1. ¹Mood and Anxiety Disorders Program, Unit on Affective Cognitive Neuroscience, National Institute of Mental Health, Bethesda, MD, USA
2. ²Department of Health and Human Services, National Institutes of Health, Bethesda, MD, USA
3. ³Laboratory of Neurogenetics, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda, MD, USA
4. ⁴Psychiatry Group, MRC Clinical Sciences Centre, Imperial College, Hammersmith Hospital, London, UK

Correspondence: Dr EC Finger, Unit on Affective Cognitive Neuroscience, Mood and Anxiety Disorders Program, National Institute of Mental Health, 15K, North Drive, MSC 2670, Bethesda, MD 20892, USA. Tel: +1 301 402 6856, Fax: +1 301 402 6100, E-mail: fingere@mail.nih.gov

Received 27 March 2006; Revised 16 June 2006; Accepted 6 July 2006; Published online 9 August 2006.

Abstract

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.