showing that serotonin is critical for punishment-induced inhibition.
  
  
  OBJECTIVE
  We sought to examine the role of serotonin in punishment-induced inhibition. We also examined the impact of induced mood on this effect to assess whether effects of tryptophan depletion on affective inhibition are moderated by mood.
  
  
  METHODS
  Healthy females consumed a balanced amino acid mixture with (N = 20) or without (N = 21) the serotonin precursor tryptophan. Each subject completed either negative or neutral mood induction. All subjects completed the reward and punishment reversal learning task adopted in the previous study.
  
  
  RESULTS
  We demonstrate a punishment prediction impairment in individuals who consumed tryptophan which was absent in individuals who were depleted of tryptophan. This effect was impervious to mood state.
  
  
  CONCLUSIONS
  Our results suggest that serotonin promotes the inhibition of responses to punishing outcomes. This may lead to reduced punishment prediction accuracy in the presence of tryptophan and may contribute to resilience to affective disorders. Reduction of serotonin via tryptophan depletion then removes this inhibition. As such, we highlight a mechanism by which reduced serotonin can contribute to disorders of impulsivity and compulsivity as well as disorders of emotion.
authors.affiliation: Department of Psychiatry and MRC/Wellcome Trust Behavioural and Clinical Neuroscience Institute, University of Cambridge, Cambridge, Addenbrooke's Hospital, P.O. Box 189, Level E4, Hills Road, Cambridge, CB2 2QQ, UK. oliver.j.robinson@gmail.com
authors.forename: Oliver J
authors.initials: OJ
authors.lastname: Robinson
authors.forename: Roshan
authors.initials: R
authors.lastname: Cools
authors.forename: Barbara J
authors.initials: BJ
authors.lastname: Sahakian
mesh_terms: D000328:Adult; D000339:Affect / Q000187:drug effects / Q000502:physiology; D000596:Amino Acids / Q000097:blood / Q000494:pharmacology; D005260:Female; D006801:Humans; D007266:Inhibition, Psychological; D009483:Neuropsychological Tests / Q000706:statistics & numerical data; D011678:Punishment / Q000523:psychology*; D055500:Resilience, Psychological / Q000187:drug effects*; D012193:Reversal Learning / Q000187:drug effects / Q000502:physiology; D012201:Reward; D014364:Tryptophan / Q000097:blood / Q000172:deficiency* / Q000502:physiology*
chemical_list: D000596:Amino Acids; D014364:Tryptophan