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Background Prolonged opioid administration leads to tolerance characterized by reduced analgesic potency. Pain management is additionally compromised by the hedonic effects of opioids, the cause of their misuse. The multifunctional protein β-arrestin2 regulates the hedonic effects of morphine and participates in tolerance. These actions might reflect µ opioid receptor up-regulation through reduced endocytosis. β-Arrestin2 also recruits kinases to µ receptors. We explored the role of Src kinase in morphine analgesic tolerance, locomotor stimulation, and reinforcement in C57BL/6 mice. Methods Analgesic (tail withdrawal latency; percentage of maximum possible effect, n = 8 to 16), locomotor (distance traveled, n = 7 to 8), and reinforcing (conditioned place preference, n = 7 to 8) effects of morphine were compared in wild-type, µ+/–, µ–/–, and β-arrestin2–/– mice. The influence of c-Src inhibitors dasatinib (n = 8) and PP2 (n = 12) was examined. Results Analgesia in morphine-treated wild-type mice exhibited tolerance, declining by day 10 to a median of 62% maximum possible effect (interquartile range, 29 to 92%). Tolerance was absent from mice receiving dasatinib. Tolerance was enhanced in µ+/– mice (34% maximum possible effect; interquartile range, 5 to 52% on day 5); dasatinib attenuated tolerance (100% maximum possible effect; interquartile range, 68 to 100%), as did PP2 (91% maximum possible effect; interquartile range, 78 to 100%). By contrast, c-Src inhibition affected neither morphine-evoked locomotor stimulation nor reinforcement. Remarkably, dasatinib not only attenuated tolerance but also reversed established tolerance in µ+/– mice. Conclusions The ability of c-Src inhibitors to inhibit tolerance, thereby restoring analgesia, without altering the hedonic effect of morphine, makes c-Src inhibitors promising candidates as adjuncts to opioid analgesics.

The tyrosine kinase, c-Src, participates in mu opioid receptor (MOP) mediated inhibition in sensory neurons in which β-arrestin2 (β-arr2) is implicated in its recruitment. Mice lacking β-arr2 exhibit increased sensitivity to morphine reinforcement; however, whether β-arr2 and/or c-Src participate in the actions of opioids in neurons within the reward pathway is unknown. It is also unclear whether morphine acts exclusively through MOPs, or involves delta opioid receptors (DOPs). We examined the involvement of MOPs, DOPs, β-arr2 and c-Src in the inhibition by morphine of GABAergic inhibitory postsynaptic currents (IPSCs) recorded from neurons in the mouse ventral tegmental area. Morphine inhibited spontaneous IPSC frequency, mainly through MOPs, with only a negligible effect remaining in MOP−/− neurons. However, a reduction in the inhibition by morphine for DOP−/− c.f. WT neurons and a DPDPE-induced decrease of IPSC frequency revealed a role for DOPs. The application of the c-Src inhibitor, PP2, to WT neurons also reduced inhibition by morphine, while the inactive PP3, and the MEK inhibitor, SL327, had no effect. Inhibition of IPSC frequency by morphine was also reduced in β-arr2−/− neurons in which PP2 caused no further reduction. These data suggest that inhibition of IPSCs by morphine involves a β-arr2/c-Src mediated mechanism.

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Phototoxicity-induced pain is a major clinical problem triggered by light acting on photosensitising drugs or endogenous porphyrins, notably protoporphyrin IX (PpIX), an intermediary in heme biosynthesis. Protoporphyrin IX accumulates in individuals with erythropoietic protoporphyria and is elevated during photodynamic therapy subsequent to application of 5-aminolevulinic acid (ALA). Pain occurs during irradiation of PpIX and responds poorly to conventional analgesics. Our objective was to develop a model of PpIX phototoxicity pain and investigate the potential of menthol as an analgesic. Application of ALA to the tails of C57 black and SWISS white mice caused PpIX accumulation and nociception during irradiation (630 nm at 3.7 J/cm). Despite similar PpIX accumulation, C57 mice exhibited less pain behavior compared with SWISS mice because of light absorption by pigmentation. Irradiation of ALA-treated dorsal root ganglion neurons caused phototoxicity-evoked action potentials (APs) in both mouse strains. The antioxidant L-tryptophan increased the light dose required to elicit such APs. By contrast, the addition of keratinocytes to neuronal cultures decreased the threshold for APs, suggesting a requirement for proliferating cells. Inhibition of fatty acid amide hydrolase, selective antagonism of TRPV1 or the application of lidocaine or its quaternary derivative QX-314, reduced AP frequency, whereas antagonism of TRPA1 had no effect. These results suggest that products of singlet oxygen-mediated lipid peroxidation trigger nociceptor activation via TRPV1. Menthol inhibited phototoxicity-evoked APs and reduced pain behavior when applied topically to mice. These findings suggest that menthol might provide pain relief in patients experiencing PpIX-phototoxicity pain caused by photodynamic therapy or erythropoietic protoporphyria.

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