https://phaidra.vetmeduni.ac.at/o:4777 [¹⁸F]Fluspidine PET/CT imaging to assess postoperative pain-associated σ1 receptor expression in female rats under analgesia Girbig, Renée M. Renée M. Girbig Tix, Leonie Leonie Tix Hartmann, Anna M. Anna M. Hartmann Liu, Wenjia Wenjia Liu Paschenda, Pascal Pascal Paschenda Florea, Alexandru Alexandru Florea Sadeghzadeh, Masoud Masoud Sadeghzadeh Becker, Karolin Karolin Becker Palme, Rupert Rupert Palme 0000-0001-9466-3662 University of Veterinary Medicine Vienna Mottaghy, Felix M. Felix M. Mottaghy Tolba, René René Tolba Kiessling, Fabian Fabian Kiessling 0000-0002-7341-0399 Rix, Anne Anne Rix Baier, Jasmin Jasmin Baier Springer journal article open access eng Background Pain assessment in animal models is challenging, as behavioral tests often lack sensitivity. Particularly under analgesia, it is unclear whether pain occurs without medication. Imaging of pain-associated pathways, such as σ1 receptor (σ1R) expression, offers a promising approach to better understand underlying mechanisms. Therefore, this study evaluated [¹⁸F]fluspidine positron emission tomography/computed tomography (PET/CT) imaging for detecting σ1R-mediated pain after partial liver resection in rats. Materials and methods Postoperative pain was assessed in eighteen female Wistar rats undergoing skin incision or partial liver resection. Nine untreated rats served as controls. Carprofen was administered for three consecutive days after surgery. PET/CT imaging was performed on postoperative days 1, 4, and 7. At each time point, organs and incision sites of three animals were harvested for histological analysis. Postoperative pain and welfare were monitored by observational score sheets, the Open Field test, Rat Grimace Scale, Von Frey test, fecal corticosterone metabolites, and hemograms. Results Despite analgesic treatment, PET/CT and immunohistochemistry revealed elevated σ1R expression at the abdominal incision site on day 1 after partial liver resection in comparison to the other groups, likely due to the additional peritoneal opening. σ1R expression normalized by day 4. No behavioral indicators of pain or distress were observed, though mechanical hypersensitivity was detected on day 4 in all groups, likely due to carprofen side effects. Conclusion [18F]Fluspidine PET/CT imaging sensitively detected postoperative pain-associated σ1R expression independent of analgesia. This imaging modality could remarkably refine pain monitoring, opening to further studies using different pain and analgesia models. Relevance statement [¹⁸F]Fluspidine PET/CT imaging demonstrates high sensitivity in detecting pain-associated σ1R upregulation despite non-steroidal anti-inflammatory drug administration. This approach offers valuable insights for refining pain assessment, improving severity grading, and enhancing the reliability and translational value of preclinical pain models. [18F]fluspidine Pain Measurement Positron Emission Tomography Computed Tomography Rats (Wistar) Sigma-1 receptor http://creativecommons.org/licenses/by/4.0/ https://phaidra.vetmeduni.ac.at/api/object/o:4777/download 10.1186/s41747-025-00646-2 https://phaidra.vetmeduni.ac.at/o:605 application/pdf European Radiology Experimental issn:2509-9280 VoR European Radiology Experimental 9 1 1.36 MB 2025