Glial-cytokine-neuronal interactions in the mechanisms of persistent pain

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There has been increasing awareness of neuroimmune interactions and their role in the etiology of diseases including stroke, Parkinson's disease, and chronic pain. Although it is now widely appreciated that glia and inflammatory cytokines affect neuronal function and behavior through a variety of cellular signaling pathways, the underlying mechanisms linking immune and neuronal functions are unknown. We propose to employ a rat model of hind paw inflammatory pain to study interactions between glia, cytokines and neurons and explore their significance in the central nervous system response to injury and the development of persistent pain. Recent studies indicate that pain processing can be vigorously facilitated by brainstem descending circuitry, a process that contributes to the development of chronic pain conditions. Abnormal pains after injury are linked to an enhanced neuronal activity in the rostral ventromedial medulla (RVM), a pivotal structure in descending pain modulation. The emerging literature strongly implicates a role for glia and inflammatory cytokines in the development of hyperalgesia. Through still unknown mechanisms, glia can be activated after injury and release chemical mediators that modulate neuronal activity. Such glial-cytokine-neuronal interactions may be critical in the chronic pain process. To date, no studies have addressed the involvement of glia and related chemicals in descending facilitation of persistent pain. We propose to identify the cellular and molecular mechanisms of descending pain facilitation after tissue injury with an emphasis on neuronal-glial interactions in the RVM circuitry. We hypothesize that 1) peripheral inflammation induces neuronal plasticity in the RVM circuitry involving activation of glia; and 2) RVM glial activation and inflammatory cytokine release facilitate neuronal plasticity through interactions with neuronal N-methyl-D-aspartate receptors (NMDAR) and contribute to the descending facilitation of hyperalgesia. Aim 1 will test the hypothesis that glial cells are activated in the RVM after inflammation and affect neuronal function through release of inflammatory cytokines. Complete Freund's adjuvant will be injected into the hind paw to produce inflammation and behavioral hyperalgesia. Aim 2 will determine whether neuron-to-glia signaling plays a role in glial activation after inflammation. Aim 3 will test the hypothesis that glial activation in the RVM and associated cytokine release facilitate neuronal plasticity through interaction with neuronal NMDAR and play a critical role in the development of hyperalgesia. Thus, we have proposed a model of reciprocal neuronal-glial interactions in the development of persistent pain. Advancing from previous studies, the model emphasizes activation of glia by injury-generated neuronal input, concomitant cytokine release, and post-translational regulation of NMDAR through cytokine signaling. The outcome of these studies will enhance our understanding of functional linkage between the immune and nervous system and help to identify novel targets and agents for management of chronic pain. We propose to employ a rat model of inflammatory pain to study interactions between glia, cytokines and neurons and explore their significance in the central nervous system response to injury and the development of persistent pain conditions. Although it is now widely appreciated that glia and inflammatory cytokines affect neuronal function and behavior through a variety of cellular signaling pathways, the underlying mechanisms linking immune and neuronal functions are largely unknown. The outcome of these studies will enhance our understanding of functional linkage between the immune and nervous system and help to identify novel targets and agents for management of chronic pain.