For many patients, chronic pain is often accompanied, and sometimes amplified, by co-morbidities such as anxiety and depression. Although it represents important challenges, the establishment of appropriate preclinical behavioral models contributes to drug development for treating chronic inflammatory pain and associated psychopathologies. In this study, we investigated whether rats experiencing persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA) developed anxiety-like behaviors, and whether clinically used analgesic and anxiolytic drugs were able to reverse CFA-induced anxiety-related phenotypes. These behaviors were evaluated over 28 days in both CFA-and saline-treated groups with a variety of behavioral tests. CFA-induced mechanical allodynia resulted in increased anxiety-like behaviors as evidenced by: 1) a significant decrease in percentage of time spent and number of entries in open arms of the elevated-plus maze (EPM), 2) a decrease in number of central squares visited in the open field (OF), and 3) a reduction in active social interactions in the social interaction test (SI). The number of entries in closed arms in the EPM and the distance travelled in the OF used as indicators of locomotor performance did not differ between treatments. Our results also reveal that in CFAtreated rats, acute administration of morphine (3 mg/kg, s.c.) abolished tactile allodynia and anxiety-like behaviors, whereas acute administration of diazepam (1 mg/kg, s.c) solely reversed anxiety-like behaviors. Therefore, pharmacological treatment of anxiety-like behaviors induced by chronic inflammatory pain can be objectively evaluated using multiple behavioral tests. Such a model could help identify/validate alternative potential targets that influence pain and cognitive dimensions of anxiety.

Apelin is the endogenous ligand of the APJ receptor, a member of the G-protein-coupled receptor family. The apelin-APJ complex has been detected in many tissues and is emerging as a promising target for several pathophysiological conditions. There is currently little information on the structure-activity relationship (SAR) of the apelin hormone. In an effort to better delineate SAR, we synthesized analogues of apelin-13 modified at selected positions with unnatural amino acids, with a particular emphasis on the C-terminal portion. Analogues were then tested in binding and functional assays by evaluating Gi/o-mediated decreases in cAMP levels and by assessing β-arrestin2 recruitment to the APJ receptor. The plasma stability of new compounds was also assessed. Several analogues were found to possess increased binding and higher stability than the parent peptide.

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Marchand (2018) Multicenter assessment of quantitative sensory testing (QST) for the detection of neuropathic-like pain responses using the topical capsaicin model,