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Salud mental

versión impresa ISSN 0185-3325


MORENO, Julia et al. Tryptophan and serotonin in blood and platelets of depressed patients: Effect of an antidepressant treatment. Salud Ment [online]. 2006, vol.29, n.4, pp.1-8. ISSN 0185-3325.

Platelets llave serotonin (5-HT) uptake and storage mechanisms similar to those from neurons. In addition, they represent nearly 99% of blood 5-HT concentration. For these characteristics, platelets are considered useful biomarkers of the serotonergic synaptic neurotransmission, particularly in psychiatric disturbances such as depression. However, most studies which have evaluated platelet 5-HT concentrations in depression have not shown similar findings.

It has been suggested that changes in plasma tryptophan (TRP) concentrations might modify 5-HT concentration in the brain, as well as in platelets. Likewise, decreased plasma concentrations of TRP have been found in depressed patients, and the selective 5-HT reuptake inhibitors (SSRIs) induce changes in platelet 5-HT concentration.

Considering the controversy surrounding platelet 5-HT concentrations in depressed patients, and the fact that blood 5-HT and TRP have not been studied in the Mexican population, we decided to study 5-HT and tryptophan concentrations in blood and platelets from depressed and control Mexican subjects to evaluate a possible correlation with the severity of depression. The effect of fluoxetine and citalopram treatment on blood and platelet 5-HT and TRP concentrations in depressed patients was also studied.

Material and methods

Depressed patients The patients of this study were carefully selected and evaluated. Scales based on semi-structured interviews were applied (MINI and SCID-II) by clinical investigators to reduce any possible bias in patient selection. The influence of the seasonal variability on the 5-HT or TRP blood concentrations was controlled by pairing depressed patients and healthy subjects according to age, gender and, in the case of women, menstrual cycle phase. Patients with a complete remission of depression symptoms (defined as a score not higher than 5 points in the Hamilton's scale, and lower than 7 points in Beck's scale) were asked for a blood sample to measure platelet and blood concentrations of 5-HT and TRP. The patients were weighted before the treatment and after their improvement.

Control subjects

The control group was integrated by 30 healthy subjects, 24 women and 6 men, with an average age of 32.3 ± 10.8 years. Participants were recruited from the overall Mexican population, interviewed by a psychiatrist, and evaluated with the structured interview MINI and the SCID-II, all these to discard any psychiatric diagnose. None of them had received any pharmacological treatment during the three weeks prior to the study. Control and depressed women were paired according to their menstrual cycle phase.

All participants received a detailed explanation of the study, and those who voluntarily accepted the stipulations signed an informed consent document. Control and patient subjects were clinically examined and studied with routine laboratory tests (blood count, blood chemistry, urinalysis, and thyroid function test). Blood sample procedures

5-HT and TRP measurements in total blood preparation were carried out according to the method described by Anderson, and were quantified by high performance liquid chromatography (HPLC).

Statistical analysis

The differences were statistically determined through an analysis of variance (ANOVA), with the assistance of the SPSS 12.00 (Statistical Software by SPPS Inc.).


Results from laboratory tests, such as blood count, blood chemistry, thyroid function (T3, T4 and TSH) and urinalysis were normal in depressed subjects, as well as in healthy volunteers. Platelet number, blood 5-HT concentration, platelet content of 5-HT, and blood tryptophan concentration showed no significant differences in depressed patients in comparison to control subjects. 5-HT values in blood and platelet were significantly lower than the initial concentrations in patients after antidepressant treatment.

Discussion and conclusions

Discrepancies between our study and those found in the literature can be explained with three different approaches: ethnical, physiological, and methodological, as is further discussed.

The significant decrease produced by the antidepressant treatment in blood and platelet serotonin concentration may be a consequence of the action of SSRIs, due to a 5-HT diminished uptake by the platelet.

Considering our results, we conclude that:

Blood and platelet 5-HT concentrations were not different between depressed patients and healthy volunteers.

Blood TRP concentrations were not different between depressed patients and healthy volunteers.

SSRIs (fluoxetine or citalopram) used in the treatment of depressed patients induced a significant decrease in blood and platelet content of 5-HT, and had no effect in TRP concentrations.

Based on these results, neither blood/platelet 5-HT nor blood tryptophan concentrations seem to be good biological markers of depressive patients status. However, 5-HT, but not tryp-tophan, might be a reference point for pharmacological treatment effect.

Palabras llave : Depression; 5-Hydroxitryptamine; platelet; tryp-tophan; antidepressants.

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