Lowering people's serotonin levels does not directly change their mood
the verdict
CONTESTED
contested - evenly split
refutedsupported
the weight of evidence
3 sources for · 1 against
The listed sources provide conflicting findings, with some noting no direct effect of acute tryptophan depletion on mood while others observe that altering tryptophan levels can affect mood and behavior.
BackgroundNeurocognitive impairment is frequently described in a number of psychiatric disorders and may be a direct consequence of serotonergic dysfunction. As impairments in executive functions are some of the most frequently described, the purpose of this study was to examine the performance of normal volunteers on a range of executive tasks following a transient reduction of central serotonin (5-HT) levels using the method of acute tryptophan depletion (ATD).MethodsFifteen healthy male subjects participated in a within-subject, double-blind, counterbalanced crossover study. ATD was induced by ingestion of a 100 g amino-acid drink. Executive function was evaluated using the Wisconsin Card Sorting Test, Stroop, Verbal Fluency and Trail Making. Visual analogue scales were administered to assess mood.ResultsPlasma free and total tryptophan concentrations were significantly reduced by the depleting drink (P < 0.001). ATD selectively improved motor speed/ attention on the Trails A test (P = 0.027), with no effect on subjective ratings of mood. Interaction effects between drink and the order of drink administration were observed on most neurocognitive tests.ConclusionsThe improvement in simple motor speed/ attention following ATD is in keeping with the ascribed role of 5-HT in the cortex, however performance on tests of executive function is not robustly altered. The presence of interaction effects on most tasks suggests that subtle changes may occur but are masked, possibly by simple learning effects, in the context of a crossover design. This has implications for the design of future studies, particularly those examining executive functions.
The effect of altered tryptophan levels on mood and behavior in normal human males. The effect of lowering or raising tryptophan levels was studied in normal males using amino acid mixtures that were tryptophan free or tryptophan supplemented. Tryptophan depletion caused a small but significant alteration in food selection in subjects allowed to select from a buffet. Although carbohydrate and total kilocalories selected were unchanged, significantly less protein was chosen. Tryptophan depletion also caused an acute lowering of mood, suggesting that low serotonin (5-HT) may be involved in the etiology of clinical depression in some patients. No effect of altered tryptophan levels was seen in a laboratory test of aggression. However, a study on vervet monkeys indicated that altered tryptophan levels can influence aggression when the animals are at a high level of arousal. High arousal is known to increase the firing rate of 5-HT neurons. In a preliminary study, tryptophan had a therapeutic effect in aggressive schizophrenic patients. The best effect was seen in impulsive patients, which may have been related to high arousal in these subjects.
Twelve healthy female subjects received the following three drinks in a double-blind, semi-balanced, cross over design: (a) 50 g of amino acids without L-tryptophan (LTP); (b) 50 g of amino acid with LTP (balanced); (c) plain water. Compared to both the balanced amino acid mixture and plain water, the LTP drink significantly lowered plasma total and free tryptophan at 4.5 h. However, compared to the two control conditions, there was no effect of the LTP drink on subjective ratings of mood or hunger. Similarly, the LTP drink did not alter significantly either total calorie intake or the macronutrient content of a test meal 5 h after drink ingestion.
Reciprocity motivates to reward those who are kind (= positive reciprocity) and to punish those who are unkind (= negative reciprocity). The neurotransmitter serotonin (5-HT) modulates human behavior in numerous social situations, such as retaliation in response to perceived unfairness. In a placebo-controlled study, we used acute tryptophan depletion (ATD) to investigate the influence of available serotonin on choice behavior and reciprocity in the Hawk-Dove game. This game illustrates a conflict situation and incorporates two potential strategies: the cooperative Dove strategy and the uncooperative, more aggressive Hawk strategy. After strategic choices, we elicited the subjects’ expectations (= beliefs) regarding the opponent’s choices and controlled for risk preferences and current mood. We defined strategy choices as negative reciprocity when the participants opted for Hawk in response to an expected Hawk. We hypothesized that the ATD-induced reduction of 5-HT availability would increase participants’ preferences for negative reciprocity. Generalized estimating equations reveal no significant main effect of ATD on assessed belief, mood, or risk attitude. But assessment of ATD’s marginal effects over beliefs suggests that ATD significantly increases the tendency for negative reciprocity, whereas positive reciprocity (Dove in response to an expected Dove) is unaffected. We could therefore demonstrate that 5-HT availability mediates (negative) reciprocal behavior in social decision-making.
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