This is the first report to demonstrate the association between plasma 5-HT levels, age, and COPD. Higher plasma 5-HT levels were found in stable COPD patients compared to the control subjects, which did, however, not correlate with the severity of the disease. Plasma 5-HT was a significant mediator of the relation between pack-years smoked and COPD. We also demonstrated a positive correlation between plasma 5-HT levels and age in the COPD patients, but not in the control subjects.
Platelet activation is one of the mechanisms that induce the release of 5-HT into the circulating blood. Nicotine, a major component in cigarette smoke, has been reported to stimulate the release of 5-HT via
platelet activation in vitro
. In agreement with the literature, we found significant higher plasma 5-HT levels in healthy ever-smokers compared to those of healthy non-smokers with <65 years of age. Such difference disappeared in those ≥65 years of age, which might be due to the lowering effect of plasma 5-HT levels in ever-smokers with an increase in age. One possible mechanism for the elevated plasma 5-HT levels in COPD patients is the persistent platelet activation found in patients with stable or acute exacerbation of COPD 
. Increased plasma 5-HT levels might be taken up by platelets via
SERT. That way, SERT would be modified and downregulated due to the increased intracellular 5-HT levels in platelets, resulting in the inhibition of 5-HT reuptake 
. These processes might further increase the plasma 5-HT levels in COPD.
Plasma 5-HT levels have been shown to be correlated with clinical severity and pulmonary function in symptomatic asthma patients 
. In contrast, we found no association between plasma 5-HT levels and disease severity in COPD patients. This discrepancy may be due to the fact that the pathogenesis between asthma and COPD is different 
. Another possible reason is that all the COPD patients in this study are stable. Here, we reported the partial mediation effect of plasma 5-HT on the relation between pack-years smoked and COPD. Ishii and colleagues have shown that cigarette smoking is a significant mediator of the relation between SERT polymorphism and COPD 
. Whether the increased plasma 5-HT levels in COPD patients are due to SERT gene polymorphism remains unclear. In a small group of male COPD non-smokers, significant higher plasma 5-HT levels were observed in comparison to healthy non-smokers, suggesting that the plasma 5-HT levels are influenced by other factors apart from cigarette smoking (). For example, COPD comorbidities such as increased pulmonary artery pressure or vascular dementia might be associated with the high plasma 5-HT levels 
. Further studies are required to shed more light on these contributing factors and their possible role in altering plasma 5-HT levels in patients with COPD.
Hypothetic diagram for the relation among SERT gene polymorphism, plasma 5-HT, pack-years smoked and COPD.
The relation between 5-HT levels and age was investigated in a study using different components of blood 
, demonstrating no correlation between plasma 5-HT levels and age, regardless of gender and smoking status. These findings are in agreement with the results obtained from our control subjects (healthy non-smokers and ever-smokers). However, their subjects were much younger (36.0±11.5) than our control subjects, and the effect of gender on the association between 5-HT and age was not addressed in the study. Another study reports a negative correlation between plasma 5-HT levels and age in women with age ranging from 40–84 years 
, regardless of smoking status, similar to male healthy ever-smokers in this study. Since there was no significant difference in age between healthy non-smokers and ever-smokers in our samples, our data suggest that healthy ever-smokers may be able to maintain the homeostasis of circulating 5-HT by lowering its level with an increase in age. In contrast, we found a positive correlation between plasma 5-HT levels and age in COPD ever-smokers. These data suggest that the regulation of plasma 5-HT levels is disturbed in COPD patients. A disturbance in any of the regulatory processes of 5-HT, including uptake, storage, release and breakdown by monoamine oxidases (MAOs) in the aging process or during the development of COPD can result in changes in the plasma levels of 5-HT.
The lung is a site at which the reuptake and removal of circulating 5-HT takes place. Different components of cigarette smoke have been shown to reduce MAOs' activity in the lung 
. An inhibition of the reuptake and removal of 5-HT may lead to the accumulation of 5-HT in the pulmonary vasculature, a process believed to be involved in the pathogenesis of lung disease 
, including COPD.
The present study has significant limitations. Firstly, it is a cross-sectional study and thereby does not allow for the conclusion of a causal link between 5-HT and the presence of COPD. Prospective cohort studies on healthy smokers are therefore required to fully address the role of 5-HT in the development and progression of COPD. Secondly, all measurements were carried out in plasma, which reflects only systemic changes and may not adequately reflect the local concentrations in the lungs. Further studies involving biological samples such as bronchoalveolar lavage (BAL), induced sputum and exhaled breath condensate, might shed more light on the local pulmonary processes. Thirdly, the comparison of plasma 5-HT levels between healthy non-smokers and COPD non-smokers was limited by the small sample size; furthermore, the large difference in sample size between the two groups may not provide a fair comparison. The age difference between COPD patients and control subjects is another limitation in this study. To ensure our results were not due to the age difference between the two groups, age was entered as a covariant in the regression analyses.
Our results demonstrated the elevation of plasma 5-HT levels in COPD patients, which is a significant mediator of the relation between cigarette smoking and the presence of COPD. We speculate that the cigarette smoke-mediated injury in COPD lungs is partially mediated by plasma 5-HT levels. Both cigarette smoking and COPD affected the association between plasma 5-HT levels and age. Our findings further support the involvement of the serotoninergic system in the pathogenesis of COPD. Further investigation of the functional role and the regulation of 5-HT in COPD is important for understanding and preventing the progression of this disease.