Identification of factors in peripheral sensory neurons that contribute to the axonal targeting and functional coupling of G protein-coupled receptors such as opioid receptors

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It is well known that G protein-coupled receptors (GPCR) in peripheral sensory neurons contribute to the excitation and sensitization as well as to their inhibition. Since ion channels are distributed not only at the nerve terminals, but also along the axolemma of peripheral sensory neurons, the question arises whether it is conceivable that GPCRs are also located and functionally coupled to the axonal membrane? The demonstration of such GPCRs, e.g. opioid receptors, integrated into the axonal membrane is yet unknown and would encourage the clinical use of drugs for such receptors, e.g. opioids. First clinical trials seem to indicate a clinical benefit for patients suffering from nerve injury. Consistently, in animal models of sciatic nerve injury perineural application of opioids elicited potent antinociceptive effects, although they were not observed in animal models of inflammatory pain. Here we investigate the hypothesis that mechanisms of mu-opioi receptors (MOR) axonal transport, membrane targeting and functional coupling are different in peripheral nerve injury versus inflammatory pain. Our first goal will identify in animals with complete Freunds adjuvant hindpaw inflammation versus animals with chronic constriction nerve injury the distinct antinociceptive effects of perineural opioids in relation to their axonal presence and functional coupling of MOR. The second goal will identify whether MOR enter the axonal initial segment, a filter for axonally delivered proteins, they colocalize with vesicle-associated membrane proteins (VAMP), they are transported via motor proteins such as kinesins along intraaxonal microtubules, and they are finally delivered to their destination via myosin Va motor proteins along F-actin filaments. By interfering with these mechanisms we will assess the individual relevance of these components. The third goal will test local mechanisms such as enhanced production of lipid rafts, increased expression of G protein-regulated inducer of neurite outgrowth (GRIN1), or up-regulation of neural cell adhesion molecule L1 (L1-CAM) of being responsible for the integration of MOR into the axonal membrane and whether they depend on the local expression of growth factors (e.g. NGF). To understand the exact mechanisms that determine the targeting and functionally coupling of GPCRs such as opiod receptors to the axonal membrane may give novel incentives and tools to regulate this process towards a therapeutic advantage. This would be of great relevance particularly for patients suffering from severe pain due to nerve injury or to tumor infiltration.