Signaling Mechanisms of Opioid-Induced Hyperalgesia and Tolerance

Signaling Mechanisms of Opioid-induced Hyperalgesia and Tolerance Project Summary The major objective of this project is to identify key signaling mechanisms responsible for the development of opioid-induced hyperalgesia and analgesic tolerance (OHT). Opioid drugs remain indispensable for treating severe pain caused by surgery, trauma, and cancer. However, acute and repeated administration of μ-opioid receptor (MOR) agonists often cause OHT, the major obstacle to adequate pain relief with opioids. OHT can also lead to unsafe opioid dose escalation, resulting in dependence, addiction, and even overdose death. Opioid signaling is complex and has been studied mostly in vitro, but the functional significance and relevance of various opioid signaling components to OHT are poorly understood. N-methyl-D-aspartate receptors (NMDARs) are a clinically validated target for treating OHT, and extracellular signal-regulated kinase (ERK) is stimulated by MOR activation and is involved in opioid-induced NMDAR hyperactivity at the spinal cord level and in OHT. At present, little is known about the upstream signaling mechanism leading to stimulation of ERK at the spinal cord level during OHT. Although BRAF, a serine/threonine-specific protein kinase, is a crucial upstream signal for ERK activation, its role in OHT has not been recognized previously. In our preliminary studies, we found that repeated treatment with opioids increased BRAF activity in the spinal cord. Furthermore, BRAF inhibition or knockdown at the spinal cord level substantially attenuated OHT and rescued the synaptic trafficking and expression of MORs and NMDARs in the spinal cord altered by opioid treatment. These initial findings suggest that BRAF- dependent signaling plays a key role in the control of synaptic MOR and NMDAR plasticity in the development of OHT. Therefore, in this competing renewal application, we will test the overall hypothesis that repeated treatment with opioids, through the BRAF-mediated signaling axis, induces (1) analgesic tolerance by inhibiting expression and activity of MORs at primary afferent central terminals and (2) hyperalgesia by promoting trafficking and activity of NMDARs at primary afferent terminals synapsing with spinal cord excitatory neurons. To test this hypothesis, we will use a multidisciplinary approach, including protein biochemistry, electrophysiological recordings in spinal cord slices, and targeted gene knockout and knockin. Our proposed studies are expected to advance our understanding of the fundamental signaling mechanisms highly relevant to the development of OHT. Our project also has important clinical implications and could lead to new strategies (e.g., using FDA-approved BRAF inhibitors) for improving opioid analgesic efficacy in patients with severe pain.