G protein-coupled estrogen receptor in the rostral ventromedial medulla contributes to the chronification of postoperative pain.

AIMS: Chronification of postoperative pain is a common clinical phenomenon following surgical operation, and it perplexes a great number of patients. Estrogen and its membrane receptor (G protein-coupled estrogen receptor, GPER) play a crucial role in pain regulation. Here, we explored the role of GPER in the rostral ventromedial medulla (RVM) during chronic postoperative pain and search for the possible mechanism. METHODS AND RESULTS: Postoperative pain was induced in mice or rats via a plantar incision surgery. Behavioral tests were conducted to detect both thermal and mechanical pain, showing a small part (16.2%) of mice developed into pain persisting state with consistent low pain threshold on 14 days after incision surgery compared with the pain recovery mice. Immunofluorescent staining assay revealed that the GPER-positive neurons in the RVM were significantly activated in pain persisting rats. In addition, RT-PCR and immunoblot analyses showed that the levels of GPER and phosphorylated µ-type opioid receptor (p-MOR) in the RVM of pain persisting mice were apparently increased on 14 days after incision surgery. Furthermore, chemogenetic activation of GPER-positive neurons in the RVM of Gper-Cre mice could reverse the pain threshold of pain recovery mice. Conversely, chemogenetic inhibition of GPER-positive neurons in the RVM could prevent mice from being in the pain persistent state. CONCLUSION: Our findings demonstrated that the GPER in the RVM was responsible for the chronification of postoperative pain and the downstream pathway might be involved in MOR phosphorylation.

Bile acids elicited endothelium-dependent vasoconstrictor hypo-activity through TRPV4 channels in the thoracic aorta of bile duct ligation rats.

Numerous studies have demonstrated the impaired cardiovascular reactivity in cholestasis patients and bile duct ligated animals. However, the underlying mechanism remains uncertain. Transient receptor potential cation V4 (TRPV4) channels are reported to be naturally expressed in the cardiovascular system, especially on endothelial cells. However, the role of TRPV4 (transient receptor potential vanilloid 4) in regulating vascular reactivity is poorly established. In this study, we first determined that bile acids elicited endothelium-dependent vasoconstrictor hypo-activity via TRPV4 channels, which further activated cyclooxygenase 2 (COX2). Myography results demonstrated that the vascular contractile response was attenuated in BDL rats when exposed to 60 mmol/L KCl. Real time PCR and western blotting results showed that bile duct ligation (BDL) induced a time-dependent increase in TRPV4 expression levels. In addition, bile acids upregulated the expression of TRPV4 protein, which proved to be located on the cell surface of endothelial cells, and induced intracellular Ca2+ events. The relaxation response was increased while the contractile response was decreased in BDL rats, and those effects were reversed by a TRPV4 inhibitor (HC067047). Contractions induced by norepinephrine were primarily inhibited by the COX2 inhibitor, but not the NOS inhibitor, and the expression of COX2 was downregulated after TRPV4 inhibition. These data indicated that TRPV4/COX2 pathways in the endothelium are involved in vasoconstrictor hypo-activity. Our current results suggested that the TRPV4 pathway is involved in the regulation of bile acids in vasoconstrictor hypo-activity in bile duct ligation rats.