| Summary: | The δ-opioid receptor (DOR) shows potential as a therapeutic target for analgesia. DOR-targeting pharmaceutics may lead to fewer side effects than conventional opioid drugs such as morphine due to relatively low expression of DOR in the central nervous system (compared to the main target for morphine, the u-opioid receptor). The analgesic efficacy of DOR agonists increases following inflammation and receptor insertion has been suggested as a possible mechanism for this. Modulation of membrane expression of functional DOR receptors may be useful in the development of analgesic drugs. Currently, expression of functional DOR at the membrane of sensory neurons is controversial. Here, patch-clamp recordings and total internal reflection fluorescence (TIRF) microscopy have been used to study functional expression and trafficking of DOR in sensory neurons from rat trigeminal ganglia (TG). In addition, the role of inflammatory mediator bradykinin (BK) in DOR membrane expression has been investigated. To determine whether neurons express functional DCR, inhibition of voltage-gated Ca2+ channels (VGCC) by DCR agonist [D-Ala2, D- Leu5]-Enkephalin (DADLE) was determined. DADLE inhibited VGCC in 23% of TG neurons by 25.3 ± 5%. Pre-treatment with BK increased the population of DCR-positive neurons to 54%, but did not significantly affect the degree of VGCC inhibition by DADLE. Real-time TIRF microscopy revealed that BK treatment caused robust trafficking of DCR to the plasma membrane in neurons transfected with GFP-tagged DOR. In contrast, DADLE and TRPV1 agonist capsaicin caused a decrease in membrane abundance of DOR, suggesting internalisation of the receptor. In Ca2+ imaging experiments 80% of cultured TG neurons responded to BK, thus, these data suggest that a majority of BK-responsive TG neurons have the potential to become DOR-positive upon inflammation, re-establishing the therapeutic potential of peripheral DOR. In a separate line of investigation, enhancement of T-type VGCC in nociceptive neurons by BK has been discovered.
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