Microglia Activation Causes Depression, Anxiety in Chronic Pain

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Brain inflammation from chronic pain increases microglia activation, which inhibits the release of dopamine and may lead to depression and anxiety, according to a study published in The Journal of Neuroscience.

Although more than half of chronic pain patients experience depression, anxiety, or substance abuse, scientists were unable to determine what caused this association until now. In this study, the researchers sought to test if chronic pain disrupted the transmission of dopamine.

The researchers demonstrated that the activation of microglia in mice with chronic pain inhibited the release of dopamine. These results shed light on why opioids, which stimulate a dopamine response, can be ineffective for chronic pain patients.

The researchers instead tested a drug that inhibited the activation of microglia. This, they found, restored normal dopamine release and reward-motivated behavior in the mice.

"For over 20 years, scientists have been trying to unlock the mechanisms at work that connect opioid use, pain relief, depression and addiction," said Catherine Cahill, PhD, of the University of California, Irvine. "Our findings represent a paradigm shift which has broad implications that are not restricted to the problem of pain and may translate to other disorders."­

In future studies, the researchers hope to explore if mood disorders are caused by similar brain alterations, regardless of the presence of chronic pain.

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Microglia Activation Causes Depression, Anxiety in Chronic Pain

Chronic pain attenuates midbrain dopamine (DA) transmission, as evidenced by a decrease in opioid-evoked DA release in the ventral striatum, suggesting that the occurrence of chronic pain impairs reward-related behaviors. However, mechanisms by which pain modifies DA transmission remain elusive. 

Using in vivo microdialysis and microinjection of drugs into the mesolimbic DA system, we demonstrate in mice and rats that microglial activation in the VTA compromises not only opioid-evoked release of DA, but also other DA-stimulating drugs, such as cocaine. Our data show that loss of stimulated extracellular DA is due to impaired chloride homeostasis in midbrain GABAergic interneurons. Treatment with minocycline or interfering with BDNF signaling restored chloride transport within these neurons and recovered DA-dependent reward behavior.

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