A group of Japanese researchers has discovered that neural inflammation caused by our innate immune system plays an unexpectedly important role in stress-induced depression. This insight could potentially lead to the development of new antidepressants targeting innate immune molecules. The findings were published in the online edition of Neuron.
The research team then developed a method to selectively block the expression of TLR2/4 in the microglia of specific areas of the brain. By blocking the expression of TLR2/4 in the microglia of the medial prefrontal cortex, they managed to suppress depressive behavior in response to repeated social defeat stress. They found that repeated stress induced the expression of inflammation-related cytokines IL-1α and TNFα in the microglia of the medial prefrontal cortex via TLR2/4. The depressive behavior was suppressed by treating the medial prefrontal cortex with neutralizing antibodies for the inflammation-related cytokines.
These results show that repeated social defeat stress activates microglia in the medial prefrontal cortex via the innate immune receptors TLR2/4. This triggers the expression of inflammation-related cytokines IL-1α and TNFα, leading to the atrophy and impaired response of neurons in the medial prefrontal cortex, and causing depressive behavior.