The impact of tryptophan depletion and 5-HTTLPR genotype on passive avoidance and response reversal instrumental learning tasks

Neuropsychopharmacology. 2007 Jan;32(1):206-15. doi: 10.1038/sj.npp.1301182. Epub 2006 Aug 9.


Transient reductions in serotonin levels during tryptophan depletion (TD) are thought to impair reward processing in healthy volunteers, while another facet of the serotonergic system, the serotonin transporter (5-HTTLPR) short allele polymorphism, is implicated in augmented processing of aversive stimuli. We examined the impact and interactions of TD and the serotonin promoter polymorphism genotype on reward and punishment via two forms of instrumental learning: passive avoidance and response reversal. In this study, healthy volunteers (n=35) underwent rapid TD or control procedures and genotyping (n=26) of the 5-HTTLPR for long and short allele variants. In the passive avoidance task, tryptophan-depleted volunteers failed to respond sufficiently to rewarded stimuli compared to the control group. Additionally, long allele homozygous individuals (n=11) were slower to learn to avoid punished stimuli compared to short allele carriers (n=15). TD alone did not produce measurable deficits in probabilistic response reversal errors. However, a significant drug group by genotype interaction was found indicating that in comparison to short allele carriers, tryptophan-depleted individuals homozygous for the long allele failed to appropriately use punishment information to guide responding. These findings extend prior reports of impaired reward processing in TD to include instrumental learning. Furthermore, they demonstrate behavioral differences in responses to punishing stimuli between long allele homozygotes and short allele carriers when serotonin levels are acutely reduced.

Publication types

  • Clinical Trial
  • Randomized Controlled Trial
  • Research Support, N.I.H., Intramural

MeSH terms

  • Adult
  • Alleles
  • Analysis of Variance
  • Avoidance Learning / physiology*
  • Conditioning, Operant / physiology*
  • Double-Blind Method
  • Female
  • Gene Frequency
  • Genotype
  • Humans
  • Male
  • Polymorphism, Genetic / genetics
  • Serotonin Plasma Membrane Transport Proteins / genetics*
  • Tryptophan / deficiency*


  • Serotonin Plasma Membrane Transport Proteins
  • Tryptophan