Our results showed that oral administration of green tea increased the reward-learning ability compared with control group in healthy young volunteers. Moreover, participants treated with green tea showed reduced scores measured in MADRS and HRSD-17 compared with placebo. With the evidence that anhedonia influence reward decision-making, we propose that green tea would probably have the potential for normalization of anhedonia through improve reward learning and have implications for the prevention of depression.
A recent study showed that green tea extract treatment can reduce hypothalamic-pituitary-adrenal (HPA) axis hyperactivity in response to stress in mice . Systemic administration of lipopolysaccharide (LPS) could induce depression in the forced swimming-induced despair behaviour model in mice. Pretreatment with green tea extract prevented LPS-induced immobility in a dose-dependent manner via COX-2 inhibition . Clinically effective antidepressant medications exert their therapeutic actions partially by modulating HPA function through the regulation of receptor expression, subsequently ameliorating many of the behavioral disturbances associated with depressive-like states [26, 27]. A further research is needed to determine the regulatory effect of green tea on glucocorticoids receptors expression and the target genes involved in the reward learning process and the improvement of depressive symptoms. Furthermore, a significant antidepressant-like effect was detected in mice that received a single intraperitoneal injection of green tea in the forced swim test when compared with the control . A cross-sectional study revealed that higher green tea and coffee consumption was associated with a lower prevalence of depressive symptoms, suggesting that higher consumption of green tea, coffee and caffeine may confer protection against depression .
Anhedonia has long been presumed as a core feature of major depressive disorder based on the Feighner criteria in 1972 . The American Psychological Association defines anhedonia as “report feeling less interest in hobbies, not caring anymore, or not feeling any enjoyment in activities that were previously considered pleasurable” . Anhedonia and depressed mood are two required symptoms for a diagnosis of major depressive disorder [31, 32]. It has been indicated previously that anhedonia is associated with impaired reward learning in depressed patients . We confirmed that chronic treatment with green tea can improve the reward learning compared with baseline in healthy subjects at time of inclusion. Our results showed that oral administration of green tea increased the reward-learning ability compared with placebo.
It was proposed thirty years ago that dopamine deficiency and the reduction in dopamine (DA) transmission were involved in anhedonia, in individual’s experience of pleasure and in response to rewarding stimuli [33–36]. The mesolimbic and nigrostriatal DA system appears to be related primarily to reward system function and responsiveness to the environment . In clinic, there is no effective intervention for anhedonia Although there are medications that alleviate several depressive symptoms, the current used pharmacotherapies (e.g., SSRIs) do not adequately address motivational and reward-processing deficits in depression [37–39]. It has been reported that the active component of green tea, EGCG, inhibited psychostimulants-induced hyperactivity in part by modulating dopaminergic transmission . Additionally, the findings from epidemiological studies revealed that consumption of green tea was inversely correlated with neurodegenerative diseases including Parkinson's disease. The further investigation showed that EGCG produced a protective effect against dichlorodiphenyl-trichloroethane (DTT)-induced cell death, and that prior exposures to EGCG activate an endogenous protective mechanism in the dopaminergic cells, which is believed to play a causative role in the etiology of Parkinson's disease . Moreover, EGCG also could reduce oxidative stress and neurotoxicity in different model systems of Alzheimer's disease and modulate the expression of cell survival and cell death genes .
In recent years, there has been considerable interest in investigating the neuroprotective effects of green tea polyphenolic constituent with its biological and pharmacological activities relevant to human health . A previous study a mouse behavioral model showed that green tea exerts antidepressant-like effects, and the underlying mechanism may involve inhibition of the HPA axis . Psychological stress activated HPA which resulted in the increasing release of glucocorticoids. It was also found that EGCG also could ameliorate the cognitive impairments induced by psychological stress possibly related with the declining glucocorticoids levels and the increasing contents of catecholamines and 5-HT . Whether the increased reward-learning activity induced by chronic green tea administration is due to the regulation on neuroendocrinology systems, such as HPA function, requires future investigation.
Regular dietary intake of green tea can maintain a stable concentration in the body and may have valuable effects on health. However, drinking a beverage containing green tea extract too much can lead to hepatotoxicity [44–46]. The hepatotoxicity is probably due to EGCG or its metabolites which, under particular conditions related to the patient's metabolism, can induce oxidative stress in the liver [47–49]. Therefore, the toxicity should be taken into consideration in the condition of high oral consumption of green tea polyphenols. Additionally, the baseline amount of tea consumed in participants was screened before enrolment, and the participants whose daily intake of tea was more than three cups were excluded from the study. In the current study, the difference of baseline amount of tea in two groups was not significant. Therefore, the data in this experiment cannot be explained by an under measurement of effect.
In summary, our results indicate that chronic treatment of green tea increased reward learning compared with placebo in monetary incentive delay task and reduced total scores in Montgomery-Asberg depression rating scale and 17-item Hamilton Rating Scale for Depression. Reward-learning deficits contributed to the anhedonic symptoms and the treatment outcome of depression. The current findings suggest that reduced reward learning might associate with the presence of depression. The regulatory role of green tea on the reward learning raised the potential for development of new treatments for depression by selecting complementary options.