Development and Characterization of a Novel, Anatomically Relevant Rat Model of Acute Postoperative Pain

Dara Bree, Orla Moriarty, Cliona M. O'Mahony, Bradley Morris, Karen Bannerton, Daniel C. Broom, John P. Kelly, Michelle Roche, David P. Finn

Acute postoperative pain remains a significant healthcare issue. Development of anatomically relevant animal models of postoperative pain, with improved predictive validity, would advance understanding of postoperative pain mechanisms and improve treatment outcomes. This study aimed to develop, characterise and validate a rat model of acute postoperative pain associated with inguinal hernia repair based on the Lichtenstein inguinal hernia repair procedure (without hernia induction). We hypothesised that the surgery would result in reduced spontaneous locomotor activity which would represent a pain-related phenotype. Post-surgical characterisation involved extensive monitoring of home cage and open field locomotor activity, as well as mechanical hypersensitivity and assessment of c-Fos expression in the dorsal horn of the spinal cord. In pharmacological validation studies, rats received morphine or carprofen 1h before, and/or immediately after, surgery. Rats that underwent hernia repair surgery exhibited significantly lower horizontal and vertical activity in the home cage and open field in the early post-surgical period, compared with sham rats or rats that underwent skin incision only. Morphine, carprofen and paracetamol attenuated the surgery-induced reductions in locomotor activity, to varying degrees. Surgery was associated with significantly increased c-Fos expression in the ipsilateral dorsal horn of the spinal cord, an effect attenuated by carprofen treatment. These results support the development and characterisation of a novel, anatomically relevant model of acute postoperative pain which may facilitate development of improved treatment regimes.