Discrete Pain Behavior Without Systemic Inflammation in a Rat Osteotomy Model: A Platform for Analgesic Screening

BackgroundThis study aimed to determine if a rat osteotomy model elicits a measurable systemic response and to utilize this profile to evaluate the mechanism of action of preemptive analgesics with translational relevance to veterinary perioperative pain management. MethodsTwenty-five male rats were randomized into five groups: Sham, Surgery Control, Robenacoxib (2 mg/kg S.C.), Amantadine (30 mg/kg P.O.), and Combination. A femoral osteotomy was performed following ARRIVE 2.0 guidelines for refinement of surgical models. Serum levels of IL-6, PGE2, and cortisol were quantified via ELISA at baseline, 1-, 3-, and 6- hours post-surgery. Postoperative pain was assessed using the Rat Grimace Scale (RGS). ResultsThe osteotomy model did not induce a significant systemic inflammatory or stress response. Serum IL-6 and cortisol levels showed no significant changes over time (IL-6: p=0.219; Cortisol: p=0.187) or between groups. While PGE2 showed a temporal increase (F=6.52, p=0.001), it was unaffected by drug treatments. In stark contrast, the model successfully produced significant pain-related behaviors in the Control group (p=0.007), which were effectively reduced by both Robenacoxib (p=0.014) and amantadine (p=0.019) monotherapies. The combination group also showed significant pain reduction compared to Control at T6 (p=0.002), with an additive effect relative to monotherapies. Correlation analysis confirmed a dissociation between systemic biomarker levels and pain scores. ConclusionThe efficacy of Robenacoxib (a COX-2 inhibitor approved for veterinary use) and amantadine in the absence of altered systemic biomarkers suggests their analgesic actions are mediated predominantly through local or neurogenic pathways, with direct implications for optimizing perioperative analgesia protocols in companion animal orthopedic surgery.