The NOP receptor, the fourth member of the opioid receptor family, has been the target of investigation since its discovery in 1993, with respect to pharmacology, anatomy, and behaviors elicited by agonists and antagonists. Despite considerable research efforts, the lack of suitable receptor antibodies has prevented the appropriate interpretation of many studies pertaining to the details of receptor location, internalization, and dimerization. In order to test hypotheses pertaining to NOP receptor function, Dr. Brigitte Kieffer and colleagues have generated knock-in mice that carry green fluorescent protein (eGFP) coupled to the NOP receptor. Similar GFP-tagged delta receptor and mCherry-tagged mu receptor knock-in mice have proven very useful in understanding the relationship among anatomy, cellular localization, and function of delta and mu opioid receptors. Like the mu receptor, NOP receptors are found in very high numbers in all of the pain-related brain regions, including PAG, RVM, MHb, thalamus, etc. However, NOP receptors are unlike the other members of the opiate receptor family in that NOP receptor agonists block opiate analgesia when administered i.c.v. while having antinociceptive activity when administered intrathecally. In addition, NOP receptor agonists appear to be more effective rather than less effective in chronic pain states. This is surprising since NOP receptor mRNA decreases in DRG and anterior cingulate cortex (ACC) and NOP receptors decrease in certain spinal cord laminae in spinal nerve ligated mice. Using NOP-eGFP(+/+) mice, immunohistochemical experiments will be carried out to better understand the circuitry and the cellular localization in brain, spinal cord, and DRG that leads to these unusual properties of N/OFQ and other NOP receptor agonists. These will be correlated with behavioral experiments subsequent to microinjections into specific brain regions to understand how NOP receptor activation modulates the sensory as well as the affective component of pain.
Specific Aim 1 will carefully examine the location of NOP-eGFP receptors in DRG as well as determine the pain modalities (heat, cold, touch) attenuated by systemic administration of NOP agonists and antagonists and determine how these parameters change during chronic neuropathic and inflammatory pain.
Specific Aim 2 will examine spinal cord NOP-eGFP expression as well as characterize spinal projections both to the brain and to the periphery. These results will be compared with the effects of NOP agonists on different pain modalities after intrathecal administration in sham and neuropathic mice.
Specific Aim 3 will examine neuropathic pain-induced changes in NOP-eGFP receptor levels in brain with particular emphasis on regions involved in the sensory (PAG) and affective (ACC) components of pain. Direct injections of NOP receptor agonists and antagonists into these brain regions will be used to better understand the NOP receptor-related circuitry that modulates thermal, tactile, and emotional pain through these brain regions. These experiments will clearly identify the role of NOP receptors in acute and chronic thermal and tactile pain, allodynia and hyperalgesia.

Public Health Relevance

The NOP receptor, the fourth member of the opioid receptor family, is involved in acute and chronic pain. Knock-in mice containing eGFP tagged NOP receptors have been produced to test hypotheses pertaining to the localization and function of NOP receptors. We will use immunohistochemical and behavioral methods to understand, at the cellular level, how NOP receptors influence the somatosensory system in the brain, the spinal cord and in the peripheral sensory system in normal mice and those in chronic pain.

National Institute of Health (NIH)
National Institute on Drug Abuse (NIDA)
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Somatosensory and Chemosensory Systems Study Section (SCS)
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Rapaka, Rao
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Torrey Pines Institute for Molecular Studies
Port Saint Lucie
United States
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