Activation of GPR18 by Resolvin D2 Relieves Pain and Improves Bladder Function in Cyclophosphamide-Induced Cystitis Through Inhibiting TRPV1

PURPOSE: Hyperalgesia and bladder overactivity are two main symptoms of interstitial cystitis/bladder pain syndrome (IC/BPS). Cannabinoid receptors participate in the modulation of pain and bladder function. GPR18, a member of the cannabinoid receptor family, also participates in the regulation of pain and bladder function, but its underlying mechanisms are unknown. In this work, we sought to study the role of GPR18 in IC/BPS. METHODS: A rat model of IC/BPS was established with cyclophosphamide (CYP). Paw withdrawal threshold (PWT) measurement and cystometry were used to evaluate pain and bladder function, respectively. RT-PCR, Western blotting and immunofluorescence were used to assess the expression and distribution of GPR18. The role of GPR18 in pain and bladder function was studied by intrathecal injection of resolvin D2 (RvD2, a GPR18 agonist) and O-1918 (a GPR18 antagonist). Calcium imaging was used to study the relationship between GPR18 and TRPV1. RESULTS: A rat model of IC/BPS, which exhibited a decreased PWT and micturition interval, was successfully established with CYP. The mRNA and protein expression of GPR18 was reduced in the bladder and dorsal root ganglia (DRG) in rats with CYP-induced cystitis. Intrathecal injection of RvD2 increased the PWT and micturition interval. However, O-1918 blocked the therapeutic effect of RvD2. GPR18 was present in bladder afferent nerves and colocalized with TRPV1 in DRG, and RvD2 decreased capsaicin-induced calcium influx in DRG. CONCLUSION: Activation of GPR18 by RvD2 alleviated hyperalgesia and improved bladder function, possibly by inhibiting TRPV1 in rats with CYP-induced cystitis.