Physical Exercise and Neuroinflammation in Major Depressive Disorder

Major depressive disorder (MDD) is a prevalent psychiatric disorder associated with varied prognosis, chronic course, and duration of illness with reduced quality of life. One factor that significantly contributes to the relevant disease burden of MDD is the heterogeneous treatment response patients experience with current treatment options. A variety of experimental protocols in humans and animals have highlighted that inflammation and neuroinflammation are relevant biological factors that interact with external stimuli and neurophysiological mechanisms, and can trigger MDD. It is well established that exercise is efficacious in treating mild to moderate depression with response rates comparable to mainstream therapies such as antidepressant medication and cognitive behavioral therapy. Several studies have shown that physical exercise is beneficial for a range of chronic diseases. Indeed, physical exercise can promote molecular changes that swerve a chronic pro-inflammatory state to an anti-inflammatory state in both periphery and central nervous system. The changes caused by physical exercise include an increase in PGC1α gene expression, a transcriptional co-activator involved in reducing the synthesis and releasing of pro-inflammatory cytokines, and an increase in anti-inflammatory cytokines. PGC1α changes the metabolism of kynurenine towards, and, in turn, it reduces glutamatergic neurotoxicity. Moreover, some studies have shown that physical exercise promotes alterations in the circuitry of monoaminergic neurotransmission, at least in some aspects, through the effects on the release of proinflammatory cytokines. This review will highlight the effects of physical exercise as therapy and its relation with the biological mechanisms involved in the pathophysiology of MDD, with particular emphasis in the interactions among physical exercise, hypothalamic-pituitary-adrenal (HPA) axis, neuroinflammation, and with the neurotransmitters underlying the main brain circuits involved in the MDD.