Brain stem descending pathways constitute a major mechanism in the modulation of pain transmission by which attentional, motivational and cognitive variables filter ascending information. Recent studies indicate that hyperalgesia in animal models of persistent pain is linked to enhanced activation of descending inhibitory and facilitatory modulatory circuits. Such mechanisms may play an important role in persistent pain conditions such as fibromyalgia, temporomandibular joint disorders, and irritable bowel syndrome by increasing endogenous facilitation leading to an enhancement and spread of the pain. In earlier studies we focused mainly on the activity-dependent plasticity and dynamic temporal changes occurring in the rostral ventromedial medulla (RVM) within the first 24h after complete Freund's adjuvant (CFA)-induced inflammation. The cellular mechanisms mainly responsible for descending inhibition included excitatory amino acid receptors. Less is known about the effects of chemical mediators that contribute to the initiation and maintenance of descending facilitation and behavioral hyperalgesia after inflammation. Therefore, a major objective of our proposal is to determine the chemical signature of RVM circuits involved in the initiation and maintenance of descending facilitation of inflammatory hyperalgesia. Our major hypothesis is that inflammation leads to an increase in the release of multiple chemical mediators whose chemical signature involves activation of a signal transduction cascade in RVM neurons that couples their activity to subunits of the N-methyl-D-aspartate (NMDA) receptor and a descending serotoninergic pathway. We will test these hypotheses utilizing behavioral, molecular, immunohistochemical and pharmacological approaches in the aims presented below.
Aim 1 will test the hypothesis that the balance between descending facilitation and inhibition and their contribution to inflammatory hyperalgesia is dependent, in part, upon the activation of multiple receptors for glutamate, proinflammatory cytokines, brain-derived neurotrophic factor (BDNF) and Substance P.
Aim 2 will test the hypothesis that activation of the receptors for these chemical mediators in the RVM involves changes in receptor expression levels and their activation of signal transduction cascades that lead to phosphorylation of NMDA receptor subunits.
Aim 3 will test the hypothesis that the time course of effect of these chemical mediators is differentially dependent upon primary afferent drive during the initiation and maintenance phases of behavioral hyperalgesia.
Aim 4 will test the hypothesis that descending facilitation produced by BDNF, SP and proinflammatory cytokines is mediated through serotonin-containing descending pathways and spinal 5-HT3 receptors. Finally, Aim 5 will test the hypothesis that these chemical mediators in the RVM contribute to the aversive component of behavioral hyperalgesia via descending circuits originating in the RVM.
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