Lowering of serotonin by rapid tryptophan depletion increases impulsiveness in normal individuals.
BACKGROUND
Reduced serotonergic activity has been associated with impulsive behavior; however, intervention studies have been scarce.


OBJECTIVE
To examine whether induced lowering of serotonin (5-HT) levels would increase behavioral measures of impulsivity.


METHODS
Twenty-four healthy young males ingested a mixture of the essential amino acids except for tryptophan in a balanced, randomized, double-blind, placebo-controlled, cross-over study design. The continuous-performance test-identical pairs was administered when the plasma concentration of tryptophan was expected to be at the lowest point. The plasma concentrations of 23 amino acids were measured at baseline and 5 h after the ingestion of the amino acid mixture.


RESULTS
The intervention led to a dramatic fall in free and total plasma tryptophan, and the tryptophan/large neutral amino acids ratio. This in turn has been shown to lower the level of 5-HT in the central nervous system. The tryptophan depletion resulted in a statistically significant more impulsive- or disinhibited response style on the continuous-performance test-identical pairs when the subjects were solving verbal tasks. Depleted subjects exposed to spatial stimuli had fewer correct responses and a decreased ability to discriminate between stimuli.


CONCLUSIONS
These results indicate that a rapid lowering of tryptophan increases impulsiveness and decreases discriminating ability in normal individuals. The effect of 5-HT depletion on discriminating ability in this study was similar to that previously reported in depressed patients.
Walderhaug
Espen
E
Department of Psychiatry, Ullevål University Hospital, 0407 Oslo, Norway. espen.walderhaug@psykiatri.uio.no
Lunde
Hilde
H
Nordvik
Jan E
JE
Landrø
Nils Inge
NI
Refsum
Helge
H
Magnusson
Andres
A
D000328:Adult; D001288:Attention / Q000502:physiology; D018592:Cross-Over Studies; D004193:Discrimination Learning / Q000502:physiology; D004311:Double-Blind Method; D006801:Humans; D007175:Impulsive Behavior / Q000503:physiopathology* / Q000523:psychology; D008297:Male; D009483:Neuropsychological Tests; D011340:Problem Solving / Q000502:physiology; D012701:Serotonin / Q000172:deficiency / Q000502:physiology*; D014364:Tryptophan / Q000097:blood / Q000172:deficiency*
D012701:Serotonin; D014364:Tryptophan