GPR4 and arthritic joint disease: Studies have shown that GPR4 antagonists reduce inflammation in the antigen-induced arthritis rat model, the DSS-induced acute colitis mouse model, and the short-term emphysema-exacerbation COPD mouse model as well as alleviate inflammatory pain in the complete Freund's adjuvant-induced hyperalgesia rat model (15, 17, 18, 29).