Persistent/chronic pain, a major public health problem, is inadequately controlled by standard analgesics. Understanding the mechanisms that underlie persistent pain is important for developing novel therapeutic strategies to improve clinical treatment. AMPA receptor GluR2 subunit, through its binding to GRIP and PICK1, may function in the synaptic AMPA receptor trafficking that is critical for many forms of neuronal plasticity. GRIP anchors GluR2 at synapses, whereas PICK1 brings PKC1 to synaptic GluR2. PKC1 phosphorylates GluR2 Ser880 to release GluR2 from GRIP and to promote GluR2 internalization in vitro. We have made novel discoveries that Complete Freund's Adjuvant (CFA)-induced inflammatory insult reduced the binding affinity of spinal GluR2 for GRIP and that activation of spinal NMDA receptors induced PKC phosphorylation of GluR2 Ser880 in dorsal horn neurons, suggesting that NMDA receptor-triggered PKC activation might lead to GluR2 phosphorylation at Ser880 and GluR2 internalization in dorsal horn neurons under persistent pain conditions. This proposal seeks to determine whether and how CFA-induced persistent inflammatory insult drives spinal GluR2 internalization and whether this internalization contributes to the central mechanism of CFA-induced persistent pain.
In Specific Aim 1, we will first determine whether CFA-induced peripheral inflammatory insult time-dependently increases spinal GluR2 phosphorylation at Ser880. Using a surface expression assay and an immunoelectron microscopic approach, we will then examine whether surface and synaptic expression of GluR2 in dorsal horn neurons is decreased under CFA-induced persistent pain conditions.
In Specific Aim 2, we will determine whether NMDA receptor triggering of PKC1 activation is required for CFA-induced spinal GluR2 phosphorylation at Ser880 and its internalization. We will examine whether PKC1 or NMDA receptor activation produces GluR2 phosphorylation at Ser880 and decreases GluR2 surface expression and whether these effects could be blocked by PKC1 inhibition in cultured dorsal horn neurons in vitro. Furthermore, we will study whether spinal PKC1 inhibition or NMDA receptor blockade attenuates the CFA-induced increase in GluR2 phosphorylation at Ser880 and decrease in GluR2 surface expression in dorsal horn in vivo.
In Specific Aim 3, we will determine whether CFA-induced spinal GluR2 internalization contributes to pain hypersensitivity under CFA-induced persistent pain conditions. Using GluR2 K882A knock-in mice, we will study whether targeted mutation of the GluR2 PKC1 phosphorylation site attenuates CFA-induced thermal and mechanical pain hypersensitivities. The proposed studies will provide a new insight into the mechanisms of persistent inflammatory pain and may lead to novel therapeutic strategies for treatment or prevention of persistent pain. PROJECT NARRATIVE: Persistent pain or chronic pain, a common clinical condition, is poorly controlled by standard analgesics, such as opioids and non-steroidal anti-inflammatory drugs. Understanding mechanisms that underlie persistent/chronic pain is important in developing therapeutic strategies. Activation of spinal NMDA receptors and AMPA receptors contributes to spinal central sensitization that is thought to be an essential mechanism underlying the development and maintenance of persistent or chronic pain. However, NMDA receptor antagonists as well as AMPA receptor antagonists produce unacceptable side effects when they are used in the clinical setting. This proposal will explore novel molecular mechanisms of both NMDA receptor and AMPA receptor actions on persistent pain. The proposal may provide an effective strategy in developing selective targeting receptor single subunit- and site-specific drugs that could be useful in treating persistent/chronic pain.
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