[Depression and inflammation in rheumatic diseases]

Abstract

It is known that the prevalence of depression in rheumatologic patients is higher than in the general population. Socioeconomic factors are not a sufficient explanation of mood disorder in these patients. Symptoms reported by patients with chronic inflammatory diseases resemble changes defined as "sickness behavior". Mood disorders among somatic patients could be explained by disturbances of the immune system according to the monoaminergic theory of depression. Inflammatory factors such as IL-1 (interleukin-1), IL-2 (interleukin-2), IL-6 (interleukin-6), TNF-α (tumor necrosis factor α), and IFN-γ (interferon-γ) act within the CNS (central nervous system). They get through from peripheral tissues as well as being synthesized de novo by neurons. This cytokine activity correlates positively with depression intensity as well as with genetic polymorphism of the serotonin (5-HT) transporter. The theory of glucocorticoid resistance-mediated depression (limbic-hypothalamic-pituitary-adrenal [LHPA] axis) is also connected with gained proinflammatory cytokines activity. It might assume the form of a vicious circle. Depressed mood is probably linked with depression in immune-mediated diseases. An elevated level of proinflammatory cytokines is able to activate IDO (indoleamine 2,3-dioxygenase)--an enzyme catabolizing tryptophan (5-HT precursor). Those reactions probably play the main role at the biochemical level. IDO metabolites extensively disturb neurotransmission. 3-Hydroxykynurenine (3OH-KYN), quinolinic acid (Quin) and kynurenic acid (KYNA) are neurotoxic by releasing oxidative stress mediators. Moreover, they activate MAO (monoamine oxidase), which degrades neurotransmitters responsible for stable mood. Bidirectional communication between the neuroendocrine and immune systems is significant for depression treatment, as well as CNS protection against incremental neurodegeneration among seemingly diverse diseases.