Disruption of Homeostatic Neuroimmune Interactions in Descending Circuitry in the Development of Pain Chronicity

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Millions of people suffer from chronic or persistent pain, which is a major medical problem. The current treatment for chronic pain conditions is unsatisfactory. In recent years, ample evidence has documented the role of glia and their interactions with neurons in the development of persistent pain. Despite overwhelming evidence from preclinical studies, clinical trials for the treatment of chronic pain with glial modulators have not been successful, which is related to our incomplete understanding of the mechanisms. While a majority of studies show the pain-facilitating aspect of the injury-related glial activity, a potential inhibitory/protective role of neuron-glial interactions in the development of persistent pain has been largely overlooked. The central nervous system (CNS) homeostasis is supported by multiple inhibitory signaling pathways, among which the CD200-CD200R signaling tandem has attracted attention. Immunoglobulin CD200 from neurons signals via its receptor CD200R on microglia to maintain microglia at the surveillance state. Loss of or reduced CD200- CD200R signaling after injury facilitates microglial activation. Descending pathways provide balanced modulation to maintain normal pain sensitivity. Facilitation or disinhibition from the rostral ventromedial medulla (RVM), a pivot structure in descending pathways, to the spinal/ trigeminal dorsal horn contributes to the development of chronic pain. Our preliminary results point to a new descending pathway from the anterior cingulate cortex (ACC) that directly projects to the RVM and is involved in the 5-HT3 receptor (5-HT3R)-dependent pain facilitation. The cellular mechanisms underlying the function of this direct ACC-RVM connection is unclear. We propose to analyze inhibitory/beneficial neuroglial interactions in the novel ACC-RVM descending pain modulatory circuitry and test the hypothesis that disrupted inhibitory glial activity contributes to the emergence of chronic pain. Our working hypothesis is that the CNS CD200-CD200R signaling axis is necessary for homeostasis and insufficient/disrupted signaling of which disturbs the balance and contributes to chronic pain conditions. Aim 1 will test the hypothesis that pain facilitation from the novel ACC-RVM pathway involves insufficient homeostatic CD200-CD200R signaling and hyperexcitability of 5-HT-containing neurons. Aim 2 will test the hypothesis that disrupted inhibitory CD200-CD200R signaling in the RVM contributes to the emergence of chronic pain. Aim 3 will test the hypothesis that the CD200/CD200R signaling is important for the anti- inflammatory phenotype of microglia in the RVM and involves downstream forkhead box P3 (Foxp3) and signal transducer and activator of transcription 6 (STAT6) activity. Exploring the beneficial effect of glial activity will fill the gap in our understanding and lead to a transformative shift in the search for improved management for chronic pain.