A deficit in peripheral serotonin levels in major depressive disorder but not in chronic widespread pain.

Objectives: It has been proposed that serotonin dysfunctions underlie the pathophysiology of various mood disorders (including major depressive disorder, MDD) and chronic pain conditions characterized by deficient pain inhibition, such as fibromyalgia (FM). There is reliable data showing that serotonin disturbances are involved in the pathophysiology of MDD. However, in the case of FM, results published so far are less consistent.

Pain inhibition is deficient in chronic widespread pain but normal in major depressive disorder.

Background: Given the complex relationships between fibromyalgia and major depressive disorder (MDD), it has been suggested that fibromyalgia is a "masked" MDD. In experimental settings, fibromyalgia is associated with lowered pain thresholds (hyperalgesia) and deficient pain inhibition. Similarly, it has been recently proposed that the proneness of patients with MDD to develop chronic pain results from a deficit in pain inhibition.