Psycho-oncologic effects of serotonin and melatonin

The relationship between mental and physical health has been discovered and studied for thousands of years. It has been reported that patients with depression may have higher incidences of several diseases including cardiovascular diseases and cancer. On the other hand, many patients suffering from these diseases are also prescribed antidepression drugs. Depression is also often found in drug abusers. It is known that serotonin (5-HT) functions as a principal neurotransmitter, resulting in the feeling of happiness, and has a strong correlation with depression. Moreover, 5-HT is a derivative of tryptophan, one of the essential amino acids in the diet. A lower level of serotonin is often detected in the patients with cancer and cardiovascular diseases than that found in healthy controls. Altered tryptophan metabolism in the tumor microenvironment skews the balance of neuroprotectants and neurotoxins, thus it has been suggested as one of the underlying mechanisms of depression development in cancer and other related diseases. The neurotoxins generated in the altered tryptophan metabolic pathways contribute to the depression. Furthermore, melatonin, the final product of the metabolism, decreases significantly in cancer patients as well. Melatonin is known as a circadian regulator, controlling the sleep and reproduction, and also functions as a powerful antioxidant related to neuron protections and antitumor immunity. It is found that both patients with cancer or depression and drug abusers are often suffering with insomnia, which is highly correlated to the levels of serum melatonin. Higher cancer incidence has been reported in night workers than that in day workers, indicating that the melatonin may play a role in protecting neurons and normal cells from oxidative stresses, one of the main causes involved in the development of cardiovascular diseases, cancer, and neuronal diseases including Alzheimer's disease and depression. In this chapter, we will discuss the correlations of 5-HT and melatonin with cardiovascular diseases, cancer, and neuronal diseases, and their roles in mental and physical health from various aspects.