Introduction: More than half of the patients who undergo surgery experience pain despite analgesic treatment. Therefore, new strategies for the treatment of postoperative pain are needed. Since laparotomy is common in abdominal surgeries, studying laparotomy-induced pain in mice could provide valuable insights.

Methods: We used female CD-1 mice. Laparotomy consisted of a 1.5 cm horizontal incision to gain access to the abdominal cavity. We studied three aspects of postoperative pain: movement-induced pain using infrared actimetry; pain at rest, analysing the facial expres- sions of the mice using artificial intelligence, and sensory hypersen- sitivity using the vonFrey test. We also studied the recruitment of immune cells to the surgical wound using fluorescence activated cell sorting (FACS).

Results: The highest nociceptive response after laparotomy was observed 3.5 hours after surgery in all parameters evaluated. Mechanical threshold showed some improvement by day 6, and returned to normal levels by day 10. Both pain at rest and move- ment-induced pain disappeared within 24 hours after surgery. The predominant immune cells in the surgical wound during the immediate postoperative period were neutrophils. Inhibition of neutrophilic infiltration, by administration of an anti-Ly6G, had a limited effect on mechanical allodynia, while it induced a full reversal of pain at rest and movement-induced pain. Several days after surgery, when no pain at rest or movement-induced pain was detected but a prominent allodynia was still found, the predominant immune cell present in the surgical site were macrophages/ monocytes.

Conclusions: Laparotomy in mice is a useful model for investi- gating different aspects of postoperative pain. Neutrophils appear to be involved in pain at rest, while macrophages/monocytes might be more related with tactile allodynia. Therefore, the different pain modalities after surgery might differ on their pathophysiological mechanisms.