Opioid actions are mediated primarily by three classic opioid receptors: mu, delta and kappa. Currently, opioid analgesics, mainly mu-opioid receptor (MOR) agonists such as morphine and fentanyl, are still the most effective pain relievers available and are widely used for the treatment of many forms of chronic pain. However, repeated use of opioids causes analgesic tolerance and physical dependence as a result of prolonged stimulation of MOR, which significantly reduces the analgesic efficacy of MOR agonists and limits their long-term clinical use for chronic pain management. In contrast to the well-characterized MOR actions, the function of delta-opioid receptors (DOR) is much less clear. DOR agonists produce little to weak analgesic effect both in animals and in humans under normal conditions. Recent studies including ours have demonstrated that the weak DOR effect is primarily due to the predominant intracellular localization of non- functional DOR in normal conditions, and more interestingly, the intracellular DOR is trafficked to plasma membrane and becomes functional after several forms of long-term neurochemical stimulation, including chronic morphine exposure. However, the molecular signal and mechanisms mediating the exocytotic DOR trafficking are completely unknown at present. In our preliminary studies exploring the issue recently, we have discovered that nerve growth factor (NGF), a neurotrophin with diverse functions in neuronal growth and differentiation, can trigger and maintain similar DOR trafficking and induce functional DOR in pain-modulating brainstem neurons under normal conditions. This represents a novel NGF function that has not been reported to date. Therefore, this proposal is aimed to identify the molecular mechanisms mediating the NGF- and chronic morphine-induced DOR trafficking and analgesic function, and to determine the synaptic and behavioral interactions of the emergent DOR with MOR in regulation of both central synaptic activity and pain inhibition in the context of chronic morphine. Findings from this project will provide the first evidence for NGF as a critical molecular mediator of membrane trafficking of G-protein-coupled receptors including DOR in central neurons, and will demonstrate the potential for DOR agonists as effective analgesics for chronic pain under chronic opioid conditions with decreased MOR efficacy.
Repeated use of opioid analgesics, the current pain reliever of choice, causes analgesic tolerance and physical dependence, which significantly reduce their analgesic efficacy and limit their clinical use for adequate control of moderate to severe pain. This research proposal investigates the cellular and molecular mechanisms underlying a new analgesic function of different opioid compounds in brain cells after repeated opioid administration. These studies may provide neuropharmacological evidence for using these opioid compounds as more effective analgesics under an opioid-tolerant and dependent state.
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