Interference of pain control employing opioids in in vivo immunological experiments

Pain control (PC) in laboratory animals is supported by ethical as well as methodological considerations, aimed at preventing an interfering reduction in food and water intake and normalizing stress hormone levels. However, little is known about the immunomodulatory attributes of analgesics, which putatively prevents the routine implementation of PC in immunological research. In an established murine model of endotoxemia we investigated the immunomodulatory properties of common clinical analgesics (the opioids fentanyl and buprenorphine). Additionally, a literature study was conducted to investigate the frequency of PC in laboratory animals used for immunological experimentation. In line with various reports, we observed interactions between the opioid analgesics and the immune system that altered the outcome of performed in vivo immunological experiments. Of 100 evaluated publications, none mentioned the use of PC, indicating its uncommon implementation. In conclusion, more studies on the interactions between the immune system and analgesics are needed to establish better criteria for adequate implementation. Finally, we propose that methodological sections in scientific journals should clearly document whether or not PC was employed. If PC is not used, the reason for not using it should be stated.