Agonists acting at the 5 opioid receptor (MOR) (e.g. morphine and its analogs) are the mainstay of pain management;however there are serious adverse effects (e.g. dependence) and social and legal issues which limit their use. Consequently there has been considerable interest in the peripheral analgesic effects of opioids. However, although opioid receptor systems are expressed in sensory neurons, they are functionally inactive under most basal conditions, but become functionally competent when some stimulus (inflammation) is present in the peripheral tissue. In addition to MOR, the 4 opioid receptor (DOR) is an attractive target for drug action since there are fewer adverse effects than with MOR activation. However, in general, the efficacy of 4 agonists to promote analgesia is weak to moderate. Here we propose to study mechanisms involved in the regulation of the functional competence of the MOR and DOR systems to inhibit adenylyl cyclase activity and neuropeptide release in primary cultures of rat trigeminal ganglion neurons and in behavioral models of peripheral analgesia.
Our specific aims are: 1) To characterize the kinetics of induction and maintenance of functional competence of MOR and DOR systems. We will delineate the time course for induction of competence, and the persistence of competence after induction, by administration of bradykinin (BK), arachidonic acid (AA) and naloxone. 2) To determine the changes in the MOR and DOR systems which underlie the development of functional competence In this aim, we will determine the mechanism for the priming-induced increase in opioid receptor-mediated G protein activation. 3) To determine if functional competence of MOR and DOR receptor systems induced by naloxone and BK pre-treatment is due to reduced constitutive desensitization.. We hypothesize that the MOR/DOR receptor systems exist in a constitutively desensitized state thus are non-responsive to agonist. Treatment with inverse agonists should reduce this constitutive desensitization leading to enhanced agonist responsiveness. Also priming with other agents (e.g. BK) may induce competence by reducing constitutive desensitization. 4) To investigate the functional competence of MOR and DOR in behavioral assays of analgesia.
This aim provides a translational extension of the foundation work of Aims 1-3. Using a newly developed model of orofacial pain, we will 1) determine if functional competence of the MOR and DOR systems can be induced by BK, AA, and naloxone, 2) investigate the time course of competence, 3) delineate the role of PKC5 in induction of functional competence induced by BK, AA and naloxone and 4) determine the role of BK receptors and PKC in mediating functional competence in an inflammatory model of pain. These experiments will increase our understanding of the regulation of 5 and 4 opioid receptor signaling in primary sensory neurons and may lead to novel therapeutic approaches for the treatment of pain.

Public Health Relevance

In the last few years many mutations in different genes have been isolated in C. elegans, D. Melanogaster and mice which plays role in vesicle fusion and synaptic plasticity. Fundamental insights into the processes and regulation of human synaptic plasticity will occur when we understand how different conserved signaling mechanism controls this process. The neurosecretion study of Drosophila as a model system provides extraordinary opportunities to use different methods of gene manipulation and of genomics with direct relevance to mammalian synaptic release and plasticity.

National Institute of Health (NIH)
National Institute on Drug Abuse (NIDA)
Research Project (R01)
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Somatosensory and Chemosensory Systems Study Section (SCS)
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Rapaka, Rao
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University of Texas Health Science Center San Antonio
Schools of Medicine
San Antonio
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Berg, Kelly A; Rowan, Matthew P; Gupta, Achla et al. (2012) Allosteric interactions between ýý and ýý opioid receptors in peripheral sensory neurons. Mol Pharmacol 81:264-72
Berg, Kelly A; Rowan, Matthew P; Sanchez, Teresa A et al. (2011) Regulation of ?-opioid receptor signaling in peripheral sensory neurons in vitro and in vivo. J Pharmacol Exp Ther 338:92-9