SAN JUAN, P.R. – Growing evidence points to an association between inflammation and depression, according to a presentation at the annual meeting of the American College of Psychiatrists.
For example, depressed patients have elevated inflammatory markers–such as interleukin-6 and C-reactive protein. In fact, the levels of proinflammatory cytokines correlate with the severity of depressive symptoms in studies. In addition, administration of cytokine antagonists can effectively reverse depressive symptoms in patients, Dr. Andrew H. Miller said.
“We really stand at a point that is very exciting in terms of novel therapies and translation of research,” Dr. Miller said. “The notion quite simply is that stress or depression affects the HPA [hypothalamic-pituitary-adrenal] axis, [affects] the endocrine system, alters the immune system, and leaves patients open to diseases.”
Physicians from many specialties already recognize that inflammation plays a key role in cardiovascular disease, diabetes, metabolic syndrome, and cancer, said Dr. Miller, professor in the department of psychiatry and behavioral sciences at Emory University, Atlanta.
“We did not want to be left out in terms of psychiatry,” said Dr. Miller, who also is director of the psychiatric oncology program at the Winship Cancer Institute at Emory.
There are multiple possible mechanisms whereby inflammation could cause depression. Inflammatory cytokines released peripherally might reach the brain through active transport, passage through leaky regions in the blood-brain barrier, or transmission through afferent nerve fibers (vagus nerve), Dr. Miller said. There is a cytokine network in the central nervous system, and glia and microglia are the richest source of cytokines in the brain. Neurons also produce and express cytokines.
“We've learned these cytokines have access to the brain and … ultimately can change behavior,” Dr. Miller said. Inflammatory cytokines cause anhedonia, fatigue, cognitive dysfunction, and other flu-like symptoms in sick patients. In addition, researchers induced behavioral changes that resemble major depression in human and animal studies with administration of proinflammatory cytokines.
Some therapeutic cytokines cause depression. For example, interferon-α (IFN-α) is used to treat viral infections and cancer because it is a potent inducer of the inflammatory cytokine network, especially interleukin-6, Dr. Miller said. “Oncologists told us early on this drug causes a lot of depression.”
A total of 60% of patients treated with IFN-α reported depressed mood in one study (Neuropsychopharmacology 2002;26:643–52). Dr. Miller and his associates also found a 45% incidence of major depression among patients with malignant melanoma treated with IFN-α (N. Engl. J. Med. 2001;344:961–6).
The good news is that paroxetine (Paxil) aggressively blocked development of depression. “Just 11% developed depression, so there was a fourfold reduction with this pretreatment.
“There is a caveat. If you give a drug that causes release of dopamine–for example, paroxetine–that dopamine becomes oxidized and in the long term can damage basal ganglia,” Dr. Miller said in response to a question from a person attending the meeting. “So we're using dopamine antagonists to block this until we get more information about what we are doing to patients.”
Physician reaction to his study varied, Dr. Miller said. “The people who got on us the most for that study with paroxetine were the ones who were treating hepatitis C. They said we'd expose a lot of people to antidepressants who don't really need them.” However, “with melanoma, many patients will not go back on interferon therapy, and giving antidepressant prophylaxis might help.”
In another study, patients with psoriasis treated with the cytokine antagonist etanercept experienced reversal of their depressive symptoms (Lancet 2006;367:29–35). Improvement in depression was independent of the drug's effect on disease progress.
The wider picture may be a link between stress, depression, and illness, Dr. Miller said. In one study in review, patients with major depressive disorder exhibited an exaggerated inflammatory response to stress.
“There is an interesting possible link between depression and a wide variety of medical disorders where inflammation plays a role,” Dr. Miller said. It “may explain high comorbidity of some medical conditions with depression.
“Psychiatry is now catching up to other medical specialties in recognizing the adverse effects of inflammation,” Dr. Miller added. “Psychiatrists need to keep an eye on this. The idea that the immune system might affect the brain and vice versa presents a lot of novel targets for treating psychiatric disorders.”