Bradykinin-induced functional competence and trafficking of the δ-opioid receptor in trigeminal nociceptors

Peripheral opioid analgesia is increased substantially after inflammation. We evaluated the hypothesis that an inflammatory mediator, bradykinin (BK), evokes functional competence of the δ-opioid receptor (DOR) for inhibiting trigeminal ganglia (TG) sensory neurons. We also evaluated whether BK evokes trafficking of the DOR to the plasma membrane. Rat TG cultures were pretreated with BK(10 μM) or vehicle, and the effects of DOR agonists ([D-Pen ^2,5]-enkephalin or [D-Ala², D-Leu⁵]-enkephalin) on BK (10 μM)/prostagladin E2 (PGE2; 1 μM)-stimulated immunoreactive calcitonin gene-related peptide (iCGRP) release or PGE2 (1 μM)-stimulated cAMP accumulation were measured. The effect of BK treatment on opioid receptor trafficking was evaluated by DOR immunohistochemistry, cell-surface DOR biotinylation, and live imaging of neurons transfected with mDOR-green fluorescent protein. BK pretreatment rapidly and significantly increased DOR agonist inhibition of evoked iCGRP release and cAMP accumulation. These effects of BK pretreatment were blocked by a B₂ receptor antagonist (HOE-140; 10 μM) or a protein kinase C (PKC) inhibitor [bisindolymaleimide (BIS); 1 μM]. Moreover, BK treatment rapidly and significantly increased the accumulation of DOR in the plasma membrane. However, BK-induced trafficking of DOR was not reversed by pretreatment with BIS, nor was trafficking evoked by application of a PKC activator PMA (200 nM). These data suggest that BK, in a PKC-dependent manner, induces rapid functional competence of DOR for inhibiting TG nociceptors and in a PKC-independent manner rapidly induces trafficking of DOR to the plasma membrane. These findings indicate that exposure to certain inflammatory mediators rapidly alters the signaling properties and neuronal localization of DOR, possibly contributing to peripheral opioid analgesia.