Mood disorders are the leading cause of disability in many countries around the world. Depression alone affects a staggering number of people, currently thought to be about 350 million people worldwide. Its prevalence and diagnosis is increasing to such an extent that the WHO expects it to be the primary cause of global disease burden in less than 15 years.
Mood disorders are commonly found in patients diagnosed with inflammatory conditions. Cardiovascular disease, diabetes, metabolic syndrome, asthma, allergies and many autoimmune diseases co-occur with these psychiatric conditions. While providers are often tempted to attribute depression, anxiety and maladaptive behaviors to the stress of having chronic health issues, a significant body of evidence firmly supports the idea that mood disorders are themselves inflammatory conditions and therefore biologically ordained. Furthermore, having a mood disorder can affect prognosis in some diseases.
A number of inflammatory molecules participate in immune response, including histamine, prostaglandins, bradykinin, leukotrienes, CRP, interferon, cortisol and cytokines. These substances are released in response to physical stresses such as infection, trauma or disease process. Psychological stress also triggers inflammatory response with increases of molecules such as IL-6, IL-1b, TNF and CRP.
Several studies have definitively found that mood symptoms are associated with increased levels of inflammatory markers. PGE2, CRP, TNF, IL-1b, IL-2 and IL-6 were all elevated in both peripheral blood and cerebrospinal fluid in patients with major depressive disorder. A massive 25-80% of hepatitis C patients experience depressive symptoms when they begin treatment with interferon, a potent inflammatory molecule. Elevated interferon and IL-2 levels have been observed early in the depressive event.
In human patients, studies have simulated an inflammatory response by inoculation with toxins, proteins associated with infectious organisms, or interferon. In one study, an inflammatory response was provoked by inoculation with Salmonella endotoxin. While they suffered no physical symptoms, anxiety, depressed mood and decreased memory function was observed along with elevated TNF, IL-6 and cortisol levels. Another study found that inoculation with LPS (a substance found in bacterial cell membranes) triggered a dose dependent increase in IL-6, IL-10, TNF, cortisol and norepinephrine, which in turn triggered a dose dependent increase in anxiety, “poor mind” and decreased long term memory functions.
Furtado M, Katzman MA. Examining the role of neuroinflammation in major depression. Psychiatry Research 2015: 229, 27-36.
Rosenblat JD, et al. Inflamed moods: a review of the interactions between inflammation and mood disorders. Progress in Neuro-Psychopharmacology & Biological Psychiatry 2014; 53, 23-34.